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Synthesis, characterization, and luminescence properties of BiVO4:Eu3+ embedded Fe3O4@mSiO2 nanoparticles
Journal of Luminescence 215 (2019) 116677
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis, characterization, and luminescence properties of BiVO4:Eu3+ embedded Fe3O4@mSiO2 nanoparticles
T
Quan Jin, Lu Liu, Yuting Zheng, Shige Wang, Mingxian Huang* Department of Chemistry, College of Science, University of Shanghai for Science and Technology, Shanghai 200093, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetic nanospheres Mesoporous silica Pechini sol-gel process Fluorescent nanoparticles
Magnetic Fe3O4 nanospheres with high dispersion, high saturation magnetization and narrow particle size distribution were synthesized by a CaF2 mediated solvothermal method. Next, a nonporous silica layer and a mesoporous silica (mSiO2) layer were uniformly coated on the surface of the magnetic nanospheres sequentially to form Fe3O4@mSiO2 nanoparticles. It was shown that the pore size and thickness of the mSiO2 shell could be fine-tuned by adjusting the composition of the double-template micelle system consisting of a partially fluorinated short-chain anionic fluorocarbon surfactant and cetyltrimethylammonium bromide. Finally, Eu3+ in the BiVO4 host was embedded on the surface of Fe3O4@mSiO2 nanoparticles with the Pechini sol-gel process, thus to form magnetic fluorescent nanoparticles. It was found that the fluorescence of the inorganic nanoparticles can be further enhanced by the complexation of 2-thenoyltrifluoroaceate (TTA) to the Eu3+ on the surface of the nanoparticles. The as-prepared nanoparticles were well characterized and the preparation process was optimized in order to establish a robust preparation method for the multiple functionalized luminescent nanoparticles.
1. Introduction In recent years, magnetic nanospheres have attracted wide attention and research activity due to their unique physical and chemical properties, including low cost, easy preparation and excellent performance [1–4]. In addition, the saturation magnetization, morphology, and surface properties of magnetic particles can be controlled and modified to have broad application prospects [5–8]. There are many methods to synthesize magnetic nanospheres, such as solvothermal method [8–10], microemulsion method [6,11], thermal decomposition method [12], chemical co-precipitation method [13,14], etc. In most cases, magnetic nanospheres have to be coated and modified [15,16] to guarantee their colloidal stability and suitable applications. Silica has become the most commonly used coating material because it provides controllable coating thickness, easy surface modification, and low toxicity [17]. Furthermore, the silica coating can be made into a porous layer with tunable pore size, and there have been many reports for preparing mesoporous silica layer, such as surface etching method [18] and surfactant templating methods [19,20]. Luminescent nanoparticles play an important role in bioimaging [21–23], biosensor [24,25], and biomedicine [26], etc. Compared with quantum dots and organic dye containing nanomaterials, rare earth element ions loaded nanoparticles have lower toxicity, better optical
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stability, and narrower emission wavelength range [27–29]. Among them, Eu3+ doped phosphors has been extensively studied [30–32]. It has been reported that Eu3+, Bi3+, VO43−, citric acid, polyethylene glycol, etc. could be chelated together by a Pechini sol-gel method to form a gel [33–35], which was subjected to high temperature treatment to form a strong luminescent material. The addition of Bi3+ can make the spectral band more sharp [36–38]. In addition, Eu3+ has frequently been complexed with organic ligands to enhance its luminescent properties [39]. In this study, we aimed to develop a robust method to prepare a core-shell composite nanosphere featuring strong magnetic response and good fluorescent characteristics by utilizing a magnetic core of around 200 nm and large pore mesoporous silica shell embedded with BiVO4:Eu3+ phosphors. Each step in the synthesis process was optimized to lay a solid foundation for preparing various multiple functionalized nanomaterials. 2. Experimental 2.1. Materials Iron chloride hexahydrate (FeCl3·6H2O), sodium acetate (anhydrous), poly (4-styrenesulfonic acid-co-maleic acid) sodium salt
Corresponding author. E-mail address: [email protected] (M. Huang).
https://doi.org/10.1016/j.jlumin.2019.116677 Received 6 March 2019; Received in revised form 24 July 2019; Accepted 2 August 2019 Available online 02 August 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Synthesis procedures of Fe3O4@mSiO2/BiVO4:Eu3+ nanoparticle.
Fig. 1. SEM images of Fe3O4 prepared without (a) and with 0.5 g of CaF2 added (c); the corresponding particle size distribution statistics (b) and (d) respectively; SEM image (e) of Fe3O4@mSiO2 obtained with 0.11 g of FS-66 added with CTAB; TEM image (f) of the corresponding Fe3O4@mSiO2.
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Fig. 2. BET and BJH measurement results for Fe3O4@mSiO2 with 0.011 g of FS66 added to the template system.
Fig. 3. Powder X-ray diffraction (XRD) pattern of Fe3O4 nanospheres containing CaF2.
