Retracted Article: Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell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

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.     

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.     

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.     

Photoswitchable probe with distinctive characteristics for selective fluorescence imaging and long-term tracing

A photo-switchable and high-contrast bio-imaging indicator 4,4′-(1E,1′E)-(4,4′-(cyclopentene-1,2-diyl)bis(5-methylthiophene-4,2-diyl))bis(methan-1-yl-1-ylidene)bis(azan-1-yl-1-ylidene)bis(2-(benzo[d]thiazol-2-yl)phenol) (BMBT) has been demonstrated, by integrating photochromophore with excited-state intramolecular proton transfer (ESIPT) moiety. The ability of reversible emission switching enables arbitrarily selective labeling or concealing of cells simply by controlling light irradiation. Besides, when the emission was switched on, BMBT is demonstrated to exhibit unique characteristics of aggregation induced emission (AIE), providing a high on–off ratio for favorable bio-imaging. Thus, the non-labeling and easily-controlled selective imaging, as well as good biocompatibility indicates BMBT to be a favorable cell probe with great potentials for functional bio-imaging fluorophore.

In this work, a photoswitchable probe was synthesized by integrating a photochromophore with an excited-state intramolecular proton transfer (ESIPT) moiety. It was explored to be a favorable fluorophore for selective fluorescence imaging and long-term tracing.

Although there are many conventional fluorescent probes used for fluorescent imaging in the past few years,^1–5 such as rhodamine,⁶ cyanine dye,⁷ quantum dots,^8,9 and lanthanide probes,¹⁰ these fluorescent probes can only respond irreversibly to one event.^11,12 In comparison, photochromophores,¹³ which can reversibly response with UV and visible light,¹⁴ are more valuable fluorescent probes for regional optical marking of interested cells.^15,16 Because of their favourable characteristics, such as excellent thermal stability, good fatigue resistance and fast response time, diarylethenes derivatives have drawn wide spread concern of researchers.^17–20 Furthermore, the special optical properties of diarylethenes enable them to be suitable for long time and real time monitoring in bioimaging.The significance of targeted imaging of fluorescence probe for bio-samples in vitro or in vivo is well-known for their high sensitivity, non-invasiveness, real-time detection and especially selectivity.^21–24 However, the synthesis of these targeting materials is usually complicated. Moreover, for the same type of cells, no selectivity is demonstrated by these complex targeting agents.^21–24 Therefore, easy-prepared and easy-controlled non-labeling fluorescence imaging agents have always been pursued by both the industry and scientific communities. Among the above-mentioned photochromophores, diarylethenes are expected to be promising photochromic imaging candidates due to their favorable characteristics described in preceding paragraph. Prospectively, they can be used for selective long-term tracing if the photostability can be ensured.In our previous work, a series of AIE-active excited-state intramolecular proton transfer (ESIPT) complexes have been demonstrated to be good bio-imaging candidate with many advantages such as simple preparation, good biocompatibility, high quantum yields, fast cell staining as well as long-term anti-photobleaching.^25–27 Sometimes, ESIPT compounds exhibit dual emission, originated from keto and enol state, respectively. This caused extremely fast four-level photophysical cycle (E–E*–K*–K–E), mediated by intramolecular H-bonds immediately after photoexcitation, enables two emissions.^28,29 Herein, we have demonstrated a new type of multifunctional bio-imaging materials based on facile synthesis design concept, by introducing a photochromic diarylethene moiety to enable regional emission turn-on, and introducing an ESIPT moiety to allow good photo stability for long-term tracing. To the best of our knowledge, this is the first example with integrating abilities of non-labeling selectively long-term regional tracing.The new diarylethene derivative (BMBT, see the molecular structure in Scheme 1) has been synthesized (Fig. S1^†) via condensation reaction of 5-amino-2-(benzo[d]thiazol-2-yl)phenol with 4,4-(cyclopentene-1,2-yl)-bis(5-methyl-thiophene-2-formaldehyde), according to a previous reported procedure.^30,31 The molecular structures and purities were confirmed by ¹H NMR spectroscopy and mass spectroscopy (Fig. S6 and S7,^† see the synthesis details and full molecular characterizations in the ESI^†).Open in a separate windowScheme 1Structure and photochromic process of BMBT.The photoirradiation-induced changes in absorption and fluorescence spectra at room temperature are investigated in THF/DMEM mixture (1.0 × 10⁻⁵ mol L⁻¹). The open-ring isomer mainly exhibits two absorption peaked at 304 and 380 nm, respectively (Fig. S2^†). This is ascribed to the internal charge transfer and π–π transition of 2-(2′-hydroxy-phenyl)benzothiazole (HBT),^10,30 coupled with the CT inside the HBT unit from the hydroxyphenyl ring to the benzothiazole ring (Scheme 1). BMBT exhibits two emissions; one blue emission peak around 458 nm and a red emission peak around 600 nm, corresponding to the enol and keto emission, respectively (Fig. 1). Upon irradiation with ultraviolet light at 365 nm, the absorption band at longer wavelength region centered at 595 nm increased obviously with irradiation time (Fig. S2^†). This is caused by the formation of the closed-ring isomer (see the photochromic reaction in Scheme 1). Correspondingly, due to spectroscopic overlap between this longer-wavelength absorbance with the red emission ranged from 560–700 nm, the relative intensity of the red emission substantially decreases with the UV irradiation (Fig. 1), because of efficient energy transfer. Upon further visible light (λ = 520 nm) irradiation, the closed-ring isomer transfers back to the initial open-ring isomer, and thus the longer-wavelength absorption decreases and the red emission restores. This indicates good reversibility of the photochromic reaction.Open in a separate windowFig. 1Fluorescence emission changes of BMBT in THF/DMEM (1 : 200, vol : vol) mixture upon irradiation with 365 nm light (λ_ex = 385 nm) and visible light (λ_ex = 520 nm).Excitingly, BMBT exhibit prominent characteristics of Aggregation-Induced Emission (AIE).^31–33 The fluorescence intensity of BMBT in THF solution was relatively weak, while the powder or nanoparticles of the material dispersed in DMEM buffer exhibited a significantly enhanced emission (Fig. 2). When the DMEM buffer fraction was increased gradually from 0% to 20%, the fluorescence intensity only slightly enhanced. When the DMEM fraction was further increased to 40%, the emission exhibits a significant enhancement. The total intensity at the blue enol emission increased more than 10 times at 100% DMEM fraction as compared with that in THF solution. This AIE property enables high signal-to-noise ratios for favorable bioimaging (Fig. S3^†).Open in a separate windowFig. 2Fluorescence spectra of 1 × 10⁻⁵ M BMBT in the THF/DMEM mixture at different water fractions (λ_ex = 385 nm).Based on the good reversibility of light response and AIE characteristics, the practical application of the BMBT as bioprobe was further investigated. The biological imaging of BMBT was observed by using confocal laser scanning microscopy (CLSM). Blue luminescence in the cytoplasm of HeLa cells was observed after incubation with a THF/DMEM (1 : 200, vol : vol) solution of BMBT (20 μM) for 30 min at 37 °C (Fig. 2 inset). The overlay of luminescent images and bright-field images confirmed that BMBT was located mainly in the cytoplasm of cells rather than the membrane and nucleus (Fig. S3^†). Intense intracellular luminescence with a high signal-to-noise ratio (I₁/I₂ > 7) was detected between the cytoplasm (regions 3 and 1) and nucleus (region 2), also implying weak even few nuclear uptake of BMBT (Fig. S4^†). Besides, BMBT has a low cytotoxicity with the cellular viabilities estimated to be greater than 85% after 24 h incubation with the highest cultural concentration of 50 μM BMBT (Fig. S5^†).The luminescence switching of BMBT can also be achieved while alternating UV and visible light illumination in fixed HeLa cells. Cells (shown in red circle, Fig. 3a) were irradiated with 488 nm light (0.5 mW) for 3 min, the blue fluorescence of the irradiated cells was lamped off while the surrounding cells remained almost unchanged. Such fluorescence quenching is likely ascribed to the intramolecular fluorescence resonance energy transfer of BMBT, due to the intensified short-wavelength absorption band (270–450 nm) of the closed-open form of BMBT with the blue emission band (420–560 nm).³⁴ Upon irradiation with 405 nm light (1.25 mW) the fluorescence of all the selected cell was rapidly recovered within 1 min, caused by decrease of the relative intensity of the 270–450 nm absorption. The fluorescence can be repeatedly erased and recovered many rounds without significant fluorescence quenching, which was hardly achieved by conventional fluorophores (Fig. 3b).^35,36Open in a separate windowFig. 3(a) CLSM image (above) and the overlay image (bottom) of fixed HeLa cells incubated with 20 μM BMBT for 30 min at 37 °C (1) and (5) in original state; (2) and (6) irradiated by 488 nm light (0.5 mW) for a single cell; (3) and (7) all cells, and; (4) and (8) recovered by 405 nm light (1.25 mW). (b) Fluorescence switching of fixed HeLa cells by alternating UV (405 nm, 1.25 mW, 10 s/time) and visible (488 nm, 0.5 mW, 3 min/time) light illumination (λ_ex = 405 nm).This characteristic of selectively opto-marking or de-marking of cells may be used for non-invasive and dynamic tracing of the interested objects in vitro. That is, BMBT can arbitrarily opto-label or de-label interested cells without affecting cell proliferation. As shown in Fig. 4, the cell marked in the red circle was treated with visible light illumination (488 nm, 0.5 mW, 3 min), and its fluorescence was effectively quenched. Cells division of the remaining “bright” cells was observed under the microscope field of vision for a long-term tracing with time up to 36 h. Multiple new cells were produced, as indicated by upper white arrow. Even, as indicated by bottom white arrow, the tacked cells were observed to be doubled. The high brightness of the fluorescence in the proliferated cells indicated the good photo stability of BMBT. This indicated that BMBT can be used as a cell marker for arbitrarily selective erasing the fluorescence of the designated cell. It can also be used for selective lighting their emission by making them as the remaining bright cells after selective photo-erasing or selective photo-recovering after full erasing. This long-term tracking with non-labeling and selective optically marking or de-marking is seldom reported by other photochromophore-based bioimaging agents.³⁷ Herein, the excellent anti-photo bleaching characteristic in the long-term tracing is attributed to high photo stability of the HBT moiety.^26,27Open in a separate windowFig. 4Cells was treated with visible (488 nm, 0.5 mW, 3 min) light illumination for erasing the fluorescence. The remaining bright cells were incubated for another 36 hours, which were observed to be amplified normally (λ_ex = 405 nm).     

Highly dispersed Pt nanoclusters supported on zeolite-templated carbon for the oxygen reduction reaction

The formation of highly dispersed Pt nanoclusters supported on zeolite-templated carbon (PtNC/ZTC) by a facile electrochemical method as an electrocatalyst for the oxygen reduction reaction (ORR) is reported. The uniform micropores of ZTC serve as nanocages to stabilize the PtNCs with a sharp size distribution of 0.8–1.5 nm. The resultant PtNC/ZTC exhibits excellent catalytic activity for the ORR due to the small size of the Pt clusters and high accessibility of the active sites through the abundant micropores in ZTC.

Electrochemically synthesized highly dispersed Pt nanoclusters (PtNCs) stabilized by the nanocages of zeolite-templated carbon (ZTC) exhibit excellent electrocatalytic performance toward the oxygen reduction reaction.

