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).     

Characterization of the surface redox process of adsorbed morin at glassy carbon electrodes

The thermodynamic and kinetics of the adsorption of morin (MOR) on glassy carbon (GC) electrodes in 0.2 mol dm⁻³ phosphate buffer solutions (PBS, pH 7.00) was studied by both cyclic (CV) and square wave (SWV) voltammetries. The Frumkin adsorption isotherm was the best to describe the specific interaction of MOR with GC electrodes. The SWV allowed to characterize the thermodynamic and kinetics of surface quasi-reversible redox couple of MOR, using the combination of the “quasi-reversible maximum” and the “splitting of SW net peaks” methods. Average values obtained for the formal potential and the anodic transfer coefficient were (0.27 ± 0.02) V and (0.59 ± 0.09), respectively. Moreover, a value of formal rate constant (k_s) of 87 s⁻¹ for the overall two-electron redox process was calculated. The SWV was also employed to generate calibration curves, which were linear in the range MOR bulk concentration from 1.27 × 10⁻⁷ to 2.50 × 10⁻⁵ mol dm⁻³. The lowest concentration experimentally measured for a signal to noise ratio of 3:1 was 1.25 × 10⁻⁸ mol dm⁻³ (3 ppb).     

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.     

Electrochemical behavior of high-pressure synthetic boron doped diamond powder electrodes

Boron doped diamond (BDD) was synthesized under high pressure and high temperature using B-doped graphite intercalation compositions (GICs) as carbon sources. The electrochemical characteristics of high-pressure synthetic BDD powder electrodes were investigated by measuring the cyclic voltammetry curves and AC impedance spectrum. For the [Fe(CN)₆]^3−/4− redox couple, the electrode reaction process is reversible or quasi-reversible at the scan rates of 0.01-1.0 V/s. At the low scan rate the linear relation between peak current and square root of scan rate indicates that the electrode process was a diffusion-controlled mass transport process. The electrochemical behavior is similar to a planar electrode. With the increasing of the scan rate the electrode process is controlled by the mass transport plus kinetic process. AC impedance spectra exhibit the porous structure characteristic of BDD powder electrode.     

Cyclic voltammetry and scanning electrochemical microscopy studies of the heterogeneous electron transfer reaction of some nitrosoaromatic compounds

The heterogeneous electron transfer reaction for the reduction of some nitroso aromatic derivatives in aqueous-alcoholic medium was studied on both mercury and glassy carbon electrodes (GCE) by using cyclic voltammetry (CV) and scanning electrochemical microscopy techniques (SECM).The nitrosoaromatic derivatives followed a two-electron two-proton mechanism producing a quasi-reversible overall process. This strongly pH dependent mechanism varied from ECCE mechanism at pH < 8.5 to ECEC mechanism at pH > 8.5.The apparent heterogeneous rate constant for the reduction of the nitroso derivatives was calculated using CV or SECM. The rate constant for the electron transfer process depends on the nature of the electrode material. The heterogeneous rate constant on the GCE is almost two orders of magnitude smaller than that on mercury electrode i.e. (3.4 ± 0.3) × 10⁻³ cm s⁻¹ on Hg and (7.0 ± 1.0) × 10⁻⁵ cm s⁻¹ on GCE, for the same nitroso compound and pH.The heterogeneous rate constant values were checked by comparison between experimental and simulated cyclic voltammograms.     

Electrochemistry of guanine and 8-oxoguanine at gold electrodes

Guanine was electrochemically oxidised at polycrystalline gold electrodes. The potential for guanine oxidation at pH 7.4 was +690 mV versus Ag|AgCl when extrapolated to a scan rate of 3 mV s⁻¹. Kinetic analysis revealed the initial irreversible 2e⁻ oxidation of guanine, achieved with fast scan cyclic voltammetry. At scan rates lower than 5 V s⁻¹, the 2e⁻ oxidation changed to the extensive 4e⁻ one. The initial 2e⁻ oxidation of guanine, observed in the first scan, resulted in a quasi-reversible 1e⁻ redox process of the oxidation product in the following scans, with a formal potential (E^o/) of +263 mV. This correlated well with the redox transformations of 8-oxo-7,8-dihydroguanine (8-oxoguanine, 8-oxoG). The kinetics of guanine oxidation on gold is discussed in detail within Laviron’s approach and compared with that of 8-oxoG.     

