Intracellular K$^+$ Determination With a Potentiometric Microelectrode Based on ZnO Nanowires

The fabrication and application of an intracellular K ⁺ -selective microelectrode is demonstrated. ZnO nanowires with a diameter of 100-180 nm and a length of approximately 1.5 μm are grown on a borosilicate glass microcapillary. The ZnO nanowires were coated by a K ⁺ -ionophore-containing membrane. The K ⁺ -selective microelectrode exhibited a K ⁺ -dependent potentiometric response versus an Ag/AgCl reference microelectrode that was linear over a large concentration range (25 μM-125 mM) with a minimum detection limit of 1 μM. The measured K ⁺ concentrations in human adipocytes and in frog oocytes were consistent with values of K ⁺ concentrations reported in the literature. The sensor has several advantages including ease of fabrication, ease of insertion into the cells, low cost, and high selectivity features that make this type of sensor suitable to characterize physiologically relevant ions within single living cells.