6710
J. Phys. Chem. C 2008, 112, 6710-6716
Preparation and Characterization of Magnetic Nanoparticles and Their Silica Egg-yolk-like Nanostructures: A Prospective Multifunctional Nanostructure Platform Minghai Chen, Yong Nam Kim, Cuncheng Li, and Sung Oh Cho* Department of Nuclear and Quantum Engineering, Korea AdVanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong, Yuseong, Daejeon 305-701, Republic of Korea ReceiVed: NoVember 11, 2007; In Final Form: February 13, 2008
A facile solvothermal method was successfully utilized to synthesize well-crystallized magnetite (Fe3O4) nanoparticles through high-temperature decomposition of intermediate complexing compound using diethylene glycol (DEG) and diethanolamine (DEA) as solvent and complexing agent. Then a novel egg-yolk-like magnetic silica nanostructure, composed of a silica core and another silica shell with magnetic nanoparticles embedded in the boundary, was successfully prepared by a facile one-step sol-gel technique. The experimental parameters were carefully investigated to optimize the preparation process. The diameters of final products could be effectively tuned in a wide range by use of easily obtained silica colloidal cores with different diameters. Shell thickness could be controlled by changing the addition amount of tetraethyl orthosilicate (TEOS). On the basis of the characterization, we propose a formation mechanism of the nanostructure. The pre-addition of acid solution played a key role in the procedure, enhancing the electrostatic attraction between magnetic nanoparticles and silica spherical cores. Because of the offered magnetic properties to silica beads, some aggregations were found resulting from the magnetic attraction, which shall be further improved. This strategy provides a facile method to integrate magnetic nanoparticles into a silica matrix. Additionally, this novel magnetic silica nanostructure presents a good platform to construct multifunctional nanostructures by the well-developed composite strategy based on silica chemistry.
1. Introduction Magnetic nanoparticles have become a hot research topic and attracted great interest in the past decades. Especially with the rapid development of biological and biomedical nanotechnology, magnetic nanoparticles have found abundant novel applications in targeted drug delivery, magnetic cell sorting, magnetic resonance imaging (MRI) contrast agents, and cancer treatments by hyperthermia and immunoassays.1-4 Among varieties of magnetic nanoparticles, Fe3O4 has been most intensively studied. A great many chemical routes have been successfully utilized to prepare well-dispersed Fe3O4 nanopoarticles, including coprecipitation,5,6 organic colloid method,7-10 reverse micelle method,11,12 ultrasound irradiation,13 hydrothermal or solvothermal method,14,15 and spray pyrolysis techniques.16 Among these, coprecipitation of iron salts in alkaline solution in the presence of stabilizers is the most common route to prepare Fe3O4 nanoparticles on a large scale because of its ease in dealing and scaling up.5,6 However, because of a too-fast reaction rate, the as-prepared nanoparticles suffer from broad size distribution, irregular crystallite shape, and poor crystallization owing to the difficulties in controlling the nucleation and growth rate in the reaction system. The recently developed organic colloid method is considered as the most successful method to synthesize monodisperse Fe3O4 nanoparticles; it is based on the decomposition of oxygen-rich ferric compound or metal carbonyls in the presence of oxygen donors in solution of high-boiling nonpolar solvents.7-10 The capping ligands and coordination reagents involved in the process can effectively maintain a stable precursor concentration, which is in favor of a uniform nucleation and coarsening rate. This method has been * Corresponding author: tel +82-42-869-3823, e-mail
[email protected].
