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.