Bifunctional Nanoparticles with Magnetization and Luminescence

Feb 12, 2009 - Proofs. Bifunctional Nanoparticles with Magnetization and Luminescence. Citing Articles; Related Content. Citation data is made availab...
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J. Phys. Chem. C 2009, 113, 3955–3959

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Bifunctional Nanoparticles with Magnetization and Luminescence Leyu Wang,* Zhihua Yang, Yi Zhang, and Lun Wang College of Chemistry and Materials Science, Anhui Key Laboratory of Chemo-Biosensing, Anhui Normal UniVersity, Wuhu 241000, P.R. China ReceiVed: September 23, 2008; ReVised Manuscript ReceiVed: December 25, 2008

The Fe3O4@LaF3:Ce3+-Tb3+ bifunctional composite nanoparticles were fabricated with a facile layer-bylayer technology. To get these bifunctional nanocomposites, the as-prepared magnetic (Fe3O4) and luminescent (LaF3:Ce3+-Tb3+) nanoparticles were modified with polyelectrolyte to get positively charged and negatively charged surfaces, respectively. The nanocomposites were then obtained by absorbing the negatively charged LaF3:Ce3+-Tb3+@(PAH/PSS)2 luminescent nanoparticles onto the positively charged Fe3O4@(PAH/PSS)2/ PAH magnetic nanocrystal surface through electrostatic interaction. The nanocomposites were characterized with transmission electron microscope (TEM), powder X-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometer, Fourier transform infrared (FTIR), and fluorescence technologies. Results indicate that these nanocomposites possess both high magnetization and strong fluorescence, which allows them great potential for bioapplications such as magnetic bioseparation, magnetic resonance imaging (MRI), and drug/gene delivery, simultaneously coupled with fluorescence immunoassays/imaging. Introduction Fluorescent labeling of biomolecules using fluorescent tags such as organic dyes and quantum dots is a common and very useful practice in biological and medical fields.1-4 Recently, rare-earth-doped nanocrystals have attracted growing interest for use as luminescent biolabels owing to their novel luminescent properties comprising long fluorescence lifetimes, high quantum yields, low photobleaching, and high chemical stability.5-10 Among the reported lanthanide-doped nanomaterials, fluoride nanocrystals, due to the low vibration energy of the Ln-F (Ln represents the lanthanide ion and F is the fluorinion) bond and, consequently, strong fluorescence and long lifetime, have drawn much attention.5,8,10 On the other hand, magnetic nanomaterials have been extensively studied in the fields of biomedical applications, including magnetic resonance imaging (MRI), gene/drug delivery, and biosensors, as well as biochemical separation and concentration of trace amount of samples.3,11-14 For example, the magnetic nanomaterial labeled biosamples could be rapidly, conveniently, and efficiently separated with an external magnetic field. Besides the magnetic separation and concentration,12,13 in a targeting drug-delivery system,11 the magnetic labeling of drug could be easily controlled and transported to the target tissue under the guidance of an external magnetic field, leading to a safer and more effective tissuespecific drug release. Also, the magnetic nanoparticle position and the drug efficacy can be checked with MRI technology by using the nanoparticles as MRI contrast agents.14 In this work, we employed the layer-by-layer (LbL) technology to fabricate the Fe3O4@LaF3:Ce3+-Tb3+ bifunctional nanocomposites, producing bifunctional (fluorescent and magnetic) biomarkers that can be used simultaneously as fluorescent markers for immunoassay/cell labeling, magnetic concentration, and separation of trace amount of biological samples, as well as MRI contrast agents. On the basis of the electrostatic interaction between the oppositely charged species, the LbL technology is a facile * Corresponding author. E-mail: [email protected].

