Preparation of Ni-microsphere and Cu-MOF using aspartic acid as coordinating ligand and study of their catalytic properties in Stille and sulfoxidation reactions

In this study, the thermal and catalytic behavior of Ni-microsphere and Cu-MOF were investigated with aspartic acid as the coordinating ligand with different morphologies. The Ni-microsphere and Cu-MOF with aspartic acid, as the coordinating ligand, were prepared via a solvothermal method. The morphology and porosity of the obtained Ni microsphere and Cu-MOF were characterized by XRD, FTIR, TGA, DSC, BET and SEM techniques. The catalytic activity of the Ni-microsphere and Cu-MOF was examined in Stille and sulfoxidation reactions. The Ni microsphere and Cu-MOF were easily isolated from the reaction mixtures by simple filtration and then recycled four times without any reduction of catalytic efficiency.

In this study, the thermal and catalytic behavior of Ni-microsphere and Cu-MOF were investigated with aspartic acid as the coordinating ligand with different morphologies.

Cross-coupling reaction is one of the most significant methods to create carbon–carbon bonds in organic synthesis. There are many approaches, including, Suzuki, Stille, and Sonogashira cross-coupling reactions, which are well recognized and highly applicable in organic synthesis. Among them, the Stille reaction, which is an increasingly versatile tool for the formation of carbon–carbon bonds, involves the coupling of aryl halides with organotin reagents.¹ However, these reactions generally require expensive transition metal catalysts such as Pd.² Therefore, it is necessary to develop a new economic, green, and efficient methodology to reduce the environmental impact of the reaction. They are also important intermediates in organic chemistry and have been widely used as ligands in catalysis. The direct oxidation of sulfides is an important method in organic chemistry. Besides, they are also valuable synthetic intermediates for the construction of chemically and biologically important molecules, which usually synthesized by transition metal complexes.³ In this regard, different transition metal complexes of mercury(ii) oxide/iodine,⁴ oxo(salen) chromium(v),⁵ rhenium(v) oxo,⁶ H₅IO₆/FeCl₃,⁷ Na₂WO₄/C₆H₅PO₃H₂,⁸ chlorites and bromites,⁹ NBS¹⁰etc. have been introduced as catalysts. However, these catalysts have several drawbacks; including, separation problems from the reaction medium, harsh reaction conditions, and generating a lot of waste. In order to solve these drawbacks, of separation and isolation of expensive homogeneous catalysts is the heterogenization of homogeneous catalysts and generation of a new heterogeneous catalytic system. Metal–organic frameworks (MOFs) are a class of porous crystalline materials, which show great advantages, i.e. their enormous structural and chemical diversity in terms of high surface area,^11,12 pore volumes,¹³ high thermal,¹⁴ and chemical stabilities,¹⁵ various pore dimensions/topologies, and capabilities to be designed and modified after preparation.¹⁶ In this sense, it is worth mentioning that these features would result in viewing these solids as suitable heterogeneous catalysts for organic transformations.^17–22 MOFs materials are prepared using metal ions (or clusters) and organic ligands in solutions (i.e. solvothermal or hydrothermal synthesis). MOF structures are affected by metal and organic ligands, leading to have more than 20 000 different MOFs with the largest pore aperture (98 Å) and lowest density (0.13 g cm⁻³).²³ Generally, surface area and pore properties of MOFs seem quite dependent on their metal and ligand type as well as synthesis conditions and the applied post-synthesis modifications. The largest surface area was measured in Al-MOF (1323.67 m² g⁻¹)^24,25 followed by ZIF-8-MOF (1039.09 m² g⁻¹),²⁶ while the lowest value was with Zn-MOF (0.86 m² g⁻¹),²⁷ followed by γ-CD-MOF (1.18 m² g⁻¹)²⁸ and Fe₃O(BDC)₃ (7.6 m² g⁻¹).²⁹ Microspheres are either microcapsule or monolithic particles, with diameters in the range (typically from 1 μm to 1000 μm),²⁹ depending on the encapsulation of active drug moieties. In this regard, there are two types of microspheres: microcapsules, defined, as spherical particles in the size range of about 50 nm to 2 mm and micro matrices.³⁰ Microsphere structures have recently attracted much attention due to their unique properties, such as large surface area,³¹ which make them suitable for tissue regenerative medicine,³²i.e. as cell culture scaffolds,³³ drug-controlled release carriers³⁴ and heterogeneous catalysis.³⁵ Many chemical synthetic methods has been developed for their synthesis, including seed swelling,³⁶ hydrothermal or solvothermal methods,³⁶ polymerization,³⁷ spray drying³⁸ and phase separation.³⁹ Among these methods, the solvothermal synthesis has been used as the most suitable methodology to prepare a variety of nanostructural materials, such as wire, rod,⁴⁰ fiber,⁴¹ mof⁴² and microsphere.⁴³ In this sense, the synthesis process involves the use of a solvent under unusual conditions of high pressure and high temperature.⁴⁴ The properties of microspheres are highly dependent on the number of pores, pore diameter and structure of pore.⁴⁵ The degree of porosity depends on various factors such as temperature, pH, stirring speed, type, and concentration of porogen, polymer, and its concentration.⁴⁶ There have been numerous studies to investigate the coordination behavior of a ligand with different metals under the same conditions.^47–49 Herein, we aim at comparing the catalytic behavior of Ni-microsphere and Cu-MOF with aspartic acid as the coordinating ligand in Stille and sulfoxidation reactions (Scheme 1).Open in a separate windowScheme 1(a) Schematic synthesis of Ni microsphere and Cu-MOF and their application as catalyst (b) topological structure of Cu-MOF (c) topological of Ni microsphere.     

Synthesis of novel benzopyran-connected pyrimidine and pyrazole derivatives via a green method using Cu(ii)-tyrosinase enzyme catalyst as potential larvicidal,antifeedant activities

A series of benzopyran-connected pyrimidine (1a–g) and benzopyran-connected pyrazole (2a–i) derivatives were synthesized via Biginelli reaction using a green chemistry approach. Cu(ii)-tyrosinase was used as a catalyst in the synthesis of compounds 1a–g and 2a–ivia the Biginelli reaction. The as-synthesized compounds were characterized by IR, ¹H NMR, ¹³C NMR, mass spectroscopy, and elemental analysis. The as-synthesized compounds were screened for larvicidal and antifeedant activities. The larvicidal activity was evaluated using the mosquito species Culex quinquefasciatus, and the antifeedant activity was evaluated using the fishes of Oreochromis mossambicus. The compounds 2a–i demonstrated lethal effects, killing 50% of second instar mosquito larvae when their LD₅₀ values were 44.17, 34.96, 45.29, 45.28, 75.96, and 28.99 μg mL⁻¹, respectively. Molecular docking studies were used for analysis based on the binding ability of an odorant binding protein (OBP) of Culex quinquefasciatus with compound 2h (binding energy = −6.12 kcal mol⁻¹) and compound 1g (binding energy = −5.79 kcal mol⁻¹). Therefore, the proposed target compounds were synthesized via a green method using Cu(ii)-enzyme as a catalyst to give high yield (94%). In biological screening, benzopyran-connected pyrazole (2h) was highly active compared with benzopyran-connected pyrimidine (1a–g) series in terms of larivicidal activity.

Cu(ii)-tyrosinase catalytic help with the synthesis of benzopyran-connected pyrimidine and pyrazole derivatives and their larvicidal activity.

Benzopyrans (coumarins) are an important group of naturally occurring compounds widely distributed in the plant kingdom and have been produced synthetically for many years for commercial uses.¹ In addition, these core compounds are used as fragrant additives in food and cosmetics.² The commercial applications of coumarins include dispersed fluorescent brightening agents and as dyes for tuning lasers.³ Some important biologically active natural benzopyran (coumarin) derivatives are shown in Fig. 1. Mosquitoes are the vectors for a large number of human pathogens compared to other groups of arthropods.⁴ Their uncontrollable breeding poses a serious threat to the modern humanity. Every year, more than 500 million people are severely affected by malaria. The mosquito larvicide is an insecticide that is specially targeted against the larval life stage of a mosquito. Particularly, the compound bergapten (Fig. 1), which shows the standard of larivicidal activity,⁵ is commercially available, and it was used as a control in this study for larvicidal screening. Moreover, the antifeedant screening defense mechanism makes it a potential candidate for the development of eco-friendly ichthyocides. Coumarin derivatives exhibit a remarkably broad spectrum of biological activities, including antibacterial,^6,7 antifungal,^8–10 anticoagulant,¹¹ anti-inflammatory,¹² antitumor,^13,14 and anti-HIV.¹⁵Open in a separate windowFig. 1Biologically active natural benzopyran compound.Coumarin and its derivatives can be synthesized by various methods, which include the Perkin,¹⁶ Knoevenagel,¹⁷ Wittig,¹⁸ Pechmann,¹⁹ and Reformatsky reactions.Among these reactions, the Pechmann reaction is the most widely used method for the preparation of substituted coumarins since it proceeds from very simple starting materials and gives good yields of variously substituted coumarins. For example, coumarins can be prepared by using various reagents, such as H₂SO₄, POCl₃,²⁰ AlCl₃,²¹ cation exchange resins, trifluoroacetic acid,²² montmorillonite clay,²³ solid acid catalysts,²⁴ W/ZrO₂ solid acid catalyst,²⁵ chloroaluminate ionic liquid,²⁶ and Nafion-H catalyst.²⁷Keeping the above literature observations, coumarin derivatives 1a–g and 2a–i are usually prepared with the conventional method involving CuCl₂·2H₂O catalysis with using HCl additive. This reduces the yield and also increases the reaction time. To overcome this drawback, we used mushroom tyrosinase as a catalyst without any additive, a reaction condition not reported previously. The as-synthesized compounds were used for the biological screening of larvicidal and antifeedant activities (marine fish). In addition, in this study, we considered the molecular docking studies study based on previous studies for performing the binding ability of hydroxy-2-methyl-4H-pyran-4-one (the root extract of Senecio laetus Edgew) with the odorant binding protein (OBP) of Culex quinquefasciatus.²⁸     

Synthesis of a metal–organic framework by plasma in liquid to increase reduced metal ions and enhance water stability

Synthesis of a metal–organic framework by plasma in liquid was demonstrated with HKUST-1 as an example. HKUST-1 synthesized by this method contains a higher amount of monovalent copper ions than that synthesized by other conventional methods. The enhanced water stability was also confirmed.

Plasma in liquid provides a method for the synthesis of HKUST-1 with increased reduced metal ions and high water stability.

Metal–organic frameworks (MOFs) are a class of hybrid materials composed of organic linkers and metal nodes. They have high surface areas with tunable pore size and functionality.^1,2 Because of these features, MOFs have potential applications in the fields of gas adsorption/storage,^3,4 catalysis,^5,6 drug delivery,^7,8 and fabrication of luminescent materials.^9,10 Many methods have been proposed for synthesizing MOFs; these include solvothermal methods,^11,12 microwave-assisted methods,^13,14 ultrasonic-assisted methods,^15,16 mechanochemical methods,¹⁷ and electrosynthesis methods.¹⁸ Different synthesis methods result in the different crystallinity and size. Further, the synthesis method can influence the physicochemical and semiconductor properties.¹⁹Recently, a method for synthesizing MOFs in liquid contacting gas-phase dielectric barrier discharges has been reported.^20,21 However, MOFs synthesized by plasma have not been sufficiently characterized yet, thereby demanding further research. Plasma is a unique reaction field and has the advantage of being able to reduce and functionalize species using electrons and radicals. These characteristics have been widely utilized even in liquid phase for the synthesis of various materials such as carbon materials^22,23 and metal nanoparticles^24,25 and for introducing functional groups in materials.²⁶ Using plasma, it may be possible to synthesize functional MOFs with reduced metal ions or additional functional groups. Plasma in liquid generally have higher density of electrons and radicals than plasma in the gas phase,²⁷ and the influence of reactive species can be observed more prominently.In this study, we focused on [Cu₃(BTC)₂]_n (BTC = 1,3,5-benzenetricarboxylate). This is known as HKUST-1 and is one of the most widely researched MOFs. This MOF is assembled from copper nodes and BTC, with each copper coordinated with four oxygen atoms. HKUST-1 is a promising candidate for applications such as H₂ and SO₂ storage.^28,29 However, the crystal structure of HKUST-1 is changed by water molecules, and its surface area decreases.^30,31 To ensure the practical applications of HKUST-1, it is important to increase its resistance to water. To improve its stability to water, strategies such as incorporation of other materials, including graphite oxide³² and carboxyl-functionalized attapulgite,³³ have been developed. In the post-processing using plasma, the adsorption of water molecules on HKUST-1 is inhibited by introducing hydrophobic groups with perfluorohexane.³⁴ In another method, the adsorption of water molecules on unsaturated metal sites was prevented by irradiating the HKUST-1 with oxygen plasma.³⁵ In addition, to improve the water tolerance, reduction of Cu(ii) ions in HKUST-1 was also proposed.³⁶ However, in the above-mentioned methods, mixing of other materials or post-treatment steps was essential.In this work, we synthesized HKUST-1 containing Cu(i) by plasma in liquid; the method involved in situ plasma treatment during the synthesis activated also by plasma. Its resistance to water was compared with HKUST-1 synthesized by conventional methods such as heating or addition of triethylamine at room temperature.HKUST-1 was synthesized using the plasma generated by applying a bipolar pulse voltage to the electrode in an ethanol–water solution containing copper nitrate trihydrate and BTC (Fig. S1 and S2^†). Hereafter in this paper, the thus-synthesized HKUST-1 will be referred to as PL-HKUST-1. For comparison, HKUST-1 was synthesized by the conventional heating method and by the addition of triethylamine at room temperature,¹⁹ and the HKUST-1 formed are referred to as CH-HKUST-1 and RT-HKUST-1, respectively. The details of the synthetic methods are described in the ESI.^†All the samples were characterized by X-ray diffraction (XRD) and thermogravimetric analysis. The XRD patterns of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 are shown in Fig. 1. In all the cases, the characteristic diffraction peaks of HKUST-1 were identified, confirming the phase-pure formation of HKUST-1 samples irrespective of the synthetic method.^37,38 Thermogravimetric analysis of HKUST-1 revealed that the thermal stability of PL-HKUST-1 was similar to those of CH-HKUST-1 and RT-HKUST-1 (Fig. S3^†).Open in a separate windowFig. 1XRD patterns of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 before and after immersion in water. Fig. 2 shows the fluorescence (FL) spectra of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 upon excitation at 310 nm at room temperature. Note that a broad peak around 520 nm was observed only for PL-HKUST-1 and not for CH-HKUST-1 and RT-HKUST-1. Such a behavior has been previously reported for Cu(i) complexes and is attributed to the metal-to-ligand charge transfer (MLCT) from Cu(i) to an empty antibonding π* orbital of the ligand.^39–42 Thus, the FL spectrum indicates that a certain amount of Cu(i) is formed in PL-HKUST-1, which highly contrasted to CH-HKUST-1 and RT-HKUST-1.Open in a separate windowFig. 2FL spectra of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 (λ_ex = 310 nm).The existence of Cu(i) in PL-HKUST-1 was also confirmed by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of PL-HKUST-1 showed peaks at 932.2 and 934.6 eV, assignable to Cu(i) and Cu(ii), respectively (Fig. S4^†).^43,44 Although the reduction to Cu(i) inevitably proceeded upon X-ray irradiation,⁴⁵ the concentration of Cu(i) in PL-HKUST-1 was apparently higher than those in CH-HKUST-1 and RT-HKUST-1, which was consistent with the FL spectra. The plasma has ample free electrons and ions, which can reduce metal ions via electrochemical reactions.⁴⁶ The temperature of the plasma in liquid could be as high as 1000–7000 K,²⁷ and the reduction of Cu(ii) to Cu(i) by heat might occur.⁴⁷To examine the water stability of the MOFs, HKUST-1 was immersed in water at room temperature for 12 h. The XRD patterns of the samples after immersion in water were compared with those of the as-synthesized samples. The water treatment resulted in the appearance of new peaks at 2θ ≈ 9.4°–9.7°, 10.1°, 11.1°, 17.3°, and 19.6° for CH-HKUST-1 and RT-HKUST-1 (Fig. 1). Notably, the XRD patterns of PL-HKUST-1 before and after immersion in water were identical, with no detectable changes.The effect of water treatment on the morphology of HKUST-1 was also investigated using scanning electron microscopy (SEM). Particles with octahedral shape were obtained by the plasma in liquid method, similar to those obtained by the conventional heating method (Fig. 3). After immersion in water, the formation of particles with a thread-like morphology was observed for CH-HKUST-1 and RT-HKUST-1, suggesting the decomposition of HKUST-1.^30,48 In contrast, although PL-HKUST-1 contained a portion of the etched corners, the particle shape and size remained unchanged after water treatment.Open in a separate windowFig. 3SEM images of (a and b) CH-HKUST-1, (c and d) RT-HKUST-1, and (e and f) PL-HKUST-1 before and after immersion in water.Fourier transform infrared (FTIR) spectra of HKUST-1 before and after the water treatment are shown in Fig. 4. All as-synthesized samples showed the typical characteristic peaks of HKUST-1. The bands from 1300 to 1700 cm⁻¹ are associated with the carboxylate group of the BTC ligand.⁴⁹ The two peaks in the range 1371–1448 cm⁻¹ correspond to the symmetric stretching vibrations of the carboxylate group.⁵⁰ The bands at 730 and 758 cm⁻¹ correspond to the in-plane C–H bending mode.⁴⁹ Peaks observed below 600 cm⁻¹ correspond to the bonds involving copper ions.⁴⁹ After immersion in water, the FTIR spectrum of PL-HKUST-1 did not show any significant change whereas those of CH-HKUST-1 and RT-HKUST-1 showed shifts in the peaks and formation of new peaks. When exposed to water, peaks corresponding to COO vibrations were observed for CH-HKUST-1 and RT-HKUST-1 between 1200 and 1700 cm⁻¹ due to a change in the environments of the linker BTC.⁴⁸ The peak around 1640 cm⁻¹ shifted to longer wavelengths, and a new peak appeared around 1570 cm⁻¹, which is consistent with previous research.³⁸ The Raman spectra of CH-HKUST-1 and RT-HKUST-1 also showed the structural change in the coordination bonding upon the water treatment (Fig. S5^†).⁵¹Open in a separate windowFig. 4FTIR spectra of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 before and after immersion in water. Fig. 5). The BET surface areas of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 were 888, 574, and 739 m² g⁻¹, respectively. After immersion in water, the amount adsorbed by CH-HKUST-1 and RT-HKUST-1 was lowered compared to that before immersion, and their BET surface areas were 407 and 351 m² g⁻¹, respectively. By contrast, in the case of PL-HKUST-1, the adsorption amount remained high even after immersion in water, and its BET surface area was 811 m² g⁻¹.Brunauer–Emmett–Teller surface areas of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 before and after immersion in water

Influence of SiO2 or h-BN substrate on the room-temperature electronic transport in chemically derived single layer graphene

The substrate effect on the electronic transport of graphene with a density of defects of about 0.5% (^0.5%G) is studied. Devices composed of monolayer ^0.5%G, partially deposited on SiO₂ and h-BN were used for transport measurements. We find that the ^0.5%G on h-BN exhibits ambipolar transfer behaviours under ambient conditions, in comparison to unipolar p-type characters on SiO₂ for the same flake. While intrinsic defects in graphene cause scattering, the use of h-BN as a substrate reduces p-doping.

Defects in graphene cause scattering and basal plane interactions shift the Dirac-point.

