Poly(Methyl Red) Modified Glassy Carbon Electrodes: Electrosynthesis,Characterization, and Sensor Behavior

Poly(methyl red), PMR, was electropolymerized on glassy carbon electrodes by potential cycling in 50 mM phosphate buffer solution at pH 7.0 and 8.0 and Britton Robinson buffer solution in the pH range 7.0‐11.0. The electrochemical behavior of PMR modified electrodes was investigated by cyclic voltammetry in Britton Robinson buffer solution at different pHs from 5.0 to 10.0 and found that the best PMR film formation was obtained at pH 9.0. Uric acid was quantitatively determined at PMR modified electrodes by cyclic voltammetry and differential pulse voltammetry in Britton Robinson buffer at pH 5.0. Both methods presented a linear dependence between the anodic peak current and the concentration of uric acid in the range of 0.4 to 60 μM and 0.08 to 100 μM with the limits of detection of 0.038 and 0.009 μM for cyclic voltammetry and differential pulse voltammetry, respectively. Poly(methyl red) as redox mediator allowed the determination of uric acid without any interferences from the substances in serum samples.     

Decolorization Potential of Some Reactive Dyes with Crude Laccase and Laccase-Mediated System

In this study, decolorization of dyestuffs, such as Reactive Red 198, Rem Blue RR, Dylon Navy 17, Rem Red RR, and Rem Yellow RR was studied using laccase and laccase-mediated system. The laccases are known to have an important potential for remediation of pollutants. Among these dyestuffs, decolorization of Rem Blue RR and Dylon Navy 17 was performed with crude laccase under optimized conditions. Vanillin was selected as laccase mediator after screening six different compounds with Rem Yellow RR, Reactive Red 198, and Rem Red RR as substrates. However, Rem Yellow RR was not decolorized by either laccase or laccase-mediated system. It is observed that the culture supernatant contained high laccase activity after treatment with catalase that was responsible for the decolorization. Besides, culture supernatant with high laccase activity as enzyme source was treated with catalase; in this way, the hypothesis that laccase was the enzyme responsible for decolorization was supported. The Rem Blue RR was decolorized with 64.84% under the optimum conditions and Dylon Navy 17 with 75.43% with crude laccase. However, using the laccase and vanillin, the decolorization of Reactive Red 198 and Rem Red RR was found to be 62% and 68%, respectively. Our study demonstrated that the decolorization abilities of laccase and/or laccase mediator systems were based on the types of mediator, the dye structure, and the standard experimental conditions. Also, the electrochemical behaviors of some samples were studied. The redox potentials of these samples were determined using cyclic voltammetry on glassy carbon electrode in phosphate buffer (pH 6) solution.     

Oriented immobilization of a fully active monolayer of histidine-tagged recombinant laccase on modified gold electrodes

The formation of a dense monolayer of histidine-tagged recombinant laccase on gold electrodes by using a short thiol-NTA linker is described, as well as a kinetic analysis of the process by cyclic voltammetry. From a detailed analysis of the catalytic reduction of dioxygen by laccase in the presence of a one-electron redox mediator it can be concluded that the immobilized enzyme remains as active as in homogeneous solution.     

金电极上偶氮腺嘌呤的电化学行为研究

本文研究了光滑金电极上偶氮腺嘌呤的电化学特性,并确定了相关动力学参数. 在含偶氮腺嘌呤的0.2 mol·L^-1的磷酸盐缓冲液(PBS,pH = 4.0 ~ 10.0）中,发现其循环伏安图上出现一对氧化还原峰. 基于对扫速和伏安峰值电位的分析,结果表明这是一个由吸附控制的可逆偶氮腺嘌呤氧化还原电化学过程. 当pH值从低到高改变时,氧化还原峰值向负电位移动,证实H⁺参与了该反应. 通过进一步实验数据分析和电极表面吸附量计算,发现该反应为分步进行的两电子两质子反应. 最后,通过快速循环伏安扫描方法确定了电化学过程的表观传递系数α和表观速率常数ks.     

Correlation with redox potentials and inhibitory effects on Epstein-Barr virus activation of azaanthraquinones

The redox potentials have been determined for nine azaanthraquinones in phosphate buffer at pH 7.2 by means of cyclic voltammetry. A definite correlation has been found between the redox potentials and the inhibitory effects of the azaanthraquinones on Epstein-Barr virus early antigen (EBV-EA) activation. It has further been shown that the correlation can be made better by introducing an electronic property, i.e., the atomic charge at O11 as an additional parameter.     

