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Preparation of magnetic polymeric particles via inverse microemulsion polymerization process
Journal of Magnetism and Magnetic Materials 257 (2003) 69–78
Preparation of magnetic polymeric particles via inverse microemulsion polymerization process Y. Denga, L. Wanga, W. Yanga, S. Fua,*, A Ela.ıssarib a
Department of Macromolecular Science, Fudan University, The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Shanghai 20043, China b Unit!e Mixte CNRS-bioM!erieux ENS-Lyon, 46, All!ee d’Italie, 69364-Lyon cedex, France Received 25 July 2002; received in revised form 26 August 2002
Abstract Hydrophilic submicron magnetic polymeric particles were prepared using inverse microemulsion polymerization process. Firstly, the magnetic properties of iron oxide nanoparticles elaborated were examined using X-ray diffraction and magnetization analysis of the chemical structure and the magnetic properties, respectively. The results obtained using stoichiometric precipitation of FeCl2 and FeCl3 revealed the magnetite iron oxide properties. Secondly, various magnetic polymeric latexes were prepared by investigating the effect of surfactant (AOT) concentration, the amount of cross-linker (MBA), the initiator nature and the microemulsion elaboration methodology. The colloidal characterization of the final magnetic polymeric latexes revealed to be submicronic in size, spherical in morphology, containing 5–23 wt% iron oxide and superparamagnetic in character. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic latex; Submicron; Inverse microemulsion; Acrylamide
1. Introduction In recent years, magnetic polymeric particles (i.e. polymer composites), due to their relatively rapid and easy magnetic separation, have been used in biomedical and bioengineering such as cell separation [1,2], immunoassay [3] and nucleic acids concentration [4]. In addition, magnetic polymeric particles offer a high potential in several areas of application such as detoxification of biological fluid and the magnetic guidance of particle systems *Corresponding author. Tel.: +86-21-65642385; fax: +8621-65640293. E-mail address: [email protected] (S. Fu).
for specific drug delivery process [5]. The pioneering work of Ugelstad et al. [6,7], based on the preparation of hydrophobic monosized polystyrene magnetic particles, has stimulated the research in this domain. The methodology used is basically based on direct precipitation of iron salt inside in the pores of the porous polystyrene seed. The particles obtained exhibit large particles size (i.e. 2.8 and 4.5 mm) with a good magnetic separation. The hydrophilic magnetic latexes have been first reported by Kawaguchi et al. [8] by using acrylamide as the main monomer. The second type of hydrophilic magnetic particles has been reported by Sauzedde et al. [9,10] using the Furazawa’s methodology [11]. The hydrophilic
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Y. Deng et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 69–78
thermally sensitive latexes have been obtained by encapsulating adsorbed iron oxide nanoparticles onto oppositely charged polystyrene-core/poly (Nisopropylacrylamide)-shell. The encapsulation has been performed using water-soluble monomers only (N-isopropylacrylamide, N,N0 methylene bisacrylamide and itaconic acid). The final particles exhibit thermal-sensitive property. In addition, various original methods (via non-conventional polymerization) have been investigated using natural polymers or proteins. In this domain, various interesting papers have been reported by Chatterjee et al. [12] such as cross-linked albumin magnetic microspheres and their uses in the separation of red blood cell from whole blood. It is worth noting that all reported methods in the elaboration of magnetic polymeric latexes lead to submicron particles size (generally above 500 nm) with appreciable iron oxide content. However, few works have been dedicated to the preparation of small hydrophilic magnetic polymeric latexes mainly appreciable in drug delivery domains. The object of this paper is to present a new approach for the preparation of hydrophilic magnetic polymeric latexes with particle size lower than 100 nm via inverse microemulsion polymerization. Magnetic polymeric particles prepared using this method, combined magnetic property and various properties of polymer particles, can be used in drug delivery area for some hydrophilic anti-cancer drugs such as adriamycin which, encapsulated in or combined, can be absorbed onto the particles.
2. Experimental 2.1. Materials Acrylamide (Am) (Shanghai LvNiao Co.) was re-crystallized twice from chloroform and then vacuum dried. The cross-linking agent N,N0 methylene bis (acrylamide) (MBA) (Shanghai Chem Reagent Co.) was re-crystallized from acetone and vacuum dried. Analytical-grade FeCl2 4H2O and FeCl3 6H2O (Merck), toluene, sodium hydroxide (NaOH) (Shanghai FeiDa Trade Company) and trisodium citrate (Shanghai Chem Reagent Co.) were all used as supplied.
