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

Plasticized poly(vinyl chloride) (PVC) optode membranes containing novel calix[4]arene chromoionophores 1 or 2 and one equivalent of a lipophilic anion respond to Pb2+ ions with high selectivity over alkali, alkaline-earth, and other heavy metal ions. This selectivity stems from the combination of ligand specificity and a unique ion exchange scheme that employs both monovalent metal ions and protons as the exchanged ions. Complexation of Pb2+ ions inside the membrane is accompanied by deprotonation of the chromoionophores, which causes a bathochromic shift of the absorption maximum h-max. The response to Pb2+ ions is modulated by pH and alkali metal ions in a fashion 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. However, these monovalent metal ions cause hypsochromic shifts of h-max instead of the bathochromic shift caused by Pb2+, because the chromoionophores remain protonated upon complexation.