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Graphene oxide functionalized Zn-based metal-organic framework (ZnMOF) indicates a sensitive and selective luminescence response to Cu2+ ions.
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A Cubic Luminescent Graphene Oxide Functionalized Zn-based MetalOrganic Framework Composite for Fast and Highly Selective Detection of Cu2+ Ions in Aqueous Solution Liying Hao, Hongjie Song, Yingying Su and Yi Lv* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x In this work, we have synthesized graphene oxide functionalized Zn-based metal-organic framework (ZnMOF) under the one-pot condition, named as ZnMGO composite, which has high luminescence and good water dispersibility. The luminescence of the aqueous ZnMGO composite could be efficiently and selectively quenched by Cu2+ ions through the interactions between Cu2+ and the ligand, and the detection limit was measured as low as 1.00 μM. Also, the robust ZnMGO composite demonstrated the fast response and high sensitivity (Ksv=3.07 × 104 M-1) for Cu2+ ions in aqueous solution. Moreover, we further explored the possible luminescence mechanism in terms of energy migration or electron transfer, and the quenching mechanism is also discovered based on the collapse of the crystal structure with the help of various characterizations. Remarkably, it is the first time that ZnMGO composite possessed the excellent ability is used to rapidly detect Cu2+ ions in aqueous solution. The work does not only contribute to extend the potential application of ZnMGO composite, but also is hopeful to make a contribution in biological areas.
Introduction 20
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Novel interesting and meritorious materials have been paid more attention by analytical researchers because of their potential analytical applications. A new group of materials is metal-organic frameworks (MOFs), which are organic-inorganic hybrids constructed from metal ion nodes linked together by organic linkers to form a three-dimensional crystal lattice.1,2 Recently, researchers have received great interest in MOFs owe to their diverse topologies, high surface areas, predictable structures, tunable pore sizes, and the capability of catalysis.3-5 Therefore, these excellent properties have been quickly developed into a fruitful research field including gas storage,6 catalysis,7 chemical separation,8 drug delivery,9 light-harvesting systems,10,11 biomedical imaging,10 stationary phases for gas chromatography12 and liquid chromatography,13,14 chemical sensing,15 and gas sensing,16 etcetera. In addition to these attractive applications, we are particularly interested in using MOFs as fluorescence probe to detect the metal ions and small molecules. Besides, a few important studies demonstrating the promise of MOFs for the detection of the ions and small molecules17 have been reported previously. For example, Qian and co-workers have reported the detection of metal ions18,19 and some organics20 through fluorescence quenching measurements. Sun group designed a fluorescent probe based Eu-MOF with chelating terpyridine sites for Fe3+.21 Also, the similar applications for MOFs have been reported by other groups.22, 23 However, most of them are performed in organic solvents and in solid state, which is very discommodious for the fluorescence detection. Additionally, This journal is © The Royal Society of Chemistry [year]
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pristine MOFs possess poor stability in the presence of humidity or upon a solvent removal as well as weak dispersibility in aqueous solution,24 which limit their further applications. Furthermore, they exhibit a little poor sensing performances such as low sensitivity and selectivity towards metal ions.18,19 Taking into account these disadvantages, many research efforts have focused on functionalized MOFs materials, such as the deposition of MOFs on various supports, in order to synthesize the MOFsbased composites with better water dispersibility and increased sensitivity and selectivity to metal ions. Another group of interesting materials is graphene and graphenebased materials, which have recently opened new possibilities of applications due to the unique structural, mechanical, electronic properties arising from one-atom-thick carbon sheets.25 Graphene oxide (GO) is the solution-dispersible oxidized form of graphene, which is commonly produced by exfoliating graphite under strong oxidizing conditions.26 The presence of epoxy, carboxyl and hydroxyl functional groups on either side of the GO imparts building block property on the material, which has been widely used to create a large number of GO-based composites, and allows it to act as structural nodes in MOFs. In this concept, GO with dense arrays of layers and numerous oxygen groups is introduced into MOFs to form composites for the sake of overcoming the drawback of MOFs and GO.27 Recently, Petit and co-workers reported the synthesis of MOF-graphite oxide composites in order to increase the dispersive forces in MOF and enhance the adsorption capacity of gas.28,29 Up to now, the MOFGO composites are only used in gas storage, and there are few [journal], [year], [vol], 00–00 | 1
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reports about the other applications, which submerge the superior properties of MOF-GO composites. Taking into account the above, we design a luminescent ZnMGO composite via interactions between the carboxyl groups of GO and the metallic center of the ZnMOF,25,30,31 as a fast response and high selectivity fluorescent probe (FL) for Cu2+ ions in aqueous solution. Moreover, the detection occurs with high sensitivity (Ksv=3.07 × 104 M-1) and selectivity towards Cu2+ ions using the ZnMGO composite compared to the other MOFs.18,19 Meanwhile, we speculate the luminescence mechanism due to energy migration (EM) or electron transfer (ET), and the quenching mechanism is explored based on some characterizations of the ZnMGO composite before and after Cu2+ ions exposure. The obtained results show a satisfied linear range and a low detection limit, which indicate that the ZnMGO composite plays an important role in the metal ions analysis from the environment and the organism.
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Preparation Method of ZnMOF and the ZnMGO composite
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Experimental Section Reagents 20
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Graphite powder was of Specpure grade, which was purchased from Tianjin Guangfu Fine Chemical Research Institute. All chemicals including zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 1,4-benzenedicarboxylic acid (H2BDC), and N,Ndimethylformamide (DMF), cupric nitrate trihydrate (Cu(NO3)2·3H2O, ≥ 99.00%), ethanol, sodium hydroxide (NaOH), and hydrochloric acid (HCl, ≥ 36.46%) were obtained from Chengdu Kelong Chemical Reagent Company (China), which were at least of analytical grade and were used without further purification. Phosphate-buffered saline (PBS) solutions (0.01 M, pH 5.00) dissolved 6.8410 g NaH2PO4·2H2O and 0.2204 g Na2HPO4·12H2O in 500 mL deionized water were used in this experiment, and different pH values of PBS solutions were adjusted with a concentrated sodium hydroxide solution (0.10 M) or hydrochloric acid solution (0.10 M) to the required pH values. Deionized (DI) water from ULUP standard solution of copper (0.01 mol L-1) was prepared from analytical grade cupric nitrate trihydrate. ULUPURE Water Purification System (Chengdu, China) was used in all the experiments. Stock standard solution of copper (0.01 mol L-1) was prepared from analytical grade cupric nitrate trihydrate.
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According to the Hummers method, Graphite oxide was prepared by oxidation of graphite.32 Next, Graphene oxide sheets were obtained by complete exfoliation of graphite oxide as an entry into the ZnMGO composite. In this work, graphite oxide (0.50 mg mL-1) was exfoliated to be graphene oxide on the basis of our previous work.33 The ZnMGO composite was prepared according to the previous papers with a little modification.28,31 Briefly, zinc nitrate hexahydrate (1.04 g) and 1, 4-benzenedicarboxylate (0.20 g) were dispersed in N, N-dimethylformamide (DMF, 14 mL) by stirring until complete dissolution of the solids. Afterwards, the mixture was transferred into a three-necked flask connected to a condenser, and 11 mL GO solution was added into the above mixed solution slowly with stirring. Finally, the mixture was heated at 118 oC for 16 h and 50 oC for 8 h in an oil bath. After cooling, the supernate was removed and the ZnMGO composite on the bottom of the flask was collected through 0.45 μm filter, washed with DMF and ethanol, and immersed in fresh chloroform overnight. At last, the composite was filtered, dried at 100 oC for 8 h in vacuum. The resulting composite was kept in a desiccator. According to the same method, ZnMOF was prepared except adding GO. In this work, 20 mg of the ZnMGO composite was dispersed into 200 mL aqueous solution by sonicated for 1.5 h, and used for the following experiment. Fluorescence spectroscopic studies
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Instrumentations XRD: X-ray diffraction (XRD) measurements were taken using standard powder-diffraction procedures. The materials were ground in a small agate mortar. Then, the materials were smearmounted onto a glass slide and measured by CuKα radiation generated in a Tongda TD-3500 X-ray powder diffractometer (Liaoning, China). SEM: The surface morphology of the materials was examined by SEM. The samples were previously dried and coated with a thin layer of gold were used to scan on a Hitachi, S3400 instrument. Energy dispersive X-ray spectroscopy (EDS) analyses were performed on the same instrument with samples previously dried and coated with gold in order to investigate elemental compositions of ZnMOF, ZnMGO and ZnMGO&Cu. From EDS analyses, the content of elements on the surface was calculated.
