Nitrate-selective electrode based on a cyclic bis-thiourea ionophore

Compounds containing urea and thiourea groups can hydrogen bond oxoanions and, as such, represent a class of potential ionophores for anion-selective electrodes. We have evaluated cyclic and linear bis-thiourea compounds as ionophores in polymer membrane ion-selective electrodes. An improvement in the selectivity coefficient of nitrate against chloride of greater than one order of magnitude was observed with the cyclic ionophore in comparison to the Hofmeister lipophilicity series (log KNO3−,Cl−pot of −3.50 with ionophore versus −2.40 with no bis-thiourea ionophore), indicating the preference of the cyclic ionophore for nitrate. This preference for nitrate over chloride may be attributed to the spatial arrangement of the thiourea hydrogen atoms, which are capable of forming hydrogen bonds with nitrate, as well as the size of the ionophore binding cavity. In the lowest-energy conformation, all thiourea hydrogen atoms point toward a putative nitrate binding site, likely contributing to enhanced coordination.