Covalent modification of a glassy carbon surface by 4-aminobenzoic acid and its application in fabrication of a polyoxometalates-consisting monolayer and multilayer films

4-Aminobenzoic acid (4-ABA) was covalently grafted on a glassy carbon electrode (GCE) by amine cation radical formation in the electrooxidation process of the amino-containing compound. X-ray photoelectron spectroscopy measurement proves the presence of 4-carboxylphenylamine monolayer on the GCE. The redox responses of various electroactive probes were investigated on the 4-ABA-modified GCE. Electron transfer to Fe(CN)(6)(3-) in solutions of various pHs was studied by both cyclic voltammetry and electrochemical impedance analysis on the modified electrode. Changes in the solution pH value result in the variation of the terminal group charge state, based on which surface pK(a) values are estimated. The 4-ABA-modified GCE was used as a suitable charged substrate to fabricate polyoxometalates-consisting (POM-consisting) monolayer and multilayer films through layer-by-layer assembly based on electrostatic attraction. Cyclic voltammetry shows the uniform growth of these three-dimensional multilayer films. Taking K10H3[Pr-(SiMo7W4O39)(2)]. H2O (abbreviated as Pr(SiMo7W4)(2)), for example, the preparation and electrochemical behavior of its monolayer and multilayer film had been investigated in detail. This modification strategy is proven to be a general one suitable for anchoring many kinds of POMs on the 4-ABA-modified GCE.

Electrochemical and spectroscopic study of vitamin D-2 by in situ thin layer CD spectroelectrochemistry

The electrooxidation of vitamin D-2 (VD2) was studied by cyclic voltammetry and in situ circular dichroic (CD) spectroelectrochemistry for the first time, The mechanism of electrooxidation and some useful kinetic and adsorption parameters were obtained. The results showed that the oxidation of VD2 in ethanol solution is an irreversible diffusion controlled process following a weak adsorption of the electroinactive product at a glassy carbon electrode, which blocks the electrochemical reaction. The electrooxidation occurs mainly at the triene moieties of the VD2 molecule. The CD spectroelectrochemical data were treated by the double logarithm method together with nonlinear regression, from which the formal potential E-0 = 1.08 V, alphan = 0.245, the standard electrochemical rate constant k(0) = 4.30( +/- 0.58) x 10(-4) cm s(-1) and the adsorption constant beta = 1.77(+/- 0.25) were obtained. (C) 2001 Elsevier Science B.V. All rights reserved.

Investigation of the self-accelerating process of ascorbic acid by in-situ circular dichroism spectroelectrochemistry

The electrode reaction process of ascorbic (Vc) was studied by in-situ circular dichroic(CD) spectroelectrochemistry with a long optical path thin layer cell on glassy carbon(GC) electrode. The spectroelectrochemical data were analyzed by the double logarithmic method together with nonlinear regression. The results suggested that the mechanism of Ve in pH 7.0 phosphate buffer solution at GC electrode was a two-electron irreversible electrooxidation followed by adsorption of the oxidation product. That is a self-accelerated process. Some kinetic parameters at free and at adsorbed electrode surface, i.e, the formal potentials, E-0' = 0.09 V, E-a(0') = 0.26 +/- 0.02 V; the electron transfer coefficient and number of transfered electron, alpha n = 0.41, alpha(a)n = 0.07;the standard heterogeneous electron transfer rate constant, k(0) = 8.0 x 10(-5) cm.s(-1), k(a)(0) = 1.9 x 10(-4) cm.s(-1) and adsorption constant, beta = 102.6 were also estimated.

Electrocatalytic oxidation of methanol on polypyrrole film modified with platinum microparticles

The electrocatalytic oxidation of methanol on polypyrrole (PPy) film modified with platinum microparticles has been studied by means of electrochemical and in situ Fourier transform infrared techniques. The Pt microparticles, which were incorporated in the PPy film by the technique of cyclic voltammetry, were uniformly dispersed. The modified electrode exhibits significant electrocatalytic activity for the oxidation of methanol. The catalytic activities were found to be dependent on Pt loading and the thickness of the PPy film. The linearly adsorbed CO species is the only intermediate of electrochemical oxidation of methanol and can be readily oxidized at the modified electrodes. The enhanced electrocatalytic activities may be due to the uniform dispersion of Pt microparticles in the PPy film and the synergistic effects of the highly dispersed Pt microparticles and the PPy film. Finally, a reaction mechanism is suggested.

