Novel Multifunctional Nanocomposites: Magnetic Mesoporous Silica

Nov 3, 2009 - Ln(DBM)3phen-MMS (Ln = Nd, Yb) nanospheres with an average size ... operating at 1.3 μm and Yb(DBM)3phen-MMS nanocomposites have ...
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Novel Multifunctional Nanocomposites: Magnetic Mesoporous Silica Nanospheres Covalently Bonded with Near-Infrared Luminescent Lanthanide Complexes Jing Feng,†,‡ Shu-Yan Song,†,‡ Rui-Ping Deng,† Wei-Qiang Fan,†,‡ and Hong-Jie Zhang*,† † State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and ‡Graduate School of the Chinese Academy of Sciences, Beijing, P. R. China

Received August 13, 2009. Revised Manuscript Received October 18, 2009 In this paper, we report the fabrication and characterization of magnetic mesoporous silica nanospheres covalently bonded with near-infrared (NIR) luminescent lanthanide complexes [denoted as Ln(DBM)3phen-MMS (Ln = Nd, Yb)]. Ln(DBM)3phen-MMS (Ln = Nd, Yb) nanospheres with an average size of 80-130 nm were synthesized via incorporation of the chelate ligand 5-[N,N-bis-3-(triethoxysilyl)propyl]ureyl-1,10-phenanthroline (phen-Si) into the framework of magnetic mesoporous silica (denoted as phen-MMS), followed by introduction of the Ln(DBM)3(H2O)2 (Ln = Nd, Yb) complexes into the nanocomposites via a ligand exchange reaction. The morphological, structural, textural, magnetic, and NIR luminescent properties were well-characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), N2 adsorption-desorption, a superconducting quantum interference device (SQUID), and photoluminescence spectra. These nanocomposites, which possess high surface area, high pore volume, and well-defined pore size, exhibit twodimensional hexagonal (P6mm) mesostructures. After ligand-mediated excitation, Ln(DBM)3phen-MMS (Ln = Nd, Yb) nanocomposites exhibit the characteristic NIR emission of Nd3þ and Yb3þ, respectively. Magnetic measurements reveal that these mulfunctional nanocomposites possess superparamagnetic properties at 300 K. The high magnetization values make the nanocomposites respond to the external magnetic field quickly. Additionally, the results indicate that Nd(DBM)3phen-MMS nanocomposites may have potential applications for laser systems or the optical amplifiers operating at 1.3 μm and Yb(DBM)3phen-MMS nanocomposites have several advantages for potential applications in drug delivery or optical imaging.

Introduction Recently, multifunctional nanomaterials have become an attractive research field. Multifunctional mesoporous composite nanomaterials with unique magnetic and luminescent properties have great potential in biological applications such as magnetic resonance imaging (MRI) contrast agents, drug delivery carriers, cell sorting, and labeling.1-8 Mesoporous silicas with unique properties (e.g., large surface area, high pore volume, controlled pore structure, narrow pore size distribution, controlled morphology, and high thermal and hydrothermal stabilities) have *To whom correspondence should be addressed. Phone: þ86-43185262127. Fax: þ86-431-85698041. E-mail: [email protected].

