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Langmuir 2007, 23, 7836-7840
Bifunctional Magnetic-Optical Nanocomposites: Grafting Lanthanide Complex onto Core-Shell Magnetic Silica Nanoarchitecture Shi-Yong Yu,†,‡ Hong-Jie Zhang,*,† Jiang-Bo Yu,† Cheng Wang,† Li-Ning Sun,†,‡ and Wei-Dong Shi†,‡ Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China, and Graduate School of the Chinese Academy of Sciences, Beijing, PR China ReceiVed March 13, 2007. In Final Form: April 23, 2007 A new class of bifunctional architecture combining the useful functions of superparamagnetism and terbium complex luminescence into one material has been prepared via two main steps by a modified Sto¨ber method and the layerby-layer (LbL) assembly technique. The obtained bifunctional nanocomposites exhibit superparamagnetic behavior, high fluorescence intensity, and color purity. The architecture has been characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), UV-vis absorption and emission spectroscopy, X-ray diffraction, and superconducting quantum interference device (SQUID) magnetometry.
Introduction Magnetic and optical materials are of great importance in the fields of chemistry, biology, and medical sciences as well as in biotechnology.1-3 Magnetic nanoparticles have widely been studied for biomedical applications,4 such as MRI contrast enhancement,5 magnetic immobilization,6 and drug targeting,7 whereas high-photostability fluorescent nanomaterials have attracted much attention in the field of biological labeling.8-10 The combination of magnetic and fluorescent properties of different materials into a single micro- or nanocomposite system might greatly enhance their applications in the biomedical and biopharmaceutical fields.11 For applications involving magnetic delivery or separation, superparamagnetic properties are more desirable than ferromagnetism because there could be no residual magnetism after the removal of the magnetic field. Among optical materials investigated for such composites, organic fluorescent compounds12 and quantum dots (QDs)13,14 are the most widely used as fluorescent labels although both of them have inherent limitations. For example, organic fluorescent compounds typically * To whom correspondence should be addressed. E-mail: hongjie@ ns.ciac.jl.cn. Tel: +86-431-85262127. Fax: +86-431-85698041. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. (1) Oh, B. K.; Park, S.; Millstone, J. E.; Lee, S. W.; Lee, K. B.; Mirkin, C. A. J. Am. Chem. Soc. 2006, 128, 11825. (2) Gunnarsson, K.; Roy, P. E.; Felton, S.; Pihl, J.; Svedlindh, P.; Berner, S.; Lidbaum, H.; Oscarsson, S. AdV. Mater. 2005, 17, 1730. (3) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (4) Meldrum, F. C.; Heywood, B. R.; Mann, S. Science 1992, 257, 522. (5) Tanaka, T.; Matsunaga, T. Anal. Chem. 2000, 72, 3518. (6) Wuang, S. C.; Neoh, K. G.; Kang, E. T.; Pack, D. W.; Leckband, D. E. AdV. Funct. Mater. 2006, 16, 1723. (7) Nasongkla, N.; Bey, E.; Ren, J.; Ai, H.; Khemtong, C.; Guthi, J. S.; Chin, S. F.; Sherry, A. D.; Boothman, D. A.; Gao, J. Nano Lett. 2006, 6, 2427. (8) Louis, C.; Bazzi, R.; Marquette, C. A.; Bridot, J. L.; Roux, S.; Ledoux, G.; Mercier, B.; Blum, L.; Perriat, P.; Tillement, O. Chem. Mater. 2005, 17, 1673. (9) Yin, X. B.; Qi, B.; Sun, X. P.; Yang, X. R.; Wang, E. K. Anal. Chem. 2005, 77, 3525. (10) Jiao, G. S.; Thoresen, L. H.; Burgess, K. J. Am. Chem. Soc. 2003, 125, 14668. (11) Kim, H.; Achermann, M.; Balet, L. P.; Hollingsworth, J. A.; Klimov, V. I. J. Am. Chem. Soc. 2005, 127, 544. (12) Lu, Y.; Yin, Y. D.; Mayers, B. T.; Xia, Y. N. Nano Lett. 2002, 2, 183. (13) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D. J.; Ying, Y. J. Am. Chem. Soc. 2005, 127, 4990. (14) Gu, H.; Zheng, R.; Zhang, X. X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 5664.
