Chronoamperometric evaluation of the antioxidant capacity of tea on a polyquercetin-modified electrode

It was established that the components of tea are oxidized on a glassy carbon electrode modified with carbon nanotubes and electropolymerized quercetin in a phosphate buffer solution (pH 7.0) as a supporting electrolyte under conditions of differential pulse voltammetry. The oxidation potentials of the individual phenolic antioxidants of tea (gallic acid, rutin, quercetin, catechin, epigallocatechin gallate, and tannin) on the modified electrode were found. A method for the chronoamperometric determination of the antioxidant capacity (AOC) of tea was developed based on the oxidation of tea antioxidants at a potential of 0.20 V. The AOC of tea was evaluated using а difference between the oxidation currents of the analyte and a supporting electrolyte after 50 s of electrolysis in terms of gallic acid. The analytical range of gallic acid was 0.25?750 μM with a detection limit of 0.063 μM. Positive correlations of the AOC with antioxidant activity in a reaction with 2,2-diphenyl-1-picrylhydrazyl and the total phenolic content were found (r = 0.700 and 0.647 at r _crit = 0.396, respectively).     

Pulsed electrochemical deposition of nickel oxides on multi-walled carbon nanotubes from EDTA alkaline solutions: a SEM,XPS, and voltammetric characterization

A study regarding the electrodeposition of nickel oxide particles on the activated multi-walled carbon nanotubes from 2 M NaOH solution containing Ni(NO₃)₂ and EDTA was carried out. The electrodeposition process was carried out using an optimized double-pulse sequence of potentials: E ₁ = ?0.2 V vs. SCE (t ₁ = 0.3 s) and E ₂ = 0.7 V vs. SCE (t ₂ = 0.03 s). Spectroscopic XPS investigations and SEM analysis were used in order to characterize the surface and morphology of the studied modified electrode. Cyclic voltammetry and chronoamperometry were used in order to evaluate the electrochemical/amperometric performance of the GC/MWCNT-Ni electrode toward the oxidation of some aliphatic alcohols in strong alkaline medium.     

Computational assisted electrochemical studies for 1,4-diazabicyclo[2,2,2]octane determination at multiwalled carbon nanotube paste electrode

A multiwalled carbon nanotube-modified carbon paste electrode (MWCNT-PE) was used for determination of 1,4-diazabicyclo[2,2,2]octane (DABCO or TEDA) in 0.1 M phosphate buffer solutions (pH 10.25). Cyclic voltammetry(CV) and differential pulse voltammetry (DPV) techniques were used to investigate the electrocatalytic oxidation of DABCO at the surface of modified electrode. The results shown that the oxidation peak current of DABCO at the surface of MWCNT-PE was 2.40 times larger than that at the bare electrode. The experimental formal redox potential (E°') of DABCO was obtained 986 mV versus SHE (Standard Hydrogen Electrode). Density functional theory (DFT) method at B3LYP/6-311++G** level of theory and a conductor-like Polarizable Continuum Model (CPCM) was used to calculate the E°' values. The highest occupied molecular orbital (E _HOMO), lowest unoccupied molecular orbital (E _LUMO) and some thermodynamic parameters such as Gibbs free energy of DABCO and its oxidation forms were calculated. Both direct and indirect methods were used to calculate the theoretical standard electrode potential for DABCO and the results were found to be in good agreement with the experimental values.     

