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Ammoniated MOF-74(Zn) derivatives as luminescent sensor for highly selective detection of tetrabromobisphenol A
Ecotoxicology and Environmental Safety 187 (2020) 109821
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Ammoniated MOF-74(Zn) derivatives as luminescent sensor for highly selective detection of tetrabromobisphenol A
T
Xiao-lei Zhangb, Su-mei Lia, Sha Chena,∗, Fan Fenga, Jin-quan Baib,∗∗, Jian-rong Lib a
Key Laboratory of Beijing on Regional Air Pollution Control, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, PR China Beijing Key Laboratory for Green Catalysis and Separation, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, PR China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Tetrabromobisphenol-A Ammoniated MOF-74(Zn) derivatives Fluorescent sensor
In this study, a porous framework MOF-74(Zn) (Zn2 (DHBDC)(DMF)(H2O)2, H4dondc = 1, 5-dioxido-2, 6naphthalenedicarboxylic acid) with open metal sites was successful synthesized. MOF-74(Zn) as a template was grafted on the open metal sites with ethylenediamine (en) named MOF-74(Zn)-en to develop a highly selective and sensitive fluorescence detector for rapid determination of tetrabromobisphenol A (TBBPA). The obtained MOF-74(Zn)-en was well characterized by Fourier Transform Infrared (FT-IR), Scanning Electron Microscopy (SEM) and showed ideal properties of photoluminescence. The fluorescence enhancement showed a good linear relationship with the concentrations of TBBPA in the range of 50–400 μg/L, and its limit of detection could reach to 0.75 μg/L. Furthermore, the possible sensing mechanism of the fluorescence enhancement could be attributed to Förster resonance energy transfer (FRET). The results will provide a convenient and quick method for detection of TBBPA. To the best of my knowledge, this is the first case to detect TBBPA by fluorescence enhancement with MOF derivatives.
1. Introduction Tetrabromobisphenol-A (TBBPA) is the most used brominated flame retardant (BFR) in recent decades. 200,000 tons of TBBPA were consumed in 2013, and its market share reached about 60% of the total BFRs (Yu et al., 2019; Cheng et al., 2019). TBBPA is usually used as an additive flame retardant, which cannot chemically interact with polymers and can be released into the environment more readily (Birnbaum and Staskal, 2004; Huang et al., 2014). So far, the presence of TBBPA has been found in aquatic food webs, soils, and sediments (Alaee et al., 2003; Covaci et al., 2006). Moreover, TBBPA has been reported in biological samples, such as breast milk, serum, cord blood, adipose tissue worldwide (Barghi et al., 2017, Malliari and Kalantzi, 2017). Recent studies showed that the potential adverse effects of TBBPA might act as an endocrine disruptor. In some cases, TBBPA has estrogenic and significantly thyroid hormonal activities (Barghi et al., 2017). Therefore, in recent years, methods for detecting TBBPA in various substrates have attracted wide attention. Traditional analytical methods have been established for TBBPA detection, including high performance liquid chromatography-ultraviolet detection (HPLC-UV), liquid chromatography-mass spectrometry
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(LC-MS), gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) (Kopp et al., 2012; Lankova et al., 2013). Although the above methods are sensitive and accurate, complex instruments and equipment, high cost, requirement of amounts of sample preparation and skilled operation of professionals, limit their use. Therefore, developing a rapid, sensitive, simple, and low-cost method to monitor trace TBBPA is essential. In recent years, fluorescence detection has received more and more attention, because of its high sensitivity, simplicity, short response time, and application in both solution and solid phase. Some fluorescent composites have been synthesized and used in the detection of trace TBBPA (Chen et al., 2012; Feng et al., 2019). However, due to multistep processing, lack of ordered molecular arrangement and non-adjustable surface functional groups, wide applications of these sensors are usually restricted. Metal–organic frameworks (MOFs) is a new type of crystalline porous material. It has a reasonably designed framework structure, adjustable molecular and pore size, tailorable inner surface of the channels and cavities, and unsaturated metal sites. Due to their unique and attractive features, MOFs and MOF derivatives have been applied in gas storage (Je Seon Yeon et al., 2015) and separation (Li et al.,
Corresponding author. Corresponding author. E-mail address: [email protected] (S. Chen).
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https://doi.org/10.1016/j.ecoenv.2019.109821 Received 2 August 2019; Received in revised form 8 October 2019; Accepted 14 October 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.
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Fig. 1. Schematic procedure for preparation of the MOF-74(Zn)-en sensing material.
fluorescence sensor have been investigated. Compared with other analytical techniques and previous studies based on photoluminescence probes for determination of TBBPA, this method was simpler and had higher efficient. Moreover, this MOF-74(Zn)-en sensor will provide a simple, convenient, and accurate method for determination of TBBPA.
