Langmuir 1999, 15, 1945-1951
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Preparation and Properties of Magnetite and Polymer Magnetite Nanoparticles Pierre A. Dresco,† Vladimir S. Zaitsev,† Richard J. Gambino,‡ and Benjamin Chu*,† Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794-3400, and Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275 Received August 4, 1998. In Final Form: December 16, 1998 A novel approach to prepare magnetic polymeric nanoparticles by synthesis of the magnetite core and polymeric shell in a single inverse microemulsion is reported. Stable magnetic nanoparticles colloid dispersion with narrow size distribution can thus be produced. The microemulsion seed copolymerization of methacrylic acid, hydroxyethyl methacrylate, and cross-linker results in a stable hydrophilic polymeric shell of the nanoparticles. The preparation of the nanoparticles was carried out also by the two-stage microemulsion process and the seed precipitation polymerization. The particle size was varied in the range 80-320 nm by changing of the monomer concentration and water/surfactant ratio. The magnetic properties and the size distribution of the nanoparticles synthesized by these three methods were compared. The polymeric nanoparticles synthesized in single microemulsion have superparamagnetic properties and the narrowest size distribution.
Introduction Magnetite (Fe3O4), due to its strong magnetic properties, was used first in biology and then in medicine for the magnetic separation of biochemical products1 and cells2 as well as the magnetic guidance of particle systems for site-specific drug delivery.3 The use of superparamagnetic iron oxide as a contrast agent for magnetic resonance imaging (MRI) has been the subject of extensive studies over the past decade.4,5 Magnetite particles have nearly a 50-fold greater magnetic susceptibility than gadolinium chelates approved for use in MR imaging.6 However, the size, charge, and surface chemistry of magnetic particles could strongly influence their biodistribution.7-10 Another important point is that the magnetic properties depend strongly on the size of the magnetite particles.11 The most common method for the synthesis of magnetite particles is by coprecipitation from a solution of Fe(II) and Fe(III) salts using alkali metal hydroxide12-14, Smaller and more uniform particles can be synthesized by using † ‡
Department of Chemistry. Department of Materials Science and Engineering.
(1) Patton, W. F.; Kim, J.; Jacobson, B. S. Biochim. Biophys. Acta 1985, 816, 83. (2) Kemshead, J. T.; Treleaven, J. G.; Gibson, F. M.; Ugllstad, J.; Rembaum, A.; Philip, T. Prog. Exp. Tumor Res. 1985, 29, 249. (3) Gupta, P. K.; Hung, C. T. Life Sci. 1989, 44, 175. (4) Weissler, R.; Papicov, M. Rev. Magn. Reson. Med. 1992, 4, 1. (5) Briggs, R. W.; Wu, Z.; J.Mladinich, C. R.; Stoupis, C.; Gauger, J.; Liebig, T.; Ros, P. R.; Ballinger, J. R.; Kubilis, P. Magn. Reson. Imaging 1997, 15, 559. (6) Stark, D. D.; Weissleder, R. Radiology. 1988, 168, 297. (7) Bengele, H. H.; Palmacci, S. Magn. Reson. Imaging 1994, 12, 433. (8) Josephson, L.; Lewis, J.; Jacobs, P.; Hanh, P. F.; Stark, D. D. Magn. Reson. Imaging 1988, 6, 647. (9) U. S. Patent 5,492, 814, 1996. (10) Papisov, M. I.; Bogdanov, A., Jr.; Schaffer, B.; Nossiff, N.; Shen, T.; Weissleder, R.; Brady. T. J. J. Magn. Magn. Mater. 1993, 122, 383. (11) Pouliquen, D.; Perroud, H.; Calza, F.; Jallet, P.; Le Jeune, J. J. Magn. Reson. Medicine 1992, 24, 75. (12) Shalafalla, E. K.; Reimers, G. W. IEEE Trans. Magn. 1980, 16, 178. (13) Massart, R. IEEE Trans. Magn. 1981, 17, 1247. (14) Sato, T.; Iijima, T.; Seki, M.; Inagaki, N. J. Magn. Magn. Meter. 1987, 65, 252.
