A single chemosensor with combined ionophore/fluorophore moieties acting as a fluorescent 'Off-On' Zn~(2+) sensor and a colorimetric sensor for Cu~(2+): Experimental, logic gate behavior and TD-DFT calculations

摘要

There is a huge scope for the exploration of the role of zinc in biological systems. Therefore, it is highly desired to develop a sensitive method for its detection. Unlike other metal ions such as Mn~(2+), Fe~(2+), or Cu~(2+), the Zn~(2+) in biological systems cannot be detected by spectroscopic methods, thus the fluorescence stands out as a method of choice. The Hqpzc, N-(quinoline-8-yl)pyrazine-2-carboxamide, as a small fluorogenic molecule with a selective "Off-On" switching behavior for detection of Zn~(2+) and a colorimetric sensor for Cu~(2+), comprises of quinoline as the fluorophore and pyrazine-2-carboxamide as the chelating group. This chemosensor exhibits a remarkable fluorescence response when investigated in acetonitrile and the fluorescence intensity enhances significantly upon addition of one equivalent of Zn~(2+). The selectivity of Hqpzc for Zn~(2+) is based on the chelation-enhanced fluorescence (CHEF) mechanism. The binding mode of the Hqpzc with Zn~(2+) was investigated through Job's plot experiment, the fluorescence and UV-vis titration, ESI-MS, and density functional theory calculations. These results revealed that the binding stoichiometric ratio between Hqpzc and Zn~(2+) in acetonitrile is 1:1. The binding constant (K_a) and limit of detection (LOD) for Zn~(2+) and Cu~(2+) complexes are calculated. Other interfering ions such as Na~+, K~+, Ca~(2+), Mg~(2+), Fe~(2+), Co~(2+), Ni~(2+), Cu~(2+), Cd~(2+), Hg~(2+), Mn~(2+), Cr~(3+) and Al~(3+), show either no or slight change in the fluorescence intensity of Hqpzc in the presence of Zn~(2+). Notably, in the presence of Zn~(2+), the Hqpzc fluorescence exhibits reversibility with SCN~-, and the fluorescent signals of Hqpzc are utilized to construct an INHIBIT type logic gate at the molecular level. Theoretical calculations, carried out with TD-DFT method, support the experimental observations.