2014), catalysis (Hwang et al., 2008), and sensing (Allendorf et al., 2009). MOFs have contributed to development in the environmental remediation field, concerning the detection of hazardous pollutants. A small number of MOFs have been applied to detect some hazardous substance, because of their luminescent properties (Gole et al., 2011; Lustig et al., 2017; Nagarkar et al., 2013). Lan et al., (2009) synthesized a luminescent 3D porous MOF ([Zn2(bpdc)2-(bpee)]2DMF, (bpdc = 4,4′-biphenyldicarboxylate; bpee = 1,2-bipyridylethene) (Lan et al., 2009), which could highly selective detection of nitro explosives. The possible reason for this MOF to have a highly selective detection of nitro explosives is its infinite 3D framework structure and inherent micro porosity. MOFs can combine fluorescence properties and contactable voids, which chemically convert host and guest into detectable luminescence changes, making it an ideal choice for chemical sensing applications (Pramanik et al., 2011; Shustova et al., 2013; Tian et al., 2014). In this work, a highly sensitive and selective MOFs-based materials sensor, named MOF-74(Zn)-en for determination of TBBPA was developed. It used MOF-74(Zn) as a template and ethylenediamine (en) as a modifying and functional group. The photoluminescence enhancement performance between TBBPA and the synthesized MOF-74(Zn)-en was investigated. The stability, responsiveness and practicability of the
2. Materials and methods 2.1. Chemicals and reagents In this study, solvents and reagents were without further purification and commercially available. Zinc acetate dihydrate was purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). 2,5Dihydroxyterephthalic Acid was purchased from J&K Scientific Ltd. (Beijing, China). N, N-dimethylformamide (DMF) were purchased from Fuchen Chemical Reagents Factory (Tianjin, China). Ethanol was purchased from Beijing Chemical Works (Beijing, China). Methanol was purchased from Beijing Chemical Works (Beijing, China). TBBPA (purity, ≥97%), 4-Nonylphenol (purity, ≥99%), Phenol (purity, ≥99%), were obtained from innochem Chemical Reagent Co., Ltd. (Beijing, China). Milli-Q Advantage A10 system (Millipore, Molsheim, France) was applied to make ultrapure water. 2
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newly synthesized MOF-74(Zn) is 440 nm. While the maximum emission wavelength of the MOF-74(Zn)-en the maximum emission wavelength shifted to 499 nm. The 59 nm red shift of the FL peak was observed, which indicated that the coordination molecule, DMF, was removed from the cavities of this as-prepared MOF-74(Zn) and the ethylenediamine was coordinated with the open metal sites of Zn. The FT-IR spectra of the evacuated at 270 °C MOF-74(Zn) (a), and MOF-74(Zn)-en (b) were compared as depicted in Fig. 2B. The broad peaks near 3327 cm−1 and 3280 cm−1 were corresponding to the RNH2 stretching vibration peak. The peak at 1579 cm−1 was the characteristic peak of the stretching vibration peak of N–H. The peak at around 1460 cm−1 could be assigned the stretching vibration peak of CH2, which is a part of ethylenediamine stretching vibration peak. Those major bands of MOF-74(Zn) and MOF-74(Zn)-en spectra shared a similar frequency. However, there should be no absorption peak of amino group of MOF-74 after activating at 270 °C. It might be due to the residual DMF, which was partially coordinated with the open sites of Zn. However, there were good agreement with a reported literature around 30% of quality loss (Nathaniel L. Rosi, 2005). Combining with FT-IR spectra, the results revealed that ethylenediamine was successfully grafted on the MOF-74(Zn). The X-ray powder diffraction (XRD) spectra of the MOF-74(Zn) simulated (a) as well as MOF-74(Zn) (b) were exhibited in Fig. 3A. The measured and simulated of diffraction peaks for MOF-74(Zn) were in a good agreement. The N2 adsorption of MOF-74(Zn) and MOF-74(Zn)-en was measured at 77 K and is shown in. Fig. 3B. The N2 adsorptiondesorption isotherm of MOF-74(Zn) displayed the type-IV isotherms with hysteresis loops of large constricted mesopores. As anticipated from the channel size, the BET surface area of MOF-74(Zn) were 232.05 m2/g, which is comparable with that reported recently by Zhang et al. (Zhang et al., 2016a,b). Amine grafting resulted in a severe reduction in BET surface area, because the pore space was occupied by the ethylenediamine. The surface area was 11.09 m2/g for MOF-74(Zn)en. This phenomenon of a significant decrease in specific surface area has been reported by Je Seon Yeon et al. (Je Seon Yeon et al., 2015). The morphology of the MOF-74(Zn) (A), MOF-74(Zn)-en (B) was measured with a scanning electron microscope (SEM). As shown in Fig. S1, particles of MOF-74(Zn)-en were smaller than those of MOF-74 (Zn) and it could be attributed to the agitation during the reaction. The TGA results (Fig. 4A) showed the change in weight of the sample when they were heated from 40 °C to 600 °C. The results of thermal stability of MOF-74 (Zn) showed that it was stable below 350 °C. The TGA results for MOF-74(Zn)-en was different from MOF-74(Zn). Since the boiling point of ethylenediamine in the pure state is about 116 °C, this change could be attributed to the evaporation of ethylenediamine. MOF74(Zn)-en were prepared by removing the solvent in the tunnel by activation and heating to remove the coordinated DMF from MOF74(Zn). Therefore, it is easy to understand that the residual quality of MOF-74 (Zn)-en was higher than that of MOF-74 (Zn).
2.2. Apparatus X-ray diffraction of MOF-74(Zn) was measured by using a D8-Focus Bragg-Brentano X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.54,178). Scanning electron microscopy (SEM) images were measured using a scanning electron microscope of Shimadzu SS550. Thermogravimetric analysis of MOF-74 (Zn) was obtained by using a TGA-50 (SHIMADZU) thermogravimetric analyzer at a heating rate of 10 °C min−1 under a N2 atmosphere. The Brunauer–Emmett–Teller (BET) surface area of MOF-74 (Zn) was obtained by a Micromeritics ASAP2020 under a nitrogen at 77 K. An IRAffinity-1 instrument with a spectral range of 4000–400 cm−1 was employed to record the Fourier transform infrared (FT-IR) spectra. Photoluminescence measurements were performed by using a Hitachi F-7000 photoluminescence spectrophotometer. 2.3. Synthesis of MOF-74(Zn)-en composite The design of this MOF-74(Zn)-en composite was based on the principle that the MOF-74 played a part of the carrier of functional groups and the selectivity for targeted molecules because of the appropriately sized tunnel, while the ethylenediamine (en) achieved recognition signal amplification as well as optical readout. Fig. 1 shows the schematic procedure for the preparation of MOF-74(Zn)-en. In briefly, 2,5-Dihydroxyterephtalic acid (239 mg, 1.20 mmol) and Zn (OAc)2•2H2O (686 mg, 3.12 mmol) were dissolved in 20 mL of dimethylformamide (DMF). The acid solution was slowly added to the salt solution and then stirred at the room temperature for 18 h to yield MOF-74 (Zn) (Tranchemontagne et al., 2008.) The MOF-74 (Zn) (100 mg) was washed for several times and dried in the air and then evacuated at 270 °C for 24 h. Subsequently, the MOF-74 (Zn) sample was added to a solution of toluene with ethylenediamine (1.14 mL) and stirred for 17 h at the room temperature. The final product was washed for several times and dried in the air to yield MOF-74(Zn)-en. The amino group (-NH2) of the ethylenediamine molecules interacted with TBBPA and then the FL intensity was enhanced. 2.4. Photoluminescence measurements Photoluminescence measurements were carried out under the following experimental conditions: the photomultiplier tube voltage and the slit width were set to 800 V and 2.5 nm, respectively. The excitation wavelength was 361 nm and the emission wavelengths were between 380 and 700 nm. A series of different concentrations of TBBPA (50–400 μg/L) were prepared in ethanol. The synthesized MOF-74(Zn)en particles were uniformly dispersed in ethanol under ultrasonic vibration to obtain a fresh stock solution (3 mg/ml). Then, 1800 μL of MOF-74(Zn)-en and 200 μL of different concentrations of TBBPA were mixed completely by stirring and then the photoluminescence intensity was measured.