the microemulsion approach.15-18 Despite the use of proteins,19-21 lipids,22 and synthetic1,23,24 or natural25-27 polymers to improve biocompatibility of magnetic particles, they are rapidly removed from circulation due to the changes in their surface characteristics,25,28 resulting in the formation of large aggregates. An advance in the use of magnetic particles for medical applications depends on new synthetic methods with better control of the size distribution and of particle surface properties. The wellknown microemulsion synthesis of polymeric microspheres29,30 was used on one occasion to develop magnetic particles,31 but they were in the micron size range. We report herein a new synthetic method for magnetic polymeric nanoparticles. The objectives are as follows: (1) to develop a convenient method to prepare nanosize (15) Mobe, G.; Kon-no, K.; Kyori, K.; Kitahara, A. J. Colloid Interface Sci. 1983, 93, 293. (16) Lee, K. M.; Corensen, M. S.; Klabunde, J. K.; Hadjipanayis, G. C. IEEE Trans. Magn. 1992, 28, 3180. (17) Liz, L.; Lo´pez-Quintela, M. A.; Mira, J.; Rivas, J. J. Mater. Sci. 1994, 29, 3797. (18) Lo´pez-Perez, J. A.; Lo´pez-Quintela, M. A.; Mira, J.; Rivas, J. IEEE Trans. Magn. 1997, 33, 4359. (19) Widder, D. J.; Greif, W. L.; Widder, K. J.; Edelman, R. R.; Brady, T. J. Am. J. Roentgenol. 1987, 148, 399. (20) Renshaw, P. F.; Owen, C. S.; McLaughlin, A. C.; Frey, T. G.; Leigh, J. S., Jr. Magn. Reson. Med. 1986, 3, 217. (21) Dickson, D. P. E.; Walton, S. A.; Mann, S.; Wong, K. Nanostruct. Mater. 1997, 9, 595. (22) Bulte, J. W. M.; Ma, L. D.; Magin, R. L.; Kmman, R. L.; Hulstaert, C. E.; Go, K. G.; The, T. H.; de Leij, L. Magn. Reson. Med. 1993, 29, 32. (23) Lee, J.; Tsobe, I.; Senna, M. Colloids Surf A 1996, 109, 121. (24) Mendenhall, G. D.; Geng, Y.; Hwang, J. J. Colloid Interface Sci. 1996, 184, 519. (25) Josephson, L.; Groman, E. V.; Menz, E.; Luis, J. M.; Bengele, H. Magn. Reson. Imaging 1990, 8, 637. (26) Molday, R. S.; Mackenzie, D. J. Immunol. Methods 1982, 52, 353. (27) Sjo¨ren, C. E.; Jhoansson, C.; Nævestad, A.; Sontum, P. C.; BrileySæbø, K.; Fahlvik, A. K. Magn. Reson. Imaging 1997, 15, 55. (28) Pouliquen, D.; Le Jeune, J. J.; Perdrisot, R.; Ermias, A.; Jallet, P. Magn. Reson. Imaging 1991, 9, 275. (29) Seijo, B.; Fattal, E.; Roblot-Treupel, L.; Couvreur, P. Int. J. Pharm. 1990, 62, 1. (30) Barton, J. Prog. Polym. Sci. 1996, 21, 399. (31) Pouliquen, D.; Perdrisot, R.; Ermias, A.; Jallet, P.; Le Jeune, J. J. Magn. Reson. Imaging 1990, 7, 619.
