Synthesis of Highly Monodisperse Particles Composed of a Magnetic

Jul 24, 2008 - Ayako Okada , Daisuke Nagao , Takuya Ueno , Haruyuki Ishii , and Mikio ... Daisuke Nagao , Carlos M. van Kats , Kentaro Hayasaka , Maki...
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Langmuir 2008, 24, 9804-9808

Synthesis of Highly Monodisperse Particles Composed of a Magnetic Core and Fluorescent Shell Daisuke Nagao, Mikio Yokoyama, Noriko Yamauchi, Hideki Matsumoto, Yoshio Kobayashi, 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 May 1, 2008. ReVised Manuscript ReceiVed June 11, 2008 Highly monodisperse particles composed of a magnetic silica core and fluorescent polymer shell were synthesized with a combined technique of heterocoagulation and soap-free emulsion polymerization. Prior to heterocoagulation, monodisperse, submicrometer-sized silica particles were prepared with the Sto¨ber method, and magnetic nanoparticles were prepared with a modified Massart method in which a cationic silane coupling agent of N-trimethoxysilylpropylN,N,N-trimethylammonium chloride was added just after coprecipitation of Fe2+ and Fe3+. The silica particles with negative surface potential were heterocoagulated with the magnetic nanoparticles with positive surface potential. The magnetic silica particles obtained with the heterocoagulation were treated with sodium silicate to modify their surfaces with silica. In the formation of a fluorescent polymer shell onto the silica-coated magnetic silica cores, an amphoteric initiator of 2,2′-azobis[N-(2-carboxyethyl)-2-2-methylpropionamidine] (VA-057) was used to control the colloidal stability of the magnetic cores during the polymer coating. The polymerization of St in the presence of a hydrophobic fluorophore of pyrene could coat the cores with fluorescent polymer shells, resulting in monodisperse particles with a magnetic silica core and fluorescent polymer shell. Measurements of zeta potential for the composite particles in different pH values indicated that the composite particles had an amphoteric property originating from VA-057 initiator.

Introduction Functional composite particles with magnetism and fluorescence have been of great interest in applications such as cell labeling,1–4 biosensing,5 and diagnostic medical devices.6,7 A large number of methods have been developed to synthesize several types of magnetic, fluorescent microspheres such as emulsion,8 polymeric micelles,9 polymer-based particles,10,11 and silica-based particles.12–18 Mandal et al. prepared oil droplets containing both magnetic iron oxide particles and fluorescent * To whom correspondence should be addressed. Telephone: +81-22795-7239. Fax: +81-22-795-7241. E-mail: [email protected]. (1) Vuu, K.; Xie, J.; McDonald, M. A.; Bernardo, M.; Hunter, F.; Zhang, Y.; Li, K.; Bednarski, M.; Guccione, S. Bioconjugate Chem. 2005, 16, 995–999. (2) Lin, Y.-S.; Wu, S.-H.; Hung, Y.; Chou, Y.-H.; Chang, C.; Lin, M.-L.; Tsai, C.-P.; Mou, C.-Y. Chem. Mater. 2006, 18, 5170–5172. (3) Guo, J.; Yang, W.; Deng, Y.; Wang, C.; Fu, S. Small 2005, 1, 737–743. (4) Selvan, S. T.; Patra, P. K.; Ang, C. Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2007, 46, 2448–2452. (5) Dubus, S.; Gravel, J.-F.; Drogoff, B. L.; Nobert, P.; Veres, T.; Boudreau, D. Anal. Chem. 2006, 78, 4457–4464. (6) Guo, J.; Yang, W.; Wang, C.; He, J.; Chen, J. Chem. Mater. 2006, 18, 5554–5562. (7) Salgueirin˜o-Maceira, V.; Correa-Duarte, M. A. AdV. Mater. 2007, 19, 4131– 4144. (8) Mandal, S. K.; Lequeux, N.; Rotenberg, B.; Tramier, M.; Fattaccioli, J.; Bibette, J.; Dubertret, B. Langmuir 2005, 21, 4175–4179. (9) Nasongkla, N.; Bey, E.; Ren, J.; Ai, H.; Khemtong, C.; Guthi, J. S.; Chin, S.-F.; Sherry, A. D.; Boothman, D. A.; Gao, J. Nano Lett. 2006, 6, 2427–2430. (10) Qiu, G.-M.; Xu, Y.-Y.; Zhu, B.-K.; Qiu, G.-L. Biomacromolecules 2005, 6, 1041–1047. (11) Holzapfel, V.; Lorenz, M.; Weiss, C. K.; Schrezenmeier, H.; Landfester, K.; Mailander, V. J. Phys.: Condens Matter 2006, 18, S2581–S2594. (12) Salgueirin˜o-Maceira, V.; Correa-Duarte, M. A.; Spasova, M.; Liz-Marza´n, L. M.; Farle, M. AdV. Funct. Mater. 2006, 16, 509–514. (13) Zhang, L.; Liu, B.; Dong, S. J. Phys. Chem. B 2007, 111, 10448–10452. (14) Sathe, T. R.; Agrawal, A.; Nie, S. Anal. Chem. 2006, 78, 5627–5632. (15) Yu, S.-Y.; Zhang, H.-J.; Yu, J.-B.; Wang, C.; Sun, L.-N.; Shi, W.-D. Langmuir 2007, 23, 7836–7840. (16) Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990–4991. (17) Lu, Y.; Yin, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 183–186. (18) Liu, Z.; Yi, G.; Zhang, H.; Ding, J.; Zhang, Y.; Xue, J. Chem. Commun. 2008, 694–696.