Hydrophobic initiator 2,20 -azobis(isobutyronitrile) (AIBN) (Aldrich) was re-crystallized from ethanol and vacuum dried. Water soluble initiator 2,20 azobis(2-amidinopropane) dihydrochloride (V50) (Aldrich) was re-crystallized from a 50:50 acetone–water mixture, surfactant aerosol OT (AOT) (Cytec) was purified as described in Ref. [13]. In this study, only distilled water was used. 2.2. Synthesis of inorganic and magnetic polymeric colloids Iron oxide dispersion was prepared using the method already described [14,15], based on the coprecipitation of FeCl2 and FeCl3 by adding a concentrated solution of base (10 M NaOH) into the mixture of iron salts with a molar ratio (FeCl2/ FeCl3) of 12: The alkaline solution was stirred for 1 h at 201C and was then heated at 901C for 1 h. The iron oxide dispersion was then stirred for 30 min at 901C upon addition of 100 ml trisodium citrate solution (0.3 M). The ultrafine magnetic particles were precipitated with acetone, and the supernatant was decanted with the help of a magnet. Then water was added to redisperse the ultrafine magnetic particles. This dispersion achieved was treated by dialysis, and then adjusted to 6%wt for further use. The monomer (Am) and cross-linker (MBA) are added to a suspension of previously synthesized iron oxide nanoparticles (6%wt) in the desired amount (as shown in Table 2). The aqueous phase is dispersed in an AOT-toluene solution to form a W/O microemulsion under nitrogen. The microemulsion was then purged with nitrogen for 30 min. The polymerization of monomers was carried out with AIBN or V50 at 601C. After polymerization, the magnetic polymeric particles were dispersed in water by two methods: (i) Precipitation–redispersion, where the particles were recovered by precipitation with an excess of methanol and thoroughly washed and vacuum dried before redispersion in water using ultrasonic bath. (ii) Phase inversion, which is based on phase inversion. The polymerized W/O dispersion is inverted to O/W structure by adding an excess of water, the solvent toluene and the emulsifier AOT were removed by magnetic separation-washing process.
Y. Deng et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 69–78
2.3. Measurements 2.3.1. Dynamic light scattering analysis Dynamic light scattering analysis (DLS, Malvern 4700) was used to measure the size and size distribution of the magnetic polymeric latexes and the magnetic polymeric particles dispersed in water. Before measurement the prepared magnetic polymeric latexes were highly diluted and filtered through a 0.45 mm filter to remove impurities; then the sample was introduced into a thermostated scattering cell at 251C. 2.3.2. X-ray diffraction analysis A crystallographic study was performed on an iron oxide powder by Rotating Anode X-ray Diffractometer (Rigaku, Japan) using Cu Ka radiation. The distances between peaks, d; were calculated according to Bragg’s law and were compared to the ASTM X-ray diffraction (XRD) data in order to deduce the crystal structure. 2.3.3. Vibrating-sample magnetometer analysis A vibrating-sample magnetometer (VSM, EG&G Princeton Applied Research Vibrating Sample Magnetometer, Model 155, Made in USA) was used at room temperature to study the magnetic properties of iron oxide nanoparticles and composite particles. 2.3.4. Transmission electron microscopy analysis Transmission electron microscopy (TEM, Hitachi HU-11B) was used to determine the particle size of the magnetite and the magnetic polymeric particles. A drop of very dilute dispersion (magnetite and magnetic polymeric latexes) was placed on a carbon-coated copper grid, and the diameter was determined from the micrographs.
3. Results and discussion 3.1. Properties of iron oxide material Many different natural structures of iron oxide exist, such as magnetite (Fe3O4), hematite (a-Fe2O3), maghemite (g-Fe2O3), martite (Fe2O3),
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ferric hydroxide (b-FeOOH) [16], and all exhibit magnetic properties. 3.1.1. XRD analysis The crystallographic structure of iron and iron oxide type was determined by XRD pattern. Fig. 1 shows the XRD pattern of inorganic magnetic particles, and experimental d spacing obtained from the XRD pattern are reported in Table 1, and the experimental data was found similar to the ASTM data cards of the Fe3O4. Based on the d spacing and on the preparation method, it can be concluded that the iron oxide particles are mainly composed of magnetite (Fe3O4). 3.1.2. Particle size analysis The particle size (D) of the magnetic nanoparticles was deduced from XRD using Scheerrer’s formula [17]: D¼
0:9 l ; D cos y
where 0.9 is a dimensionless constant of the equipment, l is the wavelength of the radiation corresponding to the Cu Ka peak, y is Bragg’s angle, and D is full-width at half-maximum. The particle size was determined to be around 10 nm, which is a common value for monodomain’s superparamagnetic iron oxide nanoparticles as reported by different authors [18]. The TEM analysis of the iron oxide nanoparticles revealed the spherical shape of the particles as shown in Fig. 2. The particle size is too small to be adequately deduced from such micrograph in which the particle clusters are clearly shown. 3.1.3. Magnetic properties The magnetization variation (M) versus the applied magnetic field (H) (Fig. 3) provides information on the magnetic properties of the iron oxide elaborated. The saturation magnetization of the prepared magnetite was found to be around 50 emu/g. In addition, the magnetization decreases from the plateau value and reaches zero (i.e. no remanence effect) when the magnetic field intensity decreases. The behavior shows that the iron oxide particles correspond to a single crystal
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Fig. 1. X-ray diffraction spectrum of the synthesized magnetite.
