Electrooxidation of Pd–Cu NP loaded porous carbon derived from a Cu-MOF

The development of new non-platinum catalysts for alcohol electrooxidation is of utmost importance. In this work, a bimetallic Pd–Cu loaded porous carbon material was first synthesized from a Cu-based metal–organic framework (MOF). The Cu loaded porous carbon was pre-synthesized through calcinating the Cu-based MOF under a N₂ atmosphere. After loading Pd onto the precursor and heating, Pd–Cu loaded porous carbon (Pd–Cu/C) was obtained for alcohol electrooxidation. Electrooxidation experiments revealed that this Pd–Cu bimetal loaded porous carbon assisted steady state electrolysis for alcohol oxidation in alkaline media. Moreover, different alcohols were electrooxidated using the present electrocatalyst for the purposes of discussing the oxidation mechanism. This electrooxidation study of Pd–Cu/C derived from a MOF demonstrates a good understanding of the electrooxidation of different alcohols, and provides useful guidance for developing new electrocatalyst materials for energy conversion and electronic devices.

We have synthesized Pd–Cu NP loaded porous carbon through the direct carbonization of a porous Cu based MOF for efficient electrooxidation.

There is an immediate need to develop direct alcohol fuel cells (DAFCs), which have been proven to be a fine source of energy, which could probably replace fossil fuels use to fulfil global energy demand. As one of the most significant electrocatalytic procedures, the electrooxidation of alcohols is an important process in DAFCs, and has gathered much attention and is attractive, due to high power density output and low pollutant emissions. Generally, Pt based materials are the most common electrocatalysts for alcohol electrooxidation reactions. However, the high cost and limited supply of Pt severely restricts its commercial application. Therefore, the development of new efficient and inexpensive non-platinum alternative materials to Pt-based catalysts is of utmost importance.Metal–organic frameworks (MOF) are assembled from metal ions linked by organic ligands, and are used in catalysis, guest molecule storage/separation, fluorescence, sensors and other devices.^1–3 Due to their highly ordered porous structures and large surface areas, MOFs can also be used as templates/precursors for preparing porous carbon materials through thermal treatment.^4–8 Several MOF derived carbon materials with good electrical conductivity are reported to show effective electrocatalytic performance,⁹ such as in the oxygen evolution reaction (OER),¹⁰ hydrogen evolution reaction (HER),¹¹ and oxygen reduction reaction (ORR).¹² Recently, a zeolitic imidazolate framework ZIF-8 was calcinated in order to prepare porous carbon with both micro- and meso-pores to support Pd electrocatalysts for methanol electrooxidation.¹³ Unfortunately, this could not efficiently limit the use of the noble metal, which gives challenges to scientists for further exploration.It has been demonstrated that the alloying of noble metals with transition metals has been used for the enhancement of catalytic activity and reduction of cost,¹⁴ because of the low cost and relatively high abundance of transition metals. The use of alloyed metal, Pd–M (where M is Cu, Co or Ni), binary electrocatalysts has been reported for effectively improving the catalytic properties.^15–17 For this purpose, introducing a non-noble metal into a noble metal bimetallic system will fulfil the demand for a new catalyst and become an area of interest nowadays. The second metal (such as Cu) will behave as a donor, while Pd has an empty d orbital to accept electrons, and it is assumed that its electronic properties will be more similar to Pt.¹⁸ Therefore, Pd–Cu bimetal loaded porous carbon derived from a Cu-MOF can provide a good electrocatalyst for alcohol oxidation.In this work, as shown in Scheme 1, we have synthesized Cu loaded porous carbon through the direct carbonization of a porous Cu based MOF. After loading Pd onto the precursor and heating, Pd–Cu bimetal loaded porous carbon (Pd–Cu/C) was obtained. Chronoamperometric studies revealed that this Pd–Cu bimetal loaded porous carbon assisted steady state electrolysis for alcohol oxidation in alkaline media. In addition, alcohols with different numbers of carbon atoms (such as ethanol, 1-propanol and 2-propanol) were also investigated for electrooxidation. It is important to note that the effectiveness of this bimetallic NP loaded carbon means that it can serve as a catalyst for the electrooxidation of low-molecular weight alcohols, which probably can be used as energy sources in portable electronic devices.Open in a separate windowScheme 1The preparation procedure for Pd–Cu/C derived from HKUST-1 and PdCl₂: (a) MOF HKUST-1; (b) Cu/C calcinated from HKUST-1; (c) PdCl₂ loaded on Cu/C; and (d) the Pd–Cu/C material.Here, a 3-D MOF, HKUST-1 (also called Cu₃(BTC)₂, BTC = 1,3,5-benzenetricarboxylate), was chosen as the precursor for preparing the Cu/C material, due to the structure having high porosity and it being a rich Cu source. The as-synthesized HKUST-1 was calcinated at 700 °C for 5 h under a N₂ atmosphere, and the Cu/C material was obtained. In addition, the guest species PdCl₂ was loaded onto the calcinated HKUST-1 through immersing the pre-calcinated HKUST-1 into a PdCl₂ ethanolic solution (1 mM) for 2 h (Scheme 1). The PdCl₂ loaded Cu/C (PdCl₂@Cu/C) was heated at 300 °C for 1 h under a N₂ atmosphere. Finally, an alloy of Pd and Cu loaded porous carbon material (Pd–Cu/C) was obtained and characterized through powder XRD, BET and XPS analyses.The PXRD data (Fig. 1a) from as-synthesized HKUST-1 powder and the bimetallic Pd–Cu NP loaded carbon porous material derived from HKUST-1 show that the samples contain bimetallic palladium and copper mostly. The XRD peak appearing at 43.3° corresponds to the (fcc) (111) facet plane of Cu. Due to Pd being dispersed homogenously at a low concentration through the sample, the XRD pattern could not display the obvious peak from Pd. However, inductively coupled plasma emission spectroscopy (ICP) data (Table S1^†) from the sample showed 0.76% Pd and 36.68% Cu, indicating the existence of Pd and Cu. The porosity of Pd–Cu/C was demonstrated through BET data, which shows N₂ adsorption of ∼150 cm³ g⁻¹. The Pd XPS spectrum showed two definite peaks at 335.5 and 341 eV, respectively assigned to 3d_5/2 and 3d_3/2 and matching well with Pd⁰. XPS peaks at 932.4 and 952.1 eV indicate the valence states of Cu ions in the Cu 2p_3/2 and Cu 2p_1/2 orbitals in the Pd–Cu/C material. Cu²⁺ is present in the porous carbon material, with respective peaks at 933.7 eV and 934.4 eV from CuO and Cu(OH)₂, with a prominent satellite observed in the 938–946 eV range. A few Pd²⁺ ions also exist in the sample due the easy oxidation of the surface. The Raman spectrum of Pd–Cu/C (Fig. S4^†) shows typical graphitic carbon. The results of the characterization studies clearly reveal that the nanoparticles have a Pd and Cu bimetallic nature.Open in a separate windowFig. 1(a) XRD data from HKUST-1 and Pd–Cu/C; (b) N₂ sorption isotherms for Pd–Cu/C; and XPS data from (c) Pd and (d) Cu in a sample of Pd–Cu/C.SEM images with EDS (Fig. 2a and b) results show that the sample contained much more copper than palladium, which clearly suggests that the presence of copper in the sample would probably be the reason for the expected electrooxidation of alcohols. It could be possible to replace the use of high-cost Pd or Pt based catalysts. The morphology of the Pd–Cu NPs was further characterized via TEM imaging and TEM element mapping (Fig. 2c, d and S5^†), demonstrating that the nano-sized NPs were dispersed homogeneously. The HR-TEM image in Fig. 2c gives insight into the bimetallic nature of the synthesized nanoparticles, with two noticeable lattice fringes (0.225 nm for Pd(111) and 0.202 nm for Cu(111)). The mean size of the Pd–Cu NPs was 7.38 nm, as shown in Fig. 2d. The homogenous distribution, with well-defined bimetallic Pd–Cu based carbon material, was good for the electrooxidation of alcohols. The electrochemical active surface area (ECSA) for Pd–Cu/C was high compared with commercial Pd/C, which suggested that the synthesized Pd–Cu/C has ample available surface area, mainly because of synergistic effects from the Cu-MOF based carbon material and the morphology of the electrocatalyst.Open in a separate windowFig. 2(a) SEM image of and (b) EDS data from Pd–Cu/C; (c) a TEM image of Pd–Cu NPs in the hybrid carbon material; and (d) the size distribution of the Pd–Cu NPs.In Fig. 3, CV profiles for commercial Pd/C and the presented Pd–Cu/C show two distinct peaks (forward (iF) and backward (iB) peaks) during the oxidation of methanol-containing 1 M KOH solution. The peak at −0.37 V indicates the oxidation of aforementioned carbonaceous species, such as Pd–CO_ads, along with newly formed alcohol adsorbates, following the removal of surface intermediates at lower potentials.¹⁹ For the forward peak potential, a shift in the iF value is observed, mainly because of Cu existing with Pd in the material. This results in the oxidation of poisonous species, such as Pd adsorbed CO, at higher potentials,²⁰ leading to such high activity. ATR-IR (Fig. S7^†) and GC analyses (Fig. S8^†) show the methanol oxidation reaction (MOR) pathway during the formation of the final CO₂ product. The catalytic activity of Pd–Cu/C is found to be ∼13 times higher than commercial Pd/C for methanol oxidation, demonstrating that the presence of Cu with Pd in Pd–Cu based catalysts increases CO oxidation because of a strong binding ability. Cu binds to CO more strongly than Pd, as a result of electronic structure differences,^21,22 thus preventing the electrode from undergoing CO poisoning, a major issue for Pd-based catalysts during the methanol oxidation reaction (MOR). The mechanism of methanol oxidation is shown in the ESI (eqn (8)–(10)).^† The high iF value for Pd–Cu/C can be ascribed to the fast formation of reactive intermediates, such as Pd–CH₂OH, Pd–COOH, Pd–H, Pd–(CHO)_ads, and Pd–(COOH)_ads.^23–25 The removal of these intermediates is necessary for a high current density. Furthermore, formaldehyde (HCHO), formic acid (HCOOH) and CO₂ would be the final products in the MOR.^26,27 Pd–Cu/C has good catalytic activity for the MOR, leading to further investigation into the electrooxidation of different alcohols, such as ethanol, 1-propanol, and 2-propanol (Fig. 4 and Table S2^†).Open in a separate windowFig. 3CV curves from Pd/C and Pd–Cu/C electrocatalysts during CH₃OH (1 M) oxidation in 1 M KOH solution, at a scan rate of 50 mV s⁻¹, at room temperature.Open in a separate windowFig. 4(a) CV curves from: the Pd–Cu/C electrocatalyst for C₁–C₃ aliphatic alcohol (1 M) oxidation in KOH (1 M) solution; and (b) Pd–Cu/C in 1 M EtOH, 1-propanol and 2-propanol at a scan rate of 50 mV s⁻¹ at room temperature.The current densities for different alcohol oxidation processes are summarized in Fig. 4a and b. The normalized iF (calculated using Pd mass) for the MOR (∼4643 mA mg⁻¹) was higher than for three other alcohols, i.e., it was ∼139, ∼94 and ∼26.5 mA mg⁻¹ for ethanol, 1-propanol and 2-propanol, respectively. The iF/iB ratio for methanol is ∼12 times higher than that for ethanol, ∼13 times that for 1-propanol and ∼4 times that for 2-propanol. The reactivity order for Pd–Cu/C is methanol > ethanol > 1-propanol > 2-propanol. 2-Propanol electrooxidation showed a lower current density on a Pd–Cu/C electrode in alkaline medium, although iF/iB is ∼5.4. The negative shift in the onset of the ethanol oxidation reaction (EOR) suggested that a high copper content with very low amount of Pd was suitable for EOR kinetics using a Pd–Cu/C catalyst. The ethoxy (CH₃CO)_ads was strongly adsorbed, and blocked hydrogen absorption/adsorption. The current intensity of iF increased due to the formation of fresh Pd–OH, through stripping carbonaceous residue from the electrode (eqn (11)–(14)^†). In addition, the increased current at high potentials sharply reached the largest value then started to decline, because a PdO layer formed on the electrode, blocking the further adsorption of reactive species.²⁸ ATR-IR spectra (Fig. S9^†) show the presence of CO₂ and CO_ads, whereas bands appear at 1670 cm⁻¹ and 1390 cm⁻¹ because of the formation of acetic acid.²⁹ The Pd–Cu/C electrocatalyst has the potential to oxidize the intermediate to the final product, CO₂, during EOR to some extent; Cu promotes oxidation through increasing the production of OH_ads⁻/H₂O to eliminate the intermediate CH₃CO_ads simultaneously on Pd.³⁰ The stability of Pd–Cu/C in all four alcohols (methanol, ethanol, 1-propanol and 2-propanol) was studied using chronoamperometry at a potential of −0.25 V, as shown in Fig. S2.^† The slow current decay showed that the stability of the Pd–Cu/C electrocatalyst in methanol is best. Comparing the results, the current for methanol oxidation was higher than that for the other three alcohols. However, the oxidation currents from ethanol and 1-propanol were larger than that from 2-propanol. This suggested that Pd–Cu/C is less stable and shows lower anti-poisoning ability during 2-propanol oxidation in an alkaline medium.During this oxidation, 1-propanol oxidizes to propanal first, and its further oxidation results in the formation of a stable product, propanoic acid (Scheme S1^†). 1-Propanol is converted, with its carboxylate as the major product, as verified using ATR-IR (Fig. S10^†). 2-Propanol forms acetone as an intermediate product, leading to the poisoning of the electrode.³¹ ATR-IR spectra (Fig. S11^†) of Pd–Cu/C also confirm that the electrocatalyst follows a dual pathway through acetone and propene intermediates to oxidize to CO₂ finally (Scheme S2^†).^32–34 However, acetone formation is kinetically favored.³⁵ The results show that the location of the –OH group in the alcohol influences the electrooxidation reaction kinetics. In contrast methanol oxidation using Pd–Cu/C has much higher catalytic activity than ethanol, 1-propanol and 2-propanol oxidation, which makes it a good candidate for direct methanol fuel cells.In summary, we have first synthesized a bimetallic Pd–Cu NP loaded porous carbon material from a Cu-based MOF for alcohol electrooxidation. The Cu loaded porous carbon was pre-synthesized by calcinating the Cu-based MOF HKUST-1 under a N₂ atmosphere. Afterwards, Pd–Cu NP loaded porous carbon was obtained for alcohol electrooxidation. Electrooxidation experiments revealed that Pd–Cu/C was suitable for steady state electrolysis for alcohol oxidation in alkaline media. In addition, different alcohols were electrooxidated using the present electrocatalyst to discuss the oxidation mechanism. This electrooxidation study of Pd–Cu/C derived from a MOF offers good understanding into the electrooxidation of different alcohols and it could provide useful guidance for the development of new electrocatalyst materials.     

Highly efficient hydrogen peroxide direct synthesis over a hierarchical TS-1 encapsulated subnano Pd/PdO hybrid

We report a hierarchical TS-1 encapsulated subnano Pd/PdO hybrid catalyst that shows unprecedented activity in H₂O₂ direct synthesis from H₂ and O₂. The macro reaction rate in 30 min is up to 35 010 mmol g_Pd⁻¹ h⁻¹ at ambient temperature. Such high catalytic activity is achieved due to the hierarchical porous structure of TS-1 and the formation of the encapsulated subnano Pd/PdO hybrid after oxidation/reduction/oxidation treatment.

A hierarchical TS-1 encapsulated subnano Pd/PdO hybrid catalyst that shows unprecedented activity in H₂O₂ direct synthesis from H₂ and O₂.

Hydrogen peroxide as a clean and strong oxidant is one of the commonly used chemicals in various fields of chemical industry, such as the pulp and paper industry, the textile industry, wastewater treatment, green chemical synthesis metallurgy, electronics manufacture, propulsion and the food industry.¹ Compared to the traditional anthraquinone process (sequential hydrogenation and oxidation of alkyl anthraquinone), the direct synthesis of hydrogen peroxide (DSHP) from hydrogen and oxygen was recognized as an efficient and environmental alternative process owing to its remarkable adherence to green chemistry perspectives, such as low energy consumption, minimized toxicity and infrastructure investment.^2–5Pd supported catalysts were the most extensively and earliest studied catalysts for the DSHP since 1914.⁶ Both DFT and experimental results indicated that subnano Pd particles were most effective for the selective oxygen hydrogenation to hydrogen peroxide,⁷ and the activity and selectivity are also highly dependent upon the oxidation state of the Pd particles.⁸ However, there were limitations in applying Pd nanoparticles catalyst to the reaction due to the thermal vulnerability in a calcination and reduction activation process.⁹ To solve this problem, many preparation methods have been adopted to stabilise Pd nanoparticles and control the particle size and morphology, such as yolk–shell structure,¹⁰ core–shell structure¹¹ and other encapsulation structure supports. But there were still problems that the size of metal particles is larger than 2.5 nm. Encapsulation of Pd species by mercaptosilane-assisted dry gel conversion (DGC) synthesis method can provide a precise control over the nanoparticle size as well as limitating the aggregation under high temperature during activation.¹² However, active sites deep inside the encapsulated nanoparticles were often hardly accessible since the internal configuration diffusion limitations of reactants and products in micropores, leading to low H₂ conversion and decomposition of the long residence time of synthetic H₂O₂.¹³ So, the role of the porous structured catalyst was essential for encapsulated metal nanoparticles.Titanium silicalite-1 (TS-1) has already been used as an excellent catalyst for a variety of selective oxidation reactions employing hydrogen peroxide as oxidant.^14,15 Moreover, in situ H₂O₂ generation coupled with these selective oxidation reactions leading to the desired products such as propylene,^16,17 benzyl alcohol,¹⁸ cyclohexene¹⁹ was a desirable, green and lower cost route. More importantly, the Ti–OOH species formed on the TS-1 during selective oxidation might improve the stability of OOH, which is a key reaction intermediate during the DSHP.²⁰ Hutchings et al. reported that hierarchical titanium silicalite supported Au–Pd catalysts showed high peroxide production rate and benzaldehyde production rate for oxidation of benzyl alcohol by in situ generated H₂O₂.²¹ In this report, the encapsulation of subnano-sized Pd metal particles within conventional (Pd@TS-1) and hierarchical titanium silicalite-1 (Pd@HTS-1) has been achieved (see Scheme 1). The Pd@HTS-1 catalyst after oxidation–reduction–oxidation pre-treatment showed unprecedented activity in direct synthesis of hydrogen peroxide from hydrogen and oxygen under ambient temperature without any promoter.Open in a separate windowScheme 1Schematic diagram of the preparation method for Pd@HTS-1.The TS-1 and HTS-1 encapsulated Pd sub-nanoparticles were first synthesized via solvent evaporation-assisted dry gel conversion method, where the Pd was encapsulated in situ through hydrothermal crystallization in assistance of 3-mercaptopropyl-trimethoxysilane (Scheme 1). The results of ICP analysis confirmed that total Pd contents in Pd@TS-1 and Pd@HTS-1 were 0.094 and 0.106 wt%, respectively. The characteristic diffraction “finger peak” on the X-ray diffraction in Fig. S1^† proved that the TS-1, Pd@TS-1 and Pd@HTS-1 had a well-crystallized MFI structure,²² which was further confirmed by the asymmetric stretching of Si–O–Ti in the spectra of Fourier Transform Infrared Spectroscopy (FT-IR, see Fig. S2^†). For all of the samples, the diffraction peak at 2θ of 25.4° was not observed. Meanwhile, the diffraction peak of crystalline Pd was also not detected for Pd@HTS-1 and Pd@TS-1, indicating that the Pd particles were well dispersed in the zeolite.⁷ Besides, the diffuse reflectance UV-vis spectra of the TS-1, Pd@TS-1 and Pd@HTS-1 were shown in Fig. S3.^† The band at 210 nm in three samples confirmed the tetrahedral structural geometry of Ti in these silicates, and the weak band at 280 nm was assigned to small amounts of penta/hexacoordinated Ti species.²³ Moreover, the absorption band around 300 nm indicated that the three samples contain anatase TiO₂.²⁴The textural properties of the synthesized Pd@TS-1 and Pd@HTS-1 were characterized by N₂ adsorption/desorption and the results were shown in Fig. 1 and Table S1.^† Notably, typical irreversible type IV adsorption isotherms with an H1 hysteresis loop were observed over the Pd@HTS-1 sample (Fig. 1b), indicating the presence of a mesoporous structure. The mesopore size of Pd@HTS-1, obtained through the BJH method, and the obtained graph peaked at about 7.0 nm. Volume of the micropores was around 0.14 cm³ g⁻¹ for both Pd@TS-1 and Pd@HTS-1, but the surface area of Pd@HTS-1 (509.9 m² g⁻¹) was 48.9 m² g⁻¹ larger than that of Pd@TS-1 (461.0 m² g⁻¹) due to its mesoporous structure, which is beneficial for the diffusion of reactants and products through the catalysts.²⁵Open in a separate windowFig. 1Nitrogen adsorption–desorption isotherms of the synthesized TS-1: (a) Pd@TS-1 and (b) Pd@HTS-1.Comparison between the experimentally obtained results from ammonia temperature-programmed desorption (NH₃-TPD) analysis (Fig. S4^†) and the previously reported data showed that the peaks observed were corresponding to weak acid sites, medium acid sites, and strong acid sites of the catalysts.²⁶ Furthermore, pyridine adsorption peak on the FT-IR spectra of these samples (Fig. S5^†) revealed that titanium silicate (TS-1) was an acidic support with a large number of Lewis acid segments and few Brønsted acid segments. As shown in scanning electron microscopy (SEM) image (Fig. 2), Pd@TS-1 particles were crystallites with a morphology close to cuboids and a mean particle size of about 3–5 μm, while the Pd@HTS-1 has spherical morphology with a particle size of about 1.3 μm.Open in a separate windowFig. 2SEM images of the synthesized Pd-modified TS-1: (a) Pd@TS-1 and (b) Pd@HTS-1.The synthesized TS-1 and HTS-1 encapsulated Pd sub-nanoparticles were then subjected to oxidation/reduction/oxidation treatment to adjust the valence states of Pd.²⁷ Such heat treatment cycle can switch off the sequential hydrogenation and decomposition reactions in the DSHP. However, Ostwald ripening, thus the migration and coalescence of metal clusters, will occur at a higher temperature. Therefore, high temperature treatments was used to emulate the conditions used in the literature mentioned before,^28,29 and the thermal stability of the encapsulated Pd@TS-1 catalysts before and after the treatments were also evaluated and compared to investigate the effect of high temperature and the thermal treatments on the catalysts. The Pd@TS-1 and Pd@HTS-1 samples after an air/H₂/air thermal treatments at 500/400/500 °C for 4/2/6 h were denoted as Pd@TS-1-O, Pd@TS-1-OR, Pd@TS-1-ORO, Pd@HTS-1-O, Pd@HTS-1-OR, Pd@HTS-1-ORO respectively with O denoting oxidation and R denoting reduction. The Pd particle size distribution after such treatments was first released by the high-resolution transmission electron microscopy (HRTEM) image in Fig. 3 and S6.^† The Pd particles encapsulated within microporous TS-1 zeolites were well dispersed and uniformly distributed throughout the zeolite crystals. The average sizes of Pd particles encapsulated in the TS-1 and HTS-1 were in the range of 1–2 nm, which, however, was bigger than those of the MFI topology channels (0.53 × 0.56 nm) and intersectional channels (∼0.9 nm). Nevertheless, the successful encapsulation of the Pd particles in the TS-1 zeolites was verified by comparing the hydrogenation rates of a mixture of nitrobenzene and 1-nitronaphthalene. As shown in Fig. S7,^† the reaction rate for the hydrogenation of nitrobenzene and 1-nitronaphthalene was much higher over the Pd@HTS-1-OR compared to the Pd@TS-1-OR. We anticipated that the slightly larger Pd size than the zeolite channels might reflect the local disruption of the crystal structures near the location of the particles during the in situ synthesis. More detailed size distributions of Pd particles encapsulated in the TS-1 and HTS-1 zeolites after air, Ar/H₂ and air treatments were shown in Fig. 3d–f and j–l, respectively. The particle sizes of most of the Pd species still remain below 2 nm on average, which indicated the absence of metal clusters migration and coalescence by Ostwald ripening even after such higher temperature treatments. The high thermal stability of the Pd subnano particles resulted from the embedding confinement.³⁰Open in a separate windowFig. 3HRTEM images and metal particle size distributions of the Pd@TS-1 and Pd@HTS-1 before and after high-temperature oxidation–reduction–oxidation treatments. (a, d Pd@TS-1-O. b, e Pd@TS-1-OR. c, f Pd@TS-1-ORO. g, j Pd@HTS-1-O. h, k Pd@HTS-1-OR. i, l Pd@HTS-1-ORO.)The Pd dispersion and average Pd nanoparticle size for Pd@TS-1 and Pd@HTS-1 after the air/H₂ treatment were further determined by CO chemisorption measurements (see Table S2^†). The dispersions of Pd in Pd@TS-1 and Pd@HTS-1 are 85% and 81%, respectively. The average Pd particle sizes for Pd@TS-1 and Pd@HTS-1 calculated by CO adsorption measurements are 1.06 nm and 1.17 nm, respectively, which was smaller than that estimated from the TEM analysis. This was probably due to the presence of Pd nanocluster or single atoms, which cannot be directly observed by HRTEM.We now turn to the Pd valence states of the catalysts after the oxidation/reduction/oxidation treatment by the XPS (see Fig. S8^†). The Pd3d spectra signals were hardly observed when the concentration of Pd atoms was low, the binding energy peaks for different oxidation states of Pd atoms were collected after peak fitting by prolonging the scanning time.³¹ The XPS results demonstrated the presence of both metallic Pd and PdO. The binding energy of peaks for Pd⁰3d_5/2 and Pd⁰3d_3/2 correspond to 335.5 and 340.6 eV, respectively, while the binding energy for Pd²⁺3d_5/2 and Pd²⁺3d_3/2 were at 337.8 and 341.9 eV, respectively.³¹ The transformation of valence state could be observed in Fig. S8a–c,^† which was derived from XPS measurements. Moreover, the ratios for Pd⁰ and Pd²⁺ atoms in Pd@TS-1 and Pd@HTS-1 were approximately 2 and 1, respectively. On the basis of these results, we proposed a reaction mechanism for the synthetic process of the catalysts, subnano-sized Pd particles might be oxidated from Pd⁰ to Pd²⁺ to form PdO on the surface of the catalysts during reoxidation.The catalytic performance of the TS-1 and HTS-1 encapsulated subnano-sized Pd/PdO hybrid in the direct synthesis of hydrogen peroxide from H₂ and O₂ were tested at ambient temperature without any promoters. Compared to the Pd supported by the active carbon, the selectivity of hydrogen peroxide was higher, the reason might be the formation of Ti–OOH³² and the confinement effect of the Pd encapsulated in the channel of the zeolite (Scheme 2). Both HTS-1 zeolite and Pd@zeolites showed significant amount of O₂ adsorption according to the O₂-TPD (Fig. S9^†), which might be the reason for high activity/selectivity. The selectivity for hydrogen peroxide on Pd@TS-1-OR is lower than that on Pd@TS-1-O, while the degradation rate of hydrogen peroxide on Pd@TS-1-OR are higher than that on Pd@TS-1-O (Fig. 4 and Table S3^†), which was attributed to the change in oxidation state from Pd²⁺ to Pd⁰ after reductive treatment, in agreement with previous reports.^27,33 The selectivity of hydrogen peroxide over Pd@TS-1 increased after an oxidation/reduction/oxidation cycle, the reason might be the weaker adsorption of O₂ and H₂, the intermediate OOH and the production H₂O₂ and the suppression of H₂O₂ decomposition.²⁰Open in a separate windowScheme 2Schematic of the mechanism for DSHP by Pd@TS-1.Open in a separate windowFig. 4H₂O₂ selectivity of DSHP over Pd@TS-1 with different oxidation states for 5 min reaction. Reaction conditions (same as Fig. 5 and ​and6):6): H₂/Ar (2.9 MPa) and air (1.35 MPa), 8.5 g solvent (2.9 g water, 5.6 g MeOH), 0.02 g catalyst, RT, 1200 rpm.The productivity of DSHP over Pd@TS-1 increased with oxidation, reduction and reoxidation treatment in 30 minutes (Fig. 5 and Table S3^†), demonstrated that PdO layer on monometallic Pd catalysts could suppress oxygen dissociation and H₂O₂ degradation,¹² the appropriate PdO formed on the surface of the catalysts after reoxidation can optimize the H₂O₂ production. The hierarchical Pd@TS-1 (35 010 mmol g_Pd⁻¹ h⁻¹) is remarkably higher than those of conventional Pd@TS-1 (3210 mmol g_Pd⁻¹ h⁻¹), the superior hydrogen peroxide production rate of Pd@HTS-1-ORO indicating that the Pd encapsulated by uniformed topology structure of TS-1 highly limited by the effect of pore-diffusion resistance.¹¹ Compared to Pd@TS-1, it was noteworthy that Pd@HTS-1 with only 0.1 wt% Pd content and subnano size after oxidative treatments showed famous reaction activity without any promoters under mild condition, which could be mainly ascribed to the presence of internal diffusion limitation within encapsulated micropore zeolites. The micropore structure limited the use of Pd metal because a part of the Pd crystal surface was blocked by zeolite supports, the hydrogen and oxygen were restricted by the configurational diffusion of zeolite to the Pd surface. Moreover, the formed and desorption H₂O₂ was also constrained by the micropore and thereby resulted in prolonged residence time of the product leading to degradation of H₂O₂. The intracrystal diffusion no longer limited the mass transport process of the hierarchical zeolite due to the presence of additional porosity. Although the physical and structural properties (including the primary particle size, the properties of the external surface and so on) were different between Pd@HTS-1 and Pd@TS-1, we may still draw a conclusion that the excellent catalytic activity is mainly attributed to the presence of mesopore favours diffusion of both reactants and products to and off the active sites in micropores.Open in a separate windowFig. 5Macro reaction rate for H₂O₂ production over Pd@TS-1 and Pd@HTS-1. ^aPd/C#C&Pd/C#Ex from Young-Min Chung;^34bPd–Sn/TiO₂ from Hutchings.²⁹The TON of H₂O₂ production at different reaction time over the six different Pd@TS-1 and Pd@HTS-1 catalysts were shown in Fig. 6. The TON increases with increasing reaction time, however, the slop of the TON–time curves (d_TON/d_t) seems decreased with increasing time, which revealed that the net productivity rate of hydrogen peroxide synthesis declined slightly with increasing time, especially for the Pd@HTS-1-OR at the reaction period of 30–60 min. The accumulative productivity of hydrogen peroxide slowed down, the reason might be the rapid decrease of hydrogen partial pressure in the medium and the ongoing H₂O₂ degradation.Open in a separate windowFig. 6The TON of H₂O₂ production with different reaction time over Pd@TS-1 and Pd@HTS-1 catalysts. TON (turnover number) = mol (H₂O₂)/mol (surface Pd).In summary, successful encapsulation of subnano-size Pd metal particles within titanium silicate (TS-1) voids was achieved via the mercaptosilane-assisted DGC synthesis method. The subnano-size Pd nanoparticles encapsulated in HTS-1 zeolites exhibited superior thermal stability after the oxidation/reduction/oxidation heat treatment process adjusting Pd/PdO hybrid owing to the embedding confinement. The synthesized high-efficiency Pd@HTS-1-ORO showed the famous hydrogen peroxide synthesis productivity, a hydrogen peroxide production rate as high as about 35 010 mmol H₂O₂ g_Pd⁻¹ h⁻¹. Our strategy brings about a finely tailored method to control particle size down to the subnano level and eliminate the diffusion inside metal encapsulated microporous zeolites, which is advantageous for catalytic activity and selectivity in direct synthesis of hydrogen peroxide. Thus, our approach opens up the possibility that the titanium-containing zeolites encapsulated noble metal catalyst can be extended further to selective oxidation reactions with H₂O₂ generated in situ from H₂ and O₂.     

Retracted Article: Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m⁻².

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m^−2.