semiconductor quantum dots.8 Salgueirin˜o-Maceira et al. synthesized composite particles with a magnetic silica core and a fluorescent silica shell.12 The authors recently created composite particles with a magnetic silica core and a fluorescent polymer shell.19 The development of the synthesis has enabled the production of magnetic, fluorescent microspheres composed of various materials. However, another important factor for further development is homogeneity in particle sizes and contents of functional components, which will be required for advanced application and/or highly quantitative analysis. Attainment of high homogeneity is the purpose of the present study that intends to prepare composite particles of a magnetic silica core/fluorescent polymer shell. In our previous report,19 magnetic silica cores were prepared with the addition of magnetic nanoparticles into a Sto¨ber system during the hydrolysis of tetraethyl orthosilicate (TEOS). The CV value of the magnetic silica particles was not lower than the criterion of monodisperse particles, resulting in inadequate monodispersity of the composite particles. In the present work, monodisperse silica particles prepared with the Sto¨ber method are heterocoagulated with oppositely charged magnetic nanoparticles to achieve high homogeneities in the particle size and the content of magnetic nanoparticles. The particles obtained with heterocoagulation are surface-modified with silica to apply a previous method19,20 to the formation of fluorescent polymer shells on the magnetic cores. The coating of the polymer shell is conducted via soap-free emulsion polymerization in which an amphoteric initiator of 2,2′-azobis(2-methylpropionamidine) (VA-057) is employed under pH control of the reactant solution, since our previous (19) Nagao, D.; Yokoyama, M.; Saeki, S.; Kobayashi, Y.; Konno, M. Colloid Polym. Sci. 2008, 286, 959–964. (20) Gu, S.; Kondo, T.; Konno, M. J. Colloid Interface Sci. 2004, 272, 314– 320.

10.1021/la801364w CCC: $40.75  2008 American Chemical Society Published on Web 07/24/2008

Fluorescent, Magnetic, Monodisperse Particles

Figure 1. Zeta potentials of different particles in a pH range of 2-12: magnetic nanoparticles prepared with the addition of TSA (9), magnetic nanoparticles without the addition (0), silica particles (O), magnetic silica particles obtained with heterocoagulation between the silica particles and the TSA-modified magnetic nanoparticles (2), and composite particles after silica coating of the magnetic silica with sodium silicate (b).

work21–24 indicated that the use of VA-057 initiator under pH control is effective for maintaining the electrostatic surface potential of polymer particles to an appropriate range that keeps particle dispersion stable without generation of new polymer particles. The use of an amphoteric initiator can also introduce weakly dissociated groups such as carboxyl groups on the surface of composite particles, which are very useful to attach other functional molecules on the particle surface.9–11,25,26 This is another reason for the employment of the amphoteric initiator to the preparation of monodisperse composite particles.

