Electrochemical impedance spectroscopy and the quartz crystal microbalance measurements are used to examine the ability of potential applied to a substrate to create, in situ, conditions favorable for the electrostatic deposition of polyelectrolytes onto a gold Substrate modified by the self-assembly of cysteine. Cysteine is a zwitterionic compound that, when confined to a substrate, has the ability to establish either a net positive or a net negative interfacial charge, depending on the conditions. As Such, cysteine modified interfaces Could possibly be used as a versatile Substrate for deposition of either cationic or anionic polyelectrolytes. The potential of zero charge of a gold electrode modified by self-assembly with cysteine in the presence of 0.10 mol L-1 KCl and buffered at pH 5 is found by differential capacitance measurement to be -0.12(+/- 0.02) V vs. Ag/AgCl. When -0.05 V vs. Ag/AgCl is applied to the substrate (a potential positive of the PZC) in the presence of different polyelectrolytes, both impedance spectroscopy and quartz crystal microbalance data Suggest the accumulation of anionic poly(sodium styrenesulfonate) along the cysteine modified interface. Conversely, when -0.40V vs. Ag/AgCl is applied to the Substrate (a potential negative of the PZC), experimental results suggest the accumulation of cationic poly(dially-dimethylammonium chloride). (C) 2009 Elsevier Inc. All rights reserved.