Highly selective naked eye pyridine-2,6-dicarbohydrazide based colorimetric and fluorescence chemosensor for Pb2+ with AIE activity

In this work, two novel fluorescein hydrazone N'2, N'6-bis[(1E)-1-(thiophen-2-yl) ethylidene] pyridine-2, 6-dicarbohydrazide (L1) and N'2, N'6-bis[(1E)-1-(1H-pyrrol-2-yl) ethylidene] pyridine-2, 6-dicarbohydrazide (L2) were synthesized and characterized by simply changing substituents in the ligand framework. Upon addition of metals, L1 did not show any characteristic change, whereas L2 selectively sensed Pb2+ ion with the change of coloration from colorless to dark yellow for the visual detection of Pb2+ ion with emission at 493 nm. The obtained limit of detection (LOD) values 2.3 µM for Pb2+ clearly revealed the considerable sensitivity of the chemosensor L2. FTIR, Mass analysis and Job's plot indicated that two L2 molecules bind to one Pb2+ ion, which might be due to chelation-enhanced fluorescence (CHEF) effect. Moreover, L2 possesses no aggregation-induced emission (AIE) characteristics but can detect Pb2+ ions selectively in the “on” mode based on the AIE of their complexes in the media of DMSO/H2O, which will provide a new strategy for target detection based on AIE. The quantum yield of L2 enhances significantly in the presence of Pb2+.