Reorientation of Tetradecylmethyl Viologen on Gold upon Coadsorption of Decanethiol and Its Mediation of Electron Transfer to Nitrate Reductase

Decanethiol was coadsorbed with tetradecylmethyl viologen (C 14 MV; 1-Methyl-1‘-tetradecyl-4,4‘-bipyridinium chloride) on gold electrodes to improve stability of C 14 MV as an electrochemical mediator for coupling to nitrate reductase enzymes. Surface-enhanced Raman spectroscopy (SERS) and in situ spectroscopic ellipsometry were used to monitor the structural properties of surface-confined C 14 MV during its redox conversion. The potential range investigated was limited to that of the first electron transfer to give V • +, as the subsequent reduction to the neutral species is irreversible and could not be used for electron-transfer mediation to redox enzymes. When C 14 MV was adsorbed by itself, in the absence of C 14 MV solution species, the in situ optical studies showed the loss of initial electroactivity was due to the bipyridinium rings being oriented parallel to the electrode plane. This configuration is thought to be unfavorable for the anion (Cl - ) transport in and out of the film, which is essential for the redox reaction. The electroactivity in the adsorbed film was restored by coadsorbing decanethiol (C 10 T) with C 14 MV. This gave an intercalated film with the end-on, bipyridinium ring oriented vertically relative to the electrode surface. In this film, the smaller methyl group is positioned closer to the electrode surface, and the bipyridinium electroactive groups are surrounded by longer decanethiol molecules. Both SERS and spectroscopic ellipsometry measurements show the presence of radical dimers in reduced surface films. Intercalated C 14 MV is stable for several thousand voltammetry scans and was found to be an efficient electron-transfer mediator to soluble nitrate reductase, despite being embedded in a decanethiolate layer.