Lead selective electrodes based on calixarene phosphine oxide derivatives

We have discovered that a series of calixarene phosphine oxide derivatives can produce ion-selective electrodes with excellent response characteristics to the Pb2+ ion. These ligands were previously known to complex lanthanides, but their complexation behavior with transition metal ions and group I/group II ions is relatively unexplored. The extreme stability of these compounds, coupled with interesting selectivity behavior, make them attractive contenders for use in potentiometric poly(vinyl chloride) membrane electrodes. Results show near-Nernstian responses to Pb2+ for electrodes based on three ligands (p-tert-butylcalix[n]arene ethyleneoxydiphenylphosphine oxide, n = 4, 5, 6) and excellent selectivity against a wide range of common interferences. Ca2+ was found to be the most important interference, but its impact dramatically decreased as the cavity size increased from the tetramer (n = 4) through the pentamer (n = 5) to the hexamer (n = 6), Electrodes based on the hexamer were found to have characteristics superior to similar devices based on several commercially available Pb2+-selective ligands. The importance of the calixarene macrocyclic backbone on the selectivity is highlighted by the poor selectivity of the electrode based on triphenylphosphine oxide. Molecular modeling has been used to provide some insight into the observed selectivity.