extensively studied and can be used to control the nanoparticle size within a variation of ∼1 nm.8 However, most of the products are hydrophobic and cannot be dissolved in water. Water solubility of magnetic nanoparticles is usually necessary for biomedical and biological applications. Although many researchers have successfully realized the hydrophilic surfaces by ligand exchange methods, surface modification itself is still a major task to functionalize the magnetic nanoparticles.17 The ideal strategy is to synthesize hydrophilic nanoparticles directly. Actually, pure magnetic nanoparticles are not popularly used in bio-applications because of easy agglomeration, less stabilization of their structures and magnetic properties, and rapid biodegradation. Before introduction into an organism, an organic or inorganic layer of polystyrene (PS), copolymer, silica, and other large organic molecules is usually coated on the surface of magnetic particles to increase their biocompatibility and magnetic stability. Silica is considered as an ideal candidate for its inert nature and ease of functionalization through varieties of commercial available silane coupling agents.18,19 Correspondingly, the well-defined magnetic silica beads have gained popularity in real bio-applications. Two strategies to prepare magnetic silica beads are generally used: aggregation and adsorption processes. In the former strategy, magnetic nanoparticles are usually trapped in the silica matrix simultaneously with the formation of silica spheres.20,21 Although the wellknown Sto¨ber sol-gel technique was widely used to prepare this kind of core/shell structure, it is not so easy to accomplish well-dispersed magnetic silica beads with uniform shape and size.22,23 Microemulsion or reverse micelle methods are very successful in preparing well-dispersed silica-coated magnetic nanoparticles.24 But some effects limit its scaling up and induces low yield, involving unfriendly organic solvents and inconve-
10.1021/jp710775j CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008
Prospective Multifunctional Nanostructure Platform nience in removing the surfactant. Additionally, there are other successful reports of magnetic beads composed of PS cores with trapped Fe3O4 nanoparticles and silica shells by combining miniemulsion/emulsion polymerization and sol-gel technique.25 Shi and co-workers26,27 introduced a multistep procedure to prepare magnetic silica spheres by coating Fe2O3 particles with a thin silica shell, followed by H2 reduction. Aggregation processes are usually poorly controllable; therefore, neither the polydispersity of the colloidal particles nor the radial position of the nanoparticles in the matrix particle can be well controlled. Comparatively, because the second strategy is based on the available colloidal cores where magnetic nanoparticles are selectively adsorbed through electrostatic attraction or coordination, it is easier to overcome those shortcomings. By this method, surface functionalization is usually required to attach the nanoparticles on the core colloid. The layer-by-layer method is widely used to prepare magnetic PS beads or silica beads through alternate adsorption of magnetic nanoparticles by polyelectrolyte.28-30 Other research groups reported on the preparation of magnetic colloidal spheres through the surface functionalization by active groups, such as -SH31 or -NH2,32,33 that have strong binding ability to magnetic nanoparticles. Graf et al.34 introduced a general method to control embedding of nanoparticles in silica colloids with the help of polyvinylpyrrolidone. For all these composites, another protective silica layer is necessary to maintain the stable nanostructure. It is of fundamental importance to develop a facile method and to increase the yield and efficiency. Herein, we first introduced a facile solvothermal method to prepare well-crystallized and well-dispersed Fe3O4 nanoparticles with hydrophilic active surfaces. Then these magnetic nanoparticles were successfully embedded in the boundary between silica cores and outer silica shells to form a novel egg-yolklike nanostructure by a one-step sol-gel technique. To the best of our knowledge, this is the first report on preparing this novel magnetic silica nanostructure by a one-step method. The silica shells serve not only as protective layers to enhance the composite nanostructure but also as active layers, enabling versatile postsynthesis treatment. This novel nanostructure shows prospective applications in constructing multifunctional nanostructures by grafting or coating other functional groups and organic molecular and inorganic materials to form magnetic silica bead-based nanostructures, for example, magnetic optical or magnetic catalytic nanostructures. 