Interfacial Investigations of a Biological Electron Transfer Model: Cytochrome c Adsorbed on Gold Electrodes Modified with Self-Assembled Monolayers

Abstract

Interfacial investigations of a protein monolayer electrochemical system, cytochrome c (cyt c) adsorbed to a carboxylic acid (COOH) terminated self-assembled monolayer (SAM) were undertaken. Previous research suggested that anomalous peak broadening observed in the voltammetry of cyt c may be the manifestation of surface effects at the SAM/solution interface (heterogeneous adsorption sites). To examine this matter further, research was directed at 1) deciphering the role of the gold substrate's topography in both SAM formation and cyt c voltammetry; 2) understanding the protein binding interactions at the SAM/solution interface that influence cyt c adsorption and electrochemical response, and 3) investigating the microscopic properties of all the surfaces involved. Electrochemical and scanning probe microscopy techniques were used to explore the influence of gold topography in cyt c / COOH SAM / Au systems. COOH SAMs (11-mercaptoundecanoic acid and 14-mercaptotetradecanoic acid) were prepared and characterized on a variety of gold surfaces including evaporated, bulk, single crystal, and expitaxially grown on mica gold substrates. Each type of gold surface exhibited specific topographical features and characteristic roughness. SAMs were found to have a decreasing number of defects as the topography of the gold became smoother, as evidenced by an increased ability to block solution probe molecules. As the SAMs become less defective on the smoother gold, the extent of adsorption and the magnitude of the electrochemical response of adsorbed cyt c decreased significantly. These results show cyt c adsorption and electrochemistry to be intimately related to the density of defects in the SAM, which, in turn, are heavily influenced by the gold topography. Additionally, as the gold roughness decreased, the double layer capacitance of the films was observed to increase. A physical model was proposed in which the structure and properties of COOH SAMs are dictated by significant endgroup interactions in addition to chain-chain interactions. The model illustrates how gold topography plays an intricate role in determining the structure and application of COOH terminated SAMs. Research was also performed on the SAM/solution interface by thermally healing SAMs on gold and utilizing SAMs on Ag-UPD modified gold. Thermal healing, by reducing the number of defects in the SAMs, was also found to affect both SAM structural properties and cyt c adsorption. SAMs that had been thermally healed exhibited a lower density of defects while, at the same time, supported lower electroactive cyt c coverage. Ag UPD layers were tested as a means of creating more stable, less defective COOH SAMs for cyt c immobilization and possibly allowing for more ideal voltammetry of the proteins. Preliminary research has shown that SAMs with Ag-UPD layers have fewer defects and greater inherent stability. Finally, scanning probe microscopy techniques were employed to investigate the structure of the gold substrate, the SAM/solution interface, and adsorbed cyt c. In addition to cyt c / SAM / Au research, results from the following investigations are presented: electrochemistry of adsorbed cyt c at indium tin oxide electrodes and the electron transfer (ET) properties of iron-sulfur metallodendrimers. The metallodendrimers exhibited attenuated ET properties with increasing generations of dendritic ligands in a solventless, polymeric media. The electrochemistry of Cu(II) in physiological pathways that may be related to neurological and ocular diseases was also explored. Research on these systems revealed that the reduction of Cu(II) by certain peptides and catabolites may play a vital role in the development of these diseases.