Cyclic voltammetry and impedance spectroscopy analysis for graphene-modified solid-state electrode transducers

The transducer of solid-state electrodes based on an epoxy-graphite composite was modified by two different methods, such as direct mixed and layer deposition of graphene (commercial and synthesized by electrochemical exfoliation of graphite). The modified electrodes were characterized by cyclic voltammetry and impedance spectroscopy. Also, scanning electron microscopy (SEM) was carried out to acquire information concerning the morphology of the composite electrode. Voltammetric measurements, in presence of [Fe(CN)₆]^3? as electroactive standard, determined a quasi-reversible electrochemical behavior under linear diffusion control. Electronic transference for modified and unmodified electrodes was compared. Solid-state electrode modified by inclusion of synthesized graphene showed a better electronic transference at electrode surface, due to the lower potential difference between anodic and cathodic peaks (ΔE = 125 mV) with respect to unmodified electrode (ΔE = 160 mV). Impedance spectroscopy characterization of electrode bodies in solid-state it was revealed a higher electronic conductivity and a supercapacitive behavior for the modified composites (values of intrinsic capacitances in the order of nanofarads) due to inclusions of graphite and graphene in the epoxy matrix. These inclusions were verified through SEM microscopy. The electronic conductivity and the supercapacitive character contributed both to the enhancement of electronic transference at electrode surface.     

Cathodic Stripping Voltammetry of 2-Mercapto-5-Methyl-1,3,4-Thiadiazol and 2,5-Dimercapto-1,3,4-Thiadiazol at a Controlled-Growth Mercury Drop Electrode

2-Mercapto-5-methyl-1,3,4-thiadiazol (MMTD) and 2,5-dimercapto-1,3,4-thiadiazol (DMTD) were studied by differential pulse cathodic stripping voltammetry (DPCSV). The influence of buffer, pH, accumulation potential (E_acc), and accumulation time (t_acc) was investigated. It was stated that the concentration of the buffer affects the height of DPCSV peaks. The best analytical signals were recorded in acetate buffer at pH 4.3 and a buffer concentration of 0.01 mol/L for MMTD and 0.02 mol/L for DMTD, E_acc = 0.2 V, and t_acc = 120 s for MMTD and 180 s for DMTD. A linear dependence was found from 1 to 8 × 10^?8 mol/L for MMTD and from 1 × 10^?8 to 1 × 10^?7 mol/L for DMTD. The influence of cations [Cu(II), Co(II)] was also considered.     

Voltammetric Characterization of Grafted Polymer Modified with ZnO Nanoparticles on Glassy Carbon Electrode

In this study, a grafted polymer (GP) with ZnO nanoparticles (GP/ZnO NPs) was attached on the surface of glassy carbon electrode (GCE), in order to produce a new modified electrode (GP/ZnO NPs-GCE). The gamma irradiation method was used to grafted polystyrene (polymer) with acrylonitrile (monomer), while slow evaporation process was used to prepare the new modified electrode. The cyclic voltammetry (CV) of K₄[Fe(CN)₆] was used to study the electrochemical properties GP/ZnO NPs-GCE. The peak separation (ΔE_pa-c) was 500 mV between the redox peaks of Fe(II)/Fe(III) in an aqueous solution of 1 M KCl and the current ratio of redox current peaks (I_pa/I_pc) was ≈ 1 for the modified electrode. This indicated that the modified electrode has s good reversibility and conductivity, wherefore; it was applied in the voltammetric filed. It was found that the modified electrode GP/ZnO NPs-GCE have a reasonable solubility and stability at various pH medium. Additionally, the sensitivity of the electrochemical analysis by cyclic voltammetric (CV) method is extensively subjected to the pH medium and the scan rate (SR). A couple of redox current peaks of K₄[Fe(CN)₆] in KCl solution was observed with a reversible process: Fe³⁺/Fe²⁺. Finally a good diffusion coefficient of electroactive species (D) for the new modified electrode was found in this study by chronoamperometry method using Cottrell equation.     

Properties of gold electrode modified with dimercaptosuccinic acid and electrochemical behavior of copper histidine complex

A self-assembled monolayer of meso-2,3-dimercaptosuccinic acid was prepared on the surface of gold disc electrode. The modified electrode was characterized using cyclic voltammetry in copper(II) solution and cyclic voltammetry and electrochemical impedance spectroscopy in the presence of potassium hexacyanoferrate( II)/(III) and hexaammineruthenium (II)/(III) chloride. Binding of copper(II) histidine complex (Cu–His) onto the electrode was successfully achieved for a wide range of tested concentrations, as shown with adsorption transfer stripping voltammetry. Electrode response (logΔI_p) was linearly proportional to logc(Cu–His) with correlation coefficient R³² = 0.9839.     