(1) Huh, Y.-M.; Jun, Y.-W.; Song, H.-T.; Kim, S.; Choi, J.-S.; Lee, J.-H.; Yoon, S.; Kim, K.-S.; Shin, J.-S.; Suh, J.-S.; Cheon, J. J. Am. Chem. Soc. 2005, 127, 12387. (2) Giri, S.; Trewyn, B. G.; Stellmarker, M. P.; Lin, V. S.-Y. Angew. Chem., Int. Ed. 2005, 44, 5038. (3) Kim, J.; Lee, J. E.; Lee, J.; Jang, Y.; Kim, S.-W.; An, K.; Yu, J. H.; Hyeon, T. Angew. Chem., Int. Ed. 2006, 45, 4789. (4) Sen, T.; Sebastianeili, A.; Bruce, I. J. J. Am. Chem. Soc. 2006, 128, 7130. (5) Kin, J.; Lee, J. E.; Lee, J.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y.; Shin, C.-H.; Park, J.-G.; Kim, J.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 688. (6) 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. (7) Lin, Y.-S.; Wu, S.-H.; Hung, Y.; Chou, Y.-H.; Chang, C.; Lin, M.-L.; Tsai, C.-P.; Mou, C.-Y. Chem. Mater. 2006, 18, 5170. (8) Lee, C. H.; Cheng, S. H.; Wang, Y. J.; Chen, Y. C.; Chen, N. T.; Souris, J.; Chen, C. T.; Mou, C. Y.; Yang, C. S.; Lo, L. W. Adv. Funct. Mater. 2009, 19, 215. (9) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (10) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (11) Ciesla, U.; Sch€uth, F. Microporous Mesoporous Mater. 1999, 27, 131. (12) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56.

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been extensively investigated because of their wide potential applications.9-16 For many applications, it is quite troublesome for mesoporous silica materials to separate small particles from liquid. As a type of important functional material, magnetic nanoparticles have continued to draw considerable interest because of their great potential applications in magnetic fluids, catalysis, biotechnology/biomedicine, MRI, magnetic recording devices, and environmental remediation.17 Among magnetic particles, magnetite (Fe3O4) has received more attention because of its potential applications in nanobiotechnology such as using it as a tag for separation, sensing, and imaging, and as an active agent for antitumor therapy.18-20 Therefore, mesoporous silica nanospheres combined with Fe3O4 magnetic particles can be conveniently separated from the aqueous phase by application of an external magnetic field. To give the composite more interesting character, we could introduce functional materials into it, such as luminescent materials. It is well-known that lanthanide complexes make up a useful class of luminophores since they exhibit high quantum efficiency, long lifetimes, (13) Trewyn, B. G.; Whitman, C. M.; Lin, V. S. Y. Nano Lett. 2004, 4, 2139. (14) Suzuki, K.; Ikari, K.; Imai, H. J. Am. Chem. Soc. 2004, 126, 462. (15) Han, Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2005, 44, 288. (16) Ying, J. Y. Chem. Eng. Sci. 2006, 61, 1540. (17) Lu, A.; Salabas, E. L.; Sch€uth, F. Angew. Chem., Int. Ed. 2007, 46, 1222. (18) Perez, J. M.; Loughlin, T. O.; Simeone, F. J.; Weissleder, R.; Josephson, L. J. Am. Chem. Soc. 2002, 124, 2856. (19) Perez, J. M.; Simeone, F. J.; Tsourkas, A.; Josephson, L.; Weissleder, R. Nano Lett. 2004, 4, 119. (20) Louie, A. Y.; Huber, M. M.; Ahrens, E. T.; Rothbacher, U.; Moats, R.; Jacobs, R. E.; Fraser, S. E.; Meade, T. J. Nat. Biotechnol. 2000, 18, 321.

