Figure 1.

Schematic diagram of the AFM apparatus and associated control electronics. When pressed against a layer of protein attached to a substrate, the silicon nitride tip of the AFM cantilever may adsorb a single protein molecule. Extension of the molecule by retraction of the piezoelectric positioner results in deflection of the cantilever. We can stretch a single molecule by using either a length-clamp mode or a force-clamp mode. In the standard length-clamp mode we set the desired position (L) and measure the resulting force (F) calculated from the laser deflection (a − b)/(a + b). In the force-clamp mode, the measured force is compared with a set value generating an error signal that is fed to a proportional, integral and differential amplifier (PID) whose output is connected directly to the piezoelectric positioner.