ORIGINAL PAPER Sheela Berchmans Æ R. G. Nirmal Æ G. Prabaharan A. K. Mishra Æ V. Yegnaraman Solution phase electron transfer versus bridge mediated electron transfer across carboxylic acid terminated thiols Received: 27 March 2005 / Revised: 19 April 2005 / Accepted: 11 May 2005 / Published online: 15 July 2005 Ó Springer-Verlag 2005 Abstract Self-assembled monolayers (SAMs) of thiols with carboxylic acid terminal groups were formed on gold substrates. The electron transfer characteristics of redox species on the above SAM-modified electrodes were studied in acid and neutral media with the help of voltammetry under two different conditions: (1) solution phase electron transfer and (2) bridge mediated electron transfer. Two redox systems, viz., [Fe(CN) 6 ] 4-/3 and Ru[(NH 3 ) 6 ] 2+/3+ were chosen for the solution phase study. Investigations of bridge mediated electron trans- fer were carried out by functionalising the SAM with redox moieties and then studying their redox behaviour. For this study, ferrocene carboxylic acid and 1,4-dia- mino anthraquinone were used and they were linked to carboxylic acid terminated thiols by covalent linkage. The voltammetric results with mercaptoundecanoic acid SAM demonstrate the difference in behaviour between solution phase and bridge mediated electron transfer processes. Keywords Self-assembled monolayers (SAM) Æ Functionalised thiols Æ Redox systems Æ Electron transfer kinetics Æ Electron tunnelling Introduction Substantial research efforts are being directed towards the miniaturisation of devices to nanoscale dimensions [1–4]. Construction of nanoscale devices involves the chemical modification of surfaces with functionalised monolayers achieved through different routes such as self-assembly, electrostatic interactions, covalent link- age etc [5]. One of the popular surface derivatisation procedure is the molecular self-assembly because of its simplicity, versatility and establishment of high level order on a molecular scale [6]. Functionalisation of self-assembled monolayers (SAM) on electrode sur- faces has yielded molecular assemblies exhibiting diode like electron transfer [7, 8] and interfaces showing sensing [9, 10] and catalytic [11] characteristics. Other applications include electron transfer kinetics [12–17], lithography [18] and electronic devices [19, 20]. The formation of highly ordered monolayers is often accomplished by the spontaneous adsorption of n-al- kane thiols, n-alkyl silanes or their derivatives to the metal or metal oxide surfaces. Molecular layers formed by the self-assembly method are invariably more stable than those formed by the Langmuir-Blodgett (LB) method because the interaction between the adsorbed molecules and the substrate is chemisorption in the former unlike physisorption in the latter. The study of electron transfer (ET) kinetics across such monolayers is of considerable importance as follows. The study of ET blocking properties in monolayers and mixed monolayers will help us to build pore free impervious monolayers, which is an essential prerequisite to de- velop diode-like interfaces. The study of bridge medi- ated ET will lead to design of thin film sensors and catalysts. Layer-by-layer assembly of redox species/ thiol or metal/thiols will lead to formation of super lattice and wire like structures [21, 22] (S. Berchmans et al, unpublished results). In this communication, investigations of ET kinetics across SAM with car- boxylic acid terminal groups and their functionalised units with redox species have been undertaken. Car- boxylic acid terminated thiols are particularly chosen for our investigations as they offer the possibility of further functionalisation with redox species, enzymes, catalysts etc. The ET mediated by the SAM molecular bridges with the redox species in solution and redox molecules covalently linked to these bridges are S. Berchmans Æ R. G. Nirmal Æ G. Prabaharan V. Yegnaraman (&) Central Electrochemical Research Institute, Karaikudi 630006, India E-mail: vyegna@rediffmail.com Tel.: +91-4565-226204 Fax: +91-4565-227779 A. K. Mishra Institute of Mathematical Sciences, Chennai 600113, India J Solid State Electrochem (2006) 10: 439–446 DOI 10.1007/s10008-005-0011-0