Mesoporous Silica Encapsulating Upconversion Luminescence Rare

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Mesoporous Silica Encapsulating Upconversion Luminescence Rare-Earth Fluoride Nanorods for Secondary Excitation Jianping Yang, Yonghui Deng, Qingling Wu, Jing Zhou, Haifeng Bao, Qiang Li, Fan Zhang, Fuyou Li, Bo Tu,* and Dongyuan Zhao* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Key Laboratory of Molecular Engineering of Polymers of the Chinese Ministry of Education, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, P. R. China Received December 7, 2009. Revised Manuscript Received January 18, 2010 Mesoporous silica encapsulating upconversion luminescence NaYF4 nanorods with uniform core-shell structures have been successfully synthesized by the surfactant-assistant sol-gel process. The thickness of ordered mesoporous silica shells can be adjusted from 50 to 95 nm by varying the amount of hydrolyzed silicate oligomer precursors from tetraethyl orthosilicate (TEOS), which further influences the BET surface area, pore volume, and the luminescence intensity. After coated with mesoporous silica shells, the hydrophobic nanorods is rendered to hydropholic simultaneously. The obtained β-NaYF4@SiO2@mSiO2 core-shell nanorods possess high surface area (71.2-196 m2 g-1), pore volume (0.07-0.17 cm3 g-1), uniform pore size distribution (2.3 nm), and accessible channels. Furthermore, the uniform core-shell nanorods show strong upconversion luminescence property similar to the hexagonal upconversion cores. The open mesopores can not only provide convenient transmission channels but also offer the huge location for accommodation of large molecules, such as fluorescent dyes and quantum dots. The secondary-excitation fluorescence of Rhodamine B is generated from the upconversion rare-earth fluoride nanorods cores to the fluorescent dyes loaded in the mesoporous silica shells.

Introduction Upconversion materials can be efficiently excited by nearinfrared (NIR) light and emit visible light; namely, they emit high-energy photons after absorbing low-energy photons. Compared with down-conversion fluorescent materials, they have many advantages, including the higher chemical stability, quantum yields and light penetration depth in tissue, lower toxicity, and background light.1,2 These unique properties are attractive for flat-panel displays,3,4 optical storage,5 solid-state lasers,6 lightemitting diodes,7 and biological labeling and imaging.8,9 Among these materials, hexagonal-phase β-NaYF4 has been reported as one of the most efficient hosts for Yb3þ/Er3þ-codoped infrared-tovisible photoconversion.10 Until now, upconversion β-NaYF4 materials with various morphologies have been synthesized, such as nanoparticles,11 nanotubes,12 disks,12 nanocubes,13 and *Corresponding author: e-mail [email protected], Tel 86-21-51630205; Fax 86-21-5163-0307. (1) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73. (2) Li, Z. Q.; Zhang, Y. Nanotechnology 2008, 19, 345606. (3) Downing, E.; Hesselink, L.; Ralston, J.; Macfarlane, R. Science 1996, 273, 1185. (4) Londahl, P.; van Driel, A. F.; Nikolaev, L. S.; Irman, A.; Overgaag, K.; Vanmaekelbergh, D.; Vos, W. L. Nature 2004, 430, 654. (5) Cheben, P.; del Monte, F.; Worsfold, D. J.; Carlsson, D. J.; Grover, C. P.; Mackenzie, J. D. Nature 2000, 408, 64. (6) Rumbles, G. Nature 2001, 409, 572. (7) Stockman, M. Nat. Mater. 2004, 3, 423. (8) Hu, H.; Xiong, L. Q.; Zhou, J.; Li, F. Y.; Cao, T. Y.; Huang, C. H. Chem.; Eur. J. 2009, 15, 3577. (9) Kumar, R.; Nyk, M.; Ohulchanskyy, T. Y.; Flask, C. A.; Prasad, P. N. Adv. Funct. Mater. 2009, 19, 1. (10) Kramer, K. W.; Biner, D.; Frei, G.; G€udel, H. U.; Hehlen, M. P.; Luthi, S. R. Chem. Mater. 2004, 16, 1244. (11) Yi, G. S.; Chow, G. M. Adv. Funct. Mater. 2006, 16, 2324. (12) Zhang, F.; Wan, Y.; Yu, T.; Zhang, F. Q.; Shi, Y. F.; Xie, S. H.; Li, Y. G.; Xu, L.; Tu, B.; Zhao, D. Y. Angew. Chem., Int. Ed. 2007, 46, 7976. (13) Wang, L. Y.; Li, Y. D. Chem. Mater. 2007, 19, 727. (14) Zeng, J. H.; Su, J.; Li, Z. H.; Yan, R. X.; Li, Y. D. Adv. Mater. 2005, 17, 2119.

