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J. Phys. Chem. C 2007, 111, 2426-2429
Fabrication of Superparamagnetic Cobalt Nanoparticles-Embedded Block Copolymer Microcapsules Ru Qiao, Xiao Li Zhang, Ri Qiu, Yan Li, and Young Soo Kang* Department of Chemistry, Pukyong National UniVersity, 599-1 Daeyeon-3-dong, Namgu, Busan 608-737, Korea ReceiVed: NoVember 14, 2006; In Final Form: December 18, 2006
A facile synthetic route for the preparation of block copolymer microcapsules with cobalt magnetic nanoparticles (MPs) embedded in the polymer shells is reported. The preparation involved first the Co2+ reduction to produce cobalt nanocrystals (∼13 nm in size). This was followed by the formation of thin polymeric coatings on the surface of Co nanoparticles owing to van der Waals attractive forces between the oxygen atoms in the block copolymer and the Co atoms and the self-assembly of capsule shells simultaneously. XRD, FT-IR, FE-SEM, TEM, and TGA were used for the characterization of the obtained hollow microcapsules. The experiments show that the reaction time and copolymer concentration are the crucial factors affecting the phase of the products. The excellent dispersibility of the products in most common solvents originates from amphiphilic nature of the block copolymer. The hybrid microcapsules are superparamagnetic at room temperature under existence of MPs and could be moved and collected with an external magnetic field, which makes these products a good candidate for multifunctional directed materials in biotechnological application.
Introduction Fabrication of micro- and nanosized inorganic/organic hybrid microcapsules,1-6 in which incorporation of the inorganic component can provide useful optical or magnetic properties, has received increasing attention due to their wide applications, ranging from targeting drug delivery7,8 and catalysis carriers9 to biomedical applications10 and electronic inks.11 To our knowledge, two strategies for preparing this type of microcapsule have been reported. One is the colloid templated electrostatic layer-by-layer self-assembly of oppositely charged inorganic nanoparticles and polymer multilayers, followed by removal of the templated core,12 and the second is an inward growth method using core-gel-shell particles as templates for preparing hollow microcapsules.13-15 Thus, recent efforts to prepare hybrid microcapsules have focused on inorganic precipitation processes against a variety of sacrificial templates.9,16-19 Currently, many efforts have been devoted to the synthesis of polymeric microcapsules containing magnetic nanoparticles (MPs) and organic components, thereby making them ideal candidates for many technological applications.20-25 A multilayer self-assembly technique onto colloidal templates has been mainly used. One advantage of this approach is that the thickness of the coating layers can be controlled. Although this technique is an exciting development in microcapsule preparation, the templates used in this method must be removed subsequently to get a hollow core structure, which complicates the method and is not always compatible with other compounds that have to be encapsulated. Herein, we report a solvothermal reaction for preparing polymeric microcapsules into whose shells Co magnetic nanoparticles are embedded. This is a process for one-step formation of microcapsules that is rarely reported in the literature. The approach involves the formation of Co magnetic nanoparticles and self-assembly of nanoparticles and block copolymer poly* Corresponding author. E-mail:
[email protected].
(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), a biocompatible amphiphilic polymer, into
hollow microcapsules. This copolymer not only affords protective coatings to prevent oxidation of Co particles, but also makes the products disperse well in polar and nonpolar solvent. We expect that MPs/copolymer microcapsules can have more potential applications in the fields of materials science and biotechnology than single MPs or copolymer capsules. Experimental Section Materials. Poly(ethylene glycol)-block-poly(propylene glycol)block-poly(ethylene glycol) (PEG-b-PPG-b-PEG), ethylene glycol (99.5%), and anhydrous ethanol were obtained from Aldrich. Cobaltous chloride hexahydrate (CoCl2‚6H2O) and sodium hydroxide (NaOH) were purchased from Samchun Chemical (Korea). All chemicals were used as received without further purification. Synthesis of Polymeric Microcapsules Embedded with MPs. CoCl2‚6H2O (2 mmol), NaOH (0.3 g, 7.5 mmol), and PEG-b-PPG-b-PEG (0.35 mmol) were dissolved in 40 mL of ethylene glycol and stirred vigorously at room temperature under nitrogen protection until homogeneous. Subsequently, the suspension was put into a 50-mL Teflon-liner autoclave and heated at 200 °C for 12 h followed by naturally cooling to room temperature. The solid products were washed by anhydrous ethanol to remove the residual reagents, separated by centrifugation, and then redispersed in ethanol. Characterization. X-ray diffraction (XRD) patterns of cobalt nanoparticles-embedded microcapsules were recorded using an X-ray diffractometer (X′ Pert-MPD, Philips) with a Cu KR radiation. The FT-IR spectrum was recorded in KBr on an FT-
10.1021/jp067534s CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007
Fabrication of Superparamagnetic Cobalt Nanoparticles
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Figure 1. Schematic illustration used to generate Co MPs-embedded microcapsules.
