A probe with hydrazinecarbothioamide and 1,8-naphthalimide groups for “turn-on” fluorescence detection of Hg2+ and Ag+ ions and live-cell imaging studies

The probe (L) having hydrazinecarbothioamide and 1,8-naphthalimide moieties was synthesized and evaluated for its metal ion sensing ability. It exhibits a selective and sensitive colorimetric as well as fluorescent recognition of Hg2+ and Ag+ ions in CH3OH - HEPES buffer solution (5 mM, 7:3, v/v, pH = 7.4) in presence of other metal ions. Probe L is weakly fluorescent upon excitation with 410 nm light, but after gradual addition of HgCl2 and AgNO3 enhancement in fluorescent intensity was observed. Detection limits of Hg2+ and Ag+ using L have been found to be 20 nM (Hg2+) and 40 nM (Ag+) over the pH range of 6 – 10 that is suitable for practical application under physiological pH conditions. The reversibility of interaction of L with Hg2+ ion was monitored using Na2EDTA by emission titration. The “OFF-ON’’ fluorescence switching can be observed with naked eye, in which fluorescent “OFF’’ is due to operation of PET process in free L. Whereas upon complexation with these two metal ions, PET is restricted and CHEF process becomes operational. Mass spectral analysis and Job’s plot yielded a binding ratio of 1:1 for both metal ions. NMR titration studies are consistent with binding of Hg2+ or Ag+ to NH group (attached with naphthalimide moiety) and sulfur atom of hydrazinecarbothioamide group. From cytotoxicity assay, 5 μM solution of L was considered in cellular imaging study and the potentiality of the probe L was established by using human breast cancer cell line (MDA-MB-231) and primary human dermal fibroblasts (HDF), through fluorescence cell imaging experiments for tracking both Hg2+ and Ag+ in living cells.