Design and Synthesis of Novel Magnetic Core−Shell Polymeric

Langmuir 2008, 24, 1801-1807

1801

Design and Synthesis of Novel Magnetic Core-Shell Polymeric Particles Kin Man Ho and Pei Li* Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic UniVersity, Hung Hom, Kowloon, Hong Kong, People’s Republic of China ReceiVed September 18, 2007. In Final Form: NoVember 15, 2007 A novel synthetic strategy was developed for the preparation of magnetic core-shell (MCS) particles consisting of hydrophobic poly(methyl methacrylate) cores with hydrophilic chitosan shells and γ-Fe2O3 nanoparticles inside the cores via copolymerization of methyl methacrylate from chitosan in the presence of vinyl-coated γ-Fe2O3 nanoparticles. The magnetic core-shell particles were characterized with transmission electron microscopy, field-emission scanning electron microscopy, particle size and ζ-potential measurements, vibrating sample magnetometry, and atomic force microscopy, respectively. The MCS particles were less than 200 nm in diameter with a narrow size distribution (polydispersity ) 1.09) and had a good colloidal stability (critical coagulation concentration ) 1.2 M NaCl at pH 6.0). Magnetization study of the particles indicated that they exhibited superparamagnetism at room temperature and had a saturation magnetization of 2.7 A m2/kg. The MCS particles were able to form a continuous film on a glass substrate, where magnetic nanoparticles could evenly disperse throughout the film. Thus, these new materials should be extremely useful in various applications.

Introduction There has been an increasing interest in the design and fabrication of magnetic polymeric particles consisting of one or more magnetic nanoparticles as a core with a coating matrix of polymer as a shell. Applications of these particles are very diverse, including information storage, electronic devices, electromagnetic interference (EMI) shielding,1,2 as well as magnetic separation, medical diagnostics, and drug delivery.3-8 Various approaches to the synthesis of magnetic polymeric particles have been investigated.4,8-13 They include the coprecipitation of ferrous and ferric salts under alkaline conditions in the presence of either polymers or surfactants,10,14,15 cross-linking of functional polymers in an emulsifier-stabilized magnetic nanoparticle dispersion,13 and layer-by-layer (LBL) self-assembly of alternating layers of polyelectrolytes and magnetic nanoparticles onto colloidal templates.16 Other approaches based on polymerization techniques have also been investigated (i.e., precipitation polymerization of vinyl monomers onto seeded polymer templates * Corresponding author. E-mail: [email protected]. (1) Nguyen, M. T.; Diaz, A. F. AdV. Mater. 1994, 6, 858-860. (2) Bidan, G.; Jarjayes, O.; Fruchart, J. M.; Hannecart, E. AdV. Mater. 1994, 6, 152-155. (3) Pankhurst, Q. A.; Connolly, J.; Jones, S. K.; Dobson, J. J. Phys. D: Appl. Phys. 2003, 36, 167-181. (4) Berry, C. C.; Curtis, A. S. G. J. Phys. D: Appl. Phys. 2003, 36, 198-206. (5) Langer, R. Science (Washington, DC, U.S.) 1990, 249, 1527-1533. (6) Kronick, P. L.; Campbell, G. L.; Joseph, K. Science (Washington, DC, U.S.) 1978, 200, 1074-1076. (7) Tartaj, P.; Morales, M. P.; Veintemillas-Verdaguer, S.; Gonzalez-Garreno, T.; Serna, C. J. J. Phys. D: Appl. Phys. 2003, 36, 182-197. (8) Lu, A.; Salabas, E. L.; Schuth, F. Angew. Chem., Int. Ed. 2007, 46, 12221244. (9) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995-4021. (10) Zhang, Y.; Kohler, N.; Zhang, M. Biomaterials 2002, 23, 1553-1561. (11) Gupta, A. K.; Curtis, A. S. G. J. Mater. Sci. 2004, 15, 493-496. (12) Shan, G. B.; Xing, J. M.; Luo, M. F.; Liu, H. Z.; Chen, J. Y. Biotechnol. Lett. 2003, 25, 1977-1981. (13) Denkbas, E. B.; Kilicay, E.; Birlikseven, C.; Ozturk, E. React. Funct. Polym. 2002, 50, 225-232. (14) Berry, C. C.; Wells, S.; Charles, S.; Curtis, A. S. G. Biomaterials 2003, 24, 4551-4557. (15) Bergemann, C.; Muller-Schulte, D.; Oster, J.; Brassard, L.; Lubbe, A. S. J. Magn. Magn. Mater. 1999, 194, 45-52. (16) Caruso, F.; Susha, A. S.; Giersig, M.; Mohwald, H. AdV. Mater. 1999, 11, 950-953.

