A water-soluble 'turn-on' fluorometric sensor for Zn2+ and Cd2+ detection, quantification, and discrimination in freshwater samples.

A 4'-hydrazineyl-2,2':6',2″-terpyridine-based fluorescent probe achieves "turn-on" detection of Zn2+ and Cd2+ in water at picomolar concentrations, far below regulatory limits. Combined experimental and computational studies reveal the nature of the metal complexes, while successful analysis of Lake Ontario freshwater samples and paper-based test strips demonstrate its potential for practical environmental monitoring.