Direct thermal noise calibration of colloidal probe cantilevers

We show that the spring constant of colloidal probes made by attaching spherical microparticles to atomic force microscope cantilevers can be directly calibrated by the thermal noise method. We show that the energy equipartition theorem used for cantilevers, which is at the origin of the thermal noise method, is applicable to colloidal probes as well. In some close-to-ideal cases the spring constants agree within 5% with the spring constants of the bare cantilevers, given that some technical precautions during preparation of the colloidal probes are taken care of. We found no major influence of materials or sizes of the microparticles used, of absolute values of the used cantilevers’ spring constants, shape, or dimensions.