Current Research
AFM for Single Molecule Force Spectroscopy
Our interest in AFM stems from its ability to measure forces on single molecules. This is accomplished by bringing the AFM tip into contact with a surface covered with biological molecules of interest. Then the tip is retracted from the surface. In some cases, this results in a molecule being tethered between the tip and the surface. As the tip is retracted further, the molecule is brought into tension. Bonds within the molecule break and domains become unfolded until either (1) the molecule detaches from the surface, or (2) the tip detaches from the molecule. This technique has given rise to an entire field of experiments called “single molecule force spectroscopy.”
Force-Extension Mode
One way of performing single molecule force spectroscopy is by retracting the cantilever at a constant velocity. This is known as force-extension mode. The resulting force curve depicts how the force exerted on the cantilever changes as a function of distance between the tip and the surface.
Force-Clamp Mode
A second way in which single molecule force spectroscopy can be performed is by retracting the cantilever until it feels a predetermined force pulling down on it, and then the instrument maintains (or “clamps”) that force until (1) bonds within the molecule break and the molecule extends and releases from the tip or surface, or (2) a few seconds have passed. The resulting force curve depicts how the piezoelectric sample stage adjusts to maintain the pulling force setpoint over time.