Electrochemical PEDOT Sensor for Antioxidantsin Beverages and Milks

Abstract

Introduction The conducting polymer PEDOT (poly(3,4-ethylenedioxythiophene)) as an electrochemical sensor element possesses redox-mediating properties that allow excellent separation of signals due to antioxidants present in beverages such as teas and coffee [1]. This includes the phenolic acids present in white wines, with specific features noted for nanostructured microelectrodes [2]. Excellent separation of signals due to ascorbic acid and sulfites, alongside the response due to the oxidation of polyphenols in beverages, has been achieved using the redox-mediating properties of PEDOT [3]. Method PEDOT films were grown by cycling to +1.2 V (Ag/AgCl) on 3 mm dia. glassy carbon electrodes and 10 µm dia. gold microelectrodes in 0.1 M EDOT/ 0.1 M LiClO4 in propylene carbonate at 100 mV s-1. The electrodes were then transferred to undiluted milks (c. pH 6.5), or to an aqueous 0.1 M sodium phosphate buffer (PBS) at pH 6.6 containing uric acid and ascorbic acid standards, prior to testing by cyclic voltammetry (CV). A background PEDOT curve was taken in the PBS solution and subtracted from the voltammograms obtained for the milk samples. Results and Conclusions The PEDOT sensor showed an excellent response for the oxidation of uric acid, with an anodic peak seen at 350 mV (Ag/AgCl), taken at pH 6.6, with both uric acid standards and undiluted milks [4]. A small peak due to ascorbic acid was at times located close to 0 mV, enabling the simultaneous detection of the two antioxidants. Very good linear relationships were obtained between peak current intensities and concentrations in the range of 6-100 μM for uric acid and 30-500 μM for ascorbic acid. The newly developed sensor was then successfully applied to 36 commercial milk samples for a fast voltammetric determination of uric acid and ascorbic acid concentration. An independent HPLC analysis was also performed immediately after running the same milk samples, with a pre-treatment step to remove fats and proteins from the milk. The average difference between the results for uric acid from the two methods was 7%, demonstrating an acceptable degree of agreement between the two measurement methods. At the same time the results were obtained much more rapidly with the electrochemical method than in the lengthy HPLC procedure.The polyphenols present in teas, chocolate and flavoured milks present additional analytical challenges, due to strong adsorption effects. PEDOT-modified electrodes have been successfully developed to quantify flavonoids present in chocolate-flavoured milks, and hydroxycinnamic acids present in coffee-flavoured milks. Strong adsorption effects were seen with regular and flavoured milks at PEDOT microelectrodes. Rather than a diffusion-controlled plateau current, peaks with current densities up to 1000 times greater than observed at the corresponding macroelectrodes were observed, consistent with an extremely high surface area and fractal-type growth of PEDOT at microelectrodes [2].