The redox thermodynamics and kinetics of flavonoid rutin adsorbed at glassy carbon electrodes by stripping square wave voltammetry

The adsorptive accumulation of rutin (RU) at glassy carbon (GC) electrodes in 10% ethanol + 90% 1 mol dm⁻³ HClO₄ aqueous solution is studied by using cyclic (CV) and square wave (SWV) voltammetries. The Frumkin adsorption isotherm best described the specific interaction of rutin with carbon electrodes. By fitting the experimental data, values of −31.9 kJ mol⁻¹ and 0.54 ± 0.02 were obtained for the Gibbs free energy of adsorption and the interaction parameter, respectively. SWV fully characterized the thermodynamics and kinetics of the surface redox process, using a combination of the “quasi-reversible maximum” and the “splitting of SW peaks” methods. Average values of 0.644 ± 0.003 V and 0.44 ± 0.02 were obtained for the formal potential and the anodic transfer coefficient, respectively. Moreover, a formal rate constant of 6.1 × 10² s⁻¹ was obtained. SWV was also employed to generate calibration curves. The lowest concentration of RU experimentally measured for a signal-to-noise ratio of 3:1 was 2 × 10⁻⁸ mol dm⁻³ (12 ppb).     

Adsorption of ochratoxin A (OTA) anodic oxidation product on glassy carbon electrodes in highly acidic reaction media: Its thermodynamic and kinetics characterization

We study the thermodynamics and kinetics of the adsorption of a redox couple having quinone nature on glassy carbon electrodes. This couple is produced by the anodic oxidation of mycotoxin ochratoxin A in 10% acetonitrile + 90% 1 M HClO₄ aqueous solution. The quasi-reversible redox couple was studied by both cyclic (CV) and square wave (SWV) voltammetric techniques. The Frumkin adsorption isotherm best described the specific interaction of the redox couple with carbon electrodes. By fitting the experimental data, we obtained values of −28.4 kJ mol⁻¹ and 0.70 ± 0.02 for the Gibbs free energy of adsorption and the interaction parameter, respectively. SWV fully characterized the thermodynamics and kinetics of the adsorbed redox couple, using a combination of the “quasi-reversible maximum” and the “splitting of SW peaks” methods. Average values of 0.609 ± 0.003 V and 0.45 ± 0.06 were obtained for the formal potential and the anodic transfer coefficient, respectively. Moreover, a formal rate constant of 10.7 s⁻¹ was obtained. SWV was also employed to generate calibration curves. The lowest concentration of mycotoxin was 1.24 × 10⁻⁸ M (5 ppb), measured indirectly with a signal to noise ratio of 3:1.     

DNA Langmuir-Blodgett modified glassy carbon electrode as voltammetric sensor for determinate of methotrexate

A highly sensitive electrochemical biosensor for the detection of trace amounts of methotrexate has been designed. Double stranded (ds)DNA molecules are immobilized onto a pretreated glassy carbon electrode (GCE(ox)) surface with Langmuir-Blodgett (LB) technique. The adsorptive voltammetric behaviors of methotrexate on DNA-modified electrode were explored by means of cyclic voltammetry (CV) and square wave voltammetry (SWV). The oxidation mechanism was proposed and discussed in this work. In addition, the optimum experimental conditions for the detection of methotrexate were explored, and the currents measured by SWV presented a good linear property as a function of the concentrations of methotrexate in the range of 2.0 × 10⁻⁸ to 4.0 × 10⁻⁶ mol L⁻¹, with an LOD of 5.0 × 10⁻⁹ mol L⁻¹. The method proposed was applied for the determination of methotrexate in pharmaceutical dosage and diluted human urine with wonderful satisfactory successfully.     

Electrochemical behaviour of americium ions in LiCl-KCl eutectic melt

This work presents a study on the electrochemical properties of AmCl₃ in a molten LiCl-KCl eutectic, at a temperature range of 733-833 K. Transient electrochemical techniques, such as cyclic voltammetry and chronopotentiometry, on inert metallic tungsten working electrode have been used to investigate the reduction mechanism of Am³⁺ ions. The results show that Am³⁺ is reduced to Am metal by a two-step mechanism corresponding to the Am³⁺/Am²⁺ and Am²⁺/Am⁰ transitions. Formal standard potentials of Am³⁺/Am²⁺ ( versus Cl₂/Cl⁻ at 733 K) and Am²⁺/Am⁰ ( versus Cl₂/Cl⁻ at 733 K) redox couples as well as diffusion coefficients of Am³⁺ and Am²⁺ (2.4 × 10⁻⁵ and 1.15 × 10⁻⁵ cm² s⁻¹ at 733 K, respectively) have been calculated at three different temperatures. In the studied range of temperature, the DAm3+/DAm2+ ratio was found to be around 2. In addition, thermodynamic properties have been calculated for Am³⁺ () and Am²⁺ () and compared to thermodynamic reference data in order to estimate activity coefficients (Am³⁺ = 4.7 × 10⁻³ and Am²⁺ = 2.7 × 10⁻² at 733 K) in the molten LiCl-KCl eutectic.     

