Modulation of fluorescence sensing properties of quinoline-based chemosensor for Zn2+: Application in cell imaging studies

An electron pushing group has been introduced to a Schiff-base compound, 2-((2-(quinolin-2-yl)hydrazono)methyl)phenol (HQP) to get another Schiff-base molecule, namely, 4-methyl-2-((2-(quinolin-2-yl)hydrazono)methyl)phenol (HMQP), to modulate the Zn2+ sensing properties of the probe. HQP and HMQP have been synthesized by Schiff-base condensation between 2-hydrazinylquinoline and the respective aldehyde in 1:1 ratio and characterized by standard methods. Both the compounds show their Zn2+ sensing ability but to different extents. The fluorescence intensity of HQP at ~490 nm (λex = 410 nm) increases by about 4.5 folds in the presence of one equivalent of Zn2+ ion. However, the fluorescence intensity is enhanced by around 30 folds in the presence of one equivalent of Zn2+ in case of HMQP. The fluorescence peak of HMQP at 502 nm (λex = 420 nm) is shifted to 515 nm in the presence of Zn2+ whereas same type of shifting for HQP is found to be negligible. Some theoretical calculations have been performed to look into the spectral transitions of HMQP. Fluorescence intensity enhancement is explained by PET mechanism. Limit of detection (LOD) value for HMQP has been determined to be 220.6 nM, showing its good sensitivity towards zinc. The color change of HMQP under visible light is not conclusive in the presence of different metal ions, but only Zn2+ can induce green coloration to HMQP solution under UV light, thus indicating the possibility of naked eye detection by the analyte. HMQP with one equivalent of Zn2+ displays high fluorescence intensity in the pH range of 7.0–9.0 indicating its suitability towards biological applications. The probe has been utilized to live cell imaging studies using C6 cells.