A Ratiometric Tetrazolylpyridine-Based “Turn-On” Fluorescent Chemosensor for Zinc(II) Ion in Aqueous Media

Zinc, an essential trace element, is the second most abundant metal in the human body that plays a vital role in numerous biological processes, such as peptide synthesis, DNA synthesis, RNA transcription, metabolism of cells, metalloenzyme regulation, and neurophysiology, and it also induces the formation of b-amyloid, which is related to neurological function. More than 100 enzymes, such as peptidases, carbonic anhydrases, and alcohol dehydrogenases, require zinc for their catalytic activity. Labile zinc is found in the cells of mammalian brain, pancreas, and prostate. Zinc and its compounds are widely used in various industries, such as electroplating, rubber, dye, wood preservatives, ointments, batteries, paint, and pharmaceuticals. Uncontrolled release of zinc from mossy fiber terminals causes brain injury, stroke, or neuronal death. Studies show that a lack of zinc in the body causes prostate cancer, diabetes, Alzheimer’s disease, night blindness, growth retardation, and skin lesions, and affects gene expression and enzyme activity. Thus, in view of its biological relevance and importance, a selective sensor for the monitoring of zinc is essential. Although there are several analyses available to detect zinc ion, such as atomic absorption spectrometry (AAS), inductively coupled plasma mass spectrometry, and voltammetry, they are all expensive and time-consuming. Recently, many zinc sensors have been developed using the quinolone moiety, cyclam, Znpyr, Schiff base, thiophene-based moieties, pyrazoline and pyrazole moieties, and rhodamine-based moieties. However, construction of selective and ratiometric sensors for Zn remains in high demand. Thus considerable attention has been focused on developing simple, inexpensive, ratiometric, “turn-on” sensors for Zn selective determination. Also, understanding the biological and environmental roles of Zn requires robust and versatile methods for quantification. The goal has been to devise “turnon” fluorescent sensors for Zn . However, these turn-on signals are insufficient for quantification. An alternative approach involves developing sensors that display a change in the ratio of multiple emission bands, thus providing quantification as a significant advantage, and only a few ratiometric fluorescent sensors for zinc were available recently. Our interest in developing chemosensors for biologically important cations, anions, and neutral molecules prompted us to develop a simple ratiometric fluorescence chemosensor involving 2-(1H-tetrazole-5-yl)pyridine (2PT) for the selective sensing of Zn , which involves excited-state intramolecular proton transfer (ESIPT). Though several ESIPT sensors for Zn have been reported, the present system is very simple and is employed in aqueous medium.