The height of biomolecules measured with the atomic force microscope depends on electrostatic interactions

In biological applications of atomic force microscopy, the different surface properties of the biological sample and its support become apparent. Observed height differences between the biomolecule and its supporting surface are thus not only of structural origin, but also depend on the different sample-tip and support-tip interactions. This can result in negative or positive contributions to the measured height, effects that are described by the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory. Experimental verification shows that the electrostatic interactions between tip and sample can strongly influence the result obtained. To overcome this problem, pH and electrolyte concentration of the buffer solution have to be adjusted to screen out electrostatic forces. Under these conditions, the tip comes into direct contact with the surface of support and biological system, even when low forces required to prevent sample deformation are applied. In this case, the measured height can be related to the thickness of the native biological structure. The observed height dependence of the macromolecules on electrolyte concentration makes it possible to estimate surface charge densities.