Fabrication of luminescent and macroporous Y2O3:Eu3+-coated silica monoliths via freeze drying

Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 481–488

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Fabrication of luminescent and macroporous Y2 O3 :Eu3+ -coated silica monoliths via freeze drying Yulei Wei 1 , Lei Qian 1 , Lu Lu, Runhua Fan ∗ Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, 17923 Jingshi Road, Jinan 250061, China

g r a p h i c a l

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 17 April 2013 Received in revised form 5 September 2013 Accepted 1 October 2013 Available online 11 October 2013 Keywords: Y2 O3 :Eu3+ Macroporous Luminescence Freeze drying Wet impregnation

a b s t r a c t In this paper, we report a new method to fabricate luminescent and macroporous Eu3+ doped Y2 O3 (Y2 O3 :Eu3+ )-coated silica monoliths. Macroporous silica monoliths were firstly prepared through freeze drying emulsions containing silica nanoparticles (silica NPs) and calcination. Then Y2 O3 :Eu3+ layers were loaded onto the macroporous silica monoliths by the wet impregnation method. The Y2 O3 :Eu3+ -coated silica monoliths were characterized by field emission scanning electron microscopy, X-ray diffraction and mercury intrusion porosimetry. The silica NPs content, emulsion composition and dip times played important roles on morphologies of the Y2 O3 :Eu3+ -coated silica monoliths. It was found that the silica monoliths still remained good macroporous structures after coating the Y2 O3 :Eu3+ layers. The loading amount of Y2 O3 :Eu3+ was also controlled by the dip times. Finally, luminescence properties of the Y2 O3 :Eu3+ -coated silica monoliths were examined and they gave a strong emission peak of 612 nm attributed to the 5 D0 → 7 F2 transition of Eu3+ . This method was simple, convenient and provided a novel route for fabrication of luminescent and macroporous silica monoliths. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Rare-earth luminescent materials containing europium (Eu) have wide applications in many fields, such as phosphors, light emitting diodes, sensors, biomedicine, etc. [1–9]. Among these materials, porous Eu3+ doped materials have received much attention in recent years. Due to their large surface area, controllable

∗ Corresponding author. Tel.: +86 531 88393396. E-mail address: [email protected] (R. Fan). 1 These authors contributed equally to this work. 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.10.008

pore structures and high loading capacity, these materials are highly suitable as catalysts, absorbents, sensing devices and drug carriers [10–20]. For example, mesoporous Eu3+ -doped silica spheres with high adsorption capacity have been used as a new type of drug carriers. The drug release behavior was monitored by the change of luminescence intensity from Eu3+ . In addition, mesoporous Eu3+ -doped materials have also been used for construction of sensors resulting from their unique structures and properties. According to the response of luminescence intensity, oxygen and moisture have been detected and the mesoporous structures obviously improve the response sensitivity and stability [19,20]. A lot of research on the porous Eu3+ -doped materials has

