pubs.acs.org/Langmuir © 2009 American Chemical Society
Repetitive Heterocoagulation of Oppositely Charged Particles for Enhancement of Magnetic Nanoparticle Loading into Monodisperse Silica Particles Hideki Matsumoto, Daisuke Nagao, and Mikio Konno* Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07, Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan Received September 1, 2009. Revised Manuscript Received September 29, 2009 Oppositely charged particles were repetitively heterocoagulated to fabricate highly monodisperse magnetic silica particles with high loading of magnetic nanoparticles. Positively charged magnetic nanoparticles prepared by surface modification with N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TSA) were used to heterocoagulate with silica particles under basic conditions to give rise to negative silica surface charge and prevent the oxidation of the magnetic nanoparticles. The resultant particles of silica core homogeneously coated with the magnetic nanoparticles were further coated with thin silica layer with sodium silicate in order to enhance colloidal stability and avoid desorption of the magnetic nanoparticles from the silica cores. Five repetitions of the heterocoagulation and the silica coating could increase saturation magnetization of the magnetic silica particles to 27.7 emu/g, keeping the coefficient of variation of particle sizes (CV) less than 6.5%. Highly homogeneous loading of the magnetic component was confirmed by measuring Fe-to-Si atomic ratios of individual particles with energy dispersive X-ray spectroscopy.
Introduction Magnetic nanoparticles are promising materials applicable to drug delivery and separation of biochemical products, since the nanoparticles can be moved with an external magnetic field. It is desirable that the nanoparticles are isolated by capping ligands1 or inert materials such as silica and polymer2-4 to avoid aggregation of the nanoparticles in the application systems. Synthesis of magnetic silica particles has been intensively studied so far. The St€ober method is effective for incorporation of the magnetic nanoparticles into silica particles formed in the hydrolysis of silicon alkoxides.1,2,5,6 The hydrolysis in water droplets in oil (w/o emulsion) has also been used to prepare the magnetic silica particles with sizes of several tens of nanometers to micrometers.7-9 However, the homogeneities in particle size and loading of magnetic component into silica particles were not high enough to be applied to highly quantitative analysis. An approach to obtain monodisperse composite particles with homogeneous loading of functional component to each particle is *To whom correspondence should be addressed: Tel þ81-22-795-7239; Fax þ81-22-795-7241; e-mail
[email protected].
(1) Barnakov, Y. A.; Yu, M. H.; Rosenzweig, Z. Langmuir 2005, 21, 7524–7527. (2) Im, S. H.; Herricks, T.; Lee, Y. T.; Xia, Y. Chem. Phys. Lett. 2005, 401, 19– 23. (3) Sun, Y.; Duan, L.; Guo, Z.; DuanMu, Y.; Ma, M.; Xu, L.; Zhang, Y.; Gu, N. J. Magn. Magn. Mater. 2005, 285, 65–70. (4) Xu, H.; Cui, L.; Tong, N.; Gu, H. J. Am. Chem. Soc. 2006, 128, 15582–15583. (5) Salgueiri~no-Maceira, V.; Correa-Duarte, M. A.; Spasova, M.; Liz-Marzan, L. M.; Farle, M. Adv. Funct. Mater. 2006, 16, 509–514. (6) Kobayashi, Y.; Saeki, S.; Yoshida, M.; Nagao, D.; Konno, M. J. Sol-Gel Sci. Technol. 2008, 45, 35–41. (7) Tartaj, P.; Serna, C. J. J. Am. Chem. Soc. 2003, 125, 15754–15755. (8) Xu, Z. Z.; Wang, C. C.; Yang, W. L.; Fu, S. K. J. Mater. Sci. 2005, 40, 4667– 4669. (9) Stjerndahl, M.; Andersson, M.; Hall, H. E.; Pajerowski, D. M.; Meisel, M. W.; Duran, R. S. Langmuir 2008, 24, 3532–3536. (10) Furusawa, K.; Nagashima, K.; Anzai, C. Colloid Polym. Sci. 1994, 272, 1104–1110. (11) Caruso, F.; Susha, A. S.; Giersig, M.; M€ohwald, H. Adv. Mater. 1999, 11, 950–953. (12) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem. Mater. 2001, 13, 109–116.
