Electrochemical behavior of ellipticine derivatives: Part I. Oxidation of 9-hydroxy-ellipticine

The electrochemical one-electron oxidation of 9-hydroxy-ellipticinium cations at a platinum electrode has been examined with particular attention to the thermodynamic redox potential and to the dimerization rate of the radical species produced. Both the reactant and the product of the electrochemical reaction are strongly adsorbed at the solution/electrode interface. The initial step of the anodic process is a reversible electron transfer accompanied by a fast deprotonation; E^0′=900?53 pH mV vs. NHE. The resulting neutral radical dimerizes, the rate constant of the surface dimerization being ca. 2.5×10⁹ mol^?1 cm² s^?1.     

Oxydation électrochimique de l'acide formique sur électrode d'or en milieu neutre

The electrochemical oxydation of formic acid is studied in 0.5 M Na₂SO₄ medium on a gold electrode, for pH greater than pK of formic acid. Its characteristics are different from that of an acid medium. The reaction begins at about 200 mV/NHE. The oxidation current shows a maximum at about 800 mV/NHE depending on the rate of potential change. A mechanism is written and one interpretation is given taking into account the aqueous species adsorbed on a gold electrode.     

An amperometric lactate sensor based on a NAD+-analog and lactate dehydrogenase coimmobilized on reticulated vitreous carbon

A NAD⁺-analog was coimmobilized with lactate dehydrogenase (LDH) on reticulated vitreous carbon (RVC) to give an amperometric lactate biosensor. Both LDH and the NAD⁺ -analog were bound covalently with carbodiimide to the surface of the porous RVC-material. The electrode was operated in a FIA-arrangement in the presence or absence of a soluble mediator. Meldola Blue. The stability was poor when the electrode was operated at +400 mV (vs. Ag/AgCl) in the absence of mediator but improved most significantly in the presence of 5 μM mediator, so that 65% of the original activity remained after 16 days. The amperometric currents were smaller with regeneration by mediator at −100 mV than with direct electrochemical oxidation at +400 mV, indicating that the additional steps slow down the reaction rate. Linear calibration plots were obtained from the detection limit, 1 μM, to 500 μM lactate with 5 μM mediator, reoxidized at −100 mV. The sample throughput was about 60 h⁻¹.     

A glucose sensor based on glucose dehydrogenase adsorbed on a modified carbon electrode

A glucose sensor is prepared by adsorption of the mediator Meldola blue (N,N-dimethyl-7-amino-1,2-benzophenoxazinium ion, as well as glucose dehydrogenase, on the surface of a carbon electrode. The nicotinamide coenzyme, whhich is present in the solution, is reduced in the enzymatic reaction and is re-oxidized amperometrically at 0 mV vs. Ag/AgCl. The properties of such electrodes depend on whether the mediator or the enzyme is adsorbed first; possible models for the molecular arrangements at the surface are discussed. The modified electrode is mounted in a flow-through cell in a flow-injection system and tested with 50-μl injections of β-d-glucose. The calibration graphs were linear in the range 5 × 10^?6—2 × 10^?3 M βd glucose with the highest sensitivity at pH 6.0. The membrane-free enzyme electrode has a fast response; peak widths are 12 s at half height (flow rate 0.7 ml min^?1, making it possible to process 100 samples h^?1.     

An Amperometric Glucose Electrode Based on Adsorbed Glucose Oxidase on Palladium/Gold Modified Graphite

Abstract

A glucose electrode was constructed by adsorbing glucose oxidase (GOD) on a modified electrode for H₂ O ₂ oxidation, consisting of Pd/Au sputtered on graphite. Maximally, 0.8 U cm^?2 of GOD could be adsorbed. The electrode was used in a f.i.a. manifold for determination of glucose. Linear calibration curves were obtained in the concentration range 3. 10^?6 4. 10^?3 mol L^?1 glucose. The applied potentials for glucose determination were + 300 mV vs. Ag/AgCl at pH 8.0, + 350 mV at pH 7.0, + 400 mV at pH 6.0 and + 500 mV at pH 5.0. The activity vs. pH profile of adsorbed GOD was broad having an optimum between pH 5 and 6. The apparent kinetic parameters for adsorbed GOD, K_M ^app and i_max, were found to be 50 mM and 160 uA at optimal pH.     

