Electrochemical sensor based on an electrode modified with porous graphitic carbon nitride nanosheets (C3N4) embedded in graphene oxide for simultaneous determination of ascorbic acid, dopamine and uric acid.

Two-dimensional porous graphitic carbon nitride (g-C3N4) nanosheets were synthesized by low-cost and direct thermal oxidation. Porous g-C3N4 assembled with graphene oxide (GO) was immobilized on a glassy carbon electrode. The sensor was applied to simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA) with high performance. Cyclic voltammetry and differential pulse voltammetry were used to investigate electrochemical and electrocatalytic properties. The results indicate that the electrochemical sensor possesses high specific surface area, hierarchical pore structure and excellent signal response to AA, DA and UA. The oxidation potentials are well separated at around 0.15, 0.34 and 0.46 V for AA, DA and UA respectively. The determination limits for AA, DA and UA are 3.7 µM, 0.07 µM and 0.43 µM, respectively. The sensor was applied to tracking the three analytes in spiked serum samples with recovery 95.1~105.5% and relation standard deviations of less than 5%. Graphical abstract Schematic representation of porous graphitic carbon nitride nanosheet embedded in graphene oxide for simultaneous determination of ascorbic acid, dopamine and uric acid.

On-plate enzyme and inhibition assay of glucose-6-phosphate dehydrogenase using thin-layer chromatography.

We performed on-plate enzyme and inhibition assays of glucose 6-phosphate dehydrogenase using thin-layer chromatography. The assays were accomplished based on different retardation factors of the substrates, enzyme, and products. All the necessary steps were integrated on-plate in one developing process, including substrate/enzyme mixing, reaction starting, and quenching as well as product separation. In order to quantitatively measure the enzyme reaction, the developed plate was then densitometrically evaluated to determine the peak area of the product. Rapid and high-throughput assays were achieved by loading different substrate spots and/or enzyme (and inhibition) spots in different tracks on the plate. The on-plate enzyme assay could be finished in a developing time of only 4 min, with good track-to-track and plate-to-plate repeatability. Moreover, we determined the Km values of the enzyme reaction and Ki values of the inhibition (Pb(2+) Cd(2+) and Cu(2+) as inhibitors), as well as the corresponding kinetics using the on-plate assay. Taken together, our method expanded the application of thin-layer chromatography in enzyme assays, and it could be potentially used in research fields for rapid and quantitative measurement of enzyme activity and inhibition.

Graphene oxide and reduced graphene oxide as novel stationary phases via electrostatic assembly for open-tubular capillary electrochromatography.

We present here the application of graphene oxide (GO) and reduced graphene oxide (GOOH) sheet as novel stationary phases for open-tubular CEC (OTCEC) separation based on electrostatic assembly. The inner walls of a bare capillary column was first modified by ionic assembly of poly (diallyldimethylammonium chloride) (PDDA), and then negatively charged GO or GOOH was easily assembled on a positively charged interior walls of the capillary by electrostatic force. Scanning Electron Microscope images showed that GO and GOOH can still maintain sheet-layer-like structure when coated onto the capillary via electrostatic assembly. The chromatographic properties of the GO and GOOH coated columns were evaluated via OTCEC separations of various kinds of analytes, including three acid nitrophenol isomers, three basic nitroaniline isomers, and four neutral PAHs. Efficient separations of all the analytes were achieved with optimized buffer pH and organic additive. The reproducibility and stability of the GO or GOOH coated columns were investigated. Our results indicate the capability of application GO or GOOH sheet in OTCEC separation, which can be coated on the inner wall of fused-silica capillary via electrostatic assembly.

Polyoxometalate@g-C3N4 nanocomposite for enhancing visible light photoelectrocatalytic performance.

The prepared g-C3N4 under morphology controlling shows better physic and chemical performance. The synergistic effect of POM and g-C3N4 in the hybrid improves its high photocatalytic capability. The research indicates that g-C3N4-based material is a potential photoelectrode for PEC degradation. Besides, the PMoV nanocomposite shows better activities in the PEC and removal efficiency of RhB. Compared with the same PEC system, the degradation time of RhB is shorter and the degradation efficiency is higher for the MCN/PMoV catalysts.

A fast and facile electrochemical method for the simultaneous detection of epinephrine, uric acid and folic acid based on ZrO2/ZnO nanocomposites as sensing material.

A novel electrochemical sensor based on ZrO2 and ZnO hybrid nanocomposites was developed for simultaneous determination of epinephrine (EA), uric acid (UA) and folic acid (FA) with the highly selective and ultrasensitive. The nanocomposites have been synthesized by chemical precipitation and thermal calcine method with economical, eco-friendly and practical nature. Their structure and electrochemical properties were investigated by X-ray diffraction (XRD), X-ray photo electron spectroscopy (XPS), scanning electron microscopy (SEM) and electrochemical techniques. The results reveal that the ZrO2/ZnO nanocomposites can possesses highly exposed catalytic sites, favorable conductivity, and the sensor excellent signal response for EP, UA and FA under the optimal condition. The electrochemical sensing platform has a low detection limit of 0.039 µM, 0.29 µM and 0.037 µM and a wide detection range of 0.8-420 µM, 10-2400 µM and 2-480 µM, respectively. It was also tested with a human blood serum sample at physiological pH with recovery 97.3-103.8% and relation standard deviation less than 5%. It indicates that the electrochemical sensors has a hopeful capacity of extensive applications in bioanalysis and diseases diagnosis.

