Journal of Electroanalytical Chemistry 516 (2001) 10 – 16 Characterisation of gold electrodes modified with self-assembled monolayers of L-cysteine for the adsorptive stripping analysis of copper Wenrong Yang, J. Justin Gooding, D. Brynn Hibbert * School of Chemistry, The Uniersity of New South Wales, Sydney, NSW 2052, Australia Received 24 June 2001; received in revised form 29 August 2001; accepted 10 September 2001 Abstract Electrochemical sensors for copper ions in environmental samples were prepared by modifying gold electrodes with L-cysteine by self-assembly. The adsorption of L-cysteine on gold electrodes was studied by electrochemical reductive desorption in 0.5 M KOH, and the interaction of L-cysteine with copper ions was investigated by cyclic voltammetry, chronoamperometry and X-ray photoelectron spectroscopy. At low concentrations the ratio of L-cysteine to bound Cu(II) is 2:1. At higher concentrations (0.1 M) copper reacts with adsorbed cysteine forming copper sulfide on the electrode surface. On a modified L-cysteine gold electrode, Osteryoung square wave voltammetric determination of Cu(II) with a detection limit below 5 ppb has been demonstrated. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Self-assembled monolayers; L-Cysteine modified gold electrode; Cu(II) analysis; XPS; Adsorptive stripping voltammetry www.elsevier.com/locate/jelechem 1. Introduction Self-assembled monolayers (SAMs) of organic thiols on gold surfaces have attracted considerable interest lately both from the fundamental point of view [1–3] and from their analytical prospects [4 – 6]. The advan- tages of SAMs include their ease of preparation, their stability and the possibility of introducing different chemical functionalities. The incorporation of the ap- propriate chemical functionality with some molecular level control into the highly ordered monolayers allows the preparation of surfaces with tailor-made properties. This interface is typically prepared over a noble metal surface, which has the potential to act as an electrode for the transduction of the recognition event; thus producing an electrochemical sensor. Much of the liter- ature related to using SAMs for the preparation of electrochemical sensors has concentrated on using bio- logical recognition elements [7–11] and some exquisitely integrated biological interfaces with analyti- cal capabilities have been produced [12 – 16]. There have been, however, surprisingly few examples of solid state metal ion sensors, which exploit biological recognition elements such as amino acids and peptides [17]. One of the earlier examples of an organised mono- layer that selectively binds redox metal ions was de- scribed by Rubinstein et al. who used mixed monolayers consisting of self-assembled tetradentate ligand receptors and passivating molecules [18]. Cyclic voltammetry was used to show that the resulting mono- layer selectively formed a 1:1 complex with Cu 2 + on the surface of the electrode with no interference from Fe(III) that was also present. Detection limits at ppt levels have been reported by Turyan and Mandler handle using gold electrodes modified with a SAM of HSC 2 H 4 COOH for the detection of Cd(II) [19] and electrodes modified with 4-(mercapto-n -alkyl) pyri- dinium monolayer for the detection of Cr(VI) from aqueous solutions [20]. Flink et al. have extended the ion recognition to electrochemically inactive cations using impedance spectroscopy [21]. In this approach self-assembled monolayers of crown ethers were pre- * Corresponding author. Tel.: +61-2-93854713; fax: +61-2- 93856141. E-mail address: b.hibbert@unsw.edu.au (D. Brynn Hibbert). 0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0022-0728(01)00649-0