Probing the electrochemical behaviour of SWCNT-cobalt nanoparticles and their electrocatalytic activities towards the detection of nitrite at acidic and physiological pH conditions

The electrochemical decoration of edge plane pyrolytic graphite electrode (EPPGE) with cobalt and cobalt oxide nanoparticles integrated with and without single-walled carbon nanotubes (SWCNTs) is described. Successful modification of the electrodes was confirmed by field emission scanning electron microscopy (FESEM), AFM and EDX techniques. The electron transfer behaviour of the modified electrodes was investigated in [Fe(CN)6]^3−/4− redox probe using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) and discussed. The study showed that cobalt nanoparticles modified electrodes exhibit faster electron transfer behaviour than their oxides. The catalytic rate constant (K) obtained at the EPPGE-SWCNT-Co for nitrite at pH 7.4 and 3.0 are approximately the same (∼3 × 10⁴ cm³ mol⁻¹ s⁻¹) while the limits of detection (LoD = 3.3δ/m) are in the μM order. From the adsorption stripping voltammetry, the electrochemical adsorption equilibrium constant β was estimated as (13.0 ± 0.1) × 10³ M⁻¹ at pH 7.4 and (56.7 ± 0.1) × 10³ M⁻¹ at pH 3.0 while the free energy change (ΔG°) due to the adsorption was estimated as −6.36 and −10.00 kJ mol⁻¹ for nitrite at pH 7.4 and 3.0, respectively.     

Preparation and characterization of osmium hexacyanoferrate films and their electrocatalytic properties

Osmium hexacyanoferrate films have been prepared using repeated cyclic voltammetry, and the deposition process and the films’ electrocatalytic properties in electrolytes containing various cations have been investigated. The cyclic voltammograms recorded the deposition of osmium hexacyanoferrate films directly from the mixing of Os³⁺ and Fe(CN)₆³⁻ ions from solutions containing various cations. An electrochemical quartz crystal microbalance, cyclic voltammetry, and UV-visible spectroscopy were used to study the growth mechanism of the osmium hexacyanoferrate films. The osmium hexacyanoferrate films showed a single redox couple, and the redox reactions included “electron transfer” and “proton transfer” with a formal potential that demonstrates a proton effect in acidic solutions up to a 12 M aqueous HCl solution. The electrochemical and electrochemical quartz crystal microbalance results indicate that the redox process was confined to the immobilized osmium hexacyanoferrate film. The electrocatalytic reduction of dopamine, epinephrine, norepinephrine, S₂O₃²⁻, and SO₅²⁻ by the osmium hexacyanoferrate films was performed. The preparation and electrochemical properties of co-deposited osmium(III) hexacyanoferrate and copper(II) hexacyanoferrate films were determined, and their two redox couples showed formal potentials that demonstrated a proton effect and an alkaline cation effect, respectively. Electrocatalytic reactions on the hybrid films were also investigated.     

Electrochemical studies on the redox mechanism of uranium chloride in molten LiCl-KCl eutectic

The reduction and oxidation processes on platinum and glassy carbon electrodes in molten LiCl-KCl eutectic containing UCl₃ were investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS) in the temperature range 660-780 K. Two redox peaks have been observed in the cyclic voltammograms corresponding to the two redox reactions U(IV)/U(III) and U(III)/U which are found to be reversible and quasi-reversible, respectively. The reduction potentials of U(IV)/U(III) and U(III)/U are −0.325 and −1.490 V versus reference electrode (0.1 mol% AgCl in the LiCl-KCl), respectively at 700 K. Chronopotentiometric measurements confirm the three-electron transfer during the reduction of U(III) to U metal. Electrochemical impedance spectroscopy data at various potentials were interpreted either as diffusion, adsorption or reduction process by nonlinear fit of the impedance data to the simulated equivalent circuits.     

