Chemical force microscopy using functionalized ZnO whisker probe tips

We have developed an atomic force microscope-based technique utilizing a zinc oxide whisker crystal as a probe tip. This technique was used for measuring interactions between the chemically modified tip and oxidized silicon substrates as a function of solution pH. Surfaces terminating in amine and methyl functional groups were prepared by covalently modifying the silicon substrate with self-assembled monolayers of (3-amino-propyl)triethoxysilane and n-butyltrichlorosilane, respectively. Patterned samples prepared by microcontact printing consisted of amine-terminated square regions surrounded by a methyl-terminated background. The contrast in lateral force images of the patterned samples obtained with amine-functionalized tips was seen to strongly depend on solution pH. High friction was observed between the attractive and strongly interacting functional groups in the methyl-terminated regions at low pH values and the amine-terminated regions at intermediate pH values. Low friction was observed between repulsive and weakly interacting functional groups. In addition, interacting force measurements on approach showed long-range attractive forces between the amine-terminated surfaces at intermediate pH values. Adhesive force measurements showed a dependency on the state of ionization of the amine groups, which was controlled by varying the pH. The pKa value estimated from the measurements was shifted to a lower pH than that of primary amine groups in solution. These results show that functionalized ZnO whisker probe tips have great potential for chemically sensitive imaging.