pre-adsorbed with magnetic nanoparticles,17-19 emulsion polymerization of vinyl monomers in surfactant-stabilized micelles containing magnetic nanoparticles,20 and miniemulsion polymerization of vinyl monomers in surfactant-stabilized nanoreactors containing magnetic nanoparticles).21 Despite the success of these approaches in preparing magnetic polymeric particles, there are still some drawbacks with respect to the synthetic methods and the properties of the particles produced. For example, the synthesis involves tedious multiple step reactions and the use of large amounts of toxic organic solvents, emulsifiers, and surfactants. Leaching of magnetic nanoparticles from the polymeric particles and nanoparticle dissolution in acidic medium are still serious concerns. In addition, magnetic polymeric particles produced through these approaches generally have broad size distributions and usually a lack of surface functional groups, which are highly desirable for further chemical modifications. To address these problems, we designed a novel magnetic core-shell (MCS) polymeric particle. The particle consists of poly(methyl methacrylate) (PMMA) and magnetic nanoparticles (γ-Fe2O3) as a core and chitosan as a shell. The PMMA core acts as a solid support and a barrier to prevent γ-Fe2O3 nanoparticles from leakage and dissolution in an acidic medium. The PMMA core also provides a good film-forming property when the MCS particles are used as a coating material. Chitosan was chosen as a shell material because it is a nontoxic, biocompatible, and biodegradable natural polymer with antibacterial properties.22 It contains hydroxyl and amine functional groups that can be used for further chemical modifications. Chitosan also has an advantage (17) Sauzedde, F.; Elaı¨ssari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277, 846-855. (18) Deng, Y. H.; Yang, W. L.; Wang, C. C.; Fu, S. K. AdV. Mater. 2003, 15, 1729-1732. (19) Gu, S.; Shiratori, T.; Konno, M. Colloid Polym. Sci. 2003, 281, 10761081. (20) Dresco, P. A.; Zaitsev, V. S.; Gambino, R. J.; Chu, B. Langmuir 1999, 15, 1945-1951. (21) Ramirez, L. P.; Landfester, K. Macromol. Chem. Phys. 2003, 204, 2231. (22) Shahidi, F.; Arachchi, J. K. V.; Jeon, Y. J. Trends Food Sci. Technol. 1999, 10, 37-51.

10.1021/la702887m CCC: $40.75 © 2008 American Chemical Society Published on Web 01/29/2008

1802 Langmuir, Vol. 24, No. 5, 2008

Ho and Li

Scheme 1. Preparation of Magnetic Core-Shell Particlesa

a (a) Synthesis of citrate-coated γ-Fe O nanoparticles; (b) synthesis of vinyl-coated γ-Fe O nanoparticles; and (c) encapsulation of 2 3 2 3 vinyl-coated γ-Fe2O3 via the hydroperoxide-induced graft copolymerization of MMA from chitosan.

of low cost because it is produced from the deacetylation of chitin, poly-β-(1,4)-N-acetyl-D-glucosamine. In fact, chitin is the second most abundant biopolymer in nature after cellulose23,24 and is usually obtained from the shell wastes of crustaceans, such as shrimp, lobster, krill, squid, and crab. Thus, chitosan offers MCS particles with antibacterial properties, surface functionality and colloidal stability. Here, we report the synthesis of magnetic core-shell particles with the previously mentioned design through a two-stage reaction: (1) preparation of surface-modified magnetic nanoparticles and (2) synthesis of the core-shell polymeric particles via the hydroperoxide-induced graft copolymerization of MMA from chitosan in the presence of the surface-modified magnetic nanoparticles in an aqueous medium. This copolymerization method is based on our recent work on the synthesis of amphiphilic core-shell particles.25 The major challenge for the preparation of such designed particles was to be able to encapsulate the magnetic nanoparticles into the hydrophobic PMMA cores during copolymerization. It was anticipated that the surface property of the magnetic nanoparticles would play a key role in enabling the encapsulation. Three different types of surface-modified magnetic nanoparticles have been investigated. They are citrate-coated γ-Fe2O3 (c-Fe2O3), vinyl-coated γ-Fe2O3 (MPS-Fe2O3), and oleate-coated γ-Fe2O3 (o-Fe2O3) nanoparticles. The rationale for using these three types of surface-modified magnetic nanoparticles was based on the following assumptions: (1) Because the citrate-coated γ-Fe2O3 nanoparticles have negative surface charges, they would be preabsorbed onto positively charged chitosan through electrostatic (23) Monteiro, J. O. A. C.; Airoldi, C. J. Colloid Interface Sci. 2005, 282, 32-37. (24) Synowiecki, J.; Al-Khateeb, N. A. Crit. ReV. Food Sci. Nutr. 2003, 43, 145-171. (25) Li, P.; Zhu, J.; Sunintaboon, P.; Harris, F. W. Langmuir 2002, 18, 86418646.