2. Experimental Section 2.1. Chemicals. Iron(III) chloride hexahydrate (FeCl3‚6H2O), iron(II) chloride tetrahydrate (FeCl2‚4H2O), diethylene glycol (DEG), diethanolamine (DEA), sodium hydroxide, ammonia solution (28%), nitric acid, acetic acid, and hydrochloric acid (35%) were purchased from Junsei Chemical Co., Ltd., Japan. Tetraethyl orthosilicate (TEOS, 98%) was purchased from Aldrich. All the chemicals were of analytical grade and used without further purification. Throughout the experiment, pure water and absolute alcohol were used, which were purchased from DC Chemical Co., Ltd. 2.2. Synthesis of Magnetic Nanoparticles. Magnetic nanoparticles were synthesized by a solvothermal coprecipitation method. In a typical experimental procedure, 4 mmol of FeCl3‚ 6H2O and 2 mmol of FeCl2‚4H2O were dissolved in 50 mL of DEG at 90 °C in an oil bath. At the same time, another 16 mmol of NaOH was dissolved in 20 mL of DEG. After 30 min, 10 mL of DEA was injected into the hot iron ion solution and
J. Phys. Chem. C, Vol. 112, No. 17, 2008 6711 a deep green turbid solution formed. Then the NaOH solution in DEG was also introduced into the mixture. After being stirred for another 10 min, the mixture was transferred to a Teflonlined autoclave of 80 mL capacity and sealed. The autoclave was loaded into an oven and maintained at a temperature range of 180-220 °C for 8 h. After being cooled to room temperature, the black precipitates were isolated by centrifugation at 10 000 rpm for 10 min and washed repeatedly with absolute alcohol. The final black product was redispersed in water and formed a 0.5 wt % suspension for the following use. 2.3. Preparation of Magnetic Silica Beads. The preparation of egg-yolk-like magnetic silica nanostructure was based on the well-known Sto¨ber technique. First, silica spheres were prepared by hydrolysis and condensation of TEOS in alcohol-ammonia solution (v/v 1/8). The as-prepared SiO2 spheres were isolated by centrifugation at 7000 rpm for 10 min and washed three times with absolute alcohol. Then these SiO2 spheres were redispersed in alcohol and formed a 0.8 wt % suspension. Then another silica shell was grown on the silica spheres with magnetic nanoparticles embedded in the outer shell. Typically, several drops of HCl solution (0.1 M) were dropped into 1-5 mL of magnetic nanoparticles in a water suspension until a transparent brown solution formed. Then water, SiO2 spheres in an alcohol suspension, alcohol, and ammonia were added into the transparent solution one by one; the final volume ratio of alcohol/water/ammonia is 40/8/1. Finally, 0.05-0.3 mL of TEOS was introduced into the mixture solution to induce the growth of another silica shell around the silica sphere core. After being stirred at room temperature for 5 h, the products were collected by filtration, washed with distilled water and absolute alcohol repeatedly, and then dried at 60 °C for 4 h in vacuum. 2.4. Characterizations. The crystal structure of the asprepared samples was characterized by powder X-ray diffraction (XRD) (Philips X’pert PRO), using Cu KR radiation. The morphologies and microstructures of the samples were observed on a FEI Tecnai F20 transmission electron microscope (TEM). Selected area electronic diffraction (SAED) pattern and energydispersive spectra (EDS) were also taken on a JEOL 2100F TEM instrument at the Korea Basic Science Institute (KBSI). X-ray photoelectron spectra (XPS) were recorded on a VG ESCA2000 spectrometer, with nonmonochromatized Mg KR X-ray as the excitation source and C1s chosen as the reference line. The magnetic hysteresis loops were measured on a vibrating sample magnetometer (Lakeshore Model 7304) at room temperature, with a powder sample packed in a capsule. 3. Results and Discussions 3.1. Magnetic Nanoparticles. Powder X-ray diffractometry was used to identify the crystal structure of as-prepared nanopowders after they were vacuum-dried at 60 °C for 4 h, and the corresponding XRD pattern is shown in Figure 1a. All the peaks show good consistency with the reported data (PDF card 65-3107) and can be indexed to a cubic inverse spinel Fe3O4 structure, indicating the pure phase of the product. The broadened peaks indicate the small size of the particles. However, because the crystal structure of Fe3O4 is very similar to that of γ-Fe2O3, usually it is very difficult to distinguish them from each other by XRD results alone. Thus, XPS was used to further analyze the composition signals. The corresponding highresolution XPS spectrum of Fe 2p was shown in Figure 1b. For the peak positions, the binding energies at 711.08 and 724.1 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively, which agree with the values of Fe3O4 in literature.35 For the peak shape, the absence of the shoulder peak between two peaks, which
6712 J. Phys. Chem. C, Vol. 112, No. 17, 2008
Chen et al.
Figure 1. (a) XRD pattern and (b) XPS pattern of magnetic nanoparticles synthesized by solvothermal route.