Cobalt hydroxide film deposited on glassy carbon electrode for electrocatalytic oxidation of hydroquinone

The electrodeposition of cobalt hydroxide film on glassy carbon electrode was prepared by electrochemical method in an alkaline aqueous solution. The electrochemical behavior of hydroquinone on cobalt hydroxide film electrode has been investigated by using cyclic voltammetry and linear sweep voltammetry. The results showed that the film electrode has good electrocatalytic ability for the oxidation of hydroquinone to p-quinone. The recovery of hydroquinone from sample ranged from 94.7% to 102.9% and a rectilinear analytical curve for hydroquinone concentration from 5.0 × 10⁻⁶ to 1.25 × 10⁻⁴ mol/L was obtained. The detection limit was 5.0 × 10⁻⁷ mol/L and the relative standard deviation was 2.63%. Various factors affecting the electrocatalytic activity of cobalt hydroxide film were investigated.     

Activation of glassy carbon electrodes by photocatalytic pretreatment

This paper describes a simple and rapid photocatalytic pretreatment procedure that removes contaminants from glassy carbon (GC) surfaces. The effectiveness of TiO₂ mediated photocatalytic pretreatment procedure was compared to commonly used alumina polishing procedure. Cyclic voltammetric and chronocoulometric measurements were carried out to assess the changes in electrode reactivity by using four redox systems. Electrochemical measurements obtained on photocatalytically treated GC electrodes showed a more active surface relative to polished GC. In cyclic voltammograms of epinephrine, Fe(CN)₆^3−/4− and ferrocene redox systems, higher oxidation and reduction currents were observed. The heterogeneous electron transfer rate constants (k^o) were calculated for Fe(CN)₆^3−/4− and ferrocene which were greater for photocatalytic pretreatment. Chronocoulometry was performed in order to find the amount of adsorbed methylene blue onto the electrode and was calculated as 0.34 pmol cm⁻² for photocatalytically pretreated GC. The proposed photocatalytic GC electrode cleansing and activating pretreatment procedure was more effective than classical alumina polishing.     

Studies of a novel conducting polymer by cyclic and square wave voltammetries: Its synthesis and characterization

A novel conducting polymer of polynaphthidine, poly(NAP), was synthesized electrochemically by direct anodic oxidation of naphthidine in aqueous media. The yield of the electropolymerization reaction depends on the temperature and pH of the solution. It was possible to differentiate two working regions: I (for pH < 0.5 and all temperatures) where the film yield tends to zero and II (for approximately 2.0 < pH < 2.8 and temperatures >15 °C) where the film production is maximum. Therefore, the naphthidine electrooxidation mechanism was studied under experimental conditions of region I by cyclic (CV) and square wave voltammetries (SWV) as well as by controlled potential electrolysis.The experimental conditions of region II were chosen to obtain the poly(NAP). The electrochemical response of the film was investigated in pH 1 HClO₄ + 0.1 M NaClO₄ electrolyte solution by CV and SWV. A plot of I_p,n/fvs. f from SW voltammograms showed the so-called “quasi-reversible maximum”. Formal potential, formal rate constant and anodic transfer coefficient for the surface redox process were also evaluated from the SWV.The poly(NAP) is insoluble in common organic solvents and shows electrochromic behaviour. Its probable structure was determined by FTIR spectroscopy.     

Powdered activated carbon and carbon paste electrodes: comparison of electrochemical behaviour

Commercial activated carbon (Norit R3ex), de-mineralised with conc. HF and HCl, was oxidised (conc. HNO₃) and heat-treated at various temperatures (180, 300 and 420 °C). The physicochemical properties of the samples obtained were characterised by selective neutralisation and pH-metric titration of surface functional groups (acid–base properties), thermogravimetry (thermal stability—TG), FTIR spectroscopy (chemical structure) and low-temperature nitrogen adsorption (BET surface area). Thermal treatment of the carbon materials caused the surface functional groups to decompose; in consequence, the chemical properties of the carbon surfaces changed. Cyclic voltammetric studies were carried out on all samples using a powdered activated carbon electrode (PACE) and a carbon paste electrode (CPE), as were electrochemical measurements in aqueous electrolyte solutions (0.1 M HNO₃ or NaNO₃) in the presence of Cu²⁺ ions acting as a depolariser. The shapes of the cyclic voltammograms varied according to the form of the electrodes (powder or paste) and to the changes in the surface chemical structure of the carbons. The electrochemical behaviour of the carbons depended on the presence of oxygen-containing surface functional groups. The peak potentials and their charge for the redox reactions of copper ions depended on their interaction with the carbon surface.     

