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One-pot synthesis of a magnetic, ratiometric fluorescent nanoprobe by encapsulating Fe3O4 magnetic nanoparticles and dual-emissive rhodamine B modified carbon dots in metal–organic framework for enhanced HClO sensing Yujie Ma, Guanhong Xu, Fangdi Wei, Yao Cen, Xiaoman Xu, Menglan Shi, Xia Cheng, Yuying Chai, Muhammad Sohail, and Qin Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05643 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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One-pot synthesis of a magnetic, ratiometric fluorescent nanoprobe by encapsulating Fe3O4 magnetic nanoparticles and dual-emissive rhodamine B modified carbon dots in metal–organic framework for enhanced HClO sensing Yujie Ma, Guanhong Xu, Fangdi Wei, Yao Cen, Xiaoman Xu, Menglan Shi, Xia Cheng, Yuying Chai, Muhammad Sohail, Qin Hu∗ School of pharmacy, Nanjing medical university, Nanjing, Jiangsu 211166, PR China

Abstract In this work, a new magnetic, ratiometric fluorescent nanoprobe has been designed and fabricated by encapsulating Fe3O4 magnetic nanoparticles (MNPs) and dual-emissive carbon dots into the cavities of metal–organic frameworks (MOFs). This one-pot method combined hybrid characteristics of MOFs with multiple properties of the encapsulated functional materials. The MOF-based nanoprobe possessed the advantages of MOFs (strong adsorption ability, accumulating the analytes), Fe3O4 MNPs (magnetic separation) and ratiometric sensors (eliminating the variabilities caused by the unstability of the instruments and environment). The MOF-based nanoprobe was dispersible and stable in aqueous solution, and the nanoprobe was applied to HClO sensing. This work will provide a promising strategy for design and synthesis of novel MOF-based composite materials.

Keywords: Metal–organic frameworks, dual-emissive carbon dots, Fe3O4 magnetic nanoparticles, HClO, ratiometric fluorescence



Corresponding author: Tel. / fax: +86-2586868468; E-mail: [email protected]

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Introduction Metal–organic frameworks (MOFs) are porous materials which are constructed by metallic ions and organic linkers. During the past two decades, MOFs have attracted great attention and interest from scientists. MOFs can be rationally designed and conveniently synthesized, and they have tunable properties which make them promising candidates for a wide range of applications in chemical sensing, catalysis, drug delivery, gas storage, separation, energy storage and optoelectronics.1-9 Recently, while the development of novel MOFs keeps rapid, MOF-based nanoparticles,10-15 particularly MOF-based emitting nanoparticles16-19 have also been greatly investigated. The tunable pores and ultrahigh porosities make MOFs potential host matrices for functional guest nanoparticles. In addition, the MOF structure can prevent guest nanoparticles from aggregation, thus improving the properties of the nanoparticles. As a result, encapsulating functional guest nanoparticles into MOF cavities is a promising strategy to obtain functionalized MOF-based composite materials with advanced properties. MOF-based composite materials have attracted much attention and have potential uses in sensing, catalysis, drug delivery, and white light emission.14,16,20,21 Among previously reported MOF hosts, the zinc 2-methyl imidazolate framework (ZIF-8) is a good candidate because of its high stability, ease of synthesis, and its proved ability as host matrix. Firstly, ZIF-8 is stable in water while many other MOFs are moisture-sensitive. Secondly, it takes only 1 hour to synthesize ZIF-8 while the synthesis of most MOFs needs much longer time. Last but not least, several nanoparticles have been successfully encapsulated into ZIF-8, demonstrating its potential to act as a host matrix.16,22 Ratiometric fluorescent sensors own the advantages of decreasing the error caused by the concentration of the probe and the variation of light source and environment.23 This method can greatly improve the detection accuracy. Up to now, ratiometric sensors have been commonly designed by the combination of two components with distinct fluorescent emission wavelengths. It is much easier to operate a ratiometric sensor with one component which has two distinct fluorescent emission wavelengths. ACS Paragon Plus Environment

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Carbon dots (CDs) have gained much attention because of their good optical properties and potential uses.24-26 Compared with other phosphors, such as organic dyes and quantum dots, CDs have advantages including tunable emission lights, high fluorescence intensity, photostability against photo-bleaching and lower toxicity. These characteristics make them promising fluorescent materials. Up to now, some dual-emissive CDs have been reported,27,28 but few of them have been applied to sensing.28 In this work, a magnetic, ratiometric fluorescent nanoprobe was designed and synthesized by encapsulating Fe3O4 MNPs and dual-emissive rhodamine B modified carbon dots (RhB-CDs) simultaneously in ZIF-8 cavities through a facile one-pot synthetic route. The dual-emissive property will endow the nanoprobe with the advantage of ratiometric sensors, thus eliminating the variability caused by the instability of the instruments and environment. The magnetic property will enable the nanoprobe have separation ability, thus reducing the influence of substrates in complex system. The strong adsorption ability of MOFs will help enrich the target analytes, thus improving the sensitivity of the nanoprobe. Overall, the nanoprobe combined the advantages of MOFs, ratiometric sensors and magnetic separation, resulting in excellent property in sensing. The nanoprobe was then applied to the detection of HClO.

