title: Scanning electrochemical microscopy determination of organic soluble MPC electron-transfer rates.
abstract: In this paper, we describe a novel method for measuring the forward heterogeneous electron-transfer rate constant (kf) through the thiol monolayer of gold monolayer protected clusters (MPCs) in solution using scanning electrochemical microscopy (SECM). Applying the equations for mixed mass-transfer and electron-transfer processes, we develop a new formula using only the diffusion coefficient and the tip radius and use it as part of a new method for evaluating SECM approach curves. This method is applied to determine the electron-transfer rates from a series of SECM approach curves for monodisperse hexanethiol MPCs and for polydisperse hexanethiol, octanethiol, decanethiol, dodecanethiol, and 2-phenyethylthiol gold MPCs. Our results show that as the alkanethiol length increases the rate of electron transfer decreases in a manner consistent with the previously proposed tunneling mechanism for the electron transfer in MPCs. However, the effective tunneling coefficient, Beta, is found to be only 0.41 A-1 for alkanethiol passivated MPCs compared to typical values of 1.1 A-1 for alkanethiols as self-assembled monolayers on two-dimensional gold substrates. Similar SECM approach curve results for Pt and Au MPCs indicate that the electron-transfer rate is dependent mostly on the composition of the thiol layer and not on differences in the core metal.
authors.affiliation: Department of Chemistry, Vanderbilt University, VU Station B 351822, Nashville, Tennessee 37235-1822, USA.
authors.forename: Rachel R
authors.initials: RR
authors.lastname: Peterson
authors.forename: David E
authors.initials: DE
authors.lastname: Cliffel
mesh_terms: D004563:Electrochemistry; D004583:Electrons; D006046:Gold / Q000737:chemistry; D007700:Kinetics; D008567:Membranes, Artificial; D016252:Microscopy, Scanning Tunneling / Q000379:methods*; D012680:Sensitivity and Specificity; D013438:Sulfhydryl Compounds / Q000032:analysis*; D013499:Surface Properties
chemical_list: D008567:Membranes, Artificial; D013438:Sulfhydryl Compounds; D006046:Gold