Tuning the structure of lariat crown ethers for ion-selective electrodes: Significant shifts in sodium/potassium selectivity

Lariat crown ethers (LCEs) offer the ability to bind selectively alkali metal ions. The presence of a coordinating functionality in the sidearm chain provides a means to further tune the selectivity of LCEs. A series of four LCEs based on a 15-crown-5 structural motif were synthesized with variations in the coordinating functionality of the sidearm and the substitution at the carbon pivot atom. Poly(vinyl chloride) liquid-membrane ion-selective electrodes were prepared with each of the four ionophores, and their response patterns to various alkali metals were determined. The results of this experimentation revealed that the coordinating ability of the sidearm is more important than the substitution at the geminal group at the pivot atom. It was also demonstrated that the change of a single atom in the sidearm structure, to induce coordinating ability to the sidearm, alters the selectivity between sodium and potassium by roughly five orders of magnitude.