2. Experimental section 2.1. Materials Iron (III) chloride hexahydrate, RhB, zinc nitrate hexahydrate and citric acid monohydrate (CA) were bought from the Sinopharm Chemical Reagent Co., Ltd (China). 2-methylimidazole was obtained from Macklin Biochemical Co., Ltd (China). Ferrous chloride tetrahydrate was obtained from Rich Joint Chemical Reagent Co., Ltd. PEG400 and NH3 aqueous solution (25wt%) were bought from Shanghai Lingfeng Chemical Reagent Co., Ltd (China). The purified water was prepared through a water purification system (Millipore, USA). The buffer solution was made from the mixture of citric acid monohydrate and disodium hydrogen phosphate stock ACS Paragon Plus Environment

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solution, the concentration of the buffer solution was 0.2 M. The preparations of ROS and RNS solutions were described in detail in supporting information.

2.2. Instruments Fluorescence emission spectra were obtained from an F-4600 fluorescence spectrometer (Japan). The morphology of RhB-CDs, Fe3O4 MNPs and the MOF-based nanoprobe was measured by a JEM-2100 transmission electron microscopy (Japan). UV-vis absorption spectrum was recorded by a UV-2450 UV-vis spectrophotometer (Japan). The magnetization hysteresis loops were measured by vibrating sample magnetometer (VSM, PPMS Dynacool).

2.3. Synthesis of dual-emissive RhB modified carbon dots (RhB-CDs) Dual-emissive RhB-CDs were prepared through a reported hydrothermal method with minor modification.27 Briefly, 1.0 g of citric acid (CA) and 0.02 g of RhB was dissolved in 10 mL of water. Subsequently, the mixed solution was put in a 30 mL autoclave. The mixture was heated at 220 °C for 16 h, then cooled to the room temperature. The products were neutralized with diluted ammonia solution and purified by dialyzing against ultrapure water through a dialysis bag (retained molecular weight: 1 kDa) for two days. Finally, the solution of the product was concentrated to 10 mL for further use.

2.4. Synthesis of Fe3O4 magnetic nanoparticles Fe3O4 MNPs were prepared through a reported method with minor modification.29 Briefly, 80 mL of water was heated to 70 °C under N2 protection, then FeCl2·4H2O (0.86 g) and FeCl3·6H2O (2.36 g) was added into the heated water. Subsequently, 5 mL of NH3 aqueous solution (25wt%) and 1 mL of PEG400 was added into the mixture dropwise, the mixture was then allowed to react at 80 °C under N2 protection and stirring for 1 h. The products were washed with water for several times and dried under vacuum at 40 °C. 100 mg of Fe3O4 MNPs was ground and dispersed in 8 mL of water for further use. ACS Paragon Plus Environment

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2.5. Preparation of the MOF-based nanoprobe The MOF-based nanoprobe was prepared through a reported one-pot method with minor modification.12 Typically, 20 mL of Zn(NO3)2·6H2O (0.9870 mmol) water solution, 20 mL of 2-methylimidazole (7.904 mmol) water solution, 1 mL of RhB-CDs solution and 500 µL of the as-prepared Fe3O4 suspension were mixed and allowed to react under stirring for 1 h (room temperature). The products were collected and washed several times with water. Finally, half of the product was vacuum-dried overnight (60 °C) and half of the products were dispersed evenly in 40 mL of water for further use. The pure ZIF-8 was prepared through the same method without adding RhB-CDs and Fe3O4 suspension. To prepare the mixture of ZIF-8+RhB-CDs+Fe3O4, 500 µL of RhB-CDs solution, 500 µL of the as-prepared Fe3O4 suspension and the synthesized pure ZIF-8 were put together and allowed to react under stirring for 1 h. The products were collected and washed several times with water, and then dispersed evenly in 40 mL of ultrapure water.

