Observation of Silver and Hydrogen Ion Binding to Self-Assembled Monolayers Using Chemically Modified AFM Tips

Abstract

Atomic force microscopy (AFM) was used to study binding of ionic species (Ag+ and H+) to planar substrates and AFM tips chemically modified with self-assembled monolayers (SAMs) formed with terminally functionalized organosiloxanes. Adhesion forces between tips and substrates modified with methylsulfanyl-terminated SAMs and immersed into aqueous Ag+ solutions increased with an increase in the Ag+ concentration up to ≈10 mM Ag+ and then decreased with a further increase in the Ag+ concentration. Contact angles on the same substrates continuously decreased as the concentration of Ag+ increased. The increase in adhesion force between methylsulfanyl groups with an increase in the Ag+ concentration up to ≈10 mM Ag+ can be explained by simultaneous binding of Ag+ to methylsulfanyl groups on the tip and on the substrate, resulting in 1:2 complexes. The decrease in the adhesion force at very high Ag+ concentrations likely reflects the formation of 1:1 complexes in the presence of an excess of Ag+, resulting in el...