Design and synthesis of sodium ion-selective ionophores based on 16-crown-5 derivatives for an ion-selective electrode

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.