Selective binding of divalent cations at the surface of self-assembled monolayers of an aromatic bifunctional molecule studied on a quartz crystal microbalance

Abstract

Self-assembled monolayers (SAMs) of a small aromatic bifunctional molecule, 4-carboxythiophenol, formed on gold films have been used to selectively bind Cd²⁺ and Pb²⁺ ions at the surface of the SAMs. SAMs formed with this molecule on Au have a surface rich in carboxylic acid functionality which can be used to bind the above-mentioned cations through an ion-exchange process. The kinetics of ion exchange, as well as the pH dependence of the binding of Cd²⁺ and Pb²⁺ ions on the SAM surface, have been followed using quartz crystal microbalance measurements. It is found that, at low pH where the carboxylic acid groups are expected to be unionized, significant mass increase is observed. At high pH, the percentage of ion exchange is found to be twice that expected from charge neutrality constraints (0.5 Cd²⁺/Pb²⁺ ion per carboxylic acid group). This is tentatively explained as being due to incorporation of Cd(OH)⁺ and Pb(OH)⁺ ions. The titration curve obtained yields an equilibrium binding constant of ~5.3 for both ions, which is close to that obtained for these ions in Langmuir monolayers and Langmuir-Blodgett multilayer films.