Table 1 ( spacings from X-ray patterns, dðexpÞ and d Experimental d (A) spacings from ASTM data cards for iron oxide [Fe3O4] synthesized y (exp)
d (exp)
d (Fe3O4)
18.300 30.120 35.480 43.120 57.020 62.620
4.8480 2.9670 2.5301 2.0979 1.6151 1.4835
4.852 2.967 2.532 2.0993 1.6158 1.4845
domain exhibiting only on orientation of the magnetic moment and magnetite in structure. 3.2. Synthesis of magnetic polymeric nanolatex Firstly, Am and MBA were added to the iron oxide suspension previously synthesized (6%wt) in a desired proportion to form a stable colloidal dispersion, then added to an AOT-toluene solution to form a W/O microemulsion in ultrasonic bath. The microemulsion was then purged with
nitrogen for 30 min. The polymerization of monomers (Am and MBA) was carried out with AIBN or V50 at 601C. The microemulsion is stable and has a black–red transparent color. A slight change in the color aspect of the microemulsion is observed a few minutes after the initiation of the polymerization. The final magnetic polyacrylamide latex with black–red color remained stable for several months without visible sedimentation. The influence of the AOT/H2O ratio and the type of initiators applied on the final magnetic polymeric latex particle morphology, size and size distribution have been investigated. Due to the different nature of AIBN and V50, two different polymerization mechanisms are possible during the polymerization. When AIBN is used, the polymerization was initiated principally from the organic phase. The formed radicals induce radical polymerization via the entry mechanism process. The Am monomer, which slightly dissolved in toluene, continuously deposited onto the latexes particles during polymerization process. However, in the case of water-soluble initiator (V50), the polymerization starts in
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Fig. 3. Magnetization curve for the synthesis of iron oxide nanoparticles (sample weight is 70.74 mg).
final particles with an average particle size about 80 nm. In addition, the TEM photo shows that the magnetite particles were encapsulated and dispersed in the polymeric latex.
Fig. 2. TEM photos of magnetite nanoparticles and magnetic polymeric particles.
aqueous phase via classical emulsion polymerization process. 3.2.1. Morphology of magnetic polymeric particles Fig. 2B shows the transmission electron microscopy micrograph of crude magnetic polymeric particles and revealed the spherical morphology of
3.2.2. Particle size and size distribution of magnetic polymeric particles To study the effect of the surfactant/water ratio and the MBA concentration on the final particle size of the composite latexes, firstly, various AOT/ H2O ratios are used in microemulsion formation. The microemulsion obtained with an AOT/H2O (molar ratio) from 0.10 to 0.25 are stable over months. However, phase separation occurs rapidly (i.e. 10 h) when AOT/H2O ratio is lower than 0.10 as reported by Dresco [19]. Then, different MBA amounts are used in the recipe to synthesize magnetic polymeric latexes. DLS analysis has shown that the hydrodynamic diameter (DH ) values of magnetic polymeric latexes obtained range from 70 to 120 nm, depending on the AOT/H2O ratio and irrespective of the MBA concentration. When the ratio of AOT/H2O increases, the particle size of the magnetic polymeric latexes decreases, as shown in Table 2, which agreed with what have been observed in the case of microemulsion polymerization of acrylamide as reported by Candau et al. [20]. When the ratio of AOT/H2O increases, the water core size of the microdroplets in W/O microemulsion decreases, and this leads to smaller magnetic latexes. The behavior observed is reported in Fig. 4, in which
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Table 2 Influence of AOT/H2O ratio, MBA amount and different treatment methods on the size of magnetic polymeric latex and particle (temperature is 251C)a Run
AOT/H2Ob
MBA (106 mol)
DH c (nm)
DH d (nm)
DH e (nm)
1201A 1106A 1222A 1222B 1222C 1110B 1109B 1110A 1109Cf 1128Ag 1201A
0.25 0.25 0.25 0.25 0.25 0.21 0.16 0.12 0.09 0.25 0.25
12.72 25.97 38.69 51.41 64.66 25.97 25.97 25.97 25.97 12.72 12.72
74 73 71 75 75 83 91 126 — 76 74
170 145 216 134 224 223 192 285 — — 170
144 124 106 114 86 166 174 186 — 260 144
a
Am: 1.41 mmol, toluene: 81.40 mmol, Fe3O4 dispersion 0.53 g (6%wt). Molar ratio of AOT/H2O. c Magnetic polymeric latex without further treatment. d Precipitation and redispersion method. e Phase inversion method. f Unstable dispersion >10 h. g Sample with V50 as initiator. b
Fig. 4. Particle size distribution profile of magnetic polymeric latex (sample 1201A) using DLS.
the average diameter size (74 nm) and size distribution of the magnetic polymer latex (sample 1201A) are illustrated. Two methods (i.e. phase inversion method and precipitation–redispersion process) are applied to disperse polyacrylamide magnetic particles in aqueous solution from W/O latexes. DLS analyses of the average hydrodynamic particle size (DH ) of the magnetic polymeric particles are listed in Table 2. It is found that the particle size and size distribution of the magnetic polymeric particles
prepared by two methods above described are different. When the phase reversion method was applied, the magnetic particles have DH values ranging from 80 to 180 nm (when AIBN was used), depending on the AOT/H2O ratio and MBA amount. In fact, the average particle size (DH ) decreases when the AOT/H2O ratio or MBA concentration increase as shown in Fig. 5A and B (curve 1). The behavior observed is mainly attributed to the reduction of particle size with
Y. Deng et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 69–78
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300 280 260 Particles Size (nm)
240 220 200
2
180 160 140
1
120 0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
0.26
AOT/H2O Mol/Mol
(A)
240 220
Particles Size (nm)
200
2
180 160 140 120
1
100 80 20
40
(B)
60
80
100
120
[MBA] (mmol/l)
Fig. 5. Influence of (A) ratio of AOT/H2O and (B) [MBA] on the hydrodynamic polymer particle size. (K) Method of phase inversion and (&) method of precipitation and redispersion.
