Effect of sample slope on image formation in scanning ion conductance microscopy

Scanning ion conductance microscopy (SICM) is a scanning probe technique that allows investigating surfaces of complex, convoluted samples such as living cells with minimal impairment. This technique monitors the ionic current through the small opening of an electrolyte-filled micro- or nanopipet that is approached toward a sample, submerged in an electrolyte. The conductance drops in a strongly distance-dependent manner. For SICM imaging, the assumption is made that positions of equal conductance changes correspond to equal tip-sample distances and thus can be utilized to reconstruct the sample surface. Here, we examined this assumption by investigating experimental approach curves toward silicone droplets, as well as finite element modeling of the imaging process. We found that the assumption is strictly true only for perpendicular approaches toward a horizontal sample and otherwise overestimates the sample height by up to several pipet opening radii. We developed a method to correct this overestimation and applied it to correct images of fixed cellular structures and living entire cells.