Kinetic parameters for anodic oxidation of hypochlorite ion on Pt and Pt oxide electrodes in alkaline solution

Kinetic parameters for the anodic oxidation of hypochlorite ion have been determined by means of normal pulse voltammetry by using a platinum disk as the working electrode. By using the working electrode that formed an oxide film by electrochemical pretreatment, the effect of the lattice oxygen of the surface oxide on the reaction was also examined. The measurement results were analyzed by the classical method, and then the analytical results were evaluated by digital simulation. The normal pulse voltammogram of the hypochlorite ion showed quasi-reversible oxidation waves. The apparent rate constant was calculated to be 5.0-8.1 × 10⁻⁴ cm s⁻¹, depending on the electrode surface state. At the low-concentration range of <4.0 mg Cl dm⁻³, the oxidation current was concentration dependent at the cathodically polarized electrode, while it became independent after the anodic polarization.     

One-electron redox reaction of di-tert-butyl nitroxide at platinum electrode in acetonitrile

The electrochemical oxidation of di-tert-butyl nitroxide (DTBN) at a platinum electrode in acetonitrile was examined. The cyclic voltammogram indicated an irreversible response during a normal time scale measurement, whereas chemically reversible voltammograms were obtained during a shorter time using a micro disk electrode with relatively fast sweep rates. The apparent formal redox potential and heterogeneous electron transfer rate constant were estimated to be 0.218 V (versus Fc⁺|Fc) and 0.035 ± 0.015 cm s⁻¹ from the digital simulation analysis.     

The characterization and bioelectrocatalytic properties of hemoglobin by direct electrochemistry of DDAB film modified electrodes

Direct electrochemistry of hemoglobin can be performed in acidic and basic aqueous solutions in the pH range 1-13, using stable, electrochemically active films deposited on a didodecyldimethylammonium bromide (DDAB) modified glassy carbon electrode. Films can also be produced on gold, platinum, and transparent semiconductor tin oxide electrodes. Hemoglobin/DDAB films exhibit one, two, and three redox couples when transferred to strong acidic, weak acidic and weak basic, and strong basic aqueous solutions, respectively. These redox couples, and their formal potentials, were found to be pH dependent. An electrochemical quartz crystal microbalance and cyclic voltammetry were used to study the in situ deposition of DDAB on gold disc electrodes and hemoglobin deposition on DDAB film modified electrodes. A hemoglobin/DDAB/GC modified electrode is electrocatalytically reduction active for oxygen and H₂O₂, and electrocatalytically oxidation active for S₂O₄²⁻ through the Fe(III)/Fe(II) redox couple. In the electrocatalytic reduction of S₄O₆²⁻, S₂O₄²⁻, and SO₃²⁻, and the dithio compounds of 2,2′-dithiosalicylic acid and 1,2-dithiolane-3-pentanoic acid, the electrocatalytic current develops from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in neutral and weakly basic aqueous solutions. Hemoglobin/DDAB/GC modified electrodes are electrocatalytically reduction active for trichloroacetic acid in strong acidic buffered aqueous solutions through the Fe(III)/Fe(II) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about −0.9 V (from the Fe(II)/Fe(I) redox couple) in weak acidic and basic aqueous solutions. The electrocatalytic properties were investigated using the rotating ring-disk electrode method.     

Study on the electrochemical behaviour of the anticancer herbal drug emodin

The electrochemical behaviour of the anticancer herbal drug emodin was investigated by cyclic voltammetry (CV) at glassy carbon electrode. In 0.05 M NH₃-NH₄Cl (50% ethanol, pH 7.2) buffer solution, a pair of quasi-reversible redox peaks at potentials of Ep1 = −0.688 V and Ep2 = −0.628 V and one irreversible anodic peak, which was a typical anodic peak of emodin, at Ep3 = −0.235 V appeared at a scan rate of 100 mV/s. The irreversible anodic peak currents are linearly related to the emodin concentrations in a range from 8.9 × 10⁻⁸ M to 7.8 × 10⁻⁶ M with a pre-concentration time of 80 s under −0.620 V. Using the established method without pretreatment and pre-separation, emodin in herbal drug was determined with satisfactory results. Moreover, the electrode process dynamics parameters were also investigated by electrochemical techniques.     

