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Graphene-based magnetic metal organic framework nanocomposite for sensitive colorimetric detection and facile degradation of phenol
Journal of the Taiwan Institute of Chemical Engineers 102 (2019) 312–320
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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Graphene-based magnetic metal organic framework nanocomposite for sensitive colorimetric detection and facile degradation of phenol Yang Wang a,∗, Mingzhen Zhao a, Chen Hou a,∗, Xiaona Yang a, Zhijian Li a,∗, Qingjun Meng a, Chen Liang b a
College of Bioresources Chemical and Materials Engineering, Shaanxi Provincial Key Laboratory of Papermaking Technology and Specialty Paper Development, Key Laboratory of Paper Based Functional Materials of China National Light Industry, National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science and Technology, Xi’an 710021, China b Key Laboratory of Clean Pulp and Papermaking and Pollution Control of Guangxi Province, Guangxi University, Nanning 543003, China
a r t i c l e
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a b s t r a c t
Article history: Received 14 November 2018 Revised 27 May 2019 Accepted 25 June 2019 Available online 3 July 2019
Phenol is a toxic and widely spread as environmental pollutants; the sensitive detection and efficient removal are quite important but challenging. Herein, we take the intrinsic advantages of graphene-based magnetic metal organic framework (MOF) composite as mimic enzyme sensor and Fenton-like catalyst for the colorimetric detection and degradation applications. In this work, magnetic Fe3 O4 decorated graphene MOF composite (Fe3 O4 /rGO/MOF) was fabricated by a simple strategy for the purpose of welldistributed porous MOF on basal planes of magnetic graphene. The adhesive coating of tannic acid film on both side of magnetic graphene was explored as the directing agent for the controllable self-assembly of MOF. Interestingly, the Fe3 O4 /rGO/MOF represent excellent mimic enzyme properties, which can detect phenol based on a visual colour change in water solution. The mimic enzyme senor can oxidize 4-aminoantipyrine in the presence of H2 O2 and exhibit strong affinity and selectivity in phenol detection. Simultaneously, Fe3 O4 /rGO/MOF also display high degradation ability towards phenol removal. These results indicate that the bifunctional composite can act both as colorimetric senor and Fenton-like catalyst, it would facilitate the real-time monitoring of water quality and wastewater treatment. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Magnetic graphene Metal organic framework Composite Colorimetric sensor Fenton-like catalyst
1. Introduction Phenol is a common water pollutant discharged from chemical plants, pharmaceuticals and various petrochemical industry [1]. Due to its high stability, poor biodegradability and toxicity at even low concentration, much greater efforts are needed for the sensitive detection and effective removal of phenol in waste water [2]. Therefore, considerable efforts were taken to develop various techniques for phenol detection, such as chromatographic [3], spectrophotometric [4], photocatalytic [5], adsorptive [6], and electrochemical [7]. Though most of these methods yield high sensitivity and high reproducibility, they are usually time consuming, expensive and operating sophisticated, thus limit their wide applications. Colorimetric detection, which can detect target analytes by the naked eye based on colour changes, is cheaper and simpler compared to the above instrumental detection methods, as well
∗
Corresponding authors. E-mail addresses: [email protected] (C. Hou), [email protected] (Z. Li).
(Y.
Wang),
[email protected]
as practical and reliable [8]. Among many developed technologies for the removal of phenols, Fenton oxidation process is utilized as a useful and environmentally friendly method for degradation of organic pollutants [9,10]. Especially, the Fenton-like heterogeneous catalyst, which is held with catalytic site of Fe2+ offer the advantages of easier separation and better reusability [11]. Herein, colorimetric sensing and Fenton-like catalysis have been considered as promising technologies in the detection and removal of phenol, respectively. Even though, most of the reported checking materials exhibited only one function as either the colorimetric sensors or the degradation materials. Systematic integration of the above two functions into one material is clearly desirable but rarely identified. Since Zare’s and Lin’s groups using enzyme-based inorganic material as a sensor for colorimetric detection of phenol in water, a great deal of excellent works about novel hybrid materials for colorimetric detection of phenol have been done [12,13]. Although promising, the application of enzyme-hybrid as a colorimetric sensor for sensitive and convenient detection of phenol from waste water has several disadvantages. First, the activity of enzyme can be hampered by environmental changes, such as temperature, pH value and ion concentration. Moreover, the high cost
https://doi.org/10.1016/j.jtice.2019.06.019 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Y. Wang, M. Zhao and C. Hou et al. / Journal of the Taiwan Institute of Chemical Engineers 102 (2019) 312–320
of enzyme and difficult to store can also limit its practical application in colorimetric detection. To overcome the above drawbacks, enzyme mimic nanomaterials based colorimetric sensor has emerged as an important colorimetric tool because of their robust properties and lower cost. Since the demonstration by Yan’s group that Fe3 O4 nanoparticles (NPs) exhibited peroxidase-like activity, numerous other artificial nanomaterials have been emerged as enzyme mimics and developed as various kinds of extremely promising colorimetric sensors [14]. Various nanomaterials, including Au NPs, Pt NPs, CdS, NPs, Co3 O4 NPs, CuO NPs, and carbon dots, have already been evaluated as enzyme mimics [15,16]. Compared to natural enzymes, nanomaterial-based enzyme mimics exhibited some robust properties such as excellent stability, tunable catalytic activity, easy to storage and recyclability, as well as low cost. Recently, metal-organic framework (MOF) have been reported to exhibit peroxidase-like catalytic performances similar to natural enzymes [17,18]. Moreover, the highly porous structure of MOFs is conducive to the preconcentration of target detector, thus enhancing the detection ability of the material. However, further exploration of organic pollutant degradation with the MOF materials as enzyme mimics have been seldom reported. It is known that ferrites are effectively heterogeneous catalyst for Fenton degradation of organics, which can produce highly active hydroxyl radicals (·OH) with the addition of H2 O2 on the basis of Fe(III)/Fe(II) redox cycle. The obtained ·OH radicals can oxidize most of organic contaminants in waste water [19, 20]. Thus, the integration of magnetic Fe3 O4 NPs with MOFs is rational and will be promising candidates for both colorimetric detection and degradation functions. Moreover, the metallic NPs loaded reduced graphene oxide (rGO) nanosheets have drawn intensive interest due to the fantastic physical and chemical properties of rGO. When applied as an enzyme mimic, the NPs loaded rGO showing catalytic performances similar to or even better than that of the natural enzyme [21, 22]. Although NPs loaded rGO has been reported as enzyme mimics, further exploration of additional functions of the rGO based nanostructures and applications in fast and sensitive detection and removal of pollutants still remains a great challenge in this field. This is might be due to the intrinsic limitation in the metal-chelation ability of graphene, which has only a few functional groups [23]. Besides, the pure rGO is prone to aggregation, and thus prevents the ordered assembly of NPs on the surface. Therefore, it is critical to develop bifunctional graphene based magnetic MOFs composite as mimic enzyme with an optimized structure and surface affinity for colorimetric detection and Fenton-like degradation of phenol. Herein, we report a double application of the rGO based magnetic MOFs composite for colorimetric detection and degradation of phenol. MOFs were assembled on basal planes of the Fe3 O4 /rGO on the aid of tannic acid adhesion and thus successfully synthesized a dispersive sandwich-like Fe3 O4 /rGO/MOF composite. Tannic acid (TA) is a polyphenol that contains five digalloylester groups with strong affinity towards surfaces [24,25]. Thus, the fantastic properties in metal chelation made TA been prominent constituent for organic-inorganic film construction. It was demonstrated the controllable construction of hierarchical Fe3 O4 /rGO/MOF composite for sensitive colorimetric detection and efficient degradation of phenol. The purpose of the Fe3 O4 /rGO/MOF composite can act as an excellent platform based on the integrate several different functionalities into one structure, and the double active sites (Fe3 O4 and metal nodes in MOFs) can boost the catalytic reaction via the synergy effect. Excellent sensitivity toward the colorimetric detection of phenol was verified. The degradation ability of Fe3 O4 /rGO/MOF was further elucidated by the Fenton-like reaction as well. Both detection and degradation of phenol can be easily conducted in the recycling experiments owning to the unique magnetic property of the catalyst. The results show that the
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sensitive detection and effective degradation of phenol by using Fe3 O4 /rGO/MOF composite as colorimetric sensor and Fenton-like catalyst. 2. Materials and methods 2.1. Materials Natural graphite powder, FeCl3 ·6H2 O, Zn(NO3 )2 ·6H2 O, 2methylimidazole, o-phenylenediamine (OPD), 4-aminoantipyrine (4-AAP), tannic acid (TA), Polyethylene glycol 60 0 0 (PEG6000 ), polyvinylpyrrolidone 10,0 0 0 (PVP10000 ), NaCH2 COOH (NaAc), CH3 COOH (HAc), H2 O2 (30%), citric acid (CA) were purchased from KeLuo Fine Chemicals Co., Ltd. (China); other chemicals and reagents were of analytical grade, obtained from Tianjing Chemical Reagent Company (China). 2.2. Synthesis of tannic acid modified magnetic graphene composite Graphene oxide (GO) was prepared from graphite powder according to a modified method [26], and a developed solvothermal strategy was used for the preparation of the magnetic Fe3 O4 /rGO composites [23]. In a typical procedure, GO (0.05 g) was added into ethylene glycol (30 mL) and ultrasounded under a drastic stirring for 1 h. Then FeCl3 ·6H2 O (0.487 g) and sodium actate (NaAc, 0.973 g) were dispersed into the aforementioned solution under a continuous stirring. After dissolved completely, the rich dark brown solution was transferred into a 50 mL Teflon reactor and heated at 200 °C for 8 h. After cooling down to room temperature, the solid product was gained by magnetic separation, and washed with deionized water and ethanol repeatedly several times. Finally, the product was dried in a vacuum at 40 °C for 8 h to obtain the uniform Fe3 O4 nanoparticles embedded in GO. For the tannic acid adhesive coating, Fe3 O4 /rGO (5 mg ml−1 ) was suspended in deionized water with TA (3 mg mL−1 ) under gental stirring. 15 min later, the composites were collected by an extenal magnetic field to obtain TA modified Fe3 O4 /rGO (Fe3 O4 /rGO/TA). The product was washed with deionized water and ethanol and evaporated under vacuum. 2.3. Preparation of the Fe3 O4 /rGO/MOF composite MOFs (zeolite imidazole framework-8, ZIF-8) were assembled on Fe3 O4 /rGO platform by chelating reaction between tannic acid and metal ions [24]. Typically, 50 mg of the as-prepared Fe3 O4 /rGO/TA was suspended in 10 mL of methanol solution containing 1 mmol Zn(NO3 )2 . After stirring together with ultrasonication at room temperature for 10 min, 10 mL of methanol solution containing 10 mmol 2-methylimidazole and 90 mg PVP10000 was added to the above suspension. Then the temperature was raised to 55 °C and stirred for another 1 h, the products were collected with a magnet and washed with methanol and deionized water. 2.4. Mimic peroxidase catalytic activity measurement and the detection of H2 O2 The mimic peroxidase properties of Fe3 O4 /rGO/MOF were examined as followed process. 200 μL of OPD (0.5 mM) and different concentrations of H2 O2 were added into NaAc buffer (0.2 M, pH = 4.0), then 2 mg of Fe3 O4 /rGO/MOF was suspended in the above solution with a total volume of 3.0 mL. The reaction was processed for 10 min at ambient temperature, then the solution was separated by an extra magnet and the absorbance of the solution was measured with UV–vis measurement at 450 nm.
