The nanopipette as a multifunctional probe: Current progress in single-entity detection through resistive pulse sensing and ion current rectification modalities and nanoscale imaging

Recent advances in nanoscale analytical technology have positioned nanopipette sensors as an unrivaled multifunctional framework for single-entity analysis in both analytical and real-world samples, as well as for high-resolution cell imaging, owing to their ease of manufacturing and tunable pore size. Moreover, relative to other nanopores, the nanoconfined space at the nanopipette pore orifice and flexible surface functionalization enhance single-entity resolution and the signal-to-noise ratio, facilitating integration with optical and fluorescence microscopy. This review provides an in-depth overview of nanopipette-based techniques for sensing single molecules, single particles, and single cells. We first discuss the fundamental principles of its two main sensing strategies: the ion current rectification (ICR) method, which leverages changes in surface charge polarity induced by the analyte, and the resistive-pulse (RPS) method, which enables single-entity probing based on the volume-exclusion effect caused by its passage through the nanopipette. We also highlight key factors that influence detection outputs in both sensing methods. We then summarize the current progress in single-entity analysis, including in vitro biomolecule detection, multidimensional single-particle characterization, and in vivo single-cell analysis. The integration of a nanopipette as a scanning probe in scanning ion conductance microscopy (SICM) was further studied, emphasizing high-resolution imaging of live cells under physiological conditions. Overall, this review provides a cutting-edge overview of nanopipette sensing strategies and an outlook on the prospects of this nanotechnology.