Improved nitrate sensing using ion selective electrodes based on urea-calixarene ionophores

Urea-calix[4]arenes 1 and 2 were synthesised and incorporated into ISE membranes for assessment as sensors for inorganic anions in water. 1 revealed a strong response to all anions following the Hofmeister selectivity order. For ISEs of 2, the response to a portion of the anion series was suppressed, increasing the margin of selectivity of nitrate over chloride, a common interferant of nitrate in fresh and marine water samples. The performance of ISEs containing 2 was compared to commercially available alkylammonium nitrate ion-exchange salts used for nitrate sensing. Our ISEs performed favourably in terms of sensitivity, linear range and LOD with an improved selectivity coefficient over chloride of log K_NO3-Cl-^pot of -3.4, an order of magnitude better than commercially available nitrate ISEs. The pre-conditioning of ISEs in non-primary chloride salt was essential for obtaining these results.