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Fig. 1. Schematic illustration of preparation of Fe3 O4 /rGO/MOF composite and colorimetric detection and degradation of phenol using Fe3 O4 /rGO/MOF.
2.5. Detection of phenol Different concentrations of phenol were added into NaAc buffer (0.2 M, pH = 4.0), which containing H2 O2 (130 × 10−3 M), 4aminoantipyrine (4-AAP, 10 × 10−3 M) and Fe3 O4 /rGO/MOF (4 mg) with a total volume of 3.0 mL. The reaction was processed for 10 min at ambient temperature, then the solution was separated by an extra magnet and the absorbance of the solution was measured with UV–vis measurement at 505 nm. 2.6. Catalytic degradation experiments In a typical experiment, 4 mg of Fe3 O4 /rGO/MOF was suspended in a mixture solution containing 10 mL of phenol (20 mg L−1 ). The solution initial pH was adjusted by using predetermined concentration of HCl. The degradation reaction was initiated with an appropriate amount of H2 O2 . The process was performed under magnetic stirring at 40 °C. Samples (0.5 mL) were taken out at given time intervals and tertiary butanol (20 μL) was added to stop the reaction. For the reaction test, magnetic Fe3 O4 /rGO/MOF was collected by an extra magnet, washed with deionized water, dried and reused for the next run. All the experiments were conducted for three times to obtain average values and the average values were shown in figures. 2.7. Characterization Fourier transform infrared (FTIR) spectra were obtained in transmission mode on a FT-IR spectrometer (Bruker VERTEX 70, Germany) using the KBr pellet technique. Powder X-ray diffraction (XRD, Bruker D8 Advance, Germany, X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 1.54056)) was used to investigate the crystal structure of the composites. All spectra were recorded under the same experimental conditions. The morphologies of the Fe3 O4 /rGO/MOF were characterized by a transmission electron microscope (TEM, FEI Tecnal G2 F30) with energydispersive X-ray spectroscopy (EDS). The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. The pore
size distributions were derived from the adsorption branches of the isotherms based on the Barrett-Joyner-Halanda (BJH) model. Magnetization measurements were characterized on a Vibrating sample magnetometer (LAKESHORE-7304, USA) at room temperature. The UV–vis spectra were obtained using a Shimadzu U-30 0 0 spectrophotometer. The samples were placed in a 1 cm × 1 cm × 3 cm quartz cuvette, and the spectra were recorded at room temperature. The quantitative analysis of the samples was measured using a high-performance liquid chromatography (HPLC). A waters C18 column (5 μm, 4.6 × 250 mm) was utilized in the separation of residual phenol concentration. The mobile phase was composed of methanol and water with 60:40 v/v with a constant flow rate at 1.0 ml min−1 . The injective volume of the sample was 100 μL. 3. Results and discussion 3.1. Preparation and characterization of the Fe3 O4 /rGO/MOF composite Fe3 O4 /rGO was prepared via a hydrothermal method, and used as scaffold to carry other functionalized components and provide magnetic property for the easily recycle and reuse. To guarantee a homogeneous growth of MOF on both sides of Fe3 O4 /rGO, tannic acid (TA) was utilized as functional component for the adhesive coating on the surface of Fe3 O4 /rGO. Thus TA functionalized Fe3 O4 /rGO (Fe3 O4 /rGO/TA) can act as the platform and template, supporting and directing the formation of the MOF, and leading to a sheet-like MOF composite(Fe3 O4 /rGO/MOF) (Fig. 1). The coordination characterization of TA would contribute to a strong metalligand interaction and make MOF a uniform growth and nucleation on the Fe3 O4 /rGO plane. As a result, Fe3 O4 /rGO/MOF with a dispersive sandwich-like structure was successfully prepared, and abundant active sites were obtained owning to the surface coating of the MOF layer on Fe3 O4 /rGO. Furthermore, the thickness of the MOF layer can be easily tuned by the concentration of the organic linker and metal ions. The size and morphologies of Fe3 O4 /rGO/MOF composite was characterized by TEM observations. As displayed in Fig. 2a,
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Fig. 2. TEM images of the as-prepared Fe3 O4 /rGO (a), Fe3 O4 /rGO/TA (b) and Fe3 O4 /rGO/MOF (c), and (d) EDS analysis of Fe3 O4 /rGO/MOF (the signals of Cu and part of C/O are from the formvar-carbon coated copper grid).
Fig. 3. (a) FTIR spectra of Fe3 O4 /rGO, Fe3 O4 /rGO/TA and Fe3 O4 /rGO/MOF; (b) XRD patterns of GO, Fe3 O4 /rGO and Fe3 O4 /rGO/MOF (∗ and # refer to MOF and Fe3 O4 , respectively).
spherical Fe3 O4 NPs with a diameter about 200–250 nm was deposited uniformly on the rGO surface. Plenty of wrinkles on Fe3 O4 /rGO can be observed obviously due to the thin structure of the rGO sheet. Compared with Fe3 O4 /rGO, there was no obvious distinction after the TA film adhesion, indicating a transparent, thin and uniform TA film formed on the surface of Fe3 O4 /rGO (Fig. 2b). The transparent TA film is different from another adhesive film, polydopamine (PDA), which is formed by oxidizing dopamine to the black quinone structure [27, 28]. After the selfassembly of MOF, the morphology of the nanosheets changed obviously (Fig. 2c). A distinct grey thin layer was well-dispersed on the surface of Fe3 O4 /rGO/TA which made it become rough and coarse, implying the formation of MOF layer. Moreover, the thickness of the MOF layer on Fe3 O4 /rGO/MOF can be controlled in virtue of the self-assembly property. Energy-dispersive
X-ray spectroscopy (EDS) confirmed the presented elements in Fe3 O4 /rGO/MOF (Fig. 2d), peaks of Zn and N is characterized by zeolite imidazole frameworks (MOF), peaks of Fe confirmed the existence of Fe3 O4 NPs, and the appearance of the partial peaks of C and O can be attributed to the co-existence of rGO and TA. To demonstrate the functional groups and crystal structure of Fe3 O4 /rGO/MOF composite, FTIR spectra and XRD spectrum was recorded. As displayed in Fig. 3a, absorption bands at 3424, 1385 and 1055 cm−1 can be assigned to the functional groups, including –OH, C=C, and C–O–C of rGO appeared in the spectra of Fe3 O4 /rGO. A typical absorption band at 560 cm−1 can be attributed to Fe-O, while the intensities of the absorption bands at 1716 cm−1 (ν C = O ) can be nearly negligible, indicating the reduction of GO during the hydrothermal process [29]. TA was used for the modification of Fe3 O4 /rGO, and the FTIR spectrum of
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Fig. 4. (a) N2 absorption-desorption isotherms of Fe3 O4 /rGO/MOF; (b) hysteresis loops of Fe3 O4 /rGO and Fe3 O4 /rGO/MOF.