(PSSMA), calcium fluoride (CaF2), cetyltrimethylammonium bromide (CTAB), triethanolamine (TEA), tetraethoxysilane (TEOS), 2-propanol, ethanol, citric acid, concentrated nitric acid, sodium fluoride (NaF), europium (III) nitrate hexahydrate (Eu(NO3)3·6H2O), bismuth(III) nitrate pentahydrate (Bi(NO3)3·5H2O), ammonium metavanadate (NH4VO3), ethylene glycol (EG), poly (ethylene glycol) (PEG) (Mn = 10000) were obtained from Alading Reagents (China). Capstone FS-66 (FS-66) and 2-thenoyltrifluoroaceate (TTA) were purchased from Sigma-Aldrich (US).
dispersion. The combined solution was ultrasonically treated for 3 min, mechanically stirred for 15 min at 500 r/min, and then stirred for 5 h at 250 r/min. After the reaction, the obtained Fe3O4@mSiO2 nanoparticles were washed with ethanol, then shaking these nanoparticles with saturated ammonium nitrate ethanol solution for 12 h. After that, the solution was washed twice with ethanol, twice with DI water. 2.5. Synthesis of BiVO4: Eu3+ phosphor precursor Briefly, 0.714 g of Eu(NO3)3·6H2O, 0.194 g of Bi(NO3)3·5H2O, 0.117 g of NH4VO3, 0.7685 g of citric acid, and 1 mL of concentrated nitric acid were added to a mixed solution of 35 mL of EG and 15 mL of DI water, (EG: water = 7:3). The mixture were heated in a water bath at 80 °C in order to facilitate the dissolution of the regents, after all the regents in the solution were dissolved, 4 g of PEG (Mn = 10000) was added to the solution. The pH of the solution was adjusted to near neutral, and the solution was heated for 3 h at 80 °C.
2.2. Synthesis of Fe3O4 nanospheres The magnetic nanospheres were synthesized according to Gao's work [8] with modification. Briefly, 1.62 g of FeCl3·6H20, 1.5 g of PSSMA, 4.5 g of anhydrous sodium acetate, and 0.5 g of calcium fluoride were dispersed in 60 mL of EG, the solution was mixed and stirred at a 60 °C water bath with a magnetic stirrer for 2 h. Next, the reddish brown uniform dispersion was transferred to a Teflon-lined autoclave, and the mixture was reacted at 220 °C for 15 h. After cooling to room temperature, the blackish precipitate was washed once with absolute ethanol and three times with deionized (DI) water. The asprepared Fe3O4 nanospheres were dispersed in anhydrous ethanol.
2.6. Loading and fixing of BiVO4: Eu3+ phosphor precursor 5 mL of ethanol dispersion of Fe3O4@mSiO2 nanoparticles (0.5 g) were washed twice with DI water, then mixed with 5 mL of the BiVO4:Eu3+ phosphor precursor. The mixture was sonicated for 5 min, then oscillated for 24 h at 700 r/min. Subsequently, the mixture was magnetically separated and the solid was aged in a drying oven at 120 °C for 5 h. After that, the sample was placed in a furnace and calcined at 600–900 °C for 3 h, and the heating rate was 1 °C/min. This loading and fixing process was repeated.
2.3. Synthesis of Fe3O4@SiO2 nanospheres Briefly, 3 mL of Fe3O4 microsphere ethanol dispersion (~0.1 g Fe3O4 microspheres) was added with 10 mL of DI water, 0.5 mL of NaF solution (wt. 1%), and 0.3 mL of TEOS, and the solution was mixed with a shaker at 500 r/min. The oscillation was continued for 15 h, followed by magnetic separation. Finally, Fe3O4@SiO2 nanospheres were generated after washing the mixture with absolute ethanol and DI water.
2.7. Complexation of Fe3O4@mSiO2/BiVO4:Eu3+ with TTA 0.1 g of TTA was dissolved in 10 mL of ethanol, and 0.05 g of the fluorescent nanoparticles were added to the above solution. Then, the solution was sonicated for 5 min, shaken for 3 h. After the reaction, the nanoparticles were washed twice with ethanol and twice with DI water.
2.4. Synthesis of Fe3O4@mSiO2 nanoparticles The coating of mesoporous silica shell on Fe3O4@SiO2 microspheres was performed according to Huang's work [20]. Briefly, a total of 5 mL of Fe3O4@SiO2 nanosphere ethanol dispersion was washed once with DI water and dispersed in 5 mL of DI water. Separately, 0.64 g of CTAB and 0.12 mL of TEA were added to 33.3 mL of DI water in a flask, the solution was stirred in a water bath at 60 °C for 15 min, then 0.11 g of FS-66 dissolved in isopropanol was added to the solution, stirring was continued for 1 h. Then 5 mL of TEOS was added to this surfactant solution, and the solution was stirred for 1–2 min. Next, 10 mL of the surfactant solution was added to above Fe3O4@SiO2 microsphere
2.8. Material characterization Transmission electron microscopy (TEM) images were taken using a JEOL JEM 2011 microscope (Japan). For TEM measurements, the samples were dispersed in ethanol and then dried on a holey carbon film Cu grid. Scanning electron microscope (SEM) images were obtained on a Philips XL30 microscope (Holland). Fluorescence measurements for the fluorescent NP were conducted using a Hitachi F7000 3
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Fig. 4. TEM images of Fe3O4@mSiO2 nanoparticles obtained without (a) and with 0.025 g (b), 0.05 g (c), 0.075 g (d) of FS-66 added with CTAB.
