The common approach to modelling NADH oxidation

In the literature, NADH oxidation at chemically modified electrodes is most commonly suggested to occur via a two-step reaction mechanism. In the first step, NADH forms a complex with an oxidised mediator site, and in the second step, this complex breaks down producing product and a reduced mediator site, Fig. 2.6. 

This type of mechanism has been proposed for a variety of different NADH oxidation systems, including phenoxazine dyes [35, 36], phenothia-zine dyes [39] and a conducting organic salt [40]. Experimental evidence for the formation of a complex during the chemical oxidation of NADH has been provided by Fukuzumi et al. [41]. These authors showed that a mixture of an NADH model compound and a quinone derivative formed a charge-transfer complex in solution as determined by UV/Vis spectros- 

Analytical models of modified electrodes for NADH oxidation 

Due to the formation of an intermediate complex, this type of reaction mechanism was described as being analogous to Michaelis-Menten kinetics [39]. A common error made when examining the behaviour of systems of this type is to use the Koutecky-Levich equation to analyse the rotation speed-dependence of the current. This is incorrect because the Koutecky-Levich analysis is only applicable to surface reactions obeying strictly first-order kinetics. Applying the Koutecky-Levich analysis to situations where the surface kinetics are non-linear, as in this case, leads to erroneous values for the rate constants. Below, we present the correct treatment for this problem based on an extension of a model originally developed by Albery et al. [42] 

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