2.6. Detection of HClO Typically, the HClO solutions with different concentrations were prepared by diluting the stock solution with buffer (pH 4). The MOF-based nanoprobe suspension (560 µL, 35 µg/mL) was added into HClO solutions (60 µL) with different concentrations (15-180 µM). The mixture was then vortexed for 2 min. After magnetic separation, the supernatant was removed, and the deposit (nanoprobe) was redispersed in 200 µL of buffer solution (pH 4). Finally, the fluorescence spectrum was measured upon 355 nm excitation.

2.7. Detection of the real sample The sample was a commercial disinfectant diluted 5000 times with buffer (pH 4). The detection of diluted disinfectant sample was performed as described above.

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3. Results and discussion 3.1. Characterization of the MOF-based nanoprobe The as-synthesized MOF-based nanoprobe appeared pink and had good dispersibility in aqueous solution (Figure 1A), it could emit orange fluorescence (Figure 1B) under ultraviolet light (365 nm). As shown in Figure 1C, the nanoprobe had fluorescence emission peaks at 415 nm and 580 nm upon 355 nm excitation. The fluorescence stability of the MOF-based nanoprobe was studied by recording the fluorescence emission spectrum every five days (Figure S1). The fluorescence intensity kept intact within 20 days, suggesting its good fluorescence stability.

Figure 1. Images of water suspensions of the MOF-based nanoprobe under natural light (A) and UV light (365 nm) (B). Fluorescence emission spectrum of the MOF-based nanoprobe suspension under excitation at 355 nm (C).

To demonstrate that the RhB-CDs were encapsulated into the MOF cavities, rather than adsorbed into the MOF cavities, the as-synthesized MOF-based composite material and the mixture of ZIF-8+RhB-CDs+Fe3O4 were washed with water for several times. After thorough rinsing, the mixture of ZIF-8+RhB-CDs+Fe3O4 had no visible fluorescence while the MOF-based nanoprobe retained strong fluorescence, which fully illustrated that the RhB-CDs were successfully encapsulated into the MOF cavities. Figure 2 showed the UV-vis absorption spectra of the pure ZIF-8 (a), Fe3O4 MNPs (b), RhB-CDs (c) and the MOF-based nanoprobe (d). RhB-CDs had absorption peaks at 350 nm and 550 nm, these two peaks were consistent with reported results.29 From 300 nm to 600 nm, Fe3O4 MNPs had broad absorption band, while pure ZIF-8

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had very weak absorption band. The MOF-based nanoprobe had strong absorption band in this range, which came from the absorption of Fe3O4 MNPs. In addition, the MOF-based nanoprobe had a specific absorption peak at 550 nm, which was in accordance with that of RhB-CDs. These results indicated that both Fe3O4 MNPs and RhB-CDs were successfully encapsulated in the cavities of ZIF-8.

Figure 2. UV-vis absorption spectra of the pure ZIF-8 (a), Fe3O4 MNPs (b), RhB-CDs (c) and the MOF-based nanoprobe (d).

The morphology of the MOF-based nanoprobe was shown in Figure S2, the as-synthesized nanoprobe displayed rhombic dodecahedral structure with an average size of approximately 40 nm, which was consistent with the reported result.30 Figure S3 and Figure S4 showed the TEM images of RhB-CDs and Fe3O4 MNPs. Figure 3 showed the powder X-ray diffraction (PXRD) results of the MOF-based nanoprobe and the pure ZIF-8. The diffraction peak positions of the MOF-based nanoprobe were in accordance with those of the pure ZIF-8, revealing that the crystalline integrity of ZIF-8 kept intact after the encapsulation of RhB-CDs and Fe3O4 MNPs.

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Figure 3. PXRD patterns of ZIF-8 and the MOF-based nanoprobe.

Vibrating sample magnetometer was used to investigate the magnetic behavior of the synthesized MOF-based nanoprobe. As is shown in Figure 4, the magnetic hysteresis loops of the MOF-based nanoprobe and Fe3O4 MNPs exhibited super paramagnetic behavior. The magnetic saturation value of the MOF-based nanoprobe was 26.5 emu g-1, which was lower than that of Fe3O4 MNPs, mainly caused by the MOF shell. The inset in Figure 4 showed the photograph of the MOF-based nanoprobe in the presence of an external magnetic field, the photograph indicated that the magnetic property of the nanoprobe was good enough for magnetic separation. The magnetic property was significant for the separation of analytes from the complicated detection system, thus reducing the influence of the interferents in the detection system.

Figure 4. The magnetization hysteresis loops of the MOF-based nanoprobe (a) and Fe3O4 MNPs (b). The inset shows the photograph of the MOF-based nanoprobe in an external magnetic field.