It is well known that inkjet printing works as precise and versatile patterning method for printed electronics.^1,2 As for its advantages, it is already being exploited widespread for printing electronics.^3,4 Those merits are easily processed from solutions and conveniently used for air-stable electrodes.^5,6Light-emitting electrochemical cells (LECs) have emerged as an active layer,^7,8 arousing tremendous attention over the years.^9–11 Nevertheless, LECs are simpler than organic light-emitting diodes (OLEDs).^12,13 Due to its single layer architecture, low fabrication price and operating voltages, LECs are considered as a promising, next-generation, emissive thin-film technology.^14–16The most efficient device used to date for LECs are biscyclometalated iridium(iii) (Ir) complexes, because they have highly efficient and stable devices spanning the whole visible range.^17–19 However, avoiding the use of Ir is strongly desired because of its high cost and limited supply.^20–22 Till now, thiazole has worked as active component in a LECs system with a long-lived charge-separation molecule, without additional ions in its active layer.^23–26 As for its advantages, it can reduce recombination of charge carriers and facilitate carrier transfer.^27–29 More and more importance has been attached to thiazole compounds, especially bithiazole-based ones.^30–33 Therefore, these have aroused interest in the syntheses of bithiazoles.^34–36Based on our previous research,^37–41 the use of bithiazole ligands already explored by our group for photocatalytic technology has the advantage of an easy transformation of neutral complexes into charged ones by substitution on the N-atom of the thiazole moiety.^42,43 Furthermore, we demonstrate the fabrication of LEC devices by a combination of inkjet printing and spin coating, on A4 paper substrates for low cost, disposable and flexible conductive pattern.^44,45 Their corresponding material characteristics were closely investigated, a detailed report of the material properties of the resulting coatings, and an envisaged proof of concept application are disclosed in this paper. These fully printed devices demonstrate the potential upscaling of the fabrication of optoelectronic devices.Herein we report the design for a range of bithiazole derivatives 1a–1c (Fig. 1, their synthesis is shown in Scheme S1^†), and explore their properties. First of all, the UV-visible diffuse-reflectance (UV-vis DRS) spectra of those specimens at room temperature are displayed in Fig. 2 and the data are collected in Table S1.^† This enhancement was ascribed to the increase of aromatic rings, which has an intense absorption in the visible-light region. And their bandgap energies (E_g) are between 2.97–3.01 eV (Table S1^†), and are estimated from these absorption spectra according to the Kubelka–Munk method. Expanding the conjugated system and electron density with these donor groups, it can lead to a larger bathochromic shift of the absorption maximum.^46,47Open in a separate windowFig. 1Chemical structures of compounds 1a–1c described in this work.Open in a separate windowFig. 2UV-vis DRS spectra of 1a–1c.All of the three compounds are photoluminescent at room temperature, the relevant fluorescence peak maximum ranges from 360 to 398 nm (Fig. 3). They emit blue light when they are being excited. In agreement with Fig. 3, the presence of electron-releasing groups would shift the emission maximum. Among them, the most electron-donating carbazole unit would give the most red-shifted peak at 398 nm for 1c (Table S1^†). The PL quantum yield (ϕ_PL) studies show that both 1a and 1b are as high as 62 and 78%, while another one is 85% (Table S1^†). The data for their fluorescent quantum yields largely depends on interaction effect with molecules 1a–1c in the crystal packing. In this context, π–π intermolecular interactions can inhibit the fluorescence.⁴⁸Open in a separate windowFig. 3Typical PL spectra of 1a–1c.It may indicate that enlarging the conjugation length and electron density with these donor groups plays an important role in increasing the ϕ_PL.⁴⁹ Moreover, those with high ϕ_PL may be suitable for application as efficient light-emitting material in LECs.⁵⁰In addition, time-resolved measurements of the donor lifetime (τ) in 1a–1c were carried out, and their corresponding values are given in Table S1.^† The bithiazole derivatives 1a–1c showed long-lived singlet fluorescence lifetimes (τ_F), ranging between 1.04 and 9.54 ns (Table S1^†). This was on average more than double the fluorescence lifetime of the bithiazole starting material.⁵¹ These three samples possess relatively high decay time, as a result of their inhomogeneity.⁵² Compound 1c exhibited the longest fluorescence lifetime decay τ_F = 9.54 ns. The sensitivity of the emission to the polarity of the solvent is beneficial for an intramolecular charge transfer (ICT)-like emission.⁵³ This difference probably accounts for the high concentration of donor units in the bithiazole backbone, low rates of intersystem crossing to reactive triplet states.^48,54Consequently, the composite 1c was the best performing LEC and chosen for the below test. The EL spectra obtained for both devices were almost identical, with a main peak at 498 nm and other peak at 504 nm wavelength as illustrated in Fig. 4a. It may account for trapping, cavity, and self-absorption effects from within the LEC device 1c multilayer-structure. The spin-coated emission is quantified by the commission Internationale de L’Eclairage (CIE) coordinates of (0.28, 0.42), and a colour rendering index (CRI) of 65 (Table S2^†). While inkjet-printed light emission with CIE coordinates of (0.34, 0.43) and a CRI value of 83 was achieved for device 1c (Table S2^†). It exhibits good colour rendering indices (CRI > 70), this device exhibited a warm-white appearance, and the colour rendering is considered sufficient for indoor lighting applications.Open in a separate windowFig. 4Comparison of device characteristics with spin-cast and inkjet printer emitting layer for 1c: (a) electroluminescent spectra, (b) luminance vs. voltage, (c) current density vs. voltage, (d) efficiency vs. luminance behaviour.The time-dependent brightness and current density under constant biases of 2.9–3.3 V for device 1c are shown in Fig. 4b and c. Interestingly, the device with the inkjet-printed emitting layer (EML) produced a light output of around 3600 cd m⁻², whereas 4600 cd m⁻² was achieved with the spin-coated EML. In other words, the device with the inkjet printed EML obtained about 78.3% of brightness compared with the reference device. The differences between the two devices regarding current efficiency (J) and power efficiency (PE) were rather closed, as listed in the Table S2.^† A luminous efficiency much larger than 10 cd A⁻¹ was achieved for the inkjet-printed EML, which tended to be 73.9%, and is similar to the spin-coated emitting layer.The performance gap between the devices (Fig. 4d) can be attributed to differences in the height and surface roughness of the emitting layer.⁵⁵ Lateral sizing histograms (Fig. S11^†) show that the spin-coated EML possessed a thickness of 35 nm and a very smooth surface. The inferior performance of the inkjet-printed EML is caused by a less homogeneous surface morphology and an overall fatter layer.⁵⁶ The depth of the inkjet-printed EML ranged from 25 nm (pixel centre) to 35 nm (pixel edges). As a consequence, it is an inkjet deposition process and the related evaporation dynamics reduces the light output and efficiency, but also postpones the stability of the device in the system.^10,57We then shifted our attention to the turn-on kinetics, efficiency and long-term stability of the LEC device for 1c; the typical evolution of current density and light emission is presented in Fig. 5a and b, showing a slow turn-on followed by a progressive decay over time.Open in a separate windowFig. 5(a) Current density (closed symbols) and brightness (open symbols) versus time at 3 V for a device with the representative sample 1c; (b) EQE versus time at an applied voltage of 3 V.The build-up of the light output is synchronous with that of the current density. This time-delayed response is one of the striking features of the operation of an electrochemical cell and reflects the mechanism of device operation.²² The champion device in this set exhibited a peak power efficiency (PE) of 17.4 lm W⁻¹ at a luminance of 18 000 cd m⁻². Meanwhile, the current density began to decrease, possibly due to the electrochemical oxidation or electro corrosion that occurred on component 1c through accumulated electroinduced holes, and then it kept up a durative datum.⁵⁸ Apparently it exhibited outstanding long-term operation stability beyond 40 h (Fig. 5b).The time-dependent external quantum efficiency (EQE) of LEC for the representative substrate 1c are shown in Fig. 5b. It also exhibited similar temporal tendency in device efficiency. When a bias was applied on the LECs, the EQE quickly increased since balanced carrier injection was achieved by the formation of the doped layers.⁵⁹ After attaining the crest value, the device current was still rising while the EQE reduced little by little. It implied that both growing the doped layers and weakening of the EQE was resulting from exciton quenching near the bithiazlole core in persistently extended doped layers.⁶⁰ Doping-induced self-absorption was rather adaptable to the temporal roll-off in device efficiency. This configuration shows the best performance (EQE = 12.8%) compared to those devices prepared with the single compounds, which is given in Table S3.^† The reason may be that the steady-state recombination zone in 1c device was close to the central active layer.⁶¹ Besides it contains fluorenes on its side chains, constructing plane structure, benefiting from good electron injection abilities, leading to enhancing the EQE itself.⁶²The fabrication of inkjet-printed bithiazole interdigitated electrode (IDE) is illustrated in Fig. 6. All the prepared varieties of bithiazole-based ink have been found to be highly stable with nil or miniscule precipitation in over 180 days being stored on the shelf (Fig. 6a). After HCl treatment of the paper substrate, the prepared inks have filled within ink cartridges of a low-cost desktop printer Canon iP1188 (Fig. 6b). Fig. 6c shows the photograph of inkjet-printed conductive patterns.Open in a separate windowFig. 6(a) The representative ink based on sample 1c was stable for 180 days; (b) electrode printing using Canon iP1188 printer; (c) scale showing size of printed electrodes; (d) interdigitated electrode (inset shows small size printed pattern).In addition, the most attractive prospect of the bilayer device structure at this stage is the possibility for patterned emission for the creation of a static display. Fig. 7 presents a photograph of a small portion of a larger static display, with a resolution of 170 PPI, which repeatedly exhibits a message in the form of the word “LEC”. As shown in Fig. 7, the pixel array produced are fairly luminous. The EL spectra were collected from each pixel on the substrate, the emission peak wavelength was about 510 nm and the full width at half maximum was about 170 nm. The low variation in emission intensity in the different pixels implies a small variation in thickness of the solution-coated layers. More specifically, if we assume a minimum diameter of 20 μm for an inkjetted electrolyte droplet, and a smallest inter-droplet distance of 10 μm, we attain a pitch of 30 μm, which corresponds to a high display resolution of 850 PPI. Finally, we draw attention to the herein presented static-display LEC that comprises solely air-stabile materials, and that we routinely fabricate the bilayer stack under ambient atmosphere.Open in a separate windowFig. 7(a) The patterned light emission from a bilayer LEC, with the emission pattern defined by the selected positions of the ink jetted electrolyte droplets. (b) The droplet diameter and pitch were 50 and 150 μm, respectively, and the device was driven at V = 3 V. The scale bar measures 300 μm.     

Mesoporous-silica-coated palladium-nanocubes as recyclable nanocatalyst in C–C-coupling reaction – a green approach

We report the straightforward design of a recyclable palladium-core–silica-shell nanocatalyst showing an excellent balance between sufficient stability and permeability. The overall process – design, catalysis and purification – is characterized by its sustainability and simplicity accompanied by a great recycling potential and ultra high yields in C–C-coupling reactions.

A green approach: in a single-step coating process a mesoporous silica shell was tailored onto palladium-nanocubes. Along with a PEG-matrix this core–shell-nanocatalyst could be recovered after C–C-coupling reactions and reused without any significant decrease in product yield.

In contrast to bulk materials, metal-nanocyrstals (NC) possess unique physical and chemical properties. Both, nano-scale and geometry can dictate their optical, electronic and catalytic behavior.^1–3 Consequently, nanomaterials have been implemented already in various fields like medicine,⁴ sensing,⁵ nano-electronics⁶ and organic synthesis.⁷ Smart strategies for a sustainable usage of limited resources such as precious metals and hydrocarbons are inevitable due to the consistently growing population and economy.^8–10 Generally, catalysis gives rise to novel and energy-saving synthetic routes. However, homogeneous catalysis is not widely used in industrial processes, due to the need of mostly toxic ligands and the costly purification along with a restricted reusability potential.^11–13 Heterogeneous catalysis based on the utilization of metal nanocrystals overcomes most of these limitations. Caused by its high surface-to-volume ratio, catalytic activities are drastically increased in comparison to bulk materials. However, the most common disadvantage in NC-based catalysis lies in the occurrence of aggregates during the reaction, which leads to a decrease of the catalytic active surface. Several studies were already presented in literature to delay that phenomenon using micelle-like- or core–shell-nanostructures as potential nanocatalytic systems.^14–20 However, these surfactants or shells are either potentially harmful for the environment or very step-inefficient to produce. Other approaches used the deposition of small NCs in a mesoporous support which offers a large active surface area.^21–24 In this context, it is crucial to find an adequate balance between permeability for small organic molecules and the overall stability of the nanocatalyst. Overcoming these obstacles can contribute to a sustainable supply of drugs and other organic substances.In the present work palladium-nanocubes (Pd-NCubes) were fabricated in aqueous solution using cetyltrimethyl-ammoniumbromide (CTAB) as surfactant. The procedure is adapted from a previously published study.^25,26 The respective transmission electron microscopy (TEM) image and the selected area electron diffraction (SAED) pattern of the as-obtained Pd-NCubes are depicted in Fig. S1.^† The SAED confirms the single-crystallinity of Pd-NCubes bound by {100}-facets. TE micrographs of Pd-NCubes revealed an average edge-length of (18 ± 2) nm (for histogram see Fig. S2^†). The formation of polyhedra and nanorods was found to be less than 1%.To the best of our knowledge, there is no procedure reported that showed the direct fabrication of a mesoporous silica (mSi) shell tailored on Pd-NCs. However, Matsuura et al. demonstrated a single-step coating approach of CTAB-capped gold-nanorods and CdSe/ZnS quantum dots obtaining a mesoporous silica shell.²⁷ The pores were determined to be 4 nm in width with 2 nm thick walls. Since the Pd-NCubes are covered by CTAB, already no surfactant exchange is necessary. Consequently, this procedure could be directly transferred to the as-obtained Pd-NCubes (∼10¹⁵ particles per L) of this study using tetraethyl orthosilicate (TEOS) as silica precursor in an alkaline solution. Here, CTAB serves as organic template for the formation of the mesoporous silica shell.TE micrographs of individual Pd-mSi-nanohybrids are depicted in Fig. 1 showing a spherical silica coating with a thickness of (17 ± 2) nm (for histogram see Fig. S3^†). The porosity is essential to ensure that vacant coordination sites on the palladium-core are present and accessible for catalysis. In contrast to other multistep approaches, pores are formed in situ with no additional etching step necessary.^28,29 This avoids the usage of harmful etching agents such as fluorides or ammonia.³⁰ The silica shell then served as platform for further surface modification using two different PEG-silanes (M_n = 5000 g mol⁻¹ and M_n = 20 000 g mol⁻¹). TE microscopy did not reveal any changes in the structure of the PEG functionalized Pd-mSi-nanohybrids opposed to the unfunctionalized nanocatalyst, since the contrast of polymer is too low (see Fig. S4^†). However, dynamic light scattering (DLS) measurements in diluted aqueous solutions proved an increased hydrodynamic radius with increasing molecular weight of the PEG-chain grafted onto the silica shell (see Fig. 2). These results provide evidence that only individual nanostructures are formed while no larger aggregates are present.Open in a separate windowFig. 1Exemplary TE micrographs of Pd-mSi-nanohybrids.Open in a separate windowFig. 2DLS results along the different stages of the hierarchal fabrication process of the nanocatalyst.The successful functionalization of the Pd-mSi-nanohybrid with PEG provides the dispersibility for the overall nanocatalyst in a PEG matrix. Due to its lack of toxicity and its simple recovery, caused by its melting point at ∼50 °C, PEG is considered as a “green” reaction medium.³¹ It has already been shown that PEG can act as suitable solvent for both homogeneous and heterogeneous catalysis.^32–34 Consequently, further experiments were performed using only the Pd-mSi-nanohybrid functionalized with PEG-silane having an average molecular weight of M_n = 5000 g mol⁻¹ (PEG-5k). For catalytic reactions, Pd-mSi-PEG-5k was dispersed in a PEG matrix (PEG-2000, M_n = 2000 g mol⁻¹) and charged into a Teflon centrifuge tube. Using only one tube for the reaction and the product separation avoids an additional transfer step and prevents any loss of the nanocatalyst between reaction cycles (see recycling Scheme 1). Here, the C–C-coupling between ethyl acrylate and p-iodoanisole served as model Heck-reaction to prove the catalytic activity of the designed catalyst (4.4 mol% overall Pd conc. equal to ca. 0.3 mol% surface-available Pd; conc. is determined by ICP-MS measurements, see Table S1^†). Sodium phosphate was used as base providing the largest product yield when compared to other bases, such as Na₂CO₃, K₂CO₃ and K₃PO₄. Once the catalysis was performed and the PEG-2000 was cooled down, diethyl ether was added to extract the product and separated from the reaction medium via centrifugation.Open in a separate windowScheme 1Recycling process of the Pd-mSi-PEG-5k-nanocatalyst and PEG-2000 as solvent after the Heck-reaction between ethyl acrylate and p-iodoanisole to form ethyl p-methoxycinnamate.To exclude any suspended compounds from the desired product, the mixture was passed through a PTFE-filter. Et₂O was removed in vacuo without the need of column chromatography. Comparing the ¹H-NMR spectra of the as-obtained product with the educts indicate a yield of 98% with only small amounts of PEG-2000 present (identified by the signal at ∼3.6 ppm, < 1 weight-%). The results show that both educts and the base Na₃PO₄ are able to diffuse through the mesoporous silica shell to the palladium core (see Fig. 3). A detailed ¹H-NMR signal assignment of the product is given in Fig. S5.^†Open in a separate windowFig. 3 ¹H-NMR spectra of the educts ethyl acrylate (top) and p-iodoanisole (center) and the product ethyl p-methoxycinnamate (bottom).After recovering the reaction mixture containing Pd-mSi-PEG-5k and PEG-2000, seven further Heck-reaction-cycles were conducted under the same conditions. Results obtained from ¹H-NMR and gravimetry indicate no significant decrease in catalytic activity (see ¹H-NMR spectra in Fig. 4). Along the eight Heck-reactions, product yields were determined between 94% and 99%. Only small traces of p-iodoanisole (identified by the signal at ∼6.75 ppm) were still present while ethyl acrylate could be fully removed in vacuo. The yields obtained after each cycle are displayed in Fig. 5. ICP-MS measurements of the catalysis product were performed to determine the palladium leaching out of the catalytic system. The results are displayed in Table S1^† indicating that leaching is strongly suppressed since the overall Pd-content in the product ranges from 0.3–5.7 ng, only. This corresponds to 0.002–0.044 ppm palladium with respect to the product mass. The data are in good agreement with the high product yields along the eight Heck-reactions. To trace the evolution of the nanocatalyst along the cycles, TE micrographs were taken after the 1^st and the 8^th Heck-cycle (see Fig. 6). It can be seen that the cubical structure of the palladium vanishes during the first reaction (left TEM image). Inside the silica shell spherical palladium particles were formed. An explanation for this rearrangement lies in the suggested Heck-mechanism.³⁵ Here, a Pd²⁺-species forms after the oxidative addition of the p-iodoanisole which can desorb from the Pd-NCube. Once the reductive elimination of the product occurs the Pd⁰-species is generated again that can re-deposit on the palladium-core.²¹ Since the spherical geometry possesses the lowest free surface energy, globules were eventually formed.³ The mesoporous silica shell is not affected significantly by the catalysis and the rearrangement of the palladium. The porosity appears to stay intact, enabling the penetration of further small organic molecules. Control experiments were performed to validate whether these observations are based on either a thermally or a chemically induced rearrangement process of the palladium core. Therefore, only the nanocatalyst was dispersed in PEG-2000 and heated at 110 °C for 24 h without any conducted catalysis reaction. TEM results showed no changes in the structure of neither the palladium core nor the silica shell (see Fig. S6^†). After the 8^th reaction, no core–shell-nanostructures could be detected anymore via TEM. Only small randomly shaped Pd-nanoparticles (≤20 nm) were found indicating a slow leaching of the palladium out of the silica shell. However, no larger aggregates were formed (see right TE micrograph in Fig. 6) which explains the continuously high catalytic activity.Open in a separate windowFig. 4 ¹H-NMR spectra of the product ethyl p-methoxycinnamate after the 1^st Heck-reaction (bottom) up to the 8^th reaction (top).Open in a separate windowFig. 5Conversions of ethyl p-methoxycinnamate obtained via NMR and gravimetry after each respective Heck-reaction (1–8).Open in a separate windowFig. 6TE micrographs of the Pd-mSi-PEG-5k-nanocatalyst taken after the 1^st (left) and the 8^th Heck-reaction (right).     

Dual enzyme-like activity of iridium nanoparticles and their applications for the detection of glucose and glutathione

Here we show that iridium nanoparticles (Ir NPs) functionally mimic peroxidase and catalase. The possible mechanism of intrinsic dual-enzyme mimetic activity of Ir NPs was investigated. Based on the excellent peroxidase-like activity of Ir NPs, a new colorimetric detection method for reduced glutathione (GSH) and glucose was proposed.

Iridium nanoparticles could functionally mimic peroxidase and catalase. The possible mechanism of intrinsic dual-enzyme mimetic activity of Ir NPs was investigated.

Recently, nanomaterials with inherent enzyme-like activity have attracted considerable attention due to their simple preparation, storage, and separation, as well as the low cost as compared with natural enzymes. Various nanomaterials have been shown to mimic the activity of oxidase, peroxidase, catalase or superoxide dismutase (SOD), ranging from metals,^1–10 metal oxides,^11–15 and metal coordination complexes^16–21 to carbon-based nanomaterials.^22–26 The ability of these nanomaterials to replace specific enzymes may offer new opportunities for enzyme-based applications. For example, nanozymes with oxidase-like or peroxidase-like activity have shown potential applications in biosensing and immunoassay, such as the detection of H₂O₂, glucose, antioxidants, antigens, antibodies and so on.²⁷ By using nanozymes with peroxidase-like activities, Wei and co-workers recently have developed novel sensor arrays to detect biothiols and proteins as well as discriminate cancer cells owing to the differential nonspecific interactions between the components of the sensor arrays and the analytes, providing a potential approach to discriminate versatile analytes.⁸ Nanomaterials with SOD-like or catalase-like activity have exhibited antioxidant activity, thus could protect aerobic cells from oxidative stress, showing potential application in inflammation therapy.^6,28–31 Also, nanozymes with high catalase-like activity was able to produce O₂ at the hypoxic tumor site, serving as efficient agents for cancer therapy.^23,32,33 Very recently, Zhang'' group has developed a simple and biocompatible platform to elevate O₂ for improving photodynamic therapeutic efficacy by combining the photosensitizer with Prussian blue nanomaterials.³⁴ Prussian blue could catalyze H₂O₂ to generate O₂, and then the photosensitizer transforms the O₂ to produce singlet oxygen (¹O₂) upon laser irradiation for cancer therapy. Besides, Qu et al. have found the porous platinum nanoparticles with catalase-like activity, which greatly enhanced radiotherapy efficacy and overcame the hypoxic tumor microenvironment.³⁵In this work, we demonstrate that Ir NPs exhibited both peroxidase-like and catalase-like activities. As shown in Scheme 1, Ir NPs can catalyze the decomposition reaction of H₂O₂ into oxygen and water, possessing potential applications in the cancer therapy as catalase mimics. On the other hand, the tiny Ir NPs with the average diameter of 2.4 nm exhibited high peroxidase-like catalytic activity. Furthermore, by using H₂O₂ as an intermediary, a simple and sensitive colorimetric detection method for GSH and glucose has been designed.Open in a separate windowScheme 1Schematic presentation for dual-enzyme mimetic activity of Ir NPs.Synthesis of Ir NPs were carried out by a simple chemical reduction process, in which sodium hexachloroiridate(iii) hydrate was used as precursor with ascorbic acid as a protecting agent and sodium borohydride as the reducing agent (see Experimental section in ESI^†). After heating and stirring at 95 °C for 15 min, the resulting homogeneous light brown Ir NPs dispersion was obtained with good stability and reproducibility. The obtained Ir NPs was thoroughly characterized by various methods. Transmission electron microscopy (TEM) images indicated that the as-prepared Ir NPs showed a narrow size distribution with the average diameter of ∼2.4 nm (Fig. 1A–C). UV-vis spectrum of Ir NPs showed an absorption peak at ∼280 nm (Fig. 1D), in agreement with the value reported earlier,²⁸ indicating the formation of Ir NPs. XPS spectra of Ir 4f in Ir NPs were presented in the Fig. S1.^† A pair of doublet peaks at 61.0 and 64.0 eV were observed, revealing that Ir in Ir NPs is mostly metallic Ir(0).³⁶ Furthermore, inductively coupled plasma-optical emission spectroscopy (ICP-OES) disclosed that the exact concentration of Ir NPs is 25 μg mL⁻¹.Open in a separate windowFig. 1(A and B) TEM images of Ir NPs at different magnification. (C) Size distribution histogram of Ir NPs. (D) UV-vis absorption spectrum of Ir NPs. The inset shows the photograph of Ir NPs dispersed in the aqueous solution.To investigate the peroxidase-like activity of Ir NPs, the peroxidase coupled assay was employed and the change in absorbance of reaction was monitored using a UV-vis absorbance spectrophotometer. As shown in Fig. 2A, exposure of Ir NPs to a colorless peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂ resulted in the fast oxidation of TMB to a blue product. However, no color change of the TMB substrate was observed only with Ir NPs or H₂O₂, indicating the intrinsic peroxidase-like activity of Ir NPs. Similar to HRP, the catalytic activity of Ir NPs is dependent on the pH, temperature and catalyst dosage. Fig. S2^† showed that the catalytic activity of Ir NPs was much higher in weakly acidic solution and reached its highest at pH 4.0, consistent with those reported for peroxidase-like NPs and HRP.^37–40 In the range of 20–80 °C, the maximum catalytic activity was obtained under 50 °C. For simplicity, we adopted pH 4.0 and room temperature (∼20 °C) for subsequent analysis of peroxidase-like activity of Ir NPs. When increasing the dosage of Ir NPs, the catalytic activities of Ir NPs clearly increased as shown in Fig. 2B. Interestingly, some small gas bubbles were observed in the tubes at the same time.Open in a separate windowFig. 2(A) The absorption spectra and digital photos of different colorimetric reaction systems: (a) TMB + H₂O₂, (b) TMB + Ir NPs, and (c) TMB + H₂O₂+Ir NPs. (B) Time-dependent absorbance changes at 652 nm of TMB reaction solutions catalyzed by the different concentrations of Ir NPs. (C) Time-dependent absorbance changes at 240 nm of 20 mM H₂O₂ catalyzed by the different concentrations of Ir NPs incubated in 0.1 M HAc–NaAc buffer (pH 9.0). (D) The effect of concentration of Ir NPs on the formation of hydroxyl radical with terephthalic acid as a fluorescence probe. Reaction condition: 0.1 M HAc–NaAc buffer (pH 6.0).In order to confirm which gas was produced and whether Ir NPs had the intrinsic catalase-like activity, the decomposition of H₂O₂ was further investigated by monitoring the changes of UV-vis absorbance at 240 nm under the basic conditions. As shown in Fig. 2C, the absorbance was obviously decreased as the dosage of Ir NPs increased, and many gas bubbles could be observed in the cuvette, indicating Ir NPs could catalyze the decomposition of H₂O₂ into O₂. Temperature and pH can also make a big effect on the catalase-like activity of Ir NPs. Under the basic conditions or higher temperature, much more and bigger gas bubbles were produced (Fig. S3^†), suggesting higher catalase-like activity of Ir NPs. Obviously, Ir NPs possessed intrinsic catalase-like activity, which could be regulated by adjusting the temperature and pH.The formation of ˙OH during the reactions was assessed to better understand the mechanism for the dual enzyme-like activity of Ir NPs. Terephthalic acid was adopted here as a fluorescence probe to trap ˙OH. As shown in Fig. 2D, in the absence of Ir NPs, terephthalic acid emitted blue. However, the fluorescence intensity was gradually decreased as the concentration of Ir NPs increased, suggesting that Ir NPs could consume ˙OH radicals rather than generate ones. The reactivity of Ir NPs appears to be different from that of the other peroxidase mimics, where ˙OH mediates the oxidation of organic substrate.As Ir NPs exhibited the peroxidase-like activity, we monitored the reaction of Ir NPs with H₂O₂ and TMB. The apparent steady-state kinetic parameters were determined by changing one substrate concentration while keeping the other substrate concentration constant. The value ε = 39 000 M⁻¹ cm⁻¹ (at 652 nm) for the oxidized product of TMB was used here to obtain the corresponding concentration term from the absorbance data. As shown in Fig. S4,^† we observed that the oxidation reaction catalyzed by Ir NPs followed the typical Michaelis–Menten behavior toward both substrates. The Michaelis–Menten constant (K_m) and the maximum initial velocity (V_m) given in Table S1^† were obtained by using Lineweaver–Burk plot (Fig. S4B and D^†). Compared with the Pd–Ir cubes,³⁸ Ir NPs presented a similar K_m for TMB and a very low K_m for H₂O₂, suggesting that Ir NPs have a higher affinity to H₂O₂. Moreover, Ir NPs presented larger V_m for both of TMB and H₂O₂, indicating the strong catalytic activity of Ir NPs. What is more important is that Ir NPs showed high stability after long-term storage. After five months of storage at room temperature, the peroxidase-like catalytic activity of Ir NPs maintained 96% (Fig. S5^†), significantly expanding their practical applications.On the basis of the high affinity and catalytic activity of Ir NPs to H₂O₂, the analytes that could consume or produce H₂O₂ could be detected indirectly by using the TMB as substrate.²⁷ Therefore, a simple colorimetric method was developed to detect GSH and glucose using Ir NPs. GSH, which plays an important role in many cellular processes including redox activities, signal transduction, detoxification, and gene regulation,^41,42 can consume H₂O₂ and result in the shallowing color of the Ir NPs-TMB-H₂O₂ system. As presented in Fig. 3A, the absorbance dropped sharply with the addition of GSH. And a linear relationship between the absorbance and logarithmic values of GSH''s concentrations was obtained in the range from 200 nM to 100 μM (Fig. 3B). The level changes of GSH have been linked to varieties of diseases, such as diabetes, psoriasis, liver damage and Parkinsons.^43,44 The proposed colorimetric biosensor provided a sensitive method to monitor GSH. Furthermore, because H₂O₂ is the main product of the glucose oxidase (GOx)-catalyzed reaction; glucose could be detected based on the combination of GOx. Fig. 3C shows typical glucose concentration–response curves with the linear range of 10 μM to 2 mM. According to the principle of S/N = 3, the calculated detection limit was 5.8 μM. Table S2^† was listed to compare the sensing performance of Ir NPs with other nanomaterials. Ir NPs are superior to other nanomaterials in lower detection limit and wider detection range for colorimetric determination of glucose.Open in a separate windowFig. 3(A) Dose–response curve for GSH detection at 652 nm. (B) Linear relationship between the absorbance and logarithmic values of GSH''s concentrations in the range from 200 nM to 100 μM. (C) Dose–response curve for glucose detection at 652 nm. (D) Determination of the selectivity of glucose detection (from left to right: blank, 0.3 mM glucose, 3 mM fructose, 3 mM maltose, 3 mM lactose and 3 mM sucrose). Inset: the color change with the different solutions. The error bars represents the standard deviation of four measurements.To explore the selectivity of above glucose sensor, 10 times concentration of control samples including fructose, maltose, lactose and sucrose were tested as shown in Fig. 3D. The color difference could be distinguished by the naked eye, suggesting the high selectivity of the biosensing system for glucose detection. Using this method, we detected glucose in 50-fold dilution fetal bovine serum to demonstrate the feasibility of this biosensor for practical applications, and the results are listed in Table S3.^† As can be seen, the recoveries of glucose fall in the range of 93.3–104% by using the standard addition method. The proposed biosensor was also applied for determining glucose concentrations in blood samples donated by healthy and diabetic persons (Fig. S6^†). According to the calibration curve, the concentration of glucose from different samples was 7.0 mM and 14.4 mM, which agrees well with that measured in the local hospital, 6.8 mM and 14.4 mM. Therefore, this colorimetric method is suitable and satisfactory for glucose analysis of real samples with high sensitivity and selectivity.In summary, Ir NPs synthesized by a simple chemical reduction process exhibited both of peroxidase-like and catalase-like activity. Moreover, the dual enzyme-like activity could be regulated by adjusting the temperature and pH. On the one hand, Ir NPs could consume ˙OH radicals exhibiting potential applications in the antioxidant therapeutics as antioxidant nanozymes. On the other hand, as peroxidase mimics, Ir NPs were successfully applied in the construction of colorimetric biosensors to detect GSH and glucose. This work will facilitate the utilization of intrinsic dual-enzyme activity and other catalytic properties of Ir NPs in analytical chemistry, biotechnology, and medicine.     

Effect of nanostructured silicon on surface enhanced Raman scattering

Non-metallic materials are often employed in SERS systems by forming composite structures with SERS-active metal materials. However, the role of the non-metallic structures in these composites and the effect of them on the SERS enhancement are still unclear. Herein, we studied the effect of silicon morphology on SERS enhancement on silver nanoparticles-coated different structured silicon surfaces. Our finding will help to further understand the SERS mechanism and pave the way for making more efficient SERS systems.

The surface morphology of non-metallic silicon has a big effect on the SERS enhancement of silver nanoparticle-coated silicon surfaces.