strategy for the nanoparticle surface modification and the drug encapsulefabrication.6,15-18 Althoughsomemagnetic@luminescent nanoparticles have been fabricated,1,3 most of them are based on the luminescent dyes whose fluorescence is easily photobleaching.19 To the best of our knowledge, there are few reports about the magnetic@luminescent nanomaterials based on the rare-earth-doped fluoride nanocrystals yet.20 Herein, both the magnetic and luminescent nanoparticles were prepared beforehand. Thereafter, the as-synthesized Fe3O4 and LaF3:Ce3+-Tb3+ nanoparticles were deposited with multilayers of polyelectrolyte to obtain the desired surface. Finally the Fe3O4@LaF3: Ce3+-Tb3+ nanocomposites were successfully fabricated by coating the negatively charged LaF3@(PAH/PSS)2 nanoparticles onto the surface of positively charged Fe3O4@(PAH/PSS)2/PAH nanoparticles. The as-prepared magnetic@fluorescent nanocomposites were well water-dispersible due to the multilayer polymer surface, which is crucial to most biological applications. Moreover, the as-prepared nanocomposites have excellent magnetic properties and can be conveniently separated and collected with an external magnetic field, which greatly simplifies the modification procedure. Experimental Section Chemicals. Poly(allylamine hydrochloride) (PAH, Mw ) 8000-11 000, Aldrich) and poly(sodium 4-styrenesulfonate) (PSS, Mw ) 13400, Fluka) were used as supplied. La(NO3)3 · 6H2O, Ce(NO3)3 · 6H2O, Tb(NO3)3 · 6H2O, NaF, dehydrated sodium acetate (NaAc), NaCl, FeCl3 · 6H2O, sodium dedcyl sulfate (SDS), ethanol, and ethylene glycol were purchased from Beijing Chemical Reagent. All the chemicals are of analytical grade and used without further purification. Deionized water was used throughout. Characterization. The phase purity and crystallinity of the samples were characterized by using a Bruker D8-advance X-ray powder diffractometer with Cu KR radiation (λ ) 1.5418 Å). The size and morphology of the as-prepared nanocrystals were observed at 100 kV by using a JEOL JEM-1200EX transmission electron microscope (TEM). A superconducting quantum in-

10.1021/jp808425k CCC: $40.75  2009 American Chemical Society Published on Web 02/12/2009

3956 J. Phys. Chem. C, Vol. 113, No. 10, 2009 terference device (SQUID) magnetometer (LakeShore 7307) was used for the magnetization measurement. Fluorescence spectra were carried out on an F-4500 fluorescence spectrophotometer (Hitachi). FTIR spectra were recorded by using a Nicolet 560 spectrophotometer. Detection of Fe concentration in the nanocomposites is carried out on an Optima5300DV-ICP (PerkinElmer) inductive coupled plasma emission spectrometer. Synthesis of Fe3O4 Nanoparticles. With a modified strategy, the magnetite nanoparticles were prepared. Typically, FeCl3 · 6H2O (1.75 g) was dissolved in ethylene glycol (40 mL) to form a clear solution, followed by the addition of NaAc (3.6 g) and SDS (1.0 g) as base and surfactants, respectively. The mixture was stirred vigorously for 30 min and then transferred into a 45 mL Teflon-lined autoclave and heated at 180 °C for 10 h. And then the reaction systems were allowed to cool to room temperature. The black products were collected and washed several times with heated deionized water and ethanol. Finally, the nanopowder was dispersed into the deionized water to form a colloidal solution with the final concentration of ∼50 mg mL-1. Modification of Fe3O4. Into deionized water (60 mL) was added the as-synthesized Fe3O4 colloidal solution (0.5 mL, ∼25 mg) and the pH value was then adjusted to 8.5. The PAH solution (80 µL, 10 mg mL-1) containing NaCl (20 mM) was added to the above solution under ultrasonication for 2 min, and then the solution was stirred for 8 min. This PAH deposition process was repeated for another two cycles. In this process, the total dosage of PAH is 240 µL. Thereafter, the PAH-coated Fe3O4 nanoparticles were collected with an external magnet and washed once with NaCl solution (20 mM) and twice with deionized water. The obtained nanoparticles were then dispersed into deionized water (60 mL). Thereafter, the PSS solution (3 × 80 µL, 10 mg mL-1) containing NaCl (20 mM) was deposited onto the PAH surface with the aforementioned procedure. By repeating the modification procedure as above, the multilayer Fe3O4@(PAH/PSS)2/PAH-functionalized nanoparticles were then obtained and dispersed into deionized water (20 mL, ∼1.25 mg mL-1) for use. Synthesis of LaF3 Nanocrystals. In brief, La(NO3)3 (1.8 mL, 0.5 mol L-1), Ce(NO3)3 (100 µL, 0.5 mol L-1), Tb(NO3)3 (100 µL, 0.5 mol L-1), and NaF (3 mL, 1.0 mol L-1) solution were added to a mixture of ethanol (20 mL) and ethylene glycol (10 mL), and then the solution was thoroughly mixed. Subsequently, the milky colloidal solution was sealed into a 45 mL Teflonlined autoclave and heated at 160 °C for 10 h. The system was then allowed to cool to room temperature. The final product was collected by means of centrifugation and washed once with ethanol and twice with deionized water to remove any possible remnants. The obtained white powder was then dispersed into deionized water with a final concentration of ∼25 mg mL-1 and stored at room temperature. Modification of LaF3 Nanocrystals. The obtained LaF3 colloidal solution (2 mL, ∼50 mg) was added to 50 mL of deionized water and the pH was adjusted to 8.5. Then 80 µL of PAH (10 mg mL-1) solution containing NaCl (20 mM) was added to the above solution under ultrasonication for 2 min followed by stirring for 18 min. This deposition process was then repeated for another two cycles. Thereafter, the positively charged LaF3@PAH nanoparticles were collected by means of centrifugation. The white powder was washed once with NaCl solution (20 mM) and twice with deionized water. The purified LaF3@PAH nanoparticles were then dispersed into deionized water (50 mL) with ultrasonication. With the same procedure as above, the negatively charged PSS was deposited onto the