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nanorods.12,13 However, most of these materials are synthesized in organic solvents12-15 or at high temperature;11,16 the obtained nanocrystals are hydrophobic and show poor dispersibility in water. The direct use of these nanocrystals for biological applications is limited owing to the low solubility in water and unsuitable surface property, which do not have functional chemical groups or appropriate sites for attachment of biomolecules. Therefore, further surface modification of these nanocrystals is much required. Up to now, several surface modification methods have been developed, including encapsulating upconversion particles with polymers,20-22 directly oxidizing oleic acid ligands to carboxylic acid-groups,23 and coating the upconversion particles with silica.17-19 In the upconversion particle/polymer composites, amphiphilic surfactants, for example polyvinylpyrrolidone (PVP),20 poly(acrylic acid),21 and polyethylenimine,22 have been used, which act as both the chelating agent and stabilizer for the NaYF4 nanocrystals with controlled size and shape, resulting in suitable surface property and solubility in water. However, the surface is still not easily modified with biofunctional molecules.17 The directly oxidizing oleic acid ligands to carboxylic acid does not change the morphologies, phases, compositions, and luminescent capabilities of the upconversion nanocrystals.23 But the method is still limited to the oleic acid-capped nanocrystals.8 (15) Shan, J. N.; Ju, Y. G. Appl. Phys. Lett. 2007, 91, 123103. (16) Yi, G. S.; Lu, H. C.; Zhao, S. Y.; Ge, Y.; Yang, W. J.; Chen, D. P.; Guo, L. H. Nano Lett. 2004, 4, 2191. (17) Liu, Z. Y.; Yi, G. S.; Zhang, H. T.; Ding, J.; Zhang, Y. W.; Xue, J. M. Chem. Commun. 2008, 694. (18) Li, Z. Q.; Zhang, Y.; Jiang, S. Adv. Mater. 2008, 20, 1. (19) Jalil, R. A.; Zhang, Y. Biomaterials 2008, 29, 4122. (20) Li, Z. Q.; Zhang, Y. Angew. Chem., Int. Ed. 2006, 45, 7732. (21) Yi, G. S.; Chow, G. M. Chem. Mater. 2007, 19, 341. (22) Wang, F.; Chatterjee, D. K.; Li, Z. Q.; Zhang, Y.; Fan, X. P.; Wang, M. Q. Nanotechnology 2006, 17, 5786. (23) Chen, Z. G.; Chen, H. L.; Hu, H.; Yu, M. X.; Li, F. Y.; Zhang, Q.; Zhou, Z. G.; Yi, T.; Huang, C. H. J. Am. Chem. Soc. 2008, 130, 3023.

Published on Web 02/02/2010

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Because of the well-known surface chemistry, benign effects on biological systems and the facile grafted other functional groups,24 silica-coated core-shell structures, are generally considered to be an ideal method for the modification of the upconversion nanocrystal surface. Zhang and co-workers18 have used a microemulsion method for coating uniform thin silica layer on the surface of hexagonal-phase β-NaYF4 nanospheres with strong upconversion fluorescence. Multicolor spheres are produced by encapsulating organic dyes or quantum dots (QDs) into the silica. Recently, ordered mesoporous materials have attracted more and more attention owing to their high surface area, uniform pore size, and the potential applications in separation, adsorption, catalysis, and sensors.26 The integration of mesoporous silica with functional nanocrystals to form core-shell structures is undoubtedly of great values because the mesoporous shells not only offer high surface area for derivation of numerous functional groups but also provide accessible large pore channels for the adsorption and encapsulation of biomolecules and even functional nanoparticles.27 Several papers have reported the synthesis of nanoparticles core/mesoporous silica shell structures, such as mesoporous silica encapsulating magnetic particles,27-30 metal nanoparticles,31 and core-satellite nanocomposite.32 However, all core particles are spherical. There are seldom papers about the nanorods core-shell nanocomposites; only mesoporous silicacoated gold nanorods have been reported.33 Owing to the luminescence intensity of β-NaYF4 nanorods is nearly 8 times higher than R-NaYF4 nanoparticles,12 the high upconversion luminescence can fulfill the secondary excitation. In this paper, ordered mesoporous silica encapsulating upconversion luminescence nanorods core-shell structures have been synthesized by the surfactant-assistant sol-gel coating method under a basic condition. The obtained silica shell mesostructure possesses uniform pore size (∼2.3 nm) and opening channels. The thickness of the mesoporous silica shells can be adjusted in the range of 50-95 nm. The hexagonal phase β-NaYF4:Yb, Er nanorods cores with uniform mesoporous silica shells still show remarkable upconversion luminescence property. The mesoporous silica shells with large pore channels can easily adsorb Rhodamine B fluorescent dye. The secondary excitation from the upconversion nanorod core to the fluorescent dye is clearly realized, resulting in an impact secondary excitation luminescence. The attractive features would develop the multicolor fluorescence and make it possessing enormous potential applications.

