Preparation and Redox Properties of N,N,N‐1,3,5‐Trialkylated Flavin Derivatives and Their Activity as Redox Catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H₂O₂ oxidation of methyl p‐tolyl sulfide. Introduction of electron‐withdrawing groups increased the stability of the reduced catalyst precursor.     

Characterization and Applications of a Bismuth Bulk Electrode

A bismuth bulk electrode (BiBE), a new solid‐state electrode, is presented. The polycrystalline metal bismuth disk‐shaped electrode was examined for its anodic stripping voltammetry performance, which was found to be well comparable to that achieved with the bismuth or mercury film electrodes. Useful potential windows of the BiBE in aqueous solutions of pH 1 to 13 were found to range from approximately ?1.7 to ?0.1 V, depending on pH, where either hydrogen evolution or anodic dissolution of metallic bismuth limit the electrochemical inertness of the BiBE. Employing cyclic voltammetry (CV), the cathodic behavior of the BiBE was examined by testing inorganic (cadmium(II) ions) and organic (2‐nitrophenol) model compounds; a CV quasi‐reversible behavior was recorded in the case of the Cd(II)‐Cd(0) couple. The characteristics of the BiBE under anodic conditions, i.e., at bismuth surface coated with a thin conductive Bi₂O₃ film, was examined by testing two well‐established redox systems, potassium hexacyanoferate(III) and ruthenium(III) hexaaminechloride; a nearly reversible behavior was recorded in the latter case. Based on the presented preliminary results, BiBE can be considered as an interesting alternative to common solid and (toxic) mercury electrodes for possible use in electrochemical studies and electroanalytical applications.     

Synthesis,electrochemistry and in situ spectroelectrochemistry of water-soluble phthalocyanines

The synthesis, characterization and voltammetric and spectroelectrochemical properties of newly synthesized metal-free and metallo phthalocyanines (M = Co, Cu, Zn) containing four dialkylaminophenoxy or trialkylammoniumphenoxy substituents on peripheral positions have been presented in this work. The new compounds have been characterized by using elemental analysis, UV–Vis, FT-IR, ¹H NMR and MS spectroscopic data. Phthalocyanines with trialkylammoniumphenoxy substituents are soluble in aqueous solution over a wide pH range, and these compounds are present as aggregated species in solution as confirmed by the blue shift of Q-bands in their electronic spectra. The electrochemical behavior of the phthalocyanines was investigated by cyclic voltammetry and differential pulse voltammetry on a platinum-working electrode in DCM and DMSO. The voltammetric and spectroelectrochemical measurements of the complexes show that while cobalt phthalocyanine gives both ligand- and metal-based redox processes, metal-free, zinc and copper phthalocyanine complexes give only ligand-based processes in harmony with common phthalocyanine complexes.     

Direct Electrochemistry with Nitrate Reductase in Chitosan Films

Stable films made from chitosan (CS) on pyrolytic graphite (PG) electrode gave direct electrochemistry for incorporated enzyme nitrate reductase (NR). Cyclic voltammetry (CV) of NR‐CS films showed a pair of well‐defined and nearly reversible redox peaks at about ?0.430 V vs. SCE at pH 7.0 phosphate buffers, which was considered as the redox of FAD and heme‐ion. The electron transfer between NR and PG electrode was greatly facilitated in CS films. Integration of reduction peaks at different scan rates from 0.01 to 0.5 V s^?;1 gave nearly constant charge values, which is characteristic of thin‐larger‐electrochemical behavior. The pH of the solution strongly affected the direct electron transfer of NR‐CS films. EDTA accelerated the electron transfer, and it was proposed as a stimulant for the system. Reflectance absorption infrared spectra demonstrated that NR retained a nearly native conformation in CS films.     

