Electrocatalytic Response of Electrodes Modified with Metal Phthalocyanines in the Cholinesterase–Thiocholine Ester System

Hydrolysis products in the cholinesterase–thiocholine ester system were determined by voltammetry using electrodes modified with metal phthalocyanines (MPc, M = Fe, Co). Specific features of the electrooxidation of the hydrolysis product (thiocholine) at a carbon-paste electrode and at a chemically modified electrode (CME) were compared. The conditions for the detection of an electrocatalytic signal in the oxidation of thiocholine at the CME were studied. Performance characteristics of the electrocatalytic determination of thiocholine at the CME were determined. The use of electrodes chemically modified with MPc reduces the detection limit for cholinesterase substrates under consideration to n × 10^–6 M.     

Electrochemical behavior and voltammetric determination of cysteine and cystine at carbon-paste electrodes modified with metal phthalocyanines

The electrochemical behavior of cysteine and cystine on carbon-paste electrodes modified with Fe(II), Co(II), Ni(II), and Cu(II) phthalocyanines is studied. Metal phthalocyanines exhibit catalytic activity in the electrooxidation of cysteine and cystine and in the reduction of cystine. The best catalyst is the cobalt complex. In the electrooxidation of cysteine, the catalysts are electrogenerated complex species of Co(II)Pc or Co(III)Pc⁺, and in the oxidation of cystine, oxidized or reduced forms of the coordinated ligand. Various versions of the determination of cysteine and cystine by the electrocatalytic reactions on the modified electrodes are proposed.     

Electrocatalytic Oxidation and Flow-Injection Determination of Sulfur-Containing Amino Acids at Graphite Electrodes Modified with a Ruthenium Hexacyanoferrate Film

The electrochemical behavior of sulfur-containing amino acids (cysteine, cystine, and methionine) at graphite electrodes modified with a ruthenium(III) hexacyanoferrate(II) film was studied. Glassy carbon and carbon paste were used as graphite materials. The electrocatalytic oxidation of amino acids at a modified electrode resulted in a decrease in the oxidation potentials of amino acids and an increase in the currents of their oxidation peaks as compared to those observed at an unmodified electrode. The voltammetric characteristics and hydrodynamic conditions for detecting the maximum catalytic current were found. A procedure is proposed for the electrocatalytic determination of cysteine, cystine, and methionine at a carbon-paste electrode modified with an inorganic film of ruthenium(III) hexacyanoferrate(II) under the conditions of flow-injection analysis.     

Electrooxidation of oxalic acid at a carbon-paste electrode with deposited palladium nanoparticles

Palladium nanoparticles deposited on the surface of a carbon-paste electrode exhibit electrocatalytic activity in the oxidation of oxalic acid. The surface topography of modified carbon-paste electrodes (CPEs) and the shapes of the metal crystallite particles were studied with atomic-force microscopy. These data were correlated with the voltammetric parameters of the electrooxidation of palladium and oxalic acid. As the dispersity of palladium nanoparticles electrodeposited on the CPE surface increased and their size diminished, the peak current of the catalytic oxidation of oxalic acid decreased, whereas the increment of this current increased as compared to the limiting current of metal oxidation. Evidently, this was due to an increase in the catalytic activity of the metal. The use of CPE modified with palladium nanoparticles instead of CPE containing palladium macrocrystals lowered the detection limit for the analyte by one order of magnitude (down to 2 × 10^?5 M).     

Voltammetric Studies of Propylthiouracil at a Carbon-Paste Electrode Modified with Cobalt(II)-4-chlorosalophen: Application to Voltammetric Determination in Pharmaceutical and Clinical Preparations

A carbon-paste electrode chemically modified with a Schiff-base complex of cobalt (cobalt(II)-4-chlorosalophen, CoClSal) is utilized to investigate the voltammetric response of propylthiouracil (PTU). The mechanism of electrocatalytic oxidation of the compound is investigated by means of cyclic voltammetric studies applying various pHs to the buffered solutions. The modified electrode exhibits effective catalytic properties that lower the anodic overpotential and enhance the rate of electron transfer for the electrochemical oxidation of PTU. The results of the cyclic voltammetric (CV) and differential pulse voltammetric (DPV) techniques showed that the modified electrode exhibits good selectivity for discriminating between the anodic wave of PTU and some biological thiols (e.g. cysteine) which may be present in human serum samples. DPV is used as a very sensitive electroanalytical method for the detection of trace amounts of PTU in pharmaceutical and clinical preparations. The results showed that the anodic peak current for the analyte in DPV varies linearly with the concentration of PTU in the range of 7.5 × 10⁻⁶ to 7.5 × 10⁻⁴ M (RSD for the slope of calibration curve <4%, n = 6).     