FT-IR: Fourier transform infrared (FT-IR) spectroscopy was carried out using a Thermo Nicolet IS10 FT-IR Spectrometer with View Article Online KBr pellets in the range 500-4000 cm-1 toDOI: analyze the surface 10.1039/C3AN01943H properties and composition of the materials. UV-Vis: The UV-Vis spectra of ZnMGO and ZnMGO&Cu composite in the region of 200-500 nm were recorded by U-2910 UV-Vis spectrophotometer. TG: Thermogravimetric (TG) measurements were operated using a NETZSCH STA 449 C. The samples were heated up to 800 °C with the heating rate 10 deg min-1 under a flow of nitrogen of 100 mL min-1.
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All of the fluorescence measurements were done using F-7000 fluorescence spectrophotometer (Hitachi Co., Tokyo, Japan). The Cu2+ detection procedures by the ZnMGO composite were described as follows: 300 μL of the ZnMGO composite aqueous solution reacted with different concentrations of Cu2+ solution (concentrations from 10-3 to 10-6 M in ultrapure water). The mixture was diluted into 2.00 mL with PBS (pH 5.00, except pH optimization experiment) and mixed thoroughly, and then reacted in a wobbler machine at 25 oC (in addition to temperature experiment) for 10 min. Afterwards, the fluorescence detection was excited at 310 nm and the emission was monitored at 442 nm, with the slot widths of the excitation and emission were both 5 nm. Under the same conditions, the effects of various parameters on the fluorescence quenching of the composites were investigated, including pH, temperature and selectivity. All the measurements were conducted in triplicate.
Results and Discussion Characterizations of the materials 110
The XRD patterns of GO, parent material and the composite are
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used to examine the phase and structure of the synthesized products, as shown in Fig. 1. The well-defined peak at 2θ about 10.40o is seen in the case of GO, which is related to an interlayer distance of about 8.50 Å.34 The major sharp diffraction peaks are presented in the case of ZnMOF, which are the feature of this material structure and similar to the data published in those previous literatures for MOF-5.4,35 Obviously, the XRD pattern of the ZnMGO composite is rather similar to the parent ZnMOF, which suggests that the presence of GO does not affect the formation of the crystalline frameworks. Even though the structure of ZnMOF is preserved in ZnMGO hybrid material, the sharpness of the peak at 2θ about 9.67o is slightly reduced and the splitting of that peak is also clearly observed for the ZnMGO composite, which is correlated with the distortion of the crystal structure of ZnMOF.25 The phenomena have been reported by Lillerud and coworkers previously.4 Of course, the major structural and chemical features of ZnMOF preserved in ZnMGO composite are expected since the composites consist of very few GO.28
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consider that GO are packed on the surface of ZnMOF through interactions between ZnMOF metallic centers and the oxygen View Article Online groups of GO.31 Besides, these two explanations are in agreement DOI: 10.1039/C3AN01943H with the SEM images. Meanwhile, a new pore space is created between the ZnMOF blocks and the GO units.
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Fig. 2 SEM images for (a, b) ZnMOF, (d, e) ZnMGO composite, (c, f) EDS spectrum and the percentage of the elements of ZnMOF and ZnMGO composite, respectively. The red square area in e represents the enlarged view of the red square area in d.
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Fig. 1 XRD patterns for GO, ZnMOF and ZnMGO composite.
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The texture of the materials studied can be observed on SEM micrographs presented in Fig. 2. For ZnMOF (Fig. 2a, b), the cubic crystals are clearly observed with some traces of amorphous phase and some cracks.28,30 The size of crystals is as big as 150 μm which is attributed to some regular cubic ZnMOF material stacked together in an organized way. For comparison, SEM images of ZnMGO (Fig. 2d, e) are relatively flat with homogenous structure, remained amorphous phases. From the enlarged view (2e) of the red square of 2d, the wrinkles formed by the GO flakes can be clearly seen. Taking into account the differences in the texture between ZnMOF and ZnMGO, it is plausible to speculate that thin GO layers are located on the surface of ZnMOF and act as dividers and protectors, which is also related to the water-dispersible of the ZnMGO composite. EDS analyses of the parent material and hybrid material are presented in Fig. 2c, f, which reveal that carbon, oxygen, and zinc are dispersed well on the surface. According to these data, the oxygen content in the ZnMGO composite increases with adding GO.25,30 In consideration of the chemistries of both components of ZnMOF and ZnMGO composite, we have assumed the building process of the ordered structure of ZnMOF and ZnMGO, as shown in Scheme 1a and b, respectively. As shown in Scheme 1a, the more regular arrangement of the ZnMOF is composed of some small cubic crystals. With regard to ZnMGO composite, we This journal is © The Royal Society of Chemistry [year]
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Scheme 1 The schematic view of the ideal material structures. (a) Schematic view of the steps of the ZnMOF formation. (b) GO and ZnMGO composite which are formed through proposed bonding between ZnMOF and GO via -COOH groups along direction; in light blue: zinc atoms; in red: oxygen atoms; in light grey: carbon atoms; in white: hydrogen atoms. 25, 28, 30
Further details and support on the three materials are provided by FT-IR spectroscopy (Fig. 3). The spectra for GO were analyzed in detail as follows: C=O, 1724 cm-1; O-H, 1384 and 3379 cm-1; the C-OH stretching, 1257 cm-1; C-O-C (epoxy group) stretching, 1058 cm-1; skeletal ring stretch, 1624 cm-1.36 As expected based on the composition of the ZnMGO composite, the FT-IR spectrum for the composites largely resembles that of ZnMOF. The asymmetric stretching of carboxylic groups in BDC ((1,4benzenedicarboxylate) were observed at 1500 cm-1 and 1578 cm-1, whereas the peak at 1385 cm-1 was assigned to the symmetric stretching of carboxylic groups in BDC.28 In the region of 1300 and 700 cm-1, some bands were found which were due to the outof-plane vibrations of BDC.37 A peak at 1675 cm-1 was contributed to the C=O stretch of carboxylate group located on the surface of GO. Compared to GO, the downshift of the C=O stretch from 1724 cm-1 to 1663 cm-1 in the spectrum of the ZnMGO composite was the result of the coordination of the carboxylate group with the zinc clusters in ZnMOF.30 Journal Name, [year], [vol], 00–00 | 3
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The TGA data revealed that the ZnMGO composite was stable up to 350 oC, as shown in Fig. S1 of the Supporting Information. On account of the good thermal property, further research should pay more attention to the potential of ZnMGO composite for the gas sensors and gas chromatographic column.
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d) FL responses of the ZnMGO composite (15.00 mg L-1) in the presence of 10 μM Cu2+ ions at different pH values (3.00 - 12.00) and temperatures ZnMGO (red (5 - 70 oC), respectively. The inset of c: the FL intensity of View Article Online with 10.1039/C3AN01943H different pH values columns) and ZnMGO + Cu2+ (reseda columns) DOI: of PBS buffer. All the experiments were performed for 15 min and excited at 310 nm. Error bars were based on three measurements.
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Fig. 3 FTIR spectra of GO, ZnMOF, and ZnMGO composite.
Optical performance of the ZnMGO composite 10
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Compared to the ZnMOF, the good water dispersibility is in favor of the fluorescence applications. Besides, having successfully prepared ZnMGO composite, we firstly demonstrated the potential application of the ZnMGO composite as fluorescent sensor to detect Cu2+ ions via the fluorescence quenching of the ZnMGO composite. To evaluate the optical properties of the ZnMGO composite, the UV-Vis absorption spectrum and FL emission spectrum were investigated (Fig. 4a). UV-Vis spectrum of the ZnMGO composite (black line) in water solution exhibited one broad band at 243 nm attributable to the π → π* transition of aromatic carbon bonds. The red shift, compared to GO (Fig. S2), may attributed to the coordination between the carboxyl on the surface of GO and the metal center (Zn) of ZnMOF. Upon excitation at 310 nm, the fluorescence spectrum of the ZnMGO composite showed a 442 nm emission peak (blue line), which was clearly different from that of ZnMOF with emission of 420 nm (insert). Intriguingly, the FL of the ZnMGO composite can be quenched through adding a certain amount of Cu2+ ions (Fig. 4b), which considers that it is possible to apply the ZnMGO composite in detecting Cu2+ ions.