10.1021/la980971g CCC: $18.00 © 1999 American Chemical Society Published on Web 02/17/1999
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magnetic particles with a narrow size distribution; (2) to study the effects of the synthetic procedure on the particle properties; (3) to compare properties of the particles synthesized by different methods. Materials and Methods Chemicals were generally reagent grade from commercial sources. Ferric chloride, ferrous chloride tetrahydrate, and tetramethylammonium hydroxide (TMAOH) were supplied by Aldrich and used as received. Aerosol OT (sodium bis(2ethylhexylsulfosuccinate) was purchased from Fluka and purified as described elsewhere.32 Toluene purchased from Aldrich (spectroscopy grade) was used as supplied. Water was deionized, doubly distilled, and deoxygenated prior to use. Methacrylic acid (MA), supplied by Aldrich, and hydroxyethyl methacrylate (HEMA), supplied by Sigma, were distilled at 40 and 97 °C (10 mmHg), respectively, in the presence of copper powder. Hydrophobic initiator AIBN (2,2′-azobis(isobutyronitrile); Aldrich) was recrystallized from ethanol, dried under vacuum, and stored in a refrigerator before use. Hydrophilic initiator potassium persulfate (K2S2O8) was purchased from Fisher Scientific and used as supplied. Cross-linker N,N′-methylenebis(acrylamide) (electrophoresis grade) was purchased from Polysciences, Inc., and used as received. Dynamic Light Scattering Analysis. Dynamic light scattering (DLS) was used to obtain information about the average hydrodynamic radius and the size distribution of the particles. The laboratory-built light scattering spectrometer had a SpectraPhysics argon ion laser (λ ) 514.5 nm) as the light source. The particles were dispersed in doubly distilled water with pH adjusted to 8.5 by using a (TMA)OH solution. After filtration through a 0.45 µm nominal pore size Millipore filter, or centrifugation to remove suspended impurities, the sample solutions were placed in a thermostated scattering cell at 25 °C with temperature control to (0.1 °C. The intensity-intensity time correlation function measurements were carried out in the self-beating mode by using a Brookhaven BI 2030 AT 128-channel digital correlator over an angular range between θ ) 30 and 135°. Correlation functions were accepted when the measured and the calculated baselines were in good agreement (with a difference of less than 0.1%). The CONTIN method33 was used for the data analysis of the measured intensity time correlation function G(2)(τ). The characteristic line width, Γ, was then reduced to the translational diffusion coefficient (D) for each scattering angle measured. The diffusion coefficient of the particles was determined by extrapolation to zero scattering angle from a plot of D versus q2, where q ) 4πnλ-1 sin(θ/2) with n, λ, and θ being the solvent refractive index, the wavelength in a vacuum, and the scattering angle, respectively. The average hydrodynamic radius RH was calculated by using the Stokes-Einstein relation from the diffusion coefficient. The variance of the line width distribution function (eq 1) gives an indication of the width of the
µ2/Γ h2
(1)
hydrodynamic size distribution (eq 2), where G(Γ) is the normal-
Γ h2 )
∫ ΓG(Γ) dΓ
(2)
ized characteristic line width distribution which can be estimated from G(2)(τ) by using the CONTIN method (eqs 3-5, where A is the baseline and β is a coherence factor).
µ2 )
∫(Γ - Γh ) G(Γ) dΓ 2
(2)
(1)
(4)
dΓ
(5)
G (τ) ) A(1 + β|g (τ)| ) |g(1)(τ)| )
∫G(Γ)e
-Γτ
(3)
2
The apparent particle size distribution profiles, each measured at a finite concentration and a fixed scattering angle, are (32) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1979, 70, 577. (33) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213.
