A single chemosensor with combined ionophore/fluorophore moieties acting as a fluorescent “Off-On” Zn2+ sensor and a colorimetric sensor for Cu2+: Experimental, logic gate behavior and TD-DFT calculations

Abstract

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 Mn2+, Fe2+, or Cu2+, the Zn2+ 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 Zn2+ and a colorimetric sensor for Cu2+, 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 Zn2+. The selectivity of Hqpzc for Zn2+ is based on the chelation-enhanced fluorescence (CHEF) mechanism. The binding mode of the Hqpzc with Zn2+ 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 Zn2+ in acetonitrile is 1:1. The binding constant (Ka) and limit of detection (LOD) for Zn2+ and Cu2+ complexes are calculated. Other interfering ions such as Na+, K+, Ca2+, Mg2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Hg2+, Mn2+, Cr3+ and Al3+, show either no or slight change in the fluorescence intensity of Hqpzc in the presence of Zn2+. Notably, in the presence of Zn2+, 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.