Design and synthesis of lithium ionophores for an ion-selective electrode by chemical modification of natural carboxylic polyether antibiotic monensin

Ion-selective electrodes were prepared with 12 kinds of monensin derivatives obtained by chemical modification of natural antibiotic monensin, and the relationship between the chemical structures of the derivatives and the ion selectivities of those electrodes was investigated for designing Li⁺-selective ionophores. Lactonization of monensin and subsequent acylation of its tertiary hydroxyl group are effective for obtaining highly Li⁺-selective ionophores. Of all the monensin derivatives synthesized, macrocyclic monensin monoisobutyrate proved to have the highest Li⁺ selectivity. The selectivity coefficient of Li⁺ to Na⁺ (log K_LINa^pot) was -1.8 in the poly(vinyl chloride) (PVC) matrix membrane electrode, which was prepared by using the macrocyclic monensin derivative and dibenzyl ether (DBE) as the membrane solvent.