pubs.acs.org/Langmuir © 2009 American Chemical Society
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Colloidosome-based Synthesis of a Multifunctional Nanostructure of Silver and Hollow Iron Oxide Nanoparticles Yue Pan,† Jinhao Gao,‡ Bei Zhang,§ Xixiang Zhang,§, and Bing Xu*,†,‡ †
Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02453, Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China, §Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China, and Advanced Nanofabrication Core Lab, King Abdullah University of Science and Technology, Thuwal 23955-6900 Kingdom of Saudi Arabia )
‡
Received August 28, 2009 Nanoparticles that self-assemble on a liquid-liquid interface serve as the building block for making heterodimeric nanostructures. Specifically, hollow iron oxide nanoparticles within hexane form colloidosomes in the aqueous solution of silver nitrate, and iron oxide exposed to the aqueous phase catalyzes the reduction of silver ions to afford a heterodimer of silver and hollow iron oxide nanoparticles. Transmission electron microscopy, selected area electron diffraction, energy-dispersive X-ray spectrometry, X-ray diffraction, UV-vis spectroscopy, and SQUID were used to characterize the heterodimers. Interestingly, the formation of silver nanoparticles helps the removal of spinglass layer on the hollow iron oxide nanoparticles. This work demonstrates a powerful yet convenient strategy for producing sophisticated, multifunctional nanostructures.
Introduction This paper reports a facile synthesis of a sophisticated, multifunctional nanostructure that consists of a silver nanoparticle and a hollow iron oxide nanoparticle. The construction of heterodimers of nanoparticles consisting of two inorganic phases provides a powerful approach for tailoring the properties of nanomaterials for a wide range of applications.1 For example, the unique structure of heterodimeric nanoparticles promises many advantages in their applications in physical science and biology.2 After the elegant demonstration of heterodimers of microparticles,3 several research groups have reported the production of heterodimers of nanoparticles with two *E-mail:
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different inorganic compositions, such as FePt-CdS/CdSe, γ-Fe2O3-CdSe/ZnS, CoPt3-Au heterodimers, and dumbbelllike Au-Fe3O4 heterodimer nanoparticles.4,5 We also reported an efficient method to form heterodimeric nanostructures based on the reactions on the colloidosome of iron oxide nanoparticles.6 “Colloidosomes”, a unique biphasic system formed by the selfassembly of nanoparticles at the interface of an organic solvent and water, allow a heterogeneous reaction to take place on the exposed surface of the nanoparticles and to produce the heterodimers of two distinct nanospheres with a simple but relatively precise control.7,8 This procedure not only controls the composi(6) Gu, H. W.; Yang, Z. M.; Gao, J. H.; Chang, C. K.; Xu, B. J. Am. Chem. Soc. 2005, 127, 34. (7) (a) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (b) Lin, Y.; Skaff, H.; Emrick, T.; Dinsmore, A. D.; Russell, T. P. Science 2003, 299, 226. (8) (a) Binder, W. H. Angew. Chem., Int. Ed. 2005, 44, 5172. Hong, L.; Jiang, S.; Granick, S. Langmuir 2006, 22, 9495. (b) Kim, B. S.; Taton, T. A. Langmuir 2007, 23, 2198. (c) Roh, K. H.; Yoshida, M.; Lahann, J. Langmuir 2007, 23, 5683. (d) Snyder, C. E.; Yake, A. M.; Feick, J. D.; Velegol, D. Langmuir 2005, 21, 4813. (e) Sun, B.; Zhang, Y.; Gu, K. J.; Shen, Q. D.; Yang, Y.; Song, H. Langmuir 2009, 25, 5969. (f) Teo, J. J.; Chang, Y.; Zeng, H. C. Langmuir 2006, 22, 7369. (9) (a) Gu, H. W.; Ho, P. L.; Tsang, K. W. T.; Yu, C. Y.; Xu, B. J. Am. Chem. Soc. 2003, 125, 15702. Ai, H.; Flask, C.; Weinberg, B.;(b) Shuai, X.; Pagel, M. D.; Farrell, D.; Duerk, J.; Gao, J. M. Adv. Mater. 2005, 17, 1949. (c) Gu, H. W.; Xu, K. M.; Xu, C. J.; Xu, B. Chem. Commun. 