Oxygen-reducing electrodes based on layer-by-layer assemblies of cytochrome c and laccasse

Electroactive multilayer assemblies combining the redox protein cytochrome c and the enzyme laccase were fabricated by the layer-by-layer adsorption technique on gold electrodes and were shown to be capable of direct oxygen reduction. Laccase from trametes versicolor was electrostatically immobilized on multilayer films consisting of cytochrome c and the polyelectrolyte polyanilinesulfonic acid. The layer formation was monitored by quartz crystal microbalance. The electrochemical behavior of the electrodes was investigated by cyclic voltammetry. The resulting assembly exhibited a catalytic oxygen reduction current. This indicates a multi-step electron transport chain from the electrode via the cytochrome c layers towards laccase, and finally, to molecular oxygen. The catalytic efficiency of the electrodes was examined in the pH range from 4.5 to 7.0, showing highest enzymatic oxygen reduction at pH 4.5. Furthermore, the catalytic current was found to correlate linearly with the oxygen content of the solution. This suggests that the overall current is limited by the catalytic reduction of oxygen by the laccase rather than by the preceding electron transfer steps.     

Large amplitude Fourier transformed AC voltammetric investigation of the active state electrochemistry of a copper/aqueous base interface and implications for electrocatalysis

The higher harmonic components available from large-amplitude Fourier-transformed alternating current (FT-ac) voltammetry enable the surface active state of a copper electrode in basic media to be probed in much more detail than possible with previously used dc methods. In particular, the absence of capacitance background current allows low-level Faradaic current contributions of fast electron-transfer processes to be detected; these are usually completely undetectable under conditions of dc cyclic voltammetry. Under high harmonic FT-ac voltammetric conditions, copper electrodes exhibit well-defined and reversible premonolayer oxidation responses at potentials within the double layer region in basic 1.0 M NaOH media. This process is attributed to oxidation of copper adatoms (Cu*) of low bulk metal lattice coordination numbers to surface-bonded, reactive hydrated oxide species. Of further interest is the observation that cathodic polarization in 1.0 M NaOH significantly enhances the current detected in each of the fundamental to sixth FT-ac harmonic components in the Cu*/Cu hydrous oxide electron-transfer process which enables the underlying electron transfer processes in the higher harmonics to be studied under conditions where the dc capacitance response is suppressed; the results support the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis. The underlying quasi-reversible interfacial Cu*/Cu hydrous oxide process present under these conditions is shown to mediate the reduction of nitrate at a copper electrode, while the mediator for the hydrazine oxidation reaction appears to involve a different mediator or active state redox couple. Use of FT-ac voltammetry offers prospects for new insights into the nature of active sites and electrocatalysis at the electrode/solution interface of Group 11 metals in aqueous media.     

Effect of electrolyte cations on the charge transfer in films of prussian blue

Charge transfer in films of Prussian blue is studied by cyclic voltammetry and chronoamperometry at potentials of first (PB/PW) and second (PB/PY) redox processes in solutions of potassium, cesium, and ammonium nitrates (pH 4.0). The current relaxation following a film perturbation by a small-amplitude voltage pulse is analyzed with theoretical relationships obtained earlier for the charge transfer in redox polymers. Parameters, characterizing the charge transfer rate (formal diffusion coefficient, instantaneous current), are calculated and analyzed as a function of the film oxidation state and the supporting-electrolyte nature     

Kinetics of redox polymer-mediated enzyme electrodes

Oxygen-reducing enzyme electrodes are prepared from laccase of Trametes versicolor and a series of osmium-based redox polymer mediators covering a range of redox potentials from 0.11 to 0.85 V. Experimentally obtained current density generated by the film electrodes is analyzed using a one-dimensional numerical model to obtain kinetic parameters. The bimolecular rate constant for mediation is found to vary with mediator redox potential from 250 s(-1) M(-1) when mediator and enzyme are close in redox potential to 9.4 x 10(4) s(-1) M(-1) when the redox potential difference is large. The value of the bimolecular rate constant for the simultaneously occurring laccase-oxygen reaction is found to be 2.4 x 10(5) s(-1) M(-1). The relationship between mediator-enzyme overpotential and bimolecular rate constant is used to determine the optimum mediator redox potential for maximum power output of a hypothetical biofuel cell with a planar cathode and a reversible hydrogen anode. For laccase of T. versicolor (E(e)(0) = 0.82), the optimum mediator potential is 0.66 V (SHE), and a molecular structure is presented to achieve this result.     

