Electrocatalytic Oxidation of Some Carbohydrates by Nickel/Poly(o‐Aminophenol) Modified Carbon Paste Electrode

This study investigates the electrocatalytic oxidation of glucose and some other carbohydrates on nickel/poly(o‐aminophenol) modified carbon paste electrode as an enzyme free electrode in alkaline solution. Poly(o‐aminophenol) was prepared by electropolymerization using a carbon paste electrode bulk modified with o‐aminophenol and continuous cyclic voltammetry in HClO₄ solution. Then Ni(II) ions were incorporated to electrode by immersion of the polymeric modified electrode having amine group in 1 M Ni(II) ion solution. Cyclic voltammetric and chronoamperometric experiments were used for the electrochemical study of this modified electrode; a good redox behavior of Ni(OH)₂/NiOOH couple at the surface of electrode can be observed, the capability of this modified electrode for catalytic oxidation of glucose and other carbohydrates was demonstrated. The amount of α and surface coverage (Γ*) of the redox species and catalytic chemical reaction rate constant (k) for each carbohydrate were calculated. Also, the electrocatalytic oxidation peak currents of all tested carbohydrates exhibit a good linear dependence on concentration and their quantification can be done easily.     

Nickel/Poly(o‐aminophenol) Film Prepared in Presence of Sodium Dodecyl Sulfate: Application for Electrocatalytic Oxidation of Carbohydrates

Poly(o‐aminophenol) (POAP) was formed by successive cyclic voltammetry in monomer solution in the presence of sodium dodecyl sulfate (SDS) on the surface of a carbon paste electrode (CPE). Ni(II) ions were incorporated into the electrode by immersion of the polymeric modified electrode having amine groups in 0.1 M Ni(II) ion solution. Electrochemical study of this modified electrode shows a good redox behavior of the Ni(III)/Ni(II) couple. The electrocatalytic oxidations of glucose and other carbohydrates at the surface of the Ni/SDS‐POAP/CPE were studied in a 0.1 M NaOH solution. Compared to POAP/CPE, the SDS‐POAP/CPE significantly enhanced the catalytic efficiency of Ni ions for carbohydrates oxidation. Finally, using chronoamperometric method, the catalytic rate constants (k) for carbohydrates were calculated.     

Electrocatalytic Oxidation of Amines on a Mediator‐Modified Electrode by Electrochemical Copolymerization of Nitroxyl Radical Precursor Containing Pyrrole Side Chain and Bithiophene

This paper describes the electrocatalytic oxidation of amines on TEMPO (2,2,6,6‐tetramethylpiperidine‐1‐oxyl)‐modified electrodes prepared by electrochemical copolymerization of TEMPO precursor containing pyrrole side chain and 2,2′‐bithiophene. The modified electrode exhibits electrocatalytic activity for the oxidation of primary and secondary amines. Cyclic voltammetric studies showed that the peak current of the cyclic voltammogram increased linearly with increasing concentration of amine in the sample solution.     

Mechanistic – Kinetic Scheme of Oxidation/Reduction of TEMPO Involving Hydrogen Bonded Dimer. RDE Probe for Availability of Protons in Reaction Environment

Mechanistic – kinetic studies on the electrochemical oxidation/reduction process of radical TEMPO (2,2,6,6‐tetramethylpiperidine‐1‐oxyl) under ionic strength (0.1 M, 1.0 M) and pH (0, 7) of aqueous perchlorate electrolyte (NaClO₄‐HClO₄) have been undertaken. Analytical and/or digital simulation methods for voltammetry at stationary (CV) and rotating electrode (RDE) have allowed one to determine numerical values of twelve parameters characterizing two electrode reactions (oxidation and reduction of the radical) and three chemical reactions (protonation, disproportionation, dimerization involving the radical and/or electrogenerated species). A potential window of the measurements was 0.6 V and it corresponded to that where the oxidation wave of TEMPO in neutral aqueous solution is situated. To account for the observed pH effect, the hydrogen bonded dimer resulting from the radical reactant and the protonation product of its reduction has been postulated to form in solution near the electrode surface. The RDE voltammetric discernables of the TEMPO process (i.e., absolute RDE wave current, zero RDE current potential, oxidation and reduction limiting RDE currents) can be considered good candidates for a use to follow acidity of complex reactive media.     

