Direct electron transfer of copper–zinc superoxide dismutase (SOD) on crystallographically oriented Au nanoparticles Mohamed S. El-Deab 1 , Takeo Ohsaka * Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received 11 October 2006; received in revised form 26 October 2006; accepted 27 October 2006 Available online 28 November 2006 Abstract The direct electron transfer (ET) of copper–zinc superoxide dismutase (SOD) has been realized, for the first time, at Au nanoparticles electrodeposited onto GC (nano-Au/GC) electrodes. Electrodeposition of Au nanoparticles in the presence of some additive (typically cysteine) resulted in the fabrication of Au nanoparticles with a high roughness morphology (enriched in the Au(1 0 0) orientation) and thus providing a favorable adsorption orientation of SOD (key–lock interaction) suitable for a facilitated direct ET without the use of mediators or so-called ET promoters. The redox reaction of the SOD confined on the nano-Au/GC electrode was found to have a formal potential of +0.02 V vs. Ag/AgCl/KCl(sat). Whereas, round-shape spherical plumbs of Au nanoparticles were electrodeposited in the presence of iodide ions (as additive) which lack for the favorable rough surface and consequently the suitable interaction with the SOD is missing and hence the ET is not realized at this surface. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Cu–Zn-superoxide dismutase; Gold nanoparticles; Electrodeposition; Superoxide anion; Charge transfer; Crystallographic orientation 1. Introduction Superoxide dismutases (SODs) are vital metallo- enzymes in all oxygen-tolerant living organisms. They pro- vide an efficient protection against the harmful effects of the in situ generated superoxide anions by catalyzing its dis- mutation into O 2 and H 2 O 2 via the redox reaction of the Cu complex moiety (Cu 2+ /Cu + ) of the SOD [1–6]. The electrochemical studies on electron transfer of SODs have received an increasing attention in view of its importance in the understanding of the intrinsic thermodynamic and kinetic properties of SODs [1–7]. This would enable the development of the so-called SOD-based third generation biosensors for the superoxide ions [8]. A facilitated electron transfer of the SODs is observed when the SOD enzyme is confined on a thiol (e.g., cysteine) self-assembled mono- layer (SAM) on the Au substrate [4,9–11]. In this case, the cysteine-SAM acts as an electron transfer promoter [4]. Also the electron transfer properties of polyethylene oxide-modified SOD could be successfully achieved at a glassy carbon (GC) electrode in the presence of methyl viologen as a redox mediator [12]. To the best of our knowledge, the direct electron transfer of SOD was not observed at bare Au electrode. Only an irreversible oxida- tion peak of Cu–Zn-SOD has been observed in phosphate buffer solution of pH 4.0 [13], suggesting a conformational change of the active sites of the SOD molecules via its adsorption on the Au electrode surface [13]. Wang et al. [14] demonstrated the electrochemical behavior of Cu– Zn-SOD on Hg electrode and attributed the voltammetric peaks at À642 and À980 mV (vs. SCE) to the reversible one-electron reduction of Cu 2+ and an irreversible two- electron reduction of Zn 2+ , respectively. Whereas, Tian et al. [4] attributed the redox reaction obtained at 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.10.054 * Corresponding author. Tel.: +81 45 924 5404; fax: +81 45 924 5489. E-mail addresses: msaada68@yahoo.com (M.S. El-Deab), ohsaka@ echem.titech.ac.jp (T. Ohsaka). 1 Permanent address: Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt. www.elsevier.com/locate/elecom Electrochemistry Communications 9 (2007) 651–656