Langmuir 2010, 26(6), 4207–4211
electrostatic heterocoagulation between oppositely charged particles.10-13 Furusawa et al. heterocoagulated negatively charged polystyrene lattices with positively charged magnetic nanoparticles at pH 2.5.10 The magnetic nanoparticles heterocoagulated with the core lattices were desorbed from the composite particles that were redispersed in solutions above pH 9. A layer-by-layer (LbL) technique has also been used to heterocoagulate Fe3O4 nanoparticles onto polystyrene particles11,12 and silica particles.13 Three polyelectrolyte layers of PDADMAC/ PSS/PDADMAC on polystyrene particles were required for homogeneous coating of the polystyrene particles with Fe3O4 nanoparticles.11-13 The authors recently developed a method for producing monodisperse, magnetic particles using heterocoagulation of silica particles with positively charged magnetic nanoparticles.14 The magnetic nanoparticles were prepared with a modified Massart method in which a silane coupling agent of N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TSA) was added to the conventional system.15 The heterocoagulation was conducted with 234 nm silica particles and ca. 8 nm magnetic particles and under basic conditions to give rise to negative surface charge of the silica particles and prevent the oxidation of the magnetic nanoparticles.14 The resultant particles of silica core homogeneously coated with the magnetic nanoparticles were further coated with silica thin layer with sodium silicate in order to enhance colloidal stability and avoid desorption of the magnetic nanoparticles from the silica cores. The composite particles had a saturation magnetization (MS) of 6.9 emu/g. The previous work examined only a single-step heterocoagulation. However, since the final silica coating step gives rise to negative surface charge on the composite particles, the heterocoagulation process may be repeated to increase the MS of the (13) Zhu, Y.; Da, H.; Yang, X.; Hu, Y. Colloids Surf., A 2003, 231, 123–129. (14) Nagao, D.; Yokoyama, M.; Yamauchi, N.; Matsumoto, H.; Kobayashi, Y.; Konno, M. Langmuir 2008, 24, 9804–9808. (15) Massart, R. IEEE Trans. Magn. 1981, Mag-17, 1247–1248.
Published on Web 10/14/2009
DOI: 10.1021/la903266e
4207
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Matsumoto et al.
composite particles. In the present work, repetitions of the heterocoagulation are examined with varying the sizes of silica and magnetic nanoparticles. We measure the size distribution and saturation magnetization of composite particles prepared by multiple processes of the heterocoagulation and the silica coating. To our knowledge, most work in the literature did not measure magnetic content in individual particles or colloidal sizes of the particles dispersed in solvent. However, the distribution of the magnetic contents and the dispersion of the particles in solvent are important, especially for advanced application and/or highly quantitative analysis. In this regard, the present work also measures the individual compositions and the colloidal sizes of the composite particles in water.
Experimental Section Chemicals. Ammonia solution (25%), ethanol (99.5%), hydrochloric acid (2 M), iron(III) chloride (FeCl3, 95%), and tetraethyl orthosilicate (TEOS, 95%) were purchased from Wako Pure Chemical Industries (Osaka, Japan) and used as received. Iron(II) chloride (FeCl2, 99.9%, Superpurification Science Laboratory, Saitama, Japan), N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TSA, 50% methanol solution, Gelest Ink., U.S.), sodium silicate (27 wt % SiO2, Sigma-Aldrich Co.), and ion-exchange resin (Dowex Monosphere 650C, The Dow Chemical Co.) were used without purification. Water was deionized (>18.2 MΩ 3 cm) and bubbled with nitrogen for 30 min. Synthesis of Positively Charged Magnetic Nanoparticles.
