In Situ Monitoring of Extracellular K Using the Potentiometric Mode of Scanning Electrochemical Microscopy with a Carbon-Based Potassium Ion-Selective Tip.

The expression of potassium channels can be related to the occurrence and development of tumors. Their change would affect K outflow. Thus, in situ monitoring of extracellular K shows a great significance. Herein, the dual-functional K ion-selective electrode as the scanning electrochemical microscopy (SECM) tip (K-ISE SECM tip) has been developed for in situ monitoring of the extracellular K. Based on multi-wall carbon nanotubes as a transduction layer, the K-ISE SECM tip realizes both the plotting of approach curves to position the tip for in situ detection and the recording of potential responses. It shows a near Nernstian response, good selectivity, and excellent stability. Based on these characteristics, it was used to in situ monitor K concentrations ([K]) of three breast cancer cell lines (MCF-7, MDA-MB-231, and SK-BR-3 cells) at 3 μm above the cell, and [K] of MDA-MB-231 cells show the highest value, followed by MCF-7 cells and SK-BR-3 cells. K outflow induced by electrical stimulation or pH changes of the culture environment (Δ[K]) was further determined, and the possible mechanism of K outflow was investigated with 4-aminopyridin (4-AP). MCF-7 cells present the largest value of Δ[K], followed by MDA-MB-231 cells and SK-BR-3 cells at all the stimulation potentials, and pH 6.50 shows the greatest impact on K outflow of the three cell lines. The pretreatment of 4-AP changed K outflow, probably due to the regulation of voltage-gated channels. These findings provide insight into a deep understanding of the microenvironment influence on K outflow, thereby reflecting the possible mechanism of potassium channels.