Fabrication of Novel Multihollow Superparamagnetic Magnetite

Langmuir 2008, 24, 10395-10401

10395

Fabrication of Novel Multihollow Superparamagnetic Magnetite/ Polystyrene Nanocomposite Microspheres via Water-in-Oil-in-Water Double Emulsions Song Yang, Huarong Liu,* and Zhicheng Zhang Department of Polymer Science and Engineering, Key Laboratory of Soft Matter Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed March 1, 2008. ReVised Manuscript ReceiVed July 3, 2008 We herein present a novel and simple synthetic strategy for fabricating multihollow superparamagnetic magnetite/ polystyrene nanocomposite microspheres via water-in-oil-in-water double emulsions. Amphipathic magnetite nanoparticles surface-modified with oleic acid act as an oil-soluble emulsifier and sodium dodecyl sulfate acts as a water-soluble surfactant in the system. The final products were thoroughly characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and field-emission scanning electron microscopy, which showed the formation of multihollow magnetite/polystyrene nanocomposite microspheres. Preliminary results of magnetic properties of multihollow magnetite/polystyrene microspheres were reported. The effect of the content of amphipathic magnetite nanoparticles on the morphology of nanocomposite microspheres was studied. Furthermore, the mechanism of formation of multihollow magnetic nanocomposite microspheres was also discussed.

Introduction Nowadays, hollow spheres with nanometer-to-micrometer dimensions, tailored structures, and optical, magnetic, and surface properties have attracted great attention because of their potential applications in various fields such as controlled-release capsules of various substances (drugs, cosmetics, dyes, and inks), protection of biologically active macromolecules, artificial cells, catalysts, fillers, coatings and pigments, lightweight structural or lowdensity materials, and waste removal.1-5 Various hollow spheres have been synthesized by using different templates including polystyrene latex spheres,6 silica sols,7 liquid drops,8 vesicles,9 * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +86-551-3601586. Fax: +86-551-3601592. (1) (a) Cochran, J. K. Curr. Opin. Solid State Mater. Sci. 1998, 3, 474. (b) Huang, H.; Remsen, E. E. J. Am. Chem. Soc. 1999, 121, 3805. (c) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (d) Fowler, C. E.; Khushalani, D.; Mann, S. J. Mater. Chem. 2001, 11, 1968–1971. (e) Li, Y.; Shi, J.; Hua, Z.; Chen, H.; Ruan, M.; Yan, D. Nano Lett. 2003, 3, 609. (f) Yang, M.; Ma, J.; Zhang, C. L.; Yang, Z. Z.; Lu, Y. F. Angew. Chem., Int. Ed. 2005, 44, 6727. (g) Shchukin, D. G.; Ustinovich, E. A.; Sukhorukov, G. B.; Mohwald, H.; Sviridov, D. V. AdV. Mater. 2005, 17, 468. (h) Jeong, U.; Wang, Y. L.; Ibisate, M.; Xia, Y. N. AdV. Funct. Mater. 2005, 15, 1907. (2) (a) Caruso, F. Chem.sEur. J. 2000, 6, 413. (b) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (3) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 206. (4) Bourlinos, A. B.; Karakassides, M. A.; Petridis, D. Chem. Commun. 2001, 16, 1518. (5) Rhodes, K. H.; Davis, S. A.; Caruso, F.; Zhang, B.; Mann, S. Chem. Mater. 2000, 12, 2832. (6) (a) Park, M. K.; Onishi, K.; Locklin, J.; Caruso, F.; Advincula, R. C. Langmuir 2003, 19, 8550. (b) Wang, D. B.; Song, C. X.; Hu, Z. S.; Fu, X. J. Phys. Chem. B 2005, 109, 1125. (c) Strohm, H.; Lobmann, P. Chem. Mater. 2005, 17, 6772. (d) Cheng, X. J.; Chen, M.; Wu, L. M.; Gu, G. X. Langmuir 2006, 22, 3858. (e) Song, X. F.; Gao, L. J. Phys. Chem. C, in press. (7) (a) Oda, Y.; Fukuyama, K.; Nishikawa, K.; Namba, S.; Yoshitake, H.; Tatsumi, T. Chem. Mater. 2004, 16, 3860. (b) Arul Dhas, N.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 2368. (c) Wang, Y.; Su, F. B.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2006, 18, 1347. (8) (a) Huang, J. X.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y. T.; Zhang, S. Y. AdV. Mater. 2000, 12, 808. (b) Fowler, C. E.; Khushalani, D.; Mann, S. Chem. Commun. 2001, 19, 2028. (c) Yin, L. W.; Bando, Y.; Li, M. S.; Golberg, D. Small 2005, 1, 1094. (9) (a) Zheng, X. W.; Xie, Y.; Zhu, L. Y.; Jiang, X. C.; Yan, A. H. Ultrason. Sonochem. 2002, 9, 311. (b) Mckelvey, C. A.; Kaler, E. W.; Zasadzinski, J. A.; Coldren, B.; Jung, H. T. Langmuir 2000, 16, 8285. (c) Hentze, H.-P.; Raghavan, S. R.; McKelvey, C. A.; Kaler, E. W. Langmuir 2003, 19, 1069. (d) Kepczynski, M.; Ganachaud, F.; Hemery, P. AdV. Mater. 2004, 16, 1861.