Effect of Bi(III) concentration on the stripping voltammetric response of in situ bismuth-coated carbon paste and gold electrodes

The effect of Bi(III) concentration (over the wide concentration range of 10⁻⁷ to 10⁻⁴ M) on the determination of Pb and Cd metal ions (in the 10⁻⁸ to 10⁻⁵ M range), by means of anodic stripping voltammetry (ASV) at in situ bismuth-coated carbon paste (CPE) and gold electrodes, has been studied. It is shown that in square wave anodic stripping voltammetry (SWASV) experiments the sensitivity of the technique generally depends on the Bi(III)-to-metal ion concentration ratio. It was found that, unlike the usually recommended at least 10-fold Hg(II) excess in anodic stripping experiments at in situ prepared mercury film electrodes, Bi(III)-to-metal ion ratios less than 10 are either optimal or equally effective at CPE and Au electrode substrates. Detection limits down to 0.1 μg L⁻¹ for Pb(II) and 0.15 μg L⁻¹ for Cd(II) were estimated at CPEs under conditions of small or moderate Bi(III) excess. Depending on Bi(III) concentration and deposition time, multiple stripping peaks attributed to Bi were recorded (especially in the case of Au substrates), indicating various forms of Bi deposits.     

An electrochemical biosensor based on Nafion-ionic liquid and a myoglobin-modified carbon paste electrode

An electrochemical biosensor was constructed based on the immobilization of myoglobin (Mb) in a composite film of Nafion and hydrophobic ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) for a modified carbon paste electrode (CPE). Direct electrochemistry of Mb in the Nafion-BMIMPF₆/CPE was achieved, confirmed by the appearance of a pair of well-defined redox peaks. The results indicate that Nafion-BMIMPF₆ composite film provided a suitable microenvironment to realize direct electron transfer between Mb and the electrode. The cathodic and anodic peak potentials were located at −0.351 V and −0.263 V (vs. SCE), with the apparent formal potential (Ep) of −0.307 V, which was characteristic of Mb Fe(III)/Fe(II) redox couples. The electrochemical behavior of Mb in the composite film was a surface-controlled quasi-reversible electrode process with one electron transfer and one proton transportation when the scan rate was smaller than 200 mV/s. Mb-modified electrode showed excellent electrocatalytic activity towards the reduction of trichloroacetic acid (TCA) in a linear concentration range from 2.0 × 10⁻⁴ mol/L to 1.1 × 10⁻² mol/L and with a detection limit of 1.6 × 10⁻⁵ mol/L (3σ). The proposed method would be valuable for the construction of a third-generation biosensor with cheap reagents and a simple procedure.     

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.     

n-Type nitrogenated nanocrystalline diamond thin-film electrodes: The effect of the nitrogenation on electrochemical properties

Nitrogenated nanocrystalline diamond thin-film electrodes with controlled conductivity are grown from microwave- or arc-plasma in CH₄-Ar-H₂-N₂ gas mixtures. Their electrochemical behavior is studied using cyclic voltammetry and electrochemical impedance spectroscopy techniques. It is concluded from Mott-Schottky plots that the studied material has n-type conductance; the donor concentration is estimated. The character of electrode behavior is controlled by the degree of nitrogenation of the material. In particular, with the increasing of nitrogen concentration in the feeding gas (0-25%) supplied to plasma-chemical reactor, the potential window in the supporting electrolyte (2.5 M H₂SO₄) becomes somewhat narrower, the reversibility of electrochemical reactions in the [Fe(CN)₆]^3−/4− redox couple becomes more pronounced. Kinetic parameters of redox reactions in this couple are determined. By and large, with the increasing of the nitrogenation the electrochemical behavior of “poor conductor” gives way to that of metal-like conductor.     