Wet-chemically prepared graphene from graphite can be stabilized in solution by covalently bound oxo-groups using established oxidation protocols.^1–3 In general, the materials obtained are termed graphene oxide (GO). However, the chemical structure varies and the carbon lattice may even be amorphous due to the evolution of CO₂ during synthesis.⁴ Thus, in this study we use oxo-functionalized graphene (oxo-G), a type of GO with a more defined structure, as proven in our previous work.³ The oxygen-containing groups on the graphene basal plane and rims of flakes and holes make GO a p-type semiconductor with a typical resistance of 10¹⁰–10¹³ Ω sq^−1 5,6 and a band gap of about 2.2 eV.^7,8 The reductive defunctionalization of GO leads to a certain type of graphene (G), often named reduced GO (r-GO).^4,9 Removal of oxo-groups from the surface can be achieved by chemical reduction,^9,10 electrochemical methods,^11,12 electron beam treatment¹³ and was observed in situ by transmission electron microscopy.¹³ Thermal processing of GO instead leads to a disproportionation reaction forming carbon with additional vacancy defects and CO₂.¹⁴ In general, the reduction of GO turns r-GO from a semi-conductive material to a semi-metal. Mobility values were determined in field effect transistor (FET) devices.^15,16 Generally, the quality of graphene strongly depends on the integrity of the hexagonal carbon lattice. Thus, mobility values of 10⁻³ and up to 10³ cm² V⁻¹ s⁻¹ were reported,^3,17,18 with the resistance fluctuating between 10³ and 10⁶ Ω sq⁻¹.^19–21 We reported on the highest mobility values of chemically reduced oxo-G (with about 0.02% of lattice defects) of 1000 cm² V⁻¹ s⁻¹,³ determined by Hall-bar measurements at 1.6 K.Hexagonal boron nitride (h-BN) has been proved to be an excellent substrate for matching graphene-based materials owing to its atomic flatness, chemical inertness and electronic insulation due to a bandgap of ∼5.5 eV.²² Up to now, most studies with graphene deposited on h-BN were restricted to measurements with virtually defect-free graphene.²³ To the best of the authors knowledge, no studies reported transport measurements based on single layers of GO or oxo-G on h-BN substrates. No studies are reported with graphene derived from GO or oxo-G on single-layer level. Recently, we found that chemical reactions can be selectively conducted close to the rims of defects.²⁴ However, before functionalized devices can be studied, the lack of knowledge on the ambient environment device performances of graphene with defects and the influence of substrates must be addressed. Therefore, we fabricated the devices composed of ^0.5%G, partially deposited on SiO₂ (SiO₂/^0.5%G) and h-BN (h-BN/^0.5%G) (Fig. 1). Areas of the same flake on both materials are used to ensure reliable measurements and to prove that the results stem from the influence of the substrate rather than from the difference between devices. Thereby, the ^0.5%G exhibits an I_D/I_G ratio of about 3–4, corresponding to 0.5% of defects, according to the model introduced by Lucchese and Cançado.^25–28 Our results demonstrate that the h-BN layer is responsible for a downshift of the Dirac point and a more narrow hysteresis, resulting in ambipolar transfer behaviours in h-BN/^0.5%G.Open in a separate windowFig. 1(a) Optical image of the fabricated h-BN/^0.5%G heterostructure on SiO₂. (b) The h-BN/^0.5%G heterostructure device. Electrodes 1 and 2 define the SiO₂/^0.5%G FET device. Electrodes 1 and 3 define the ^0.5%G on overlapped SiO₂/h-BN hetero-substrate device. Electrodes 3 and 4 define the h-BN/^0.5%G FET device. Distance between the electrodes 1–2 and 3–4 is 1.5 μm and 3 μm, respectively. (c) 3D illustration of the h-BN/^0.5%G transistor device.     

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

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

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

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

Renewable juglone nanowires with size-dependent charge storage properties

Inspired by the biological metabolic process, some biomolecules with reversible redox functional groups have been used as promising electrode materials for rechargeable batteries, supercapacitors and other charge-storage devices. Although these biomolecule-based electrode materials possess remarkable beneficial properties, their controllable synthesis and morphology-related properties have been rarely studied. Herein, one dimensional nanostructures based on juglone biomolecules have been successfully fabricated by an antisolvent crystallization and self-assembly method. Moreover, the size effect on their electrochemical charge-storage properties has been investigated. It reveals that the diameters of the one dimensional nanostructure determine their electron/ion transport properties, and the juglone nanowires achieve a higher specific capacitance and rate capability. This work will promote the development of environmentally friendly and high-efficiency energy storage electrode materials.

Renewable juglone nanowires have been successfully fabricated, and their size effect on electrochemical charge-storage properties has been investigated.

Currently, the development of high-performance electrochemical energy-storage materials and devices is attracting intensive interest. Conventional electrode materials involving transition metal compounds,^1–4 elementary substances,^5–8 and conductive polymers,^9–12 with superior charge storage properties have been widely investigated. However, the poor biocompatibility, rising prices and depletion issues limit their sustainable applications due to their intrinsic material properties.¹³ Thus, exploring naturally abundant and renewable charge-storage materials with promising electrochemical performance is of great significance.In the biological system, its metabolic process mainly relies on ions transport and energy exchanges of redox-active biomolecules with special functional groups such as carbonyl groups, carboxyl groups, and pteridine centres.¹⁴ Due to their abundance, sustainability, environmental benignity, these renewable and nature-derivable biomolecules with well-defined charge-storage behaviors are ideal alternatives to conventional electrode materials for the next-generation green energy-storage devices.^15–17 For instance, biomolecules such as lignin,¹⁸ melanin,¹⁹ riboflavin,²⁰ juglone²¹ and humic acid²² have been demonstrated as promising electrode materials for the rechargeable batteries, supercapacitors and other charge-storage devices. Although these biomolecule-based electrode materials possess remarkable beneficial properties, they are still confronted with several serious problems of poor conductivity and high electrochemical reaction impedance.²³ For some conventional inorganic and organic active electrode materials, decreasing their size has been demonstrated to be effective strategies to enhance the electrochemical reaction kinetics by exposing more active sites to electrolytes and conductive agent.^8,24,25 These results have strongly motivated us to develop biomolecule-based nanostructures, and investigated their size-correlative charge storage behavior.^26,27Herein, one dimensional (1D) nanostructures based on juglone, a renewable redox-active biomolecule which can be derived from matured fruits of black walnut and the green peel of juglandaceae, have been successfully fabricated by an antisolvent crystallization and self-assembly method.^28–30 The size effect on the electrochemical charge-storage properties of these biomolecule-based 1D nanostructures have been investigated. It reveals that the electronic/ionic transport properties and charge-storage performance can be modulated by the size of self-assembled 1D nanostructures, and the samples with smaller diameter realize the higher specific capacitance and rate capability. Our work will provide insights for the development of high-performance biomolecule-based green energy-storage materials and devices.Juglone, also called 5-hydroxy-1,4-naphthalenedione, is a nature-derivable biomolecule, and displays a well-defined redox behavior in acetonitrile due to its quinone groups (Fig. 1a and b).³¹ As organic molecule inherently, it is soluble in organic solvent and difficult to dissolve in water, so its nano-architectures could be condonably fabricated by an antisolvent crystallization strategy (Fig. 1c) and would carry out stably for charge storage in an aqueous electrolyte.^29,30 Firstly, the juglone-biomolecule-based 1D nanostructures with different size were synthesized. The juglone micropillars with a mean diameter around 12 μm were prepared by directly recrystallizing a water/acetonitrile mixed solution of juglone at room temperature (Fig. 2a and d). Compared to commercially available raw juglone materials (Fig. S1^†), the juglone micropillars could increase its charge storage performance, but its relative large size would still confine its contact with electrolyte, and thus remarkably reduce the reaction kinetics.³² To further improve the potential reaction kinetics, the juglone microwires with an average diameter about 1 μm (Fig. 2b and e) and juglone nanowires with a mean diameter about 550 nm (Fig. 2c and f) were fabricated at room temperature using the reprecipitation method.³³ In the specific synthesis process, a high-concentration juglone acetonitrile solution is injected into water, which is a poor solvent for juglone molecules. Under stirring, juglone started to crystallize within a few seconds owing to its poor solubility in the water/acetonitrile mixture solvent and self-assembled into 1D nanostructures, and this procedure could be resulted from the π–π interaction.²⁹ It can be found that the diameter of 1D juglone materials is adjustable by tuning the concentration of juglone in acetonitrile, and the higher concentration of juglone acetonitrile solution yields the smaller size of 1D juglone materials. The insert panels show the percentage of juglone micropillar, microwire, nanowire with different diameter coverage (Fig. 2d–f).Open in a separate windowFig. 1(a) Chemical structural formula of juglone biomolecules which can be derived from the bark of black walnuts. (b) Juglone redox activity verified in a mixed solution of acetonitrile/deionized water by a three-electrode system using Pt foils as both the counter and working electrodes, Ag/AgCl as the reference electrode, and 2.3 M H₂SO₄ as the electrolyte. (c) Schematically illustration of the fabrication of the juglone nanowire/microwire.Open in a separate windowFig. 2SEM images of juglone 1D nanostructures at different magnification. (a and d) Juglone micropillar, (b and e) juglone microwire, (c and f) juglone nanowire. The insert images in panels (d–f) are corresponding mathematical statistic results of juglone samples with different diameter.Generally, the covalent bond, hydrogen bond, van der Waals forces, electrostatic forces, surface tension forces/dewetting, and π–π stacking interactions are considered as the effective factors in the self-assembly procedure of organic compounds.^34–36 The juglone biomolecule has a α-naphthol backbone with two carbonyl groups, which may induce the molecules self-assembly by the π–π interactions.^29,37–39 Fourier Transform Infrared Spectroscopy (FTIR) of such samples was utilized to examine the ingredient of these samples. As shown in Fig. S2,^† all the samples show similar characteristic peaks, the peak at 1640 cm⁻¹ is attributed to the stretching vibration of carbonyl groups, which is the main reversible redox center presented in juglone molecules. Meanwhile, Raman spectra also corroborated the results (Fig. S4^†). The crystal structures of these samples are further characterized by X-ray diffraction (XRD) as shown in Fig. S3.^† No impurity peak is observed in these patterns, demonstrating that these samples have the same crystal structure. And the peak intensity of juglone nanowire is stronger than juglone microwire and micropillar, suggesting that juglone nanowires have high crystallinity and relatively uniform crystal size.³⁰To investigate the redox behavior of 1D juglone micro/nanostructures, the cyclic voltammetry (CV) measurements were firstly performed (Fig. 3a), and the result reveals that these samples show a superior reversible redox performance, implying a potential application as energy storage materials. Meanwhile, compared with the CV curves of raw juglone, juglone micropillar and juglone microwire, the juglone nanowire exhibits the strongest redox peak, suggesting that the juglone nanowire-based electrode has a better charge storage behavior. To further evaluate the electrochemical performance of these samples, galvanostatic charge–discharge (GCD) measurements were carried out (Fig. 3b). First, juglone nanowire shows a higher specific capacity than that of microwire and micropillar. Besides, the plot of juglone 1D-based electrodes exhibit symmetric triangular shape with one pair of charge–discharge voltage plateau, and the voltage plateau of juglone micropillar, microwire and nanowire appears at 0.28 V, 0.26 V, 0.22 V, which is roughly consistent with the results of CV curves. Furthermore, it further reveals that the 1D nanostructure with smaller size exhibits higher specific capacity during the scan rates increasing from 10 to 200 mV s⁻¹ (Fig. 3c). In detail, at a low scan rate of 10 mV s⁻¹, the specific capacity delivered by juglone nanowire, juglone microwire, juglone micropillar are 389 F g⁻¹, 375 F g⁻¹, 116 F g⁻¹, respectively, and these samples possess the specific capacities of 232 F g⁻¹, 205 F g⁻¹, 80 F g⁻¹, when the scan rate are enhanced to 200 mV s⁻¹. The general perception is that nanomaterials with higher specific surface areas usually possess better electrochemical performance.⁴⁰ Obviously, the higher specific capacitance and superior rate capability are achieved by juglone nanowire electrode.Open in a separate windowFig. 3Electrochemical performance of the juglone samples with different diameter. (a) CV curves of juglone micropillar, microwire, nanowire electrodes in the potential range of −0.1 to 0.7 V (vs. Ag/AgCl) with a scan of 50 mV s⁻¹, (b) galvanostatic charge–discharge curves of juglone electrodes with different diameter at 2 A g⁻¹. (c) Statistical study of the rate performance conducted at each scan rate based on the cyclic voltammetry capacity of five electrodes as one batch. (d) Impedance phase angle as a function of frequency for juglone micropillar, microwire, nanowire electrodes. Inset is the Nyquist plot of them. (e) Correlation between the diameter and the specific capacity of the juglone samples at various scan rates.Then, the reaction kinetics of juglone with different sizes were investigated by electrochemical impedance spectroscopy (EIS) (Fig. 3d and S5^†). The crooked curve at high frequency denotes the interface resistance, involving contact and charge transfer resistances, while the low-frequency line represents ion diffusion resistance. The interface resistances of juglone nanowire, microwire, micropillar, raw material electrodes are 3.45 Ω, 3.80 Ω, 3.95 Ω, 4.02 Ω. The plot of the phase angle against the frequency reveals the characteristic frequency f₀ at the phase angle of −45° is 7.46 Hz for juglone nanowire-based electrode, and it is relatively higher than that for juglone microwire (7.36 Hz), micropillar (7.25 Hz) and raw material (6.48 Hz). Accordingly, the time constant t₀ (t₀ = 1/f₀) that represents the minimum time to discharge ≥50% of all the energy from the electrode was 0.134 s for juglone nanowire, whereas 0.136 s, 0.138 s, 0.154 s were required for juglone microwire, micropillar and raw material. The low charge transfer resistance, and short time constant validated the excellent charge and discharge capability of the juglone nanowire-based electrode.⁴¹In addition, for solid-state diffusion of H⁺ in electrode materials, the mean diffusion time is proportional to the square of the diffusion path length according to the following equation:t ≈ L²/D_H⁺1where L is the diffusion length and D_H⁺ the diffusion constant.^42,43 It has been found that the diffusion pathway will be shortening by nanostructuring of electrode materials when the diffusion constant D is same,⁴⁴ indicating that the smaller size nanostructures of juglone facilitate rapid ion/electron transport. Thus, 1D juglone nanowires possess higher specific capacitance and show slower capacity decay during the increased scan rates (Fig. 3e).Furthermore, the CV and GCD for juglone nanowire-based electrodes were tested under various scan rates and current densities. The CV profiles show that the redox activity can be maintained very well when increasing the scan rate from 10 to 100 mV s⁻¹ (Fig. 4a). Moreover, the GCD curves collected at different current densities exhibit symmetric triangular shape with one pair of charge–discharge voltage plateau, which is similar to the results of CV test (Fig. 4b and c). And the specific capacity is 342 F g⁻¹ at a current density of 2 A g⁻¹, it is approximately equivalent to the 345 F g⁻¹ at a scan rate of 20 mV s⁻¹. Next, the specific capacitance of our juglone nanowire is further compared to those of the reported conventional electrode materials.^45–51 As shown in Fig. 4d, the capacity of N/rGO (reduced graphene oxide doping with nitrogen) is 138 F g⁻¹, and our juglone nanowire have about 2-fold higher specific capacity than N/rGO, 1.5-fold higher than Fe₃O₄/rGO (154 F g⁻¹).Open in a separate windowFig. 4(a) CV curves of the juglone nanowire electrode at different scan rates from 10 mV s⁻¹ to 100 mV s⁻¹. (b) Charge and discharge curves in the current density range from 2 A g⁻¹ to 10 A g⁻¹. (c) Various specific capacity of the juglone nanowire electrode when increasing the scan rate from 10 mV s⁻¹ to 200 mV s⁻¹. (d) Specific capacity of juglone nanowire in comparison with different materials.     

Rutin-modified silver nanoparticles as a chromogenic probe for the selective detection of Fe3+ in aqueous medium

The development of nanoprobes for selective detection of metal ions in solution has attracted great attention due to their impact on living organisms. As a contribution to this field, this paper reports the synthesis of silver nanoparticles modified with rutin in the presence of ascorbic acid and their successful use as a chromogenic probe for the selective detection of Fe³⁺ in aqueous solution. Limits of detection and quantification were found to be 17 nmol L⁻¹ and 56 nmol L⁻¹, respectively. The sensing ability is proposed to proceed via an iron-induced nanoparticle growth/aggregation mechanism. A practical approach using image analysis for quantification of Fe³⁺ is also described.

The use of rutin-modified silver nanoparticles for selective detection and sensitive quantification of Fe³⁺ in aqueous solution is described.