presented as a plot of Γ*G(Γ) versus RH, with Γi*G(Γi) being proportional to the scattered intensity of particle i having a hydrodynamic radius RHi. Vibrating-Sample Magnetometry Analysis. The vibratingsample magnetometer (VSM Model 1660, Digital Measurement System, Inc.) was used to study the magnetic properties of magnetite particles in fields up to 16 kOe. Atomic Force Microscopy Analysis. Atomic force microscopy was carried out with a nanoscope dimension microscope (Digital Instruments). Images were performed using the tapping mode with a Si2N3 tip. The samples were prepared by dipping a silica wafer in an aqueous suspension of the particles which were then attached to the hydrophilic plate. Synthesis of Magnetite Particles in Solution. The ferric and ferrous chlorides (molar ratio 2:1) were dissolved in deoxygenated water at a concentration of 0.1 M of iron ions. The solution was used immediately after preparation. Chemical precipitation was achieved by using a 1M deoxygenated solution of ammonium hydroxide. The reaction was carried out at low temperatures (4-6 °C) under vigorous stirring. The final pH of the solution was maintained between 9 and 9.5.34 Particles were washed several times with distilled water by decantation. They were then precipitated with acetone and dried in a vacuum oven. Some samples were washed by HNO3 (2M) and then a solution of TMAOH (5 wt %) was used to prepare the colloidal magnetite particles. Synthesis of Magnetite Particles in Microemulsion. Method 1. Two microemulsions were prepared separately. The first one was made of a 0.05 mol/L aqueous solution of iron chloride salts (Fe(II)/Fe(III) molar ratio 1:2) dispersed in a deoxygenated solution of AOT-toluene. The second one was made of a solution of (TMA)OH (10 wt %) dispersed in an identical AOT-toluene solution. The water/AOT/toluene ratio was 1:8:15 by weight. Both microemulsions were purged with nitrogen before reaction. They were then mixed together with ultrasonic agitation under nitrogen. The solution should turn intense black. The final mixture was further stirred for another 20 min under nitrogen. The particles were recovered by precipitation in an excess of an acetone-methanol mixture (9:1 ratio), followed by several washings, centrifuged and dried under vacuum before characterization. Method 2. The microemulsion was made of 0.1 mol/L of an aqueous solution of iron chloride salts (Fe(II)/Fe(III) molar ratio 1:2) dispersed in a deoxygenated solution of AOT-toluene. The water/AOT/toluene ratio was 1:8:15 by weight. A deoxygenated solution (3 mL) of 10 wt % TMAOH was then added to the microemulsion. The final mixture was further stirred for 20 min under nitrogen. The particles were recovered by precipitation in an excess of an acetone-methanol mixture (9:1 ratio), followed by several washings, centrifuged, and dried under vacuum before characterization. Synthesis of Polymeric Magnetic Particles by Seed Precipitation Polymerization. The magnetite particles (1 wt %) were dispersed in ethyl acetate by sonication for 30 min. Monomers (2.5 wt %), cross-linker (5 mol % of monomers), and initiator (0.025 wt %) were dissolved in the reaction mixture under nitrogen for 30 min. The polymerization of monomers was carried out with AIBN at 55 °C for 8 h. After polymerization, the particles were recovered by magnetic separation followed by several washings with acetone and dried under vacuum before characterization. Synthesis of Polymeric Shells with Previously Synthesized Magnetite Particles in Microemulsion. The microemulsion was prepared by adding an aqueous suspension of previously synthesized magnetite particles (0.45 wt %), in some cases with a hydrophilic initiator, to a solution of methacrylic acid, hydroxyethyl methacrylate (MAA/HEMA ) 9:1 molar ratio), cross-linker, and AOT in toluene in the chosen proportions. Water/ AOT/toluene ratio was 2:5:25 by weight. The monomer concentration was about 2.3 wt % with a cross-linker/monomer molar ratio of 1:200. The microemulsion was then purged with nitrogen for 45 min, and a hydrophobic initiator (0.2 wt %) was added to the solution in toluene. The polymerization of monomers was carried out with either AIBN or potassium persulfate at 55 °C. (34) Massart, R.; Cabuil, V. J. Chim. Phys. 1987, 84, 967.