Published on Web 11/03/2009

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narrow-band emission, and high color purity under ultraviolet excitation, via protection of lanthanide ions from vibrational quenching and an increasing light absorption cross section by the well-known “antenna effect”.21-27 These characteristics make them particularly valuable in sensors and displays, and they also are employed for applications such as fluoroimmunoassay and fluorescence microscopy.28-30 There has been growing interest in the use of near-infrared (NIR) light, which is one of the best strategies for acquiring high-resolution pictures of deep tissues as NIR light diffracts much less than visible light.31-33 For the choice of the NIR luminescent lanthanide ions, we selected Nd3þ and Yb3þ ions, for the following reasons. First, Nd-containing systems have been regarded as the most popular infrared luminescent materials for application in laser systems (the basis of the common 1064 nm laser).34,35 Furthermore, the emission band at 1337 nm of Nd-based materials is suitable for the optical amplifiers operating at 1.3 μm.36,37 Second, for in vivo imaging, it is necessary that the emitted light be at a wavelength where biological tissue is transparent; for example, the Yb3þ ion emission occurs in the NIR region (∼1000 nm) where biological tissues and fluids (e.g., blood) are relatively transparent. Thus, the development of Yb3þ ion luminescence for various analytical and chemosensor applications is promising.38-40 To the best of our knowledge, the syntheses and characterization of magnetic mesoporous silica nanospheres covalently bonded with nearinfrared (NIR) luminescent lanthanide complexes have not been explored in the open literature so far. Herein, we report the synthesis of novel Ln(DBM)3phen-MMS (Ln = Nd, Yb) nanocomposites, in which Fe3O4 magnetic particles were embedded in the mesoporous silica nanospheres and Ln(DBM)3phen complexes were covalently bonded to the framework of mesoporous silica based on the modified 1,10-phenanthroline (phen-Si). Full characterization and detailed studies of magnetic and NIR luminescent properties of these novel nanocomposites were investigated and discussed. The synthetic protocol is represented in Scheme 1. (21) Weissman, S. I. J. Chem. Phys. 1942, 10, 214. (22) Sato, S.; Wada, M. Bull. Chem. Soc. Jpn. 1970, 43, 1955. (23) Sabbatini, N.; Guardingli, M.; Lehn, J. M. Coord. Chem. Rev. 1993, 123, 201. (24) Bekiari, V.; Lianos, P. Adv. Mater. 1998, 10, 1455. (25) De Sa, G. F.; Malta, O. L.; De Mello Donega, C.; Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; Da Silva, E. F., Jr. Coord. Chem. Rev. 2000, 196, 165. (26) Reisfeld, R. Struct. Bonding (Berlin) 2004, 106, 209. (27) Carlos, L. D.; Ferreira, R. A. S.; Bermudez, V. D.; Ribeiro, S. J. L. Adv. Mater. 2009, 21, 509. (28) Binnemans, K.; Gorller-Walrand, C. Chem. Rev. 2002, 102, 2303. (29) Comby, S.; B€unzli, J.-C. G. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., B€unzli, J.-C. G., Jr, Pecharsky, V. K., Eds.; Elsevier: New York, 2007; pp 217-254. (30) Binnemans, K. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., B€unzli, J.-C. G., Jr., Pecharsky, V. K., Eds.; Elsevier: Amsterdam, The Netherlands, 2005; pp 107-272. (31) Kim, S.; Lim, Y. T.; Soltesz, E. G.; De Grand, A. M.; Lee, J.; Nakayama, A.; Parker, J. A.; Mihaljevic, T.; Laurence, R. G.; Dor, D. M.; Cohn, L. H.; Bawendi, M. G.; Frangioni, J. V. Nat. Biotechnol. 2004, 22, 93. (32) Zhang, J.; Badger, P. D.; Geib, S. J.; Petoud, S. Angew. Chem., Int. Ed. 2005, 44, 2508. (33) Sun, L. N.; Zhang, H. J.; Yu, J. B.; Yu, S. Y.; Peng, C. Y.; Dang, S.; Guo, X. M.; Feng, J. Langmuir 2008, 24, 5500. (34) Weber, M. J. ACS Symp. Ser. 1980, 131, 275. (35) Ryo, M.; Wada, Y.; Okubo, T.; Hasegawa, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 11302. (36) Klink, S. I.; Alink, P. O.; Grave, L.; Peters, F. G. A.; Hofstraat, J. W.; Geurts, F.; Van Veggel, F. C. J. M. J. Chem. Soc., Perkin Trans. 2 2001, 363. (37) Lai, W. P.-W.; Wong, W.-T. New J. Chem. 2000, 24, 943. (38) Horrocks, W. D., Jr.; Bolender, J. P.; Smith, W. D.; Supkowski, R. M. J. Am. Chem. Soc. 1997, 119, 5972. (39) Shavaleev, N. M.; Pope, S. J. A.; Bell, Z. R.; Faulkner, S.; Ward, M. D. Dalton Trans. 2003, 808. (40) Davies, G. M.; Aarons, R. J.; Motson, G. R.; Jefferym, J. C.; Adams, H.; Faulkner, S.; Ward, M. D. J. Chem. Soc., Dalton Trans. 2004, 1136.

