Anodic voltammetric behavior of inosine on a glassy carbon electrode

The anodic voltammetric behavior of inosine (I) was investigated by linar-sweep voltammetry, differential-pulse voltammetry and cyclic voltammetry at a glassy carbon electrode. In a medium of 0.1 mol/L N2HPO4, inosine showed a well defined anodic peak. The peak potential was about 1.42 V (vs. Ag/AgCl). A linear relationship held between the peak current and the concentration of inosine in the rang of 5 x 10(-4) similar to 8 x 10(-2) g/L. The peak potential decreased with the decrease of the acidity of the solution. The four anodic peaks of inosine with hypoxanthine, xanthine and uric acid were obtained. Their peak potentials were about at 1.42, 1.07, 0.72 and 0.26 Vt vs. Ag/AgCl). The method has been used for the direct determination of inosine in injections. Recoveries of inosine in urine samples were about 85%. Experimental result proved that the electrode reaction was diffusion-controlled and irreversible.

Amperometric enzyme electrode for the determination of hydrogen peroxide based on sol-gel/hydrogel composite film

A new type of organic-inorganic composite material was prepared by sol-gel method, and a peroxidase biosensor was fabricated by simply dropping sor-gel-peroxidase mixture onto glassy carbon electrode surface. The sol-gel composite film and enzyme membrane were characterized by Fourier-transform infrared (FT-IR) spectroscopy and EQCM, the electrochemical behavior of the biosensor was studied with potassium hexacyanoferrate(II) as a mediator, and the effects of pH and operating potential were explored for optimum analytical performance by using amperometric method. The response time of the biosensor was about 10 s; the linear range was up to 3.4 mM with a detection limit of 5 x 10(-7) M. The sensor also exhibited high sensitivity (15 mu A mM(-1)) and good long-term stability. In addition, the performance of the biosensor was investigated using flow injection analysis (FIA), and the determination of hydrogen peroxide in real samples was discussed. (C)2000 Elsevier Science B.V. All rights reserved.

Electrochemical behavior of redox species at carbon fibre microdisk array electrode modified with mixed-valent molybdenum(VI, V) oxide

In this study, electrode responses to a large number of electroactive species with different standard potentials at the molybdenum oxide-modified carbon fibre microdisk array (CFMA) electrode were investigated. The results demonstrated that the electrochemical behavior for those redox species with formal potentials more positive than similar to 0.0 V at the molybdenum oxide-modified CFMA electrode were affected by the range and direction of the potential scan, which were different from that at a bare CFMA electrode. If the lower limit of the potential scan was more positive than the reduction potential of the molybdenum oxide film, neither the oxidation nor the reduction peaks of the redox species tested could be observed. This indicates that electron transfer between the molybdenum oxide film on the electrode and the electroactive species in solution is blocked due to the existence of a high resistance between the film and electrolyte in these potential ranges. If the lower limit of the potential scan was more negative than the reduction potential of the molybdenum oxide film (similar to - 0.6 V), the oxidation peaks of these species occurred at the potentials near their formal potentials. In addition, the electrochemical behavior of these redox species at the molybdenum oxide-modified CFMA electrode showed a diffusionless electron transfer process. On the other hand, the redox species with formal potentials more negative than similar to - 0.2 V showed similar reversible voltammetric behaviors at both the molybdenum oxide-modified CFMA electrode and the bare electrode. This can be explained by the structure changes of the film before and after reduction of the film. In addition we also observed that the peak currents of some redox species at the modified electrode were much larger than those at a bare electrode under the same conditions, which has been explained by the interaction between these redox species and the reduction state of the molybdenum oxide film. (C) 2000 Elsevier Science Ltd. All rights reserved.

