(2) Intermolecular Interactions measured by "Subpiconewton
Intermolecular Force Microscopy"n

Takaaki AOKI¹, Michio HIROSHIMA², Toshio YANAGIDA¹ and Makio TOKUNAGA (¹Department of Physiology I, Osaka University Medical School, ²Department of Biophysical Engineering, Osaka University)

Force sensitivity of scanning probe force microscopy was increased by incorporating a cantilever with very low stiffness, 0.1pN/nm, which is over 1000-fold more flexible than is typically used in conventional atomic force microscopy. Thermal bending motions of the cantilever were reduced to less than 1 nm by exerting feed-back positioning with laser radiation pressure. The system was tested by measuring electrostatic repulsive forces or hydrophobic attractive forces in aqueous solutions. Subpiconewton intermolecular forces were resolved at controlled gaps in the nanometer range between the probe and a material surface. Interaction forces between hydrophobic surfaces of a probe and a coverslip were measured, and a long-range attractive force was observed out to 100 nm in pure water. This result suggests that very long range hydrophobic forces also operate between objects of the size of a protein molecule. For details, see Ref. 2 and 5.