Fig. 5. (a) UV–vis absorption spectra of OPD solutions containing Fe3 O4 /rGO/MOF upon the addition of 2–150 μM H2 O2 ; (b) the plot of A450 versus H2 O2 concentration; (c) colorimetric change of OPD solutions containing Fe3 O4 /rGO/MOF upon the addition of 2–150 μM H2 O2 .
Fe3 O4 /rGO/TA provides the new evidence of the adhesive coating of TA film. In the spectrum of Fe3 O4 /rGO/TA, the enhanced band at 3424 cm−1 is attributed to the -OH stretching of the phenol group and methylol group of TA. The absorption band near 2924 and 2846 cm−1 is assigned to its aromatic C–H stretching mode of vibration. The absorption band at 1710 cm−1 is attributed to C=O stretching frequency of carboxyl group of TA. Intensities of the bands at 1434, 1346 and 1196 cm−1 corresponding to aromatic C–C and phenolic C–O stretching vibrations of TA [30]. After the self-assembly of MOF (ZIF-8), new absorption bands of
Fe3 O4 /rGO/MOF were presented compared with Fe3 O4 /rGO/TA. The presence of new band at around 442 cm−1 is assigned to ZnN stretching, bands at 1422 cm−1 and 10 0 0–130 0 cm−1 (1080, 1146 and 1308 cm−1 ) can be associated with the stretching and plane bending of the imidazole ring, and the absorption bands at 2924 and 2846 cm−1 are attributed to the aromatic and aliphatic C–H stretching, respectively. The FTIR analysis demonstrated the functional groups of the Fe3 O4 /rGO/MOF composite. Also, the XRD spectrum of pure GO shown a clear diffraction peak at 10.1°, which is related to the (002) reflection (Fig. 3b). After decorated with
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Fig. 6. (a) UV–vis absorption spectra of 4-AAP solutions containing Fe3 O4 /rGO/MOF upon the addition of 10–100 μM phenol; (b) the plot of A505 versus phenol concentration; (c) colorimetric change of 4-AAP solutions containing Fe3 O4 /rGO/MOF upon the addition of 10–100 μM phenol; (d) value at A505 of the solutions containing 40 μM phenol or various species. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fe3 O4 NPs by the solvothermal process, many new peaks can be observed in the pattern of Fe3 O4 /rGO. These peaks are typical diffraction peaks of magnetite (JCPDS NO. 19–0629). Compared with the XRD pattern of Fe3 O4 /rGO, several distinct diffraction peaks in Fe3 O4 /rGO/MOF corresponding to crystalline MOF (ZIF-8) can be observed, suggesting the successful deposition of MOF on the Fe3 O4 /rGO plane [31]. The nitrogen adsorption/desorption isotherm of the Fe3 O4 /rGO/MOF composite demonstrated that the composite exhibited large surface area (380.33 m2 g−1 ) (Fig. 4a). The total pore volume of Fe3 O4 /rGO/MOF is about 0.38 m2 g−1 , and the average pore diameter is 4.0 nm. Importantly, the high surface area of the Fe3 O4 /rGO/MOF would contribute to the affinity ability in practical catalysis. Compared to the aggregated graphene and bulk MOF materials, the thin MOF layers were assembled on the surface of graphene composite for Fe3 O4 /rGO/MOF. Thus, a unique mesoporous MOFs surface with affinitive graphene internal would accelerate the fast transport and diffusion of the substrates and the products. Especially, the porous MOF shell can provide sufficient surface area to facilitate the preconcentration of substrate, thus accelerate permeation of the reactants to access the active catalytic sites and enhance the reaction efficiency. The introduction of magnetic Fe3 O4 NPs on the rGO provide another advantage of the Fe3 O4 /rGO/MOF composite, which the magnetism of the as-prepared composite would promote the fast and
convenient separation in the process of catalyst recovery and reuse. The magnetic hysteresis curves of Fe3 O4 /rGO and Fe3 O4 /rGO/MOF were shown in Fig. 4b. The Fe3 O4 /rGO/MOF exhibited a relatively strong magnetism (35.2 emu g−1 ) at room temperature. Compare with that of Fe3 O4 /rGO (50.0 emu g−1 ), the decrease of the saturation magnetization value indicated the incorporation of nonmagnetic components. 3.2. Colorimetric detection of H2 O2 and phenol with Fe3 O4 /rGO/MOF A typical colorimetric detection process involving the oxidation of OPD in the presence of H2 O2 has been performed to test the peroxidase-like activity of the as-prepared Fe3 O4 /rGO/MOF composite. By monitoring the absorbance changes with UV–vis spectrum, a typical H2 O2 concentration-response curve in acetate buffer solution (pH = 4.0) was shown in Fig. 5a. An obvious increase in the adsorption peak at 450 nm was observed by increasing H2 O2 concentration. The high activity of the Fe3 O4 /rGO/MOF composite nanomaterial allowed the fast oxidation reaction with the reaction time of 10 min. Fig. 5b displayed the corresponding calibration plot. It can be calculated that H2 O2 can be detected in the linear range from 2 to 60 μM (R2 = 0.997), and the detection limit was estimated to be about 6.67 × 10−7 M (S/N = 3). Similarly, another application of Fe3 O4 /rGO/MOF composite as a colorimetric sensor to determine phenol was demonstrated
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Fig. 7. The effect of various parameters on degradation of phenol: (a) initial pH; (b) H2 O2 concentration; (c) catalyst dose; (d) reusability of Fe3 O4 /rGO/MOF. Conditions: (a) phenol concentration = 20 mg L−1 , initial H2 O2 concentration = 3.0 mmol L−1 , catalyst dose = 0.4 g L−1 , ambient temperature; (b) phenol concentration = 20 mg L−1 , initial pH = 3.0, catalyst dose = 0.4 g L−1 , ambient temperature; (c) phenol concentration = 20 mg L−1 , initial H2 O2 concentration = 3.0 mmol L−1 , initial pH = 3.0, catalyst dose = 0.4 g L−1 ; (d) phenol concentration = 20 mg L−1 , initial H2 O2 concentration = 3.0 mmol L−1 , initial pH = 3.0, catalyst dose = 0.4 g L−1 .