Metal ions are key species in nature due to their essential functions in living organisms.^1,2 On the other hand, heavy metals as well as essential metals at abnormally high levels are toxic.² Iron, for instance, in addition to its popular use in industry and construction, is essential to the human body and active in biological processes. Although the trivalent form of iron is particularly important for oxygen transport in blood and the mitochondrial respiratory chain, high levels of this cation are associated with important pathologies.^3,4 The detection of metal ions in aqueous solution is traditionally performed by methods including atomic absorption spectrometry,⁵ electrochemical measurements,^6,7 and inductively coupled plasma techniques,⁸ among others. However, these techniques have important drawbacks, notably the need for sophisticated instrumentation, in addition to being time-consuming and requiring laborious procedures. To overcome these issues, the development of chromogenic and fluorogenic chemosensors for the selective detection of metal-targets has attracted great attention, especially due to the possibility of fast, sensitive and non-expensive analysis.^9,10 In the last decade, nanoscaled materials have been reported as selective probes for metal ions, including Fe³⁺.^11–24Silver nanoparticles (AgNPs) are of particular interest because of the affordable price of starting materials, ease of controlling size and morphology, possibility to functionalize their surface with organic molecules, and optical properties that enable detection of a variety of analytes via simple UV-vis spectroscopy and digital image analysis. Furthermore, applications of AgNPs are also biotechnologically relevant due to the possibility of green synthetic protocols, including the use of plant extracts,²⁵ natural sources,²⁶ glycerol,²⁷ among others.Flavonoids are secondary metabolites naturally found in fruits and other vegetables with relevant roles due to their nutritional, pharmaceutical and medicinal properties.²⁸ Because of their adequate structural features, flavonoids are candidates to be employed in the synthesis of AgNPs.²⁶ Rutin (RU), a sugar-based flavonoid, may be employed as reducing agent in the synthesis of AgNPs along with a stabilizer such as polyvinylpyrrolidone (PVP)²⁶ or used as crude plant extract component.^29,30This paper reports the use of RU-modified AgNPs (RU-AgNPs) as a chromogenic probe for Fe³⁺ in aqueous medium in the presence of ascorbic acid (AA). Sensing ability of RU-AgNPs for the selective detection of Fe³⁺ toward other metal cations was investigated with UV-vis spectroscopy analysis. These data and transmission electronic microscopy (TEM) results allowed a mechanistic proposal involved in the selective detection of Fe³⁺ by RU-AgNPs. Furthermore, a practical approach based on correlation of images of solutions obtained with a conventional smartphone and chemometrics was employed for a simpler quantification of Fe³⁺ in aqueous medium.Initially, the order of reagent combination was investigated in the synthesis of RU-AgNPs. Concentration of RU ranged from 0.10 to 0.50 mmol L⁻¹, while concentrations of other components were fixed at 0.20 mmol L⁻¹ AgNO₃, 0.10 mmol L⁻¹ AA and 0.10 M NaOH. Water was used as solvent in all cases. Narrower surface plasmon resonance (SPR) bands were obtained from adding a solution of AA and NaOH to a solution containing RU and AgNO₃ (Fig. 1a) against the addition of RU, AA, and NaOH to AgNO₃ solution (Fig. S1a – ESI^†), or the addition of RU and NaOH to a solution of AA and AgNO₃ (Fig. S1b^†). RU-AgNPs obtained from the condition presented in Fig. 1a are small (4.1 nm average diameter) and considerably polydisperse (standard deviation of 4.7 nm), however, presenting only one population (Fig. 1b and S2^†). A study of the influence of RU concentration (0.10 to 0.50 mmol L⁻¹) on the stability of RU-AgNPs over time indicated that 0.10 mmol L⁻¹ RU generates more stable RU-AgNPs (Fig. S3^†). Next, a study on the influence of pH indicated that RU-AgNPs are only stable under strong alkaline conditions (pH 12.5 or higher) (Fig. S4^†).Open in a separate windowFig. 1(a) UV-vis analysis of RU-AgNPs under strong alkaline condition (pH > 12.5) based on the order of adding reagents (RU, 0.10 to 0.50 mmol L⁻¹; AgNO₃, 0.20 mmol L⁻¹; AA, 0.10 mmol L⁻¹; NaOH, 0.10 mmol L⁻¹); (b) TEM image of RU-AgNPs under the selected condition.The ability of RU-AgNPs to sense metal cations was investigated by both naked-eye and UV-vis spectroscopy analysis (Fig. 2). The separate addition of 10 μmol L⁻¹ of several metal cations (Fe³⁺, Co²⁺, Zn²⁺, Sr²⁺, Cu²⁺. Al³⁺, Ba²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Mg²⁺, Hg²⁺, Cu⁺ and Cr³⁺) to solutions of RU-AgNPs (prepared according to Fig. 1a) indicated that only Fe³⁺ induces a significant colorimetric change in the final aspect of solution after 50 minutes (Fig. 2a).Open in a separate windowFig. 2Naked-eye (a) and UV-vis (b) analysis of RU-AgNPs in absence (control, C) and presence of 10 μmol L⁻¹ of selected metal cations (Fe³⁺, Co²⁺, Zn²⁺, Sr²⁺, Cu²⁺. Al³⁺, Ba²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Mg²⁺, Hg²⁺, Cu⁺ and Cr³⁺) after 50 min; (c) calibration curve obtained by the addition of different amounts (0 to 10 μmol L⁻¹) of Fe³⁺ to solutions of RU-AgNPs; (d) TEM image of RU-AgNPs after addition of Fe³⁺ (10 μmol L⁻¹). In all experiments, RU-AgNPs were prepared in the presence of ascorbic acid.The results presented in Fig. 2a are consistent with the UV-vis spectroscopy analysis (Fig. 2b). Co²⁺ ions also induce some change in the system, however at a considerably smaller extension than Fe³⁺. In this study, AA played a crucial role in the selective detection of Fe³⁺ by the referred nanoprobe. In the absence of AA, Co²⁺ (mainly) as well as other cations induce stronger colorimetric and spectral changes (Fig. S5a and b,^† respectively) in the analysis of solutions of RU-AgNPs when compared to the system containing AA. This selectivity may arise from two possible reasons: (i) preservation of RU by avoiding its oxidation in the reduction of Ag⁺ ions; (ii) coordination of ascorbate anion to cations other than Fe³⁺.Interaction of RU-AgNPs and Fe³⁺ (10 μmol L⁻¹) stabilizes after approximately 40 minutes (Fig. S6^†). Next, a calibration curve was built from the direct relationship between the absorbance at 396 nm and the concentration of Fe³⁺ (Fig. 2c), presenting a good correlation (R² = 0.9929). The limits of detection and quantification were found to be 17 nmol L⁻¹ and 56 nmol L⁻¹, respectively, which is very satisfactory.^13,14 The influence of other cations in the detection of Fe³⁺ was investigated by UV-vis spectroscopy. Fig. S7^† clearly demonstrates that there are only small changes when a second cation (30 μmol L⁻¹) is added together with Fe³⁺ to the RU-AgNPs solution.Mechanistically, the detection of Fe³⁺ by RU-AgNPs in aqueous medium proceeds via a growth/aggregation-combined process. This proposal is first evidenced by UV-vis analysis due the suppression of SPR band (Fig. 2b), a behavior consistent with the literature.^31,32 Interestingly, TEM analysis clearly shows a growth in AgNPs size after addition of Fe³⁺ to the solution (Fig. 2d), resulting in a final single AgNPs population with average diameter of 14.7 nm ± 8.9 nm. Due to the strong alkaline medium, the main specie responsible for the behavior of NPs is likely to be Fe(OH)₃. A schematic illustration of the mechanism involving aggregation of RU-AgNPs induced by the addition of Fe³⁺ is presented in Fig. 3. AgNPs are initially formed by adsorption of anionic RU to the silver surface via deprotonated 5-hydroxychromen-4-one moiety. This is supported by the literature³³ and confirmed by alteration in the 1800–1500 cm⁻¹ region of the RU infrared spectra before and after coordination with silver (Fig. S9^†). Afterwards, the addition of Fe³⁺ induces the formation of a coordination complex through an anionic catechol group, in which at least 2 : 1 ligand–Fe³⁺ stoichiometry is required for an aggregated effect.Open in a separate windowFig. 3Mechanistic proposal for growth/aggregation of RU-AgNPs in the presence of Fe³⁺. Insert: binding model for RU-AgNPs.Due to increasing interest in image processing as an analytical tool for many purposes,^34,35 Multiple Linear Regression was employed to verify the capacity of the RU-AgNPs to probe Fe³⁺ standards at distinct concentrations, as presented in Fig. 4. The curve was obtained by plotting the color absorbances RGB-based values versus the concentrations of Fe³⁺ standards after RU-AgNPs interaction. Predicted iron is a vector based on RGB values that were then extracted from the filtered images and inserted in the equation described by Beer–Lambert law in order to generate the absorbances for the construction of the analytical curve. A linear behavior between the predicted response and the measured concentrations was observed (R² = 0.9806).Open in a separate windowFig. 4Calibration curve for Fe³⁺ analysis showing the predicted iron (RGB) vs. iron(iii) concentration (1 to 8 μmol L⁻¹). Adjusted R² = 0.9806.Regression coefficients of the calibration model (using the R, G and B channels simultaneously) obtained by MLR method are shown in eqn (1):[Fe³⁺] = 44.5R − 4.9G − 16.4B + 5.11where Fe³⁺ concentration is the dependent variable (Predicted Iron), 5.1 is the intercept (β₀), 44.5, −4.9 and −16.4 are the regression coefficients of the independent variables (R, G and B channels, respectively).It is possible to observe an interesting performance of the method using the three RGB color channels allied with MLR to quantify the Fe³⁺ content, which presented reasonable deviations in its responses considering that it is simple and low-cost. The good linearity is similar to other colorimetric methods such as sodium determination in seawater and coconut water (R² > 0.91) by Moraes and coauthors,³⁶ or iron(ii) in simulated seawater (R² = 0.9993) by Gasparotto et al.³⁷Second order regression was applied to the dataset to obtain a better adjustment, resulting in an adjusted R-squared of 0.9955. RGB values were then inserted in eqn (2) in order to generate the construction of the correlation:[Fe³⁺] = 14.4R + 30.7G − 23.4B − 398RG − 348RB − 0.8BG − 978R² + 695G² + 26.9B² + 4.52This paper reports the use of AgNPs functionalized with RU as nanoprobes for selective detection and sensitive quantification of Fe³⁺ in aqueous solution. The synthesis of RU-AgNPs is reproducible, easily performed and requires no stabilizer agent other than RU. AA has a crucial role in the selectivity by either the avoidance of oxidation of RU by silver and/or coordination of ascorbate with other cations. The literature brings relevant examples of chromogenic and fluorogenic chemosensors for selective detection of Fe³⁺ in solution. Many of these artificial organic receptors present high selectivity and relevant limits of detection, requiring, however, very specific reagents and laborious synthetic procedures.^38–41 Metal-based nanoparticles have emerged as potential probes for detection of Fe³⁺.^11–16 Although effective in Fe³⁺ sensing, the synthesis of these nanoprobes require some toxic reagents, such as NaBH₄ or PVP, use plant extracts, which may lead to some drawbacks, such as the understanding of the sensing mechanism. In contrast, our method is based on commercially available, nontoxic, low-cost reagents. Fe³⁺ sensing performed satisfactorily in the 1–10 μmol L⁻¹ range, and the limit of detection obtained with this method (17 nmol L⁻¹) is comparable to the most sensitive methods reported in literature. A mechanism for the detection of Fe³⁺ by RU-AgNPs involves a combined growth/aggregation of the NPs. There is a still limited number of nanoscaled systems reported as being selective and sensitive in the detection of Fe³⁺, which reinforces the relevance of the method reported herein. The linearity range obtained by both UV-vis spectroscopy and image analysis comprises the maximum of residual Fe³⁺ in drinking water according to the European and US legislations.^15,42     

Nano-Fe3O4@walnut shell/Cu(ii) as a highly effective environmentally friendly catalyst for the one-pot pseudo three-component synthesis of 1,3-oxazine derivatives under solvent-free conditions

Fe₃O₄@walnut shell/Cu(ii) as an eco-friendly bio-based magnetic nano-catalyst was prepared by adding CuCl₂ to Fe₃O₄@walnut shell in alkaline medium. A series of 2-aryl/alkyl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazines were synthesized by the one-pot pseudo three-component reaction of β-naphthol, formaldehyde and various amines using nano-Fe₃O₄@walnut shell/Cu(ii) at 60 °C under solvent-free conditions. The catalyst was removed from the reaction mixture by an external magnet and was reusable several times without any considerable loss of its activity. This protocol has several advantages such as excellent yields, short reaction times, clean and convenient procedure, easy work-up and use of an eco-friendly catalyst.

Fe₃O₄@walnut shell/Cu(ii) as an eco-friendly bio-based magnetic nano-catalyst was prepared by adding CuCl₂ to Fe₃O₄@walnut shell in alkaline medium.

Biopolymers, especially cellulose and its derivatives, have some unparalleled properties, which make them attractive alternatives for ordinary organic or inorganic supports for catalytic applications.¹ Cellulose is the most abundant natural material in the world and it can play an important role as a biocompatible, renewable resource and biodegradable polymer containing OH groups.² Walnut shell is a natural, cheap, and readily available source of cellulose. Fe₃O₄ nanoparticles are coated with various materials such as surfactants,³ polymers,^4,5 silica,⁶ cellulose⁷ and carbon⁸ to form core–shell structures. Magnetic nanoparticles as heterogeneous supports have many advantages such as high dispersion in reaction media and easy recovery by an external magnet.⁹ Cu(ii) as a safe and ecofriendly cation is a good Lewis acid and can activate the carbonyl group for nucleophilic addition reactions.¹⁰1,3-Oxazines moiety has gained great attention from many organic and pharmaceutical chemists due to their broad range of biological activities such as anticancer,¹¹ anti-bacterial,¹² anti-tumor¹³ and anti-Parkinson''s disease.¹⁴Owing to the biological importance of benzo-fused 1,3-oxazines, various methods have been developed for the synthesis of these compounds. Some shown protocols for the synthesis of various 2-aryl/alkyl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazines via a Mannich type condensation between a 2-naphthol, formaldehyde and a primary amine were reported. This protocol has been catalyzed by KAl(SO₄)₂·12H₂O (alum),¹⁵ ZrOCl₂,¹⁶ polyethylene glycol (PEG),¹⁷ thiamine hydrochloride (VB₁)¹⁸ and CCl₃COOH.¹⁹ Other methods of synthesis of oxazines are aza-acetalizations of aromatic aldehydes with 2-(N-substituted aminomethyl) phenols in the presence of an acid as catalyst²⁰ and electrooxidative cyclization of hydroxyamino compounds.²¹However, some of these catalysts have limitations such as inefficient separation of the catalyst from reaction mixtures, unrecyclable and environmental limitations. Therefore, the development of green and clean methodology for the preparation of 2-aryl/alkyl-2,3-dihydro-1H-naphtho[1,2-e][1,3]oxazine derivatives is still an interesting challenge.Herein, we wish to report the preparation of Fe₃O₄@nano-walnut shell/Cu(ii) as a new and bio-based magnetic nanocatalyst and its using for one-pot synthesis of 1,3-oxazine derivatives via condensation of β-naphthol, primary amine and formaldehyde.     

Egyptian blue: from pigment to battery electrodes

Herein we report on using Egyptian blue as an anode material for Li-ion batteries. A 1^st cycle lithiation capacity of 594 mA h g⁻¹ and reversible capacity of 210 mA h g⁻¹ at 20 mA g⁻¹, and at 500 mA g⁻¹ a reversible capacity of 120 mA h g⁻¹ (stable over 1000 cycles) were achieved with coulombic efficiency more than 99.5%. Using X-ray diffraction, and FTIR and X-ray absorption spectroscopies we found that the material goes through a conversion reaction during the 1^st cycle that results in the formation of amorphous mixed oxides with copper nanoclusters.

Herein we report on using a well known pigment that was used by ancient Egyptians and called Egyptian blue as an anode material for Li-ion batteries (background photo by Kokhanchikov via Adobe Stock).

Calcium copper silicate (CaCuSi₄O₁₀), known as Egyptian Blue (EB), was first used about 5000 years ago by the ancient Egyptians as a blue pigment for pottery, jewelry and paintings.¹ Even under the harsh conditions that some of the artifacts from that era have been in, the EB remained without significant degradation mostly due to its high physical and chemical stability.² Even though this material has been thoroughly characterized for identification purposes in ancient pottery, Renaissance era paintings and other art pieces, recent studies on EB have yielded newly discovered properties such as near infrared photoluminescence.^3–5 Even more recently, the discovery of exfoliating EB into nanosheets has breathed new life into studies with EB as a functional nanomaterial.⁶ By combining the luminescent properties and the ability to process the nanosheets in ink-jet printing, EB may have a promising future in security inks and near-IR-based biomedical imaging.^6,7 With its new momentum, studies into EB may produce results never expected of an ancient pigment.⁸As shown in Fig. 1, the crystal structure of EB consists of corner shared SiO₄ tetrahedra rings linked together by square-planar coordinated copper, and the copper silicates blocks are interleaved by calcium atoms. Copper silicates with elements other than Ca between the blocks such as BaCuSi₄O₁₀ and SrCuSi₄O₁₀ were also reported.⁹Open in a separate windowFig. 1Atomistic model for the CaCuSi₄O₁₀ (Egyptian blue) structure. Ca, Cu, Si, and O atoms are represented by blue, red, yellow, and black spheres, respectively.With the need to find new electrode materials for Li-ion batteries, we decided to explore EB as an electrode material. The structure contains two hypothetical sites for hosting Li by intercalation or insertion. The first site is similar to other silicate structures that can host lithium ions (e.g. Li₂MnSiO₄).¹⁰ In EB, this storage mechanism would be an intercalation of the lithium ions between the silicate layers by replacing Ca or coexisting with it. The coexistence of Ca and Li ions has been reported before by Pruvost et al. for graphite.¹¹ In addition, Li insertion in the silicate rings similar to Li₃VO₄ is the second hypothetical site.¹²Herein we show that, using X-ray and infrared based techniques, none of these hypothetical mechanisms take place but rather that EB goes through a conversion reaction that irreversibly changes the crystalline structure into amorphous composite of oxides decorated with copper nanoclusters. Conversion reactions are of interest to the battery community due to their high capacities and low costs.¹³X-ray diffraction (XRD) pattern of the as-received EB powder (Fig. 2a) shows a predominantly single phase of CaCuSi₄O₁₀ [PDF#12-0512].^1,3 Scanning electron microscopy (SEM) image of the as received EB (Fig. 2b-​-1)1) shows large particles (10–100 μm). The galvanostatic cycling (at 100 mA g⁻¹), of the as-received EB (Fig. 2c) shows 1^st cycle lithiation and delithiation capacities of 290 and 100 mA h g⁻¹, respectively. Both values are lower than that of commercial graphite anodes (372 mA h g⁻¹). This can be explained by the large bulky particles (>45 μm) of the as-received EB.Open in a separate windowFig. 2(a) XRD patterns for the as-received EB powder (red) and after milling (blue). The black markers represent the diffraction peaks position of CaCuSi₄O₁₀ [PDF#12-0512]. The insets are photographs of the EB powders as received (bottom) and after milling (top) (b) SEM images of EB before (1) and after (2) milling. (c) The voltage profiles of EB at 100 mA g⁻¹ before and after milling. (d) Nyquist plots before and after milling. The inset is the equivalent electrical circuit used to fit EIS data.To reduce the particle size (PS), the as-received EB was milled as described in the ESI.^† XRD pattern of the milled powder showed broader diffraction peaks that can be explained by the smaller PS, which agrees with the SEM image in Fig. 2b-2. Using Scherrer formula,¹⁴ the average PS after milling was estimated to be ∼100 nm. It is worth noting that by milling the EB, its color lightened up significantly (insets in Fig. 2a), which can be explained by the PS reduction. Peak broadening, splitting and impure peaks measured after the milling process can be attributed to changes in the lattice structure induced by the stress of mechanical milling.¹⁵ As shown in Fig. 2c, this simple milling step resulted in a large increase in the capacity. Thus, the rest of the study was performed using milled EB. At 100 mA g⁻¹, the 1^st lithiation capacity increased from 290 to 520 mA h g⁻¹ by milling and the delithiation capacity was 150 mA h g⁻¹ after milling. The smaller PS facilitates ion transport by reducing the diffusion length of the ions and increases specific surface area.^16–18To understand the kinetics before and after milling, electrochemical impedance spectroscopy (EIS) was conducted at 5 mV vs. Li/Li⁺. As shown in Fig. 2d, Nyquist plots before and after milling exhibit the similar shape, including two semicircles in the high and medium/low frequency regions, and a straight line at the low frequency range. The EIS data were fitted using the inset equivalent electrical circuit, indicating two time constants of solid electrolyte interphase (SEI) and charge transfer processes. The same system resistances of 3.7 Ω and the resistance from SEI of 18.7 Ω are delivered before and after milling. However, the charge transfer resistances are 19.6 and 8.9 Ω before and after milling, respectively. Combining the feature of charger transfer resistance and straight lines, we can clearly see the attribution to the phase transformation or the formation of new phases after intercalating Li⁺. In addition, the diffusion coefficients before and after milling are calculated to ∼1.3 × 10⁻¹² and 8.8 × 10⁻¹² cm² s⁻¹, respectively (details for the calculations can be found in the ESI and Fig. S1^†). The smaller charge transfer resistance and fast diffusion coefficient facilitate the better electrochemical performance after milling.The cyclic voltammogram of milled EB is shown in Fig. S2.^† Large irreversible lithiation peak below 0.3 V vs. Li/Li⁺ was observed and it is discussed in more details below. To further investigate the electrochemical performance of milled EB, galvanostatic cycling at 20, 100 and 500 mA g⁻¹ was conducted. A 1^st cycle lithiation capacities of 598, 504 and 267 mA h g⁻¹ were measured at 20, 100 and 500 mA g⁻¹, respectively (Fig. 3a). Even though the first cycle irreversibility is high due to irreversible reactions taking place in the 1^st cycle (discussed latter), the coulombic efficiencies after the first cycle were over 90% for the length of cycling. At 20 mA g⁻¹, reversible specific capacity of 210 mA h g⁻¹ was delivered. As shown in Fig. 3b, at 500 mA g⁻¹, a stable (over 1000 cycles) reversible capacity of 120 mA h g⁻¹ was achieved. It is worth noting that the coulombic efficiency was larger than 99.5% after 100 cycles. After 100 cycles, the capacity at 100 mA g⁻¹ increased with extended cycling (Fig. 3b and c). One explanation for this initial decrease in capacity can be attributed to extensive structural changes that occur during the first 100 cycles. As these changes begin to form a unique electrode structure, the reactivation of lithium sites is made possible and more active lithium sites over time are developed as observed in other electrode materials.^19–22 More work is needed to fully explain this observation.Open in a separate windowFig. 3(a) The voltage profiles of the EB at 20 mA g⁻¹ (red), 100 mA g⁻¹ (blue), and 500 mA g⁻¹ (black) (b) cycling performance of 100 mA g⁻¹ and 500 mA g⁻¹ (c) the voltage profile and number of lithium ions participating electrochemically per mole of EB of select cycles at 100 mA g⁻¹. The inset shows the lithiation and delithiation profiles for the first cycle.As shown in Fig. 4a, XRD pattern after the first lithiation cycle (discharging to 5 mV) showed no new peaks, beside the copper foil current collector peak, but rather a significant drop in the diffraction peaks intensities of the EB, almost vanished, without any measurable shift in peaks position, compared to pristine EB (i.e., before any electrochemical cycling). These results suggest that the material became completely amorphous after lithiation. Thus, we eliminated the possibility of either intercalation or insertion reactions taking place, but rather a conversion reaction that results in the amorphization of the material.Open in a separate windowFig. 4(a) XRD of pristine EB electrode on Cu foil (black) and fully lithiated electrode-discharged to 5 mV (red) and fully delithiated electrode-recharged back to 3 V (blue). (b) The FTIR spectra of pristine (black) and fully lithiated-discharged to 5 mV (red). (c) FTIR spectra of pristine (I), partially discharged to 0.3 V (II) and 0.075 V (III), and fully discharged to 5 mV (IV), and fully discharged then charged to 3 V (V) EB electrodes.To further understand this conversion reaction, FTIR was performed to characterize changes taking place during lithiation (Fig. 4b and c). Si–O–R bending and stretching vibrations usually appear at around 1000–1100 cm⁻¹ (with a shoulder around 1200 cm⁻¹).²³ The absorption peaks at both 1000 and 1050 cm⁻¹ can be attributed to Si–O–Si and Cu–O–Si bonds in the EB and presumably with the peak at 1160 cm⁻¹.²⁴ This peak splitting behavior has been observed in other silicates such as CuMeSiO₂.^25,26 The absorption peak at 880 cm⁻¹ before lithiation is associated with the C–C–C asymmetric stretching vibration and CF stretching vibration of the polyvinylidene fluoride (PVDF) binder.²⁷ The peaks emerged after lithiation at 845 and 1407 cm⁻¹ indicate the formation of Li₂CO₃ during SEI formation.²⁸ Normally, absorption peaks around 660 cm⁻¹ represent Cu(i)–O vibrations, but in the case of the EB, the spacing in the crystal structure causes the Cu(ii)–O bonds to have a wavenumber indicative of Cu(i)–O bonds (i.e. 650 cm⁻¹).²⁹In the fully lithiated state, a noticeable loss in the Cu–O bonding peak (at 650 cm⁻¹) has occurred along with the formation of absorption peaks at 845 and 938 cm⁻¹ and smaller peaks after 1300 cm⁻¹. The peaks at 845 cm⁻¹ and at 1300 cm⁻¹ and beyond can be attributed to the formation of organo-lithium compounds and Li₂CO₃ and SEI formation.³⁰ The disappearance of the Cu–O peak at 650 cm⁻¹ indicates vanishing the Cu–O bonds through an irreversible reaction. The change in the relative ratio between the peaks at 1000 and 1050 cm⁻¹ and the formation of a peak at 938 cm⁻¹ indicates that Si–O–Si and Si–O–Cu bonds are giving way to the formation of Li–Si and Li–Si–O bonds.³⁰We used X-ray absorption spectroscopy (XAS) to study the changes in the Cu K-edge peak of EB upon lithiation. Fig. 5a shows the XAS spectra of pristine EB compared to CuO and Cu foil. The Cu K-edge spectra features of pristine EB are comparable to what was reported by Pagès-Camagna et al. for archaeological EB.³¹ As shown in Fig. 5a, oxidation state of Cu in EB is very close to what we measured for copper oxide, CuO, reference material, but after fully lithiation the edge peak was shifted to lower oxidation state and became comparable to copper metal, which agrees with the FTIR results discussed above. After delithiation, the Cu K-edge did not change any further and remained comparable to Cu metal. Fig. 5b shows the Cu K-edge for electrodes lithiated from OCV to 0.3 V and 0.075 V vs. Li/Li⁺. A gradual shift toward lower oxidation state from pristine EB (Fig. 5a) to lithiated to 0.3 V then 0.075 V (Fig. 5b) and finally fully lithiated to 5 mV (Fig. 5a) was observed. The absence of pronounced peaks around 8994 eV and 9003 eV in the fully lithiated EB compared to what was observed for Cu foil can be explained by formation of atomic clusters of Cu that result in smearing these peaks.³²Open in a separate windowFig. 5Cu K-edge XAS for EB electrodes compared to reference Cu foil and CuO. (a) Pristine EB electrode (blue), fully lithiated electrode by discharging to 5 mV (purple) and fully delithiated electrode by recharging back to 3 V (gray) compared to Cu foil reference (black) and CuO reference (red). (b) Partially lithiated EB electrodes by discharging to 0.3 V (purple) and 0.075 V (blue) compared to Cu foil reference (black) and CuO reference (red).There are definitive evidences from the XRD, FTIR and XAS results that the structure of the EB is changing irreversibly during the initial lithiation, and three proposed reactions are taking place to explain the three slopes observed in the voltage profile of the 1^st cycles (Fig. 3a and c). First, the SEI is formed by electrolytic reduction of carbonate solvents. This reaction can clearly be seen in FTIR spectra upon lithiation by the presence of newly formed lithium compounds such as LiF and Li₂CO₃, which make up the SEI layer.³³ Moreover, the Cu–O bonds are breaking and forming copper metal that was confirmed using XAS. During this process, a combination of copper nanoclusters and Li₂O is formed.^34,35 The new-formed copper nanoclusters can act as conductive material enhancing the electrochemical performance. Finally, lithium reacts reversibly with the conversion reaction products (e.g., CaSiO₃, SiO₂ or SiO_x). The exact reversible reaction mechanism of the mixed oxides with Li is not well understood. Even for SiO₂, its lithiation mechanism is still debated.³⁶To find out why does the lithiation of EB is through a conversion reaction, we employed density functional theory (DFT) calculations to determine the energies of the two following reactions:8Li + 2CaCu(Si₂O₅)₂ = Li₂Si₂O₅ + 2CaSiO₃ + 3Li₂SiO₃ + Si + 2Cu,12Li₄CaCu(Si₂O₅)₂ = Li₂Si₂O₅ + 2CaSiO₃ + 3Li₂SiO₃ + Si + 2Cu.2As shown in Table S1,^† both reactions are notably exothermic, indicating that both a mixture of Li and EB (with a 4 : 1 mol ratio) or a well relaxed imaginary lithiation compound Li₄CaCu(Si₂O₅)₂ (Fig. S3^†) are obviously not stable thermodynamically. In both reactions, the reactants tend to decompose into the resultant phases that are all low temperature stable phases determined using Pymatgen and the Materials Project.^37,38 This explains why a conversion reaction takes place rather than insertion/intercalation in the EB crystal.Confirmed by XRD, FTIR and XAS, as Li-ion electrode material, Egyptian blue hosts Li through a conversion reaction that results in a composite of copper nanoclusters and amorphous oxides. DFT suggest that the lithiated crystal of EB is thermodynamically unstable and therefore conversion reaction takes place. In lieu of the newly formed amorphous composite, EB exhibits a decent reversible capacity (∼210 mA h g⁻¹ at 20 mA g⁻¹), high coulombic efficiencies, and an increase in capacity with cycling time. At 500 mA g⁻¹ a reversible capacity of 120 mA h g⁻¹ was stable over 1000 cycles. Considering that this is the first report on using EB as electrode material, further optimization is inevitable and better electrochemical performance is possible. Also, the presence of Ca ions between the copper silicate layers suggests that EB could be used in Ca-ion batteries.     

