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 titra tion is reported here. A self-assembled monolayer (SAM) patterned with COOH and CH3 groups was used as the model surface, and its local dissociation p roperty was studied by both chemical force titration and contact angle titr ation. The dissociation constant (pK(1/2)) estimated by chemical force titr ation 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, th e force curves varied greatly from site to site even in the same pH solutio n, 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 titra tion indeed has a spatially resolved capability, with a lateral spatial res olution of better than 1 mu m, and is more effective for detecting the loca l properties of chemically inhomogeneous surfaces.