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

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