A fluorescein-2-(Pyridin-2-ylmethoxy) benzaldehyde conjugate for fluorogenic turn-ON recognition of Hg2+ in water and living cells with logic gate and memory device applications

An effective Hg2+ specific probe, fluorescein-2-(Pyridin-2-ylmethoxy)-benzaldehyde conjugate (FO511), was designed, synthesized and characterized by various instrumental techniques. The sensing behavior of FO511 was investigated by fluorescence technique which clearly established the high selectivity towards Hg2+ through OFF–ON fluorescence response in the presence of other metal ions like Zn2+, Mg2+, Co2+, Ni2+, Cu2+, Pb2+, Al3+, Cr3+, Cd2+, Pd2+, Fe2+ and Fe3+ in aqueous medium (pH 7.2, 10 mM HEPES buffer). The sensing mechanism could be attributed to the Hg2+ triggered spirolactam ring opening of the fluorescein moiety and simultaneous formation of a 1:1 FO511–Hg2+ complex. The interaction and formation of FO511–Hg2+ species was supported by the observations gained from fluorescence titrations, Job’s plot, 1H NMR and HRMS, and other spectroscopic studies. For the Hg2+ interaction towards FO511 the binding constant was evaluated to be (3.21 ± 0.05) × 104 M−1 with detection limit 92.7 nM. On addition of S2− to the FO511–Hg2+complex, the fluorescence intensity was totally quenched due to removal of Hg2+ from the complex by S2− ion arising out of stronger affinity of Hg2+ towards S2− resulting concomitant formation of ring closed non-fluorescent form, FO511. The tentative coordination environment in the FO511–Hg2+complex was established by DFT studies. FO511 exhibits low cytotoxicity and cell permeability, which makes it capable for bioimaging applications in living HepG2 cells.