Demonstration of high-resolution capability of chemical force titration via study of acid/base properties of a patterned self-assembled monolayer

An experimental approach for probing the resolution of chemical force titration is reported here. A self-assembled monolayer (SAM) patterned with COOH and CH₃ groups was used as the model surface, and its local dissociation property was studied by both chemical force titration and contact angle titration. The dissociation constant (pK_1/2) estimated by chemical force titration was found to be sensitive to the surface location. In the COOH region, chemical force titration gave a pK_1/2 value of 5.4, identical with the value obtained on a pure, unpatterned COOH SAM, while in the CH₃ region, the force curves varied greatly from site to site even in the same pH solution, indicating the mixed film nature being originated from the microcontact printing process. In contrast, contact angle titration generates a fixed pK_1/2 value of 11.0 on the patterned surface, completely different from the force titration results. This study demonstrates that chemical force titration indeed has a spatially resolved capability, with a lateral spatial resolution of better than 1 μm, and is more effective for detecting the local properties of chemically inhomogeneous surfaces.