Simultaneous determination of both the calibration constant in an electrochemical quartz crystal microbalance and the active surface area of a polycrystalline gold electrode

A novel method is employed for the simultaneous determination of both the calibration constant of an electrochemical quartz crystal microbalance (EQCM) and the active surface area of a polycrystalline gold electrode. A gold electrode: is immersed into a 1 mM KI/1 M H2SO4 solution and on which forms a neutral monolayer. The adsorbed iodine can then be completely oxidized into IO3-. The active surface area of a gold electrode can be obtained from the net electrolytic charge of the oxidation process, and the calibration constant in the EQCM can be calculated from the corresponding frequency shift. The result shows that this method is simple, convenient and valid. (C) 2000 Elsevier Science S.A. All rights reserved.

Construction of a heteropolyanion-modified electrode by a two-step sol-gel method and its electro catalytic applications

The sol-gel technique was used here to construct heteropolyanion-containing modified electrodes. This involves two steps, i.e. the first forming a functionalized sol-gel thin film on the surface of the glassy carbon electrode and then immersing the electrode into a heteropolyanion solution to incorporate the heteropolyanion into the sol-gel film. Here a Dawson-type heteropolyanion, K6P2W18O62 (P2W18), was used as a representative to illuminate the behavior of the as-prepared composite film. The electrochemical performance of the P2W18-modified electrode was studied with respect to the pH effect and long-term stability. The modified electrode exhibited a high electrocatalytic response for the reduction of BrO3- and NO2-. Steady-state amperometry was applied to characterize the electrode as an amperometric sensor for the determination of NO2-. The sensor had a linear range from 0.02 to 34 mM and a detection limit of 5 x 10(-6) M. (C) 2001 Elsevier Science B.V. All rights reserved.

Oxidation of ascorbic acid by rutin at a glassy carbon electrode modified with lipid films

A stable film was prepared by casting dipalmitoylphosphatidylcholine (DPPC) and rutin onto the surface of a glassy carbon (GC) electrode. The electrochemistry behavior of rutin in the DPPC film was investigated. The modified electrode coated with rutin shows a quasi-reversible reduction-oxidation peak on the cyclic voltammogram in phosphate buffer (pH 7.4). This model of biological membrane was not only used to provide biological environment but also to investigate the oxidation of ascorbic acid by rutin. The DPPC-rutin modified electrode behaves as electrocatalytic oxidation to ascorbic acid. The oxidation peak current of ascorbic acid increases drastically and the peak potential of 4 x 10(-4) mol L-1 ascorbic acid shifts negatively about 100 mV compared with that obtained at a bare glassy carbon electrode. The catalytic current increased linearly with the ascorbic acid concentration in the range of 2 x 10(-4) mol L-1 and 1.4 x 10(-3) mol L-1 at a scan rate of 50 mV s(-1).

Multilayer assembly of Mn-substituted Keggin-type [ZnW11O39Mn(H2O)](8-) on chemically modified glassy carbon electrode and its electrocatalytic properties

Through layer-by-layer assembly, undecatungstozincates monosubstituted by transition metals Mn, ZnW11 Mn (H2O) O-39(8-) was successfully immobilized on a glassy carbon electrode surface grafted covalently by 4-aminobenzoic acid. The electrochemical behavior of these polyoxometalates was investigated. Cyclic voltammetry proves the uniform growth of the film. They exhibit some special electrochemical properties in the films, different from those in homogeneous aqueous solution. The effect of pH on the redox behavior of ZnW11Mn(H2O)O-39(8-) in the film was discussed in detail. The multilayer film electrodes have an excellent electrocatalytic response to the reduction of H2O2 and BrO3-, and to the oxidation of ascorbic acid.

