Non-contact scanning probe microscopy with sub-piconewton force sensitivity

Abstract

Aiming at detecting and visualizing weak interaction forces between biological macromolecules in non-contact mode, we have refined scanning probe microscopy to have approximately 100-fold higher sensitivity than conventional atomic force microscopy (AFM). The essential features of this system are that the force sensitivity is in the sub-piconewton range and the gap distance between interacting stylus and the sample surfaces can be controlled with nanometer accuracy. We have achieved these technical improvements by using flexible handmade cantilevers with the spring constant of approximately 0.1 pN nm −1. Thermal bending motions have been reduced to less than 1 nm in root-mean-square amplitude by exerting feedback forces with a laser radiation pressure. The performance of this newly developed scanning probe microscopy was tested by measuring electrostatic repulsive forces in solution. Intermolecular forces in the sub-piconewton range were resolved at controlled gaps in the nanometer range between an amino-silanized stylus and an amino-silanized glass surface. Force curves as a function of gap distance coincided well with the theory of electricity. Debye lengths calculated at various ionic strengths were in good agreement with the Debye-Hückel theory. Furthermore, non-contact two-dimensional images mapped with electrostatic forces within the piconewton range could also be obtained. This system should prove useful not only for constructing the surface topographies of soft biomaterials but also for mapping the surfaces with intermolecular forces which are closely related to their functions.