Experimental Section Chemicals. All chemicals were analytical grade and used without further purification. NaOH, NaF, ethanol, triton X-100 (24) L€u, Q.; Guo, F. Y.; Sun, L.; Li, A. H.; Zhao, L. C. J. Appl. Phys. 2008, 103, 123533. (25) Selvan, S. T.; Tan, T. T.; Ying, J. Y. Adv. Mater. 2005, 17, 1620. (26) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (27) Deng, Y. H.; Qi, D. W.; Deng, C. H.; Zhang, X. M.; Zhao, D. Y. J. Am. Chem. Soc. 2008, 130, 28. (28) Kim, J.; Kim, H. S.; Lee, N.; Kim, T.; Kim, H.; Yu, T.; Song, I. C.; Moon, W. K.; Hyeon, T. Angew. Chem., Int. Ed. 2008, 47, 8438. (29) Yang, X. Y.; Li, Y.; Tendeloo, G. V.; Xiao, F. S.; Su, B. L. Adv. Mater. 2009, 21, 1. (30) Zhao, W. R.; Gu, J. L.; Zhang, L. X.; Chen, H. R.; Shi, J. L. J. Am. Chem. Soc. 2005, 127, 8916. (31) Joo, S. H.; Park, J. Y.; Tsung, C. K.; Yamada, Y.; Yang, P. D.; Somorjai, G. A. Nat. Mater. 2009, 8, 126. (32) Ge, J. P.; Zhang, Q.; Zhang, T. R.; Yin, Y. D. Angew. Chem., Int. Ed. 2008, 47, 1. (33) Gorelikov, I.; Matsuura, N. Nano Lett. 2008, 8, 369.

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(tert-octylphenoxypolyoxyethylene), tetraethyl orthosilicate (TEOS), hexadecyltrimethylammonium bromide (CTAB), and acetone were purchased from Sinopharm Chemical Reagent Co. (China). Oleic acid was obtained from Alfa Aesar. Rare earth nitrates [Ln(NO3)3, Ln: Y, Yb, Er] were prepared by dissolving the corresponding oxides (Y2O3, Yb2O3, and Er2O3 from Beijing Lansu Co. China) in 20% nitric acid solution and then evaporating the water completely. Deionized water was used in all experiments. Synthesis of Upconversion Nanorods. The hexagonal phase β-NaYF4 nanorods codoped with 20 wt %Yb and 2 wt % Er (denoted β-NaYF4:Yb/Er) were prepared similar to the previous report.12 In a typical synthesis, 0.70 g (17.5 mmol) of NaOH, 7.1 g (22.6 mmol) of oleic acid (90 wt %), and 10.0 g of ethanol were well mixed at room temperature to get a white viscous solution. 8.3 mL (7.2 mmol) of 0.87 M NaF solution was added with vigorous stirring until a translucent solution was obtained. Then 1.1 mL (0.88 mmol) of 0.80 M Y(NO3)3, 0.35 mL (0.22 mmol) of 0.63 M Yb(NO3)3, and 0.05 mL (0.02 mmol) of 0.40 M Er(NO3)3 were poured into the above solution with vigorous stirring. After aging for 20 min, the mixture was transferred to a 40 mL of Teflon-lined autoclave and heated at 230 °C for 12 h.