Stabilization of beryllium-containing planar pentacoordinate carbon species through attaching hydrogen atoms

The diagonal relationship between beryllium and aluminum and the isoelectronic relationship between BeH unit and Al atom were utilized to design nine new planar and quasi-planar pentacoordinate carbon (ppC) species CAl_nBe_mH_x^q (n + m = 5, q = 0, ±1, x = q + m − 1) (1a–9a) by attaching H atoms onto the Be atoms in CAl₄Be, CAl₃Be₂⁻, CAl₂Be₃²⁻, and CAlBe₄³⁻. These ppC species are σ and π double aromatic. In comparison with their parents, these H-attached molecules are more stable electronically, as can be reflected by the more favourable alternative negative–positive–negative charge-arranging pattern and the less dispersed peripheral orbitals. Remarkably, seven of these nine molecules are global energy minima, in which four of them are kinetically stable, including CAl₃Be₂H (2a), CAl₂Be₃H⁻ (4a), CAl₂Be₃H₂ (5a), and CAlBe₄H₄⁺ (9a). They are the promising target for the experimental realization of species with a ppC.

The destabilization issues, like high charges, small HOMO–LUMO gaps, and dispersed MOs, etc. can be eliminated via attaching hydrogen atoms.

Planar hypercoordinate carbon chemistry can be dated back to 1968 when Monkhorst proposed the first planar tetracoordinate carbon (ptC) in a transition state structure.¹ In 1970, Hoffmann and co-workers sponsored the project of stabilizing the ptC in equilibrium structure.² After the proposal of first ptC energy minimum 1,1-dilithiocyclopropane (C₃H₄Li₂) by Schleyer and Pople group in 1976,³ the most significant advance has been the conceptual extension of ptC to planar pentacoordinate or hexacoordinate carbon (ppC or phC),⁴ which promoted the extension of the number of planar coordination to values as high as ten and the central planar hypercoordinate atom from carbon to other main group elements or even transition metals.⁵ Nevertheless, the ppC are the most witnessed in all these extended type of structures,⁶ possibly due to the geometrically nice fit between the peripheral ligand atoms and center carbon and the electronically easy satisfaction of the 18 electron (18e) rule.In 2001, Wang and Schleyer reported a family of ppC-containing molecules, i.e. the milestone “hyparenes”, which were designed by substituting the –(CH)_n– moieties in aromatic or even anti-aromatic hydrocarbons with ppC building blocks –C₃B₃–, –C₂B₄–, and –CB₅–.^4b Subsequently, ppC species such as CCu₅H₅,⁷ CBe₅ and CBe₅⁴⁻,⁸ wheel-like C₂B₈, C₃B₉³⁺ and C₅B₁₁^+ 9 were also proposed. However, it is still unknown to date whether these small ppC clusters are global minima on their potential energy surfaces. Hence, people cannot evaluate their experimental viability. Nonetheless, situation changed when Zeng and Schleyer group reported the first ppC global minimum D_5h CAl₅⁺ in 2008, which features not only the 18e structure, but also the σ and π double aromaticity.¹⁰ Taking CAl₅⁺ as the seed structure and considering the periodicity as well as the diagonal relationship, etc., people had designed a series isoelectronic ppC structures, including CAl_nBe_m^1−m (m = 1–3),¹¹ CAl₄E⁺ (E = Al, Ga, In, Tl),¹² CBe₅E⁻ (E = Al, Ga, In, Tl),¹³ CAl₄TmX₂ (Tm = Ti, Zr, Hf; X = F, Cl, Br, I, and cyclopentadienyl anion),¹⁴ and CGa_nBe_m^1−m (n + m = 5, m = 1–4).¹⁵During the design of these species, when each Al or its heavier congeners was replaced by a Be atom, a negative charge was added to maintain the isoelectronic relationship. However, it would be hard to experimentally realize such ppC structures due to the high molecular charges and the exposure of the electron deficient metal atoms. As a result, people had tried to stabilize the highly negatively charged ppC ions through attaching the certain number of auxiliary atoms. For example, H atoms had been attached to CAl₄⁻ and CAl₄²⁻ in a joint experimental-theoretical study, leading to the new ptC species CAl₄H and CAl₄H⁻.¹⁶ For ppC species, the most studied seed structure should be CBe₅⁴⁻,⁸ which is even not an eligible minimum.¹⁷ Nevertheless, through attaching the auxiliary atoms, like alkali metals, hydrogen, halogens, and even transition metal gold, a family of ppC species can be designed, including CBe₅Li_nⁿ⁻⁴ (n = 1–5),^17a CBe₅H_nⁿ⁻⁴ (n = 2, 3, 5),^17b CBe₅E₅⁺ (E = F, Cl, Br, Li, Na, K),¹⁸ and CBe₅Au_nⁿ⁻⁴ (n = 2–5).¹⁹ In these ppC species, the auxiliary atoms play the roles of both compensating the deficient electrons and reducing the high negatively charges. Remarkably, H atoms were also employed to stabilize the ppC-containing C–Be double chain nanoribbon, where its CBe₅H₂ ppC unit possesses the similar electronic structures to that of CAl₅⁺.²⁰Very recently, using Li atoms to balance the high negative charge of Be-doped ppC structures results in neutral or mono-anionic species [(CAl₂Be₃)Li]^− 11b and CGa_nBe_mLi_m−1 (n + m = 5, m = 1–4),¹⁵ where Li atoms generally show high affinity to Be atoms. In addition, our recent study revealed that Be and H can be bounded together through favourable covalent and ionic bonding,^5c,21 thus H should be a better choice than Li as the auxiliary atoms around CBe₅ ppC core.^17b Herein, we wonder whether H atoms can be attached onto the Be-doped ppC structures to design the new ppC molecules possessing the molecular charges with ±1|e|, which facilitates the experimental generation and accurate calculation. The answer is positive and presented in this computational study, which designed nine new ppC molecules (see Fig. 1), in which four of them are kinetically stable global minima, thereby providing promising targets for experimental realization.Open in a separate windowFig. 1Optimized structures 1a–9a at the B3LYP/aug-cc-pVTZ level. Bond distances and NBO charges are given in black and italic blue fonts, respectively.     

Fish-scale-derived carbon dots as efficient fluorescent nanoprobes for detection of ferric ions

Herein, highly fluorescent carbon dots (CDs) with the incorporation of N and O functionalities were prepared through a facile and cost-effective hydrothermal reaction using fish scales of the crucian carp as the precursor. The as-prepared CDs exhibit strong fluorescent emissions at 430 nm with a relative quantum yield of 6.9%, low cytotoxicity, and robust fluorescence stability against photobleaching and good ionic strength. More significantly, the fluorescence of these CDs can be effectively and selectively quenched by Fe³⁺ ions, which enables the application of CDs as fluorescent Fe³⁺ nanoprobes with a linear range of 1–78 μmol L⁻¹ and a detection limit of 0.54 μmol L⁻¹. The proposed fluorescent CD nanoprobes can also be used for the assay of spiked Fe³⁺ in real water samples and human serums with high recoveries and low standard deviations. Hence, CDs can be potentially applied as safe and reliable fluorescent nanoprobes for environmental and clinical Fe³⁺ analyses.

Herein, highly fluorescent carbon dots (CDs) with the incorporation of N and O functionalities were prepared through a facile and cost-effective hydrothermal reaction using fish scales of the crucian carp as the precursor.

Fe³⁺ ions are a versatile chemical that have been widely used in the purification of drinking water, catalyzing certain reactions,^1,2 and electronic industry. In addition, Fe³⁺ is also an essential metal ion in hemoglobin, myosin, and cytochrome, which can regulate normal oxygen uptake, cellular metabolism, and enzymatic reactions in physiological DNA and RNA syntheses in human bodies.^3,4 The deficiency of Fe³⁺ causes various diseases such as iron deficiency, anemia, aplastic anemia, complicating further into limb weakness and reduced immune functions,⁵ whereas excessive Fe³⁺ can result in dysfunctions (kidney, heart, and liver), Alzheimer''s disease, and even cancer.^6–8 Hence, the accurate analysis of Fe³⁺ is of significant clinical importance. On the other hand, the overutilization of Fe resources and the discharge of Fe³⁺-containing sewage into aquatic environments result in a considerable threat to the natural environment; therefore, the accurate and reliable determination of Fe³⁺ in aquatic media is also imperative.⁹ Conventional Fe³⁺ detection techniques mainly include flame atomic absorption spectrophotometry (FAAS),¹⁰ inductively coupled plasma mass spectrometry (ICP-MS),¹¹ voltammetry,¹² and spectrometry.¹³ Although worthwhile detection accuracy can be achieved, these techniques usually require tedious sample preparation procedures or sophisticated instruments, which limit the rapid and handy analysis of Fe³⁺ in practice. Hence, it is essential to develop cost-effective analysis means for the quantitative detection of Fe³⁺.Because of its high sensitivity, rapid response, and simplicity in sample preparation, the fluorescent probe technique is recognized as an alternative means to quantitatively determine Fe³⁺ ions through fluorescence quenching interactions.^14–16 Currently, the major fluorescent probe materials for Fe³⁺ detection include organic fluorescent dyes,^17,18 noble metal nanoclusters,^19,20 and heavy-metal chalcogenide semiconductor quantum dots.^21,22 Nevertheless, organic fluorescent dyes suffer from photobleaching, and semiconductor quantum dots suffer from potential elemental toxicity and complicated size/morphology control issues. Therefore, it is essential to investigate alternative fluorescent probes for the sensitive and rapid detection of Fe³⁺ with high photostability, low cytotoxicity, and easy availability that can be suitable for widespread applications.Carbon quantum dots (CDs) represent a newly emerged fluorescent nanomaterial with the doping or decoration of heteroelements containing functionalities within/onto the tiny carbonaceous motif (below 10 nm).²³ Besides the high fluorescence intensity, CDs also exhibit high solubility, robust chemical inertness, high resistance toward photobleaching, and good biocompatibility; hence, CDs can serve as efficient and safe fluorescent nanomaterials for bioimaging,²⁴ anti-faking,²⁵ biosensing,²⁶ and photocatalysis.²⁷ In particular, CDs can serve as electron donors in contact with guest acceptors through electrostatic or coordination interactions: the excitation state electrons of CDs can be nonradiatively transferred to the guest analytes, and therefore, the fluorescence of CDs is effectively quenched. Hence, CDs are widely investigated as credible sensing nanoprobes for the analyses of various guest ions/molecules with high sensitivities and reliabilities.In general, CDs can be prepared through top-down and bottom-up strategies. The former approach mainly includes the chemical or electrochemical cutting of graphitic motifs (graphite, carbon nanotubes, or graphene) into tiny nanodots with tunable bandgaps, but this approach commonly necessitates harsh synthesis conditions and modification of functionalities to tune the electron structure, structural energy traps, and further the fluorescent activities. In contrast, the latter approach can more easily yield CDs through the pyrolysis, solvothermal, or wet-chemical condensation of small molecular organics, polymers, and other easily available biomass low-cost raw materials, which cater toward the cost expectation for practical applications. Up to now, various natural biomass precursors, such as fruit juices,^28–30 vegetables,^31,32 grass,³³ milk,³⁴ beer,³⁵ and even waste³⁶ have been successfully employed to fabricate fluorescent CDs. Fish scale is a commonly discarded biomass waste containing rich proteins, which is considered to be a potential precursor for the cost-effective preparation of CDs in a safe manner. The further utilization of the afforded fluorescent CDs as fluorescent nanoprobes for Fe³⁺ detection deserves to be investigated.In this work, the easily available fish scales of the crucian carp was employed as a precursor for the hydrothermal synthesis of CDs. The afforded water-soluble CDs exhibit strong bluish fluorescence with a quantum yield (QY) of 6.9%, high resistances against photobleaching, high ion strength, and low cytotoxicity. The fluorescence of CDs can be effectively quenched by Fe³⁺ ions via static quenching, which facilitates its application as fluorescent Fe³⁺ nanoprobes with a wide linear detection range and low detection limit. Such CD-based fluorescent Fe³⁺ nanoprobes can be further used in the determination of real water samples and human serum with high reproducibility, exhibiting their potential as reliable fluorescent nanoprobes for environmental and clinical Fe³⁺ detection.     

Characterization of biosurfactant lipopeptide and its performance evaluation for oil-spill remediation

Biosurfactant lipopeptide is a promising dispersant over varieties of chemical ones in oil-spill remediation. The toxicity, biodegradability and performance of the biosurfactant lipopeptide are studied in this paper.

Biosurfactant lipopeptide is a promising dispersant over varieties of chemical ones in oil-spill remediation.