Electrooxidation of hydrazine catalyzed by 4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy (TEMPOL)

The electrocatalytic oxidation of hydrazine (N₂H₄) by TEMPOL on a glassy carbon electrode has been studied. The kinetic parameters of the electrode reaction were measured and the electrocatalytic reaction mechanism for the electrooxidation of hydrazine in the presence of TEMPOL was proposed. TEMPOL undergoes a reversible single electron transfer process at a glassy carbon electrode (GCE) at pH 1.2–8.0, and the electrochemical oxidation of N₂H₄ at a GCE can be catalyzed by TEMPOL. The catalytic current is affected by the concentration of catalyst and pH. The overall number of electrons involved in the catalytic oxidation of N₂H₄ and the number of electrons involved in the rate determining step (rds) are 4 and 1, respectively. The catalytic oxidation obeys the first-order kinetics with respect to N₂H₄. The proposed mechanism is consistent with the experimental data, and a cation intermediate [> N---O---N₂H₄⁺], formed by reaction of oxoammonium salt with N₂H₄, is involved in the reaction.     

Coupled Electron‐Proton Transport in Electropolymerized Methylene Blue and the Influences of Its Protonation Level on the Rate of Electron Exchange with β‐Nicotinamide Adenine Dinucleotide

This paper describes electrochemical characteristics of poly(methylene blue) electrolytically deposited on glassy carbon and examines the electrocatalytic activity of the polymer toward oxidation of the coenzyme NADH. Redox‐active properties of the cationic polyelectrolyte arose from both electron self‐exchange between electroactive sites and a high ionic film‐conductivity. The diffusion coefficient of charge carriers in the film increased with decreasing solution pH, indicating the pH dependence of the electron diffusion coefficient. The electrocatalytic oxidation of NADH at the polymer‐modified electrode proceeded via an intermediate charge‐transfer complex of the reduced polymer with the oxidized coenzyme. The complex dissociated more rapidly into the oxidation products as the reduced polymer protonated. Thus, the rate constant for the cross‐exchange reaction rose with a decrease in pH. For NADH oxidation, the polyelectrolyte exhibited an electrocatalytic activity higher than the monomeric dye because of a stronger oxidizing power of the second oxidized form of the polymer.     

Electrochemical Characterization of Pyrocatechol Sulfonephthalein-Modified Glassy Carbon Electrode and Separation of Electrocatalytic Responses for Ascorbic Acid and Dopamine Oxidation

A pyrocatechol sulfonephthalein- (PS-) modified glassy carbon (PS/GC) electrode has been prepared by adsorption of PS on a glassy carbon electrode surface. Cyclic voltammograms of the PS/GC electrode indicate the presence of a couple of well-defined redox peaks, and the formal potential shifts in the negative direction with increasing solution pH. The relation between formal potential,E^0′, and solution pH can be fit to the equationE^0′(mV) = −51.4 pH + 538.7. The PS/GC electrode shows high electrocatalytic activity toward ascorbic acid oxidation, with an overpotential ca. 380 mV less than that of the bare electrode and a drastic enhancement of the anodic currents. The electrocatalytic reaction rate constant (k), which was decreased with increasing concentration of H₂A, was determined using rotating disk electrode measurements. The values ofkwas also affected by the solution pH. The electrode can also separate the electrochemical responses of ascorbic acid and dopamine. The separation between the anodic peak potentials of ascorbic acid and dopamine is more than 50 mV by the differential pulse voltammetry.     

Electrochemical determination of homocysteine using carbon nanotubes modified paste electrode and isoprenaline as a mediator

Homocysteine (HCy) is an important amino acid containing thiol group and is known as a risk factor in relation to ischemic heart disease and stroke. In this study the electrochemical determination of homocysteine (HCy) has been described using isoprenaline hydrochloride (ISP) as a mediator on multiwall carbon nanotubes modified paste electrode (MWCNTPE). Electrochemical behavior of homocysteine was investigated by cyclic voltarrtmetry and chronoamperometry. The cyclic voltammograms showed that the electrocatalytic oxidation of homocysteine occurs in the presence of ISP on the surface of MWCNTPE at a potential about 640 mV. Also, results showed that the oxidatation peak current of HCy at the modified carbon nanotubes electrode was more than unmodified electrode. The diffusion coefficient and the kinetic parameters including electron transfer coefficient and rate constant of electrocatalytic reaction were determined using electrochemical approaches. Linear sweep voltammetry results showed that electrocatalytic oxidation peak current of HCy had linear dynamic range in the range of 5.0 to 800 μmol L^?1 with a detection limit of 3.3 μmol L^?1 in pH 3.5 (universal buffer).     