Voltammetric determination of pyrocatechol at a mechanically renewed electrode made of a graphite-epoxy composite

The reversibility of a two-electron pyrocatechol/o-benzoquinone system (ΔE _p = 28 ± 1 mV) is found by cyclic voltammetry at a mechanically renewed electrode from a graphite-epoxy composite. The closeness of the found and theoretical values of ΔE _p indicates the high electrochemical activity of the graphite-epoxy composite electrode renewed in situ by cutting a thin surface layer. The potentials of pyrocatechol anodic and cathodic peaks are proportional to E ₀ ± 0.059pH in the pH range of 1–9. The pyrocatechol anodic and cathodic peak currents remain linear functions of pyrocatechol concentration in the concentration range from 0.01 to 0.08 and from 0.08 to 0.9 g/L, respectively. A procedure for the voltammetric determination of pyrocatechol in its individual solutions and in the presence of hydroquinone is proposed. Good precision and the absence of a systematic error in the determination of pyrocatechol by measuring its cathodic peak are demonstrated.     

Electroanalytical study of guanine synthetic derivatives with antiviral activity

Experimental information on the electrochemical behavior of guanine synthetic derivatives (acyclovir, valacyclovir, ganciclovir, and famciclovir) with the strong antiviral activity on presynthesized activated carbosital electrode (CSE) in aqueous solutions of different acidity is obtained. The mechanism of irreversible oxidation of substances containing guanidine group (–NH–C(NH–)=N–) is discussed and the dependences of current and potential of experimental anodic peaks on the potential scan rate, the nature and acidity of supporting electrolyte solution, and also on the concentration of depolarizer and the time of its accumulation on the electrode surface are found. It is shown that all other factors being equal (0.1 М phosphate buffer, рН 6), the position of the observed anodic peak depends substantially on the nature of acyclic substituent at N(9) of imidazole ring in the aminopurine molecule: the susceptibility to oxidation decreases in the series Gua (E_pa = 0.84 V), ACV (E_pa = 1.00 V), VACV (E_pa = 1.04 V), GCV (E_pa = 1.07 V), FCV (E_pa = 1.20 V). It is shown that the electrocatalytic activity and the high absorbability of the activated CSE with respect to substances tested make it possible to reach their lower detection limits (20–40 nM) in multicomponent solutions.     

Nickel(II) complex with 1,4,7-tris(2-aminoethyl)-1,4,7-triazacyclononane

A nickel(II) complex, [Ni(taetacn)](ClO₄)₂ ? H₂O, where taetacn = 1,4,7-tris(2-aminoethyl)-1,4,7-triazacyclononane was synthesized. The crystal structure was determined by the single-crystal X-ray diffraction method at 293 K. The complex crystallizes in the orthorhombic space group Pna2₁ with a = 16.004(2) Å, b = 10.186(1) Å, c = 13.937(2) Å, V = 2271.9(5) ų, D_x = 1.56 g cm^?3, D_m = 1.59 g cm^?3 (floatation method), and Z = 4. The R₁ [I > 2σ(I)] and wR₂ (all data) values are 0.0636 and 0.1672, respectively, for all 4845 independent reflections. The compound is composed of octahedral nickel(II) cation with three 2-aminoethyl pendant groups of taetacn, tetrahedral ClO ₄ ^? anion, and a water molecule of crystallization. Electronic spectra are consistent with the octahedral geometry. Temperature dependence of the magnetic susceptibility (4.5–300 K) can be interpreted considering the zero-field splitting of the nickel(II) ion (g = 2.14, D = 3.72 cm^?1, and Nα = 300 × 10^?6 cm³ mol^?1). Cyclic voltammetry in DMF showed quasi-reversible and irreversible oxidation waves (E_pa = 0.54 V, E_pc = 0.45 V; E_pa = 1.16 V, E_pc = 0.71 V vs. Ag/Ag⁺).     

