Permselectivity and High Sensitivity at Ultrathin Monolayers. Effect of Film Hydrophobicity

Abstract

The effect of monolayer structure and solution composition on electrochemical response at ultrathin monolayers formed by self-assembly has been investigated. The monolayers studied consisted of thioctic acid (1,2-dithiolane-3-pentanoic acid) and 1-hexanethiol, where different degrees of surface hydrophobicity were obtained through the coassembly of these two molecules. On the more hydrophilic thioctic acid monolayers, fast kinetics and high electrochemical sensitivity were obtained for hydrophilic probes Fe(CN) 6 3- and Ru(NH3) 6 3+ . Permselectivity at the hydrophilic monolayer electrodes could be achieved by controlling the extent of dissociation of the monolayer COOH head groups. As the hydrophobicity of the film increased with the coadsorption of hexanethiol, the kinetics of the hydrophilic probes became slower. For more hydrophobic catecholamines and quinone probes, slower kinetics (lower sensitivity) were consistently observed at the monolayer electrodes, independent of the monolayer composition. A simple modified Stern model is proposed to represent the relationship between the probe response, the monolayer structure, and the electrolyte composition. The role of the Donnan potential in controlling film permselectivity is addressed, and the roles of monolayer stability and substrate quality on the monolayer electrode response are discussed.