Electrochemical,In Situ Spectroelectrochemical,In Situ Electrocolorimetric and Electrocatalytic Characterization of Metallophthalocyanines Bearing Four Dioctylaminocarbonyl Biphenyloxy Substituents

This study describes the electrochemical, in situ spectroelectrochemical, in situ electrocolorimetric and electrocatalytic characterization of metallophthalocyanines bearing four dioctylaminocarbonyl biphenyloxy groups (MPc's). While CoPc gives both metal‐based and ring‐based redox processes, ZnPc and CuPc show only ring‐based reduction and oxidation processes. In‐situ electrocolorimetric method was applied to investigate color of the electrogenerated anionic and cationic forms of the complexes. Perchloric acid titrations monitored by cyclic voltammetry and spectrophotometry represent possible electrocatalytic activities of the complexes for hydrogen evolution reaction. CuPc having inactive metal center incorporated into a Nafion film on GCE decreases overpotential of the electrode for H⁺ reduction in aqueous solution.     

Preparation and redox properties of N,N,N-1,3,5-trialkylated flavin derivatives and their activity as redox catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H2O2 oxidation of methyl p-tolyl sulfide. Introduction of electron-withdrawing groups increased the stability of the reduced catalyst precursor.     

胆红素LB膜的电化学行为研究

胆红素（BR）是存在于动物体内的一种重要生物物质，也是人和绝大多数哺乳动物体内血红蛋白等含铁卟啉化合物分解代谢的产物和代谢中间体。它是一种内源性抗氧化剂，对肝细胞的再生具有积极的作用，BR常以线状四吡咯或类卟啉结构的形式存在。胆红素及其金属配合物在水溶液犤１～６犦与有机介质（如DMF和DMSO）犤７～９犦中的电化学行为，文献上已有较多报道。前文犤１０～１２犦我们报道了胆红素可在不同亚相（酸性、中性及部分金属离子）表面形成Langmuir－Blodgett（LB膜）；在有磷脂存在时，其成膜性能更佳犤１３犦。由于LB膜同生…     

液流电池正极活性物质钛铁试剂初探

应用循环伏安法研究钛铁试剂(4,5-二羟基-1,3-苯二磺酸钠)电对(石墨基底)在水溶液中的电化学行为.实验表明,溶液pH<4时,该试剂的氧化还原反应电位较高,可逆性好.以其作正极与V(III)/V(II)负极组成液流电池,在硫酸介质中,充放电库仑效率可达90%以上,而且循环寿命较好,性能衰减小,具备作为液流电池正极活性物质的可行性.     

[SiCu(H2O)W11O39]6-在玻碳电极上的多层组装及电催化性能

The electrochemical behavior of SiCu W11 heteropolyacide in acidic aqueous solution was studied. The effect of solution pH on the electrochemical behavior of SiCu W11 was discussed and the mechanism was suggested. New electrode was modified by muhilayer films composed of heteropolyanion (SiCu W11 and cationic polymer poly (diallyldimethylammonium chloride). Cyclic vohammetry showed the uniform growth of the film. The modified electrodes exhibited some special electrochemical properties in the films, different from those in homogeneous aqueous solutions. The effect of pH on the redox behavior of SiCu W11 in the films was discussed in details. The muhilayer film electrodes have an excellent electrocatalytic response to the reduction of BrO3^- and NO2^-.     

Reversible Electrochemistry of Cytochrome c on Recompressed,Binderless Exfoliated Graphite Electrodes

The direct electrochemistry of cytochrome c (cyt‐c) has been investigated on exfoliated graphite (EG) electrodes. The as‐polished and roughened (using SiC emery sheet) EG surfaces are inactive for the direct electron transfer. However, when the EG electrode was sonicated before the experiment, a pair of redox waves were obtained for freely diffusing cyt‐c in the solution phase. The formal potential was found to be 0.01 V (vs. SCE) in 0.1 M phosphate buffer at a pH of 7.1. The electrochemical response for the adsorbed cyt‐c on sonicated EG electrodes, which is shown to have carbonyl functional groups on its surface, shows nearly reversible voltammograms in the same electrolyte. However, the formal potential in the adsorbed state is more negative than that observed for the solution phase cyt‐c. A structure based on an open heme conformation proposed by Hildebrandt and Stockburger is probably present on the EG surface. It is suggested that the electrochemistry at the EG electrode is essentially governed by favourable electrostatic interactions.     