Highly sensitive electrocatalytic determination of pyrazinamide using a modified poly(glycine) glassy carbon electrode by square-wave voltammetry

A new voltammetric sensor based on electropolymerization of glycine at glassy carbon electrode (GCE) was developed and applied to determine of pyrazinamide (PZA) by square-wave voltammetry (SWV). The initial cyclic voltammetric studies showed an electrocatalytic activity of poly(Gly)/GCE on redox system of pyrazinamide in 0.1 mol L^?1 phosphate buffer solution pH 7.5, with E _Pc and E _Pa in ?0.85 and ?0.8 V (versus E _Ag/AgCl), respectively. Studies at different scan rates suggest that the redox system of pyrazinamide at poly(Gly)/GCE is a process controlled by diffusion in the interval from 10 to 100 mV s^?1. Square-wave voltammetry-optimized conditions showed a linear response of PZA concentrations in the range from 0.47 to 6.15 μmol L^?1 (R?=?0.998) with a limit of detection (LOD) of 0.035 μmol L^?1 and a limit of quantification (LOQ) of 0.12 μmol L^?1. The developed SWV-poly(Gly)/GCE method provided a good intra-day (RSD?=?3.75 %) and inter-day repeatability (RSD?=?4.96 %) at 4.06 μmol L^?1 PZA (n?=?10). No interference of matrix of real samples was observed in the voltammetric response of PZA, and the method was considered to be highly selective for the compound. In the accuracy test, the recovery was found in the range of 98.2 and 104.0 % for human urine samples and pharmaceutical formulation (tablets). The PZA quantification results in pharmaceutical tablets obtained by the proposed SWV-poly(Gly)/GCE method were comparable to those found by official analytical protocols.     

Sensitive detection of sulfanilamide by redox process electroanalysis of oxidation products formed in situ on glassy carbon electrode

This paper reported a simple method for sulfanilamide determination by redox process electroanalysis of oxidation products (SFDox) formed in situ on glassy carbon electrode. The CV experiments showed a reversible process after applied E _acc = + 1.06 V and t _acc = 1 s, in 0.1 mol L^?1 BRBS (pH = 2.0) at 50 mV s^?1. Different voltammetric scan rates (from 10 to 450 mV s^?1) suggested that the redox peaks of SFDox on the glassy carbon electrode (GCE) is an adsorption-controlled process. Square-wave voltammetry (SWV) method optimized conditions showed a linear response to SFD from 3.00 to 250.0 μmol L^?1 (R = 0.998) with a limit of detection of 0.638 μmol L^?1 and limit of quantification of 2.0 μmol L^?1. The developed the SWV method was successfully used in the determination of SFD pharmaceutical formulation and human serum. The SFD quantification results in pharmaceutical obtained by SWV-GCE were comparable to those found by official analytical protocols.     

Electrochemical behavior of phthaloyl peroxide in aqueous media

The electrochemical behavior of phthaloyl peroxide C₈H₄O₄ on an Au disk electrode in a 0.05 M aqueous solution of Na₂SO₄ was studied by cyclic voltammetry (CVA). It demonstrated a high activity in cathodic reduction with the formation of an irreversible peak on the CVA curve at E =–0.81 V. Additionally, during the anodic oxidation of C₈H₄O₄, the surface of the Au electrode became passivated by compounds which prevented its oxidation during the registration of repeated cycles. Apparently, these compounds are surface complexes of phthaloyl peroxide with a gold cation.     

