Electron and ion transport through multilayers of Au nanoclusters covered by self-assembled monolayers

Two types of gold nanoclusters (GNCs), one covered by self-assembled monolayers of mercaptoundecanoic acid (MUA), hexanethiol (C6SH), and ferrocenylhexanethiol (FcC6SH), MHF-GNC, and the other with MUA and C6SH, MH-GNC, were used for the construction of ten GNC layers of two different sequences on an MUA modified Au(1 1 1) surface based on the carboxylate/polycation (poly(allylamine hydrochloride), PAH)/carboxylate electrostatic interaction. MHF-GNC was placed either as the layer closest to the gold electrode, i.e. the first layer, or as the outermost layer with MH-GHC in the other layers. A quasi-reversible redox peak with a constant charge corresponding to the redox reaction of the ferrocene moiety of the MHF-GNC monolayer was observed at both electrodes, showing electrons and perchlorate ions could be transferred through the MH-GNC/PAH multilayers. Cross-sectional transmission electron microscopy (TEM) revealed that the size of the immobilized GNC was almost same as that in solution and well-separated GNCs were dispersed rather uniformly within the polymeric matrix. The incorporation of perchlorate ion upon the oxidation of the ferrocene moiety of the MHF-GNC was demonstrated by electrochemical quartz crystal microbalance measurement. Based on the above results, the charge transfer mechanism in the GNC multilayers was discussed.