In past decades, increasing attention has been attracted to surface enhanced Raman scattering (SERS) due to the dramatically enhanced detection sensitivity of Raman scattering (down to single-molecule sensitivity). The Raman intensity of the molecules located in the vicinity of SERS nanostructures can be enhanced up to 10¹⁰ to 10¹¹ times,^1,2 largely extending the application of SERS in the fields of physics, chemistry and biology, etc.^3–9 Metal materials are mainly employed in fabricating SERS structures, especially gold or silver ones for visible spectrum excitation.^10,11 To date, many nanostructures have been reported that can enhance Raman scattering enormously, leading to so called Raman hot spots,^12–15 including nanogaps,^16,17 nanostars,¹⁸ nanotriangles and nanorods,¹⁹ mainly due to the introduction of a localized electromagnetic field under illumination. This kind of enhancement is referred to as the electromagnetic mechanism, which dominates the SERS enhancement in most cases.Non-metallic structures can also contribute to Raman enhancement, although the enhancement factor is usually very low. It has been reported that Cu₂O,²⁰ TiO₂ ²¹ and ZnS²² nanoparticles can enhance the Raman intensity of adsorbed molecules. Graphene has also been proved to be an efficient platform for Raman enhancement.²³ Although non-metallic materials can be used directly for SERS applications, they are usually used by forming a composite with SERS-active metal material, where they act as supporting materials or borrow the SERS activity from the metallic Raman hot spots. It has been reported that SERS activity can be borrowed from SERS-active materials through ultrathin SERS-inactive transition metals (e.g., Pt, Ni, Co and Pd)²⁴ or dielectric (e.g., SiO₂, Al₂O₃)²⁵ layer. Tian et al. reported the shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) by using the gold nanoparticles coated with ultra-thin silica or aluminum oxide shell.^26,27 Raman enhancement can be achieved at the silica shell surface by borrowing the SERS activity from the gold core. However, the role of non-metallic structures in enhancing Raman scattering and the interactions between these two kinds of materials are still fuzzy.Silicon nanostructures fabricated by catalytic etching method can be easily metalized with silver or gold by electroless deposition for SERS applications.^28–32 However, up to now, only limited types of silicon nanostructures have been reported for SERS applications.^17,30,31 In addition, the role of the nanostructured silicon surface in Raman enhancement is still unclear.In this report, we fabricated SERS structures on two types of silicon surfaces, flat silicon and nanoporous silicon, by metallizing the silicon structure with silver nanoparticles (AgNPs). Compared to the fabricated SERS structure on flat silicon surface, the one fabricated on nanoporous silicon surface showed obvious enhancement on the Raman spectrum of adsorbed probe molecules. The effect of pore size and depth of nanoporous silicon on Raman enhancement was investigated in detail.To investigate the role of silicon nanostructures in Raman enhancement, we compared the Raman spectra of probe molecules adsorbed on AgNPs-coated flat silicon and nanoporous silicon surfaces, respectively. The nanoporous silicon was fabricated by following a modified reported procedure (see details described in Experimental section and scheme shown in Fig. S1 in (ESI^†)).²⁸ Vertical nanopores were produced on silicon surface, and the pore size and pore depth can be easily tuned by varying the reaction parameters. Then a flat silicon and a nanoporous silicon substrates were both metallized with silver by immersing them into a mixed aqueous solution of AgNO₃ and HF,^29,33 forming AgNPs with size of 60 ± 30 nm. The as-prepared AgNPs-coated silicon surfaces (see scanning electron microscopy (SEM) images shown in Fig. S2 in ESI^†) served as SERS-active substrates. After p-aminothiophenol (PATP) molecules (Raman probe) were adsorbed on the AgNPs-coated silicon structures, both AgNPs-coated substrates showed uniform and strong Raman enhancements (Fig. 1). On AgNPs-coated nanoporous silicon surface, the Raman bands of PATP molecules at 1076 and 1142 cm⁻¹ are 4.2 and 7.4 times stronger compared to those on AgNPs-coated flat silicon surface (Fig. 1), respectively, demonstrating the vital role of silicon morphology in the obtained Raman enhancement. There are two widely accepted mechanism for SERS enhancement, electromagnetic mechanism and charge transfer mechanism.¹ Both flat silicon and nanoporous silicon substrates are composed of same material and the only difference between them is the silicon morphology. Thus the charge transfer mechanism should contribute similar effect in both conditions. Moreover, both flat and nanoporous silicon surfaces were covered with a thin layer of silicon dioxide,^28,34 which limit the charge transfer between AgNPs and silicon surface. This is also confirmed by the XPS measurement on the nanoporous silicon surface (Fig. S3 in ESI^†). Therefore, the observed different enhancement may attribute to the electromagnetic mechanism, which will be discussed latter. In addition, the Raman enhancement is uniform over the whole substrate. This is probably due to the uniform coating of AgNPs on high-density silicon nanopore structures. The enhancement factor (EF) can be calculated by using the following equation, EF = (I_SERS/I_bulk)(N_bulk/N_SERS),³⁵ where I_SERS and I_bulk represent the Raman intensities in SERS and bulk Raman measurements, respectively; N_SERS and N_bulk represent the number of probe molecules located in the excitation volume under these two conditions. For Raman band at 1076 cm⁻¹ (represents a₁ vibration mode of PATP,¹¹ which sits at 1089 cm⁻¹ for bulk,³⁶ Fig. S4 in ESI^†), the average EFs over the whole surface were calculated as 6.7 × 10⁵ and 2.8 × 10⁶ for SERS structures on flat silicon and nanoporous silicon, respectively. The strong Raman band at 1142 cm⁻¹ indicates a chemical conversion from PATP to 4,4′-dimercaptoazobenzene (DMAB) upon light irradiation.¹¹Open in a separate windowFig. 1Raman spectra of PATP molecules adsorbed on the AgNPs-coated (a) flat silicon and (b) nanoporous silicon. A nanoporous silicon with pore depth of 220 nm was used here. The schemes at the bottom right and top right show the structures of the AgNPs-coated flat silicon surface and AgNPs-coated nanoporous silicon surface, respectively. The size of AgNPs was not drawn to scale.As discussed, electromagnetic mechanism dominates the observed SERS enhancement. To confirm the role of silicon morphology, we did numerical simulation using the finite-difference time-domain (FDTD) method to investigate the localized electromagnetic field distributions on AgNPs-coated flat and nanoporous silicon surfaces. Note that, for the AgNPs-coated nanoporous silicon surface, many AgNPs sit on the edge of silicon nanopores (Fig. S2D in ESI^†). In this case, the electromagnetic field around the AgNPs is more localized. Compared with the AgNPs-coated flat silicon, the electromagnetic field is five times more localized on the AgNPs-coated nanoporous silicon surface (Fig. 2), which in principal could introduce 25 times stronger Raman enhancement.³⁷ However, in real case, only a proportional of the AgNPs locates on the edge of silicon nanopores and the shapes of the coated AgNPs are not exactly same with the ones we used in simulation, which explains the smaller SERS enhancement we observed on nanoporous silicon surface.Open in a separate windowFig. 2Schemes (side views) and FDTD simulations on the AgNPs-coated flat silicon (A and B) and nanoporous silicon (C and D). Dashed circles in (D) indicate the positions of silicon pores. The schemes in (A) and (C) were not drawn to scale.As discussed above, the morphology of nanoporous silicon contributes to the enhanced Raman signal. By varying the pore size and pore depth of the nanoporous silicon, different Raman enhancement should be observed.First, we studied the effect of pore depth of nanoporous silicon on Raman enhancement. The depth of silicon nanopores can be easily tuned by varying the period of catalytic etching of silicon. The Raman intensity of the probe molecules increased continuously with increased silicon nanopore depth (from 40 to 220 nm, Fig. 3). When PTAP molecules were adsorbed on the AgNPs-coated nanoporous silicon surface, the Raman intensity measured on silicon with 220 nm pore depth was increased about 2 times compared to that on silicon with 40 nm pore depth. Further increasing the pore depth to 900 nm, the Raman intensity dropped instead (Fig. 3). These results indicate the important role of the pore depth in Raman enhancement. FDTD simulations were carried out to investigate the mechanism behind (Fig. S5 in ESI^†). As the pore depth increases, the electromagnetic field becomes more localized, which is consistent with the experimental data. However, the Raman intensity decreased on surface with very deep silicon nanopores (900 nm), which can be explained by the enhanced light trapping.^38,39 In this case, part of the Raman scattering light cannot escape from the nanopores (confirmed by the dark black color of the sample, Fig. S6 in ESI^†), leading to a weaker Raman signal. This can also be double confirmed by studying the Raman scattering from nanoporous silicon samples with AgNPs located at the bottom of the nanopores (discussed in ESI and Fig. S7^†).Open in a separate windowFig. 3Raman intensity variation (peak at 1076 cm⁻¹) on the AgNPs-coated nanoporous silicon (pore size of ∼40 nm was used) surface with four different depths. The scheme on top was not drawn to scale.Second, the size of silicon nanopores also plays a role in the Raman enhancement. The pore size on silicon surface can be tuned by controlling the size of catalysts (AgNPs) deposited on silicon wafer (Fig. S1B in ESI^†), whose size was replicated by the nanopores in subsequent catalytic etching process (Fig. S1C^†). By varying the deposition time, nanoporous silicon samples with four different pores sizes, 31 ± 10, 41 ± 11, 80 ± 24 and 160 ± 50 (Fig. 4A–D), were fabricated, respectively. For the AgNPs-coated nanoporous silicon samples, the Raman intensity of probe molecules slightly changed while increasing the pore size (Fig. 4E), indicating a weak effect of pore size on Raman enhancement. As aforementioned discussion, the Raman enhancement is mainly contributed by the AgNPs that locate on the nanopore edges. Therefore, the Raman enhancement is strongly dependent on the perimeter of all the nanopores and the number of AgNPs that locate on the edge of silicon nanopores. While increasing the pore size, the perimeter of single pore increases. However, many pores are fused together, compensating the increase of the perimeter of single pore. Thus, the total perimeter of all nanopores does not change much when increasing the pore size. In this case, the amount of the AgNPs locating on the edge of silicon nanopores may not change too much, which may explain the less dependency of the pore size on SERS enhancement. To investigate the structure of the AgNPs-coated nanoporous silicon, we deposited gold nanoparticles (AuNPs) onto it. In this case, however, a stronger Raman scattering was observed due to the formation of AgNP–AuNP nanogaps and the enhancement varied on different sized silicon nanopores. When increasing the pore size from 31 ± 10 to 80 ± 24 nm, the Raman scattering became stronger. Further increasing the pore size to 160 ± 50 nm, the Raman intensity decreased. As known, two particles formed nanogap shows a more localized electromagnetic field when the polarization of incident light is parallel to the center to center axis of the two particles.^16,40 Therefore, horizontal positioned two-particle nanogaps will give much stronger Raman enhancement. If the size of the silicon nanopores is too small, it is difficult for the AuNPs (13 nm in diameter) to enter the pore, limiting the number of AgNP–AuNP nanogaps that are horizontally positioned, and in turn limiting the Raman enhancement. While increasing the nanopore size, we have a better chance to form the ideally positioned AgNP–AuNP nanogaps to improve the Raman enhancement (Fig. S8 in ESI^†). However, when the pore size is too big, only a small part of the nanostructures locates inside the excitation volume during Raman measurement, leading to a weaker Raman signal.Open in a separate windowFig. 4(A–D) Nanoporous silicon with different pore sizes obtained by varying the silver deposition time described in Fig. 1A and B. Scale bars = 200 nm. The pore depth here was set as 220 nm. (E) Raman intensity variation (peak at 1076 cm⁻¹) on the AgNPs-coated nanoporous (four different pore sizes shown in A–D) silicon surface without (magenta bars) and with (cyan bars) the adsorption of AuNPs.The Raman enhancement can also be affected by the size of the AgNPs coated on nanoporous silicon. The AgNP size can be tuned by varying the AgNP deposition time shown in Fig. S1E.^† It has been reported that AgNPs with several tens of nanometers showed optimized plasmon resonance with excitation wavelength of 632.8 nm.⁴¹ In this work, the 40–75 nm AgNPs coated on nanoporous silicon show higher Raman enhancement than those with smaller or bigger AgNPs (Fig. S9^†), since the size of these AgNPs fall into the optimized range for Raman enhancement, which is consistent with the reported work.     

Protein-activated transformation of silver nanoparticles into blue and red-emitting nanoclusters

Proteins are very effective capping agents to synthesize biocompatible metal nanomaterials in situ. Reduction of metal salts in the presence of a protein generates very different types of nanomaterials (nanoparticles or nanoclusters) at different pH. Can a simple pH jump trigger a transformation between the nanomaterials? This has been realized through the conversion of silver nanoparticles (AgNPs) into highly fluorescent silver nanoclusters (AgNCs) via a pH-induced activation with bovine serum albumin (BSA) capping. The BSA-capped AgNPs, stable at neutral pH, undergo rapid dissolution upon a pH jump to 11.5, followed by the generation of blue-emitting Ag₈NCs under prolonged incubation (∼9 days). The AgNPs can be transformed quickly (within 1 hour) into red-emitting Ag₁₃NCs by adding sodium borohydride during the dissolution period. The BSA-capping exerts both oxidizing and reducing properties in the basic solution; it first oxidizes AgNPs into Ag⁺ and then reduces the Ag⁺ ions into AgNCs.

Protein capping can trigger nanoparticle to nanocluster transformation at elevated pH.

Noble metal nanomaterials, especially silver (Ag) and gold (Au), have witnessed exceptional research exploration in the last couple of decades from both fundamental and application perspectives.¹ These nanomaterials mainly exist in two distinct size regimes with unique optical characteristics. Ultra-small nanoclusters (NCs) (size typically <3 nm) contain only a handful of atoms (few to hundred), while relatively large nanoparticles (NPs) may comprise thousands of atoms. NPs may display strong extinction (absorption or scattering) spectra in the UV-vis region but are generally non-fluorescent.² In contrast, metal nanoclusters (MNCs) exhibit bright emission but not so noteworthy absorption spectra.^3,4 The distinct optical characteristics of the two nanomaterials have been exploited in various applications. For example, metal nanoparticles (MNPs) are extensively used in photothermal therapy⁵ and imaging,⁶ while NCs are more suited in fluorescence imagining⁷ and sensing⁸ applications. A facile transformation between the two nanomaterials could enable us to combine the complementary optical properties in a single system. Moreover, the kinetics of transformation can provide insights on various intermediate processes like dissolution, etching and digestive ripening etc.^9–11Silver nanoparticles (AgNPs) and nanoclusters (AgNCs) are of particular interest, as it not only possess the intriguing physicochemical properties of MNPs and MNCs, but also feature unique properties pertaining to silver.^3,12,13 For example, metallic silver has been well known for its capability to prevent infection since the ancient times, while recent studies revealed that ultrasmall AgNCs exhibit even superior antibacterial properties towards a broad spectrum of bacteria.^13,14 Moreover, due to superior plasmonic properties and bright fluorescence, AgNPs and AgNCs are preferred over other metal nanomaterials.^15,16The fluorescence properties of AgNCs mainly be attributed to the quantum confinement effect or surface ligand effect.¹⁷ The strong fluorescence generally arises from the electronic transition between occupied d band and states above the Fermi level (sp bands) or the electronic transition between highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).¹⁸ Several reviews have been devoted for the fundamental understanding of the fluorescence origin of AgNCs.^17,19 Recently, it was demonstrated that aggregation-induced emission (AIE) may also contribute to the luminescence pathway of MNCs.^19,20 The origin of AIE from MNCs could be attributed to the restriction of intramolecular vibration and rotation of ligand on the surface of MNCs after aggregation, which facilitates the radiative energy relaxation via inhibiting of non-radiative relaxations.^21,22Protein capping is quite common for obtaining both NPs^23,24 and NCs.^25–29 Serum proteins, bovine serum albumin (BSA) and human serum albumin (HSA) are the most popular among trials with different proteins.^26–29 BSA is a large protein which provides steric stabilization to the MNCs with its various functional group like –OH, –NH₂, –COOH, –SH.^25,30 The disulfide bond of BSA may have strong interaction with the MNCs where sulfur may be covalently bonded to the MNCs core.^24,31 The nanomaterials are synthesized within the protein template at very different pHs. AgNPs are obtained from the reduction of silver salts at neutral pH (6–8),²⁴ whereas the same process at a higher pH (>11) leads to AgNCs.^30,32 The protein capping itself may reduce Ag⁺; AgNCs are formed without any external reducing agent.^25,33 However, an external reducing agent may change the nature and kinetics of the NCs significantly.³⁰Thus, the influence of pH on the protein structure may govern the selective synthesis of AgNPs or AgNCs. BSA can achieve several conformations – N (native), B (basic), A (aged) and U (unfolded) as the pH of the medium gradually changes from neutral to highly alkaline.^34,35 It may be possible that a specific type of nanomaterial is stable within a particular conformation dictated by the pH of the medium. Hence, by simply changing the pH, we may expect a significant modulation of the morphology of the nanomaterial. Herein, we applied this concept to show an effortless transformation from AgNP to AgNC. Although BSA template is exceptionally popular in the preparation of both AgNPs and AgNCs, however, to the best of our knowledge, no report is available on the conversion from AgNP to AgNC within the protein capping.The BSA-capped AgNPs (BSA-AgNPs) were first synthesized at a neutral pH (pH = 6) using sodium borohydride reduction (see ESI^†). The AgNPs show a sharp surface plasmon resonance (SPR) band at 415 nm (Fig. 1a) and have uniform diameters of 12.5 ± 1.5 nm (Fig. 1b). The AgNPs are quite stable at this pH with no apparent change in the SPR band even after 15 days (Fig. S1^†).Open in a separate windowFig. 1(a) UV-vis spectrum and (b) TEM image of BSA-protected AgNPs synthesized at pH 6. The insets show the appearance of the AgNP solution under regular and UV light (left panel), and size distribution histogram (right panel).However, when the BSA-AgNPs were treated with NaOH to elevate the pH to 11.5, we observed a remarkable decrease in the SPR band at 415 nm and a color change from dark to light brown within 2 h of the pH jump (Fig. 2a). The observations indicate the dissolution of AgNPs, which was further confirmed from the TEM images taken quickly (∼10 min) after the NaOH treatment (Fig. S2^†). Heterogeneous distribution of AgNPs was obtained with sizes varying from 2.6 nm to 17 nm, which is in sharp contrast to the uniform AgNPs before the addition NaOH (cf.Fig. 1b). Upon further incubation (6 h), the light brown color gradually faded to light yellow with a further decrease in the SPR band absorbance (Fig. S3^†).Open in a separate windowFig. 2Time evaluation of the (a) UV-visible and (b) emission spectra (λ_ex = 370 nm) of BSA-capped AgNPs after enhancement of the pH from 6 to 11.5 (by addition of NaOH at t = 0). The inset shows the snapshot of the final blue-emitting AgNC solution under normal and UV lights. (c) TEM of the blue-emitting AgNCs along with the HRTEM image and the analyzed size distribution in the inset. (d) MALDI-mass spectra of the BSA protein and BSA-capped blue-emitting AgNCs.Interestingly, the solution also develops distinct fluorescence with a maximum at ∼460 nm after the addition of NaOH (Fig. 2b). The fluorescence intensity gradually grows up upon incubation, and finally, an intense blue fluorescence was developed within ∼9 days. The final NaOH-treated AgNP solution appears to be light yellow under normal light and blue-fluorescent when viewed under a hand-held UV lamp (Fig. 2b, inset). The blue-emitting AgNCs exhibit a single band excitation spectrum with a maximum at 372 nm (Fig. S4^†).TEM image of the optimized NCs (after 9 days incubation at 37 °C) exclusively reveals uniform AgNCs of ∼2.10 ± 0.28 nm diameter without any trace of large NPs (Fig. 2c). The mass of the BSA-capped AgNCs (67 375 Da) was shifted by 845 Da from that of native BSA (66 530 Da) (Fig. 2d). Thus, the new species should correspond to Ag₈ cluster. The characteristics of the blue-AgNCs were quite similar to the human serum albumin (HSA)-protected blue-AgNCs, directly prepared from silver salt.³³ However, the formation time of those AgNCs was significantly less (∼10 h) than the present method (∼9 days).³³ Thus, the initial dissolution process, although quite fast, may have a crucial role in the kinetics of the protein-protected NCs. When we performed a similar pH jump experiment on a citrate-stabilized AgNP,³⁶ the extinction spectrum of the AgNPs showed much less variation compared to the BSA-AgNPs. Instead of a strong decrease, the SRP band showed a red-shift with an extended tail indicating aggregation rather than dissolution of NPs (Fig. S5^†).Furthermore, a red-emitting cluster was generated when an external reducing agent, sodium borohydride (NaBH₄), was added during the dissolution process. NaBH₄ was added after ∼11 min of the NaOH addition when the SPR band of BSA-AgNP was already decreased by half (Fig. 3a). The SPR band (λ_max = 415 nm) of AgNP continues to diminish similarly before and after the addition of NaBH₄ (Fig. S3^†). Thus, NaBH₄ may not have any significant effect on the dissolution process of AgNP. However, it has a strong impact on the modulation of the fluorescence; a new fluorescence band was developed at ∼650 nm within a much shorter duration (1 h) (Fig. 3b). The solution exhibits a bright-red fluorescence under a UV lamp (Fig. 3b, inset) with a quantum yield of 3.5%.Open in a separate windowFig. 3Early time evolution of (a) UV-visible and (b) emission spectra (λ_ex = 370 nm) of the BSA-protected AgNPs upon subsequent treatments with NaOH (pH 11.5) and NaBH₄ at t = 0 and 11 min, respectively. The decrease of the SPR band at 415 nm and a concomitant increase of the fluorescence band at ∼650 nm indicates dissolution of the AgNPs and formation of the red-emitting cluster. The inset shows the visuals of the AgNCs formed after 1 h under normal light and UV light. (c) TEM of the red-emitting AgNCs along with the HRTEM image and the analyzed size distribution in the inset. (d) MALDI-mass spectra of the BSA protein and BSA-capped red-emitting AgNCs.TEM measurements of the red-emitting species show homogeneous distribution of AgNCs with ∼2.25 ± 0.25 nm diameter (Fig. 3c). MALDI-mass experiment further assigned the red-emitting species as Ag₁₃ cluster (Fig. 3d). The excitation spectrum (λ_em = 650 nm) displays two distinct peaks at 370 nm and 470 nm, which match closely to the reported excitation peaks of the Ag_13–15 clusters within BSA/HSA capping (Fig. S4^†).^30,32,33 Moreover, the fluorescence decay of the red-emitting-AgNCs converted from AgNP almost matches with those prepared directly from AgNO₃; both display very similar average lifetimes (0.95 ns vs. 0.89 ns) (Fig. S6 and Table S1^†).³⁷Another important observation is that the red-emitting AgNCs have only transient stability at 37 °C. With further incubation, the absorbance at ∼470 nm (characteristic excitation peak of the red-emitting cluster) reduces and the absorbance at 370 nm (excitation peak of the blue-emitting cluster) increases simultaneously (Fig. 4a). The red-emission at 650 nm also decreases gradually with a concomitant increase of a blue emission band at 465 nm (Fig. 4b). Thus, both absorption and emission measurements clearly indicate transformation of red- to blue-emitting clusters which takes up to ∼15 days for completion. The solution finally becomes light yellow and exhibits a bright blue fluorescence under UV light similar to the blue-emitting cluster obtained earlier from the AgNP in the absence of NaBH₄. Interestingly, other characteristics of the regenerated blue-emitting AgNCs (converted from Ag₁₃NCs) also match quite nicely with the directly prepared blue-AgNCs (converted from AgNPs in the absence of NaBH₄). The size of this blue cluster was 2.04 ± 0.12 nm, which is similar to the previously obtained direct blue-emitting cluster (2.10 ± 0.28) (Fig. 4c). Furthermore, MALDI-mass measurement reveals that both the blue-emitting clusters may have the same composition, Ag₈ (Fig. 4d). In addition to this, the average lifetime (0.53 ns) of the blue-emitting AgNCs synthesized from AgNP agrees well to the average lifetime (0.40 ns) of the blue-emitting AgNCs converted from the red-emitting AgNCs (Fig. S7 and Table S2^†). However, the quantum yield (23%) of blue-emitting AgNCs, converted from red-emitting AgNCs, was higher than the quantum yield (18%) of the blue-emitting AgNCs, converted from AgNPs. Since, the emission characteristics of the blue and the red-emitting clusters nearly matches with earlier report, we expect that silver may be present in the zero oxidation state as determined in those studies.^30,31Open in a separate windowFig. 4Transformation of red-emitting to blue-emitting cluster: (a) UV-visible and (b) emission spectra (λ_ex = 370 nm) showing transformation of the BSA-protected red-emitting Ag₁₃NCs (obtained at 1 h) to blue-emitting AgNC upon prolonged incubation. Red and blue arrows respectively denote the decrease/increase of the red/blue cluster absorbance and emission intensity with time. The inset (b) shows a magnified wavelength region in 580–720 nm of the emission spectra. (c) TEM image of the blue-emitting silver nanocluster while its inset shows HRTEM image with size histogram of corresponding silver nanocluster. (d) MALDI-mass spectra of BSA and BSA-containing blue-emitting silver nanocluster synthesized from Ag₁₃NCs.Moreover, the atomic composition of the NCs can be also be estimated from the Jellium model using the equation^38,39E_em = E_Fermi/N^0.33where E_Fermi is the Fermi energy of the metal (Ag), E_em is the emission energy of the MNCs and N is the number of atoms constituting a MNC. Using the model equation, the number of silver atoms for the blue-emitting AgNCs can be predicted as 8.45 (∼8) Ag atoms which is a good agreement with our MALDI data (8 Ag atoms). However, the theoretical calculation estimated as N ∼ 24 for the red-emitting AgNCs, which is not in agreement with the MALDI data (13 Ag atoms). This is because of the well-known deviation of the Jellium model for higher number of Ag atoms in AgNCs because of increase in the electronic screening effects and the harmonic distortion in the potential energy well.¹⁹Although the red-emitting cluster is not very stable at the experimental condition (pH 11.5, 37 °C), it may be easily stabilized by lowering the temperature or pH. The fluorescence intensity of the red-emitting and blue-emitting cluster kept at 4 °C, was almost preserved for more than 15 days (Fig. S8^†). On the other hand, lowering the pH to 6, also inhibits the red to blue-cluster transformation (Fig. S9^†). The observations indicate that the red-blue transformation has a moderate activation barrier and the conversion may be governed by the change in the structure of the protein in the alkaline condition. Acidification of the solution can stop the transformation of the protein conformation and inhibits the process.From these observations, we may conclude that the conversion from NPs to NCs occurs in two steps. First, a rapid dissolution of AgNP occurs in the alkaline medium. The kinetics of the dissolution process can be monitored through a time-dependent decrease of the SPR band and the time constant was found to be ∼13 min (Fig. S10^†). Dissolution of AgNPs is an important issue and assumed to be the leading cause of toxicity of AgNPs in biological mediums.⁴⁰ The dissolution is commonly favored at a low pH but drastically inhibited at high pH.⁴¹ The swift dissolution of the BSA-protected AgNP observed here at a high pH (11.5) is unprecedented. Thus, the BSA capping may have an active role in the dissolution process. We comprehend that the oxidation power of protein may be activated in the basic medium.Organothiols (R-SH) are known to promote dissolution of AgNPs; R-SH progressively reacts with Ag atoms to form RS-Ag complex.⁴² Since cysteine is also an organothiol, it is expected to play an essential role in the dissolution of AgNPs. Gondikas et al. showed that excess cysteine could favor the dissolution process of AgNPs, whereas another amino acid, serine (S–H bond is replaced by O–H bond), has no effect.⁴³ Zang and coworkers showed that only the isolated or reduced cysteine in a protein has a dominant role in the dissolution of NPs.⁴⁴ Although BSA contains as many as 35 cysteine residues; 34 of them are involved in S–S bond formation and only a single cysteine is present in free form (S–H). Hence, the dissolution of AgNPs at neutral pH may be negligible.Most proteins rich in sulfur-containing residues (cysteine and methionine) may degrade in alkaline solution. Florence reported that about 5 of 17 S–S bridges in BSA may be cleaved in the presence of 0.2 M NaOH.⁴⁵ Thus, at higher pH, some disulfide bonds may be cleaved and more cysteine residues may participate in the dissolution of BSA-capped AgNPs.In the second step, Ag⁺ ions generated from the dissolution of AgNPs, can be reduced either by the protein capping itself or by an external reducing agent to form NCs (Scheme 1). The tyrosine residues may be responsible for the reduction of the metal ions to NCs.^25,33 At a pH, higher than the pKa (10.46) of tyrosine, the reduction capability of tyrosine is enhanced by deprotonation of the phenolic group.^25,33,46 Moreover, the addition of a strong reducing agent (e.g., NaBH₄) may lead to a faster reduction, which favors quicker nucleation and growth of Ag atoms forming the bigger NCs (Ag₁₃NCs). However, the large Ag₁₃NCs may not be adequately stabilized by the protein conformation at that condition and hence may transform into the more stable blue-emitting Ag₈NCs.Open in a separate windowScheme 1Schematic representation of the transformation of the BSA-capped AgNPs to blue- and red-emitting AgNCs.The conformation change of the protein capping during the conversion was also supported by the circular dichroism (CD) measurements (Fig. S11^†). The formation of AgNPs results in a negligible change in the protein conformation (Table S3^†). However, the formation of red Ag₁₃ cluster results in a substantial modification in the BSA conformation. The α helix content reduces from 57% to 49%, whereas coil randomness increases from 17% to 21% without a major change in the β sheet. Interestingly, blue-emitting Ag₈ cluster perturbed the conformation of the BSA to a much larger extent (Table S3^†). As the cysteine disulfide bond has a direct role on maintaining the folded conformation of BSA, its breaking may change the protein conformation. The addition of NaOH induces breaking of S–S bond, which leads to formation of AgNCs with subsequent change in protein secondary structure.In conclusion, we report an unprecedented fast dissolution of AgNPs through activation of the protein (BSA) capping by elevating the pH of the medium to 11.5. At higher pH, the disulfide bonds may be cleaved, and the free cysteine may activate the dissolution process. The protein capping also plays a crucial role in the formation of fluorescent nanocluster after the completion of the dissolution process. Thus, we explored multiple roles of the BSA capping – (1) a stable capping agent at neutral pH to stabilize the AgNPs (2) activates the dissolution process probably via oxidative dissolution of the AgNPs (3) adsorbing the nascent silver ions within its scaffold and (4) finally reducing them to fluorescent nanocluster.     

A top-down design for easy gram scale synthesis of melem nano rectangular prisms with improved surface area

An unprecedented top-down design for the preparation of melem by 1 h stirring of melamine-based g-C₃N₄ in 80 °C concentrated sulfuric acid (95–98%) was discovered. The melem product was formed selectively as a monomer on the gram scale without the need for controlled conditions, inert atmosphere, and a special purification technique. The as-prepared air-stable melem showed a distinctive nano rectangular prism morphology that possesses a larger surface area than the melems achieved by traditional bottom-up designs making it a promising candidate for catalysis and adsorption processes.

A novel practical method for the gram scale preparation of melem possessing a nano rectangular prism morphology and improved specific surface area through a top-down depolymerization design was developed.