Mechanism of the decomposition of the surface oxide film on polycrystalline palladium

The decomposition of thin surface oxide films on polycrystalline palladium Pd(poly) at 500–1300 K was investigated by mathematical modeling. This process was analyzed in terms of a model including O₂ desorption from the chemisorbed oxygen layer (O_ads) and the passage of oxygen inserted under the surface layer of the metal (O_abs) and oxygen dissolved in metal subsurface layers (O_dis) to the surface. O₂ desorption was modeled on a surface with a square lattice of adsorption sites, with account taken of the energy of the lateral repulsive interactions between adjacent O_ads atoms (ε_aa). At ε_aa = 10 kJ/mol and when the activation energy of O₂ desorption for a chemisorbed-oxygen surface coverage of θ ≈ 0 is E_des⁰ = 230 kJ/mol, the calculated spectra are in agreement with the oxygen temperature-programmed desorption (TPD) spectra obtained for Pd(poly) at θ ≤ 0.5. The passage of O_abs and O_dis atoms to the surface was calculated using a first-order equation, with account taken of the activation energy for these atoms coming out to the surface (E₂ and E₃, respectively). As the oxide film is heated, O₂ desorption is accompanied by the passage of O_abs and then O_dis to the surface, which leads to an increase in the O_ads surface coverage and, accordingly, to a buildup of lateral surroundings in the adsorbed layer. Owing to this fact and to the repulsive interactions between O_ads atoms, the bonds between O_ads and the surface weaken and E_des decreases. As a consequence, the O₂ desorption rate increases and a low-temperature peak with T_max ≈ 710 K, which is due to the passage of O_abs atoms to the surface, and then a high-temperature peak with T_max ≈ 770 K, which is due to the passage of O_dis atoms to the surface, appear in the TPD spectrum. At ε_aa = 10 kJ/mol, E_des⁰ = 230 kJ/mol, E₂ = 145 kJ/mol, and E₃ = 160 kJ/mol and when the number of inserted oxygen monolayers is θ_abs ≤ 0.3 and the number of oxygen monolayers dissolved in subsurface layers is θ_dis ≤ 10, the TPD spectra calculated for the given model are in agreement with the O₂ TPD spectra that are observed for Pd(poly) and are due to the decomposition of surface oxide films.     

Theoretical and experimental prediction of the redox potentials of metallocene compounds

The standard redox electrode potential (E°) values of metallocene compounds are obtained theoretically with density functional theory (DFT) method at B3LYP/6-311++G(d,p) level and experimentally with cyclic voltammetry (CV). The theoretical E° values of metallocene compounds are in good agreement with experimental ones. We investigate the substituent effects on the redox properties of metallocene compounds. Among the four metallocene compounds, the E° values is largest for titanocene dichloride and smallest for ferrocene.     

Promising electrochemical performance of high-surface-area boron-doped diamond/carbon nanotube electroanalytical sensors

Porous boron-doped diamond (p-BDD) electrodes of high-surface-area have been prepared on vertically aligned carbon nanotube substrates, and their electrochemical performance has demonstrated promising results for application in electroanalysis. The electrochemical features of the p-BDD electrodes were investigated and compared with those of a conventional flat BDD electrode (f-BDD). From cyclic voltammetry studies performed for the electrochemical probes [Fe(CN)₆]^3? and N,N,N′,N′-tetramethyl-para-phenylenediamine (TMPD), a fast charge transfer was observed at the p-BDD/electrolyte interface. For the [Fe(CN)₆]^3? redox probe, the heterogeneous electron-transfer rate constant (k ⁰) value obtained for p-BDD was 10.9 times higher than that obtained using a f-BDD electrode. Moreover, the p-BDD electrodes also gave a smaller peak potential separation, ΔE _p, and larger analytical signal magnitude for different biomolecules, such as dopamine (DA), acetaminophen (AC), and epinephrine (EP). These set of results demonstrated that the p-BDD electrode is a suitable candidate for applications in electroanalytical chemistry.     

