Langmuir 2007, 23, 5143-5147
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Surfactant Templating Effects on the Encapsulation of Iron Oxide Nanoparticles within Silica Microspheres Tonghua Zheng,† Jiebin Pang,† Grace Tan,† Jibao He,‡ Gary L. McPherson,§ Yunfeng Lu,*,† Vijay T. John,*,† and Jingjing Zhan† Department of Chemical and Biomolecular Engineering, Coordinated Instrumentation Facility, and Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118 ReceiVed December 29, 2006. In Final Form: February 19, 2007 Hollow silica microspheres encapsulating ferromagnetic iron oxide nanoparticles were synthesized by a surfactantaided aerosol process and subsequent treatment. The cationic surfactant cetyltrimethyl ammonium bromide (CTAB) played an essential role in directing the structure of the composite. Translation from mesoporous silica particles to hollow particles was a consequence of increased loading of ferric species in the precursor solution and the competitive partitioning of CTAB between silicate and ferric colloids. The hypothesis was that CTAB preferentially adsorbed onto more positively charged ferric colloids under acidic conditions. At a critical Fe/Si ratio, most of the CTAB was adsorbed onto ferric colloids and coagulated the colloids to form larger clusters. During the aerosol process, a silica shell was first formed due to the preferred silicate condensation on the gas-liquid interface of the aerosol droplet. Subsequent drying concentrated the ferric clusters inside the silica shell and resulted in a silica shell/ferric core particle. Thermal treatment of the core shell particle led to encapsulation of a single iron oxide nanoparticle inside each silica hollow microsphere.
Introduction The synthesis of iron oxide nanoparticles is an area of extensive research in applications ranging from data storage and magnetic resonance imaging (MRI) to the magnetic separation of biomolecules, drug delivery, and environmental remediation.1-5 In several synthetic schemes, surfactants and/or polymers are commonly added to stabilize the nanoparticles and prevent them from aggregating.6,7 However, the poor thermal stability of the organic ligands presents a problem with high-temperature processing of such magnetic materials. Applying ceramic coatings or encapsulating magnetic nanoparticles within silica to protect the particles from intersystem dipolar interactions and sintering has been demonstrated to be an effective way to enhance stability.8 The commonly employed method for incorporation of iron oxide nanoparticles into the silica matrix is the solution condensation of silica in an aqueous sol of iron oxide, resulting in the encapsulation of the nanoparticles * Corresponding author. E-mail:
[email protected]; phone: (504) 865-5883; fax: (504) 865-6744. † Department of Chemical and Biomolecular Engineering. ‡ Coordinated Instrumentation Facility. § Department of Chemistry. (1) (a) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. (b) Park, J.; Kim M.; Jun, Y.; Lee, J.; Lee, W.; Cheon, W. J. Am. Chem. Soc. 2004, 126, 9072. (2) (a) Babes, L.; Denizot, B.; Tanguy, G.; Le Jeune, J. J.; Jallet, P. J. Colloid Interface Sci. 1999, 212, 474. (b) Byrne, S. J.; Corr, S. A.; Gun’ko, Y. K.; Kelly, J. M.; Brougham, D. F.; Ghosh, S. Chem. Commun. 2004, 22, 2560. (3) (a) Bucak, S.; Jones, D. A.; Laibinis, P. E.; Hatton, T. A. Biotechnol. Prog. 2003, 19, 477. (b) Bruce, I. J.; Sen, T. Langmuir 2005, 21, 7029. (c) Yu, W. W.; Falkner, J. C.; Yavuz, C. T.; Colvin, V. L. Chem. Commun. 2004, 20, 2306. (d) Yang, H.-H.; Zhang, S.-Q.; Chen, X.-L.; Zhuang, Z.-X.; Xu, J.-G.; Wang, X.-R. Anal. Chem. 2004, 76, 1316. (e) Son, S. J.; Reichel, J.; He, B.; Schuchman, M.; Lee, S. B. J. Am. Chem. Soc. 2005, 127, 7316. (4) Jain, T. K.; Morales, M. A.; Sahoo, S. K.; Leslie-Pelecky, D. L.; Labhasetwar, V. Mol. Pharmaceutics 2005, 2, 194. (5) (a) Martin, T. A.; Kempton, J. H. EnViron. Sci. Technol. 2000, 34, 3229. (b) Jang, M.; Shin, E. W.; Park, J. K.; Choi, S. I. EnViron. Sci. Technol. 2003, 37, 5062. (c) Guo, X.; Chen, F. EnViron. Sci. Technol. 2005, 39, 6808. (6) Redl, F. X.; Black, C. T.; Papaefthymiou, G. C.; Sandstrom, R. L.; Yin, M.; Zeng, H.; Murray, C. B.; O’Brien, S. P. J. Am. Chem. Soc. 2004, 126, 14583. (7) Rockenberger, J.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 1999, 121, 11595. (8) Jolivet, J. P.; Chaneac, C.; Tronc, E. Chem. Commun. 2004, 5, 477.