Electrochemistry of adenosine-5′-diphosphate on ionic liquid modified carbon electrode and its detection

A sensitive electrochemical method was proposed for the determination of adenosine-5′-diphosphate (ADP) on an ionic liquid (IL) 1-(3-chloro-2-hydroxy-propyl)-3-methylimidazole chloride modified carbon paste electrode (CPE) in a pH 4.5 Britton-Robinson (B-R) buffer solution. Compared with CPE, IL modified CPE (CILE) showed strong electrocatalytic ability to promote the electrochemical oxidation of ADP. A well-defined irreversible oxidation peak of ADP appeared at +1.381 V with an adsorption-controlled process, which was due to the presence of high conductive IL on the electrode. The experimental conditions were optimized and the electrochemical parameters of ADP were calculated with the electron transfer coefficient (α) as 0.293, the electron transfer number (n) as 1.23, the apparent heterogeneous electron transfer rate constant (k ^s) as 3.325 × 10^?6 s^?1 and the surface coverage (Γ_T) as 0.92 × 10^?8 mol/cm². Under the optimum conditions, the oxidation peak current was linear to ADP concentration in the range from 3.0 to 1000.0 μmol/L with the detection limit as 2.78 μmol/L (3σ) by differential pulse voltammetry. The CILE also eliminated the interferences of commonly coexisting substances and was successfully applied to detect the ADP artificial samples.     

Al Electrode Modified by Au Atoms as a Novel Electrode for Electrocatalytic Oxidation of Thiosulfate

An aluminum electrode modified with gold atoms was introduced as a novel electrode. Gold atoms were deposited both chemically and electrochemically onto the aluminum electrode to make an aluminum/gold (Al/Au) modified electrode (ME). The experimental results showed that the Al/Au modified electrode prepared by chemical deposition, exhibits much more current than the electrochemical deposition method. The electrochemical behavior of the Al/Au modified electrode was studied by cyclic voltammometry. This modified electrode showed two pairs of peaks, a₁c₁ and a₂c₂, with surface‐confined characteristics in a 0.5 M phosphate buffer. The dependence of E_pa of the second peak (a₂c₂) on pH shows a Nernestian behavior with a slope of 55 mV per unit pH. The effect of different supporting electrolytes, solution's pH and different scan rates on electrochemical behavior of Al/Au modified electrode was studied. Au deposited electrochemically on a Pt electrode (Pt/Au) was also used as another modified electrode. A comparative study of electrochemical behavior of bare Al, Pt/Au and Al/Au modified electrodes showed that both Pt/Au and Al/Au electrodes have the ability of electrocatalytic oxidation of S₂O₃^2?, but the electrocatalytic oxidation on the latter was better than the former. The kinetics of the catalytic reaction was investigated by using cyclic voltammetry and chronoamperometry techniques. The average value of the rate constant for the catalytic reaction and the diffusion coefficient were evaluated by means of chronoamperometry technique.     

Electrocatalytic study of an electrode modified with Reactive Blue 4 dye covalently immobilized on amine-functionalized silica

In the present work, we investigated the immobilization and electrochemical behavior of Reactive Blue 4 dye on 3-aminopropyl-functionalized silica. The electrochemical behavior of the modified electrode and the electro-oxidation of dipyrone were studied by cyclic voltammetry. The modified electrode showed a well-defined redox coupling with a formal potential of 0.45 V (vs. saturated calomel reference electrode) assigned to anthraquinone/anthrahydroquinone redox process (pH?=?2). The modified electrode also demonstrated electrocatalytic activity and an increased peak current towards the oxidation of dipyrone at a reduced overall potential. The electrocatalytic process was found to be highly dependent on the pH of the supporting electrolyte. The voltammetric responses for dipyrone were linear in the concentration range of 49.9 to 440 μmol L^?1 at a pH of 2.0 with a detection limit and sensitivity of 22.0 μmol L^?1 and 0.0278 μA mmol L^?1, respectively.     

Voltammetric Behavior of Antileukemia Drug Glivec. Part I – Electrochemical Study of Glivec

The electrochemical behavior of the antileukemia drug glivec was investigated at a glassy carbon electrode (GCE). The oxidation is a complex, pH‐dependent, irreversible electrode process involving the transfer of 2 electrons and 2 protons and the formation of an electroactive product, P_glivec, which strongly adsorbs on the GCE surface and undergoes reversible oxidation. The adsorption of P_glivec at the GCE surface yields a compact monolayer that inhibits further oxidation of glivec. The electrochemical reduction is a simple pH dependent irreversible process involving the transfer of 2 electrons and 2 protons and occurs with the formation of a nonelectroactive product. The diffusion coefficient of glivec was calculated to be D_O=7.35×10^?6 cm² s^?1 in pH 4.5 0.1 M acetate buffer.     