polymer micelles,10 and emulsion,11 microemulsion,12 and miniemulsion13 droplets. It is well-known that magnetic materials have extensive applications in the biomedicine,14 bioengineering, and biotechnology fields, such as in protein and enzyme immobilization, immunoassays, RNA and DNA purification, cell isolation, and magnetic drug delivery.15-19 If (multi)hollow materials could be introduced with magnetic properties, then they would be targetguided to their desired locations by an external magnetic field; therefore, drug transportation efficiency could be improved significantly, and toxic effects could be reduced as well. Such (multi)hollow magnetic nanomaterials might be ideal candidates for controlled drug delivery. However, to our knowledge, the approach to (multi)hollow superparamagnetic nanocomposite spheres is scarce. Caruso et al.20 prepared magnetic/polymer hollow microspheres through the colloid-templated electrostatic layer-by-layer self-assembly of oppositely charged inorganic nanoparticles and polymer multilayers, followed by removal of (10) (a) Ma, Y. R.; Ma, J. M.; Cheng, H. M. Langmuir 2003, 19, 4040. (b) Liu, T.; Xie, Y.; Chu, B. Langmuir 2000, 16, 9015. (c) Duan, H. W.; Kuang, M.; Wang, J.; Chen, D. Y.; Jiang, M. J. Phys. Chem. B 2004, 108, 550. (11) (a) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (b) Yu, C. Z.; Tian, B. H.; Fan, J.; Stucky, G. D.; Zhao, D. Y. Chem. Lett. 2002, 31, 62. (c) Bao, J. C.; Liang, Y. Y.; Xu, Z.; Si, L. AdV. Mater. 2003, 15, 1832. (12) (a) Walsh, D.; Mann, S. Nature 1995, 377, 320. (b) Jafelicci, M.; Davolos, M. R.; Dos Santos, F. J.; De Andrade, S. J. J. Non-Cryst. Solids 1999, 247, 98. (c) Wu, D. Z.; Ge, X. W.; Zhang, Z. C.; Wang, M. Z.; Zhang, S. L. Langmuir 2004, 20, 5192. (13) Putlitz, B.; Landfester, K.; Fischer, H.; Antonietti, M. AdV. Mater. 2001, 13, 500. (14) (a) Ugelstad, J.; Stenstad, P.; Kilaas, L.; Prestvik, W. S.; Herje, R.; Bererge, A.; Hornes, E. Blood Purif. 1993, 11, 349. (b) Elaı¨ssari, A.; Veyret, R.; Mandrand, B.; Chatterjee, J. J. Surfactant Sci. Ser. 2004, 116, 1. (15) Bı´lkova´, Z.; Slova´kova´, M.; Hora´k, D.; Lenfeld, J.; Chura´cek, J. J. Chromatogr., B 2002, 770, 177. (16) Sy¨afarˇ´ık, I.; Sy¨afarˇ´ıkova´, M. J. Chromatogr., B 1999, 722, 33. (17) Yu, H.; Raymonda, J. W.; McMahon, T. M.; Campagnari, A. A. Biosens. Bioelectron. 2000, 14, 829. (18) Ha¨feli, U.; Schu¨tt, W.; Teller, J.; Zborowski, M. Scientific and Clinical Applications of Magnetic Carriers; Plenum: New York, 1997; p 269. (19) Lu¨bbe, A. S.; Bergemann, C.; Huhnt, W.; Fricke, T.; Riess, H.; Brock, J. W.; Huhn, D. Cancer Res. 1996, 56, 4694. (20) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem. Mater. 2001, 13, 109.

