1023-1935/02/3810- $27.00 © 2002 åÄIä “Nauka /Interperiodica” 1093 Russian Journal of Electrochemistry, Vol. 38, No. 10, 2002, pp. 1093–1096. Translated from Elektrokhimiya, Vol. 38, No. 10, 2002, pp. 1212–1215. Original Russian Text Copyright © 2002 by Abalyaeva, Eﬁmov. INTRODUCTION The aim of this work was to develop a chemically- modiﬁed electrode for the determination of the overall concentration of antioxidants (vitamins C and E and polyphenols, say, ﬂavonoids), which enter a human organism with food (fruits, vegetables, wines, teas). Antioxidants have antioxidant properties and are capa- ble of acting as “sacriﬁcial” reactants, which rapidly react with free radicals, thus protecting more important cell elements from damage. The starting point for this work was the chemical method of analysis suggested by the authors of [1, 2]. The method utilized the reduc- tion of radical cation, 2,2'-azino-bis(3-ethylbenzthiazo- line-6-sulfo) (ABTS), which rapidly reacted with active and slowly, with rather inert antioxidants. This property permitted to use ABTS + as a model radical when titrat- ing mixtures of natural antioxidants and quantitatively determine their overall content (antioxidant equiva- lent). According to preliminary experiments, electro- chemical reduction–oxidation of ABTS + is a quasi- reversible reaction, speciﬁcally, ABTS + + e ABTS. This property of ABTS + suggested its use as a redox catalyst for creating a chemically-modiﬁed electrode and then as an amperometric sensor for the antioxidant concentration determination. The matrix for immobi- lizing ABTS + on the electrode surface can be made of a conductive polymer, for example, polyaniline (PAn). Electrosynthesized polyaniline may capture a radical in the form of an anion containing dissociated groups, from an electrolyte. Examples of such immobilization of redox catalysts and enzymes in conductive polymers are given in [3]. Moreover, we felt it justiﬁed to expect that redox catalyst ABTS + would substantially affect the polyaniline electrosynthesis (ES). The role of such a redox catalyst was studied in [4, 5]. The aim of this work was to search for the condi- tions of electrosynthesis of a polyaniline coating on an electrode surface with ABTS molecules incorporated into it and to determine how the current would depend on the concentration of model antioxidants in solution at a speciﬁed electrode potential. EXPERIMENTAL Polyaniline was electrosynthesized in a three-cham- ber electrochemical cell of glass, in air. The compart- ment of the working electrode (glassy-carbon disk 0.5 cm in diameter soldered into a glass tube) was sep- arated from that of the auxiliary electrode (glassy-car- bon plate of area 2 cm 2 ) by a porous glass diaphragm. Electrochemical measurements were done using a PI-50-1 potentiostat coupled with a PR-8 programmer. A copper current lead contacted glassy carbon through Hg. The reference electrode was a silver–silver chloride electrode. Measurements were done in a 0.1 M aniline sulfate solution in 0.1 M H 2 SO 4 . In the presence of ABTS, the electrosynthesis was carried out either from an aqueous suspension in an argon ﬂow (technique 1) or from a homogeneous solution electrolyte : EtOH = 1 : 1 (technique 2). The antioxidants studied: trolox and gallic, uric, and ascorbic acids were of reagent grade. Before taking a measurement, a weight of an antioxi- dant would be dissolved in a buffer containing 0.025 M KH 2 PO 4 and 0.025 M Na 2 HPO 4 (pH 6.8) in an argon ﬂow. The dependence of the current on the antioxidant concentration was measured in this buffer in an argon ﬂow. RESULTS AND DISCUSSION Interaction between ABTS and Aniline. Production of Polyaniline–ABTS Composite Upon mixing an aqueous solution of ABTS and a sulfuric acid solution of aniline, there immediately forms a ﬁnely crystalline precipitate. The results of an element analysis of C 30 H 30 N 6.2 S 4.2 O 6.2 correspond best to the composition ABTS ⋅ 2An (An is aniline), which conforms to the formula C 30 H 32 N 6 S 4 O 6 . Mixing an alcohol solution of ABTS and a sulfuric acid solution of aniline produced a solution. With aniline polymerized via technique 1, composites showed a higher activity in measurements of antioxidant concentrations. An analy- sis of the ﬁltrate obtained after isolating the precipitate formed by a complex of ABTS and aniline showed that a small amount of the complex remained in solution. Polyaniline Electrode for Assaying Antioxidants V. V. Abalyaeva and O. N. Efimov Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia Received November 14, 2001 Abstract—It is shown that an electrochemical sensor for determining concentration of antioxidants in various solutions and mixtures can be created. An organic or inorganic oxidant introduced into electrosynthesized poly- aniline interacts with antioxidants in solution at a speciﬁed potential. The antioxidant concentration is deter- mined from the magnitude of the current.