Chemical manipulations of nanoscale electron transfers

The recent progress in nanoscale electron transfers is reviewed. Here, we focus on quantized capacitance charging of monolayerprotected nanoparticle molecules. For gold nanoparticles larger than 1.6 nm in diameter, this novel charge transfer phenomenon is represented by a series of voltammetric peaks that are evenly separated around 0 V; whereas for smaller-sized particles, uneven spacings of the discrete charging peaks are observed. Additionally a sizeable bandgap starts to evolve with decreasing particle core size, due to the quantum size effect. This bandgap can be further manipulated by the interactions between the particle core and surface ligands. When nanoparticles are immobilized onto electrode surfaces by bifunctional chemical linkages, one system that is of particular interest is the observation of ion-induced rectification of nanoparticle quantized charging in aqueous media in the presence of hydrophobic electrolyte ions. This is interpreted on the basis of the ion-pair formation between "soft" electrolyte ions and particle molecules which leads to the variation of the electrode interfacial double-layer capacitance (Randles equivalent circuit). Further control of nanoscale electron transfers can be accomplished by magnetic and photochemical interactions. Fundamentally, these studies offer a rare glimpse of the molecular origin and mechanistic regulation of electron transfers at nanoscale interfaces. (C) 2003 Elsevier B.V. All rights reserved.