in biological buffer medium. A colorimetric system involving Fe3 O4 /rGO/MOF, co-oxidation of H2 O2 and 4-AAP was developed for the determination of phenol. The quantitative detection of phenol samples was performed upon adding different concentrations of phenol to the above reaction system at room temperature with the reaction time of 10 min. When the concentration of phenol increased from 10 to 100 μM, the absorbance of the samples increased (Fig. 6a) and the color of the reaction solution gradually changed from colorless to pink (Fig. 6c). A linear plot was found at the phenol concentration in the range of 10–80 μM (R2 = 0.994) at A505 (Fig. 6b), and its corresponding detection limit is estimated to be about 3.33 × 10−6 M (S/N = 3). The above results imply by the fact that the as-prepared Fe3 O4 /rGO/MOF composite exhibited excellent peroxidase-like activity, thus resulting in slow, gradual changes in color and wide linear range, and showing potential applications in biosensing and environmental monitoring. The selective detection of phenol in water system is highly desirable for practical applications. Above results demonstrate that Fe3 O4 /rGO/MOF exhibit high colorimetric detection efficiencies toward phenol, and the detection selectivity for phenol needs to be checked furthermore. In a control experiment, the UV–vis spectra of solution with the presence of phenol against other control molecules was examined. As displayed by Fig. 6d, the presence of the equivalent control molecules caused slight changes of the spectra. Upon introducing phenol into the mixture of the
control molecules and Fe3 O4 /rGO/MOF, the UV–vis spectra were significantly increased. This result reveals that the interference from above control molecules can be neglected, convincing the high colorimetric detection selectivities of Fe3 O4 /rGO/MOF toward phenol. The highly selective detections in water system make Fe3 O4 /rGO/MOF reliable sensing materials for phenol. 3.3. Degradation reaction of phenol Besides detection, the removal of phenol from wastewater is also important in water treatment. Reported results have shown that Fe3 O4 -based nanocomposite can be used as good Fenton-like catalyst for the degradation of organic contaminants from water [32, 33]. However, the relevant studies on phenol are still mean to be challenged up to now. Thus, the degradation performances of Fe3 O4 /rGO/MOF composite for phenol were checked. The influence of pH on the degradation reaction was shown in Fig. 7a. It is obvious that the elimination of phenol was nearly pHdependent. For example, at pH 3.0, phenol can be degraded 80% within 2 min, while the degradation efficiency decreased to 60% at pH 4.0. It can be explained by the reason that the decreased iron ions in the reaction system as pH increases [34]. When the pH decreased from 3.0 to 2.0, the degradation efficiency of phenol decreased sharply from 80% to 35%. According to previous reports, a futher increased acidity of the solution could make a less
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effective degradation [35]. Thus we can speculated that the formation of proton shell on the catalytic Fe(H2 O)6 3+ at lower acid environment, which can release hydration shell, thus affecting the catalytic reaction of H2 O2 . Even though, total removal of phenol can eventually be achieved in 16 min under different acid condition. The effect of initial H2 O2 concentration on phenol degradation was displayed by Fig. 7b. 20% of phenol can be converted within 2 min when 1.0 mM of H2 O2 was added into the solution. And the degradation of phenol increased rapidly (from 20% to 40%) if the H2 O2 concentration was 3.0 mM. This is due to an increased H2 O2 concentration was contributed to eliminate the given amount of phenol. When 5.0 mM of H2 O2 was added, a slightly decreased degradation rate can be observed. It was due to that excessive H2 O2 can react with ·OH, thus leading to the loss of hydroxyl radical which played a key role in phenol degradation. The following equations are shown to explain the undesired negative effects [36–38]:
H2 O2 + ·OH → H2 O + ·O2 H
(1)
·O2 H + ·OH → H2 O + O2
(2)
In order to explore the role of each component of the catalyst in the Fenton-like reaction, control experiments with an equivalent dosage of Fe3 O4 /rGO/TA composite, sandwich-like Fe3 O4 /rGO/MOF, and bare MOFs as catalysts were carried out with 3.0 mM of H2 O2 at pH 3.0. As shown in Fig. 7c, the Fe3 O4 /rGO/TA composite adsorbed less than 20% of phenol within 16 min in the absence of H2 O2 . The H2 O2 alone could only trigger small amount of phenol to be degraded. About 35% of phenol was removed when the bare MOFs was used as catalyst, indicating that MOFs could not efficiently catalyze the formation of ·OH from H2 O2 . The Fe3 O4 /rGO/TA without MOFs shell could catalyze complete degradation of phenol as well. But the achieved degradation rate was much lower than that obtained by the Fe3 O4 /rGO/MOF. This result indicated that both the target phenol and H2 O2 were able to access to the active center. But the porous MOFs shell on Fe3 O4 /rGO/MOF facilitate the increase of the local concentration of the reactive molecules via the adsorption effect, thus leading to a faster degradation reaction. The above results suggested that the excellent catalytic performance of the Fe3 O4 /rGO/MOF composite originated from the synergistic effects of porous MOFs shell and the inner Fe3 O4 /rGO. The reusability of the Fe3 O4 /rGO/MOF was tested by recovering the catalysts when the reaction finished and reusing it in the next run. Owning to the magnetic property, the Fe3 O4 /rGO/MOF can be recovered by an extra magnet easily. As shown in Fig. 7d, the catalytic activity of Fe3 O4 /rGO/MOF retained at 96% after five times reuse. It can be demonstrated by the negligible loss of Fe from Fe3 O4 /rGO/MOF composite. The stability of the catalyst can be attributed to the layer by layer protection of TA film on Fe3 O4 /rGO and the outer MOFs shell. Because TA is well known as a polydentate ligand to form complexes with metal ions [24]. As a result, Fe3 O4 /rGO/MOF showed high reusability and activity stability in phenol degradation. 4. Conclusion In this work, a facile and effective strategy was developed for controllable synthesis of sandwich-like graphene based magnetic metal organic frameworks. Due to the mild adhesive property of tannic acid, MOF layer can be self-assembled on both sides of magnetic graphene via the complexation between tannic acid and metal nods of MOF. Thus endowing a well-dispersed and porous MOF structure on the Fe3 O4 /rGO. The obtained Fe3 O4 /rGO/MOF composite were used as a mimic enzyme sensor for the colorimetric detection of phenol and achieved a superior catalytic ac-
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tivity. The colorimetric sensor can detect H2 O2 and phenol with a low detection limit, wide linear range and good selectivity. In addition, Fe3 O4 /rGO/MOF also can be used as effective Fenton-like catalyst to degrade phenol. It was found that the porous structure of the composite plays a key role in the preconcentration of phenol around the catalyst, which can enhance the detection and degradation efficiency. Due to the facile operation, low cost, high efficiency and easy separation, the bifunctional Fe3 O4 /rGO/MOF would provide a new sight into the simultaneously detection and degradation of contaminants, and showing potential applications in wastewater monitoring and treatment. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21806096), and the Open Project Program of National Demonstration Center for Experimental Light Chemistry Engineering Education (Grant no. 2018QGSJ02-19), Shaanxi University of Science & Technology, and the Guangxi Key Laboratory of Clean Pulping, Papermaking and Pollution Control Opening Fund (KF 201711). References [1] Gao J, Liu M, Song H, Zhang S, Qian Y, Li A. Highly-sensitive electrocatalytic determination for toxic phenols based on coupled cMWCNT/cyclodextrin edge– functionalized graphene composite. J Hazard Mater 2016;318:99–108. [2] Ni Y, Xia Z, Kokot S. A kinetic spectrophotometric method for simultaneous determination of phenol and its three derivatives with the aid of artificial neural network. J Hazard Mater 2011;192:722–9. [3] Zhao T, He J, Wang X, Ma B, Wang X, Zhang L, Li P, Liu N, Lu J, Zhang X. Rapid detection and characterization of major phenolic compounds in Radix Actinidia chinensis Planch by ultra-performance liquid chromatography tandem mass spectrometry. J Pharm Biomed Anal 2014;98:311–20. [4] Han XX, Chen L, Kuhlmann U, Schulz C, Weidinger IM, Hildebrandt P. Magnetic titanium dioxide nanocomposites for surface-enhanced resonance Raman spectroscopic determination and degradation of toxic anilines and phenols. Angew Chem Int Ed 2014;53:2481–4. [5] Ali MM, Sandhya KY. Visible light responsive titanium dioxide-cyclodextrin– fullerene composite with reduced charge recombination and enhanced photocatalytic activity. Carbon 2014;70:249–57. [6] Ma W, Li J, Tao X, He J, Xu Y, Yu JC, Zhao J. Efficient degradation of organic pollutants by using dioxygen activated by resin-exchanged iron(II) bipyridine under visible irradiation. Angew Chem Int Ed 2003;42:1059–62. [7] Li H, Chen Y, Zhang Y, Han W, Sun X, Li J, Wang L. Preparation of Ti/PbO2 -Sn anodes for electrochemical degradation of phenol. J Electro Anal Chem 2013;689:193–200. [8] Yao ZY, Yang YB, Chen XL, Hu XP, Zhang L, Liu L, Zhao YL, Wu HC. Visual detection of copper (II) ions based on an anionic polythiophene derivative using click chemistry. Anal Chem 2013;85:5650–3. [9] Wang H, Jing M, Wu Y, Chen W, Ran Y. Effective degradation of phenol via Fenton reaction over Cunife layered double hydroxides. J Hazard Mater 2018;353:53–61. [10] Dhakshinamoorthy A, Navalon S, Alvaro M, Garcia H. Metal nanoparticles as heterogeneous Fenton catalysts. Chem Sus Chem 2012;5:46–64. [11] He JC, Zhang Y, Zhang XD, Huang YM. Highly efficient Fenton and enzyme-mimetic activities of NH2 -MIL-88B(Fe) metal organic framework for methylene blue degradation. Sci Rep 2018;8:5159–66. [12] Zhu L, Gong L, Zhang Y, Wang R, Ge J, Liu Z, Zare RN. Rapid detection of phenol using a membrane containing laccase nanoflowers. Chem Asian J 2013;8:2358–60. [13] Lin ZA, Xiao Y, Yin YQ, Hu WL, Liu W, Yang HH. Facile synthesis of enzyme-inorganic hybrid nanoflowers and its application as a colorimetric platform for visual detection of hydrogen peroxide and phenol. ACS Appl Mater Interfaces 2014;6:10775–82. [14] Gao L, Zhuang J, Nie L, Zhang J, Zhang Y, Gu N, Wang T, Feng J, Yang D, Perrett S, Yan XY. Intrinsic peroxidase-like activity of ferromagnetic nanoparticles. Nat Nanotechnol 2007;2:577–83. [15] Cormode DP, Gao L, Koo H. Emerging biomedical applications of enzyme-like catalytic nanomaterials. Trends Biotechnol 2017;36:15–29. [16] Zhou Y, Liu B, Yang R, Liu J. Filling in the gaps between nanozymes and enzymes: challenges and opportunities. Bioconjugate Chem 2017;28(12):2903–9. [17] Hou C, Wang Y, Ding Q, Jiang L, Li M, Zhu W, Pan D, Zhu H, Liu MZ. Facile synthesis of enzyme-embedded magnetic metal-organic frameworks as a reusable mimic multi-enzyme system: mimetic peroxidase properties and colorimetric sensor. Nanoscale 2015;7:18770–9. [18] Wang Q, Zhang X, Huang L, Zhang Z, Dong S. GOx@ZIF-8(NiPd) nanoflower: an artificial enzyme system for tandem catalysis. Angew Chem Int Ed 2017;56(50):16082–5.