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been reported, but their development still faces large challenges, such as how to better control pore structures and morphology; how to produce materials with complex pore structures, various functions and shapes. Freeze drying method is often used in food preservation, pharmaceuticals and tissue engineering. Recently, it has been reported that freeze drying method can also be used to produce porous polymeric and inorganic materials with special morphologies [21–28]. The materials from freeze drying usually are macroporous monoliths with high porosity. Aligned porous materials are also produced via controlling the growth of ice crystals [29–31]. Freeze drying method is simple, convenient and suitable for large-scale industrial production. Moreover, the produced samples do not require subsequent processing such as cleaning, filtering, removing templates and so on. We previously used freeze drying to prepare various porous inorganic materials [32,33]. For example, silica, alumina and zirconia materials with adjustable pore volume and structures were prepared by combination of freeze drying and emulsion-templating (freeze drying emulsions) [33]. Compared with the common freeze drying, the material porosity can be significantly improved. Moreover, the emulsion composition including volume ratio of the oil and aqueous phase, surfactant content also influences the pore structures of materials. Currently, much research on porous Eu3+ -doped materials is concentrated on mesoporous materials. Moreover, most of the porous materials have been prepared as powders or thin films, and this limits their certain practical applications. Macroporous Eu3+ -doped monoliths have many advantages including good permeability, low diffusion resistance, easy recycling, operation and modification and so on [34,35]. Unfortunately, less related research has been reported until now. Considering the characters of freeze drying emulsions such as forming monolithic materials, tunable pore structures and high porosity, here luminescent and macroporous Eu3+ doped Y2 O3 (Y2 O3 :Eu3+ )coated silica monoliths were prepared by combining freeze drying emulsions and the wet impregnation method. Our aim is to develop a simple and novel method for the production of luminescent macroporous silica monoliths. Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and mercury intrusion porosimetry were used to characterize the Y2 O3 :Eu3+ -coated silica monoliths. Effects of the content of silica nanoparticles (silica NPs), emulsion composition and the dip times were studied. Finally, research on luminescence properties of the macroporous monoliths was also conducted. 2. Experimental 2.1. Chemical and reagents Polyvinyl alcohol (PVA, 80% hydrolyzed, Mw 9000–10,000) was purchased from Sigma–Aldrich. Silica NPs were from Hangzhou Wanjing New Materials Co., Ltd. Sodium dodecyl sulfate (SDS) was from Shanghai Zhanyun Chemical Co., Ltd. Cyclohexane (CHE) was obtained from Tianjin Fuyun Fine Chemical Co., Ltd. Yttrium oxide (Y2 O3 , 99.99%) and Europium Oxide (Eu2 O3 , 99.99%) were purchased from Shanghai Yuelong Rare Earth New Chemical Company. All chemical and reagents used in this experiment were analytical grade and all aqueous solutions were prepared with distilled water. 2.2. Characterization Morphologies of the porous materials were observed with a SU70 Field emission scanning electron microscopy with an energy dispersive spectrometer (EDS). The samples were adhered to the studs, and then coated with platinum using a sputter-coater. XRD

data were obtained on Rigaku Dmax-2500 X-ray diffractometer with Cu K␣ radiation operated at 50 kV and 80 mA. Macropore size distributions were obtained using a PoreMaster 60 G (Quantachrome) mercury porosimeter. Photoluminescent spectra were recorded with a FLS920 fluorescence spectrometer. 3. Procedures 3.1. Preparation of porous silica monoliths Porous silica composites were firstly prepared from freeze drying oil-in-water (O/W) emulsions containing silica NPs according to our reported method [33]. In order to prepare the emulsions with silica NPs, a 2.5% (w/v) PVA solution was prepared by dissolving 0.25 g PVA in 10 ml distilled water with vigorous stirring. Then 0.5 g SDS and 0.45 g silica NPs were added to the PVA solution. The silica NPs were dispersed with ultrasonication for some time and the mixed solution was obtained as the aqueous phase of emulsions. CHE (2.5 ml, 7 ml, 15 ml) was slowly dropped into the silica NPs suspension (the aqueous phase) with stirring of 600 rpm. After the adding of CHE, the stirring was continued for 2 min to get the stable emulsions. Different volume percentage of the oil phase in the emulsions was from 20%, 40% to 60%. In order to compare with the common freeze drying, the silica NPs suspensions and emulsions were all placed in a container filled with liquid nitrogen to freeze these samples. After these samples were completely frozen, the samples were quickly put in a freeze drier (FD-18, Beijing Detianyou Company) for 48 h to ensure that the samples were thoroughly dried. After that, the porous silica composites were calcined in a furnace at 1000 ◦ C in air for 4 h to obtain porous silica monoliths. 3.1.1. Preparation of Y2 O3 :Eu3+ coated porous silica monoliths The wet impregnation method was used to prepare Y2 O3 :Eu3+ coated porous silica monoliths. 1.016 g Y2 O3 and 0.1759 g Eu2 O3 were dissolved with 7 M HNO3 with vigorous stirring. The porous silica monoliths were dipped into the HNO3 solution containing Y3+ and Eu3+ for 30 min at room temperature. After that, the porous silica monoliths were taken out and washed with a lot of water to remove physically adsorbed rare-earth ions. The resulted porous silica monoliths were dried in an oven at 100 ◦ C for 1 h. Above process was repeated for several times to obtain porous silica monoliths loading with different amount of Y2 O3 :Eu3+ . Then they were calcined in the furnace to produce Y2 O3 :Eu3+ coated porous monoliths. The calcining conditions: heat at 1 ◦ C/min in air to 1000 ◦ C, hold for 4 h and then cool down to room temperature. 4. Results and discussion 4.1. Porous silica monoliths from freeze drying silica NPs suspensions In order to compare with the porous materials from the common freeze drying method, the silica NPs suspensions were also frozen. For the silica NPs suspension, PVA and SDS were used as stabilizers to well disperse silica NPs in the aqueous solution. Moreover, the PVA and SDS molecules were in favor of formation of monoliths with porous structures. Morphologies of the freeze dried PVA-SDSsilica NPs composites were characterized by FESEM. Fig. 1A and C show FESEM images of the freeze dried silica composites. As observed in these images, large amounts of pores were formed in the silica composites. The pore wall was consisted of many clusters of silica NPs, as shown in the corresponding magnification image (insert of Fig. 1A) while 0.45 g silica NPs was used (4.5% silica NPs). However, for 0.36 g silica NPs (3.6% silica NPs), lots of flakes