3. Results and discussion The synthetic process of the fluorescent nanoparticle was shown in Scheme 1. As the core of the composite functional nanoparticle, magnetic nanospheres with a particle size of about 180 nm were synthesized first by a solvothermal method. The particle size of around 200 nm provides the balance benefits of strong magnetic response and stable suspension, as well as the suitability for the biological system [5–7]. In the preparation process, we noticed that the addition of CaF2 into the synthesis mixture made the magnetic particles size more uniform and their magnetic response more stable, as shown in Fig. 1 a-d. In the absence of CaF2, the average particle size is 144.29 ± 32.35 nm. In contrast, the average size increased to 187.26 ± 22.63 nm with narrow distribution by adding CaF2. The possible interpretation of this result is that there is an enhanced aggregation of PSSMA-coated iron oxide (hematite) nanoparticles in the presence of calcium ions [40], nanoparticles become more compact and uniform during the growth process due to the formation of monodispersed pseudocubic hematite nanocrystals [41] and some calcium iron oxide nanocrystals (CaFe2O4) [42]. It is known that the solubility of CaF2 is low, and trace amount of calcium ions are produced and consumed slowly in above nanoparticle formation process, thus lead to the final magnetic nanospheres with improved distribution uniformity and magnetic responsive performance. To obtain the mesoporous silica coating layer on the surface of the magnetic nanosphere, we found that it was necessary to coat a thin layer of silica on the surface of the magnetic nanosphere in advance. Otherwise, non-uniform and sticky coatings were observed. The deposition of the first layer of SiO2 on the Fe3O4 surface was realized by adding NaF to the coating solution following the reported method [43,44]. The first silica coating layer is nonporous, which makes better
Fig. 5. The M − H hysteresis loop results for the magnetic particles obtained under normal temperature conditions, Fe3O4 (solid line) and Fe3O4@mSiO2 (dashed line).
fluorescence spectrophotometer (Japan). The hysteresis loop data were obtained by Quantum Design PPMS-9 (US). The X-ray diffraction (XRD) pattern results of the magnetic particles were obtained by Bruker/D8 ADVANCE (DE). The Berunauer- Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) measurements for Fe3O4@mSiO2 nanoparticles were conducted using a micromeritics TriStar II 3020 (US).
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Fig. 6. The excitation (a) and fluorescence emission (b) spectra of Fe3O4@mSiO2 embedded with BiVO4:Eu3+ at different temperatures. The excitation (c) and emission (d) spectra of Fe3O4@SiO2 and Fe3O4@mSiO2 embedded with BiVO4:Eu3+ phosphors with one (denoted as −1) or two (denoted as −2) Pechini sol-gel processes at 700 °C and with TTA complexation (denoted as -TTA).
(Fig. 4 a); when the amount of FS-66 was 0.025 g, the pore size increased under the action of double template, and larger pore and thicker shell were obtained (Fig. 4 b). When 0.05 g (Figs. 4 c) and 0.075 g (Fig. 4 d) of FS-66 were added, respectively, the pore sizes become larger and larger. Thus, the pore size of the mSiO2 coating layer can be fine-tuned by controlling the amount of FS-66 added into the templating system. Pore size became larger as the amount of FS-66 increased, the reason is that FS-66 enlarged the CTAB micelle templating system because of the bulky and rigid fluorocarbon chains, which further lowered the surface tension of aqueous solutions. This phenomenon is consistent with our previous work [20]. The M − H hysteresis loop for the magnetic particles was measured by a magnetometer at 300 K, as shown in Fig. 5. It can be seen that the saturation magnetizations of Fe3O4 and Fe3O4@mSiO2 are 60.3 emu/g and 42.6 emu/g, respectively. After the MSN shell formation, the saturation magnetization of the magnetic microspheres is reduced. Both hysteresis loops are in S shape, which indicates that the magnetic particles are superparamagnetic. In order to obtain fluorescent magnetic nanoparticles, we chose a preparation method based on the embedding of BiVO4:Eu3+ on the surface of Fe3O4@mSiO2. BiVO4:Eu3+ is a proven substance to produce stable and strong fluorescence [34–38], and the porous mSiO2 layer is believed to hold more fluorescent materials due to its porosity and high pore volume. Fig. 6 shows the excitation spectrum and the emission spectrum of the as-prepared nanoparticles. The typical emission peaks of Eu3+ are located mainly in the red region of spectrum (612 and 620 nm), and are attributable to the 5D0-7F2 transition, when excited at 278 nm. The result is in agreement with the characteristic absorption of the VO43− group at 278 nm. It is noticed that there are three weak emission peaks located at 593, 654 and 706 nm, which are attributable
protection for the magnetic particle and facilitates the formation of the mesoporous silica (mSiO2) layer in the next step. From the SEM image (e) and TEM image (f) in Fig. 1, we can see that a porous silica layer was uniformly coated around the magnetic particles, and the pores were found to be dendritic from the core. Thus, Fe3O4@mSiO2 nanoparticles were successfully prepared. To further characterize the as-prepared Fe3O4@mSiO2 nanoparticles, the Berunauer-Emmett-Teller (BET) and Barrett-JoynerHalenda (BJH) measurements corresponding to Fe3O4@mSiO2 nanoparticles with the addition of 0.011 g of FS-66 to the template system were conducted, and Fig. 2 shows the results. The measured BET surface area, pore volume and pore size were counted as 231.84 m2/g, 0.38 cm3/g, and 3–4 nm, respectively. The large surface area can provide sufficient functional groups for labeling with biomolecular after surface modification [33,45], and the large pore volume can potentially contain more guest