Synthesis of β-NaYF4@SiO2@mSiO2 Core-Shell Nanorods. The core-shell nanorods were synthesized via the

surfactant-assistant sol-gel coating method by using CATB as a mesostructual template in ammonia aqueous solution. For a typical procedure, 0.10 g of upconversion nanorods was added into 20 mL of Triton X-100 solution, after ultrasonic for 10 min to form transparent solution, and then 80 mL of water was poured into the above solution with continuous stirring for 6 h. After centrifugated and washed with water, the precipitation was dispersed in the mixture of ethanol (160 mL), water (40 mL), and 2 mL of ammonia aqueous solution (28 wt %), and then 0.06 g of TEOS was added. After stirring for another 6 h, the product was separated by centrifugation and then washed with ethanol and water and redispersed in a mixed solution containing 0.30 g of CTAB, 80 mL of deionized water, 60 mL of ethanol, and 1.00 g of ammonia aqueous solution (28 wt %). After being ultrasonic for 30 min, 0.30-0.40 g of TEOS was added dropwise to the dispersion with vigorous stirring. After the reaction for 6 h, the product was collected by centrifugation and washed with ethanol and water for several times. Finally, the precipitation was redispersed in 70 mL of acetone and refluxed at 80 °C for 48 h to remove the CTAB templates. The extraction was repeated for three times, the products were washed with deionized water, and β-NaYF4@SiO2@mSiO2 nanorods were finally obtained. Adsorption of Rhodamine B Organic Dye. 10 mg of Rhodamine B was dissolved in 10 mL of ethanol, and then 10 mg of the obtained β-NaYF4@SiO2@mSiO2 nanorods was added. After ultrasonication for 10 min, the solution was slowly stirred at room temperature to evaporate the ethanol. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D4 X-ray diffractometer (Germany) with Ni-filtered Cu KR radiation (40 kV, 40 mA). Nitrogen sorption isotherms were measured at 77 K with a Micromeritcs Tristar 3000 analyzer. Before measurements, the samples were degassed in a vacuum at 180 °C for at least 6 h. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas (SBET), using adsorption data in a relative pressure range from 0.04 to 0.2. The pore volume and pore size distributions were derived from the adsorption branches of isotherms by using Barrett-Joyner-Halenda (BJH) model. The total pore volume, Vt, was estimated from the amount adsorbed at a relative pressure P/P0 of 0.95. Scanning electron microscopic (SEM) images were obtained on a Philip XL30 microscope. A thin film of gold was sprayed on the sample before this characterization. Field-emission scanning electron microscopy (FESEM) images were obtained on a Hitachi S-4800 microscope. Transmission electron microscopy (TEM) measurements were carried out DOI: 10.1021/la904596x

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Figure 1. Schematic illustration of the formation of β-NaYF4@SiO2@mSiO2 nanorods. When the upconversion nanorod cores are excited by NIR light at 980 nm, the cores can emit visible light due to upconversion process. Then the appropriate wavelength green light is adsorbed by the dye molecules (Rhodamine B) that are loaded in the mesoporous silica shell, and then these molecules are excited to emit red light and form secondary-excitation fluorescence. on a JEOL 2011 microscope operated at 200 kV. All samples were first dispersed in ethanol and then collected using copper grids covered with carbon films for measurements. Energy-dispersive X-ray spectroscopy (EDX) was performed on a JEOL 2011 EDX instrument. Confocal luminescence images were made with an Olympus FV1000, with a CW NIR laser at λ = 980 nm as the excitation source. For β-NaYF4@ SiO2@mSiO2 nanorods and after the adsorption of Rhodamine B (Rh-B) organic dyes powder samples, the CW NIR laser at λ=980 nm was used to provide excitation, and emissions were collected in the range of λ = 520-550, 600-630, and 500-700 nm, respectively. All the measurements were carried out in the same condition.

