Study on Contact-Free Scanning and Imaging Reliability for Low Electric Field SICM With Dual-Barrel Pipette

Scanning ion-conductance microscope (SICM) is a versatile, non-contact, and high-resolution scanning probe microscope technique, it has been extensively used in the detection of cell topography and surface charge. In SICM measurements, it is an effective method to desensitize the surface charge effect on topography imaging by employing a dual-barrel pipette as an SICM probe and ion current between the two apertures to measure the charged sample. However, one of the crucial but still unclear issues is the safe ion-current feedback threshold. This parameter, especially for the slope, affects the reliability, as well as the approach speed of the probe. In this study, we first employ finite element method (FEM) simulation to study the effect of the tip parameters including the opening inner and outer radius, their ratio, and half-cone angle on the allowable current threshold for the slope and tall step structure topography, respectively. Based on this, we then study the effect of the concentration gradient between the dual-barrel pipette and bath solution on this threshold. The approach curves and contact-free scanning experiment are conducted using these conventional identical and concentration gradient-based methods, the experimental results illustrate that the scanning and imaging reliability of the conventional approach is low in comparison to the concentration gradient approach, especially for the surface with a larger slope and height difference. Adapting the positive feedback mode can significantly increase the imaging reliability due to its longer working distance.