Electrochemical Condensation of Methylviologen on Specifically‐adsorbed Anion Layers

The adsorption of methylviologen dications (MV2+) on single-crystalline Au electrodes in both H2SO4 and HClO4 was examined. MV2+ strongly interacted with sulfate and bisulfate anions adsorbed on the Au(111) electrode surface in 0.05 M H2SO4 under a controlled potential of 1.25 V vs. the reversible hydrogen electrode (RHE). A characteristic non-Faradaic current was observed at 1.10 V vs. RHE. When adsorption of MV2+ was carried out in 0.1 M HClO4, the electrochemical response of MV2+ was less than that obtained in H2SO4. The results show that the formation of a highly ordered sulfate/bisulfate adlayer plays an important role in the formation of condensed MV2+ layers. Examination of polycrystalline Au and Au(100) electrodes revealed a poor electrochemical response due to the surface roughness of the Au substrate, but the electrochemical detection was applicable to polycrystalline Au electrodes. A systematic investigation of the structural dependency of viologen derivatives showed that molecular size is important for electrostatic interactions with a highly ordered sulfate/bisulfate adlayer. The findings of the present study demonstrate successful detection of MV2+ at a concentration of <= 1 pM with a non-Faradaic current.