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Fig. 1. FESEM images of PVA-SDS-silica NPs composites from freezing in liquid nitrogen and then freeze drying in the freeze drier (A and C), and the corresponding porous silica monoliths after calcination (B and D). (A) prepared from 10 ml of 2.5% PVA with 5% SDS and 4.5% silica NPs, (C) prepared from 10 ml of 2.5% PVA with 5% SDS and 3.6% silica NPs. The scale bar of the insert in (A) was 500 nm.

appeared in the silica composites (Fig. 1C). The results indicated that the content of silica NPs could affect the morphologies of silica composites. During the freezing process, silica NPs, PVA and SDS molecules were excluded from the ice crystals due to their low solubility in ice crystals. The excluded silica NPs, PVA and SDS were assembled into the silica composites between ice crystals. After the ice crystals were removed via sublimation in the freeze drier, the silica composites were left and this resulted in the formation of porous silica composites. In order to improve mechanical strength of the materials, the porous silica composites were calcined at 1000 ◦ C to sinter and bind silica NPs together. Fig. 1B and D show the FESEM images of the porous silica monoliths after calcination, and the calcined silica monoliths still exhibited good porous structures. Silica NPs in the pore walls were not observed as they were sintered and bond together. The amount of silica NPs in the freeze dried composites also obviously influenced mechanical strength of the porous silica monoliths. It was found that when the amount of silica NPs was above 0.45 g, the porous silica monoliths exhibited good mechanical strength and were not easily broken. Thus 0.45 g silica NPs was chosen to prepare porous silica monoliths from freeze drying. 4.2. Porous silica monoliths from freeze drying emulsions It has been reported that freeze drying can be incorporated with emulsion-templating to produce materials with high porosity [33]. This method has the advantage of easy tuning pore structures of materials through emulsion composition including volume ratio of oil and aqueous phase, surfactant content, etc. Here porous silica

materials were also prepared by freeze drying emulsions containing silica NPs. Fig. 2 corresponded to FESEM images of the porous silica materials before and after calcination. Some obvious large pores (10–50 ␮m) and aligned pores were observed from Fig. 2A. The large pores were attributed to the emulsion templating [33]. The oil phase (CHE) and aqueous phase of emulsions were frozen during the freezing process in liquid nitrogen, and the “solid” CHE was removed via freeze drying which resulted in appearance of large pores. The aligned pores were resulted from the water templates, and this was consistent with that reported in literatures [36]. The volume ratio of oil and aqueous phase also affected the pore structures. When the volume percentage ratio was increased from 20, 40 to 60%, more and more oil droplets (the oil phase) appeared in the emulsions which mean that the amount of emulsion templates was enhanced. Thus more pores were formed, especially for the volume ratio of 60%, and highly porous structures were formed (Fig. 2C). Porous silica monoliths could be obtained after calcining these porous silica composites. As shown in Fig. 2D, the pore structures still were remained and porous silica monoliths were produced. These results indicated that more macropores could be introduced into the materials by freeze drying emulsions. Moreover, by adjusting the volume ratio between oil and aqueous phase, the porous silica monoliths with different pore amounts and volume could be obtained. 4.3. Y2 O3 :Eu3+ coated porous silica monoliths Surfaces of silica materials contain large amounts of hydroxyl groups, and these hydroxyl groups easily adsorb inorganic ions,

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Fig. 2. FESEM images of the PVA-SDS-silica NPs from freezing the emulsions and the resulted porous silica monoliths. (A)–(C) prepared from the emulsions with different volume percentage of oil phase from 20%, 40% to 60%, respectively. (D) the porous silica monolith from the emulsion with 60% oil phase and calcination at 1000 ◦ C. The aqueous phase was 10 ml of 2.5% PVA with 5% SDS and 4.5% silica NPs.