Wang et al. LaF3@PAH particle surface by adding 80 µL of PSS (10 mg mL-1) solution containing NaCl (20 mM), and the deposition process was repeated for another two cycles. Finally, the LaF3@(PAH/PSS)2 multilayer functional nanoparticles were obtained by repeating the aforementioned modification process. The as-prepared nanoparticles were then dispersed into deionized water (60 mL, ∼0.8 mg mL-1) for use. Fabrication of Magnetic@Luminescent Nanoparticles. The as-prepared Fe3O4@(PAH/PSS)2/PAH (5 mL, ∼1.25 mg mL-1) solution was diluted into 50 mL of deionized water with ultrasonication. The LaF3@(PAH/PSS)2 colloidal solution (2.5 mL, ∼0.8 mg mL-1) was then dripped into the above solution under ultrasonication for 5 min followed by stirring for another 25 min. This procedure was repeated for another three cycles before magnetic separation. Thereafter, the Fe3O4@LaF3 composite nanoparticles were collected with an external magnet and washed once with NaCl solution (20 mM) and twice with deionized water. The purified composite nanoparticles were dispersed into deionized water (50 mL) and then 80 µL of PAH solution (10 mg mL-1) containing NaCl (20 mM) was added into the Fe3O4@LaF3 composite nanoparticle colloidal solution under ultrasonication for 5 min followed by stirring for another 15 min. This PAH modification procedure was repeated for another two cycles. The obtained Fe3O4@LaF3@PAH composite nanoparticles were then collected and purified as above. For further deposition of the LaF3 nanoparticles, these as-prepared Fe3O4@LaF3@PAH composite nanoparticles were then dispersed into deionized water (50 mL) by ultrasonication. Thereafter, another 10 mL of LaF3@(PAH/PSS)2 colloidal solution was used to coat the Fe3O4@LaF3@PAH composite nanoparticles by repeating the aforementioned LaF3 deposition procedure. Finally, the obtained nanocomposites were magnetically collected, washed, and dispersed into deionized water to form an aqueous solution (∼0.5 mg mL-1) for later use. Results and Discussion Whereas several methods have been developed for the preparation of Fe3O4 nanoparticles,21-24 some of them use toxic and expensive organometallic compounds as precursors,22,23 and the others get nanoparticles with poor particle size distribution, poor crystallization, and, consequently, low magnetization by using normal inorganic salts as precursors.24 In this work, we prepared Fe3O4 with solvothermal technology by using FeCl3 · 6H2O as precursors.25 The as-synthesized nanoparticles are ∼150 nm with high crystallinity. Using a facile wet chemical technology,6 the luminescent LaF3 nanoparticles were successfully prepared by co-doping Tb3+ and Ce3+ ion-pair into the LaF3 matrix. A TEM image shows that the luminescent nanoparticle is a single crystal with ∼10 nm of size. For the fabrication of the bifunctional nanocomposites, both the magnetic and luminescent nanoparticles were modified with the polyelectrolyte via the LbL technology. As mentioned before,6,26 the as-prepared nanoparticles possess negatively charged surface under weak base conditions (pH 8.5); it is therefore important to deposit the positively charged electrolyte (PAH) first and then deposit the negatively charged polymer (PSS). To make the polymer layer stable and consequently, well dispersible in water, the deposition is repeated many times as desired (see ExperimentalSection).Thereafter,multifunctionalmagnetic@luminescent nanocomposites were obtained by depositing the negatively charged LaF3:Ce3+-Tb3+@(PAH/PSS)2 nanoparticles onto the outerface of positively charged Fe3O4@(PAH/PSS)2/PAH nanoparticles. The as-prepared bifunctional Fe3O4@LaF3:Ce3+-Tb3+ nanoparticles were first characterized with TEM. Figure 1 shows

