Electrooxidation of the antiviral drug valacyclovir and its square-wave and differential pulse voltammetric determination in pharmaceuticals and human biological fluids

The electrochemical properties of valacyclovir, an antiviral drug, were investigated in pH range 1.8-12.0 by cyclic, differential pulse and square-wave voltammetry. The drug was irreversibly oxidized at a glassy carbon electrode in one or two oxidation steps, which are pH-dependent. For analytical purposes, a very resolved diffusion controlled voltammetric peak was obtained in Britton-Robinson buffer at pH 10.0 using differential pulse and square-wave modes. Limits of detection were 1.04 × 10⁻⁷ and 4.60 × 10⁻⁸ M for differential pulse and square-wave voltammetry, respectively. The applicability to direct assays of tablets, spiked human serum and simulated gastric fluid, was described.     

Simultaneous determination of N-acetyl-p-aminophenol and p-aminophenol with poly(3,4-ethylenedioxythiophene) modified glassy carbon electrode

A sensitive and selective method was developed for the determination of N-acetyl-p-aminophenol (APAP) and p-aminophenol (PAP) using poly(3,4-ethylenedioxythiophene) (PEDOT)-modified glassy carbon electrode (GCE). Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical reaction of APAP and PAP at the modified electrode. Both APAP and PAP showed quasireversible redox reactions with formal potentials of 367 mV and 101 mV (vs. Ag/AgCl), respectively, in phosphate buffer solution of pH 7.0. The significant peak potential difference (266 mV) between APAP and PAP enabled the simultaneous determination both species based on differential pulse voltammetry. The voltammetric responses gave linear ranges of 1.0 × 10⁻⁶-1.0 × 10⁻⁴ mol L⁻¹ and 4.0 × 10⁻⁶-3.2 × 10⁻⁴ mol L⁻¹, with detection limits of 4.0 × 10⁻⁷ mol L⁻¹ and 1.2 × 10⁻⁶ mol L⁻¹ for APAP and PAP, respectively. The method was successfully applied for the determination of APAP and PAP in pharmaceutical formulations and biological samples.     

A novel poly(cyanocobalamin) modified glassy carbon electrode as electrochemical sensor for voltammetric determination of peroxynitrite

This report described the direct voltammetric detection of peroxynitrite (ONOO⁻) at a novel cyanocobalamin modified glassy carbon electrode prepared by electropolymeriation method. The electrochemical behaviors of peroxynitrite at the modified electrode were studied by cyclic voltammetry. The results showed that this new electrochemical sensor exhibited an excellent electrocatalytic activity to oxidation of peroxynitrite. The mechanism of catalysis was discussed. Based on electrocatalytic oxidation of peroxynitrite at the poly(cyanocobalamin) modified electrode, peroxynitrite was sensitively detected by differential pulse voltammetry. Under optimum conditions, the anodic peak current was linear to concentration of peroxynitrite in the range of 2.0 × 10⁻⁶ to 3.0 × 10⁻⁴ mol L⁻¹ with a detection limit of 1.0 × 10⁻⁷ mol L⁻¹ (S/N of 3). The proposed method has been applied to determination of peroxynitrite in human serum with satisfactory results. This poly(cyanocobalamin) modified electrode showed high selectivity and sensitivity to peroxynitrite determination, which could be used in quantitative detection of peroxynitrite in vivo and in vitro.     

Study of the electrochemical behavior of isorhamnetin on a glassy carbon electrode and its application

The electrochemical behavior of isorhamnetin (ISO) at a glassy carbon electrode was studied in a phosphate buffer solution (PBS) of pH 4.0 by cyclic voltammetry (CV) and differential pulse voltammetric method (DPV). A well-defined redox wave of ISO involving one electrons and one proton appeared. The electrode reaction is a reactant weak adsorption-controlled process with a charge transfer coefficient (α) of 0.586. Based on the understanding of the electrochemical process of ISO at the glassy carbon electrode, analysis of ISO can be realized. Under optimal conditions, the oxidation peak current showed linear dependence on the concentration of ISO in the range of 1.0 × 10⁻⁸ to 4.0 × 10⁻⁷ M and 1.0 × 10⁻⁶ to 1.0 × 10⁻⁵ M. The detection limit is 5.0 × 10⁻⁹ M. This method has been successfully applied to the detection of ISO in tablets.     

A novel bimediator amperometric sensor for electrocatalytic oxidation of gallic acid and reduction of hydrogen peroxide

A novel bimediator amperometric sensor is fabricated for the first time by surface modification of graphite electrode with thionine (TH) and nickel hexacyanoferrate (NiHCF). The electrochemical behavior of the TH/NiHCF bimediator modified electrode was characterized by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The TH/NiHCF bimediator modified electrode exhibited a pair of distinct redox peaks for NiHCF and TH with formal potentials of 0.33 V and −0.27 V vs. SCE at a scan rate of 50 mV s⁻¹ in 0.1 M NaNO₃ and 0.1 M NH₄NO₃ respectively. The electrocatalytic activity of the bimediator modified electrode towards oxidation of gallic acid with NiHCF and reduction of hydrogen peroxide with TH was evaluated and it was observed that the modified electrode showed an electrocatalytic activity towards the oxidation of gallic acid in the concentration range of 4.99 × 10⁻⁶–1.20 × 10⁻³ M with a detection limit of 1.66 × 10⁻⁶ M (S/N = 3) and reduction of H₂O₂ in the concentration range of 1.67 × 10⁻⁶–1.11 × 10⁻³ M with a detection limit of 5.57 × 10⁻⁷ M (S/N = 3). The bimediator modified electrode was found to exhibit good stability and reproducibility.     