二茂铁基1,3,4-三唑席夫碱衍生物合成及生物活性和电化学性质

首次合成了二茂铁基-1,3,4-三唑Schiff碱衍生物（5a~5e）,利用IR、1H NMR和HRMS等手段对其进行了结构表征。 用黄瓜子叶生根法以及小麦芽鞘法,分别测试化合物的细胞分裂素活性和生长素活性。 结果表明,在10 mg/L浓度下,有4个化合物的子叶生根促进作用为46.9%~73.6％,比对参照物的细胞分裂活性更高,但生长素活性不明显,化合物5c仅提高25％;化合物的除草活性在68%~72%之间。 以石墨烯修饰的玻碳电极为工作电极,在Britton-Robison缓冲溶液中,研究了目标化合物电化学行为。 结果表明,在pH值为2或3的溶液中,合成目标化合物具有良好的氧化还原可逆性,氧化峰电流对数（lg Ipc）与扫描速度对数(lg v)、浓度对数(lg c)均呈现良好线性关系,目标化合物(5a~5d)的电子转移过程均受吸附-扩散混合控制,该类化合物的循环伏安法最低检出限为10-5 mol/L。     

Adsorptive Square‐Wave Voltammetry for the Analysis and Speciation of Complexes of Cd(II)‐Ferron

The theory of adsorptive stripping square‐wave voltammetry (SWV) for relatively low ligand concentrations is employed to determine the reduction mechanism of Cd(II)‐ferron complexes accumulated on a static mercury drop electrode at different pH values. The electrochemical behavior of ferron molecules indicated that the adsorptive concentration of Cd(II) is possible in solutions with 3.5<pH<11, providing a wide pH range where the interference of other ligands present in real samples would be not so critical. Cyclic voltammetry experiments were also performed for the purpose of comparison. Fitting between experimental and theoretical square‐wave voltammograms shows that the prevailing species at the reaction layer coincide with the equilibrium bulk distribution. The simulation procedure indicated that the electrochemical rate constants of Cd(II)‐ferron complexes varied from (6±1) s^?1 to (0.17±0.01) s^?1 for solutions analyzed at pH 3.9 and 10.8, respectively. Changes at the surface concentrations are discussed considering the ligand to complex ratios at the electrode surface and at the solution bulk. From this analysis it is possible to infer that the oxidized metal species are produced in the electrolytic solution instead of on the electrode surface.     

Carbon Paste Electrode Incorporating 1‐[4‐(Ferrocenyl Ethynyl) Phenyl]‐1‐Ethanone for Electrocatalytic and Voltammetric Determination of Tryptophan

A carbon paste electrode spiked with 1‐[4‐ferrocenyl ethynyl) phenyl]‐1‐ethanone (4FEPE) was constructed by incorporation of 4FEPE in graphite powder‐paraffin oil matrix. It has been shown by direct current cyclic voltammetry and double step chronoamperometry that this electrode can catalyze the oxidation of tryptophan (Trp) in aqueous buffered solution. It has been found that under optimum condition (pH 7.00), the oxidation of Trp at the surface of such an electrode occurs at a potential about 200 mV less positive than at an unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and rate constant for the chemical reaction between Trp and redox sites in 4FEPE modified carbon paste electrode (4FEPEMCPE) were also determined using electrochemical approaches. The electrocatalytic oxidation peak current of Trp showed a linear dependent on the Trp concentrations and linear calibration curves were obtained in the ranges of 6.00×10^?6 M–3.35×10^?3 M and 8.50×10^?7 M–6.34×10^?5 M of Trp concentration with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods, respectively. The detection limits (3σ) were determined as 1.80×10^?6 M and 5.60×10^?7 M by CV and DPV methods. This method was also examined as a selective, simple and precise new method for voltammetric determination of tryptophan in real sample.     

Electrochemical Behavior of Chloranil Chemically Modified Carbon Paste Electrode. Application to the Electrocatalytic Determination of Ascorbic Acid

A p‐chloranil modified carbon paste electrode was constructed and the electrochemical behavior of this electrode was studied in the aqueous solution with different pH. From the E_1/2–pH diagram for this compound the values of formal potential E^0' and pK_a of some different redox and acid‐base couples depending on the solution pH were estimated. The diffusion coefficient, D, value for p‐chloranil was estimated 1.5×10^?7 cm² s^?1. It has been shown by direct current cyclic voltammetry and double potential step chronoamperometry, that this p‐chloranil incorporated carbon paste electrode, can catalyze the oxidation of ascorbic acid in the aqueous buffered solution. Under the optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such an electrode occurs at a potential about 325 mV less positive than that at an unmodified carbon past electrode. The catalytic oxidation peak currents was linearly dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range of 7×10^?5 M–4×10^?3 M of ascorbic acid with a correlation coefficient of 0.9998. The limit of detection (3σ) was determined as 3.5×10 ^?5 M. This method was used as simple, selective and precise voltammetric method for determination of ascorbic acid in pharmaceutical preparations.     