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Sensitivity of the Sensing System
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Fig. 4 (a) UV-Vis spectra (black line) and FL emission spectra (blue line, excitation at 310 nm) of the ZnMGO composite. Insert: FL emission spectra of ZnMOF. (b) FL responses of the ZnMGO composite (15.00 mg L-1) upon addition of different amounts of Cu2+ ions (5.0×10-6, 1.0×10-5, 5.0×10-5, 5.0×10-4, 1.0×10-3 M) in aqueous solution (excited at 310 nm). (c,
As mentioned above, the fluorescence quenching of the ZnMGO composite by Cu2+ ions was evaluated with fluorescence spectra. Moreover, we investigated the key factors including the effects of pH and temperature for the sensing system as follows. As depicted in inserted columns of Fig. 4c, the presence of Cu2+ leads to the FL quenching of the ZnMGO composite over the wide pH range from 3.00 to 12.00; most remarkably, the quenching efficiencies (F0 - F / F) at these pH values are various (Fig. 4c). We can see that in the condition of the addition of Cu2+ ions in strongly acidic media (pH 3.00), the quenching efficiency is very low, which may be due to the fact that the carboxyl groups at the surfaces of the ZnMGO composite are well protonated, indicating that it is impossible to complex with Cu2+ to form FL-quenching cupric moieties. In the case of alkaline solutions (pH > 7.00), the quenching efficiencies are not satisfied either, which may attributed to partial hydrolysis of Cu2+ ions in the alkaline media.38 By comparison, in the weakly acidic medium (pH 5.00), the quenching efficiency is better than others, suggesting that the weakly acid medium is suitable for this sensing system for the sensitive detection of Cu2+ ions. Fig. 4d shows the effect of the temperature. We can distinctly observe that the quenching efficiency (F0 / F) decreased along with the temperature increased (> 25oC), especially at high temperature. According to the experiment, the room temperature (25 oC) was chosen as the optimal condition.
Fig. 5 (a) FL response of the ZnMGO composite upon addition of various concentrations of Cu2+ ions (from top to bottom: 0, 1, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, and 1000 μM) in a pH 5.00 PBS solution. (b) The relationship between the concentrations of Cu2+ and F0 / F. Inset: The linear calibration plot for Cu2+ detection. All the experiments were performed for 15 min and excited at 310 nm. Error bars were based on three measurements.
Under the optimized conditions discussed above, the sensitivity, the linear response range, and the detection limit of the ZnMGObased sensing system are measured as follows. As shown in Fig. 5a, the FL intensity of the ZnMGO composite at 442 nm is sensitive to Cu2+ ions and decreases with the increase of concentration of Cu2+ ions. There is a good linear correlation between the quenching efficiency (F0 / F) and the concentrations of Cu2+ ions in the range from 1.00 to 70.00 μM (see inset of Fig. 6b) via the following equation:
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Establishment of the FL Sensing Method for Cu2+
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F0 / F =0.0307C+0.9987 (R=0.9963) Where F0 and F are the FL intensity of the ZnMGO composite in the absence and presence of Cu2+ ions, respectively, and C is the concentration of Cu2+ ions. The detection limit (DL) was calculated to be 1.00 μM through the equation DL = 3 σ/k, where σ is the relative standard deviation and k is the slope of the calibration graph (n=11). Therefore, we have demonstrated a fluorescent turn-off sensor based on the ZnMGO composite for detection of Cu2+ ions with a high sensitivity. Moreover, the quenching effect can be quantitatively described by the Stern-Volmer equation:
F0 1 Ksv[C] F
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(1)
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KSV is the quenching effect coefficient of the metal ion, which can be calculated as 3.07 × 104 M-1 via luminescent data (inset in Fig. 6b) and equation 1. Fortunately, the value of KSV is much larger than that of 89.4 M-1 in [Eu(pdc)1.5(dmf)]·(DMF)0.5(H2O)0.518 and 528.7 M-1 in Eu2 (FMA)2(OX)(H2O)4·4H2O for Cu2+ sensing,19 indicating that it is a much more sensitive luminescent ZnMOFbased sensor and has the strongest quenching effect on the luminescence intensity of the ZnMGO composite.
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Specificity of the Sensing System
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Besides sensitivity, selectivity was another key factor to assess the performance of a new proposed sensor. Except for Cu2+ ions, the effects of 12 other kinds of cations, including alkali metal ions (K+, Na+), alkaline earth metal ions (Ca2+, Mg2+), light metal ion (Al3+), and heavy metal ions (Co2+, Ni2+, Mn2+, Zn2+, Pb2+, Cd2+, Cr3+) on the FL response of the ZnMGO composite were also investigated. As illustrated in Fig. 6, alkali and alkaline earth metal ions have no apparent effect on the fluorescence intensity, while Al3+ and some heavy metal ions have different quenching degrees on the luminescence intensity. Although the concentrations of these ions were as ten times as that of Cu2+ ions, there were no apparent fluorescence quenching compared with adding Cu2+ ions, which indicates the present sensing system exhibits excellent selectivity for the detection of Cu2+ ions, and could further be used to the practical applications.
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Fig. 6 Selectivity of the ZnMGO-based sensor for over other ions in pH 5.00 PBS solution: the concentration was 10 μM for Cu2+ and 100 μM for other metal ions. The treatment reactions were performed for 15 min at room temperature, and excited at 310 nm.
Study on the mechanisms
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Recently, many researchers have paid more attention to the applications of MOFs for the development of sensing systems Articleof Online based on the changes of fluorescence intensity View because the DOI: 10.1039/C3AN01943H energy migration (EM), electron transfer (ET), or other interactions between MOFs and the analyte. In our study, we speculate the possible luminescence mechanisms on account of EM or ET for the ZnMGO composite. In the case of EM (Scheme 2.1), Zn is the luminescence center, while ligand BDC functions as both a linker and a receptor. When the light is provided, the energy transfers from BDC ligands to Zn2+ ions, which makes the ZnMGO composite emit fluorescence.11,19,39,40 In the case of ET (Scheme 2.2), GO is helpful to electron transfer, owing to its chemical bonds with MOF and the conductivity of distorted graphene oxide layers. Under the condition of light, the electrons of the ZnMGO composite are excited, and the excited electrons from the ZnMGO composite are transferred to the unoccupied states of GO-Oepoxy. Then, these electrons undergo a transition down to metal center (Zn), leading to fluorescence emission.41 For the quenching mechanism, we consider that the addition of Cu2+ ions, weakened the bonds between the metallic sites and the organic ligands,31 results in the collapse of the ZnMGO composite because of the coordination between Cu2+ ions and carboxyl groups of ligand, which forbids the EM or ET from the organic ligands (BDC) to the metallic sites (Zn), leading to the fluorescence quenching.
Scheme 2 The schematic view of the possible mechanisms. (1 a, b) The proposed luminescence and fluorescence quenching mechanisms of the ZnMGO composite based on energy migration, respectively. (2 a, b) The proposed luminescence and fluorescence quenching mechanisms of the ZnMGO composite based on electron transfer, respectively.
For the sake of elucidating the possible quenching mechanism by Cu2+ ions, some characterizations were used to monitor the structure change of the ZnMGO composite before and after adding Cu2+ ions. As shown in Fig. S3, the differences between the ZnMGO composite and ZnMGO&Cu indicate the structure of ZnMGO&Cu has changed compared to the ZnMGO composite. The SEM (Fig. 7) images clearly show the damage of the crystal form, which has changed from the cubic (Fig. 7a) to the plane structure (Fig. 7b). Also, the XRD (Fig. S4) can verify the collapse of the ZnMGO composite. In comparison with the ZnMGO composite, the peak at 6.25o disappears and the other Journal Name, [year], [vol], 00–00 | 5
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peaks are in disorder, which suggest the damage of the crystal form. Therefore, it is possible that the addition of Cu2+ ions leads the collapse of the ZnMGO composite and prohibits the EM or ET from the organic ligands (BDC) to the metallic sites (Zn)， which lead to the fluorescence quenching.