Triamino-s-heptazine or 2,5,8-triamino-tri-s-triazine known as “melem”, is a mysterious molecule and invaluable intermediate in the density of melamine rings to graphitic carbon nitride (g-C₃N₄) with a rigid heptazine structure with three pendant amino substituents.¹ Melem does not bear two of the strongest emission quenchers, namely C–H and O–H groups;² In this way it has unique optical properties³ and is known as an efficient metal-free luminescent material.⁴ High stability, the possibility for supramolecular self-assembly, tunable band gap, and an already rich physicochemical chemistry are some of the known properties for melem.^1b For this reason, melem has the potential to be used in photocatalysts, MOFs, COFs, electrochemistry sensors, flame retardants, TADF and related OLEDs, and liquid crystals.¹ The use of melem in solar hydrogen evolution⁵ and bioimaging⁶ is also known.Very few reports of its catalytic application are available, nevertheless, in recent years it has attracted much attention because of exploring its unique properties. Metal-free g-C₃N₄/melem hybrid photocatalysts have been used for visible-light-driven hydrogen evolution.⁷ Lei et al. used melem single crystal nanorods as a photocatalyst with modulated charge potentials and dynamics.⁸ Recently, Liu et al. improved the photocatalytic properties of carbon nitride for water splitting by attaching melem to Schiff base bonds.⁹ In another report, a promotion in photocatalytic activity was obtained by construction of melem/g-C₃N₄ vermiculite hybrid photocatalyst for photo-degradation of tetracycline.¹⁰ Lei et al. reported that H₂ evolution activity of melem derived g-C₃N₄ was 18 times higher than g-C₃N₄.¹¹ Melem was also utilized as a precursor for the preparation of rod-like g-C₃N₄/V₂O₅ heterostructure with enhanced sonophotocatalytic degradation for tetracycline antibiotics.¹² CO₂ cycloaddition into cyclic carbonates,¹³ non-sacrificial photocatalytic H₂O₂ production,³ water treatment,¹⁴ simultaneous reductions of Cr(vi) and degradation of 5-sulfosalicylic acid,¹⁵ are some of the catalytic applications of melem at various fields of sciences.The main protocol of preparing melem is the annealing of cyanamide, dicyanamide, or melamine, which requires precise temperature control under an inert atmosphere such as N₂ or argon. Just recently, the synthesis approaches for molecular s-heptazines as well as their applications and properties have been reviewed by Audebert et al.^1b Most of the reported methods do not lead to the preparation of pure monomer melem and are often mixed with its oligomers and polymerized derivatives,¹⁶ meanwhile the possibility of forming triazine oligomers or oligomers between melem and triazine cannot be precluded.^5,16c Complete polymerization of melamine at 500–550 °C leads to g-C₃N₄ and at 400–450 °C leads to melem-like derivatives,¹⁷ mostly a mixture of different products requiring careful attention during isolation and purification.⁵ Recently, Kessler and his colleague investigated the thermolysis of melamine, the formation of melem, and the formation of poly(triazine imide) from melem precursor via ionothermal as well as thermal condensation (conventional synthesis) as the back reaction of the melem condensation.¹⁸The growing demands for employing melem in new applications besides the serious problems in preparing pure samples necessitate the development of a simple and operational scale-up method that does not have any acute and controlled conditions.It is well-known that the polycondensation mode of g-C₃N₄ and consequently the chemical and thermal stability as well as texture properties strongly depend on the nitrogen rich precursors (cyanamide, dicyandiamide, urea, and melamine) as well as annealing temperature.¹⁹The interaction between the molecular precursors and/or intermediate compounds are critical factors.¹⁷ Due to some drawbacks associated with the g-C₃N₄ such as low electronic conductivity, a high rate of photogenerated electron–hole pairs, a low surface area, poor visible-light absorption, low quantum yield, and low solubility in almost all of the traditional solvents,²⁰ it has been subjected to various acid treatments, to promote its properties and photochemical activity.²¹ Various nanosheets with different properties and morphologies have been obtained depending on the precursor used, acid nature and concentration, as well as reaction temperature and time.²² However, the oxidation products such as cyameluric or cyanuric acids (Scheme 1) under high reaction temperatures and times have been reported.^21cOpen in a separate windowScheme 1The selective production of the monomer melem from melamine-based g-C₃N₄ presented in this work. Other molecules are possible decomposition and/or oxidation products of g-C₃N₄.Inspired by the previous reports to prepare the acidified g-C₃N₄, we started with melamine to synthesize the g-C₃N₄ by calcining at 550 °C under air,²³ followed by the treatment with H₂SO₄. Nevertheless, we discovered that stirring the melamine-based g-C₃N₄ at concentrated H₂SO₄ (95–98%) at 80 °C for a limited time (1 h), afforded selectively monomer melem in high yield (Scheme 1). Following the intercalation, chemical exfoliation, and protonation of nitrogen atoms of the g-C₃N₄ sheets at concentrated H₂SO₄,^22,24 the bridging C–NH–C groups between s-heptazine units breaks which releases the triamino-s-heptazine (melem) molecules as monomer (Scheme S1^†). Under these conditions the formation of oligomers was precluded because of the effective breaking of the bridging amino groups, however, the limited reaction time and moderate temperature prevented the tri-s-triazine ring-opening as well as the formation of the oxidation products such as cyameluric (or cyanuric) acids.^21c Thus, we developed a facile and easy gram-scale synthesis of melem from acidic depolymerization of melamine-based g-C₃N₄ with no need for controlled conditions, and inert atmosphere. The air-stable white powder was insoluble in most common solvents (H₂O, C₂H₅OH, CH₃OH, DMF, CH₃CN, acetone, etc.) and only dissolved in DMSO with a very limited solubility exactly like that reported for the isolated pure monomer melem.^5,25 A new and distinctive rectangular prism morphology with an improved surface area was detected for the as-prepared melem,^5,8,26 which makes our study even more unique and novel.^5,8,27 It is well known that both morphology and specific surface area play important roles in affecting the photocatalytic activity of semiconductors.^22,28 Thus, our study not only provides a novel practical method for the preparation of nanostructured monomer melem, but also paves a new pathway for increasing its surface area. The chemical structure and purity of the as-prepared melem were verified by the combination of different techniques including FT-IR, ¹H and ¹³C NMR, mass spectra, elemental analysis, XRD, XPS, DRS, and photoluminescence spectroscopy.FT-IR spectrum of g-C₃N₄ and the as-prepared melem are depicted in Fig. 1. While the peaks at 803 and 796 cm⁻¹ exhibited the vibrations of tri-s-triazine moieties in g-C₃N₄ and melem respectively, two intense bands at 1622 and 1471 cm⁻¹, consistent with those of monomer melem. The lack of obvious C–NH–C vibrations at around 1230 cm⁻¹ featured the absence or negligible amount of dimelem or further melem-oligomers in the product.^29a,5,8 In the region of NH-stretching frequencies, a spectrum characteristic of amides is observed: three diffuse absorption bands (3415, 3360, and 3106 cm⁻¹) indicate the presence of strong intermolecular hydrogen bonds and strong interaction between the amino-groups and the ring.^29a Inspection of the characteristic bands of melem presented in Fig. 1, no evidence for the formation of cyameluric acid or other oxidation products (Scheme 1) was detected.²⁹Open in a separate windowFig. 1FT-IR of the as-synthesized g-C₃N₄ and Melem.In the ¹³C NMR spectrum (Fig. S1^†), two signals at 165.8 and 156 ppm are assigned to carbon atoms adjacent to the amino groups and CN3 groups in heptazine rings, respectively.^1a,5 The ¹H NMR (Fig. S1^†) showed a sharp signal at 7.4 ppm assigned to six protons of the terminal amino groups of melem along with two weak broad signals at ∼8 ppm which can be attributed to the partial protonation of some nitrogens. The lack of the signal at 149.37 ppm in ¹³C NMR^29b and a high-field signal at ¹H NMR (10.9 ppm or higher)³⁰ strongly confirmed that our method precludes the formation of cyameluric acid accompanied by the desired melem.The mass spectrometry depicted in Fig. S2^† shows that the bulk material contains almost entirely monomer melem evidenced by the main peak at m/z 218 pertinent to a single unit of melem and a very little peak at m/z 419 corresponding to dimelem and nothing of higher mass.³¹ Also, no trace of the oxidation products was observed in mass spectra (m/z 129 and 221 for cyanuric and cyameluric acids, respectively).The C/N atomic ratio is one of the most significant clues to prove the successful formation of melem. The ratio of 0.605 found for the produced melem is very close to the theoretical value in the monomer melem (C/N = 0.6).^5,8A substantial evidence for the exclusive formation of monomer melem was achieved by the XRD pattern. Fig. 2 shows XRD patterns of melem and its polymeric graphitic carbon nitride used in this work. A great match with literature was observed.⁵ Two characteristic peaks of g-C₃N₄ at 2θ = 13.28° (100) and 27.47° (002) related to the in-plane structural packing motif, and interlayer-stacking of aromatic systems respectively, significantly changed after treatment with 80 °C concentrated sulfuric acid for 1 h and showed strong evidence for the formation of the monomer melem.^5,8 The former peak (100) became pronounced and shifted to a lower angle of 12.52°, while the latter one (002) was shortened in the melem and shifted to the higher angle of 2θ = 27.6° caused by decreased stacking distance between the melem inter-layers. More important is the emergence of a new intense peak at 6.16°, which is the unique characteristic of monomer melem,⁵ while, other weak peaks located at about 19, 23, 25, 29 and, 31 are almost looked at in the XRD patterns of both monomer and oligomers.^5,8 No trace of cyameluric acid as the possible oxidation product was detected in the XRD pattern of the resulting product.^21c,30Open in a separate windowFig. 2XRD patterns of g-C₃N₄ and the as-synthesized melem.Next, XPS was used to identify the chemical environments of the product as shown in Fig. 3 and S3.^† Only C, N, and trace amounts of O and S caused by the negligible remaining sulfuric acid and water can be detected (Fig. S3^†). The C1s signals (Fig. 3 left) can be fitted into five components with binding energies of 284.5 eV, 285.18 eV, 287.78 eV, 288.58 eV, and 293.58 eV. The C signal at 284.5 eV is exclusively assigned to carbon atoms (C–C bonding) in a pure carbon environment, such as graphitic or amorphous carbons.^26,32 The signals at 285.16 and 287.78–288.3 eV attributed to graphitic carbon sp² C–C, and the sp² trigonal C–N bonding (s-triazine ring), respectively, characteristic of melem structure.^8,33 The advent of a high-energy satellite at 293.7 eV corresponds to the Π-electron delocalization in the heptazine system of melem.²⁷ The N 1s signals (Fig. 3 right) were deconvolved into five peaks. The signals with binding energies of 398.4, 400, and 401–404 eV are associated with the sp²-hybridized nitrogen (C N–C), tertiary nitrogen (N–(C)₃), and protonated amino groups (C–N–H) in melem, respectively.^33a,34 The emergence of (N–(C)₃) undoubtedly indicated the preservation of tri-s-triazine units (C₆N₇, basic part of melem molecule) during treatment with 80 °C concentrated acid. Thus, no significant changes in the carbon nitride heterocycles such as the oxidation transformation of terminal C–NH–C to C–OH–C and/or tri-s-triazine ring-opening reactions occurred.^21c,36 The advent of a satellite at high binding energy of 406 eV corresponds to the partial protonation of some nitrogens (N–H⁺).³⁵Open in a separate windowFig. 3XPS spectra of the as-prepared melem, left: C 1s and right: N 1s.The morphology of the as-synthesized product was determined by FESEM (Fig. 4A). The FESEM images clearly show microsized rectangular prisms with thickness ranging from ∼50 to 350 nm, which was completely different with carbon nitride with the main nanosheets morphology.³⁷ To the best of our knowledge, this is the first report for such a morphology for melem,⁸ that aroused our curiosity to assess its surface properties. The porosity of the samples was determined by N₂ physisorption experiments. The N₂ adsorption/desorption isotherms and pore size distributions of the as-prepared melem are given in Fig. 4B. The sample exhibited typical type IV isotherms with H3 hysteresis loop according to the IUPAC classification,²⁷ suggesting mesoporous structures with slit-shaped pores resulting from the aggregation of plate-like particles.³⁸ The BET specific surface area of the as-synthesized melem was found to be 19.54 m² g⁻¹ which is about 3–4 folds larger than those reported for bulk melems as 5.63 m² g⁻¹,²⁷ and 7.02 m² g⁻¹,¹² as well as melem nanorods as 4.87 m² g⁻¹,⁸ obtained from the condensation of melamine. These results clearly show the superiority of our easy-to-make melem over the other samples obtained by the traditional bottom-up design under quite controlled conditions.^5,8,27 The mesoporous nature of the as-synthesized melem was further supported by the pore-size distribution analysis depicted as an inset of isotherm (in Fig. 4B) indicating an average diameter of pore size at 2.1 nm.Open in a separate windowFig. 4(A) FESEM image and (B) BET N₂ adsorption/desorption isotherms of the as-synthesized melem.The TG analysis of the as-synthesized melem exhibited three mass loss steps (Fig. S4^†). At the first step, the sample lost about 10% of its weight at less than 200 °C caused by removing water and ethanol molecules absorbed during the elution process. The second one was begun at around 240 °C and continued to 500 °C with the evolution of ammonia and small amounts of HCN, attributed to the condensation polymerization of the monomer.The third thermal decomposition was accelerated above 500 °C (with releasing HCN and C₂N₂),⁶ rendering strong evidence for the absence of triazine derivatives (or lower) in the as-prepared product and once again ruled out the tri-s-triazine ring-opening reactions during the synthesis of melem in this work.^21c,39 The high thermal stability of the produced melem,⁶ is comparable with the parent g-C₃N₄, making it more appropriate for comparative studies and applied goals that add further benefits to our sample.Lastly, the optical properties of the sample were evaluated using UV-Vis diffuse reflectance spectroscopy (DRS). As shown in Fig. S5,^† the absorption maximum wavelength of the resulting melem locates at 310 nm coincides with that reported in the literature.^5,40 The band edge of melem shifted to the lower wavelength (380 nm) compared to the polymer g-C₃N₄ (460 nm), caused by decreasing in Π-electron delocalization in the heptazine system of melem which stretches the band gap from 2.7 eV (polymer) to 3.45 eV (monomer melem) in excellent agreement with reported theoretical value for monomer melem (3.497 eV).^5,27 Further support for this claim was obtained by fluorescence spectra. Fig. 5 shows the comparative fluorescence spectra of the as-synthesized melem, under 355 nm light excitation. As shown in Fig. 5, it is found that the fluorescence emission of polymer g-C₃N₄ peaked at 476 nm,^40a shifted to 412 nm in the melem coincide with Ricci report (415 nm).⁴¹ In addition, the photoluminescence intensity of the resulting melem increased significantly compared to polymer g-C₃N₄ in broad agreement with literature indicating that the condensation of melem to g-C₃N₄ causes the weaker photoluminescence.^40aOpen in a separate windowFig. 5Photoluminescence spectra of g-C₃N₄ and the as-synthesized melem under 355 nm light excitation.Finally, our formulation is very simple and robust with respect to processing conditions to overcome the potential scale-up problems to make it operational and amenable to scalability readily. As an example, a 5 fold semi-scaled-up procedure using 1.0 g g-C₃N₄ led to the isolation of the related pure monomer melem in 95% yield within 1 h.In summary, we developed a novel operational protocol for easy gram scale preparation of air-stable monomer melem through a top-down synthesis design with no need for any control conditions and further purification. Our analyses ruled out the presence of the starting polymer as well as the formation of oligomers and oxidation products in the final product highlighting the selectivity of the method toward the monomer of melem. The distinctive nano rectangular prism morphology with desired surface area and thermal stability, as well as the appropriate photoluminescence property qualifies our synthesized melem for applied goals and makes it a promising alternative for catalysis and adsorption processes which is under investigation in our lab.     

Luminometric dosimetry of X-ray radiation by a zwitterionic uranium coordination polymer

A novel X-ray dosimeter based on a uranium coordination polymer U-Cbdcp was obtained by the judicious synergy between the luminescent uranyl centres and zwitterionic tritopic ligands. Notably, U-Cbdcp exhibits luminescence quenching upon increasing X-ray dose, which in combination with its excellent radiolytic stability, makes it suitable for X-ray dosimetry.

A novel X-ray dosimeter based on a uranium coordination polymer has been developed by the judicious synergy between the luminescent uranyl centres and zwitterionic tritopic ligands.

X-ray radiation has been extensively used in medical diagnosis and treatment, security screening, quality control inspection, scientific instrumentation, etc.^1–3 Overexposure to X-ray radiation cause damage to human cells, which could result in skin burn, tissue damage, and increased incidence of cancer.^4,5 Moreover, X-ray dosimetry is required in many industrial fields, including food irradiation, sterilization, and material modification.⁶ Thus, different types of radiation dosimeters, including ionization chamber, scintillator, semiconductor, thermoluminescence dosimeter, chemical dosimeter, and so on, have been commercialized to quantify the incident X-ray dose.⁶ The former three types of dosimeters are more frequently applied to record the dose-rate of incident radiation.^7–10 Thermoluminescence dosimeters and chemical dosimeters are suitable for dosimetry of accumulated dose, but they suffer from critical drawbacks such as cumbersome reading processing, instrument-demand, or cost-ineffectiveness.^11,12 Therefore, further development of new types of X-ray dosimeters remains essential.Coordination polymers, which are assembled from metal ions and organic ligands, have been met with great interest in diverse fields including catalysis, sensing, sorption, separation, and luminescence.^13–18 Their tunability in terms of chemical composition, structure, and more importantly photophysical property, makes them promising for radiation detection. Indeed, pioneering works by Allendorf and co-workers have demonstrated that scintillating metal–organic frameworks (MOFs) assembled from metal cations and radioluminescent organic ligands can function as a new type of radiation detection materials.¹³ Furthermore, coordination polymers or cluster species showing radiochromism, radio-photoluminescence, fluorochromism, and photoluminescence quenching upon accumulated doses of ionizing radiation have been documented, making them as promising candidates of radiation dosimeters.^19–26 Notably, the abundance of luminescent centers or radio-responsive moieties in some of these materials renders higher saturation point in response to radiation dose. This attribute allows for wider operation ranges or higher upper limits of detection compared with those of traditional metal-ion-doped inorganic dosimeters, e.g. Ag-doped phosphate glass and Mg²⁺-doped LiF (LiF:Mg).²⁷We have recently undertaken a study focused on developing actinide-based coordination polymers or cluster materials for their promising applications in ionizing radiation detection.^19,21,28 The large coordination numbers and diverse coordination geometries of actinide cations engender a myriad of topologies of these materials.^29–31 Moreover, the intrinsically intense green emission from the uranyl cation can be utilized as a radio-luminescent center.^23,26 In addition, the slight radioactivity of ²³⁸U (t_1/2 = 4.47 billion years) can be neglected in the course of high dose radiation detection.^32,33 Herein, a novel zwitterionic uranium coordination polymer is reported, showing rather unique fluorescence quenching response to X-ray radiation. This radio-responsive feature, in combination with its high radiolytic stability, points to the potential implementation of uranium-bearing materials for radiation dosimetry.Solvothermal reaction between UO₂(NO₃)₂·6H₂O, zwitterionic N-(4-carboxybenzyl)-(3,5-dicarboxyl)pyridinium bromide (H₃CbdcpBr), and CH₃COOH in DMF/H₂O mixed solution at 100 °C afforded yellow crystals of UO₂(OH)(H₂Cbdcp)(HCbdcp)·4H₂O (U-Cbdcp) with a yield of 63% based on U.Single crystal X-ray diffraction (SCXRD) analysis revealed that U-Cbdcp crystalizes in the monoclinic P2₁/n space group (Table S1^†). The asymmetric unit of U-Cbdcp network consists of one crystallographically independent UO₂²⁺ cation, two Cbdcp ligands, and one hydroxide group (Fig. S1^†). The coordination geometry of uranyl cation can be best described as a typical pentagonal bipyramid, of which four O atoms on the pentagonal plane are donated from three Cbdcp ligand and the rest one is from a hydroxide group (Fig. 1a).^34–38 One of the organic linkers coordinates with one uranyl cation in a μ₁–η₁ bridging mode, while the other one interconnects with two uranyl cations in a μ₂–η₁:η₂ manner (Fig. 1b). Therefore, these two crystallographically unique ligands can be assigned as H₂Cbdcp⁻ and HCbdcp²⁻ with one and two carboxylate group being deprotonated, respectively. The torsion angles between the carboxybenzyl and (3,5-dicarboxyl)pyridinium moieties are measured to be 110.909° and 112.447° for H₂Cbdcp⁻ and HCbdcp²⁻, respectively, as defined by ∠N–C–C. The assembly of uranyl cations, H₂Cbdcp⁻, and HCbdcp²⁻ ligands results in the formation of a one-dimensional infinite chain extending along the c axis (Fig. 1c). The afforded chains are further extended into a 3D supramolecular network via π–π interactions and hydrogen bonds between the ligands (Fig. 1d). The phase purity of bulky U-Cbdcp sample was confirmed by powder X-ray diffraction (PXRD), showing that the measured pattern matches well with the simulated one (Fig. S2^†).Open in a separate windowFig. 1(a) The coordination environment of UO₂²⁺ cation. (b) The coordination modes of two crystallographically independent ligands. (c) The 1D chain of U-Cbdcp extending along the c axis. (d) Representation showing the network of U-Cbdcp. In figure (a)–(c), U atoms are in green, O atoms are in red, N atoms are in blue, and C atoms are in grey.The solid-state luminescence spectrum (λ_ex = 365 nm) was collected on a tablet of U-Cbdcp, that was fabricated from finely ground powder. As expected, U-Cbdcp exhibits five characteristic bands of uranyl cation centring at 488, 508, 531, 556, and 583 nm (Fig. 2a). This intense green emission can be attributed to the HOMO-LUMO transition occurring in the uranyl bonds upon UV excitation.^38,39 Strikingly, the uranyl-based luminescence is strongly quenched after X-ray radiation (4.7 kGy) as shown by the photographs of U-Cbdcp tablet (Fig. 2a inset). Concomitantly, the intensities of characteristic UO₂²⁺ emission bands, which were measured from the tablet exposed to specific interval of X-ray dose, gradually diminished upon continuous X-ray irradiation (Cu-K_α, 120 Gy min⁻¹). More specifically, approximately 44% luminescence intensity was retained after being exposed to 260 Gy X-ray radiation (Fig. 2a). Further increasing the dose to 4.7 kGy resulted in nearly 90% emission quenching. Interestingly, I₀/I as a function of radiation dose can be well fitted with a linear correlation with R² of 0.9988, where I₀ and I are the luminescence intensities monitored at 508 nm before and after irradiation, respectively. This excellent linearity allows for quantifying X-ray dose in a wide dynamic range spanning from 10 to 4700 Gy via a luminescence “turn-off” manner. To obtain limit of detection (LOD), the calibration curve was established by plotting the quenching rate (I₀ − I)/I₀ as a function of dose at the low dose range (0–30 Gy) (Fig. S3^†). The limit of detection (LOD) is calculated to be 0.093 Gy based on the method reported by Zang and coworkers.⁴⁰ Markedly, this LOD is comparable to 0.047 Gy of the most sensitive photochromic sensor Htpbz@Th-SINAP-2.²¹Open in a separate windowFig. 2(a) X-ray dose-dependent fluorescence spectra and optical micrographs (inset) of a U-Cbdcp tablet. (b) The plot showing the linear correlation between I₀/I and X-ray dose.To decipher the quenching mechanism, the structures of U-Cbdcp before and after X-ray irradiation (5 kGy) were thoroughly characterized by combined techniques including PXRD and SCXRD. The PXRD patterns of U-Cbdcp remained approximately unchanged upon irradiation, ruling out our initial speculation of radiation induced damage to the bulky sample (Fig. S4^†). This supposition is additionally supported by the nearly identical FTIR spectrum of irradiated U-Cbdcp with the nonirradiated one (Fig. S5^†). Furthermore, SCXRD analysis before and after X-ray radiation was conducted on the same single crystal of U-Cbdcp and revealed that the overall network derived from these two datasets retain unchanged as well (Table S1^†). In detail, the local structure as represented by the bond length and bond angle of U-Cbdcp changes slightly, which can be attributed to the standard deviations of these parameters obtained from SCXRD (Table S2^†). This observation further excludes the quenching mechanism via decomposition of U-Cbdcp crystal.There is precedence in literature that the luminescence quenching can be associated with the generation of radicals via radio-induced bond break or electron transfer.^23,26,41,42 Therefore, electron paramagnetic resonance (EPR) spectrum of irradiated U-Cbdcp was collected and indeed shows an intense EPR signal with a g-tensor of 2.0197, corresponding to the value (g = 2.0023) of a free electron (Fig. 3).⁴³ The freshly synthesized sample, however, is EPR silent for comparison. To identify the location of radical species in the coordination polymer, EPR spectra of H₃CbdcpBr ligand before and after irradiation were recorded as well. As shown in Fig. S6,^† the irradiated H₃CbdcpBr exhibits a relatively weak resonance with a g factor of 2.0198, which is comparable with that of U-Cbdcp. In the light of aforementioned results, we may conclude that continuous X-ray radiation generates ligand-based radical species, which functions as a quencher via a nonradiative energy transfer pathway.^44–46Open in a separate windowFig. 3EPR spectra of U-Cbdcp before and after 5 kGy X-ray radiation.Encouraged by the structural integrity of U-Cbdcp upon 5 kGy X-ray irradiation, we further investigated its radiolytic stability by irradiating the sample with high dose β-ray and γ-ray radiations. The radiations were provided by a custom-built electron cyclotron (1.2 MeV) and a ⁶⁰Co irradiation source (2.22 × 10¹⁵ Bq) with dose rates of 150 and 11.8 kGy per h, respectively. PXRD study indicated that no obvious changes in long-range order or loss of crystallinity of U-Cbdcp were observed after radiations, implying excellent radiation resistance of U-Cbdcp (Fig. S7^†).In summary, a new 1D uranium coordination polymer built from uranyl cations and zwitterionic Cbdcp ligands were obtained solvothermally. One of the most intriguing properties of U-Cbdcp is the occurrence of luminescence quenching upon X-ray radiation. This unique radio-induced luminometric response can be utilized as a strategy for X-ray dosimetry. Notably, the quenching response can be well fitted with a linear correlation and the detection limit was calculated to be 0.093 Gy. This finding, in conjunction with the excellent radiation resistance of U-Cbdcp, point to potential applications of uranium bearing materials for radiation detection.     

Facile one-pot solvothermal preparation of two-dimensional Ni-based metal–organic framework microsheets as a high-performance supercapacitor material

We report a facile one-pot solvothermal way to prepare two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) using only Ni precursor and ligand without any surfactant. The Ni-MOFms exhibit good specific capacities (91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively) and long-term stability in 5000 cycles when used for a supercapacitor electrode.

Two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) were synthesized via a facial one-pot solvothermal approach and exhibited good specific capacities and excellent long-term stability when used for a supercapacitor electrode.

With the continuous growth of energy demand worldwide, high-performance, environmental-friendly, and low-cost energy storage devices have attracted extensive research interest.^1–3 Among them, supercapacitors are considered most promising because of their high power density, long lifespan, and fast charging/discharging speed.^4–6 To date, numerous materials have been explored for fabricating supercapacitors. Carbon materials have been usually used for electrical double-layer capacitors (EDLCs), including carbon fibers, carbon nanotubes, carbon spheres, carbon aerogels, and graphene,^7–12 while conducting redox polymers and transition metal oxides/hydroxides are widely explored as active materials for pseudocapacitance and battery-type electrodes.^13–16Metal–organic frameworks (MOFs), a porous crystalline material composed of metal nodes and organic linkers, have been widely applied in versatile fields including chemical sensors, catalysis, separation, biomedicine, and gained more and more attention in the area of energy storage.^17–25 Recently, two-dimensional (2D) MOFs have aroused great interest as a new kind of 2D materials.^26,27 Compared with traditional bulk MOFs, 2D MOFs possess distinctive properties, such as short ion transport distances, abundant active sites, and high aspect ratios, making them exhibit better performance than their bulk counterparts.^28–32 Bottom-up methods are generally adopted to prepare 2D MOFs with the addition of surfactants to control the growth of MOFs in a specific direction.^33–35 However, the use of surfactants inevitably blocks part of the active sites at the expense of the performance of materials. Therefore, it is highly necessary to explore and develop a direct solvothermal synthesis of 2D MOFs with the advantages of additive-free, simple operation, and easy scale-up.Herein, we report a facile one-pot solvothermal method to synthesize 2D Ni-based MOF microsheets (denoted as Ni-MOFms) by treating nickel chloride hexahydrate (NiCl₂·6H₂O, the metal precursor) together with the trimesic acid (H₃BTC, the ligand) in a mixed solvent of N,N-dimethylformamide (DMF), ethanol (EtOH) and H₂O. During the whole preparation process, only Ni precursor and the ligand are used while no surfactant is added. When used as active materials for a supercapacitor electrode, the obtained Ni-MOFms displayed excellent reversibility and rate performance. It also exhibited specific capacities of 91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively. Besides, they showed a good cycling performance in 5000 cycles with about 70% of the specific capacity and almost 100% of the coulombic efficiency maintained.Morphologies of the Ni-MOFms were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 1a and b, the Ni-MOFms were successfully fabricated via the facile one-pot solvothermal method with varying lateral sizes on the micron scale. Energy dispersive spectroscopy (EDS) mapping indicated that the obtained microsheets were mainly composed of C, O, and Ni. A trace amount of N was also observed, which could be attributed to the residual DMF in the mixed solvent (Fig. 1c). These elements were uniformly distributed throughout the whole microsheet. To measure the exact thickness of the Ni-MOFms, atomic force microscopy (AFM) was used. Fig. 1d showed that the thickness of the microsheet was about 58 nm. Considering the large lateral size, even such thickness could produce a relatively high aspect ratio, which is beneficial to the electrochemical performance.Open in a separate windowFig. 1(a) TEM image, (b) SEM image, (c) EDS mapping, and (d) AFM image and the corresponding height profile of the Ni-MOFms.The composition information of the Ni-MOFms was analyzed by X-ray diffraction (XRD) and the resulting diffraction pattern was shown in Fig. 2a. It was clear that the sample was a crystalline material. However, the exact structure was difficult to determine because no matching MOF structure has been found. Therefore, the structure of the Ni-MOFms was further confirmed by Fourier transform infrared spectroscopy (FT-IR). As shown in Fig. 2b, there was a sharp peak at 1721 cm⁻¹ for H₃BTC, which could be ascribed to the stretching vibration of C O in the nonionized carboxyl group.³⁶ For the Ni-MOFms, the peak at this location disappeared while four new peaks appeared. Bands at 1634 and 1557 cm⁻¹ were related to the asymmetric stretching vibration of carboxylate ions (–COO⁻) and peaks at 1433 and 1371 cm⁻¹ were the characteristic peaks of the symmetric stretching vibration of –COO⁻.^37,38 All these changes indicate that the ligand interacted well with the metal precursor.Open in a separate windowFig. 2(a) XRD pattern of the Ni-MOFms. (b) FT-IR spectra of H₃BTC and the Ni-MOFms.The chemical status and surface composition of the Ni-MOFms were further examined by X-ray photoelectron spectroscopy (XPS). From Fig. S1a^† we could see that the Ni-MOFms were composed of C, O, Ni, and N, which was consistent with the result of EDS mapping. High-resolution spectra of C 1s, Ni 2p, O 1s, and N 1s were shown in Fig. S1b–e.^† Characteristic peaks of C 1s at 288.27, 286.50, 285.85, and 284.80 eV were related to O C–OH, C–O, C–C, and C C, respectively, suggesting the presence of H₃BTC (Fig. S1b^†).³⁹ The Ni 2p spectrum showed two peaks at 873.32 and 855.77 eV, which could be ascribed to Ni 2p_1/2 and Ni 2p_3/2, respectively, together with two satellite peaks at 879.26 and 861.05 eV, verifying the existence of Ni²⁺ (Fig. S1c^†).⁴⁰ In the O 1s region, bands positioned at 532.94 and 531.40 eV could be ascribed to the adsorbed H₂O molecules on the surface of Ni-MOFms and typical metal–oxygen bonds, respectively, further corroborating the coordination between H₃BTC and Ni²⁺ (Fig. S1d^†).³⁹ Finally, the high-resolution spectrum of N 1s was also analyzed (Fig. S1e^†). There were two main peaks at 400.18 and 402.21 eV that could be ascribed to neutral amine and charged nitrogen, respectively,⁴¹ further proving the residual DMF on the Ni-MOFms surface.To explore the crucial factors in the formation process of the Ni-MOFms, the reaction time and temperature, the solvent, the ligand addition amount, and the ligand type were studied. As shown in Fig. S2,^† different crystalline materials were obtained at different reaction times. With the increase of reaction time, the material gradually changed from sphere to sheet. The reaction temperature is another crucial factor. At 120 °C, the material was amorphous and spherical. When the temperature rose, the crystal formed and appeared as microsheets (Fig. S3^†). The effect of solvent was illustrated in Fig. S4.^† Microsheets could not be synthesized in DMF or DMF with a small amount of EtOH. In the mixed solvent of DMF and H₂O, crystals could be prepared, indicating the vital role of H₂O. However, spheres existed in the products. Only when a mixture of DMF, EtOH, and H₂O with a certain proportion was used as the solvent, the Ni-MOFms could be obtained. Furthermore, we investigated the effect of the ligand addition amount. From Fig. 1 and S5^† we can see that the Ni-MOFms crystals formed when the molar ratio of Ni precursor and H₃BTC was 1 : 2 (Fig. 1). We speculated that ligands could simultaneously act as regulators to adjust the morphology of materials, avoiding the use of additional surfactants. When the ligand was replaced with 2-methylimidazole (2-MI) or terephthalic acid (H₂BDC), flower-like crystals rather than microsheets were obtained (Fig. S6^†), indicating the importance of the ligand type. Taking the above factors into account, we could finally determine the suitable conditions for preparing the Ni-MOFms (see the experimental section in ESI^†).The potential application of the Ni-MOFms in supercapacitors was first tested by cyclic voltammetry (CV) in 3 M KOH between 0 and 0.4 V (vs. saturated calomel electrode, SCE). As can be seen from Fig. 3a, all CV curves had similar shapes and the peak currents improved gradually as the scan rate increased, suggesting the good capacitive behavior of the Ni-MOFms electrode.⁴² When the scan rate was as high as 150 mV s⁻¹, redox peaks could still be observed, which indicated the excellent rate performance and kinetic reversibility.⁴³ Besides, as the scan rate went up from 10 to 150 mV s⁻¹, the reduction and oxidation peaks moved towards negative and positive potential, respectively, demonstrating the electrode polarization at large scan rates.⁴⁴Open in a separate windowFig. 3Electrochemical measurements of the Ni-MOFms. (a) CV curves at different scan rates. (b) GCD curves at various current densities and (c) corresponding specific capacities. (d) The EIS Nyquist plot at the bias potential of 0.4 V and the equivalent circuit model with the fitted plots (the red dots).The galvanostatic charge–discharge (GCD) behavior was further investigated to assess the coulombic efficiency and the specific capacity of the Ni-MOFms (see the ESI^† for detailed calculation method).^45,46 As shown in Fig. 3b, the shape of GCD curves was highly symmetric during charging and discharging, indicating that the coulombic efficiency of Ni-MOFms was almost 100% at various current densities. The specific capacities of 91.4, 78.4, 71.4, 64.0, and 60.0 C g⁻¹ were achieved at current densities of 2, 4, 6, 8, and 10 A g⁻¹ (Fig. 3c), respectively, demonstrating the excellent rate capability with about 65.6% of the specific capacity maintained from 2 to 10 A g⁻¹. The specific capacity at 2 A g⁻¹ was comparable with or even superior to that of some MOF materials reported in the literatures (Table S1^†).^47–50The kinetics of the electroanalytical process was then investigated by electrochemical impedance spectroscopy (EIS). Fig. 3d showed the Nyquist plot of Ni-MOFms from 0.01 to 100000 Hz and the corresponding equivalent circuit model (inset) with the fitted plots. CPE was the constant phase element related to the double layer capacity.⁵¹ The equivalent series resistance was denoted by R_s and its value obtained from the x-axis intercept was about 2.1 Ω, indicating the low resistance of the solution.⁴³R_ct represented the charge-transfer resistance at the interface of the electrode and electrolyte.⁵² For Ni-MOFms, the value of R_ct was up to 147.1 Ω, which could be attributed to the poor conductivity of MOF materials.The long-term stability of Ni-MOFms was also explored by charging–discharging at 10 A g⁻¹ for 5000 consecutive cycles. From Fig. 4 we could see that the specific capacity retention remained about 70% after 5000 cycles and the coulombic efficiency was maintained at almost 100% throughout the whole process. Furthermore, the inset in Fig. 4 exhibited that the GCD curves of the last 10 cycles were the same as the first 10 cycles, indicating excellent cycling stability.Open in a separate windowFig. 4Cycle property of Ni-MOFms at 10 A g⁻¹. Inset: GCD curves of the first 10 cycles (left) and the last 10 cycles (right).     

A novel fluorescent probe for imaging the process of HOCl oxidation and Cys/Hcy reduction in living cells

A new on-off-on fluorescent probe, CMOS, based on coumarin was developed to detect the process of hypochlorous acid (HOCl) oxidative stress and cysteine/homocysteine (Cys/Hcy) reduction. The probe exhibited a fast response, good sensitivity and selectivity. Moreover, it was applied for monitoring the redox process in living cells.

A new on–off–on fluorescent probe, CMOS, was designed and applied to detect the process of HOCl oxidation and Cys/Hcy reduction.