in a matrix of silica.9 The porosity of the silica matrix can be tuned by adding a structure directing agent in the silica sources, leading to a mesoporous silica shell/magnetic iron oxide core.10 The presence of the silica coating effectively protects the iron oxide nanoparticles from aggregating and sintering at temperatures as high as 1000 °C. An additional advantage of using a silica coating is the fact that the surface silanol groups can also serve as functional sites to graft biomolecules, such as protein and drugs on magnetic particles.11 This makes such silica coated nanoparticles a promising candidate for applications in bioseparations and targeted drug delivery under magnetic gradients.1,10 The discovery of mesoporous silica templated by surfactants12 has led to interest in the incorporation of magnetic particles into porous silica matrices. Ionic solutions of an iron salt (e.g., ferric chloride) can be used as precursors that are either coprecipitated with the silica source or infiltrated into porous silica prepared by condensation of the silica source.13 The subsequent drying and thermal treatment results in random distribution of iron oxide nanoparticles inside the silica matrix. Our previous work has demonstrated that an aerosol-assisted process is a simple and efficient approach to prepare ordered (9) (a) Garcia, C. B. W.; Zhang, Y.; Mahajan, S.; DiSalvo, F.; Wiesner, U. J. Am. Chem. Soc. 2003, 125, 13310. (b) Santra, S.; Tapec, R.; Theodoropoulou, N.; Dobson, J.; Hebard, A.; Tan, W. Langmuir 2001, 17, 2900. (c) Moreno, E. M.; Zayat, M.; Morales, M. P.; Serna, C. J.; Roig, A.; Levy, D. Langmuir 2002, 18, 4972. (d) Kim, D. K.; Mikhaylova, M.; Zhang, Y.; Muhammed, M. Chem. Mater. 2003, 15, 1617. (10) Zhao, W.; Gu, J.; Zhang, L.; Chen, H.; Shi, J. J. Am. Chem. Soc. 2005, 127, 8916. (11) (a) Li, H.; Perkas, N.; Li, Q.; Gofer, Y.; Koltypin, Y.; Gedanken, A. Langmuir 2003, 19, 10409. (b) Bourlinos, A. B.; Simopoulos, A.; Boukos, N.; Petridis, D. J. Phys. Chem. B 2001, 105, 7432. (c) Vallet-Regi, M.; Ra´mila, A.; Del Real, R.; Pe´rez-Pariente, J. Chem. Mater. 2001, 13, 308. (12) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) 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.; Schlenkert, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (c) Yang, H.; Coombs, N.; Ozin, G. AdV. Mater. 1997, 9, 811. (d) Tanev, P.; Pinnanvaia, T. Science 1996, 271, 1267. (13) Samanta, S.; Giri, S.; Sastry, P. U.; Mal, N. K.; Manna, A.; Bhaumik, A. Ind. Eng. Chem. Res. 2003, 42, 3012. (b) Froba, M.; Kohn, R.; Bouffaud, G.; Richard, O.; van Tendeloo, G. Chem. Mater. 1999, 11, 2858.
10.1021/la063761+ CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007
5144 Langmuir, Vol. 23, No. 9, 2007
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Figure 1. Experimental set up of the aerosol reactor.
Figure 2. (a) TEM image of an as-synthesized particle; (b) TEM image of a calcined particle; (c) TEM of calcined particles at low magnification; (d) SEM image of calcined particles at a magnification comparable to panel c; (e) TEM image of a particle reduced by H2/N2; and (f) high-resolution TEM image of the encapsulated Fe3O4 nanoparticle from panel e; inset: corresponding [011] zone axis electron diffraction of the nanoparticle.
mesoporous silica particles.14 As compared to the irregular shape of the particles synthesized by solution condensation of the silica source, silica particles prepared by the aerosol process show a well-defined spherical shape and highly ordered mesostructures. More recently, the one-step aerosol process has been utilized to incorporate magnetic nanoparticles into disordered microporous silica particles.15 Hollow particles randomly incorporating many supermagnetic Fe2O3 nanoparticles (