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Figure 3. FTIR characterization of the LaF3:Ce3+-Tb3+, Fe3O4, and Fe3O4@LaF3:Ce3+-Tb3+ nanoparticles.

Figure 1. TEM images of the Fe3O4@LaF3:Ce3+-Tb3+ nanocomposites.

Figure 4. Magnetization curves of (1) Fe3O4 core and (2) Fe3O4@LaF3: Ce3+-Tb3+ nanocomposite at room temperature.

Figure 2. XRD patterns of the as-synthesized nanocrystals: (1) Fe3O4 nanocrystals, (2) the Fe3O4@ LaF3:Ce3+-Tb3+ composite nanocrystals, and (3) LaF3:Ce3+-Tb3+ nanocrystals.

the TEM images of the as-prepared magnetic@luminescent nanocomposites. It can be clearly seen that the LaF3:Ce3+-Tb3+ nanocrystals are coated on the surface of the Fe3O4 nanoparticles. The results indicated that the fabrication is successful. The composition and crystallinity of the as-synthesized Fe3O4, LaF3:Ce3+-Tb3+ and Fe3O4@LaF3:Ce3+-Tb3+ nanoparticles were checked with X-ray powder diffraction (XRD) technology. XRD patterns of the as-prepared nanocrystals are shown in Figure 2. From pattern 1, it can be seen that the magnetite core is easily indexed to Fe3O4 (JCPDS 82-1533) with good crystallinity. In the meantime, all peak positions and relative intensities of pattern 3 shown in Figure 2 were in good agreement with those of hexagonal phase LaF3 crystal (JCPDS 72-1435). The pattern 2 is for the magnetic@luminescent nanocomposite, and it clearly demonstrates the existence of both Fe3O4 and LaF3:Ce3+-Tb3+. Due to the strong peak intensity of Fe3O4 nanocrystals, the signal of LaF3 is not so clear, but the special peaks of LaF3 still can be seen. The arrows in pattern 2 clearly show the (220) and (440) reflections of Fe3O4. The stars in pattern 2 indicate the two overlap positions of the reflections of both Fe3O4 and LaF3:Ce3+-Tb3+ nanocrystals. One is the overlap peak of the (311) reflection (Fe3O4) and the (112) reflection (LaF3:Ce3+-Tb3+). The other is the overlap of the (333) reflection of Fe3O4 and the (222) reflection of LaF3: Ce3+-Tb3+. The XRD results further indicate that the formation of the magnetic@luminescent nanocomposite is successful. The magnetic@luminescent nanocomposites were characterized by means of FTIR spectroscopy. The FTIR spectra of both