such as metal and rare-earth ions. We recently reported that Ni nanoparticles could be produced on porous alumina by the wet impregnation and the resulted composites exhibited negative permittivity and negative permeability [37]. By the wet impregnation method, inorganic ions can be loaded onto silica surfaces and inorganic oxide coated silica materials are produced after heat treatment [38]. In our experiment, the obtained porous silica monoliths had high porosity and good porous structures, which resulted in the high adsorption capacity. It has been reported that the porous polymers from freeze drying had high adsorption capacity for organic reagents and mineral oil [36]. Here, the porous silica monoliths with good porous structures were used to load rareearth oxide by the wet impregnation method. The Y2 O3 :Eu3+ coated porous silica monoliths were analyzed by XRD and the results were shown in Fig. 3. From the XRD patterns, it was found that for Y2 O3 :Eu3+ powders obtained from calcining the dip solution, four strong diffraction peaks at about 2Â = 28.9◦ , 33.6◦ , 48.3◦ and 57.2◦ could be observed (curve b), respectively, corresponding to the diffraction of Y2 O3 with different surface (2 2 2), (4 0 0), (4 4 0) and (6 2 2) (JCPDS No 17-0341). The Eu3+ doping had no effect on the diffraction peaks of Y2 O3 , indicating that Eu3+ ions were mixed into the Y2 O3 lattices which did not result in distortion of Y2 O3 lattices. For Y2 O3 :Eu3+ coated porous silica monoliths (curve a), besides the corresponding diffraction peaks of Y2 O3 , four diffraction peaks from silica were also detected corresponding to the different crystal face. No diffraction peak from other substances was found and this showed that pure Y2 O3 :Eu3+ coated porous silica materials were obtained.

The morphologies of porous Y2 O3 :Eu3+ coated silica monoliths were characterized by FESEM. Fig. 4 shows that FESEM images of the porous silica monoliths coated with Y2 O3 :Eu3+ . While loading with Y2 O3 :Eu3+ , the silica monoliths still kept good porous structures.

Fig. 3. XRD patterns from Y2 O3 :Eu3+ -coated silica monoliths prepared from freezing the emulsion with 60% oil phase and dipping once (curve a), and Y2 O3 :Eu3+ powder (curve b) from the dipping solution.

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Fig. 4. (A) and (B) FESEM images of Y2 O3 :Eu3+ -coated silica monoliths prepared from dipping once in the 7 M HNO3 with Y3+ and Eu3+ and the corresponding magnification image. (C) Element analysis of Y2 O3 :Eu3+ -coated silica monoliths by EDS. (D) and (E) FESEM images of the Y2 O3 :Eu3+ -coated silica monoliths prepared from dipping three and five times. The Y2 O3 :Eu3+ -coated silica monoliths were obtained from freezing the emulsion with 60% oil phase.

Some particles or networks on the silica pore walls were observed from the magnification image (Fig. 4B). EDS was used to further define the composition of the materials. And the result showed that they were consisted of Eu, Y, Si and O elements (Fig. 4C). This suggested that the Y2 O3 :Eu3+ layer was loaded onto the silica walls. The weight ratio of Y and Eu element was about 16.73% and 2.79%, respectively. The similar result was obtained from the porous silica monoliths produced by freeze drying silica NPs suspensions (Fig. 5). However, the quantity of Y (5.34%) and Eu (1.38%) were lower than that from freeze drying emulsions. After one dip, the weight of porous silica monolith from emulsions increased about 64%, which was higher than that from silica NPs suspensions (about

28%). This was attributed to higher porosity and loading capacity of the silica monoliths form freeze drying emulsions. Fig. 4D and E corresponded to images of the Y2 O3 :Eu3+ coated porous silica monoliths after three and five dipping times. After three and five dipping times, obvious changes in the microstructure of the samples were also observed. The Y2 O3 :Eu3+ layers gradually formed on the pore walls and large amounts of pore walls were covered with Y2 O3 :Eu3+ layers, resulting in the formation of dendritic structures. The quality of Y and Eu increased to 32.87% and 7.31% after five dipping. The result indicated that by changing the dip times, the loading amount of Y2 O3 :Eu3+ on porous silica monoliths could be adjusted and controlled.

