Redox Site-Mediated Charge Transport in a Hg#SAM//Ru(NH3)63+/2+//SAM#Hg Junction with a Dynamic Interelectrode Separation: Compatibility with Redox Cycling and Electron Hopping Mechanisms

Abstract

This paper describes the formation and electrical properties of a new Hg-based metal−molecules−metal junction that incorporates charged redox sites into the space between the electrodes. The junction is formed by bringing into contact two mercury-drop electrodes whose surfaces are covered by COO^{−}-terminated self-assembled monolayers (SAMs) and immersed in a basic aqueous solution of Ru(NH_3)_6Cl_3. The electrical behavior of the junction, which is contacted at its edges by aqueous electrolyte solution, has been characterized electrochemically. This characterization shows that current flowing through the junction on the initial potential cycles is dominated by a redox-cycling mechanism and that the rates of electron transport can be controlled by controlling the potentials of the mercury electrodes with respect to the redox potential of the Ru(NH_3)_6^{3+/2+} couple. On repeated cycling of the potential across the junction, the current across it increases by as much as a factor of 40, and this increase is accompanied by a large (>300 mV) negative shift in the formal potential for the reduction of Ru(NH_3)_6^{3+}. The most plausible rationalization of this behavior postulates a decrease in the size of the gap between the electrodes with cycling and a mechanism of conduction dominated by physical diffusion of Ru(NH_3)_6^{3+/2+} ions (at larger interelectrode spacing), with a possible contribution of electron hopping to charge transport (at smaller interelectrode spacing). In this rationalization, the negative shift in the formal potential plausibly reflects extrusion of the solution of electrolyte from the junction and an increase in the effective concentration of negatively charged species (surface-immobilized COO^{−} groups) in the volume bounded by the electrodes. This junction has the characteristics required for use in screening and in exploratory work, involving nanogap electrochemical systems, and in mechanistic studies involving these systems. It does not have the stability needed for long-term technological applications.