Quantitative dynamic-mode scanning force microscopy in liquid

We describe a method to perform dynamic-mode scanning force microscopy in liquid with true atomic resolution. A frequency-modulation technique is used to maintain constant amplitude, phase, and frequency shift of the cantilever oscillation. As a consequence, the tip-sample interaction force is well defined and quantitative. The force sensitivity is demonstrated by imaging and deliberate bending of a peptide loop connecting transmembrane helices of the membrane protein bacteriorhodopsin. The experimental setup allows further enhancement of the force sensitivity by the use of small cantilevers.