undergo rapid photobleaching15 whereas QDs are less chemically stable and are potentially toxic16 and show fluorescence intermittence.17,18 Such problems would hinder their applications from biological detections and medical diagnosis. Therefore, it is essentially necessary to find substitutes for current luminescently labeled materials. Lanthanide ions have been well known for their unique luminescence properties.19-25 Also, their complexes show narrow band photoluminescence and have a high luminescence quantum yield, which makes them interesting candidates for luminescence applications such as biochemical sensors and fluoroimmunoassays.26 To make magnetic-optical materials with high selectivity, good sensitivity, and manipulation capability, we introduced superparamagnetism Fe3O4 nanoparticles to couple with the lanthanide complexes. The method developed here is simple and straightforward and could be described as the grafting of a lanthanide complex through covalent bonding onto the silica shell of preformed core-shell magnetic silica microspheres. Because the terbium ion (Tb3+) is one of the most strongly emitting ions in lanthanide ions, the following studies will focus on the system incorporating the Tb complex onto magnetic silica nanoparticles as a result of its several distinct advantages: (i) Tb3+ is resistant to photobleaching, nontoxic, biocompatible, monochromatic, highly luminescent, and, most importantly, ultrasensitive both in in vitro and in vivo bioassays, so it could (15) Schrum, K. F.; Lancaster, J. M.; Johnston, S. E.; Gilman, S. D. Anal. Chem. 2000, 72, 4317. (16) Brokmann, X.; Hermier, J. P.; Desbiolles, P.; Bouchaud, J. P.; Dahan, M. Phys. ReV. Lett. 2003, 90, 120601. (17) Hohng, S.; Ha, T. J. Am. Chem. Soc. 2004, 126, 1324. (18) Fischer, H. C.; Liu, L. C.; Pang, K. S.; Chan, W. C. W. AdV. Funct. Mater. 2006, 16, 1299. (19) Heer , S.; Ko¨mpe, K.; Gudel, H. U.; Haase, M. AdV. Mater. 2004, 16, 2102. (20) Lehmann, O.; Ko¨mpe, K.; Haase, M. J. Am. Chem. Soc. 2004, 126, 14935. (21) Sa´Ferreira, R. A.; Carlos, L. D.; Goncu¨alves, R. R.; Ribeiro, S. J. L.; Bermudez, V. Z. Chem. Mater. 2001, 13, 2991. (22) Fu, L. S.; Sa´, Ferreira, R. A.; Silva, N. J. O.; Carlos, L. D.; Bermudez, V. Z.; Rocha, J. Chem. Mater. 2004, 16, 1507. (23) Nogami, M.; Suzuki, K. AdV. Mater. 2002, 14, 923. (24) Chengelis, D. A.; Yingling, A. M.; Badger, P. D.; Shade, C. M.; Petoud, S. J. Am. Chem. Soc. 2005, 127, 16752. (25) Petoud, S.; Muller, G.; Moore, E. G.; Xu, J.; Sokolnicki, J.; Riehl, J. P.; Le, U. N.; Cohen, S. M.; Raymond, K. N. J. Am. Chem. Soc. 2007, 129, 77. (26) Wolfbeis, O. S.; Durkop, A.; Wu, M.; Lin, Z. H. Angew. Chem., Int. Ed. 2002, 41, 4495.
