Distance-dependent electron transfer at passivated electrodes decorated by gold nanoparticles

The phenomenon of nanoparticles attached to an electrode passivated by an organic layer allowing efficient electron transfer between redox species in solution and the underlying electrode to be restored has resulted in Chazalviel and Allongue proposing a theory [Chazalviel, J.-N.; Allongue, P. J. Am. Chem. Soc.2011, 133, 762-764] to explain this phenomenon. The theory suggests that with electrode-organic layer-nanoparticle constructs, high exchange current densities, compared with when the nanoparticles are absent, results in the rate of electron transfer being independent of the thickness of the organic layer until a threshold thickness is exceeded. Thereafter, the thicker the organic layer, the slower the rate of electron transfer. Herein we provide the first experimental data to support this theory using a single experimental system that can show the transition from thickness independent electron transfer kinetics to distant dependent kinetics. This was achieved using ethylenediamine electrodeposited on a glassy carbon electrode. Different numbers of deposition cycles were applied in order to fabricate different thicknesses of the organic film. The deposited films showed progressively greater blocking abilities toward ruthenium hexamine, as a redox active probe in solution, as the films got thicker. Electron transfer kinetics of nanoparticle-decorated surfaces showed a change from thickness independent to thickness dependent as the organic layer exceeded an average thickness of 20 Å. Electrochemical impedance spectroscopy, cyclic voltammetry, scanning electron microscopy, ellipsometry, and atomic force microscopy were used to characterize the fabricated surfaces.