Structural Changes of 4,4′-(Dithiodibutylene)dipyridine SAM on a Au(111) Electrode with Applied Potential and Solution pH and Influence of Alkyl Chain Length of Pyridine-Terminated Thiolate SAMs on Cytochrome <i>c</i> Electrochemistry

Abstract

From the standpoint of cytochrome c electrochemistry, it is important to understand the relationship between the electron transfer reaction and alkyl chain length of pyridine-terminated self-assembled monolayers (SAMs) at the molecular level. In the present study, SAMs of 4,4′-(dithiodibutylene)dipyridine ((PyC 4 S) 2 ) formed on a Au(111) surface were investigated in aqueous electrolyte solutions by cyclic voltammetry (CV), surface-enhanced infrared absorption spectroscopy (SEIRAS), and in situ scanning tunneling microscopy (STM). Structural changes in the PyC 4 S-SAM were driven by changes in the applied electrode potential or pH of the electrolyte solution. In 0.05 M HClO 4, a structural change in PyC 4 S-SAM was drastically induced by the potential manipulation from the open circuit potential to 0.10 V versus a reversible hydrogen electrode (RHE). A high-resolution STM image showed that the PyC 4 S-SAMs formed a p (12 × √3 R –30°) adlattice at ∼0.15 V versus RHE. A semisquarely arranged adlattice was newly formed at 0 V versus RHE. The reductive peaks that appeared in the CV profile were thus attributed to be a phase transition in the PyC 4 S-SAM. By changing the electrolyte in the solution from HClO 4 to KClO 4, PyC 4 S-SAM having a p (12 × √3 R –30°) lattice was changed to a p (8 × √3 R –30°) adlattice in 0.05 M KClO 4 at pH 5.5, indicating that rearrangement of PyC 4 S-SAM took place. This difference in the adlayer structure between acidic and neutral solutions resulted from the protonation and deprotonation of the pyridine moieties in PyC 4 S-SAMs. Electrochemical investigations on the electron transfer reaction of cytochrome c revealed that the alkyl chain length in a Py-terminated thiolate SAM plays an important role in the heterogeneous electron transfer rate constant, k o ′, but not in the diffusion coefficient, D, of cytochrome c . These results show that the structure of PyC 4 S-SAM can be controlled by changing the applied electrode potential and pH of the electrolyte solution. In addition, the effect of alkyl chain length in the Py-terminated SAM on cytochrome c electrochemistry is clearly demonstrated. Long-range electron transfer kinetics is discussed on the basis of the Marcus theory.