Reactive oxygen species (ROS) are indispensable products and are closely connected to various physiological processes and diseases.¹ For instance, endogenous hypochlorous acid (HOCl) as one of the most important ROS, which is mainly produced from the reaction of hydrogen peroxide with chloride catalyzed by myeloperoxidase (MPO), is a potent weapon against invading pathogens of the immune system.^2,3 However, excess production of HOCl may also give rise to oxidative damage via oxidizing or chlorinating the biomolecules.⁴ The imbalance of cellular homostasis will cause a serious pathogenic mechanism in numerous diseases, including neurodegenerative disorders,⁵ renal diseases,⁶ cardiovascular disease,⁷ and even cancer.⁸ Fortunately, cells possess an elaborate antioxidant defense system to cope with the oxidative stress.⁹ Therefore, it is necessary and urgent to study the redox process between ROS and antioxidants biosystems.Fluorescence imaging has been regarded as a powerful visual methodology for researching various biological components as its advantages of high sensitivity, good selectivity, little invasiveness and real-time detection.^10,11 To date, amounts of small molecular fluorescent probes have been reported for detection and visualization of HOCl in vivo and in vitro.^12–22,29 The designed strategies of HOCl sensitive probes are based on various HOCl-reactive functional groups, such as p-methoxyphenol,¹³p-alkoxyaniline,¹⁴ dibenzoyl-hydrazine,¹⁵ selenide,¹⁶ thioether,¹⁷ oxime,¹⁸ hydrazide,¹⁹ hydrazone.²⁰ But, many of these probes display a delayed response time and low sensitivity. And, only few fluorescent probes can be applied for investigating the changes of intracellular redox status.²¹ Besides, it''s worth noting that most of the redox fluorescent probes rely on the organoselenium compounds.²² Even though these probes are well applied for detection of cellular redox changes, excessive organic selenium is harmful to organisms and the synthesis of organoselenium compounds is high requirement and costly. Additionally, almost all the reports have only investigated the reduction effects of glutathione (GSH) as an antioxidant in the redox events. While, there are the other two important biothiols, cysteine (Cys) and homocysteine (Hcy), which not only present vital antioxidants, but also are tightly related to a wide variety of pathological effects in biosystem, such as slowed growth, liver damage, skin lesions,²³ cardiovascular,²⁴ and Alzheimer''s diseases.²⁵ However, the fluorescent probes for specially studying internal redox changes between HOCl and Cys/Hcy are rarely reported. In this respect, a novel redox-responsive fluorescent probe, CMOS, was designed and synthesized in this work, and we hope that it can be a potential tool for studying their biological relevance in living cells.Based on literature research, the aldehyde group has excellent selectivity in identification of Cys/Hcy, and the thiol atom in methionine can be easily oxidized to sulfoxide and sulfone by HOCl.^26,27 Considering these two points, we utilized 2-mercaptoethanol to protect the 3-aldehyde of 7-diethylamino-coumarin as the recognition part of HOCl, meaning that two kinds of potential recognition moieties are merged into one site. Fluorescent probe CMOS can be easily synthesized by the acetal reaction in one step (Scheme S1^†). A control molecule CMOS-2 was also prepared by 3-acetyl-7-diethylaminocoumarin (CMAC) similarly. The structure of all these compounds have been convinced by ¹H NMR, ¹³C NMR, and HR-MS (see ESI^†).As shown in Scheme 1a, we estimated that both CMOS and CMOS-2 can be rapidly oxidized in the appearance of HOCl. The oxidation product CMCHO of CMOS, which has the aldehyde moiety, can further react with Cys/Hcy to obtain the final product CMCys and CMHcy, respectively. In contrast, the oxidation product CMAC of CMOS-2 cannot combine with Cys/Hcy or other biothiols anymore (Scheme 1b).Open in a separate windowScheme 1Proposed reaction mechanism of CMOS and CMOS-2 to HOCl and Cys/Hcy.In order to confirm our design concept, the basic photo-physical characteristics of CMOS, CMCHO, CMOS-2 and CMAC were tested (Table S1, Fig. S1^†). Under the excitation wavelength 405 nm, CMOS and CMOS-2 exhibited strong fluorescence centred at 480 nm in PBS buffer solution, while the fluorescence of CMCHO and CMAC was weak around this band. The emission properties of CMOS and CMCHO were also investigated at the excitation wavelength 448 nm under the same experimental conditions as well (Fig. S2^†). After careful consideration, we chose 405 nm as the excitation wavelength in the follow-up experiments in vitro and in vivo.Next, the sensitivity of CMOS and CMOS-2 to HOCl and Cys/Hcy were investigated. As we expected, both the CMOS and CMOS-2 exhibited good response to HOCl. The fluorescence intensity of CMOS and CMOS-2 decreased gradually with addition of NaOCl (Fig. 1a, S3a^†), indicating that the fluorescence was switched off obviously in the presence of HOCl. The variation of intensity displayed good linearity with concentration of HOCl in the range of 0–20 μM (R² = 0.993, Fig. S4^†), and the detection limit of CMOS to HOCl was calculated to be 21 nM (S/N = 3). Subsequently, when Cys/Hcy was added to the final solution in Fig. 1a, the fluorescence intensity increased gradually within 180 min (Fig. 1b, S5^†). However, the fluorescence cannot be recovered by addition thiols to the CMOS-2 solution with excess HOCl (Fig. S3b^†). These results indicate that the probe CMOS can response to HOCl and Cys/Hcy in a fluorescence on-off-on manner, and can be used for monitoring the redox process with high sensitivity.Open in a separate windowFig. 1(a) Fluorescence responses of CMOS (2 μM) to different concentrations of NaOCl (0–200 μM). (b) Fluorescence responses of the CMOS solution (2 μM) with HOCl (200 μM) to Cys/Hcy (5 mM). (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_ex = 405 nm).To further identify the recognizing mechanism of probe CMOS, high performance liquid chromatography (HPLC) and mass spectral (MS) analysis were used to detect the redox process. Initially, probe CMOS displayed a single peak with a retention time at 3.7 min (Fig. 2a, S6^†) while reference compound CMCHO produced a single peak with a retention time at 2.5 min (Fig. 2b, S7^†). Upon the addition of HOCl to the solution of CMOS, the peak at 3.7 min weakened while 2.5 min and 2.2 min appeared (Fig. 2c). According to corresponding mass spectra, the new main peak at 2.5 min is related to compound CMCHO (Fig. S8^†). The other new peak of 2.2 min corresponds to the compound C3, which can be predicted as an intermediate in the oxidation process (Fig. S8^†).²⁸ The addition of Cys to the solution of CMCHO also caused a new peak with a retention time at 2.1 min, which has been confirmed to be the thioacetal product CMCys (Fig. S9^†). The possible sensing mechanism is depicted in Fig. S10.^†Open in a separate windowFig. 2The reversed-phase HPLC with absorption (400 nm) detection. (a) 10 μM CMOS. (b) 10 μM CMCHO. (c) 10 μM CMOS in the presence of 50 μM HOCl for 30 s. (d) 10 μM CMCHO in the presence of 1 mM Cys for 30 min. (Eluent: CH₃CN containing 0.5% CH₃COOH; 100% CH₃CN (0–7 min), 0.5 ml min⁻¹, 25 °C; injection volume, 5.0 μL).To study the selectivity of CMOS towards HOCl, we performed fluorescence response to different reactive oxygen species (ROS), reactive nitrogen species (RNS) and reactive sulfur species (RSS). As shown in Fig. 3a, CMOS exhibited significant change of fluorescence intensity only in the presence of HOCl, while other ROS and RNS, such as singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), hydroxyl radical (HO·), superoxide anion (O₂⁻), nitric oxide (NO), tert-butylhydroperoxide (t-BuOOH) and tert-butoxy radical (t-BuOO·) had no obvious fluorescence emission changes. Additionally, RSS which are abundant in biological samples, showed no influence in this process under the identical condition. The detection of reducing process was also investigated. As displayed in Fig. 3b, only cysteine and homocysteine induced excellent fluorescence recovery towards other reducing materials, such as RSS and various amino acids. Furthermore, the selectivity of CMOS-2 was also studied in the same condition. As expected, CMOS-2 could selectively detect HOCl, and not alter fluorescence intensity under various kinds of biothiols (Fig. S11^†). Therefore, our design strategy for the on–off–on probe is confirmed by results obtained above, with which CMOS can be utilized for detecting the redox process between HOCl and Cys/Hcy with high selectivity.Open in a separate windowFig. 3(a) Fluorescence response of CMOS (2 μM) to different ROS, RNS and RSS (200 μM). Bars represent emission intensity ratios before (F₀) and after (F₁) addition of each analytes. (a) HOCl; (b) KO₂; (c) H₂O₂; (d) ¹O₂; (e) HO·; (f) t-BuOOH; (g) t-BuOO·; (h) NO₂⁻; (i) NO₃⁻; (j) NO; (k) GSH; (l) Cys; (m) Hcy; (n) Na₂S; (o) Na₂S₂O₃; (p) Na₂S₂O₈; (q) NaSCN; (r) DTT; (s) Na₂SO₃. (b) Fluorescence response of the solution added HOCl in (a) to different RSS and amino acids. Bars represent emission intensity ratios before (F₂) and after (F₃) addition of each analytes (5 mM). (a) Cys; (b) Hcy; (c) Na₂S; (d) Na₂S₂O₃; (e) Na₂S₂O₈; (f) NaSCN; (g) DTT; (h) Na₂SO₃; (i) Ala; (j) Glu; (k) Gly; (l) His; (m) Ile; (n) Leu; (o) Met; (p) Phe; (q) Pro; (r) Ser; (s) Trp; (t) Vc; (u) GSH. (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_ex/λ_em = 405/480 nm).Subsequently, the influence of pH on probe CMOS was measured. The fluorescence intensity of CMOS and CMCHO perform no significant variances in wide pH ranges (pH = 4–11, Fig. S12a^†). Fluorescence intensity changes could be observed immediately when HOCl was added into the solution of probe CMOS, especially in alkaline condition (Fig. 4a). Considering the pK_a of HOCl is 7.6,²⁹CMOS is responsive to both HOCl and OCl⁻. Alkaline condition was also benefit for the fluorescence recovery of CMOS from Cys/Hcy (Fig. S12b^†). It is reasonable to consider that thiol atom displays higher nucleophilicity in alkaline condition. From the stop-flow test, the UV-visible absorbance of probe CMOS sharply decreased at the wavelength of 400 nm (Fig. 4b). The response time was within 10 s and the kinetic of the reaction was fitted to a single exponential function (k_obs = 0.67 s⁻¹). The ability of instantaneous response is extremely necessary to intracellular HOCl detection.Open in a separate windowFig. 4(a) Fluorescence responses of CMOS to HOCl under different pH values. Squares represent emission intensity ratios after (F₁) and before (F₀) addition of 200 μM HOCl (λ_ex/λ_em = 405/480 nm). (b) Time-dependent changes in the absorption intensity of CMOS (1 μM) before and after addition of HOCl. (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_abs = 400 nm).With these data in hand, we next applied CMOS for fluorescence imaging of the redox changes with HOCl and Cys/Hcy in living cells. After incubation with 5 μM CMOS at 37 °C for 30 min, intense fluorescence was observed of the SKVO-3 cells in the optical window 425–525 nm (Fig. 5a and d), indicating the probe can easily penetrate into cells. Treating the cells with 100 μM NaOCl led to remarkable fluorescence quenching as the probe sensed the HOCl-induced oxidative stress (Fig. 5b and e). After 3 min, the cells were washed with PBS buffer three times, and added 5 mM Cys/Hcy for 1 h, respectively. Then the fluorescence was recovered obviously (Fig. 5c and f). Experimental results clearly declare that the probe CMOS was successfully used to detect the process of HOCl oxidative stress and Cys/Hcy reducing repair in living cells.Open in a separate windowFig. 5Fluorescence imaging of the process of HOCl oxidative stress and thiols repair in CMOS-labeled SKVO-3 cells. Fluorescence images of SKVO-3 cells loaded with 5 μM CMOS at 37 °C for 30 min (a and d). Dye-loaded cells treated with 100 μM NaOCl at 25 °C for 3 min (b and e). Dye-loaded, NaOCl-treated cell incubated with 5 mM Cys (c), 5 mM Hcy (f) for 1 h. Emission intensities were collected in an optical window 425–525 nm, λ_ex = 405 nm, intensity bar: 0–3900.     

Green synthesis of crystalline bismuth nanoparticles using lemon juice

Lemon juice effectively served as a reducing and capping agent for an easy, cost-effective, and green synthesis of crystalline bismuth nanoparticles (BiNPs) in basic aqueous media. Spherical BiNPs with a rhombohedral crystalline structure are capped by phytochemicals and stably dispersible in aqueous media. The BiNPs effectively catalyze the reduction of 4-nitrophenol to 4-aminophenol by NaBH₄.

Lemon juice effectively served as a reducing and capping agent for an easy, cost-effective, and green synthesis of crystalline bismuth nanoparticles (BiNPs) in basic aqueous media.

Nanostructures of bismuth, the heaviest element among the ‘safe ones’ earning the status of a ‘green element’,¹ are particularly interesting due to their large magnetoresistance and excellent thermoelectric properties.^2–4 Bismuth nanoparticles (BiNPs) are the most extensively used nanostructures of bismuth. For example, BiNPs are utilized as contrast agents for computed tomography, photoacoustic imaging and infrared thermal imaging,^5,6 catalysts for the reduction of nitroaromatic compounds,^7,8 and removal of NO from air.⁹ Furthermore, BiNPs can act as intermediates for the synthesis of other nanostructures of bismuth, such as thermoelectric Bi₂Te₃ (ref. 10) and seeds for the solution–liquid–solid growth of nanowires.^11,12 Techniques for the synthesis of pure BiNPs could be categorized into (i) thermal decomposition,¹² (ii) mechanochemical processing,¹³ (iii) photochemical reduction,¹⁴ and (iv) solution-phase chemical reduction methods.^6,15–19 The thermal decomposition method requires harsh preparation conditions, expensive organometallic precursors, high temperature, and long reaction time, while producing high-quality monodispersed BiNPs. The mechanochemical processing technique is advantageous in terms of using inexpensive and nontoxic bulk bismuth pellets as precursors. However, the energy consumption and costly instrumentations are the limitations. The photochemical reactions typically require long time for sufficient conversion to bismuth nanoparticles, while they can also produce highly monodispersed BiNPs. The solution-phase chemical reduction methods are most popular due to the facile procedures and accessible reagents. However, stabilizers and toxic reducing agents often used are the limitations. To resolve the above-mentioned limitations, simple and green alternative methods are highly demanded.The use of abundant plant sources, which has been applied for synthesis of various metal nanoparticles such as Ag and Au, is a promising solution.^20,21 Plant sources contain a wide variety of biomolecules potentially serving as reducing and capping agents. Edible plant sources are obviously safest. To the best of our knowledge, there is no report on the synthesis of crystalline BiNPs using plant sources, while P. Poltronieri et al. reported synthesis of amorphous BiNPs using hydroalcoholic extract of Moringa oleifera.²² We focused on lemon, a very common fruit containing abundant antioxidants such as polyphenols, limonoids, citric acid, ascorbic acid, and vitamins potentially reduce ions with high oxidation states. Some of the phytochemicals, namely carbohydrates and proteins bearing ionic moieties, can be capping agents. Accordingly, lemon juice was successfully applied for the formation and in situ stabilization of silver and gold nanoparticles in aqueous media.^23,24 We presumed that a similar mechanism also works for BiNPs.In this communication, we introduce a greener strategy for the synthesis of crystalline BiNPs using lemon juice as a reducing and capping agent. For example, the synthesis was carried out using Bi(NO₃)₃·5H₂O (0.25 mmol) and freshly prepared lemon juice (25 mL) at 80 °C for 2 h under an aerobic basic condition. The X-ray diffraction (XRD) pattern (Fig. 1) of the obtained product was indexed to the pure rhombohedral phase of elemental bismuth (JCPDS no. 44-1246), indicating that the obtained product is BiNPs without detectable oxide phases. The average crystallite size was calculated to be 20 nm applying the Scherrer''s equation on the peaks of the (012), (104) and (110) plane. This result indicates that the phytochemicals present in lemon juice have ability to reduce bismuth salts to form BiNPs. The plausible phytochemicals for reduction are ascorbic acid, citric acid, and sugars. The reduction of Bi³⁺ with glucose⁸ and ascorbic acid¹⁷ was reported, and we accordingly performed the control experiment using possible reducing agents contained in lemon juice, namely ascorbic acid, glucose, and starch without any stabilizing agents. All of them could form elemental bismuth in highly aggregated forms as confirmed by the XRD patterns (ESI, Fig. S9^†) and SEM images (ESI, Fig. S10^†).Open in a separate windowFig. 1XRD pattern of obtained BiNPs synthesized using lemon juice with that of authentic Bi (JCPDS no. 44-1246).Electron microscopy was employed to confirm the size and the morphology of synthesized BiNPs. The scanning electron micrography (SEM) image (Fig. 2a) shows spherical objects having the size in the range of 50 to 100 nm agglomerated and covered with amorphous substances presumably originating from lemon juice. This agglomeration occurred during drying as confirmed by the stable water dispersibility of BiNPs with an average hydrodynamic diameter (D_h) of 255 nm investigated by dynamic light scattering (DLS) (ESI, Fig. S1a^†). The high colloidal dispersibility of the BiNPs and the coating layer observed in the SEM image suggest that phytochemicals present in lemon juice act also as capping agents. The larger D_h measured by DLS than the size observed by SEM can be attributed to the surrounding hydration layer and swelled phytochemicals attached to the surface of the BiNPs. We then performed transmission electron microscopy (TEM) analysis to get the actual size of the Bi cores. The TEM images of the BiNPs (Fig. 2b and c) indicated the presence of heavy elements in the matrix of light elements. The particles are spherical, and the diameter ranges from 8 to 30 nm. The high-resolution TEM image (Fig. 2d) shows the lattice fringes of 0.298 nm, 0.253 nm, and 0.224 nm of the typical crystallite agreeing well with the distance of the (012), (104) and (110) plane, respectively, of the rhombohedral Bi(0).Open in a separate windowFig. 2(a) SEM and (b–d) TEM images of BiNPs synthesized using lemon juice.The EDX spectrum (ESI, Fig. S2^†) reveals that obtained BiNPs comprise bismuth (Bi), carbon (C), and oxygen (O). The strongest signal of bismuth testifies the successful synthesis of BiNPs, whereas the C and O signals demonstrate the presence of the capping layer on the BiNPs.The FT-IR spectrum of the synthesized BiNPs was analyzed to presume the possible components on the BiNPs surface with the comparison with the solid content of lemon juice (Fig. 3). The FT-IR spectrum of the BiNPs shows the following major absorption bands. The broad absorption band at 3050–3500 cm⁻¹ is assignable to the O–H stretching of alcohol moieties. The absorption band around 2840–2940 cm⁻¹ is assignable to the C–H stretching of alkane, and the absence of the sharp absorption around 2950–3050 cm⁻¹ suggests the absence of trace contents of unsaturated C–H groups. The absence of the adsorptions originating from COO–H around 2400–3100 cm⁻¹ and C O around 1700–1730 cm⁻¹ indicates that citric acid^25,26 and other carboxylic acids contained sufficiently in lemon juice are not the major components on the BiNPs, while the presence of the strong absorption around 1600 cm⁻¹ is assignable to carboxylate moieties. The absorption bands around 930–1130 cm⁻¹ assignable to C–C and C–O stretching and around 1200–1420 cm⁻¹ assignable to O–C–H, C–C–H and C–O–H bending suggest the presence of sugars on BiNPs.^25,26 The content of the organic moieties was estimated approximately 14% by thermogravimetric analysis from weight loss that occurred at the temperature range from 130 to 450 °C (ESI, Fig. S3^†), at which the negligible C and O signals were observed in the EDX spectrum of the residue after TGA (ESI, Fig. S2^†).Open in a separate windowFig. 3FT-IR spectra of solid content of lemon juice and obtained BiNPs synthesized using lemon juice.The ¹H and ¹³C NMR and FTIR spectroscopic analysis (ESI, Fig. S4–S8^†) of the ethanol and chloroform extracts of the BiNPs suggests that the major capping phytochemicals are polysaccharides and fatty acid derivatives. Other minor possible components may include amino acids, terpenes and phenolic compounds contained in lemon juice.The successful formation of BiNPs made us enthusiastic to investigate their catalytic performance in the reduction of nitroaromatic compounds, problematic pollutants arising from explosives, analgesic, and antipyretic drugs and dyes.²⁷ Herein, the catalytic performance of our BiNPs was investigated by selecting the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH₄ at room temperature as a model reaction.^7,8,28 The progress of the reduction was monitored by UV-vis absorption spectroscopy. The colour of the 4-NP aqueous solution changed from light yellow to deep yellow upon the addition of NaBH₄ to produce 4-nitrophenolate ion and an intense absorption peak at 403 nm appeared instead of the original absorption peak of 4-NP at 316 nm in the UV-vis spectrum.⁷ In the absence of BiNPs, the colour of the solution and the intensity of the peak were retained even after 12 h. In the presence of BiNPs, the deep yellow mixture became almost colourless within 180 min (Fig. 4a). The intensity of the absorption peak at 403 nm decreased overtime, and simultaneously a new absorption peak appeared and grew at 299 nm, indicating the formation of 4-AP (Fig. 4a).^8,27,29 The reaction rate almost follows pseudo-first-order kinetics agreeing with the Langmuir–Hinshelwood mechanism; in which two reactants react after adsorption on the solid surface, and then the product desorbs. The rate constant (k) was determined from the plot of ln(A/A₀) vs. time (Fig. 4b) according to the previous reports.^7,8,28,29 The k value of 0.0134 min⁻¹ (activity factor = 0.1 s⁻¹ g⁻¹) is lower than previously reported PVP-coated bismuth nanodots (6.033 s⁻¹ g⁻¹)⁸ and starch coated BiNPs (0.02751 s⁻¹).⁷ The lower catalytic rate of our BiNPs can be attributed to the electrostatic repulsion of 4-nitrophenolate ions with negatively charged BiNPs (zeta potential value = −31.7 mV) in the reaction mixture. The initial rate is slower than the rate after 50 min. Possible factors are (i) the inductive period for the surface activation of the nanoparticles at the initial stage as reported for bismuth nanodots by Liang et al.⁸ and (ii) partial and gradual release of phytochemicals under the basic conditions. After the catalytic reaction, the D_h value measured by DLS (195 nm) became smaller than the original one maintaining the good water dispersibility, suggesting unravelling of primary particles fused by the capping layer (ESI, Fig. S1b^†). In addition, the weight loss in the TGA curve became lower (ESI, Fig. S3^†), and the SEM image implies the partial loss of the coating (ESI, Fig. S11^†). This decrease of the organic moieties is attributable to the partial removal of coating substances such as fatty acids during the catalytic reaction under the basic and reductive conditions, supported by the disappearance of the absorption band around 1600 cm⁻¹ assignable to carboxylate moieties in the FTIR spectrum of BiNPs after the catalytic reaction (ESI, Fig. S12^†). Carboxylic acids and their esters are reported to be reduced to alcohol by NaBH₄ in the presence of electrophiles.^30–33 The initial presence of the amphiphilic and anionic layer plausibly delays the catalytic reaction.Open in a separate windowFig. 4(a) Optical images and absorption spectra of catalytic reduction of 4-NP ([4-NP] = 15 ppm) by NaBH₄ (4.28 × 10⁻⁴ M) in presence of BiNPs (142 mg L⁻¹); (b) pseudo-first order kinetic plot of catalytic reduction.In conclusion, we have demonstrated a green, cost-effective, and successful approach for synthesis of crystalline BiNPs using lemon juice as a reducing as well as capping agent. The obtained BiNPs effectively catalysed the reduction of 4-NP to 4-AP by NaBH₄. The importance of this method lies in the simple synthetic procedure, uses of safe and low-cost lemon, and good dispersibility over conventional chemical approaches.     

Insulating 3D-printed templates are turned into metallic electrodes: application as electrodes for glycerol electrooxidation

We turned printed plastic pieces into a conductive material by electrochemical polymerization of aniline on the plastic surface assisted by graphite. The conductive piece was then turned into a metallic electrode by potentiodynamic electrodeposition. As a proof-of-concept, we built indirect-3D-printed Pd, Pt and Au electrodes, which were used for glycerol electrooxidation.

Insulating printed plastics are turned into metallic pieces by electrochemical polymerization of aniline followed by metal electrodeposition.

Additive manufacturing, or 3D-printing, is a revolutionary technique which allows for the easy fabrication of three-dimensional objects, enabling the construction of complex devices with minimum waste and very low cost, which previously could only have been fabricated with sophisticated equipment and facilities.^1,2 This technology has attracted attention from industry and academia due to the myriad architectures, materials and applications.^2–4 Among the various 3D-printing methods, extrusion through fused deposition modeling (FDM) is the most commonly used. This method, in which a thermoplastic is forced through a heated nozzle head, was invented in 1989 by Scott Crump,⁵ and is currently the most affordable and commercialized way of 3D-printing.The field of electrochemistry has benefited from 3D-printing technologies. Ambrosi and Pumera have detailed the advances brought by additive manufacturing to electrochemistry, including instruments, sensors and materials.² In particular, the ability to rapidly prototype new parts and devices boosted the advancement in instrumentation used for electrochemistry.^2,6,7Our group printed a three-parts electrolyzer with an approximate cost of US $5.⁸ This device was used with Pd-nanocubes-modified glassy carbon as working electrode to convert glycerol into tartronate.⁸ Concerning microfluidics, 3D-printing is already considered a revolution in microfluidics, due to the ability of partially or completely replacing poly(dimethylsiloxane) pieces built by soft lithography.⁹FDM technique has recently opened up the possibility of printing conductors, which can be used as electrodes for a variety of applications. The biggest challenge is to print ready-to-use electrodes, since the extrusion of composite thermoplastic/metal is still rare. In this sense, much effort has been spent to build and print plastic/carbon materials^10–12 and to modify existing printed objects.^4,11 Wei et al. produced graphene oxide (GO) blended with acrylonitrile butadiene styrene (ABS) or polylactic acid (PLA) to build a GO-thermoplastic filament for the first time.¹⁰ The authors also reduced GO to rGO with hydrazine hydrate in order to make a rGO-thermoplastic filament.¹⁰ Another remarkable achievement was made by Rymansaib et al.¹² These authors made a new filament of polystyrene/carbon-nanofiber/graphite simultaneously printed embedded in an insulating material, composing a monolithic object, which is electrochemically stable and prospective for electroanalysis.¹²Palenzuela et al. developed a simple protocol to achieve the theoretical electrical conductivity predicted for printed graphene/PLA electrodes.¹¹ They exposed the printed electrodes to dimethylformamide in order to dissolve the fused polymer on the electrode surface. This procedure allows the authors to access the active sites of the electrode, which is used for electroanalysis.¹¹ Aside from thermoplastic/carbon materials, there is an impending need to build metallic 3D-printed electrodes. A successful post-printed modification was made by Díaz-Marta et al. to produce Pd- and Cu-based materials for heterogeneous organic synthesis.⁴ By following complex steps, the authors printed a SiO₂ material, which was chemically treated in sequential steps to be finally modified with CuI and Pd(AcO)₂ to produce Cu and Pd catalysts.⁴ The development of metal-based 3D-printed materials for a wide range of electrochemistry applications using simple protocols is still necessary.Here, we used FDM to print PLA templates, which are modified with precursors of a conducting polymer and graphite (GR). This material is submitted to electrochemical polymerization, allowing further metallic electrodeposition. As proof-of-concept, we prepared Pd, Pt and Au electrodes built on 3D-printed templates. These electrodes were then used for glycerol electrooxidation to investigate their applicability as anodes of glycerol fuel cells and electrolyzers.^13–15We used. PLA as the thermoplastic filament in a commercially available 3D-printer (Sethi 3D, model S3). A CAD model of the electrode was drawn using software Autodesk Inventor 2017 and further sliced using software Simplify 3D. The electrodes are chosen to have dimensions of 3 × 3 × 5 mm³ so that they could be connected to a regular alligator connector, as shown in Fig. 1. The template is modified in aniline and HCl solution and further modified with graphite 99.69% (ashes 0.31%; humidity 0.06%; 13.33% retained in 100# grid and 97.40% in 325# grid). The procedures for such modifications are detailed through the text.Open in a separate windowFig. 1Illustrative scheme of the graphite-assisted electrochemical polymerization of polylactic acid template electrodes. The PLA template is covered by aniline precursor and graphite, which is further electrochemically polymerized to emeraldine (and other PANI structures in different oxidation states).The printing parameters of the working electrode template are summarized in Table S1.^† The total building time is only 1 min and material cost is 0.01 US $, which is a very low price and a fast way of obtaining the templates. It is worth noting that the printing parameters found in Table S1^† change whether one changes the size and shape of the desired template.The post-printing treatment developed here is based on a surface polymerization reaction for growing polyaniline (PAni) on PLA. The 3D-printed template is immersed in a mixture of 1 mL aniline with 2 mL 1 mol L⁻¹ HCl in order to place the polymer precursor, aniline chloride (C₆H₅NH₃⁺Cl⁻) on the PLA surface. The aim is to polymerize the precursor on the template to become conducting. The electrochemical synthesis of PAni has been previously described.^16,17 This conducting polymer is structurally different depending on the oxidation state. The reduced form, which has a yellow color, is leucoemeraldine. This form can be electrooxidized to emeraldine, which is blue in an alkaline medium or green in an acidic medium, as shown in Fig. S1.^† Emeraldine can be further oxidized to pernigraniline (violet color). In order to maintain structural stability during the electrochemical measurements, the reduced form leucoemeraldine/emeraldine is desirable, while further oxidation of emeraldine may accelerate degradation. This process has been widely discussed in the literature, as those for metal polymerization via electrochemistry.^18,19 For these previous studies, the substrate of the polymerization was already a conducting material, facilitating the electric contact with the voltage source (e.g. potentiostat/galvanostat), represented by an “aligator” in Fig. 1. However, in the present case, the challenge is that the widely used 3D-printed PLA is an insulating material.To surpass this issue, we developed GR-assisted polymerization. Immersing the template in the aniline chlorite solution smoothly melts the PLA surface. The template is then transferred to a glass container with GR and gently shaken until the PLA is completely covered. The GR-modified printed piece is finally dried at room temperature for 20 h. After dried, the template is washed with D.I. water under stirring for 10 min. This process is repeated five times and it is imperative to spread out unattached GR. Finally, the electrode is transferred to an electrochemical cell for the electrochemical polymerization of aniline in 1 mol L⁻¹ HCl. Once impossible electrochemical polymerization of aniline over PLA, is now achievable by cyclic voltammetry. The electrolyte accesses the template surface covered with C₆H₅NH₃⁺Cl⁻ as it goes through the GR flakes, while GR makes the connection between the electric connector (alligator connected to the potential source) and the electrolyte, allowing for electrochemical polymerization, as illustrated in Fig. 1.The GR-assisted electrochemical polymerization of the GR/C₆H₅NH₃⁺Cl⁻/PLA template is shown in Fig. 2. The growth of PAni is evidenced through the increase of two redox coupled peaks at low (I/I′) and high (II/II′) potentials. The oxidation of leucoemeraldine to emeraldine starts at 0.35 V, displaying an evident peak centered at 0.48 V (peak I in Fig. 2). Emeraldine is further oxidized to pernigraniline, displaying a peak at 0.73 V (peak II in Fig. 2). During the negative potential scan, pernigraniline is reduced to emeraldine at ∼0.7 V and further reduced to leucoemeraldine with a cathodic transient centered at 0.25 V, as shown by peaks II′ and I′ respectively in Fig. 2. The potential cycles were performed until the redox couple I/I′ is evident, which takes place after ∼30 cycles. At this point, the PLA is mainly covered by emeraldine, making it a conducting electrode.Open in a separate windowFig. 2Graphite-assisted electrochemical polymerization of GR/C₆H₅NH₃⁺Cl⁻/PLA template in 1 mol L⁻¹ HCl at 0.05 V s⁻¹.The electrode resistance of the polymerized material is 45 Ω, which is acquired by measuring the high frequency intercept of the Nyquist plot in an electrochemical impedance spectroscopy analysis performed at 106 Hz. The electrochemical profile shown in Fig. 2 associated to the low resistance demonstrates the success of the GR-assisted polymerization. Furthermore, the resistance measured with a multimeter is ∼36 Ω cm⁻¹, which reveals a remarkable conducting material. GR has a double contribution on the electrode construction; one is assisting polymerization, working as an electron collector (previously discussed) while also contributing to the increase in conductivity after polymerization. It is worth noting that GR used with other organochlorides does not show similar performance. As an example, we smoothly melted the PLA template in dichloromethane and dispersed GR on it. In such case, the resulting composite showed high resistance, in the MΩ scale. Therefore, PAni coverage is imperative to the increase in electrical conductivity of the template.After turning the PLA into an electrical conductor to form a GR/PAni/PLA template, the printed piece was immersed in a 6 mmol L⁻¹ PdCl₂ in 0.5 mol L⁻¹ H₂SO₄ for Pd electrodeposition. The metallic deposition was achieved by applying successive potential cycles between 0.019 and 1.319 V. Potentiodynamic deposition allows us to observe the formation of a Pd electrode through the appearance of a Pd profile, i.e. we can in situ evaluate the growth of Pd on the template. In other words, once a characteristic profile of Pd in acidic media is identified, we are certain of a success deposition. The experimental details of the electrochemical measurements are found in the ESI Section II.^† Fig. 3A shows an increase in anodic and cathodic currents wherein successive potential cycles are applied. Pd²⁺ is reduced to Pd on the template during the cycles and the currents related to the Pd surface reaction in aqueous acid solution become more evident. After identifying an obvious profile of Pd, the electrode was transferred to an electrochemical cell containing 0.1 mol L⁻¹ KOH in order to measure a profile in a controlled system.Open in a separate windowFig. 3(A) Potentiodynamic electrodeposition process achieved by successive potential cycles of GR/PAni/PLA template in 6 mmol L⁻¹ PdCl₂ in 0.5 mol L⁻¹ H₂SO₄, measured between 0.019 and 1.31 V. (B) Cyclic voltammogram of the indirect-3D-printed Pd electrode in 0.1 mol L⁻¹ KOH, measured between 0.15 and 1.27 V and (C) in the presence of 0.2 mol L⁻¹ glycerol between 0.35 and 1.29 V. All measurements are performed at 0.05 V s⁻¹. (D) and (E) show representative SEM images, while (F) and (G) show EDS Kα elemental composition maps (indicated in the figure). (H) EDS spectrum of the indirect-3D-printed Pd electrode. Fig. 3B undoubtedly evidences a characteristic profile of Pd through the cyclic voltammogram in the range of 0.15–1.27 V. Surface oxide formation starts at ∼0.7 V during the positive potential sweep, which is reduced producing a cathodic current, forming a peak centered at ∼0.7 V. Another pivotal characteristic is the hydrogen under potential deposition (H_UPD) region at around 0.15–0.5 V. Overall, this electrochemical profile shows that a Pd electrode was indeed manufactured.²⁰The indirect-3D-printed Pd electrode was then used as a working electrode in the presence of 0.2 mol L⁻¹ glycerol in 0.1 mol L⁻¹ KOH, as shown in Fig. 3C. The cyclic voltammograms display anodic currents during the positive and negative potential sweeps. The current density increases with successive cycles due to the initial cleaning of the surface, until a stable profile is reached at the fifth cycle. Such a phenomenon was previously reported for Pd catalyst.²¹ During the positive scan, electrooxidation starts at ∼0.6 V, reaching a maximum at ∼1.08 V. During the reverse scan, the surface reactivation after the reduction of Pd surface oxides promotes an accentuated oxidation peak centered at 0.76 V; these potentials match the electrocatalytic parameters reported for commercial Pd/C.¹³The Pd electrode was also investigated by EDS mapping of the Pd electrode in order to qualitative characterize the chemical composition (experimental details in ESI Section III^†). Firstly, the morphology of the electrode investigated via SEM reveals nanostructures built onto the GR (Fig. 3D). The Pd particles do not have well-defined shapes and are slightly agglomerated. The size of the particles ranges between approximately 100–450 nm. A section of an SEM image (Fig. 3E) was then used to qualitatively investigate the chemical composition of a Pd electrode using EDS. Fig. 3F shows an image highlighting the presence of carbon on the electrode surface, whereas Fig. 3G highlights the presence of Pd. The large region with high presence of Pd (Fig. 3G) overlaps with the region of Pd particles from the SEM image (Fig. 3E). Moreover, the wide carbon region (Fig. 3E) is indicated by the high-intensity green colored region on the EDS mapping (Fig. 3F). These findings assure the presence of the metal and carbon on the printed template. Finally, an EDS spectrum shows the presence of Pd through a peak at ∼2.84 keV (Fig. 3H). Aiming the manufacturing of noble metal-based indirect-3D-printed, we built Pt and Au electrodes, which were both applied for glycerol electrooxidation, as described in ESI Section IV.^†In summary, we successfully turned an insulating 3D-printed piece into a mostly metallic piece with low resistivity. The present research shows a low-cost alternative to build metallic electrodes or metallic pieces suitable for different sizes and shapes of a 3D-printed template with multipurpose. This sustainable protocol allows for the modification of any existing printed material, so it is an alternative for recycle and/or reuse of existing materials, showing that low-cost and widely available thermoplastic filaments of FDM 3D-printers can be used as source of template. This work opens up the possibility of indirect-3D-printing any metallic piece in any shape.     