Analytical Modelling of Electron-coupled Ion Transfers with Immobilized vs Soluble Redox Transducer at Thick Film-modified Electrodes

Analytical theoretical solutions are deduced for the current-potential response, concentration profiles and interfacial potentials of electron-coupled ion transfers in the cyclic voltammetry with thick film-modified electrodes. The theory covers a wide variety of possible situations, namely, the redox transducer can be either immobilized on the working electrode or freely diffusing in the organic film, and the ion transfer can be either simple or complicated by homogeneous chemical processes. A comparative and comprehensive study of each case is performed, establishing the key parameters that define the behaviour of the system, as well as guidelines to analyse its electrochemical signal.     

Determination of Imidacloprid in Tomato Grown in Greenhouse Based on Copper(II) Phthalocyanine Modified Carbon Ceramic Electrode by Differential Pulse Voltammetry

A new sol‐gel derived electrocatalytic carbon ceramic electrode was prepared by incorporating copper(II) phthalocyanine (CuPc) in a carbon ceramic network. This electrode was used as a sensitive electrochemical sensor for determination of the insecticide Imidacloprid (1‐(6‐chloro‐3‐pyridylmethyl)‐N‐nitro‐imidazolidin‐2‐ylideneamine) by differential pulse voltammetry (DPV). The resulting modified electrode exhibits a cathodic peak potential shifted positively and an increasing in cathodic peak current in comparison with unmodified electrode. The redox properties of this modified electrode at various pH values and CuPc percentage were investigated. The catalytic current obtained from differential pulse voltammetry is linearly dependent on Imidacloprid concentration over the two linear ranges of 0.67‐17 μM and 17‐93 μM with correlation coefficient of R² = 0.9999 and R² = 0.990, respectively. The detection limit for Imidacloprid was found to be 0.28 μM according to lower linear range. Possible interferences from several common pesticides were also evaluated. The inherent stability, high sensitivity, low detection limit and low cost for each preparation are advantages of this sensor. Determination of Imidacloprid in commercial formulation and residual Imidacloprid in tomato grown in greenhouse (protected cultivation) was also conducted. The results obtained from commercial formulation were completely consistent with those obtained through the standard high‐performance liquid chromatography (HPLC) method.     

Mechanism of Chicoric Acid Electrochemical Oxidation and Identification of Oxidation Products by Liquid Chromatography and Mass Spectrometry

Electrochemical oxidation of chicoric acid (ChA) was investigated using cyclic voltammetry and chronoamperometry at a glassy carbon electrode. Chicoric acid generates single quasi‐reversible redox wave in cyclic voltammetry over a wide pH range, and an ECEC‐dimerization mechanism is proposed. Effect of glutathione (GSH) on the electrochemical oxidation of chicoric acid (ChA) was investigated in Britton−Robinson buffer solution. Ultra‐high performance liquid chromatography (UPLC) coupled with mass spectrometry (MS) was used to show that the naturally occurring chicoric acid (ChA) underwent an electrochemical oxidation in the presence of glutathione (GSH) to form mono‐, bi‐, tri‐, and four‐glutathione conjugates of chicoric acid and a mono‐glutathione conjugate of a chicoric acid dimer. The obtained results are useful for understanding and predicting the oxidative degradation pathway of chicoric acid.     