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Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Graphene-based magnetic metal organic framework nanocomposite for sensitive colorimetric detection and facile degradation of phenol Yang Wang a,∗, Mingzhen Zhao a, Chen Hou a,∗, Xiaona Yang a, Zhijian Li a,∗, Qingjun Meng a, Chen Liang b a
College of Bioresources Chemical and Materials Engineering, Shaanxi Provincial Key Laboratory of Papermaking Technology and Specialty Paper Development, Key Laboratory of Paper Based Functional Materials of China National Light Industry, National Demonstration Center for Experimental Light Chemistry Engineering Education, Shaanxi University of Science and Technology, Xi’an 710021, China b Key Laboratory of Clean Pulp and Papermaking and Pollution Control of Guangxi Province, Guangxi University, Nanning 543003, China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 14 November 2018 Revised 27 May 2019 Accepted 25 June 2019 Available online 3 July 2019
Phenol is a toxic and widely spread as environmental pollutants; the sensitive detection and efficient removal are quite important but challenging. Herein, we take the intrinsic advantages of graphene-based magnetic metal organic framework (MOF) composite as mimic enzyme sensor and Fenton-like catalyst for the colorimetric detection and degradation applications. In this work, magnetic Fe3 O4 decorated graphene MOF composite (Fe3 O4 /rGO/MOF) was fabricated by a simple strategy for the purpose of welldistributed porous MOF on basal planes of magnetic graphene. The adhesive coating of tannic acid film on both side of magnetic graphene was explored as the directing agent for the controllable self-assembly of MOF. Interestingly, the Fe3 O4 /rGO/MOF represent excellent mimic enzyme properties, which can detect phenol based on a visual colour change in water solution. The mimic enzyme senor can oxidize 4-aminoantipyrine in the presence of H2 O2 and exhibit strong affinity and selectivity in phenol detection. Simultaneously, Fe3 O4 /rGO/MOF also display high degradation ability towards phenol removal. These results indicate that the bifunctional composite can act both as colorimetric senor and Fenton-like catalyst, it would facilitate the real-time monitoring of water quality and wastewater treatment. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Magnetic graphene Metal organic framework Composite Colorimetric sensor Fenton-like catalyst
1. Introduction Phenol is a common water pollutant discharged from chemical plants, pharmaceuticals and various petrochemical industry [1]. Due to its high stability, poor biodegradability and toxicity at even low concentration, much greater efforts are needed for the sensitive detection and effective removal of phenol in waste water [2]. Therefore, considerable efforts were taken to develop various techniques for phenol detection, such as chromatographic [3], spectrophotometric [4], photocatalytic [5], adsorptive [6], and electrochemical [7]. Though most of these methods yield high sensitivity and high reproducibility, they are usually time consuming, expensive and operating sophisticated, thus limit their wide applications. Colorimetric detection, which can detect target analytes by the naked eye based on colour changes, is cheaper and simpler compared to the above instrumental detection methods, as well
∗
Corresponding authors. E-mail addresses: [email protected] (C. Hou), [email protected] (Z. Li).
(Y.
Wang),
[email protected]
as practical and reliable [8]. Among many developed technologies for the removal of phenols, Fenton oxidation process is utilized as a useful and environmentally friendly method for degradation of organic pollutants [9,10]. Especially, the Fenton-like heterogeneous catalyst, which is held with catalytic site of Fe2+ offer the advantages of easier separation and better reusability [11]. Herein, colorimetric sensing and Fenton-like catalysis have been considered as promising technologies in the detection and removal of phenol, respectively. Even though, most of the reported checking materials exhibited only one function as either the colorimetric sensors or the degradation materials. Systematic integration of the above two functions into one material is clearly desirable but rarely identified. Since Zare’s and Lin’s groups using enzyme-based inorganic material as a sensor for colorimetric detection of phenol in water, a great deal of excellent works about novel hybrid materials for colorimetric detection of phenol have been done [12,13]. Although promising, the application of enzyme-hybrid as a colorimetric sensor for sensitive and convenient detection of phenol from waste water has several disadvantages. First, the activity of enzyme can be hampered by environmental changes, such as temperature, pH value and ion concentration. Moreover, the high cost
https://doi.org/10.1016/j.jtice.2019.06.019 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Y. Wang, M. Zhao and C. Hou et al. / Journal of the Taiwan Institute of Chemical Engineers 102 (2019) 312–320
of enzyme and difficult to store can also limit its practical application in colorimetric detection. To overcome the above drawbacks, enzyme mimic nanomaterials based colorimetric sensor has emerged as an important colorimetric tool because of their robust properties and lower cost. Since the demonstration by Yan’s group that Fe3 O4 nanoparticles (NPs) exhibited peroxidase-like activity, numerous other artificial nanomaterials have been emerged as enzyme mimics and developed as various kinds of extremely promising colorimetric sensors [14]. Various nanomaterials, including Au NPs, Pt NPs, CdS, NPs, Co3 O4 NPs, CuO NPs, and carbon dots, have already been evaluated as enzyme mimics [15,16]. Compared to natural enzymes, nanomaterial-based enzyme mimics exhibited some robust properties such as excellent stability, tunable catalytic activity, easy to storage and recyclability, as well as low cost. Recently, metal-organic framework (MOF) have been reported to exhibit peroxidase-like catalytic performances similar to natural enzymes [17,18]. Moreover, the highly porous structure of MOFs is conducive to the preconcentration of target detector, thus enhancing the detection ability of the material. However, further exploration of organic pollutant degradation with the MOF materials as enzyme mimics have been seldom reported. It is known that ferrites are effectively heterogeneous catalyst for Fenton degradation of organics, which can produce highly active hydroxyl radicals (·OH) with the addition of H2 O2 on the basis of Fe(III)/Fe(II) redox cycle. The obtained ·OH radicals can oxidize most of organic contaminants in waste water [19, 20]. Thus, the integration of magnetic Fe3 O4 NPs with MOFs is rational and will be promising candidates for both colorimetric detection and degradation functions. Moreover, the metallic NPs loaded reduced graphene oxide (rGO) nanosheets have drawn intensive interest due to the fantastic physical and chemical properties of rGO. When applied as an enzyme mimic, the NPs loaded rGO showing catalytic performances similar to or even better than that of the natural enzyme [21, 22]. Although NPs loaded rGO has been reported as enzyme mimics, further exploration of additional functions of the rGO based nanostructures and applications in fast and sensitive detection and removal of pollutants still remains a great challenge in this field. This is might be due to the intrinsic limitation in the metal-chelation ability of graphene, which has only a few functional groups [23]. Besides, the pure rGO is prone to aggregation, and thus prevents the ordered assembly of NPs on the surface. Therefore, it is critical to develop bifunctional graphene based magnetic MOFs composite as mimic enzyme with an optimized structure and surface affinity for colorimetric detection and Fenton-like degradation of phenol. Herein, we report a double application of the rGO based magnetic MOFs composite for colorimetric detection and degradation of phenol. MOFs were assembled on basal planes of the Fe3 O4 /rGO on the aid of tannic acid adhesion and thus successfully synthesized a dispersive sandwich-like Fe3 O4 /rGO/MOF composite. Tannic acid (TA) is a polyphenol that contains five digalloylester groups with strong affinity towards surfaces [24,25]. Thus, the fantastic properties in metal chelation made TA been prominent constituent for organic-inorganic film construction. It was demonstrated the controllable construction of hierarchical Fe3 O4 /rGO/MOF composite for sensitive colorimetric detection and efficient degradation of phenol. The purpose of the Fe3 O4 /rGO/MOF composite can act as an excellent platform based on the integrate several different functionalities into one structure, and the double active sites (Fe3 O4 and metal nodes in MOFs) can boost the catalytic reaction via the synergy effect. Excellent sensitivity toward the colorimetric detection of phenol was verified. The degradation ability of Fe3 O4 /rGO/MOF was further elucidated by the Fenton-like reaction as well. Both detection and degradation of phenol can be easily conducted in the recycling experiments owning to the unique magnetic property of the catalyst. The results show that the
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sensitive detection and effective degradation of phenol by using Fe3 O4 /rGO/MOF composite as colorimetric sensor and Fenton-like catalyst. 2. Materials and methods 2.1. Materials Natural graphite powder, FeCl3 ·6H2 O, Zn(NO3 )2 ·6H2 O, 2methylimidazole, o-phenylenediamine (OPD), 4-aminoantipyrine (4-AAP), tannic acid (TA), Polyethylene glycol 60 0 0 (PEG6000 ), polyvinylpyrrolidone 10,0 0 0 (PVP10000 ), NaCH2 COOH (NaAc), CH3 COOH (HAc), H2 O2 (30%), citric acid (CA) were purchased from KeLuo Fine Chemicals Co., Ltd. (China); other chemicals and reagents were of analytical grade, obtained from Tianjing Chemical Reagent Company (China). 2.2. Synthesis of tannic acid modified magnetic graphene composite Graphene oxide (GO) was prepared from graphite powder according to a modified method [26], and a developed solvothermal strategy was used for the preparation of the magnetic Fe3 O4 /rGO composites [23]. In a typical procedure, GO (0.05 g) was added into ethylene glycol (30 mL) and ultrasounded under a drastic stirring for 1 h. Then FeCl3 ·6H2 O (0.487 g) and sodium actate (NaAc, 0.973 g) were dispersed into the aforementioned solution under a continuous stirring. After dissolved completely, the rich dark brown solution was transferred into a 50 mL Teflon reactor and heated at 200 °C for 8 h. After cooling down to room temperature, the solid product was gained by magnetic separation, and washed with deionized water and ethanol repeatedly several times. Finally, the product was dried in a vacuum at 40 °C for 8 h to obtain the uniform Fe3 O4 nanoparticles embedded in GO. For the tannic acid adhesive coating, Fe3 O4 /rGO (5 mg ml−1 ) was suspended in deionized water with TA (3 mg mL−1 ) under gental stirring. 15 min later, the composites were collected by an extenal magnetic field to obtain TA modified Fe3 O4 /rGO (Fe3 O4 /rGO/TA). The product was washed with deionized water and ethanol and evaporated under vacuum. 2.3. Preparation of the Fe3 O4 /rGO/MOF composite MOFs (zeolite imidazole framework-8, ZIF-8) were assembled on Fe3 O4 /rGO platform by chelating reaction between tannic acid and metal ions [24]. Typically, 50 mg of the as-prepared Fe3 O4 /rGO/TA was suspended in 10 mL of methanol solution containing 1 mmol Zn(NO3 )2 . After stirring together with ultrasonication at room temperature for 10 min, 10 mL of methanol solution containing 10 mmol 2-methylimidazole and 90 mg PVP10000 was added to the above suspension. Then the temperature was raised to 55 °C and stirred for another 1 h, the products were collected with a magnet and washed with methanol and deionized water. 2.4. Mimic peroxidase catalytic activity measurement and the detection of H2 O2 The mimic peroxidase properties of Fe3 O4 /rGO/MOF were examined as followed process. 200 μL of OPD (0.5 mM) and different concentrations of H2 O2 were added into NaAc buffer (0.2 M, pH = 4.0), then 2 mg of Fe3 O4 /rGO/MOF was suspended in the above solution with a total volume of 3.0 mL. The reaction was processed for 10 min at ambient temperature, then the solution was separated by an extra magnet and the absorbance of the solution was measured with UV–vis measurement at 450 nm.