Figure 2. FT-IR patterns of Co MPs-embedded microcapsules.
IR spectrometer (Perkin-Elmer), and thermogravimetric analysis (TGA) measurement was performed with a thermal analyzer (TGA-7, Perkin-Elmer) under a nitrogen atmosphere at a temperature range of 50-700 °C; the heating rate was 10 °C/ min. The hollow structure and size distribution of produced microcapsules were observed with a field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and transmission electron microscopy (TEM, JEOL TEM-100). Superconducting quantum interference device (SQUID) magnetic characterization of the microcapsules was performed on a Quantum Design magnetometer (MPMS XL 7). Results and Discussion Synthesis of Cobalt Nanoparticles-Embedded Copolymer Microcapsules. The general procedure used to produce these Co nanoparticles-embedded microcapsules is schematically depicted in Figure 1. During the reaction, Co nanoparticles were prepared by reducing CoCl2‚6H2O with ethylene glycol in the presence of NaOH. Meanwhile, PEG-b-PPG-b-PEG copolymer was adsorbed on the surface of the Co nanoparticles in the basic medium due to van der Waals attractive forces. Then the microcapsules were formed by the aggregation of Co nanocrystals because of the activity of the block copolymer to the inorganic surface. To demonstrate the possible acting forces between Co nanoparticles and block copolymer, FT-IR measurements were carried out. The FT-IR spectra of PEG-b-PPGb-PEG and Co-embedded copolymer microcapsules are displayed in Figure 2. For Co-embedded microcapsules (Figure 2b), the absorption peaks at 1458.8 and 1383.0 cm-1 can be assigned to bending vibrations of CH2 groups and CH3 groups, respectively, while the absorption peak at 2925.1 cm-1 can be ascribed to stretching vibration of C-H bonds. The absorption peak around 1653.7 cm-1 suggests that the generation of CdO bonds is because of the oxidation of the -OH groups in part of the block copolymers. Compared to the FT-IR spectrum of PEGb-PPG-b-PEG (Figure 2a), the absorption peak of O-H stretching at 3489.7 cm-1 shifts to 3446.9 cm-1 and the absorption peak of C-O bonds at 1110.3 cm-1 shifts to 1057.8 cm-1, suggesting formation of intermolecular weak bonds between
Figure 3. TGA curve of Co MPs-embedded microcapsules in N2.
oxygen atoms in block copolymer and Co atoms. Furthermore, the TGA (Figure 3) also reveals the presence of PEG-b-PPGb-PEG copolymer in the hollow microcapsules. The TGA curve shows two weight loss steps. The first one between 200 and 300 °C is attributed to the pyrolysis of the copolymer and the PEG block, while the second step higher than 340 °C corresponds to the decomposition of the residue polymer especially the PPG block. Simultaneously, the curve shows there is a phenomenon of mass re-increase at the end of two weight loss steps. The reason is that cobalt oxides were formed by the reaction between cobalt particles and water, which was obtained by the decomposition of the copolymer although the TGA measurement was carried out under a nitrogen atmosphere. Morphology of Hybrid Microcapsules. FE-SEM was used to characterize the morphology and size of hybrid microcapsules in the solid state. A spherical morphology with a size distribution of 2-4 µm is observed (Figure 4a,b). The broken and collapsed capsule shells point out the existence of an inner hollow core, as revealed by FE-SEM images of the dried microcapsules. The shell surface is not smooth due to the aggregation of Co crystals. The XRD spectrum for hybrid microcapsules (Figure 4c) gives a clear crystalline pattern of Co nanoparticles, which indicates that Co nanoparticles are loaded into the hollow capsules and located in the shells. The mean crystallite size of the crystalline Co nanoparticles was calculated using the Scherrer equation (eq 1)
D)
0.9λ β cos θ
(1)
where λ is the characteristic wave length of the X-ray (Cu KR ) 0.154056 nm), θ is the Bragg diffraction angle, and β is the full width at half-maximum (111) peak. The crystallite size determined from the Scherrer equation is 13 nm. Meanwhile, the capsule morphology can be changed by varying the reaction time and copolymer concentration. When decreasing the reaction time to 6 h, XRD analysis (Figure 5a) indicates an amorphous structure of Co nanoparticles, while the
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Figure 5. XRD patterns of Co MPs-embedded microcapsules with (a) 6 h of reaction time and (b) 0.175 mmol of PEG-b-PPG-b-PEG.