interactions. The adsorbed magnetic nanoparticles would become more hydrophobic due to the charge neutralization and thus would facilitate encapsulation. (2) The terminal double bonds on the MPS-Fe2O3 nanoparticle surfaces would allow copolymerization between the vinyl groups on the surface of the nanoparticles and MMA monomer. (3) The hydrophobic oleate coating of γ-Fe2O3 nanoparticles would be favorable for encapsulation through hydrophobic compatibility between the nanoparticles and the MMA monomer. Experimental Procedures Materials. Anhydrous iron(III) chloride (FeCl3, Junsei Chemical Co. Ltd), iron(II) chloride tetrahydrate (FeCl2‚4H2O, Aldrich), ammonium hydroxide solution (25 w/w %, Acros), and trisodium citrate-2-hydrate (Riedel-de Hae¨n) were used as received. Methanol, ethanol, tetraethyl orthosilicate [Si(OC2H5)4, TEOS], and 3-(trimethoxysilyl)propyl methacrylate (MPS) were obtained from International Laboratory USA. Chitosan (medium, Aldrich) was purified by being dissolved in a dilute acetic acid solution (1 v/v %) at 60 °C, followed by precipitating in a NaOH solution (10%) under stirring at room temperature. The chitosan was then filtered off and washed with distilled water to neutrality and dried in a vacuum oven at 60 °C. Dilute solution viscosity measurements suggested that the viscosity average molecular weight (Mv) of chitosan was approximately 80 000. The degree of deacetylation of chitosan, as estimated by proton-nuclear magnetic resonance spectroscopy (1H NMR) based on the literature,26 was 74%. t-Butyl hydroperoxide (TBHP, 70% solution in water) was obtained from Aldrich Chemical Co. and used without further purification. The phenolic inhibitor in methyl methacrylate (Aldrich) was removed by being washed 3 times with a 10% sodium hydroxide solution and then with deionized water until the pH of the water layer dropped to 7. It was further purified by vacuum distillation. Freshly deionized and distilled water or Milli-Q water was used as the dispersion medium. (26) Hirai, A.; Odani, H.; Nakajima, A. Polym. Bull. 1991, 26, 87-94.

