CHANNEL MIMETIC SENSING MEMBRANES FOR ALKALI-METAL CATIONS BASED ON ORIENTED MONOLAYERS OF CALIXARENE ESTERS

As a fundamental study of the development of channel mimetic sensing m embranes, guest-induced permeability changes were examined for oriente d monolayers composed of 4-tert-butylcalix[6]arene-O, O', O '', O''' O ''''-hexaacetic acid hexaethyl ester and the cone conformers of 4-tert -butyl- and 4-(1,1,4,4-tetramethylpentyl)calix[4]arene-O,O',O '',O'''- tetraacetic acid tetraethyl esters. The permeabilities through these m onolayers were evaluated by cyclic voltammetry using cationic, anionic and neutral electroactive compounds as the permeability markers. The permeability was evaluated on the basis of both voltammogram area and limiting semi-integral current. The voltammetric measurements were car ried out for either a monolayer formed at the air / water interface un der an applied surface pressure, or a monolayer transferred to an elec trode by the Langmuir-Blodgett method. Alkali metal cations induced an increase in the permeability for an anionic marker ion such as [Fe(CN )(6)](4-) and a decrease in the permeability for cationic marker ions such as [Co(1,10-phenanthroline)(3)](2+) and methyl-4,4'-bipyridinium] (2+)(methylviologen(2+)). These permeability changes can be ascribed t o an attractive or repulsive electrostatic interaction between the mar ker ions and the membrane calixarene hosts bearing positive charges by complexation with alkali metal ions. A cationic guest-induced increas e in the membrane permeability was also observed when a neutral marker (1,4-benzoquinone) was used, possibly because of an increase in the i ntermolecular voids (or microscopic defects) owing to conformational c ontraction upon host-guest complexation. Selectivity was observed for the calix[6]arene monolayer, with the magnitude of the response being in the order Cs+ Rb+> K+> Na+, Li+, which is consistent with the selec tivities in solvent extraction or transport and in potentiometric resp onse.