Electrocatalytic Determination of Ampicillin Using Carbon-Paste Electrode Modified with Ferrocendicarboxylic Acid

Abstract

A carbon-paste electrode spiked with ferrocenedicarboxylic acid (FDCMCPE) was constructed by incorporation of ferrocenedicarboxylic acid in a graphite powder–paraffin oil matrix. It has been shown by direct current cyclic voltammetry and double-step chronoamperometry that this electrode can catalyze the oxidation of ampicillin (AMPC) in aqueous buffered solution. It has been found that under the optimum condition (pH 10.0) in cyclic voltammetry, the oxidation of AMPC occurred at a potential of about 480 mV on the surface of the modified carbon-paste electrode. The kinetic parameters such as electron-transfer coefficient, α, and rate constant for the chemical reaction between AMPC and redox sites in FDCMCPE were also determined using electrochemical approaches. Under the optimized conditions, the electrocatalytic oxidation peak current of AMPC showed two linear dynamic ranges with a detection limit of 0.67 µmol L^?1 AMPC. The linear calibration was in the range of 2.34–30 µmol L^?1 and 40–700 µmol L^?1 AMPC using the differential pulse voltammetric method. Finally, this method was also examined as a selective, simple, and precise electrochemical sensor for the determination of AMPC in real samples such as drugs and urine.     

Electrocatalytic oxidation of captopril using a carbon-paste electrode modified with copper-cobalt hexacyanoferrate

A carbon-paste electrode was modified with copper-cobalt hexacyanoferrate by consecutive potential cycling. The kinetic parameters were calculated for the electroactive species. The resulting electrode exhibited electrocatalytic activity towards the oxidation of captopril. The kinetics of the electrocatalytic reaction was studied. A linear relationship was observed between anodic current and the concentration of captopril in the range of 5.0 × 10^?6–3.1 × 10^?5 μM with a detection limit of 4.2 μM (S/N = 3). The modified electrode was used in the analysis of captopril tablets successfully.     

The Ag+-G interaction inhibits the electrocatalytic oxidation of guanine--a novel mechanism for Ag+ detection

The heavy metal ions-nucleobases interaction is an important research topic in environmental and biochemical analysis. The presence of the silver ion (Ag⁺) may influence the formation of oxidation intermediate and the electrocatalytic oxidation activity of guanine (G), since Ag⁺ can interact with guanine at the binding sites which are involved in the electrocatalytic oxidation reaction of guanine. According to this principle, a new electrochemical sensor for indirectly detecting Ag⁺ based on the interaction of Ag⁺ with isolated guanine base using differential pulse voltammetry (DPV) was constructed. Among the heavy metal ions examined, only Ag⁺ showed the strongest inhibitory effect on the electrocatalytic oxidation of guanine at the multi-walled carbon nanotubes modified glassy carbon electrode (CNTs/GC). And the quantitative study of Ag⁺ based on Ag⁺-G sensing system gave a linear range from 100 nM to 2.5 μM with a detection limit of 30 nM. In addition, this modified electrode had very good reproducibility and stability. The developed electrochemical method is an ideal tool for Ag⁺ detection with some merits including remarkable simplicity, low-cost, and no requirement for probe preparation.     