Simultaneous detection of ascorbate and uric acid using a selectively catalytic surface

The direct and selective detection of ascorbate at conventional carbon or metal electrodes is difficult due to its large overpotential and fouling by oxidation products. Electrode modification by electrochemical reduction of diazonium salts of different aryl derivatives is useful for catalytic, analytical and biotechnological applications. A monolayer of o-aminophenol (o-AP) was grafted on a glassy carbon electrode (GCE) via the electrochemical reduction of its in situ prepared diazonium salts in aqueous solution. The o-aminophenol confined surface was characterized by cyclic voltammetry. The grafted film demonstrated an excellent electrocatalytic activity towards the oxidation of ascorbate in phosphate buffer of pH 7.0 shifting the overpotential from +462 to +263 mV versus Ag/AgCl. Cyclic voltammetry and d.c. amperometric measurements were carried out for the quantitative determination of ascorbate and uric acid. The catalytic oxidation peak current was linearly dependent on the ascorbate concentration and a linear calibration curve was obtained using d.c. amperometry in the range of 2-20 μM of ascorbate with a correlation coefficient 0.9998, and limit of detection 0.3 μM. The effect of H₂O₂ on the electrocatalytic oxidation of ascorbate at o-aminophenol modified GC electrode has been studied, the half-life time and rate constant was estimated as 270 s, and 2.57 × 10⁻³ s⁻¹, respectively. The catalytically selective electrode was applied to the simultaneous detection of ascorbate and uric acid, and used for their determination in real urine samples. This o-AP/GCE showed high stability with time, and was used as a simple and precise amperometric sensor for the selective determination of ascorbate.     

Single‐Site Copper(II) Water Oxidation Electrocatalysis: Rate Enhancements with HPO42− as a Proton Acceptor at pH 8

The complex Cu^II(Py₃P) ( 1 ) is an electrocatalyst for water oxidation to dioxygen in H₂PO₄^?/HPO₄^2? buffered aqueous solutions. Controlled potential electrolysis experiments with 1 at pH 8.0 at an applied potential of 1.40 V versus the normal hydrogen electrode resulted in the formation of dioxygen (84 % Faradaic yield) through multiple catalyst turnovers with minimal catalyst deactivation. The results of an electrochemical kinetics study point to a single‐site mechanism for water oxidation catalysis with involvement of phosphate buffer anions either through atom–proton transfer in a rate‐limiting O? O bond‐forming step with HPO₄^2? as the acceptor base or by concerted electron–proton transfer with electron transfer to the electrode and proton transfer to the HPO₄^2? base.     

Caffeic acid modified glassy carbon electrode for electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH)

A new modified electrode was prepared by electrodeposition of caffeic acid (CFA) at the surface of an activated glassy carbon electrode. Cyclic voltammetry was used to investigate the redox properties of this electrode at various solution pH values and at various scan rates. The pH dependence of the electrode response was found to be 58.5 mV/pH, which is very close to the expected Nernstian value. The electrode was also employed to study electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH), using cyclic voltammetry, chronoamperometry and rotating disk voltammetry as diagnostic techniques. It was found that the modified electrode exhibits potent and persistent electrocatalytic properties toward NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 450 mV compared to the process at an unmodified electrode. The electrocatalytic current increases linearly with NADH concentration in the range tested from 0.05 to 1.0 mM. The apparent charge transfer rate constant and transfer coefficient for electron transfer between the electrode surface and immobilized CFA were calculated as 11.2 s⁻¹ and 0.43, respectively. The heterogeneous rate constant for oxidation of NADH at the CFA-modified electrode surface was also determined and found to be about 3 × 10³ M⁻¹ s⁻¹. Finally, the diffusion coefficient of NADH was calculated as 3.24 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometric results. Received: 6 January 1999 / Accepted: 11 May 1999     

掺杂双(N-2-甲基苯基-水杨醛亚胺)合铜(II)配合物的碳糊电极电催化测量过氧化氢HosseinKhoshro,HamidR.Zare,RasoulVafazadeh简