Experimental Section Materials. Iron(III) chloride (FeCl3), tetraethyl orthosilicate (TEOS, 95%), ethanol (99.5%), styrene (99%), 2,2′-azobis[N-(2carboxyethyl)-2-2-methylpropionamidine] (VA-057), pyrene, ammonia hydroxide (25%), methylamine (MA, 40%), and hydrochloric acid (25%) were obtained from Wako Pure Chemical Industries (Osaka, Japan) and used as received. 3-Methacryoxypropyltrimethoxysilane (MPTMS, 95%, Shinetsu Chemicals, Tokyo) was used without further purification. Iron(II) chloride (FeCl2) and sodium silicate (Na2O(SiO2)3-5, 27 wt % SiO2) were purchased from Superpurification Science Laboratory (Saitama, Japan) and SigmaAldrich Corporation, respectively. A cationic silane coupling agent of N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TSA, 50 wt % methanol solution) was purchased from Gelest Inc. Styrene was distilled at a reduced pressure under a nitrogen atmosphere. Water was deionized and distilled to have an electric resistance higher than 18 MΩ · cm. Preparation of Magnetic Silica Particles. The positively charged magnetic nanoparticles were prepared with a modified Massart method.27 Two aqueous solutions of 2 M FeCl2 (5 cm3) and 1 M FeCl3 (20 cm3) were added to 212 cm3 of water that had been bubbled with nitrogen for 30 min. The NH3 solution with a volume of 11.9 cm3 was then added to the mixture. The 1.9 g methanolic solution of TSA was injected into the suspension 30 s after addition of the NH3 solution. Black precipitate obtained after mixing for 3 h was washed in centrifugal processes with ethanol and water. The silica particles used for heterocoagulation were prepared with the Sto¨ber method using 0.2 M TEOS, 11 M H2O, and 0.05 M MA (21) Gu, S.; Inukai, S.; Konno, M. J. Chem. Eng. Jpn. 2002, 35, 977–981. (22) Gu, S.; Inukai, S.; Konno, M. J. Chem. Eng. Jpn. 2003, 36, 1231–1235. (23) Yamada, Y.; Sakamoto, T.; Gu, S.; Konno, M. J. Colloid Interface Sci. 2005, 281, 249–252. (24) Gu, S.; Akama, H.; Nagao, D.; Kobayashi, Y.; Konno, M. Langmuir 2004, 20, 7948–7951. (25) Lee, H.; Yu, M. K.; Park, S.; Moon, S.; Min, J. J.; Jeong, Y. Y.; Kang, H.-W.; Jon, S. J. Am. Chem. Soc. 2007, 129, 12739–12745. (26) Holzapfel, V.; Musyanovych, A.; Landfester, K.; Lorenz, M. R.; Mailander, V. Macromol. Chem. Phys. 2005, 206, 2440–2449. (27) Massart, R. IEEE Trans. Magn. 1981, MAG-17, 1247–1248.

