Chemical force titration of conjugated pyridyl group-terminated self-assembled monolayers

Chemical force microscopy (CFM) was used to follow the dissociation process of 4-mercapt-opyridine (4-MP) self-assembled monolayers (SAMs) on gold. The force probe was an Au-coated Si3N4 tip modified with 4-MP SAM. The adhesion force between tip and substrate was found to be a function of solution pH. The pK1/2 value of surface pyridyl groups of 4-MP SAM obtained from force titration curve was 5.4, remarkably different from the conventional contact angle titration result (pK1/2=3.3). This difference is believed to arise from the difference in spatial resolution of two approaches. The force titration would probe the local dissociation properties of surface groups, while the contact angle titration reflects the average dissociation properties underneath the water droplet