Photoelectrochemical photocurrent switching effect on a pristine anodized Ti/TiO2 system as a platform for chemical logic devices

We report here the effect of the photoelectrochemical photocurrent switching (PEPS) observed on highly-ordered pristine anodized Ti/TiO₂ for the first time. At negative potential bias, blue irradiation gives cathodic photocurrent, whereas anodic photocurrent was observed for ultraviolet irradiation. We believe this phenomenon is due to the electron pathway provided by Ti³⁺ defect states.

We report here the effect of the photoelectrochemical photocurrent switching (PEPS) observed on highly-ordered pristine anodized Ti/TiO₂ for the first time.

Titanium dioxide, being one of the most studied materials, still draws much attention from researchers.^1,2 It is considered to be a very promising material due to its high chemical stability, nontoxicity, and its unique properties. Due to stable and robust photoactivity, titania is widely used in the design of solar cells³ and photocatalytic applications.⁴ In addition to the fact that titanium dioxide occurs in several crystalline modifications, it can also be obtained in various forms, such as, for example, nanotubes,⁵ nanofibers,⁶ and nanosheets.⁷ The photocatalytic performance of TiO₂ is highly dependent on crystallinity,⁸ phase content, form, and preparation method.⁹ It was reported that highly ordered arrays of TiO₂ nanotubes are characterized by short charge transport distance and little carrier transport loss.⁵ Therefore, electrochemically fabricated TiO₂ nanotube arrays are preferable compared to random non-oriented titania.¹⁰ Great varieties of photoelectrochemical behaviour can be achieved by doping¹¹ and surface modification.^12,13An interesting feature has recently been demonstrated for highly ordered arrays of TiO₂ nanotubes obtained by double stepwise electrochemical anodization of a titanium foil (Ti/TiO₂). Together with our colleagues observed that localized illumination of Ti/TiO₂ surface in water solution triggers proton flux from irradiated area.¹⁴ The photocatalytic activity of TiO₂ is based on photogenerated electron–hole pairs. Under the electric field of Ti/TiO₂ Schottky junction and due to upward surface band bending, efficient spatial charge separation occurs, and photoexcited holes (h⁺) reach TiO₂ – solution interface. The h⁺, which is a strong oxidizing agent, can react with water, and a pronounced pH gradient arises due to water photolysis. Thus, titanium dioxide can be used to trigger local ion fluxes, and proton release is associated with anodic photocurrent. The use of the light-pH coupling effect to control pH-sensitive soft matter was previously demonstrated.^15,16 Complementary species, H⁺ and OH⁻, annihilating when occurring simultaneously, extend chemical arithmetic with subtraction operation opening way to pure chemical calculations.¹⁷ Ion fluxes consideration as information transducers in solution were proposed¹⁸ and performing simple logic operations was demonstrated.¹⁹ This phenomenon opens perspectives to biomimetic information processing and developing effective human–machine interfaces.²⁰Photoelectrodes using light and potential as inputs and yielding photocurrents are being considered as the basis for logic devices. In this way, optical computing compatible with existing silicon-based devices may be performed.Logic operations are described by Boolean algebra operating with truth values denoted 0 (false) and 1 (true). Elementary logical operations are modelled by logic gates producing single binary output from multiple binary inputs and physically implemented by some switch. As for photoelectrode based information processing, the photoelectrochemical photocurrent switching (PEPS) effect is utilized. This effect is that under appropriate external polarization or/and illumination by light with appropriate photon energy, switching between anodic and cathodic photocurrent may be observed for n-type semiconductors and the opposite for p-type.^21,22Without further modification, this effect was observed for a very limited number of materials, such as bismuth orthovanadate, lead molybdate, V–VI–VII semiconductors, and some others. To show this effect, the majority of semiconductors require electronic structure perturbation creating new electron pathways. A convenient solution is specific modifier adsorption onto the semiconductors'' surface, providing a sufficient level of electronic coupling. Photoelectrodes made of nanocrystalline TiO₂ modified by cyanoferrate,^13,23 and ruthenium²⁴ complexes, thiamine, folic acid,²⁵ and carminic acid²⁶ demonstrated PEPS behavior.Surprisingly, we observed the PEPS effect on non-modified Ti/TiO₂ obtained by anodation of Ti plates.Highly ordered arrays of anatase Ti/TiO₂ were obtained. Crystallinity was proved by XRD (Fig. S1a^†). Fig. 1a shows a SEM image of TiO₂ nanotube arrays obtained as described above. According to SEM image, an average pore diameter is ca. 60 nm. As reported, highly ordered TiO₂ nanotubes possess a short charge transport distance and little carrier transport loss. Therefore, highly ordered TiO₂ nanotube arrays fabricated by electrochemical anodization of titanium may exhibit some enhanced capacity of electron transfer than non-oriented ones of random mixture.¹⁰Open in a separate windowFig. 1(a) SEM image of the TiO₂ nanotubes array. The inset shows cross-section view. (b) Scheme of a cell for photocurrent measurements experiment, CE – counter electrode, RE – reference electrode, WE – working electrode.According to Mott–Schottky analysis, at potential bias more positive than −0,697 V vs. Ag/AgCl reference electrode upwards band bending occurs (Fig. S2^†). Heat treatment in a nonoxidizing atmosphere leads to Ti³⁺ formation. Appearance of Ti³⁺ self-doping was proved by EDX analysis (Fig. S1b^†). It was previously reported that Ti³⁺ introduces gap states which act as recombination centers and pathways for electron transfer.^27–29 Ti³⁺ species in reduced TiO₂ introduce a gap state between valence and conduction bands.^27,28We studied dependence of photocurrent on applied potential. Ultraviolet irradiation (365 nm) gave positive photocurrent for all potentials studied in range from −0.6 V to 0.6 V vs. Ag/AgCl reference electrode (Fig. S3^†). The photocurrent increases as the potential becomes more positive, but eventually saturates. The dependence of the current on the potential under blue irradiation (405 nm) had a different character. Sigmoid function with inflection point at 0–0.2 V was observed for blue light.It should be noticed that photocurrent plotted against time on Fig. 2–4 as well as against potential on Fig. S3^† is ΔI = I_{under illumination} − I_{in darkness}. Steady state current values were used for calculations.Open in a separate windowFig. 2Photocurrent curves under chopped irradiation by (a) 365 nm UV LED, (c) 405 nm blue LED at applied potential bias +300 mV vs. Ag/AgCl, and corresponding scheme of electron pathway at +300 mV polarization under irradiation by (b) 365 nm UV LED and (d) 405 nm blue LED.Open in a separate windowFig. 3Photocurrent curves under chopped irradiation by (a) 365 nm UV LED, (c) 405 nm blue LED at applied potential bias −300 mV vs. Ag/AgCl, and corresponding scheme of electron pathway at −300 mV polarization under irradiation by (b) 365 nm UV LED and (d) 405 nm blue LED.Open in a separate windowFig. 4(a) XOR logic realized on negatively polarized (−0.3 V) pristine Ti/TiO₂ by two source irradiation, input A – UV light (365 nm), input B – blue light (405 nm); blue light gives anodic photocurrent, UV – cathodic photocurrent. The current, significantly different from the dark one, is taken as output 1, otherwise – 0. When irradiated by blue and UV light simultaneously, anodic and cathodic current compensate each other, and no total photocurrent observed. Thus output 0, when both inputs are 1 (b) OR logic realized on positively polarized (+0.3 V) non-modified Ti/TiO₂ by two sources of irradiation. Irradiation by any of them, blue or UV, gives anodic photocurrent.At +300 mV vs. Ag/AgCl irradiation by both blue and ultraviolet light give anodic photocurrent (Fig. 2a and c). The UV-irradiation (λ = 365 nm, 5 mW cm⁻²) excites electron directly to the conduction band (CB) of TiO_2, which is further transferred to conducting titanium support (Fig. 2b). When Ti/TiO₂ electrode in thermodynamic equilibrium with electrolyte, an upward surface band bending occurs at the semiconductor–liquid junction. This phenomenon obstructs electron injection from the conduction band into the electrolyte and forces electron drift to conducting substrate. The fast and steady photocurrent production/extinction upon light on/off indicates efficient charge separation and low recombination.Blue light (λ = 405 nm, 70 mW cm⁻²) is characterized by lower energy than UV-irradiation, which is not sufficient to excite the electron to CB. But electron excited by blue light can be trapped by Ti³⁺ located close to the conduction band and transferred to conduction support from these levels (Fig. 2c). An initial current spike following by an exponential decrease suggesting a fast recombination process. It should be also noticed than when irradiation is switched off photocurrent ‘overshoots’ as the remaining surface holes continue to recombine with electrons.At more negative potential (−300 mV vs. Ag/AgCl, for example) applied to non-modified anodized Ti/TiO₂ photoelectrode, we observed anodic photocurrent during irradiation by UV light (Fig. 3a) whereas blue irradiation gave anodic photocurrent (Fig. 3c). Excitation within bandgap by UV-irradiation leads to cathodic photocurrent (Fig. 3b). In the case of irradiation by blue light, electron trapping by Ti³⁺ occurs in the same manner as at +300 mV polarization. But at negative polarization, the energy landscape is such that electron transport to electron donor in solution is preferable (Fig. 3d). As a result, cathodic current occurs.Thereby, photoelectrode activity of non-modified anodized Ti/TiO₂ can be switched from anodic to cathodic and vice versa by applying various potentials and various photon energies. This is the effect of photoelectrochemical photocurrent switching.Thereby, when Ti/TiO₂ is irradiated simultaneously by blue and UV light being negatively polarized, competition between cathodic and anodic photocurrents occurs. Returning to Boolean logic, the PEPS effect allows us to perform annihilation of two input signals and implement optoelectronic XOR logic gate. XOR logic operation outputs true (1) only when input values are different and yield zero otherwise.It is necessary to assign logic values to input and output signals to analyse the system based on Ti/TiO₂ PEPS effect in terms of Boolean logic. Logical 0 and 1 are assigned to off and on states of the LEDs, respectively. Different wavelengths (365 and 405 nm) correspond to two different inputs of the logic gate. In the same way, we can assign logic 0 to the state when photocurrent is not generated and logic 1 to any nonzero photocurrent intensity irrespectively on its polarization (cathodic or anodic). Fig. 4 demonstrates how different types of Boolean logic are realized by irradiation of Ti/TiO₂. Light sources are denoted here as inputs, UV light – A and blue light – B. If the corresponding light source is switched ON and illuminates photoelectrode Ti/TiO₂, this input is ‘1’, otherwise, it''s ‘0’. The photocurrent is read as output. It''s considered to be ‘1’ if significantly differs from dark value and ‘0’ otherwise.At −300 mV vs. Ag/AgCl, pulsed irradiation with UV diode (365 nm, 5 mW cm⁻²) results in anodic photocurrent, which is consistent with electron excitation to CB and transfer to conducting support. Irradiation with blue LED (405 nm) gives cathodic photocurrent due to electron capture by Ti³⁺ states following by transferring to electron acceptor in solution. Simultaneous irradiation with two LEDs with adjusted intensity yields zero net current as anodic and cathodic photocurrents compensate effectively (Fig. 4a).At positive potentials, pulsed irradiation with UV diode gives anodic photocurrent pulses, as well as the blue one. It is interesting to note that when two sources of light are simultaneously irradiated, the photocurrents created by each of them individually do not summarize. At +300 mV, photocurrent output under the influence of two light inputs (365 nm and 405 nm) follows OR logic giving positive output if at least one of inputs is positive (Fig. 4b). Fig. 5 demonstrates the reconfigurable logic system which characteristics can be changed via an appropriate polarization of the photoelectrode regarded as programming input. Two irradiation sources are considered as inputs. OR/XOR logic is realized depending on programming input.Open in a separate windowFig. 5A reconfigurable logic system based on non-modified Ti/TiO₂. Light sources are inputs. The choice between XOR and OR function is determined by programming input of potential bias. At +300 mV OR logic is realized, at −300 – XOR logic. Corresponding truth table is presented.In summary, PEPS effect on modified nanocrystalline TiO₂ was previously discussed a lot.^13,23–26 In this work we report the same phenomenon for pristine anodized Ti/TiO₂ system. Due to substructure of Ti/TiO₂ system, it shows characteristic response to various range of illumination, including visible range and polarization. The Ti/TiO₂ system is a simple and robust model of chemical logic gates. Suggested mimicking of logic functions in aqueous solutions allows further integration of element into communication with living objects¹⁶vs. intrinsically associated photooxidation and degradation, but rather activation for needed function.³⁰     

Preparation of solution processed photodetectors comprised of two-dimensional tin(ii) sulfide nanosheet thin films assembled via the Langmuir–Blodgett method

We report the manufacture of fully solution processed photodetectors based on two-dimensional tin(ii) sulfide assembled via the Langmuir–Blodgett method. The method we propose can coat a variety of substrates including paper, Si/SiO₂ and flexible polymer allowing for a potentially wide range of applications in future optoelectronic devices.

Norton et al. report the manufacture of fully solution processed photodetectors based on two-dimensional tin(ii) sulfide assembled via the Langmuir–Blodgett method.

Two-dimensional (2D) materials are condensed matter solids formed of crystalline atomic layers held together via weak van der Waals forces.¹ They have a wide range of applications including use as channel materials in transistors,² absorber layers in solar cells,³ light emission,⁴ energy storage⁵ and drug delivery⁶ among others. 2D materials often have different properties from their bulk counterparts such as increased strength⁷ and electrical conductivity.⁸ 2D semiconductors may exhibit a change in electronic states from confinement in 1D.⁹ Thin films are often required for the creation of devices from nanomaterials for practical applications and can often be made into flexible devices such as thin film solar cells¹⁰ or photodetectors.^11,12 Thin film solar cells in particular have several advantages over conventional solar cells including lower materials consumption and are lightweight, yet have the potential for high power conversion efficiency.¹⁰Many of the two-dimensional materials produced thus far have been derived from mechanical exfoliation, where Scotch tape or an equivalent is manually used to remove single crystalline layers from a bulk van der Waals solid followed by transfer to a substrate. Whilst this method in general produces extremely high quality crystalline atomic layers,¹³ and is therefore often used to produce prototype devices, it inherently lacks scalabilty. In order to address the problem of mass manufacture of two dimensional materials, liquid phase exfoliation (LPE) was introduced as a cost effective method for producing two dimensional nanomaterials¹⁴ with the possibility of 100 L scales being produced and production rates up to 5.3 g h⁻¹ demonstrated by Coleman et al. with both NMP and aqueous surfactant solutions utilised.¹⁵ This method also does not require the high temperatures needed for methods such as CVD¹⁶ or transfer between the growth and final substrates. Liquid phase exfoliated nanomaterials are also directly processable from solution.¹⁵ Furthermore, LPE has been shown to be effective for the production of a wide range of 2D materials such as graphene,¹⁵ transition metal dichalcogenides¹⁷ and monochalcogenides such as SnSe.¹⁸Tin(ii) sulfide (SnS) is a van der Waals solid with a puckered ab structure consisting of alternating Sn and S atoms, and is isostructural and isoelectronic with black phosphorus.¹⁹ The bulk material has attracted interest due to its indirect band gap energy of 1.07 eV,²⁰ similar to bulk silicon at 1.14 eV. This band gap energy for SnS is useful for applications such as photodetection²¹ and due to its higher theoretical Shockley–Queisser efficiency limit (24%) for solar cells.²² The liquid phase exfoliation method established by Coleman et al. enables nanosheets to be separated from the bulk into solution utilising matching surface energies of the material and solvent.²³ Liquid phase exfoliation of SnS was first reported by Lewis et al. it was established that as layer number reduced, band gap energy increased, and by tuning layer number the onset of photon absorption can be tuned over the near infrared²³ to visible range.²⁴ Overall, LPE is capable of creating large quantities of nanosheets, with potential for industrial scale production. Liquid phase exfoliated SnS has, for example, recently been used in the creation of photoelectrochemical systems with strong stability under both acidic and alkali conditions.²⁵ Many of the functional devices produced thus far have been derived from micromechanical exfoliation and manual nanomanipulation. A far more elegant solution to producing functional devices is to assemble them from solution, for example Kelly et al. recently reported a transistor based on exfoliated WSe₂ nanosheets.²The Langmuir–Blodgett method involves the use of a trough with a layer of water and controllable barriers to compress the film. Nanomaterials in solution are added to the surface of the water and spread evenly to reduce their surface energy,²⁶ often by using a low surface tension spreading solvent such as chloroform.²⁷ The surface pressure is measured as the film is compressed with the substrate being withdrawn when the film becomes solid.²⁸ The Langmuir–Blodgett method has the advantages of large area deposition and improved control of the film at the nanoscale in comparison to vacuum filtration as well as the advantage of requiring no volatile solvents in comparison to liquid–liquid assembly methods. The use of movable barriers also allows for greater film compression.²⁶This method has been used to assemble large scale films of exfoliated MoS₂ by Zhang et al. MoS₂ was exfoliated using n-butyl lithium followed by solvent exchange. MoS₂ was deposited onto the water surface using a 1 : 1 mix of DMF and dichloroethane. Substrates up to 130 cm² were coated with a surface coverage of 85–95%.²⁶ Collapse mechanisms of MoS₂ Langmuir films have also been studied²⁹ alongside MoS₂ deposition on the surface of water with an upper hexane layer.³⁰ Graphene films have also been prepared using the Langmuir–Blodgett method.³¹ The Langmuir–Blodgett method has been used for the assembly of organo-clay hybrid films via the coating of octadecylammonium chloride in a 4 : 1 chloroform : ethanol solution onto a 2D nanoclay liquid phase exfoliated film using an electrospray method.³² A solvent mix of chloroform and NMP has also been utilised for the deposition of nanosheet films.³³ Recently the Langmuir–Blodget method has been used for the assembly of unmodified clay nanosheets,³⁴ Ti₃C₂Tx MXene nanosheet films for the removal of Cr(vi) and methyl orange from an aqueous environment³⁵ as well as for the growth of rGO wrapped nanostructures for use in electrocatalysts.³⁶Given the chemical similarity of the basal planes of inorganic 2D materials, we hypothesised that the assembly of group IV–VI nanomaterials such as SnS should also be possible at the air water interface. Due to their interesting semiconducting and properties described, it should also be possible to produce prototype optoelectronic devices from a fully solution processed pathway. In this paper we now communicate a methodology to assemble thin films comprised of 2D SnS nanosheets using the Langmuir–Blodgett technique (Scheme 1a). We report the use of these films in simple photodetectors. This represents a scalable methodology to produce fully solution processed devices based on 2D materials.Open in a separate windowScheme 1Preparation of SnS nanosheet thin films via the Langmuir–Blodgett method. (a) Cartoon of Langmuir–Blodgett film preparation. (b) Image of Langmuir–Blodgett trough with compressed SnS film. (c) Surface pressure profile during film compression. (d) Image of sample prepared on Si/SiO₂ substrate with edges masked (scale bar 1.5 cm). Scheme 1(a) shows the step by step process of film preparation. Firstly, bulk SnS is broken down by liquid phase exfoliation from the bulk material to produce a stable dispersion of crystalline nanosheets. Characterisation of the exfoliated nanomaterials was undertaken using atomic force and electron microscopy yielding average sheet dimensions of 23.9 nm height × 224 nm longest side length (Fig. S1^†). The nanosheets were then deposited onto the water air interface. The film is then compressed whilst an immersed substrate is withdrawn, leading to the creation of a densely packed nanosheet film. Scheme 1(b) shows that SnS can be successfully deposited on the water–air interface via the addition of chloroform as a spreading solvent, as shown previously with other Langmuir based films.²⁷Scheme 1(a) shows a z-type deposition of SnS as the hydrophilic glass and Si with a 300 nm oxide layer is withdrawn through the film at 1 atm pressure. The film compression occurred at a rate of 5.88 cm² s⁻¹. No further treatments were performed to change the hydrophilicity of the substrates, the oxide layer present was sufficient to provide hydrophilicity to the substrate.³⁷Scheme 1(c) shows a gradual increase in surface pressure as the area was decreased from 1175 cm² to 298 cm² before a sharp increase in pressure, indicating the film has reached full compression. The sharp increase in surface pressure during compression is common in Langmuir–Blodgett assembled films of nanomaterials.³⁸ In response to compression the surface pressure profile in Scheme 1(c) rises rapidly until it reaches a maximum due to the size of the sheets and the potential difficulty in sliding over each other compared to polymers or smaller nanomaterials. Scheme 1(d) shows that the film is capable of being coated onto Si/SiO₂ with a mask defining the areas covered.We characterised the resulting structural and electronic properties of the thin film of SnS nanosheets deposited via the Langmuir–Blodgett method using a range of techniques. Fig. 1(a) shows a height profile AFM image of a film edge with an average on-film roughness (R_a) of 31.9 nm and an average film thickness of 78.6 nm (Fig S3^† provides an additional film profile). Previous work on Langmuir–Blodgett deposition has produced thinner films. The use of high centrifugation speeds yielded 7 nm thick films for a single deposition³¹ whilst the use of lithium ion intercalation before exfoliation enabled film thicknesses of under 2 nm per layer to be realised.²⁶ The average film thickness is above the average sheet thickness, suggesting that the film is made up of overlapping flake multilayers. However, the thickness of the films is significantly lower than those grown via chemical bath deposition (e.g. 290 nm (ref. 39)) indicating that thinner films can be produced compared to chemical bath methods, and potentially at a much lower cost than methods such as CVD. Images of the film morphology in plan view SEM (Fig. 1(b)) suggest no notable alignment of the nanosheets in the lateral dimension as the film is formed and deposited (see Fig S4^† for statistical analysis of sheet angle measurement). The coverage of the film is 94.6% as determined by image thresholding using imagej software to determine the area left uncovered. This gives a coverage of 0.0142 gm⁻² as calculated from average thickness, SnS density and % coverage of the substrate. Preliminary SEM results also suggest that the Langmuir–Blodgett method is effective at coating SnS onto a variety of substrates including polyolefin films (Parafilm®), aluminium foil and paper (Fig S6^†). We also probed the structure of the thin films by powder X-ray diffraction (XRD). After exfoliation and film assembly, the diffraction peak associated with the (400) of SnS is still the most intense reflection but is characterised by a much larger FWHM compared to that of bulk SnS under the same recording conditions (0.442° ± 18.5% compared to 0.175° ± 5%). This indicates a successful breakdown of the crystal structure and thinning of the material in the (400) plane during exfoliation due to the reduction in long range order⁴⁰ (reflections for bulk SnS are assigned to orthorhombic SnS and indexed in Fig S2^†). The lack of any additional peaks indicates that there has not been any significant degradation of the material to the corresponding oxide which is in agreement with previous works.^24,25 The reflections at 88° and 94° are unlikely to be from crystalline silicon⁴¹ due to the thick oxide layer and low angle of incidence used. We tentatively ascribe these peaks to the 3,0,−3 and 3,2,4 peaks for SnS.⁴¹ However a confident assignment of this reflection requires further studies.Open in a separate windowFig. 1Structural characterisation of SnS nanosheet thin films assembled by the Langmuir-Bllodgett method. (a) AFM image of LB assembled SnS film edge. Inset film profile, scale bar = 10 μm. (b) SEM image of LB assembled film on Si/SiO₂ at 3 kV using secondary electron imaging, scale bar = 1 μm. (c) XRD pattern of coated film and bulk SnS powder, (additional peaks labelled in Fig. S2^†). (d) Raman spectra and for bulk and Langmuir–Blodgett assembled SnS nanosheets. (e) UV-Vis spectra of SnS suspension and deposited SnS film on glass (f) Tauc plot of SnS solution and film.We also characterised the optical properties of the nanosheet thin films using Raman and UV-Vis-NIR absorption spectroscopy. No shifts in the Raman peak positions B₃g, Ag and B₃u from bulk SnS to Langmuir–Blodgett film were observed. The broad feature at around 300 cm⁻¹ for the LB film may potentially be due to SnS₂ and Sn₂S₃ impurities.⁴² It is predicted that due to the lower density compared to SnS⁴³ the impurities may increase in concentration compared to the bulk after centrifugation. These impurities may have significant effects on the efficiency of the devices produced.⁴⁴A shift in peak positions is typically observed in nanomaterials which exhibit quantum confinement,⁴⁵ this occurs at 14 nm for SnS.⁴⁶Fig. 1(e) shows a UV-Vis spectra from which the absorption coefficients at fixed wavelengths may be obtained, for 350 nm, 405 nm, 450 nm, 500 nm, 600 nm and 800 nm the values obtained were: 2.26 × 10⁵ cm⁻¹, 2.21 × 10⁵ cm⁻¹, 2.16 × 10⁵ cm⁻¹, 2.04 × 10⁵ cm⁻¹, 1.67 × 10⁵ cm⁻¹ and 1.05 × 10⁵ cm⁻¹ respectively, this matches well to the absorption coefficients of SnS in literature (greater than 10⁴ cm⁻¹).⁴⁷ It also suggests there may be a greater response at shorter wavelengths. Fig. 1(f) shows a band gap of 0.92 eV for the exfoliated SnS in NMP which is below the expected value of 1.07 eV (ref. 20) although lies within the reasonable error introduced by the use of Tauc plots.⁴⁸ The band gap also matches well with SnS exfoliated in NMP in previous work.²⁴ The band gap of the film appears to change from nanosheet suspension to film in 1(f). This has been observed previously for Langmuir–Blodgett⁴⁹ and other deposited films. It has also been observed that apparent decreases in band gap may occur due to the presence of scattering artefacts within films of nanoscale objects.⁵⁰We then produced simple prototype photodetectors via the printing of Ag nanoparticles to form interdigitated electrodes on top of the SnS nanosheet film. Additionally, SnS films were deposited onto lithographically defined Au interdigitated electrodes for characterisation and referencing to the printed devices.Previously SnS photodetectors have been created via methods such as electron beam deposition,⁵¹ thermal evaporation⁵² and chemical bath deposition.⁵³ The Langmuir–Blodgett method allows SnS to be directly processed into a film from a liquid phase exfoliated solution, allowing them to be produced cheaply and with the potential for scalability.Inset to Fig. 2(a) is an image of an interdigitated Ag electrode SnS photodetector device with an area of 6.4 × 10⁻⁵ m². The electrodes can be clearly identified with an average spacing of 99 μm, and an average RMS edge roughness value of 1.89 μm (determined for individual contact lines using the imageJ ‘analyze_stripes’ plugin⁵⁴ (Fig S7^†)). Fig. 2(a) shows an increase in the slope of the I–V curve in the third quadrant indicating a reduction in resistance under 1 sun illumination (1000 W m⁻²) with the AM1.5 spectrum. No short circuit current under illumination was observed indicating that the device functions as a photoconductor. The non-linear response upon negative biasing is due to initial trap filling which once equilibrium has been reached results in linear device operation. Previously it has been shown that silver diffusion into SnS has an interstitial doping effect, neutralising defect states and lowering the film resistivity.^55,56 It is also possible that the Ag ink morphology and the concentration of nanoparticles in the ink may play an effect on the device properties.⁵⁷ A resistivity of 2.85 × 10⁶ Ω sq⁻¹ was obtained for the device which is significantly higher than SnS films prepared by physical vapour deposition (250 Ω sq⁻¹),⁵⁸ likely due to poor carrier mobility between flakes.Open in a separate windowFig. 2(a) IV curves of printed contacts SnS device under darkness and AM1.5 illumination with inset photograph of pseudo Langmuir–Blodgett device with printed Ag contacts scale bar 5 mm. (b) Device under +40 V bias under fixed darkness/illumination cycle. Fig. 2(b) indicates that a clear response is present under illumination when an external bias is applied (giving a field strength of 0.4 V μm⁻¹). Closer inspection shows a fast and slow decay component following the illumination being blocked. This biexponential decay indicates the capture of trapped carriers and the presence of trap states within the device.^59,60 This again supports the photoconductive nature of the device operation with a rise time of ∼0.22 s and a fall time of ∼2.83 s,⁶¹ both being longer than the shutter closing/opening time of 3.7 ms (which was considered negligible). The rise time is the time taken to get from 10% to 90% of the light current with the fall time being the time taken from 90% of the light current to 10%.Previous work performed by Jiang et al. has shown a slow fall time in Ag/SnS photoconductor devices arising from carrier trapping.⁶² Similarly, in our devices the large rise time may also be due to the presence of a high trap density which must be filled upon light exposure.The mean dark current is 2.78 × 10⁻¹⁰ A with a standard deviation of 2.02 × 10⁻¹¹ A. The mean light current was found to be 3.92 × 10⁻¹⁰ A with a standard deviation of 4.03 × 10⁻¹¹ A. A poor signal to noise ratio appears to be present within the device, possibly due to the large number of SnS nanosheets involved in charge carrier transit, leading to a low signal, hence a low signal to noise ratio. The noise could be reduced via surface passivation⁶³ or the use of a diode like structure to reduce leakage current under reverse bias.⁶⁴ A low responsivity of 2.00 × 10⁻⁹ A W⁻¹ ± 1.5 × 10⁻¹⁰ A W⁻¹ was found for energies above the band gap energy of 0.6 eV for the deposited film.The low responsivity may be due to poor bridging between individual SnS nanosheets and the poor transport of holes between adjacent flakes (hopping) relative to the higher mobility within each flake.⁶⁵ There are potentially hundreds of nanosheets between the contacts as determined by the average length obtained (Fig S1^†). To confirm that the optical response was due to the presence of the SnS a reference device was tested (without SnS deposition, Fig S8^†) with no photoresponse observed. Despite the low responsivity, it is notable that the SnS devices fabricated are one of the few examples of a thin film photodetector device based on 2D materials requiring only solution processing at ambient temperature and atmospheric pressure.To demonstrate that the observed behaviour originates from the photoresponse of the SnS flakes a second device was fabricated by pseudo Langmuir–Blodgett deposition on to lithographically defined Au interdigitated electrodes (15 μm separation) on fused silica (inset Fig. 3(b)). This enabled us to remove any effect of photoinduced Ag migration from the observed behaviour as well as eliminating the issue of potential printing irregularities. Fig. 3(a) shows that the devices display a similar photoresponse to the devices with printed Ag electrodes when exposed to modulated AM1.5 illumination. The dark current remains similar at ∼0.3 nA, though during illumination the current is higher (0.7 nA vs. 0.4 nA). This increase directly correlates to the higher electric field strength (0.66 V μm⁻¹vs. 0.4 V μm⁻¹) between the interdigitated electrodes. The responsivity of the device was determined to be 1.79 × 10⁻⁸ A W⁻¹, with a photoresponse rise and fall time of 0.77 s and 0.85 s respectively. The responsivity is lower than for photodetectors prepared by Guo et al.⁶⁶ Improvements to the device to improve the responsivity could include methods to improve the lateral size of nanosheets such as intercalation.⁶⁷ Other routes to improve the device may include doping^68,69 or a change in architecture to a phototransistor type device.⁷⁰ The removal of potential SnS₂ and Sn₂S₃ impurities via methods such as annealing at 500 °C, 500 mbar pressure under argon or the use of higher quality starting material may also be a key route to improve the efficiency of the device.⁴²Open in a separate windowFig. 3(a) Device under 30 s off, 30 s on solar simulator illumination at 1 sun and 10 V bias (b) IV curves under darkness and 350 nm illumination with inset optical microscopy image of contacts (c) monochromatic illumination responses under 10 V bias mapped onto UV-Vis transmission spectra (d) device response under fixed 10 V bias under 350 nm and 405 nm monochromatic illumination.It is also noticeable that the level of noise present in Fig. 3(a) is reduced compared to that in Fig. 2(b), indicating that the Ag electrodes themselves (in addition to the SnS sheets) also affect the performance.When exciting using AM1.5 illumination it is possible that thermal effects may be present which could give rise to the observed behaviour.In order to demonstrate a true photoresponse monochromatic illumination was used to determine if illumination energies above the band gap generated a photocurrent response in the device. Fig. 3(b) shows a small response under 350 nm (3.54 eV) illumination. (IV curves for other wavelengths are available in Fig. S9^†). Fig. 3(c) shows an increased response for 350 nm wavelength as determined via the IV curves. This increased response is likely due to increased absorption as shown in the UV-Vis spectra (Fig. 1e), the signal at longer wavelengths is difficult to observe due to the low responsivity. A higher response at lower wavelength has been observed previously for SnS.⁵³ Fig. 3(d) shows that an increase in current is present under 350 nm and 405 nm illumination which can be cycled on and off. A rise and fall time of 1.09 of 1.44 seconds respectively was observed for 405 nm illumination. A light/dark current ratio of 1.03 was obtained under 405 nm. To account for noise the on and off section had their current averaged using origin software. A drift in current during measurement was observed, this was considered as the reason for the significant difference between the dark current for 350 nm and 405 nm. To further reduce noise surface passivation may also be used to improve the device properties.⁶³ Alternatively, an increase in bias voltage or an increase in monochromatic illumination intensity may improve the signal: noise ratio though may risk damage to the device. A magnified off/on cycle for 405 nm is shown in Fig. S10.^†In conclusion, we report here a methodology for the assembly of 2D SnS nanosheets into thin films using the Langmuir–Blodgett method, and the testing of the films as prototype all-solution processed photodetectors. Tin(ii) sulfide was successfully exfoliated with an average sheet thickness of 33 nm with the average longest side length of 224 nm. A nanosheet based film was coated onto a variety of substrates via the Langmuir–Blodgett method with the addition of chloroform as a spreading solvent. The films were found to be polycrystalline with an average thickness of 78.6 nm with a high surface coverage up to 94.6% for an Si/SiO₂ substrate. The films were found to be semiconductive with the ability to respond to light under bias as shown by AM1.5 and monochromatic illumination. Proof-of-concept photodetectors have been successfully produced. It was also confirmed that the response was due to the photoresponse as opposed to a heating effect. This deposition method could potentially be used to create a variety of SnS films using different exfoliated nanosheet sizes separated via cascade centrifugation as well as the potential for future flexible photodetector devices. Despite the low responsivity, large rise and fall times further work could allow the gain to be optimised. We also note that the use of the Langmuir–Blodgett trough is an easily scalable technology and could provide coatings over very large area substrates not only for photodetectors but for other devices such as thin film solar cells.     