Electrochemical Characteristics of Poly(<Emphasis Type="Italic">o</Emphasis>-Aminobenzoic Acid) Modified Glassy-Carbon Electrode and Its Electrocatalytic Activity towards Oxidation of Epinephrine

Poly(o-aminobenzoic acid) (o-ABA) film is deposited on glassy-carbon electrode (GCE) by electropolymerization in pH 7.0 phosphate buffer solution (PBS). Electrochemical behavior of modified electrode is investigated by electrochemical impedance spectroscopy (EIS), different pulse voltammetry (DPV), and cyclic voltammetry (CV). The results indicate that there is a greater resistance during the electron transfer process in poly(o-ABA) film than in bare GCE for the redox of [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻. Further research indicates that epinephrine (EP) can be strongly absorbed on the surface of the poly(o-ABA) film-modified electrode. The modified electrode shows an excellent electrocatalytical activity on EP oxidation. The EP cathodic peak potential shifts negatively with a slope of −53.5 mV/pH, indicating that equal amounts of proton and electron are involved in the electrode reaction process. In pH 7.0 PBS, the peak current of EP and the concentration has a linear relationship from 0 to 65 μM by amperometric current-time curve. __________ From Elektrokhimiya, Vol. 41, No. 9, 2005, pp. 1059–1065. Original English Text Copyright ? 2005 by Cheng, Jin, Zhang. The text was submitted by the authors in English.     

Direct electrochemistry of myoglobin in a layer-by-layer film on an ionic liquid modified electrode containing CeO2 nanoparticles and hyaluronic acid

We describe an ionic liquid modified electrode (CPE-IL) for sensing hydrogen peroxide (HP) that was modified by the layer-by-layer technique with myoglobin (Mb). In addition, the surface of the electrode was modified with CeO₂ nanoparticles (nano-CeO₂) and hyaluronic acid. UV-vis and FTIR spectroscopy confirmed that Mb retains its native structure in the composite film. Scanning electron microscopy showed that the nano-CeO₂ closely interact with Mb to form an inhomogeneously distributed film. Cyclic voltammetry reveals a pair of quasi-reversible redox peaks of Mb, with the cathodic peak at ?0.357?V and the anodic peak at ?0.269?V. The peak separation (??E _p) and the formal potential (E ^0??) are 88?mV and ?0.313?V (vs. Ag/AgCl), respectively. The Mb immobilized in the modified electrode displays an excellent electrocatalytic activity towards HP in the 0.6 to 78.0???M concentration range. The limit of detection is 50?nM (S/N?=?3), and then the Michaelis-Menten constant is 71.8???M. We believe that such a composite film has potential to further investigate other redox proteins and in the fabrication of third-generation biosensors.

Use of adsorptive square-wave anodic stripping voltammetry at carbon paste electrode for the determination of amlodipine besylate in pharmaceutical preparations

The antihypertensive drug amlodipine has been characterized voltammetrically in a carbon paste electrode by means of anodic stripping voltammetry. An adsorptive stripping method in a carbon paste electrode for trace determination of amlodipine has been described. Cyclic voltammetric studies indicated the oxidation of amlodipine besylate at the electrode surface through a single two-electron irreversible step fundamentally controlled by adsorption. A study of the variation in the peak current with solution variables such as pH, ionic strength, concentration of amlodipine, possible interference, and instrumental variables, such as preconcentration time and accumulation potential, has resulted in the optimization of the oxidation signal for analytical purposes. By anodic adsorptive anodic stripping voltammetry, the calibration plot was linear in the range 9.9 × 10^?9 ? 1.4 × 10^?7 M with a detection limit of 2 × 10^?10 M in a carbon paste electrode at pH 11.0. The procedure was successfully applied to the assay of amlodipine besylate in some commercial products in the market (Amlopres®, Amlodipine, and Norvasc®). The percentage recoveries were in agreement with those obtained by the reference method.