The dynamics of electron self-exchange between nanoparticles

The rate of electron self-exchange reactions between discretely charged met al-like cores of nanoparticles has been measured in multilayer films of nan oparticles by an electrochemical method. The nanoparticles an Au monolayer- protected clusters with mixed monolayers of hexanethiolate and mercaptoun-d ecanoic acid ligands, linked to each other and to the Au electrode surface with carboxylate-metal ion-carboxylate bridges. Cyclic voltammetry of the n anoparticle films exhibits a series of well-defined peaks for the sequentia l, single-electron, double-layer charging of the 1.6-nm-diameter Au cores. The electron self-exchange is measured as a diffusion-like electron-hopping process, much as in previous studies of redox polymer films on electrodes. The average electron diffusion coefficient is D-E = 10(+/-5) X 10(-8) cm(2 )/S, With no discernible dependence on the state of charge of the nanoparti cles or on whether the reaction increases or decreases the core charge. Thi s diffusion constant corresponds to an average first-order rate constant k( HOP) of 2(+/-1) X 10(6) s(-1) and an average self-exchange rate constant, k (EX), of 2(+/-1) x 10(8) M-1 s(-1), using a cubic lattice hopping model. Th is is a very large rate constant, considering the nominally lengthy linking bridge between the Au cores.