increasing AOT/H2O by affecting the microemulsion surface tension as is well known in such polymerization process. When the concentration of MBA increases, the cross-linking density of the polymer particles also increases and consequently, the particles are more difficult to be swollen by water. However, when the precipitation–redispersion method is applied, the particle size of the magnetic polymeric range from 140 to 300 nm, which is
much larger than those prepared via phase inversion process, and found to be unrelated to AOT/H2O ratio and the concentration of MBA as shown in Fig. 5A and B (curve 2). The observed behavior may be attributed to the influence of precipitation–redispesion and dryness on irreversible aggregated particles formation which induces uncertain particle size measurement. In fact, the large clusters formed during the process are difficult to be redispersed.
Y. Deng et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 69–78
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In addition to hydrophobic initiator AIBN, a water-soluble initiator 2,20 azobis-amidinoptopane di-hydrochlorides (V50) was also used in such W/O microemulsion polymerization. DLS analysis showed that the hydrodynamic diameter (DH ) value of the latex (sample 1128A) obtained is about 76 nm (Fig. 6A), which is of the same range as that of sample 1201A. The method of phase
inversion was also performed on sample 1128A, and it is found that the DH value of the swelled particles is about 260 nm (Fig. 6B), which is much larger than the size of the swelled sample 1201A with AIBN as initiator. Such a size difference between the two swelled magnetic polymeric particles is due to the fact that V50 is a charged initiator. The distribution of the positive charges
Intensity Size distribution (s)
% in class
10
5
5
10
(A)
100 50 Diameter (nm)
500
1000
Intensity Size distribution (s)
% in class
10
5
5 (B)
10
100 50 Diameter (nm)
500
1000
Fig. 6. Particle size distribution profile of sample 1128A: (A) the magnetic polymeric latex (w/o); and (B) the swollen magnetic polymeric particle using DLS.
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4. Conclusion
Fig. 7. Magnetization curve of magnetic polymeric particles (sample 1106A weight: 34.63 mg).
Table 3 The saturation magnetizations of systems having different magnetite concentration
1219A 1222D 1106A
ss (emu/g)
mFe3 O4 =mðFe3 O4 þAmþMBAÞ
2.7 7.5 11.8
5.45 14.8 23.4
in the polyacrylamide matrix may enhance the swelling ability of the magnetic particles via intraelectrostatic repulsion and hydration process, and consequently large hydrodynamic size was obtained after phase inversion. 3.2.3. Magnetic properties of magnetic polymeric particles The magnetic polymeric particles have been studied using a vibrating sample magnetometer. The magnetization curve of a sample is shown in Fig. 7. The magnetic polymeric particles also exhibit superparamagnetic properties: no remanence was observed when the magnetic field is removed. The saturation magnetizations obtained for systems having different magnetite concentration are shown in Table 3, which is found to vary from ss ¼ 2:7 to 11.8 emu/g. Consequently, the magnetite concentrations in polymer particles are found between 5.0% and 23% (wt/wt), which is in agreement with the magnetite concentration in the initial recipes and confirms the total incorporation of iron oxide nanoparticles.
The results have demonstrated that the inverse microemulsion is an ideal method to synthesize magnetic polymeric nanoparticles with a good control over iron oxide amount and magnetic properties. Using the phase inversion technique, the magnetic polymeric nanoparticles can be well dispersed in water phase. The particle size (i.e. diameter), ranging from 80 to 180 nm, is controlled by the water-soluble cross-linker concentration and surfactant/water ratio. The magnetic polymeric particles obtained are found to have narrow size distributions. The magnetic property of the final polymer composite particles can be controlled by the magnetite concentration in the polymer particles. In addition, using an appropriate ratio of FeCl2/FeCl3, superparamagnetic polymeric particles are obtained as needed as an ideal carrier in numerous biomedical applications.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (No. 29874010) and The Key Laboratory of Molecular Engineering of Polymers of Minister of Education for the financial support.
References [1] Y. Haik, V. Pai, C.J. Che, J. Magn. Magn. Mater. 194 (1999) 254. [2] K. Sugibayashi, Y. Morimoto, T. Nadai, Y. Kato, Chem. Pharm. Bull 25 (1977) 3433. [3] M. Mary, in: U. Hafeli, W. Schutt, M. Zborowski (Eds.), Scientific and Clinical Applications of Magnetic Carriers, Plenum Press, New York, 1997, p. 303. [4] A. Elaissari, M. Rodrigue, F. Meunier, C. Herve, J. Magn. Magn. Mater. 225 (2001) 127. [5] P.K. Gupta, C.T. Hung, Life Sci. 44 (1989) 175. [6] J. Ugelstad, P. Stenstad, L. Kilaas, W.S. Prestvik, R. Herje, A. Berge, E. Hornes, Blood Purif. 11 (1993) 349. [7] J. Ugelstad, P.C. Mork, R. Schmid, T. Ellingsen, A. Berge, Polym. Int. 30 (1993) 157. [8] H. Kawaguchi, K. Fujimoto, Y. Nakazawa, M. Sakagawsa, Y. Ariyoshi, M. Shidara, H. Okazaki, Y. Ebisawa, Colloid Surf. A 109 (1996) 147.
