Characterization of Charge Storage in Redox-Active Self-Assembled Monolayers

Abstract

Self-assembled monolayers (SAMs) of thiol-derivatized alkyl ferrocenes and Zn tetraarylporphyrins on a gold surface have been characterized under open circuit conditions using fluorescence imaging microscopy and a varietyof electrochemical methods. The fluorescence of the neutral form of the Zn tetraarylporphyrin SAM and the electrode cell potential are monitored simultaneously after oxidation of the SAM and subsequent establishment of an open circuit condition. These experiments capitalize on the fact that the neutral Zn tetraarylporphyrin SAM is fluorescent whereas emission from the oxidized Zn tetraarylporphyrin SAM is quenched. Accordingly, the fluorescence recovery after disconnection from an oxidizing potential provides an independent measure of the change in oxidation state of the porphyrin SAM as a function of time after the cell is open-circuited. This measurement demonstrates that the porphyrin SAM remains oxidized for an extended period (hundreds of seconds) even though the electrochemical cell potential decays rapidly to the open circuit potential (OCP). Cyclic voltammetry was used to confirm the electrochemical properties of the redox SAMs and to determine the surface coverage of the redox-active molecules. A new method, designated open circuit potential amperometry, was used to read the charge of the oxidized SAM at the OCP after charging currents have decayed away. Additionally, open circuit potential voltammetry yields a sigmoidal response with a characteristic half-wave potential identical to that observed with cyclic voltammetry, indicative of a Nernstian process. Collectively, these studies show that the electrochemical cell discharges to the OCP quickly, that the SAM remains oxidized under these conditions, and that these methods can be used to quantitatively determine the amount of stored charge in the SAM.