To develop an ionophore that is highly selective for sodium for use in an ion-selective electrode, we propose a model based on 16-crown-5 which has a cavity just the size of Na+ and has a "block" subunit to prevent complex formation with ions larger than Na+. Based on this molecular model, eight kinds of 16-crown-5 derivatives have been synthesized, and their structural ion selectivity has been evaluated in detail. The 16-crown-5 derivatives having two bulky "block" subunits showed high Na+ selectivity relative to K+. In particular, the derivative with two decalino subunits (DD16C5) exhibited the highest Na+ selectivity of all the ionophores examined. When a phosphate ester-type membrane plasticizer, tris(ethyl-hexyl) phosphate, was used as the membrane solvent for the ion-sensing membrane based on pory(vinyl chloride), the electrode using DD16C5 exhibited a Na+ selectivity of over 1000 times relative to alkali metal and alkaline earth metal ions, including K+, which is the most serious interferant The evaluation of the relationship between the ionophore chemical structures and the ion-selective features contributes to the host-guest chemistry to give a highly selective ionophore for an alkali metal ion.
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