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Fig. 7 SEM images for (a) ZnMGO composite, (b) ZnMGO&Cu. Inset of b: EDS spectrum of ZnMGO&Cu.
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In summary, a highly luminescent metal-organic frameworkbased composite (ZnMGO) has been prepared by mixing the precursors and GO, which illustrates unprecedented sensor for fluorescence “turn-off ” detection of Cu2+ ions. On the basis of the fluorescence quenching of the ZnMGO composite by Cu2+ ions, the Cu2+ ions can be detected as low as 1.00 μM with a detection range of 1.00 to 70.00 μM, without the apparent interferences from other metal ions. Moreover, we also elaborate the possible luminescence mechanisms on base of energy migration or efficient electron transfer. Also, the quenching mechanism is explored by means of some characterizations based on the collapse of the ZnMGO composite leading the impossible of the EM or ET from the organic ligands (BDC) to the metallic sites (Zn). Taking advantage of the phenomenon, MOFs-based sensor may be applicable to the detection of a wide range of metal ions through properly functionalizing the MOFs. Further efforts will be focused on the function of porous luminescent MOFs to enhance their recognition sensitivity and selectivity. It is noteworthy that we are now exploring some luminescent and innocuous MOFs-based composites, and developing the strategies to apply to the biological system.
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Acknowledgment This work was supported by the National Nature Science Foundation of China (21375089). The authors also would like to show gratitude for Yanyan Meng and Feng Yang from College of Chemistry at Sichuan University for their assistance in the structure of the materials and the XRD analysis, respectively.
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Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. Fax: 86 28 8541 2798; Tel: 86 28 8541 2798; E–mail: [email protected] 1. 2.
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Férey, G. Chem. Soc. Rev. 2008, 37, 191-214. Yaghi, O. M., O'Keeffe, M., Ockwig, N. W., Chae, H. K., Eddaoudi, M. and Kim, J. Nat.,2003, 423, 705-714. Furukawa, H.; Ko, N., Go, Y. B., Aratani, N., Choi, S. B., Choi, E.,
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Yazaydin, A. Ö.; Snurr, R. Q., O’Keeffe, M. and Kim, J. Science, 2010, 329, 424-428. Hafizovic, J.; Bjørgen, M., Olsbye, U., Dietzel, P. D.,View Bordiga, Article S., Online DOI: Prestipino, C., Lamberti, C. and Lillerud, K. P. 10.1039/C3AN01943H J. Am. Chem. Soc., 2007, 129, 3612-3620. Li, J.-R., Sculley, J.and Zhou, H.-C. Chem. Rev., 2011, 112, 869-932. Zhao Y., Seredych Mykola, Zhong Q. and Bandosz T. J., ACS Appl. Mater. Interfaces, 2013, 5, 4951-4959. Bromberg L., Su X. and Hatton T. A., ACS Appl. Mater. Interfaces, 2013, 5, 5468-5477. S. S. Liu, C. X. Yang, S. W. Wang and X. P. Yan, Analyst, 2012, 137, 816-818 Horcajada, P., Serre, C., Vallet‐Regí, M.; Sebban, M.; Taulelle, F., and Férey, G. Angew. Chem. 2006, 118, 6120-6124. Della Rocca, J., Liu, D.and Lin, W., Acc. Chem. Res. 2011, 44, 957968. Kent, C. A., Liu, D., Ma, L., Papanikolas, J. M., Meyer, T. J.and Lin, W., J. Am. Chem. Soc. 2011, 133, 12940-12943. Chang, N., Gu, Z.-Y., Wang, H.-F.and Yan, X.-P. Anal. Chem. 2011, 83, 7094-7101. Yang, C.-X. and Yan, X.-P., Anal. Chem. 2011, 83, 7144-7150. Gu, Z.-Y., Yang, C.-X., Chang, N.and Yan, X.-P., Acc. Chem. Res. 2012, 45, 734-745. Pramanik, S., Zheng, C., Zhang, X., Emge, T. J. and Li, J. J. Am. Chem. Soc. 2011, 133, 4153-4155. Kreno, L. E., Hupp, J. T. and Van Duyne, R. P. Anal. Chem. 2010, 82, 8042-8046. Chen, B., Xiang, S. and Qian, G., Acc. Chem. Res. 2010, 43, 11151124. Chen, B., Wang, L., Xiao, Y., Fronczek, F. R., Xue, M., Cui, Y.and Qian, G., Angew. Chem. Int. Ed. 2008, 48, 500-503. Xiao, Y., Cui, Y, Zheng, Q., Xiang, S., Qian, G. and Chen, B. Chem. Comm. 2010, 46, 5503-5505. Xu, H., Liu, F., Cui, Y., Chen, B.and Qian, G., Chem. Comm. 2011, 47, 3153-3155. Zheng, M., Tan, H., Xie, Z., Zhang, L., Jing, X. and Sun, Z., ACS Appl. Mater. Interfaces, 2013, 5, 1078-1083. Lan, A., Li, K., Wu, H., Olson, D. H., Emge, T. J., Ki, W., Hong, M.and Li, J., Angew. Chem. Int. Ed., 2009, 48, 2334-2338. S. H. Huo and Xiu-Ping Yan, Analyst, 2012, 137, 3445–3451. Petit, C., Mendoza, B., O’Donnell, D.and Bandosz, T. J., Langmuir, 2011, 27, 10234-10242. Petit, C.and Bandosz, T. J. Adv. Mater,. 2009, 21, 4753-4757. Song, H., Zhang, L., He, C., Qu, Y.; Tian, Y.and Lv, Y., J. Mater. Chem., 2011, 21, 5972-5977. Huang, Z.-H., Liu, G.and Kang, F. ACS Appl. Mater. Interfaces, 2012, 4, 4942-4947. Petit, C.and Bandosz, T. J. J. Mater. Chem., 2009, 19, 6521-6528. Petit, C.and Bandosz, T. J. Adv. Funct. Mater., 2009, 20, 111-118. Jahan, M., Bao, Q., Yang, J.-X.and Loh, K. P. J. Am. Chem. Soc., 2010, 132, 14487-14495. Bandosz, T. J. Dalton Trans., 2012, 41, 4027-4035. Hummers Jr, W. S.and Offeman, R. E. J. Am. Chem. Soc., 1958, 80, 1339-1339. Hao, L., Song, H., Zhang, L., Wan, X., Tang, Y.and Lv, Y. J. Colloid Interface Sci., 2011, 369, 381-387. Song, H., Hao, L. Tian, Y., Wan, X., Zhang, L. and Lv, Y. ChemPlusChem, 2012, 77, 379-386. Kaye, S. S., Dailly, A., Yaghi, O. M.and Jeffrey, R., J. Am. Chem. Soc., 2007, 129, 14176-14177. Xu, Y., Bai, H., Lu, G., Li, C.and Shi, G. J. Am. Chem. Soc., 2008, 130, 5856-5857. Hermes, S., Schröder, F., Amirjalayer, S., Schmid, R.and Fischer, R. A., J. Mater. Chem., 2006, 16, 2464-2472. Dong, Y., Wang, R., Li, G., Chen, C., Chi, Y. and Chen, G. Anal. Chem., 2012, 84, 6220-6224. Kent, C. A., Mehl, B. P., Ma, L., Papanikolas, J. M., Meyer, T. J. and Lin, W. ,J. Am. Chem. Soc., 2010, 132, 12767-12769. Son, H.-J., Jin, S., Patwardhan, S., Wezenberg, S., Jeong, N. C., So, M., Wilmer, C. E., Sarjeant, A. A., Schatz, G. C.and Snurr, R. Q. J. Am. Chem. Soc., 2012, 135, 862-869.
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Graphene oxide functionalized Zn-based metal-organic framework (ZnMOF) indicates a sensitive and selective luminescence response to Cu2+ ions.
