Estimating the Geometry of Scanning Ion Conductance Microscope Pipettes from Resistance Variation with Breakage

The spatial resolution and non-contact working conditions of a scanning ion conductance microscope (SICM) are largely determined by the size and shape of the scanning probe tip. Estimating the tip geometry has traditionally required the use of scanning electron microscopy (SEM), a difficult, time-consuming process that, even if successful, provides little information about the crucial inner geometry of the probe. As a result, such measurements are not routinely made. Instead, tip sizes are often crudely estimated from pipette resistances. We have developed a simple method of more precisely estimating the geometry (tip radius and inner cone angle) from multiple resistance measurements recorded during quasi-controlled breakage of the tip. Such measurements can be easily obtained using only the standard SICM apparatus. Results compare favourably with SEM estimates, are more informative and avoid some of the assumptions necessary for SEM estimation.