Magnetic Core-Shell Polymeric Particles

Langmuir, Vol. 24, No. 5, 2008 1803

Figure 1. The pH dependence of ζ-potential values measured in a 1 mM NaCl solution at 25 °C. (Deep blue triangles) Citrate-coated γ-Fe2O3 nanoparticles; (red circles) vinyl-coated γ-Fe2O3 nanoparticles; (green squares) PMMA/chitosan core-shell particles; and (blue diamonds) γ-Fe2O3-encapsulated PMMA/chitosan core-shell particles. Synthesis of Citrate-Coated γ-Fe2O3 Nanoparticles (c-Fe2O3). γ-Fe2O3 nanoparticles were synthesized based on a reported method with minor modifications.27,28 FeCl2‚4H2O (1.99 g) and anhydrous FeCl3 (3.25 g) were dissolved in water (20 mL) separately, followed by the two iron salt solutions being mixed under vigorous stirring (800 rpm). A NH4OH solution (0.6 M, 200 mL) was then added to the stirring mixture at room temperature, immediately followed by the addition of a concentrated NH4OH solution (25 w/w %, 30 mL) to maintain the reaction pH between 11 and 12. The resulting black dispersion was continuously stirred for 1 h at room temperature and then heated to reflux for 1 h to yield a brown dispersion. The magnetic nanoparticles were then purified by a repeated centrifugation (30006000 rpm, 20 min), decantation, and redispersion cycle 3 times, until a stable brown magnetic dispersion (pH 9.4) was obtained. A total of 100 mL of the γ-Fe2O3 nanoparticle dispersion (2.42 w/w %) prepared as stated previously was acidified with a HNO3 solution (4 M, 100 mL) and then centrifuged at 3000 rpm for 20 min to collect the magnetic nanoparticles. The collected magnetic nanoparticles were redispersed in water (200 mL), and the dispersion was heated to reflux. Trisodium citrate dihydrate (11.7 g) was then added, followed by heating the mixture for 1 h under reflux to produce citrate-coated iron oxide nanoparticles. The brown dispersion was purified by being placed into a dialysis tube (10 kDa molecular weight cutoff, Sigma-Aldrich) and dialyzed against water for 8 days with a daily change of water until the conductivity of water was comparable to that of purified H2O used. Synthesis of Vinyl-Coated γ-Fe2O3 Nanoparticles (MPSFe2O3). Coating of a layer of silica on the surface of the c-Fe2O3 nanoparticles was achieved by premixing a dispersion of the purified citrate-coated nanoparticles (8.5 w/w %, 20 mL) obtained previously with methanol (80 mL) for 1 h at 40 °C. Concentrated ammonia solution (25 w/w %, 1.8 mL) was added, and the resulting mixture was stirred at 40 °C for 30 min. Subsequently, tetraethyl orthosilicate (TEOS, 1.0 mL) was charged to the reaction vessel, and the mixture was continuously stirred at 40 °C for 24 h. Finally, 3-(trimethoxysilyl)propyl methacrylate (MPS, 5.3 mL) was added, and the mixture was allowed to react at 40 °C for 24 h to give vinyl-coated γ-Fe2O3 nanoparticles. The MPS-coated nanoparticles were collected by placing a permanent magnet (∼4 T) next to the container wall, followed by discarding the solution. The collected magnetic nanoparticles were redispersed in ethanol (20 mL), and the dispersion was transferred into a dialysis tube (10 kDa molecular weight cutoff, Sigma-Aldrich) and dialyzed against ethanol for 1 week with a daily change of ethanol to remove the unreacted MPS, TEOS, and NH3. The amounts of unreacted MPS molecules removed through the (27) Massart, R.; Dubois, E.; Cabuil, V.; Hasmonay, E. J. Magn. Magn. Mater. 1995, 149, 1-5. (28) Tang, B. Z.; Geng, Y.; Lam, J. W. Y.; Li, B.; Jing, X.; Wang, X.; Wang, F.; Pakhomov, A. B.; Zhang, X. X. Chem. Mater. 1999, 11, 1581-1589.

Figure 2. Comparison of FT-IR spectra of (a) trisodium citrate; (b) citrate-coated γ-Fe2O3nanoparticles; (c) vinyl-coated γ-Fe2O3 nanoparticles; and (d) MPS. dialysis were monitored with ultraviolet (UV) measurements using a PerkinElmer UV-vis spectrophotometer (Lambda 35) at 203.5 nm. Finally, the purified dispersion was concentrated to a 10.0 w/w % solid content for subsequent reactions. Synthesis of Oleate-Coated γ-Fe2O3 (o-Fe2O3) Nanoparticles. The o-Fe2O3 nanoparticles were prepared via a thermal decomposition approach, according to the method reported by Hyeon et al.29 Synthesis of Magnetic Core-Shell (MCS) Particles. For a total of 25 mL of solution, 1 mL of MPS-Fe2O3 nanoparticle dispersion (10 w/w % in ethanol) was mixed with ethanol and then with 22 mL of chitosan solution containing 0.25 g of chitosan and 0.6 v/v % acetic acid (99 w/w %) using a homogenizer (Sonics VC130PB, output wattage ) 6 W), giving a final volume ratio of H2O/ethanol of 12.5:1. The dispersion was homogenized for another 10 min and then transferred into a water-jacketed flask equipped with a thermometer, a condenser, a magnetic stirrer, and a nitrogen inlet. The dispersion was purged with nitrogen for 20 min and stirred at 80 °C prior to the addition of MMA (0.6 g) and TBHP (final concentration was 0.1 mM). The resulting mixture was continuously stirred at 80 °C for 2 h under nitrogen. After the reaction, the particle dispersion was filtered to remove precipitates (if any) generated during the polymerization. The MMA conversion (conv %) was determined gravimetrically. The particle dispersion was purified by a repeated centrifugation (13 000 rpm), decantation, and redispersion cycle until the conductivity of the supernatant was close to that of distilled water used. Measurement and Characterization. The crystallographic structures of iron oxide nanoparticles (γ-Fe2O3) were studied with a rotating anode Bruker D8 Avance X-ray diffractometer. Infrared spectra were recorded on a Nicolet Avatar 360 FT-IR spectrophotometer using KBr disks. Zeta-potential measurements were conducted with a Malvern Zetasizer 3000HS in a 1 mM NaCl aqueous solution as the suspension liquid. Volume average (Dv) and number (29) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798-12801.

