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 characteristic s to the Pb2+ ion. These ligands were previously known to complex lanthanid es, but their complexation behavior with transition metal ions and group I/ group II ions is relatively unexplored. The extreme stability of these comp ounds, coupled with interesting selectivity behavior, make them attractive contenders for use in potentiometric poly(vinyl chloride) membrane electrod es. Results show near-Nernstian responses to Pb2+ for electrodes based on t hree ligands (p-tert-butylcalix[n]arene ethyleneoxydiphenylphosphine oxide, n = 4, 5, 6) and excellent selectivity against a wide range of common inte rferences. Ca2+ was found to be the most important interference, but its im pact dramatically decreased as the cavity size increased from the tetramer (n = 4) through the pentamer (n = 5) to the hexamer (n = 6), Electrodes bas ed on the hexamer were found to have characteristics superior to similar de vices based on several commercially available Pb2+-selective ligands. The i mportance of the calixarene macrocyclic backbone on the selectivity is high lighted by the poor selectivity of the electrode based on triphenylphosphin e oxide. Molecular modeling has been used to provide some insight into the observed selectivity.