Correlation of the Structural Decomposition and Performance of Pyridinethiolate Surface Modifiers at Gold Electrodes for the Facilitation of Cytochrome c Heterogeneous Electron-Transfer Reactions

Abstract

This paper describes the results of an electrochemical and spectroscopic (infrared reflection and X-ray photoelectron spectroscopies) investigation of the modified gold electrode surfaces prepared from dilute ethanolic solutions of 4-mercaptopyridine (PySH) and 4,4‘-dipyridyl disulfide (PySS). Both precursors have been used extensively as facilitators for the electron transfer of redox proteins like cytochrome c (cyt c). During the course of an investigation of the interfacial architectures formed from the two different precursors, a previously unreported structural instability in the adlayers was discovered. This instability manifests itself as a decrease in the ability of the modified surfaces to facilitate the electron transfer of cyt c that correlates with an increase of the immersion time in the precursor solutions. Results are presented that delineate the decrease in facilitator performance and probe the structural changes resulting in the decrease in performance. Together, the electrochemical and surface spectroscopic findings reveal that the modified surfaces spontaneously decompose to yield an adlayer composed largely of adsorbed atomic and oligomeric sulfur, an adlayer that we found to be ineffective in the facilitation of the electron transfer reaction of cyt c. The implications of these findings on the use of this type of modifier to studies of electron transfer reactions of redox proteins and to issues of the general structural stability of organosulfur-based monolayers are briefly discussed.