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Article Scheme 1. Synthesis of Ln(DBM)3phen-MMS (Ln = Nd, Yb) Nanospheres

Experimental Section Materials. FeCl3 3 6H2O (99%), FeCl2 3 7H2O (99%), an ammonia solution (NH3 3 H2O, 25-28%) 1,10-phenanthroline monohydrate (phen 3 H2O, 99%,), cetyltrimethylammonium bromide (CTAB, 99%), fuming nitric acid, and anhydrous ethanol were purchased from Beijing Fine Chemical Co. and used as received. Tetraethoxysilane (TEOS, 98%) was purchased from Aldrich, and 3-(triethoxysilyl)propyl isocyanate was purchased from TCI. The solvent chloroform (CHCl3) was used after desiccation with anhydrous calcium chloride. Neodymium oxide (Nd2O3, 99.99%) and ytterbium oxide (Yb2O3, 99.99%) were purchased from Yue Long Chemical Plant (Shanghai, China). The LnCl3 (Ln = Nd, Yb) ethanol solution was prepared as follows. Ln2O3 (Ln = Nd, Yb) was dissolved in concentrated hydrochloric acid (HCl), and the surplus HCl was removed by evaporation. The residue was dissolved in anhydrous ethanol. The concentration of the Ln3þ (Ln = Nd, Yb) ion was determined by titration with a standard ethylenediaminetetraacetic acid (EDTA) aqueous solution. Synthesis of Phen-Functionalized Magnetic Mesoporous Silica Nanospheres (denoted as phen-MMS). Phen-Si was synthesized by the reaction of phen-NH2 and 3-(triethoxysilyl)propyl isocyanate in CHCl3 according to the procedure described in the literature.41 Oleic acid-modified Fe3O4 nanoparticles were prepared according to the literature.42 Then, the phen-MMS nanospheres were synthesized as follows: 3.0 mL of Fe3O4 nanoparticles dispersed in CHCl3 (10 mg mL-1) was added to 10 mL of an aqueous solution containing 0.2 g of CTAB. A homogeneous oil-in-water microemulsion was obtained after vigorous stirring. The resulting solution was heated at 65 °C for 20 min to evaporate the CHCl3, resulting in a black Fe3O4/CTAB solution; 0.05 g of CTAB and 0.7 mL of NaOH (2 M) were dissolved in 86 mL of water and stirred at room temperature. Then 10 mL of the Fe3O4/CTAB solution described above was added to the system and the mixture heated to 80 °C. After that, 1.35 mL of TEOS and 0.085 g of phen-Si were added to the reaction solution while it was being stirred vigorously. The solution was stirred for 2 h. The product was collected, washed with deionized water several times, and dried at room temperature. To remove the CTAB, 80 μL of HCl was added to the dispersion of the product in ethanol (40 mL, pH ∼1.4) and the mixture stirred for 3 h at 60 °C. (41) Li, H. R.; Lin, J.; Zhang, H. J.; Fu, L. S.; Meng, Q. G.; Wang, S. B. Chem. Mater. 2002, 14, 3651. (42) Xu, H.; Cui, L. L.; Tong, N. H.; Gu, H. C. J. Am. Chem. Soc. 2006, 128, 15582.