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Cite this: DOI: 10.1039/c0xx00000x
DOI: 10.1039/C3AN01943H
PAPER
A Cubic Luminescent Graphene Oxide Functionalized Zn-based MetalOrganic Framework Composite for Fast and Highly Selective Detection of Cu2+ Ions in Aqueous Solution Liying Hao, Hongjie Song, Yingying Su and Yi Lv* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x In this work, we have synthesized graphene oxide functionalized Zn-based metal-organic framework (ZnMOF) under the one-pot condition, named as ZnMGO composite, which has high luminescence and good water dispersibility. The luminescence of the aqueous ZnMGO composite could be efficiently and selectively quenched by Cu2+ ions through the interactions between Cu2+ and the ligand, and the detection limit was measured as low as 1.00 μM. Also, the robust ZnMGO composite demonstrated the fast response and high sensitivity (Ksv=3.07 × 104 M-1) for Cu2+ ions in aqueous solution. Moreover, we further explored the possible luminescence mechanism in terms of energy migration or electron transfer, and the quenching mechanism is also discovered based on the collapse of the crystal structure with the help of various characterizations. Remarkably, it is the first time that ZnMGO composite possessed the excellent ability is used to rapidly detect Cu2+ ions in aqueous solution. The work does not only contribute to extend the potential application of ZnMGO composite, but also is hopeful to make a contribution in biological areas.
Introduction 20
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Novel interesting and meritorious materials have been paid more attention by analytical researchers because of their potential analytical applications. A new group of materials is metal-organic frameworks (MOFs), which are organic-inorganic hybrids constructed from metal ion nodes linked together by organic linkers to form a three-dimensional crystal lattice.1,2 Recently, researchers have received great interest in MOFs owe to their diverse topologies, high surface areas, predictable structures, tunable pore sizes, and the capability of catalysis.3-5 Therefore, these excellent properties have been quickly developed into a fruitful research field including gas storage,6 catalysis,7 chemical separation,8 drug delivery,9 light-harvesting systems,10,11 biomedical imaging,10 stationary phases for gas chromatography12 and liquid chromatography,13,14 chemical sensing,15 and gas sensing,16 etcetera. In addition to these attractive applications, we are particularly interested in using MOFs as fluorescence probe to detect the metal ions and small molecules. Besides, a few important studies demonstrating the promise of MOFs for the detection of the ions and small molecules17 have been reported previously. For example, Qian and co-workers have reported the detection of metal ions18,19 and some organics20 through fluorescence quenching measurements. Sun group designed a fluorescent probe based Eu-MOF with chelating terpyridine sites for Fe3+.21 Also, the similar applications for MOFs have been reported by other groups.22, 23 However, most of them are performed in organic solvents and in solid state, which is very discommodious for the fluorescence detection. Additionally, This journal is © The Royal Society of Chemistry [year]
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pristine MOFs possess poor stability in the presence of humidity or upon a solvent removal as well as weak dispersibility in aqueous solution,24 which limit their further applications. Furthermore, they exhibit a little poor sensing performances such as low sensitivity and selectivity towards metal ions.18,19 Taking into account these disadvantages, many research efforts have focused on functionalized MOFs materials, such as the deposition of MOFs on various supports, in order to synthesize the MOFsbased composites with better water dispersibility and increased sensitivity and selectivity to metal ions. Another group of interesting materials is graphene and graphenebased materials, which have recently opened new possibilities of applications due to the unique structural, mechanical, electronic properties arising from one-atom-thick carbon sheets.25 Graphene oxide (GO) is the solution-dispersible oxidized form of graphene, which is commonly produced by exfoliating graphite under strong oxidizing conditions.26 The presence of epoxy, carboxyl and hydroxyl functional groups on either side of the GO imparts building block property on the material, which has been widely used to create a large number of GO-based composites, and allows it to act as structural nodes in MOFs. In this concept, GO with dense arrays of layers and numerous oxygen groups is introduced into MOFs to form composites for the sake of overcoming the drawback of MOFs and GO.27 Recently, Petit and co-workers reported the synthesis of MOF-graphite oxide composites in order to increase the dispersive forces in MOF and enhance the adsorption capacity of gas.28,29 Up to now, the MOFGO composites are only used in gas storage, and there are few [journal], [year], [vol], 00–00 | 1
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reports about the other applications, which submerge the superior properties of MOF-GO composites. Taking into account the above, we design a luminescent ZnMGO composite via interactions between the carboxyl groups of GO and the metallic center of the ZnMOF,25,30,31 as a fast response and high selectivity fluorescent probe (FL) for Cu2+ ions in aqueous solution. Moreover, the detection occurs with high sensitivity (Ksv=3.07 × 104 M-1) and selectivity towards Cu2+ ions using the ZnMGO composite compared to the other MOFs.18,19 Meanwhile, we speculate the luminescence mechanism due to energy migration (EM) or electron transfer (ET), and the quenching mechanism is explored based on some characterizations of the ZnMGO composite before and after Cu2+ ions exposure. The obtained results show a satisfied linear range and a low detection limit, which indicate that the ZnMGO composite plays an important role in the metal ions analysis from the environment and the organism.
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Preparation Method of ZnMOF and the ZnMGO composite
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Experimental Section Reagents 20
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Graphite powder was of Specpure grade, which was purchased from Tianjin Guangfu Fine Chemical Research Institute. All chemicals including zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 1,4-benzenedicarboxylic acid (H2BDC), and N,Ndimethylformamide (DMF), cupric nitrate trihydrate (Cu(NO3)2·3H2O, ≥ 99.00%), ethanol, sodium hydroxide (NaOH), and hydrochloric acid (HCl, ≥ 36.46%) were obtained from Chengdu Kelong Chemical Reagent Company (China), which were at least of analytical grade and were used without further purification. Phosphate-buffered saline (PBS) solutions (0.01 M, pH 5.00) dissolved 6.8410 g NaH2PO4·2H2O and 0.2204 g Na2HPO4·12H2O in 500 mL deionized water were used in this experiment, and different pH values of PBS solutions were adjusted with a concentrated sodium hydroxide solution (0.10 M) or hydrochloric acid solution (0.10 M) to the required pH values. Deionized (DI) water from ULUP standard solution of copper (0.01 mol L-1) was prepared from analytical grade cupric nitrate trihydrate. ULUPURE Water Purification System (Chengdu, China) was used in all the experiments. Stock standard solution of copper (0.01 mol L-1) was prepared from analytical grade cupric nitrate trihydrate.
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According to the Hummers method, Graphite oxide was prepared by oxidation of graphite.32 Next, Graphene oxide sheets were obtained by complete exfoliation of graphite oxide as an entry into the ZnMGO composite. In this work, graphite oxide (0.50 mg mL-1) was exfoliated to be graphene oxide on the basis of our previous work.33 The ZnMGO composite was prepared according to the previous papers with a little modification.28,31 Briefly, zinc nitrate hexahydrate (1.04 g) and 1, 4-benzenedicarboxylate (0.20 g) were dispersed in N, N-dimethylformamide (DMF, 14 mL) by stirring until complete dissolution of the solids. Afterwards, the mixture was transferred into a three-necked flask connected to a condenser, and 11 mL GO solution was added into the above mixed solution slowly with stirring. Finally, the mixture was heated at 118 oC for 16 h and 50 oC for 8 h in an oil bath. After cooling, the supernate was removed and the ZnMGO composite on the bottom of the flask was collected through 0.45 μm filter, washed with DMF and ethanol, and immersed in fresh chloroform overnight. At last, the composite was filtered, dried at 100 oC for 8 h in vacuum. The resulting composite was kept in a desiccator. According to the same method, ZnMOF was prepared except adding GO. In this work, 20 mg of the ZnMGO composite was dispersed into 200 mL aqueous solution by sonicated for 1.5 h, and used for the following experiment. Fluorescence spectroscopic studies
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Instrumentations XRD: X-ray diffraction (XRD) measurements were taken using standard powder-diffraction procedures. The materials were ground in a small agate mortar. Then, the materials were smearmounted onto a glass slide and measured by CuKα radiation generated in a Tongda TD-3500 X-ray powder diffractometer (Liaoning, China). SEM: The surface morphology of the materials was examined by SEM. The samples were previously dried and coated with a thin layer of gold were used to scan on a Hitachi, S3400 instrument. Energy dispersive X-ray spectroscopy (EDS) analyses were performed on the same instrument with samples previously dried and coated with gold in order to investigate elemental compositions of ZnMOF, ZnMGO and ZnMGO&Cu. From EDS analyses, the content of elements on the surface was calculated.