Figure 6. FE-SEM images of Co MPs-embedded microcapsules with (a) 6 h of reaction time and (b) 0.175 mmol of PEG-b-PPG-b-PEG.
Figure 4. Characterization of Co MPs-embedded microcapsules. (a and b) FE-SEM images of hybrid microcapsules. The inset is the amplified image of a broken microcapsules. (c) XRD pattern.
shape of the capsules and their main size distribution remain unchanged (Figure 6a). Then, on the other hand, 600-800-nm hollow capsules are obtained with a smoother surface by decreasing the copolymer concentration to 0.175 mmol (Figure 6b). As shown in Figure 5b, the XRD pattern indicates the inorganic components are the same Co nanoparticles with a crystalline structure. Magnetic Properties. The magnetic property of the products was measured using a SQUID. The field-dependence hysteresis loop of the hybrid microcapsules at 300 K is presented in Figure
7. From Figure 7, it can be seen that the hysteresis and coercivity are almost undetectable, suggesting that the hybrid microcapsules have superparamagnetic properties at room temperature, and the saturation magnetization is 155 emu g-1, which is lower than that for the reported bulk Co metal (164 emu g-1).26 The difference in the magnetization value between the bulk and our produced materials can be attributed to the small particle size effect. It is known that the magnetic properties of nanoparticles, such as saturation magnetization and magnetic hyperfine field value, are smaller than those of the corresponding bulk materials.27 Thus, the crystallite size of the embedded Co nanoparticles, about 13 nm, reduces the total magnetic moment. In addition, the nonmagnetic organic components can also reduce it. The microcapsules exhibit relatively low magnetization consequently. However, a clear magnetic response is evident and can readily be used to move and collect the capsules in an external magnetic field (Figure S1). Amphiphilic Properties of the Microcapsules. The amphiphilic nature of PEG-b-PPG-b-PEG block copolymer gives the products an advantage for excellent dispersibility in most
Fabrication of Superparamagnetic Cobalt Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 6, 2007 2429 Conclusions We report a facile route for the fabrication of polymeric microcapsules embedded with Co magnetic nanoparticles. This type of microcapsule demonstrates a magnetic response with the presence of MPs. The use of the block copolymer not only prevents oxidation of Co particles for long-term use under ambient conditions, but also makes the microcapsules biocompatible and amphiphilic. Further work will be devoted to controlling particle size during the solvothermal reaction and assembly of the permeable capsules on the interface of emulsion droplets using this type of MPs-embedded microcapsule as building blocks. Overall, we can predict that these amphiphilic magnetic microcapsules will be useful in biomedicine and biotechnology.
Figure 7. Magnetization curve of Co MPs-embedded microcapsules measured at 300 K.
Acknowledgment. This work was financially supported by a Korea Research Foundation Grant for foreign graduate student and the Brain Korea 21 program. Supporting Information Available: Photograph of Co MPembedded microcapsule dispersion and the capture of the microcapsules by a magnet. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 8. (a) Suspensions of Co MPs-embedded microcapsules in water and hexane. (b) Thin microcapsule film formed onto the water/ hexane interface. (c and d) TEM images of the sintered microcapsules. (e) Schematic illustration of the permeable capsule fabricated by MPsembedded microcapsules.
polar and nonpolar solvents. As shown in Figure 8a, the microcapsules can be well-dispersed and remain stable in water and hexane. In the mixed water/hexane phase, gentle sonication easily moves the amphiphilic microcapsules to the water/hexane interface (Figure 8b), and a gray thin film forms at the interface, where the interstices between the microcapsules form holes that can provide a selective permeability (Figure 8c,d). Dinsmore and co-workers28 reported the preparation of permeable capsules fabricated by the self-assembly of colloidal spheres. Herein, these Co-embedded microcapsules can also be used as excellent building blocks onto the interface of emulsion droplets for permeable capsules with controlled permeability by varying the microcapsule diameter, compatibility, and especially capability of oriented movement under ambient magnetic field (Figure 8e). The flexibility will allow a wide range of potential applications to be explored.
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