Voltammetry of a Benzoquinone–Hydroquinone Redox Couple at Electrodes Modified with a Polyvinylpyridine Film Doped with Cobalt Phthalocyanine

The electrochemical properties of a glassy-carbon electrode coated with a polyvinylpyridine film doped with incorporated cobalt phthalocyanine were studied in a reaction involving a benzoquinone–hydroquinone redox couple. It was found that poly-(2-vinylpyridine) film applied to the electrode and cobalt phthalocyanine deposited onto it or incorporated in the polymeric film exhibited electrocatalytic activity on the oxidation of hydroquinone. Conditions were selected for obtaining a polyvinylpyridine film doped with cobalt phthalocyanine on the electrode surface providing a maximum catalytic effect. The current of the hydroquinone oxidation peak and the current of the reverse benzoquinone reduction peak at the chemically modified electrode were linear functions of their concentrations in the range from 1 × 10^–6 to 1 × 10^–3 M.     

Accumulation voltammetry of copper(II) at carbon-paste electrode containing salicylideneamino-2-thiophenol

Summary Accumulation voltammetry of copper(II) was investigated with a carbon-paste electrode containing salicylideneamino-2-thiophenol(SATP). Copper(II) was accumulated as the copper(II)-SATP complex on the electrode without an applied potential by immersing the electrode in 0.01 mol/l acetate buffer (pH 3.8) containing copper(II). The reduction peak of the copper(II)-SATP complex was observed at –0.12 V (vs. SCE) in 0.01 mol/l acetate buffer (pH 3.8) by scanning the potential in a negative direction. The calibration curve for copper(II) was linear in the range of 2×10^–9–1×10^–7 mol/l. Since the accumulation of copper(II) is based on a chemical reaction between copper(II) and SATP, copper(II) was selectively accumulated on the electrode. The presented method was applied to the determination of copper(II) in standard reference materials prepared by the National Institute for Environmental Studies.     

Electrocatalytic Response of a Glassy-Carbon Electrode Modified with a Polyvinylpyridine Film with Electrodeposited Palladium in the Oxidation of Oxalic Acid

The electrochemical properties of a glassy-carbon electrode modified with a polyvinylpyridine film with electrodeposited palladium were studied. Conditions were selected for preparing a composite film on a glassy-carbon surface. It was found that palladium particles deposited on the polyvinylpyridine film exhibited electrocatalytic activity in the oxidation of H₂C₂O₄. Compared to an unmodified electrode, the oxidation potential of oxalic acid decreased and the current of its oxidation multiply increased. The catalytic current of oxalic acid oxidation was a linear function of its concentration in the range from 1 × 10^–2 to 1 × 10^–6 M.     

Electrochemical activity of rhodium complexes supported on fibrous-carbon material

The redox properties of heterogenized Rh^I, Rh^II, and Rh^III complexes with different, particularly organophosphorus, ligands were studied by cyclic voltammetry (CVA). The support is a carbon-paste electrode based on a fibrous-carbon material and activated carbon. The electrochemical reduction of Rh^III produces Rh metal, which further catalyzes hydrogen evolution. After the reduction of water-soluble binuclear Rh^II complexes, the CVA curves exhibit peaks of electrocatalytic hydrogen evolution and irreversible Rh^I→Rh^II oxidation. The Rh^II complexes with organophosphorus ligands are characterized only by the peak of Rh^I→Rh^II oxidation. After reduction, the Rh^I complexes behave as a pseudo-reverse Rh⁰/Rh^I pair. The electron-donating properties of the ligand determine the reversibility of the system. The degree of structurization of the carbon matrix and the presence of phosphorus(v) atoms in it affect the electrochemical activity of the Rh^II and Rh^I complexes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 908–914, May, 1999.     

Substituents Effects to the Electrochemical,and In Situ Spectroelectrochemical Behavior of Metallophthalocyanines: Electrocatalytic Application for Hydrogen Evolution Reaction

This study describes the effects of the number of fluoro substituents to the electrochemical, in situ spectroelectrochemical, and electrocatalytic activities of metallophthalocyanines for the hydrogen evolution reaction. Tuning of the number of fluoro groups shifts the redox processes and affects the aggregation tendencies of the complexes. An in situ electrocolorimetric method was applied to investigate the color of the electrogenerated anionic and cationic forms of the complexes. Cobalt phthalocyanines incorporated into Nafion film on a glassy carbon electrode decreases the overpotential of the working electrode for H⁺ reduction. The number and position of the electron withdrawing substituents and nature of the metal center change the electrocatalytic activities for the hydrogen evolution reaction in aqueous solutions.     

