Highly selective recognition of mercury ions through the "naked-eye"

A new sensor 2-((E)-(3-(1H-imidazol-1-yl)propylimino)methyl)-5- (diethylamino)phenol (1) based on the combination of diethylaminosalicylaldehyde and imidazole groups was designed and synthesized as a Hg^{2 +} selective colorimetric chemosensor. Upon treatment of 1 with mercury ions, sensor 1 showed a color-change from colorless to yellow in a mixture of H ₂O/DMF (5:95), while many other ions such as Al^{3 +}, Zn^{2 +}, Cd^{2 +}, Cu^{2 +}, Fe^{2 +}, Mg ^{2 +}, Cr^{3 +}, Ag⁺, Co^{2 +}, Ni ^{2 +}, Na⁺, K⁺, Ca^{2 +}, Mn ^{2 +} and Pb^{2 +} had no influence. Notably, this chemosensor could distinguish clearly Hg^{2 +} from Cu^{2 +}, Ag ⁺, Fe^{2 +} and Pb^{2 +}. The underlying signaling mechanism is a ligand-to-metal charge-transfer (LMCT) process. Compound 1 showed the serial formation of the 1:1 and the 1:2 complexation between 1 and Hg ^{2 +}. This two-step binding mode was proposed based on the UV titration, ¹H NMR titration and ESI-mass studies.