Langmuir, Vol. 24, No. 17, 2008 9805 in ethanol at 35 °C. At an ambient temperature, a given amount of suspension of the magnetic nanoparticles was mixed with the silica particle suspension (2.28 silica wt %) during heterocoagulation. After mixing for 30 min, the mixture was centrifuged to remove magnetic nanoparticles that were not attached onto silica surfaces. The final residual suspension volume of magnetic silica particles was 10 cm3. The magnetic silica particles were coated with a silica thin layer formed in sodium silicate solution with pH adjustment. A 40 cm3 sodium silicate solution at 0.12 M (pH ) 10) was added to the 10 cm3 suspension of magnetic silica particles, and hydrochloric acid was added to lower the solution pH to 9. The solution was diluted with 200 cm3 of ethanol and kept under stirring for 24 h. After several centrifuge processes for washing, the volume of the final residual suspension of silica-coated magnetic silica particles was adjusted to 10 cm3 with water. Formation of Fluorescent Polymer Shells onto the SilicaCoated Magnetic Silica Particles. The formation of polymer shells was performed in a batch reactor (500 cm3) equipped with a fourbladed 45° pitched paddle impeller with a diameter of 6 cm. The formation was mainly performed by the following procedure. MPTMS was mixed with a suspension of the silica-coated magnetic silica particles, and the mixed solution was kept at 35 C° for 30 min. The concentrations of MPTMS and silica-coated magnetic silica particles were 0.43 mol/m3 and 0.15 vol %, respectively. After heating up to 70 C°, a styrene monomer dissolving pyrene was injected into the mixed solution, and 20 min later an aqueous solution of VA-057 was added to initiate polymerization. The polymerization was continued for 8 h under a nitrogen atmosphere and stirring at 300 rpm. Characterization of Particles. Transmission electron microscopy (TEM, Zeiss, LEO 912 OMEGA) and field emission (FE)-TEM (Hitachi, HF-2000) were used for particle observation. More than 100 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 amount of TSA introduced onto the magnetic nanoparticles was measured via thermal gravity analysis (TGA, Seiko Instruments Inc., TG/DTA). 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 synthetic step. Suspensions of those particles were centrifuged and dried in vacuum conditions for 12 h before the VSM measurement. The dry powder was also subjected to XRD measurement (Rigaku, RU-200A). Fluorescence spectra were measured with an excitation light of 338 nm in a Hitachi F-4500 fluorophotometer. Before the measurement, the composite particles were washed twice via centrifuging processes, and the suspension of the composite particles was diluted 1000-fold. The electrophoretic mobility of the particles was measured at ambient temperature with an electrophoresis light scattering (ELS-8000, Otsuka Electronics) instrument equipped with an apparatus for dynamic light scattering (DLS). The Smoluchowski equation was used to convert the electrophoretic mobility into the zeta potential.

Results and Discussion Synthesis of Magnetic Silica Particles with Low Polydispersity. Magnetic nanoparticles used for heterocoagulation were prepared with the modified Massart method conducted with the cationic coupling agent of TSA. The average diameter of the magnetic nanoparticles was 8 nm slightly smaller than the conventional nanoparticles prepared without the addition of TSA. A TEM image and XRD pattern of the magnetic nanoparticles are shown in the Supporting Information (Figures S-1 and S-2, respectively). Since XRD measurement cannot distinguish between Fe3O4 and Fe2O3, we conducted Raman spectroscopy analysis of the magnetic nanoparticles with a 488 nm Ar+ laser. The Raman spectrum corresponding to that of Fe3O4 (not γ-Fe2O3) was detected, although the peaks also corresponding to R-Fe2O3 were included in the spectrum due to the rapid oxidation by the

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Figure 2. TEM images of particles obtained by heterocoagulation at different weight ratios of magnetic nanoparticles to silica particles. The weight ratios were 10% (A), 20% (B), and 30% (C).

Figure 3. TEM image of composite particles obtained after the silica coating of the magnetic silica particles in Figure 2C.

laser irradiation. At the beginning of the irradiation, Fe3O4 was the major component of the nanoparticles. However, determination of purity was difficult due to the rapid oxidation. Zeta potentials of the nanoparticles formed with and without the addition of TSA are shown with square symbols in Figure 1. The TSA-modified nanoparticles were positively charged in pHs lower than 11, whereas the magnetic nanoparticles prepared without the addition had an isoelectric point (IEP) around 6. The addition of TSA shifted the IEP and broadened the positive region of the nanoparticles. TGA of the TSA-modified nanoparticles indicated that approximately 30% of the TSA added to the system was introduced onto the nanoparticles. The weight ratio of the TSA-modified nanoparticles to suspension was 1.1% that was determined from the residual weight of the suspension at 600 °C in the TGA. TGA profiles of the powder and suspension of TSAmodified nanoparticles are shown in the Supporting Information. The saturation magnetization (MS_nano) of a dried sample of the TSA-modified nanoparticles was 65.8 ((10%) emu/g, comparable to the one prepared without TSA addition. The suspension of the positively charged nanoparticles was mixed with another suspension of negatively charged silica particles (dV ) 234 nm, CV ) 3.1%) at different volume ratios of suspensions. Figure 2 shows TEM images of particles obtained after heterocoagulation at the three different ratios of magnetic

