DOI: 10.1002/cplu.201402126 A Ratiometric Tetrazolylpyridine-Based “Turn-On” Fluorescent Chemosensor for Zinc(II) Ion in Aqueous Media Palanichamy Kaleeswaran, [a] Ismail Abulkalam Azath, [a] Vairaperumal Tharmaraj, [a] and Kasi Pitchumani* [a, b] Introduction Zinc, an essential trace element, is the second most abundant metal in the human body [1] that plays a vital role in numerous biological processes, such as peptide synthesis, DNA synthesis, RNA transcription, [2] metabolism of cells, metalloenzyme regu- lation, and neurophysiology, and it also induces the formation of b-amyloid, which is related to neurological function. More than 100 enzymes, such as peptidases, [3] carbonic anhydrases, [4] and alcohol dehydrogenases, [5] require zinc for their catalytic activity. [6] 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 pharma- ceuticals. [7] Uncontrolled release of zinc from mossy fiber terminals causes brain injury, stroke, or neuronal death. [8] Studies show that a lack of zinc in the body causes prostate cancer, diabetes, Alzheimer’s disease, [9] night blindness, [10] growth retardation, and skin lesions, and affects gene expression and enzyme ac- tivity. [11] Thus, in view of its biological relevance and impor- tance, 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), [12] induc- tively coupled plasma mass spectrometry, [13] and voltamme- try, [14] they are all expensive and time-consuming. Recently, many zinc sensors have been developed using the quinolone moiety, [15] cyclam, [16] Znpyr, [17] Schiff base, [18] thiophene-based moieties, [19] pyrazoline and pyrazole moieties, [20] and rhoda- mine-based moieties. [21] However, construction of selective and ratiometric sensors for Zn 2 + remains in high demand. Thus considerable attention has been focused on developing simple, inexpensive, ratiometric, “turn-on” sensors for Zn 2 + se- lective determination. Also, understanding the biological and environmental roles of Zn 2 + requires robust and versatile methods for quantification. The goal has been to devise “turn- on” fluorescent sensors for Zn 2 + . However, these turn-on sig- nals are insufficient for quantification. [22] An alternative ap- proach involves developing sensors that display a change in the ratio of multiple emission bands, thus providing quantifica- tion as a significant advantage, and only a few ratiometric fluo- rescent sensors for zinc were available recently. [23] Our interest in developing chemosensors for biologically im- portant cations, [24] anions, [25] and neutral molecules [26] prompt- ed us to develop a simple ratiometric fluorescence chemosen- sor involving 2-(1H-tetrazole-5-yl)pyridine (2PT) for the selec- tive sensing of Zn 2 + , which involves excited-state intramolecu- lar proton transfer (ESIPT). Though several ESIPT sensors for Zn 2 + have been reported, [27] the present system is very simple and is employed in aqueous medium. Results and Discussion Synthesis and characterization of the probe Compounds 2PT and 4-(1H-tetrazole-5-yl)pyridine (4PT) were prepared by following the literature procedure [28] using picoli- nonitrile and sodium azide in the presence of ZnCl 2 as a cata- lyst (Scheme 1), and the products were characterized by NMR spectroscopy (Figures S1–S4 in the Supporting Information) and ESI-MS (Figures S5 and S6). As ZnCl 2 is used as a catalyst during the synthesis of the probe, care is taken to remove it completely from the reaction mixture and the synthesized 2PT The highly selective ratiometric “turn-on” fluorescent sensing of Zn 2 + ion involving 2-(1H-tetrazole-5-yl)pyridine (2PT) in aqueous medium is reported, which is not observed when other metal ions are present. Upon 2PT binding selectively with Zn 2 + ion, a fluorescence enhancement is observed that is attributed to an enhancement of localized emission and sup- pression of excited-state intramolecular proton transfer. The described sensing system involving 2PT is also successfully ap- plied to the detection of Zn 2 + ion in real samples with a detec- tion limit of 7.5  10 À7 m. [a] P. Kaleeswaran, Dr. I. A. Azath, Dr. V. Tharmaraj, Prof. K. Pitchumani School of Chemistry, Madurai Kamaraj University Madurai 625021 (India) Fax: (+ 91) 452 2459181 E-mail : pit12399@yahoo.com [b] Prof. K. Pitchumani Centre for Green Chemistry Processes School of Chemistry, Madurai Kamaraj University Madurai 625021 (India) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402126.  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPlusChem 0000, 00,1–7 &1& These are not the final page numbers! ÞÞ CHEMPLUSCHEM FULL PAPERS