10.1021/la700735m CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007
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Scheme 1. Synthesis Procedure to Obtain Bifunctional Magnetic-Optical Nanoparticles
serve as a more efficient biological label than traditional organic fluorescent compounds or QDs.27 (ii) The luminescence of Tb3+ is characterized by narrow emission bandwidths independent of particle size. (iii) The silanol groups on the surface of the composites make further functionalization possible. The bifunctional nanocomposite particles were prepared via three consecutive steps by a modified Sto¨ber method combined with the layer-by-layer (LbL) assembly technique. The first step involved the controlled addition of tetraethoxysilane (TEOS) to an aqueous dispersion of Fe3O4 nanoparticles in order to cover the nanoparitcles with silica layers. The resulting Fe3O4 nanoparticle/silica core-shell nanoparticles were designated as Fe3O4@SiO2. The second step was to graft the Tb-PABI (PABI ) N-(4-benzoic acid-yl),N′-(propyltriethoxysilyl) urea) complex onto the surface of Fe3O4@SiO2 core-shell nanospheres. The last step was to coat an extra silica shell on the bifunctional nanoparticles with similar targets as described in step one (Scheme 1C). The purpose of coating with an outer layer of silica is to improve the wettability and biocompatibility of bifunctional nanoparticles further. The whole procedure is depicted in Scheme 1. Experimental Section All chemicals (analytical grade) were purchased from Beijing Chemicals Corp., China, and used as received without further purification. Synthesis of Fe3O4 Nanoparticles. Monodisperse magnetite nanoparticles were prepared according to the method reported28 with minor modification, which was based on the coprecipitation of ferrous and ferric ion solutions (1:2 molar ratio). Ten milliliters of aqueous 1 M FeCl3 and 2.5 mL of 2 M FeSO4‚7H2O in 2 M HCl were added to 125 mL of 0.7 M NH3‚H2O (28%) under rapid mechanical stirring. A black precipitate formed quickly, and stirring was continued for 30 min. The product was collected from the suspension by centrifugation and washed with distilled water three times. It was then redispersed into 100 mL of distilled water. Synthesis of Silica-Coated Magnetite Particles (Fe3O4@SiO2). An ethanol solution of TEOS (98%) (1 mL of TEOS in 30 mL of EtOH) was added to a mixture of 3 mL NH3‚H2O (28%), 38 mL of H2O, 38 mL of EtOH, and 10 mL of the previously washed magnetic nanoparticles in aqueous solution under mechanical stirring. The hydrolysis and condensation of TEOS onto the magnetic (27) Wang, L. Y.; Yan, R. X.; Huo, Z. Y.; Wang, L.; Zeng, J. H.; Bao, J.; Wang, X.; Peng, Q.; Li, Y. D. Angew. Chem., Int. Ed. 2005, 44, 6054. (28) Jiang, W. Q.; Yang, H. C.; Yang, S. Y.; Horng, H. E.; Hung, J. C.; Chene, Y. C.; Hongd, C. Y. J. Magn. Magn. Mater. 2004, 283, 210.
nanoparticles were completed within 4 h. The precipitates were collected by centrifugation, washed with water and ethanol three times, and redispersed in 250 mL of ethanol. Synthesis of N-(4-Benzoic acid-yl),N′-(propyltriethoxysilyl) Urea (PABI). PABI was synthesized using the procedure described in a previous report.29 A brief description is as follows: 5 mmol of p-aminobenzoic acid was dissolved in 15 mL of dry tetrahydrofuran (THF). To this solution, 7.5 mmol of ICPTES was then added. The resultant solution was refluxed for 8 h. A white precipitate was observed when 50 mL of dry petroleum ether was added to the above solution. The precipitates were filtered and washed with petroleum ether to remove excess ICPTES. Finally, the collected precipitates were dried at 60 °C under vacuum for 6 h. Covalent Immobilization of Tb-PABI onto Silica-Coated Magnetite Particle Surfaces (Fe3O4@SiO2@PABI-Tb). Fifty milligrams of PABI dissolved in 10 mL of chloroform was added to 100 mL of an ethanol solution containing 25 mL of the previously washed silica-coated magnetite nanoparticle (Fe3O4@SiO2) under vigorous stirring under room temperature; then the temperature was increased to 80°C to initiate the reaction, and the mixture was held at this temperature for 2 h. After reaction, the PABI-modified silicacoated magnetite nanospheres (Fe3O4@SiO2@PABI) were obtained by centrifugation and were