Acetylcholinesterase amperometric detection system based on a cobalt(II) tetraphenylporphyrin-modified electrode

An acetylcholinesterase (AChE) activity detection system was fabricated based on the electrocatalysis of cobalt(II) tetraphenylporphyrin of the electrooxidation of thiocholine chloride, which is the product of the hydrolysis of acetylthiocholine chloride by AChE. A simple modified method was used to form the base electrode. AChE was cross-linked on the base electrode by glutaraldehyde. The optimum working conditions are discussed and the characteristics of the detection system are evaluated.

ATTEMPTS TO MODIFY AN ELECTRODE WITH A CONDUCTING PLATINUM CLUSTER COMPOUND

A conducting platinum cluster compound K1.64Pt(C2O4)(2) was electrochemically synthesized on a glassy carbon electrode through the electrooxidation of K2Pt(C2O4)(2) in an aqueous medium using single potential step and cyclic voltammetry methods. The precursor K2Pt(C2O4)(2) was prepared by a ligand exchange reaction between C2O42- and PtCl42-. During single potential step experiments, the electrolytic current corresponding to the oxidation of K2Pt(C2O4)(2) increased dramatically after a sharp decrease at the beginning (due to the formation of conducting K1.64Pt(C2O4)(2) on the surface of the working electrode). Two kinds of mechanism account for the current transients at the different applied potentials. Cyclic voltammetry was conducted with K1.64Pt(C2O4)(2) on the surface of the working electrode and a steady-state diffusion current was observed. Since the material grew in a fibrous manner, each conducting fiber which was in contact with the electrode could serve as an ultramicroelectrode. The behavior of the working electrode was thus changed from a plain to an ultramicroelectrode array.

THE EXTRAORDINARY PROPERTIES OF ELECTROCHEMICALLY SYNTHESIZED CLUSTER COMPOUND K1.64PT(C2O4)2

The conducting platinum cluster compound K1.64Pt(C2O4)2 was electrochemically synthesized on a glassy carbon electrode through electrooxidation of K2Pt(C2O4)2 in an aqueous medium using a single potential step and cyclic voltammetry methods. Two kinds of

ANODIC-OXIDATION OF HYDRAZINE ON GLASSY-CARBON ELECTRODES IN ACETONITRILE

The anodic oxidation kinetics of hydrazine on glassy carbon electrodes in acetonitrile were examined by cyclic voltammetry, a rotating ring-disc electrode technique and chronoamperometry. The experimental results of the rotating ring-disc electrode prove that hydrazine is oxidized to HN=NH, which cannot be oxidized further in acetonitrile. Hydrazine molecules are adsorbed on the electrode surface. One-third of the adsorbed hydrazine molecules are oxidized to HN=NH and the other two thirds act as proton acceptors. A possible mechanism of hydrazine oxidation is proposed.

ELECTROCATALYTIC OXIDATION AND FLOW DETECTION OF HYDRAZINE COMPOUNDS IN LIQUID-CHROMATOGRAPHY AT A VITAMIN-B-12 ADSORBED GLASSY-CARBON ELECTRODE

A vitamin B-12 chemically modified electrode (CME) was constructed by adsorption of vitamin B-12 onto a glassy carbon surface. The electrode catalyzes the electrooxidation of hydrazine compounds over a wide pH range. The electrocatalytic behavior of hydrazines is elucidated with respect to the CME preparation conditions, solution pH, operating potential, mobile phase flow rate, and other variables. When applied to liquid chromatographic detection of the analytes, the vitamin B-12 CME yielded a linear response range over 2 orders of magnitude, and detection limits at the picomole level. The vitamin B-12 CME offers acceptable catalytic stability in both batch and flow systems.

AMPEROMETRIC FLOW-INJECTION ANALYSIS OF HYDRAZINE BY ELECTROCATALYTIC OXIDATION AT COBALT TETRAPHENYLPORPHYRIN MODIFIED ELECTRODE WITH HEAT-TREATMENT

Chemically modified electrodes prepared by treating the cobalt tetraphenylporphyrin modified glassy-carbon electrode at 750-degrees (HCME) are shown to catalyze the electrooxidation of hydrazine. The oxidation occurred at +0.63 V vs. Ag/AgCl (saturated potassium chloride) in pH 2.5 media. The catalytic response is evaluated with respect to solution pH, potential scan-rate, concentration dependence and flow-rate. The catalytic stability of the HCME is compared with that of the cobalt tetraphenylporphyrin adsorbed glassy-carbon electrode. The stability of the HCME was excellent in acidic solution and even in solutions containing organic solvent (50% CH3OH). When used as the sensing electrode in amperometric detection in flow-injection analysis, the HCME permitted sensitive detection of hydrazine at 0.5 V. The limit of detection was 0.1 ng. The linear range was from 50 ng to 2.4-mu-g. The method is very sensitive and selective.

