Rapid electroanalysis of uric acid and ascorbic acid using a poly(3,4-ethylenedioxythiophene)-modified sensor with application to milk

Here we present a sensitive and selective electrochemical sensor that has been developed for the analysis of uric acid and ascorbic acid in milk with minimum interference from each other. A conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), was prepared electrochemically as a thin layer on a glassy carbon electrode and then acclimatized to an aqueous buffer before sample analysis. The modified sensor showed an excellent catalytic response towards the oxidation of uric acid, with an anodic peak during cyclic voltammograms at around 350 mV (Ag/AgCl), taken at pH 6.6 as typical of untreated milk samples. A small peak due to ascorbic acid was located close to 0 mV, enabling the simultaneous detection of the two antioxidants. A stronger response for ascorbic acid was produced at pH 3, a pH value obtained after treating milk with acids to remove protein material prior to HPLC analysis. The electrochemical oxidation of uric acid at the modified electrode involved pre-adsorption of uric acid and a form of stripping voltammetry. This was not the case with ascorbic acid where the oxidation proceeded under diffusion control, with a diffusion coefficient of 6.9 × 10−6 cm2 s−1 obtained from cyclic voltammograms. Good linear relationships between peak current intensities and concentrations were obtained in the range of 6-100 μM for uric acid and 30-500 μM for ascorbic acid. The sensitivity and limit of detection (S/N = 3) for the PEDOT-modified electrode were 2.5 μA μM−1 cm−2 and 7 μM for uric acid, and 0.6 μA μM−1 cm−2 and 45 μM for ascorbic acid, respectively. The newly developed sensor was successfully applied to milk samples for a fast voltammetric determination of uric acid and ascorbic acid concentration.