Direct observation of potassium ions in HeLa cell with ion-selective nano-pipette probe

The local concentration of potassium ion in a single HeLa cell was observed with an ion-selective nano-pipette probe. Ion selectivity was achieved by using a polyvinyl chloride film with selected ionophores placed within the nano-pipette. Both alternating and constant bias voltages were applied to the counter electrode for the observation of local ion concentrations with a response time of less than 0.1 s. These measurements were enabled by a low-current detection system prepared specifically for this study. The difference in local potassium concentrations between inside a living HeLa cell and the surrounding solution was approximately 100 mM, while no difference in potassium ion concentration was observed between the interior of dead cells and the surrounding solution.