nanoparticles to silica particles. The surface occupancy of magnetic nanoparticles on silica particles in the TEM images seemed to be increased with the concentration of magnetic nanoparticles. VSM measurements showed that saturation magnetizations of the particles in Figure 2A, B, and C were 1.8, 4.4, and 6.9 emu/g, respectively (Supporting Information, Figure S-4). According to the saturation magnetization values, magnetic contents in the magnetic silica particles were 2.7, 6.7, and 10% for Figure 2A, B, and C, respectively, and almost 30% of the magnetic nanoparticles added to the silica suspension were adsorbed onto the silica particles in each case of Figure 2. Prior to coating the magnetic silica particles with polymer shells, they were surface-modified with sodium silicate in water to introduce a thin silica layer on their surfaces. Figure 3 shows a TEM image of composite particles obtained after the silica coating. The dV and CV values of the composite particles were 268 nm and 4.3%, respectively. The inset of Figure 3 presents a high magnification image of the particle surface. The magnetic nanoparticles of dark gray attached to the core were surrounded with a substance of light gray which seemed to be thin silica layers. Size distribution measurements with DLS revealed that no coagulation of the silica-coated particles in Figure 3 occurred in water. On the other hand, the uncoated particles (Figure 2C) were not perfectly stable and partially coagulated with time (1 or 2 days), generating a separated peak of large particles in DLS measurements (see the Supporting Information, Figure S-5). The existence of the silica layer on the magnetic silica particles was examined with electrokinetic measurements. Three series of zeta potentials of the silica particles used for heterocoagulation and the magnetic silica particles before and after the silica coating are shown in Figure 1. The silica particles showed an IEP around 2. Heterocoagulation with the magnetic nanoparticles shifted the IEP of silica toward the IEP of the positively charged nanoparticles. Furthermore, the IEP of the magnetic silica particles after silica coating was shifted back toward the IEP of the silica particles, indicating that the surface occupancy of silica on the particles was increased with the silica coating. The saturation magnetization of the composite particles, MS_core, was 6.7 emu/ g. The magnetic silica particles with silica surfaces were used in a succeeding step of the formation of fluorescent polymer shells. Formation of Fluorescent Polymer Shell onto the Magnetic Silica Cores. Figure 4A shows a TEM image of the core-shell particles formed in soap-free emulsion polymerization in the presence of pyrene and the silica-coated magnetic silica particles

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Figure 4. TEM images of the core-shell particles formed during polymerization in the presence of pyrene and the magnetic core particles of Figure 3: core-shell particles obtained without centrifuge pretreatment to remove MPTMS in water solvent (A) and with the centrifuge pretreatment (B, low magnification; C, high magnification).

Figure 5. Magnetization curve of the core-shell particles obtained in Figure 4C.

of Figure 3. Prior to this experiment, appropriate conditions of initiator, pyrene, and pH for shell formation were examined using magnetic-nanoparticle-free silica particles (see the Supporting Information, Figures S6-S9). According to the preparatory experiments, the polymerization of the particles shown in Figure 4A was conducted at a styrene concentration of 0.2 M and a VA-057 concentration of 5 mM. A pH value of 8.8 and a pyrene concentration of 1.0 mmol/L polymer were selected to homogeneously form the fluorescent polymer shells. The dV and CV values of the core-shell particles in Figure 4A were 547 nm and 4.1%, respectively. The core particles were completely covered

with a polymer shell, although a small number of polymer particles were generated. The polymer shell of the particles in Figure 4A had a rough surface. Some researchers have reported that MPTMS molecules in a reactant solution affected the morphology of the polymer formed during polymerization.19,28 Therefore, another synthetic experiment in which residual molecules of MPTMS were removed with a centrifuge process prior to polymerization was conducted. Figure 4B and C shows TEM images of the core-shell particles observed with low and high magnifications after polymerization. The magnetic core particles were found to be covered with smoother polymer shell than that in Figure 4A. The particle size distribution of the core-shell particles in Figure 4B (dV ) 583 nm, CV ) 4.6%) was similar to the one in Figure 4A. Figure 5 shows the magnetization curve of the core-shell particles obtained in Figure 4C. The core-shell particles exhibited superparamagnetic properties: no remnant magnetism was observed when the magnetic field was removed. The saturation magnetization, MS_core/shell, was 1.26 emu/g, similar to 1.34 emu/g of silica-coated iron oxide nanoparticles (without fluorescence) reported by Salgueirin˜o-Maceira et al.12 Figure 6 shows fluorescent spectra of the core-shell particles in Figure 4C and magnetic-nanoparticle-free core-shell particles. (28) Sacanna, S.; Rossi, L.; Philipse, A. P. Langmuir 2007, 23, 9974–9982.