substance. In addition, the X-ray diffraction (XRD) pattern results of the magnetic particles of Fe3O4 prepared with CaF2 in the solvothermal reaction mixture are shown in Fig. 3. It can be seen that the diffraction peaks in the figure matched well with standard Fe3O4 (JCPDS No. 19–0629) and CaF2 (JCPDS No. 99-0051). The results showed that the main components of the synthesized magnetic particles were Fe3O4 (red line) and CaF2 crystals (purple line). This result indicates that the Fe3O4 magnetic nanospheres prepared in this work indeed contain CaF2, which is helpful for improving the stability and uniformity of the magnetic particles. As described in the experimental section, different amounts of FS-66 were added to the template solution containing a certain amount of CTAB. Under weak alkaline conditions (pH ≈ 7.4), mSiO2 coating layer was formed with fine worm like pores without the addition of FS-66 5
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to the 5D0-7F1, 5D0-7F3, and 5D0-7F4 transitions of Eu3+, respectively. No emission from the VO43− group is observed, indicating that the excitation of Eu3+ is mainly through energy transfer from VO43− to Eu3+ [25,33,35]. From Fig. 6 a and b, we can see that the calcination temperature of 700 °C produced the strongest fluorescence emission. At this high calcination temperature, the Fe3O4 core can be oxidized to some extent, leading to lower magnetic response. We found that the problem can be solved by a new solvothermal process at 220 °C in ethylene glycol, which can restore the magnetic response of the nanoparticles without affecting the fluorescence. From Fig. 6 c and d, we also noticed that Fe3O4@mSiO2 indeed produced stronger fluorescence than Fe3O4@ SiO2 nanospheres, indicating that the mSiO2 shell layer is helpful for the loading of more fluorescent materials. In addition, when the embedding process was repeated, the fluorescence intensity was significantly enhanced. Furthermore, since some of the Eu3+ ions were exposed on the particle surface, the fluorescence emission increased further when TTA was added for additional organic ligand complexation. The UV absorption of both the inorganic host and the organic ligand transferred the energy to Eu3+ ions, resulting in the fluorescence enhancement for the magnetic fluorescent nanoparticles.
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4. Conclusions In summary, this work has shown that the addition of CaF2 improved the magnetic response and the uniformity of the magnetic nanospheres prepared in the solvothermal process, the nonporous silica coating layer allowed the uniform coating of the mesoporous silica layer by a double-template micelle system consisting of a partially fluorinated short-chain anionic fluorocarbon surfactant and cetyltrimethylammonium bromide, and the mSiO2 layer promoted the embedding of rare earth element ion based fluorescent materials on the surface of the magnetic particles. In this work, the optimized conditions for preparing Fe3O4@mSiO2/BiVO4:Eu3+ fluorescent nanoparticles were via a repeated Pechini sol-gel process at the calcination temperature of 700 °C, with the additional complexation with organic ligand TTA. We consider this work as a stepstone to move forward to preparing more multiple functionalized nanoparticles for biological detection and biomedical applications in the future. Acknowledgment The authors thank the Science and Technology Commission of Shanghai Municipality for financial support under grand number 14440502300 and The foundation of Hujiang under the grand number D15011. References [1] X. Zhou, F. Cao, J. Li, F. Chen, J. Liu, W. Shen, Three-dimensional cobalt microspheres composed of nanosheets assembled via facile hydrothermal method, Mater. Lett. 182 (2016) 269–272. [2] L. Zhen, T. Bien, A. Mathieu, A.I. Cooper, M.J. Rosseinsky, Direct coprecipitation route to monodisperse dual-functionalized magnetic iron oxide nanocrystals without size selection, Small 4 (2010) 231–239. [3] W. Wu, Z. Wu, T. Yu, C. Jiang, W.S. Kim, Recent progress on magnetic iron oxide nanoparticles: synthesis, surface functional strategies and biomedical applications, Sci. Technol. Adv. Mater. 16 (2015) 023501. [4] M. Han, Q. Liu, J. He, Y. Song, Z. Xu, J.M. Zhu, Controllable synthesis and magnetic properties of cubic and hexagonal phase nickel nanocrystals, Adv. Mater. 19 (2007) 1096–1100. [5] N.R. Jana, Y. Chen, X. Peng, Size-and shape-controlled magnetic(Cr,Mn,Fe,Co,Ni) oxide nanocrystals via a simple and general approach, Chem. Mater. 16 (2004) 11–22. [6] X. Hong, C. Longlan, T. Naihu, G. Hongchen, Development of high magnetization Fe3O4/polystyrene/silica nanospheres via combined miniemulsion/emulsion polymerization, J. Am. Chem. Soc. 128 (2006) 15582–15583. [7] J. Ge, Y. Hu, M. Biasini, W.P. Beyermann, Y. Yin, Superparamagnetic magnetite colloidal nanocrystal clusters, Angew. Chem., Int. Ed. Engl. 46 (2007) 4342–4345. [8] J. Gao, X. Ran, C. Shi, H. Cheng, T. Cheng, Y. Su, One-step solvothermal synthesis of highly water-soluble, negatively charged superparamagnetic Fe3O4 colloidal
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Cryst. Growth Des. 12 (2012) 18–28. [42] H. Abd El-Wahab, A.M. Hassan, A.M. Naser, et al., Preparation and evaluation of nanosized mixed calcium iron oxide (CaFe2O4) as high heat resistant pigment in paints, Pigment Resin Technol. 44/3 (2015) 172–178. [43] L.U. Khan, D. Muraca, H.F. Brito, O. Moscoso-Londoño, M.C.F.C. Felinto, K.R. Pirota, E.E.S. Teotonio, et al., Optical and magnetic nanocomposites containing Fe3O4@SiO2 grafted with Eu3+ and Tb3+ complexes, J. Alloy. Comp. 686 (2016) 453–466. [44] W. Stöber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69. [45] B. Chang, J. Guo, C. Liu, J. Qian, W. Yang, Surface functionalization of magnetic mesoporous silica nanoparticles for controlled drug release, J. Mater. Chem. 20 (2010) 9941–9947.
