Catechol, electrooxidation

A Case Study Catechol Electrooxidation at RuOz/Nafion Composite Films... 

The combination of anodic oxidation of benzene using the Ag(I)/Ag(II) mediator with cathodic oxidation of benzene using the Cu(I)/Cu(II) mediator in a single electrolytic cell produces p-benzoquinone selectively in both the anodic and the cathodic chambers [242]. Silver-mediator promoted electrooxidation of hydrocarbon has been attempted [243]. The kinetics of indirect oxidation of catechol and L-dopa with IrCl6 has been studied in polymer-coated glassy carbon [244]. 

Several publications on electrochemical mechanistic studies of the oxidative transformations of catecholamines followed the contribution by R. N. Adam s group (256) and involved a-methyldopamine, a-methylnor-adrenaline, dopamine (257), a-methyldopa, 5,6-dihydroxy-2-methylin-dole (255), and dopa (259). These studies (257) (Scheme 5), which confirmed the validity of the melanization scheme by Mason and Raper (Ref. 7, p. 50), explored the pH effect on the sequence of events that characterize the electrooxidation of catecholamines. Thus, the cyclic voltammogram in I M HCIO4 (pH 0.6) shows only peaks corresponding to the catechol-quinone redox couple as the protonation of the amino group prevents the cyclization step. 

This move from the modification of electrodes to the modification of nonconducting surfaces in contact with electrodes is also seen in systems in which electrodes are modified using nonconduct ive particles. For example, Zak and Kuwana showed that the basic surface of alpha-alumina imbedded in glassy carbon electrodes caused catalysis of the electrooxidation of catechol and other organic species requiring loss of a proton for their oxidation JJ ), Similar results were obtained by Shaw and Creasy (78) using alumina or layered-double hydroxides in composite electrodes (vide infra). 

We now examine a specific problem, the electrooxidation of catechol at a Nafion-coated electrode containing dispersed RUO2 particles. This work was recently published by Lyons and coworkers. Typical cyclic voltammograms recorded for the Ru02/Nafion composite material in 0.2 M H2SO4 are shown in Fig. 2.33. The broad nature of the current response across the entire potential window examined... 

The main difference between the two types of sensors is that the first type (based on phenol oxidases or peroxidase) uses a low applied potential (-100 - 0 mV vs. Ag/AgCl) (3), whereas the second type (based on CDH or GDH) needs a higher potential (300 - 400 mV V5. Ag/AgCl) (9,11,13) to be able to oxidize the phenols and therefore the risk of electrooxidizing interfering compounds in the sample is higher. Tyrosinase, laccase and peroxidase can all be used for both phenolic and diphenolic compounds, however, for laccase and tyrosinase the sensitivity is much higher for catecholic compounds (3). CDH and GDH require a quinone, therefore a diphenol or aminophenol is needed. 

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