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(ethylhexyl) phosphate, was used as the membrane solvent for the ion-sensing membrane based on poly(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.