Figure 2. TEM images and SAED pattern of as-prepared magnetic nanoparticles at 220 °C for 8 h.
shows primarily in γ-Fe2O3, indicates the characteristic of Fe3O4.15,36 From the combined XRD and XPS results, it is confirmed that the as-prepared product is pure Fe3O4. As suggested by the broadened XRD peaks, the nanosized particles can be clearly seen in the TEM images, as shown in Figure 2. The TEM images show well-dispersed nanoparticles prepared at 220 °C for 8 h with a spherical morphology and an average diameter of ∼8 nm. A slight decrease of nanoparticle size can be achieved by decreasing the solvothermal temperature (not shown here). The easily distinguishable rings in the polycrystalline SAED pattern in Figure 2a indicate good crystallization of the product. Furthermore, a high-magnification TEM image in Figure 2b clearly shows crystal plane stripes.
The as-prepared Fe3O4 nanoparticles can be easily dispersed in weak acid solution, and the transparent brown solution formed is stable for several months. The TEM sample was prepared by dropping a drop of this solution on the copper grid. Presently, polyol compounds are widely used to synthesize nanocrystals with well-defined shape and good crystallization, owing to their good compatibilities with inorganic and organic compounds, high boiling points, less agglomeration, and ease of use. Consequently, nanocrystals can be synthesized at a higher temperature and well crystallized.37,38 DEG has chelating ability toward iron ions and slows down the formation of hydroxide or oxide counterparts.39 So it serves as both solvent and complex agent. DEA has a stronger complexing ability than DEG. In this work, both DEG and DEA chelate iron ions and form a stable complex compound at low temperature, which is confirmed by the color change to deep green with the addition of DEA. Although we cannot identify the exact structure of this complex, it can be speculated that this intermediate compound plays a very important role in obtaining high-purity and wellcrystallized nanoparticles. When the complex is loaded into an autoclave and maintained at high temperature, its stability decreases with increasing in temperature. The iron ions (both Fe2+ and Fe3+) are slowly released into the solvent by decomposition of the complex compound at high temperature. At the same time, the released Fe2+ and Fe3+ readily combine with OH- ions and coprecipitate at once. This is a hightemperature coprecipitation process, and thus a slow nucleation
Figure 3. TEM images of (a, b) bare SiO2 spheres, (c, d) magnetic silica beads with an egg-yolk-like structure, (e) high magnification image of the magnetic nanoparticles entrapped in silica matrix, and (f) hollow structures after treatment with HNO3 solution.
Prospective Multifunctional Nanostructure Platform
J. Phys. Chem. C, Vol. 112, No. 17, 2008 6713
Figure 4. EDS analysis results: (a) EDS element distribution map; (b) Fe; (c) Si; (d) EDS spectrum.
speed can be obtained. In the meantime, the high viscosity of DEG inhibits the mass transformation in the reaction system and results in a slow crystal growth speed, which is helpful for the mass transformation to the right sites. The combination of slow nucleation speed and slow growth speed at high temperature accomplishes well-crystallized nanocrystals with small size. Generally, the synthesis of magnetic nanoparticles requires either some surfactants in the aqueous phase, to prevent agglomeration, or the use of nonpolar organic conditions. The surfaces of these nanoparticles are always coated with an organic layer, which makes their surfaces inert and limits their applications. Our work was performed under hydrophilic polyol conditions and introduced a facile method to prepare welldispersed magnetic nanoparticles with reactive surfaces. 3.2. Egg-yolk-like Structural Magnetic Silica Beads. The novel magnetic silica bead is composed by a silica spherical core and another silica shell with magnetic nanoparticles embedded in their boundary. Figure 3 shows TEM images of (a, b) bare silica spheres and (c, d) magnetic silica beads. The bare silica spheres synthesized by the Sto¨ber method are of narrow size distribution with an average diameter of ∼180 nm and a smooth surface, as shown in Figure 3a,b. After they are coated with another silica shell, an egg-yolk-like structure appears. In the meantime, because of the offered magnetic properties to silica beads, some aggregation can be found resulting from the magnetic attraction. In Figure 3c,d, a thin silica shell can be distinguished from the core from the contrast difference in the sample prepared with the composition of 3.5 mL of Fe3O4 and 0.12 mL of TEOS. All the silica spheres were homogeneously grown with a thin silica shell of ∼25 nm average thickness. A dark rim divides the inner core and the outer shell. Further observation indicates that this dark rim is composed of magnetic nanoparticles; these assemble at the boundary between inner core and outer shell, as shown in Figure 3d. Although the high-magnification image of the magnetic
Figure 5. TEM images of the comparative experiments results: (a, b) without addition of HCl and (c, d) with addition of CH3COOH.
