Kinetic modelling at electrodes modified by films

In this section, we present a kinetic model for the oxidation of NADH within a polymer film at a rotating disc electrode. In this model, we assume that the rate of electron transfer between the mediating species in the film and the electrode is sufficiently fast so that it is not rate limiting [43]. We also assume that the substrate diffuses into the film where it is adsorbed at the reaction sites within the film and where it undergoes oxidation to the product. The possibility that the product is able to compete for these reaction sites with the film in a reversible manner is also considered. 

Thus the steps in the reaction scheme are Mass transport to the electrode surface 

Analytical models of modified electrodes for NADH oxidation 

This type of diffusion/reaction mechanism has been treated semi-analyti-cally by Albery et al. [42, 44, 45], under steady-state conditions and its applications to amperometric chemical sensors has been described by Lyons et al. [46]. In both models, only diffusion and reaction within a boundary layer is considered, while the effect of concentration polarisation in the solution is neglected. Thus, to apply the model to an experimental system it is necessary to be able to accurately determine the concentration of substrate at the polymer/solution interface. Assuming that the system is in the steady state, the use of the rotating disc electrode allows simple determination of the substrate concentration at the interface from the bulk concentration and the experimentally determined flux using [47] 

The approximate analytical treatment described by Albery can now be applied. In the absence of any inhibition, and assuming rapid desorption of product, an expression for the flux can be determined by combining 

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