Dispersants were globally applied to physico-chemically enhance the dispersion of oil in water and were assumed to stimulate oil biodegradation by indigenous microorganisms and to reduce the environmental impact of oil spills.^1,2 Since the 1960s,^3,4 chemical dispersants have been applied as an emergency response to oil spills in marine ecosystems,⁵ and have showed effectiveness at removing oil slicks from the coast.^3,6,7 However, most of the chemically synthesized dispersants are inherently toxic to various aquatic species and hardly biodegradable in the natural environment.^2,8 The application of chemical dispersants in the 2010 Gulf of Mexico oil spill also raised concerns regarding the toxicity and the potential environmental impact,^9,10 and caused a debate about the effectiveness of chemical dispersants on the rates of oil biodegradation.¹¹ Biosurfactants are promising dispersants in oil-spill remediation, owning to their environmentally friendly and biodegradable properties.¹² Chemical surfactants could be replaced with biosurfactants and this change would diminish the environmental impact of traditional dispersants.^8,13,14 Lipopeptide produced by microorganisms is one of the representative biosurfactants and has showed great potential applications in food,¹⁵ medicine,¹⁶ microbial enhanced oil recovery,¹⁷ and other fields.¹⁸ Nevertheless, the knowledge about the application of biosurfactant lipopeptide in marine oil-spill remediation is still limited.In the present work, the dispersion effectiveness, aquatic toxicity, biodegradability and environmental compatibility of the biosurfactant lipopeptide were determined using recognized standardized methods,^19–23 and the biosurfactant lipopeptide used as a bio-dispersant for marine oil-spill remediation were studied, which is, to the best of our knowledge, the first report about biosurfactant lipopeptide used in oil-spill remediation.The lipopeptide samples were isolated from cell-free broth of B. subtilis HSO121 at our laboratory.²⁴ The typical chemical structure of the lipopeptide used in the study was shown in Fig. 1 and its critical micelle concentration (CMC) was 8.69 × 10⁻⁵ mol L⁻¹.Open in a separate windowFig. 1Typical chemical structure of lipopeptides (a) and the surface tensions of lipopeptides respect to concentrations (b).Dispersion effectiveness (DE) of lipopeptides was examined at different surfactant-to-oil ratios (SORs), temperatures, pH values, and salinities. It indicated in Fig. 2 that DE of lipopeptides reached 70.23% at SORs of 1 : 10 (w/w) at 25 °C, pH 7 and the present of 3% NaCl (w/v). It should be noticed that the DE of lipopeptides was almost kept when SORs dropped to 1 : 250 w/w. The increase in DE with increasing SORs can be attributed to the generation of emulsions with smaller droplets and lower rising velocity.² Sharp drop off in DE was observed for lipopeptides when SORs below 1 : 500 (w/w), and DE value was 36.45% at an extreme SORs of 1 : 1250 (w/w). It had been reported that the abrupt decline for 80 : 20 lecithin : Tween 80 (w/w) surfactant happened when SORs below 1 : 100 v/v, from 77% (SORs 1 : 100 v/v) to 15% (SORs 1 : 200 v/v),²⁵ indicating a lower SOR in lipopeptides usage could reach its maximum effectiveness. Lipopeptides exhibited >70% DE values with temperatures ranged from 15 °C to 25 °C, and an increasing DE values when pH values raised, the largest DE was 77.45% at pH 11. DE of lipopeptides increased from 56.12% to 71.14% with increase in salinity. Higher DE at higher salinity was observed for anionic biosurfactants, which can be attributed to the electrostatic repulsion between polar head groups reduced by ions, and a close-packed arrangement of surfactant molecules at the oil–water interface were formed.²Open in a separate windowFig. 2The dispersion effectiveness (DE) of lipopeptides under different SORs (), temperatures (), pH values () and salinities ().Mortalities of zebrafish under different concentrations of different surfactants were shown in Fig. 3. It was evident that the toxicity of lipopeptides was far less than those of sodium dodecyl sulfate (SDS) and 3-(N,N-dimethyl palmityl ammonio) propane sulfonate (Betaine). The 24 h median lethal concentration (LC₅₀) values were calculated and showed in 26 in which an 96 h LC₅₀ of 1.9 mg L⁻¹ for Cyprinodon variegatus was reported. Low toxicities of lipopeptides on whiteleg shrimp and copepods were also evaluated that the 96 h LC₅₀ of lipopeptides from Bacillus sp. GY19 were 1050 mg L⁻¹ and 1174 mg L⁻¹, respectively.²⁷Open in a separate windowFig. 3Mortality of zebrafish (M_z) after a 24 h exposure to Betaine, SDS or lipopeptides.Ecotoxicity of tested surfactants to the zebrafish

Synthesis of 3-selanylbenzo[b]furans promoted by SelectFluor®

A simple and practical protocol for the synthesis of 3-selanyl-benzo[b]furans mediated by the SelectFluor® reagent was developed. This novel methodology provided a greener alternative to generate 3-substituted-benzo[b]furans via a metal-free procedure under mild conditions. The intramolecular cyclization reaction was carried out employing an electrophilic selenium species generated in situ through the reaction between SelectFluor® and organic diselenides. The formation of this electrophilic selenium species (RSe-F) was confirmed by heteronuclear NMR spectroscopy, and its reactivity was explored.

This novel methodology provided a greener alternative to generate 3-substituted-benzo[b]furans mediate by Selectfluor® reagent. The formation of this electrophilic selenium species (RSe-F) was confirmed by heteronuclear NMR spectroscopy.

The benzo[b]furan scaffold is an important structural motif that is present in natural products and in synthetic compounds with therapeutic proprieties.¹ Substituted benzo[b]furans have shown a broad range of biological activities,² being found in a variety of pharmaceutical targets, such as Viibryd® and Ancoron® (Fig. 1).³ These drugs are used for treatment of depression and for cardiac arrhythmias, respectively. An efficient method to obtain substituted benzo[b]furans is the intramolecular cyclization reaction between 2-alkynylphenol or 2-alkynylanisole derivatives with different electrophilic species to generate a wide variety of 3-substituted-benzo[b]furans. This strategy is especially useful because of the atom-economic synthesis under mild conditions.⁴Open in a separate windowFig. 1Substituted benzo[b]furans in commercial drugs.Organoselenium compounds have attracted great interest due the large number of biological applications and their versatile reactivity.⁵ From a synthetic point of view, the ease cleavage of the Se–Se bond in diselenide compounds can generate species with different reactivity, as radical, electrophile, and nucleophile. This ample usefulness becomes the diselenides in key synthetic intermediates to introduce selenium moiety in organic compounds or to catalyse organic transformations.^5,6Despite the recent advances in the synthesis of 3-selanyl-benzo[b]furans, new electrophiles and reactional conditions were explored (Scheme 1).^{7–9,11–15} Initially, the establishing work by Larock and co-workers toward the synthesis of 3-selanyl-benzo[b]furans through the intramolecular cyclization of 2-(phenylethynyl)anisole with PhSeCl in CH₂Cl₂ at room temperature.⁷ In 2009, Zeni and co-workers demonstrated the synthesis of 3-selanyl-benzo[b]furans employing PhSeBr as an active electrophile.⁸ A pioneering protocol was reported by Zeni and co-workers, which employed FeCl₃ (1.0 equiv.) and diorganyl diselenides in CH₂Cl₂ at 45 °C.⁹ Additionally, Lewis acids have been used as effective catalysts in Se–Se bonding cleavage to access functionalized selenium compounds.¹⁰ Afterward, alternative methods were developed, such as the synthesis of 3-selanyl-benzo[b]furans mediated by PdCl₂/I₂, I₂/water, and CuI (1.5 equiv.).^11–14 More recently, Liu and co-workers reported a radical cyclization reaction using selenium powder as selenium source and AgNO₃ as catalyst in DMSO at 100 °C.¹⁵Open in a separate windowScheme 1Methodologies to prepare 3-selanyl-benzo[b]furans.Although, there are different methodologies to prepare 3-selanyl-benzo[b]furans and other functionalized selenium compounds through the reaction between diselenides compounds with oxidant reagents or Lewis acids, alternative electrophilic selenium species should be employed to avoid metals and/or toxic reagents.^9–15 Furthermore, RSeCl and RSeBr,^7,8 obtained from the reaction of diselenides with SO₂Cl₂ (or Cl₂) and Br₂ respectively, are commercially available and largely used as selenylating agent. However, these species present a low stability under moisture, and the high nucleophilicity of chloride and bromide leaving groups can lead to undesirable side reactions.On the other hand, SelectFluor® is a versatile reagent used for different applications, such as fluorination reactions,¹⁶ C–H functionalization¹⁷ and organic function transfer.¹⁸ In addition, SelectFluor® has been used as an efficient method for intramolecular annulation reactions, due its higher reactivity.¹⁹ This ample application together with the desirable characteristics of the SelectFluor®, such as the higher stability, non-hydroscopic solid and hazard-free source of fluorine,²⁰ promoted new possibilities to investigate fluorine chemistry. In 2004, Poleschner and Seppelt prepared PhSeF derivatives by the reaction between diorganyl diselenides and XeF₂ in CH₂Cl₂ as a solvent at −40 °C.²¹ The products were characterized by low-temperature ¹⁹F and ⁷⁷Se NMR, and it was the first confirmation of this type of electrophilic selenium compound. Although electrophilic selenium catalysis (ESC) with electrophilic fluoride reagents as oxidants has been demonstrated in the functionalization of alkenes,²² fewer knowledge about the reactivity of this selenium electrophilic species is available in the literature.²³Based on the development of new electrophilic selenium reagents,^9–14,24 herein, we describe a metal-free synthesis of 3-selanyl-benzo[b]furans under mild conditions using this very reactive electrophilic selenium species (RSe-F), generated in situ at room temperature by the reaction of diorganyl diselenides with SelectFluor® reagent (Scheme 1). Moreover, the higher reactivity of RSe-F species could be explored for the insertion of selenium moiety in other building blocks because the environmentally friendly reactional condition, and the replacing chlorine and bromine by the non-nucleophilic fluorine counter ion, can partially circumvented some side reactions.     

Insight into trifluoromethylation – experimental electron density for Togni reagent I

3,3-Dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole represents a popular reagent for trifluoromethylation. The σ hole on the hypervalent iodine atom in this “Togni reagent” is crucial for adduct formation between the reagent and a nucleophilic substrate. The electronic situation may be probed by high resolution X-ray diffraction: the experimental charge density thus derived shows that the short intermolecular contact of 3.0 Å between the iodine and a neighbouring oxygen atom is associated with a local charge depletion on the heavy halogen in the direction of the nucleophile and visible polarization of the O valence shell towards the iodine atom. In agreement with the expectation for λ3-iodanes, the intermolecular O⋯I–C_aryl halogen bond deviates significantly from linearity.

The experimentally observed electron density for the “Togni reagent” explains the interaction of the hypervalent iodine atom with a nucleophile.

A “halogen bond” denotes a short contact between a nucleophile acting as electron density donor and a (mostly heavy) halogen atom as electrophile;^1,2 halogen bonds are a special of σ hole interactions.^3–5 Such interactions do not only play an important role in crystal engineering;^6–11 rather, the concept of a nucleophile approaching the σ hole of a neighbouring atom may also prove helpful for understanding chemical reactivity.The title compound provides an example for such a σ hole based reactivity: 3,3-dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole, 1, (Scheme 1) commonly known as “Togni reagent I”, is used for the electrophilic transfer of a trifluoromethyl group by reductive elimination. The original articles in which the application of 1¹² and other closely related “Togni reagents”¹³ were communicated have been and still are highly cited. Trifluoromethylation is not the only application for hypervalent iodine compounds; they have also been used as alkynylating¹⁴ or azide transfer reagents.^15,16 The syntheses of hypervalent iodanes and their application in organofluorine chemistry have been reviewed,¹⁷ and a special issues of the Journal of Organic Chemistry has been dedicated to Hypervalent Iodine Catalysis and Reagents.¹⁸ Recently, Pietrasiak and Togni have expanded the concept of hypervalent reagents to tellurium.¹⁹Open in a separate windowScheme 1Lewis structure of the Togni reagent, 1.Results from theory link σ hole interactions and chemical properties and indicate that the electron density distribution associated with the hypervalent iodine atom in 1 is essential for the reactivity of the molecule in trifluoromethylation.²⁰ Lüthi and coworkers have studied solvent effects and shown that activation entropy and volume play relevant roles for assigning the correct reaction mechanism to trifluoromethylation via1. These authors have confirmed the dominant role of reductive elimination and hence the relevance of the σ hole interaction for the reactivity of 1 in solution by ab initio molecular dynamics (AIMD) simulations.^21,22 An experimental approach to the electron density may complement theoretical calculations: low temperature X-ray diffraction data of sufficient resolution allow to obtain the experimental charge density and associate it with intra- and inter-molecular interactions.^23–25 Such advanced structure models based on aspherical scattering factors have also been applied in the study of halogen bonds.^26–30 In this contribution, we provide direct experimental information for the electronic situation in Togni reagent I, 1; in particular, we analyze the charge distribution around the hypervalent iodine atom.Excellent single crystals of the title compound were grown by sublimation.^§ The so-called independent atom model (IAM), i.e. the structure model based on conventional spherical scattering factors for neutral atoms, confirms the solid state structure reported by the original authors,¹² albeit with increased accuracy. As depicted in Fig. 1, two molecules of 1 interact via a crystallographic center of inversion. The pair of short intermolecular O⋯I contacts thus generated can be perceived as red areas on the interaction-sensitive Hirshfeld surface.³¹Open in a separate windowFig. 1Two neighbouring molecules of 1, related by a crystallographic center of inversion. The short intermolecular I⋯O contacts show up in red on the Hirshfeld surface³² enclosing the left molecule. (90% probability ellipsoids, H atoms omitted, symmetry operator 1 − x, 1 − y, 1 − z).The high resolution of our diffraction data ^¶ for 1 allowed an atom-centered multipole refinement^33,34 and thus an improved model for the experimental electron density which takes features of chemical bonding and lone pairs into account. Fig. 2 shows the deformation density, i.e. the difference electron density between this advanced multipole model and the IAM in the same orientation as Fig. 1.^|| The orientation of an oxygen lone pair (blue arrow) pointing towards the σ hole of the heavy halogen in the inversion-related molecule and the region of positive charge at this iodine atom (red arrow) are clearly visible. Single-bonded terminal halides are associated with one σ hole opposite to the only σ bond, thus resulting in a linear arrangement about the halogen atom. Different geometries and potentially more than a single σ hole are to be expected for λ3-iodanes such as our target molecule, and as a tendency, the resulting halogen bonds are expected to be weaker than those subtended by single-bonded iodine atoms.³⁵ In agreement with these theoretical considerations, the closest I⋯O contacts in 1 amount to 2.9822(9) Å. This distance is significantly shorter than the sum of the van-der-Waals radii (I, 1.98 Å; O, 1.52 Å (ref. 36)) but cannot compete with the shortest halogen bonds between iodine and oxygen^37,38 or iodine and nitrogen.^39–41Fig. 1 and ​and22 show that the C_aryl–I⋯O contacts are not linear; they subtend an angle C10–I1⋯O1′ of 141.23(3)° at the iodine atom. On the basis of theoretical calculations, Kirshenboim and Kozuch³⁵ have suggested that the split σ holes should be situated in the plane of the three substituents of the hypervalent atom and that halogen and covalent bonds should be coplanar. Fig. 3 shows that the C_aryl–I⋯O interaction in 1 closely matches this expectation, with the next oxygen neighbour O1′ only 0.47 Å out of the least-squares plane through the heavy halogen I1 and its three covalently bonded partners C1, O1 and C10.Open in a separate windowFig. 2Deformation density for the pair of neighbouring molecules in 1; the dashed blue and red arrows indicate regions of opposite charge. (Contour interval 0.10e Å⁻³; blue lines positive, red lines negative, green lines zero contours, symmetry operator 1 − x, 1 − y, 1 − z).Open in a separate windowFig. 3A molecule of 1, shown [platon] along O1⋯C1, and its halogen-bonded neighbour O1ⁱ. Symmetry operator 1 − x, 1 − y, 1 − z.The Laplacian, the scalar derivative of the gradient vector field of the electron density, emphasizes local charge accumulations and depletions and it allows to assess the character of intra- and inter-molecular interactions. A detailed analysis of all bonds in 1 according to Bader''s Atoms In Molecules theory⁴² is provided in the ESI.^‡ We here only mention that the electron density in the bond critical point (bcp) of the short intermolecular I⋯O contact amounts to 0.102(5)e Å⁻³; we are not aware of charge density studies on λ3-iodanes, but both the electron density and its small positive Laplacian match values experimentally observed for halogen bonds involving O and terminal I in the same distance range.⁴³The crystal structure of 1 necessarily implies additional contacts beyond the short halogen bond shown in Fig. 1 and ​and2.2. The shortest among these secondary interactions is depicted in Fig. 4: it involved a non-classical C–H⋯F contact with a H⋯F distance of 2.55 Å.Open in a separate windowFig. 4C–H⋯F contact in 1; additional information has been compiled in the ESI.^‡ Symmetry operators i = 1 − x, 1 − y, 1 − z; ii = 1 − x, 1 − y, −z.The topological analysis of the experimental charge density reveals that this non-classical C–H⋯F hydrogen bond and all other secondary contacts are only associated with very small electron densities in the bcps. Table S8 in the ESI^‡ provides a summary of this analysis and confirms that the I⋯O halogen bond discussed in Fig. 1 and ​and22 represents by far the most relevant intermolecular interaction.The relevance of this halogen bond extends beyond the crystal structure of 1: Insight into the spatial disposition of electrophilic and nucleophilic regions and hence into the expected reactivity of a molecule may be gained from another electron-density derived property, the electrostatic potential (ESP). The ESP for the pair of interacting molecules in 1 is depicted in Fig. 5.Open in a separate windowFig. 5Electrostatic potential for a pair of molecules in 1 mapped on an electron density isosurface (ρ = 0.5e Å⁻³; program MoleCoolQt^44,45). Fig. 5 underlines the complementary electrostatic interactions between the positively charged iodine and the negatively charged oxygen atoms. One can easily imagine to “replace” the inversion-related partner molecule in crystalline 1 by an incoming nucleophile.The ESP tentatively obtained for a single molecule in the structure of 1 did not differ significantly from that derived for the inversion-related pair (Fig. 5), and even the results from theoretical calculations in the gas phase for an isolated molecule²⁰ are in good qualitative agreement with our ESP derived from the crystal structure. In the absence of very short contacts, polarization by neighbouring molecules only has a minor influence on the ESP. The experimentally observed electron density matches the proven reactivity for the title compound, and we consider it rewarding to extend our charge density studies on related hypervalent reagents.     

Determination of berberine in Rhizoma coptidis using a β-cyclodextrin-sensitized fluorescence method

In this work, a method for the determination of berberine in Rhizoma coptidis using β-cyclodextrin-sensitized fluorescence technology is established. Berberine is the main extract of Rhizoma coptidis, a medicinal material, which causes an envelope reaction with β-cyclodextrin to generate fluorescence sensitization. In the environment of its own aqueous extract, with 0.0065 mol L⁻¹ of β-cyclodextrin, a fluorescence excitation wavelength (λ_ex) of 345 nm and an emission wavelength (λ_em) of 540 nm were selected to avoid interference from other distractors. The fluorescent sensor for the detection of berberine exhibits a low limit of detection (3.59 × 10⁻⁹ mol L⁻¹) and a wide linear range from 2.7 × 10⁻⁷ mol L⁻¹ to 2.7 × 10⁻⁶ mol L⁻¹. Our sensor can be also used to detect berberine in real medicinal materials. The content of berberine in Rhizoma coptidis medicinal material was found to be 7.60% using this method with an average recovery rate of 99.5%. The result obtained by thin-layer chromatography with fluorescence detection was 7.61%, which is consistent with the result from the β-cyclodextrin sensitized fluorescence method. This method is simple and environmentally friendly with high sensitivity and good selectivity and gives reliable results, which is promising for practical application.

In this work, a method for the determination of berberine in Rhizoma coptidis using β-cyclodextrin-sensitized fluorescence technology is established.