Noncovalent Immobilization of Molecular Electrocatalysts for Chemical Synthesis: Efficient Electrochemical Alcohol Oxidation with a Pyrene–TEMPO Conjugate

Electrocatalytic methods for organic synthesis could offer sustainable alternatives to traditional redox reactions, but strategies are needed to enhance the performance of molecular catalysts designed for this purpose. The synthesis of a pyrene‐tethered TEMPO derivative (TEMPO=2,2,6,6‐tetramethylpiperidinyl‐N ‐oxyl) is described, which undergoes facile in situ noncovalent immobilization onto a carbon cloth electrode. Cyclic voltammetry and controlled potential electrolysis studies demonstrate that the immobilized catalyst exhibits much higher activity relative to 4‐acetamido–TEMPO, an electronically similar homogeneous catalyst. In preparative electrolysis experiments with a series of alcohol substrates and the immobilized catalyst, turnover numbers and frequencies approach 2 000 and 4 000 h⁻¹, respectively. The synthetic utility of the method is further demonstrated in the oxidation of a sterically hindered hydroxymethylpyrimidine precursor to the blockbuster drug, rosuvastatin.     

Electrocatalytic Oxidation of Vitamin B6 by 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl

Introduction Nitroxides such as 2,2,6,6-tetramethylpiperidine-N- oxyl (TEMPO) are well-known stable free radicals which have been extensively used in spin labeling,1 spin trapping2 and as antioxidants.3 Nitroxides are also easy to undergo reversible one-electron oxidation to form the corresponding oxoammonium ions.4 Oxoammonium ions are mild one-electron oxidants which have been used in organic synthesis5-8 and to generate radical cations.9 We10 found recently that electrochemically generat…     

β‐Cyclodextrin‐Functionalized Gold Nanoparticles/Poly(L‐cysteine) Modified Glassy Carbon Electrode for Sensitive Determination of Metronidazole

A simple electrochemical method was developed to determine metronidazole based on β‐cyclodextrin‐functionalized gold nanoparticles/poly(L ‐cysteine) modified glassy carbon electrode (β‐CD‐GNPs/poly(L ‐cys)/GCE). The electropolymerized film of poly(L ‐cys) provides a stable matrix for the fabrication of a sensing interface. β‐CD‐GNPs can form inclusion complexes with metronidazole and act as a modifier with catalytic function. The modified electrode exhibited excellent electrocatalytic activity towards metronidazole. The reaction of metronidazole at the modified electrode was an irreversible process controlled by diffusion. Under optimum experimental conditions, the logarithm of catalytic currents shows a good linear relationship with that of the metronidazole concentration in the range of 0.1–600 µmol/L with a low detection limit of 14 nmol/L. In addition, the modified electrode exhibited satisfactory stability, sensitivity and reproducibility, and could be applied to the determination of metronidazole in an injection solution.     

Preparation,Electrochemistry, and Electrocatalytic Activity of Lead Pentacyanonitrosylferrate Film Immobilized on Carbon Ceramic Electrode

Lead pentacyanonitrosylferrate (PbPCNF), a new Prussian blue analog, was immobilized on the surface of a carbon ceramic electrode (CCE) prepared by sol‐gel method. The immobilization process consists of adding a certain amount of metallic lead to the electrode matrix before gelation, and chemical derivatization of Pb on the electrode surface to a PbPCNF solid film by immersing the electrode in a solution of sodium pentacyanonitrosylferrate (PCNF). The composition of the synthesized PbPCNF was characterized by FTIR, scanning electron microscopy (SEM), and energy‐dispersive X‐ray (EDX) techniques. The resulting modified electrode showed electroactivity at two redox centers. The electrochemical behavior of the PbPCNF modified carbon ceramic electrode (PbPCNF|CCE) was studied by cyclic voltammetry. Under optimized conditions the peak‐to‐peak separation is only 39 mV, indicative of a surface reaction. Ion effects of the supporting electrolyte suggest that cations have a considerable effect on the electrochemical behavior of the modified electrode. The transfer coefficient (α) and the charge transfer rate constant at the modifying film|electrode interface (k_s) were calculated. The electrocatalytic activity of the modified electrode toward the electro‐reduction of peroxodisulfate was studied in details.     