FT-IR: Fourier transform infrared (FT-IR) spectroscopy was carried out using a Thermo Nicolet IS10 FT-IR Spectrometer with View Article Online KBr pellets in the range 500-4000 cm-1 toDOI: analyze the surface 10.1039/C3AN01943H properties and composition of the materials. UV-Vis: The UV-Vis spectra of ZnMGO and ZnMGO&Cu composite in the region of 200-500 nm were recorded by U-2910 UV-Vis spectrophotometer. TG: Thermogravimetric (TG) measurements were operated using a NETZSCH STA 449 C. The samples were heated up to 800 °C with the heating rate 10 deg min-1 under a flow of nitrogen of 100 mL min-1.
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All of the fluorescence measurements were done using F-7000 fluorescence spectrophotometer (Hitachi Co., Tokyo, Japan). The Cu2+ detection procedures by the ZnMGO composite were described as follows: 300 μL of the ZnMGO composite aqueous solution reacted with different concentrations of Cu2+ solution (concentrations from 10-3 to 10-6 M in ultrapure water). The mixture was diluted into 2.00 mL with PBS (pH 5.00, except pH optimization experiment) and mixed thoroughly, and then reacted in a wobbler machine at 25 oC (in addition to temperature experiment) for 10 min. Afterwards, the fluorescence detection was excited at 310 nm and the emission was monitored at 442 nm, with the slot widths of the excitation and emission were both 5 nm. Under the same conditions, the effects of various parameters on the fluorescence quenching of the composites were investigated, including pH, temperature and selectivity. All the measurements were conducted in triplicate.
Results and Discussion Characterizations of the materials 110
The XRD patterns of GO, parent material and the composite are
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used to examine the phase and structure of the synthesized products, as shown in Fig. 1. The well-defined peak at 2θ about 10.40o is seen in the case of GO, which is related to an interlayer distance of about 8.50 Å.34 The major sharp diffraction peaks are presented in the case of ZnMOF, which are the feature of this material structure and similar to the data published in those previous literatures for MOF-5.4,35 Obviously, the XRD pattern of the ZnMGO composite is rather similar to the parent ZnMOF, which suggests that the presence of GO does not affect the formation of the crystalline frameworks. Even though the structure of ZnMOF is preserved in ZnMGO hybrid material, the sharpness of the peak at 2θ about 9.67o is slightly reduced and the splitting of that peak is also clearly observed for the ZnMGO composite, which is correlated with the distortion of the crystal structure of ZnMOF.25 The phenomena have been reported by Lillerud and coworkers previously.4 Of course, the major structural and chemical features of ZnMOF preserved in ZnMGO composite are expected since the composites consist of very few GO.28
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consider that GO are packed on the surface of ZnMOF through interactions between ZnMOF metallic centers and the oxygen View Article Online groups of GO.31 Besides, these two explanations are in agreement DOI: 10.1039/C3AN01943H with the SEM images. Meanwhile, a new pore space is created between the ZnMOF blocks and the GO units.
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Fig. 2 SEM images for (a, b) ZnMOF, (d, e) ZnMGO composite, (c, f) EDS spectrum and the percentage of the elements of ZnMOF and ZnMGO composite, respectively. The red square area in e represents the enlarged view of the red square area in d.
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Fig. 1 XRD patterns for GO, ZnMOF and ZnMGO composite.
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The texture of the materials studied can be observed on SEM micrographs presented in Fig. 2. For ZnMOF (Fig. 2a, b), the cubic crystals are clearly observed with some traces of amorphous phase and some cracks.28,30 The size of crystals is as big as 150 μm which is attributed to some regular cubic ZnMOF material stacked together in an organized way. For comparison, SEM images of ZnMGO (Fig. 2d, e) are relatively flat with homogenous structure, remained amorphous phases. From the enlarged view (2e) of the red square of 2d, the wrinkles formed by the GO flakes can be clearly seen. Taking into account the differences in the texture between ZnMOF and ZnMGO, it is plausible to speculate that thin GO layers are located on the surface of ZnMOF and act as dividers and protectors, which is also related to the water-dispersible of the ZnMGO composite. EDS analyses of the parent material and hybrid material are presented in Fig. 2c, f, which reveal that carbon, oxygen, and zinc are dispersed well on the surface. According to these data, the oxygen content in the ZnMGO composite increases with adding GO.25,30 In consideration of the chemistries of both components of ZnMOF and ZnMGO composite, we have assumed the building process of the ordered structure of ZnMOF and ZnMGO, as shown in Scheme 1a and b, respectively. As shown in Scheme 1a, the more regular arrangement of the ZnMOF is composed of some small cubic crystals. With regard to ZnMGO composite, we This journal is © The Royal Society of Chemistry [year]
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Scheme 1 The schematic view of the ideal material structures. (a) Schematic view of the steps of the ZnMOF formation. (b) GO and ZnMGO composite which are formed through proposed bonding between ZnMOF and GO via -COOH groups along direction; in light blue: zinc atoms; in red: oxygen atoms; in light grey: carbon atoms; in white: hydrogen atoms. 25, 28, 30
Further details and support on the three materials are provided by FT-IR spectroscopy (Fig. 3). The spectra for GO were analyzed in detail as follows: C=O, 1724 cm-1; O-H, 1384 and 3379 cm-1; the C-OH stretching, 1257 cm-1; C-O-C (epoxy group) stretching, 1058 cm-1; skeletal ring stretch, 1624 cm-1.36 As expected based on the composition of the ZnMGO composite, the FT-IR spectrum for the composites largely resembles that of ZnMOF. The asymmetric stretching of carboxylic groups in BDC ((1,4benzenedicarboxylate) were observed at 1500 cm-1 and 1578 cm-1, whereas the peak at 1385 cm-1 was assigned to the symmetric stretching of carboxylic groups in BDC.28 In the region of 1300 and 700 cm-1, some bands were found which were due to the outof-plane vibrations of BDC.37 A peak at 1675 cm-1 was contributed to the C=O stretch of carboxylate group located on the surface of GO. Compared to GO, the downshift of the C=O stretch from 1724 cm-1 to 1663 cm-1 in the spectrum of the ZnMGO composite was the result of the coordination of the carboxylate group with the zinc clusters in ZnMOF.30 Journal Name, [year], [vol], 00–00 | 3
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The TGA data revealed that the ZnMGO composite was stable up to 350 oC, as shown in Fig. S1 of the Supporting Information. On account of the good thermal property, further research should pay more attention to the potential of ZnMGO composite for the gas sensors and gas chromatographic column.
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d) FL responses of the ZnMGO composite (15.00 mg L-1) in the presence of 10 μM Cu2+ ions at different pH values (3.00 - 12.00) and temperatures ZnMGO (red (5 - 70 oC), respectively. The inset of c: the FL intensity of View Article Online with 10.1039/C3AN01943H different pH values columns) and ZnMGO + Cu2+ (reseda columns) DOI: of PBS buffer. All the experiments were performed for 15 min and excited at 310 nm. Error bars were based on three measurements.
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Fig. 3 FTIR spectra of GO, ZnMOF, and ZnMGO composite.
Optical performance of the ZnMGO composite 10
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Compared to the ZnMOF, the good water dispersibility is in favor of the fluorescence applications. Besides, having successfully prepared ZnMGO composite, we firstly demonstrated the potential application of the ZnMGO composite as fluorescent sensor to detect Cu2+ ions via the fluorescence quenching of the ZnMGO composite. To evaluate the optical properties of the ZnMGO composite, the UV-Vis absorption spectrum and FL emission spectrum were investigated (Fig. 4a). UV-Vis spectrum of the ZnMGO composite (black line) in water solution exhibited one broad band at 243 nm attributable to the π → π* transition of aromatic carbon bonds. The red shift, compared to GO (Fig. S2), may attributed to the coordination between the carboxyl on the surface of GO and the metal center (Zn) of ZnMOF. Upon excitation at 310 nm, the fluorescence spectrum of the ZnMGO composite showed a 442 nm emission peak (blue line), which was clearly different from that of ZnMOF with emission of 420 nm (insert). Intriguingly, the FL of the ZnMGO composite can be quenched through adding a certain amount of Cu2+ ions (Fig. 4b), which considers that it is possible to apply the ZnMGO composite in detecting Cu2+ ions.
