Silver ion-selective electrodes based on pi-coordinating ionophores without heteroatoms

Ion-selective electrodes (ISEs) were constructed by using spherical hydrocarbons (cyclophanes) as pi-coordinating ionophores in solvent polymeric membranes. Four structurally similar cyclophanes, i.e., [2.2.2]p,p,p-cyclophane, [2.2.2]m,p,p-cyclophane, [2.2.1]p,p,p-cyclophane and [2.2.1]m,p,p-cyclophane were studied as ionophores for Ag+. The ion-selective membranes were composed of the corresponding ionophore (1%), potassium tetrakis(4-chlorophenyl)borate (0.5%), 2-nitrophenyl octyl ether (65 - 66%) and PVC (32 - 33%). The ion-selective membrane was placed on top of a layer of the conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), working as ion-to-electron transducer. The selectivity pattern of the solid-contact ISEs was found to depend strongly on the structure of the cyclophane. The results show that ionophores that do not contain any heteroatoms such as sulfur, nitrogen or oxygen can be highly selective to "soft" cations like Ag+, based solely on cation-pi interactions between Ag+ and benzene rings. The lipohilic tetrakis(4-chlorophenyl)borate, which also contains pi-coordinating phenyl groups, and which is used as the anionic additive in the ion-selective membrane, shows a remarkable selectivity for Hg2+. The selectivity is discussed in terms of cation-pi interactions between Ag+ and the ionophores.