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Synthesis of Magnetic Mesoporous Silica Nanospheres Covalently Bonded with Lanthanide Complexes [denoted as Ln(DBM)3phen-MMS (Ln = Nd, Yb)]. The Ln(DBM)3(H2O)2 (Ln = Nd, Yb) complex was prepared according to our previous report.43 Phen-MMS was soaked in an excess of the Ln(DBM)3(H2O)2 (Ln = Nd, Yb) ethanol solution while being stirred. The mixture was refluxed for 6 h and recovered by filtration. The resulting products were washed with ethanol and acetone completely to remove the excess Ln(DBM)3(H2O)2 (Ln = Nd, Yb) complex and dried at room temperature under vacuum for 12 h. Characterization. Fourier transform infrared (FTIR) spectra were recorded within the 4000-400 cm-1 wavenumber range using a Perkin-Elmer model 580B IR spectrophotometer with the KBr pellet technique and operating in the transmittance mode. X-ray diffraction pattern (XRD) measurements were performed on a D8 FOCUS X-ray diffractometer (Bruker) using Cu KR radiation (λ = 1.5416 A˚) at 40 kV and 40 mA. Field-emission scanning electron microscopy (FE-SEM) images were obtained with a Hitachi S-4800 microscope. Transmission electron microscopy (TEM) images were characterized with a Tecnai G2 (FEI) transmission electron microscope with an accelerating voltage of 200 kV. Nitrogen (N2) adsorption-desorption isotherms were measured by using a Nova 1000 analyzer with nitrogen. The samples were outgassed for 4 h at 120 °C before the measurements. Specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method and pore sizes by the Barrett-Joyner-Halenda (BJH) methods. Magnetic measurements were performed using a Quantum Design MPMS-XL SQUID magnetometer equipped with a 7 T magnet. The luminescence excitation and emission spectra were recorded with a HORIBA Jobin Yvon FluoroLog-3 spectrofluorometer equipped with a 450 W Xe lamp as an excitation source and a liquid nitrogen-cooled R5509-72 PMT as a detector. The time-resolved measurements were performed by using the third harmonic (355 nm) of a Spectra-physics Nd:YAG laser with a 5 ns pulse width and 5 mJ of energy per pulse as a source; the NIR emission lines were dispersed by the emission monochromator of the HORIBA Jobin Yvon FluoroLog-3 instrument equipped with a liquid nitrogen-cooled R5509-72 PMT, and the data were analyzed with a LeCroy WaveRunner 6100 1 GHz oscilloscope. The luminescence lifetimes were calculated with Origin version 7.0. All the measurements were taken at room temperature.

Results and Discussion FE-SEM and TEM images of phen-MMS and Ln(DBM)3phenMMS (Ln = Nd, Yb) nanocomposites are shown in Figure 1. It can be seen that the obtained nanospheres are uniform in size. Particles with a diameter ranging from 80 to 130 nm could be observed from FE-SEM and TEM images. The TEM image of the Yb(DBM)3phen-MMS nanocomposite is presented as an example. It can be seen that Fe3O4 nanoparticles are successfully embedded in the final sample, and mesoporous silicas have two-dimensional hexagonal mesostructures. Figure 2 shows the FTIR spectra of as-synthesized phen-MMS and surfactant-extracted phen-MMS. The formation of the Si-O-Si framework is evidenced by the bands located at 1065 and 453 cm-1 and at 1078 and 458 cm-1 in parts a and b of Figure 2, respectively.44 The peaks at 1654 and 1541 cm-1 in Figure 2a and 1636 and 1542 cm-1 in Figure 2b, originating from the CONH group of phen-Si, can be observed, which is consistent with the fact that the phen group in the Si-O-Si framework remains intact after both the hydrolysis-condensation reaction (43) Sun, L. N.; Zhang, H. J.; Peng, C. Y.; Yu, J. B.; Meng, Q. G.; Fu, L. S.; Liu, F. Y.; Guo, X. M. J. Phys. Chem. B 2006, 110, 7249. (44) Peng, C. Y.; Zhang, H. J.; Yu, J. B.; Meng, Q. G.; Fu, L. S.; Li, H. R.; Sun, L. N.; Guo, X. M. J. Phys. Chem. B 2005, 109, 15278.

