A highly-selective chloride microelectrode based on a mercuracarborand anion carrier

The chloride gradient plays an important role in regulating cell volume, membrane potential, pH, secretion, and the reversal potential of inhibitory glycine and GABA(A) receptors. Measurement of intracellular chloride activity, [Formula: see text] , using liquid membrane ion-selective microelectrodes (ISM), however, has been limited by the physiochemical properties of Cl(−) ionophores which have caused poor stability, drift, sluggish response times, and interference from other biologically relevant anions. Most importantly, intracellular [Formula: see text] may be up to 4 times more abundant than Cl(−) (e.g. skeletal muscle) which places severe constraints on the required selectivity of a Cl(−) – sensing ISM. Previously, a sensitive and highly-selective Cl(−) sensor was developed in a polymeric membrane electrode using a trinuclear Hg(II) complex containing carborane-based ligands, [9]-mercuracarborand-3, or MC3 for short. Here, we have adapted the use of the MC3 anion carrier in a liquid membrane ion-selective microelectrode and show the MC3-ISM has a linear Nernstian response over a wide range of a(Cl) (0.1 mM to 100 mM), is highly selective for Cl(−) over other biological anions or inhibitors of Cl(−) transport, and has a 10% to 90% settling time of 3 sec. Importantly, over the physiological range of a(Cl) (1 mM to 100 mM) the potentiometric response of the MC3-ISM is insensitive to [Formula: see text] or changes in pH. Finally, we demonstrate the biological application of an MC3-ISM by measuring intracellular a(Cl), and the response to an external Cl-free challenge, for an isolated skeletal muscle fiber.