3.2. Optimization of recognition and detection conditions 2.5. Specificity In order to achieve the highest sensitivity and the shortest analysis time, the concentrations of MOF-74(Zn)-en solution and incubation time were optimized. The photoluminescence intensity was obtained in the presence of TBBPA (150 μg/L). All of the optimization procedures were performed at the room temperature (25 °C).
Nonylphenol, phenol and bisphenol A were involved to evaluate the specificity of MOF-74(Zn)-en sensor for TBBPA. The selective experiments were carried out by using the same concentrations of TBBPA, nonylphenol, phenol, bisphenol A on the MOF-74(Zn)-en sensors, respectively. The procedures of measurement were as described above.
3.2.1. The concentration effect of MOF-74(Zn)-en A series of MOF-74(Zn)-en solution (80, 100, 125, 160, 200, 300, 375, 400 and 450 mg/L) were investigated for the sensitive detection of TBBPA. As we can see in Fig. 4B, the photoluminescence enhancement ratio was represented by F/F0, where F0 and F were the photoluminescence intensities before and after the addition of TBBPA, respectively. This ratio of F/F0 increased along with the concentrations of MOF-74(Zn)-en solution in the range of 80–300 mg/L. Then as the
3. Results and discussion 3.1. Synthesis and characterization of MOF-74(Zn)-en As shown in Fig. 2A, there was a notable difference in the photoluminescence spectra of the prepared MOF-74(Zn)-en compared to MOF-74(Zn). Specifically, the maximum emission wavelength of the 3
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Fig. 2. (A) Photoluminescence spectra of MOF-74(Zn) (a), and MOF-74(Zn)-en (b). (B) FT-IR spectra of MOF-74(Zn)-en (a), MOF-74(Zn) (b).
Fig. 3. (A) XRD patterns from MOF-74(Zn) simulated (a) and synthesized MOF-74(Zn) (b). (B) Nitrogen sorption-desorption isotherms image of the synthesized MOF74(Zn) (a) and MOF-74(Zn)-en (b).
Fig. 4. (A) TGA: weight changes of MOF-74(Zn) (a) and ethylenediamine functionalized MOF-74(Zn)-en (b). (B) Photoluminescence enhancement ratio of the fluorescent sensor with different concentrations of MOF-74(Zn)-en solution in the presence of TBBPA (150 μg/L).
Fig. 5. (A) Photoluminescence spectra of MOF-74(Zn)-en in the presence of TBBPA (150 μg/L) at different incubation time. (B) Photoluminescence enhancement ratio of MOF-74(Zn)-en in the presence of TBBPA (150 μg/L) at different incubation time.
3.2.2. Effect of contact time In order to ensure the completion of the interaction between TBBPA and MOF-74(Zn)-en, the contact time in the range of 0–55 min was studied. As shown in Fig. 5A, as the time increased, the FL intensity of
concentrations of MOF-74(Zn)-en were higher than 300 mg/L, the enhancement factor decreased slightly. Therefore, a MOF-74(Zn)-en solution of 300 mg/L was finally chosen for the determination of TBBPA in the further studies. 4
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Fig. 6. (A) Photoluminescence spectra of MOF-74(Zn)-en at the optimum conditions in the presence of different concentrations of TBBPA (0–400 μg/L) and (B) their corresponding calibration curves.
Fig. 7. (A) Fluorescence spectra of ligand, MOF-74, Activated MOF-74 and MOF-74-en and (B)photoluminescence emission spectrum of TBBPA and UV–vis absorption spectrum of MOF-74-en .
The pKa of BPA is 9.5, while the TBBPA pKa is 7.5, it may be easier with MOF-74(Zn)-en interaction.
MOF-74(Zn)-en was significantly enhanced. Fig. 5B shows that at the beginning, the enhancement rate of fluorescence increased rapidly with time, and the growth rate decreased until equilibrated at 40 min. So, the optimal contact time is 40 min. It was noted that the slit width of this measurement was 5 nm.
3.5. Method validation In order to test the accuracy and precision of the method, spiked samples with TBBPA at three different concentrations (100, 200 and 400 μg/L) were investigated. The satisfactory recoveries (89.28–106.67%) were obtained, indicating that the method has good accuracy and can be used for the determination TBBPA (see Table S1).
