Impedance spectroscopy of self-assembled monolayers on Au(111): Sodium ferrocyanide charge transfer at modified electrodes

The Faradaic impedance of a Au(lll) electrode modified by three different self-assembled monolayers has been studied over a wide frequency range in aqueous NaClO4 solutions in the presence of 1 mM [Fe(CN)(6)](4-/3-). The impedance characteristics of the bare electrode are compared with those of the same electrode modified with decanethiol, omega-hydroxydecanethiol, and 4'-hydroxy-4-mercaptobiphenyl. The 4'-hydroxy-4-mercaptobiphenyl molecule is susceptible to pi-pi interactions and is of special interest as a mediator of electron transfer. In the case of the decanethiol and omega-hydroxydecanethiol systems the interfacial impedance can be modelled by the Randles circuit. However, for the 4'-hydroxy-4-mercaptobiphenyl system, the impedance behavior is more complex with a resistance in the monolayer parallel to and distinguishable from the expected Faradaic impedance. This behavior is quite different from that discussed to date in the literature and assumed in the interpretation of voltammetric data for modified electrodes. The parameters for these systems are reported here as a function of the type of electrode modification. The consequences of the present observations are discussed with respect to the phenomena observed.