Molecular design of calixarene-based ion-selective electrodes

In order to obtain insights into relationships between the calix4]arene structure and the ion selectivity in the electrode system, 20 ionophoric calix4]arenes were synthesized and their ion selectivity (with Na⁺ as a standard) estimated. Among these ionophoric calix4]arenes, 25,26,27,28-tetrakis(ethoxycarbonyl)methoxy]-p-t-octylcalix4]arene afforded the highest logK _NA,M ^pot value (–3.1) in the presence of 2-fluorophenyl-2-nitrophenylether (10) as the best of 13 plasticizers. This is the first example in which the Na⁺/K⁺ selectivity exceeds a factor of 10³ in the electrode system based on the neutral carrier. The high Na⁺ selectivity is attributed to modification of the upper rim which ostensibly has no relation with the component of the cavity. This paper demonstrates the potential relationships between the unique structure of the calix4]arene-based ligands and selectivity performance for the design of ion-selective electrodes.