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The aforementioned characterizations could confirm that the MOF-based nanoprobe with good fluorescent property, magnetic property and the characteristic of ZIF-8 was successfully synthesized.

3.2. The interaction between the MOF-based nanoprobe and HClO To demonstrate that the MOF structure had the ability in accumulating the analyte and improving sensing sensitivity, 80 µM of HClO was added into RhB-CDs solution and MOF-based nanoprobe suspension, respectively. Figure 5 showed that the fluorescence quenching intensity (∆F) of the MOF-based nanoprobe was much higher than that of the RhB-CDs because of the strong adsorption ability of MOFs. Such result indicated that the MOF structure was important for improving sensing sensitivity.

Figure 5. Fluorescence spectra of RhB-CDs (0.5 µg/mL) and MOF-based nanoprobe (35 µg/mL) in the presence and absence of 80 µM of HClO.

The reason that the fluorescence of RhB-CDs quenched by HClO might be that RhB part in RhB-CDs was oxidized by HClO and the π-electron density (δ) in the conjugated system decreased, thus causing the fluorescence quenching.31,32 The parameters that may affect the reaction between the MOF-based nanoprobe and HClO, such as pH and incubation time, were optimized. The results were shown in Figure S5, pH 4 and 2 min of reaction time were chosen as optimal conditions.

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3.3. Selectivity In order to illustrate that the MOF-based nanoprobe had good selectivity to HClO, the influence of several ROS and RNS species and some common interferents on the MOF-based nanoprobe were tested. 150 µM of HClO and 1 mM of interferes were added into the MOF-based nanoprobe, respectively. Figure 6 and Figure S6 showed that the fluorescence at 580 nm of the nanoprobe was enormously quenched by 150 µM of HClO. However, almost no changes were observed in the fluorescence response (F415/F580) after the addition of interferes into the nanoprobe, illustrating good selectivity.

Figure 6. Selectivity of the MOF-based nanoprobe for HClO over interferents. The concentrations of HClO and interferents were 150 µM and 1 mM.

3.4. Method validation Figure 7 showed that the fluorescent intensity of the MOF-based nanoprobe at 580 nm decreased obviously with the addition of increasing amount of HClO, while the fluorescent intensity at 415 nm kept almost unchanged. There was a good linear relationship between the quenching efficiency (F415/F580-F0415/F0580) and HClO concentration

(15-180

µM)

using

the

following

equation:

F415/F580-F0415/F0580=0.006C-0.046, where F415/F580 and F0415/F0580 was the ratio of fluorescent intensity at 415 nm to 580 nm in the presence and absence of HClO, respectively. Limit of detection (LOD=3σ/K, where σ represents the standard deviation (SD) of the blank measurements of the nanoprobe (n = 10), K is the slope of

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the calibration curve) was 6.7 µM. As shown in Table 1, the method built had good precision since the inter-day and intra-day relative standard deviations (RSDs) were all below 10%.

Figure 7. Fluorescence spectra of the MOF-based nanoprobe in the presence of several concentrations of HClO (15-180 µM). The inset is the calibration curve of HClO detection.

Table 1 Inter-day and intra-day precisions of the detection of HClO (n = 3). Concentration of HClO (µM)

intra-day (µM)

20.0

19.2 17.6

60.0

inter-day (µM)

4.6

21.5 20.1

68.3

64.3 3.5

64.1

64.3

65.0

126.0

120.0

124.7

RSD (%)

18.5

18.0

68.4

120.0

RSD (%)

4.3

116.4

116.2

7.6

3.3

4.0

121.3

3.5. Determination of HClO The MOF-based nanoprobe was used for the detection of HClO in commercial disinfectant sample. The samples came from 5000-fold-diluted disinfectant. Table 2 showed that the recoveries of the spiked HClO were between 95.4 and 106%, the RSDs were below 5%, indicating the satisfied accuracy and precision for HClO

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detection in real samples. Table 2 Determination of HClO in real samples with the method built (n = 3). Sample

Spiked (µM)

Found (µM)

Recovery (%)

RSD (%)

0

35.1

-

4.6

25.0

61.7

106

3.7

50.0

82.8

95.4

1.4

Disinfectant

4. Conclusion In summary, different kinds of functional materials (dual-emissive RhB-CDs and Fe3O4 MNPs) were encapsulated simultaneously into the MOF cavities through a simple and quick one-pot synthesis approach for the first time. This strategy combined the strong adsorption ability of MOFs with multiple properties of encapsulated functional materials. The as-synthesized MOF-based nanoprobe had the advantages of ratiometric sensors, magnetic separation and strong adsorption ability of MOFs and performed well in HClO sensing. The experimental results indicated that this could be a simple and efficient strategy to fabricate novel functional MOF-based materials. Supporting Information Preparation of ROS and RNS solutions; fluorescence stability of the MOF-based nanoprobe; TEM images of the MOF-based nanoprobe, RhB-CDs and Fe3O4 MNPs; influence of pH and incubation time to the reaction between the MOF-based nanoprobe and HClO Acknowledgement This work was financially supported by the Natural Science Foundation of Jiangsu Province (No. BK20171487, BK20171043) and National Natural Science Foundation of China (No. 21705080, 61775099).