Rhizoma coptidis¹ is the dried rhizome of Coptis chinensis (Chuan Lian), Coptis chinensis (Ya Lian) or Yunlian (Chuan Lian), and is an important traditional Chinese medicine commonly used in clinics. Rhizoma coptidis has the functions of clearing heat and dampness, purging fire and detoxifying.² Its active ingredients are isoquinoline alkaloids,³ such as berberine hydrochloride (also known as berberine), coptisine hydrochloric (also known as coptisine), palmatine hydrochloride (also known as palmatine), and jatrorrhizine hydrochloride (also known as jatrorrhizine). Berberine is the main effective component of Rhizoma coptidis. Antibacterial,⁴ antiviral,⁵ and anti-diabetic⁶ effects of berberine have been reported, as well as its use for the treatment of liver cancer .⁷ Thus, the content of berberine is critical to the quality control of Rhizoma coptidis.At present, many methods are used for the determination of berberine in Rhizoma coptidis, such as UV-vis spectroscopy, capillary electrophoresis with field amplified sample stacking, high-performance liquid chromatography, high-performance liquid chromatography-mass spectrometry, near-infrared spectroscopy, proton nuclear magnetic resonance and nuclear magnetic resonance proton spectroscopy.^8–14 However, there are no relevant reports on the systematic study of fluorescence spectroscopy for berberine. Fluorescence analysis is the direct measurement of the secondary light emission of a substance. Fluorometric analysis has the advantages of high sensitivity and good selectivity compared to photometric methods and is simple, rapid and inexpensive compared to chromatographic methods. It is gradually receiving increasing attention for the analysis of proprietary Chinese medicines.^15–18Cyclodextrins (CDs), as shown in Fig. 1, are cyclic oligosaccharides consisting of (α-1,4)-linked α-d-glucopyranose units. The primary hydroxyl group extends at one end and the secondary hydroxyl group extends at the other end. Because the primary hydroxyl group can rotate freely to cover a part of the ring, while the secondary hydroxyl group is relatively rigid, CDs are not cylindrical but slightly tapered. The glycoside oxygen atom with an unbonded lone pair of electrons in the glucose group points to the center of the molecule, which has a high density. As a result of these factors, the center of the molecule is hydrophobic and the surface is hydrophilic. As shown in Fig. 1(b), it can match the size and cavity of the molecule, and the hydrophobic guest molecules form an inclusion complex.¹⁹ Cyclodextrin can form an inclusion complex with dapoxyl sodium sulfonate (DDS). A large fluorescence enhancement of DSS was observed upon formation of the inclusion complex.^20,21 The formation of inclusion complexes by cyclodextrin has the potential for tuning the optical properties of other chromophoric molecules by varying the cavity size of the macrocyclic host molecule.²²Open in a separate windowFig. 1Molecular structure (a) and molecular shape (b) of CDs.Berberine itself has weak fluorescence but produces stronger fluorescence when it forms an inclusion complex with other substances. Berberine can form an inclusion complex with cucurbit⁷ uril and produce a highly sensitive fluorescence response.²³ Berberine can form an inclusion complex with β-CD and a new spectrofluorimetric method for the determination of berberine in the presence of β-CD was developed. The linear range was 1.00–4.00 μg mL⁻¹ with a detection limit of 5.54 ng mL⁻¹.²⁴Both the collision between the fluorescent molecules and the quenchers around the fluorescent molecules can cause the inactivation of the fluorescent molecules and produce non-radiation. When the fluorescent molecule berberine enters the hydrophobic cavity of β-CD to form the inclusion compound, β-CD plays a shielding role, making berberine reduce the non-radiation deactivation process and quenching process, thus improving the fluorescence quantum yield.^25,26 It has been proved that both ends of the berberine molecule are matched with the pores of β-CD molecules, and 1 : 1 and 2 : 1 type inclusion compounds can be formed, as shown in Fig. 2.¹⁹ The experiment found that β-CD had no fluorescence enhancement effect on other isoquinoline alkaloids, so based on the sensitization and selectivity of β-CD for berberine, a new fluorescence analysis method for the determination of berberine content in the Chinese medicine Rhizoma coptidis was proposed.Open in a separate windowFig. 2Molecular structure of berberine (a) and models of the β-CD–BH inclusion mixtures: 1 : 1 inclusion mixture (b), 1 : 1 inclusion mixture (c), 2 : 1 inclusion mixture (d).In this work, a method for the determination of berberine in Rhizoma coptidis using β-cyclodextrin-sensitized fluorescence technology is established. Berberine is the main extract of Rhizoma coptidis medicinal material, which causes an envelope reaction with β-cyclodextrin to generate fluorescence sensitization. The fluorescent sensor for the detection of berberine exhibits a low limit of detection (1.33 ng mL⁻¹) and a wide linear range from 0.1 μg mL⁻¹ to 1.0 μg mL⁻¹. Our sensor can be also used to detect berberine in real medicinal materials. The content of berberine in Rhizoma coptidis medicinal material was found to be 7.60% with this method and the average recovery rate was 99.5%. The result obtained by thin-layer chromatography with fluorescence detection was 7.61%, which is consistent with the result from the β-cyclodextrin-sensitized fluorescence method.     

Ultra-high adsorption of cationic methylene blue on two dimensional titanate nanosheets

In this work, we examined the performance of 2D titanate nanosheets for dye adsorption. Their adsorption capacity for methylene blue (MB) is up to 3937 mg g⁻¹, which is more than 10 times higher than active carbon and occupies the highest place among all the reports.

2D titanate nanosheets show ultra-high adsorption for methylene blue.

Dyes are important materials in many industries such as the textile, paper, leather, printing, and plastic industries. However, the dye effluents released by these industries give rise to major environmental issues because the dyes are generally toxic and/or carcinogenic to human beings.¹ Dye removal from the wastewater is essential before release in these industries. There are several methods to remove dye contaminants from wastewaters, such as adsorption, coagulation, chemical oxidation, membrane separation processes, etc.¹ Among the technologies, the adsorption process has been generally considered to be the most efficient method of quickly lowering the concentration of dissolved dyes in an effluent.² Various adsorbents have been studied for dye removal purposes, such as activated carbons (ACs),^3–6 clay,^7,8 sawdust,⁷ beer brewery waste,² chitosan,⁹ metal oxides,^10–12 carbon nanotubes and graphene oxide.^13,14 A general strategy to enhance the dye adsorption is to increase the surface area of the adsorbents, so the nanosized materials have been attracting much attention for the dye removal application because they exhibit much large surface area.^14–20Two dimensional (2D) materials are gaining great attention since discovery of graphene.^21–23 The 2D materials consist of large number of groups, such as graphene, transition metal dichalcogenides (TMDs), transition metal oxides (TMOs), layered double hydroxides (LDHs), MXene et al.^21,24–26 One may divide the 2D materials into three groups based on the charge of the host layer: neutral, negatively charged, and positively charged. The charged 2D materials must couple with counterions to compensate the charges. As a sample, 2D titanium oxide, also called titanate nanosheet (TONS) is negatively charged. The TONS is generally synthesized by the chemical exfoliation.^27,28 These chemical exfoliated TONS is monolayer nanocrystals with crystallographic thickness of 0.75 nm.²⁹ The nanosheets in aqueous solution behave as individual molecular entities, and they are surrounded by positively charged ions (TBA⁺ and H⁺).^28,30,31 The colloidal system is governed by electrostatic interaction.³¹As an emerging class of materials, the charged 2D materials are potentially useful for treatment of the dyes with opposite charges. This has motivated us to study the performance of the titanate nanosheets, a charged 2D material, for dye treatment.We synthesized the titanate nanosheets via top down approach report by Sasaki et al.^32,33Fig. 1a and b show the XRD pattern and SEM image of the protonated titanate. The XRD pattern is consistent with the previous report by Sasaki et al.^32,33 The SEM image shows that the crystal size is up to 20 μm. Fig. 1c shows the UV-Vis spectrum of the exfoliated titanate nanosheets. The peak is located at 265 nm, which is consistent with the value in the previous reports.^28,29,34Fig. 1d shows the STEM image of the nanosheets obtained by chemical exfoliation, and provides an evidence for the successful exfoliation. The AFM image of the nanosheets transferred to Si substrate by simple adsorption shown in Fig. S1^† shows the lateral size of the adsorbed nanosheets is generally less than 1 μm.Open in a separate windowFig. 1The XRD (a), SEM image (b) of protonated titanate; UV-Vis spectrum (c) and STEM image (d) of 2D titanate nanosheets. The dash line in (c) indicates the position of 265 nm.We first evaluated the adsorption capacity of the exfoliated nanosheets comparing with its bulk counterparts K_0.8[Ti_1.73Li_0.27O₄] (KLTO) and its protonated form (HTO). Fig. 2a shows that the adsorption capacity and adsorption percentage of the TONS in the function of the initial concentration of MB. The observed adsorption capacity increased up to 2236 mg g⁻¹ as the initial MB concentration was 4000 mg l⁻¹. It is noted that this value is underestimated because we didn''t consider the MB loss in experimental process. We analyzed the adsorption isotherm by fitting it with the Freundlich and the Langmuir models and the fitting curves are shown in Fig. 2b. The Langmuir model was fitting better than the Freundlich model, and the correlation coefficients R² for the Langmuir and the Freundlich models are 0.97 and 0.93, respectively. The better Langmuir model fitting indicates that the adsorption of MB on TONS surface is homogenous.¹¹ The maximum adsorption capacity fitted by the Langmuir model is 3937 mg g⁻¹. This adsorption capacity is, to the best of our knowledge, the largest for the MB adsorption among all the reports (see Table S1 in the ESI^†).^1,15–17,35 However, the dye removal efficiency is low even at the low initial concentration. Only half even less dye can be adsorbed as shown in Fig. 2a. Fig. 2c shows the morphology of the TONS after the MB adsorption at initial concentration of 1000 mg l⁻¹. The 2D feature of the TONS is clearly presented, indicating its good structural stability. In contrast to the large adsorption capacity of the TONS, its layered counterparts KLTO and HTO show limited adsorption capacity as shown Fig. 2d (see the adsorption data for KLTO in Fig. S2^†). Our data show that the adsorption capacities of the KLTO and HTO are 3.45 and 2.63 mg g⁻¹, respectively.Open in a separate windowFig. 2(a) The adsorption capacity and adsorption efficiency of titanate nanosheets at different initial concentrations; (b) the adsorption isotherm fitting with the Freundlich and the Langmuir models; (c) the typical morphology of TONS after MB adsorption (the initial concentration at 1000 mg l⁻¹); (d) the UV-Vis spectra of MB solution with concentration of 10 mg l⁻¹ before and after HTO adsorption. Fig. 3a presents the UV-Vis spectra of the MB (10 mg l⁻¹) and the TONS solution centrifuged at different time. The TONS shows rapid adsorption of the MB molecules. The concentration of the MB monomers dropped to a negligible concentration within 1 min, and a new peak appeared at ∼575 nm (purple by naked eyes) which can be assigned to the trimer form of the MB.^36–38 It is noted that the adsorption of MB monomers onto the TONS is much faster than other material systems, such as clay and graphene oxide.^14,39 The reason beyond this phenomenon is probably due to the large surface area and high charge density. It has been reported that the external adsorption of the MB is a fast process comparing with the internal adsorption in the clay system.³⁹ In this work, the TONS were dispersed in aqueous solution, so the external adsorption was dominating in the adsorption process. Another interesting feature is that the concentration of the MB trimers increased as the time proceeded. Because the MB monomers had been adsorbed onto the TONS within 1 min, the increase of the MB trimer concentration after 1 min suggests that the MB trimers came from desorption of the MB from the TONS. We analyzed the kinetic of trimer formation by assuming that the MB⁺ ions participating formation of trimer are irrelevant to the kinetic of desorption of MB–TONS complex, and we found the third order kinetic model is the best fitting to the experimental data among first, second and third order kinetic models, see Fig. 3b. The third order kinetic model yielded the correlation coefficient value of 0.965, while the first and second order kinetic models yielded the correlation coefficient values of 0.909 and 0.948, respectively. The data suggest that recombinative molecular desorption occurred, that is, multiple MB molecules desorbed and formed trimers. The desorption of counterions on the TONS was also observed with the TBA⁺ ions because the TBA–TONS complex went through an ionization process in the course of time.⁴⁰ The data here suggest the MB–TONS complex also went through an ionization process in the course of time after the rapid combination.Open in a separate windowFig. 3(a) UV-Vis spectra of MB and titanate nanosheets solution centrifuged at different time. (b) The trimer adsorption intensity vs. t for describing the desorption kinetics of methylene blue (10 mg l⁻¹) on the TONS.We also investigated the effect of the initial concentration of the MB on adsorption. Fig. 4a shows the UV-Vis spectra of residual (equilibrium) solution at different initial concentrations. The data show that the trimers were the main product after desorption at low concentrations (<100 mg l⁻¹), while the dimers (peak position at ∼615 nm) appeared and became more and more pronounced at high concentrations (>300 mg l⁻¹). We plotted the intensity change of monomers and dimers in reference to trimers in Fig. 4b. It is clear that the intensity of monomers didn''t change significantly in reference to trimers, but the intensity of the dimers did when the initial concentration of the MB solution increased, especially at 500 mg l⁻¹. It is noted that one should not compare the absolute values instead of the trend in the curves. The right vertical axis in the Fig. 4b presents the ratio of active surface area of adsorbed MB : TONS in the function of initial MB concentration in our study. We calculated the values assuming the MB molecules have only one side active to occupy the surface of the TONS. Fig. 4b shows that the number of the adsorbed MB⁺ ions was sufficient to cover the entire TONS surface and the second layer of the MB⁺ formed on the TONS surface when the initial concentration of MB solution reached 300 mg l⁻¹. Taking into consideration that the dimer started forming at the initial concentration of 300 mg l⁻¹, (see Fig. 4a) and the dimer formation enhanced further as the MB⁺ concentration increased, these data suggest that the formation of first layer MB⁺ on TONS surfaces suppresses the formation of trimers and promotes the formation of dimers. This is reasonable because the formation of first layer MB⁺ reduces the charge density of the MB–TONS complex. Bujdák et al. report that the high charge density of clay induces high order agglomeration of MB dye and low charge density suppresses the agglomeration.⁴¹ In our study, the charge density of the TONS remains constant, but the surface charge density of nanosheet complex changes with different amount of MB adsorption. The charge density of the MB–TONS complex determines the yield either trimer or dimer.Open in a separate windowFig. 4(a) UV-Vis spectra of residual (equilibrium) solution, (b) the intensity change of the monomer and dimer in the reference with trimer and the ratio of active surface area of adsorbed MB : TONS (the red dash line marks the position of left vertical axis at 1).In these colloidal systems, the TONS complex is governed by electrostatic force.³¹ According to the double layer theory, the TONS should be tightly bounded with the counterions such as MB⁺, TBA⁺ and H⁺ in first layer. In our solutions which are very basic, the concentration of these ions is in following order, TBA⁺ (0.0129 mol l⁻¹) > MB⁺ (0.00313 mol l⁻¹ for 1000 mg l⁻¹) ≫ H⁺. So, the TBA⁺ and MB⁺ ions are the main contributors to build the first layer. Given the size of the MB⁺ molecules 1.7 × 0.76 × 0.33 nm, the charge density is calculated to be 1.78q_e/nm² (where q_e is the electron charge) in the form of perpendicular to the TONS.⁴² In reality, the charge density should be smaller than this value because MB molecules are preferable to bound with TONS at certain angle. For example, the angle was found to be 65–70 °C on mica surface.⁴¹ The charge density of the TBA⁺ and NSTO is 1.63 and 4.72q_e/nm², respectively.²⁸ Therefore, one layer of the TBA⁺ and MB⁺ ions is insufficient to compensate the charge on the TONS surface. The TBA⁺ and MB⁺ ions in second and outer layers are necessary, but are loosely attracted to the TONS by the Coulomb force. The large dye adsorption capacity is probably due to this multilayer dye molecule adsorption behavior and the large surface area of the TONS. The mechanism of the ultra-high adsorption is to be verified.In summary, we studied the charged TONS for the MB adsorption treatment. We found that the TONS possess an ultra-high dye adsorption, and the adsorption capacity of titanate nanosheets is found up to 3937 mg g⁻¹. However, our data show that the dye removal efficiency of titanate nanosheets is low as less than 50%. Based on our analysis, we conclude that the large dye adsorption capacity is due to the large surface area of the TONS and the multilayer dye molecule adsorption behavior because of the presence of the surface charge. Furthermore, our study shows that the adsorption is a rapid process, and the loosely adsorbed MB molecules go through desorption to form high order agglomerates in the course of time. Our analysis indicates the desorption process a recombinative molecular desorption. What''s more, the product after desorption varies with the initial concentration of MB. Low concentration yields trimers, and high concentration promotes formation of dimers.It is well known that MB is a well-used redox indicator for photocatalysis study, at the end, we would like to highlight that one should use MB as the indicator more carefully because the color change may not be always due to the catalytic effect.     

Three-dimensional macroporous graphene monoliths with entrapped MoS2 nanoflakes from single-step synthesis for high-performance sodium-ion batteries

Layered metal sulfides (MoS₂, WS₂, SnS₂, and SnS) offer high potential as advanced anode materials in sodium ion batteries upon integration with highly-conductive graphene materials. However, in addition to being costly and time-consuming, existing strategies for synthesizing sulfides/graphene composites often involve complicated procedures. It is therefore essential to develop a simple yet scalable pathway to construct sulfide/graphene composites for practical applications. Here, we highlight a one-step, template-free, high-throughput “self-bubbling” method for producing MoS₂/graphene composites, which is suitable for large-scale production of sulfide/graphene composites. The final product featured MoS₂ nanoflakes distributed in three-dimensional macroporous monolithic graphene. Moreover, this unique MoS₂/graphene composite achieved remarkable electrochemical performance when being applied to Na-ion battery anodes; namely, excellent cycling stability (474 mA h g⁻¹ at 0.1 A g⁻¹ after 100 cycles) and high rate capability (406 mA h g⁻¹ at 0.25 A g⁻¹ and 359 mA h g⁻¹ at 0.5 A g⁻¹). This self-bubbling approach should be applicable to delivering other graphene-based composites for emerging applications such as energy storage, catalysis, and sensing.

A single-step, template-free, high-throughput synthesis method is developed to produce graphene/MoS₂ composites for improved performances in sodium-ion batteries.

Sodium-ion batteries (NIBs) have been proposed as promising alternatives to lithium-ion batteries (LIBs) in the megawatt- and kilowatt-scale energy storage scenarios (i.e.; electric vehicles, stationary grids) for their high cost-effectiveness, sustainability, and environmental benignity.¹ Since the operation chemistry of NIBs is very similar to that of LIBs, knowledge gained from developing LIB technology can be mostly applied to NIBs with the exception of electrode materials.^2,3 In particular, the larger ionic radius of Na⁺ (0.102 nm) than that of Li⁺ (0.076 nm) makes graphite, the most commonly used anode in LIBs, unable to accommodate sodium ions in a satisfactory regime.⁴ Inspired by the findings on LIBs, scientists have tested carbonaceous materials,^5,6 alloy materials (Sn, Sb),^7,8 and metal oxides (Fe₂O₃, CuO, TiO₂)^9–11 as anode materials for NIBs. Unfortunately, due to the large volume change and/or the sluggish kinetics during charge/discharge cycles, these materials delivered either low reversible capacity or poor cyclability.¹² Consequently, layered metal sulfides (MoS₂, WS₂, SnS₂ and SnS) have also been explored as anode materials in NIBs due to their unique structural characteristics.¹³ For example, molybdenum sulfide (MoS₂), stemming from its large interlayer spacing (0.62 nm, compared to 0.34 nm for graphite) and high capacity for hosting foreign species, has been recently highlighted as a possible candidate for anode material in NIBs.^14–19 According to the intercalation and conversion reaction between one MoS₂ molecule and four Na⁺, the theoretical capacity of MoS₂ is as high as 670 mA h g⁻¹.²⁰However, there are two major issues when using MoS₂ as anodes in large-scale applications: poor electronic conductivity and drastic volume expansion upon conversion reaction from MoS₂ to Mo and Na₂S.^18,20–27 One effective approach to address the problems and thus improve the electrochemical performance of MoS₂ in NIBs is by supporting MoS₂ with conductive scaffolds to create porous composites, so as to simultaneously improve its conductivity as well as buffer the volumetric variation.¹² In this regard, carbon materials, especially graphene, have been repeatedly confirmed to be an efficient conductive additive in electrode materials in resolving the above issues.^28,29 Some examples of such effective treatment on electrode materials include sulfur/graphene cathode in lithium–sulfur batteries,³⁰ lithium metal phosphates/carbon cathode materials in LIBs,^31–34 and various metal oxides/graphene anode materials in LIBs.³⁵To improve the electrical conductivity and enhance the structural integrity of MoS₂ anode, MoS₂/graphene composites have been synthesized via several methods and applied in NIBs.^{17,18,21,23,26,36–41} For instance, David et al. prepared MoS₂/graphene composite paper through vacuum filtration of homogeneous dispersions consisting of exfoliated MoS₂ and graphene oxide sheets, followed by thermal reduction at elevated temperatures.¹⁸ Wang et al. and Xie et al. also synthesized MoS₂/graphene composites via hydrothermal reactions plus thermal annealing, respectively.^23,26 In spite of the significant synthetic achievements made, the existing strategies for synthesizing MoS₂/graphene composites present a few shortcomings as these methods often involve complicated procedures (graphene oxide preparation, MoS₂ preparation, compositing or mixing step, thermal treatments, etc.) in addition to be costly and time-consuming.²⁸ Another issue with existing MoS₂/graphene compositing methods is that some of them do not ensure the intimate contact between MoS₂/graphene interfaces, an unfavorable condition for electrochemical applications (charge-transfer process).²⁶ Finally, most of the present MoS₂/graphene compositing methods are faced with the issue of low yield, ranging from several tens to hundreds milligrams of powders under laboratory conditions.Herein, we report a single-step, template-free, high-throughput “self-bubbling” method for synthesizing MoS₂/graphene composite. Our method is cost-effective, simple and scalable. The synthesis utilizes the thermal decomposition of solid precursor to generate MoS₂; meanwhile, the released gas from the decomposition reaction blows premixed, melted glucose into crowded bubbles, which then evolve into graphene structures during annealing. The final product is microscopically featured as highly crystalline MoS₂ nanoflakes distributed in three-dimensional (3D) macroporous monolithic graphene. With the additional assistance of intimate interfacial contacts between MoS₂ and graphene, our composite demonstrates considerably improved electrochemical performance when compared with those of conventional MoS₂/graphene composite upon application in NIBs. It is expected that such a unique MoS₂/graphene composite should hold potential in promoting the development of practical MoS₂ anode in NIBs, while the straightforward self-bubbling method could offer the opportunity in producing MoS₂/graphene composites in industrial scale as well as synthesizing other advanced graphene-based composites.     

