Jumping mode AFM imaging of biomolecules in the repulsive electrical double layer

We present a method to image single biomolecules in aqueous media by atomic force microscope (AFM) without establishing any mechanical contact between the tip and the sample. It works by placing the feedback set point in the repulsive electrical double-layer curve just before the mechanical instability occurs. We use the jumping operation mode, where the set point is controlled at every image point and a stable imaging is achieved for several hours. This is a necessary condition for this method to be operative, otherwise the tip can fall in contact in a short time. The method is applied to image single-avidin protein molecules deposited on cleaved mica. In addition, the dependence of the height of avidin molecules as a function of ion concentration, due to differences in surface charge density of mica and avidin, is tentatively used to deduce relative values of these quantities.