Electrochemical behavior and determination of L-tryptophan on carbon ionic liquid electrode

A carbon ionic liquid electrode (CILE) was fabricated by mixing N-butylpyridinium hexafluoro-phosphate (BPPF ₆ ) with graphite powder and further used for the investigation on the electrochemical behavior of L-tryptophan (Trp). The fabricated CILE showed good conductivity, inherent electrocatalytic ability and strong promotion to the electron transfer of Trp. On the CILE, an irreversible oxidation peak appeared at 0.948 V (vs. saturated calomel reference electrode). For 5.0 × 10⁻⁵ M Trp the oxidation peak current increased about 5 times and the oxidation peak potential decreased on 0.092 V compared to carbon paste electrode. The results indicated that an electrocatalytic reaction occurred on CILE. The conditions for the electrochemical detection were optimized and the electrochemical parameters of Trp on CILE were carefully investigated. Under the selected conditions, the oxidation peak current showed linear relationship with Trp concentration in the range of 8.0 × 10⁻⁶ ∼1.0 × 10⁻³ M for cyclic voltammetry and the detection limit was estimated as 4.8 × 10⁻⁶ M (3σ). The interferences of other amino acids or metal ions on the determination were tested and the proposed method was successfully applied to the synthetic sample analysis.     

Electrocatalytic Oxidation of Hydrogen Peroxide on Poly(m‐toluidine)‐Nickel Modified Carbon Paste Electrode in Alkaline Medium

The poly(m‐toluidine) film was prepared by using the repeated potential cycling technique in an acidic solution at the surface of carbon paste electrode. Then transition metal ions of Ni(II) were incorporated to the polymer by immersion of the modified electrode in a 0.2 M NiSO₄, also the electrochemical characterization of this modified electrode exhibits stable redox behavior of the Ni(III)/Ni(II) couple. The electrocatalytic ability of Ni(II)/poly(m‐toluidine)/modified carbon paste electrode (Ni/PMT/MCPE) was demonstrated by electrocatalytic oxidation of hydrogen peroxide with cyclic voltammetry and chronoamperometry methods in the alkaline solution. The effects of scan rate and hydrogen peroxide concentration on the anodic peak height of hydrogen peroxide oxidation were also investigated. The catalytic oxidation peak current showed two linear ranges with different slopes dependent on the hydrogen peroxide concentration and the lower detection limit was 6.5 μM (S/N=3). The catalytic reaction rate constant, (k_h), was calculated 5.5×10² M^?1 s^?1 by the data of chronoamperometry. This modified electrode has many advantages such as simple preparation procedure, good reproducibility and high catalytic activity toward the hydrogen peroxide oxidation. This method was also applied as a simple method for routine control and can be employed directly without any pretreatment or separation for analysis cosmetics products.     

Voltammetric study on the complex of thiram-copper(II) and its application

Based on the formation of tetramethylthiuram disulfide (thiram)-copper(II) complex by square-wave voltammetry on a glassy carbon electrode, a new method is proposed for determination of thiram. There is a good linear relationship between peak current and concentration of thiram in the range of  mol l⁻¹ (r=0.999). The method is applied to the determination of thiram residue in plant with satisfactory results, compared with results achieved by using HPLC. Two oxidation peaks and two reduction peaks are obtained by cyclic voltammetry on a glassy carbon electrode in 0.2 mol l⁻¹ HAc-NaAc (pH 4.0) solution containing thiram-copper(II) complex. The electrode reaction process is quasi-reversible with adsorptive characteristics. The mechanism of electrode reaction is discussed.     

Fabrication of highly sensitive cysteine electrochemical sensor based on nanostructured compound and carbon nanotube modified electrode

This work describes the electrochemical behavior of copper(II)-bis[5-((4-ⁿ-decyloxyphenyl)azo)-N-(ⁿethanol)-salicylaldiminato]film immobilized on the surface of multiwall carbon nanotube glassy carbon electrode and its electrocatalytic activity toward the oxidation of L-cysteine. The surface structure and composition of the sensor was characterized by scanning electron microscopy. Electrocatalytic oxidation of L-cysteine on the surface of modified electrode was investigated with cyclic voltammetry, chronoamperometry and hydrodynamic amperometery methods and the results showed that the Cu-Schiff base film displays excellent electrochemical catalytic activities towards L-cysteine oxidation. The modified electrode indicated reproducible behavior and high level of stability during the electrochemical experiments.     

