Channel mimetic sensing membranes for alkali metal cations based on oriented monolayers of calixarene esters

Abstract

As a fundamental study of the development of channel mimetic sensing membranes, guest-induced permeability changes were examined for oriented monolayers composed of 4-tert-butylcalix[6]arene- O, 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 monolayers 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 carried out for either a monolayer formed at the air|water interface under an applied surface pressure, or a monolayer transferred to an electrode 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 [1,1′-dimethyl-4,4′-bipyridinium] 2+ (methylviologen 2+). These permeability changes can be ascribed to an attractive or repulsive electrostatic interaction between the marker ions and the membrane calixarene hosts bearing positive charges by complexation with alkali metal ions. A cationic guest-induced increase in the membrane permeability was also observed when a neutral marker (1,4-benzoquinone) was used, possibly because of an increase in the intermolecular voids (or microscopic defects) owing to conformational contraction 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 selectivities in solvent extraction or transport and in potentiometric response.