3.3. Analytical performances of MOF-74(Zn)-en for the detection of TBBPA The photoluminescence intensities of MOF-74(Zn)-en with different concentrations of TBBPA (0–400 μg/L) were studied under the optimized experimental conditions. As shown in Fig. 6A, the fluorescence intensity of MOF-74(Zn)-en increased significantly as the concentrations of TBBPA increased. The results showed that the prepared MOF74(Zn)-en could be used as an effective and simple method for detecting TBBPA. In this detection system, FL enhancement of MOF-74(Zn)-en can be quantified by the Stern-Volmer type equation (Zhang et al., 2016a,b): F/F0 = 1+KSV [CTBBPA].where F0 and F are the photoluminescence intensities before and after the addition of TBBPA, respectively. And [CTBBPA] was the concentration of TBBPA while KSV was the SternVolmer constant. As shown in Fig. 6B, for MOF-74(Zn)-en, there was a good linear relationship between F/F0 and TBBPA with a concentration range from 0 to 400 μg/L. The linear regression equation was F/ F0 = 0.004[c]+1(R^2 = 0.998).
3.6. Comparison of the analytical performances for MOF-74(Zn)-en and other methods As described in Table S2, the analytical performance of methods of MOF-74(Zn)-en fluorescence sensor and other techniques for detecting TBBPA reported were compared. This method is the first case to detect TBBPA by fluorescence enhancement of MOF derivatives, which has a wide linear range (50–400.0 μg/L) with the low limit of detection (LOD = 0.75 μg/L). In addition, the fluorescence sensor is simpler, more convenient, and more economical than the methods of chromatography and mass spectrometry. Compared to other fluorescence detection methods, it is a fluorescence enhancement method, which is more accurate in detecting higher concentration solutions. Therefore, the developed method shows a high potential for simple, rapid, high sensitivity and selective determination of TBBPA.
3.4. Selectivity of the MOF-74(Zn)-en sensor for TBBPA
3.7. Possible mechanism
In order to detect the selectivity of MOF-74(Zn)-en to TBBPA, we tested the TBBPA and its analogues fluorescence enhancement fractions F/F0 under at the same condition. Fig S2 shows the relationship between the fluorescence enhancement fractions F/F0 of the MOF-74(Zn)en (300 mg/L) sensors and the concentrations (150 μg/L) of TBBPA and its analogues, BPA, NP, and Phenol. Compared with NP and Phenol, the high selectivity of the MOF-74(Zn)-en fluorescent probe is due to size selection effect. For BPA the selective effect due to the pKa different.
To demonstrate the fluorescence enhancement phenomena, we investigated the fluorescence spectra of the ligand, MOF-74, MOF-74(Zn) after the activation and MOF-74(Zn)-en (Fig. 7A). The ligand 2,5-dihydroxyterephthalic acid emits broad wavelength with a maximum at 454 nm. Under the same conditions, the maximum emission wavelength of MOF-74(Zn) was reduced by 17 nm at 437 nm. This could be 5
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explained by a change in the band structure due to coordination with the metal cations (Drache et al., 2016). While MOF-74(Zn)-en showed broad photoluminescence peak centered at 499 nm. This phenomenon could be explained by the introduction of an amino group could promote the charge transfer interactions in the MOF (Fu et al., 2012; Gomes Silva et al., 2010). Förster resonance energy transfer, a popular mechanism for explaining fluorescence quenching or enhancement phenomena, is dipole–dipole coupling between excited donor and ground state acceptor chromophores, which allows an excited state to be transferred from the donor to the acceptor chromophore without electron exchange in short range (1–10 nm). This resonance process requires spectral overlap between the donor species' emission spectrum and acceptor species’ absorption spectrum. So, we investigated TBBPA fluorescence spectra (Fig. 7B) under the same conditions. At the excitation wavelength of 361 nm, TBBPA also has fluorescence emission with an emission range of 370–550 nm and a maximum emission wavelength of 440 nm. MOF74(Zn)-en also has a certain absorption in this range. The overlap between the TBBPA emission spectrum and MOF-74(Zn)-en absorption spectrum could prove that the mechanism of the fluorescence enhancement could be explained by Förster resonance energy transfer (FRET). And we speculated that TBBPA could interact with amino groups in MOF-74(Zn)-en and that could promote fluorescence enhancement, because FRET is a short range (1–10 nm) interaction (Lustig et al., 2017).
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4. Conclusions In this study, based on the properties of TBBPA, a MOF-based fluorescent probe MOF-74(Zn)-en was synthesized and applied to detect TBBPA with the high sensitivity and selectivity, which was the first case to detect TBBPA by using the property of fluorescence enhancement of MOFs. The mechanism of the fluorescence enhancement could be explained by Förster resonance energy transfer. This optical sensing method based on MOF-74(Zn)-en will provide a new method for detecting TBBPA. In the future, combined with pretreatment technologies (such as solid-phase extraction), it will be more widely used in the environmental monitoring. Acknowledgements Starting fund for doctoral research of Beijing University of Technology [005000514119050]; Foundation research fund of Beijing University of Technology [005000546318514] Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecoenv.2019.109821. References Alaee, M., Arias, P., Sjodin, A., Bergman, A., 2003. An overview of commercially used brominated flame retardants, their applications, their use patterns in different countries/regions and possible modes of release. Environ. Int. 29, 683–689. Allendorf, M.D., Bauer, C.A., Bhakta, R.K., Houk, R.J., 2009. Luminescent metal-organic frameworks. Chem. Soc. Rev. 38, 1330–1352. Barghi, M., Shin, E.S., Kim, J.C., Choi, S.D., Chang, Y.S., 2017. Human exposure to HBCD and TBBPA via indoor dust in Korea: estimation of external exposure and body burden. Sci. Total Environ. 593–594, 779–786. Birnbaum, L.S., Staskal, D.F., 2004. Brominated flame retardants: cause for concern? Environ. Health Perspect. 112, 9–17. Cheng, H., Hua, Z., Wang, L., Wang, Y., Xie, Z., Zhu, T., 2019. Relative effects of windinduced disturbances and vegetation on tetrabromobisphenol A cycling in shallow lakes: direct and indirect effects. Environ. Pollut. 252, 794–803.