nanoparticles entrapped in the silica matrix is not very clear due to the shrinkage of silica sphere under the irradiation of the electron beam, the magnetic nanoparticles with a average diameter of ∼8 nm still can be distinguished (Figure 3e). After they are dipped in strong HNO3 solution for 20 min, the magnetic nanoparticles can be dissolved and removed, as shown in Figure 3f, indicating a bright ring. EDS spectrum analysis was further used to confirm the egg-yolk structure, as shown in Figure 4. Except for the Si signal from the silica matrix, clear Fe peaks can be seen in the EDS spectrum in Figure 4d. The
6714 J. Phys. Chem. C, Vol. 112, No. 17, 2008
Chen et al.
Figure 6. TEM images of the samples with different added amounts of TEOS: (a, b) 0.05 mL; (c, d) 0.1 mL; (e, f) 0.3 mL.
EDS maps in Figure 4b,c show that elemental Fe is uniformly dispersed in the silica spheres. As far as we know, this is the first report of this novel egg-yolk-like magnetic structure prepared by a one-step method. The available report on this structure is concentrated on the layer-by-layer method or other surface modification route.28-33 The present method is a facile and effective way to construct magnetic silica beads. Although the exact formation mechanism is not very clear, we still try to deduce a possible formation procedure based on the experiments and this novel structure. In this work, the stability of the nanoparicles was influenced by two facts: steric hindrance and surface charge. The adsorption and complexing of DEG on the nanoparticles can effectively separate the nanoparticles by steric hindrance. The status of surface charge is even more prominent for the stability of these oxide nanoparticles, and the same kind of charged particles can form a very stable colloid solution through electrostatic repulsion. Good stability of the nanoparticles is the principal condition to prepare this novel egg-yolk nanostructure. Because of the adsorption and complexing by DEA, the surfaces of the asprepared Fe3O4 nanoparticles are terminated with many -NH2 groups, which offer the surface of the nanoparticles with positive charges. In the early stage, magnetic nanoparticles are dispersed in weak acid solution and form a transparent colloid solution. Although this phenomenon is not very clear, this effect really enhances the surface charge density. While in this condition, the surfaces of silica spheres are negatively charged because the isoelectric point of silica sphere with pH of 1∼2.30 So, these two kinds of electrically charged particles are apt to attract each other and form a composite nanostructure. Some researchers reported that this adsorption is sensitive to pH value and desorption takes place easily after a slight change of pH condition. Claesson and Philipse31 tried to construct a magnetic silica composite through electrostatic adsorption alone, but the result was not as good as expected. We also found a similar result. However, if the hydrolysis and condensation of TEOS can take place at the same time, the weak adsorption composite structure can be maintained by coating with another protective silica shell, which is successfully realized by quick addition of ammonia solution and TEOS. Although the adsorption of Fe3O4 nanoparticles on the silica spheres can take place through two routesselectrostatic attraction and complexing by -NH2 groupss the former effect takes the vital role. Some comparative experiments were carried out to validate this hypothesis, as
Figure 7. TEM images of the samples with different added amounts of Fe3O4 nanoparticles: (a, b) 1 mL; (c, d) 5 mL.
