Glassy carbon electrodes reactions

Cyclic voltammetry, square-wave voltammetry, and controlled potential electrolysis were used to study the electrochemical oxidation behavior of niclosamide at a glassy carbon electrode. The number of electrons transferred, the wave characteristics, the diffusion coefficient and reversibility of the reactions were investigated. Following optimization of voltammetric parameters, pH, and reproducibility, a linear calibration curve over the range 1 x 10 6 to 1 x 10 4 mol/dm3 niclosamide was achieved. The detection limit was found to be 8 x 10 7 mol/dm3. This voltammetric method was applied for the determination of niclosamide in tablets [33]. 

A flow injection optical fibre biosensor for choline was also developed55. Choline oxidase (ChOX) was immobilized by physical entrapment in a photo-cross-linkable poly(vinyl alcohol) polymer (PVA-SbQ) after adsorption on weak anion-exchanger beads (DEAE-Sepharose). In this way, the sensing layer was directly created at the surface of the working glassy carbon electrode. The optimization of the reaction conditions and of the physicochemical parameters influencing the FIA biosensor response allows the measurement of choline concentration with a detection limit of 10 pmol. The DEAE-based system also exhibited a good operational stability since 160 repeated measurements of 3 nmol of choline could be performed with a variation coefficient of 4.5%. 

Consideration of these primary processes together with the voltammetric results for the M/ OH systems (Figures 1-3), the potentiometric titration data (Figure 4), and the voltammetric data for O2 reduction at metal electrodes (Figure 5) and in the presence of metal ions at a glassy carbon electrode (Figures 6 and 7), prompts the formulation of self-consistent reaction Schemes for the three metals in combination with OH and O2 (Schemes I,... 

The reaction was investigated under a variety of conditions. As noted in Table 2, both the yield and stereo-selectivity varied in response to changes in the electrode and proton donor (HD). Environmental factors clearly make the use of a glassy carbon electrode preferable to mercury or lead electrodes. Yields range from 58 to 87%, and the stereo-selectivity from a low of 2.6 1 trans/cis (6a/6b) to as high as 14.8 1 when the reaction is conducted in the presence of cerium (III) chloride. 

Figure 3.24 Schematic representation of the analytical protocol (A) Capture of the ALP-loaded CNT tags to streptavidin-modified magnetic beads by a sandwich DNA hybridization (a) or Ab-Ag-Ab interaction (b). (B) Enzymatic reaction. (C) Electrochemical detection of the product of the enzymatic reaction at the CNT-modified glassy carbon electrode MB, Magnetic beads P, DNA probe 1 T, DNA target P2, DNA probe 2 Abl, first antibody Ag, antigen Ab2, secondary...

Zhao et al. prepared magnetite (FesO nanoparticles modified with electroactive Prussian Blue [44]. These modified NPs were drop-cast onto glassy-carbon electrodes. They observed the redox processes commonly observed for PB (similar to that seen in Figure 4.8), and also demonstrated that the Prussian White material produced by PB reduction at 0.2 V served as an electrocatalyst for Fi202 reduction. They also prepared LbL films in which PB NPs and glucose oxidase were alternated between PD DA layers [99]. These were demonstrated to act as electrocatalysts for Fi202 reduction. Based on the ability to sense the product of the enzymatic reaction, these structures were shown to act as glucose sensors. 

The voltammetric reduction of a series of dialkyl and arylalkyl disulfides has recently been studied in detail, in DMF/0.1 M TBAP at the glassy carbon electrode The ET kinetics was analyzed after addition of 1 equivalent of acetic acid to avoid father-son reactions, such as self-protonation or nucleophilic attack on the starting disulfide by the most reactive RS anion. Father-son reactions have the consequence of lowering the electron consumption from the expected two-electron stoichiometry. Addition of a suitable acid results in the protonation of active nucleophiles or bases. The peak potentials for the irreversible voltammetric reduction of disulfides are strongly dependent on the nature of the groups bonded to the sulfur atoms. Table 11 summarizes some relevant electrochemical data. These results indicate that the initial ET controls the electrode kinetics. In addition, the decrease of the normalized peak current and the corresponding increase of the peak width when v increases, point to a potential dependence of a, as discussed thoroughly in Section 2. 

The importance of 1,4-dihydropyridine nucleotides in biological systems prompted the increasing interest in their electrochemical oxidation.238-243 The mechanistic aspects of the electrochemical oxidation of NADH involving removal of two electrons and one proton to form NAD4 has been examined in aqueous and DMSO media at a glassy carbon electrode.242 The reaction occurs according to an ECE mechanism ... 

The electrochemical oxidation of 4-dimethylaminoantipyrine (4-dimethyl-amino-2,3-dimethyl-l-phenyl-A3-pyrazolin-5-one) has been investigated in CH3CN-NaC104 at a glassy carbon electrode.421 The first step is a quasi-reversible electron transfer from the lone-pair electrons on the 4-dimethyl-amino nitrogen to form the radical-cation. The second-order disappearance of the radical-cation is presumably due to a disproportionation reaction. The oxidation at the potential of the plateau of the first wave gave the protonated 4-dimethylaminoantipyrine in 60% yield, but other products were not identified. 

A cyclic voltammogram of the [FeBrJ-72- electrode reaction at a Pyrex glass/glassy carbon electrode in molten AlBr3-l-methyl-3-ethylimidazolium bromide at 60°C is displayed in Figure 17.5. 

Recent studies describe the use of cyclic voltammetry in conjunction with controlled-potential coulometry to study the oxidative reaction mechanisms of benzofuran derivatives [115] and bamipine hydrochloride [116]. The use of fast-scan cyclic voltammetry and linear sweep voltammetry to study the reduction kinetic and thermodynamic parameters of cefazolin and cefmetazole has also been described [117]. Determinations of vitamins have been studied with voltammetric techniques, such as differential pulse voltammetry for vitamin D3 with a rotating glassy carbon electrode [118,119], and cyclic voltammetry and square-wave adsorptive stripping voltammetry for vitamin K3 (menadione) [120]. 

If one wishes to verify whether the prewave can form the basis for an amperometric sensor, one would preferably dispose of as much information as possible concerning the nature and the properties of this wave. An obvious technique for diagnosis is cyclic voltammetry. Hydrogen peroxide can be oxidised as well as reduced at glassy-carbon electrodes however, the potential ranges within which the reactions occur are situated relatively distant from each other, as can be seen in Fig. 4.3 and Fig. 4.5. 

McCreery and co-workers have investigated the redox reactions for several redox analytes at glassy carbon electrodes, and have summarized the categorization of redox systems according to the effects of surface modification on electrode kinetics [1-3]. These redox analytes in the present study are known to be sensitive or insensitive to the electronic properties, surface microstructure, and surface termination of the carbon electrodes. 

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