Effects of ion-pairing on rate of electron transfer between immobilized gold nanoclusters and soluble redox probes Daniel A. Lowy a,1 , Sulay D. Jhaveri a,1 , Edward E. Foos c,2 , Leonard M. Tender b, * , Mario G. Ancona d,3 , Arthur W. Snow c,2 a Nova Research, Inc., 1900 Elkin Street, Alexandria, VA 22308, USA b Center for Bio/Molecular Science and Technology, Code 6900, Naval Research Laboratory, Wasington, DC 20375, United States c Chemistry Division, Code 6123, Naval Research Laboratory, United States d Electronics Science and Technology Division, Code 6876, Naval Research Laboratory, United States Received 30 May 2006; received in revised form 29 June 2006; accepted 29 June 2006 Available online 8 September 2006 Abstract Hydrophilic gold nanoclusters were tethered onto gold electrodes modified with mixed 1-octane thiol/1,9-nonane dithiol monolayers. The heterogeneous electron transfer (ET) kinetics of soluble redox species in the supporting electrolyte were investigated at these elec- trodes by cyclic voltammetry (CV) in the presence and absence of the ion-pairing anions PF  6 and NO  3 . The redox species investigated, [Fe(CN) 6 ] 3/4 and [Co(C 12 H 8 N 2 ) 3 ] 3+/2+ where oppositely charged. The results presented here reveal that the rate of ET for the nega- tively charged redox species decreases with decreasing ionic charging time constant of the electrolyte. The opposite trend is observed for the positively charged redox species. Published by Elsevier B.V. Keywords: Gold nanocluster; Quantized charging; Mixed monolayer; Electron transfer kinetics; Blocked surface; Cobalt tris(phenanthroline) 1. Introduction Due to their optical and electronic properties, gold nanoclusters are widely recognized for potential value in nano-scale electronics, chemical sensing, and electro-optics applications [1–3]. Their most striking electrochemical property is non-linearity (the so called ‘‘Coulomb stair- case’’), which is evident in voltammetry as a series of cur- rent peaks evenly spaced in voltage (DV = e/C, where e is the electron charge and C is the capacitance of a single immobilized nanocluster). These peaks are attributed to stepwise charging of the clusters, a process in which addi- tion or removal of an additional electron per cluster requires application of a potential that is more reducing or oxidizing, respectively [4,5]. The electrochemical properties of gold nanoclusters encapsulated with various alkanethiol-based self assembled monolayers (SAMs) and immobilized onto electrode sur- faces has been extensively studied [6–11]. Here, 1.8 nm- diameter gold nanoclusters encapsulated with a SAM comprised of tri(ethylene oxide)thiol (HS(CH 2 CH 2 O) 3 CH 3 , referred to as EO3) were immobilized onto gold electrodes modified by a SAM comprised of a mixture of 1-octane- thiol (HS(CH 2 ) 7 CH 3 ) and 1,9-nonanedithiol, HS(CH 2 ) 9 SH (the mixed SAM referred to here as OT/NDT). A previous electrochemical characterization [12] of these immobilized gold clusters (abbreviated as AuEO3) revealed the follow- ing trend. The stepwise charging current was enhanced sig- nificantly by a large ion-pairing anion such as PF  6 and enhanced to lesser extent by a smaller ion-pairing anion such as NO  3 compared to the absence of an ion-paring anion in the electrolyte. The greater enhancement for 1388-2481/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.elecom.2006.06.029 * Corresponding author. Tel.: +1 202 404 6029; fax: +1 202 404 7946. E-mail address: Tender@NRL.NAVY.MIL (L.M. Tender). 1 Tel.: +1 202 404 6067. 2 Tel.: +1 202 767 5341. 3 Tel.: +1 202 404 7195. www.elsevier.com/locate/elecom Electrochemistry Communications 8 (2006) 1821–1824