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[9] F. Sauzedde, A. Elaissari, C. Pichot, Colloid Polym. Sci. 277 (1999) 846. [10] F. Sauzedde, A. Elaissari, C. Pichot, Colloid Polym. Sci. 277 (1999) 1041. [11] K. Furusawa, K. Nagashima, C. Anzai, Colloid Polym. Sci. 272 (1994) 1104. [12] J. Chatterjee, Y. Haik, C.J. Chen, J. Magn. Magn. Mater. 225 (2001) 21. [13] H. Kunieda, K.J. Shinoda, Colloid Inter. Sci. 70 (1979) 577. [14] F.A. Tourinho, R. Franck, R. Massart, J. Mater. Sci. 25 (1990) 3249.
[15] F.A. Tourinho, R. Franck, R. Massart, Prog. Colloid Polym. Sci. 79 (1989) 128. [16] R.V. Upadhyay, K.J. Davies, S. Wells, S.W. Charles, J. Magn. Magn. Mater. 139 (1995) 249. [17] R. Massart, V. Cabuil, J. Chim. Phys. 84 (1987) 967. [18] M. Shinkai, H. Honda, T. Kobayashi, Biocatalysis 5 (1991) 61. [19] P.A. Dresco, V.S. Zaitsev, R.J. Gambio, B. Chu, Langmuir 15 (1999) 1945. [20] M.T. Carver, U. Dreyer, F. Candau, R.M. Fitch, J. Polym. Sci. A 27 (1989) 2161.
Preparation of magnetic polymeric particles via inverse microemulsion polymerization process Y. Denga, L. Wanga, W. Yanga, S. Fua,*, A Ela.ıssarib a
Department of Macromolecular Science, Fudan University, The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Shanghai 20043, China b Unit!e Mixte CNRS-bioM!erieux ENS-Lyon, 46, All!ee d’Italie, 69364-Lyon cedex, France Received 25 July 2002; received in revised form 26 August 2002
Abstract Hydrophilic submicron magnetic polymeric particles were prepared using inverse microemulsion polymerization process. Firstly, the magnetic properties of iron oxide nanoparticles elaborated were examined using X-ray diffraction and magnetization analysis of the chemical structure and the magnetic properties, respectively. The results obtained using stoichiometric precipitation of FeCl2 and FeCl3 revealed the magnetite iron oxide properties. Secondly, various magnetic polymeric latexes were prepared by investigating the effect of surfactant (AOT) concentration, the amount of cross-linker (MBA), the initiator nature and the microemulsion elaboration methodology. The colloidal characterization of the final magnetic polymeric latexes revealed to be submicronic in size, spherical in morphology, containing 5–23 wt% iron oxide and superparamagnetic in character. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic latex; Submicron; Inverse microemulsion; Acrylamide
1. Introduction In recent years, magnetic polymeric particles (i.e. polymer composites), due to their relatively rapid and easy magnetic separation, have been used in biomedical and bioengineering such as cell separation [1,2], immunoassay [3] and nucleic acids concentration [4]. In addition, magnetic polymeric particles offer a high potential in several areas of application such as detoxification of biological fluid and the magnetic guidance of particle systems *Corresponding author. Tel.: +86-21-65642385; fax: +8621-65640293. E-mail address: [email protected] (S. Fu).
for specific drug delivery process [5]. The pioneering work of Ugelstad et al. [6,7], based on the preparation of hydrophobic monosized polystyrene magnetic particles, has stimulated the research in this domain. The methodology used is basically based on direct precipitation of iron salt inside in the pores of the porous polystyrene seed. The particles obtained exhibit large particles size (i.e. 2.8 and 4.5 mm) with a good magnetic separation. The hydrophilic magnetic latexes have been first reported by Kawaguchi et al. [8] by using acrylamide as the main monomer. The second type of hydrophilic magnetic particles has been reported by Sauzedde et al. [9,10] using the Furazawa’s methodology [11]. The hydrophilic
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Y. Deng et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 69–78
thermally sensitive latexes have been obtained by encapsulating adsorbed iron oxide nanoparticles onto oppositely charged polystyrene-core/poly (Nisopropylacrylamide)-shell. The encapsulation has been performed using water-soluble monomers only (N-isopropylacrylamide, N,N0 methylene bisacrylamide and itaconic acid). The final particles exhibit thermal-sensitive property. In addition, various original methods (via non-conventional polymerization) have been investigated using natural polymers or proteins. In this domain, various interesting papers have been reported by Chatterjee et al. [12] such as cross-linked albumin magnetic microspheres and their uses in the separation of red blood cell from whole blood. It is worth noting that all reported methods in the elaboration of magnetic polymeric latexes lead to submicron particles size (generally above 500 nm) with appreciable iron oxide content. However, few works have been dedicated to the preparation of small hydrophilic magnetic polymeric latexes mainly appreciable in drug delivery domains. The object of this paper is to present a new approach for the preparation of hydrophilic magnetic polymeric latexes with particle size lower than 100 nm via inverse microemulsion polymerization. Magnetic polymeric particles prepared using this method, combined magnetic property and various properties of polymer particles, can be used in drug delivery area for some hydrophilic anti-cancer drugs such as adriamycin which, encapsulated in or combined, can be absorbed onto the particles.
