A novel chemosensor with visible light excitability for sensing Zn2+ in physiological medium and in HeLa cells

Abstract

In the present study a novel imine-hydrazone based fluorescent chemosensor (L₁) for efficient and selective sensing of Zn²⁺ over other biologically important metal ions under physiological conditions is reported. An enhancement in fluorescence emission intensity of the developed probe with a red shift of ∼25 nm was observed for Zn²⁺, whereas other metal ions failed to reveal any significant change in the emission spectra. Interestingly, the receptor functioned under completely physiological conditions (99.7% HEPES buffer) and has visible light excitability. Sensing of Zn²⁺ was investigated in detail by absorption spectroscopy, emission spectroscopy, DFT calculation, ¹H-NMR titration experiment and ESI-MS experiment. The association constant between L₁ and Zn²⁺ was found to be 5.58 × 10⁵ M⁻¹. The receptor could detect as low as 69 ppb Zn²⁺. Sensing of Zn²⁺ is proposed through switch-on of intramolecular charge transfer (ICT) and chelation enhanced fluorescence (CHEF) processes after the introduction of Zn²⁺ into the free ligand. The developed receptor was non-toxic and rendered intracellular sensing of Zn²⁺ in HeLa cells through fluorescence imaging studies.