ORIGINAL PAPER Enhancement of electron transfer kinetics on a polyaniline-modified electrode in the presence of anionic dopants S. Ghasemi & M. F. Mousavi & M. Shamsipur Received: 22 March 2007 / Revised: 20 June 2007 / Accepted: 3 July 2007 / Published online: 28 July 2007 # Springer-Verlag 2007 Abstract The electrocatalytic redox behavior of 1,4- naphthoquinone (NQ) has been studied on a polyaniline- modified platinum electrode (PANI) using cyclic voltammetry and rotating disc electrode (RDE) as diagnostic techniques. The modified electrode was prepared by electropolymeriza- tion of aniline in different acidic solutions. The PANI showed electrocatalytic activity toward the redox behavior of NQ. This process includes the participation of PANI to the redox reaction of NQ via the surface catalysis phenomena. The cyclic voltammograms of NQ in HCl on the PANI-Cl- modified electrode showed an overlapped oxidation peak, the peak potential of which did not change with increasing scan rate. The influence of other anions including ClO  4 and SO 2 4 as dopant was also studied and compared with Cl - . The use of HClO 4 as a supporting electrolyte resulted in well- separated redox peaks. The RDE voltammogram was used to obtain a quantitative assessment of reaction rate at the PANI- modified electrode. It was found that PANI acts as an electrocatalyst for NQ reduction with decreasing ΔE p and increasing k°. Keywords Polyaniline-modified electrode . 1,4-Naphthoquinone . Anion effect . Electrocatalysis Introduction A promising approach for minimizing overvoltage effects in electrocatalytic processes is the use of chemically modified electrodes [1–6]. A thin layer of a modifier, deposited on the surface of the electrode, is able to enhance the kinetics of electrode process of some solution species which give rise to an attractive area of investigation in various fields of electrochemistry such as gas sensors [7], biosensors [8, 9], corrosion control [10], and batteries [11–13]. Among different materials used for the modification of electrode surfaces, the conducting polymers have been shown to be of critical interest [14–16]. Polyaniline (PANI) as an intrinsi- cally conducting polymer can catalyze some electrode processes with heterogeneous electron transfer between the electrode and polymer [17–22]. Quinone–hydroquinone redox couples [23, 24], organic compounds of analytical interest such as ascorbic acid [25], reduced form of nicotinamide adenine dinucleotide phosphate [NADPH (H)] [26], H 2 O 2 [27], N 2 H 4 [28] on PANI and reduction in O 2 [29], electrooxidation of methanol on PANI with dispersed metals [30], glucose [31, 32], nitrite [33], urea [34], choline [35], and glycerol [36] are examples of compounds that have been studied for their electrocatalytic behaviors. Numerous quinones play vital roles in the bioelectro- chemistry of living cells and exert relevant biological and physiological activities in animals and plants [37–39]. 1,4- Naphthoquinone (NQ) composes a backbone for many compounds such as phthiocol, various pigments, vitamin K, and many antitumor drugs which are involved in biological activities [40, 41]. A series of investigations have been carried out on the electrochemical characterization of substituted NQs. Lebedev et al. [42] observed the surprising effect of Ca 2+ on the redox reaction of Echinochrome A (6-ethyl-2,3,5,7,8- pentahydroxyquinone), as a pigment of sea urchins. Ferraz et al. [43] investigated the electrochemical aspects of the reduction of the biological active 2-hydroxy-3-alkyl-1,4- naphthoquinone. Oyama et al. [44] have revealed that, in general, it is thermodynamically possible for species to be J Solid State Electrochem (2008) 12:259–268 DOI 10.1007/s10008-007-0386-1 S. Ghasemi : M. F. Mousavi (*) Department of Chemistry, Tarbiat Modares University, P. O. Box: 14115-175, Tehran, Iran e-mail: mousavim@modares.ac.ir M. Shamsipur Department of Chemistry, Razi University, Kermanshah, Iran