Magnetite and Polymer Magnetite Nanoparticles
Figure 1. Magnetization curve for magnetite particles synthesized in solution: (a) without treatment; (b) (TMA)OH treated. After polymerization, the particles were recovered by precipitation in an excess of an acetone-methanol mixture (9:1 ratio), followed by several washings. The magnetic particles were recovered by magnetic separation of the particle suspension in methanol by using an electromagnet (ERIEZ Magnetics Model SL4, maximum magnetic field 2 kOe, magnetic field gradient at 2 cm from the poles 0.2 kOe/cm) and dried under vacuum before characterization. Synthesis of Magnetite and Polymeric Magnetic Particles in the Same Microemulsion. First, the magnetite particles were synthesized by either microemulsion method 1 or method 2. Then, water, monomers (MAA/HEMA ) 9:1 molar ratio), cross-linker, and an initiator were added to the reaction mixture in the desirable proportion under nitrogen. Several ratios of water/AOT were used. The microemulsion was then purged with nitrogen for 15 min. The polymerization of monomers was carried out with AIBN at 55 °C. After polymerization, the particles were recovered by precipitation in an excess of an acetonemethanol mixture (9:1 ratio), followed by several washings. The magnetic particles were recovered by magnetic separation as described above and dried under vacuum before characterization.
Results Synthesis of Magnetite Particles. An appearance of the black color was immediate for all three methods of particle preparation, with the molecular exchange between the microdroplets being very fast during the mixing process. The colloidal solutions obtained with TMAOH were stable for months. Microemulsions of colloidal magnetite particles were very stable. No sedimentation was observed after a few months. A dry powder of (TMA)OH-treated magnetite particles which had been stored for several months formed a colloidal suspension spontaneously, whereas untreated magnetite particles which had been stored for even several weeks did not form a stable colloidal suspension. Characterization of Magnetite Particles. The magnetite particles were analyzed by VSM. Figure 1 shows a typical magnetization curve. The saturation magnetization of synthetic magnetite particles, which was found to be equal to 84 emu/g, is comparable to the theoretical value of 92 emu/g. The 84 emu/g value was one of the highest values found in the literature35 for synthetic magnetite particles, suggesting that the synthesized magnetite particles had a high purity. The hysteresis loop which is characteristic of the ferrimagnetic behavior is clearly observed in the expanded view of the center of the curve inserted in Figure 1 curve a (Hc ) 85 Oe). The magnetization curve for particles coated with (TMA)OH is shown in Figure 1 curve b. The saturation magnetization (35) Lin, C. H.; Kuo, P. C.; Pan, J. L.; Huang, D. R. J. Appl. Phys. 1996, 79, 6035.
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Figure 2. Magnetization curve obtained for a colloidal suspension of (TMA)OH treated magnetite particles. Table 1. Average Hydrodynamic Radius RH and Variance of Magnetite Particles sample
RH, nm variance
magnetite particles magnetite particles coated with (TMA)OH magnetite particles synthesized in microemulsion
27 15 7.3
0.4 0.2 0.24
was about 80 emu/g. The magnetization curve for a colloidal suspension of magnetite coated with (TMA)OH is shown in Figure 2. The negative slope at high field is from the diamagnetic contribution of water and glass sample capsules used in the measurement. The saturation magnetization obtained for the ferrofluid was found to be equal to 0.03 emu/g after subtraction of the diamagnetic background, representing a magnetite concentration of 0.45 wt %. Dynamic Light Scattering Analysis. Size distribution profiles of the magnetite particle suspensions are shown in Figures 3. The average hydrodynamic radius RH and the variance of the magnetite particles are presented in Table 1. The corresponding angular dependence of the average diffusion coefficient (eq 6) from Figure 3C for magnetite particles synthesized in microemulsion is shown in Figure 4 curve a.
DH ) Γ h q2
(6)
Synthesis of Polymeric Magnetic Particles. At first, the polymeric magnetic particles were synthesized by precipitation polymerization in ethyl acetate. The average hydrodynamic radius was 200 nm with a variance of 0.7 signifying a fairly broad size distribution. The diffusion coefficient of polymeric magnetic particles also revealed a significant angular dependence, as shown in Figure 4 curve c. This behavior usually suggests the presence of larger particles in a relatively more polydisperse size distribution. As the measured diffusion coefficient is a form of z-average diffusion coefficient, the larger particles with size approaching the incident light wavelength scatter more strongly with decreasing scattering angles, resulting in a decrease in the average diffusion coefficient DH. A stable solution of magnetite particles peptized by (TMA)OH was used as the water phase for microemulsion synthesis of polymeric magnetic particles. The concentration of magnetite in the colloidal solution was 0.45 wt %. The microemulsion was stable and had a yellow transparent color. No settling was observed even with the solution placed on top of an electromagnet (ERIEZ Magnetics Model SL4, maximum magnetic field 2 KOe, magnetic field gradient at 2 cm from the poles 0.2 kOe/ cm) for 48 h.