washed with chloroform and ethanol three times to remove excess PABI molecules. The purified Fe3O4@SiO2@PABI was redispersed into 100 mL of ethanol. Then, an excess solution of TbCl3 (10 mM in ethanol) was added to the above solution with vigorous stirring and was allowed to react at 80 °C for 6 h. The resultant Fe3O4@SiO2@PABI-Tb nanocomposites were purified by further centrifuging and washing with ethanol three times. Synthesis of Silica-Coated Fe3O4@SiO2@PABI-Tb Particles. An ethanol solution of TEOS (98%) (0.3 mL of TEOS in 30 mL of EtOH) was added to a mixture of 2 mL of NH3‚H2O (28%), 10 mL of H2O, 10 mL of EtOH, and 10 mL of the previously obtained Fe3O4@SiO2@PABI-Tb nanoparticles in aqueous solution under mechanical stirring. The hydrolysis and condensation of TEOS onto the Fe3O4@SiO2@PABI-Tb nanoparticles were completed at room temperature within 4 h. The precipitates were collected by centrifugation, washed with water and ethanol three times, and redispersed in 100 mL of ethanol. Characterization. The morphologies and structures of the assynthesized products were characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). All of the samples for the FESEM and TEM characterization were prepared by directly transferring the suspended products onto the ITO glass slide and a standard copper grid coated with an (29) Liu, F. Y.; Fu, L. S.; Wang, J.; Liu, Z.; Li, H. R.; Zhang, H. J. Thin Solid Films 2002, 419, 178.
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Figure 1. (a) SEM and (b, c) TEM images of obtain bifunctional magnetic-optical nanoparticles amorphous carbon film, respectively. FESEM analysis was conducted on a Philips XL-30 field-emission scanning electron microscope operated at 15 kV, whereas TEM observation was carried out on a JEOL-JEM-2010 at 200 kV. Magnetite was characterized by X-ray diffraction (XRD) using a Rigaku-D/max 2500 V X-ray diffractometer equipped with a Cu KR radiation source (λ ) 1.54178 Å) at a step width of 0.02°. The photoluminescence spectra of the samples were measured in aqueous solution (0.02 wt %) using 1 cm quartz cuvettes on a Hitachi 4500 fluorescence spectrophotometer at room temperature. The quantum yield was measured by an integrating sphere (Power Technology, Inc.) using a 325 nm HeCd laser (Kimmom, Japan) as the excitation source. Luminescence lifetimes were measured with a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a 290 nm laser (pulse width of 4 ns) as the excitation source (Continuum Sunlite OPO). The magnetic characterizations of Fe3O4@SiO2@PABI-Tb nanocomposites were performed by using a superconducting quantum interference device (SQUID) magnetometer with fields of up to 5 T. All measurements were performed at room temperature.
Results and Discussion The FESEM and TEM images of bifunctional magneticoptical nanoparticles are shown in Figure 1. Particles with diameter ranging from 120 to 160 nm could be observed from the FESEM picture (Figure 1a). The presence of core Fe3O4 nanoparticles is confirmed by the TEM image (Figure 1b,c), which shows obvious difference in terms of contrast between the core and its surroundings. However, because of the aggregation of Fe3O4 nanoparticles prior to or during the coating process, more than one magnetic nanoparticle (Figure 1b) was trapped in the composites, which is advantageous when manipulating the core-shell nanoparticles with an external magnetic field. Uniformly sized 3-5 nm magnetite nanoparticles were prepared following the reported method28 with minor modifications as shown in the Experimental Section. Such magnetite nanoparticles could be dispersed in a mixture of distilled water and ethanol. This mixed solvent facilitates the subsequent wrapping process where magnetic nanoparticels were covered by silica spheres using a sol-gel method based on the hydrolysis and condensation of TEOS, the well-known Sto¨ber method.30 The advantages of encapsulating Fe3O4 nanoparticles within silica include compatibility in biological systems,31 functionality, and high colloidal stability under different conditions. Moreover, the surface silanol (30) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (31) Yoon, T. J.; Kim, J. S.; Kim, B. G.; Yu, K. N.; Cho, M. H.; Lee, J. K. Angew. Chem., Int. Ed. 2005, 44, 1068.
