Critical evaluation of voltammetric techniques for antioxidant capacity and activity: Presence of alumina on glassy-carbon electrodes alters the results

Abstract

Voltammetric techniques have been widely applied to determine the food antioxidant capacity/activity using different electrochemical parameters (peak current, peak potential, charge) and by calculating the electrochemical index (EI) using glassy-carbon electrodes (GCE). A crucial step in this analysis involves electrode cleaning that typically involves polishing of the GCE surface on alumina slurry. Probably because this procedure is well established, many works have not mentioned how residual alumina is efficiently removed after polishing, which suggests that alumina residues may be present on the GCE surface. This work demonstrates that residual alumina on GCE affects the electrochemical parameters of antioxidants (caffeic acid, gallic acid, chlorogenic acid, catechin, rutin and quercetin) and food samples (tea and wine) resulting in misleading results. We critically compare the responses of unmodified (properly cleaned) with alumina-modified GCE. The results obtained by cyclic voltammetry, differential-pulse voltammetry and square-wave voltammetry showed substantial current (and charge) increase on alumina-modified electrodes and the calculated EI was dramatically increased. Alumina immobilized on GCE likely provides adsorption sites that change the electrochemical oxidation pathway of phenolic antioxidants and may adsorb the antioxidant species or their intermediates facilitating the electron transfer between antioxidants and GCE. This explanation was evidenced by the change of the rate-determining step of the electrochemical oxidation of all antioxidants after alumina modification from diffusion-controlled to adsorption-controlled processes. The enhanced detectability provided by alumina may suggest the GCE modification for improved determination of the antioxidant capacity and activity. Considering this possibility, inter- and intra-electrode precision were evaluated and RSD values lower than 5% confirms the feasibility of the alumina-modified GCE for this purpose. On the other hand, this work highlights the need for proper electrode cleaning when a bare GCE is used for this aim.