Rectifying Nanoscale Electron Transfer by Viologen Moieties and Hydrophobic Electrolyte Ions

The rectifying effects of electrolyte ions on interfacial electron transfers were investigated with a gold nanoparticle monolayer anchored by bifunctional chemical bridges with viologen moieties. Gold nanoparticle monolayers were fabricated by using a sequential anchoring mechanism with viologen dithiols as the chemical linkers. For viologen monolayers, the voltammetric currents were found to be sensitive to electrolyte ions. Due to the ion-pair formation between the bipyridinium moieties and hydrophobic anions, the electron-transfer kinetics of the viologen functional groups seems impeded by the increasing concentration of “soft” ions, leading to the diminishment of the faradaic currents. This response to electrolyte ions is in great contrast to that of gold nanoparticle monolayers, where the quantized charging currents become better-defined with increasing concentration of hydrophobic ions. By hybridizing these conventional and nanoscale redox-active entities into the interfacial organized structure, one can manipulate interfacial charge transfer, where the voltammetric responses behave analogously to Coulomb blockade with the currents rectified at either negative or positive electrode potentials, depending on the nature of the electrolyte ions.