Electrochemical investigations into the binding of some nonredox active metal ions to surface-bound glutamic acid conjugates

A number of ferrocene glutamic acid cystamine conjugates suitable for the preparation of thin film on gold were prepared and fully characterized electrochemically and spectroscopically. In this study, we evaluate the interactions of nonelectroactive metal ions such as Ca2+ and Tb3+ ions using a combined electrochemical and spectroscopic approach. Cyclic voltammetry studies show a reversible one-electron redox behavior associated with the ferrocene group. These peptide interfaces exhibit a considerable affinity to bind Ca2+ and Tb3+ ions from aqueous solutions with log K similar to 3. In the presence of metal ion, the charge associated with oxidation of the ferrocene group decreases in line with a blockage of the formation of Fc(+)/ClO4- ion pair by the formation of an overlayer on top of the film, which essentially shuts down the oxidation of the Fc group. The resulting increase in the film thickness is monitored by X-ray photoelectron spectroscopy (XPS) and by a decrease in the capacitance as monitored by electrochemical impedance spectroscopy. XPS measurements show that the metal ions bind exclusively to the carboxylic acid and that the amide group is not involved in metal binding.