Hemoglobin immobilized on whisker-like carbon composites and its direct electrochemistry

We developed a novel mesoporous carbon/whisker-like carbon (MCWC) composite which can promote the direct electron transfer of hemoglobin (Hb) immobilized on its surface. The cyclic voltammetric results showed that Hb immobilized on the surface of the MCWC composite could undergo a direct quasi-reversible electrochemical reaction. Its formal redox potential, E^0′ is −0.313 V in the phosphate buffer solution (pH 6.9) at a scan rate of 200 mV/s and is almost independent of the scan rate in the range of 100-600 mV/s. The dependence of E^0′ on the pH of phosphate buffer solution indicated that the redox reaction of Hb includes a one-electron-transfer reaction process coupled with one-proton-transfer. The experiment obtained larger value of electron transfer rate constant, k_s, than that of Hb immobilized on other carriers reported previously due to its special structure of loosely packed nanometer-scale carbon whiskers and thus formed “V-type” nano-pores. Furthermore, Hb immobilized on the surface of the MCWC composite can retain the stable bioelectrocatalytic activity for the reduction of H₂O₂.     

Electrochemical evaluation of avidin-biotin interaction on self-assembled gold electrodes

The avidin-biotin interaction on 11-mercaptoundecanoic acid self-assembled gold electrodes was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The interfacial properties of the modified electrodes were evaluated in the presence of the Fe(CN)₆^3−/4− couple redox as a probe. A simple equivalent circuit model with a constant phase element was used to interpret the obtained impedance spectra. The results of cyclic voltammetry showed that the voltammetric behavior of the redox probe was influenced by the electrode surface modification. It is evident that the accumulation of treated substances and the binding of biotin to avidin on the electrode surface resulted in the increasing electron-transfer resistance and the decreasing capacitance. The changes in the electron-transfer resistance on the avidin-modified electrodes were more sensitive than that in the capacitance while detecting biotin over the 2-10 μg/mL concentration. The detection amount can be as low as 20 ng/mL based on the electron-transfer resistance that presented the change of 4.3 kΩ without the use of labels. The development of a rapid, facile, and sensitive method for the quantitation of nanogram quantities of biomolecules utilizing EIS may be achieved.     

A study of adsorption kinetics and thermodynamics of ω-mercaptoalkylhydroquinone self-assembled monolayer on a gold electrode

The adsorption kinetics and thermodynamics for the formation of redox active self-assembled monolayer (SAM) of 2-(n-mercaptoalkyl)hydroquinone (abbreviated as H₂Q(CH₂)_nSH, where n = 4, 6, 8, 10, and 12) on gold electrode has been investigated by cyclic voltammetry to study the effects of concentration and alkyl chain length. The time dependence of surface coverage, differential capacitance, and formal potential of electroactive hydroquinone(H₂Q) moiety supports that the adsorption of H₂Q(CH₂)_nSH molecules typically processes with a two-step adsorption consisted of a fast initial adsorption and a slowly following reorganization. The adsorption processes can be satisfactorily described by simple Langmuir adsorption kinetics, irrespective of concentration and alkyl chain length of adsorbate molecule. Based on Langmuir kinetics, the adsorption rate constant was determined at the initial step for the formation of all H₂Q(CH₂)_nSH-SAMs studied in this work. The rate constant value was found to be decreased with increasing alkyl chain length and decreasing bulk solution concentration (≤10 μM). The dependence of a surface coverage (Γ_e) at adsorption equilibrium on the bulk concentration is accurately described by the Langmuir isotherm at several concentrations ranging from 8 × 10⁻⁶ to 1 × 10⁻⁵ M for all H₂Q(CH₂)_nSH molecules. Parameters characterizing the adsorption thermodynamics, such as Γ_s, adsorption coefficient (β), and adsorption free energy (ΔG_ads) were determined from this isotherm.