Specific interaction
(a) The tip of an atomic force microscope (black) is covered in a vitamin-B molecule called biotin (red) and then covered in a protein called streptavidin (blue), which binds strongly to it. The bead-shaped substrate is also coated with biotin. Usually just one bond is formed when the tip and substrate are brought into contact. (b) The final jump in the forceextension curve corresponds to the force needed to break a single (or only a few) molecular bonds.