2006, 941. (d) Wang, L.; Yang, Z. M.; Gao, J. H.; Xu, K. M.; Gu, H. W.; Zhang, B.; Zhang, X. X.; Xu, B. J. Am. Chem. Soc. 2006, 128, 13358. (e) Jun, Y. W.; Choi, J. S.; Cheon, J. Chem. Commun. 2007, 1203. (f) Kim, J. S.; Valencia, C. A.; Liu, R. H.; Lin, W. B. Bioconjugate Chem. 2007, 18, 333. (g) Shevchenko, E. V.; Kortright, J. B.; Talapin, D. V.; Aloni, S.; Alivisatos, A. P. Adv. Mater. 2007, 19, 4183. (h) Bin Na, H.; Lee, I. S.; Seo, H.; Il Park, Y.; Lee, J. H.; Kim, S. W.; Hyeon, T. Chem. Commun. 2007, 5167. (i) Lee, K. S.; Lee, I. S. Chem. Commun. 2008, 709. (j) Lee, J.; Lee, Y.; Youn, J. K.; Bin Na, H.; Yu, T.; Kim, H.; Lee, S. M.; Koo, Y. M.; Kwak, J. H.; Park, H. G.; Chang, H. N.; Hwang, M.; Park, J. G.; Kim, J.; Hyeon, T. Small 2008, 4, 143. (k) Latham, A. H.; Williams, M. E. Acc. Chem. Res. 2008, 41, 411. (l) Hsia, C. H.; Chen, T. Y.; Son, D. H. Nano Lett. 2008, 8, 571. (m) Xu, X. L.; Friedman, G.; Humfeld, K. D.; Majetich, S. A.; Asher, S. A. Chem. Mater. 2002, 14, 1249. (n) Binks, B. P.; Desforges, A.; Duff, D. G. Langmuir 2007, 23, 1098. (o) Ge, J. P.; He, L.; Goebl, J.; Yin, Y. D. J. Am. Chem. Soc. 2009, 131, 3484. (p) Kim, D.; Lee, N.; Park, M.; Kim, B. H.; An, K.; Hyeon, T. J. Am. Chem. Soc. 2009, 131, 454. (q) Lim, J.; Eggeman, A.; Lanni, F.; Tilton, R. D.; Majetich, S. A. Adv. Mater. 2008, 20, 1721. (r) An, K.; Kwon, S. G.; Park, M.; Bin Na, H.; Baik, S. I.; Yu, J. H.; Kim, D.; Son, J. S.; Kim, Y. W.; Song, I. C.; Moon, W. K.; Park, H. M.; Hyeon, T. Nano Lett. 2008, 8, 4252. (s) Messersmith, P. B.; Textor, M. Nat. Nanotechnol. 2007, 2, 138. (t) Yu, S. Y.; Zhang, H. J.; Yu, J. B.; Wang, C.; Sun, L. N.; Shi, W. D. Langmuir 2007, 23, 7836.
Published on Web 12/11/2009
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tions of heterodimers of nanoparticles, but also allows functional molecules to be attached on specific parts of the heterodimers.6 Despite its promising potentials, this useful and versatile method remains less explored. Thus, we are interested in studying the scope of the colloidosome-based synthesis for making heterodimeric or hybrid nanostructures and fully characterizing the resulted nanostructures. Among various nanocrystals, magnetic nanoparticles, especially iron oxide nanoparticles, have attracted broad attention because they promise new applications in the rapidly advancing field of biofunctional nanomaterials.9 Recently, Sun et al. and Alivisatos et al. have shown that Kirkendall effect10 at nanoscale can lead to the production of hollow iron oxide nanoparticles,11,12 and their works demonstrated that hollow iron oxide nanoparticles with controlled interior void and thickness of the shell are an important class of nanoporous materials. We have shown that hollow iron oxide nanoparticles can exhibit high relaxivity for MRI enhancement.13 These attractive features of hollow iron oxide nanoparticles make them ideal building blocks of heterodimeric nanostructures for the further exploration and expansion of their functions. Here, we report the use of the colloidosome approach to synthesize the heterodimers of silver and hollow iron oxide nanoparticles based on the reactions at a liquid-liquid interface. Besides being the first example of heterodimers that contain hollow nanoparticles and further demonstrating the versatility of this method for constructing sophisticated nanostructures at the liquid-liquid interface, these heterodimeric nanostructures could provide a new class of nanomaterials for useful applications. Silver nanoparticles have excellent surface plasma resonance properties and are themselves a Raman enhancer,14 and hollow iron oxide nanoparticles are superparamagnetic at room temperature. Therefore, potentially, the silver part can serve as optical tags and the hollow iron oxide as magnetic resonance imaging (MRI) contrast and hyperthermia therapy agents.
