Remarkable thallium(I) selectivity for ion sensors based on π-coordination of calix[4]arene neutral carriers

Calix[4]arene derivatives have been applied as neutral carriers for thallium(I)-selective electrodes, taking advantage of their π-coordination. Calix[4]arene derivatives without any t-butyl group at the upper rim, such as calix[4]arene tetra(propyl ether) and tetra(allyl ether), provided the thallium(I)-selective electrodes with high sensitivity and ion selectivity. t-Butylcalix[4]arene derivatives employed here did not afford very good results as for the thallium(I) neutral carrier. 1H-NMR spectroscopy indicated that thallium(I) complexation by the calixarene neutral carriers is made primarily by π-coordination of their skeletal aromatic rings. The ion-selective electrode based on calix[4]arene tetra(propyl ether) was successfully applied to thallium(I) assays in human urine.