Results and Discussion The ordered mesoporous silica encapsulating upconversion rare-earth fluoride nanorods core-shell structures can be synthesized by a surfactant-assistant sol-gel coating process (Figure 1). First, the uniform hexagonal NaYF4: Yb/Er nanorods with hydrophobic oleic acid ligand synthesized by the hydrothermal method were modified with amphiphilic nonionic surfactant Triton X-100 to obtain hydrophilic surface. Second, the modified nanorods were coated with a thin silica layer through St€ober method to form thin silica encapsulating β-NaYF4 nanorods (designated as β-NaYF4@SiO2). Third, through a surfactanttemplating sol-gel approach by using CTAB as a template, the mesostructured CTAB/silica composites were coated on the β-NaYF4@SiO2 nanorods. Finally, the CTAB templates were removed in a mild way of acetone extraction to obtain the mesoporous silica/rare-earth fluoride nanorods core-shell structures (assigned as β-NaYF4@SiO2@mSiO2). The uniform β-NaYF4-codoped Yb3þ and Er3þ nanorods were prepared by using a hydrothermal method according to the paper reported recently.12 Oleic acid was used as a stabilizing agent, and Ln(NO3)3 and NaF were used as precursors at 230 °C under a basic condition. The high F- concentration is in favor of dissolution-reconstruction process, which causes the fast growth along [0001] direction and ultimately to form the nanorods.12 SEM images show that the products are composed of uniform nanorods with diameters of about 110 nm and lengths of about 8852 DOI: 10.1021/la904596x

2 μm (Figure 2a). The FESEM image shows that the top/bottom surfaces are hexagonal (Figure 2a, inset). The TEM image (Figure 2b) further confirms the uniform nanorods with smooth surface. The electron diffraction (ED) pattern (Figure 2b, inset) indicates that the nanorods are single-crystal. Energy-dispersive X-ray (EDX) analysis of the uniform nanorods (Figure 2c) reveals the exist of Yb3þ and Er3þ cations, suggesting the Er3þ/Yb3þ ions are codoped into the NaYF4 lattice. The XRD pattern (Figure 2d) shows that the nanorods consist of pure hexagonal-phase β-NaYF4 crystals. The high intensity of diffraction peaks further demonstrates that the nanorods are well crystallized. Triton X-100 is selected as an amphiphilic surfactant to modify the hydrophobic surface of the β-NaYF4 nanorods. The hydrophobic segment of Triton X-100 can interact with the alkyl chain of oleic acid located outside surface of the nanorods, resulting that the hydrophilic group of poly(ethylene oxide) (PEO) is dispersed in external surface. Therefore, after the modification, the surface of the nanorods changes to be hydrophilic which can be directly coated by the silica layer by using the mature St€ober sol-gel method. After being coated with a thin silica layer, uniform core-shell β-NaYF4@SiO2 nanorods are obtained. The SEM image (Figure 3a) shows that the surface of nanorods becomes rough, implying the nanorods are coated by silica layer. Furthermore, our careful SEM measurements clearly prove that all of the nanorods are coated, and no silica nanoparticles are formed with the low amount of silica precursor and under vigorous stirring condition. TEM images clearly show that the β-NaYF4 nanorods are coated with thin silica layer (Figure 3b). The thickness is measured to be thin about 10-20 nm. The hydrophilic property and successful coating indicate that Triton X-100 modification is an efficient method to change the hydrophobic nanocrystals into hydrophilic. On the coating process, Triton X-100 amphiphilic surfactant not only modifies the hydrophobic surface into hydrophilic but also makes some porous formed during the subsequent extraction process. The CTAB/SiO2 mesophase can be coated on the surface of the β-NaYF4@SiO2 nanorods by the surfactant-templating sol-gel process. The surfactant template can be removed by extraction Langmuir 2010, 26(11), 8850–8856

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Figure 2. (a) SEM image of the as-made hexagonal β-NaYF4 nanorods prepared by hydrothermal synthesis method at 230 °C, and the inset is the FESEM image. (b) TEM image of the nanorods and corresponding ED pattern (inset). (c) EDX pattern of the nanorods. (d) Wide-angle XRD pattern of as-made β-NaYF4 nanorods.

Figure 3. SEM (a) and TEM (b) images of the β-NaYF4@SiO2 core-shell structure prepared by the St€ ober method.