Scheme 1. Illustration of the Synthetic Steps of the Heterodimers
oleylamine was mixed with the solution of silver nitrate (2 mL, 30 mg/mL) in a small vial. Ultrasonic emulsification afforded a stable brown oil-in-water emulsion of the two phases. The mixture was shaken frequently to make the two liquid phases to mix well. After reacting for 5 h (temperature of the vial: ≈40 °C), the mixture was precipitated by adding ethanol. The heterodimers were separated by centrifugation (6000 rpm). Then, the heterodimers were dispersed in hexane in the presence of oleylamine. In the final step, the heterodimer nanoparticles were further purified by magnetic harvesting and redispersed in hexane for the further analysis. Characterization. The nanostructures were characterized by transmission electron microscopes (TEM) (JEOL 2010, 200 kV), high-resolution TEM, and correlative Energy-dispersive X-ray spectrometric (EDX) (JEOL 2010F, 200 kV). The UV-vis absorbance spectra were obtained on a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer. The magnetic properties of the nanostructures were measured by a superconducting quantum interference device (SQUID) magnetometer.
Results and Discussion Materials and Methods General Data. Iron pentacarbonyl (Fe(CO)5), oleylamine (70%), and 1-octadecene (90%) were purchased from Sigma Aldrich, and silver nitrate from Fisher Chemical. All the reactions were carried out at ambient conditions unless otherwise stated. Synthesis. Hollow iron oxide nanoparticles were synthesized using a reported procedure.13 Typically, oleylamine (0.3 mL) and 1-octadecene (20 mL) were heated at 120 °C for 30 min under argon atmosphere before Fe(CO)5 (0.7 mL) was injected into the hot solution. Then, the solution was kept at 180 °C for 20 min and afforded the Fe nanoparticles as the intermediate. Then, the dispersion was moved to ambient atmosphere and heated up to 180 °C with an O2 gas flow at a rate of 2 m3/h for 2 h. The Fe nanoparticles were completely oxidized to iron oxide. After the black-brown colored solution was cooled to room temperature, the hollow iron oxide nanoparticles were precipitated by adding isopropanol followed by centrifugation (6000 rpm) and wash with pure ethanol. The hollow nanoparticles were then dispersed in hexane in the presence of oleylamine. In a typical synthesis of the heterodimers, the hollow iron oxide nanoparticles (2.5 mg) in 2 mL of hexane in the presence of (10) (a) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (b) Gao, J. H.; Zhang, B.; Zhang, X. X.; Xu, B. Angew. Chem., Int. Ed. 2006, 45, 1220. (11) Peng, S.; Sun, S. H. Angew. Chem., Int. Ed. 2007, 46, 4155. (12) Cabot, A.; Puntes, V. F.; Shevchenko, E.; Yin, Y.; Balcells, L.; Marcus, M. A.; Hughes, S. M.; Alivisatos, A. P. J. Am. Chem. Soc. 2007, 129, 10358. (13) Gao, J. H.; Liang, G. L.; Cheung, J. S.; Pan, Y.; Kuang, Y.; Zhao, F.; Zhang, B.; Zhang, X. X.; Wu, E. X.; Xu, B. J. Am. Chem. Soc. 2008, 130, 11828. (14) Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241.