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Sensitivity of the Sensing System
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Fig. 4 (a) UV-Vis spectra (black line) and FL emission spectra (blue line, excitation at 310 nm) of the ZnMGO composite. Insert: FL emission spectra of ZnMOF. (b) FL responses of the ZnMGO composite (15.00 mg L-1) upon addition of different amounts of Cu2+ ions (5.0×10-6, 1.0×10-5, 5.0×10-5, 5.0×10-4, 1.0×10-3 M) in aqueous solution (excited at 310 nm). (c,
As mentioned above, the fluorescence quenching of the ZnMGO composite by Cu2+ ions was evaluated with fluorescence spectra. Moreover, we investigated the key factors including the effects of pH and temperature for the sensing system as follows. As depicted in inserted columns of Fig. 4c, the presence of Cu2+ leads to the FL quenching of the ZnMGO composite over the wide pH range from 3.00 to 12.00; most remarkably, the quenching efficiencies (F0 - F / F) at these pH values are various (Fig. 4c). We can see that in the condition of the addition of Cu2+ ions in strongly acidic media (pH 3.00), the quenching efficiency is very low, which may be due to the fact that the carboxyl groups at the surfaces of the ZnMGO composite are well protonated, indicating that it is impossible to complex with Cu2+ to form FL-quenching cupric moieties. In the case of alkaline solutions (pH > 7.00), the quenching efficiencies are not satisfied either, which may attributed to partial hydrolysis of Cu2+ ions in the alkaline media.38 By comparison, in the weakly acidic medium (pH 5.00), the quenching efficiency is better than others, suggesting that the weakly acid medium is suitable for this sensing system for the sensitive detection of Cu2+ ions. Fig. 4d shows the effect of the temperature. We can distinctly observe that the quenching efficiency (F0 / F) decreased along with the temperature increased (> 25oC), especially at high temperature. According to the experiment, the room temperature (25 oC) was chosen as the optimal condition.
Fig. 5 (a) FL response of the ZnMGO composite upon addition of various concentrations of Cu2+ ions (from top to bottom: 0, 1, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, and 1000 μM) in a pH 5.00 PBS solution. (b) The relationship between the concentrations of Cu2+ and F0 / F. Inset: The linear calibration plot for Cu2+ detection. All the experiments were performed for 15 min and excited at 310 nm. Error bars were based on three measurements.
Under the optimized conditions discussed above, the sensitivity, the linear response range, and the detection limit of the ZnMGObased sensing system are measured as follows. As shown in Fig. 5a, the FL intensity of the ZnMGO composite at 442 nm is sensitive to Cu2+ ions and decreases with the increase of concentration of Cu2+ ions. There is a good linear correlation between the quenching efficiency (F0 / F) and the concentrations of Cu2+ ions in the range from 1.00 to 70.00 μM (see inset of Fig. 6b) via the following equation:
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Establishment of the FL Sensing Method for Cu2+
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F0 / F =0.0307C+0.9987 (R=0.9963) Where F0 and F are the FL intensity of the ZnMGO composite in the absence and presence of Cu2+ ions, respectively, and C is the concentration of Cu2+ ions. The detection limit (DL) was calculated to be 1.00 μM through the equation DL = 3 σ/k, where σ is the relative standard deviation and k is the slope of the calibration graph (n=11). Therefore, we have demonstrated a fluorescent turn-off sensor based on the ZnMGO composite for detection of Cu2+ ions with a high sensitivity. Moreover, the quenching effect can be quantitatively described by the Stern-Volmer equation:
F0 1 Ksv[C] F
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KSV is the quenching effect coefficient of the metal ion, which can be calculated as 3.07 × 104 M-1 via luminescent data (inset in Fig. 6b) and equation 1. Fortunately, the value of KSV is much larger than that of 89.4 M-1 in [Eu(pdc)1.5(dmf)]·(DMF)0.5(H2O)0.518 and 528.7 M-1 in Eu2 (FMA)2(OX)(H2O)4·4H2O for Cu2+ sensing,19 indicating that it is a much more sensitive luminescent ZnMOFbased sensor and has the strongest quenching effect on the luminescence intensity of the ZnMGO composite.
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Besides sensitivity, selectivity was another key factor to assess the performance of a new proposed sensor. Except for Cu2+ ions, the effects of 12 other kinds of cations, including alkali metal ions (K+, Na+), alkaline earth metal ions (Ca2+, Mg2+), light metal ion (Al3+), and heavy metal ions (Co2+, Ni2+, Mn2+, Zn2+, Pb2+, Cd2+, Cr3+) on the FL response of the ZnMGO composite were also investigated. As illustrated in Fig. 6, alkali and alkaline earth metal ions have no apparent effect on the fluorescence intensity, while Al3+ and some heavy metal ions have different quenching degrees on the luminescence intensity. Although the concentrations of these ions were as ten times as that of Cu2+ ions, there were no apparent fluorescence quenching compared with adding Cu2+ ions, which indicates the present sensing system exhibits excellent selectivity for the detection of Cu2+ ions, and could further be used to the practical applications.
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Fig. 6 Selectivity of the ZnMGO-based sensor for over other ions in pH 5.00 PBS solution: the concentration was 10 μM for Cu2+ and 100 μM for other metal ions. The treatment reactions were performed for 15 min at room temperature, and excited at 310 nm.
Study on the mechanisms
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Recently, many researchers have paid more attention to the applications of MOFs for the development of sensing systems Articleof Online based on the changes of fluorescence intensity View because the DOI: 10.1039/C3AN01943H energy migration (EM), electron transfer (ET), or other interactions between MOFs and the analyte. In our study, we speculate the possible luminescence mechanisms on account of EM or ET for the ZnMGO composite. In the case of EM (Scheme 2.1), Zn is the luminescence center, while ligand BDC functions as both a linker and a receptor. When the light is provided, the energy transfers from BDC ligands to Zn2+ ions, which makes the ZnMGO composite emit fluorescence.11,19,39,40 In the case of ET (Scheme 2.2), GO is helpful to electron transfer, owing to its chemical bonds with MOF and the conductivity of distorted graphene oxide layers. Under the condition of light, the electrons of the ZnMGO composite are excited, and the excited electrons from the ZnMGO composite are transferred to the unoccupied states of GO-Oepoxy. Then, these electrons undergo a transition down to metal center (Zn), leading to fluorescence emission.41 For the quenching mechanism, we consider that the addition of Cu2+ ions, weakened the bonds between the metallic sites and the organic ligands,31 results in the collapse of the ZnMGO composite because of the coordination between Cu2+ ions and carboxyl groups of ligand, which forbids the EM or ET from the organic ligands (BDC) to the metallic sites (Zn), leading to the fluorescence quenching.
Scheme 2 The schematic view of the possible mechanisms. (1 a, b) The proposed luminescence and fluorescence quenching mechanisms of the ZnMGO composite based on energy migration, respectively. (2 a, b) The proposed luminescence and fluorescence quenching mechanisms of the ZnMGO composite based on electron transfer, respectively.
For the sake of elucidating the possible quenching mechanism by Cu2+ ions, some characterizations were used to monitor the structure change of the ZnMGO composite before and after adding Cu2+ ions. As shown in Fig. S3, the differences between the ZnMGO composite and ZnMGO&Cu indicate the structure of ZnMGO&Cu has changed compared to the ZnMGO composite. The SEM (Fig. 7) images clearly show the damage of the crystal form, which has changed from the cubic (Fig. 7a) to the plane structure (Fig. 7b). Also, the XRD (Fig. S4) can verify the collapse of the ZnMGO composite. In comparison with the ZnMGO composite, the peak at 6.25o disappears and the other Journal Name, [year], [vol], 00–00 | 5
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peaks are in disorder, which suggest the damage of the crystal form. Therefore, it is possible that the addition of Cu2+ ions leads the collapse of the ZnMGO composite and prohibits the EM or ET from the organic ligands (BDC) to the metallic sites (Zn)， which lead to the fluorescence quenching.