shown in Figure 5. In Figure 5a,b, well-defined egg-yolk nanostructure cannot be formed without the addition of acid solution, even when the other conditions are maintained. Because of high affinity between Fe3O4 nanoparticles and silica, silica films are easy to coat on Fe3O4 nanoparticle surfaces through the hydrolysis and condensation of TEOS without any further surface functionality. But because of the small size of the magnetic nanoparticles and heavy agglomeration without addition of acid solution, it is difficult to obtain monodispersed core/shell nanostructure as reported in other papers.23 The silicacoated magnetic nanoparticles aggregate and cannot combine with the silica spheres to form well-defined spherical shapes. In the meantime, the diameter of the silica spheres is slightly increased due to the coarsening in the alcohol-ammonia-TEOS system. But it is not easy to distinguish the further grown silica shell from the silica core because of the same nature. This result shows obvious difference from Figure 3c,d and further confirms the embedment of magnetic nanoparticles in the outer silica shell. In the process, the pre-addition of acid solution plays a
Prospective Multifunctional Nanostructure Platform
Figure 8. TEM images of the sample prepared based on silica colloidal cores with different sizes: (a, b) 240 nm; (c, d) 110 nm.
key role in the formation of this nanostructure. We also investigated the effect of other acids on the nanostructure. Figure 5c,d shows the as-prepared sample when HCl was replaced by CH3COOH while the other conditions were kept constant, indicating that egg-yolk nanostructures were still formed. This suggests that weak acid solution is the precondition to get magnetic nanoparticles with good dispersion and for surface activation, which is very helpful for magnetic nanoparticles to attach to the silica spheres’ surfaces. For the following applications, the thickness of the silica shell should be carefully controlled, which can be effectively tuned by controlling the addition amount of TEOS. As shown in Figure
J. Phys. Chem. C, Vol. 112, No. 17, 2008 6715 6, with the increase of addition amount of TEOS, the thickness of silica shell is gradually increased. The shell thicknesses of 10, 20, and 32 nm correspond to the TEOS added amounts of 0.05, 0.1, and 0.3 mL, respectively. Additionally, the amount of TEOS is also important to fully pack the magnetic nanoparticles in the silica shells. When only 0.05 mL of TEOS was introduced into the reaction system, some nanoparticles individually coated with thin silica shells were not embedded but stuck onto the silica spheres (Figure 6a,b). Though this nanostructure still has a well-defined spherical shape, its surface is not as smooth as those samples with larger added amounts of TEOS. Thus, increasing the TEOS addition can effectively improve the surface smoothness. As shown in Figure 3c,d and Figure 6c-f, a smooth surface can be obtained when the TEOS addition is beyond 0.1 mL. Further increasing the TEOS addition increased the shell thickness, which is similar to the well-known seeded-growth Sto¨ber procedure. The ratio of magnetic nanoparticles to silica beads also has some effect on this kind of nanostructure (Figure 7). When the added amount of Fe3O4 aqueous suspension is only 1 mL, although there are many well-defined egg-yolk nanostructures that can be found, there are still many bare silica spheres in the sample (Figure 7a,b). Magnetic nanoparticles were not evenly shared with all the silica spheres and thus an appropriate ratio of magnetic nanoparticles to silica beads is necessary to construct a uniform magnetic silica bead. But if the ratio is increased more than necessary, such as 5 mL of Fe3O4 suspension, the result in Figure 7c,d is similar to that with less TEOS addition shown in Figure 6a,b. Excessive magnetic nanoparticles cannot be attracted onto silica spheres’ surfaces owing to insufficient charge density. These nanoparticles are individually coated with thin silica shells and stuck on the surfaces of egg-yolk-like silica spheres. Furthermore, because these excessive nanoparticles have consumed much TEOS, the resultant thickness of the silica shells on silica spheres is obviously reduced.
Figure 9. Magnetic properties of (a) Fe3O4 nanoparticles and (b) a typical egg-yolk-like magnetic silica sphere; optical photos of magnetic silica beads suspension (c) with and (d) without exterior magnetic field.