with refluxing acetone. The SEM image clearly shows that the obtained β-NaYF4@SiO2@mSiO2 core-shell composites still retain the uniform nanorod morphology (Figure 4a). The TEM image shows that all nanorods are coated with a thin uniform mesoporous silica layer (Figure 4b). High-magnification TEM image shows that the thickness of the silica shell is very uniform and about 50 nm (Figure 4c,d), and the uniform ordered mesopores can be observed. The mesopore channels are all open. FESEM images (Figure 4e) further reveal that the top of β-NaYF4@SiO2@mSiO2 nanorods consist of the opened uniform mesopores on the surface. These results clearly indicate that mesoporous rod-shape silicate/CTAB composites can assemble with oriental alignment on the surface of β-NaYF4@SiO2 nanorods. Wide-angle XRD patterns of β-NaYF4@SiO2 @mSiO2 nanorods (Figure 5) show well-resolved diffraction peaks similar to that of their parent β-NaYF4 nanorods, indicating that the Langmuir 2010, 26(11), 8850–8856

hexagonal phase crystalline is well retained. The decreased diffraction intensity is ascribed to the influence of the silica shell. The low-angle XRD pattern (Figure 6) of the β-NaYF4@SiO2@mSiO2 nanorods shows one intense diffraction peak at 2θ=2.4°, suggesting an ordered mesostructure. Combined with the TEM results, it further confirms that the β-NaYF4@SiO2@mSiO2 nanorods have ordered mesostructures. N2 adsorption/desorption isotherms of the β-NaYF4@SiO2@mSiO2 nanorods after the removal of CTAB template show typical type IV curves (Figure 7), further suggesting a uniform mesopore. The mesopore size distribution (Figure 7, inset) calculated from the BJH model exhibits a sharp peak centered at the mean value of 2.3 nm, indicating the uniform mesopores. The BET surface area and the total pore volume are calculated to be 71.2 m2 g-1 and 0.07 cm3 g-1, respectively. Because the rare-earth fluoride nanorods cores have high density, much larger than that for the silica DOI: 10.1021/la904596x

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Figure 4. SEM (a) and TEM (b-d) images of the obtained β-NaYF4@SiO2@mSiO2 core-shell nanorods prepared by using surfactanttemplating sol-gel method and (e) FESEM image in the top view of the β-NaYF4@SiO2@mSiO2 nanorods.

Figure 7. N2 adsorption-desorption isotherms and pore size dis-

Figure 5. Wide-angle XRD patterns of (a) as-made β-NaYF4 nanorods and (b) the β-NaYF4@SiO2@mSiO2 core-shell nanorods.

Figure 6. Low-angle XRD pattern of obtained mesoporous β-NaYF4@SiO2@ mSiO2 core-shell nanorods.

shells, such low specific surface area and small pore volume are reasonable. The unclosed adsorption-desorption hysteresis loop is attributed to a few residual surfactants inside of the silica shells, which can interact with N2 molecules and further result in the incomplete desorption process. The relationship between the content of silicate species and the thickness of the shell has been considered. As TEOS content increases, the silica shell thickness can increase to about 95 nm 8854 DOI: 10.1021/la904596x

tribution curve (the inset) of the mesoporous β-NaYF4@SiO2@mSiO2 core-shell nanorods.

(Figure S1a-c). The core-shell nanorods become more agglomeration, and the silica nanoparticles appeared under such high silica source condition (Figure S1a). A typical sandwich structure with β-NaYF4 nanorod cores and ordered mesoporous silica shells with accessible channels can be clearly observed (Figure S1c). Wide-angle XRD patterns of β-NaYF4@SiO2@mSiO2 nanorods show the same diffraction peaks (Figure S2) to that of the β-NaYF4 nanorods, but the diffraction intensity becomes much weak as increase of the TEOS content. It can be ascribed to the thick mesoporous silica shells which can decrease the mass fraction of upconversion nanorods in the β-NaYF4@SiO2@mSiO2 sample and then further reduce the X-ray intensity. Furthermore, the intensity of incident X-ray can also be decreased when it goes through the thick mesoporous silica layer of nanorods. Similarly, the low-angle XRD pattern (Figure S3) shows two wide diffraction peaks at 2θ=0.76° and 2.5°, suggesting an ordered mesostructure. N2 adsorption-desorption isotherms also show typical type IV curves (Figure S4). A narrow pore size distribution (Figure S4, inset) is observed at a maximum mean value of 2.3 nm. The BET surface area and pore volume are calculated to be 196 m2 g-1 and 0.17 cm3 g-1, respectively, clearly indicating an increase with the thickness of the mesoporous silica shells. The upconversion luminescence spectrum excited with a 980 nm laser diode for the β-NaYF4 nanorods codoped with Langmuir 2010, 26(11), 8850–8856

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Figure 8. Luminescence spectra of (a) the β-NaYF4 codoped with 20 wt % Yb3þ and 2 wt % Er3þ nanorods (red line), (b) the β-NaYF4@SiO2 nanorods (blue line) after coated thin silica by the St€ ober method, and (c) the mesoporous β-NaYF4@SiO2@mSiO2 core-shell nanorods (black line) prepared by the surfactanttemplating sol-gel process. A 980 nm laser diode was used as excited light.