ANODIC-OXIDATION OF HYDRAZINE ON GLASSY-CARBON MODIFIED BY MACROCYCLIC TRANSITION-METAL COMPLEXES .1. COBALT PROTOPORPHYRIN DIMETHYL ESTER MODIFIED ELECTRODE

The electrocatalytic oxidation of hydrazine (N2H4) on a glassy carbon electrode (GC) modified by monolayer and polymer films of cobalt protoporphyrin dimethyl ester (CoPP) has been studied. Both the monolayer and polymer films of CoPP are very active to the anodic oxidation of N2H4. The activity of CoPP for the anodic oxidation of N2H4 is dependent on the pH of the solution, and the thickness of polymerized CoPP film. The oxidation kinetics were examined by methods of cyclic voltammetry, rotating disc electrodes and steady-state polarization measurement.

Size-controllable synthesis of monodispersed SnO2 nanoparticles and application in electrocatalysts

Size-controllable tin oxide nanoparticles are prepared by heating ethylene glycol solutions containing SnCl2 at atmospheric pressure. The particles were characterized by means of transmission electron microscopic (TEM), X-ray diffraction (XRD) studies. TEM micrographs show that the obtained material are spherical nanoparticles, the size and size distribution of which depends on the initial experimental conditions of pH value, reaction time, water concentration, and tin precursor concentration. The XRD pattern result shows that the obtained powder is SnO2 with tetragonal crystalline structure. On the basis of UV/vis and FTIR characterization, the formation mechanism of SnO2 nanoparticles is deduced. Moreover, the SnO2 nanoparticles were employed to synthesize carbon-supported PtSnO2 catalyst, and it exhibits surprisingly high promoting catalytic activity for ethanol electrooxidation.

Mechanistic studies of the electro-oxidation pathway of ammonia in several room-temperature ionic liquids

A mechanistic study of the direct oxidation of ammonia has been reported in several room-temperature ionic liquids (RTILs), namely, [C(4)mim][BF4], [C(4)mim][OTf], [C(2)mim][NTf2], [C(4)mim][NTf2], and [C(4)mim][PF6], on a 10 mu m diameter Pt microdisk electrode. In four of the RTILs studied, the cyclic voltammetric analysis suggests that ammonia is initially oxidized to nitrogen, N-2, and protons, which are transferred to an ammonia molecule, forming NH4+ via the protonation of the anion(s) (A(-)). In contrast, NH4+ is formed first in [C(4)mim][PF6], followed by the protonated anion(s), HA. In all five RTILs, both HA and NH4+ are reduced at the electrode surface, forming hydrogen gas, which is then oxidized. The effect of changing the RTIL anion is discussed, and this may have implications in the defining of pK(a) in RTIL media. This work also has implications in the possible amperometric sensing of ammonia gas.

A mechanistic study of the electro-oxidation of bromide in acetonitrile and the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide at platinum electrodes

The oxidation of bromide has been investigated by linear sweep and cyclic voltammetry at platinum electrodes in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, ([C(4)mim][NTf2]), and the conventional aprotic solvent. acetonitrile, (MeCN). Similar voltammetry was observed in both solvents, despite their viscosities differing by more than an order of magnitude. DigiSim(R) was employed to simulate the voltammetric response. The mechanism is believed to involve the direct oxidation of bromide to bromine in a heterogeneous step, followed by a homogenous reaction to form the tribromide anion: 2Br(-) --> Br-2 + 2e(-)

Determination of ammonia based on the electro-oxidation of hydroquinone in dimethylformamide or in the room temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

The results detail a novel methodology for the electrochemical determination of ammonia based on its interaction with hydroquinone in DMF. It has been shown that ammonia reversibly removes protons from the hydroquinone molecules, thus facilitating the oxidative process with the emergence of a new wave at less positive potentials. The analytical utility of the proposed methodology has been examined with a linear range from 10 to 95 ppm and corresponding limit-of-detection of 4.2 ppm achievable. Finally, the response of hydroquinone in the presence of ammonia has been examined in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluormethylsulfonyl)imide, [EMIM][N(Tf)(2)]. Analogous voltammetric waveshapes to that observed in DMF were obtained, thereby confirming the viability of the method in either DMF or [EMIM][N(Tf)(2)] as solvent. (C) 2003 Elsevier B.V. All rights reserved.