Active resonance control for imaging and force spectroscopy

Mervyn Miles, Andrew Humphris, Javier Tamayo, Rachel Owen, Andrew Round

H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL U.K.

A major advantage of atomic force microscopy (AFM) is its ability to image biological molecules and structures in their natural state in aqueous environments of physiological relevance. Such imaging requires the force interaction between the probe and the specimen to be low, typically less than 100 pN, in order to avoid distortion and damage to the specimen. Tapping mode decreases the lateral force applied to the specimen during scanning, but in the liquid environment the oscillation of the cantilever is heavily damped resulting in a decrease in quality factor (Q) to about 3. Greater forces are therefore involved in imaging in liquid than in air. The use of an active resonance control (ARC) allows the effective value of Q to be increased in liquid to values of about 300 with the resulting decrease in tip-specimen force. This results not only in less distortion of soft specimens, higher lateral resolution and stronger contrast in phase imaging but also the ability to image specimens that could not be imaged otherwise such as swollen gels. The use of the ARC technique will be illustrated with comparative images of single molecules, gels, and living cells. In addition, ARC can be used for enhancement of force spectroscopy measurements by providing complex mechanical properties of single molecules during extension, providing important information in the process of protein unfolding.