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Ammoniated MOF-74(Zn) derivatives as luminescent sensor for highly selective detection of tetrabromobisphenol A
T
Xiao-lei Zhangb, Su-mei Lia, Sha Chena,∗, Fan Fenga, Jin-quan Baib,∗∗, Jian-rong Lib a
Key Laboratory of Beijing on Regional Air Pollution Control, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, PR China Beijing Key Laboratory for Green Catalysis and Separation, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, PR China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Tetrabromobisphenol-A Ammoniated MOF-74(Zn) derivatives Fluorescent sensor
In this study, a porous framework MOF-74(Zn) (Zn2 (DHBDC)(DMF)(H2O)2, H4dondc = 1, 5-dioxido-2, 6naphthalenedicarboxylic acid) with open metal sites was successful synthesized. MOF-74(Zn) as a template was grafted on the open metal sites with ethylenediamine (en) named MOF-74(Zn)-en to develop a highly selective and sensitive fluorescence detector for rapid determination of tetrabromobisphenol A (TBBPA). The obtained MOF-74(Zn)-en was well characterized by Fourier Transform Infrared (FT-IR), Scanning Electron Microscopy (SEM) and showed ideal properties of photoluminescence. The fluorescence enhancement showed a good linear relationship with the concentrations of TBBPA in the range of 50–400 μg/L, and its limit of detection could reach to 0.75 μg/L. Furthermore, the possible sensing mechanism of the fluorescence enhancement could be attributed to Förster resonance energy transfer (FRET). The results will provide a convenient and quick method for detection of TBBPA. To the best of my knowledge, this is the first case to detect TBBPA by fluorescence enhancement with MOF derivatives.
1. Introduction Tetrabromobisphenol-A (TBBPA) is the most used brominated flame retardant (BFR) in recent decades. 200,000 tons of TBBPA were consumed in 2013, and its market share reached about 60% of the total BFRs (Yu et al., 2019; Cheng et al., 2019). TBBPA is usually used as an additive flame retardant, which cannot chemically interact with polymers and can be released into the environment more readily (Birnbaum and Staskal, 2004; Huang et al., 2014). So far, the presence of TBBPA has been found in aquatic food webs, soils, and sediments (Alaee et al., 2003; Covaci et al., 2006). Moreover, TBBPA has been reported in biological samples, such as breast milk, serum, cord blood, adipose tissue worldwide (Barghi et al., 2017, Malliari and Kalantzi, 2017). Recent studies showed that the potential adverse effects of TBBPA might act as an endocrine disruptor. In some cases, TBBPA has estrogenic and significantly thyroid hormonal activities (Barghi et al., 2017). Therefore, in recent years, methods for detecting TBBPA in various substrates have attracted wide attention. Traditional analytical methods have been established for TBBPA detection, including high performance liquid chromatography-ultraviolet detection (HPLC-UV), liquid chromatography-mass spectrometry
∗
(LC-MS), gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) (Kopp et al., 2012; Lankova et al., 2013). Although the above methods are sensitive and accurate, complex instruments and equipment, high cost, requirement of amounts of sample preparation and skilled operation of professionals, limit their use. Therefore, developing a rapid, sensitive, simple, and low-cost method to monitor trace TBBPA is essential. In recent years, fluorescence detection has received more and more attention, because of its high sensitivity, simplicity, short response time, and application in both solution and solid phase. Some fluorescent composites have been synthesized and used in the detection of trace TBBPA (Chen et al., 2012; Feng et al., 2019). However, due to multistep processing, lack of ordered molecular arrangement and non-adjustable surface functional groups, wide applications of these sensors are usually restricted. Metal–organic frameworks (MOFs) is a new type of crystalline porous material. It has a reasonably designed framework structure, adjustable molecular and pore size, tailorable inner surface of the channels and cavities, and unsaturated metal sites. Due to their unique and attractive features, MOFs and MOF derivatives have been applied in gas storage (Je Seon Yeon et al., 2015) and separation (Li et al.,
Corresponding author. Corresponding author. E-mail address: [email protected] (S. Chen).
∗∗
https://doi.org/10.1016/j.ecoenv.2019.109821 Received 2 August 2019; Received in revised form 8 October 2019; Accepted 14 October 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.
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Fig. 1. Schematic procedure for preparation of the MOF-74(Zn)-en sensing material.
fluorescence sensor have been investigated. Compared with other analytical techniques and previous studies based on photoluminescence probes for determination of TBBPA, this method was simpler and had higher efficient. Moreover, this MOF-74(Zn)-en sensor will provide a simple, convenient, and accurate method for determination of TBBPA.