Platinum (Pt) is currently considered one of the best electrocatalysts for the oxygen reduction reaction (ORR), which occurs at the cathode of a fuel cell and is the key process determining the overall performance.^1–5 However, the high cost and scarcity of Pt limit its wide commercialization in this field. According to the US Department of Energy, the total Pt loading is required to be below 0.125 mg cm⁻², in contrast to a presently used Pt loading of 0.4 mg cm⁻² or more for fuel cell application.⁴ Therefore, reducing the Pt loading without loss or with an improvement of the cathode performance has received significant interest in electrocatalytic research for fuel cell systems.^6–10 In this regard, reducing the size of Pt particles to a nanocluster scale (size < 2 nm) and maximizing the Pt dispersion may offer an efficient way to achieve maximum utilization of the Pt electrocatalyst with appropriate consumption.^4,11–15The size of nanomaterials generally plays a critical role in controlling the physical and chemical properties for catalytic applications.^16–20 With a decrease in the particle size to the nanoscale, quantum size effects are induced, which alter the surface energy of the material due to unsaturated coordination and change in the energy level of the d orbital of metal atoms, leading to spatial localization of the electrons.^17–20 This size-induced effect on the electronic structures at the active sites modifies the capability of binding the reactant molecules in catalytic reactions, thereby altering the activity of the nanocatalyst.²⁰ When the particle contains a few to several dozens of atoms with sizes, ranging from sub-nanometer to 2 nm often termed as nanocluster that bridges nanoparticle and a single atom.²¹ However, the Pt single atom is not an appropriate electrocatalyst for the ORR in a fuel cell system as the fast four-electron (4e⁻) pathway for the reduction of O₂ to H₂O requires at least two neighboring Pt atoms.^22,23 Anderson''s group demonstrated that the ratio between the production of H₂O (product of 4e⁻ process) and H₂O₂ (2e⁻) in the ORR strongly depends on the number of atoms in the Pt cluster. Typically, it requires more than 14 atoms in a Pt cluster to produce H₂O efficiently through the 4e⁻ pathway of the ORR.²⁴ Therefore, Pt nanoclusters having more than a dozen atoms have proven to be highly efficient ORR electrocatalysts for fuel cell systems.^13–15 Upon decreasing the size of the nanoparticles to a nanocluster, the electronic state and structure are known to be changed, leading to an increase of the catalytic activity in the ORR. Therefore, it is highly desirable to synthesize a Pt nanocluster-based material as an ORR electrocatalyst with high catalytic performance. To date, several synthesis strategies, such as wet-chemical, atomic-layer deposition, and photochemical methods, have been applied for the preparation of well-dispersed Pt nanoclusters on different types of support, such as dendrimer, metal oxide, and carbon materials.^{13–15,25–31}An alternative approach to synthesize Pt nanocluster (PtNC) is the encapsulation of the cluster within nanosized pores, for example, by utilizing microporous (diameters less than 2 nm) carbon materials.³² Among the microporous carbons, zeolite-templated carbon (ZTC) has been attractive for supporting Pt clusters due to its ordered microporous structure.^33–37 ZTC is a potentially promising material as catalyst support as it offers the advantages of extremely large surface area and high electrical conductivity of graphene-like carbon frameworks constituting a three-dimensional (3D) interconnected pore structure.³⁶ Moreover, the micropores of ZTC can serve as nanocages for stabilization of the Pt nanoclusters. Coker et al. used Pt²⁺ ion-exchanged zeolite as a carbon template to synthesize Pt nanoparticles in ZTC with size in a range of 1.3 to 2.0 nm.³³ Recently, atomically dispersed Pt ionic species was synthesized via a simple wet-impregnation method on ZTC containing a large amount of sulfur (17 wt%).²³ Itoi et al. synthesized PtNC consisting of 4–5 atoms and a single Pt atom in ZTC using the organoplatinum complex.³⁷ Although these methods produced Pt nanoclusters with narrow size distribution and atomic dispersion, they required multi-step processes and/or high-temperature treatment (>300 °C). High-temperature treatment often induces the sintering of nanoclusters to aggregated clusters. Therefore, it is highly desirable to develop a simple and low-cost method for the preparation of PtNC supported on ZTC (PtNC/ZTC) for use as an efficient ORR electrocatalyst. The electrochemical reduction approach offers an alternate and efficient route for the synthesis of PtNC in the micropores of ZTC. The electrochemical method is one of the popular ways to prepare electrocatalysts because it is a simple single-step procedure and ensures electrical contact between the nanoparticles and the support.^38,39Herein, we report a facile electrochemical method for the formation of PtNC with a narrow size range of 0.8–1.5 nm supported on ZTC. The resultant PtNC/ZTC shows higher electrocatalytic activities towards ORR compared to that of commercial Pt/C. Here, ZTC plays two important roles: (i) it provides nanocages to stabilize the PtNC and (ii) it accelerates the ORR activity by enhancing the accessibility of active sites through its abundant micropores. Fig. 1a shows a schematic representation of the typical electrochemical synthesis of PtNC/ZTC. In the first step, ZTC was impregnated with a Pt-precursor dissolved in a water–ethanol mixture. As ZTC possess ordered micropores (Fig. S1a^†) with high Brunauer–Emmett–Teller (BET) surface area of 3400 m² g⁻¹ (vide infra), the uniform adsorption and anchorage of PtCl₆²⁻ ions into the micropores of ZTC was favored. After impregnating and drying, the resultant ZTC–PtCl₆²⁻ was mixed with water–ethanol and Nafion to make the ink for the preparation of the electrode. Using the prepared electrode, a potential of 0.77 V vs. reversible hydrogen electrode (RHE) (Fig. 1b) was applied followed by potential cycling between 1.12 to −0.02 V vs. RHE until the cyclic voltammogram was stabilized. The Pt content of PtNC/ZTC was determined to be ∼10 wt% (Fig. S2^†) by thermogravimetric analysis (TGA). The obtained PtNC/ZTC was electrochemically characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The cyclic voltammogram (Fig. 1c) after potential cycling in fresh KOH electrolyte shows the characteristic Pt peaks corresponding to hydrogen adsorption and desorption. The Nyquist plots (Fig. 1d) demonstrate that PtNC/ZTC has lower electrolyte resistance (42 Ω) than that of ZTC (70 Ω), implying an improvement in the conductivity of ZTC by the presence of PtNC. Due to the increase in the conductivity, PtNC/ZTC could facilitate the electron transfer more effectively than ZTC, enhancing its electrocatalytic activity.Open in a separate windowFig. 1(a) Illustration for the formation of PtNC/ZTC:Pt-precursor was impregnated into ZTC micropores, and then a potential (0.77 V vs. RHE) was exerted on the ZTC–PtCl₆²⁻ composite in a 0.1 M KOH solution to form PtNC/ZTC (b) Chronoamperometric response of ZTC–PtCl₆²⁻ at a constant potential of 0.77 V (vs. RHE) in 0.1 M KOH electrolyte. (c) Cyclic voltammogram of PtNC/ZTC in a fresh 0.1 M KOH at a scan rate of 20 mV s⁻¹. (d) Nyquist plots of ZTC and PtNC/ZTC in 0.1 M KOH. Fig. 2a and b show images from aberration-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF). The HAADF-STEM images exhibit the typical morphology of the final product (PtNC/ZTC) after electrochemical reduction. As shown in Fig. 2a, it is very clear that isolated PtNCs are uniformly dispersed in ZTC. These PtNCs have a homogeneous distribution with a narrow size range (0.8–1.5 nm, Fig. 2b). On further magnification, the STEM image shows a cluster-like structure of Pt (Fig. 2c). The STEM image of selected PtNC (Fig. 2d) reveals that it consists of ∼20 atoms. The number of atom content in PtNC was further determined by matrix-assisted laser-desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry using trans-2-[3-(4-test-butylphenyl)-2-methyl-2-propenylidene]malononitrile as the matrix.^40,41 As shown in Fig. S3,^† MALDI-TOF measurement produces a mass spectra with a predominant peak centered at ∼3700 Da corresponding to the Pt₁₉ cluster. The TEM image (Fig. S4 a and b^†) validates the formation of PtNC with an average size of 0.9 nm. In addition, the energy dispersive X-ray spectrometer (EDS) mapping images clearly shows the uniform dispersion of Pt nanocluster in ZTC (Fig. S4c^†). The X-ray powder diffraction (XRD) pattern (Fig. 2e) of PtNC/ZTC showed three broad peaks associated with small size metallic Pt corresponds to (111), (200), and (311) planes (Fig. 2e, inset), along with peaks of ZTC at 2θ = 7.8° and 14.9° corresponding to the ordered microporous structure. Along with the structural analysis, the porous texture of PtNC/ZTC was examined by Ar adsorption (Fig. 2f). PtNC/ZTC had a high BET surface area of 2360 m² g_ZTC⁻¹, which is 1.4 times lower than that of pristine ZTC (3400 m² g_ZTC⁻¹). The decrease in Ar adsorption capacity after the formation of PtNC in ZTC is interpreted as a result of the filling of ZTC micropores by PtNC. This micropore filling was confirmed in the pore size distributions of the pristine ZTC and the metal-loaded carbon (inset of Fig. 2f). The X-ray photoelectron spectroscopy (XPS) results reveal the signature of Pt in ZTC (Fig. S5^†). The elemental survey (Fig. S5a^†) shows the signature of C 1s, O 1s, F 1s (Nafion), and Pt 4f. The chemical nature of Pt in PtNC/ZTC was inspected by a detailed Pt 4f XPS analysis. The deconvoluted Pt 4f XPS spectra (Fig. S5b^†) reveals the presence of both metallic and ionic Pt species. The peaks observed at 71.0 (4f_7/2) and 74.2 (4f_5/2) eV correspond to metallic Pt whereas the other peaks positioned at 72.6 (4f_7/2) and 76.0 (4f_5/2) are attributed to Pt²⁺ and the peaks at 74.9 (4f_7/2) and 77.8 (4f_5/2) eV are attributed to Pt⁴⁺ originating from the surface oxidation of metallic Pt.⁴²Open in a separate windowFig. 2(a–d) Representative spherical aberration-corrected HAADF-STEM images of PtNC/ZTC at various magnifications. (e) XRD pattern of PtNC/ZTC and (f) Ar adsorption–desorption isotherms of ZTC and PtNC/ZTC. Inset in (e) shows a 30 times magnified high-angle region of XRD of PtNC/ZTC. Inset in (f) shows the pore size distributions of the ZTC and PtNC/ZTC.The formation of narrow sized PtNC by the electrochemical method can be ascribed to the stabilization of PtNC in the ZTC micropores, which serve as cages to impose a spatial limitation on the size of the Pt clusters. For comparison, Pt supported on ZTC was also prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction at high temperature (300 °C). The Pt obtained by this incipient wetness impregnation method shows the formation of Pt nanoparticles on the exterior surface of ZTC (PtNP/ZTC) (Fig. S6^†). The formation of larger Pt nanoparticles is due to the sintering at high temperature, showing that even ZTC micropores could not prevent the aggregation of PtNCs at high temperatures. Fig. 3 shows the electrochemical ORR activity of PtNC/ZTC using linear sweep voltammetry (LSV) technique on a rotating disc electrode (RDE) in a 0.1 M KOH solution saturated with O₂ at a scan rate of 5 mV s⁻¹. The ORR activity of ZTC (without PtNC) was measured for comparison as well. As shown in Fig. 3a, PtNC/ZTC exhibited higher diffusion limiting current density and higher positive onset and half-wave potential compared to ZTC alone, indicating that PtNC is the active center for the ORR. To investigate the effect of the Pt loading amount on the ORR activity, PtNC/ZTC with various Pt loadings, 2–20 wt%, was used for the measurement of LSV at 1600 rpm. With an increase in Pt content, both the onset and half-wave potential shifted towards more positive potential up to 10 wt% loading of Pt (Fig. 3a and S7^†). Upon further increase of loading of Pt on ZTC to 20 wt%, both the onset and half-wave potential of PtNC/ZTC shifted towards less positive potential along with a slight decrease in the diffusion limiting current density (Fig. 3a). The decrease in the ORR activity of PtNC/ZTC at high loading of Pt (20 wt%) was attributed to the decrease in the electrochemically active surface area (Fig. S8^†) and decrease in the specific surface area (Fig. S9^†). The STEM image clearly shows that the aggregated Pt clusters were formed on the exterior surface of ZTC at 20 wt% loading of Pt (Fig. S10c^†), blocking the accessibility of active sites. Therefore, PtNC/ZTC with the optimum loading of 10 wt% of Pt leads to superior ORR activity with a high positive onset potential of 0.99 V, which is similar to commercial Tanaka Pt/C (Pt/C-TKK) (Fig. 3b), and a half-wave potential of 0.87 V, which is ∼10 mV more positive than that of commercial Pt/C-TKK (0.86 V) (Fig. 3b). Compared to the case of PtNC/ZTC, both the onset and half-wave potential of PtNP/ZTC prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction with the same loading of Pt exhibited a less positive value (Fig. S11^†). The poorer activity of PtNP/ZTC is due to the blockage of active sites by larger PtNPs formed on the exterior surface of ZTC (Fig. S6^†).Open in a separate windowFig. 3(a) RDE ORR polarization curves of PtNC/ZTC with different mass loading of Pt. (b) Comparison of PtNC/ZTC (PtNC_10%/ZTC) with commercial Pt/C-TKK at the same loading of 40 μg_Pt cm⁻². (c) RDE ORR polarization curves of PtNC/ZTC at different rotation speeds. Inset in (c) shows the corresponding K–L plots at different potentials. (d) Represents the kinetic current density values of Pt/C-TKK and PtNC/ZTC at the potential of 0.8 V vs. RHE.To investigate the kinetics of the ORR activity of PtNC/ZTC, LSV measurements were performed with RDE at different rotating rates (Fig. 3c), and the kinetics was analyzed using a Koutecký–Levich (K–L) plot (Fig. 3c, inset). From Fig. 3c, it was observed that the current density increases with the increasing speed of rotation of the electrode, which is characteristic of a diffusion-controlled reaction. The corresponding linear K–L plots (Fig. 3c, inset) with a similar slope at different potentials reveal that the number of transferred electrons was ∼4, indicating that O₂ is directly reduced to OH⁻ and the ORR is dominated by the H₂O₂-free 4e⁻ pathway. To estimate the amount of produced peroxide ion, rotating ring-disc electrode (RRDE) measurement was performed and the produce peroxide ion calculated from RRDE curve was < 4% (Fig. S12^†). The kinetic current density (J_k) obtained from K–L plot at the potential of 0.8 V (Fig. 3d) for PtNC/ZTC (J_k = 50 mA cm⁻²) is 2.2 times higher than that of commercial Pt/C-TKK (J_k = 22 mA cm⁻²).As Pt-based electrocatalysts are known to be highly active in an acidic medium, the ORR activity of PtNC/ZTC in O₂-saturated 0.1 M HClO₄ was also evaluated by comparing it with that of commercial Pt/C-TKK with the same loading of Pt on the electrode surface using RDE at a scan rate of 5 mV s⁻¹. The PtNC/ZTC-based electrode exhibited ORR activity with an onset potential of 0.96 V (Fig. 4), which is close to that of Pt/C-TKK (0.98 V), and half-wave potentials of 0.84 V, which is 20 mV more positive than that of Pt/C-TKK (0.82 V). PtNC/ZTC showed a slightly higher diffusion-limiting current density of ∼5.9 mA cm⁻² (0.4–0.7 V) compared with that of the Pt/C-TKK catalyst (∼5.6 mA cm⁻²). The kinetics of the ORR in an acidic medium was further analyzed using RDE at different rotation rates (Fig. S13^†) and it was observed that the current density increases with the increasing speed of rotation of the electrode, as in the case of the alkaline medium. The number of electron involved and the amount of produced H₂O₂ estimated by RRDE measurement were ∼4 and < 5%, respectively (Fig. S14^†). The mass activity of PtNC/ZTC obtained using the mass transport corrected kinetic current at 0.8 V is 0.15 A mg⁻¹, which is 3.2 times higher than that of Pt/C-TKK (0.046 A mg⁻¹).Open in a separate windowFig. 4(a) RDE ORR polarization curves at 1600 rpm and (b) mass activity at 0.8 V of PtNC/ZTC and Pt/C-TKK in 0.1 M HClO₄.Furthermore, the methanol tolerance of PtNC/ZTC was assessed by intentionally adding methanol to the oxygen saturated electrolyte solution (both in alkaline and acidic media). The commercial Pt/C-TKK was used for comparison as well. The peak current densities for methanol oxidation with PtNC/ZTC were ∼2.8 and ∼3 times lower than that of Pt/C-TKK in alkaline (Fig. 5a) and acidic (Fig. 5b) media, respectively. These results indicate that PtNC/ZTC has much higher tolerance towards methanol than Pt/C-TKK does. This higher methanol tolerance of PtNC/ZTC can be attributed to the small size of the Pt cluster, which may not be sufficient to catalyze the oxidation of methanol efficiently, as the oxidation of methanol requires Pt ensemble sites.⁴³Open in a separate windowFig. 5ORR polarization curves of PtNC/ZTC and Pt/C-TKK in the absence (solid line) and presence (dotted line) of 0.1 M of CH₃OH at a rotation rate of 1600 rpm in (a) alkaline and (b) acid media.The durability of PtNC/ZTC was also investigated by the amperometric technique. The test was performed at a constant voltage of the half-wave potential in an O₂-saturated alkaline medium and at 0.7 V in an O₂-saturated acidic medium at a rotation rate of 1600 rpm (Fig. S15a and b^†). The durability of the PtNC/ZTC catalyst in the alkaline medium was higher than that of Pt/C-TKK, exhibiting a 30% decrease compared to a 40% decrease of Pt/C-TKK in 5.5 h of ORR operation (Fig. S15a^†). The higher durability of PtNC/ZTC compared to Pt/C-TKK in the alkaline medium may be due to the stabilization of PtNC by pore entrapment. In the acidic medium, however, PtNC/ZTC exhibited a 54% decrease in the initial current after 5.5 h of operation while a 33% decrease was observed in the case of Pt/C-TKK (Fig. S15b^†). The decrease in ORR activity in the acidic medium may be due to the leaching out of tiny Pt nanoclusters in acid electrolyte from the ZTC micropores. To understand the decrease in the ORR durability with time, STEM measurements of PtNC/ZTC after 5.5 h of ORR operation were performed. In the alkaline medium, the STEM image of post-ORR PtNC/ZTC shows a slight change in the size of PtNC (Fig. S15c^†) while the STEM image of PtNC/ZTC after ORR in the acidic medium exhibited sintering of PtNC into large particles with an average size of 30 nm (Fig. S15d^†), resulting in a decrease of the ORR activity. In the alkaline medium, the decrease in ORR activity with time may be due to the oxidation of the ZTC support in KOH.⁴⁴We attributed the excellent ORR activity of PtNC/ZTC to the interplay between the following: (1) the structure of the Pt cluster possessing a high ratio of surface atoms that benefits the surface reactions,^45–47 (2) the microporous 3D graphene-like structure of the ZTC support that enables easy access of O₂ and electrolyte molecules to the active sites,⁴⁸ and (3) the high conductivity and large accessible surface area of ZTC that facilitates the electron transfer.^49–51     