Relative stabilities of Ce(IV) and Ce(III) limiting carbonate complexes at 5-50 °C in Na aqueous solutions, an electrochemical study

The normal potential of the Ce(IV)/Ce(III) redox couple was determined by square wave voltammetry (SWV) at different temperatures in solutions with a constant ratio [CO₃²⁻]/[HCO₃⁻] ≈10 for high ionic strengths (3.29 mol dm⁻³ at 4.39 mol dm⁻³): varies from 259.5 to 198.0 mV/S.H.E. in the 15-50 °C range. Linear variations were found for versus (RT/F)ln(mCO₃²⁻), leading to the stoichiometry, Ce(CO₃)₆⁸⁻ for the Ce(IV) limiting complex. But the slopes of these linear variations were actually found in the range 1.8-1.9, not exactly 2. This was interpreted as dissociation of the Ce(IV) limiting complex following the reaction: Ce(CO₃)₅⁶⁻ + CO₃²⁻ → Ce(CO₃)₆⁸⁻ and as dissociation of the Ce(III) limiting complex following the reaction: Ce(CO₃)₃³⁻ + CO₃²⁻ → Ce(CO₃)₄⁵⁻; for which maximum possible values of log₁₀ K_IV,6 and log₁₀ K_III,4 were estimated via fitting in the 15-50 °C temperature range (log₁₀ K_IV,6 = 0.42 (0.97) and log₁₀ K_III,4 = 0.88 (7.00) at 15 °C (50 °C). The normal potential was found to decrease linearly with T, these variations correspond to , with T⁰ = 298.15 K and . The apparent diffusion coefficient of Ce(IV) was determined by direct current polarography (DCP), cyclic voltammetry (CV) and square wave voltammetry. It was found to depend on the ionic strength and to be proportional to T.     

Electrochemical study of the Eu(III)/Eu(II) system in molten fluoride media

The electrochemical behaviour of the Eu(III)/Eu(II) system was examined in the molten eutectic LiF–CaF₂ on a molybdenum electrode, using cyclic voltammetry, square wave voltammetry and chronopotentiometry. It was observed that EuF₃ is partly reduced into EuF₂ at the operating temperatures (1073–1143 K). The electrochemical study allowed to calculate both the equilibrium constant and the formal standard potential of the Eu(III)/Eu(II) system. The reaction is limited by the diffusion of the species in the solution; their diffusion coefficients were calculated at different temperatures and the values obey Arrhenius’ law. The second system Eu(II)/Eu takes place out of the electrochemical window on an inert molybdenum electrode, which inhibits the extraction of Eu species from the salt on such a substrate.     

Immobilization of Nafion-ordered mesoporous carbon on a glassy carbon electrode: Application to the detection of epinephrine

A stable suspension of ordered mesoporous carbon (OMC) was obtained by dispersing OMC in a solution of Nafion. By coating the suspension onto glassy carbon (GC) electrode, cyclic voltammetry was used to evaluate the electrochemical behaviors of Nafion-OMC-modified GC (Nafion-OMC/GC) electrode in 0.1 mmol L⁻¹ hexaammineruthenium(III) chloride (Ru(NH₃)₆Cl₃)/0.1 mol L⁻¹ KCl solution, where Nafion-OMC/GC electrode shows a faster electron transfer rate as compared with OMC/GC, Nafion/GC and GC electrodes. Due to the unique properties of Nafion-OMC, an obvious decrease in the overvoltage of the epinephrine (EP) oxidation (ca. 100 mV at pH 4.1 and 115 mV at pH 7.0) as well as a dramatic increase in the peak current (12 times at pH 4.1 and 6 times at pH 7.0) was observed at Nafion-OMC/GC electrode compared to that seen at GC electrode. By combining the advantages of OMC with those of Nafion, the anodic peak of EP and that of ascorbic acid (AA) were separated successfully (by ca. 144-270 mV) in the pH range of 2.0-10.0, which may make Nafion-OMC/GC electrode potential for selective determination of EP in the presence of AA at a broad pH range. As an EP sensor, the EP amperometric response at Nafion-OMC/GC electrode in pH 7.0 PBS is extremely stable, with 99% of the initial activity remaining (compared to 32% at GC surface) after 120 min stirring of 0.20 mmol L⁻¹ EP. And Nafion-OMC/GC electrode can be used to readily detect the physiological concentration of EP at pH 7.0. These make Nafion-OMC/GC electrode potential candidates for stable and efficient electrochemical sensor for the detection of EP. The solubilization of OMC by Nafion may provide a route to more precise manipulation, and functionalization for the construction of OMC-based sensors, as well as allowing OMC to be introduced to biologically relevant systems.     