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Figure 1. SEM images of phen-MMS (a), Nd(DBM)3phen-MMS (b), and Yb(DBM)3phen-MMS (c) nanospheres and (d) TEM image of Yb(DBM)3phen-MMS nanospheres.

Figure 2. FTIR spectra of as-synthesized phen-MMS (a) and surfactant-extracted phen-MMS (b).

and the surfactant extraction. In Figure 2b, the peaks in the range of 2800-3000 cm-1, originating from the asymmetric strenching vibrations of -CH3 and -CH2 aliphatic moieties of the surfactant cations,45 become very weak, which confirms that the surfactant has been removed. More importantly, it can be seen from the TEM image that Fe3O4 particles have been well-preserved after the surfactant extraction with acidified ethanol (pH ∼1.4).6 Figure 3 shows the XRD patterns of phen-MMS and Ln(DBM)3phen-MMS (Ln = Nd, Yb) nanocomposites. In the wide-angle XRD pattern of phen-MMS (Figure 3, inset), the characteristic diffraction peaks of Fe3O4 with a space group Fd3m structure are clearly observed, which is in agreement with TEM observations. In the small-angle range, all samples show a 2θ = 2.0° reflection, which can be assigned to the (100) reflection of a two-dimensional hexagonal (P6mm) mesostrcture, like that exhibited by a MCM-41 type material.9,46 It can be seen that (45) Wong, T. C.; Wong, N. B.; Tanner, P. A. J. Colloid Interface Sci. 1997, 186, 325. (46) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834.

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Figure 3. Small-angle XRD patterns of surfactant-extracted phenMMS (a), Nd(DBM)3phen-MMS (b), and Yb(DBM)3phen-MMS (c). The inset shows the wide-angle XRD pattern of surfactantextracted phen-MMS.

Figure 4. N2 adsorption-desorption isotherms (inset, pore size distribution) of surfactant-extracted phen-MMS (a), Nd(DBM)3phen-MMS (b), and Yb(DBM)3phen-MMS (c).

the uniform mesoporous structures of the nanocomposites are quite maintained after the extraction procedure and the introduction of lanthanide complexes. N2 adsorption-desorption isotherms were recorded to assess the textural properties of these nanocomposites (Figure 4). The isotherms of the samples display type IV isotherms according to the IUPAC classification.47 The specific area and the pore size are calculated by the Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) model, respectively. The samples display a narrow pore size distribution (Figure 4, inset). The textural parameters of these samples are listed in Table 1. As expected, the surface area, pore volume, and pore size of the Ln(DBM)3phen-MMS (Ln = Nd, Yb) nanocomposites decrease considerably after introduction of the Ln(DBM)3(H2O)2 (Ln = Nd, Yb) complexes into the phen-MMS, which is consistent with the presence of anchored Ln(DBM)3phen (Ln = Nd, Yb) in the pore channels of Ln(DBM)3phen-MMS (Ln = Nd, Yb) nanocomposites. The magnetization curves of the obtained nanocomposites registered at 300 K show no detectable hysteresis loop (Figure 5), which means that the nanocomposites exhibited the superparamagnetic characteristics. The saturation magnetization values of phen-MMS, Nd(DBM)3phen-MMS, and Yb(DBM)3phen-MMS are 4.31, 3.19, and 3.46 emu/g, respectively. These nanocomposites (47) Everett, D. H. Pure Appl. Chem. 1972, 31, 577.

