Sequential displacement strategy for selective and highly sensitive detection of Zn 2+ , Hg 2+ and S 2- ions: An approach toward a molecular keypad lock.

A thiocarbonohydrazone locked salicylidene based macrocycle ligand L has been synthesized and its ion sensing properties were examined by UV–visible and fluorescence spectroscopy. The macrocycle serves as a highly selective colorimetric sensor for Hg2+ ions while it acts as an excellent fluorescent sensor for Zn2+ ions by exhibiting a green fluorescence at 498 nm even in the presence of interfering ions. A detailed analysis of binding characteristics such as complex stoichiometry, association constant and detection limits of L toward Hg2+ and Zn2+ ions were evaluated by UV–visible and fluorescence experiments which revealed a stronger binding affinity and higher detection limit of L toward the mercury ions. Further, the sequential displacement strategy for the chromofluorogenic detection of Zn2+, Hg2+ and S2− ions by ligand L, has been studied comprehensively. Finally, the ion-responsive fluorescence output signal of L were employed to design a molecular keypad lock which could be accessible by two users having two different set of chemical passwords (inputs) through distinguishable optical trajectories.