Synthesis and magnetic properties of sub-nanosized iron carbides on a carbon support

Iron carbide clusters with near-sub-nanometer size have been synthesized by employing a tetraphenylmethane-cored phenylazomethine dendrimer generation 4 (TPM-DPAG4) as a molecular template. Magnetic measurements reveal that these iron carbide clusters exhibit a magnetization–field hysteresis loop at 300 K. The data indicate that these iron carbide clusters are ferromagnets at room temperature.

This study reports the synthesis and ferromagnetism of iron carbide clusters with near-subnanometer size by employing a dendrimer template and carbothermal hydrogen reduction (CHR).

Iron carbide is a well-established material that is typically generated during the steelmaking process. Research into the phase diagram of the Fe–C system was conducted as early as the 1890s.¹ According to this phase diagram, iron and carbon atoms can be mixed in arbitrary proportions up to 0.095 atom% of C at temperatures below 1000 K; above this ratio iron carbide cementite (Fe₃C) is formed.² As with metallic iron, iron carbides are also known to exhibit ferromagnetism;^3,4 therefore, there have been many studies reported on the ferromagnetism of bulk iron carbides and iron carbide nanoparticles.^5–19 The size effect in nanomaterials is also of particular interest because the properties of the bulk materials can be significantly changed. For example, melting-point depression,²⁰ catalyst activation,²¹ and the alloying of non-mixable metals²² have been reported to occur as the particle size decreases into the nanosize range. We have recently reported atomicity-dependent changes in the catalytic activity^23,24 and size-dependent phase transformations of near-sub-nanometer particles.²⁵ The properties of many substances are thus sensitively affected by particle size, particularly in the near-sub-nanosize range. In this context, the smallest iron carbide nanoparticles reported to date are as small as ca. 2 nm,^7,16 aside from the gas phase experiments^26,27 and the theoretical studies.^28–32 In these cases, the iron carbide nanoparticles exhibit superparamagnetism, i.e., they do not act as magnets at ambient temperature. However, sub-nanosized iron carbide particles have remained elusive to date. In the present study, we have synthesized near-sub-nanometer iron carbide particles/clusters, and these iron carbide clusters are ferromagnets, even at room temperature, thereby countering superparamagnetism. Bulk iron carbide is an old material; however, the iron carbide clusters synthesized in this work are the smallest room temperature magnets reported to date. Fig. 1 shows the strategy employed for the synthesis of near sub-nanometer-sized iron carbide clusters. The macromolecular tetraphenylmethane-cored dendritic phenylazomethine dendrimer generation 4 (TPM-DPAG4) was used as a molecular template. This DPA-type dendrimer coordinates to metal ions in solution via its imine sites, and complexation proceeds stepwise from the center of the dendrimer to its periphery due to its basicity gradient.^33–37 Stepwise complexation was confirmed in the present study by UV-Vis titrations. Upon the addition of FeCl₃ to a solution of TPM-DPAG4, spectral changes and shifts in the isosbestic point were observed (Fig. S1^†); these changes reached saturation after the combined addition of 60 eq. of FeCl₃. Different isosbestic points were observed in the ranges of 0–4, 6–12, 16–28, and 32–60 eq., respectively, which is consistent with the number of imines at each type of site and reflects the stepwise complexation from the central to the peripheral sites. The in situ-prepared dendrimer complexes, i.e., TPM-DPAG4 with 4, 12, 28, or 60 eq. of FeCl₃ incorporated were then adsorbed onto a graphitic carbon support (graphitized mesoporous carbon: GMC). Carbothermal hydrogen reduction (CHR), which is a synthetic method used to obtain metal carbides, was subsequently applied.^25,38,39 After CHR at 773 K for 30 min, the samples (Fe₁₂/C, Fe₂₈/C, and Fe₆₀/C) were examined using transmission electron microscopy (TEM), and the results are shown in Fig. 2 and S2.^† There are several reports for TEM observations of iron carbide nanoparticles larger than 2 nm diameter without atomic-resolution.^5–19 Very fine particles dispersed over the carbon support were observed as blurry black dots in the TEM images. The mean particle diameter and standard deviation of the size distribution were estimated to be 0.9 ± 0.2 nm (Fe₁₂/C), 1.0 ± 0.3 nm (Fe₂₈/C), and 1.3 ± 0.3 nm (Fe₆₀/C), respectively. The average particle size consistently increased with the FeCl₃ content in the TPM-DPAG4 template. These samples represent the first examples of near-sub-nanometer-sized iron carbide particles. However, we could not observe any individual particles in the Fe₄/C sample, because the particle size was too small. In this case, the particle size was estimated to be ca. 0.6 nm using the tetra-nuclear cluster model of the [Fe₄C(CO)₁₂]²⁻ carbidocarbonyl complex reported by Boehme et al. (Fig. S3a^†)⁴⁰ as well as the theoretical studies.^28–32 It should be noted that atomic-resolution images that would project the clusters could not be obtained, because these samples exhibit ferromagnetism, even at room temperature (vide infra).Open in a separate windowFig. 1Chemical structure of the TPM-DPAG4 and illustration of metal ions assembly (4, 12, 28, 60 eq.).Open in a separate windowFig. 2TEM images of Fe₁₂/C, Fe₂₈/C, and Fe₆₀/C after 30 min of CHR at 773 K.Powder X-ray diffraction (PXRD) analysis cannot be applied to the characterization of such sub-nanosized particles on solid supports, as they do not adopt any long-range-ordering crystal structure. On the other hand, X-ray absorption fine structure (XAFS) is a powerful tool to clarify the local structure around the metal atoms.⁴¹ We found that the X-ray absorption near edge structure (XANES) spectra of Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C after CHR are very similar to those of metallic iron (Fe foil) and Fe₃C, whereas they are substantially different from those of iron oxides such as Fe₃O₄ and α-Fe₂O₃ as well as from that of the FeCl₃ starting material (Fig. S4^†). Therefore, it can be concluded that these samples are not oxides. XANES spectrum of Fe₃C and metallic iron can clearly be distinguished in their first derivatives form (Fig. 3). Metallic iron and Fe₃C have a pre-edge peak in common at ca. 7111 eV (3s → 4d transitions). Metallic iron exhibits two maxima in the range of 7115–7130 eV (3s → 4p transitions), while Fe₃C exhibits one maximum and several shoulders in this region. The spectra of Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C after CHR had a pre-edge peak at ca. 7111 eV, together with a maximum peak in the 7115–7130 eV region, which indicates the iron carbide nature. Therefore, it can be concluded that Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C are iron carbides rather than metals. The white-line peak was slightly shifted to the higher energy side with downsizing (Fig. 3b), i.e., 7129.4 eV (Fe₃C), 7130.2 eV (Fe₆₀/C), 7130.1 eV (Fe₂₈/C), 7131.2 eV (Fe₁₂/C), and 7131.4 eV (Fe₄/C). The experimental error was estimated to be ±0.3 eV based on the applied energy resolution. This shift tendency supported that the cluster samples (Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C) are very fine particles with high specific surface. In addition, the assignment as iron carbides is decisively supported by their Curie temperatures (T_C), which were measured to be 483–488 K (Fig. 4). These T_C values are comparable to that of Fe₃C (483 K, Fig. S5 and S6^†),⁹ which suggests that the ferromagnetic interactions originate from iron carbides. The Curie temperature of Fe₃C is far from those of metallic iron (1043 K)³ and iron oxides e.g. Fe₃O₄ (850 K) and γ-Fe₂O₃ (820–986 K).⁴² It should also be noted that Fe₃C forms a complicated crystal structure that involves nine types of Fe–Fe bonds (2.455–2.714 Å; Fig. S3b^†).^43,44 The presence of the corresponding Fe–Fe bonds in Fe₆₀/C was suggested by extended X-ray absorption fine structure (EXAFS) measurements conducted in transmission mode (Fig. S7^†). The Curie temperature for Fe₃C mainly represents the average of the direct exchange interactions between the Fe–Fe bonds, similar to that in amorphous ferromagnets such as the Fe–C–P system,^45,46 and thus, T_C would be considered not to show a significant size dependence.Open in a separate windowFig. 3Fe K-edge XANES spectra. (a) First derivatives of normalized XANES spectra for Fe₆₀/C, Fe₂₈/C, Fe₁₂/C, and Fe₄/C after CHR at 773 K for 30 min, together with those for Fe foil (metallic iron), Fe₃C, Fe₃O₄, and α-Fe₂O₃. The spectra for Fe₂₈/C, Fe₁₂/C, and Fe₄/C were recorded in fluorescence mode, whereas the others were recorded in transmission mode. (b) Magnifications around the white-line peak. The experimental error was estimated to be ±0.3 eV based on the applied energy resolution.Open in a separate windowFig. 4Temperature-dependent magnetization curves for (a) Fe₆₀/C, (b) Fe₂₈/C, (c) Fe₁₂/C, and (d) Fe₄/C obtained by application of a magnetic field (5000 Oe) and measurement of the magnetization in increments of 10 K (300–420 K) or 5 K (420–600 K). The blue lines are smoothed trend lines. The Curie point (T_C) was determined from the maximum of the second derivative (insets) and calibrated using T_C = 483 K for Fe₃C.⁹ The error in the maxima of the second derivatives was estimated to be 5 K. Fig. 5 shows magnetization–field (M–H) loops for the iron carbide clusters, and the magnetic data are summarized in Table S1.^† The M per the sample weight data are shown in Fig. S8–S12.^† The four cluster samples show hystereses in their M–H loops at 1.9 K (Fig. 5a), which indicates that they are ferromagnets with an associated coercivity (H_c). The H_c value increased with a decrease in the cluster size, i.e., 603 Oe (Fe₆₀/C), 939 Oe (Fe₂₈/C), 1856 Oe (Fe₁₂/C), and 2697 Oe (Fe₄/C). In contrast, bulk iron carbide cementite (Fe₃C) with an average crystal size of 39 nm has a smaller hysteresis with H_c values of 166 and 21 Oe at 1.9 and 300 K, respectively (Fig. S8^†). The magnetic behavior of bulk Fe₃C indicates ferromagnetism with a multi-magnetic-domain structure.³ On the contrary, iron carbide nanoparticles have been reported to exhibit more pronounced hysteresis at room temperature than bulk Fe₃C, with H_c values of 700 Oe (15 nm) and 544 Oe (14.1 ± 0.8 nm) by Grimes et al.⁵ and Hou et al.,⁶ respectively, which suggests a single-magnetic-domain structure. Therefore, the smaller iron carbide clusters in this study are considered to have a single-magnetic-domain structure. The increase in coercivity with the decrease in single-magnetic-domain particle size has been reported by Lartigue et al. for iron carbide nanoparticles with sizes of 15.1 nm (H_c = 331 Oe), 7.4 nm (H_c = 405 Oe), 5.5 nm (H_c = 625 Oe), and 2.8 nm (H_c = 1009 Oe) at 2.5 K.⁷ On the other hand, the iron carbide clusters exhibit hysteresis loops at 300 K (Fig. 5b), i.e., H_c = 140 Oe (Fe₆₀/C), 163 Oe (Fe₂₈/C), 367 Oe (Fe₁₂/C), and 666 Oe (Fe₄/C), which indicates that they are ferromagnets, even at room temperature. Clusters or near-sub-nanosize particles generally exhibit superparamagnetism with a complete loss of coercivity at room temperature.³τ_N ∝ exp(E_a/k_BT)1E_a ≃ K_effV2Open in a separate windowFig. 5Magnetization–field (M–H) loops for Fe₆₀/C (magenta), Fe₂₈/C (brown), Fe₁₂/C (green), and Fe₄/C (blue) at (a) 1.9 K and (b) 300 K. Magnetization (M) was normalized with respect to the saturation magnetization (M_s). The inset shows the magnification in the region near zero field. Eqn (1) is the Néel–Arrhenius equation,⁴⁷ where τ_N, E_a, k_B, T, K_eff, and V are the Néel relaxation time, magnetic anisotropy energy, Boltzmann constant, temperature, effective magnetic anisotropy constant, and volume of a single-magnetic-domain particle, respectively. The term for the angle between the magnetic moment and the easy magnetic axis was not introduced (eqn (2)), because these were powder samples. Therefore, superparamagnetism emerges with a decrease in size (V) because E_a becomes comparable to the thermal energy (k_BT). Lartigue et al. have reported superparamagnetism for iron carbide nanoparticles with sizes less than 5.5 nm.⁷ Fig. S13^† shows field-cooling (FC) and zero-field-cooling (ZFC) magnetization curves used to determine the blocking temperature (T_B) at which the magnets completely lose their coercivity. Fe₃C shows a T_B of 467 K which is near the Curie point (483 K). On the other hand, the T_B of Fe₆₀/C is clearly lower (ca. 385 K) than the Curie point, which was attributed to the influence of superparamagnetism in light of the results of Lartigue et al. In contrast, Fe₂₈/C has higher T_B values close to the Curie point at 473 K. This behavior is contrary to superparamagnetism and cannot be explained without increased effective magnetic anisotropy (K_eff). The interactions between the iron carbide clusters and the graphitic carbon surface may be a mechanism to afford large K_eff, because the ratio of the interacting Fe atoms increases by decreasing size. The oxidation of Fe₄/C in air at 553 K for 30 min significantly decreased the magnetization and coercivity (Fig. S14^†). Carbides can be the magnets at sub-nano scale, while oxides would not. Due to the measurement sensitivity limit and noise, the T_Bs of Fe₁₂/C and Fe₄/C were roughly estimated to be 410–470 K and 350–470 K, respectively, which indicates that their T_Bs were at least above room temperature. It should also be noted here that the magnetic moment per Fe atom of these cluster samples (1.0–2.3 μ_B atom_Fe⁻¹) was almost identical to that of Fe₃C (1.5 μ_B atom_Fe⁻¹), regardless of the Fe content (wt%) over an order of magnitude (Table S1^†). The variation in the magnetic moment may involve not only experimental errors, but also atomicity. Becker et al. reported that Fe clusters exhibit an atomicity-dependent variation in their magnetic moment in the gas phase, especially below 100 atoms.⁴⁸ Additionally, the density functional theory studies have reported that the iron carbide clusters show the magnetic moment of ca. 1–3.5 μ_B atom_Fe⁻¹,^30–32 which is consistent with those in this study. Therefore, with consideration of the magnetic moment and the Curie temperature, it was concluded that the magnetic behavior of the iron carbide cluster samples is derived from the carbides themselves, and not from impurities. The reproducibility of the M–H hysteresis loop at 300 K for Fe₄/C was certainly confirmed including another batch sample (sample B: Fig. S15^†). It was also confirmed that a blank sample (GMC) showed diamagnetism measured at both 1.9 and 300 K (raw M data shown in Fig. S16–S23^†).Nanoparticle magnets have a single magnetic-domain structure and exhibit hysteresis at room temperature; however, they lose this hysteresis upon downsizing by superparamagnetism. There have been no reports of sub-nanoparticle magnets (diameter: ∼1 nm or less) that exhibit coercivity above room temperature;^3–19,49 neither for e.g. Fe–Pt bimetallic nanoparticles⁵⁰ nor iron oxide nanoparticles.⁵¹ The iron carbide clusters in this study are unique magnets that are different from both nanoparticle magnets. They do not have a long-range-ordering crystal structure such as nanoparticles and bulk substances on account of their sub-nanometer size. The iron carbide clusters in this study were carefully characterized by XAFS (Fig. 3) as well as by the Curie temperature (Fig. 4). The magnetic measurements (Fig. 5) revealed that the iron carbide clusters represent room-temperature magnets. Therefore, the iron carbide clusters discussed in this study can be regarded as a new class of magnets, i.e., sub-nano magnets.This work has synthesized the first examples of sub-nanosized iron carbides on a graphitic carbon support. These iron carbide clusters act as magnets at room temperature. This study would open up the new research field of sub-nano magnets.     

Size effect studies in catalysis: a simple surfactant-free synthesis of sub 3 nm Pd nanocatalysts supported on carbon

Supported Pd nanoparticles are prepared under ambient conditions via a surfactant-free synthesis. Pd(NO₃)₂ is reduced in the presence of a carbon support in alkaline methanol to obtain sub 3 nm nanoparticles. The preparation method is relevant to the study of size effects in catalytic reactions like ethanol electro-oxidation.

A simple surfactant-free synthesis of sub 3 nm carbon-supported Pd nanocatalysts is introduced to study size effects in catalysis.