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Figure 4. Angular dependence of diffusion coefficient of magnetic particles: (a) magnetite particles synthesized in microemulsion, as shown in Figure 3C; (b) polymeric magnetite particles synthesized in one microemulsion; (c) polymeric magnetic particles synthesized by precipitation polymerization; (d) sample no. 23 of polymeric magnetic particles synthesized in microemulsion. Table 2. Sizes of Polymeric Magnetite Particles Synthesized with Hydrophilic Initiator and Monomer Concentration 1 wt %
a
sample no.
water, wt %
H2O/AOT, molar ratio
RH , nm
16 19 15 14 13 6 11 7 8 12 17a 9b
4 4.2 5.5 5.8 6.2 6.2 7 7 7 7.7 8.5 7.8
4.8 5.1 6.8 7.1 7.7 7.7 8.7 8.8 8.8 9.7 10.8 11.6
119 91 158 122 165 172 188 198 205 208 289 324
Solution unstable >18 h. b Solution unstable >12 h. Table 3. Sizes of Polymeric Magnetite Particles Synthesized with Hydrophilic Initiator
Figure 3. Size distribution profiles of magnetite particles: (A) without treatment, (B) (TMA)OH treated; (C) synthesized in microemulsion.
A slight change in the transparency of the microemulsion was observed a few minutes after the initiation of the polymerization process. The final product remained transparent with a yellow-beige color and remained stable over months. The solution viscosity was similar to that prior to the polymerization process, providing indirect supporting evidence for the chemical reaction that had occurred mainly inside the microdroplets. Numerous water/surfactant ratios were tried in order to obtain a range of shell thickness. Microemulsions with a water/surfactant ratio from 5 to 10 were stable over months, revealing no sedimentation. Characterization of Polymeric Magnetite Particles. Dynamic light scattering was used to obtain information on the average size and size distribution of polymeric magnetite particles. Figure 5A,B shows a typical size distribution profile and the corresponding angular dependence of diffusion coefficient, respectively. The average hydrodynamic radiuses RH of the polymeric magnetic particles obtained by using different water/ surfactant ratios are presented in Tables 2 and 3. DLS studies have shown that the microspheres obtained had
sample no.
water, wt %
monomers, wt %
H2O/AOT, molar ratio
RH, nm
21 23 24 22 25
7 6 6 6 5
1 2 2 2 3
8.9 7.5 7.5 7.5 6.4
220 270 311 316 320
RH values ranging from 90 to 320 nm, depending on the water/AOT ratio. The particles have been studied by using a vibratingsample magnetometer. The magnetization curves obtained for systems having different magnetite concentrations are shown in Figure 6 curves a-c. The saturation magnetization obtained varied from σs ) 4.4 × 10-3 to 3.6 × 10-1 emu/g for the different systems studied. Consequently, the magnetite concentration was found to be equal to 5 × 10-3 to 0.4 wt %. One should notice that systems having a magnetite concentration equal to 0.4% are close to the maximum value obtained for the initial aqueous solution of magnetite coated with TMAOH. For a magnetite concentration of 5 × 10-3 wt % as shown Figure 6 curve c, the minimum limit of detection for the vibrating-sample magnetometer had almost been reached. Only the diamagnetic response of the polymeric shell should be observed for systems with a lower magnetite concentration.
Magnetite and Polymer Magnetite Nanoparticles
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Figure 7. Magnetization curve for polymeric magnetic particles synthesized in one microemulsion.