2. Experimental 2.1. Materials Acrylamide (Am) (Shanghai LvNiao Co.) was re-crystallized twice from chloroform and then vacuum dried. The cross-linking agent N,N0 methylene bis (acrylamide) (MBA) (Shanghai Chem Reagent Co.) was re-crystallized from acetone and vacuum dried. Analytical-grade FeCl2 4H2O and FeCl3 6H2O (Merck), toluene, sodium hydroxide (NaOH) (Shanghai FeiDa Trade Company) and trisodium citrate (Shanghai Chem Reagent Co.) were all used as supplied.
Hydrophobic initiator 2,20 -azobis(isobutyronitrile) (AIBN) (Aldrich) was re-crystallized from ethanol and vacuum dried. Water soluble initiator 2,20 azobis(2-amidinopropane) dihydrochloride (V50) (Aldrich) was re-crystallized from a 50:50 acetone–water mixture, surfactant aerosol OT (AOT) (Cytec) was purified as described in Ref. [13]. In this study, only distilled water was used. 2.2. Synthesis of inorganic and magnetic polymeric colloids Iron oxide dispersion was prepared using the method already described [14,15], based on the coprecipitation of FeCl2 and FeCl3 by adding a concentrated solution of base (10 M NaOH) into the mixture of iron salts with a molar ratio (FeCl2/ FeCl3) of 12: The alkaline solution was stirred for 1 h at 201C and was then heated at 901C for 1 h. The iron oxide dispersion was then stirred for 30 min at 901C upon addition of 100 ml trisodium citrate solution (0.3 M). The ultrafine magnetic particles were precipitated with acetone, and the supernatant was decanted with the help of a magnet. Then water was added to redisperse the ultrafine magnetic particles. This dispersion achieved was treated by dialysis, and then adjusted to 6%wt for further use. The monomer (Am) and cross-linker (MBA) are added to a suspension of previously synthesized iron oxide nanoparticles (6%wt) in the desired amount (as shown in Table 2). The aqueous phase is dispersed in an AOT-toluene solution to form a W/O microemulsion under nitrogen. The microemulsion was then purged with nitrogen for 30 min. The polymerization of monomers was carried out with AIBN or V50 at 601C. After polymerization, the magnetic polymeric particles were dispersed in water by two methods: (i) Precipitation–redispersion, where the particles were recovered by precipitation with an excess of methanol and thoroughly washed and vacuum dried before redispersion in water using ultrasonic bath. (ii) Phase inversion, which is based on phase inversion. The polymerized W/O dispersion is inverted to O/W structure by adding an excess of water, the solvent toluene and the emulsifier AOT were removed by magnetic separation-washing process.
Y. Deng et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 69–78
2.3. Measurements 2.3.1. Dynamic light scattering analysis Dynamic light scattering analysis (DLS, Malvern 4700) was used to measure the size and size distribution of the magnetic polymeric latexes and the magnetic polymeric particles dispersed in water. Before measurement the prepared magnetic polymeric latexes were highly diluted and filtered through a 0.45 mm filter to remove impurities; then the sample was introduced into a thermostated scattering cell at 251C. 2.3.2. X-ray diffraction analysis A crystallographic study was performed on an iron oxide powder by Rotating Anode X-ray Diffractometer (Rigaku, Japan) using Cu Ka radiation. The distances between peaks, d; were calculated according to Bragg’s law and were compared to the ASTM X-ray diffraction (XRD) data in order to deduce the crystal structure. 2.3.3. Vibrating-sample magnetometer analysis A vibrating-sample magnetometer (VSM, EG&G Princeton Applied Research Vibrating Sample Magnetometer, Model 155, Made in USA) was used at room temperature to study the magnetic properties of iron oxide nanoparticles and composite particles. 2.3.4. Transmission electron microscopy analysis Transmission electron microscopy (TEM, Hitachi HU-11B) was used to determine the particle size of the magnetite and the magnetic polymeric particles. A drop of very dilute dispersion (magnetite and magnetic polymeric latexes) was placed on a carbon-coated copper grid, and the diameter was determined from the micrographs.
3. Results and discussion 3.1. Properties of iron oxide material Many different natural structures of iron oxide exist, such as magnetite (Fe3O4), hematite (a-Fe2O3), maghemite (g-Fe2O3), martite (Fe2O3),
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ferric hydroxide (b-FeOOH) [16], and all exhibit magnetic properties. 3.1.1. XRD analysis The crystallographic structure of iron and iron oxide type was determined by XRD pattern. Fig. 1 shows the XRD pattern of inorganic magnetic particles, and experimental d spacing obtained from the XRD pattern are reported in Table 1, and the experimental data was found similar to the ASTM data cards of the Fe3O4. Based on the d spacing and on the preparation method, it can be concluded that the iron oxide particles are mainly composed of magnetite (Fe3O4). 3.1.2. Particle size analysis The particle size (D) of the magnetic nanoparticles was deduced from XRD using Scheerrer’s formula [17]: D¼
0:9 l ; D cos y
where 0.9 is a dimensionless constant of the equipment, l is the wavelength of the radiation corresponding to the Cu Ka peak, y is Bragg’s angle, and D is full-width at half-maximum. The particle size was determined to be around 10 nm, which is a common value for monodomain’s superparamagnetic iron oxide nanoparticles as reported by different authors [18]. The TEM analysis of the iron oxide nanoparticles revealed the spherical shape of the particles as shown in Fig. 2. The particle size is too small to be adequately deduced from such micrograph in which the particle clusters are clearly shown. 3.1.3. Magnetic properties The magnetization variation (M) versus the applied magnetic field (H) (Fig. 3) provides information on the magnetic properties of the iron oxide elaborated. The saturation magnetization of the prepared magnetite was found to be around 50 emu/g. In addition, the magnetization decreases from the plateau value and reaches zero (i.e. no remanence effect) when the magnetic field intensity decreases. The behavior shows that the iron oxide particles correspond to a single crystal
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Fig. 1. X-ray diffraction spectrum of the synthesized magnetite.