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Fig. 1. Schematic illustration of preparation of Fe3 O4 /rGO/MOF composite and colorimetric detection and degradation of phenol using Fe3 O4 /rGO/MOF.
2.5. Detection of phenol Different concentrations of phenol were added into NaAc buffer (0.2 M, pH = 4.0), which containing H2 O2 (130 × 10−3 M), 4aminoantipyrine (4-AAP, 10 × 10−3 M) and Fe3 O4 /rGO/MOF (4 mg) with a total volume of 3.0 mL. The reaction was processed for 10 min at ambient temperature, then the solution was separated by an extra magnet and the absorbance of the solution was measured with UV–vis measurement at 505 nm. 2.6. Catalytic degradation experiments In a typical experiment, 4 mg of Fe3 O4 /rGO/MOF was suspended in a mixture solution containing 10 mL of phenol (20 mg L−1 ). The solution initial pH was adjusted by using predetermined concentration of HCl. The degradation reaction was initiated with an appropriate amount of H2 O2 . The process was performed under magnetic stirring at 40 °C. Samples (0.5 mL) were taken out at given time intervals and tertiary butanol (20 μL) was added to stop the reaction. For the reaction test, magnetic Fe3 O4 /rGO/MOF was collected by an extra magnet, washed with deionized water, dried and reused for the next run. All the experiments were conducted for three times to obtain average values and the average values were shown in figures. 2.7. Characterization Fourier transform infrared (FTIR) spectra were obtained in transmission mode on a FT-IR spectrometer (Bruker VERTEX 70, Germany) using the KBr pellet technique. Powder X-ray diffraction (XRD, Bruker D8 Advance, Germany, X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 1.54056)) was used to investigate the crystal structure of the composites. All spectra were recorded under the same experimental conditions. The morphologies of the Fe3 O4 /rGO/MOF were characterized by a transmission electron microscope (TEM, FEI Tecnal G2 F30) with energydispersive X-ray spectroscopy (EDS). The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. The pore
size distributions were derived from the adsorption branches of the isotherms based on the Barrett-Joyner-Halanda (BJH) model. Magnetization measurements were characterized on a Vibrating sample magnetometer (LAKESHORE-7304, USA) at room temperature. The UV–vis spectra were obtained using a Shimadzu U-30 0 0 spectrophotometer. The samples were placed in a 1 cm × 1 cm × 3 cm quartz cuvette, and the spectra were recorded at room temperature. The quantitative analysis of the samples was measured using a high-performance liquid chromatography (HPLC). A waters C18 column (5 μm, 4.6 × 250 mm) was utilized in the separation of residual phenol concentration. The mobile phase was composed of methanol and water with 60:40 v/v with a constant flow rate at 1.0 ml min−1 . The injective volume of the sample was 100 μL. 3. Results and discussion 3.1. Preparation and characterization of the Fe3 O4 /rGO/MOF composite Fe3 O4 /rGO was prepared via a hydrothermal method, and used as scaffold to carry other functionalized components and provide magnetic property for the easily recycle and reuse. To guarantee a homogeneous growth of MOF on both sides of Fe3 O4 /rGO, tannic acid (TA) was utilized as functional component for the adhesive coating on the surface of Fe3 O4 /rGO. Thus TA functionalized Fe3 O4 /rGO (Fe3 O4 /rGO/TA) can act as the platform and template, supporting and directing the formation of the MOF, and leading to a sheet-like MOF composite(Fe3 O4 /rGO/MOF) (Fig. 1). The coordination characterization of TA would contribute to a strong metalligand interaction and make MOF a uniform growth and nucleation on the Fe3 O4 /rGO plane. As a result, Fe3 O4 /rGO/MOF with a dispersive sandwich-like structure was successfully prepared, and abundant active sites were obtained owning to the surface coating of the MOF layer on Fe3 O4 /rGO. Furthermore, the thickness of the MOF layer can be easily tuned by the concentration of the organic linker and metal ions. The size and morphologies of Fe3 O4 /rGO/MOF composite was characterized by TEM observations. As displayed in Fig. 2a,
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Fig. 2. TEM images of the as-prepared Fe3 O4 /rGO (a), Fe3 O4 /rGO/TA (b) and Fe3 O4 /rGO/MOF (c), and (d) EDS analysis of Fe3 O4 /rGO/MOF (the signals of Cu and part of C/O are from the formvar-carbon coated copper grid).
Fig. 3. (a) FTIR spectra of Fe3 O4 /rGO, Fe3 O4 /rGO/TA and Fe3 O4 /rGO/MOF; (b) XRD patterns of GO, Fe3 O4 /rGO and Fe3 O4 /rGO/MOF (∗ and # refer to MOF and Fe3 O4 , respectively).
spherical Fe3 O4 NPs with a diameter about 200–250 nm was deposited uniformly on the rGO surface. Plenty of wrinkles on Fe3 O4 /rGO can be observed obviously due to the thin structure of the rGO sheet. Compared with Fe3 O4 /rGO, there was no obvious distinction after the TA film adhesion, indicating a transparent, thin and uniform TA film formed on the surface of Fe3 O4 /rGO (Fig. 2b). The transparent TA film is different from another adhesive film, polydopamine (PDA), which is formed by oxidizing dopamine to the black quinone structure [27, 28]. After the selfassembly of MOF, the morphology of the nanosheets changed obviously (Fig. 2c). A distinct grey thin layer was well-dispersed on the surface of Fe3 O4 /rGO/TA which made it become rough and coarse, implying the formation of MOF layer. Moreover, the thickness of the MOF layer on Fe3 O4 /rGO/MOF can be controlled in virtue of the self-assembly property. Energy-dispersive
X-ray spectroscopy (EDS) confirmed the presented elements in Fe3 O4 /rGO/MOF (Fig. 2d), peaks of Zn and N is characterized by zeolite imidazole frameworks (MOF), peaks of Fe confirmed the existence of Fe3 O4 NPs, and the appearance of the partial peaks of C and O can be attributed to the co-existence of rGO and TA. To demonstrate the functional groups and crystal structure of Fe3 O4 /rGO/MOF composite, FTIR spectra and XRD spectrum was recorded. As displayed in Fig. 3a, absorption bands at 3424, 1385 and 1055 cm−1 can be assigned to the functional groups, including –OH, C=C, and C–O–C of rGO appeared in the spectra of Fe3 O4 /rGO. A typical absorption band at 560 cm−1 can be attributed to Fe-O, while the intensities of the absorption bands at 1716 cm−1 (ν C = O ) can be nearly negligible, indicating the reduction of GO during the hydrothermal process [29]. TA was used for the modification of Fe3 O4 /rGO, and the FTIR spectrum of
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Fig. 4. (a) N2 absorption-desorption isotherms of Fe3 O4 /rGO/MOF; (b) hysteresis loops of Fe3 O4 /rGO and Fe3 O4 /rGO/MOF.