Fabrication of Trifunctional Polyoxometalate-Decorated Chitosan Nanofibers for Selective Production of 2,5-Diformylfuran.

Trifunctional catalysts based on polyoxometalate (POM) decorating chitosan nanofibers (H5 PMo10 V2 O40 /chitosan nanofibers, abbreviated as HPMoV/CS-f), synthesized by using the electrospinning method, realized highly efficient oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF). Decorating chitosan nanofibers with POMs generated enhanced catalytic activity by merging their unique individual properties of redox ability, Brønsted acidity, basicity, and nanofiber structure with higher surface area. As a result, HPMoV/CS-f(25) (with 25 representing the POM amount) was found to be the most active catalyst in the aerobic oxidation of HMF, resulting in 94.1 % DFF yield at 96.2 % conversion in DMSO at 120 °C for 6 h, whereas 56.2 % DFF yield at 95.0 % conversion was obtained in water at 140 °C for 8 h. Importantly, DFF could be produced in one pot in one step to give 61.9 and 31.4 % yield, respectively, directly from fructose and glucose under the reaction conditions of 140 °C, 6 h in DMSO, which was owing to the suitable balance of Brønsted acidity and basicity of the trifunctional HPMoV/CS-f(25). Moreover, HPMoV/CS-f showed good stability and ability to be reused at least ten times without leaching of the POMs from the chitosan nanofibers.

Biodiesel production from high acid value waste frying oil catalyzed by superacid heteropolyacid.

Transesterification of waste cooking oil with high acid value and high water contents using heteropolyacid H3PW12O40 x 6H2O (PW12) as catalyst was investigated. The hexahydrate form of PW(12) was found to be the most promising catalyst which exhibited highest ester yield 87% for transesterification of waste cooking oil and ester yield 97% for esterification of long-chain palmitic acid, respectively. The PW12 acid catalyst shows higher activity under the optimized reaction conditions compared with conventional homogeneous catalyst sulfuric acid, and can easily be separated from the products by distillation of the excess methanol and can be reused more times. The most important feature of this catalyst is that the catalytic activity is not affected by the content of free fatty acids (FFAs) and the content of water in the waste cooking oil and the transesterification can occur at a lower temperature (65 degrees C), a lower methanol oil ratio (70:1) and be finished within a shorter time. The results illustrate that PW12 acid is an excellent water-tolerant and environmentally benign acid catalyst for production of biodiesel from waste cooking oil.

Fabrication of mesoporous POMs/SiO2 nanofibers through electrospinning for oxidative conversion of biomass by H2O2 and oxygen.

Polyoxometalate H5PMo10V2O40/SiO2 (HPMoV/SiO2) nanofibers with a mesoporous structure were fabricated through combined electrospinning and surfactant-directing pore formation technique. These heterogeneous nanofibers were characterized by a wide variety of techniques to show mesoporous structure, high surface area, excellent stability, nanofiber morphology, highly efficient activity and selectivity towards the oxidation of starch and 5-hydroxymethylfurfural (5-HMF) either by H2O2 or by oxygen. POM/SiO2 nanofibers were suitable for oxidation of starch by H2O2 and oxidation of 5-HMF by O2 with high efficiency of 0.58 mol per 100 g carboxyl content and 89.2% yield of DFF; these results are comparable with those obtained using homogeneous H5PMo10V2O40 and HPMoV/SiO2 without nanofiber morphology. Furthermore, HPMoV/SiO2 nanofibers could be easily recycled and reused at least ten times with no significant loss of catalytic activity due to their nanofiber morphology.

Incorporation of Ce3+ ions into dodecatungstophosphoric acid for the production of biodiesel from waste cooking oil.

A series of cerium-exchanged dodecatungstophosphates CexH3-3xPW12O40 (CexH3-3xPW, x = 0.4, 0.6, 0.7, 0.8, 0.9, and 1.0) were designed and characterized by Fourier transform infrared spectroscopy (FT-IR), pyridine adsorption IR spectra, X-ray diffraction (XRD), and temperature programmed desorption of ammonia (NH3-TPD). The activity of these catalysts was evaluated for the generation of biodiesel from waste cooking oil (WCO) with 27 wt% of free fatty acids (FFAs) and 1 wt% of water. Compared to theri parent H3PW12O40, CexH3-3xPW showed higher activity for esterification of FFAs and transesterification of triglyceride to mono-alkyl esters of fatty acids (FAMEs) in one-pot. The acidic properties of CexH3-3xPW depended on the amount of Ce3+ ions in the secondary structure of Keggin heteropolyacids, while conversion of triglycerides and FFAs depended on their increasing acid contents. Among CexH3-3xPW, Ce0.7H0.9PW showed significant activity due to its high Brønsted acidity and Lewis acidity with 98% conversion of WCO and almost 100% selectivity to FAME at the molar ratio of methanol to WCO = 21:1 and 65 °C for 12 h. The reaction adhered to first-order kinetics with the activation energy (Ea) of 71 kJ/mol and the frequency factor (A) of 1.8 × 108 min-1, while the reaction rates were not influenced by the internal mass transport. The catalyst behaved as a heterogeneous catalyst, which can achieve the regeneration and be used more than five runs but without obvious decrease in activity. The characteristics of the WCO methyl ester were found to be close to the engine requirement.