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.     

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.     

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.     

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.     

A stable tunnel-type NaGe3/2Mn1/2O4 anode for Na-ion batteries

Tunnel-type NaGe_3/2Mn_1/2O₄ was fabricated for anode of sodium ion batteries, delivering a discharge capacity of 200.32 mAh g⁻¹ and an ultra-low potential platform compared with that of pure Na₄Ge₉O₂₀ (NGO). The results of X-ray photoelectron spectroscopy (XPS) demonstrate that Ge redox occurs, and partial substitution of Mn effectively improves the Na-storage properties compared to those of NGO.

We investigated tunnel-type NaGe_3/2Mn_1/2O₄; the main structure is Na₄Ge₉O₂₀. NaGe_3/2Mn_1/2O₄ electrodes as anodes for sodium ions batteries deliver a discharge capacity of 200.32 mAh g⁻¹ and satisfactory capacity retention after 50 cycles.

In terms of the high abundance and ready availability of sodium, sodium-ion batteries (SIBs) have been generally regarded as a better alternative to lithium-ion batteries for power stations.^1–4 Hard carbon is widely recognized as one of the most attractive and ideal anode materials for SIBs.^5,6 However, the potential required for sodium ions to insert into hard carbon is very close to that for sodium plating, resulting in sodium dendrites, which raise safety concerns.^7,8 Moreover, the reaction of electrode materials with sodium through alloying or conversion mechanisms always results in serious volume changes in the process of sodium insertion and extraction.⁹ Therefore, insertion-type transition-metal oxides as anodes have attracted much attention owing to their suitable operating potentials and minor volume expansion.^10,11 Recently, embedded titanium/vanadium/molybdenum based oxides with layered structures have been studied as anode materials for SIBs,¹² such as layered Na₂Ti₃O₇,¹³ tunnel Na₂Ti₆O₁₃ (ref. 14) and spinel Li₄Ti₅O₁₂.¹⁵ In addition, post-spinel structured materials have been proposed, which show ultra-stable cycle performances via highly reversible sodium-ion insertion/desertion through large-size tunnels. Recently, in Zhou''s group, NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄ (ref. 17) have been prepared and they have been shown to possess robust cycle lifetimes (more than 10 000 cycles) and discharge plateaus of 0.84 V and 0.7 V, respectively. Meanwhile, in our group, Na_0.76Mn_0.48Ti_0.44O₂ has been developed, which holds an initial discharge capacity of 103.4 mAh g⁻¹, shows a superb rate capability and retains 74.9% capacity after 600 cycles.¹⁸ The large radius of the redox active metal center could optimize the tunnel size and thus boosting the electrochemical performance. It is also a big challenge to find further suitable active centers for insertion-type transition metal oxides as anodes of SIBs. Besides, a host of published reports have said that germanium-based materials can be used as alloy anodes for SIBs with highly reversible sodium storage properties and satisfactory ionic/electronic conductivity.¹⁹ However, it is unclear to us whether Ge could act as an active center in a transition-metal oxide anodes.In this work, we fabricated a tunnel-type NaGe_3/2Mn_1/2O₄ (NGMO) material. When used as the anode of SIBs, it delivers a sustained discharge capacity of 200.32 mAh g⁻¹. Compared with NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄,¹⁷ NGMO delivers a lower safety voltage of 0.36 V. Pure Na₄Ge₉O₂₀ (NGO) as a comparative sample, only exhibits a capacity of 24.8 mAh g⁻¹, which is far inferior to that of NGMO. During discharge and charge process, reversible redox reactions around Ge center occur, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The introduction of Mn in the NGMO improves the reversibility of the Ge redox performance.The structure of NGMO was carefully characterized by XRD, and Rietveld refinement was performed as depicted in Fig. 1. The main Bragg peaks of NGMO could be assigned to space groups of P1(2) and I4₁/a(88), which were fitted to give lattice parameters of a = 10.56/15.04 Å, b = 11.18/15.04 Å, and c = 9.22/7.39 Å, and a volume of 811.2/1672.2 ų, respectively. Na₄Ge₉O₂₀ has a typical tunnel structure, which consists of polymerized Ge/MnO₄ tetrahedra connected with Ge/MnO₆ octahedra. Four Ge/MnO₆ octahedra are connected together by sharing edges to form a tetrameric (Ge/Mn)₄O₁₆ cluster. Each cluster is connected to six GeO₄ tetrahedra, and adjacent clusters are connected by GeO₄ tetrahedra. Na atoms are located in the channels and have elongated Na–O bonds.¹⁹ This highly stable crystal structure can effectively accelerate the migration of sodium ions.²⁰Fig. 2 shows the low and high magnification scanning electron microscopy (SEM) images of NGMO, which is composed of particles of different sizes from 1 to 3 μm; the larger particles are the result of sintering at high temperature. SEM images of NGO with different magnifications are given in Fig. S1,^† and show that the average particle size of NGO is 1 μm.Open in a separate windowFig. 1The Rietveld refinement spectra of NGMO.Open in a separate windowFig. 2(a) and (b) SEM images of NGMO at different magnifications.The morphology and fine structure were studied by transmission electron microscopy (TEM). Fig. 3a and b show the low magnification TEM images. It can be seen from the images that NGMO has an irregular sheet-like morphology with particle sizes from 250 nm to 2 μm. As shown in Fig. 3c, the lattice spacing of the (200) plane is 4.55 Å. In the SAED pattern of Fig. 3d, the red line corresponds to the (020) plane in NGMO, and the lattice spacing is 13.100 Å. These results clearly demonstrate that NGMO exhibits good crystallinity. The corresponding energy dispersive X-ray spectroscopy (EDS) results, and Raman and infrared spectra (IR) are provided in Table S1 and Fig. S2.^† The results indicate that the atomic ratio of Na : Ge : Mn is close to 1 : 1.5 : 0.5 and that there is little sodium loss. The Raman and IR peaks in the high frequency region are attributed to stretching vibrations of Ge–O–Ge and the peaks between 600 and 400 cm⁻¹ are attributed to the bending vibrations of Ge–O–Ge in NGMO.Open in a separate windowFig. 3(a) and (b) Low resolution TEM images, (c) a HRTEM image and (d) a SAED image of NGMO.Galvanostatic electrochemical measurements were evaluated in a voltage range of 0.05–2.0 V, with the current density of 20 mA g⁻¹. Fig. 4a and b show the discharge and charge profiles of NGMO and NGO, respectively. Because of the formation of a solid electrolyte interface (SEI) layer in the initial cycle, the electrochemical behaviour tends stabilize in the second cycle, so voltage profiles are given from the second cycle; the first cycles of the discharge–charge curves of NGMO and NGO materials are given in Fig. S3.^† It can be seen intuitively that both NGMO and NGO have low voltage platforms, while NGMO has the smaller polarization. In Fig. 4a, we notice a reversible voltage profile in the second cycle with discharge capacity of 200.32 mAh g⁻¹ for NGMO. Only NGMO has a flat potential platform and delivers an ultra-low plateau potential. It can be seen from Fig. 4b that NGO shows a capacity of 24.8 mAh g⁻¹, obvious polarization at the 20^th cycle and increased capacity due to the surface side-effect. Fig. 4c indicates that the capacity retention of the NGMO electrode after 50 cycles is 86.2%, which is superior to that of NGO; the coulombic efficiency of NGMO is also provided in Fig. S4.^† To further understand the redox reactions along with the discharge/charge process in NGMO, Fig. 4d displays the differential capacity versus voltage (dQ/dV) curve. The clear anodic peak at 0.33 V and cathodic peak at 0.81 V correspond well with the redox reactions of NGMO.Open in a separate windowFig. 4Electrochemical performance: (a) and (b) voltage profiles of NGMO and NGO, respectively; (c) cycling performance; (d) dQ/dV profile of NGMO. The current density was controlled at 20 mA g⁻¹ over a voltage range of 0.05–2.0 V.The electrochemical impedance spectra (EIS) of fresh and cycled electrodes of NGMO and NGO, with a frequency range of 0.01 Hz to 100 kHz, are shown in Fig. 5. From Fig. 5a, it is obvious that the charge-transfer resistance of the fresh NGMO electrode is lower than that of NGO. This indicates that the migration of charges in the NGMO material occurs more easily than in NGO, which also facilitates the shifting of ions on the surface and inside of the electrodes of NGMO. In Fig. 5b, NGMO electrode in its 10^th cycle exhibits a smaller charge-transfer resistance than both NGO and the NGMO fresh electrode, indicating that the surface of NGMO more readily forms a stable SEI film.¹⁸ The EIS results were fitted by the model shown in Fig. S5.^† The fitting results are provided in Table S2.^† The resistances of the fresh and cycled electrodes of NGMO and NGO are composed of an internal resistance (R_s), the resistance of the surface film (SEI) (R_f; a small semicircle in the high frequency region), the resistance of the charge transfer (R_ct; another opposite semicircle in the middle frequency region), and the Warburg resistance (W; an oblique line in the low frequency region).²¹ Both the fresh and cycled electrodes of NGMO deliver lower charge transfer resistance than NGO. Meanwhile, the transfer resistance of the cycled NGMO electrode is lower than that of the fresh electrode and its slope at low frequency is higher than that of the fresh one (Table S2^†). All these results show that NGMO has lower resistance and better electronic/ionic conductivity than NGO.Open in a separate windowFig. 5Nyquist plots of (a) fresh NGMO and NGO electrodes, and (b) NGMO and NGO electrodes after ten cycles.The evolution of the chemical valence states of the 150-times discharged electrodes were observed by XPS and SEM as provided in Fig. S6.^† It is generally clear that the electrode surface was covered with a thick SEI layer after discharging. Ar plasma etching was used to obtain the internal information. The Ge 3d core-level of the discharged NGMO electrode with and without etching is shown in Fig. S7.^† Before etching, the peaks of the Ge 3d core-level could be fitted to Ge¹⁺ and Ge²⁺.²² After etching, (Fig. S7c^†), the peaks at 30.8 eV and 30.2 eV were also associated with Ge¹⁺ and Ge²⁺, respectively. This result indicates that the valence of Ge decreases as a whole and that there is no obvious difference between the etched and non-etched samples. The reversible redox reactions of Ge remain stable even after cycling. Meanwhile, the Mn 2p core-level spectra are shown in Fig. S7d–f.^† For the Mn 2p core level, owing to the spin orbit coupling, the valence states of Mn comprise two couples including Mn³⁺ and Mn²⁺ (Fig. S7e and f^†). The binding energies of Mn³⁺ are 642.37 eV and 654.04 eV, and the binding energies of Mn²⁺ are 640.71 eV and 652.18 eV. Similarly, after discharging, the binding energies of Mn³⁺ are 642.40 eV and 654.06 eV, and those for Mn²⁺ are 640.69 eV and 652.20 eV, indicating that there are no changes in Mn binding energies before and after etching. This is in good agreement with results in previous reports.^23,24 All results also show that a thin SEI layer has been formed, favoring ions transfer on the repeatedly cycled electrode. It can be seen from the refined XRD results that NGMO consists of Ge⁴⁺, Mn²⁺ and Mn³⁺, and combined with XPS analysis, the results show that the valence states of Mn does not change for the discharged NGMO electrode. Ge displays electrochemical activity in NGMO, and Mn exhibits good chemical stability in the framework.     