6716 J. Phys. Chem. C, Vol. 112, No. 17, 2008 This method also shows a strong ability to construct differentsized magnetic silica beads based on different-sized silica spherical cores, which suggests a general method to prepare magnetic silica spheres with a wide diameter range. Figure 8 shows TEM images of samples with different sizes. Silica cores with average diameters of 110 and 210 nm were uniformly coated with silica shells of average thickness 25 nm. These results indicate the method’s superiority in controlling the final product’s size in a wide range because of the ease of obtaining monodispersed silica colloidal spheres in a wide diameter range from several tens of nanometers to several micrometers. 3.3. Magnetic Properties. The magnetization curves measured at room temperature for the Fe3O4 nanoparticles and their silica composites are shown in Figure 9a,b. Both curves present very narrow hysteresis loops that are hardly distinguishable. The saturation magnetization values (Ms) of Fe3O4 nanoparticles and egg-yolk-like silica beads are 45.3 and 6.35 emu/g, respectively. The decrease of Fe3O4 nanoparticle fraction in the composite results in the decrease of Ms. According to the proportion 3.5 mL of Fe3O4, 5 mL of SiO2, and 0.12 mL of TEOS in the experimental stage, the calculated Fe3O4 fraction in the magnetic silica bead is about 30 wt %. This content and its corresponding Ms are similar to the reported results (27 wt %, Ms ) ∼6.1 emu/g).40 The optical images in Figure 9c,d show the good response of these silica beads toward exterior magnetic field. Generally, when the size of ferromagnetic nanoparticle is decreased to a critical size, these nanoparticles can exhibit superparamagnetic behavior with very low remnant magnetization (Mr) and coercivity (Hc).1 For Fe3O4, the critical size usually is around 20 nm.38 In the present study, the as-prepared Fe3O4 nanoparticles have an average diameter of ∼8 nm, and the very low Mr confirms the good superparamagnetic behavior. This signifies that the magnetic nanoparticles have essentially single domains. Based on these magnetic nanoparticles, the egg-yolklike magnetic silica beads also show good superparamagnetic behavior, which make them ideal components vehicles for biological applications. 4. Conclusions A facile solvothermal method was successfully utilized to synthesize well-crystallized Fe3O4 nanoparticles through hightemperature decomposition of intermediate complexing compound by use of DEG and DEA as solvent and complexing agent. Then a novel egg-yolk-like magnetic silica nanostructure, composed of a silica core and another silica shell with magnetic nanoparticles embedded in their boundary, was successfully prepared by a facile one-step sol-gel method. This method shows its superiority in high yield, high efficiency, and ease of controlling the final product’s size and shell thickness in a wide range. Because of the magnetic attraction between the magnetic nanoparticles in the silica matrix, some aggregations appeared as an imperfection that should be further improved. This novel magnetic silica nanostructure presents a good platform to construct multifunctional nanostructure with the help of the well-developed composite strategy based on silica chemistry. Acknowledgment. This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MOST) (2007-00543).