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by the particles. The resemblance in shapes between the two spectra in Figure 6 indicates that the existence of the magnetic cores scarcely affected the fluorescent behavior of the composite particles. To examine the surface features of the core-shell particles prepared with the amphoteric initiator, the zeta potentials of the core-shell particles were measured in solutions at different pHs, which are shown in Figure 7. The zeta potentials were positive in a strongly acidic region and negative in weakly acidic and basic regions, which was very similar to the case of PSt particles prepared with VA-057 in the absence of core particles.30 Consequently, the use of VA-057 in the formation of fluorescent polymer shells on the magnetic core particles could create magnetic, fluorescent core-shell particles with an amphoteric property. Figure 6. Fluorescent spectra of magnetic core-shell particles in Figure 4C (solid line) and magnetic-nanoparticle-free core-shell particles with sizes similar to the particles in Figure 4C (dashed line).

Figure 7. Zeta potentials of the core-shell particles with magnetism and fluorescence.

The magnetic-nanoparticle-free cores with an average size similar to the one in Figure 3 were prepared with the Sto¨ber method. Main peaks at 375 (I1), 385 (I3), and 394 nm in both spectra can be attributed to intrinsic peaks of pyrene molecules. It is reported that intensity ratios of I1/I3 depend on the dielectric constant of the disperse medium surrounding the pyrene molecules.29 The intensity ratios for the two spectra were in a range of 0.89-0.91, which are close to the specific value of 0.95 reported for pyrene incorporated into PSt and clearly different from the value of 1.9 reported for pyrene in water. After the synthesis of the composite particles of Figure 4C, the water phase of the resultant suspension was separated from the particles by centrifuge treatment. No detectable level of the fluorescent pyrene remained in the water phase, which indicated that almost all of the pyrene was absorbed (29) Wilhelm, M.; Zhao, C.-L.; Wang, Y.; Xu, R.; Winnik, M. A.; Mura, J.-L.; Riess, G.; Croucher, M. D. Macromolecules 1991, 24, 1033–1040.

Conclusion Heterocoagulation between positively charged magnetic nanoparticles and negatively charged silica particles could prepare composite particles with a high monodispersity and a high homogeneity in magnetic content. The treatment with sodium silicate to form a thin silica layer on the magnetic silica particles was required for the succeeding step of polymer shell formation. Polymerization with VA-057 initiator in the presence of pyrene succeeded in the formation of fluorescent polymer shells onto the silica-coated magnetic silica particles. Zeta potentials of the core-shell particles indicated an amphoteric property of the particle surface. Acknowledgment. The authors are grateful to T. Miyazaki for FE-TEM observation and Dr. Watanabe (Institute of Multidisciplinary Research for Advanced Materials, Tohoku University) for the characterization of TSA-modified magnetic nanoparticles. This research was partially supported by the Ministry of Education, Culture, Sports, Science and Technology (20360354 and 20681011) and the New Energy and Industrial Technology Development Organization program in Japan. Supporting Information Available: TEM image and XRD profile of the TSA-modified nanoparticles (Figures S-1 and S-2); TGA profiles of the powder and suspension of TSA-modified nanoparticles (Figure S-3); magnetization curves of the particles in Figure 2 (Figure S-4); size distributions of the magnetic silica particles before and after the silica coating with sodium silicate (Figure S-5); and discussion of preparatory experiments for appropriate conditions of initiator, pyrene, and pH in the formation of polymer shells using magnetic-nanoparticlefree silica particles (Figures S-6–S-9). This material is available free of charge via the Internet at http://pubs.acs.org.. LA801364W (30) Nagao, D.; Sakamoto, T.; Konno, H.; Kobayashi, Y.; Konno, M. Langmuir 2006, 22, 10958–10962.