2014), catalysis (Hwang et al., 2008), and sensing (Allendorf et al., 2009). MOFs have contributed to development in the environmental remediation field, concerning the detection of hazardous pollutants. A small number of MOFs have been applied to detect some hazardous substance, because of their luminescent properties (Gole et al., 2011; Lustig et al., 2017; Nagarkar et al., 2013). Lan et al., (2009) synthesized a luminescent 3D porous MOF ([Zn2(bpdc)2-(bpee)]2DMF, (bpdc = 4,4′-biphenyldicarboxylate; bpee = 1,2-bipyridylethene) (Lan et al., 2009), which could highly selective detection of nitro explosives. The possible reason for this MOF to have a highly selective detection of nitro explosives is its infinite 3D framework structure and inherent micro porosity. MOFs can combine fluorescence properties and contactable voids, which chemically convert host and guest into detectable luminescence changes, making it an ideal choice for chemical sensing applications (Pramanik et al., 2011; Shustova et al., 2013; Tian et al., 2014). In this work, a highly sensitive and selective MOFs-based materials sensor, named MOF-74(Zn)-en for determination of TBBPA was developed. It used MOF-74(Zn) as a template and ethylenediamine (en) as a modifying and functional group. The photoluminescence enhancement performance between TBBPA and the synthesized MOF-74(Zn)-en was investigated. The stability, responsiveness and practicability of the
2. Materials and methods 2.1. Chemicals and reagents In this study, solvents and reagents were without further purification and commercially available. Zinc acetate dihydrate was purchased from Macklin Biochemical Co., Ltd. (Shanghai, China). 2,5Dihydroxyterephthalic Acid was purchased from J&K Scientific Ltd. (Beijing, China). N, N-dimethylformamide (DMF) were purchased from Fuchen Chemical Reagents Factory (Tianjin, China). Ethanol was purchased from Beijing Chemical Works (Beijing, China). Methanol was purchased from Beijing Chemical Works (Beijing, China). TBBPA (purity, ≥97%), 4-Nonylphenol (purity, ≥99%), Phenol (purity, ≥99%), were obtained from innochem Chemical Reagent Co., Ltd. (Beijing, China). Milli-Q Advantage A10 system (Millipore, Molsheim, France) was applied to make ultrapure water. 2
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newly synthesized MOF-74(Zn) is 440 nm. While the maximum emission wavelength of the MOF-74(Zn)-en the maximum emission wavelength shifted to 499 nm. The 59 nm red shift of the FL peak was observed, which indicated that the coordination molecule, DMF, was removed from the cavities of this as-prepared MOF-74(Zn) and the ethylenediamine was coordinated with the open metal sites of Zn. The FT-IR spectra of the evacuated at 270 °C MOF-74(Zn) (a), and MOF-74(Zn)-en (b) were compared as depicted in Fig. 2B. The broad peaks near 3327 cm−1 and 3280 cm−1 were corresponding to the RNH2 stretching vibration peak. The peak at 1579 cm−1 was the characteristic peak of the stretching vibration peak of N–H. The peak at around 1460 cm−1 could be assigned the stretching vibration peak of CH2, which is a part of ethylenediamine stretching vibration peak. Those major bands of MOF-74(Zn) and MOF-74(Zn)-en spectra shared a similar frequency. However, there should be no absorption peak of amino group of MOF-74 after activating at 270 °C. It might be due to the residual DMF, which was partially coordinated with the open sites of Zn. However, there were good agreement with a reported literature around 30% of quality loss (Nathaniel L. Rosi, 2005). Combining with FT-IR spectra, the results revealed that ethylenediamine was successfully grafted on the MOF-74(Zn). The X-ray powder diffraction (XRD) spectra of the MOF-74(Zn) simulated (a) as well as MOF-74(Zn) (b) were exhibited in Fig. 3A. The measured and simulated of diffraction peaks for MOF-74(Zn) were in a good agreement. The N2 adsorption of MOF-74(Zn) and MOF-74(Zn)-en was measured at 77 K and is shown in. Fig. 3B. The N2 adsorptiondesorption isotherm of MOF-74(Zn) displayed the type-IV isotherms with hysteresis loops of large constricted mesopores. As anticipated from the channel size, the BET surface area of MOF-74(Zn) were 232.05 m2/g, which is comparable with that reported recently by Zhang et al. (Zhang et al., 2016a,b). Amine grafting resulted in a severe reduction in BET surface area, because the pore space was occupied by the ethylenediamine. The surface area was 11.09 m2/g for MOF-74(Zn)en. This phenomenon of a significant decrease in specific surface area has been reported by Je Seon Yeon et al. (Je Seon Yeon et al., 2015). The morphology of the MOF-74(Zn) (A), MOF-74(Zn)-en (B) was measured with a scanning electron microscope (SEM). As shown in Fig. S1, particles of MOF-74(Zn)-en were smaller than those of MOF-74 (Zn) and it could be attributed to the agitation during the reaction. The TGA results (Fig. 4A) showed the change in weight of the sample when they were heated from 40 °C to 600 °C. The results of thermal stability of MOF-74 (Zn) showed that it was stable below 350 °C. The TGA results for MOF-74(Zn)-en was different from MOF-74(Zn). Since the boiling point of ethylenediamine in the pure state is about 116 °C, this change could be attributed to the evaporation of ethylenediamine. MOF74(Zn)-en were prepared by removing the solvent in the tunnel by activation and heating to remove the coordinated DMF from MOF74(Zn). Therefore, it is easy to understand that the residual quality of MOF-74 (Zn)-en was higher than that of MOF-74 (Zn).
2.2. Apparatus X-ray diffraction of MOF-74(Zn) was measured by using a D8-Focus Bragg-Brentano X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.54,178). Scanning electron microscopy (SEM) images were measured using a scanning electron microscope of Shimadzu SS550. Thermogravimetric analysis of MOF-74 (Zn) was obtained by using a TGA-50 (SHIMADZU) thermogravimetric analyzer at a heating rate of 10 °C min−1 under a N2 atmosphere. The Brunauer–Emmett–Teller (BET) surface area of MOF-74 (Zn) was obtained by a Micromeritics ASAP2020 under a nitrogen at 77 K. An IRAffinity-1 instrument with a spectral range of 4000–400 cm−1 was employed to record the Fourier transform infrared (FT-IR) spectra. Photoluminescence measurements were performed by using a Hitachi F-7000 photoluminescence spectrophotometer. 2.3. Synthesis of MOF-74(Zn)-en composite The design of this MOF-74(Zn)-en composite was based on the principle that the MOF-74 played a part of the carrier of functional groups and the selectivity for targeted molecules because of the appropriately sized tunnel, while the ethylenediamine (en) achieved recognition signal amplification as well as optical readout. Fig. 1 shows the schematic procedure for the preparation of MOF-74(Zn)-en. In briefly, 2,5-Dihydroxyterephtalic acid (239 mg, 1.20 mmol) and Zn (OAc)2•2H2O (686 mg, 3.12 mmol) were dissolved in 20 mL of dimethylformamide (DMF). The acid solution was slowly added to the salt solution and then stirred at the room temperature for 18 h to yield MOF-74 (Zn) (Tranchemontagne et al., 2008.) The MOF-74 (Zn) (100 mg) was washed for several times and dried in the air and then evacuated at 270 °C for 24 h. Subsequently, the MOF-74 (Zn) sample was added to a solution of toluene with ethylenediamine (1.14 mL) and stirred for 17 h at the room temperature. The final product was washed for several times and dried in the air to yield MOF-74(Zn)-en. The amino group (-NH2) of the ethylenediamine molecules interacted with TBBPA and then the FL intensity was enhanced. 2.4. Photoluminescence measurements Photoluminescence measurements were carried out under the following experimental conditions: the photomultiplier tube voltage and the slit width were set to 800 V and 2.5 nm, respectively. The excitation wavelength was 361 nm and the emission wavelengths were between 380 and 700 nm. A series of different concentrations of TBBPA (50–400 μg/L) were prepared in ethanol. The synthesized MOF-74(Zn)en particles were uniformly dispersed in ethanol under ultrasonic vibration to obtain a fresh stock solution (3 mg/ml). Then, 1800 μL of MOF-74(Zn)-en and 200 μL of different concentrations of TBBPA were mixed completely by stirring and then the photoluminescence intensity was measured.