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.     

Proton transfer network with luminescence display controls OFF/ON catalysis that generates a high-speed slider-on-deck

A three-component network for OFF/ON catalysis was built from a protonated nanoswitch and a luminophore. Its activation by addition of silver(i) triggered the proton-catalyzed formation of a biped and the assembly of a fast slider-on-deck (k₂₉₈ = 540 kHz).

Upon addition/removal of silver(i) ions and due to efficient inter-component communication, a supramolecular multicomponent network acts as an OFF/ON proton relay with luminescence display enabling switchable catalysis.

Proton relays in living systems¹ are often coupled to bio-machinery with enzymatic catalysis.² Frequently, such proton transfer systems comprise intricate networks with information exchange^3,4 amongst multiple components. Using manmade molecules,⁵ however, it is still arduous to create networks that approach the complexity⁶ and functions of biosystems.Although activation via proton transfer plays an eminent role in molecular logic,⁷ switching,^8–10 and machine operation,^11–15 proton transfer networks in combination with the regulation of catalysis remain scarce.^16,17 Beyond that, we present the in situ catalytic synthesis of a highly dynamic supramolecular device by a new proton transfer network requiring interference-free information handling,¹⁸ self-sorting^19–21 and communication²² among multiple components.^23–25Recently, our group has developed various chemically actuated networks of communicating components and used them for the ON/OFF regulation of transition metal catalysis.^26–28 Herein, we describe a proton relay system that reversibly toggles between two networked states and is useful for regulation of acid catalysis. It catalyzes an amine deprotection which was utilized to fabricate in situ a high-speed slider-on-deck,^29,30 for the first time based on aniline → zinc-porphyrin³¹ (N_an → ZnPor) interactions.The communicating network consists of nanoswitch 1 (ref. 32) and luminophore 2 (ref. 33) as receptors (Fig. 1). In the self-sorted networked state (NetState-I), the added proton is captured in the cavity of nanoswitch 1 resulting in [H(1)]⁺, while luminophore 2 remains free. NetState-I is catalytically inactive as the encapsulated proton is unable to catalyze the deprotection of the trityl protected slider biped 4. Upon addition of AgBF₄ to NetState-I, the silver(i) coordinates to switch 1 and the proton is released onto luminophore 2 thus leading to formation of [Ag(1)]⁺ and [H(2)]⁺ in NetState-II (see Scheme 1), which may be followed by characteristic ratiometric emission changes. If NetState-II is formed in presence of 3 and 4, the catalytic trityl deprotection of 4 produces the free biped 5 that eventually binds to deck 3 furnishing the slider-on-deck [(3)(5)].Open in a separate windowFig. 1Chemical structures and cartoon representations of the ligands used in the study.Open in a separate windowScheme 1Schematic representation of the reversible proton relay network. Green circle represents catalytic site.The concept of the catalyzed biped formation through a proton transfer network relies on the following considerations: (a) the initial networked state should be catalytically inactive as expected from a self-sorting of the proton inside the nanoswitch leaving the luminophore free. (b) Addition of silver(i) ions should release the proton from the protonated switch thus forming the protonated luminophore which we anticipated to act as catalyst. (c) A di-protected biped should undergo acid catalyzed deprotection by the protonated luminophore and eventually form a slider-on-deck by coordination to a deck. (d) Finally, the product of the catalysis should not intervene in any state of the operation with the main system.As proton receptor we chose the known phenanthroline–terpyridine nanoswitch 1 (ref. 32) in order to tightly capture the proton/silver(i) as HETTAP (HETeroleptic Terpyridine And Phenanthroline) complexes.³⁴ Luminophore 2 was based on the 2,4,6-triarylpyridine scaffold which is known to fluoresce strongly upon protonation due to intramolecular charge transfer (ICT).³⁵To investigate the self-sorting within the network, we mixed nanoswitch 1, luminophore 2 and TFA in 1 : 1 : 1 ratio in CD₂Cl₂ which furnished NetState-I = [H(1)]⁺ + 2. ¹H NMR shifts of the 4/7-H proton signals to 8.43 and 8.69 ppm and of 9-H and e-H proton peaks to 6.60 and 8.48 ppm confirmed formation of [H(1)]⁺. Moreover, proton signals of c′-H and d′-H at 7.07 and 7.92 ppm corroborated presence of the free luminophore 2 (Fig. 2). Addition of one equiv. of AgBF₄ to NetState-I instantaneously generated NetState-II = [Ag(1)]⁺ + [H(2)]⁺. In the ¹H NMR of NetState-II, the 9-H and e-H proton signals shifted to 6.55 and 8.23 ppm, respectively, which substantiated the formation of [Ag(1)]⁺ whereas the shift of proton peaks c′-H and d′-H to 7.12 and 8.00 ppm, respectively, established the protonated luminophore [H(2)]⁺ (Fig. 2). Upon excitation at 300 nm, NetState-I exhibited emissions at λ = 346, 372 and 393 nm which represent an overlap of the emission peaks of the free luminophore 2 at λ = 346 and 372 nm and of [H(1)]⁺ at λ = 376 and 393 nm (Fig. 3c). In NetState-II, the fluorescence spectrum showed a single emission at λ = 461 nm, which is attributed to [H(2)]⁺ (Fig. 3). In contrast, [Ag(1)]⁺ in NetState-II is nonfluorescent as advocated by the full quenching of the emission of [H(1)]⁺ at λ = 376 and 393 nm upon titration with silver(i) ions (ESI, Fig. S18^†). When one equiv. of tetra-n-butylammonium iodide (TBAI) was added to NetState-II, complete restoration of NetState-I was achieved as confirmed from ¹H NMR data. TBAI acted as a scavenger for silver(i) by removing AgI through precipitation thus reversing the translocation sequence. Multiple switching of NetState-I ⇆ NetState-II was readily followed by ¹H NMR (Fig. 2) and luminescence changes. Upon successive addition of AgBF₄ to NetState-I, the emission changed to λ = 461 nm (sky blue emission of [H(2)]⁺) in NetState-II (Fig. 3a and b) in a clean ratiometric manner that allowed monitoring of the amount of liberated protons. Alternating switching between the NetStates was followed by emission spectroscopy over three cycles with a small decline in emission intensity (Fig. 3c and d), which may be due to the progressive accumulation of AgI.Open in a separate windowFig. 2Partial ¹H NMR spectra (500 MHz, CD₂Cl₂, 298 K) showing reliable switching between NetState-I and II over 3 cycles. (a) After mixing of TFA, 1, and 2 in 1 : 1 : 1 ratio; (b) after adding 1.0 equiv. of AgBF₄ furnishing [Ag(1)]⁺ and [H(2)]⁺; (c) after addition of 1.0 equiv. of TBAI; (d) after adding 1.0 equiv. of AgBF₄; (e) after addition of 1.0 equiv. of TBAI, (f) after adding 1.0 equiv. of AgBF₄; (g) after addition of 1.0 equiv. of TBAI. Blue asterisk marked proton signals represent the tetra-butylammonium ion.Open in a separate windowFig. 3(a) Ratiometric emission titration of 2 (2.1 × 10⁻⁵ M) with TFA (5.3 × 10⁻³ M) in CH₂Cl₂. (b) Change of emission upon protonation of 2. (c) Reversibility of the network NetState-I ⇆ II over three cycles upon addition of silver(i) and TBAI (see text, blue arrow assigns direction of switching) monitored by luminescence changes at λ = 461 nm. (d) Multiple cycles monitored by the change of the fluorescence intensity.For probing the catalytic efficiency of each state towards a simple amine deprotection reaction we chose the trityl-protected aniline 6 (Fig. 4) performing two NetState-I ⇆ NetState-II cycles (Fig. 4b). We first prepared NetState-I by mixing 1, 2 and TFA (1 : 1 : 1) in CD₂Cl₂ and then added 6 and TMB (1,3,5-trimethoxybenzene as an internal standard) in a 10 : 10 ratio. After heating for 2 h at 40 °C, the ¹H NMR spectrum revealed no product 7. Clearly, the proton in [H(1)]⁺ is catalytically inactive (ESI, Fig. S10b^†). Addition of one equiv. of AgBF₄ and probing the catalysis again for 2 h at 40 °C revealed formation of product 7 in (30 ± 2)% yield (ESI, Fig. S10c^†). When the catalytic experiment was probed with only [H(2)]⁺ under identical reaction conditions, it provided the same yield (ESI, Fig. S16^†). This suggested that the reversible proton relay network (Scheme 1) functioned reliably also in presence of 6 with complex [H(2)]⁺ as the catalytically active species. Addition of one equiv. of TBAI to the above NetState-II mixture and probing for catalysis, revealed no further product formation. Apparently, addition of TBAI translocated back the proton to nanoswitch 1 thus restoring NetState-I, which resulted in the OFF state for catalysis. Adding one equiv. of AgBF₄ to the mixture and probing again the catalytic activity resulted in formation of further (29 ± 2)% of product 7, which was basically identical to the yield produced in the first cycle of NetState-II (ESI, Fig. S10e^†). Adding TBAI and the consumed amount of substrate showed no further deprotection. In sum, the robust catalytic performance of the proton relay network as reflected in two successive catalytic cycles resulted in no significant decline of the catalytic activity (Fig. 4b).Open in a separate windowFig. 4(a) Representation of the OFF/ON regulation of the trityl deprotection reaction in NetState-I and II. (b) Reversible switching between the network states furnished reproducible amounts of product 7 in NetState-II (two independent runs). Consumed amounts of substrates were added (green asterisk).The OFF–ON switching of the catalytic trityl deprotection of aniline 6 by the proton relay network inspired us to utilize the system to catalytically fabricate a slider-on-deck based on the N_an (= aniline) → ZnPor interaction. Deprotection of the protected biped 4 should afford the bis-aniline biped 5 and enable its attachment to the tris-zinc porphyrin deck 3 thus forming the slider-on-deck [(3)(5)].Addition of one equiv. of biped 5 (for synthesis and characterization, see ESI^†) to deck 3 (ref. 25) in CD₂Cl₂ quantitatively generated the slider-on-deck [(3)(5)], which was unambiguously characterized by ¹H NMR, ¹H DOSY NMR and elemental analysis (ESI, Fig. S5 and S6^†). In the ¹H NMR-spectrum, several diagnostic shifts of biped and deck proton signals attested the binding of biped 5 to the deck. Formation of the slider-on-deck was further confirmed from a single set in the ¹H DOSY in CD₂Cl₂ (ESI, Fig. S9^†).The single set of protons (p-H, r-H, s-H, t-H, u-H, v-H) in the ¹H NMR spectrum for all three-zinc porphyrin (ZnPor) stations of deck 3 unmistakably suggested rapid intrasupramolecular exchange^29a of the aniline biped across all three ZnPor sites requiring fast N_an → ZnPor bond cleavage. At low temperature (−65 °C) the ZnPor p-H proton signals separated into two sets (1 : 2) (ESI, Fig. S7 and S8^†). Kinetic analysis over a wide temperature range provided the exchange frequency (k) with k₂₉₈ = 540 kHz at 298 K (Fig. 5a) and the activation data as ΔH^‡ = 41.5 ± 0.7 kJ mol⁻¹, ΔS^‡ = 2.8 ± 0.7 J mol⁻¹ K⁻¹ and ΔG^‡₂₉₈ = 40.7 kJ mol⁻¹ (Fig. 5).Open in a separate windowFig. 5(a) Experimental and theoretical splitting of p-H proton signal of nanoslider [(3)(5)] in VT ¹H-NMR (600 MHz) furnishing rate data in CD₂Cl₂. (b) Eyring plot providing kinetic parameters.After determining the kinetic parameters of the slider-on-deck [(3)(5)], we investigated its in situ catalyzed formation by the proton relay network. Therefore, we mixed nanoswitch 1, luminophore 2, TFA, trityl-protected biped 4, deck 3 and TMB in 1 : 1 : 1 : 5 : 5 : 5 ratio in CD₂Cl₂ and heated the reaction mixture for 12 h at 40 °C. The ¹H NMR spectrum suggested no formation of any slider-on-deck in NetState-I (ESI, Fig. S13^†). Addition of one equiv. of AgBF₄ to the same mixture (converting NetState-I to NetState-II) and heating it at 40 °C for 12 h revealed full formation of the slider-on-deck as indicated by the disappearance of the 2′-H, 3′-H and 4′-H proton signals of biped 4 at 6.60, 6.31 and 6.51 ppm (ESI, Fig. S11^†), respectively, and the quantitative emergence of 4′-H proton signal of biped 5 bound to deck 3 at 5.84 ppm (ESI, Fig. S12^†). The upfield shift of the deck''s meso t-H protons from 10.36 to 10.29 ppm allowed monitoring of the formation of [(3)(5)], e.g. by the gradually increased chemical shift difference of proton signal t-H (Δδ_t-H) (Fig. 6a; ESI, Fig. S11^†). In sum, the protonated [H(2)]⁺ in NetState-II catalyzed the deprotection of the trityl-protected biped 4 with the effect that the resultant bis-aniline biped 5 would quantitatively bind to deck 3 affording the slider-on-deck [(3)(5)]. Furthermore, the OFF/ON switching of catalysis was probed for shortened time periods of 3 h. As illustrated in Fig. 6b, the process could be readily followed using the proton signal t-H (Δδ_t-H) (ESI, Fig. S14^†) and the growth of proton signal 4′-H at 5.84 ppm of bound biped 5 (ESI, Fig. S15^†). One clearly sees recurring OFF/ON regulation in NetState-I/II.Open in a separate windowFig. 6Plot of the shift difference of proton signal t-H (Δδ_t-H) vs. time for (a) the formation of [(3)(5)] from 3 and 4. Red curve: formation of [(3)(5)] over 12 h catalyzed by NetState-II. Black curve: OFF state of catalysis in NetState-I. (b) OFF/ON catalytic cycles by switching between NetState-I and NetState-II. Addition of Ag⁺, see blue asterisk; addition of TBAI: red asterisk.In conclusion, we have designed a supramolecular multicomponent network³⁶ that acts as an OFF/ON proton relay with a luminescence display³⁷ and is useful for switchable catalysis.³⁸ The network is toggled by chemical input and intercomponent communication^15,16 and as a result is a conceptual complement to photoacids driving networks.^7,8The release and capture of protons is demonstrated by the ON/OFF trityl deprotection of anilines. To demonstrate functioning in a complex setting, the network was utilized to catalyze formation of a high-speed slider-on-deck assembly based on N_an → ZnPor interactions (sliding frequency of k₂₉₈ = 540 kHz). The robust operation of the proton relay furnishing dynamic machinery³⁹ through acid catalysis followed by self-assembly is a valuable step mimicking sophisticated biological proton relays.²     

