Potentiometric and theoretical studies of the carbonate sensors based on 3-bromo-4-hexyl-5-nitrotrifluoroacetophenone

Novel carbonate ionophore, trifluoroacetophenone derivative (TFA) substituted by two acceptor substituents in the phenyl ring (3-bromo-4-hexyl-5-nitrotrifluoroacetophenone), was synthesized. Solvent polymeric membrane sensors based on this ionophore exhibited heightened selectivity to carbonate ions in the presence of the most important interfering anions. A wide range of potentiometric properties were studied and compared with those of sensors based on mono-substituted ionophores. Special attention was paid to pH dependence of sensor responses and to elaboration of appropriate conditions for carbonate analysis. A segmented-sandwich membrane method was applied for determination of the stoichiometry of ionophore–carbonate complexes, which was determined to be 1:3, and apparent complex formation constants which were 14.4 and 13.6 for DOS- and NPOE-plasticized membranes, respectively. Theoretical studies on TFA derivatives by semi-empirical (AM1 and PM3) and ab initio (6-31+G*) methods were performed, considering different types of possible ionophore–ion interactions. The formation of hydrogen bonds between carbonate and hydrated TFA was proved to be much more favourable in terms of energy compared to tetrahedral nucleophilic adducts that earlier were postulated to being formed in the membrane phase. The final conclusion on the mechanism of carbonate sensing by TFA-based solvent polymeric membrane sensors was made on the basis of computational data and detailed analysis of the literature.