Fabrication and Characterization of a K⁺‑Selective Nanoelectrode and Simultaneous Imaging of Topography and Local K⁺ Flux Using Scanning Electrochemical Microscopy

A nanopipette containing a solution of bis­(benzo-15-crown-5) dissolved in 1,6-dichlorohexane was used as an ion-selective electrode (ISE) to probe K⁺ for shear force-based constant-distance scanning electrochemical microscopy (SECM). In a previous study, the ISE responded only at low K⁺ concentrations ([K⁺] < 1 mM), due to the depletion of the bis­(benzo-15-crown-5) at the oil/water interface at high K⁺ concentrations and the unstable response of the tip at the oil/water interface for shear force and current detection. In the present study, a nanopipette reshaped by heating and with the hydrophobic layer removed was used as the ISE. This modified ISE enabled a rapid response to changes in K⁺ flux at a physiological concentration of K⁺ and allowed SECM imaging on a nanometer scale. The fabricated nano-ISE was used as a probe for shear force-based SECM. Topography and K⁺ flux images were obtained simultaneously at a polycarbonate membrane filter with 5 μm pores and human embryonic kidney 293 cells (HEK293). Several areas containing a K⁺ flux larger than the surrounding areas were found in the SECM images of the HEK293 cells, which indicated the existence of K⁺ channels