Automated setpoint adjustment for biological contact mode atomic force microscopyimaging

Abstract

Contact mode atomic force microscopy (AFM) is the most frequently used AFM imagingmode in biology. It is about 5–10 times faster than oscillating mode imaging (inconventional AFM setups), and provides topographs of biological samples withsub-molecular resolution and at a high signal-to-noise ratio. Unfortunately, contact modeimaging is sensitive to the applied force and intrinsic force drift: inappropriate force appliedby the AFM tip damages the soft biological samples. We present a methodologythat automatically searches for and maintains high resolution imaging forces. Wefound that the vertical and lateral vibrations of the probe during scanning arevaluable signals for the characterization of the actual applied force by the tip.This allows automated adjustment and correction of the setpoint force during anexperiment. A system that permanently performs this methodology steered the AFMtowards high resolution imaging forces and imaged purple membrane at molecularresolution and live cells at high signal-to-noise ratio for hours without an operator.