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with superparamagnetic characteristics and high magnetization values can quickly respond to the external magnetic field and quickly redisperse once the external magnetic field is removed. This is advantageous for targeted drug delivery and bioseparation applications.48 We measured the NIR luminescence behavior of the Ln(DBM)3phen-MMS (Ln = Nd, Yb) nanocomposites. The excitation and emission spectra of Ln(DBM)3phen-MMS (Ln = Nd, Yb) nanocomposites are presented in Figures 6 and 7, respectively. In the excitation spectrum of Nd(DBM)3phenMMS (Figure 6), the broad excitation band in the UV region can be assigned to the absorption of the organic ligands. The excitation edge of the organic ligands overlaps some absorption bands corresponding to the characteristic transition of the Nd3þ ion. These f-f transitions correspond to 4I9/2 f 2G9/2 þ 2 K13/2 (524 nm) and 4I9/2 f 2G7/2 (580 nm) transitions, respectively. In the emission spectrum of Nd(DBM)3phen-MMS, the typical emission lines for Nd3þ ion at 907 (4F3/2 f 4I9/2), 1067 (4F3/2 f 4I11/2), and 1337 nm (4F3/2 f 4I13/2) are presented. The emission line at 1067 nm (4F3/2 f 4I11/2) is the strongest, which is potentially applicable for laser systems.34,35 Moreover, the emission band at 1337 nm offers the opportunity to develop new materials that are suitable for the optical amplifiers operating at 1.3 μm.36,37 For Yb(DBM)3phen-MMS nanocomposites, after ligandmediated excitation, the intense NIR luminescence of Yb(DBM)3phen-MMS can be observed (Figure 7). The Yb3þ ion emits in the range of 910-1160 nm, with a sharp peak around 980 nm assigned to the 2F5/2 f 2F7/2 transition of the Yb3þ ion broader vibronic components at longer wavelength.49 This may be the splitting of the energy levels of Yb3þ ion as a consequence of ligand field effects.50 The Yb3þ ion plays an important role in laser emission because of its very simple f-f energy level structure. There is no excited-state absorption upon reduction of the effective laser cross section, no up-conversion, no concentration quenching, and no absorption in the visible range. The intense Yb3þ ion absorption lines are well suited for laser diode pumping in this range, and the smaller Stokes shift between absorption and emission reduces the thermal loading of the material during laser operation.51 These properties of the Yb3þ ion and the obtained emission make Yb(DBM)3phen-MMS very important for various photonic applications in ionic crystals and glasses.52 More importantly, the relative transparency of human tissue at approximately 1000 nm suggests that in vivo luminescent probes operating at this wavelength of Yb-based emission could have diagnostic value.40 The Yb(DBM)3phen-MMS nanocomposite is a potential material for applications in drug delivery or optical imaging, which has the following advantages. (1) Mesoporous silica with a nontoxic and biocompatible nature, a large surface area, a large pore volume, and a tunable pore diameter with abundant Si-OH bonds on the pore surface is a promising candidate for use as a carrier in drug delivery system. (2) NIR emission at 980 nm from Yb(DBM)3phen-MMS is critical for in vivo optical imaging since the blood and tissues are relatively transparent in the range of 700-1000 nm, thereby minimizing complications resulting from (48) Zhang, L.; Qiao, S. Z.; Jin, Y. G.; Yang, H. G.; Budihartono, S.; Stahr, F.; Yan, Z. F.; Wang, X. L.; Hao, Z. P.; Lu, G. Q. Adv. Funct. Mater. 2008, 18, 3203. (49) Comby, S.; Imbert, D.; Vandevyver, C.; B€unzli, J.-C. G. Chem.;Eur. J. 2007, 13, 936. (50) Wolbers, M. P. O.; Van Veggel, F. C. J. M.; Snellink-Ru€el, B. H. M.; Hofstraat, J. W.; Geurts, F. A. J.; Reinhoudt, D. N. J. Chem. Soc., Perkin Trans. 2 1998, 2141. (51) Boulon, G.; Collombet, A.; Brenier, A.; Cohen-Adad, M. T.; Yoshikawa, A.; Lebbou, K.; Lee, J. H.; Fukuda, T. Adv. Funct. Mater. 2001, 11, 263. (52) Reinhard, C.; G€udel, H. U. Inorg. Chem. 2002, 41, 1048.