A new method for electrocatalytic oxidation of ascorbic acid at the Cu(II) zeolite-modified electrode

A new method is developed for the catalytic oxidation of ascorbic acid at graphite zeolite-modified electrode, doped with copper(II) (Cu²⁺A/ZCME). Copper(II) exchanged in zeolite type A acts as catalyst to oxidize ascorbic acid. The modified electrode lowered the overpotential of the reaction by ∼400 mV. First, the electrochemical behavior of copper(II), incorporated in the zeolite type A modified electrode, was studied. The results illustrate that diffusion can control the copper(II)/copper(0) redox process at the Cu²⁺A/ZCME. Then, the behavior of electrocatalytic oxidation reaction for ascorbic acid was researched. The electrode was employed to study electrocatalytic oxidation of ascorbic acid, using cyclic voltammetry and chronoamperometry as diagnostic techniques. The diffusion coefficient of ascorbic acid was equal to 1.028 × 10⁻⁵ cm² s⁻¹. A linear calibration graph was obtained over the ascorbic acid with a concentration range of 0.003-6.00 mmol L⁻¹. The detection limit (DL) of ascorbic acid was estimated as 2.76 × 10⁻⁷ mol L⁻¹. The relative standard deviations of 10 replicate measurements (performed on a single electrode at several ascorbic acid concentrations between 3.0 and 200 μmol L⁻¹) were measured between 1.0 and 2.4%.     

Nickel–poly(o-aminophenol)-modified carbon paste electrode; an electrocatalyst for methanol oxidation

In this work, a modified carbon paste electrode consisting of Nickel dispersed in poly(ortho-aminophenol) was used for electrocatalytic oxidation of methanol in alkaline solution. A carbon paste electrode bulk modified with o-aminophenol was used for polymer preparation by cyclic voltammetry method; then, Ni(II) ions were incorporated by immersion of the modified electrode in 1 M Ni(II) ion solution at open circuit. The electrochemical characterization of this modified electrode exhibits stable redox behavior of the Ni(III)–Ni(II) couple. Electrocatalytic oxidation of methanol on the surface of modified electrode was investigated with cyclic voltammetry and chronoamperometry methods, and the dependence of the oxidation current and shape of cyclic voltammograms on methanol concentration and scan rate were discussed. Also, long-term stability of modified electrode for electrocatalytic oxidation of methanol was investigated.     

Electrochemical behavior of Ni(II) incorporated in zeolite Y-modified carbon electrode: application for electrocatalytic oxidation of methanol in alkaline solution

Nickel ions were incorporated in NaY zeolite according to cation exchange mechanism. Then NiY zeolite was used as modifier for preparation of modified carbon paste electrode. The electrochemical behavior of NiY-modified carbon paste electrode (NiY/CPE) was studied in alkaline solution using cyclic voltammetry method. Ability of different electrodes containing NiY/CPE, Ni-NiY/CPE, Ni-NaY/CPE, and Ni/CPE for electrocatalytic oxidation of methanol was compared (three last electrodes prepared by open circuit accumulation of Ni(II) ions on the surface of NiY/CPE, NaY/CPE, and bare CPE, respectively). Results show that Ni-NiY/CPE is best catalyst for the electrochemical oxidation of methanol in alkaline solution and both process of earlier Ni ion incorporation through cation exchange in NaY zeolite and open circuit accumulation of Ni ion on the surface of electrode are essential to have good catalyst. Effect of graphite–zeolite ratio on electrocatalytic current was studied and 3:1 ratio of graphite–zeolite was selected as optimum ratio for preparing electrode. Ni-NiY/CPE has very good stability toward the methanol oxidation in concentration range of 0.005 to 0.5 M. Finally, using chronoamperometric method, the catalytic rate constant (k) for methanol was found to be 1.56 × 10⁴ cm³ mol⁻¹ s⁻¹.