3.2. Optimization of recognition and detection conditions 2.5. Specificity In order to achieve the highest sensitivity and the shortest analysis time, the concentrations of MOF-74(Zn)-en solution and incubation time were optimized. The photoluminescence intensity was obtained in the presence of TBBPA (150 μg/L). All of the optimization procedures were performed at the room temperature (25 °C).
Nonylphenol, phenol and bisphenol A were involved to evaluate the specificity of MOF-74(Zn)-en sensor for TBBPA. The selective experiments were carried out by using the same concentrations of TBBPA, nonylphenol, phenol, bisphenol A on the MOF-74(Zn)-en sensors, respectively. The procedures of measurement were as described above.
3.2.1. The concentration effect of MOF-74(Zn)-en A series of MOF-74(Zn)-en solution (80, 100, 125, 160, 200, 300, 375, 400 and 450 mg/L) were investigated for the sensitive detection of TBBPA. As we can see in Fig. 4B, the photoluminescence enhancement ratio was represented by F/F0, where F0 and F were the photoluminescence intensities before and after the addition of TBBPA, respectively. This ratio of F/F0 increased along with the concentrations of MOF-74(Zn)-en solution in the range of 80–300 mg/L. Then as the
3. Results and discussion 3.1. Synthesis and characterization of MOF-74(Zn)-en As shown in Fig. 2A, there was a notable difference in the photoluminescence spectra of the prepared MOF-74(Zn)-en compared to MOF-74(Zn). Specifically, the maximum emission wavelength of the 3
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Fig. 2. (A) Photoluminescence spectra of MOF-74(Zn) (a), and MOF-74(Zn)-en (b). (B) FT-IR spectra of MOF-74(Zn)-en (a), MOF-74(Zn) (b).
Fig. 3. (A) XRD patterns from MOF-74(Zn) simulated (a) and synthesized MOF-74(Zn) (b). (B) Nitrogen sorption-desorption isotherms image of the synthesized MOF74(Zn) (a) and MOF-74(Zn)-en (b).
Fig. 4. (A) TGA: weight changes of MOF-74(Zn) (a) and ethylenediamine functionalized MOF-74(Zn)-en (b). (B) Photoluminescence enhancement ratio of the fluorescent sensor with different concentrations of MOF-74(Zn)-en solution in the presence of TBBPA (150 μg/L).
Fig. 5. (A) Photoluminescence spectra of MOF-74(Zn)-en in the presence of TBBPA (150 μg/L) at different incubation time. (B) Photoluminescence enhancement ratio of MOF-74(Zn)-en in the presence of TBBPA (150 μg/L) at different incubation time.
3.2.2. Effect of contact time In order to ensure the completion of the interaction between TBBPA and MOF-74(Zn)-en, the contact time in the range of 0–55 min was studied. As shown in Fig. 5A, as the time increased, the FL intensity of
concentrations of MOF-74(Zn)-en were higher than 300 mg/L, the enhancement factor decreased slightly. Therefore, a MOF-74(Zn)-en solution of 300 mg/L was finally chosen for the determination of TBBPA in the further studies. 4
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Fig. 6. (A) Photoluminescence spectra of MOF-74(Zn)-en at the optimum conditions in the presence of different concentrations of TBBPA (0–400 μg/L) and (B) their corresponding calibration curves.
Fig. 7. (A) Fluorescence spectra of ligand, MOF-74, Activated MOF-74 and MOF-74-en and (B)photoluminescence emission spectrum of TBBPA and UV–vis absorption spectrum of MOF-74-en .
The pKa of BPA is 9.5, while the TBBPA pKa is 7.5, it may be easier with MOF-74(Zn)-en interaction.
MOF-74(Zn)-en was significantly enhanced. Fig. 5B shows that at the beginning, the enhancement rate of fluorescence increased rapidly with time, and the growth rate decreased until equilibrated at 40 min. So, the optimal contact time is 40 min. It was noted that the slit width of this measurement was 5 nm.
3.5. Method validation In order to test the accuracy and precision of the method, spiked samples with TBBPA at three different concentrations (100, 200 and 400 μg/L) were investigated. The satisfactory recoveries (89.28–106.67%) were obtained, indicating that the method has good accuracy and can be used for the determination TBBPA (see Table S1).