A key achievement in the design of catalytic materials is to optimise the use of resources. This can be done by designing nanomaterials with high surface area due to their nanometre scale. A second achievement is to control and improve catalytic activity, stability and selectivity. These properties are also strongly influenced by size.^1–3 To investigate ‘size effects’ it is then important to develop synthesis routes that ensure well-defined particle size distribution, especially towards smaller sizes (1–10 nm).Metal nanoparticles are widely studied catalysts. In several wet chemical syntheses, NP size can be controlled using surfactants. These additives are, however, undesirable for many applications^4,5 since they can block active sites and impair the catalytic activity. They need to be removed in ‘activation’ steps which can negatively alter the physical and catalytic properties of the as-produced NPs. Surfactant-free syntheses are well suited to design catalysts with optimal catalytic activity⁶ but their widespread use is limited by a challenging size control.³Palladium (Pd) NPs are important catalysts for a range of chemical transformations like selective hydrogenation reactions and energy applications.^7–9 It is however challenging to obtain sub 3 nm Pd NPs, in particular without using surfactants.² Surfactant-free syntheses are nevertheless attracting a growing interest due to the need for catalysts with higher performances.^10–14Promising surfactant-free syntheses of Pd NPs were recently reported.^8,15 The NPs obtained in these approaches are in the size range of 1–2 nm and show enhanced activity for acetylene hydrogenation⁸ and dehydrogenation of formic acid.¹⁵ Enhanced properties are attributed to the absence of capping agents leading to readily active Pd NPs. The reported syntheses consist in mixing palladium acetate, Pd(OAc)₂, in methanol and the reduction of the metal complex to NPs occurs at room temperature. The synthesis is better controlled in anhydrous conditions to achieve a fast reaction in ca. 1 hour. Another drawback is that the synthesis must be stopped to avoid overgrowth of the particles. Therefore, a support material needs to be added after the synthesis has been initiated and no simple control over the NP size is achieved.^8,15In this communication a more straightforward surfactant-free synthesis leading to sub 3 nm carbon-supported Pd NPs in alkaline methanol at ambient conditions is presented. A solution of Pd(OAc)₂ in methanol undergoes a colour change from orange to dark, indicative of a reduction to metallic Pd, after ca. a day. However, only ca. 1 hour is needed with Pd(NO₃)₂, Fig. 1 and UV-vis data in Fig. S1.^† The fast reduction of the Pd(NO₃)₂ complex in non-anhydrous conditions is a first benefit of the synthesis presented as compared to previous approaches.Open in a separate windowFig. 1Pictures of 4 mM Pd metal complexes in methanol without or with a base (as indicated).For particle suspensions prepared with Pd(OAc)₂ or Pd(NO₃)₂ the NPs agglomerate and quickly sediment leading to large ‘flake-like’ materials. When the reduction of Pd(NO₃)₂ in methanol is performed in presence of a carbon support and after reduction the solution is centrifuged and washed in methanol, a clear supernatant is observed indicating that no significant amount of NPs are left in methanol. Transmission electron microscope (TEM) analysis confirms that NPs are formed and well-dispersed on the carbon support surface and no unsupported NPs are observed, Fig. 2a. Likely, the reduction of the NPs proceeds directly on the carbon support. However, the size of the NPs is in the range 5–25 nm, which is still a relatively large particle size and broad size distribution.Open in a separate windowFig. 2TEM micrographs of Pd NPs obtained by stirring 4 mM Pd(NO₃)₂ in methanol and a carbon support for 3 hours, (a) without NaOH and (b) with 20 mM NaOH. Size distribution histograms are reported in Fig. S4.^† The same samples after electrochemical treatments are characterised in (c) and (d) respectively. Size distribution histograms are reported in Fig. S7.^†Assuming a ‘nucleation and growth’ mechanism, the NPs should become larger over time.¹⁶ But the reaction is so fast that by stopping the reaction before completion, size control is not achieved and unreacted precious metal is observed, Fig. S2.^† To achieve a finer size control and more efficient use of the Pd resources, a base was added to the reaction mixture, e.g. NaOH.³ In alkaline media, the formation of Pd NPs is slower; it takes ca. 60 minutes to observe a dark colour for a 5 mM Pd(NO₃)₂ solution with a base/Pd molar ratio of 10 in absence of a support, Fig. 1.Also in alkaline methanol, the NPs agglomerate over time in absence of a support material. However, if the alkaline solution of Pd(NO₃)₂ is left to stir in presence of a carbon support the desired result is achieved, i.e. Pd NPs with a significantly smaller size and size distribution of ca. 2.5 ± 1.0 nm, Fig. 2b. The NPs homogeneously cover the carbon support and no unsupported NPs are observed by TEM suggesting that the NPs nucleate directly on the carbon surface. Furthermore, the supernatant after centrifugation is clear, indicating an efficient conversion of the Pd(NO₃)₂ complex to NPs, Fig. S3.^† Furthermore, there is no need for an extra reducing agent as in other approaches, for instance in alkaline aqueous solutions.⁹The benefits of surfactant-free syntheses of Pd NPs for achieving improved catalytic activity have been demonstrated for heterogeneous catalysis.^8,15 Surfactant-free syntheses are also well suited for electrochemical applications where fully accessible surfaces are required for fast and efficient electron transfer. Several reactions for energy conversion benefit from Pd NPs. An example is the electro-oxidation of alcohols,⁷ in particular ethanol¹⁷ (see also Table S1^†).Previous studies optimised the activity of Pd electrocatalysts by alloying,^18–20 by using different supports^17,21–23 or crystal structures.^24,25 Investigating NPs with a diameter less than 3 nm was challenging.^2,26,27 The surfactant-free synthesis method presented here allows to further study the size effect on Pd NPs supported on carbon (Pd/C) for electrocatalytic reactions.In Fig. 3, results for ethanol electro-oxidation in 1 M ethanol solution mixed with 1 M KOH aqueous electrolyte are reported based on cyclic voltammetry (CV) and chronoamperometry (CA) with Pd/C catalysts exhibiting 2 significantly different size distributions. The electrode preparation, the measurement procedure and the sequence of electrochemical treatments are detailed in the ESI.^† In order to highlight size effects, we compare geometric and Pd mass normalized currents (Fig. 3a and c) as well as the oxidation currents normalized to the Pd surface area (Fig. 3b).Open in a separate windowFig. 3Electrochemical characterisation of carbon supported Pd NPs with 5–25 nm (grey) and 2.5 nm (dark) size in 1 M KOH + 1 M ethanol aqueous electrolyte. (a) 2^nd CV before chronoamperometry (CA), (b) current normalised by the electrochemically active surface area of Pd, (c) CA recorded at 0.71 V vs. RHE after 50 cycles between 0.27 and 1.27 V.It is clearly seen that based on the geometric current density, the smaller Pd NPs exhibit significantly higher currents for ethanol oxidation than the larger NPs. To differentiate if this observation is a sole consequence of the different surface area, the electrochemically active surface area (ECSA) has been estimated based on “blank” CVs (without ethanol) recorded between 0.27 and 1.27 V vs. RHE in pure 1 M KOH aqueous electrolyte and integrating the area of the reduction peak at ca. 0.68 V, Fig. S5.^† As conversion factor, 424 μC cm⁻² was used.²⁸Using this method, the smaller NPs with a size around 2.5 nm exhibit an ECSA of 92 m² g⁻¹ whereas the larger NPs with a size in the range 5–25 nm exhibit an ECSA of 47 m² g⁻¹, consistent with a larger size. Normalising the ethanol electro-oxidation to these ECSA values instead of the geometric surface area, Fig. 3b, still indicates a size effect. It is clearly seen that the smaller Pd NPs exhibit higher surface specific ethanol oxidation currents, in particular at low electrode potentials. Furthermore, a clear difference in the peak ratios in the CVs is observed. The ratio in current density of the forward anodic peak (j_f, around 0.9 V) and the backward cathodic peak (j_b, around 0.7 V vs. SCE) is around one for the smaller NPs, whereas it is about 0.5 for the larger NPs. The forward scan corresponds to the oxidation of chemisorbed species from ethanol adsorption. The backward scan is related to the removal of carbonaceous species not fully oxidised in the forward scan. The higher j_f/j_b ratio therefore confirms that the smaller NPs are more active for ethanol electro-oxidation and less prone to poisoning, e.g. by formation of carbonaceous species that accumulate on the catalyst surface.^29,30 This observation is further supported by a chronoamperometry (CA) experiment, Fig. 3c, at 0.71 V performed after continuous cycling (50 cycles between 0.27 and 1.27 V at a scan rate of 50 mV s⁻¹). In the CA testing of the thus aged catalysts at 0.71 V, the ethanol oxidation current on the two catalysts starts at around the same values, however, its decay rate is significantly different. The Pd mass related oxidation currents for the smaller NPs are after 30 minutes almost twice as high (ca. 200 A g_Pd⁻¹) as for the larger ones (ca. 130 A g_Pd⁻¹), confirming that the small Pd NPs are less prone to poisoning. In particular a factor up to 4 in the Pd mass related ethanol oxidation currents after 1800 s of continuous operation is achieved compared to a recently characterised commercial Pd catalyst on carbon,²⁰ Table S1.^† Despite different testing procedure reported in the literature, it can be concluded from these investigations that the surfactant-free synthesis presented shows promising properties for electrocatalytic ethanol oxidation even after extended cycling.The extended cycling, however, has different consequences for the two catalysts. For the small (2.5 nm) NPs of the Pd/C catalyst, a massive particle loss, but only moderate particle growth is observed as highlighted in Fig. 2 (see also Fig. S6^†). TEM micrographs of the two Pd/C samples recorded before and after the complete testing (CVs and CAs, for details see Fig. S7^†) show that the catalyst with small Pd NPs exhibits a pronounced particle loss as well as a particle growth to ca. 6 nm probably due to sintering. By comparison, for the Pd/C catalyst with the large Pd NPs, no significant influence of the testing on particle size or particle density is apparent.     

Leucomethylene blue probe detects a broad spectrum of reactive oxygen and nitrogen species

Reactive oxygen and nitrogen species (ROS, RNS) are ubiquitous in biology with a variety of physiological and pathological functions. Here we describe a broad spectrum ROS/RNS detecting fluorogenic probe with red fluorescence emission and up to 100-fold gain. Hence these modified probes are useful for in vivo non-invasive quantification of ROS/RNS.

A broad spectrum ROS/RNS sensing butylated phenol tethered leucomethylene blue is presented. This probe detects a variety of ROS/RNS, with up to 100-fold gain in fluorescence in the red range, is suitable for microscopic and macroscopic in vivo fluorescence imaging.

Reactive oxygen species (ROS: O₂˙⁻, H₂O₂, HO˙, OCl⁻, and HO₂˙) and reactive nitrogen species (RNS: ONOO⁻, NO˙, NO₃⁻) are classes of short-lived molecules produced in biological environments, e.g. in cellular metabolism, and in neutrophil actions.^1,2 They function as messengers, pathogen neutralizers, and play key roles in inducing inflammation,^3,4 and cancers.^5–7 Therefore, early non-invasive detection of ROS/RNS enable the onset of diseases. Several strategies, methods and probes are employed currently for the detection of ROS and RNS.^2,8–10 Electron paramagnetic resonance (EPR)¹¹ and fluorescence-based techniques are prevalent,¹² followed by photometric, chemiluminescent and electrochemical methods. In all these techniques, the applied probes change their properties post-reaction (or trapping) with ROS/RNS, and enable the detection.Typically, many of above probes detect only one of the particular ROS/RNS species, and “overlook” those formed in earlier or subsequent metabolic steps, e.g. in EPR only radical species, in fluorescence and colorimetric techniques one type of ROS/RNS species. However, in biological systems the primarily produced ROS species (e.g. O₂˙⁻) undergoes a variety of dissociation pathways forming secondary ROS species (see Fig. 1) and cause oxidative burden.¹³ Thus, to estimate an overall oxidative stress, it is necessary to accurately assess the pathogenic levels production of all ROS/RNS species using a single probe.Open in a separate windowFig. 1Schematic overview for generation of a variety of ROS/RNS species in biological conditions and their interconversions.Highly studied ROS detecting fluorogenic probes, 2′-7′-dichlorodihydrofluorescein (DCFH), its diacetate (DCFH-DA),¹⁴ and dihydrorhodamine (DHR),¹⁵ produce O₂˙⁻ from O₂ after reaction with one-electron-oxidizing ROS/RNS species, via the radical ion (DCFH˙⁻ or DHR˙⁻). This catalytic formation of O₂˙⁻, and its further conversion to other reactive species leads to an inaccurate detection of cellular ROS.¹⁶ Further, the absorption and emissions wavelengths are incompatible for in vivo use.¹⁷ It is optimal to have probes exhibiting optical properties in red-to-near infrared (NIR) range, where tissue auto fluorescence and phototoxicity are minimal. Towards such red-to-NIR emitting probes, previous studies employed reduced hydrocyanines, which undergo HO˙ and O₂˙⁻ mediated oxidation yielding red-to-NIR emitting cyanine dyes.¹⁸ However the polymethine chain in cyanine itself is sensitive to oxidative cleavage with ROS species,^19,20 thus could lead to underestimation of ROS. Thus, new probes are of interest, which show high stability towards ROS, simultaneously exhibit fluorescence in red-to-NIR range.Methylene blue (MB) dye has attracted our attention, as it exhibits absorption and emission in the biocompatible region, at 665 nm and 686 nm, respectively.²¹ Further, MB core is chemically modifiable,^22,23 and FDA-approved for therapeutic use,^24,25 and has been applied in a variety of applications, in disinfecting blood,²⁶ in treatment a variety of cancers,²⁷ as an antidote for methemoglobinemia²⁸ and in the photodynamic inactivation of bacteria,²⁵ funguses²⁹ and viruses.^30,31 These biological uses coupled with in vivo suitable optical properties make MB an attractive core the detection of ROS/RNS.Recently, a reduced leucomethylene blue (LMB) was applied to selectively detect HOCl by formylating 10-N site (N¹⁰-CHO) of LMB.³² In another study, the N¹⁰-site was modified with an enzyme³³ or light cleavable synthon,³⁴ which upon applying the respective trigger, yielded LMB and spontaneously oxidized to MB. We therefore are interested in modifying the LMB for detection of a broad spectrum of ROS/RNS species.In our previous studies, we have identified that 2,6-di-tert-butyl phenol (BHP) moiety substituted dyes like porphyrins,³⁵ BODIPYs³⁶ reacted with a variety of ROS/RNS species, undergoing changes in their optical properties. The reactivity of BHP towards a variety of ROS/RNS is similar to that of the antioxidant 2,6-di-tert-butyl hydroxy toluene (BHT, a food additive),³⁷ as both share the same reactive site. Extending this strategy to LMB, we conceived to prepare a N¹⁰-BHP appended LMB (BHP-LMB; 1, 2) for the ROS detection. As the BHP unit is hydrophobic, we approached a modification of LMB with polar groups like 1-butanesulfonate. Here we report the synthesis, and ROS detection characteristics as well as imaging applications of both constructs.Towards the synthesis of the target BHP-LMB 1, we employed a Buchwald–Hartwig C–N coupling, using reduced LMB and 4-iodo-2,6-tert-butylphenol (6). This method is appealing for its simplicity, as the C–N coupling chemistry has been extensively studied.³⁸ The C–N coupling reaction was performed in toluene with Pd₂(dba)₃ catalyst at 120 °C under nitrogen atmosphere (Scheme 1A). The BHP-LMB 1 could be isolated in 10% yield after column chromatography. Low yield was due to solubility of LMB in toluene, further sensitivity towards dissolved oxygen. Thus for purification of the coupling product 1, N₂-gas was employed in chromatography, and the dye 1 could be obtained as pale-turquoise coloured solid after removal of the solvent. The HPLC and NMR showed >98% purity. The air-oxidation can be prevented by using reducing agents like citric acid, especially for spectroscopic measurements.Open in a separate windowScheme 1(A) Synthesis of hydrophobic BHP-LMB (1), (B) bis-sultone added hydrophilic BHP-LMB (2). (a) K₂CO₃, Na₂S₂O₄ in DCM/H₂O at 40 °C under N₂, (60%) (b) Pd₂(dba)₃, DPPF, 6, NaO^tBu in toluene at 110 °C under N₂, (10%, air sensitive) (c) NaIO₄ in DCM/AcOH, BHP, at 40 °C (34%); (d) BH₃·SMe₂ in toluene at 110 °C (87%); (e) 1,4-butanesultone, DIPEA in MeCN at 80 °C under N₂ (15%).For the hydrophilic 1-butanesulfonylated BHP-LMB 2, we have developed a second generation multi-step synthetic route starting from phenothiazine (PTZ). Towards 2, we have first prepared a 2,8-diaminoacyl phenothiazine derivative (PTZ-NHAc, 3) from 3,7-dinitro-10H-phenothiazine-5-oxide (see ESI^†) as reported previously.³⁹ Then, for introducing the BHP unit at 3, a convenient metal-free dehydrogenative amination method was applied based on a recently published route (Scheme 1B).⁴⁰ An advantage of this route is also the high stability of 3 and 4 towards oxygen. The following reduction of the acetyl groups in 4 was achieved using BH₃·SMe₂ complex, obtaining mono ethyl BHP-PTZ derivative (5), which sets the stage for the final reaction towards the desired product 2. Treating 5 with 1,4-butanesultone under basic conditions, by nucleophilic ring opening of the sultone, yielded 1-butanesulfonate attached BHP-LMB (2), along with mono sultone addition product. The hydrophilic product 2 was isolated by preparative RP-HPLC (reverse phase, with CH₃CN/H₂O gradient) as white solid after lyophilisation.Having obtained two BHP-tethered probes 1 & 2, we performed a screening with a variety of ROS/RNS by optical methods (Fig. 2; Fig. S1 and S2 in ESI^† for titrations). The absorption (UV-vis) spectra of compounds were obtained, 1 in DMSO and of 2 in water; both BHP-LMBs showed a similar reactivity towards ROS, yielding MB as the final compound, though some peculiarities were observed. The O₂˙⁻ was found to be highly aggressive especially in DMSO with 1, compared to 2 in water. In DMSO, the O₂˙⁻ reaction yielded MB instantly, that underwent decomposition in the presence of excess O₂˙⁻. We investigated this reaction by NMR spectroscopy to identify any intermediates towards MB, using 1 titration with O₂˙⁻ and MB with O₂˙⁻, but found no stable species (Fig. S5^†). However, during the treatment with OH˙ and ^tBuO˙ species, it was found that 2 in water gave a persistent far-red absorbing species which was not observed in DMSO with 1 (Fig. S3^†versus Fig. S1^†). Similar spectral features were observed with H₂O₂ and tBuOOH towards 2 in H₂O albeit with a lower intensity, but not with 1 in DMSO. This is presumably, a relatively stable BHP-radical appended LMB formed in water but not in DMSO (Fig. S3^†). For anionic ROS species, O₂˙⁻, ONOO⁻ and OCl⁻, the absorption and fluorescence emission are consistent in both solvents with a small discrepancy in the gain ratio. This can be attributed to a different rate of reaction for formation of MB. In treating 1 with RNS species, NO₂⁻ and NO₃⁻ in DMSO, resulted MB (Fig. S2^†). These optical characteristics confirm that 1 and 2 exhibit a good sensitivity towards most of the ROS/RNS species and are thus suitable for estimating overall oxidative stress.Open in a separate windowFig. 2Optical properties of MB-BHT (25 nmol mL⁻¹) after treating sufficient amount of ROS (>5 equiv.). Fluorescence emission (concentration 12 nmol mL⁻¹) at the maximum of 689 nm for excitation at 645 nm (†: due to fast decomposition of the formed MB).Mechanistic investigations into the formation of intermediates were conducted by HPLC and NMR (Fig. 3). Here, 1 was treated with ONOO⁻, OCl⁻, O₂˙⁻ as they were found to be consistent in both solvents. Adding ONOO⁻ and OCl⁻ in excess to 1 gave a clean MB product without any trace of other species (Fig. 3A). Treatment of 1 with a minimal amount of O₂˙⁻ showed existence of both 1 and MB traces in HPLC. For NMR titrations, to a DMSO-d₆ solution of 1, ROS species in D₂O were added. For addition of O₂˙⁻, the chemical shifts corresponding to MB are downfield shifted slightly compared to ONOO⁻, OCl⁻. Furthermore, the NMR signals of the cleaved BHP unit were upfield shifted for O₂˙⁻ than ONOO⁻, OCl⁻ addition (Fig. 3B). This could indicate that the formed by-product, ^tBu-hydroquinol, might have been further oxidized to a ^tBu-quinone with O₂˙⁻, but with ONOO⁻, OCl⁻ persistent ^tBu-hydroquinol. Nevertheless, as no other intermediate species were detected in HPLC, and all ROS produced MB as the final compound, 1 could be suitable for detecting all the ROS species.Open in a separate windowFig. 3(A) HPLC chromatograms (detection at 254 nm; along with MB and neat 1) and (B) NMR-spectra (in DMSO-d₆) for addition of O₂˙⁻, ONOO⁻, OCl⁻ species to 1 in D₂O/H₂O to the DMSO-d₆.Having characterized the ROS reactions of 1 and 2 in the formation of MB, we focused on imaging the cellular produced ROS by confocal laser scanning microscopy. For this, we chose the established ROS-producing macrophage cell line J774.A1, which express ROS when treated with lipopolysaccharide (LPS) and interferon-γ (IFN-γ).The cells attached to coverslips were treated with LPS and IFN-γ, incubated for 24 hours to induce ROS. To these activated cells in 1 mL medium, 10 nmol of BHP-LMB 1 or 2 was added as DMSO or phosphate buffered saline (PBS) solution, respectively, and incubated for 6 h. The cells were fixated with 4% formalin, washed PBS and mounted on glass slides with Mowiol for microscopy. The membrane labelling WGA-488 probe was used additionally for reference. For control cells, no ROS induction was carried out with LPS/IFN-γ (Fig. 4, top). In addition, the hydrophobic 1 incorporated controlled cells, after formalin fixation, were treated with ONOO⁻ for 30 min, and washed with PBS (Fig. 4, bottom) to characterize the activation of intracellularly accumulated 1. This confirmed intra-cellularization of 1 and trapping the cellular produced ROS (Fig. 4, middle panel) or the added ROS by BHP-LMB 1 yielding red-emitting MB. A similar procedure was performed with 2, and microscopy imaging confirmed that there was no fluorescence (Fig. S4^†) indicating the dye 2 was not intracellularized.Open in a separate windowFig. 4Confocal microscopy images using 1 and J774.A1 cells for the detection of ROS; top: without ROS induction; middle: ROS expression induced by LPS/IFN-γ; bottom with ONOO⁻ addition to the fixed cells (conc: 10 nmol mL⁻¹ of 1, 6 h incubation; green: membrane labelling probe WGA-488 (exc. 488 nm), red: 1 after ROS reaction (exc. 660 nm).In summary, here we described broad-spectrum ROS/RNS detecting probes based on 2,6-di-tert-butyl phenol (BHP) appended leuco-methylene blue (BHP-LMBs, 1, 2). Hydrophobic 1 and hydrophilic 2 are sensitive towards a variety of ROS and RNS, producing the red fluorescent MB. In vitro titration with a variety of ROS showed formation of MB from 1 in DMSO, and also a persistent far red-absorbing radical species from 2 in H₂O. The NMR and HPLC analysis of 1 in DMSO with O₂˙⁻, ONOO⁻ and OCl⁻ showed no stable intermediates en route to MB. Then 1 was employed in detecting cellular produced ROS by fluorescence microscopy, which confirmed its intracellularization and suitability for detecting cellular generated ROS. The hydrophilic 2 found to be not intracellularized, and hence it may be used for extracellularly diffused ROS detection. These results suggest the suitability of 1, 2 in broad spectrum in vivo detection of ROS, which will be explored in future studies.