A highly selective fluorescence turn-on chemosensor for Zn2+, and its application in live cell imaging, and as a colorimetric sensor for Co2+: experimental and TD-DFT calculations

Abstract

A small fluorogenic molecule Hbpq, N-(quinoline-8-yl)pyridine-2-carboxamide, for monitoring Zn(II) ions in vitro has been synthesized. Hbpq, prepared by a facile eco-friendly one-pot synthesis under mild reaction conditions with high efficiency, selectively exhibits a ∼120-fold increase in fluorescence intensity upon addition of half an equivalent of Zn²⁺. The chemoselective response of Hbpq to the Zn²⁺ ion in the presence of interfering ions in acetonitrile has also been examined. The results of the Job's plot experiment, the fluorescence and UV-vis titration, and ESI-MS, indicate that the binding stoichiometric ratio between Hbpq and Zn²⁺ in acetonitrile is 2 : 1. Single crystal X-ray structure determination of the zinc complex, synthesized independently, also confirms the formation of mononuclear Zn(bpq)₂. In addition, Hbpq acts as a sensitive optical chemosensor for Co²⁺ ions in acetonitrile. The association constant (K_a) and limit of detection (LOD) for the Zn²⁺ and Co²⁺ complexes are also calculated. The cytotoxicity of Hbpq has been examined, and its applicability for visualizing intracellular Zn²⁺ has been demonstrated. Importantly, the MTT assay and cell imaging experiments show that Hbpq exhibits low cytotoxicity, good cell membrane permeability, and high potential for monitoring Zn²⁺ ions in vitro. The weak emission of Hbpq in MeCN solution is highly sensitive to the presence of water, being almost quenched, however this sensor behaves like a molecular light switch after entering the living cell and being surrounded by a hydrophobic environment. DFT/TD-DFT calculations have also been employed to support the binding mode and photophysical properties of Hbpq towards Zn²⁺ and the enhancement of its fluorescence by Zn²⁺ in living cells.