Applications of Nanopipettes in Scanning Ion Conductance Microscopy for High-Spatial-Resolution Topographic Imaging...

Membrane structures and cellular phenomena have been studied using scanning ion conductance microscopy (SICM). Conventional techniques for studying single cells, such as optical microscopy, fluorescence microscopy, electron microscopy (EM), and atomic force microscopy (AFM), have provided a wealth of information on the architecture of cell membranes, but they could potentially be invasive to live cells due to reasons including phototoxicity, electron beam damage, and cantilever-mediated damage to the cell membrane. While super-resolution approaches such as stimulated emission depletion (STED) microscopy have extended the capabilities of optical imaging, conventional optical microscopy remains limited by the diffraction limit in resolving intricate structures on cell membranes. In this review article, we discuss SICM as a technique that allows noninvasive imaging of live single cells in aqueous solutions, including cell culture media. We also discuss the fabrication and characterization of nanopipettes, advances in instrumentation and scanning regimes used in SICM, and applications of nanopipettes in the technique for topography mapping, high spatial resolution imaging, precise delivery of molecules to cells, biopsy, and surface charge measurements. We also discuss how nanopipettes are functionalized for applications in the simultaneous mapping of cell topography and high spatial resolution sensing, such as extracellular pH mapping. SICM has also been combined with scanning electrochemical microscopy (SECM) to enable the measurement of electroactive species at the cell membrane, and applied in cell surface charge mapping, where membrane charge is implicated in many cellular events. Advances in SICM imaging speed will allow the capture of fast cellular phenomena, and because the application of nanopipettes in SICM for high spatial resolution topographic imaging and sensing is still in its infancy, these developments could open new opportunities for imaging the distribution of analytes around live single cells.