1804 Langmuir, Vol. 24, No. 5, 2008

Ho and Li

Figure 4. γ-Fe2O3-encapsulated PMMA/chitosan core-shell particles. (a) FE-SEM image and (b) particle size and size distribution (blue diamonds) Dv and (pink squares) Dn.

Figure 3. TEM images of γ-Fe2O3-encapsulated PMMA/chitosan core-shell particles. (a) Without staining and (b) stained with a 0.5 w/w % PTA solution for 2 min. average (Dn) diameters of the MCS particles were measured with a Coulter LS 230 Particle Size Analyzer using a polarization intensity differential scattering (PIDS) module. Hydrodynamic diameters (〈Dh〉) of MCS particles were measured with a Malvern Zetasizer 3000HS using a He-Ne laser at a wavelength of 632.8 nm. The measurements were performed at 25 °C with a fixed angle of 90°. Sample concentrations were between 100 and 300 mg/L. 〈Dh〉 was calculated based on the relation of D ) kT/3πη〈Dh〉, where k is the Boltzmann constant, η is the refractive index of the solvent used as the dispersant, T is the temperature (K), and D is the diffusion coefficient obtained from the decay rate of the intensity correlation function of the scattered light (i.e., correlogram). The correlogram was expressed by the relation of G(τ) ) ∫I(t)I(t + τ)dt. Results obtained were averages of at least three repeated measurements. Transmission electron microscopy (TEM) micrographs were obtained using a JEOL 2010 TEM instrument at an accelerating voltage of 160 kV. Samples were prepared by wetting carbon-coated grids with a small drop of dilute particle dispersion (100-200 mg/L). Upon drying, the particles were stained with a small drop of 0.5 w/w % phosphotungstic acid solution (PTA) for 2 min and then dried at room temperature before analysis. The morphologies of MCS particles were also examined by field-emission scanning electron microscopy (FE-SEM, JEOL JSM 6335F) after coating the dried sample with a thin layer of gold to a depth of approximately 5 Å under vacuum. Colloidal stabilities of MCS particles were studied by two methods: (1) Measuring the changes in the hydrodynamic diameter (〈Dh〉) of MCS particles at various pH values and sodium chloride (NaCl) concentrations (0-2.0 M) using dynamic light scattering (Malvern Zetasizer 3000HS). (2) Measuring the turbidity changes of MCS particle dispersion (at pH 6.0) at various NaCl concentrations (0-2.0 M) using UV-vis spectroscopy. A typical experimental

procedure for the UV-vis measurement is described as follows:30 for a total of 5 mL of solution, the dispersion of MCS particles was first diluted to 100 mg/L, followed by the addition of various NaCl solutions with known concentrations. These suspensions were gently shaken for 10 min and then allowed to stand for 60 min at room temperature to reach equilibrium. The suspensions were centrifuged at 2000 rpm for 15 min. The supernatants were then collected for measurement with a PerkinElmer UV-vis spectrophotometer (Lambda 35) at an incident wavelength of 450 nm. Turbidity changes (A/A0) were plotted against c, where c is the electrolyte concentration. A0 and A are the absorbances of the MCS particle dispersion before and after the addition of an electrolyte. The critical coagulation concentration (CCC) was determined by extrapolating the sharply decreasing portion of the turbidity curve. Compositions of γ-Fe2O3 nanoparticles in MCS particles were determined with a PerkinElmer thermogravimetry analyzer TGA7 (TGA) at a heating rate of 20 °C/min and calculated based on the following equation. The residue weight is the weight of materials remained at 900 °C, while the weight of MCS particles is the dry weight of particles used γ-Fe2O3 (%) )