Electrochemical oxidation of glutathione at well-aligned carbon nanotube array electrode

Electrochemical oxidation and determination of glutathione (GSH) were investigated with well-aligned carbon nanotube (CNT) arrays. Square wave voltammetric and amperometric results suggest that aligned-CNT electrode exhibits excellent electrochemical activity and good anti-fouling property for direct electrochemical oxidation of glutathione. Also, the preliminary application of the aligned-CNT electrode for amperometric determination of glutathione was evaluated.     

Electrochemical mechanism and kinetics studies of haloperidol and its assay in commercial formulations

The kinetics and mechanism for electrochemical reduction of haloperidol, a psychotherapeutic drug used in the treatment of schizophrenia, were studied using square wave and cyclic voltammetries allied to a hanging mercury drop electrode. The experimental and voltammetric parameters were optimized at 0.04 mol L⁻¹ Brinton–Robinson buffer (pH 10), with a pulse potential frequency of 100 s⁻¹, a pulse amplitude of 30 mV and scan increment of 2 mV. Two well-defined peaks were observed, which exhibited properties of fast electron transfer with a strong adsorption process of reactants and products on the electrode surface. The first peak was related to a fast and reversible anion-radical formation originating from the reduction of the carbonyl group, and the second was related to the irreversible reduction of the anion-radical previously formed. Analytical parameters such as: linearity range, equation of the analytical curves, correlation coefficients, detection and quantification limits, recovery efficiency, and relative standard deviation for intraday and interday were compared to similar results obtained by use of the UV–vis spectrophotometry technique, and the analytical results obtained in commercial formulations show that the voltammetric procedure using a hanging mercury drop electrode is suitable for analyzing haloperidol in complex commercial formulation samples.     

Thiols deposition onto the surface of glassy carbon electrodes mediated by electrical potential

Electrochemical oxidation of thiols in acetonitrile and application of this process for modification of glassy carbon electrodes were studied. Addition of strong deprotonating agent, tetrabutylammonium hydroxide, was found to facilitate oxidation of thiols as well as their deposition onto the carbon surface. Thus, in the presence of 1 mM tetrabutylammonium hydroxide, glassy carbon electrode can be grafted with 1 mM 3-(nitrobenzyl)mercaptan at as low as +200 mV (vs. SCE). The modified electrodes were characterized by electrochemical methods and XPS confirming the stable binding of thiols which were absent on the surface of unmodified and control treated electrodes. Surprisingly, surface modification occurs independent of RS radicals formation and is explained by nucleophilic addition of deprotonated thiols to the surface of carbon electrode. The electrode potential plays an important role in this process presumably modulating electrophilic properties of the carbon surface.     

铋膜玻碳电极阳极溶出伏安法测定异烟肼的研究

研究了异烟肼在铋膜玻碳电极上的电化学行为检测方法。在0.1mol/L pH=4.5的HAc—NaAc缓冲溶液的底液中,通过富集,用铋膜玻碳电极进行阳极溶出伏安法测定异烟肼。异烟肼的阳极峰电位为-1.02V(vs.SCE),峰电流与异烟肼的浓度在1.6×10~(-7)～8.0×10~(-5)mol/L范围内呈良好的线性关系。该方法的检出下限为8.0×10~(-8)mol/L。对异烟肼含量用本法进行了测定,获得了满意的结果。本方法的优点是成本低,操作方便,重现性好以及检测下限低。     

Electrooxidation of sulfide by cobalt pentacyanonitrosylferrate film on glassy carbon electrode by cyclic voltammetry

A glassy carbon (GC) electrode was modified with cobalt pentacyanonitrosylferrate (CoPCNF) film. Cyclic voltammetry (CV) of the CoPCNF onto the GC (CoPCNF/GC) shows a redox couple (Fe^III/Fe^II) with a standard potential (E^0′) of 580 mV. The current ratio I_pa/I_pc remains almost 1, and a peak separation (ΔE_p) of 106 mV is observed in 0.5 M KNO₃ as the supporting electrolyte. Anodic peak currents were found to be linearly proportional to the scan rate between 10 and 200 mV s⁻¹, indicating an adsorption-controlled process. The redox couple of the CoPCNF film presents an electrocatalytic response to sulfide in aqueous solution. The analytical curve was linear in the concentration range of 7.5 × 10⁻⁵ to 7.7 × 10⁻⁴ M with a detection limit of 4.6 × 10⁻⁵ M for sulfide ions in 0.5 M KNO₃ solution.     