Cu and Ni Nanoparticles Deposited on ITO Electrode for Nonenzymatic Electrochemical Carbohydrates Sensor Applications

A novel non‐enzymatic carbohydrates sensor which was an indium tin oxide (ITO) glass electrode modified by nickel and copper nanoparticles (Cu/Ni/ITO) was developed by an electrochemical method. The crystallinity, morphology, electrochemical measurements and amperometric response of the as‐prepared ITO modified electrode were examined by the X‐ray diffraction (XRD), scanning electron microscopic (SEM), cyclic voltammetry (CV) and chronoamperometry, respectively. The Cu/Ni/ITO electrode had better electroactivity for glucose oxidation than that obtained using Cu/ITO, Ni/ITO, and Ni/Cu/ITO. The logistic regression equation, I_pa = (A ₁ – A ₂)/[1 + (C_glucose /x ₀)^p ] + A ₂, was used to fit the calibration curves of glucose aqueous solution concentrations and responsive current intensity. In research of other saccharides, such as fructose, lactose, sucrose, and maltose, which were detected by the Cu/Ni/ITO electrode, it was obvious that the Cu/Ni/ITO electrode was more sensitive to monosaccharides than disaccharides. Monosaccharides and disaccharides can be detected because the saccharides themselves had aldehyde group or be isomerized to an isomer having an aldehyde group in alkaline environment, and then aldehyde group produced carboxylic acid in the catalytic oxidation of the electrode, which lead to the change of electrode surface conductivity and the appearance of oxidation peak, and the alkaline environment further promotes the above reaction.     

A Colloidal Au Monolayer Modulates the Conformation and Orientation of a Protein at the Electrode/Solution Interface

The orientation and conformation of adsorbed cytochrome c (cyt c) at the interface between an electrode modified with colloidal Au and a solution were studied by electrochemical, spectroscopic, and spectroelectrochemical techniques. The results indicate that the colloidal Au monolayer formed via preformation of an organic self‐assembled monolayer (SAM) can increase the electronic coupling between the SAM and cyt c in the same manner as bifunctional molecular bridges, one functional group of which is bound to the electrode surface while the other interacts with the protein surface. The approach of cyt c to the modified electrode/solution interface can be assisted by strong interactions of the intrinsic charge of colloidal particles with cyt c, while the heme pocket remains almost unchanged due to the screening effect of the negatively charged field created by the intrinsic charge. The conformational changes of cyt c induced by its adsorption at a bare glassy carbon electrode/solution interface and the effect of the electric field on the ligation state of the heme can be avoided at the colloidal‐Au‐modified electrode/solution interface. Finally, a possible model for the adsorption orientation of cyt c at the colloidal‐Au‐modified electrode/solution interface is proposed.     

Electrochemical Glutathione Sensor Based on Electrochemically Deposited Poly‐m‐aminophenol

Preparation and characterization of electrodes suitable for determination of glutathione is reported in this study. For this poly‐m‐aminophenol (PmAP), poly‐o‐aminophenol, and poly‐p‐aminophenol were electrochemically deposited from aqueous solution on the surface of glassy carbon (GC) electrode by potential cycling in the range of +0.2–+1.0 V. The modified GC electrodes were characterized by cyclic voltammetry, electrochemical impedance spectroscopy, contact angle measurement and ellipsometry. It was found that poly‐m‐aminophenol modified GC electrode (PmAP/GC‐electrode) is most suitable for electroanalytical determination of glutathione. An electroanalytical system for the determination of glutathione based on the PmAP/GC‐electrode was developed. The analytical system was characterized by low limit of detection, good stability, high sensitivity and wide linear detection range.     