20 wt % Yb3þ and 2 wt % Er3þ, the β-NaYF4@SiO2 nanorods and β-NaYF4@SiO2@mSiO2 nanorods with silica shell thickness of 50 nm are shown in Figure 8. All spectra feature three distinct Er3þ emission bands. A dominant red emission originated from the 4F9/2 to 4I15/2 transition is observed at 650-680 nm.34 The green emissions between 514-534 and 534-560 nm are attributed to the transitions from 2H11/2 and 4S3/2 to 4I15/2 of Er3þ, respectively.35 By a rough comparison, coating porous silica layer does not obviously affect the luminescence intensity and spectra, except in the green region (Figure 8). The reason is ascribed to the comparatively thin layer of the mesoporous silica shells and large β-NaYF4:Yb3þ/Er3þ nanorods cores. The volume ratio of the coated silica layer to β-NaYF4 core is roughly evaluated to be 0.8. The low ratio causes less change in ion density, resulting that the coated mesoporous silica shell has less effect on luminescence intensity.15 The upconversion luminescence spectra of the β-NaYF4:Yb3þ/Er3þ nanorod powder sample and obtained β-NaYF4@SiO2@mSiO2 core-shell nanorods with silica shell thickness of 95 nm are shown in Figure S5. The thick shell β-NaYF4@SiO2@mSiO2 nanorods still have the feature Er3þ emission bands. However, the luminescence intensity is evidently lower than the original β-NaYF4 nanorods, which is attributed to the depression effect from the thick silica shell. The β-NaYF4 nanorod cores possess superiorly upconversion luminescence property, and the mesoporous silica shells have attractive adsorption ability due to the opening mesochannels and hydrophilic property with abundant hydroxyl in the surface. So the β-NaYF4@SiO2@mSiO2 nanorods can provide a venue for secondary excitation from the upconversion nanorods core to the fluorescent dyes. In the case, the upconversion nanorods cores are used as energy donors and the down-conversion materials (such as fluorescent dyes or quantum dots) are used as energy acceptors. Once the upconversion nanorods cores are excited by NIR light (such as 980 nm laser), the cores emit visible light due to upconversion. Then the appropriate wavelength visible light is adsorbed by the down-conversion materials which are loaded in the mesoporous silica shell, and then the down-conversion materials are excited to emit fluorescent light, as shown in Figure 1. The commonly adequate fluorescence dye, Rhodamine B, which is (34) Auzel, F. Chem. Rev. 2004, 104, 139. (35) Page, R. H.; Schaffers, K. I.; Waide, P. A.; Tassano, J. B.; Payne, S. A. J. Opt. Soc. Am. B 1998, 15, 996.

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Figure 9. Confocal luminescence images of the β-NaYF4@SiO2@mSiO2 nanorods with silica shell thickness of 50 nm before (a-c) and after (d-f) the adsorption of Rhodamine B organic dyes. All images were excited at λex = 980 nm, and emissions were collected in the range of λ = 520-550 nm (a, d) and λ = 600630 nm (b, e). (c) Overlay images of (a) and (b); (f) overlay images of (d) and (e). All the measurements were carried out in the same conditions.

