Coumarin-based Hg2+ fluorescent probe: Synthesis and turn-on fluorescence detection in neat aqueous solution
Graphical abstract
Introduction
The heavy metal ions pollution has attracted widely attention in worldwide owing to their damage to human health and environment. Among those heavy metals, Hg2+ is one of the most harmful elements. First, it can be formed from Mercury vapour (Hg0) by aerial oxidation and enter to the living environment. [1], [2]. Second, Hg2+ easily convers to the highly toxic methyl mercury, passes through biological membranes and accumulates in the human body by the food chain, resulting in the human’s neurological system damage, DNA damage, various cognitive and motion disorders [3], [4], [5], [6]. In addition, some industry processes such as coal and gold mining, solid-waste incineration and the combustion of fossil fuels, making the Hg2+ pollution possible in water, air and soil of people living environment [7], [8]. Therefore, the development of techniques for effective detection of Hg2+ in aqueous environment is of great importance.
Recently, many methods have been reported to sensitively and reliably detect toxic Hg2+, such as atomic absorption spectrometry, inductively coupled plasma-atomic emission spectrometry (ICP-MS) and capillary electrophoresis-ICP–MS [9], [10], [11]. However, these techniques usually need the expensive instrumentations, complicated and time consuming pre-treatment steps. Sometimes, it is difficult to sensitively detect concentration of Hg2+ on the ppb levels range. Compared with the above techniques, fluorescent probe based on the optical signal change for analytical target has attracted much attention due to its several outstanding advantages such as low cost, simplicity and high sensitivity [12], [13], [14]. Hence, a wide variety of fluorescent probes have been developed for detection of Hg2+ [15]. Among these probes, the key part of designing probe is the combine model between probes and Hg2+, including the heteroatom-based coordination interaction [16], [17], [18], [19], [20], [21], [22] and specific Hg2+-promoted reaction [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37]. In contrast to the coordination interaction based probe, the reaction-based probes have more superiority in term of detection selectivity over the competing metal ions. Thus, many Hg2+-induced reactions were applied in fluorescent probes for detection of Hg2+, involving the desulfation [23], [24], [25], alkyne or vinyl ether oxymercuration [26], [27], selective mercuration reaction [28], [29], [30], thioacetal deprotection [31], [32], [33] and coordination-induced ring-opening [34], [35], [36], [37].
According to the US Environmental Protection Agency requirement, the limit of Hg2+ concentration for sake drinking water was set as 2 ppm [38]. So, it is very important to detect water quality for safe drinking water. For this purpose, many reported fluorescent probes have same drawbacks in testing system due to using of organic solvent or higher amount of the same as a co-solvent with water. Therefore, there is a strong interest in fluorescent probe for recognition of Hg2+ in neat aqueous environment with high selectivity and sensitivity.
Coumarin and its derivatives with excellent chromogenic and fluorogenic properties are widely applied in fluorescent probes [39], [40], [41]. In this work, we designed and synthesized a fluorescence “turn on” probe VC, in which Coumarin serves as fluorophore core and vinyl ether group as a reaction unit (Scheme 1). The probe VC could be synthesized through a two-step procedure with 7-hydroxy-4-methylcoumarin (compound 1) as the starting materials, where the key elimination reaction was optimized. The solution of probe VC in neat water showed almost no fluorescence. After treatment with Hg2+, the solution displayed remarkable fluorescence, which can be easily observed by naked eyes under a hand-held UV lamp. In addition, probe VC could be applied to monitor Hg2+ with high selective and sensitive in real water sample.
Section snippets
Reagents and instrumentation
Solvents for organic synthesis are reagent grade, and are dried prior to use. 7-hydroxy-4-methylcoumarin was prepared as described in the literature [42]. Other chemicals are purchased from commercial sources and used as received. Double distilled water was used throughout the experiments. 1H and 13C NMR spectra were measured in CDCl3 with a Varian operating at 400 MHz and 100 MHz, respectively and chemical shifts were reported in ppm using tetramethylsilane (TMS) as internal standard. Mass
Result and discussion
The synthesis route of the probe VC is depicted in Scheme 1. The probe VC was synthesized through a two-step procedure with 7-hydroxy-4-methylcoumarin (compound 1) as the starting materials. The etherification of compound 1 was achieved using 1,2-dibromoethane and K2CO3 in CH3CN at 90 °C to give compound 2 with 57% yield. For the last step, the elimination reaction of compound 2 did not work when the common condition was used (t-BuOK as base in DMSO). The result may be due to the fact that the
Conclusions
In summary, a novel Coumarin-based Hg2+ fluorescent probe VC is synthesized by elimination reaction using 7-bromoethaneoxy-4-methylcoumarin with organic base DBU. After treatment with Hg2+, the probe VC shows 110-fold fluorescence enhancement at 450 nm in HEPES buffer (20 mM, pH 7.0). Based on 1H NMR and MS analysis, the turn on fluorescence response is attributed to Hg2+-promoted reaction with VC to generate a strong fluorescent product 2-methyl-7-hydroxy-quinoline. Owing to its good water
Acknowledgments
This work was supported by Foundation of the Program for Science and Technology of Zhejiang Province (Grant No. 2016C37050), Zhejiang Provincial Natural Science Foundation of China (Grant Nos. LQ15B060005 and LY14B020008), University Student Research Innovation Team of Zhejiang Province (Grant No. 2016R417024) and the National Natural Science Foundation of China (Grant No. 21606102).
Chenjun Wu is an undergraduate in College of Biological, Chemical Sciences and Engineering, Jiaxing University.
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Chenjun Wu is an undergraduate in College of Biological, Chemical Sciences and Engineering, Jiaxing University.
Jianbo Wang received his Ph.D from University of Science and Technology of China in 2012. After working at WuXi AppTec Co., Ltd for a year, he joined in College of Biological, Chemical Sciences and Engineering, Jiaxing University in 2013. His current research interests are the design and synthesis of florescence chemosensors.
Jinjin Shen is an undergraduate in College of Biological, Chemical Sciences and Engineering, Jiaxing University.
Cheng Bi is an undergraduate in College of Biological, Chemical Sciences and Engineering, Jiaxing University.
Hongwei Zhou is a professor in College of Biological, Chemical Sciences and Engineering, Jiaxing University. His current research interests include the organic synthesis, industry catalysis and chemical sensors.