Amperometric glycolate sensors based on glycolate oxidase and polymeric electron transfer mediators

Using experiments involving cyclic voltammetry and stationary potential measurements, it was shown that siloxane polymers with covalently attached ferrocene and 1,1'-dimethylferrocene relays efficiently mediate electron transfer from reduced glycolate oxidase to a conventional carbon-paste electrode. Sensors containing these polymeric relay systems and glycolate oxidase respond rapidly to low (< 0.1 mM) glycolate concentrations, with steady-state current responses achieved in less than 1 min. The dependence of the sensor response on the nature of the siloxane polymer backbone, the type of polymer-bound redox mediator used and the amount of redox mediator present is discussed. From these considerations, sensors have been designed which can operate efficiently at low applied potential and can avoid decreased current response due to dissolved oxygen.     

Electrochemical behavior of α1/α2-[Fe(H2O)P2W17O61](7-) isomers in solution: experimental and DFT studies

The unusual redox behavior displayed by the two isomers of the Wells-Dawson phosphotungstate anion [Fe(H(2)O)P(2)W(17)O(61)](7-) is presented. The electrochemical measurements have been performed in aqueous media at different pH values from 0.5 up to 8.0. The cyclic voltammetry has also been carried out in organic media to get additional experimental data to establish the effect of the protonation on the redox properties of both isomers. At high pH values (pH ≥ 6) or in an organic medium, the reduction of the Fe center is easier in the case of the alpha-1 isomer, whereas for the alpha-2 isomer such reduction takes place at more negative potentials, as expected. In contrast, at lower pH values (pH ≤ 5), an inversion of this trend is observed, and the reduction of the Fe center becomes easier for the alpha-2 isomer compared to the alpha-1. We were able to highlight the influence of the pH and the pK(a) of the electrolyte on POM-based redox potentials given the pK(a) of the latter. A complementary theoretical study has also been performed to explain the experimental data obtained. In this sense, the results obtained from the DFT study are in good agreement with the experimental data mentioned above and have provided additional information for the electrochemical behavior of both isomers according to their different molecular orbital energies. We have also shown the influence of protonation state of the iron derivative on the relative reduction potentials of both isomers.     

Mediatorless catalytic oxygen reduction at boron-doped diamond electrodes

Most approaches to electron conduction from electrode to the enzyme requires the use of mediators – molecular relays which can take electrons from the electrode and deliver them to the redox sites of the enzyme. In the present paper, the biocatalytic reduction of oxygen to water in the presence of laccase is shown to proceed on the boron-doped diamond at highly positive potentials and without any additional mediator. The onset of catalytic reduction current appears at 0.805 V vs. NHE in solutions of pH 5.2. Laccase is either dissolved in the solution or trapped on the BDD electrode in a thin film of lipidic cubic phase. The remarkable stability of the modified electrode, avoiding the use of mediators and positive potential of the dioxygen reduction process make the BDD–laccase system especially interesting for applications in electrochemical sensing and microbiofuel cells.     

Electrochemical Behavior from Pig Spleen Ferritin and Bacterial Ferritin of Azotobacter vinelandii

Being electrochemical studies, a rate of the core potential shifting to negative as a form of -205mV+(-115mV/pH) from horse spleen ferritin (HSF) has been measured by microcoulometry in the presence of mediator such as methyl viologen^[1]. It was well indicated that the mediator played an important role in transferring the electrons between the ferritin and the electrode. However, using cyclic voltammetry of pulse polarography, HSF appeared appreciable currentless at the mercury electrode at scan rate 5 mV.s^-1, moreover, and its mineral core isolated from protein shell showed the reductive current^[2]. Evidently, these results was known that the ferritin shell be no a redox protein due to it no exhibited the current at the electrode at low potential.     

Electron transfer between ferrocene-modified Au/octadecanethiol/lipid BLM electrode and redox couples in solution

Bilayers incorporated with ferrocene consisting of self-assembled octadecanethiol and lipid monolayer on gold substrates were fabricated. Its electrochemical behaviors in solutions containing different redox couples were investigated by cyclic voltammetry and ac impedance. The transmembrane electron transfer reaction across octadecanethiol self-assembled film and an adsorbed phospholipid layer mediated by ferrocene have been observed in the solution of Fe(CN)6(3-/4-). The formal potential difference between mediator in bilayer lipid membrane (BLM) and redox couple in solution has a great impact on the transmembrane electron transfer behavior. The ferrocene-modified BLM electrodes might be useful for constructing a bilayer-based electrochemical current rectifying device.     