Rapid detection of donor-dependent photocatalytic hydrogen evolution by NMR spectroscopy

Understanding molecular processes at nanoparticle surfaces is essential for designing active photocatalytic materials. Here, we utilize nuclear magnetic resonance (NMR) spectroscopy to track photocatalytic hydrogen evolution using donor molecules and water isotopologues. Pt–TiO₂ catalysts were prepared and used for isotopic hydrogen evolution reactions using alcohols as electron donors. ¹H NMR monitoring revealed that evolution of the H₂ and HD species is accompanied by the oxidation of donor molecules. The isotopic selectivity in the hydrogen evolution reaction gives rise to formal overpotential. Based on a comparison of the rates of hydrogen evolution and donor oxidation, we propose the use of ethanol as an efficient electron donor for the hydrogen evolution reaction without re-oxidation of radical intermediates.

Isotopic molecule processes at photocatalytic hydrogen evolution reactions observed by NMR clarify the importance of the choice of electron donors for efficient chemical energy conversions at electrified interfaces.

The conversion of light energy to chemical energy requires a combination of electronic excitation and sequential electron transfer.^1–3 Efficient electronic excitation is achieved by choosing materials with suitable optical properties, while efficient electron transfer can be achieved by rational design of catalytically active surface sites.⁴ To achieve high catalytic performance, an understanding of the molecular processes occurring at the catalyst surface is required.Photocatalytic hydrogen evolution is accompanied by oxidation of the electron donor. Most studies on this reaction have been conducted using in-line mass spectrometry measurements⁵ or oxygen-quenching methods.⁶ However, monitoring the whole reaction cycle using one methodology remains challenging.Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for observing chemical reactions. This method is mainly used to confirm small-molecule conversions in organic synthesis. However, NMR spectroscopy can also be used to gain information of nanoparticle surfaces⁷ or even for the detection of photocatalytic reactions.^8–10 Furthermore, NMR spectroscopy can be used to determine the nuclear spin states of product molecules.¹¹ Nevertheless, there are very few reported studies on the in situ observation of photocatalytic hydrogen evolution using NMR spectroscopy.Accordingly, in the present study, we utilized NMR spectroscopy to observe the photocatalytic hydrogen evolution reaction. We employed Pt–TiO₂, which is frequently used for the photocatalytic hydrogen evolution reaction, as a model catalyst for this study. NMR spectroscopy enabled sub-micromole-scale detection of reaction products within one minute. We investigated the dependence of isotopic hydrogen evolution reactions on the donor molecules. The effects of efficient donors on the photocatalytic hydrogen evolution reaction are discussed.Pt–TiO₂ nanoparticles were prepared by a typical chemical reduction method (see ESI^†). The morphologies of TiO₂ and Pt–TiO₂ nanoparticles were characterized by transmission electron microscopy, as shown in Fig. S1 and S2.^† The average size of the Pt nanoparticles is approximately 5 nm. The average size of the TiO₂ nanoparticles is within the range 20–30 nm. Typically, 5 mg of catalysts and 0.6 mL of reaction mixture were introduced to an NMR tube under Ar for observation of the photocatalytic hydrogen evolution reaction by NMR spectroscopy. Fig. 1a shows the ¹H NMR spectra of Pt–TiO₂/2-propanol/D₂O before and after light irradiation. Before light irradiation, three peaks are observed (Fig. 1a, black). The single peak observed at 4.81 ppm is assigned to HDO.¹²¹H signals from the methine and methyl groups of 2-propanol are observed at 4.01 and 1.20 ppm, respectively.Open in a separate windowFig. 1 ¹H NMR spectra of Pt–TiO₂/2-propanol/D₂O. The black lines are the ¹H NMR spectra before light irradiation. The red lines are the ¹H NMR spectra after light irradiation for 15 min. (a) Full spectrum. (b) Enlargement of the oxidation product. (c) Enlargement of the area for the H₂ and HD species.After light irradiation, additional species are observed (Fig. 1a, red). We confirmed the photocatalytic response of the Pt–TiO₂ nanoparticles from ON–OFF experiments (Fig. S3^†). The oxidation product is acetone resulting from two-electron and two-proton oxidation of 2-propanol. The signal at 2.25 ppm is assigned to the methyl group in the acetone (Fig. 1b).¹² Hydrogen evolution is observed as a reduction reaction. Four peaks are observed between 4.5 and 4.7 ppm (Fig. 1c). The single peak at 4.63 ppm can be assigned to H₂ dissolved in the solvent.¹³ Other peaks at 4.66, 4.59, and 4.52 ppm are assigned to the HD.^14–16 The observed coupling constant for HD is 43 Hz, which is a typical value for HD.^14–16 The difference in the chemical shifts of H₂ and HD is due to variation of the nuclear magnetic screening constants with interatomic separation as a consequence of the zero-point energy in vibration.^17,18Importantly, NMR spectroscopy can detect H₂ and HD species from the photocatalytic hydrogen evolution reaction. The observed peak splitting of the three peaks is due to the heteronuclear coupling between hydrogen and deuterium atoms.^14–16 The observed chemical shift for H₂ in methanol/D₂O is 4.56 ppm. The observed chemical shifts of HD in methanol/D₂O are 4.60, 4.53, and 4.46 ppm (Fig. S4^†). These values are similar to those for 2-propanol/D₂O. The observed chemical shift of H₂ in ethanol/D₂O is 4.61 ppm. Those for HD in ethanol/D₂O are 4.65, 4.57, and 4.50 ppm (Fig. S5^†). The slight shift in the H₂ and HD signals is due to the difference in the shielding effect depending on the solvation environment.^19–21 The coupling constant between hydrogen and deuterium in HD is 43 Hz, and it is 43 Hz in methanol/D₂O and ethanol/D₂O. The similarity in the coupling constants for the different solvents indicates that the chemical bonding between hydrogen and deuterium is consistent.^14–16 Interestingly, the fullwidth at half maximum (FWHM) values for the H₂ and HD signals are dependent on the solvent. The FWHM values for the H₂ signal are 1.48, 2.24, and 3.51 Hz in methanol/D₂O, ethanol/D₂O, and 2-propanol/D₂O, respectively. The FWHM values for the HD signal are 1.59, 2.00, and 3.32 Hz for methanol/D₂O, ethanol/D₂O, and 2-propanol/D₂O, respectively. H₂ and HD show similar FWHM values in the same solvent. However, the FWHM value is solvent-dependent. In general, a wider peak indicates lower mobility.¹⁶ Therefore, it is expected that 2-propanol induces lower mobility for the hydrogen, probably because of the rotation or diffusional freedom of hydrogen molecules. The solvation environment of hydrogen influences the molecular mobility of hydrogen species in the photocatalytic hydrogen evolution reaction.Oxidation products of the donor molecules are observed in the NMR spectra, as shown in Fig. S4 and S5.^† The number of product molecules is quantified on the basis of the hydrogen atoms in the alkyl chain groups in 2-propanol, ethanol, and methanol as reactants. For methanol, the signals for methylene glycol, 1-methoxymethanol, and methyl formate are observed as shown in Fig. S4.^† For ethanol, acetaldehyde and acetic acid are observed as the products, as shown in Fig. S5.^† As described above, the oxidation product of 2-propanol is limited to acetone. This is due to the unstable intermediate formed in the oxidation of 2-propanol.²² Conversely, the reaction products of methanol^23–26 and ethanol²⁷ are complicated owing to the sequential oxidation and/or hydration reactions.We evaluated the isotopic selectivity of the hydrogen evolution reaction depending on the donor molecules. Fig. 2 shows the typical isotopic selectivity of the hydrogen evolution reaction. The amounts of H₂ and HD were quantified from the NMR spectra. The HD/H₂ ratios were calculated to be 4.1, 3.4, and 1.9 for 2-propanol, ethanol, and methanol, respectively, where the mixture ratio of D₂O and alcohol is 1 : 1. In both cases, attenuation of the hydrogen evolution reaction is specifically observed for methanol. This is probably due to poisoning of the Pt surface with carbon monoxide molecules evolved from the oxidation of methanol at the TiO₂ surface.²⁸Open in a separate windowFig. 2Isotopic selectivity for HD (black) and H₂ (red) from the photocatalytic hydrogen evolution reaction using a 1 : 1 mixture of D₂O and the corresponding alcohol upon light irradiation for 15 min.H₂ is classified as o-H₂ or p-H₂ depending on the nuclear spin isomer.^29,30o-H₂ is observable and p-H₂ is not by NMR because of the Zeeman splitting of the nucleus spin momentum. Because of the spin statistic, the ratio of o-H₂ and p-H₂ is 3 : 1.^29,30 D₂ is not included in the observation because of the low sensitivity to D atoms, even in ²H NMR spectroscopy measurements. Similarly, we observed an increase in the oxidation products of methanol and ethanol. Importantly, the selectivity for the oxidation and hydrogen evolution reaction were continuously monitored, as shown in Fig. S6–S11.^† In addition, the maximum concentration of H₂ in this photocatalytic reaction is approximately 1 mmol L⁻¹, which is below the solubility limits of water and alcohol.^31–34 These results suggest that a robust photocatalytic process continues throughout the catalytic cycle.Isotopic hydrogen evolution provides information about the reaction mechanism at the metal surface.^35–39 The reaction follows an electrochemical adsorption and desorption cycle. The adsorption of atomic hydrogen from the proton donor (Volmer step)⁴⁰ is followed by either desorption via recombination of adsorbed hydrogens (Tafel step)⁴¹ or desorption of atomic hydrogen with a proton donor (Heyrovsky step).⁴² The enrichment of hydrogen over deuterium is observed for the Heyrovsky, Tafel, and Volmer step sequence.^40–42 Generally, the Tafel step is rate-limiting in the hydrogen evolution process for Pt surfaces. Therefore, isotopic selectivity is not dependent on electrochemical potential.As shown in Fig. 2, the isotopic selectivity is similar for the reactions using 2-propanol and ethanol. This suggests that the formal potential of the hydrogen evolution reaction is similar for these two conditions. Additionally, we evaluated self-diffusion of water molecules and each alcohol molecule as shown in Table S1.^† We determined diffusion coefficients for the alcohols and HDO. These results suggest that the diffusion of the reactant in the hydrogen evolution reaction is not the rate-determining step in the photocatalytic reaction cycle.⁴³Interestingly, the efficiency of the multi-electron transfer is dependent on the donor molecule. Fig. S12^† shows the time-course of the oxidation and reduction reactions obtained by accounting for the half-reaction. Linearity in the time-course plot is observed, indicating stable photocatalysis. Therefore, the reaction rate was calculated from the slope of each reaction. Fig. 3 and S12^† show the rates of oxidation and reduction obtained by accounting for the number of electrons in the half-reaction, defined as r_ox and r_red. For a 3 : 1 ratio of D₂O and alcohol, r_red is nominally low. This is probably due to the small number of donor molecules in the catalytic reaction. Importantly, r_red shows the highest value of 0.26 μmol min⁻¹ for the combination of 2-propanol/D₂O (1 : 1). This value is comparable with that for ethanol/D₂O (1 : 1), which is 0.22 μmol min⁻¹. Conversely, the r_ox values for 2-propanol and ethanol are not comparable. Indeed, r_ox for 2-propanol is seven times higher than that for ethanol.Open in a separate windowFig. 3Rates of the oxidation reaction (black) and hydrogen evolution reaction (red) using a 1 : 1 mixture of D₂O and the corresponding alcohol.Finally, we discuss the effect of donor molecules on the efficiency of the photocatalytic hydrogen evolution reaction. The stability of the radical derived from the alcohol plays an important role in the reaction efficiency. 2-Propanol is oxidized to the tertiary carbocation radical intermediate, which is consumed by spontaneous oxidation at the TiO₂ surface (Fig. 4a).^44–47 For ethanol (Fig. 4b), the oxidized carbocation radical species is expected to be unstable compared with that for 2-propanol. Therefore, the rate of the hydrogen evolution reaction is comparable with the rate of the oxidation reaction. For methanol (Fig. 4c), the carbon monoxide evolved is expected to attenuate the hydrogen evolution reaction.²⁸ Thus, the efficiency of the redox reaction can be evaluated from the NMR spectroscopy results.Open in a separate windowFig. 4Schematic representations of photocatalytic hydrogen evolution reactions over Pt–TiO₂ using (a) 2-propanol, (b) ethanol, and (c) methanol.In conclusion, we used NMR spectroscopy to track the photocatalytic hydrogen evolution reaction using Pt–TiO₂ as a model catalyst. We performed rapid detection of dissolved hydrogen molecules in the solvent and the oxidized product at the sub-micromole scale by ¹H NMR. The method is useful for observation of the dynamic state of molecules in solution and product-based determination of the reaction mechanism. This method is also applicable to the screening of photocatalysts under given conditions. In addition, we found that an efficient multi-electron-transfer photocatalytic reaction is possible using ethanol as the donor molecule. This study demonstrates the utility of NMR for the clarification of the hydrogen evolution reaction mechanism as a means to evaluate potential catalysts, from organic molecular catalysts to inorganic nanocrystals.     

Colour-tunable ultralong organic phosphorescence upon temperature stimulus

A new class of single-component molecular crystal with colour-tunable ultralong organic phosphorescence (UOP) was designed and synthesized through alkyl chain engineering. Forming a more rigid environment at 77 K, the colour-tunable UOP from yellow-white to blue-green is achieved through dual-emission of crystal and amorphous states.

A new class of single-component molecular crystal with colour-tunable ultralong organic phosphorescence (UOP) was designed and synthesized through alkyl chain engineering.

Ultralong phosphorescence, a kind of phosphorescence that can be observed by the naked eye after removing the excitation source, has received great attention in the fields of sensing,¹ displays,² imaging,³ anti-counterfeiting⁴ and so on during the past years. Unfortunately, it is limited to inorganic materials because of the weak spin–orbit coupling (SOC) and strong non-radiative transition of pure organic materials.⁵ Compared with inorganic materials, however, organic materials have some excellent merits, such as inexpensive cost, relative safety to the environment, and soft preparation conditions.⁶ The realization of UOP becomes very significant. As mentioned above, UOP can be achieved by promoting intersystem crossing (ISC) through enhancing SOC and suppressing non-radiative transitions. In view of these factors, various strategies such as polymerization,⁷ H-aggregation,⁸ crystallization,⁹ host–guest doping,¹⁰ and freezing conditions¹¹ have been explored to achieve UOP with the unremitting efforts of scientific researchers.Intelligent-response organic luminescent materials can change their luminescent properties such as colour, lifetime, intensity, etc. after being stimulated by the external factor, such as mechanical forces, temperature, pH, light, solvent, etc., which have caught great attraction. The sensitivity of luminescent molecule to excitation wavelength, temperature or oxygen can be applied in sensors, optical recording and so on.^12–14However, the luminescence of these materials with intelligent-response are mostly limited to fluorescence or room temperature phosphorescence (RTP) with relatively short lifetimes.^15–18 Few samples display persistent luminescent feature.¹⁹ Most organic compounds can only tune their afterglow properties by changing molecular side groups or multi-component doping.^20–23 Therefore, the development of single component UOP materials with intelligent response remains a challenge. Inspired by the alkyl-chain engineering²⁴ and freezing conditions, herein we speculate that temperature response UOP might be induced by controlling the activity of the alkyl chain to regulate non-radiative transition rate. By means of reducing the temperature to restrict the molecular motions at amorphous state, colour-tunable UOP with temperature-response can be realized by dual-emission of molecules at both amorphous and crystalline states.In our previous study, MCzT crystals showed yellow UOP at RT.²⁵ The crystals showed the same UOP at 77 K with that at RT (Fig. 1a). Here, 9-(2-((4,6-dimethoxy-1,3,5-triazin-2-yl)oxy)ethyl)-9H-carbazole (MTOD) was designed and via alkyl chain attaching carbazole with a triazine core. The target molecule was characterized by ¹H NMR and ¹³C NMR (Fig. S1–S3^†). Its melting point reaches 130 °C. (Fig. S4^†) MTOD achieved a lifetime of up to 860 ms under ambient conditions. Surprisingly, the UOP changed from yellow-white to blue-green after the removal of the UV-lamp for several seconds at 77 K (Fig. 1b), demonstrating colour-tunable property of UOP with a low temperature stimulus.Open in a separate windowFig. 1Molecular structures and UOP photographs of MCzT (a) and MTOD (b).In order to explore the reasons for the colour-tunable UOP of MTOD at low temperature. The photophysical properties of MTOD in the crystal state were first investigated under ambient conditions. As shown in Fig. 2a, the photoluminescence (PL) spectrum of MTOD shows two main emission peaks at 375 and 413 nm and a shoulder at 435 nm. From the lifetime decay profiles (Fig. S5a^†), it was confirmed that they were all assigned to the fluorescence. Notably, the crystals of MTOD presented yellow afterglow after turning off the UV lamp. From the delayed phosphorescence spectrum, the main emission of MTOD was located at 414, 556 and 600 nm with lifetimes of 681.91, 860.56 and 860.59 ms, respectively (Fig. 2b and S5b^†). Among these, the emission around 414 nm is assigned to triplet–triplet annihilation (TTA) fluorescence originating larger π–π overlaps of carbazole groups from the crystal data (Fig. S6^†).²⁶ Remaining two emission peaks are attributed to UOP emission. From the phosphorescence excitation-emission spectra of MTOD, UOP can be efficiently excited from 260 to 380 nm, with the optimal excitation at 360 nm (Fig. S7^†).Open in a separate windowFig. 2Photophysical properties of MTOD at room temperature and at 77 K. (a) Steady-state photoluminescence (PL, blue dashed line) and phosphorescence (red solid line) spectra at RT and 77 K. Inset: photographs taken after removing excitation. (b) Time-resolved phosphorescence decay of the emission bands at 556 and 600 nm at room temperature, respectively. (c) Time-resolved phosphorescence decay of the emission bands at 478, 564 and 612 nm at 77 K, respectively. (d) PL (blue dashed line) and phosphorescence (red solid line) spectra at molten state.Subsequently, we measured the PL and phosphorescence spectra of MTOD at 77 K (Fig. 2a). The steady-state PL spectrum showed four peaks at 361, 376, 410 and 436 nm, with little change compared with its corresponding spectrum at RT. However, a new phosphorescence peak appears at about 480 nm with an intense emission than others, which displays an ultralong lifetime of 2.5 s. Obviously, the new emission peak plays an indispensable role in the colour-changed UOP at 77 K. The yellowish-white UOP observed by naked eyes at 77 K is generated by the combination of three phosphorescence peaks. However, as time goes by, only the blue-green afterglow at about 480 nm due to the shorter lifetime of long-wavelength phosphorescence can be observed.To find out the origin of this new peak at 480 nm, the PL and phosphorescence spectra of MTOD at molten state were measured as amorphous emission. As shown in Fig. 2d, the PL peak is located at 380 nm and the phosphorescence spectrum shows a broad band with a main peak at 484 nm. It is suggested that the new phosphorescence emission peak at 77 K may be attributed to amorphous state. Taken together, we deduce that the colour-tunable property of the crystal is possibly due to the presence of an amorphous state at low temperature.The X-ray single crystal diffraction of MTOD crystal was taken to explore the mechanism of UOP at room temperature. Abundant intermolecular and intramolecular interactions (Fig. 3a, S6, S8 and S9a^†) in the crystals strongly restrict the torsional molecular configuration. The dihedral angle between the triazine and carbazole groups is about 75° (Fig. S10^†). In the crystal of MTOD, the single molecule is limited by multiple intermolecular interactions, including C–H⋯N (2.681 Å), π-H⋯π (2.791, 2.876 Å), π-H⋯N (2.703, 2.711 Å) (Table S3^†). The rich intermolecular interactions are beneficial to limit molecular motions to suppress non-radiative transitions of excited molecules, leading to UOP. However, the amorphous molecules around the crystals with weak restriction displayed negligible phosphorescence at room temperature due to the strong motions of alkyl chains.Open in a separate windowFig. 3Intermolecular interactions (a) at room temperature and (b) at 100 K.Comparatively, the single crystal of MTOD molecule at 100 K was measured in order to explore the colour-changing mechanism of UOP at low temperature (Fig. 3b and S9b^†). By comparison, MTOD crystals exhibit more intermolecular interactions at 100 K and the distance become shorter. Molecular conformation of MTOD changed slightly, the dihedral angle between triazine and carbazole changed from 74.95° to 75.32° (Fig. S10^†). These increased interactions can constrain the molecules more effectively and the stronger restriction of alkyl chain and carbazole will further suppress non-radiative transitions, resulting in the much longer phosphorescence lifetime of over 1.0 s at low temperature. Compared with the molecules in the crystalline state, freezing condition can provide a more rigid environment to minimize the movement of the alkyl chain, greatly reducing the non-radiative transition rate at amorphous state, resulting in the lifetime of short wavelength phosphorescence at 478 nm up to 2.5 s.According to the above results, the photophysical process of colour-tunable phosphorescence can be described by Jablonski diagram as shown in Fig. 4. Upon photoexcitation, both electrons in amorphous and crystalline molecules transforms to lowest singlet states (S₁). Then, the electrons in S₁ would further transform to the lowest triplet (T₁) through ISC. At room temperature, amorphous molecules show strong molecular motions to facilitate the non-radiative transitions. However, crystalline molecules due to closely arrangement can exhibit phosphorescence through radiative decay. At 77 K, both excitons in amorphous and crystal states are dominated by radiative transitions, leading to colour-tunable UOP.Open in a separate windowFig. 4(a) Jablonski diagram of the relevant photophysical processes illustrating amorphous and crystalline UOP process at room temperature (top) and 77 K (bottom). (b) Phosphorescence spectra of MTOD at different temperature and (c) corresponding coordinates in CIE.In view of the interesting luminescent phenomenon, we have investigated a series of phosphorescent spectra of MTOD crystals at different temperatures ranged from 183 to 273 K. As shown in Fig. 4b, as the temperature increases, the phosphorescence intensity of the amorphous molecules gradually decreases. The colour variations of the MTOD crystals in response to the environmental temperatures are shown in the Commission International de l’Eclairage (CIE) coordinate diagram (Fig. 4c). As the temperature was gradually changed from 183 to 273 K, the UOP changed from green to yellow with good linearity of the CIE coordinates. This demonstrated that MTOD crystals may have potential in low temperature sensing.In conclusion, we synthesized a colour-tunable single-component UOP compound through alkyl chain engineering. Combined the spectral and single crystal analyses, it is indicated that colour-tunable UOP comes from dual-emission of molecules at amorphous and crystalline states. Low temperature provides better rigid effect on UOP of amorphous molecules than crystals, resulting in the UOP colour changed from yellow-white to blue-green. More interestingly, red-shifted UOP of MTOD crystals with the increase of temperature can be achieved, demonstrating its potential for temperature sensing. This study will provide a platform for the design of single-component UOP molecules with tunable colour emission and broadens its application field.     

Self-assembled microrings of Au nanoparticle and Au nanorod clusters formed at the equators of Janus particles

A method for fabricating polymer Janus particles with microring structures at their equators has been developed. This method allows gold nanoparticles and nanorods to be aligned and densely packed along the microrings.

A method for fabricating polymer Janus particles with metal nanoparticle microring structures at their equators has been developed.

The recent development of plasmonic nanomaterials has revealed the interaction between metallic nanostructures and light. Microring structures composed of metallic nanoparticles (NPs) are particularly interesting since they exhibit a strong electromagnetic reaction due to plasmonic coupling among neighboring NPs and the plasmon polariton resonance of the microring structure.^1,2 This is important for realizing optical metamaterials, and microring arrays of metallic NPs on 2D substrates have been fabricated using state-of-art nanoscale lithographic techniques.^3,4 However, it is still challenging to produce 3D arrays of microring structures comprising metallic NPs.Some approaches have been reported to successfully produce 3D metal NP clusters in the form of colloidal metamaterials, or so-called “metafluids”. The production of 3D metal NP clusters has been achieved by controlling the electrostatic and hydrophobic interactions among NPs.^5–7 Manna and co-workers studied optical magnetism in silver (Ag) NP clusters prepared by assembling the NPs on thiol-terminated silica particles.⁸ Formation of Au NP cluster shells on core–shell particles with amino-terminated polybutadiene shells and polystyrene cores has also been reported; strong plasmonic coupling and near infrared (NIR) absorption was observed.⁹ Thus, three-dimensionally symmetric metal NP clusters have been successfully prepared on spherical particles. However, fabrication of metal microring structures comprising metal NPs, which have intrinsically asymmetric structures, has not been realized yet in colloidal materials.Recent developments in polymer particle preparation techniques has allowed the formation of various nanostructured particles including patchy,¹⁰ striped,¹¹ core–shell¹² and Janus structures.^13,14 In particular, Janus polymer colloids have attracted considerable attention due to their potential as pigments in electronic paper,¹⁵ optical switches,¹⁶ anisotropic micrometers¹⁷ and other applications.¹⁸ Recently, we have developed a method for fabricating Janus and core–shell particles by simple evaporation of a volatile organic solvent from a polymer blend solution containing water as a poor solvent.¹⁹ After solvent evaporation, the solubility of the polymer decreases with increasing content of the poor solvent, and the polymer eventually precipitates in the form of spherical particles containing two separated phases. When the interfacial tension between the two blended polymers is balanced, Janus type phase separated particles are selectively obtained using this self-organizing precipitation (SORP) process.²⁰In previous work, polymer stabilized inorganic NPs were incorporated into such Janus particles.^21,22 Janus type phase separated particles have an interface between the two polymers and water. A three-phase contact line is formed at the equator of the particles, leading to energetic instability. If suitable nanoparticles were incorporated at the equator, they could be aligned at the interface and act as compatibilizers between the water and the two polymer domains.Here, we report the preparation of Au NPs and nanorods (Au NRs) covered with amino-terminated polystyrene (PS-NH₂) and amino-terminated polybutadiene (PB-NH₂) prepared by ligand exchange. These are incorporated into PS/PB polymer blended particles having a Janus type phase separated structure prepared by SORP. The effect of Au NP surface ligands on the composite particles is discussed.PB (M_n = 50 kg mol⁻¹, M_w/M_n = 1.06), PS-NH₂ (M_n = 25 kg mol⁻¹, M_w/M_n = 1.04) and PB-NH₂ (M_n = 41.5 kg mol⁻¹, M_w/M_n = 1.4) were purchased from Polymer Source Inc. Co. Ltd. (Montreal, Canada). PS (M_w = 29.3 kg mol⁻¹) was purchased from Aldrich, United States. Tetrahydrofuran (THF/GR) was purchased from WAKO Chemical Industries, Co. Ltd., Japan. An aqueous dispersion of Au NPs (diameter = 20 nm) was purchased from BBI solutions, Inc. An aqueous dispersion of Au NRs (short axis = 25 nm, long axis = 100 nm) was synthesized using a method reported in the literature.²³The fabrication process for polymer-stabilized Au NPs, Au NRs and composite Janus particles is shown in Scheme 1. PS-NH₂ or PB-NH₂ was dissolved in THF to prepare a 10 mg mL⁻¹ solution. The same amounts of the respective solutions were mixed to prepare PS-NH₂ and PB-NH₂ mixed solutions. 5 mL of an aqueous dispersion of Au NPs was mixed with 5 mL of PS-NH₂, PB-NH₂, and PS-NH₂/PB-NH₂ solutions, and the mixed solution was sonicated for 5 min to exchange the ligands from citrate to polymers. Saturated NaCl aq. was mixed into the opaque dispersion after sonication, and the THF dispersion of ligand-exchanged Au NPs was then separated from the water phase. Polymer-stabilized Au NPs were collected from the THF phase by centrifugation (12 000 rpm, 15 min) and dried at room temperature. Polymer-stabilized Au NRs were also prepared using the same procedure.Open in a separate windowScheme 1Schematic illustration of preparation of polymer stabilized Au NPs and composite Janus particles.UV-Vis spectra of the THF dispersions of Au NPs and Au NRs were measured using a UV-Vis-NIR spectrometer (V-760DS, Jasco, Japan). Fourier transform infrared (FT-IR) spectra of dried Au NPs and Au NRs were measured using a FT-IR spectrometer (FT/IR-6100, Jasco, Japan) equipped with an attenuated total reflection (ATR) unit. Thermogravimetric (TG) analysis was performed using Thermo plus Evo2, RIGAKU, Japan. The shapes of the NPs and NRs were observed using transmission electron microscopy (TEM, H-7650, Hitachi, Japan).0.5 mL of THF solutions of PS and PB were mixed with 0.1 mL of a THF dispersion of Au NPs whose optical density was 0.8 at λ = 530 nm. 1 mL of membrane filtered water was added to the mixed THF dispersion in a glass bottle at a rate of 1 mL min⁻¹ with stirring. After stirring was stopped, the THF was evaporated by immersing the bottle in a water bath at 40 °C for 12 h. The aqueous dispersion of composite particles was annealed at 100 °C for 1 h in a microwave heater (Analytic Jena, Germany). The interior structure of the composite particles was observed using TEM after staining the PB moieties with OsO₄. For sample preparation, 0.1 mL of 2 wt% OsO₄ aq. was added to 1 mL of dispersion and allowed to stand for 2 h. After staining, the composite particles were collected by centrifugation (12 000 rpm, 15 min). The sample was washed using membrane filtered water (3 cycles of washing and centrifugation) and finally, one drop of the aqueous dispersion of stained composite particles was placed on a Cu grid with an elastic carbon membrane. After drying at room temperature, the interior structure of the composite particles was imaged using TEM. Fig. 1(a) shows a typical TEM image of Au NPs@PS-NH₂/PB-NH₂. Monodisperse Au NPs covered with polymer shells can be seen. In order to confirm ligand exchange, UV-Vis and FT-IR ATR spectra of the original Au NPs, Au NPs@PS-NH₂, Au NPs@PB-NH₂ and Au NPs@PS-NH₂/PB-NH₂ were measured (Fig. 1(b)–(d)). In the UV-Vis absorption spectrum of the original Au NPs, a broad plasmonic resonance around 530 nm (Fig. 1(b)(i)) is clearly observed. On the other hand, absorption peaks at 260 nm attributed to the aromatic moiety in polystyrene (Fig. 1(b)(ii)) and two peaks attributed to double bonds of 3,4-addition and 1,2-addition in polybutadiene (Fig. 1(b)(iii)) appear. Furthermore, all of the absorption peaks present for these samples (i)–(iii) are observed in the case of Au NPs@PS-NH₂/PB-NH₂. These results imply that Au NPs were amino-terminated polymers selectively modified by ligand exchange. This is also confirmed by the FT-IR spectra. There are no clear absorption peaks observed for the original Au NPs, but peaks attributed to aromatic (iv), and alkyl chains (v) are seen in the case of Au NPs@ PS-NH₂, and peaks attributed to alkyl chains (iv) and double bonds (vii) are observed in the case of Au NPs@ PB-NH₂ (Fig. 1(c) and (d), respectively). All of these peaks appear in the spectrum of Au NPs@PS-NH₂/PB-NH₂. These results confirm the modification of Au NPs with amino-terminated polymers. This is also supported by the TG measurement results shown in the ESI (Fig. S1^†).Open in a separate windowFig. 1TEM image of Au NPs@PS-NH₂/PB-NH₂ (a), UV-Vis spectra (b) and FT-IR spectra (c), (d) of original Au NPs, Au NPs@PS-NH₂, Au NPs@PB-NH₂ and Au NPs@PS-NH₂/PB-NH₂, respectively.To check the effect of surface polymer ligands on the distribution of Au NPs in the composite particles, Janus particles containing PS, PB and polymer-stabilized Au NPs were prepared using SORP. Fig. 2(a) shows a TEM image of a Janus particle comprising PS, PB and Au NPs@PS-NH₂. The dark gray region represents the PB domain and the brighter gray region represents the PS domain. The small dark spots are Au NPs and are visible only in the PS domain. Same as case, Au NPs@PS-NH₂, Au NPs located only at the PB domain of Janus particles comprised of PS, PB and Au NPs@PB-NH₂. These results indicate that the location of Au NPs can be controlled by changing the surface polymer ligands, as has been previously reported.²¹Open in a separate windowFig. 2Schematic illustration of mixed polymers and NPs, TEM images and model of composite Janus particles of PS/PB/Au NPs@PS-NH₂ (a), PS/PB/Au NPs@PB-NH₂ (b) and PS/PB/Au NPs@PS-NH₂/PB-NH₂ (c).On the other hand, when both types of polymers were present on the surface of the Au NPs, the NPs did not disperse in the two different phases, and most of them spontaneously self-assembled in the form of a ring at the three-phase contract line among PS, PB and water (Fig. 2(c)). It is noteworthy that Au NPs seldom assemble at the interface between PS and PB in Janus particles. Russell et al. reported that metal NPs have intrinsic amphiphilic properties and stabilize interfaces between hydrophobic and hydrophilic materials. Due to the amphiphilic nature of Au NPs, they are naturally present at the surface of the Janus particles. Since both PS and PB are immobilized at the surface of the Au NPs, they have equal affinity to both phases, and the interface between PS and PB is energetically stable for them. The spacing between the Au NPs is almost constant since steric hindrance by the polymer chains on the surface prevents them from aggregating.Based on these findings, Au NRs were also employed to form a ring structure. Fig. 3(a) shows a TEM image of Au NRs modified with PS-NH₂ and PB-NH₂ using the same procedure as that for Au NPs. These modified Au NRs were mixed with a THF solution of PS and PB, and Janus particles were prepared by SORP. Before thermal annealing, the Au NRs were randomly located inside the Janus particles (See ESI, S2^†). On the other hand, after thermal annealing, the Au NRs were aligned along the equator of the Janus particles, forming a ring structure as in the case of Au NPs.Open in a separate windowFig. 3Schematic illustration of mixed polymers, NPs and TEM images of Au NRs@PS-NH₂/PB-NH₂ (a) and PS/PB/Au NPs@PS-NH₂/PB-NH₂ after (b) annealing.There are two kinds of alignment of Au NRs at the interface: one along the equator (head-to-tail), and the other perpendicular to that (side-by-side). Most NRs are aligned side-by-side, in order to maximize the packing density at the equator. Since Au NRs have high anisotropy (aspect ratio ∼4), four times fewer can be accommodated for the head-to-tail arrangement. Since the 1D alignment of Au NRs has an important effect on plasmonic resonance,^24,25 controlling this alignment in a confined geometry may represent a way to tune the plasmonic properties of Au NRs.     