Table 1 ( spacings from X-ray patterns, dðexpÞ and d Experimental d (A) spacings from ASTM data cards for iron oxide [Fe3O4] synthesized y (exp)
d (exp)
d (Fe3O4)
18.300 30.120 35.480 43.120 57.020 62.620
4.8480 2.9670 2.5301 2.0979 1.6151 1.4835
4.852 2.967 2.532 2.0993 1.6158 1.4845
domain exhibiting only on orientation of the magnetic moment and magnetite in structure. 3.2. Synthesis of magnetic polymeric nanolatex Firstly, Am and MBA were added to the iron oxide suspension previously synthesized (6%wt) in a desired proportion to form a stable colloidal dispersion, then added to an AOT-toluene solution to form a W/O microemulsion in ultrasonic bath. The microemulsion was then purged with
nitrogen for 30 min. The polymerization of monomers (Am and MBA) was carried out with AIBN or V50 at 601C. The microemulsion is stable and has a black–red transparent color. A slight change in the color aspect of the microemulsion is observed a few minutes after the initiation of the polymerization. The final magnetic polyacrylamide latex with black–red color remained stable for several months without visible sedimentation. The influence of the AOT/H2O ratio and the type of initiators applied on the final magnetic polymeric latex particle morphology, size and size distribution have been investigated. Due to the different nature of AIBN and V50, two different polymerization mechanisms are possible during the polymerization. When AIBN is used, the polymerization was initiated principally from the organic phase. The formed radicals induce radical polymerization via the entry mechanism process. The Am monomer, which slightly dissolved in toluene, continuously deposited onto the latexes particles during polymerization process. However, in the case of water-soluble initiator (V50), the polymerization starts in
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Fig. 3. Magnetization curve for the synthesis of iron oxide nanoparticles (sample weight is 70.74 mg).
final particles with an average particle size about 80 nm. In addition, the TEM photo shows that the magnetite particles were encapsulated and dispersed in the polymeric latex.
Fig. 2. TEM photos of magnetite nanoparticles and magnetic polymeric particles.
aqueous phase via classical emulsion polymerization process. 3.2.1. Morphology of magnetic polymeric particles Fig. 2B shows the transmission electron microscopy micrograph of crude magnetic polymeric particles and revealed the spherical morphology of
3.2.2. Particle size and size distribution of magnetic polymeric particles To study the effect of the surfactant/water ratio and the MBA concentration on the final particle size of the composite latexes, firstly, various AOT/ H2O ratios are used in microemulsion formation. The microemulsion obtained with an AOT/H2O (molar ratio) from 0.10 to 0.25 are stable over months. However, phase separation occurs rapidly (i.e. 10 h) when AOT/H2O ratio is lower than 0.10 as reported by Dresco [19]. Then, different MBA amounts are used in the recipe to synthesize magnetic polymeric latexes. DLS analysis has shown that the hydrodynamic diameter (DH ) values of magnetic polymeric latexes obtained range from 70 to 120 nm, depending on the AOT/H2O ratio and irrespective of the MBA concentration. When the ratio of AOT/H2O increases, the particle size of the magnetic polymeric latexes decreases, as shown in Table 2, which agreed with what have been observed in the case of microemulsion polymerization of acrylamide as reported by Candau et al. [20]. When the ratio of AOT/H2O increases, the water core size of the microdroplets in W/O microemulsion decreases, and this leads to smaller magnetic latexes. The behavior observed is reported in Fig. 4, in which
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Table 2 Influence of AOT/H2O ratio, MBA amount and different treatment methods on the size of magnetic polymeric latex and particle (temperature is 251C)a Run
AOT/H2Ob
MBA (106 mol)
DH c (nm)
DH d (nm)
DH e (nm)
1201A 1106A 1222A 1222B 1222C 1110B 1109B 1110A 1109Cf 1128Ag 1201A
0.25 0.25 0.25 0.25 0.25 0.21 0.16 0.12 0.09 0.25 0.25
12.72 25.97 38.69 51.41 64.66 25.97 25.97 25.97 25.97 12.72 12.72
74 73 71 75 75 83 91 126 — 76 74
170 145 216 134 224 223 192 285 — — 170
144 124 106 114 86 166 174 186 — 260 144
a
Am: 1.41 mmol, toluene: 81.40 mmol, Fe3O4 dispersion 0.53 g (6%wt). Molar ratio of AOT/H2O. c Magnetic polymeric latex without further treatment. d Precipitation and redispersion method. e Phase inversion method. f Unstable dispersion >10 h. g Sample with V50 as initiator. b
Fig. 4. Particle size distribution profile of magnetic polymeric latex (sample 1201A) using DLS.