The electrochemical behavior of a bis(N-2-methylphenyl-salicyldenaminato)copper(Ⅱ) complex spiked in a carbon paste electrode (BMPSCu-CPE) and its electrocatalytic reduction of H₂O₂ were examined using cyclic voltammetry, chronoamperometry, and differential pulse voltammetry. Cyclic voltammetry was used to study the redox properties of BMPSCu-CPE at various potential scan rates. The apparent charge transfer rate constant and the transfer coefficient for the electron transfer between BMPSCu and the carbon paste electrode (CPE) were 1.9±0.1 s^-1 and 0.43, respectively. BMPSCu-CPE had excellent electrocatalytic activity for H₂O₂ reduction in 0.1 mol/L phosphate buffer solution (pH 5.0), and it decreased the overpotential by 300 mV as compared to CPE alone. The diffusion coefficient and kinetic parameters such as the heterogeneous catalytic electron transfer rate constant and electron transfer coefficient for the reduction of H₂O₂ at the BMPSCu-CPE surface were also determined using electrochemical methods. Differential pulse voltammetry showed two linear dynamic ranges of 1.0-10.0 and 10.0-300.0 μmol/L and a detection limit of 0.63 μmol/L H₂O₂. The BMPSCu-CPE has excellent reproducibility and long term stability, and it was successfully applied for the determination of H₂O₂ in two pharmaceutical samples: an antiseptic solution and a hair dying cream.     

Electrocatalytic Characteristics of Ferrocenecarboxylic Acid Modified Carbon Paste Electrode in the Oxidation and Determination of L‐Cysteine

The electrochemical behavior of L ‐cysteine studied at the surface of ferrocenecarboxylic acid modified carbon paste electrode (FCMCPE) in aqueous media using cyclic voltammetry and double step potential chronoamperometry. It has been found that under optimum condition (pH 7.00) in cyclic voltammetry, the oxidation of L ‐cysteine is occurs at a potential about 580 mV less positive than that an unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and catalytic reaction rate constant, K′_h were also determined using electrochemical approaches. The electrocatalytic oxidation peak current of L ‐cysteine showed a linear dependent on the L ‐cysteine concentration and linear calibration curves were obtained in the ranges of 10^?5 M–10^?3 M and 4.1×10^?8 M–3.7×10^?5 M of L ‐cysteine concentration with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods respectively. The detection limits (2δ) were determined as 2.4×10^?6 M and 2.5×10^?8 M by CV and DPV methods. This method was also examined for determination of L ‐cysteine in some samples, such as Soya protein powder, serum of human blood by using recovery and standard addition methods.     

Carbon paste electrode modified with a binuclear manganese complex as a sensitive voltammetric sensor for tryptophan

We report on a carbon paste electrode that was modified with a binuclear manganese(II) complex by the drop-coating method. A study on the mechanism of the electro-oxidation of tryptophan (Trp) at this electrode indicated that it enables Trp to be determined with good sensitivity and selectivity. Second-order derivative linear sweep voltammetry at pH 4.1 revealed that a sensitive anodic peak appears at 812?mV (vs. SCE) whose current is proportional to the concentration of Trp in the concentration range from 0.1 to 1.0???mol?L^?1 and 1.0 to 80???mol?L^?1, with a detection limit (S/N?=?3) of 0.08???mol?L^?1 (60?s of accumulation). The method was applied to the determination of Trp in amino acid injection solutions with satisfactory results.

Electrocatalytic oxidation of phytohormone salicylic acid at copper nanoparticles-modified gold electrode and its detection in oilseed rape infected with fungal pathogen Sclerotinia sclerotiorum

Salicylic acid (SA) is a biological substance that acts as a phytohormone and plays an important role in signal transduction in plants. It is important to accurately and sensitively detect SA levels. A gold electrode modified with copper nanoparticles was used to assay the electrocatalytic oxidation of salicylic acid. It was found that the electrochemical behavior of salicylic acid was greatly improved at copper nanoparticles, indicating that anodic oxidation could be catalyzed at copper nanoparticles. And the pH had remarkable effect on the electrochemical process, a very well-defined oxidation peak appeared at pH 13.3 (0.2 M NaOH). The kinetics parameters of this process were calculated and the heterogeneous electron transfer rate constant (k) was determined to be 1.34 × 10⁻³ cm s⁻¹, and (1 − α)n_α was 1.22. The gold electrode modified with copper nanoparticles could detect SA at a higher sensitivity than common electrodes. The electrode was used to detect the SA levels in oilseed rape infected with the fungal pathogen Sclerotinia sclerotiorum. The results showed that the SA concentration reached a maximum during the 10th-25th hours after infection. This result was very similar to that determined by HPLC, indicating that the gold electrodes modified with copper nanoparticles could be used as salicylic acid sensors.