A bifunctional chromogenic and fluorogenic probe for F− and Al3+ based on azo-benzimidazole conjugate☆
Introduction
The development of optical signaling systems based on organic scaffold for sensing and recognition of specific ions has been a recent area of focus owing to their potential applications in environmental detection, molecular catalysis and monitoring biological processes [1], [2], [3]. Nevertheless efficient techniques are required to detect specific metal ions or anions as often analytes tend to interfere with each other. In recent years, Fluorogenic and chromogenic sensors have been emerging as a valid alternative to conventional analytical methods [4], [5], [6] due to simplicity, high efficiency in detecting even low analyte concentration and application to bioimaging [7], [8], [9]. Moreover, design of multifunctional sensors with varying response towards different analytes is cost effective and convenient for real applications. Nevertheless, designing sensors with multiple analyte recognition capability is a challenging task.
Among common anions, fluoride ion received the most attention from chemists because of its unique properties [10]. For instance, fluoride plays crucial roles in dental care [11] and the treatment of osteoporosis [12]. Beyond recommended doses, fluoride has been associated with fluorosis [13] and urolithiasis [14]. On the other hand, Aluminum, the most abundant metallic element is extensively used as additives in food, in drugs (e.g., antacids), in consumer products (e.g., cooking utensils and aluminum foil) and in the treatment of drinking water (e.g., coagulants) [15], [16], [17]. However if the amount required is greatly exceeded by our incidental intake, it exerts adverse effects, for instance, neurological diseases such as Alzheimer׳s disease, Parkinson׳s dementia and also to some bone disorders such as osteoporosis and osteomalacia [18], [19], [20]. Aluminum toxicity is a major constraint to crop production in almost 67% of the total acid soil area [21], as it influences agricultural production in soils of pH≤5.5 [22].
Azo compounds have been extensively utilized in electronic applications such as photochromic, proton-responsive, photon-mode high density information storage, photo-switching devices, optical computing and optical data storage [23], [24], [25], due to their good stability and suitable absorption band. Being an optical group, azobenzene has been exploited well as signaling unit of chemosensor in literature [26], [27]. In addition, imidazole group has also attracted interest as metal ion sensors [28], [29], [30] and can be usually found in active site of many metalloproteins [31].
Interestingly, various fluorescent sensors specific for Fluoride [32], [33], [34], [35], [36], [37], [38], [39], [40] or Aluminum [41], [42], [43] have been reported, but to the best of our knowledge, a single molecular sensor that shows varied dual response to fluoride and aluminum is still unexplored. In our previous reports, we have utilized benzimidazole platform for the selective recognition of Al3+ ion [44]. In continuing our research to develop new fluorescent chemosensors [45], [46], [47], [48], [49], [50], we have designed an azophenol linked benzimidazole unit that not only behaves as chromogenic and fluorogenic chemosensor but also selectively recognizes Al3+ and F− individually with “turn-on” fluorescent response. Recently, azo based Schiff base has been utilized for the selective recognition of oxalic acid via counterion displacement assay by Singh et al. [51]. It is interesting to observe that formation of quinazoline nucleus rather than Schiff base [51] opens a new channel for fluorogenic and chromogenic response, which demonstrated subtle structural changes of sensors, can have a remarkable effect on recognizing analytes. In addition, a single molecular sensor that can selectively detect and distinguish more than one species has rarely been reported. To the best of our knowledge, this is the first bifunctional fluorescent chemosensor that allows the detection of Al3+ and F−.
Section snippets
Chemicals and starting materials
All the chemicals used in the present study were of analytical reagent grade and purchased from Sigma-Aldrich Chemical Company. Solvents used were of HPLC grade unless otherwise stated. Binding property of metal ion with probe AZIM was evaluated by using Chloride salts of metal ions such as Na+, K+, Mg2+, Ca2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+, Al3+ and Pb2+ ions. All anions ( F−, Cl−, Br−, I−, OAc−, NO3−, HSO4−, H3PO4− and CN− ) were used in the form of
Results and discussion
We have synthesized chemosensor AZIM by condensing ethanolic solution of 2-hydroxy-5-((4-nitrophenyl)diazenyl)benzaldehyde with 2-(2-aminophenyl)-1H-benzimidazole in 1:l mole ratio (Scheme 1). To monitor the recognition event, nitrylazoscaffold is appended with benzimidazole moiety through quinazoline nucleus. In AZIM, nitrylazobenzene acts as chromogenic unit while benzimidazole functions as fluorogenic unit. Hence AZIM may undergo intramolecular charge Transfer (ICT) from benzimidazole moiety
Conclusion
In summary, we have designed a new azo based chromogenic and “turn on” fluorogenic chemosensor for selective recognition of Al3+ and F− over other competing ions. To the best of our knowledge, this is the first bifunctional chromogenic and fluorogenic chemosensor that allows the detection of Al3+ and F− with varied responses. In other words, the probe exhibits remarkably enhanced fluorescent intensity and significant color change. The coordination of AZ with Al3+ was found to be reversible and
Acknowledgment
M.I., D.J and K.K thank UGC-BSR (F4-1/2006(BSR)/7-119/2007(BSR)) for research fellowship. M.I., D.J., K.K. and D.C. also acknowledge DST-IRHPA, FIST and PURSE for instrumental facilities. The authors gratefully acknowledge DBT-IPLS, School of Biological Sciences, MKU for providing instrumentation facilities.
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Electronic Supplementary Information (ESI) available: [NMR and MS Spectral data, UV–vis data and computational details]. See DOI: 10.1039/b000000x/