residual wt × 100% wt of MCS particles

The saturation magnetizations were determined with a vibrating sample magnetometer (VSM) (LDJ MODEL 9500) at room temperature. An exact amount of sample (usually larger than 50 mg) was weighed and closely packed into a custom-made Teflon holder. The magnetization measurements were all studied at room temperature using external magnetic fields ranging from 0 to 5 KA/m. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) measurements were performed on a Digital Instruments NanoScope IV in tapping mode, using a magnetic etched silicon probe (MESP). The sample dispersion (2 mL, 2 w/w %) was placed onto a glass substrate and heated at 155 °C for 2 h to form a film. All measurements were performed under ambient conditions. (30) Laukkanen, A.; Wiedmer, S. K.; Varjo, S.; Riekkola, M. L.; Tenhu, H. Colloid Polym. Sci. 2002, 280, 65-70.

Magnetic Core-Shell Polymeric Particles

Langmuir, Vol. 24, No. 5, 2008 1805

Figure 6. TGA thermograms of (a) MCS particles containing 8.4 w/w % γ-Fe2O3 and (b) PMMA/chitosan core-shell particles without magnetic nanoparticles.

Figure 5. (a) Effect of pH on the hydrodynamic diameter (〈Dh〉) of MCS particles at 25 °C using a 1 mM Tris-HCl as a buffer solution and (b) effect of NaCl concentration on the hydrodynamic diameter (〈Dh〉) (black triangles) and turbidity (black squares) of MCS particles dispersed at pH 6.0. The critical coagulation concentration (CCC) determined was 1.2 M.

Results and Discussion Citrate-coated γ-Fe2O3, a water-based magnetic nanoparticle with negative surface charges, was first synthesized via coprecipitation of the solutions of Fe(II) and Fe(III) chloride (molar ratio ) 1:2) in an ammonium hydroxide solution at pH 11-12 according to a reported method (Scheme 1a).27,28 X-ray diffraction (XRD) analysis of the obtained nanoparticles indicated that γ-Fe2O3 nanoparticles were produced (Figure S1, Supporting Information). TEM images showed that the γ-Fe2O3 nanoparticles had particle diameters between 10 and 30 nm (Figure S2, Supporting Information). The nanoparticles were then coated with trisodium citrate to generate c-Fe2O3 nanoparticles. Zetapotential measurements of the nanoparticles confirmed that they had negative surface charges ranging from -20 to -50 mV at pH values between 3 and 10.5 (Figure 1). They were stable in water for a long period of time. The saturation magnetization (Ms) of the nanoparticles as measured with a VSM was 41 A m2/kg, and they exhibited superparamagnetism (Figure S3, Supporting Information). The c-Fe2O3 nanoparticles were subsequently used in the graft copolymerization of MMA from chitosan induced with a small amount of TBHP (0.1 mM) at 80 °C for 2 h. A stable brown dispersion was obtained, and the MMA conversion was up to 80%. However, TEM images of the products showed that almost all the magnetic nanoparticles were located outside the PMMA/ chitosan particles (Figure S4, Supporting Information). The results indicated that encapsulation of c-Fe2O3 nanoparticles into the PMMA cores was not feasible. There were two possible reasons for this result: (1) structural incompatibility between the hydrophilic surface of c-Fe2O3 nanoparticles and the hydrophobic PMMA and (2) electrostatic complexation between the highly negatively charged nanoparticles and positively charged chitosan, which confined the nanoparticles to the shell.