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Fig. 5. FESEM image of Y2 O3 :Eu3+ -coated silica monoliths prepared from 2.5% PVA with 5% SDS and 4.5% silica NPs and dipping once (A), and the corresponding magnification image (B).

The mercury intrusion porosimetry was also used to analyze the pore size distribution of the Y2 O3 :Eu3+ coated porous silica monoliths, and the corresponding result was shown in Fig. 6A. The size of pores in the porous silica monoliths was from 0.14 to 14 ␮m with the average pore size of 1.5 ␮m (black line). Some pores from 20 to 45 ␮m also were observed. After coating of Y2 O3 :Eu3+ , the pore sizes from 0.05 to 16 ␮m with the average size of 1.0 ␮m were observed (green line). The result indicated that the Y2 O3 :Eu3+

coated porous silica materials still kept good porous structures and the pore size became smaller after loading of Y2 O3 :Eu3+ layers. Luminescent properties of the Y2 O3 :Eu3+ coated porous silica monoliths were also studied and characterized. Fig. 6B showed the emission spectra of the Y2 O3 :Eu3+ coated porous silica monoliths excited by 253 nm. A strong emission peak at 612 nm in the luminescence spectroscopy, the characteristic luminescence of Eu3+ was observed. Insert of Fig. 6B was the luminescence photo and the

Fig. 6. (A) Pore size distributions of the porous silica materials before (black line) and after coating with Y2 O3 :Eu3+ (green line). (B) Luminescence spectra of Y2 O3 :Eu3+ -coated silica monolith excited at 253 nm. Insert: the corresponding luminescence photo of the coated silica monolith under 253 nm UV light. (C) Decay curves of Y2 O3 :Eu3+ powder (curve a) and Y2 O3 :Eu3+ -coated silica monoliths (curve b) at room temperature. These Y2 O3 :Eu3+ -coated silica monoliths were obtained from the emulsions with 60% oil phase and dipping once. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Scheme 1. Schematic representation for the preparation of macroporous Y2 O3 :Eu3+ -coated silica monoliths.

sample emitted red light under 253 nm UV light irradiation. The peak of 612 nm corresponded to the 5 D0 → 7 F2 transition of Eu3+ . In addition, other weak peaks from 575 to 600 nm and about 700 nm also appeared, which were attributed to the 5 D0 → 7 F0 , 5 D0 → 7 F3 , 5 D → 7 F , 5 D → 7 F transitions, etc. [39–41]. The similar lumines0 1 0 4 cence spectra were also observed from Y2 O3 :Eu3+ coated porous silica monoliths prepared by three and five dip, and this suggested that the dip times had no effect on their luminescence properties. The fluorescence decay curves of Y2 O3 :Eu3+ powders and Y2 O3 :Eu3+ coated porous silica monoliths at room temperature were also examined and the corresponding results were shown in Fig. 6C. According to the decay curves, the fluorescence lifetime was calculated to be 0.664 and 1.311 ms for Y2 O3 :Eu3+ powders and coated silica monoliths. The lifetime was 1.830 ms for Y2 O3 :Eu3+ coated silica monoliths from freeze drying silica NPs suspensions. The longer fluorescence lifetime was obtained from the coated silica monoliths indicating that no fluorescence quenching appeared. According to above results, it was concluded that Y2 O3 :Eu3+ layer could be successfully coated onto the porous silica monoliths and the resulted materials still exhibited good luminescent properties (Scheme 1).

5. Conclusions In summary, the Y2 O3 :Eu3+ -coated silica monoliths were successfully produced by freeze drying and wet impregnation. FESEM, XRD and mercury intrusion porosimetry were used to characterize the coated silica monoliths. These results indicated that the Y2 O3 :Eu3+ -coated silica monoliths exhibited macroporous structures, and this was in favor of their practical applications compared with mesoporous powders and thin films. The silica NPs content, volume ratio between oil phase and aqueous phase and the dip times obviously affected the morphologies and structures of the coated silica monoliths. It was found that the pore amount and loading amount of Y2 O3 :Eu3+ could be tuned by the volume ratio of oil phase and aqueous phase in emulsions and the dip times. The Y2 O3 :Eu3+ -coated silica monoliths emitted red light and gave the strong emission peak of 612 nm attributed to 5 D0 → 7 F2 transition of Eu3+ . This suggested that the coated silica monoliths still remained luminescence properties of Eu3+ . Due to the fabrication process of Y2 O3 :Eu3+ -coated silica monoliths is simple, rapid and low-cost, this method provides a novel route for the production