Fig. 5. (a) UV–vis absorption spectra of OPD solutions containing Fe3 O4 /rGO/MOF upon the addition of 2–150 μM H2 O2 ; (b) the plot of A450 versus H2 O2 concentration; (c) colorimetric change of OPD solutions containing Fe3 O4 /rGO/MOF upon the addition of 2–150 μM H2 O2 .
Fe3 O4 /rGO/TA provides the new evidence of the adhesive coating of TA film. In the spectrum of Fe3 O4 /rGO/TA, the enhanced band at 3424 cm−1 is attributed to the -OH stretching of the phenol group and methylol group of TA. The absorption band near 2924 and 2846 cm−1 is assigned to its aromatic C–H stretching mode of vibration. The absorption band at 1710 cm−1 is attributed to C=O stretching frequency of carboxyl group of TA. Intensities of the bands at 1434, 1346 and 1196 cm−1 corresponding to aromatic C–C and phenolic C–O stretching vibrations of TA [30]. After the self-assembly of MOF (ZIF-8), new absorption bands of
Fe3 O4 /rGO/MOF were presented compared with Fe3 O4 /rGO/TA. The presence of new band at around 442 cm−1 is assigned to ZnN stretching, bands at 1422 cm−1 and 10 0 0–130 0 cm−1 (1080, 1146 and 1308 cm−1 ) can be associated with the stretching and plane bending of the imidazole ring, and the absorption bands at 2924 and 2846 cm−1 are attributed to the aromatic and aliphatic C–H stretching, respectively. The FTIR analysis demonstrated the functional groups of the Fe3 O4 /rGO/MOF composite. Also, the XRD spectrum of pure GO shown a clear diffraction peak at 10.1°, which is related to the (002) reflection (Fig. 3b). After decorated with
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Fig. 6. (a) UV–vis absorption spectra of 4-AAP solutions containing Fe3 O4 /rGO/MOF upon the addition of 10–100 μM phenol; (b) the plot of A505 versus phenol concentration; (c) colorimetric change of 4-AAP solutions containing Fe3 O4 /rGO/MOF upon the addition of 10–100 μM phenol; (d) value at A505 of the solutions containing 40 μM phenol or various species. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fe3 O4 NPs by the solvothermal process, many new peaks can be observed in the pattern of Fe3 O4 /rGO. These peaks are typical diffraction peaks of magnetite (JCPDS NO. 19–0629). Compared with the XRD pattern of Fe3 O4 /rGO, several distinct diffraction peaks in Fe3 O4 /rGO/MOF corresponding to crystalline MOF (ZIF-8) can be observed, suggesting the successful deposition of MOF on the Fe3 O4 /rGO plane [31]. The nitrogen adsorption/desorption isotherm of the Fe3 O4 /rGO/MOF composite demonstrated that the composite exhibited large surface area (380.33 m2 g−1 ) (Fig. 4a). The total pore volume of Fe3 O4 /rGO/MOF is about 0.38 m2 g−1 , and the average pore diameter is 4.0 nm. Importantly, the high surface area of the Fe3 O4 /rGO/MOF would contribute to the affinity ability in practical catalysis. Compared to the aggregated graphene and bulk MOF materials, the thin MOF layers were assembled on the surface of graphene composite for Fe3 O4 /rGO/MOF. Thus, a unique mesoporous MOFs surface with affinitive graphene internal would accelerate the fast transport and diffusion of the substrates and the products. Especially, the porous MOF shell can provide sufficient surface area to facilitate the preconcentration of substrate, thus accelerate permeation of the reactants to access the active catalytic sites and enhance the reaction efficiency. The introduction of magnetic Fe3 O4 NPs on the rGO provide another advantage of the Fe3 O4 /rGO/MOF composite, which the magnetism of the as-prepared composite would promote the fast and
convenient separation in the process of catalyst recovery and reuse. The magnetic hysteresis curves of Fe3 O4 /rGO and Fe3 O4 /rGO/MOF were shown in Fig. 4b. The Fe3 O4 /rGO/MOF exhibited a relatively strong magnetism (35.2 emu g−1 ) at room temperature. Compare with that of Fe3 O4 /rGO (50.0 emu g−1 ), the decrease of the saturation magnetization value indicated the incorporation of nonmagnetic components. 3.2. Colorimetric detection of H2 O2 and phenol with Fe3 O4 /rGO/MOF A typical colorimetric detection process involving the oxidation of OPD in the presence of H2 O2 has been performed to test the peroxidase-like activity of the as-prepared Fe3 O4 /rGO/MOF composite. By monitoring the absorbance changes with UV–vis spectrum, a typical H2 O2 concentration-response curve in acetate buffer solution (pH = 4.0) was shown in Fig. 5a. An obvious increase in the adsorption peak at 450 nm was observed by increasing H2 O2 concentration. The high activity of the Fe3 O4 /rGO/MOF composite nanomaterial allowed the fast oxidation reaction with the reaction time of 10 min. Fig. 5b displayed the corresponding calibration plot. It can be calculated that H2 O2 can be detected in the linear range from 2 to 60 μM (R2 = 0.997), and the detection limit was estimated to be about 6.67 × 10−7 M (S/N = 3). Similarly, another application of Fe3 O4 /rGO/MOF composite as a colorimetric sensor to determine phenol was demonstrated
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Fig. 7. The effect of various parameters on degradation of phenol: (a) initial pH; (b) H2 O2 concentration; (c) catalyst dose; (d) reusability of Fe3 O4 /rGO/MOF. Conditions: (a) phenol concentration = 20 mg L−1 , initial H2 O2 concentration = 3.0 mmol L−1 , catalyst dose = 0.4 g L−1 , ambient temperature; (b) phenol concentration = 20 mg L−1 , initial pH = 3.0, catalyst dose = 0.4 g L−1 , ambient temperature; (c) phenol concentration = 20 mg L−1 , initial H2 O2 concentration = 3.0 mmol L−1 , initial pH = 3.0, catalyst dose = 0.4 g L−1 ; (d) phenol concentration = 20 mg L−1 , initial H2 O2 concentration = 3.0 mmol L−1 , initial pH = 3.0, catalyst dose = 0.4 g L−1 .
