Reorientation of tetradecylmethyl viologen on gold upon coadsorption of decanethiol and its mediation of electron transfer to nitrate reductases

Decanethiol was coadsorbed with tetradecylmethyl viologen (C14MV; 1-Methyl- 1'-tetradecyl-4,4'-bipyridinium chloride) on gold electrodes to improve sta bility of C14MV as an electrochemical mediator for coupling to nitrate redu ctase enzymes. Surface-enhanced Raman spectroscopy (SERS) and in situ spect roscopic ellipsometry were used to monitor the structural properties of sur face-confined C14MV during its redox conversion. The potential range invest igated was limited to that of the first electron transfer to give V.+, as t he subsequent reduction to the neutral species is irreversible and could no t be used for electron-transfer mediation to redox enzymes. When C14MV was adsorbed by itself, in the absence of C14MV solution species, the in situ o ptical studies showed the loss of initial electroactivity was due to the bi pyridinium rings being oriented parallel to the electrode plane. This confi guration is thought to be unfavorable for the anion (Cl-) transport in and out of the film, which is essential for the redox reaction. The electroacti vity in the adsorbed film was restored by coadsorbing decanethiol (C10T) wi th C14MV. This gave an intercalated film with the end-on, bipyridinium ring oriented vertically relative to the electrode surface. In this film, the s maller methyl group is positioned closer to the electrode surface, and the bipyridinium electroactive groups are surrounded by longer decanethiol mole cules. Both SERS and spectroscopic ellipsometry measurements show the prese nce of radical dimers in reduced surface films. Intercalated C14MV is stabl e for several thousand voltammetry scans and was found to be an efficient e lectron-transfer mediator to soluble nitrate reductase, despite being embed ded in a decanethiolate layer.