Post-synthetically modified metal–organic framework as a scaffold for selective bisulphite recognition in water
Introduction
The versatile role played by anions both in biological and environmental aspects makes anion recognition an important area of scientific interest. Hence anion recognition chemistry has seen tremendous development over the past few decades [1]. Sulphur dioxide (SO2), a known major air pollutant further on hydrolysis produces its oxo-anion namely bisulfite (HSO3−) [2], [3]. Bisulfite anion plays a crucial role in regulation of different cardiovascular processes in our body at low concentration [4]. Also it is used extensively in food, beverage and pharmaceutical industries as an antioxidant and antimicrobial agent [5]. However, at high concentration it is well established to have marked adverse effects [6], [6](a), [6](b), [6](c). This includes attack in the cells and tissues of human respiratory system leading to asthmatic and allergic tendencies [7], [7](a), [7](b), [7](c). Further it is also found that bisulfite anion affects the human cardiovascular and alimentary systems leading to abdominal pain, diarrhea, dermatitis, hypertension as well as other biological disorders [8], [8](a), [8](b). Based on all these fact some countries have imposed serious restrictions on usage of bisulfite in food products either to a very low level or prohibited it [9], [9](a), [9](b). So from a bigger perspective recognition of bisulfite in water medium is very important. Many analytical techniques which include electrophoresis and electrochemical methods, chromatography etc., have been developed for determination of bisulfite [10], [10](a), [10](b), [10](c), [10](d). Due to lack of simplicity to a great extent and time consuming procedure have hindered the application of aforementioned techniques. On the other hand fluorescence spectroscopy is a simple, cost effective and sensitive technique with fast response time and can be an alternate powerful tool to serve this purpose [11]. Various luminescent organic probes with specific functionalities have been designed in the past for selective sensing of bisulfite anions. This includes inclusion of functional moieties like aldehyde and levulinate group or group containing unsaturated covalent bonds into the probe [12], [12](a), [12](b), [12](c), [12](d), [12](e), [12](f), [12](g), [12](h), [12](i). The reason behind choice of such groups is the facile chemical reactions of the bisulfite anion with all these functionalities which results in achieving a fast and accurate response in fluorescence intensity. But low water solubility often causes difficulty for such organic probes to detect bisulfite in water medium [13]. So designing luminescent probes for selective anion recognition in water medium is indeed required.
Metal–organic frameworks (MOFs) are a distinguished class of crystalline porous material that has emerged over the last decade [14], [14](a), [14](b), [14](c), [14](d), [14](e), [14](f). Constructed from organic struts and metal nodes, MOFs represent an excellent tunable structure–property co-relationship. The choice of metal nodes as well as the design of the organic counterpart can be devised strategically to target and obtain any specific functionality. This can be utilized for the usage of MOFs in terms of molecular storage and separation, sensors, ion-conduction, molecular transport and various other applications [15], [15](a), [15](b). Luminescent MOFs (LMOFs) are a subclass of MOFs with wide range of applications in terms of molecular sensing, fabrication of optical devices etc [16], [16](a), [16](b), [16](c), [16](d). The long range ordering and porosity generated from the coordination nanospace within these MOFs makes them an excellent platform for host–guest chemistry [17], [17](a), [17](b). The strategic design and incorporation of luminescent building blocks tune the electronic properties of the framework while the presence of secondary functional sites imparts selectivity to the host framework towards a particular incoming guest inducing high sensitivity towards the analyte. Linker modulation plays a huge role in design of luminescent MOFs which is carried out via two approaches: (a) pre-synthetic approach where MOFs are constructed from linkers bearing targeted functional sites and (b) post-synthetic approach where modifications are carried out in synthesized MOFs to render desired functionalities within the framework [18], [18](a), [18](b), [18](c). Hence with all these attributes MOFs can perform as an excellent luminescent sensor to incoming guest molecules. For recognition of a particular analyte in water medium water stability of the luminescent probe is a primary criterion. MIL-series (MIL stands for ‘Materials Institut Lavoisier’) are a well-known family of MOFs for its chemical stability, high porosity and availability of free tunable functionality within the system [19], [20], [21], [22], [23], [24]. Keeping all these things in mind herein we have chosen a carboxylate based MOF NH2-MIL-68(In) bearing free accessible amine moiety [25], [25](a), [25](b), [25](c), [25](d). Aldehyde groups were attached to it through post synthetic modification approach [26], [27], [28], [29]. The facile reaction between bisulfite anion and aldehyde group is known to generate turn on response in the fluorescence intensity [30]. Turn on response is desired for its high signal to noise ratio and also to minimize false response as the detection occurs relative to a dark background [31], [31](a), [31](b), [31](c), [31](d). Herein we report selective aqueous phase detection of bisulfite anion with a post synthetically modified MOF NH2-MIL-68(In)@CHO (Fig. 1). To the best of our knowledge detection of this anion by post-synthetically modified MOF is demonstrated for the first time in the literature.
Section snippets
Materials and measurements
Materials: All the chemicals used in this work were obtained commercially. 2-Aminoterepthalic acid, glyoxal solution (40 wt% in water) and In(NO3)3·xH2O was purchased from Sigma Aldrich and are used without any further purification.
Physical measurements: Powder X-ray diffraction (PXRD) data were recorded at Bruker D8 advanced X-ray diffractometer using Cu Kα radiation (λ = 1.5406 Å) in 5–30° 2θ range. The IR-spectra were acquired by using NICOLET 6700 FT-IR spectrophotometer using KBr pellet in
Results and discussion
The NH2-MIL-68(In)(1-NH2) was synthesized via solvothermal reaction according to protocol reported in the literature from its precursors In(NO3)3·xH2O and 2-aminoterepthalic acid [32]. After synthesis of the compound the pristine phase (1′-NH2) was collected by filtration and air-dried. PXRD analysis was done and the patterns of 1′-NH2 matched well with the simulated pattern of MIL-68(In) confirming the bulk phase purity of 1′-NH2. Then the compound was dipped into methanol for 3–4 days and
Conclusion
In conclusion we have synthesized a post-synthetically modified MOF (1-CHO) from a water stable robust porous parent MOF NH2-MIL-68(In)(1-NH2). 1-CHO is used for selective recognition of bisulfite anion in aqueous medium. Selective recognition of bisulfite anion through such reaction based approach by a MOF based luminescent probe is rarely explored. We believe that this work can contribute in the development process of constructing MOF based sensors for practical applications.
Acknowledgements
A.S., A.D. and S.D. are thankful to IISER-Pune for research fellowship. P.S. is thankful to UGC for research fellowship. S.L. is thankful to CSIR for research fellowship. We are grateful to IISER Pune and DST-SERB (Grant EMR/2016/000410) for research facilities and financial support.
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These authors contributed equally.