Complement activation by gold nanoparticles passivated with polyelectrolyte ligands

Gold nanoparticles passivated by polyelectrolyte ligands are widely used to confer stability and biofunctionality. While nanoparticles and polyelectrolytes have been reported as activators, their ability to activate the complement system as hybrid polyelectrolyte-coated nanoparticles is poorly characterized. Here, we found that gold nanoparticles passivated by common polyelectrolytes activated the system differently. The surface area of AuNPs appeared to be a major determinant of complement activation level as it determined the amount of adsorbed polyelectrolytes. Although a moderate negative correlation between AuNP surface hydrophilicity and their activation level was observed, the surface charge and functional group of polyelectrolyte ligands also influenced the final complement activation level.

We reported that the surface area and hydrophilicity of polyelectrolyte-coated gold nanoparticles influence their complement activation, a biological response not well understood to date.

Nanomaterials may elicit various biological responses upon contact with blood, of which activation of the complement system in innate immunity is one of the earliest.^1–3 The complement system is a collection of over 40 soluble and membrane-bound proteins, which acts in any of three distinct enzymatic cascades: classical, lectin, and alternative leading to the formation of a C3 convertase complex, and its accompanying range of biological responses, including inflammation, opsonization and cytolysis.^2,4While these responses could lead to undesirable physiological responses from over-activation and rapid clearance of nanomaterials from circulation, the intricate link between complement activation and adaptive immunity^5–7 also presents opportunities for exploitation in immune-related applications such as vaccines development. Therefore, complement activation by nanomaterials in biomedical applications has attracted great attention recently.^3,8–10We previously reported complement activation by gold nanoparticles (AuNPs) of different shapes in their as-synthesized citrate and CTAB coatings.¹¹ Polyelectrolyte ligands are also widely used in the preparation of AuNPs not only to stabilize AuNPs against aggregation, but to also enhance their solubility and confer additional surface functionalities.^12–14 These polyelectrolytes with different functional groups have been reported as complement activators in both their particulate and planar surface-immobilized forms.^15–21 However, the ability of polymer-passivated AuNPs to activate the complement system and the underlying mechanism remain poorly understood, although our previous study demonstrated that passivating the surface of AuNPs by poly(ethylene glycol) (PEG) modulated the activation level.¹¹Herein, we stabilized different shapes of AuNPs with different polyelectrolytes and examined their levels of complement activation and underlying mechanism. This would provide rational guidelines on the use of polyelectrolytes to modulate complement activation. All polyelectrolytes used in this study are hydrophilic and widely used as delivery platforms^5,22 and surface passivating ligands of nanomaterials²³ (Scheme 1).Open in a separate windowScheme 1Schematic illustration of gold nanoparticles (AuNPs) core, polyelectrolytes and methods used in this study. AuNPs with spherical shape of 20 nm (Au20) and 40 nm (Au40) as well as rod-shape (AuNR) were used as AuNP cores. These AuNPs were passivated with various polyelectrolyte ligands, examined for their hydrophilicities and levels of complement activation.Spherical AuNPs of two diameters, 20 nm (Au20) and 40 nm (Au40), were synthesized by well-known citrate reduction method.²⁴ AuNPs of rod-like shape (AuNR) were synthesized by hexadecyltrimethylammonium bromide (CTAB)-mediated method²⁵ with the CTAB ligands subsequently replaced by citrate using previously established protocol.²⁶ All AuNPs were then passivated by different polyelectrolytes following previously established layer-by-layer protocols (see Experimental section, ESI^†). With this library of AuNPs, we sought to examine the effects of size and shape of AuNP core as well as polyelectrolyte shell on the level of complement activation.The TEM images showed that Au20 and Au40 were spherical and homogenous while AuNR was rod-shaped with dimensions of approximately 40 × 10 nm (Fig. 1a–c). Hydrodynamic diameters, D_h, (Fig. 1d) were consistent with sizes of 20.2, 41.8, and 10.6 × 40.2 nm obtained from TEM images for Au20, Au40, and AuNR respectively (Fig. S1b–e, ESI^†). The UV-Vis absorption spectra showed surface plasmon resonance (SPR) peaks of 523 nm and 530 nm for Au20 and Au40, respectively while AuNR had transverse peak of 509 nm and longitudinal peak of 800 nm (Fig. S1a, ESI^†).^27,28 All citrate-capped Au20, Au40 and AuNRs had similar zeta potentials of −30 mV, which became nearly neutral after passivated with poly(vinyl alcohol) (ζ_AuNP-PVA ≈ −5 mV) and polyamidoamine (ζ_AuNP-PAMAM ≈ −10 mV) (Fig. 1e). In contrast, surface passivation with poly(acrylic acid) (AuNP-PAA), poly(styrenesulfonate) (AuNP-PSS), and heparin (AuNP-Heparin) conferred AuNPs a more negative zeta potential of ≈−40 mV (Fig. 1e), while coating with poly(ethyleneimine) (AuNP-PEI) conferred AuNPs a positive surface charge of ≈+40 mV. The presence of polyelectrolytes on AuNPs was further confirmed by the increase in D_h of polyelectrolyte-passivated AuNPs (Fig. 1d).Open in a separate windowFig. 1Physical properties of library of AuNPs. TEM images of (a) citrate-capped Au20, (b) Au40, and (c) AuNR. (d) Hydrodynamic diameter, D_h, and (e) zeta potential, ζ, of citrate-capped AuNPs and polyelectrolyte-passivated AuNPs. Each data point represents mean ± standard deviation (SD) of triplicate experiments. Scale bar in the TEM images represents 50 nm.When these polyelectrolyte-passivated AuNPs were incubated in normal human serum, the generation of SC5b-9 as an endpoint biomarker showed complement activation regardless of the activation pathway.^3,4,8 Here, elevated levels of SC5b-9 were detected in all AuNPs-treated sera (Fig. 2), indicating activation of the complement system. Prior to surface passivation by polyelectrolytes, we observed an expected complement activation by AuNP-citrate, although the level was significantly lower than our positive control zymosan (a well-known complement activator derived from the wall of yeast cell). Also, Au40-citrate induced more SC5b-9 than Au20-citrate and AuNR-citrate (Fig. 2).Open in a separate windowFig. 2Detection of endpoint product of complement activation, SC5b-9, using ELISA kit. 1× PBS and zymosan (10 mg ml⁻¹) were used as negative (−) and positive (+) controls, respectively. Each data point represents the mean ± standard deviation of triplicate experiments.The level of complement activation was dependent on both the AuNP core and polyelectrolyte ligand. Amongst the polyelectrolytes, we observed the highest level of complement activation from PEI, comparable or even higher than zymosan (Fig. 2). This agreed with previously published results where positively charged polymers carrying primary amino groups were shown to interact strongly with complement proteins to activate the complement system.^18,21Between the three AuNP cores, Au40-PEI had the highest level of complement activation ([SC5b-9] = 1.80 μg ml⁻¹), followed by Au20-PEI ([SC5b-9] = 1.29 μg ml⁻¹) and AuNR-PEI ([SC5b-9] = 1.16 μg ml⁻¹), which had comparable levels of complement activation (Fig. 2). The same trend was true not only for AuNPs-PEI but also other polyelectrolyte-passivated AuNPs. While complement activation by nanomaterials have been shown to depend on their sizes and shapes,^8,29 the differences observed here were more likely attributed to the amount of PEI adsorbed on the surface of AuNPs, which was in turn dictated by their surface area, since PEI by itself has been shown to activate the complement system in concentration-dependent manner.¹⁸ Here, Au20 and AuNR had comparable surface areas (1256 and 1413 nm², respectively) and hence similar levels of complement activation, while Au40 had the largest surface area (5024 nm²), thus accounting for the highest level of complement activation (Fig. 2).Unlike PEI, AuNPs passivated by both PVA and PAA induced the lowest levels of SC5b-9 (Fig. 2). While PVA is widely known as a biocompatible ligand on AuNRs,²³ its complement activation was not totally avoided probably due to its nucleophilic hydroxyl groups,^16,17 similar to PEI with nucleophilic amine groups. However, its complement activation level was much lower, and was likely due to its near neutral surface charge which did not promote interaction with many negatively charged complement proteins unlike the positively charged PEI.Similarly, the highly negative charge AuNP-PAA due to high density of carboxyl groups did not promote their interaction with negatively charged complement proteins, thus inducing an equally low level of complement activation as AuNP-PVA. Therefore, PAA is one of the most widely used water-soluble polyelectrolyte, superabsorbent polymer as well as food additive. Nonetheless, PAA conjugated to IgG has been found to specifically interact with positively charged C1q complement protein to activate the classical pathway.^30,31 Hence, AuNP-PAA has been found to promote inflammation via activation with fibrinogen.³² In this study, the AuNP-PAA could have activated the complement system via the classical pathway due to their interactions with C1q complement protein as reported.^30,31Owing to its nature as an anticoagulant, heparin has been reported to minimize complement activation.^33–35 However, we found that AuNP-heparin still activated the complement system although their level of complement activation was comparable to PVA and PAA, and much lower than PEI, PSS and PAMAM.Interestingly, despite the presence of nucleophilic amine groups on PAMAM similar to PEI, AuNP-PAMAM activated the complement system at much lower levels compared to AuNP-PEI. PAMAM dendrimers were known to be generation-dependent complement activators with stronger complement activation observed in higher generations.¹⁵ Here, we used PAMAM of generation 2.0 with aminoethanol surface and reasoned that coupling of amine with hydroxyl group helped to reduce activation level of AuNP-PAMAM. In fact, the activation level of AuNP-PAMAM was between that of AuNP-PVA with hydroxyl groups, and AuNP-PEI with amine groups. Furthermore, since primary amine groups adsorbed more C3b, a major component of complement proteins in serum, than secondary or tertiary amino groups,²⁰ the lack of primary amine group in PAMAM also explained its weaker complement activation than PEI which possesses a mixture of primary, secondary, and tertiary amine groups.The amount of SC5b-9 induced by AuNP-PSS was comparable to that of AuNP-PAMAM (Fig. 2). PSS is a widely used polyelectrolyte building block in layer-by-layer assembly,³⁶ and we used it as an intermediate ligand to prepare AuNR-citrate (Fig. S2, ESI^†). PSS by itself interacted with complement proteins of the classical pathway to activate the complement system.¹⁹ Our results not only confirmed complement activation by PSS passivation but also highlight the potential side effects of block-copolymer containing PSS commonly used as drug delivery platform in activating the complement system.Since the hydrophilicity of nanoparticles has been shown to modulate non-specific protein adsorption³⁷ and dictate immune response,³⁸ we further examined for possible correlation between the hydrophilicity of polyelectrolyte-passivated AuNPs and their level of complement activation. We measured relative hydrophilicity of polyelectrolyte-passivated AuNPs by dye absorption using a hydrophilic dye Nile Blue which interacted with a hydrophilic moiety (see Experimental section, ESI). On mixing the dye with AuNPs passivated with different polyelectrolyte ligands, we determined the partitioning quotient (PQ) as the ratio of dye bound on nanoparticles surface to the amount of free dye. In a plot of PQ versus surface area of nanoparticles, the slope of this linear regression line represented relative surface hydrophilicity.³⁹We observed different levels of hydrophilic dye adsorption on all polyelectrolyte-passivated AuNPs after 3 h incubation, indicating differences in their hydrophilicities (Fig. S8–10, ESI^†). AuNPs-PEI was the least hydrophilic (Fig. 3) as determined from the smallest slope of the linear regression line (Table S1, ESI^†), while AuNPs passivated with PVA, PAA, and heparin are amongst the most hydrophilic as given by the larger slope of the PQ vs. nanoparticle surface area plot (Fig. 3 and Table S1, ESI^†).Open in a separate windowFig. 3Negative correlation between surface hydrophilicity and complement activation by polyelectrolyte-passivated (a) Au20, (b) Au40 and (c) AuNR. X scale bar was plotted as ln of the value of relative hydrophilicity.We also observed a negative correlation between surface hydrophilicity and complement activation level across all AuNPs regardless of shape or size (Fig. 3). However, this correlation was only moderate with Pearson correlation coefficient, r = −0.2730, −0.4101, and −0.5489 for Au20, Au40 and AuNR, respectively (Fig. 3). Since Moyano et al. reported a positive correlation between nanoparticle surface hydrophobicity and their immune response previously,³⁸ the similarity in our observations suggested that the complement system could be a potential mediator between surface hydrophilicity/hydrophobicity and downstream immune response. Nonetheless, our results also demonstrated the complexity of complement activation by engineered nanomaterials, as it was not dependent solely on any one physical property such as shape, size, surface charge, surface functional group or surface hydrophilicity, but the interplay of these properties to influence the complement proteins adsorption.In summary, AuNPs passivated with different polyelectrolyte ligands activated the complement system at different levels, as characterized by the presence of endpoint product of complement activation, SC5b-9. The surface area of AuNPs appeared as a major determinant of complement activation level as it determined the amount of adsorbed polyelectrolytes. Although a moderate negative correlation between AuNPs surface hydrophilicity and their activation level was observed, the surface charge and functional group of polyelectrolyte ligands also influenced the final complement activation level. These findings provide new insights to rational selection and design guidelines for the use of polyelectrolytes to either suppress complement activation and downstream immune response for nanoparticulate drug delivery systems or to enhance complement activation and immune response for vaccine development.     