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Figure 2. IR spectra of (a) Fe3O4@SiO2 nanocomposites, (b) PABI molecules, and (c) Fe3O4@SiO2@PABI-Tb nanocomposites.
groups could easily react with a silane coupling agent, in our case, the PABI-Tb complex, to yield bifunctional nanocomposites designated as Fe3O4@SiO2@PABI-Tb. (Scheme 1B). Unfortunately, this new anchored layer is not discernible by microscopy. The presence of PABI-Tb covalently bonded onto the surface of Fe3O4@SiO2 nanoparticles was therefore characterized by FTIR spectroscopy. Figure 2a-c shows the IR spectra of Fe3O4@SiO2 nanocomposites, PABI molecules, and Fe3O4@SiO2@PABI-Tb nanocomposites, respectively. The corresponding characterized peaks at 576 and 1094 cm-1 prove the presence of Fe3O4 and SiO2 (Figure 2a). The peaks appearing at 1685 and 1292 cm-1 could be assigned to νCdO and νC-O of the -COOH in PABI, respectively, as shown in Figure 2b. The sharp peaks located at 1654 and 1593 cm-1 correspond to the NH-CO-NH group and give strong evidence that isocyanatopropyltriethoxysilane (ICPTES) has been successfully grafted onto p-aminobenzoic acid. Compared with the FTIR spectroscopy of Fe3O4@SiO2 nanoparticles (Figure 2a), the peaks in Figure 2c located at 1652 and 1595 cm-1 corresponding to the NHCO-NH group of PABI appear, which may prove the presence of PABI on the bifunctional nanoparticles. In Figure 2c, the characteristic stretchings of νCdO and νC-O of the carboxylic acid function disappear, and two new peaks appear at 1553 and 1417 cm-1 and are attributed to νas(-COO-) and νs(-COO-) correspondingly, which suggests the formation of coordinated bonds between the carboxylic group and Tb3+. Such results might induce the successful grafting of the PABI-Tb complex onto the surface of magnetic core-shell silica nanoparticles.29,32-35 Direct proof of successful grafting could be found in the following designed experiments. Upon UV light irradiation, welldispersed aqueous Fe3O4@SiO2@PABI-Tb nanoparticles emit bright-green light orignating from the characteristic emission of Tb3+ as shown in the digital photographs of Figure 3b. Besides UV light, the nanocomposites are also inductive to the external magnetic field. When a handheld magnet was placed close to the glass vial, the nanocomposite particles were attracted to the magnet very quickly (Figure 3c). We can easily drag the (32) Fu, L. S.; Ferreira, R. A. S.; Valente, A.; Rocha, J.; Carlos, L. D. Microporous Mesoporous Mater. 2006, 94, 185. (33) Skrovanek, J.; Painter, P. C.; Coleman, M. M. Macromolecules 1986, 19, 699. (34) Zhao, X. S.; Lu, G. Q.; Whittaker, A. K.; Millar, G. J.; Zhu, H. Y. J. Phys. Chem. B 1997, 101, 6525. (35) Lu, L. H.; Liu, F. Y.; Sun, G. Y.; Zhang, H. J.; Xi, S. Q.; Wang, H. S. J. Mater. Chem. 2004, 14, 2760.
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Figure 3. Photographs of the obtained bifunctional magneticoptical nanoparticles before (a, c, e) and after (b, d, f) UV light irradiation. (a, b) Aqueous suspension of the obtained bifunctional magnetic-optical nanoparticles; (c, d) after magnetic capture; and (e, f) movement of the magnet dragging the concentrated nanoparticles.