To address the problem of a hydrophilic surface of the c-Fe2O3 nanoparticles, the nanoparticles were further modified by introducing hydrophobic moieties and reactive terminal double bonds on their surfaces. It was assumed that the reactive double bonds on the surface of the nanoparticles could be copolymerized with MMA, thus chemically encapsulating the nanoparticles into the PMMA cores. The modification was achieved via a two-step reaction (Scheme 1b): first, the c-Fe2O3 nanoparticles were treated with ammonium hydroxide and tetraethylorthosilicate (TEOS) in a methanol/water mixture, based on the Sto¨ber method with minor modifications.31 They were then reacted with a silane coupling agent, 3-(trimethoxysilyl)propyl methacrylate (MPS), to form terminal vinyl-coated magnetic nanoparticles.18,32 The chemical structure of the MPS-coated nanoparticles was identified with FT-IR spectroscopy. Figure 2c displays both the characteristic peaks of citrate-coated iron oxides [the stretching vibrations of asymmetric COO- (1633 cm-1), symmetric COO(1404 cm-1),33,34 and Fe-O (400-650 cm-1)], and MPS molecules [the stretching vibrations of C-H (2800-3000 cm-1), CdO (1726 cm-1), and Si-O (1127 cm-1)]. The results indicated that the MPS molecules have been grafted onto the surface of c-Fe2O3 nanoparticles. Zeta-potential measurements indicated that the negative charge density of the MPS-Fe2O3 nanoparticles was lower than that of the c-Fe2O3 nanoparticles (Figure 1). The reduction of the negative charge density indicated that some carboxylic acid groups of trisodium citrate on the nanoparticle surface had been converted to neutral vinyl groups. VSM data showed that the MPS-Fe2O3 nanoparticles had a Ms value of 33 A m2/kg with superparamagnetic property (Figure S3, Supporting Information). Having produced the MPS-Fe2O3 nanoparticles, they were first mixed with a small amount of ethanol to improve their dispersion in water. The nanoparticle dispersion was then mixed with a chitosan solution using a homogenizer for 10 min to form a stable brown dispersion. The stability of the dispersion may be attributed to the adsorption of cationic chitosan molecules onto the negatively charged MPS-Fe2O3 nanoparticles and the hydrogen-bonding between the hydroxyl groups of chitosan and the nanoparticles. The brown dispersion was subsequently subjected to the graft copolymerization of MMA at 80 °C for 2 h, as illustrated in Scheme 1c. Stable latexes were obtained, and the MMA conversion was up to 80%. The TEM micrographs of the purified particles (Figure 3a) clearly revealed that the (31) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-69. (32) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293308. (33) Wu, N.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P. Nano Lett. 2004, 4, 383-386. (34) Huignard, A.; Buissette, V.; Laurent, G.; Gacoin, T.; Boilot, J. P. Chem. Mater. 2002, 14, 2264-2269.

1806 Langmuir, Vol. 24, No. 5, 2008

Ho and Li

Figure 7. Room-temperature magnetization measurement of MCS particles containing 8.4 w/w % γ-Fe2O3 nanoparticles.

Figure 8. Film formation of MCS particles through annealing the particle dispersion at 155 °C. (a) Surface topographic image (5 µm × 5 µm) and (b) top view of magnetic force microscopy image (5 µm × 5 µm).

MPS-Fe2O3 nanoparticles were successfully encapsulated into the polymer cores. When staining the particles with a dilute PTA solution, the core-shell nanostructures of the particles were observed. Figure 3b clearly displays that the PMMA cores were coated with chitosan shells and contained magnetic nanoparticles. Therefore, modification of γ-Fe2O3 to MPS-Fe2O3 nanoparticles containing terminal vinyl groups on the nanoparticle surface is a crucial step for the encapsulation of magnetic nanoparticles into the PMMA cores. Figure 4a shows the SEM image of the magnetic core-shell particles. They had a quite uniform size distribution, but some of them were not spherical. Furthermore, the particles tended to form aggregates, which may be attributed to the strong magnetic dipole-dipole interaction exerted between the particles during drying. Although the MCS particles tended to aggregate in the dried state, they were readily dispersed in an aqueous medium. Particle size measurement indicated that they had a number average diameter (Dn) of 185 nm. The polydispersity index (Dv/ Dn) of the particles was 1.09, indicating a very narrow size distribution in aqueous solution (Figure 4b). Formation of the chitosan shells was verified by zeta-potential measurements as a function of pH in a 1 mM NaCl solution at

25 °C. Results in Figure 1 show that the positive zeta-potential values of MCS particles decreased from +50 to nearly 0 mV as the pH increased from 3.0 to 8.0. This effect was probably attributed to the higher degree of deprotonation of the protonated amine from the chitosan. When compared with PMMA/chitosan core-shell particles without magnetic nanoparticles in the cores, the zeta-potential values of the MCS particles at various pH values were slightly lower. This may be due to the charge neutralization between some negatively charged MPS-Fe2O3 nanoparticles and chitosan. The colloidal stabilities of MCS particles at pH values between 4.5 and 7.5 were studied by measuring the hydrodynamic diameter (〈Dh〉) of the particles as a function of pH in a buffer solution containing tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl) (Figure 5a). Increasing the pH of the particle dispersion from 4.3 to 7.0 had little influence on the 〈Dh〉 of the particles (393-413 nm), but a further increase of the pH from 7.0 to 7.9 significantly increased their average diameter from 413 to 3304 nm. This effect was probably due to the coagulation of MCS particles that had low surface charge densities (