The polymeric magnetite particles were also studied by atomic force microscopy (AFM). Figure 8A,B shows the results obtained in the phase and the height modes, respectively. Discussion
Figure 5. Size distribution of polymeric magnetic particles: (A) synthesized in microemulsion (sample no. 23 with RH ) 270 nm); (B) synthesized in one microemulsion.
Figure 6. Magnetization curve of polymeric magnetite particles: (a) magnetite concentration 0.4 wt %; (b) magnetite concentration 0.06 wt %; (c) magnetite concentration of 0.005 wt %.
Characterization of Polymeric Magnetite Particles Synthesized in One Microemulsion. Figure 5B and Figure 4 curve b show respectively the particle size distribution and the angular dependence of the diffusion coefficient of polymeric magnetic particles synthesized in a single microemulsion. The average hydrodynamic radius RH for the sample was found to be equal to 80 nm with a variance for the measured characteristic line width distribution function of 0.02. These particles have a very narrow size distribution when compared with that of a “monodisperse” polystyrene latex suspension that is often used as a reference standard. The saturation magnetization was found to be equal to 2.72 emu/g, representing a magnetic content of 3.3 wt % by using the reference value for the previously synthesized pure magnetite particles of σs to be 81 emu/g. Its superparamagnetic character can be seen in Figure 7.
The magnetite particles synthesized in solution and in microemulsion showed fairly narrow size distributions. The particle formation in a microemulsion can occur through collisions between microdroplets via the oil phase in method 1, while, in method 2, the particle formation occurs only by diffusion of the hydrophilic base to the microemulsion droplets containing the iron salts. Although the surfactant walls of the microdroplets act as cages for the growing particles and thereby reduce the average size of the particles, the exchange of iron ions between the microdroplets by either a diffusion process or a collision of microdroplets affects the particle formation, resulting in a similar narrow particle size distribution. However, the magnetite particles synthesized by using the microemulsion process exhibited no angular dependence in the measured average diffusion coefficient at different scattering angles, but the magnetite particles synthesized in solution showed an apparent angular dependence in the measured average diffusion coefficient because particle aggregation could be prevented better in a microemulsion. From the VSM studies, the magnetic cores formed in microdroplets gave a relatively narrow particle size distribution having a hydrodynamic radius of the order of 7.3 nm (small enough to exhibit superparamagnetic behavior). Superparamagnetism signifies that the magnetite particles have essentially single domains, i.e., all magnetic moments in the particle are aligned in one direction. The stabilization of magnetite particles by (TMA)OH decreased the value of saturation magnetization. It is known that magnetic properties can be altered both by size and by sorption of a surfactant.36 Stable magnetite microemulsions could be formed only within a limited range of water/AOT ratio. The encapsulation of magnetite particles was found to be low when the water/AOT ratio became lower than 4.5. The system became unstable with the ratio higher than 10. The water core radius of the microdroplets of water/AOT/toluene microemulsion changed from 1 to 1.6 nm when a water/ AOT ratio changed from 5 to 10. At a water/AOT ratio of less than 4, the apparent water pool density was lower than that of bulk water.37 The radius of water sphere at all ratios of water/AOT used in our experiment was lower (36) Morup. S. J. Magn. Magn. Mater. 1983, 39, 45. (37) Day, R. A.; Robinson, B. H.; Clarke, J. H. R.; Doherty, J. V. Faraday Trans. 1979, 75, 133.
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Figure 8. Atomic force microscopy image of polymeric magnetite particles synthesized in one microemulsion: (A) phase mode; (B) height mode.