Fe3O4 and LaF3 nanoparticles without surface modification have also been depicted in Figure 3. From the pattern of Fe3O4, we cannot see any obvious vibration peak except for the Fe-O vibration located at around 600 cm-1. At the same time, in the pattern of LaF3, the strong absorption around 3441 cm-1 is assigned to O-H vibration resulting from absorbed ethylene glycol on the LaF3 surface during the particle synthesis. Meanwhile, the absorption peaks around 1400-1600 cm-1 are attributed to the Ln-O vibration (Ln is a rare-earth ion). In the pattern of nanocomposites, the strong peak at 3441 cm-1 is attributed to the NH-H (PAH) or O-H. In comparison with the pattern of LaF3, the apparent difference is the transmission band at 3194 cm-1, which can be assigned to dC-H (PSS) stretching vibration. The transmission band at 2955 cm-1 is assigned to the asymmetric stretching vibration of methylene (-CH2) in the long alkyl chain of polyelectrolytes (see Supporting Information for the FTIR spectrum with high magnification, Figure S1). The bands located around 1400 cm-1 result from the CdC vibrations of the aromatic skeleton of PSS. Meanwhile, the absorption at 1643 cm-1 is attributed to the scissoring vibration absorption of N-H in the NH2 group from PAH. The band at 1126 cm-1 can be assigned to the C-N stretching vibration. The FTIR characterization results therefore apparently indicate that the polyelectrolyte has been successfully deposited onto the nanoparticle surface. The magnetic properties of both the magnetite core and the Fe3O4@LaF3:Ce3+-Tb3+ composite nanocrystals were recorded using a superconducting quantum interference device (SQUID) magnetometer with fields up to 1 T. Figure 4 shows the saturation magnetization curves of the as-prepared nanocrystals. Hysteresis loops of the samples were registered at room temperature (298.15K). The magnetite nanoparticles reached a saturation moment of 77.3 emu g-1. After the LaF3:Ce3+-Tb3+

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Figure 5. Fluorescence spectra of the nanoparticle colloidal solution. (1) pure LaF3:Ce3+-Tb3+ nanoparticles (0.1 mg mL-1) without modification, (2) the mixture of pure Fe3O4 (0.06 mg mL-1) and pure LaF3:Ce3+-Tb3+ nanoparticles (0.2 mg mL-1), and (3) the Fe3O4@LaF3: Ce3+-Tb3+ nanocomposite (0.1 mg mL-1).

coating, the saturation magnetization value dropped to 44.8 emu g-1. The decrease of the saturation magnetization, compared to the value of the magnetite core, can be explained by taking into account the diamagnetic contribution of the LaF3 shell surrounding the magnetite cores. It also can be identified by the inductive coupled plasma (ICP) analysis results for Fe concentration in the nanocomposite. The ICP analysis results indicate that the ratio of Fe in the nanocomposite is about 61% of that in pure Fe3O4 core. That means the Fe3O4 is about 61 wt % in the nanocomposites. This is in good agreement with the decrement of saturation magnetization, which has dropped about 42% compared to that of the pure magnetite core. Although the saturation magnetization value decreased about 42%, the magnetization is still strong enough for the bioseparation and magnetic resonance imaging test. The luminescent property of the bifunctional nanoparticles dispersed in water was recorded using a Hitachi F-4500 fluorescence spectrophotometer. Figure 5 shows the emission spectrum of the LaF3:Ce3+-Tb3+, the simple mixture of Fe3O4 and LaF3:Ce3+-Tb3+, and Fe3O4@LaF3:Ce3+-Tb3+ colloidal solution. Under excitation at 250 nm, the bifunctional nanocomposite emits highly typical green fluorescence corresponding to the 5D4-7F5 transition of Tb3+ ions.6 The dominant green band emission is centered at ∼549 nm. The results indicate that the as-prepared nanocomposites still possess good fluorescence, in spite of partial quenching by the black magnetite core. In order to investigate the stability of the binding between the magnetite nanoparticles and the luminescent nanoparticles, we considered the influence of pH value and ionic strength. We dispersed 2 mL of the nanocomposite (0.5 mg mL-1) into 1.5 mL of the buffer solution with different pH value. The mixture was then tuned to 10 mL by adding deionized water and incubated for 0.5 h at room temperature. The fluorescence spectra of the nanocomposite colloidal solution in the buffer were then collected before magnetic separation. The nanocomposites were then magnetically separated, and the fluorescence intensity of the supernatant was also detected. As shown in Figure 6, the fluorescence intensity of both the nanocomposite dispersed into different pH buffer and the corresponding supernatant has no obvious change. According to the pretty weak fluorescence of supernatant and slightly fluctuation of the fluorescence intensity of nanocomposites in buffer, it is clear that only few luminescent nanoparticles have dropped off from the magnetite nanoparticle surface after incubation in the buffer. In other words, the nanocomposites are stable and insusceptible

