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 atomicforce microscopy (AFM). The essential features of this system are that the force sensitivity is in the sub-piconewton rangeand the gap distance between interacting stylus and the sample surfaces can be controlled with nanometer accuracy. Wehave achieved these technical improvements by using flexible handmade cantilevers with the spring constant ofapproximately 0.1 pN nm-1. Thermal bending motions have been reduced to less than 1 nm in root-mean-squareamplitude by exerting feedback forces with a laser radiation pressure. The performance of this newly developed scanningprobe microscopy was tested by measuring electrostatic repulsive forces in solution. Intermolecular forces in thesub-piconewton range were resolved at controlled gaps in the nanometer range between an amino-silanized stylus and anamino-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-Hiickel theory. Furthermore,non-contact two-dimensional images mapped with electrostatic forces within the piconewton range could also beobtained. This system should prove useful not only for constructing the surface topographies of soft biomaterials but alsofor mapping the surfaces with intermolecular forces which are closely related to their functions.
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