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Feng et al. Table 1. Textural Parameters of Phen-MMS, Nd(DBM)3phen-MMS, and Yb(DBM)3phen-MMS Samplesa sample

d100 (nm)

a0 (nm)

SBET (m2 g-1)

V (cm3 g-1)

D (nm)

hw (nm)

phen-MMS 4.42 5.10 1364 1.58 2.80 2.30 4.47 5.16 847 0.91 2.56 2.60 Nd(DBM)3phen-MMS 4.47 5.16 826 0.92√ 2.55 2.61 Yb(DBM)3phen-MMS a d100 is the d(100) spacing (2d sin θ = kλ, where k = 1 and λ = 1.5416 A˚), a0 the cell parameter (a0 = 2d100/ 3), SBET the BET surface area, V the total pore volume, D the average pore diameter calculated using the BJH method, and hw the wall thickness, calculated by a0 - D.

Figure 5. Magnetization curves of surfactant-extracted phenMMS (a), Nd(DBM)3phen-MMS (b), and Yb(DBM)3phenMMS (c) at 300 K.

Figure 7. Excitation (EX) and emission (EM) spectra of Yb(DBM)3phen-MMS.

of 355 nm and monitored around the most intense emission line of their corresponding emission spectra. The decay curves for Nd(DBM)3phen-MMS and Yb(DBM)3phen-MMS nanocomposites are fitted by double-exponential functions. The lifetimes of the 4F3/2 f 4I11/2 transition of Nd(DBM)3phen-MMS are 0.75 (99.90%) and 0.08 μs (0.10%). The lifetimes of the 2F5/2 f 2 F7/2 transition of Yb(DBM)3phen-MMS are 0.66 (17.49%) and 3.88 μs (82.51%).

Conclusions

Figure 6. Excitation (EX) and emission (EM) spectra of Nd(DBM)3phen-MMS.

intrinsic background interference.53 (3) Yb3þ ion emits relative intense NIR luminescence compared to the other lanthanide ions. (4) It can be guided to target sites by means of an external magnetic field. Therefore, we suggest that the Yb(DBM)3phenMMS nanocomposite may have great potential in drug delivery or optical imaging. Furthermore, time-resolved measurements on Nd(DBM)3phenMMS and Yb(DBM)3phen-MMS nanocomposites were conducted at room temperature by using an excitation wavelength (53) Li, Z. B.; Cai, W.; Chen, X. J. Nanosci. Nanotechnol. 2007, 7, 2567.

3600 DOI: 10.1021/la903008z

In conclusion, we have combined the advantages of mesoporous silica, magnetic Fe3O4 particles, and NIR luminescent lanthanide complexes to fabricate two novel nanocomposites [Nd(DBM)3phen-MMS and Yb(DBM)3phen-MMS] with a large surface area, a large pore volume, a well-defined pore size, magnetic separability, and NIR luminescent properties. The Nd(DBM)3phen-MMS nanocomposite may have potential applications for laser systems or the optical amplifiers operating at 1.3 μm, and the Yb(DBM)3phen-MMS nanocomposites have several advantages for potential applications in drug delivery or optical imaging. Furthermore, this technique could be extended to the other lanthanide complexes that display visible or NIR luminescence, thus opening a door for the development of new multifunctional nanocomposites possessing magnetic, luminescent properties and mesostructures. This work is currently being conducted in our group. Acknowledgment. We are grateful for the financial aid from the National Natural Science Foundation of China (Grants 20631040 and 20771099) and the MOST of China (Grants 2006CB601103 and 2006DFA42610).

Langmuir 2010, 26(5), 3596–3600