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.     

Coumarin-based fluorescent probe for the rapid detection of peroxynitrite ‘AND’ biological thiols

A coumarin-based novel ‘AND’ logic fluorescent probe ROS-AHC has been developed for the simultaneous detection of ONOO⁻ and biological thiols. ROS-AHC was shown to exhibit only a very small fluorescence response upon addition of a single GSH or ONOO⁻ analyte. Exposure to both analytes, however, resulted in a significant fluorescence enhancement.

A coumarin-based novel ‘AND’ logic fluorescent probe ROS-AHC has been developed for the simultaneous detection of ONOO⁻ and biological thiols.

Peroxynitrite (ONOO⁻) is a short-lived reactive oxygen and reactive nitrogen species (ROS and RNS) produced intracellularly by the diffusion-controlled reaction of nitric oxide (NO˙) with superoxide (O₂˙⁻).^1–3 Despite playing a key role as a physiological regulator,⁴ it is commonly known for its high reactivity towards most types of biomolecules, causing deleterious effects and irreversible damage to proteins, nucleic acids, and cell membranes.^5,6 ONOO⁻ is therefore a central biological pathogenic factor in a variety of health conditions such as strokes, reperfusion injuries or inflammatory and neurodegenerative diseases (Parkinson''s disease, Alzheimer''s disease).^7–9 Conversely, biothiols such as glutathione and cysteine are endogenous reducing agents, playing a central role in the intracellular antioxidant defence systems.^10–12 Glutathione (GSH), in particular, is the most abundant biothiol in mammalian cells, and exists as both its reduced GSH form, and as the oxidised disulphide form GSSG.^13–15 Peroxynitrite and biothiols such as GSH are intimately linked, as abnormal levels of highly oxidising ONOO⁻ can perturb the delicate GSH/GSSG balance, causing irreversible damage to key processes such as mitochondrial respiration.¹⁶ Thus, abnormal levels of GSH are common in cells undergoing oxidative stress, in which the regulation of and interplay between ONOO⁻ and GSH is closely associated with physiological and pathological processes.^17,18 One such example is drug-induced liver injury (DILI), in which upregulation of ONOO⁻ occurs in hepatotoxicity. Treatment with GSH could be used to remediate this type of cell injury by depletion of ONOO⁻.^19–22One of our core research interests lies in the development of dual analyte chemosensors capable of detecting two distinct analytes such as biological reactive oxygen species and biothiols.^23–26 Although a wide range of single-analyte probes exist for the detection of ROS and thiols separately,^27–30 ‘AND’ logic sensors for their simultaneous detection are still rare.^31–33 We are therefore interested in developing such probes, containing two distinct sensing units, one for each analyte, working simultaneously or in tandem to elicit a fluorescence response.³⁴ This approach allows the monitoring of multiple biomolecular events and factors involved in specific disease pathologies, in order to achieve optimal predictive accuracy for diagnosis and prognostication.³⁵Using these principles, our group has recently focused on developing a range of ‘AND’ logic based sensors exploiting a variety of sensing units and mechanisms of fluorescence. Two such probes are shown below: GSH-ABAH (Fig. 1a), an ESIPT probe with a 4-amino-2-(benzo[d]thiazol-2-yl)phenol (ABAH) core, employing a maleic anhydride thiol-acceptor group;³¹ and JEG-CAB (Fig. 1b), a coumarin-based probe, this time with a salicylaldehyde homocysteine-reactive unit.²⁴ Both of these sensors employ a benzyl boronate ester as their peroxynitrite-reactive unit.Open in a separate windowFig. 1(a) GSH-ABAH, previously reported probe for simultaneous detection of ONOO⁻ and GSH. (b) JEG-CAB, previously reported probe for simultaneous detection of ONOO⁻ and GSH. (c) AHC – a core fluorescent unit that enables the synthesis of ‘AND’ based fluorescent probe for the detection of ONOO⁻ and GSH (d) ROS-AHC, a novel probe detailed in this work for simultaneous detection of ONOO⁻ and GSH.Herein, we set out to develop an ‘AND’ logic gate based fluorescence probe for simultaneous detection of ONOO⁻ and GSH. 3-Amino-7-hydroxy-2H-chromen-2-one (AHC) was chosen as a suitable coumarin fluorophore core for the development of an ‘AND’ logic based sensor, as its free phenol and amine functional groups provided a good opportunity for independent derivatization (Fig. 1).^36–39Previous literature reports show that protection of AHC with a maleic anhydride group results in quenching of the coumarin''s fluorescence intensity due to photoinduced electron transfer (PeT) processes. This fluorescence is rapidly restored in the presence of biological thiols, however, due to their fast addition to this functional group.⁴⁰ Therefore, we suggested that functionalization of the free phenol of this sensor using a benzyl boronic ester should further block the fluorescence, whilst serving as reporter unit for ONOO⁻. The greatly increased reactivity of peroxynitrite over other ROS towards boronate esters^41–43 should allow this functionality to act as a peroxynitrite-selective reporter, leading to an ‘AND’ logic based probe for the detection of ONOO⁻ and biological thiols (Fig. 1, Scheme 1). ROS-AHC was synthesized in 5 steps, starting with a 4-step synthesis of compound 1 adapted from literature procedures,^40,44 followed by the addition of the benzyl boronic pinacol ester (see Scheme S1 ESI^†).Open in a separate windowScheme 1Fluorescence ‘turn on’ mechanism of ROS-AHC in the presence of ONOO⁻ and GSH.The UV-Vis behaviour of ROS-AHC before and after exposure to both GSH and ONOO⁻ was evaluated in pH 7.40 buffer solution, showing a maximum absorption peak at 340 nm for both the unreacted probe and the probe following exposure to GSH, shifting to 350 nm with the addition of ONOO⁻ to the probe and 365 nm after sequential additions of GSH and ONOO⁻ to the probe (Fig. S1 ESI^†). Fluorescence experiments were then carried out. As expected, ROS-AHC was initially non-fluorescent, with a small fluorescence increase upon addition of ONOO⁻ (6 µM) (Fig. 2 and S2 ESI^†). Incremental additions of GSH (0–4.5 µM) resulted in a much larger increase in fluorescence intensity (>69-fold, see Fig. 2 and S3 ESI^†), demonstrating the need for both GSH and ONOO⁻ in order to achieve a significant ‘turn on’ fluorescence response.Open in a separate windowFig. 2Fluorescence spectra of ROS-AHC (5 µM) with addition of ONOO⁻ (6 µM), wait 5 min then incremental addition of GSH (0–4.5 µM), 5 min incubation before measurements in PBS buffer solution (10 mM, pH = 7.40). Fluorescence intensities were measured with λ_ex = 400 nm (bandwidth 8 nm). The green line represents the highest intensity after addition of GSH (4 µM).Similar fluorescence experiments were then carried out in reverse order, with the addition of GSH (6 µM) to ROS-AHC resulting in only a small increase in fluorescence intensity (Fig. 3 and S4 ESI^†). As before, incremental addition of the second analyte, in this case ONOO⁻ (0–5.5 µM), resulted in a large increase in fluorescence intensity (>46-fold, Fig. 3 and S5 ESI^†), confirming that ROS-AHC requires both GSH and ONOO⁻ for a full fluorescence ‘turn on’ response.Open in a separate windowFig. 3Fluorescence spectra of ROS-AHC (5 µM) with addition of GSH (6 µM), wait 5 min then incremental addition of ONOO⁻ (0–5.5 µM) with 5 min incubation before measurements in PBS buffer solution (10 mM, pH = 7.40). Fluorescence intensities were measured with λ_ex = 400 nm (bandwidth 8 nm). The orange line shows the highest intensity after addition of ONOO⁻ (5 µM).Subsequently, the selectivity of this probe towards both analytes was evaluated. A range of amino acids were evaluated (Fig. S6 ESI^†), with only thiol-containing analytes (glutathione, cysteine and homocysteine) eliciting significant fluorescence response, whilst non-thiol amino acids led to no changes in fluorescence intensity. A broad screen of ROS analytes was also carried out, demonstrating excellent selectivity for ONOO⁻, even over H₂O₂ (Fig. S7 ESI^†).The time-dependent response of ROS-AHC with both ONOO⁻ and GSH was also examined (Fig. S8 and S9 ESI^†). After initial addition of GSH or ONOO⁻ to the probe, subsequent addition of the second analyte triggered a rapid and significant increase in fluorescence, achieving maximum fluorescence intensity within 78 s in both cases. Furthermore, LC-MS experiments confirmed the formation of the suggested non-fluorescent intermediates, as well as the final fluorescent species shown in Scheme 1 (Fig S10, S11 and S12^†).In summary, we have developed a coumarin-based dual-analyte ‘AND’ logic fluorescent sensor, ROS-AHC, for the simultaneous detection of ONOO⁻ and biological thiols. ROS-AHC has shown high sensitivity and selectivity towards both ONOO⁻ and biological thiols.     