Investigation of Methanol Behavior at the Designed Electrochemical Sensor based on Ni(II) Complex and Graphene Nanosheets

In this work, the electrochemical oxidation of methanol was investigated by different electrochemical methods at a carbon paste electrode (CPE) modified with (N‐5‐methoxysalicylaldehyde, N´‐2‐hydroxyacetophenon‐1, 2 phenylenediimino nickel(II) complex (Ni(II)–MHP) and reduced graphene oxide (RGO), which is named Ni(II)‐MHP/RGO/CPE, in an alkaline solution. This modified electrode was found to be efficient for the oxidation of methanol. It was found that methanol was oxidized by the NiOOH groups generated by further electrochemical oxidation of nickel(II) hydroxide on the surface of the modified electrode. Under optimum conditions, some parameters of the analyte (MeOH), such as the electron transfer coefficient (α), the electron transfer rate constant) k_s), and the diffusion coefficient of species in a 0.1 M solution (pH = 13), were determined. The designed sensor showed a linear dynamic range of 2.0–100.0 and 100.0–1000.0 μM and a detection limit of 0.68 μM for MeOH determination. The Ni(II)‐MHP/RGO/CPE sensor was used in the determination of MeOH in a real sample.     

An efficient way to functionalize graphene sheets with presynthesized polymer via ATNRC chemistry

Graphene nanosheets offer intriguing electronic, thermal and mechanical properties and are expected to find a variety of applications in high‐performance nanocomposite materials. The great challenge of exfoliating and dispersing pristine graphite or graphene sheets in various solvents or matrices can be achieved by facilely and properly chemical functionalization of the carbon nanosheets. Here we reported an efficient way to functionalize graphene sheets with presynthesized polymer via a combination of atom transfer nitroxide radical coupling chemistry with the grafting‐onto strategy, which enable us to functionalize graphene sheets with well‐defined polymer synthesized via living radical polymerization. A radical scavenger species, 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), was firstly anchored onto ? COOH groups on graphene oxide (GO) to afford TEMPO‐functionalized graphene sheets (GS‐TEMPO), meanwhile, the GO sheets were thermally reduced. Next, GS‐TEMPO reacted with Br‐terminated well‐defined poly(N‐isopropylacrylamide) (PNIPAM) homopolymer, which was presynthesized by SET‐LRP, in the presence of CuBr/N,N,N′,N′,N″‐pentamethyldiethylenetriamine to form PNIPAM‐graphene sheets (GS‐PNIPAM) nanocomposite in which the polymers were covalently linked onto the graphene via the alkoxyamine conjunction points. The PNIPAM‐modified graphene sheets are easily dispersible in organic solvents and water, and a temperature‐induced phase transition was founded in the water suspension of GS‐PNIPAM. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Selective Electrochemical Determination of Paracetamol Using Nanostructured Film of Functionalized Thiadiazole Modified Electrode

This paper describes the selective electrochemical determination of paracetamol (PA) in the presence of important interferent, ascorbic acid (AA) using an ultrathin electropolymerized film of 5‐amino‐1,3,4‐thiadiazole‐2‐thiol (p‐ATT) modified glassy carbon (GC) electrode in 0.20 M phosphate buffer solution (pH 7.20). Bare GC electrode failed to resolve the voltammetric signals of AA and PA in a mixture. On the other hand, the p‐ATT modified electrode not only separated the voltammetric signals of AA and PA but also enhanced their peak currents. We achieved the lowest detection limit of 0.34 nM (S/N=3) for PA at p‐ATT modified electrode.     