excited by green light and emits red light, is adsorbed into the mesoporous silica shell of β-NaYF4@SiO2@mSiO2 nanorods and used as an example to prove the secondary excitation concept. Confocal laser scanning microscopy (CLSM) can provide an intuitive way to display the result. From the confocal luminescence images (Figure 9), the luminescence dots are not regular rod-like, causing from the light scatter and at different views. At the same condition, the intensity of green light collected in the range of λ= 520-550 nm in the image (a) is stronger than that in the image (d), and the intensity of red light collected in the range of λ = 600630 nm in the image (b) is weaker than that in the image (e). These indicate that the Rhodamine B fluorescence dyes in the mesoporous silica shell (Figure 9d,e) have already adsorbed partly green light and emitted red light. The red light collected in the range of λ=600-630 nm, to avoid the effect of the high intensity red light from the upconversion core itself, the overlay image of the β-NaYF4@SiO2@mSiO2 core-shell sample (Figure 9c) shows that the intensity of green light is much stronger than that for red light. In the inner of the core-shell nanorods presents yellow color, which is the superposition color of green and red. In the surface of the nanorods, they emitted excessive green color. When Rhodamine B fluorescence dye is adsorbed, from the overlay image (Figure 9f), all nanorods display yellow color and little red color in the surface, indicating that the latter color is stronger than the former. The luminescence spectra of the β-NaYF4@SiO2@mSiO2 core-shell nanorods after adsorption of Rhodamine B (Figure 10) show that the characteristic green emission between 514 and 560 nm disappears, while a new emission at 570-600 nm appears because the sensitivity and solution condition for the luminescence spectra measurements are much different with that powder samples for the confocal laser scanning microscopy. From confocal laser scanning microscopy, the green emission light can be observed around the surface of nanorods. However, the Rhodamine B dye molecules loading on the surface of the mesoporous silica shells are partially dissolved in solution; the green emission light is quenched by the Rhodamine B molecules in solution. As a result, the green emission light is much weaker and has no obvious peak in the spectrum compared with the stronger green emission peak of the upconversion nanorods. For the emission peak in the DOI: 10.1021/la904596x

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Figure 10. Luminescence spectra of the β-NaYF4@SiO2@mSiO2 nanorods before (black line) and after adsorption of Rhodamine B fluorescence dye (pink line) excited with a 980 nm laser diode.

600-630 nm range, the weak red emission light is hidden by the strong emission red light in the 650-680 nm range, therefore, no obvious peak can be observed in the luminescent spectra; the intensity is also obviously improved. These results clearly indicate that Rhodamine B dyes can efficiently absorb green light from the upconversion rare-earth fluoride cores and emit red light, suggesting an efficient secondary excitation is accomplished from nanorods core to Rhodamine B fluorescence dyes. In conclusion, we have demonstrated a successful synthesis of uniform mesoporous β-NaYF4@SiO2@mSiO2 core-shell nanorods via the surfactant-assistant approach. The core-shell structures have high surface area (up to 195 m2 g-1), pore volume (∼0.17 cm3 g-1), uniform pore size distribution (at ∼2.3 nm), and open ordered mesopores. The thickness of mesoporous silica shells can be adjusted by varying the amount of TEOS. With the increase of the thickness, the BET surface area and pore volume increased, but the luminescence intensity decreased. However, the functional β-NaYF4@SiO2@mSiO2 core-shell

8856 DOI: 10.1021/la904596x

Yang et al.

nanorods still possess strong upconversion luminescence property, similar to their parent nanorods. The upconversion β-NaYF4:Yb3þ/Er3þ cores that emit higher energy green light after absorbing lower energy NIR light show significant luminescence and imaging. On the other hand, the surface property of the upconversion β-NaYF4 nanorods can be changed after being coated with the porous silica. The mesoporous silica shells can provide a large venue for the effective modification of biomacromolecules, nanoparticles, and organic dyes. The strong secondary excitation from the unique upconversion@mesopore core-shell structure is produced, which can accomplish re-excitation from the upconversion core to the fluorescence dyes adsorbed in mesoporous shells. These properties may provide great applications in photoinducing reaction, light-operated switch, multiple light responses, and so on. Acknowledgment. This work was supported by NSF of China (20890121, 20721063, 20821140537, 20871030), State Key Basic Research Program of PRC (2009CB930400), Shanghai Leading Academic Discipline Project (B108). Supporting Information Available: TEM images of the mesoporous β-NaYF4@SiO2@mSiO2 core-shell nanorods (Figure S1); wide-angle XRD patterns of as-made β-NaYF4 nanorods and the mesoporous β-NaYF4@SiO2@mSiO2 core-shell nanorods (Figure S2); XRD pattern of the mesoporous β-NaYF4@SiO2@mSiO2 core-shell nanorods (Figure S3); N2 adsorption-desorption isotherms and pore size distribution plots of the mesoporous β-NaYF4@SiO2@mSiO2 core-shell nanorods (Figure S4); luminescence spectra of as-made β-NaYF4 nanorods and (b) the mesoporous β-NaYF4@SiO2@mSiO2 core-shell nanorods (Figure S5). This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(11), 8850–8856