New Aspects of Protein‐film Voltammetry of Redox Enzymes Coupled to Follow‐up Reversible Chemical Reaction in Square‐wave Voltammetry

Protein‐film square‐wave voltammetry of uniformly adsorbed molecules of redox lipophilic enzymes is applied to study their electrochemical properties, when a reversible follow‐up chemical reaction is coupled to the electrochemically generated product of enzyme's electrode reaction. Theoretical consideration of this so‐called “surface ECrev mechanism” under conditions of square‐wave voltammetry has revealed several new aspects, especially by enzymatic electrode reactions featuring fast electron transfer. We show that the rate of chemical removal/resupply of electrochemically generated Red(ads) enzymatic species, shows quite specific features to all current components of calculated square‐wave voltammograms and affects the electrode kinetics. The effects observed are specific for this particular redox mechanism (surface ECrev mechanism), and they got more pronounced at high electrode kinetics of enzymatic reaction. The features of phenomena of “split net‐SWV peak” and “quasireversible maximum”, which are typical for surface redox reactions studied in square‐wave voltammetry, are strongly affected by kinetics and thermodynamics of follow‐up chemical reaction. While we present plenty of relevant voltammetric situations useful for recognizing this particular mechanism in square‐wave voltammetry, we also propose a new approach to get access to kinetics and thermodynamics of follow‐up chemical reaction. Most of the results in this work throw new insight into the features of protein‐film systems that are coupled with chemical reactions.     

Electrochemical,microscopic, and EQCM studies of cathodic electrodeposition of ZnO/FAD and anodic polymerization of FAD films modified electrodes and their electrocatalytic properties

Two kinds of chemically modified electrodes were prepared. In the first type of electrodes, zinc oxide (ZnO) and flavin adenine dinucleotide (FAD) molecules were deposited onto the glassy carbon-, gold-, and SnO₂-coated glass electrodes by using cyclic voltammetry from the bath solution containing aqueous 0.1 M zinc nitrate, 0.1 M sodium nitrate, and 1 × 10⁻⁴ M FAD. It was called as ZnO/FAD modified electrodes. The second type of modified electrode was prepared by the electropolymerization method. Electrochemical polymerization of FAD was carried out from the acidic solution containing 1 × 10⁻⁴ M FAD monomers onto electrode surfaces. This poly(FAD)-modified electrode yields a new redox couple in addition to the monomers redox couple. The influence of the concentrations, pH, and electrocatalytic properties of the ZnO/FAD- and poly(FAD)-modified electrodes are investigated by means of the in situ technique electrochemical quartz–crystal microgravimetry (EQCM) combined with cyclic voltammetry and the ex situ technique scanning electron microscopy. From these studies, it appears that the cathodic deposition of ZnO/FAD-modified electrodes gives only one redox couple, and the anodically polymerized FAD film-modified electrodes gives two reversible redox couples. The pH dependence of the redox responses were investigated and the kinetics of electron transfer was evaluated. In addition, the EQCM technique was employed to follow the deposition process of both kinds of modified electrodes in real time as well as the characteristics of the charge transfer associated with the surface-confined redox-active couples. The electrocatalytic activity of the poly(FAD)-modified electrode towards the reduction of hydrogen peroxide and the oxidation of dopamine and ascorbic acid was explored. The important electrocatalytic properties of poly(FAD)-modified electrode were observed for simultaneous separation of dopamine and ascorbic acid in neutral solution. This poly(FAD)-modified electrode has several advantages than the previously reported FAD-modified electrodes.     

Electroanalytical Studies of Biologically Active Azosalicylic Acid at a Hanging Mercury Drop Electrode

The electrochemical behavior of disodium 3,3′‐azobis‐(6‐hydroxy‐)benzoate (olsalazine sodium), 2‐hydroxy‐5‐[(4‐sulfophenyl)azo]benzoic acid and 2‐hydroxy‐5‐azo‐benzoic acid was studied as a simple model of the metabolic reduction of azosalicylic acids using cyclic and square‐wave voltammetry. The stability of the hydrazo intermediate generated in the electroreduction process is strongly dependent on pH and the effect of different substituents in the aromatic ring adjacent to the azo bridge. The electrode mechanism of azosalicylic acids at hanging mercury drop electrode exhibits characteristic properties of a surface redox reaction. The kinetic parameters of azo/hydrazo reduction, such as the standard redox reaction rate constant and the electron transfer coefficient were estimated applying the methods of a split SW peak and a quasireversible maximum. A sensitive analytical method for the determination of trace level of biologically active azosalicylic acids by square‐wave voltammetry based on adsorptive stripping technique is described.