in biological buffer medium. A colorimetric system involving Fe3 O4 /rGO/MOF, co-oxidation of H2 O2 and 4-AAP was developed for the determination of phenol. The quantitative detection of phenol samples was performed upon adding different concentrations of phenol to the above reaction system at room temperature with the reaction time of 10 min. When the concentration of phenol increased from 10 to 100 μM, the absorbance of the samples increased (Fig. 6a) and the color of the reaction solution gradually changed from colorless to pink (Fig. 6c). A linear plot was found at the phenol concentration in the range of 10–80 μM (R2 = 0.994) at A505 (Fig. 6b), and its corresponding detection limit is estimated to be about 3.33 × 10−6 M (S/N = 3). The above results imply by the fact that the as-prepared Fe3 O4 /rGO/MOF composite exhibited excellent peroxidase-like activity, thus resulting in slow, gradual changes in color and wide linear range, and showing potential applications in biosensing and environmental monitoring. The selective detection of phenol in water system is highly desirable for practical applications. Above results demonstrate that Fe3 O4 /rGO/MOF exhibit high colorimetric detection efficiencies toward phenol, and the detection selectivity for phenol needs to be checked furthermore. In a control experiment, the UV–vis spectra of solution with the presence of phenol against other control molecules was examined. As displayed by Fig. 6d, the presence of the equivalent control molecules caused slight changes of the spectra. Upon introducing phenol into the mixture of the
control molecules and Fe3 O4 /rGO/MOF, the UV–vis spectra were significantly increased. This result reveals that the interference from above control molecules can be neglected, convincing the high colorimetric detection selectivities of Fe3 O4 /rGO/MOF toward phenol. The highly selective detections in water system make Fe3 O4 /rGO/MOF reliable sensing materials for phenol. 3.3. Degradation reaction of phenol Besides detection, the removal of phenol from wastewater is also important in water treatment. Reported results have shown that Fe3 O4 -based nanocomposite can be used as good Fenton-like catalyst for the degradation of organic contaminants from water [32, 33]. However, the relevant studies on phenol are still mean to be challenged up to now. Thus, the degradation performances of Fe3 O4 /rGO/MOF composite for phenol were checked. The influence of pH on the degradation reaction was shown in Fig. 7a. It is obvious that the elimination of phenol was nearly pHdependent. For example, at pH 3.0, phenol can be degraded 80% within 2 min, while the degradation efficiency decreased to 60% at pH 4.0. It can be explained by the reason that the decreased iron ions in the reaction system as pH increases [34]. When the pH decreased from 3.0 to 2.0, the degradation efficiency of phenol decreased sharply from 80% to 35%. According to previous reports, a futher increased acidity of the solution could make a less
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effective degradation [35]. Thus we can speculated that the formation of proton shell on the catalytic Fe(H2 O)6 3+ at lower acid environment, which can release hydration shell, thus affecting the catalytic reaction of H2 O2 . Even though, total removal of phenol can eventually be achieved in 16 min under different acid condition. The effect of initial H2 O2 concentration on phenol degradation was displayed by Fig. 7b. 20% of phenol can be converted within 2 min when 1.0 mM of H2 O2 was added into the solution. And the degradation of phenol increased rapidly (from 20% to 40%) if the H2 O2 concentration was 3.0 mM. This is due to an increased H2 O2 concentration was contributed to eliminate the given amount of phenol. When 5.0 mM of H2 O2 was added, a slightly decreased degradation rate can be observed. It was due to that excessive H2 O2 can react with ·OH, thus leading to the loss of hydroxyl radical which played a key role in phenol degradation. The following equations are shown to explain the undesired negative effects [36–38]:
H2 O2 + ·OH → H2 O + ·O2 H
(1)
·O2 H + ·OH → H2 O + O2
(2)
In order to explore the role of each component of the catalyst in the Fenton-like reaction, control experiments with an equivalent dosage of Fe3 O4 /rGO/TA composite, sandwich-like Fe3 O4 /rGO/MOF, and bare MOFs as catalysts were carried out with 3.0 mM of H2 O2 at pH 3.0. As shown in Fig. 7c, the Fe3 O4 /rGO/TA composite adsorbed less than 20% of phenol within 16 min in the absence of H2 O2 . The H2 O2 alone could only trigger small amount of phenol to be degraded. About 35% of phenol was removed when the bare MOFs was used as catalyst, indicating that MOFs could not efficiently catalyze the formation of ·OH from H2 O2 . The Fe3 O4 /rGO/TA without MOFs shell could catalyze complete degradation of phenol as well. But the achieved degradation rate was much lower than that obtained by the Fe3 O4 /rGO/MOF. This result indicated that both the target phenol and H2 O2 were able to access to the active center. But the porous MOFs shell on Fe3 O4 /rGO/MOF facilitate the increase of the local concentration of the reactive molecules via the adsorption effect, thus leading to a faster degradation reaction. The above results suggested that the excellent catalytic performance of the Fe3 O4 /rGO/MOF composite originated from the synergistic effects of porous MOFs shell and the inner Fe3 O4 /rGO. The reusability of the Fe3 O4 /rGO/MOF was tested by recovering the catalysts when the reaction finished and reusing it in the next run. Owning to the magnetic property, the Fe3 O4 /rGO/MOF can be recovered by an extra magnet easily. As shown in Fig. 7d, the catalytic activity of Fe3 O4 /rGO/MOF retained at 96% after five times reuse. It can be demonstrated by the negligible loss of Fe from Fe3 O4 /rGO/MOF composite. The stability of the catalyst can be attributed to the layer by layer protection of TA film on Fe3 O4 /rGO and the outer MOFs shell. Because TA is well known as a polydentate ligand to form complexes with metal ions [24]. As a result, Fe3 O4 /rGO/MOF showed high reusability and activity stability in phenol degradation. 4. Conclusion In this work, a facile and effective strategy was developed for controllable synthesis of sandwich-like graphene based magnetic metal organic frameworks. Due to the mild adhesive property of tannic acid, MOF layer can be self-assembled on both sides of magnetic graphene via the complexation between tannic acid and metal nods of MOF. Thus endowing a well-dispersed and porous MOF structure on the Fe3 O4 /rGO. The obtained Fe3 O4 /rGO/MOF composite were used as a mimic enzyme sensor for the colorimetric detection of phenol and achieved a superior catalytic ac-
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tivity. The colorimetric sensor can detect H2 O2 and phenol with a low detection limit, wide linear range and good selectivity. In addition, Fe3 O4 /rGO/MOF also can be used as effective Fenton-like catalyst to degrade phenol. It was found that the porous structure of the composite plays a key role in the preconcentration of phenol around the catalyst, which can enhance the detection and degradation efficiency. Due to the facile operation, low cost, high efficiency and easy separation, the bifunctional Fe3 O4 /rGO/MOF would provide a new sight into the simultaneously detection and degradation of contaminants, and showing potential applications in wastewater monitoring and treatment. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21806096), and the Open Project Program of National Demonstration Center for Experimental Light Chemistry Engineering Education (Grant no. 2018QGSJ02-19), Shaanxi University of Science & Technology, and the Guangxi Key Laboratory of Clean Pulping, Papermaking and Pollution Control Opening Fund (KF 201711). References [1] Gao J, Liu M, Song H, Zhang S, Qian Y, Li A. Highly-sensitive electrocatalytic determination for toxic phenols based on coupled cMWCNT/cyclodextrin edge– functionalized graphene composite. J Hazard Mater 2016;318:99–108. [2] Ni Y, Xia Z, Kokot S. A kinetic spectrophotometric method for simultaneous determination of phenol and its three derivatives with the aid of artificial neural network. J Hazard Mater 2011;192:722–9. [3] Zhao T, He J, Wang X, Ma B, Wang X, Zhang L, Li P, Liu N, Lu J, Zhang X. Rapid detection and characterization of major phenolic compounds in Radix Actinidia chinensis Planch by ultra-performance liquid chromatography tandem mass spectrometry. J Pharm Biomed Anal 2014;98:311–20. [4] Han XX, Chen L, Kuhlmann U, Schulz C, Weidinger IM, Hildebrandt P. Magnetic titanium dioxide nanocomposites for surface-enhanced resonance Raman spectroscopic determination and degradation of toxic anilines and phenols. Angew Chem Int Ed 2014;53:2481–4. [5] Ali MM, Sandhya KY. Visible light responsive titanium dioxide-cyclodextrin– fullerene composite with reduced charge recombination and enhanced photocatalytic activity. Carbon 2014;70:249–57. [6] Ma W, Li J, Tao X, He J, Xu Y, Yu JC, Zhao J. Efficient degradation of organic pollutants by using dioxygen activated by resin-exchanged iron(II) bipyridine under visible irradiation. Angew Chem Int Ed 2003;42:1059–62. [7] Li H, Chen Y, Zhang Y, Han W, Sun X, Li J, Wang L. Preparation of Ti/PbO2 -Sn anodes for electrochemical degradation of phenol. J Electro Anal Chem 2013;689:193–200. [8] Yao ZY, Yang YB, Chen XL, Hu XP, Zhang L, Liu L, Zhao YL, Wu HC. Visual detection of copper (II) ions based on an anionic polythiophene derivative using click chemistry. Anal Chem 2013;85:5650–3. [9] Wang H, Jing M, Wu Y, Chen W, Ran Y. Effective degradation of phenol via Fenton reaction over Cunife layered double hydroxides. J Hazard Mater 2018;353:53–61. [10] Dhakshinamoorthy A, Navalon S, Alvaro M, Garcia H. Metal nanoparticles as heterogeneous Fenton catalysts. Chem Sus Chem 2012;5:46–64. [11] He JC, Zhang Y, Zhang XD, Huang YM. Highly efficient Fenton and enzyme-mimetic activities of NH2 -MIL-88B(Fe) metal organic framework for methylene blue degradation. Sci Rep 2018;8:5159–66. [12] Zhu L, Gong L, Zhang Y, Wang R, Ge J, Liu Z, Zare RN. Rapid detection of phenol using a membrane containing laccase nanoflowers. Chem Asian J 2013;8:2358–60. [13] Lin ZA, Xiao Y, Yin YQ, Hu WL, Liu W, Yang HH. Facile synthesis of enzyme-inorganic hybrid nanoflowers and its application as a colorimetric platform for visual detection of hydrogen peroxide and phenol. ACS Appl Mater Interfaces 2014;6:10775–82. [14] Gao L, Zhuang J, Nie L, Zhang J, Zhang Y, Gu N, Wang T, Feng J, Yang D, Perrett S, Yan XY. Intrinsic peroxidase-like activity of ferromagnetic nanoparticles. Nat Nanotechnol 2007;2:577–83. [15] Cormode DP, Gao L, Koo H. Emerging biomedical applications of enzyme-like catalytic nanomaterials. Trends Biotechnol 2017;36:15–29. [16] Zhou Y, Liu B, Yang R, Liu J. Filling in the gaps between nanozymes and enzymes: challenges and opportunities. Bioconjugate Chem 2017;28(12):2903–9. [17] Hou C, Wang Y, Ding Q, Jiang L, Li M, Zhu W, Pan D, Zhu H, Liu MZ. Facile synthesis of enzyme-embedded magnetic metal-organic frameworks as a reusable mimic multi-enzyme system: mimetic peroxidase properties and colorimetric sensor. Nanoscale 2015;7:18770–9. [18] Wang Q, Zhang X, Huang L, Zhang Z, Dong S. GOx@ZIF-8(NiPd) nanoflower: an artificial enzyme system for tandem catalysis. Angew Chem Int Ed 2017;56(50):16082–5.
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