Long-range electronic communication between metal nanoparticles and electrode surfaces separated by polyelectrolyte multilayer films

The dynamics of electron transfer across An electrodes modified by ultrathin polyelectrolyte multilayers (PEM) and a diluted monolayer of An nanoparticle was investigated as a function of the film thickness. An electrodes were sequentially modified by a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA), followed by alternate adsorption Of poly-L-lysine (PLL) and poly-L-glutamic acid (PGA) layers. Submonolayer coverage of citrate stabilized 19.2 +/- 2.1 nm An nanoparticles was achieved by electrostatic adsorption on PLL terminated surfaces. In the absence of nanoparticles, cyclic voltammetry and electrochemical impedance spectroscopy of the hexacyanoferrate redox probe showed that the charge-transfer resistance is independent of the number of adsorbed polyelectrolyte layers. These results revealed that the redox species can penetrate the PEM film and the electrochemical responses are controlled by the electron tunneling across the initial monolayer of MUA. The phenomenological charge-transfer resistance decreased by more than 2 orders of magnitude upon adsorption of the An nanoparticles. Normalization of the electrochemical responses with the number density of particles revealed that the PEM thickness introduces insignificant effects on the charge-transfer resistance. The effective distance independent electron-transfer kinetic was observed for film thickness up to 6.5 nm. Furthermore, in situ atomic force microscopy studies show that the Au nanoparticles do not introduce measurable local deformation (compression) of the PEM films. The unique long-range electronic communication in this system is interpreted in terms of a resonant transport process involving the density of states of trapped redox species at the redox Fermi energy.