Sol-gel-derived carbon ceramic electrode containing 9,10-phenanthrenequinone, and its electrocatalytic activity toward iodate

9,10-Phenanthrenequinone (PQ) supported on graphite powder by adsorption was dispersed in propyltrimethoxysilane-derived gels to yield a conductive composite which was used as electrode material to fabricate a PQ-modified carbon ceramic electrode. In this configuration, PQ acts as a catalyst, graphite powder guarantees conductivity by percolation, the silicate provides a rigid porous backbone, and the propyl groups endow hydrophobicity and thus limit the wetting region of the modified electrode. Square-wave voltammetry was exploited to investigate the pH-dependent electrochemical behavior of the composite electrode and an almost Nernstian response was obtained from pH 0.42 to 6.84. Because the chemically modified electrode can electrocatalyze the reduction of iodate in acidic aqueous solution (pH 2.45), it was used as an amperometric sensor for the determination of iodate in table salt. The advantages of the electrode are that it can be polished in the event of surface fouling, it is simple to prepare, has excellent chemical and mechanical stability, and the reproducibility of surface-renewal is good.

Electrocatalytic oxidation of ascorbic acid by ferrocene in lipid film cast on a glassy carbon electrode

A ferrocene-dimyristoyl phosphatidylcholine (DMPC) film electrode was prepared by casting the solution of ferrocene and DMPC in chloroform onto a glassy carbon electrode surface. Ferrocene retained in the biological membrane gave a couple of irreversible peaks of cyclic voltammogram. The electrode exhibited good electrocatalytic activity for the oxidation of ascorbic acid (H(2)A) in phosphate buffer (pH 6.64) with an anodic peak potential of +340 mV (vs. Ag/AgCl). The anodic current was directly proportional to the square root of the scan rate below 150 mV s(-1). The influence of the pH value was investigated and it was observed that pH 6.64 was the suitable value to the anodic peak potential and current. The thickness of the film and the interference of uric acid were also studied. The electrode can be used to determine H(2)A in the presence of equimolar uric acid. The catalytic peak current increased linearly with the concentration of H(2)A in the range of 1 X 10(-4)-5 X 10(-3) mol L-1.

Electrocatalytic oxidation of ascorbic acid by norepinephrine embedded in lipid cast film at glassy carbon electrode

A stable lipid cast film was made by casting a lipid in chloroform onto a glassy carbon electrode. We imbedded a new mediator norepinephrine into this lipid cast film, which was considered as a biological membrane model. Through electro catalytic oxidation of ascorbic acid by this system, the anodic overpotential was reduced by about 250 mV compared with that obtained at a bare glassy carbon electrode. The electrochemical behavior of norepinephrine in the cast film was controlled by diffusion. The obtained diffusion coefficient of ascorbic acid was 1.87 x 10(-5) cm 2 s(-1). The catalytic current increased linearly with the concentration of ascorbic acid in the range from 0.5 to 10 mM. Using cyclic voltammetry, we obtained two peaks for ascorbic acid and uric acid in the same solution. The separation between the two peaks is about 147 mV. (C) 2001 Elsevier Science Ltd. All rights reserved.

Multilayer assemblies of tungstodiphosphate anions on 1,7-diaminoheptane modified glassy carbon electrode and the electrocatalytic reduction to iodate

1,7-Diaminoheptane (DAH) had been covalently grafted on glassy carbon electrode by amino cation radical formation, which resulted in a stable cationic monolayer under proper pH conditions. Dawson-type tungstodiphosphate anion, P2W18O626- and small molecule, Ru(NH3)(6)(3+) were alternately assembled on the DAH modified electrode through layer-by-layer electrostatic interaction. Thus-prepared multilayer film had been characterized by cyclic voltammetry and X-ray photoelectron spectroscopy. The P2W18O626- multilayers exhibit high electrocatalytic response and sensitivity towards the reduction of iodate. With the increase of the number of P2W18O626- the catalytic current was enhanced and the catalytic potential shifted positively. Iodate in table salt was determined at the modified electrode containing three layers of P2W18O626- with satisfactory results. The multilayer electrode is promising as an electrochemical sensor for the detection of trace iodate.