Chen et al. References and Notes (1) Jeong, U.; Teng, X.; Wang, Y.; Yang, H.; Xia, Y. AdV. Mater. 2007, 19, 33. (2) Safarik, I.; Safarikova, M. Biomagn. Res. Technol. 2004, 2, 7. (3) Gu, H. W.; Xu, K. M.; Xu, C. J.; Xu, B. Chem. Commun. 2006, 941. (4) Gupta, A. K.; Gupta, M. Biomaterials 2005, 26, 3995. (5) Massart, R. IEEE Trans. Magn. 1981, 17, 1247. (6) Kang, Y. S.; Risbud, S.; Rabolt, J. F.; Stroeve, P. Chem. Mater. 1996, 8, 2209. (7) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (8) Park, J.; Lee, E.; Hwang, N. M.; Kang, M.; Kim, S. C.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hyeon, T. Angew. Chem., Int. Ed. 2005, 44, 2872. (9) Sun, S.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. (10) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 12798. (11) Lopez Perez, J. A.; Lopez Quintela, M. A.; Mira, J.; Rivas, J.; Charles, S. W. J. Phys. Chem. B 1997, 101, 8045. (12) Lee, Y.; Lee, J.; Bae, C. J.; Park, J. G.; Noh, H. J.; Park, J. H.; Hyeon, T. AdV. Funct. Mater. 2005, 15, 503. (13) Suh, W. H.; Suslick, K. S. J. Am. Chem. Soc. 2005, 127, 12007. (14) Si, S.; Li, C.; Wang, X.; Yu, D.; Peng, Q.; Li, Y. Cryst. Growth Des. 2005, 5, 391. (15) Daou, T. J.; Pourroy, G.; Begin-Colin, S.; Greneche, J. M.; UlhaqBouillet, C.; Legare, P.; Bernhardt, P.; Leuvrey, C.; Rogez, G. Chem. Mater. 2006, 18, 4399. (16) Li, D.; Teoh, W. Y.; Selomulya, C.; Woodward, R. C.; Amal, R.; Rosche, B. Chem. Mater. 2006, 18, 6403. (17) Lattuada, M.; Hatton, T. A. Langmuir 2007, 23, 2158. (18) Lu, C. W.; Hung, Y.; Hsiao, J. K.; Yao, M.; Chung, T. H.; Lin, Y. S.; Wu, S. H.; Hsu, S. C.; Liu, H. M.; Mou, C. Y.; Yang, C. S.; Huang, D. M.; Chen, Y. C. Nano Lett. 2007, 7, 149. (19) Yoon, T. J.; Yu, K. N.; Kim, E.; Kim, J. S.; Kim, B. G.; Yun, S. H.; Sohn, B. H.; Cho, M. H.; Lee, J. K.; Park, S. B. Small 2006, 2, 209. (20) Lu, Y.; Yin, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 183. (21) Fang, H.; Ma, C. Y.; Wan, T. L.; Zhang, M.; Shi, W. H. J. Phys. Chem. C 2007, 111, 1065. (22) Barnakov, Y. A.; Yu, M. H.; Rosenzweig, Z. Langmuir 2005, 21, 7524. (23) Deng, Y. H.; Wang, C. C.; Hu, J. H.; Yang, W. L.; Fu, S. K. Colloids Surf., A 2005, 262, 87. (24) Santra, S.; Tapec, R.; Theodoropoulou, N.; Dobson, J.; Hebard, A.; Tan, W. Langmuir 2001, 17, 2900. (25) Xu, H.; Cui, L.; Tong, N.; Gu, H. J. Am. Chem. Soc. 2006, 128, 15582. (26) Zhao, W.; Gu, J.; Zhang, L.; Chen, H.; Shi, J. J. Am. Chem. Soc. 2005, 127, 8916. (27) Zhao, W.; Shi, J.; Chen, H.; Zhang, L. J. Mater. Res. 2006, 21, 3080. (28) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Chem. Mater. 2001, 13, 109. (29) Zhu, Y. H.; Da, D.; Yang, X. L.; Hu, Y. Colloids Surf., A 2003, 231, 123. (30) Salgueirino-Maceira, V.; Spasova, M.; Farle, M. AdV. Funct. Mater. 2005, 15, 1036. (31) Claesson, E. M.; Philipse, A. P. Langmuir 2005, 21, 9412. (32) Stoeva, S. I.; Huo, F.; Lee, J. S.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 15362. (33) Kim, J.; Park, S.; Lee, J. E.; Jin, S. M.; Lee, J. H.; Lee, I. S.; Yang, I.; Kim, J. S; Kim, S. K.; Cho, M. H.; Hyeon, T. Angew. Chem., Int. Ed. 2006, 45, 7754. (34) Graf, C.; Dembski, S.; Hofmann, A.; Ruhl, E. Langmuir 2006, 22, 5604. (35) Wang, L. Y.; Luo, J.; Fan, Q.; Suzuki, M.; Suzuki, I. S.; Engelhard, M. H.; Lin, Y.; Kim, N.; Wang, J. Q.; Zhong, C. J. J. Phys. Chem. B 2005, 109, 21593. (36) Teng, X. X.; Yang, H. J. Mater. Chem. 2004, 14, 774. (37) Deng, H.; Li, X.; Peng, Q.; Wang, X.; Chen, J.; Li, Y. Angew. Chem., Int. Ed. 2005, 44, 2782. (38) Zhu, Y. F.; Zhao, W. R.; Chen, H. R.; Shi, J. L. J. Phys. Chem. C 2007, 111, 5281. (39) Caruntu, D.; Caruntu, G.; Chen, Y.; O’Connor, C. J.; Goloverda, G.; Kolesnichenko, V. L. Chem. Mater. 2004, 16, 5527. (40) Wu, W.; Caruntu, D.; Martin, A.; Yu, M. H.; O’Connor, C. J.; Zhou, W. L.; Chen, J. F. J. Magn. Magn. Mater. 2007, 311, 578.