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Figure 6. pH influence on the stability of the nanocomposites. The upper fluorescence spectra are for nanocomposites in buffer solution, and the lower fluorescence spectra are for the supernatant after magnetically removing the nanocomposites. The nanocomposites were magnetically separated, washed, and redispersed into deionized water after incubation in buffer for 30 min at room temperature.

Figure 7. Affect of ionic strength on the stability of nanocomposites: (black square) fluorescence intensity of the nanocomposites in buffer before magnetic separation, (red circle) fluorescence intensity of the nanocomposites after magnetic separation, washing, and redispersion into deionized water.

to the buffer change. In addition, the magnetically separated nanocomposites were washed and redispersed into deionized water with a final concentration of 0.1 mg mL-1 and their fluorescence intensities were detected. The fluorescence spectra of nanocomposite colloidal solution after magnetically separation are shown in the Supporting Information (Figure S2) and further indicated that the pH value has no obvious influence on the stability of nanocomposites. The influence of ionic strength on the stability of the nanocomposites was also investigated under different NaCl concentration. In brief, into each 10-mL of flask were added 2 mL of nanocomposite colloidal solution (0.5 mg mL-1) and a different volume of NaCl solution (0.25 M). The final volume of the mixture was tuned to 10 mL by adding deionized water. The mixture was then thoroughly mixed by ultrasonication and incubated for 30 min at room temperature. Thereafter, the nanocomposites were magnetically separated and washed with deionized water. The purified nanocomposites were redispersed into 10 mL of deionized water with ultrasonication. The fluorescence intensities of nanocomposites before and after magnetic separation were detected. As shown in Figure 7, almost no difference can be detected in the fluorescence intensity of nanocomposites dispersed in buffer before magnetic separation. For the nanocomposites after separation, it can be seen that there is no apparent affect on the fluorescence under low NaCl concentration. Even though the NaCl concentration reaches 75 mmol L-1, the fluorescence intensity decreases slightly. The

Bifunctional Nanoparticles results indicate that the ionic strength has no great influence on the stability of the nanocomposites. Conclusion In summary, we fabricated the water-soluble Fe3O4@LaF3: Ce3+-Tb3+ magnetic@luminescent nanocomposites with LbL technology. The nanocomposites simultaneously exhibit excellent magnetic and luminescent properties and can be easily separated from solution using an external magnet. In a word, the fluorescent, magnetic, and water-soluble properties of the nanocomposites would allow them to find great potential applications, including fluorescence cell imaging/immunoassay, magnetic bioseparation/concentration, and magnetic resonance imaging, despite the disadvantages, including large particle size and UV excitation. With this technology, the fabrication of other magnetic/luminescent bifunctional nanocomposites with smaller size and near-IR excitation are desirable. Moreover, this technology can be easily extended for the fabrication of other multifunctional nanomaterials based on these magnetic nanocrystal templates. Acknowledgment. This work was supported by the National Natural Science Foundation of P.R. China (20605001, 20871004), the Natural Science Foundation of Anhui Province (050460304), and the Ph.D. Start-Up Funds of Anhui Normal University. Supporting Information Available: High-magnification FTIR spectrum (Figure S1) and fluorescence spectra of nanocomposite colloidal solution after magnetic separation (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Li, M. J.; Chen, Z. F.; Yam, V. W. W.; Zu, Y. B. Acs Nano 2008, 2, 905. (2) Wang, L. Y.; Li, P.; Zhuang, J.; Bai, F.; Feng, J.; Yan, X. Y.; Li, Y. D. Angew. Chem.-Int. Ed. 2008, 47, 1054.