The pH-dependence of oxygen reduction on quinone-modified glassy carbon electrodes

The electrochemical reduction of oxygen has been studied on quinone-modified glassy carbon (GC) electrodes as a function of solution pH using the rotating disk electrode (RDE) technique. The surface of GC was grafted with anthraquinone (AQ) and phenanthrenequinone (PQ) by electrochemical reduction of their diazonium derivatives and the oxygen reduction measurements were carried out at different pHs (pH 7-14). The redox-potentials of surface-bound quinones were determined using cyclic voltammetry (CV). The kinetic parameters of oxygen reduction on GC/AQ and GC/PQ electrodes were determined considering a surface redox catalytic cycle model for quinone-modified electrodes.     

Influence of pencil lead hardness on voltammetric response of graphite reinforcement carbon electrodes

This work studied the voltammetric response of graphite reinforcement electrodes made of different pencil lead hardness. The studies showed that harder graphite leads, regardless of their manufacturer, are more appropriate as electrode material for voltammetric purposes due to their higher peak currents, increasing sensitivity and reproducibility, with ΔEp closer to the theoretical value for a reversible system.     

Effect of decreasing film thickness on the electrochemical responses of cations adsorbed in clay-modified electrodes

Clay-modified electrodes ranging in thickness from 3.4 μm to 8 nm, as estimated from the clay loadings, were prepared using three different smectites by spin-coating, solvent evaporation or electrophoretic deposition. For all three clays, the voltammetic waves obtained for [Ru(bpy)₃]²⁺ or [Os(bpy)₃]²⁺ adsorbed in these CMEs were independent of the film thickness for all films thicker than 100 nm. Only in very thin films, ≤40 nm were significant decreases in the peak currents observed. However, when the contributions to the peak currents from the electroactive concentrations, C* and effective diffusion coefficients, D_eff were separated, the values of C* were found to increase with decreasing film thickness, while D_eff decreased by several orders of magnitude. This was attributed to increase contributions to the electrochemical responses from less mobile electrostatically bound cations in the thinner films. Similar variations in C* and D_eff were obtained in films prepared by solvent evaporation. However, C* obtained in 20 nm thick electrodeposited films were significantly lower than in 40 nm spin-coated films. For [Ru(NH₃)₆]³⁺, the peak currents increased rapidly with the film thickness. However, no significant changes in the values of C* and D_eff with film thickness were found for this ion. This is consistent with the greater mobility of [Ru(NH₃)₆]³⁺ in clays films that allows a larger fraction of the adsorbed ions to remain electroactive even in thicker films. Results obtained for [Fe(bpy)₃]²⁺ were intermediate. While, the peak currents were independent of film thickness, the values of C* or D_eff obtained for this ion were also independent of the clay loadings.     

Electrocatalytic reduction of oxygen at glassy carbon electrode modified by polypyrrole/anthraquinones composite film in various pH media

The electrocatalytic reduction of dioxygen by one mono and four dihydroxy derivatives of 9,10-anthraquinone (AQ) incorporated in polypyrrole (PPy) matrix on glassy carbon electrode has been investigated. The electrochemical behaviour of the modified electrodes was examined in various pH media and both the formal potential of anthraquinones and reduction potential of dioxygen exhibited pH dependence. AQ and PPy composite film showed excellent electrocatalytic performance for the reduction of O₂ to H₂O₂. pH 6.0 was chosen as the most suitable medium to study the electrocatalysis by comparing the peak potential of oxygen reduction and enhancement in peak current for oxygen reduction. The diffusion coefficient values of AQ at the modified electrodes and the number of electrons involved in AQ reduction were evaluated by chronoamperometric and chronocoulometric techniques, respectively. In addition, hydrodynamic voltammetric studies showed the involvement of two electrons in O₂ reduction. The mass specific activity of AQ used, the diffusion coefficient of oxygen and the heterogeneous rate constants for the oxygen reduction at the surface of modified electrodes were also determined by rotating disk voltammetry.