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Compliance with ethical standards The authors declare no competing interests.

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Metal−Organic Framework Derived Mesoporous TiO2 as Photoanodes for High-Performance Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 22201−22212. (20) Wen, Y. H.; Sheng, T. L.; Zhu, X. Q.; Zhuo, C.; Su, S. D.; Li, H. R.; Hu, S. M.; Zhu, Q. L.; Wu, X. T. Introduction of Red-Green-Blue Fluorescent Dyes into a Metal-Organic Framework for Tunable White Light Emission. Adv. Mater. 2017, 29, 1700778. (21) Li, S. Z.; Huo, F. W. Metal–organic framework composites: from fundamentals to applications. Nanoscale 2015, 7, 7482−7501. (22) Huang, Y. B.; Zhang, Y. H.; Chen, X. X.; Wu, D. S. ; Yi, Z. G.; Cao, R. Bimetallic alloy nanocrystals encapsulated in ZIF-8 for synergistic catalysis of ethylene oxidative degradation. Chem. Commun. 2014, 50, 10115−10117. (23) Song, W.; Duan, W. X.; Liu, Y. H.; Ye, Z. J.; Chen, Y. L.; Chen, H. L.; Qi, S. D.; Wu, J.; Liu, D.; Xiao, L. H.; Ren, C. L.; Chen, X. G. Ratiometric Detection of Intracellular Lysine and pH with One-Pot Synthesized Dual Emissive Carbon Dots. Anal. Chem. 2017, 89, 13626−13633. (24) Lim, S. Y.; Shen, W.; Gao, Z. Q.; Carbon quantum dots and their applications. Chem. Soc. Rev. 2015, 44, 362−381. (25) Gao, X. H.; Du, C.; Zhuang, Z. H.; Chen, W. Carbon quantum dot-based nanoprobes for metal ion detection. J. Mater. Chem. C 2016, 4, 6927−6945. (26) Fernando, K. A.; Sahu, S.; Liu, Y.; Lewis, W. K.; Guliants, E. A.; Jafariyan, A.; Wang, P.; Bunker, C. E.; Sun, Y. P. Carbon Quantum Dots and Applications in Photocatalytic Energy Conversion. ACS Appl. Mater. Interfaces 2015, 7, 8363−8376. (27) Dong, Y. Q.; Chen, Y. M.; You, X.; Lin, W.; Lu, C. H.; Yang, H. H.; Chi, Y. W. High photoluminescent carbon based dots with tunable emission color from orange to green. Nanoscale 2017, 9, 1028−1032. (28) Shangguan, J. F.; He, D. G.; He, X. X.; Wang, K. M.; Xu, F. Z.; Liu, J. Q. Tang, J. L.; Yang, X.; Huang, J. Label-Free Carbon-Dots-Based Ratiometric Fluorescence pH Nanoprobes for Intracellular pH Sensing. Anal. Chem. 2016, 88, 7837−7843. (29) Xie, J.; Xu, C. J.; Kohler, N.; Hou, Y. L.; Sun, S. H. Controlled PEGylation of monodisperse Fe3O4 nanoparticles for reduced non-specific uptake by macrophage cells. Adv. Mater. 2007, 19, 3163−3166. (30) Pan, Y. C.; Li, T.; Lestari, G.; Lai, Z. P. Effective separation of propylene/propane binary mixtures by ZIF-8 membranes. J. Membr. Sci. 2012, 390, 93−98. (31) Liu, J. M.; Huang, Q. T.; Cai, P. Y.; Lin, C. Q.; Zhang, L. H.; Zheng, Z. Y. Design of a highly sensitive fluorescent sensor and its application based on inhibiting NaIO4 oxidizing rhodamine 6G. Anal. Methods 2014, 6, 5957−5961. (32) Chen, G. N.; Huang, C. S. A study of the chemiluminescence of some acidic triphenylmethane dyes. Talanta 1988, 35, 625−631.

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