Attractive-mode imaging of biological materials with dynamic force microscopy

We have applied the non-contact dynamic force microscopy method to investigate soft biological materials such as hexagonally-packed intermediate layers, DNA, and tobacco mosaic virus under ambient conditions. This method, where a stiff cantilever is oscillated close to its resonance frequency with an amplitude of 0.3-1.5 nm above the sample, allows highly reliable investigation of soft organic matter with minimized normal and lateral forces between tip and sample. The vertical and lateral resolution are determined to be <1 AA and 1-2 nm, respectively, comparing favorably to established results from repulsive-mode scanning force microscopy experiments on adsorbate covered surfaces in liquids. The interaction forces are found to be attractive, dominated by damping mechanisms and attractive force gradients of capillary and van der Waals interactions.