Lateral forces in AFM imaging and immobilization of cells and organelles

Lateral forces are inevitable in contact mode AFM imaging and they contribute significantly to the image formation under certain conditions. In cases where the objects are comparable in size to the cantilever tip and particularly in cases where the tips have a high aspect ratio, the lateral force may exceed the vertical force and may impose a severe limitation to the stability of the sample during imaging. Here we have calculated the relation between the exerted lateral force and the applied vertical force as a function of the friction coefficient, the geometry of the tip, and the stiffness of the cantilever. We present a strategy to immobilize larger particles by sucking them into the pores of nucleopore filters and binding them by chemical cross linking. High resolution images of nematocysts which were immobilized with this strategy are presented. The images reveal the supra-molecular arrangement of the noncollagen of the capsule wall.