Ion-exchange voltammetry of dopamine at Nafion-coated glassy carbon electrodes:: Quantitative features of ion-exchange partition and reassessment on the oxidation mechanism of dopamine in the presence of excess ascorbic acid

The incorporation/exclusion features of dopamine (DA), ascorbic acid (AA) and uric acid (UA) are evaluated for Nafion (NA)-coated glassy carbon electrodes (GCE) of different thicknesses. The ion-exchange partition of DA(+) between the NA film and the sodium phosphate electrolyte is evaluated by determining the partition 2 - 1 coefficient (k(D)) and the apparent diffusion coefficient (D-app) in thick NA films which were 401 and 1.5 x 10(-9) cm(2) s(-1), respectively. The solution diffusion coefficient was found to be 6.0 x 10(-6) cm(2) s(-1). Also, the effect of NA loading and of the voltammetric timescale on DA voltammetry in the presence of excess AA is assessed, at physiologic like conditions. It is demonstrated that, although AA is excluded at the NA coating, a catalytic regeneration of DA, induced by AA, occurs at the interface NA film/electrolyte resulting from the diffusion of the o-quinone product of DA oxidation from the electrode surface to that interface. The interference of AA in the voltammetric signal of DA is eliminated using 18 mu g mm(-2) NA films and v >= 0.5 V s(-1). Therefore, fast, selective and sensitive voltammetric analysis of DA at concentrations < 100 mu M in the presence of excess AA, e.g., 1 mM is achieved. (c) 2006 Elsevier B.V. All rights reserved.