Observation of silver and hydrogen ion binding to self-assembled monolayers using chemically modified AFM tips

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 electrostatic repulsion between tips and substrates that both bear positive charges due to Ag⁺ binding. On the other hand, for amino- or 2-imidazolin-1-yl-terminated SAMs, adhesion forces and contact angles similarly decreased from pH 10 to 2. This decrease in adhesion force with increasing H⁺ concentration seems to reflect preferential formation of 1:1 complexes, resulting in repulsion between the positively charged tip and substrate surfaces. Adhesion due to the formation of 1:2 complexes was not observed for H⁺, which is consistent with the low stability of such complexes in solution. These results demonstrate that adhesion forces can be used to observe sandwich-type binding of metal ions between ligands immobilized on solid substrates and chemically modified AFM tips.