10.1021/la800657k CCC: $40.75  2008 American Chemical Society Published on Web 08/21/2008

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the templated core. Li and co-workers21 prepared amphiphilic superparamagnetic ferrite/block copolymer hollow submicrospheres through the solvothermal method at high temperature and under high pressure. Zhao and co-workers22 prepared magnetic nanocomposite spheres with a magnetic core and mesoporous silica shell and studied the released curve of ibuprofen. Kim and co-workers23 synthesized mesoporous silica spheres embedded with monodisperse magnetic and semiconductor nanocrystals and investigated the released curve of ibuprofen. Zhou and co-workers24 prepared porous magnetic hollow silica nanospheres by the sol-gel method. However, the calcination is necessary to obtain the porous structure in the above methods, which leads to the complexity of the preparation process. Thus, for chemists, the development of simple and effective synthetic methods for obtaining (multi)hollow superparamagnetic nanocomposite spheres is a very impending task. Double emulsions are complex soft colloidal systems in which droplets of the dispersed phase themselves contain even smaller droplets. Typical examples are water-in-oil-in-water (W/O/W) and oil-in-water-in-oil (O/W/O). These are thermodynamically stabilized by a set of lipophilic and hydrophilic surfactants dissolved in the intermediate phase and external phase. It should be simple and facile to prepare the (multi)hollow spheres through the W/O/W double emulsions, because etching the templated core can be avoided. Kim et al.25 prepared multihollow-structured poly(methyl methacrylate) particles employing the W/O/W emulsion polymerization technique. Fujiwara et al.26 prepared the silica microcapsules (hollow spheres) by a simple interfacial reaction using a W/O/W emulsion system. Subsequently, they fabricated the silica hollow particles with nanomacroholes that resembled natural diatom cells in morphology using a W/O/W interfacial reaction.27 Koo et al.28 prepared polymeric microcapsules with iron oxide (γ-Fe2O3) magnetic particles embedded in the polymer shell by the W/O/W double emulsions. In our recent study, we synthesized the amphipathic property of magnetic nanoparticles (MPs) with active hydrophilic hydroxyl groups and hydrophobic oleic ester groups existing at the surface, with the aid of which hollow superparamagnetic magnetite/ polystyrene nanocompostie microspheres were prepared via the interfacial polymerization technique29 or inverse miniemulsion polymerization.30 The polymerization was initiated by the free radicals at the surface of the MPs which were located at the oil/water interface at room temperature and under ambient pressure. These radicals were produced by the attack of •OH etc. generated in the radiolysis of water against the active hydroxyl groups of the MPs. In this paper, we report a facile approach to the preparation of novel multihollow superparamagnetic magnetite/polystyrene nanocomposite microspheres via the W/O/W double emulsion system in which such amphipathic MPs act as an oil-soluble (21) Li, X. H.; Zhang, D. H.; Chen, J. S. J. Am. Chem. Soc. 2006, 128, 8383. (22) Zhao, W. R.; Gu, J. L.; Zhang, L. X.; Chen, H. R.; Shi, J. L. J. Am. Chem. Soc. 2005, 127, 8916. (23) Kim, J. Y.; Lee, J. E.; Lee, J. W.; Yu, J. H.; Kim, B. C.; An, K.; Hwang, Y. S.; Shin, C.-H.; Park, J.-G.; Kim, J. B.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 688. (24) Zhou, J.; Wu, W.; Caruntu, D.; Yu, M. H.; Martin, A.; Chen, J. F.; O’ Connor, C. J.; Zhou, W. L. J. Phys. Chem. C 2007, 111, 17473. (25) Kim, B.; Kim, J.; Suh, K. J. Appl. Polym. Sci. 2000, 76, 38. (26) Fujiwara, M.; Shiokawa, K.; Tanaka, Y.; Nakahara, Y. Chem. Mater. 2004, 16, 5420. (27) Fujiwara, M.; Shiokawa, K.; Sakakura, L.; Nakahara, Y. Nano Lett. 2006, 6, 2925. (28) Koo, H. Y.; Chang, S. T.; Choi, W. S.; Park, J.-H.; Kim, D.-Y.; Velev, O. D. Chem. Mater. 2006, 18, 3308. (29) Yang, S.; Liu, H. R. J. Mater. Chem. 2006, 16, 4480. (30) Yang, S.; Liu, H. R.; Zhang, Z. C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3900.

Yang et al. Scheme 1. Schematic Illustration of the Procedure for Preparing Multihollow Superparamagnetic Nanocomposite Microspheres

emulsifier and sodium dodecyl sulfate (SDS) acts as a watersoluble surfactant. The overall synthetic procedure is shown in Scheme 1. The formation mechanism of hollow microspheres in this paper will be discussed later. The effect of the different amounts of MPs on the morphology of the final products is studied. Otherwise, preliminary results of magnetic properties of the multihollow magnetite/polystyrene microspheres are demonstrated.