3.3. Analytical performances of MOF-74(Zn)-en for the detection of TBBPA The photoluminescence intensities of MOF-74(Zn)-en with different concentrations of TBBPA (0–400 μg/L) were studied under the optimized experimental conditions. As shown in Fig. 6A, the fluorescence intensity of MOF-74(Zn)-en increased significantly as the concentrations of TBBPA increased. The results showed that the prepared MOF74(Zn)-en could be used as an effective and simple method for detecting TBBPA. In this detection system, FL enhancement of MOF-74(Zn)-en can be quantified by the Stern-Volmer type equation (Zhang et al., 2016a,b): F/F0 = 1+KSV [CTBBPA].where F0 and F are the photoluminescence intensities before and after the addition of TBBPA, respectively. And [CTBBPA] was the concentration of TBBPA while KSV was the SternVolmer constant. As shown in Fig. 6B, for MOF-74(Zn)-en, there was a good linear relationship between F/F0 and TBBPA with a concentration range from 0 to 400 μg/L. The linear regression equation was F/ F0 = 0.004[c]+1(R^2 = 0.998).
3.6. Comparison of the analytical performances for MOF-74(Zn)-en and other methods As described in Table S2, the analytical performance of methods of MOF-74(Zn)-en fluorescence sensor and other techniques for detecting TBBPA reported were compared. This method is the first case to detect TBBPA by fluorescence enhancement of MOF derivatives, which has a wide linear range (50–400.0 μg/L) with the low limit of detection (LOD = 0.75 μg/L). In addition, the fluorescence sensor is simpler, more convenient, and more economical than the methods of chromatography and mass spectrometry. Compared to other fluorescence detection methods, it is a fluorescence enhancement method, which is more accurate in detecting higher concentration solutions. Therefore, the developed method shows a high potential for simple, rapid, high sensitivity and selective determination of TBBPA.
3.4. Selectivity of the MOF-74(Zn)-en sensor for TBBPA
3.7. Possible mechanism
In order to detect the selectivity of MOF-74(Zn)-en to TBBPA, we tested the TBBPA and its analogues fluorescence enhancement fractions F/F0 under at the same condition. Fig S2 shows the relationship between the fluorescence enhancement fractions F/F0 of the MOF-74(Zn)en (300 mg/L) sensors and the concentrations (150 μg/L) of TBBPA and its analogues, BPA, NP, and Phenol. Compared with NP and Phenol, the high selectivity of the MOF-74(Zn)-en fluorescent probe is due to size selection effect. For BPA the selective effect due to the pKa different.
To demonstrate the fluorescence enhancement phenomena, we investigated the fluorescence spectra of the ligand, MOF-74, MOF-74(Zn) after the activation and MOF-74(Zn)-en (Fig. 7A). The ligand 2,5-dihydroxyterephthalic acid emits broad wavelength with a maximum at 454 nm. Under the same conditions, the maximum emission wavelength of MOF-74(Zn) was reduced by 17 nm at 437 nm. This could be 5
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explained by a change in the band structure due to coordination with the metal cations (Drache et al., 2016). While MOF-74(Zn)-en showed broad photoluminescence peak centered at 499 nm. This phenomenon could be explained by the introduction of an amino group could promote the charge transfer interactions in the MOF (Fu et al., 2012; Gomes Silva et al., 2010). Förster resonance energy transfer, a popular mechanism for explaining fluorescence quenching or enhancement phenomena, is dipole–dipole coupling between excited donor and ground state acceptor chromophores, which allows an excited state to be transferred from the donor to the acceptor chromophore without electron exchange in short range (1–10 nm). This resonance process requires spectral overlap between the donor species' emission spectrum and acceptor species’ absorption spectrum. So, we investigated TBBPA fluorescence spectra (Fig. 7B) under the same conditions. At the excitation wavelength of 361 nm, TBBPA also has fluorescence emission with an emission range of 370–550 nm and a maximum emission wavelength of 440 nm. MOF74(Zn)-en also has a certain absorption in this range. The overlap between the TBBPA emission spectrum and MOF-74(Zn)-en absorption spectrum could prove that the mechanism of the fluorescence enhancement could be explained by Förster resonance energy transfer (FRET). And we speculated that TBBPA could interact with amino groups in MOF-74(Zn)-en and that could promote fluorescence enhancement, because FRET is a short range (1–10 nm) interaction (Lustig et al., 2017).
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4. Conclusions In this study, based on the properties of TBBPA, a MOF-based fluorescent probe MOF-74(Zn)-en was synthesized and applied to detect TBBPA with the high sensitivity and selectivity, which was the first case to detect TBBPA by using the property of fluorescence enhancement of MOFs. The mechanism of the fluorescence enhancement could be explained by Förster resonance energy transfer. This optical sensing method based on MOF-74(Zn)-en will provide a new method for detecting TBBPA. In the future, combined with pretreatment technologies (such as solid-phase extraction), it will be more widely used in the environmental monitoring. Acknowledgements Starting fund for doctoral research of Beijing University of Technology [005000514119050]; Foundation research fund of Beijing University of Technology [005000546318514] Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecoenv.2019.109821. References Alaee, M., Arias, P., Sjodin, A., Bergman, A., 2003. An overview of commercially used brominated flame retardants, their applications, their use patterns in different countries/regions and possible modes of release. Environ. Int. 29, 683–689. Allendorf, M.D., Bauer, C.A., Bhakta, R.K., Houk, R.J., 2009. Luminescent metal-organic frameworks. Chem. Soc. Rev. 38, 1330–1352. Barghi, M., Shin, E.S., Kim, J.C., Choi, S.D., Chang, Y.S., 2017. Human exposure to HBCD and TBBPA via indoor dust in Korea: estimation of external exposure and body burden. Sci. Total Environ. 593–594, 779–786. Birnbaum, L.S., Staskal, D.F., 2004. Brominated flame retardants: cause for concern? Environ. Health Perspect. 112, 9–17. Cheng, H., Hua, Z., Wang, L., Wang, Y., Xie, Z., Zhu, T., 2019. Relative effects of windinduced disturbances and vegetation on tetrabromobisphenol A cycling in shallow lakes: direct and indirect effects. Environ. Pollut. 252, 794–803.
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