Electroreduction of the antifouling agent TCMTB and its electroanalytical determination in tannery wastewaters

The electrochemical reduction of antifouling agent 2-thiocyanomethylthiobenzothiazole (TCMTB) was investigated by cyclic and pulse differential voltammetry. The irreversible electrode reduction of TCMTB proceeded by ECEC reaction mechanism by two electrons transfer with one irreversible wave. Upon the basis of electrochemical evidence, the electrodic reaction mechanism was suggested to formation of mercaptobenzothiazole (MTB) in solution.Subsequently, a pulse differential method is described for the formation of TCMTB based on this electroreduction. Having been obtained a detection limit of 1.0 × 10⁻⁷ mol L⁻¹ and recovery to 98% to concentration of 2.0 × 10⁻⁶ mol L⁻¹. Therefore, the proposed method in this study is practical, sensitive and accurate for the analysis of TCMTB in tannery wastewater samples.     

Application of manganese (II) phthalocyanine synthesized in situ in the SiO2/SnO2 mixed oxide matrix for determination of dissolved oxygen by electrochemical techniques

This work describes the in situ immobilization of Mn(II) phthalocyanine (MnPc) in a porous SiO₂/SnO₂ mixed oxide matrix obtained by the sol gel processing method. The chemically modified matrix SiO₂/SnO₂/MnPc, possessing an estimated amount of 8 × 10⁻¹⁰ mol cm⁻² of MnPc on the surface, was used to prepare an electrode to analyze dissolved oxygen in water by an electrochemical technique. The electrode was prepared by mixing the material with ultrapure graphite and evaluated using differential pulse voltammetry. Dissolved O₂ was reduced at −0.31 V with a limit of detection (LOD) equal to 7.0 × 10⁻⁴ mmol L⁻¹. A mechanism involving four electrons in O₂ reduction was determined by the rotating disk electrode technique.     

Selective response of dopamine in the presence of ascorbic acid on carbon paste electrode modified with titanium phosphated silica gel

Titanium phosphate grafted on the surface of silica gel (devoted briefly as Si-TiPH) was synthesized and used as bulk modifier to fabricate a renewable three-dimensional chemically modified electrode. The Si-TiPH bulk modified carbon paste electrode was used for the selective determination of dopamine (DA) in the presence of ascorbic acid (AA). The modified electrode offers an excellent and stable response for the determination of DA in the presence of AA. The differential pulse voltammetry peak current was found to be linear with the DA concentration in the range 2 × 10⁻⁷ to 1 × 10⁻⁶ and 2 × 10⁻⁶ to 6 × 10⁻⁵ mol L⁻¹. The detection limit of the proposed method in the presence of 2.0 × 10⁻⁵ M of AA was found to be 4.3 × 10⁻⁸ mol L⁻¹ for DA determination. The proposed method was successfully applied for the determination of DA in injections.     

Characterization of a carbon paste electrode modified with tripolyphosphate-modified kaolinite clay for the detection of lead

We report about the use of carbon paste electrode modified with kaolinite for analytical detection of trace lead(II) in domestic water by differential pulse voltammetry. Kaolinite clay was modified with tripolyphosphate (TPP) by impregnation method. The results show that TPP in kaolinite clay plays an important role in the accumulation process of Pb(II) on the modified electrode surface. The electroanalytical procedure for determination of Pb(II) comprised two steps: chemical accumulation of the analyte under open-circuit conditions, followed by electrochemical detection of the pre-concentrated species using differential pulse voltammetry. The analytical performance of this system has been explored by studying the effects of preconcentration time, carbon paste composition, pH, supporting electrolyte concentration, as well as interferences due to other ions. The calculated detection limit based on the variability of a blank solution (3s_b criterion) for 10 measurements was 8.4 × 10⁻⁸ mol L⁻¹, and the sensitivity determined from the slope of the calibration graph was 0.910 mol L⁻¹. The reproducibility (RSD) for five replicate measurements at 1.0 mg L⁻¹ lead level was 1.6%. The results indicate that this electrode is sensitive and effective for the determination of Pb²⁺.     

Gold nanoparticles modified indium tin oxide electrode for the simultaneous determination of dopamine and serotonin: Application in pharmaceutical formulations and biological fluids

A new rapid, convenient and sensitive electrochemical method based on a gold nanoparticles modified ITO (Au/ITO) electrode is described for the detection of dopamine and serotonin in the presence of a high concentration of ascorbic acid. The electrocatalytic response was evaluated by differential pulse voltammetry (DPV) and the modified electrode exhibited good electrocatalytic properties towards dopamine and serotonin oxidation with a peak potential of 70 mV and 240 mV lower than that at the bare ITO electrode, respectively. The selective sensing of dopamine is further improved by applying square wave voltammetry (SWV) which leads to the lowering of its detection limit. A similar effect on the detection limit of serotonin was observed on using SWV. Linear calibration curves are obtained in the range 1.0 × 10⁻⁹-5.0 × 10⁻⁴ M and 1.0 × 10⁻⁸-2.5 × 10⁻⁴ M with a detection limit of 0.5 nM and 3.0 nM for dopamine and serotonin, respectively. The Au/ITO electrode efficiently determines both the biomolecules simultaneously, even in the presence of a large excess of ascorbic acid. The adequacy of the developed method was evaluated by applying it to the determination of the content of dopamine in dopamine hydrochloride injections. The proposed procedure was also successfully applied to simultaneously detect dopamine and serotonin in human serum and urine.