Hydrogen peroxide biosensor with enzyme entrapped within electrodeposited polypyrrole based on mediated sol-gel derived composite carbon electrode

A novel amperometric biosensor for the detection of hydrogen peroxide was described. The biosensor was constructed by electrodepositing HRP/PPy membrane on the surface of ferrocenecarboxylic acid mediated sol-gel derived composite carbon electrode. The biosensor gave response to hydrogen peroxide in a few seconds with detection limit of 5.0 x 10(-5) M (based on signal:noise = 3). Linear range was upto 0.2 mM. The biosensor exhibited a good stability. (C) 2001 Elsevier Science B.V. All rights reserved.

Electrochemistry and electrochemiluminescence of stable tris(2,2 '-bipyridyl)ruthenium(II) monolayer assembled on benzene sulfonic acid modified glassy carbon electrode

An electrochemically stable monolayer of tris(2,2'-bipyridyl)ruthenium(II) was obtained for the first time. It was based on the electrostatic attachment of Ru(bpy)(3)(2+) to the benzene sulfonic acid monolayer film, which was covalently bound onto glassy carbon electrode by the electrochemical reduction of diazobenzene sulfonic acid. The surface-confined Ru(bpy)(3)(2+) underwent reversible surface process, and reacted with the coreactant, tripropylamine, to produce electrochemiluminescence. In view of the stability of the electrode, the results strongly suggested that light was emitted from the surface-confined Ru(bpy)(3)(2+), not from the detached Ru(bpy)(3)(2+). The Ru(bpy)(3)(2+) modified electrode was used to the determination of tripropylamine. It showed good linearity in the concentration range from 5 muM to 1 muM with a detection limit of 1 muM (S/N = 4). The good stability of the Ru(bpy)(3)(2+) modified electrode also showed that the benzene sulfonic acid monolayer film prepared can be served as an excellent support to construct multilayers. (C) 2001 Elsevier Science B.V. All rights reserved.

Renewable manganous hexacyanoferrate-modified graphite organosilicate composite electrode and its electrocatalytic oxidation of L-cysteine

Manganous hexacyanoferrate (MnHCF) supported on graphite powder was dispersed into methyltrimethoxysilane-derived gels to yield a conductive composite, which was used as electrode material to construct a renewable three-dimensional MnHCF-modifed electrode. MnHCF acts as a catalyst, graphite powder ensures conductivity by percolation, the silicate provides a rigid porous backbone, and the methyl groups endow hydrophobicity and thus limit the wetting section of the modified electrode. Cyclic voltammetry was exploited to investigate the dependence of electrochemical behavior on supporting electrolytes containing various cations. The chemically modified electrode can electrocatalytically oxidize L-cysteine, and exhibits a distinct advantage of polishing in the event of surface fouling, as well as simple preparation, good chemical and mechanical stability, and good repeatability of surface renewal.

Investigation of the model compounds at 4-aminobenzoic acid modified glassy carbon electrode by scanning electrochemical microscopy

In this paper, we studied the reactions of both potassium ferricyanide and hexaammineruthenium(III) chloride at a 4-aminobenzoic acid (4-ABA) modified glassy carbon electrode (GCE) by scanning electrochemical microscopy (SECM) in different pH solutions. The surface of the modified electrode has carboxyl groups, the dissociation of which are strongly dependent upon the solution pH values. The rate constant kb of the oxidation of ferrocyanide on the modified electrode can be obtained by fitting the experimental tip current-distance (I-T-d) curves with the theoretical values. The surface pK(a) of the 4-ABA modified GCE was estimated from the plot of standard rate constant k(o) versus the solution pH and is equal to 3.2, which is in good agreement with the reported result. The SECM approach curves for Ru(NH3)(6)(3+) both on the bare and the modified electrodes show similar diffusion control processes. These results can be explained by the electrostatic interactions between the modified electrode surface and the model compounds with different charges. (C) 2001 Elsevier Science BN. All rights reserved.