Preparation, Electrochemical Behavior and Electrocatalytic Activity of a Copper Hexacyanoferrate Modified Ceramic Carbon Electrode

A copper hexacyanoferrate modified ceramic carbon electrode （CuHCF/CCE） had been prepared by two-step sol-gel technique and characterized using electrochemical methods. The resulting modified electrode showed a pair of well-defined surface waves in the potential range of 0.40 to 1.0 V with the formal potential of 0.682 V （vs. SCE） in 0.050 mol·dm^-3 HOAc-NaOAc buffer containing 0.30 mol·dm^-3 KCl. The charge transfer coefficient （a） and charge transfer rate constant （ks） for the modified electrode were calculated. The electrocatalytic activity of this modified electrode to hydrazine was also investigated, and chronoamperometry was exploited to conveniently determine the diffusion coefficient （D） of hydrazine in solution and the catalytic rate constant （kcat）. Finally, hydrazine was determined with amperometry using the resulting modified electrode. The calibration plot for hydrazine determination was linear in 3.0 × 10^-6--7.5 × 10^-4 mol·dm^-3 with the detection limit of 8.0 × 10^-7 molodm^-3. This modified electrode had some advantages over the modified film electrodes constructed by the conventional methods, such as renewable surface, good long-term stability, excellent catalytic activity and short response time to hydrazine.     

Direct Electrochemistry and Electrocatalytic Properties of Hemoglobin Immobilized on Carbon Nanotubes Ionic Liquid Electrode

Hemoglobin (Hb) was directly immobilized on a multiwalled carbon nanotubes ionic liquid electrode by immersing this electrode in a solution consisting of Hb and 1‐octyl‐pyridinium chloride as an ionic liquid. The immobilized Hb displayed a pair of well‐defined cyclic voltammetric peaks with a formal potential of ?0.187 V in acetate buffer solution, pH 5.0. This modified electrode exhibits fast response, a long linearity range, a low detection limit, high stability and excellent sensitivity through hydrogen peroxide detection with a detection limit (3σ) of 3.18 µM. The electrode was also used for electrocatalysis and voltammetric determination of oxygen and trichloroacetic acid.     

Electrochemical properties of niosomes‐modified Au electrode and its interaction with 1‐anilino‐8‐naphthalene‐sulfonate

The gold electrode was modified by noisomes, prepared by sonicating the mixed solution of PEG 6000/Tween 80/Span 80/H₂O. Electrochemical cyclic voltammetry (CV) and impedance spectroscopy (EIS) were performed for characterizing the modification. The construction of multilayer films of octadecanethiol (ODT)‐niosomes had almost no pinholes, which elicited that niosomes‐coated electrode through ODT assembly was more effective immobilization compared to direct modification method. A large semicircle formation in the entire range of frequency indicated the complete electron‐transfer control for the redox reaction, implying a perfect blocking behavior. The Nyquist plot was consisting of two depressed semicircles after storage for 36 h in air, which implied a progressive and uniform construction manner, as expected for porous coatings. The modified electrode was used to investigate the interaction between 1‐anilino‐8‐naphthalene‐sulfonate (ANS) and noisomes. It was found that the low‐frequency semicircular arcs increase in diameter, indicating that the increase in the resistance R_p attributed mainly to the niosomes/solution interface. It was due to the binding of ANS to the niosomes‐modified electrode surface, exhibited against diffusing species through the pores. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis of poly(chloromethylstyrene‐b‐styrene) block copolymers by controlled free‐radical polymerization

The controlled free‐radical polymerization of styrene and chloromethylstyrene monomers in the presence of 2,2,6,6‐tetramethyl‐1‐piperidinyloxyl (TEMPO) has been studied with the aim of synthesizing block copolymers with well‐defined structures. First, TEMPO‐capped poly(chloromethylstyrene) was prepared. Among several initiating systems [self‐initiation, dicumyl peroxide, and 2,2′‐azobis(isobutyronitrile)], the last offered the best compromise for obtaining a good control of the polymerization and a fast polymerization rate. The rate of the TEMPO‐mediated polymerization of chloromethylstyrene was independent of the initial concentration of TEMPO but unexpectedly higher than the rate of the thermal self‐initiated polymerization of chloromethylstyrene. Transfer reactions to the chloromethyl groups were thought to play an important role in the polymerization kinetics and the polydispersity index of the resulting poly(chloromethylstyrene). Second, this first block was used as a macroinitiator in the polymerization of styrene to obtain the desired poly(chloromethylstyrene‐b‐styrene) block copolymer. The kinetic modeling of the block copolymerization was in good agreement with experimental data. The block copolymers obtained in this work exhibited a low polydispersity index (weight‐average molecular weight/number‐average molecular weight < 1.5) and could be chemically modified with nucleophilic substitution reactions on the benzylic site, opening the way to a great variety of architectures. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3845–3854, 2000     