Micro-supercapacitors based on oriented coordination polymer thin films for AC line-filtering

Reported herein is a facile solution-processed substrate-independent approach for preparation of oriented coordination polymer (Co-BTA) thin-film electrodes for on-chip micro-supercapacitors (MSCs). The Co-BTA-MSCs exhibited excellent AC line-filtering performance with an extremely short resistance–capacitance constant, making it capable of replacing aluminum electrolytic capacitors for AC line-filtering applications.

Micro-supercapacitors exhibiting excellent AC line-filtering with oriented coordination polymer thin-film electrodes are fabricated based on a substrate-independent electrode fabrication strategy.

Micro-supercapacitors (MSCs), as important Si-compatible on-chip electrochemical energy storage devices, have attracted rapidly growing attention due to their rapid energy-harvesting features and burst-mode power delivery.^1,2 In the past few years, a variety of materials including carbon nanotubes,³ graphene,⁴ graphene oxide and mesoporous conducting polymers,^5,6 have already been explored to fabricate the electrodes of MSCs for improving their electrochemical performance. Unfortunately, fabrication procedures of most of these active materials suffer from high cost, harsh and complicated processing conditions, as well as easy cracking and delamination of active films,^1,4 extremely limiting their commercial applications. Moreover, their performances are unsatisfactory for alternating current (AC) line-filtering, which is a key parameter to implement high-frequency operation in most line-powered devices.^7–9For AC line-filtering, capacitors need to respond harmonically at 120 Hz to attenuate the leftover AC ripples on direct current voltage busses.¹⁰ Notably, the development of more compact and miniaturized capacitors to replace traditional aluminum electrolytic capacitors (AECs) for AC line-filtering has become one of the major tasks for future electronics.¹¹ However, typical supercapacitors are incapable for AC line-filtering at this frequency due to their limited ion diffusion and charge transfer efficiency, corresponding to the unsuitable architectures and low conductivity of electrode materials.^10–12 Therefore, the design and fabrication of highly conductive electrodes with optimized architectures for facial electron/ion transportation is crucial for improving the performance of MSCs in AC line-filtering.^12,13 It is worth mentioning that great advancements have been achieved by utilizing vertically oriented graphene sheets as well as 3-dimensional graphene/carbon nanotube carpets prepared by chemical vapor deposition (CVD),^7,8 yielding efficient filtering of 120 Hz AC with short resistance–capacitance (RC) time constants of less than 0.2 ms, which is competitive with those of porous carbon-based supercapacitors (RC time constant = 1 s) as well as AECs (RC time constant = 8.3 ms).⁸ However, the CVD method necessitated in the fabrication of graphene/carbon nanotube electrodes suffers from high cost and complicate procedures.Coordination polymers with an unrivalled degree of structural and property tunability which could be realized by facial procedures, are promising candidates for energy storage.^14,15 Recently, a remarkable achievement which demonstrated a facile and low-cost solution-processed method towards on-chip MSCs based on an azulene-bridged coordination polymer framework (PiCBA) on a Si wafer-supported Au surface was reported.¹⁴ Nevertheless, the reported preparation of coordination polymer film exhibited strong dependence on the surface chemistry (functionality) of the substrate and further improvement of their electrochemical stability was needed. Therefore, the development of substrate-independent fabrication strategies of large-scale and uniform coordination polymer films is in great need not only for fundamental studies, but also for technological applications especially in electronics.Herein, we demonstrate a facial solution-based substrate-independent approach to fabricate oriented coordination polymer (Co-BTA) thin-film electrodes. Remarkably, rigid and flexible Co-BTA-based MSCs with excellent electrochemical stability and AC line-filtering performance were realized, indicating great application potential in micro-supercapacitors.As demonstrated in Fig. 1a–c, a large scale and continuous Co-BTA coordination polymer film composed of one dimensional (1D) molecules ([Co(1,2,4,5-bta)]_n) was prepared at the air–liquid interface through a coordination reaction between 1,2,4,5-benzenetetramine tetrahydrochloride (BTA) and cobalt acetate tetrahydrate (Co(CH₃COO)₂·4H₂O). Notably, the preparation of Co-BTA film is basing on mild conditions and independent of any substrates. The resulting film can be transferred onto any supports including rigid silicon (Si) wafer, glass, as well as flexible PET substrate, indicating great substrate-independence and making it practically applicable for various applications. Besides of a brown film formed at the air–liquid interface, a powder product is also obtained at the bottom of the reaction bottle.Open in a separate windowFig. 1(a) Synthesis of Co-BTA through the coordination reaction between BTA and cobalt ions. (b) Illustration of the gas–liquid interface growth of Co-BTA film. (c) Photographs of the reaction system before and after the coordination reaction.To study the morphology of the resulting Co-BTA film, the brown film was transferred onto a SiO₂/Si wafer by immersing the wafer down to the reaction mixture and subsequently lifting the film up. The scanning electron microscopy (SEM) image reveals a highly uniform and large-scale distribution of the obtained film without cracks or wrinkles (Fig. 2a), which is superior to other reported coordination polymer films obtained via a similar method.¹⁶ An average thickness of approximately 60 nm of the Co-BTA film is observed from the cross-sectional SEM image as shown in Fig. 2b. Interestingly, thickness of the obtained coordination polymer film could be well controlled and Co-BTA films with thicknesses up to several hundred nanometers could be well prepared by adjusting the ratio of raw materials (Fig. 2c and d).Open in a separate windowFig. 2(a) Planar SEM image of Co-BTA film. Cross-sectional SEM images of Co-BTA films with a thickness of (b) 60 nm, (c) 160 nm and (d) 260 nm.To investigate the structure information of the resulting Co-BTA and further explore the coordination reaction, characterizations including powder X-ray diffraction measurements (PXRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were carried out. The PXRD pattern of Co-BTA powder shown in Fig. S1a^† is in great agreement with that simulated from the crystal structure of Ni(dhbq)·nH₂O (Fig. S1b^†), suggesting that Co-BTA and Ni(dhbq)·nH₂O is isostructural and forms 1D structures with straight infinite chain extends.¹⁷ More interestingly, PXRD measurements employing two different scattering geometries (Fig. S2^†) on the Co-BTA thin-film demonstrate two quite different diffraction patterns. As observed in Fig. 3a, the diffraction pattern observed for the out-of-plane scattering geometry exhibits three characteristic peaks of the Co-BTA film at ∼12°, 24° and 36°, which are corresponding to (001), (002) and (003), respectively. In contrast, the in-plane PXRD profile employing grazing-incidence XRD (GIXRD) technique at an incident angle (α) of 0.2° demonstrates a main peak at ∼18°, which is assigned to the (110) diffraction peak. Different diffraction peaks observed through these two XRD scattering geometries indicate an orientation nature of the as-prepared Co-BTA film,¹⁸ which exhibits better crystallinity compared with the powder Co-BTA product. In addition, the N 1s core level spectrum for Co-BTA film exhibit one typical peak at 399.1 eV, which is corresponding to the amido coordinated with Co^II, indicating the strong coordination between Co^II and BTA (Fig. 3b). The weak peak at ∼401 eV is assigned to N–O due to the oxidation of ligand BTA in ambient environment before reaction. The atomic ratio of N : Co is calculated to be 3.53 : 1 for Co-BTA film and 3.71 : 1 for Co-BTA powder respectively (Fig. S3 and Table S1^†), which is close to the theoretical stoichiometric ratio (4 : 1) for Co-BTA structure, suggesting a high degree of coordination in the resulting product through one Co cation and two benzenetetramine groups. Moreover, the disappearance of two characteristic N–H stretching modes from –NH₂ after the coordination reaction whereas the phenyl-related vibration still exists, further confirms the existence of –NH– in the product through the loss of one H per –NH₂ (Fig. S4^†).¹⁹Open in a separate windowFig. 3(a) PXRD profiles of out-of-plane XRD, in-plane XRD and simulated PXRD pattern of Ni(dhbq)·nH₂O,¹⁷ respectively. *SiO₂/Si substrate. (b) N 1s core level spectra of the Co-BTA film.On the basis of facial fabrication, substrate independence, highly orientation nature, low band gap (1.68 eV, calculated from Fig. S5^†) and excellent stability in acid environment (Fig. S6^†), the resulting Co-BTA film is considered as a promising candidate for MSCs application. Fig. 4a schematically depicts the stepwise fabrication of a planar Co-BTA film based MSC on a SiO₂/Si wafer and its electrochemical performance is first examined by cyclic voltammetry (CV) with scan rates ranging from 50 mV s⁻¹ to 1000 V s⁻¹ (Fig. 4b and c). At a low scan rate of 50 mV s⁻¹, the 60 nm-thick Co-BTA film based MSC exhibited a pronounced pseudocapacitive effect, implying the occurance of faradaic reaction.²⁰ With the increase of scan rate, a gradual transition of the CV curves from the pseudocapacitive to the typical electrical double-layer capacitive behavior with a nearly rectangular CV shape was observed. Remarkably, the device exhibited a maximum volumetric capacitance of 23.1 F cm⁻³ at 50 mV s⁻¹, which is comparable with those of reported carbon- or graphene-based MSCs (Table S2^†), e.g., onion-like carbon,²¹ vertically oriented graphene,⁸ and carbon nanotubes/graphene.⁷ Even though a trend that C_V decreased gradually with increasing scan rate was observed, the Co-BTA-based electrode still delivered a C_V of 2.7 F cm⁻³ even at a high scan rate of 1000 V s⁻¹, suggesting an excellent capacitive performance of this Co-BTA-based MSC device.⁷Open in a separate windowFig. 4(a) Schematic illustration of the fabrication of MSC device with the Co-BTA film electrode. (b) CV curves of Co-BTA-based MSCs in the H₂SO₄–PVA gel electrolyte at different scan rates. (c) C_V evolution of the MSCs at different scan rates.Electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the charge transport properties within the Co-BTA-based MSCs. The Nyquist plot shown in Fig. S7^† indicated the kinetic features of electron transfer/ion diffusion at the electrode, from which an almost straight line especially in the low frequency region is observed. Notably, the plot shows a closed 90° slope without a charge transport semicircle at high frequency which is corresponding to an almost ideal capacitive ion diffusion mechanism, due to the excellent charge transfer property of the oriented Co-BTA electrode film. Moreover, this microdevice exhibited a low equivalent series resistance of 13.48 Ω (Fig. S7^† (inset)), further suggesting the ultrafast ion diffusion characteristic in such a Co-BTA-based-MSC.²² It''s suggested that the unique kinetic feature of fast ion diffusion and charge transfer benefits from the intrinsic characteristics of the oriented polymer film composed of 1D molecules, which can not only facilitate rapid ionic diffusion but also facilitate the interfacial charge transfer and faradaic redox reaction between the electrode material and electrolyte.What''s more, the dependence of the phase angle on frequency shown in Fig. 5a delivered a high characteristic frequency f₀ of 6812 Hz at the phase angle of −45° (the resistance and reactance of the device have equal magnitudes),¹⁰ which is much higher than that of an active carbon supercapacitor (5 Hz),²³ sulfur-doped graphene MSCs (3836 Hz),²² or an azulene-bridged coordination polymer framework based MSCs (PiCBA-MSCs) (3620 Hz) and so on,¹⁴ as summarized in Table S2.^† Moreover, a max phase angle of −80° at a frequency of 18 Hz is observed, indicating the performance of this Co-BTA based MSCs is 89% of that of an ideal capacitor. Importantly, a large impedance phase angle of −78.6° was obtained at a frequency of 120 Hz, which is the largest reported value for coordination polymer based MSCs (Table S2^†), suggesting an excellent AC line-filtering performance of our microdevice.⁷ To further conform the ultrahigh fast ion diffusion in Co-BTA-based-MSCs, the relaxation time constant τ₀ (τ₀ = 1/f₀, the minimum time needed to discharge all the energy from the device with an efficiency of greater than 50% of its max. value) was calculated to be only 0.15 ms (6812 Hz), which is orders of magnitude higher than that of conventional electrical double-layer capacitors (1 s),⁸ activated or onion-like carbon MSCs (<200 ms, <10 ms),^21,23 and much shorter than those of MSCs based on carbon nanotubes/reduced graphene oxide (4.8 ms) as well as azulene-bridged PiCBA coordination polymer framework film (0.27 ms).^14,24 Moreover, a short RC time constant (τ_RC) of 0.32 ms was obtained (Fig. 5b) through a series-RC circuit model, making it capable of replacing AECs for AC line-filtering application. To the best of our knowledge, this is the first report of coordination polymer-based MSCs exhibiting such a small relaxation time constant and RC time constant.Open in a separate windowFig. 5(a) Impedance phase angle on the frequency for the Co-BTA-based microdevices. (b) Plot of capacitance (C_V′ = volumetric real capacitance and C_V′′ = imaginary capacitance) versus the frequency of Co-BTA-based microdevices. (c) Cycling stability of Co-BTA film with 10 000 cycles at the scan rate of 50 V s⁻¹. Inset displays the CV curves at the first, five thousandth and ten thousandth cycle, respectively. (d) Ragone plots for Co-BTA film, compared with commercially applied Li-thin-film batteries,²¹ electrolytic capacitors,² CNT-graphene carpets,²⁴ PiCBA coordination polymer and MXene-reduced graphene oxide.^14,25Impressively, this oriented electrode structure exhibits excellent long-term electrochemical stability with ∼96.3% capacitance retention even after 10 000 cycles of charging/discharging at a scan rate of 50 V s⁻¹ (Fig. 5c), which has also been confirmed by comparing the CV curves before and after testing for 10 000 cycles (inset of Fig. 5c). It''s worth pointing out that the as-made Co-BTA-based MSCs exhibit the best electrochemical stability among reported MSCs with coordination polymer electrodes.¹⁴ On the basis of the above discussion, it is reasonable to conclude that the ultrahigh fast ion diffusion/charge transfer in Co-BTA-based-MSCs attributed to the oriented architecture of Co-BTA thin-film electrodes, the excellent AC line-filtering performance, as well as remarkable electrochemical stability contributes to the excellent performances of Co-BTA-based-MSCs. Moreover, the power density and energy density of the as-made device is calculated and compared with that of MSCs based on other electrode materials to evaluate the energy storage performance of the Co-BTA based MSCs. The Ragone plot in Fig. 5d reveals a high power density of 1056 W cm⁻³ for our device, which is at least five orders of magnitude higher than that of commercial thin-film lithium batteries. What''s more, our device exhibits an energy density of up to 1.6 mW h cm⁻³ at 50 mV s⁻¹, which is at least one order of magnitude higher than that obtained for MSCs based on CNTs-graphene carpet and high-power electrolytic capacitors.^2,24To further demonstrate the substrate independence of this fabrication strategy, flexible Co-BTA-based-MSC device was fabricated and investigated basing on a flexible polyethylene terephthalate (PET) substrate instead of rigid Si substrate in the same way (Fig. S8–S10^†). The as-fabricated device exhibited a maximum volumetric capacitance of 22.0 F cm⁻³ at 50 mV s⁻¹, a short relaxation time constant τ₀ of 0.15 ms and a RC time constant (τ_RC) of 0.42 ms, which are close to the properties of devices with a Si substrate, confirming the substrate independence of this fabrication scheme. As a proof-of-concept application, bending tests were carried out and the bended device (radius = 1 cm) exhibited a small relaxation time constant τ₀ of 0.21 ms and RC time constant (τ_RC) of 0.42 ms, suggesting that the Co-BTA-based MSC with PET substrate in a bended state still delivers a good ion diffusion and AC line-filtering performance.In conclusion, we have demonstrated a facile method that can be used to construct large scale and highly oriented uniform Co-BTA coordination polymer thin films using a very convenient and fast process. With this method, Co-BTA-based MSCs are fabricated without any dependence of the substrate. The as-fabricated MSCs on Si substrate exhibit high specific capacitance, energy density as well as excellent electrochemical stability. Particularly, the fabricated Co-BTA based MSCs deliver excellent AC line-filtering performance with an extremely short RC time of 0.32 ms, attributed to the facilitated ion diffusion beneficial from the oriented architecture of Co-BTA thin film. The high-performance electrochemical properties of Co-BTA-MSCs makes Co-BTA films promising materials to provide more compact AC filtering units for future electronic devices.     

Ratiometric rapid distinction of two structurally similar fluoroquinolone antibiotics by a Tb/Eu hydrogel

Norfloxacin and ofloxacin are two frequently prescribed second-generation fluoroquinolone antibiotics with an identical 4-quinolone chromophore and hence, are difficult to distinguish by conventional methods (UV or fluorescence). We have designed a Tb³⁺/Eu³⁺/cholate cocktail that enabled us to differentiate these two drugs and rapidly measure their concentrations when present together. Additionally, a Tb³⁺-cholate gel-based paper sensor was developed to detect and quantify them in a single drug containing system with a limit of detection (LOD) well below 100 nM.

Rapid distinction of structurally similar fluoroquinolone antibiotics norfloxacin and ofloxacin, and their quantification in a mixture, were achieved using a supramolecular Tb/Eu gel cocktail.

Fluoroquinolone antibiotics belong to one of the most important classes of human and veterinary medicines owing to their broad-spectrum activity and effective oral absorption. Norfloxacin and ofloxacin (Fig. 1) are among the most prescribed second generation fluoroquinolone antibiotics^1,2 used for the treatment of respiratory and urinary tract infections, ocular and skin infections, pelvic inflammatory disease, gonococcal urethritis, infectious diarrhoea, etc. However, potential adverse reactions such as tendinitis, tendon rupture, arthralgia, myalgia, peripheral neuropathy, hematuria, and central nervous system effects due to their excess intake have been well documented in the literature.³ Extensive use and misuse of the antibiotics as veterinary medicines have led to their appearance in milk,^4,5 chicken⁶ and fish⁷ imposing adverse side effects for the consumer. Contamination of municipal wastewater, surface water and even groundwater results from their extensive usage, and from their excretion from human and animals unchanged.⁸ All these can cause the propagation of antibiotic-resistant micro-organisms⁹ leading to a disturbed ecosystem. Therefore, the development of a robust, rapid and straightforward analytical protocol for screening of Norfloxacin (NFLX) and Ofloxacin (OFLX) residues in biological fluids, sewage water, edible tissues and foodstuff is highly desirable for the control of their appropriate dosage, maintaining their pharmacokinetics and avoid environmental crisis.¹⁰ Various analytical methods for this purpose have been reported in the literature including spectrophotometry,¹¹ spectrofluorimetry,¹² voltammetry,^13,14 capillary electrophoresis,¹⁵ chemiluminescence,^16,17 and high performance liquid chromatography (HPLC).¹⁸ Several fluorescent probes have utilized molecular imprinting chemiluminescence,¹⁹ Neodymium-modified micellar media,²⁰ carbon dots etc.^21,22 Although fluorescence-based reporters provide greater intrinsic sensitivity, many of them are, however, incompatible with biological samples because of complications arising from background scattering and autofluorescence from endogenous components.²³ Trivalent lanthanide ions have long radiative lifetimes allowing background-free measurement using time-gating. They also have sharp, well-defined long-wavelength emission fingerprints²⁴ ideal for biological applications and can therefore serve as a strong sensing platform. They can be sensitized by proximate chromophores (‘‘antennae’’) and may act as attractive fluorogenic (turn on) and fluorolytic (turn-off) bio-probes. This group has extensively investigated photoluminescent lanthanide cholate supramolecular hydrogels^25–32 by utilizing their in-built hydrophobic pockets to encapsulate small organic ‘antennas’. The self-assembly brings the antenna in close proximity to the acceptor lanthanide leading to energy transfer to the lanthanide ion. This strategy thus eliminates the need for the synthesis of antenna-linker-ligand type of probes to achieve lanthanide sensitization. A variety of lanthanide-luminescence based fluoroquinolone-assays^17,33–38 have been reported in the literature. However, tedious synthesis of the probe, complex sample treatments, non-aqueous detection media, strict maintenance of pH have restricted the practical applicability of many of them.^34–37 We have found that when doped in micromolar concentrations in Tb-Ch (5/15 mM) or Eu-Ch (5/15 mM) gels, both Norfloxacin and Ofloxacin significantly enhanced the luminescence intensities. Based on this finding we explored cholate-hydrogel based detection and quantification of these two fluoroquinolones. Initial experiments were performed in Tb (5 mM) cholate (15 mM) hydrogel by doping it with increasing concentrations of norfloxacin (NFLX). The emission intensity of Tb³⁺ showed a linear relationship with concentration (Fig. S1 in ESI^†). Even 4 μM of NFLX resulted in 30-fold enhancement of sensitization of Tb³⁺ luminescence. Atomic force microscopy showed that the fibrous network of Tb³⁺-cholate gel remained unaltered (Fig. S2 in ESI^†) when NFLX was doped in micromolar concentrations, proving the robustness and stability of the sensing gel system. Fluorescence microscopic image of semi-dried NFLX doped Tb-cholate gel (Fig. S2c in ESI^†) showed uniform green luminescent gel fibres, indicating that the NFLX was distributed and embedded on the hydrophobic fibres, promoting the energy transfer process making the fibres green luminescent.Open in a separate windowFig. 1Structures of Norfloxacin and Ofloxacin.In order to simplify the sensing method, we chose a paper-based system. Such sensors^39–41 are affordable, biodegradable, sensitive, specific, user-friendly, rapid, robust, and deliverable to end-users. For this purpose, Tb-cholate (5/15 mM) gel doped with NFLX (4 μM) was prepared, sonicated for 5–6 s to reduce the viscosity, and then applied (20 μL) on a 3 mm paper disc (Fig. 2). SEM images showed dried flakes of gel on the cellulose fibre network (Fig. S3 in ESI^†). The thickness of the gel layer coated on the paper surface was about 8 μm, as determined by tilt-SEM (Fig. S3c in ESI^†). Time gated fluorescence (TRF) was measured on the coated discs using a plate reader. After 5 min of drying, a significantly higher Tb³⁺ luminescence output was observed, compared to that measured in wet gels (Fig. 3b, 8 vs. 7). This increase may be attributed to the efficient wicking of water molecules from the gel in the hydrophilic cellulose frameworks of paper which in turn helps reduce the OH-mediated luminescence quenching.⁴² Increased sensitization on gel-coated paper discs was observed for both NFLX and OFLX. We also confirmed that the extent of sensitization of Tb³⁺ remained constant as a function of time (Fig. S4 in ESI^†). The limit of detection (LOD) for NFLX and OFLX were measured based on Tb³⁺-luminescence output as a function of the drug concentration and were found to be 13.6 nM (Fig. S5a in ESI^†) and 67 nM (Fig. S5b in ESI^†), respectively. The paper-based method was used to quantify the NFLX spiked in milk samples (cow raw milk and a commercial homogenized toned milk) and in human blood serum. The LOD values measured were 68 nM, 100 nM and 100 nM, respectively (Fig. S12 and S13 in SI^†). The time-gated detection method eliminated any interference from the matrix (serum or milk). This method is therefore simple, rapid, sensitive, autofluorescence and background emission free, and may thus serve as a practical tool for analytical screening.Open in a separate windowFig. 2Design of paper-based sensors (paper disc cutting, placing them on plate groove, gel dropcasting).Open in a separate windowFig. 3Tb³⁺ -luminescence (λ_em 545 nm, λ_ex 330 nm) enhancement by NFLX (4 μM) (Ac, Ch, D: acetate solution, cholate matrix and coated paper disc, respectively).Distinguishing two structurally similar compounds of the same class is a fundamental objective of analytical research.Dennany et al. recently reported a [Ru(bpy)₃]²⁺ based ECL sensor⁴³ to distinguish Atropine and Scopolamine, two similar alkaloids. Encouraged by this report, we focused on the differentiation of norfloxacin and ofloxacin which have comparable 4-quinolone chromophores with similar functional groups attached (Fig. 1). With both drugs, the luminescence enhancement for Tb³⁺ was greater than that for Eu³⁺ (Fig. S8 & S9 in ESI^†). Therefore, single lanthanide (Tb³⁺ or Eu³⁺) derived gels cannot differentiate them. For this purpose, heterobimetallic ensemble strategy was explored by adopting the ratiometric detection technique.^44,45 In order to identify optimum conditions, a set of five cholate gels samples containing varying ratios of Tb³⁺ & Eu³⁺ (maintaining total concentration of 5 mM) was prepared. This set was doped with 1 μM NFLX. An identical set was prepared with 1 μM OFLX as the dopant. After drop casting the gels on paper discs, luminescence intensities were quantified on a plate reader (Fig. 4a and b). From these data, the sensitization bias I_Tb/I_Eu, as measured by I₅₄₅/I₆₁₇ (λ_ex 330 nm), was calculated and plotted against the concentration of Tb³⁺ in the mixed gel. Tb³⁺/Eu³⁺ cocktail with [Tb³⁺] ≥ 2.5 mM doped with NFLX showed sensitization bias (I₅₄₅/I₆₁₇) values greater than 1, whereas with OFLX the bias was always less than 1 (Fig. 4c). Based on these observations we chose Tb³⁺/Eu³⁺/cholate (4.5 mM/0.5 mM/15 mM) cocktail as the probe to differentiate these two drugs over a wider concentration range (Fig. S15 in ESI^†). Another set of measurements were made at low μM concentrations of the drugs (Fig. S14 in ESI^†). The Terbium sensitization bias (I_Tb/I_Eu) was calculated and plotted against the concentration of FLXs and found to be greater than 1 for NFLX but less than 1 for OFLX (Fig. S14c in ESI^†). In the Tb³⁺/Eu³⁺ mixed cholate hydrogel matrix three concurrent energy transfer processes can occur–energy transfer from sensitizer (i) to Tb³⁺, (ii) to Eu³⁺, and (iii) energy transfer from Tb³⁺ to Eu³⁺. While one of the emission bands of Tb³⁺ has overlap with the most intense 617 nm peak of Eu³⁺, the Eu³⁺ emission peak at 690 nm is solely contributed by Eu³⁺. The 690 nm emission intensity in 15 μM NFLX doped mixed gels was measured and found to linearly increase with the Tb³⁺ concentration (even though Eu³⁺ concentration is decreasing) indicating Tb³⁺– Eu³⁺ energy transfer (Fig. S19 in ESI^†). Significant reduction of Tb³⁺-luminescence lifetime from 1.53 ms in NFLX doped Tb³⁺-Ch (4.5 mM/15 mM) gel to 0.55 ms in NFLX doped Tb³⁺/Eu³⁺-Ch (4.5 mM/0.5 mM/15 mM) gel provided additional evidence. In the Tb³⁺/Eu³⁺/cholate (4.5 mM/0.5 mM/15 mM) cocktail, the three concurrent energy transfer processes are optimized to show Tb3+/Eu3+ emission intensity ratio exactly reversed for the two drugs (Fig. S14, S16 in ESI^†). The optimized cocktail was subsequently used to determine the ratio of the two drugs in a mixture. The drug mixtures (total concentration 20 μM) were doped in the cocktail and the photophysical behavior was investigated. The excitation spectra corresponding to Eu³⁺ 617 nm emission showed a unique pattern depending on the ratio of NFLX and OFLX in the mixture. The intensity (Fig. 5a) at 280 nm reflects NFLX excitation, whereas the intensity at 300 nm reflects OFLX excitation. Based on these findings, the Eu³⁺ emission intensity at 617 nm from the drug mixtures at two different excitation wavelengths 280 nm and 300 nm were evaluated. Ratiometric response I₆₁₇ (λ_ex 280)/I₆₁₇(λ_ex 300) was found to linearly increase with the ratio of NFLX concentration to the total concentrations of both the drugs in the mixture (Fig. 5c). To validate the need for using the cocktail in this study, the excitation spectra corresponding to Eu³⁺ at 617 nm in (NFLX + OFLX) doped Eu³⁺-Ch gel (no Tb) was recorded in which no such NFLX & OFLX ratio-dependent pattern was observed (Fig. S17 in ESI^†). For further exploration of the cocktail, Terbium emission sensitization bias was calculated and found to be directly proportional to the ratio of NFLX concentration to the total concentrations of both the drugs in the mixture (Fig. 6b). This protocol can thus be used as a platform to determine the ratio and individual concentration of the two drugs when present in a mixture.Open in a separate windowFig. 4Emission intensities from Tb³⁺ at 545 nm and Eu³⁺ at 617 nm (λex 330 nm of 1 μM (a) NFLX and (b) OFLX doped mixed cholate {[Tb³⁺] + [Eu³⁺] = 5 mM} gel coated discs for varying ratios of Tb³⁺& Eu³⁺ (c) Terbium sensitization bias I₅₄₅/I₆₁₇ (λ_ex 330 nm) as a function of Tb³⁺ concentration.Open in a separate windowFig. 5(a) Excitation spectra for Eu³⁺ emission (λ_em 617 nm) with respect to the ratio of two drugs (b) Intensity of Eu³⁺ emission at 617 nm for two excitation wavelengths (280 nm, 300 nm) in Tb³⁺/Eu³⁺/cholate (4.5 mM/0.5 mM/15 mM) gel coated discs with increasing concentrations of NFLX in a mixture of OFLX & NFLX, (c) intensity ratio plotted against the concentration of NFLX in the mixture.Open in a separate windowFig. 6(a) Tb³⁺ emission at 545 nm and Eu³⁺ emission 617 nm (b) Tb³⁺ sensitization bias in Tb³⁺/Eu³⁺/cholate (4.5 mM/0.5 mM/15 mM) gel coated discs with the ratio of NFLX concentration to the total drug concentration in the mixture (λ_ex 330 nm).In conclusion, a hetero bimetallic ensemble Tb³⁺/Eu³⁺/cholate was designed for the differentiation of structurally similar NFLX and OFLX. Till date, only physical separation by HPLC is known to quantify norfloxacin and ofloxacin concentrations in a mixture. Additionally, a paper-based sensor has also been developed for the detection and quantification of them in single drug containing system with a lower nanomolar range. The method allowed autofluorescence free detection with biological samples without the need of deproteinization and sample processing. Nonetheless, our preliminary investigations employed to utilize ratiometric bis-lanthanide strategy will serve as the foundation for the fundamental need to differentiate structurally similar molecules. Further studies to utilize this supramolecular paper-based methodology in real diagnostic fields is underway in our lab.