Coumarin-based fluorescent ‘AND’ logic gate probes for the detection of homocysteine and a chosen biological analyte

With this research we set out to develop a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys. Probe JEG-CAB was constructed by attaching the ONOO⁻ reactive unit, benzyl boronate ester, to a HCys/Cys reactive fluorescent probe, CAH. Similarly, the core unit CAH was functionalised with the nitroreductase (NTR) reactive p-nitrobenzyl unit to produce probe JEG-CAN. Both, JEG-CAB and JEG-CAN exhibited a significant fluorescence increase when exposed to either HCys and ONOO⁻ (JEG-CAB) or HCys and NTR (JEG-CAN) thus demonstrating their effectiveness to function as AND logic gates for HCys and a chosen analyte.

With this research we set out to develop of a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys.

Homocysteine (HCys) is a non-proteinogenic amino acid, formed from the de-methylation of methionine,¹ which is then converted into cysteine (Cys) via a vitamin B₆ cofactor. Typical physiological concentrations of HCys range between 5–15 μmol L⁻¹.² However, elevated levels of HCys (>15 μmol L⁻¹), which is known as hyperhomocysteinemia (hHCys),³ have been associated with pregnancy disorders, Alzheimer''s disease, cardiovascular disease and neurodegenerative diseases (NDs).^4–6 It is believed that the main cause of HCys induced toxicity is through the non-enzymatic modification of proteins. This is achieved through irreversible covalent attachment of the predominant metabolite of HCys, homocysteine thiolactone (HTL), to lysine residues; a phenomenon known as ‘protein N-homocysteinylation’ that results in the loss of a proteins structural integrity leading to loss of enzymatic function and aggregation.⁷A number of fluorescent sensors have been developed for the detection of HCys to help improve our understanding of its role in biological systems.^8–11 However, these fluorescent probes have focused on the detection of a single biomarker (HCys), however, processes associated with HCys induced toxicity often involve more than one biochemical species. For example, it has been reported that peroxynitrite (ONOO⁻) and nitric oxide (NO˙) play a significant role in HCys-mediated apoptosis in trigeminal sensory neurons¹² and HCys has been reported to induce cardiomyocytes cell death through the generation of ONOO⁻.¹³ The production of ONOO⁻ is believed to be the result of an increased production of superoxide (O₂˙⁻) by HCys activating the enzyme NADPH oxidase.^14–16 This increased production of O₂˙⁻ leads to a reduction in the bioavailability of NO˙ by increasing the formation of ONOO⁻ (NO˙ + O₂˙⁻ → ONOO⁻).¹⁷ The reported ONOO⁻ concentrations in vivo are believed to be approximately 50 μM but, higher concentrations of 500 μM have been found in macrophages.^18,19 Furthermore, hypoxia has been reported to facilitate HCys production in vitamin-deficient diets²⁰ where hypoxia leads to an upregulation of nitroreductase (NTR) – a reductive enzyme upregulated in cells under hypoxic stress.^21,22 Therefore, the development of tools that enable an understanding of the relationship of HCys with these biologically important species would be highly desirable.To achieve this, a number of fluorescent probes have been developed that are capable of detecting two or more analytes.²³ These include AND logic gate based-fluorescence probes, which require both analytes to work in tandem to produce a measurable optical output.^24–28 In our group, we have developed several ‘AND’ reaction-based probes for the detection of ROS/RNS and a second analyte.^29–32 These ‘AND’ logic scaffolds have been used to detect two analytes within the same biological system.^24,33Owing to the pathological role of HCys, we set out to develop the first example of a fluorescent probe for the detection of HCys and biological related analyte. Aiming towards that target, we became interested in a previously reported coumarin-based fluorescent probe developed by Hong et al.CAH, with a salicylaldehyde (Fig. 1).³⁴ Salicylaldehyde is a known reactive unit towards HCys/Cys, therefore we believed CAH could be used as a scaffold for the development of ‘AND’-based systems for the detection of HCys/Cys and a second analyte.³⁴ In the presence of HCys, CAH exhibited a ‘turn-on’ fluorescence response which is attributed to the nucleophilic nature of the nitrogen and sulfur atoms resulting in thiazine ring formation (Scheme S1, Fig. S1 and S2^†).^34–36Open in a separate windowFig. 1(a) CAH – a core fluorescent unit that enables the synthesis of ‘AND’ based fluorescent probe for the detection of HCys/Cys and a second analyte. (b) JEG-CAB enables the detection of HCys/Cys and (ROS/RNS) while (c) JEG-CAN enables the detection of HCys/Cys and NTR.We believed that CAH was a useful core unit that can be used to introduce the chosen reactive chemical trigger on the phenol for the detection of the corresponding analyte with HCys/Cys. Owing to the relationship between HCys and ONOO⁻/NTR, we set out on the development of a HCys AND ONOO⁻ probe and a HCys AND NTR probe.Therefore, we set out to prepare JEG-CAB and JEG-CAN, which are able to detect HCys/Cys and peroxynitrite (ONOO⁻) or nitroreductase (NTR), respectively (Scheme 1). For JEG-CAB, a benzyl boronate ester was introduced as a ONOO⁻ reactive unit.³⁷ For JEG-CAN, a p-nitrobenzyl group was installed as it is known to be an effective substrate for NTR.^38–40Open in a separate windowScheme 1Synthesis of target probe JEG-CAB and JEG-CAN.To afford CAH, compound 2 was synthesized by refluxing umbelliferone and acetic anhydride at 140 °C. Compound 2 was then dissolved in trifluoroacetic acid at 0 °C followed by the addition of hexamethylenetetramine (HMTA). The mixture was heated to reflux overnight and the solvent was then removed. The intermediate was then hydrolyzed in H₂O for 30 min at 60 °C. Upon isolating CAH, it was then alkylated using 4-bromomethylphenylboronic acid pinacol ester and K₂CO₃ in DMF at r.t. to afford JEG-CAB in 51% yield. JEG-CAN was produced by alkylating CAH using 4-nitrobenzyl bromide and K₂CO₃ in DMF at r.t. to give 49% yield (Scheme 1).We then evaluated the ability of JEG-CAB to detect ONOO⁻ ‘AND’ HCys in PBS buffer solution (10 mM, pH 7.40). The maximum absorption of JEG-CAB at 336 nm shifted to 323 nm with the addition of HCys and then slightly shifted to 328 nm following the addition of ONOO⁻ (Fig. S3^†). As shown in Fig. 2, JEG-CAB was initially non-fluorescent, but the addition of HCys (1 mM) led to a small increase in fluorescence intensity, the subsequent additions of ONOO⁻ (0–24 μM) led to a significant increase in fluorescence intensity (>17-fold, see Fig. S5^†). These results demonstrated the requirement for both ONOO⁻ ‘AND’ HCys to obtain a significant turn ‘‘on’’ fluorescence response.Open in a separate windowFig. 2Fluorescence spectra of JEG-CAB (15 μM) with addition of HCys (1 mM) and incubated for 40 min then measured. Followed by incremental additions of ONOO⁻ (0–24 μM). The data was obtained in PBS buffer solution (pH 7.40, 10 mM) at 25 °C. λ_ex = 371 (bandwidth 20) nm. Dashed line represents JEG-CAB and Hcys addition only. Blue line represents highest intensity after addition of ONOO⁻.The addition of HCys and ONOO⁻ were then performed in reverse where JEG-CAB exhibited a negligible increase in fluorescence intensity upon addition of ONOO⁻ (16 μM). However, in a similar manner to that shown in Fig. 2, a large increase in fluorescence intensity was produced after the subsequent addition of HCys (0–5.5 mM) (Fig. 3 and S6^†). LC-MS experiments were carried out to ascertain the reaction mechanism and the results confirmed the sequential formation of the thiazine ring in the presence of HCys followed by boronate ester cleavage in the presence of ONOO⁻ or vice versa (Scheme S2 and Fig. S19–S21^†).Open in a separate windowFig. 3Fluorescence spectra of JEG-CAB (15 μM) with addition of ONOO⁻ (16 μM) and followed by incremental additions of HCys (0–5.5 mM) measurements were taken after 40 min of both additions. The data was obtained in PBS buffer solution (pH 7.40, 10 mM) at 25 °C. λ_ex = 371 (bandwidth 20) nm. Dashed line represents JEG-CAB and ONOO⁻ addition only. Blue line represents highest intensity after addition of HCys.As expected, probe JEG-CAB was shown to have excellent selectivity with ONOO⁻ against other ROS in the presence of HCys (1 mM) (Fig. S9 and S10^†). Furthermore, JEG-CAB exhibited a high degree of selectivity towards a series of amino acids where only HCys and Cys led to a fluorescence response in the presence of ONOO⁻. This is due to the formation of stable five or six-membered thiazine rings (Fig. S7 and S8^†).³⁴We then evaluated the changes in the fluorescence of JEG-CAN with both HCys and NTR in PBS buffer solution (10 mM, pH 7.40, containing 1% DMSO). As shown in Fig. 4, addition of HCys led to a small increase in fluorescence intensity. However, subsequent addition of NTR (4 μg mL⁻¹) led to a large time dependant increase in fluorescence intensity. To ensure both analytes were required, NTR and NADPH was kept constant (4 μg mL⁻¹ and 400 μM respectively) resulting in a 3.4 fold fluorescence increase (Fig. 5). We attribute the large initial increase to background fluorescence of NADPH.⁴¹ NTR then facilitates reduction of the nitro group of JEG-CAN releasing the core probe CAHvia a fragmentation cascade (Scheme S3^†).^38,42 Subsequent addition of HCys (2.0 mM) led to a 2 fold increase in fluorescence intensity. Again, LC-MS experiments confirmed the proposed reaction mechanism (Fig. S22^†).Open in a separate windowFig. 4Fluorescence spectra of JEG-CAN (15 μM) with initial addition of HCys (2 mM) and incubated for 60 min. Followed by addition of nitroreductase (4 μg mL⁻¹) and NADPH (400 μM) and measured over 90 min in PBS buffer solution (pH = 7.40, 10 mM, containing 1% DMSO). λ_ex = 363 nm. Ex slit: 5 nm and em slit: 5 nm. Dashed line represents JEG-CAN and HCys addition only. Blue line represents highest intensity after addition of NTR.Open in a separate windowFig. 5Fluorescence spectra of JEG-CAN (15 μM) with initial addition of nitroreductase (4 μg mL⁻¹) and NADPH (400 μM) and incubated for 60 min. Followed by addition HCys (2 mM) and measured over 90 min in PBS buffer solution (pH = 7.40, 10 mM, containing 1% DMSO). λ_ex = 363 nm. Ex slit: 5 nm and em slit: 5 nm. Dashed line represents JEG-CAN and NTR addition only. Blue line represents highest intensity after addition of HCys.Kinetic studies for JEG-CAN with both NTR and HCys were carried out (Fig. S11–S18^†) where it is clear that JEG-CAN exhibits a dose dependant fluorescence increase in response of both HCys and NTR.Unfortunately, the probes failed to give good data in cells, which could be due to their short excitation wavelengths or the extremely low intracellular HCys concentrations (5–15 μM). We are now pursuing the development ‘AND’ logic fluorescence probes with longer excitation and emission wavelengths suitable for in vitro and in vivo applications.In summary, we have developed two coumarin-based ‘AND’ logic fluorescence probes (JEG-CAB and JEG-CAN) for the detection of HCys/Cys and ONOO⁻ or NTR, respectively. CAH is a useful platform that enables easy modification for the development of ‘AND’-based fluorescent probes for the detection of HCys/Cys and a second analyte. Both JEG-CAB and JEG-CAN were shown to be ‘AND’-based fluorescent probes.     