nanoparticles from the bottom to the middle of the glass vial (Figure 3e) by elevating the magnets. Meanwhile, corresponding bright-green light emissions could be observed at these two positions under UV light irradiation (Figure 3d,f). This observation further proves our successful preparation of bifunctional materials combined by core-shell Fe3O4@SiO2 magnetic materials and grafted Tb-complex optical materials. The magnetic property of Fe3O4@SiO2@PABI-Tb nanocomposites was characterized using a superconducting quantum interference device (SQUID) magnetometer with fields of up to 5 T. The hysteresis loops of the samples were registered at temperatures of 5 and 300 K. The field-dependent magnetization plots (Figure 4a) illustrate that bifunctional magnetic-optical nanocomposites are superparamagnetic at 300 K and hysteretic at 5 K. The magnified view of the magnetization curves at 300 K shows no hysteresis (inset of Figure 4a), which means that the bifunctional nanoparticles exhibit superparamagnetic behavior at room temperature. Such behavior is believed to be derived from the well-dispersed magnetite nanoparticles in the silica spheres. The saturation magnetization of the bifunctional magnetic-optical nanoparticles at room temperature reaches a saturation moment of 7.44 emu/g. This low saturation magnetization is mainly attributed to the decrease in interparticle interactions originating from the increased distance between the magnetic cores and silica shells.36 At 5 K, the bifunctional magnetic-optical nanoparticles exhibit ferromagnetic characteristics, including coercivity (Hc ) 250 Oe; 1 Oe ) 1000/4π Am-1) and remanence. The temperature dependence of the zerofield-cooled and field-cooled (ZFC/FC) magnetization is shown in Figure 4b. Two curves coincide at high temperature and begin to separate as the temperature decreases, with a maximum at 120 K in the ZFC curve, which is a characteristic of superparamagnetism and is due to a progressive deblocking of particles as the temperature increases.37 This kind of magnetic property enables the bifunctional nanoparticles to be used in biomedical applications because they have enough strong magnetization for efficient magnetic separation under an applied external magnetic field. (36) Salgueirin˜o-Maceira, V.; Correa-Duarte, M. A.; Spasova, M.; Liz-Marza´n, L. M.; Farle, M. AdV. Funct. Mater. 2006, 16, 509. (37) Morup, S.; Bodker, F.; Hendriksen, P. V.; Linderoth, S. Phys. ReV. B 1995, 52, 287.
Figure 4. (a) Magnetization-applied magnetic field (M-H) at 300 and 5 K and (b) zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of the obtained bifunctional magnetic-optical nanoparticles at 100 Oe. The inset in plot a is a magnified view of the magnetization curves at low applied fields.
Figure 5. UV-vis absorption spectra of a suspension of the bifunctional nanoparticles before (-) and after (‚‚‚) the removal of bifunctional nanoparticles by magnetic separation.
The UV-vis absorption spectrum of a suspension of the bifunctional nanoparticles is shown as a solid line in Figure 5. The maximum absorption of PABI-Tb is located around λ ) 278 nm,35 which further indicates the formation of bifunctional nanocomposites on the surface of Fe3O4@SiO2 nanoparticles. Surprisingly, when an external handheld magnet is put at the bottom of the suspension, the supernatant solution shows no absorption peak (dotted line in Figure 5) because the nanocomposites precipitated as a result of the presence of the magnetic field. This result demonstrates the magnetic separation characteristics of the bifunctional nanoparticles. Figure 6a shows the excitation spectrum of Fe3O4@SiO2@PABI-Tb nanocomposites obtained by monitoring the Tb3+ ion emission at 544 nm. A maximum absorption peak appears at 307 nm in Figure 6a, which is the characteristic absorption
7840 Langmuir, Vol. 23, No. 14, 2007
Figure 6. (a) Excitation (λem ) 544 nm) and (b) emission spectra (λex ) 307 nm) of bifunctional nanocomposites.
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absorb energy and transfer it efficiently to the Tb3+ ions. In addition, the halfwidth of the strongest band is less than 10 nm, indicating that the obtained Fe3O4@SiO2@PABI-Tb nanocomposites exhibit high fluorescence intensity and color purity. Compared with the quantum yields of the Tb complex, which are reported in refs 40-42,40-42 the bifunctional nanomaterials have a higher quantum yield of 5.6%. The luminescence decay curve of bifunctional nanoparticles is shown in Figure 7. The curve obtained from time-resolved luminescence experiments could be fitted single-exponentially, thus confirming that the chemical environment of Tb3+ is uniform in the materials.43-45 The corresponding lifetime of the bifunctional nanoparticles is determined to be 0.9 ms. To improve the chemical stability further, the composite magnetic luminescent bifunctional nanoparticles were coated with an outer layer of silica (Scheme 1C). The objective for doing so is to improve the wettability and biocompatibility of these bifunctional nanocomposites through such silica shell. Furthermore, this layer of silica shell can also be linked through further surface modification with bioconjugators such as avidin, which is endowed with interesting biofunctionalities.46-48
Conclusions
Figure 7. Luminescence decay curve of the bifunctional materials.
