A novel 2,6-diformyl-4-methylphenol based chemosensor for Zn(ii) ions by ratiometric displacement of Cd(ii) ions and its application for cell imaging on human melanoma cancer cells

A new chelating ligand [4-methyl-2,6-bis-(pyridin-2-yl-hydrazonomethyl)-phenol] (1) was prepared by the condensation of 2-hydrazinylpyridine with 2,6-diformyl-p-cresol. Compound 1 exhibits weak fluorescence due to intramolecular photoinduced electron transfer (PET). The sensor (1) demonstrates Zn²⁺-specific emission enhancement due to the “PET off” process through a 1:1 binding mode with the metal ion. The fluorescence quantum yield of chemosensor 1 is only 0.020, and it increases more than 14-fold (0.280) in the presence of one equivalent of the zinc ion. Interestingly, the introduction of other metal ions causes the fluorescence intensity to remain either unchanged or weakened except for Cd²⁺. The new sensor showed ‘naked-eye’ detection of Zn²⁺ ions: a color change of the solution from colorless to yellow. Ratiometric displacement of Cd²⁺ ions from the complex by Zn²⁺ ions supports the formation of a more stable sensor–Zn²⁺ complex over the sensor–Cd²⁺ complex. The experimental findings have been correlated with theoretical results using the B3LYP functional and 6-31G (d, p), LANL2DZ basis set for Cd²⁺ (2) and Zn²⁺ (3) complexes, respectively, by the Density Functional Theory (DFT) method. Moreover, the ability of probe 1 to sense Zn²⁺ within human melanoma cancer cells has been explored, and the Zn²⁺-probing process in living cells was found to be reversible with zinc chelator solution of N, N, N, N-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) or EDTA.