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).     

Microwave-assisted synthesis of mutually embedded Rh concave nanocubes with enhanced electrocatalytic activity

Novel mutually embedded Rh concave nanocubes were synthesized by reducing Rh(acac)₃ in tetraethylene glycol in the presence of benzyldimethylhexadecylammonium, KI and polyvinylpyrrolidone under microwave irradiation for 120 s. KI and HDBAC were crucial to the formation of mutually embedded nanostructures. The as-prepared Rh nanocrystals exhibited higher electrocatalytic activity and stability.

Mutually embedded Rh concave nanocubes were synthesized by reducing Rh(acac)₃ with tetraethylene glycol (TEG) as both a solvent and a reducing agent under microwave irradiation for 120 s.

As important catalytic materials, the controlled syntheses of platinum group metal nanocatalysts have attracted wide attention for many years. The catalytic performances of platinum group metal nanomaterials are highly dependent upon their morphologies, compositions and surface structures. Their nanocrystals with controlled shapes have been extensively explored in order to promote their catalytic performance and reduce their cost because of their scarcity and high prices. As an important platinum group metal, rhodium (Rh) is often used as a typical catalyst with high activity and selectivity in hydroreduction,¹ hydroformylation,^1f,2 NO_x reduction,³ CO oxidation,⁴ cross coupling,⁵ and fuel cell^1g,4d,6 and other chemical reactions.⁷ In addition, Rh has a strong resistance to acids and bases as well as a high melting point. However, nanoscaled Rh exhibits high thermodynamic instability owing to its high surface free energy, although it is more stable than many other catalytically active metals. So, the shape-controlled synthesis of Rh nanocrystals is still one of the challenges in this field, though Pt and Pd nanocrystals with many different morphologies have been obtained. In the past decade, great efforts have been devoted to tailoring the sizes, morphologies and surface structures of Rh nanoparticles to improve their catalytic efficiency because of the scarcity and preciousness. Up to now, many Rh nanomaterials with various morphologies such as nanosheets,^1f,4a,8 nanotetropods,⁹ hierarchical dendrites,⁶ hyperbranched nanoplates,^1g,10 ultrathin nanosheet assemblies,^1e,7a and cubic,¹¹ tetrahedral,^1d icosahedral,¹² tetrahexahedral,^4d concave cubic,¹³ concave tetrahedral nanocrystals,¹⁴ as well as cubic nanoframes,^5b,5c truncated octahedral nanoframes,¹⁵ multipods¹⁶ and mesoporous³ Rh nanoparticles have been successfully synthesized. Moreover, all the obtained Rh nanoparticles were monodispersed and displayed enhanced catalytic activities. Although various nanoparticles with concave, frame, branched, or hierarchical structures as well as normal flat or convex surfaces have been created, it is still worthy to develop further novel Rh nanostructures.Herein, we report a facile microwave-assisted strategy for a one-pot synthesis of mutually embedded Rh concave nanocubes, a unique hierarchical nanostructure with several identical concave nanocubes embedded in each other. The co-adsorption of I⁻ ions and benzyldimethylhexadecylammonium (HDBAC) was dominantly responsible for the generation of the hierarchical concave cubic Rh nanocrystals. The as-prepared Rh nanocrystals displayed an enhanced electrochemical activity for formic acid electro-oxidation. Fig. 1a and b as well as Fig. S1 (ESI^†) display TEM images of the typical Rh nanocrystals synthesized under microwave irradiation for 120 s in the presence of PVP and an appropriate amount of KI and HDBAC. Interestingly, as can be seen, the resulting Rh nanocrystals were present in mutually embedded concave nanocubic morphologies under TEM, showing a unique hierarchical nanostructure feature. This hierarchical structure was consisted of at least two concave cubes (Fig. 1b) which were embedded in each other. If considering an individual concave nanocube, the average diameter was about 85 nm. The high-resolution TEM image, as shown in Fig. 1c and d, showed the lattice fringes with an interplanar spacing of 0.192 and 0.135 nm, which can be indexed to the (200) and (220) planes of face-centered cubic (fcc) Rh. The corresponding fast Fourier transform (FFT) pattern for the selected box area in Fig. 1c is shown in the inset in Fig. 1d, indicating a single crystal structure and good crystallinity. In fact, the as-prepared concave cube demonstrated octapod characteristics because of its showing both concave faces and concave edges. The SEM image further confirmed hierarchical structure feature. As shown in Fig. 1e and the inset for partially enlarged picture as well as Fig. S2 (ESI^†), the concave nanocubic structures and their mutually embedded feature are present clearly.Open in a separate windowFig. 1TEM (a and b), HRTEM (c and d) and SEM (e) images of the as-prepared mutually embedded Rh concave nanocubes. (d) shows the selected box area in (c). The insets in (d and e) show the FFT pattern and a partially enlarged SEM picture, respectively.The typical XRD pattern of the as-synthesized mutually embedded Rh concave nanocubes is shown in Fig. 2. The characteristic peaks at 41.28, 48.05, 70.18 and 84.45° are corresponding well with the (111), (200), (220) and (311) lattice planes according to the standard diffraction file (JCPDS 05-0685), respectively. The sharp and strong (111) diffraction peak, which indicated the preferential orientation of (111) planes and the consistency with the HRTEM observation, suggested its high purity and crystallinity of the obtained Rh nanocrystals. In addition, XPS measurement demonstrated the binding energy of Rh 3d_5/2 and Rh 3d_3/2 at 307.16 and 311.91 eV (Fig. 3), respectively, with an interval of 4.75 eV, which was coincident with the reference values (307.0 and 311.75 eV),¹⁷ indicating Rh⁰ with zero oxidation for the as-prepared mutually embedded concave nanocubes.Open in a separate windowFig. 2XRD pattern of the typical mutually embedded Rh concave nanocubes.Open in a separate windowFig. 3XPS spectrogram of the typical mutually embedded Rh concave nanocubes.It was worth noting that the use of KI was much essential for creating the mutually embedded Rh concave nanocubes. As shown in Fig. 4a, except flower ball structures connected with each other, neither concave cube nor hierarchical structure was observed in the absence of KI. When 0.4 mmol of KI was added, the embedded Rh concave nanocubes accompanying with some irregular nanostructures were generated (Fig. 4b). With further increasing the amount of KI from 0.8 to 1.2 mmol, complex inter-embedded nanostructures with obscure polyhedral outlines were formed (Fig. 4c and d). Accordingly, an excessive amount of KI was unfavorable for the generation of the mutually embedded Rh concave nanocubes. According to the previous report,¹⁸ the addition of KI would manipulate the reducing kinetics to generate Rh concave nanostructures under microwave irradiation. In the presence of KI, the precursor was transformed to a more stably coordinated anion [RhI₆]³⁻. As a result, the reducing rate of Rh(iii) to Rh(0) as well as both the nucleation and growth rate of Rh nanoparticles decreased, which would be favorable for the oriented growth of Rh concave cubes. The role of I⁻ ions was elucidated by using an equivalent amount of KBr or KCl in stead of KI, respectively, under the same other conditions. As can be seen (Fig. S3, ESI^†), no single or embedded concave nanocubes but amorphous Rh nanoparticles with agglomeration were observed under these two alternative experiments. These results suggested that different halides would result in different Rh nanostructures and the existence of an appropriate amount of I⁻ ions was beneficial for the formation of the mutually embedded Rh concave nanocubes. According to the literature,¹⁹ halides tend to selectively adsorb to {100} planes. Generally, the six surfaces of a Rh cube are 〈100〉 oriented. So, we suggested that the selective adsorption of I⁻ ions on Rh {100} planes confined a growth along 〈100〉 direction and facilitated the formation of Rh concave structures with growth along {111} facets.Open in a separate windowFig. 4TEM images of the products prepared with different amount of KI while keeping the same other conditions. (a) Without KI; (b) 0.4 mmol KI; (c) 1.2 mmol KI; (d) 1.6 mmol KI.Moreover, the effect of HDBAC on the creation of the embedded Rh concave nanocubes was also investigated. As shown in Fig. 5a, no shaped Rh nanocrystal was produced except agglomerated nanoparticles without using HDBAC. While 0.1 mmol of HDBAC was used relatively to the parameters in the typical experimental procedure, Rh nanostructures with an nonuniform cross-sectional dimension and overly embedded each other were generated (Fig. 5b). With further increasing the amount of HDBAC from 0.4 to 0.6 mmol, the cross-section dimension and the concavity of the concave Rh nanocubes decreased gradually though embedded Rh nanostructures were still generated (Fig. 5c and d). Whereas no concave nanostructure was observed with using an equivalent amount of CTAB or CTAC in stead of HDBAC, respectively (Fig. S4, ESI^†). These results implied that the formation as well as the size and surface structure of the mutually embedded Rh concave nanocubes were dependent upon the confinement effect of HDBAC. On the one hand, the existence of HDBAC would contribute to creation of the concave cubes and their mutually embedded structures, on the other hand, the growth of shaped Rh nanoparticles was confined and the adsorption of I⁻ ions on Rh {100} planes was disturbed, resulting in less concavity and smaller size, due to an excessive amount of HDBAC.Open in a separate windowFig. 5TEM images of the products prepared with different amount of HDBAC while keeping the same other conditions. (a) Without HDBAC; (b) 0.1 mmol HDBAC; (c) 0.4 mmol HDBAC; (d) 0.6 mmol HDBAC.In order to investigate whether its formation was related to oxidation etching of Rh surface by I⁻ ion/O₂,^19,20 nitrogen was filled into the reaction bottle to remove oxygen before reaction and the same results were obtained. So, the oxidative etching can be negligible, which may be ascribed to the extremely short time under microwave irradiation.According to the previous report,²¹ based on the results of the experiments with dependent I⁻ ions and HDBAC, the formation of the mutually embedded Rh concave nanocubes may be ascribed to symmetry breaking due to asymmetric passivation and attachment of Rh nuclei. I⁻ ions were responsible for retarding the growth of {100} and {110} facets of cubic nuclei and promoting the preferential overgrowth on {111} planes, resulting in the formation of the concave structure with concave faces and edges. However, the existence of an appropriate amount of HDBAC may retard the deposition of Rh atoms on one or two corners with (111) facets due to the confinement, resulting in symmetry breaking. Meanwhile, with the confinement of HDBAC, the inevitable collision of nuclei leads to attachment of nuclei one another along the confined corners due to surface defects or dislocations. As a result, the mutually embedded Rh concave nanocubes would be generated with the growth of nuclei.The electrochemical performances of the as-synthesized Rh nanocrystals were examined by electrocatalytic oxidation of formic acid. The specific current density was normalized to the electrochemical surface area (ECSA). According to the cyclic voltammetry (CV) curves (Fig. S5^†), the ECSAs were calculated as 66.5 and 52.3 m² g⁻¹ for the mutually embedded Rh concave nanocubes and the commercial Rh black, respectively. Fig. 6a shows the CV curves for the electro-oxidation of formic acid in HClO₄ by the as-prepared mutually embedded Rh concave nanocubes and the commercial Rh black. The peak current density for the mutually embedded Rh concave nanocubes was measured to be 2.988 mA cm⁻² at 0.667 V, while it was 1.379 mA cm⁻² at 0.674 V for Rh black. The electrocatalytic activity of the hierarchical Rh nanostructures, though with a larger size, was about 2.2 times that of Rh black. Obviously, the mutually embedded Rh concave nanocubes exhibited an enhanced electrocatalytic activity for formic acid comparing with the commercial Rh black, which should be ascribed to their special surface structure with more edges, corners and terraces. Fig. 6b shows the CA curves of the electrocatalytic oxidation of formic acid for these catalysts. Compared with Rh black, a slower current attenuation as well as a higher retention of current after 800 s was observed for the as-prepared embedded Rh concave nanocubes, revealing a good electrochemical stability.Open in a separate windowFig. 6The CV (a) and CA (b) curves of the mutually embedded Rh concave nanocubes and Rh black in 0.1 M HClO₄ + 0.5 M HCOOH solutions with the cyclic potential between −0.2 and 1.0 V at a sweep rate of 50 mV s⁻¹.In summary, a novel hierarchical Rh nanostructure with several concave nanocubes embedded mutually could be rapidly prepared by reducing Rh(acac)₃ in TEG under microwave irradiation for 120 s in the presence of PVP and an appropriate amount of KI and HDBAC. In the preparing process, TEG was used as both a solvent and a reducing agent. The existence of KI and HDBAC was critical to the formation of the mutually embedded Rh concave nanocubes. The as-prepared mutually embedded Rh concave nanocubes demonstrated higher electrocatalytic activity and stability than commercial Rh black in the electro-oxidation of formic acid.