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The construction of the heterodimeric nanoparticles involves an initial synthesis of the hollow iron oxide nanoparticles as the seeds and a subsequent reduction of AgNO3 in the presence of the seeds. According to the typical synthetic route illustrated in Scheme 1, the following process could contribute to the formation of the heterodimers: Ultrasonic agitation and frequent shaking cause the formation of a heterogeneous microemulsion of organic droplet in silver nitrate water solution. In this biphasic system, the hydrophobic hollow iron oxide nanoparticles self-assemble at the water-organic interface7,15 and provide the catalytic sites onto which the Agþ ions can be reduced by oleylamine16 to form silver nanoparticles. In this process, oleylamine serves as the mild reducing agent as well as the surfactant.17 Most likely, the small defects on the surfaces of the hollow iron oxide nanoparticles that are exposed to the aqueous phase catalyze the reduction of the Agþ ions to provide the initial nucleation sites of silver.18 As a result, the heterodimers with gradually grown silver spheres on the surface of the hollow iron oxide form while the reaction progresses. We used TEM to follow the progress of the synthesis and to characterize the products. TEM image in Figure 1a shows the assynthesized hollow iron oxide nanoparticles with uniform hollow (15) Huang, W. A.; Lan, Q.; Zhang, Y. Prog. Chem. 2007, 19, 214. (16) Hiramatsu, H.; Osterloh, F. E. Chem. Mater. 2004, 16, 2509. (17) Xu, Z.; Hou, Y.; Sun, S. J. Am. Chem. Soc. 2007, 129, 8698. (18) Rodriguez-Sanchez, L.; Blanco, M. C.; Lopez-Quintela, M. A. J. Phys. Chem. B 2000, 104, 9683.
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Figure 1. (a) TEM (inset: EDP image) and (b) HRTEM images of the as-prepared hollow iron oxide seeds.
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Figure 3. X-ray diffraction patterns of the heterodimers of silver and hollow iron oxide nanoparticles with representative index on typical peaks.
Figure 4. UV-vis spectra of (a) hollow iron oxide nanoparticles and (b) the heterodimers of silver and hollow iron oxide nanoparticles.
Figure 2. (a) TEM; (b) HRTEM images of the heterodimers of silver and hollow iron oxide nanoparticles; (c) EDX; (d) EDP analysis of the heterodimers.
spheres. The HRTEM image (Figure 1b) of the hollow iron oxide reveals that they have the overall diameter of 12 nm and an oxide shell of around 3 nm thick. Furthermore, EDP (electron diffraction patterns) analysis (Figure 1a,inset) of the nanoparticles confirms its excellent crystallinity: the diffraction rings of the hollow nanoparticles are consistent with the crystal planes of iron oxide phase ({220}, {311}, {400}, {422,} and {440}).6 Being oxidized by pure O2, the hollow iron oxide nanoparticles likely are maghemite.13 The TEM image (Figure 2a) reveals that the yield of the heterodimers of silver and hollow iron oxide nanoparticles is about 80%. The HRTEM image (Figure 2b) of a single heterodimer suggests that both the iron oxide part and the silver part are crystalline. The HRTEM image also reveals that the size of the hollow iron oxide remains at about 12 nm and the size of the silver part about 4 nm. The overall diameter of the heterodimer is about 16 nm. According to Figure 2, parts a and b, the ratios of the hollow iron oxide particles to silver particles mostly are 1:1. Furthermore, the EDX analysis (Figure 2c) confirms that the heterodimers are composed of Fe in the hollow sphere and Ag in the solid sphere. The corresponding EDP analysis (Figure 2d) of the lattice image shows that the diffractogram spots and the diffractogram rings match with the standard diffractogram 4186 DOI: 10.1021/la904067q
patterns of iron oxide and silver, which confirms crystalline iron oxide and silver parts. Figure 3 shows the X-ray diffraction patterns of the heterodimers of silver and hollow iron oxide nanoparticles. From the XRD data, the peak of Ag (111) is at 38.0°. The other peaks belong to iron oxide. The positions and relative intensities of all diffraction peaks match well with the standard diffraction data of iron oxide phase and silver powders, indicating that this synthetic procedure produces crystalline iron oxide and silver, which agrees with the TEM data. Figure 4 shows the UV-Vis spectra of the hollow iron oxide nanoparticles and the heterodimeric nanoparticles dispersed in hexane, respectively. Contrasting to the hollow iron oxide seeds that show largely a silent feature in the visible region, the heterodimers of silver and hollow iron oxide nanoparticles exhibit a peak at 413 nm in the absorption spectrum, belonging to the surface plasma resonance peak of silver. This result is similar to the surface plasma of iron oxide-silver heterodimer.6 In addition, the broadness of the surface plasma peak reflects the difference in the local environment of the Ag nanoparticles on the hollow iron oxide shells. The temperature-dependent magnetization (ZFC/FC) curves of the hollow iron oxide nanoparticles and the heterodimers under 100 Oe field were measured by a superconducting quantum interference device (SQUID) magnetometer. As shown in Figure 5, the distinct blocking temperature indicates that the hollow iron oxide nanoparticles and the heterodimeric nanoparticles are superparamagnetic at temperature higher than 56 K. Langmuir 2010, 26(6), 4184–4187
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Figure 5. ZFC/FC curves of (a) hollow iron oxide nanoparticles and (b) the heterodimers of silver and hollow iron oxide nanoparticles.