The voltammetric behaviour of solid 2,2-diphenyl-1-picrylhydrazyl (DPPH) microparticles

The electrochemical behaviour of 2,2-diphenyl-1-picrylhydrazyl (DPPH) microparticles, attached to a graphite electrode and adjacent to an aqueous electrolyte solution, has been studied by cyclic voltammetry. DPPH exhibits one reversible redox couple with a formal potential of 0.340 V versus Ag|AgCl (pH=7.0). At more positive potentials, a redox couple appears with a formal potential E_f=0.733 V versus Ag|AgCl. The oxidation at this potential is followed by an irreversible chemical reaction generating a product which gives a redox couple with a formal potential at 0.177 V versus Ag|AgCl. The reduction process of this couple is followed by a slow chemical reaction in the course of which DPPH is reformed.     

Direct Observation of Radical Intermediates While Investigating the Redox Behavior of Thiamin Coenzyme Models

The redox behavior of “active aldehydes” 1 ⁻ derived from 3-benzylthiazolium salts and simple aldehydes in the presence of a base has been examined with low-temperature cyclic voltammetry and EPR spectroscopy. The highly negative oxidation potentials of 1 ⁻ and the spin distribution of the intermediate radicals 1 ^. indicate that the active aldehyde can act as an efficient electron mediator in thiamin-dependent enzymatic redox systems.     

Kinetic Study of the Electro-Catalytic Oxidation of Hydrazine on Cobalt Hydroxide Modified Glassy Carbon Electrode

Electrocatalytic oxidation of hydrazine was investigated on a cobalt hydroxide modified glassy carbon (CHM-GC) electrode in alkaline solution. The process of oxidation involved and its kinetics were established by using cyclic voltammetry, chronoamperometry techniques as well as steady state polarization measurements. In cyclic voltammetry (CV) studies, in the presence of hydrazine the peak current increase of the oxidation of cobalt hydroxide is followed by a decrease in the corresponding cathodic current. This indicates that hydrazine is oxidized on the redox mediator that is immobilized on the electrode surface via an electrocatalytic mechanism. A mechanism based on the electrochemical generation of Co(IV) active sites and their subsequent consumption by the hydrazine in question was also investigated.     

氯过氧化物酶在碳纳米管修饰电极上的直接电化学研究

将氯过氧化物酶固定到单壁碳纳米管修饰玻碳电极,其循环伏安曲线出现一对准可逆氧化还原峰,说明碳纳米管能够很好地促进氯过氧化物酶在电极表面的直接电子传递.该过程与溶液pH值有关,可指认氯过氧化物酶在电极表面发生的是一电子一质子传递反应.该修饰电极制作简单,性能稳定,且对氧还原具有很好的电催化效应.     

Spectroelectrochemical detection of phenothiazine and phenoxazine derivatives covalently bound to self-assembled cystamine monolayers

The phenothiazine derivatives, Toluidine Blue O and Azur A, and the phenoxazine derivative Nile Blue were bound covalently to self-assembled cystamine monolayers chemisorbed on gold electrodes by derivatization of the surface amino groups with two different bifunctional spacers: terephthaloyl chloride and 1,6-hexamethylene-diisocyanate.The formation of the amido- and urea-derivatives of the parent compounds after covalent immobilization induces a shift of their redox potentials towards more positive values which can easily be detected by cyclic voltammetry.UV-Vis difference spectroelectrochemistry has been used to characterize the electroactive species immobilized onto transparent gold electrode surfaces in both oxidation states. In every case, the oxidized-minus-reduced (and reduced-minus-oxidized) difference spectra of the immobilized redox species show a shift of the UV maxima towards longer wavelengths and a shift of the Vis maximum towards shorter wavelengths when compared with their parent compounds. Each redox species showed different optical characteristics depending on the spacer used for immobilization.For phenothiazine derivatives immobilized with 1,6-hexamethylene di-isocyanate the total surface coverages obtained by optical methods were close to those obtained by cyclic voltammetry. However, for the same derivatives immobilized with terephthaloyl chloride, and for Nile Blue, independently of the spacer used, higher surface coverages were found by optical methods than by cyclic voltammetry.     

Voltammetric Study and Direct Analytical Determination of the Antiparkinson Drug Benserazide

For the first time, a simple differential pulse voltammetry methodology for direct determination of benserazide in presence of levodopa in tablets was developed without any redox mediator, modified electrodes, or the aplication of mathematic deconvolution of signals. Benserazide was studied by differential pulse voltammetry using glassy carbon electrode in aqueous media. The drug exhibited a main well-defined oxidation signal in a broad pH range (2–10), and two poorly resolved signals at higher potentials. We have found that levodopa does not interfere on the electrochemical response of benserazide at pH 6.0. Thus, at this pH value, the developed analytical method exhibited adequate repeatability and reproducibility (RSD < 2%), recoveries >98.5%, which permitted its successful application to both the assay and the uniformity content of benserazide. Also, hydrolytic degradation studies of benserazide were carried out by differential pulse voltammetry.