the average diameter size (74 nm) and size distribution of the magnetic polymer latex (sample 1201A) are illustrated. Two methods (i.e. phase inversion method and precipitation–redispersion process) are applied to disperse polyacrylamide magnetic particles in aqueous solution from W/O latexes. DLS analyses of the average hydrodynamic particle size (DH ) of the magnetic polymeric particles are listed in Table 2. It is found that the particle size and size distribution of the magnetic polymeric particles
prepared by two methods above described are different. When the phase reversion method was applied, the magnetic particles have DH values ranging from 80 to 180 nm (when AIBN was used), depending on the AOT/H2O ratio and MBA amount. In fact, the average particle size (DH ) decreases when the AOT/H2O ratio or MBA concentration increase as shown in Fig. 5A and B (curve 1). The behavior observed is mainly attributed to the reduction of particle size with
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300 280 260 Particles Size (nm)
240 220 200
2
180 160 140
1
120 0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
0.26
AOT/H2O Mol/Mol
(A)
240 220
Particles Size (nm)
200
2
180 160 140 120
1
100 80 20
40
(B)
60
80
100
120
[MBA] (mmol/l)
Fig. 5. Influence of (A) ratio of AOT/H2O and (B) [MBA] on the hydrodynamic polymer particle size. (K) Method of phase inversion and (&) method of precipitation and redispersion.
increasing AOT/H2O by affecting the microemulsion surface tension as is well known in such polymerization process. When the concentration of MBA increases, the cross-linking density of the polymer particles also increases and consequently, the particles are more difficult to be swollen by water. However, when the precipitation–redispersion method is applied, the particle size of the magnetic polymeric range from 140 to 300 nm, which is
much larger than those prepared via phase inversion process, and found to be unrelated to AOT/H2O ratio and the concentration of MBA as shown in Fig. 5A and B (curve 2). The observed behavior may be attributed to the influence of precipitation–redispesion and dryness on irreversible aggregated particles formation which induces uncertain particle size measurement. In fact, the large clusters formed during the process are difficult to be redispersed.
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In addition to hydrophobic initiator AIBN, a water-soluble initiator 2,20 azobis-amidinoptopane di-hydrochlorides (V50) was also used in such W/O microemulsion polymerization. DLS analysis showed that the hydrodynamic diameter (DH ) value of the latex (sample 1128A) obtained is about 76 nm (Fig. 6A), which is of the same range as that of sample 1201A. The method of phase
inversion was also performed on sample 1128A, and it is found that the DH value of the swelled particles is about 260 nm (Fig. 6B), which is much larger than the size of the swelled sample 1201A with AIBN as initiator. Such a size difference between the two swelled magnetic polymeric particles is due to the fact that V50 is a charged initiator. The distribution of the positive charges
Intensity Size distribution (s)
% in class
10
5
5
10
(A)
100 50 Diameter (nm)
500
1000
Intensity Size distribution (s)
% in class
10
5
5 (B)
10
100 50 Diameter (nm)
500
1000
Fig. 6. Particle size distribution profile of sample 1128A: (A) the magnetic polymeric latex (w/o); and (B) the swollen magnetic polymeric particle using DLS.
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4. Conclusion
Fig. 7. Magnetization curve of magnetic polymeric particles (sample 1106A weight: 34.63 mg).
Table 3 The saturation magnetizations of systems having different magnetite concentration
1219A 1222D 1106A
ss (emu/g)
mFe3 O4 =mðFe3 O4 þAmþMBAÞ
2.7 7.5 11.8
5.45 14.8 23.4
in the polyacrylamide matrix may enhance the swelling ability of the magnetic particles via intraelectrostatic repulsion and hydration process, and consequently large hydrodynamic size was obtained after phase inversion. 3.2.3. Magnetic properties of magnetic polymeric particles The magnetic polymeric particles have been studied using a vibrating sample magnetometer. The magnetization curve of a sample is shown in Fig. 7. The magnetic polymeric particles also exhibit superparamagnetic properties: no remanence was observed when the magnetic field is removed. The saturation magnetizations obtained for systems having different magnetite concentration are shown in Table 3, which is found to vary from ss ¼ 2:7 to 11.8 emu/g. Consequently, the magnetite concentrations in polymer particles are found between 5.0% and 23% (wt/wt), which is in agreement with the magnetite concentration in the initial recipes and confirms the total incorporation of iron oxide nanoparticles.
The results have demonstrated that the inverse microemulsion is an ideal method to synthesize magnetic polymeric nanoparticles with a good control over iron oxide amount and magnetic properties. Using the phase inversion technique, the magnetic polymeric nanoparticles can be well dispersed in water phase. The particle size (i.e. diameter), ranging from 80 to 180 nm, is controlled by the water-soluble cross-linker concentration and surfactant/water ratio. The magnetic polymeric particles obtained are found to have narrow size distributions. The magnetic property of the final polymer composite particles can be controlled by the magnetite concentration in the polymer particles. In addition, using an appropriate ratio of FeCl2/FeCl3, superparamagnetic polymeric particles are obtained as needed as an ideal carrier in numerous biomedical applications.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (No. 29874010) and The Key Laboratory of Molecular Engineering of Polymers of Minister of Education for the financial support.
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