Characterization of optimized Na⁺ and Cl⁻ liquid membranes for use with extracellular, self-referencing microelectrodes

Abstract

Self-referencing with ion-selective microelectrodes (ISMs) is a useful approach for monitoring near-real-time ion flux near single cells and across epithelia. While ISMs for H⁺, Ca²⁺, and K⁺ have been optimized for use with self-referencing, ISMs for two other primary inorganic ions, Na⁺ and Cl⁻, have not. In this study, we have characterized ISMs based on three Na⁺ ionophores (I, VI, and X) and one Cl⁻ ionophore to assess their suitability for use with self-referencing. ISMs constructed with Na⁺ ionophore VI have short response times (≈100 ms) but possess nearly an order of magnitude less selectivity for Na⁺ over K⁺ than ISMs constructed with Na⁺ ionophore X. The Na⁺ ionophore X mixture was enhanced to give it a shorter response time while not compromising its selectivity. A Cl⁻-selective microelectrode was constructed and characterized with superior anionic selectivity compared with previously reported Cl⁻ ISMs used with self-referencing. This Cl⁻-selective microelectrode, however, has a relatively slow response time (≈3 s), thus requiring changes to the self-referencing protocol. Self-referencing with these ISMs will enable near-real-time ion flux measurements for Na⁺ and Cl⁻.