The nanoparticles were prepared with a modified coprecipitation method.14,15 An aqueous solution (200 g) containing 3.24 g of FeCl3 and 1.27 g of FeCl2 was bubbled with nitrogen for 30 min. Another solution of 11.2 g of ammonia solution and 35.0 g of water was added to the ferrous solution under stirring at 35 °C. To modify surface of the magnetic nanoparticles, 1.93 g of TSA was injected into the suspension after addition of the ammonia solution. The injection time to control the size of magnetic nanoparticles was varied from 0 to 60 s after the ammonia addition. Black precipitate obtained after mixing for 3 h was washed with ethanol in centrifugal processes (22600g). A vacuumdried sample of the nanoparticles (50 °C, 1 h) was redispersed in water at the concentration of 0.91 wt %. The magnetic nanoparticles were mainly composed of Fe3O4 with Fe2O3 as residual component.14 Heterocoagulation of Oppositely Charged Particles. A suspension of the positively charged magnetic naoparticles was mixed with a suspension of silica particles prepared with the St€ ober method.16 The ratio of magnetic nanoparticle weight to unit SiO2 surface area was 20 mg/m2. The pH values of the mixed suspensions were in a range of 9-10. Residual magnetic nanoparticles that did not adsorb onto the silica surface were removed with several centrifuge processes (10000g, 10 min). Final suspension weight in the heterocoagulation was adjusted to 30 g by adding water to the suspension. Silica Coating of Heterocoagulates. Approximately 2 g of ion-exchange resin was added to 70 g of aqueous solution containing 1.85 g of sodium silicate to adjust the pH of silicate solution to 10. The silicate solution was mixed with the 30 g suspension of heterocoagulates, and the pH of the mixed solution was lowered to 9 by the addition of hydrochloric acid. After an hour under stirring the suspension at room temperature, 200 g of ethanol was added to lower the solubility of silica in the suspension. After another hour of stirring, the suspension was centrifuged several times (10000g, 10 min) to redisperse the particles in water. Final suspension weight in the silica coating step was adjusted to 30 g. 80% of the aqueous suspension was used for the next heterocoagulation, and the remaining 20% was used for characterization such as saturation magnetization. (16) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69.
4208 DOI: 10.1021/la903266e
Figure 1. TEM images of Fe3O4 nanoparticles prepared with the addition of TSA that was added (A) simultaneously with and (B) added 30 s after the ammonia addition for the preparation of nanoparticles.
Characterization. Transmission electron microscopy (TEM, Zeiss, LEO 912 OMEGA) was used for particle observation. More than 200 diameters were measured for each distribution and used to calculate the volume-averaged diameter, dV, and coefficient of variation of the particle diameter, CV. The electrophoretic mobility of the particles was measured at ambient temperature with ELS-8000 (Otsuka Electronics) equipped with an apparatus for dynamic light scattering (DLS). The Smoluchowski equation was used to convert the electrophoretic mobility into the zeta potential. A vibrating-sample magnetometer (VSM, BHV50HM, Riken Denshi, Japan) was used at room temperature to measure the magnetic properties of nanoparticles and the particles in each step. Energy dispersive X-ray spectroscopy (EDX, NORAN Instruments) was applied to more than 10 particles formed after the silica coating to examine the atomic compositions of Fe and Si in each particle.
Results and Discussion Figure 1 shows TEM images of magnetic nanoparticles prepared with the addition of silane coupling agent of TSA. TSA was injected into a mixed solution of Fe2þ and Fe3þ simultaneously with the ammonia addition for Figure 1A and injected 30 s after the ammonia addition for Figure 1B. It is reported that the magnetic nanoparticles were composed of Fe3O4 and Fe2O3.14 The magnetic nanoparticles of Figure 1B had an average size of 9 nm and MS value of 67 emu/g comparable to that of magnetic nanoparticles prepared with the conventional Massart method,15 whereas those of Figure 1A had an average size of 4 nm and 30 emu/g MS. The size of magnetic nanoparticles could be varied by the injection time in an early stage of reaction (