Simultaneous evaluation of one‐electron reducing systems and radical reactions in cells by nitroxyl biradical as probe

In the present study, a novel probe for the simultaneous evaluation of one‐electron reducing systems (electron transport chain) and one‐electron oxidizing systems (free radical reactions) in cells by electron chemical detection was developed. Six‐membered cyclic nitroxyl radicals (2,2,6,6‐tetramethylpiperidine‐1‐oxyl; TEMPO series) are sensitive to one‐electron redox systems, generating the hydroxylamine form [TEMPO(H)] via one‐electron reduction, and the secondary amine form [TEMPO(N)] via one‐electron oxidation in the presence of thiols. In contrast, the sensitivities of five‐membered cyclic nitroxyl radicals (2,2,5,5‐tetramethylpyrrolidine‐1‐oxyl; PROXYL series) to the one‐electron redox systems are comparatively low. The electron chemical detector can detect 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO), TEMPO(H) and PROXYL but not TEMPO(N). Therefore, nitroxyl biradical, TEMPO‐PROXYL, as a probe for the evaluation of one‐electron redox systems was employed. TEMPO‐PROXYL was synthesized by the conjunction of 4‐amino‐TEMPO with 3‐carboxyl‐PROXYL via the conventional dicyclohexyl carbodiimide reaction. TEMPO‐PROXYL, TEMPO(H)‐PROXYL and TEMPO(N)‐PROXYL were simultaneously quantified by HPLC with Coularray detection. Calibration curves for the quantification of TEMPO‐PROXYL, TEMPO(H)‐PROXYL and TEMPO(N)‐PROXYL were linear in the range from 80 nm to 80 μm , and the lowest quantification limit of each molecule was estimated to be <80 nm . The relative standard deviations at 0.8 and 80 μm were within 10% (n = 5). Copyright © 2015 John Wiley & Sons, Ltd.     

Decomposition of model alkoxyamines in simple and polymerizing systems. I. 2,2,6,6‐tetramethylpiperidinyl‐ N‐oxyl‐based compounds

The thermal decomposition of five alkoxyamines labeled TEMPO–R, where TEMPO was 2,2,6,6‐tetramethylpiperidinyl‐N‐oxyl and R was cumyl (Cum), 2‐tert‐butoxy‐carbonyl‐2‐propyl (PEst), phenylethyl (PhEt), 1‐tert‐butoxy‐carbonylethyl (EEst), or 1‐methoxycarbonyl‐3‐methyl‐3‐phenylbutyl (Acrylate‐Cum), was studied with ¹H NMR in the absence and presence of styrene and methyl methacrylate. The major products were alkenes and the hydroxylamine 1‐hydroxy‐2,2,6,6‐tetramethyl‐ piperidine (TEMPOH), and in monomer‐containing solutions, unimeric and polymeric alkoxyamines and alkenes were also found. Furthermore, the reactions between TEMPO and the radicals EEst and PEst were studied with chemically induced dynamic nuclear polarization. In comparison with coupling, TEMPO reacted with the radicals Cum, PEst, PhEt, and EEst and their unimeric styrene adducts by disproportionation to alkenes and TEMPOH only to a minor extent (0.6–3%) but with the radical adducts to methyl methacrylate to a considerable degree (≥20%). Parallel to the radical cleavage, TEMPO–EEst (but not the other alkoxyamines or TEMPO–Acrylate‐Cum) underwent substantial nonradical decay. The consequences for TEMPO‐mediated living radical polymerizations are discussed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3604–3621, 2001