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis, characterization, and luminescence properties of BiVO4:Eu3+ embedded Fe3O4@mSiO2 nanoparticles
T
Quan Jin, Lu Liu, Yuting Zheng, Shige Wang, Mingxian Huang* Department of Chemistry, College of Science, University of Shanghai for Science and Technology, Shanghai 200093, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetic nanospheres Mesoporous silica Pechini sol-gel process Fluorescent nanoparticles
Magnetic Fe3O4 nanospheres with high dispersion, high saturation magnetization and narrow particle size distribution were synthesized by a CaF2 mediated solvothermal method. Next, a nonporous silica layer and a mesoporous silica (mSiO2) layer were uniformly coated on the surface of the magnetic nanospheres sequentially to form Fe3O4@mSiO2 nanoparticles. It was shown that the pore size and thickness of the mSiO2 shell could be fine-tuned by adjusting the composition of the double-template micelle system consisting of a partially fluorinated short-chain anionic fluorocarbon surfactant and cetyltrimethylammonium bromide. Finally, Eu3+ in the BiVO4 host was embedded on the surface of Fe3O4@mSiO2 nanoparticles with the Pechini sol-gel process, thus to form magnetic fluorescent nanoparticles. It was found that the fluorescence of the inorganic nanoparticles can be further enhanced by the complexation of 2-thenoyltrifluoroaceate (TTA) to the Eu3+ on the surface of the nanoparticles. The as-prepared nanoparticles were well characterized and the preparation process was optimized in order to establish a robust preparation method for the multiple functionalized luminescent nanoparticles.
1. Introduction In recent years, magnetic nanospheres have attracted wide attention and research activity due to their unique physical and chemical properties, including low cost, easy preparation and excellent performance [1–4]. In addition, the saturation magnetization, morphology, and surface properties of magnetic particles can be controlled and modified to have broad application prospects [5–8]. There are many methods to synthesize magnetic nanospheres, such as solvothermal method [8–10], microemulsion method [6,11], thermal decomposition method [12], chemical co-precipitation method [13,14], etc. In most cases, magnetic nanospheres have to be coated and modified [15,16] to guarantee their colloidal stability and suitable applications. Silica has become the most commonly used coating material because it provides controllable coating thickness, easy surface modification, and low toxicity [17]. Furthermore, the silica coating can be made into a porous layer with tunable pore size, and there have been many reports for preparing mesoporous silica layer, such as surface etching method [18] and surfactant templating methods [19,20]. Luminescent nanoparticles play an important role in bioimaging [21–23], biosensor [24,25], and biomedicine [26], etc. Compared with quantum dots and organic dye containing nanomaterials, rare earth element ions loaded nanoparticles have lower toxicity, better optical
*
stability, and narrower emission wavelength range [27–29]. Among them, Eu3+ doped phosphors has been extensively studied [30–32]. It has been reported that Eu3+, Bi3+, VO43−, citric acid, polyethylene glycol, etc. could be chelated together by a Pechini sol-gel method to form a gel [33–35], which was subjected to high temperature treatment to form a strong luminescent material. The addition of Bi3+ can make the spectral band more sharp [36–38]. In addition, Eu3+ has frequently been complexed with organic ligands to enhance its luminescent properties [39]. In this study, we aimed to develop a robust method to prepare a core-shell composite nanosphere featuring strong magnetic response and good fluorescent characteristics by utilizing a magnetic core of around 200 nm and large pore mesoporous silica shell embedded with BiVO4:Eu3+ phosphors. Each step in the synthesis process was optimized to lay a solid foundation for preparing various multiple functionalized nanomaterials. 2. Experimental 2.1. Materials Iron chloride hexahydrate (FeCl3·6H2O), sodium acetate (anhydrous), poly (4-styrenesulfonic acid-co-maleic acid) sodium salt
Corresponding author. E-mail address: [email protected] (M. Huang).
https://doi.org/10.1016/j.jlumin.2019.116677 Received 6 March 2019; Received in revised form 24 July 2019; Accepted 2 August 2019 Available online 02 August 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Synthesis procedures of Fe3O4@mSiO2/BiVO4:Eu3+ nanoparticle.
Fig. 1. SEM images of Fe3O4 prepared without (a) and with 0.5 g of CaF2 added (c); the corresponding particle size distribution statistics (b) and (d) respectively; SEM image (e) of Fe3O4@mSiO2 obtained with 0.11 g of FS-66 added with CTAB; TEM image (f) of the corresponding Fe3O4@mSiO2.
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Fig. 2. BET and BJH measurement results for Fe3O4@mSiO2 with 0.011 g of FS66 added to the template system.
Fig. 3. Powder X-ray diffraction (XRD) pattern of Fe3O4 nanospheres containing CaF2.