of PABI-Tb arising from the efficient energy transition based on the conjugated double bonds of the aromatic ligand. The comparison between the excitation spectrum (Figure 6a) and the UV-vis spectrum (solid line in Figure 5) of bifunctional nanocomposites shows clearly that Tb3+ can be efficiently sensitized by the heterocycle ligand, which is an antenna effect that is defined as a light-conversion process via an absorptionenergy transfer-emission sequence involving distinct absorption by the PABI ligands and emission by Tb3+.38,39 Upon excitation at 307 nm, the obtained bifunctional nanocomposites gives characteristic Tb3+ emission (Figure 6b), which confirms that Tb3+ is chelated by PABI ligands. The emissions at 490, 544, 584, and 620 nm of Fe3O4@SiO2@PABI-Tb nanocomposites are assigned accordingly to the transitions from the 5D4 level to the 7FJ (J ) 6, 5, 4, 3) levels, of which the 5D4 f 7F5 emission is the most prominent one. No emission from the ligand could be observed, which indicates that the surrounding aromatic ligands (38) Bekiari, V.; Lianos, P. AdV. Mater. 1998, 10, 1455. (39) Sabbatini, N.; Mecati, A.; Guardigli, M.; Balazani, N.; Lehn, V. J. M. Coord. Chem. ReV. 1993, 123, 201.
In summary, a simple and straightforward synthetic method has been developed for the preparation of bifunctional nanoparticles that combine both magnetic and luminescent properties. The silica shells formed via the Sto¨ber method played significant roles in terms of trapping superparamagnetic Fe3O4 nanoparticles, grafting the Tb complex, and endowing the final materials with desirable physical and biocompatible properties. Various techniques have been utilized to characterize the bifunctional material, and the results show the desired properties as designed. The pursuit to use such materials as biological luminescent labels is being carried out, and the extension of this method to other lanthanide ions is underway in order to obtain broader optical properties and potential applications. Acknowledgment. We are grateful for financial aid from the National Natural Science Foundation of China (Grant Nos. 20490210, 206301040, and 20602035) and the MOST of China (Grant Nos. 2006CB601103 and 2006DFA42610). LA700735M (40) Ferreira, R.; Pires, A. P.; Castro, B. D.; Sa´ Ferreira, R. A.; Carlosc, L. D.; Pischel, U. New J. Chem. 2004, 28, 1506. (41) Bettencourt-Dias, A. D.; Viswanathan, S. Dalton Trans. 2006, 4093. (42) Bettencourt-Dias, A. D.; Viswanathan, S. Inorg. Chem. 2006, 45, 10138. (43) Sun, L. N.; Yu, J. B.; Zheng, G. L.; Zhang, H. J.; Meng, Q. G.; Peng, C. Y.; Fu, L. S.; Liu, F. Y.; Yu, Y. N. Eur. J. Inorg. Chem. 2006, 3962. (44) Li, H. R.; Lin, J.; Zhang, H. J.; Fu, L. S.; Meng, Q. G.; Wang, S. B. Chem. Mater. 2002, 14, 3651. (45) Li, H. R.; Lin, J.; Zhang, H. J.; Li, H. C.; Fu, L. S.; Meng, Q. G. Chem. Commun. 2001, 1212. (46) Fu, W. Y.; Yang, H. B.; Hari, B.; Liu, S. K.; Li, Zou, M. H., G. T. Mater. Chem. Phys. 2006, 100, 246. (47) Huo, Q. S.; Liu, J.; Wang, L. Q.; Jiang, Y. B.; Lambert, T. N.; Fang, E. J. Am. Chem. Soc. 2006, 128, 6447. (48) Jovanovic, A. V.; Flint, J. A.; Varshney, M.; Morey, T. E.; Dennis, D. M.; Duran, R. S. Biomacromolecules 2006, 7, 945.