Figure 6. Field dependent magnetization of (a) hollow iron oxide nanoparticles and (b) the heterodimers of silver and hollow iron oxide nanoparticles.
The similarity of the two ZFC/FC curves and the almost identical blocking temperature also confirm that the growth of silver nanoparticles has little adverse effect on the magnetic properties of the hollow iron oxide particles. On the surface of the hollow nanoparticles, there may be a very thin layer whose magnetic order is different from the inner part of the particle and behaves as spinglass. The spinglass phase is very difficult to be saturated magnetically, which is clearly seen in Figure 6a at low temperature (5 K curve). After the deposition of silver, the spinglass phase might be nearly removed. Because the spinglass phase becomes paramagnetic at the temperature higher than about 30 K,19 it hardly affects the blocking behavior of the nanoparticle. This is why two materials have the same blocking temperature. Figure 6 shows the field-dependent magnetization of the hollow iron oxide nanoparticles and the heterodimers of silver and hollow iron oxide nanoparticles measured at 5 and 150 K. With additional silver, the coercive field of the heterodimer at 5 K decreases a little from 810 to 700 Oe. The decrease in coercive field is due to the removal of the spingalss layer whose spins are difficult to be aligned by magnetic field. The heterodimers show similar behavior with that of the hollow iron oxide nanoparticles. Notably, the heterodimers reach magnetic saturation at a lower field than the hollow iron oxide nanoparticles at both 5 and 150 K, suggesting that the deposition of silver helped remov the spingalss layer (due to defects) on the surface of the heterodimers. (19) (a) Kodama, R. H.; Berkowitz, A. E.; E. J. McNiff, J.; Foner, S. Phys. Rev. Lett. 1996, 77, 394. (b) Martinez, B.; Obradors, X.; Balcells, L.; Rouanet, A.; Monty, C. Phys. Rev. Lett. 1998, 80, 181.
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As shown in the ZFC/FC curves (Figure 5), when the temperature is higher than the blocking temperature, the ZFC curve and FC curve nearly overlap. Furthermore, for M-H curves at 150 K (Figure 6), the almost zero coercivity (∼10 Oe) also indicates that the magnetic interaction between the nanoparticles is very weak, suggesting an excellent dispersion. More importantly, it also indicates that the particles before and after forming Ag-Fe2O3 are very uniform. These results suggest, above the blocking temperature, both the hollow iron oxide nanoparticles and the heterodimers exhibit superparamagnetic behavior.
Conclusion In summary, we have demonstrated a facile method for making the heterodimers of silver and hollow iron oxide nanoparticles with distinct properties based on the reactions at liquid-liquid interface. Furthermore, each part of the heterodimer should be possibly modified by a different surface chemistry.6 This work offers a new way to produce a sophisticated, multifunctional nanostructure that is biocompatible, responsive to magnetic forces, and optically active. We anticipate that such heterodimers, after proper surface modification, would be useful nanomaterials for biomedicine, nanoelectronics, and catalysis Acknowledgment. The authors acknowledge the financial support from start-up grant from Brandeis University and Hong Kong Research Grant Council (RGC).
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