(PSSMA), calcium fluoride (CaF2), cetyltrimethylammonium bromide (CTAB), triethanolamine (TEA), tetraethoxysilane (TEOS), 2-propanol, ethanol, citric acid, concentrated nitric acid, sodium fluoride (NaF), europium (III) nitrate hexahydrate (Eu(NO3)3·6H2O), bismuth(III) nitrate pentahydrate (Bi(NO3)3·5H2O), ammonium metavanadate (NH4VO3), ethylene glycol (EG), poly (ethylene glycol) (PEG) (Mn = 10000) were obtained from Alading Reagents (China). Capstone FS-66 (FS-66) and 2-thenoyltrifluoroaceate (TTA) were purchased from Sigma-Aldrich (US).
dispersion. The combined solution was ultrasonically treated for 3 min, mechanically stirred for 15 min at 500 r/min, and then stirred for 5 h at 250 r/min. After the reaction, the obtained Fe3O4@mSiO2 nanoparticles were washed with ethanol, then shaking these nanoparticles with saturated ammonium nitrate ethanol solution for 12 h. After that, the solution was washed twice with ethanol, twice with DI water. 2.5. Synthesis of BiVO4: Eu3+ phosphor precursor Briefly, 0.714 g of Eu(NO3)3·6H2O, 0.194 g of Bi(NO3)3·5H2O, 0.117 g of NH4VO3, 0.7685 g of citric acid, and 1 mL of concentrated nitric acid were added to a mixed solution of 35 mL of EG and 15 mL of DI water, (EG: water = 7:3). The mixture were heated in a water bath at 80 °C in order to facilitate the dissolution of the regents, after all the regents in the solution were dissolved, 4 g of PEG (Mn = 10000) was added to the solution. The pH of the solution was adjusted to near neutral, and the solution was heated for 3 h at 80 °C.
2.2. Synthesis of Fe3O4 nanospheres The magnetic nanospheres were synthesized according to Gao's work [8] with modification. Briefly, 1.62 g of FeCl3·6H20, 1.5 g of PSSMA, 4.5 g of anhydrous sodium acetate, and 0.5 g of calcium fluoride were dispersed in 60 mL of EG, the solution was mixed and stirred at a 60 °C water bath with a magnetic stirrer for 2 h. Next, the reddish brown uniform dispersion was transferred to a Teflon-lined autoclave, and the mixture was reacted at 220 °C for 15 h. After cooling to room temperature, the blackish precipitate was washed once with absolute ethanol and three times with deionized (DI) water. The asprepared Fe3O4 nanospheres were dispersed in anhydrous ethanol.
2.6. Loading and fixing of BiVO4: Eu3+ phosphor precursor 5 mL of ethanol dispersion of Fe3O4@mSiO2 nanoparticles (0.5 g) were washed twice with DI water, then mixed with 5 mL of the BiVO4:Eu3+ phosphor precursor. The mixture was sonicated for 5 min, then oscillated for 24 h at 700 r/min. Subsequently, the mixture was magnetically separated and the solid was aged in a drying oven at 120 °C for 5 h. After that, the sample was placed in a furnace and calcined at 600–900 °C for 3 h, and the heating rate was 1 °C/min. This loading and fixing process was repeated.
2.3. Synthesis of Fe3O4@SiO2 nanospheres Briefly, 3 mL of Fe3O4 microsphere ethanol dispersion (~0.1 g Fe3O4 microspheres) was added with 10 mL of DI water, 0.5 mL of NaF solution (wt. 1%), and 0.3 mL of TEOS, and the solution was mixed with a shaker at 500 r/min. The oscillation was continued for 15 h, followed by magnetic separation. Finally, Fe3O4@SiO2 nanospheres were generated after washing the mixture with absolute ethanol and DI water.
2.7. Complexation of Fe3O4@mSiO2/BiVO4:Eu3+ with TTA 0.1 g of TTA was dissolved in 10 mL of ethanol, and 0.05 g of the fluorescent nanoparticles were added to the above solution. Then, the solution was sonicated for 5 min, shaken for 3 h. After the reaction, the nanoparticles were washed twice with ethanol and twice with DI water.
2.4. Synthesis of Fe3O4@mSiO2 nanoparticles The coating of mesoporous silica shell on Fe3O4@SiO2 microspheres was performed according to Huang's work [20]. Briefly, a total of 5 mL of Fe3O4@SiO2 nanosphere ethanol dispersion was washed once with DI water and dispersed in 5 mL of DI water. Separately, 0.64 g of CTAB and 0.12 mL of TEA were added to 33.3 mL of DI water in a flask, the solution was stirred in a water bath at 60 °C for 15 min, then 0.11 g of FS-66 dissolved in isopropanol was added to the solution, stirring was continued for 1 h. Then 5 mL of TEOS was added to this surfactant solution, and the solution was stirred for 1–2 min. Next, 10 mL of the surfactant solution was added to above Fe3O4@SiO2 microsphere
2.8. Material characterization Transmission electron microscopy (TEM) images were taken using a JEOL JEM 2011 microscope (Japan). For TEM measurements, the samples were dispersed in ethanol and then dried on a holey carbon film Cu grid. Scanning electron microscope (SEM) images were obtained on a Philips XL30 microscope (Holland). Fluorescence measurements for the fluorescent NP were conducted using a Hitachi F7000 3
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Fig. 4. TEM images of Fe3O4@mSiO2 nanoparticles obtained without (a) and with 0.025 g (b), 0.05 g (c), 0.075 g (d) of FS-66 added with CTAB.