J. Phys. Chem. C, Vol. 113, No. 10, 2009 3959 (3) Quarta, A.; Di Corato, R.; Manna, L.; Argentiere, S.; Cingolani, R.; Barbarella, G.; Pellegrino, T. J. Am. Chem. Soc. 2008, 130, 10545. (4) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016. (5) Wang, L. Y.; Li, Y. D. Chem. Mater. 2007, 19, 727. (6) Wang, L. Y.; Li, Y. D. Chem.-Eur. J. 2007, 13, 4203. (7) Wang, Z. L.; Quan, Z. W.; Jia, P. Y.; Lin, C. K.; Luo, Y.; Chen, Y.; Fang, J.; Zhou, W.; O’Connor, C. J.; Lin, J. Chem. Mater. 2006, 18, 2030. (8) Sivakumar, R.; van Veggel, F.; Raudsepp, M. J. Am. Chem. Soc. 2005, 127, 12464. (9) Riwotzki, K.; Meyssamy, H.; Schnablegger, H.; Kornowski, A.; Haase, M. Angew. Chem.-Int. Ed. 2001, 40, 573. (10) Diamente, P. R.; Burke, R. D.; van Veggel, F. Langmuir 2006, 22, 1782. (11) Giri, S.; Trewyn, B. G.; Stellmaker, M. P.; Lin, V. S. Y. Angew. Chem.-Int. Ed. 2005, 44, 5038. (12) Gu, H. W.; Xu, K. M.; Xu, C. J.; Xu, B. Chem. Commun. 2006, 941. (13) Gu, H. W.; Ho, P. L.; Tsang, K. W. T.; Wang, L.; Xu, B. J. Am. Chem. Soc. 2003, 125, 15702. (14) Brahler, M.; Georgieva, R.; Buske, N.; Muller, A.; Muller, S.; Pinkernelle, J.; Teichgraber, U.; Voigt, A.; Baumler, H. Nano Lett. 2006, 6, 2505. (15) Wood, K. C.; Boedicker, J. Q.; Lynn, D. M.; Hammon, P. T. Langmuir 2005, 21, 1603. (16) Radt, B.; Smith, T. A.; Caruso, F. AdV. Mater. 2004, 16, 2184. (17) Shenoy, D. B.; Antipov, A. A.; Sukhorukov, G. B.; Mohwald, H. Biomacromolecules 2003, 4, 265. (18) Tang, Z. Y.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203. (19) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (20) Zhang, M. F.; Shi, S. G.; Meng, J. X.; Wang, X. Q.; Fan, H.; Zhu, Y. C.; Wang, X. Y.; Qian, Y. T. J. Phys. Chem. C 2008, 112, 2825. (21) Ge, J. P.; Hu, Y. X.; Biasini, M.; Beyermann, W. P.; Yin, Y. D. Angew. Chem.-Int. Ed. 2007, 46, 4342. (22) Xu, C. J.; Xu, K. M.; Gu, H. W.; Zheng, R. K.; Liu, H.; Zhang, X. X.; Guo, Z. H.; Xu, B. J. Am. Chem. Soc. 2004, 126, 9938. (23) Yu, H.; Chen, M.; Rice, P. M.; Wang, S. X.; White, R. L.; Sun, S. H. Nano Lett. 2005, 5, 379. (24) Harris, L. A.; Goff, J. D.; Carmichael, A. Y.; Riffle, J. S.; Harburn, J. J.; St Pierre, T. G.; Saunders, M. Chem. Mater. 2003, 15, 1367. (25) Deng, H.; Li, X. L.; Peng, Q.; Wang, X.; Chen, J. P.; Li, Y. D. Angew. Chem.-Int. Ed. 2005, 44, 2782. (26) Wang, L. Y.; Li, Y. D. Chem. Commun. 2006, 16, 2557.

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