Highly selective optical-sensing membranes, containing calix[4]arene chromoionophores, for Pb2+ ions

Plasticized poly(vinyl chloride) (PVC) optode membranes containing novel ca lix[4]arene chromoionophores 1 or 2 and one equivalent of a lipophilic anio n respond to Pb2+ ions with high selectivity over alkali, alkaline-earth. a nd other heavy metal ions. This selectivity stems from the combination of l igand specificity and a unique ion exchange scheme that employs both monova lent metal ions and protons as the exchanged ions. Complexation of Pb2+ ion s inside the membrane is accompanied by deprotonation of the chromoionophor es, which causes a bathochromic shift of the absorption maximum lambda (max ) The response to Pb2+ ions is modulated by pH and alkali metal ions in a f ashion that is consistent with the proposed ion-exchange mechanism. Of all of the other metal ions tested. only Cs+ and Ag+ produce a color change. Ho wever. these monovalent metal ions cause hypsochromic shifts of lambda (max ) instead of the bathochromic shift caused by Pb2+. because the chromoionop hores remain protonated upon complexation.