3. Results and discussion The synthetic process of the fluorescent nanoparticle was shown in Scheme 1. As the core of the composite functional nanoparticle, magnetic nanospheres with a particle size of about 180 nm were synthesized first by a solvothermal method. The particle size of around 200 nm provides the balance benefits of strong magnetic response and stable suspension, as well as the suitability for the biological system [5–7]. In the preparation process, we noticed that the addition of CaF2 into the synthesis mixture made the magnetic particles size more uniform and their magnetic response more stable, as shown in Fig. 1 a-d. In the absence of CaF2, the average particle size is 144.29 ± 32.35 nm. In contrast, the average size increased to 187.26 ± 22.63 nm with narrow distribution by adding CaF2. The possible interpretation of this result is that there is an enhanced aggregation of PSSMA-coated iron oxide (hematite) nanoparticles in the presence of calcium ions [40], nanoparticles become more compact and uniform during the growth process due to the formation of monodispersed pseudocubic hematite nanocrystals [41] and some calcium iron oxide nanocrystals (CaFe2O4) [42]. It is known that the solubility of CaF2 is low, and trace amount of calcium ions are produced and consumed slowly in above nanoparticle formation process, thus lead to the final magnetic nanospheres with improved distribution uniformity and magnetic responsive performance. To obtain the mesoporous silica coating layer on the surface of the magnetic nanosphere, we found that it was necessary to coat a thin layer of silica on the surface of the magnetic nanosphere in advance. Otherwise, non-uniform and sticky coatings were observed. The deposition of the first layer of SiO2 on the Fe3O4 surface was realized by adding NaF to the coating solution following the reported method [43,44]. The first silica coating layer is nonporous, which makes better
Fig. 5. The M − H hysteresis loop results for the magnetic particles obtained under normal temperature conditions, Fe3O4 (solid line) and Fe3O4@mSiO2 (dashed line).
fluorescence spectrophotometer (Japan). The hysteresis loop data were obtained by Quantum Design PPMS-9 (US). The X-ray diffraction (XRD) pattern results of the magnetic particles were obtained by Bruker/D8 ADVANCE (DE). The Berunauer- Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) measurements for Fe3O4@mSiO2 nanoparticles were conducted using a micromeritics TriStar II 3020 (US).
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Fig. 6. The excitation (a) and fluorescence emission (b) spectra of Fe3O4@mSiO2 embedded with BiVO4:Eu3+ at different temperatures. The excitation (c) and emission (d) spectra of Fe3O4@SiO2 and Fe3O4@mSiO2 embedded with BiVO4:Eu3+ phosphors with one (denoted as −1) or two (denoted as −2) Pechini sol-gel processes at 700 °C and with TTA complexation (denoted as -TTA).
(Fig. 4 a); when the amount of FS-66 was 0.025 g, the pore size increased under the action of double template, and larger pore and thicker shell were obtained (Fig. 4 b). When 0.05 g (Figs. 4 c) and 0.075 g (Fig. 4 d) of FS-66 were added, respectively, the pore sizes become larger and larger. Thus, the pore size of the mSiO2 coating layer can be fine-tuned by controlling the amount of FS-66 added into the templating system. Pore size became larger as the amount of FS-66 increased, the reason is that FS-66 enlarged the CTAB micelle templating system because of the bulky and rigid fluorocarbon chains, which further lowered the surface tension of aqueous solutions. This phenomenon is consistent with our previous work [20]. The M − H hysteresis loop for the magnetic particles was measured by a magnetometer at 300 K, as shown in Fig. 5. It can be seen that the saturation magnetizations of Fe3O4 and Fe3O4@mSiO2 are 60.3 emu/g and 42.6 emu/g, respectively. After the MSN shell formation, the saturation magnetization of the magnetic microspheres is reduced. Both hysteresis loops are in S shape, which indicates that the magnetic particles are superparamagnetic. In order to obtain fluorescent magnetic nanoparticles, we chose a preparation method based on the embedding of BiVO4:Eu3+ on the surface of Fe3O4@mSiO2. BiVO4:Eu3+ is a proven substance to produce stable and strong fluorescence [34–38], and the porous mSiO2 layer is believed to hold more fluorescent materials due to its porosity and high pore volume. Fig. 6 shows the excitation spectrum and the emission spectrum of the as-prepared nanoparticles. The typical emission peaks of Eu3+ are located mainly in the red region of spectrum (612 and 620 nm), and are attributable to the 5D0-7F2 transition, when excited at 278 nm. The result is in agreement with the characteristic absorption of the VO43− group at 278 nm. It is noticed that there are three weak emission peaks located at 593, 654 and 706 nm, which are attributable
