Cantilever-based optical interfacial force microscope in liquid using photothermal actuation.

We developed a novel cantilever based optical interfacial force microscope (COIFM) that employs photothermal actuation of a silicon cantilever and an optical detection method to establish the measurement of the single molecular interactions in liquid environments using the force feedback technique. The newly developed COIFM employs photothermal actuation, where the difference in thermal expansion between silicon and the cantilever's metallic coating induces bimetallic bending. The force resolution is determined to be ∼1.5 pN from the standard deviation measured in deionized water when the probe is off the surface. The performance of the COIFM was then demonstrated by measuring the force as a function of the distance between the probe and the surface. Both force-distance curves taken on approach and retraction showed an attractive force (-50 pN) at a distance of 30 nm from the surfaces. The direct measurement of force-distance curves using the photothermal COIFM has demonstrated that the COIFM is capable of unveiling structural and mechanical information at the single molecular level across the overall distances between two surfaces in a liquid environment.