PERMSELECTIVITY AND HIGH-SENSITIVITY AT ULTRATHIN MONOLAYERS - EFFECTOF FILM HYDROPHOBICITY

The effect of monolayer structure and solution composition on electroc hemical response at ultrathin monolayers formed by self-assembly has b een investigated. The monolayers studied consisted of thioctic acid (1 ,2-dithiolane-3-pentanoic acid) and 1-hexanethiol, where different deg rees 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(NN3)6(3+). Permselectivity at t he hydrophilic monolayer electrodes could be achieved by controlling t he extent of dissociation of the monolayer COOH head groups. As the hy drophobicity of the film increased with the coadsorption of hexanethio l, the kinetics of the hydrophilic probes became slower. For more hydr ophobic catecholamines and quinone probes, slower kinetics (lower sens itivity) were consistently observed at the monolayer electrodes, indep endent 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, an d the roles of monolayer stability and substrate quality on the monola yer electrode response are discussed.