protection for the magnetic particle and facilitates the formation of the mesoporous silica (mSiO2) layer in the next step. From the SEM image (e) and TEM image (f) in Fig. 1, we can see that a porous silica layer was uniformly coated around the magnetic particles, and the pores were found to be dendritic from the core. Thus, Fe3O4@mSiO2 nanoparticles were successfully prepared. To further characterize the as-prepared Fe3O4@mSiO2 nanoparticles, the Berunauer-Emmett-Teller (BET) and Barrett-JoynerHalenda (BJH) measurements corresponding to Fe3O4@mSiO2 nanoparticles with the addition of 0.011 g of FS-66 to the template system were conducted, and Fig. 2 shows the results. The measured BET surface area, pore volume and pore size were counted as 231.84 m2/g, 0.38 cm3/g, and 3–4 nm, respectively. The large surface area can provide sufficient functional groups for labeling with biomolecular after surface modification [33,45], and the large pore volume can potentially contain more guest substance. In addition, the X-ray diffraction (XRD) pattern results of the magnetic particles of Fe3O4 prepared with CaF2 in the solvothermal reaction mixture are shown in Fig. 3. It can be seen that the diffraction peaks in the figure matched well with standard Fe3O4 (JCPDS No. 19–0629) and CaF2 (JCPDS No. 99-0051). The results showed that the main components of the synthesized magnetic particles were Fe3O4 (red line) and CaF2 crystals (purple line). This result indicates that the Fe3O4 magnetic nanospheres prepared in this work indeed contain CaF2, which is helpful for improving the stability and uniformity of the magnetic particles. As described in the experimental section, different amounts of FS-66 were added to the template solution containing a certain amount of CTAB. Under weak alkaline conditions (pH ≈ 7.4), mSiO2 coating layer was formed with fine worm like pores without the addition of FS-66 5
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to the 5D0-7F1, 5D0-7F3, and 5D0-7F4 transitions of Eu3+, respectively. No emission from the VO43− group is observed, indicating that the excitation of Eu3+ is mainly through energy transfer from VO43− to Eu3+ [25,33,35]. From Fig. 6 a and b, we can see that the calcination temperature of 700 °C produced the strongest fluorescence emission. At this high calcination temperature, the Fe3O4 core can be oxidized to some extent, leading to lower magnetic response. We found that the problem can be solved by a new solvothermal process at 220 °C in ethylene glycol, which can restore the magnetic response of the nanoparticles without affecting the fluorescence. From Fig. 6 c and d, we also noticed that Fe3O4@mSiO2 indeed produced stronger fluorescence than Fe3O4@ SiO2 nanospheres, indicating that the mSiO2 shell layer is helpful for the loading of more fluorescent materials. In addition, when the embedding process was repeated, the fluorescence intensity was significantly enhanced. Furthermore, since some of the Eu3+ ions were exposed on the particle surface, the fluorescence emission increased further when TTA was added for additional organic ligand complexation. The UV absorption of both the inorganic host and the organic ligand transferred the energy to Eu3+ ions, resulting in the fluorescence enhancement for the magnetic fluorescent nanoparticles.
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4. Conclusions In summary, this work has shown that the addition of CaF2 improved the magnetic response and the uniformity of the magnetic nanospheres prepared in the solvothermal process, the nonporous silica coating layer allowed the uniform coating of the mesoporous silica layer by a double-template micelle system consisting of a partially fluorinated short-chain anionic fluorocarbon surfactant and cetyltrimethylammonium bromide, and the mSiO2 layer promoted the embedding of rare earth element ion based fluorescent materials on the surface of the magnetic particles. In this work, the optimized conditions for preparing Fe3O4@mSiO2/BiVO4:Eu3+ fluorescent nanoparticles were via a repeated Pechini sol-gel process at the calcination temperature of 700 °C, with the additional complexation with organic ligand TTA. We consider this work as a stepstone to move forward to preparing more multiple functionalized nanoparticles for biological detection and biomedical applications in the future. Acknowledgment The authors thank the Science and Technology Commission of Shanghai Municipality for financial support under grand number 14440502300 and The foundation of Hujiang under the grand number D15011. References [1] X. Zhou, F. Cao, J. Li, F. Chen, J. Liu, W. Shen, Three-dimensional cobalt microspheres composed of nanosheets assembled via facile hydrothermal method, Mater. Lett. 182 (2016) 269–272. [2] L. Zhen, T. Bien, A. Mathieu, A.I. Cooper, M.J. Rosseinsky, Direct coprecipitation route to monodisperse dual-functionalized magnetic iron oxide nanocrystals without size selection, Small 4 (2010) 231–239. [3] W. Wu, Z. Wu, T. Yu, C. Jiang, W.S. Kim, Recent progress on magnetic iron oxide nanoparticles: synthesis, surface functional strategies and biomedical applications, Sci. Technol. Adv. Mater. 16 (2015) 023501. [4] M. Han, Q. Liu, J. He, Y. Song, Z. Xu, J.M. Zhu, Controllable synthesis and magnetic properties of cubic and hexagonal phase nickel nanocrystals, Adv. Mater. 19 (2007) 1096–1100. [5] N.R. Jana, Y. Chen, X. Peng, Size-and shape-controlled magnetic(Cr,Mn,Fe,Co,Ni) oxide nanocrystals via a simple and general approach, Chem. Mater. 16 (2004) 11–22. [6] X. Hong, C. Longlan, T. Naihu, G. Hongchen, Development of high magnetization Fe3O4/polystyrene/silica nanospheres via combined miniemulsion/emulsion polymerization, J. Am. Chem. Soc. 128 (2006) 15582–15583. [7] J. Ge, Y. Hu, M. Biasini, W.P. Beyermann, Y. Yin, Superparamagnetic magnetite colloidal nanocrystal clusters, Angew. Chem., Int. Ed. Engl. 46 (2007) 4342–4345. [8] J. Gao, X. Ran, C. Shi, H. Cheng, T. Cheng, Y. Su, One-step solvothermal synthesis of highly water-soluble, negatively charged superparamagnetic Fe3O4 colloidal
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