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Fig. 7 SEM images for (a) ZnMGO composite, (b) ZnMGO&Cu. Inset of b: EDS spectrum of ZnMGO&Cu.
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In summary, a highly luminescent metal-organic frameworkbased composite (ZnMGO) has been prepared by mixing the precursors and GO, which illustrates unprecedented sensor for fluorescence “turn-off ” detection of Cu2+ ions. On the basis of the fluorescence quenching of the ZnMGO composite by Cu2+ ions, the Cu2+ ions can be detected as low as 1.00 μM with a detection range of 1.00 to 70.00 μM, without the apparent interferences from other metal ions. Moreover, we also elaborate the possible luminescence mechanisms on base of energy migration or efficient electron transfer. Also, the quenching mechanism is explored by means of some characterizations based on the collapse of the ZnMGO composite leading the impossible of the EM or ET from the organic ligands (BDC) to the metallic sites (Zn). Taking advantage of the phenomenon, MOFs-based sensor may be applicable to the detection of a wide range of metal ions through properly functionalizing the MOFs. Further efforts will be focused on the function of porous luminescent MOFs to enhance their recognition sensitivity and selectivity. It is noteworthy that we are now exploring some luminescent and innocuous MOFs-based composites, and developing the strategies to apply to the biological system.
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Acknowledgment This work was supported by the National Nature Science Foundation of China (21375089). The authors also would like to show gratitude for Yanyan Meng and Feng Yang from College of Chemistry at Sichuan University for their assistance in the structure of the materials and the XRD analysis, respectively.
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Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. Fax: 86 28 8541 2798; Tel: 86 28 8541 2798; E–mail: [email protected] 1. 2.
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3.
Férey, G. Chem. Soc. Rev. 2008, 37, 191-214. Yaghi, O. M., O'Keeffe, M., Ockwig, N. W., Chae, H. K., Eddaoudi, M. and Kim, J. Nat.,2003, 423, 705-714. Furukawa, H.; Ko, N., Go, Y. B., Aratani, N., Choi, S. B., Choi, E.,
110
38. 39. 40.
115
Yazaydin, A. Ö.; Snurr, R. Q., O’Keeffe, M. and Kim, J. Science, 2010, 329, 424-428. Hafizovic, J.; Bjørgen, M., Olsbye, U., Dietzel, P. D.,View Bordiga, Article S., Online DOI: Prestipino, C., Lamberti, C. and Lillerud, K. P. 10.1039/C3AN01943H J. Am. Chem. Soc., 2007, 129, 3612-3620. Li, J.-R., Sculley, J.and Zhou, H.-C. Chem. Rev., 2011, 112, 869-932. Zhao Y., Seredych Mykola, Zhong Q. and Bandosz T. J., ACS Appl. Mater. Interfaces, 2013, 5, 4951-4959. Bromberg L., Su X. and Hatton T. A., ACS Appl. Mater. Interfaces, 2013, 5, 5468-5477. S. S. Liu, C. X. Yang, S. W. Wang and X. P. Yan, Analyst, 2012, 137, 816-818 Horcajada, P., Serre, C., Vallet‐Regí, M.; Sebban, M.; Taulelle, F., and Férey, G. Angew. Chem. 2006, 118, 6120-6124. Della Rocca, J., Liu, D.and Lin, W., Acc. Chem. Res. 2011, 44, 957968. Kent, C. A., Liu, D., Ma, L., Papanikolas, J. M., Meyer, T. J.and Lin, W., J. Am. Chem. Soc. 2011, 133, 12940-12943. Chang, N., Gu, Z.-Y., Wang, H.-F.and Yan, X.-P. Anal. Chem. 2011, 83, 7094-7101. Yang, C.-X. and Yan, X.-P., Anal. Chem. 2011, 83, 7144-7150. Gu, Z.-Y., Yang, C.-X., Chang, N.and Yan, X.-P., Acc. Chem. Res. 2012, 45, 734-745. Pramanik, S., Zheng, C., Zhang, X., Emge, T. J. and Li, J. J. Am. Chem. Soc. 2011, 133, 4153-4155. Kreno, L. E., Hupp, J. T. and Van Duyne, R. P. Anal. Chem. 2010, 82, 8042-8046. Chen, B., Xiang, S. and Qian, G., Acc. Chem. Res. 2010, 43, 11151124. Chen, B., Wang, L., Xiao, Y., Fronczek, F. R., Xue, M., Cui, Y.and Qian, G., Angew. Chem. Int. Ed. 2008, 48, 500-503. Xiao, Y., Cui, Y, Zheng, Q., Xiang, S., Qian, G. and Chen, B. Chem. Comm. 2010, 46, 5503-5505. Xu, H., Liu, F., Cui, Y., Chen, B.and Qian, G., Chem. Comm. 2011, 47, 3153-3155. Zheng, M., Tan, H., Xie, Z., Zhang, L., Jing, X. and Sun, Z., ACS Appl. Mater. Interfaces, 2013, 5, 1078-1083. Lan, A., Li, K., Wu, H., Olson, D. H., Emge, T. J., Ki, W., Hong, M.and Li, J., Angew. Chem. Int. Ed., 2009, 48, 2334-2338. S. H. Huo and Xiu-Ping Yan, Analyst, 2012, 137, 3445–3451. Petit, C., Mendoza, B., O’Donnell, D.and Bandosz, T. J., Langmuir, 2011, 27, 10234-10242. Petit, C.and Bandosz, T. J. Adv. Mater,. 2009, 21, 4753-4757. Song, H., Zhang, L., He, C., Qu, Y.; Tian, Y.and Lv, Y., J. Mater. Chem., 2011, 21, 5972-5977. Huang, Z.-H., Liu, G.and Kang, F. ACS Appl. Mater. Interfaces, 2012, 4, 4942-4947. Petit, C.and Bandosz, T. J. J. Mater. Chem., 2009, 19, 6521-6528. Petit, C.and Bandosz, T. J. Adv. Funct. Mater., 2009, 20, 111-118. Jahan, M., Bao, Q., Yang, J.-X.and Loh, K. P. J. Am. Chem. Soc., 2010, 132, 14487-14495. Bandosz, T. J. Dalton Trans., 2012, 41, 4027-4035. Hummers Jr, W. S.and Offeman, R. E. J. Am. Chem. Soc., 1958, 80, 1339-1339. Hao, L., Song, H., Zhang, L., Wan, X., Tang, Y.and Lv, Y. J. Colloid Interface Sci., 2011, 369, 381-387. Song, H., Hao, L. Tian, Y., Wan, X., Zhang, L. and Lv, Y. ChemPlusChem, 2012, 77, 379-386. Kaye, S. S., Dailly, A., Yaghi, O. M.and Jeffrey, R., J. Am. Chem. Soc., 2007, 129, 14176-14177. Xu, Y., Bai, H., Lu, G., Li, C.and Shi, G. J. Am. Chem. Soc., 2008, 130, 5856-5857. Hermes, S., Schröder, F., Amirjalayer, S., Schmid, R.and Fischer, R. A., J. Mater. Chem., 2006, 16, 2464-2472. Dong, Y., Wang, R., Li, G., Chen, C., Chi, Y. and Chen, G. Anal. Chem., 2012, 84, 6220-6224. Kent, C. A., Mehl, B. P., Ma, L., Papanikolas, J. M., Meyer, T. J. and Lin, W. ,J. Am. Chem. Soc., 2010, 132, 12767-12769. Son, H.-J., Jin, S., Patwardhan, S., Wezenberg, S., Jeong, N. C., So, M., Wilmer, C. E., Sarjeant, A. A., Schatz, G. C.and Snurr, R. Q. J. Am. Chem. Soc., 2012, 135, 862-869.
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41. Son, D. I., Kwon, B. W., Park, D. H., Seo, W.-S., Yi, Y., Angadi, B., Lee, C.-L.and Choi, W. K. Nat. Nanotech., 2012, 7, 465-471. View Article Online
Analyst Accepted Manuscript
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DOI: 10.1039/C3AN01943H
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