Active quality factor control in liquids for force spectroscopy

Molecular processes are both dynamic and energetic in nature and thus exhib it both conservative (elastic) and dissipative (viscous) forces. We present a method suitable for application to a conventional atomic force microscop e (AFM) that is capable of measuring this complex quantity and use it to st udy the force-extension response of a single molecule. The dynamic force se nsitivity of the AFM cantilever was increased with a positive feedback syst em producing a stable effective quality factor of over 300 in a liquid envi ronment compared to the nominal value of similar to 1. This facilitated tra cking of the resonant frequency and the separation of conservative (elastic ) and dissipative (viscous) forces. With access to this extra information, not available by conventional static force spectroscopy, it is possible to estimate the effective viscosity of a single dextran molecule to-be 6000 Pa s.