Experimental Section Materials. Ferric chloride (FeCl3), ferrous sulfate heptahydrate (FeSO4 · 7H2O), sodium dodecyl sulfate (SDS), cetyl alcohol (CA), aqueous ammonia [25% (w/w)], sodium hydrosulfite (NaHSO3), oleic acid, and ethanol were all of analytical grade and were used without any further treatment. Potassium peroxodisulfate (KPS) was removed of impurities by recrystallization. Styrene was of reagent grade from commercial sources and was distilled under a reduced pressure before use. Preparation of MPs Surface-Modified with Oleic Acid. MPs were synthesized by the conventional coprecipitation method with some modification as in our previous work,29 which in this work is the same as Mag-1 mentioned in that paper. Synthesis of Multihollow Superparamagnetic Magnetite/ Polystyrene Nanocomposite Microspheres. In a typical experiment, MPs (1 g) and cetyl alcohol (0.31 g) were dispersed in styrene (5 g). The obtained stable oil-based dispersion was added to a solution of SDS (0.12 g) in water (45 mL). The mixture was stirred for 1 h, followed by ultrasonication for 10 min. During ultrasonication, the mixture was kept cold to avoid polymerization. Then the mixture

Fabrication of Magnetite/Polystyrene Microspheres

Langmuir, Vol. 24, No. 18, 2008 10397

Table 1. Experimental Conditions and Different Morphologies of the Samples sample 1 2 3 4 5

experimental conditions KPS initiation at 70 °C under stirring KPS initiation at 70 °C without stirring KPS and NaHSO3 initiation at room temperature under stirring

mass of MPs added (g)

morphology of the microspheres

av pore size (nm)

0.25 0.5 1 1 1

hollow or several-hollow multihollow multihollow solid solid with a few-hollow

500 161 145

was transferred to a 250 mL three-necked flask equipped with a condenser, a nitrogen inlet, and a mechanical stirrer and agitated for 30 min. After the temperature increased to 70 °C, 0.05 g of KPS was added to initiate the polymerization, which proceeded for 22 h at 70 °C. The resulting brown magnetic microspheres were separated by magnetic decantation, washed with deionized water and ethanol several times, and then dried in air at ambient temperature. To study the effects of stirring and temperature on the formation of multihollow nanocomposite microspheres, two contrastive experiments were carried out. For studying the effect of stirring, the emulsion polymerization was initiated by KPS at 70 °C without stirring. For studying the effect of temperature, reducer NaHSO3 was added to reduce the activation energy of the decomposition of KPS, inducing polymerization of styrene at room temperature with KPS as a water-soluble redox initiator because KPS could not initiate the polymerization of styrene at room temperature. The polymerization of the same emulsion was induced by water-soluble redox initiators (KPS and NaHSO3; the molar ratio is 1:1) under continuous stirring at room temperature for 22 h. The experimental conditions and main results in this paper are listed in Table 1. Characterization of MPs Surface-Modified with Oleic Acid and Multihollow Superparamagnetic Magnetite/Polystyrene Nanocomposite Microspheres. Powder X-ray diffraction (XRD) patterns of as-synthesized samples were recorded with a Japan Rigaku D/max γA X-ray diffractometer equipped with graphite-monochromatized Cu KR irradiation (λ ) 0.154178 nm), employing a scanning rate of 0.02 deg/s in the 2θ range from 20° to 80°. Transmission electron microscopy (TEM) images were observed on a Hitachi model H-800 transmission electron microscope and JEOL 2010 microscope with an accelerating voltage of 200 kV. For microtoming samples, the sample powder was embedded into epoxy resin, which was then ultramicrotomed to a thickness of ca. 70 nm. Field-emission scanning electron microscopy (FESEM) images were obtained on a JEOL JSM-6700 field-emission scanning electron microanalyzer. Fourier transform infrared (FTIR) spectra were obtained on a Bruker Vector-22 FTIR spectrometer using the KBr method. Thermogravimetric analysis (TGA) was performed with a Shimadzu TGA-50H instrument under an atmosphere of nitrogen with a gas flow of 25 cm3 · min-1. The sample was heated from 40 to 700 at 10 °C · min-1. The magnetic properties of the powder samples were evaluated on an MPMS XL magnetometer (Quantum Design Corp.). During M-H measurement, the applied magnetic field successively varied in the sequence 0/1/-1/1 T, and magnetic susceptibilities were not corrected for the background signal of the sample holder and for diamagnetic susceptibilities of all atoms, because they were very small (