Image formation, resolution, and height measurement in scanning ion conductance microscopy

Scanning ion conductance microscopy (SICM) is an emerging tool for the noncontact investigation of biological samples such as live cells. It uses an ion current through the opening of a tapered nanopipette filled with an electrolyte for topography measurements. Despite its successful application to numerous systems no systematic investigation of the image formation process has yet been performed. Here, we use finite element modeling to investigate how the scanning ion conductance microscope images small particles on a planar surface, providing a fundamental characterization of the imaging process. We find that a small particle appears with a height that is only a fraction of its actual height. This has significant consequences for the quantitative interpretation of SICM images. Furthermore, small and low particles are imaged as rings in certain cases. This can cause small, closely spaced particles to appear with a lateral orientation that is rotated by 90 degrees. Considering both real space and spatial frequency space we find that a reasonable and useful definition of lateral resolution of SICM is the smallest distance at which two small particles can clearly be resolved from each other in an image. We find that this resolution is approximately equal to three times the inner radius of the pipette tip opening. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3122007]