Tailoring the Synergistic Bronsted-Lewis acidic effects in Heteropolyacid catalysts: Applied in Esterification and Transesterification Reactions.

In order to investigate the influences of Lewis metals on acidic properties and catalytic activities, a series of Keggin heteropolyacid (HPA) catalysts, HnPW11MO39 (M = Ti(IV), Cu(II), Al(III), Sn(IV), Fe(III), Cr(III), Zr(IV) and Zn(II); for Ti and Zr, the number of oxygen is 40), were prepared and applied in the esterification and transesterification reactions. Only those cations with moderate Lewis acidity had a higher impact. Ti Substituted HPA, H5PW11TiO40, posse lower acid content compared with Ti(x)H(3-4x)PW12O40 (Ti partial exchanged protons in saturated H3PW12O40), which demonstrated that the Lewis metal as an addenda atom (H5PW11TiO40) was less efficient than those as counter cations (Ti(x)H(3-4x)PW12O40). On the other hand, the highest conversion reached 92.2% in transesterification and 97.4% in esterification. Meanwhile, a good result was achieved by H5PW11TiO40 in which the total selectivity of DAG and TGA was 96.7%. In addition, calcination treatment to H5PW11TiO40 make it insoluble in water which resulted in a heterogeneous catalyst feasible for reuse.

Ultra-deep desulfurization via reactive adsorption on peroxophosphomolybdate/agarose hybrids.

A catalyst system composed of peroxophosphomolybdates as catalytic center and agarose as matrix material had been designed. The [C16H33N(CH3)3]3[PO4{MoO(O2)2}4]/agarose (C16PMo(O2)2/agarose) hybrid was found to be active for oxidation desulfurization (ODS) of dibenzothiophene (DBT) or real fuel into corresponding sulfone by H2O2 as an oxidant, while the sulfur content could be reduced to 5ppm. The higher activity comes from its components including [PO4{MoO(O2)2}4] catalytic sites, the hydrophobic quaternary ammonium cation affinity to low polarity substrates, and agarose matrix affinity to H2O2 and sulfone. During the oxidative reaction, the mass transfer resistance between H2O2 and organic sulfurs could be decreased and the reaction rate could increase by the assistance of agarose and hydrophobic tails of [C16H33N(CH3)3]3[PO4{MoO(O2)2}4]. Meanwhile, the oxidative products could be adsorbed by agarose matrix to give clean fuel avoiding the post-treatment. In addition, the hybrid was easily regenerated to be reused.

Borate-templated self-assembly of multinuclear nickel(II)-containing POMs.

Two unprecedented nickel-boron cluster-containing POMs have been isolated through a facile approach by the reactions of trivacant [A-α-SiW9O34](10-) and Ni(2+) in borate buffer. The {BO3} and {B2O4(OH)2} units enclosed in the Ni8 cluster of 1 and the Ni12 cluster of 2 play important templating roles in the assembly of the target compounds.

Assembly and spontaneous resolution of the chiral inorganic polyoxometalates-based frameworks via helical chains association.

Two pairs of enantiomerically pure three-dimensional (3D) chiral polyoxometalate (POM) framework materials l,d-[K(H2O)]6·[H2GeW12O40]3·35H2O (1a and 1b) and l,d-[K(H2O)]6·[H2SiW12O40]3·29H2O (2a and 2b) based on inorganic achiral building blocks, have been synthesized and characterized by X-ray crystallography, elemental analysis, powder X-ray diffraction (PXRD), UV-Vis spectroscopy, circular dichroism (CD) spectra. Single-crystal X-ray diffraction analyses revealed that 1a and 1b, 2a and 2b are enantiomers, respectively. In 1a and 1b, 2a and 2b, {K(H2O)}n link terminal oxygen and µ2-bridging oxygen of Keggin-type polyanion moieties to generate 1D 31 helical infinite chiral chains, which are further connected by the achiral Keggin-type polyoxoanions to form 3D 4,8-connected chiral self-assembly frameworks with {4(12)·6(10)·8(6)}{4(6)}2 topology. 1 and 2 are isostructural. They are obtained by spontaneous resolution upon crystallization in the absence of any chiral source. They represent new examples of chiral self-penetrating pure inorganic frameworks known for POM systems. Compounds 1 and 2 display adsorption activity toward volatile organic compounds (VOCs).