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erization.39 The polymeric magnetite particles were 10 times larger than the polyacrylamide particles synthesized in a water/AOT/toluene microemulsion39 and 3 times larger than co-poly(acrylamide-sodium acrylate) synthesized in a microemulsion with a nonionic surfactant.38 The polymeric magnetite particle size increased with increasing monomer concentration. If the microdroplet size were decreased by reducing the water/AOT ratio (Table 2), the polymeric magnetite particle size would still increase with increasing monomer concentration, signifying that it was the monomer concentration which was the dominant factor in determining the final polymeric magnetite particle size. For each system, the microspheres obtained showed a fairly low polydispersity in size, due to the choice of carrying out the polymerization process by using the microemulsion approach. Polydispersity effects have been noted qualitatively in several studies on water-in-oil microemulsions.40,41 If the surfactant layer on the microemulsion droplet were flexible, the radius of the droplets could not be strictly fixed but would fluctuate around an average. The variance of the characteristic line width distribution function of the polymeric magnetite particles was on the order of 0.1. In the single microemulsion process, the measured diffusion coefficient showed no apparent angular dependence and the variance was on the order of 0.05. As a reference, the variance value obtained for microspheres synthesized by the precipitation polymerization method was 0.7. Thus, by synthesis of both the magnetite core and the polymeric shell in the same microemulsion droplet, more uniform microspheres could be produced. Atomic force microscopy was used to detect the coreshell structure of the polymeric magnetite particles synthesized in a single microemulsion (Figure 8). One can see that only one core is contained in each polymeric magnetite particle. The AFM micrograph permits us to determine both the core size and the shell size of each individual polymeric magnetic particle. It is noted that the uncovered magnetic particles could not remain stable in an aqueous solution at pH ∼ 7. They would aggregate. As our sample produced a stable colloidal suspension, it is reasonable to assume that the magnetic particles were essentially covered. Figure 8 clearly shows that the magnetic particles were mostly covered. Conclusions
Figure 9. Variation of average hydrodynamic radius, RH, with water to AOT molar ratio. The dotted line is given as a guide for the eyes.
than the radius of the magnetite particles (15 nm). At a water/AOT ratio of higher than 10, the microemulsion became unstable and the aggregation of the magnetite particles during polymerization could lead to multicore particle formation. The radius of polymeric particles increased by a factor of about 3 when the water/surfactant ratio changed by a factor of 2.2, as shown in Figure 9, in good agreement with what was observed for the microemulsion copolymerization of acrylamide and sodium acrylate (3 and 2.6, respectively).38 In our experiments, the radius of the polymeric shell after polymerization was 2 orders of magnitude larger than the radius of the microemulsion droplets before polymerization. It is known that the radius of polymeric particles increased throughout the polym-
The results reported here demonstrate that different methods can be used to synthesize polymeric magnetite nanoparticles. Polymerization in microemulsions offers the most promising versatile technique for the synthesis of a wide variety of polymeric nanoparticles with the ability to control precisely both the size of the core and that of the particles formed. In this respect, microemulsions offer the best reaction media utilizing the small size and uniform distribution of the microdroplets to produce ultrafine particles. The polymeric shell size can be controlled in the range 80-320 nm by varying the size of the microdroplets. The technique of synthesizing a core and a shell in a single (38) Candau, F.; Zekhnini, Z.; Durand, J. J. Colloid Interface Sci. 1986, 114, 398. (39) Carver, M. T.; Hirsch, E.; Wittmann, J. C.; Fitch, R. M.; Candau. F. J. Phys. Chem. 1989, 93, 4867. (40) Dvolaitzky, M.; Lague¨s, M.; Le Pesant, J. P.; Ober, R.; Sauterey, C.; Taupin, C. J. Phys. Chem. 1980, 84, 1532. (41) Meyer, C. T.; Poggi, Y.; Maret, G. J. Physique 1982, 43, 827.
Magnetite and Polymer Magnetite Nanoparticles
microemulsion has improved the particle structure and size distribution. New magnetic hydrogel particles, made of a superparamagnetic magnetite core coated with a polymeric shell of PMA-PHEMA random copolymer, have been synthesized. The smallest radius for the polymeric magnetite particles that had been synthesized was about 160 nm, with a magnetite concentration as high as 3.3 wt
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%. Further studies to reduce the particle size to below 160 nm are in progress. Acknowledgment. B.C. gratefully acknowledges the support of this work by the National Science Foundation (DMR9612386 and DMR9632525). LA980971G