of macroporous silica monoliths loading with different inorganic oxides with optical, electronic and magnetic properties, which will show promising applications in sensing devices, absorbents, catalyst carriers and so on. Acknowledgments This work was supported by the National Nature Science Foundation of China (No. 51202130), the Shandong Provincial Natural Science Foundation, China (No. ZR2012BQ001) and the Independent Innovation Foundation of Shandong University (No. 2011TB015). References [1] G. Wakefield, E. Holland, P.J. Dobson, T.L. Hutchison, Luminescence properties of nanocrystalline Y2 O3 :Eu, Adv. Mater. 13 (2001) 1557–1560. [2] H. Huang, G.Q. Xu, W.S. Chin, L.M. Gan, C.H. Chew, Synthesis and characterization of Eu:Y2 O3 nanoparticles, Nanotechnology 13 (2002) 318–323. [3] P.P. Sun, J.P. Duan, H.T. Shih, C.H. Cheng, Europium complex as a highly efficient red emitter in electroluminescent devices, Appl. Phys. Lett. 81 (2002) 792–794. [4] G. Vicentini, L.B. Zinner, J. Zukerman-Schpector, K. Zinner, Luminescence and structure of europium compounds, Coord. Chem. Rev. 196 (2000) 353–382. [5] T. Myint, R. Gunawidjaja, H. Eilers, Spectroscopic properties of nanophase Eudoped ZrO2 and its potential application for fast temperature sensing under extreme conditions, J. Phys. Chem. C 116 (2012) 21629–21634. [6] R.M.P. Colodrero, K.E. Papathanasiou, N. Stavgianoudaki, P. Olivera-Pastor, E.R. Losilla, M.A.G. Aranda, L. Leon-Reina, J. Sanz, I. Sobrados, D. Choquesillo-Lazarte, J.M. Garcia-Ruiz, P. Atienzar, F. Rey, K.D. Demadis, A. Cabeza, Multifunctional luminescent and proton-conducting lanthanide carboxyphosphonate open-framework hybrids exhibiting crystalline-to-amorphous-to-crystalline transformations, Chem. Mater. 24 (2012) 3780–3792. [7] B.K. Gupta, T.N. Narayanan, S.A. Vithayathil, Y. Lee, S. Koshy, A.L.M. Reddy, A. Saha, V. Shanker, V.N. Singh, B.A. Kaipparettu, A.A. Marti, P.M. Ajayan, Highly luminescent-paramagnetic nanophosphor probes for in vitro high-contrast imaging of human breast cancer cells, Small 8 (2012) 3028–3034. [8] B.-I. Lee, E.-S. Lee, S.-H. Byeon, Assembly of layered rare-earth hydroxide nanosheets and SiO2 nanoparticles to fabricate multifunctional transparent films capable of combinatorial color generation, Adv. Funct. Mater. 22 (2012) 3562–3569. [9] S.S. Huang, X.J. Kang, Z.Y. Cheng, P.A. Ma, Y. Jia, J. Lin, Electrospinning preparation and drug delivery properties of Eu3+ /Tb3+ doped mesoporous bioactive glass nanofibers, J. Colloid Interface Sci. 387 (2012) 285–291. [10] S. Sandoval, J. Yang, J.G. Alfaro, A. Liberman, M. Makale, C.E. Chiang, I.K. Schuller, A.C. Kummel, W.C. Trogler, Europium doped TiO(2) hollow nanoshells: twophoton imaging of cell binding, Chem. Mater. 24 (2012) 4222–4230. [11] A. Feinle, F. Lavoie-Cardinal, J. Akbarzadeh, H. Peterlik, M. Adlung, C. Wickleder, N. Husing, Novel sol–gel precursors for thin mesoporous Eu3+ -doped silica coatings as efficient luminescent materials, Chem. Mater. 24 (2012) 3674–3683. [12] J.W. Shi, H.J. Cui, X. Zong, S.H. Chen, J.S. Chen, B. Xu, W.Y. Yang, L.Z. Wang, M.L. Fu, Facile one-pot synthesis of Eu, N-codoped mesoporous titania

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