Preparation of poly N,N-dimethylaniline/ferrocyanide film modified carbon paste electrode: Application to electrocatalytic oxidation of L-cysteine Reza Ojani * , Jahan-Bakhsh Raoof, Ebrahim Zarei Electroanalytical Chemistry Research Laboratory, Faculty of Chemistry, Mazandaran University, Babolsar, Iran article info Article history: Received 21 May 2009 Received in revised form 18 October 2009 Accepted 2 November 2009 Available online 6 November 2009 Keywords: Poly N,N-dimethylaniline Ferrocyanide Carbon paste electrode L-Cysteine Electrocatalysis Cyclic voltammetry abstract Functionalized poly N,N-dimethylaniline film was prepared by adsorption of ferrocyanide onto the poly- mer forming at the surface of carbon paste electrode (CPE) in aqueous solution. The electrocatalytic abil- ity of poly N,N-dimethylaniline/ferrocyanide film modified carbon paste electrode (PDMA/FMCPE) was demonstrated by oxidation of L-cysteine. Cyclic voltammetry and chronoamperometry techniques were used to investigate this ability. In the optimum pH (6.00), the electrocatalytic ability about 480 mV and the catalytic reaction rate constant, (k h ), can be seen 3.08  10 3 M 1 s 1 . The catalytic oxidation peak current determined by cyclic voltammetry method was linearly dependent on the L-cysteine concentra- tion and the linearity range obtained was 8.00  10 5 –2.25  10 3 M. Detection limit of this method was determined as 6.17  10 5 M (2r). At a fixed potential under hydrodynamic conditions (stirred solution), the calibration plot was linear over the L-cysteine concentration range 7.40  10 6 M–1.38  10 4 M. The detection limit of the method was 6.38  10 6 M (2r). Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Polymer modified electrodes (PMEs) prepared by electropoly- merization have received interest in the detection of analytes be- cause of its selectivity, sensitivity and homogeneity in the electrochemical deposition, strong adherence to electrode surface and chemical stability of the film [1,2]. Selectivity of PMEs as a sen- sor can be attained by different mechanisms such as size exclusion [3], ion exchange [4] and hydrophobic interaction [5]. In this con- text, some electroactive conducting polymers themselves exhibit catalytic effects for any redox reactions, e.g. reduction of nitrite on poly(ortho-toluidine) [6] and oxidation of hydrazine on polyan- iline [7], but in order to increase this catalytic activity, several strategies have been developed by incorporating chemical species into the polymer matrix, which can mediate the electron transfer process. These mediators can be metal particles [8], enzymes [9] and doping ions [10]. Among the doping ions ferrocyanide was used as an ion incorporated into polymer films in order to, e.g. electrocatalysis of ascorbic acid oxidation [11] and simultaneously determination of dopamine and ascorbic acid [12]. Electropolymerization of N,N-dialkylaniline (DAA) for example dimethyl on electrode surface has been studied by many group [13–15]. The anion exchange property and electrode kinetics of metal incorporated into the polymer films were studied [13,14]. Also by using poly N,N-dimethylaniline (PDMA) the simultaneous detection of dopamine in the presence of ascorbic acid was carried out [16]. On the other hand, cysteine is of great importance in biological and biomedical processes. Being an important amino acid in pro- tein structures, cysteine participates in a number of biochemical processes which depend directly on the particular reactivity of thi- ols. Its oxidized derivatives have additional metabolic functions. Many methods for its determination have been reported including spectrophotometry [17–19], fluorimetry [20–22], electrophoresis [23], high performance liquid chromatography [24] and electro- chemical methods [25–27]. Of these, voltammetric techniques offer high sensitivity and selectivity for cysteine determination. However, at most conventional electrodes, the oxidation of cys- teine occurs at high positive potentials. Therefore, several chemi- cally modified electrodes (CMEs) have been designed for the electrocatalytic oxidation of cysteine [28–30]. Recently, we have prepared some modified carbon paste electrodes and used for elec- trocatalytic oxidation and determination of some biologically important compounds [31–33]. In the present paper, we prepared poly N,N-dimethylaniline (PDMA) film by the anodic oxidation of N,N-dimethylaniline (DMA) at the surface of carbon paste electrode (CPE). The PDMA is shown to be an ionene polymer with positively sites as quatern- ized ammonium groups in the polymeric backbone (Scheme 1). Therefore, the positively charged sites of this polymer film strongly attract the multiply charged anions, e.g. FeðCNÞ 4 6 by anion exchange with the supporting electrolytic anions, e.g. SO 2 4 initially 0022-0728/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2009.11.005 * Corresponding author. Tel.: +98 112 5242025 7; fax: +98 112 5242029/ 5242002. E-mail address: fer-o@umz.ac.ir (R. Ojani). Journal of Electroanalytical Chemistry 638 (2010) 241–245 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem