Potentiometric evaluation of solvent polymeric carbonate-selective membranes based on molecular tweezer type neutral carriers

Potentiometric properties of the ion-selective electrodes) based on highly plasticized PVC membranes doped with the carbonate-selective cholic acid (C A) derivatives have been measured. The carbonate-selective neutral carriers have been prepared by coupling one to three trifluoroacetobenzoyl (TFAB) g roups to a cholic acid derivative which has three hydroxyl linkers lining o n the C3, C7, and C12 positions of its rigid steroidal ring structure. The membranes based on cholic acid derivatives with two TFABs [3,7-bis(TFAB)CA, 3,12-bis(TFAB)CA, and 7,12-bis(T-FAB)CA] exhibited remarkably improved car bonate selectivity, indicating that the bis(TFAB)CAs behave like molecular tweezers for the carbonate ion. For example, 3,12-bis(TFAB)CA resulted in 1 0-300-fold-enhanced carbonate selectivity over other anions (e.g., salicyla te, ClO4-, SCN-, HPO42-, NO3-, NO2-, Br-, and Cl-) compared to that of the neutral carriers with a single TFAB group. The distances between the carbon ate binding centers of bis(TFAB)CAs, i.e., the carbonyl carbons of the two TFAB groups, are in the 7.3-7.9-Angstrom range at the AM1 level semiempiric al calculation, which is too far for the carbonate ion to form direct coval ent bonding. The fast atom bombardment mass spectra of bis(TFAB)CAs show th at significant fractions of the compounds are either mono- or dihydrated be fore complexing the carbonate ion. These findings seem to suggest that bis( TFAB)CAs recognize the incoming carbonate ion by forming both covalent and hydrogen bonding between the hydrated and unhydrated TFAB groups. The analy tical utility of the carbonate-selective electrode based on 3,12-bis(TFAB)C A has been demonstrated by measuring the total carbon dioxide in human seru m in the presence of lipophilic anion interferents, e.g., salicylate.