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Controlled Synthesis of Magnetite-Silica Nanocomposites via a Seeded Sol-Gel Approach Dong Yang, Jianhua Hu,* and Shoukuan Fu Department of Macromolecular Science, Laboratory of Molecular Engineering of Polymers (Ministry of Education) and Laboratory of AdVanced Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: January 29, 2009; ReVised Manuscript ReceiVed: March 10, 2009

Magnetite-silica nanocomposites were controllably synthesized by a sol-gel approach, using electrostatically stabilized magnetite nanoparticles as seeds. By virtue of the complexation occurring between the iron ions on the magnetic nanoparticles and the citrate groups, highly stable magnetic fluid (MF) containing well-dispersed magnetite nanoparticles can be obtained. Controlling the surface modification degree, magnetite nanoparticles charged with different amount of citrate groups can be obtained for coating process with silica through the seeded sol-gel approach, and magnetite-silica nanocomposites with morphologies including irregular clusters, core-shell particles can be synthesized. Moreover, silica-magnetite particles with well-defined core-shell structure and controllable silica thickness can be synthesized. 1. Introduction In recent decades, magnetic nanoparticles, especially Fe3O4 and γ-Fe2O3, have attracted increasing interest because of their outstanding properties including superparamagnetism and low toxicity and, as a result, their potential applications in various fields,1 especially in biotechnology and biomedicine, such as cell sorting,1 enzymeimmobilization,2,3 biosensingandbioelectrocatalysis,4-8 separation and purification,9-16 and so on.17-23 Various approaches have been explored for synthesis of high-quality magnetic iron oxide nanoparticles.24-30 In the past few years, much work has been done to fabricate magnetic composite particles by encapsulating magnetite nanoparticles with organic polymers through monomer polymerization or by coating magnetite nanoparticles with inorganic matrixes such as silica and titanium dioxides through a sol-gel approach.31-34 Encapsulating magnetite nanoparticles with organic polymers through monomer polymerization usually results in magnetic polymeric microspheres with dirty surfaces because of the introduction of stabilizers or surfactants during polymerization. Moreover, due to the inherent immiscibility between inorganic magnetite and organic monomers, it is difficult to achieve a satisfying encapsulation, i.e., blank polymer spheres or naked magnetite nanoparticles often coexist in the product. The sol-gel approach has been frequently employed for coating magnetite nanoparticles with inorganic matrix, especially silica, and proved to be a robust way to fabricate magnetic nanocomposites, because the surface of magnetite has strong affinity toward silica. It is currently a promising approach in the development of magnetic microspheres for technological and biomedical applications, because the silica coatings on magnetite nanoparticles not only prevent magnetic nanoparticles from aggregation in wide pH ranges in aqueous solution but also provide them with a silica-like surface, which is easily further modified with various functionalities to meet various practical applications. Furthermore, the silica coatings provide magnetite nanoparticles with a chemically inert surface, which is especially important in biological applications.35,36 Much work has been done to prepare silica-coated magnetite nanoparticles * To whom correspondence should be addressed. Tel: +86-215-5665280. Fax: +86-216-5640293. E-mail: [email protected].

through a sol-gel approach. Wang et al.37 synthesized magnetite particles by a coprecipitation method and directly coated them with silica in a basic alcohol/water mixture, and large aggregates of silica-coated magnetite particles were obtained due to coagulation of magnetite nanoparticles during the coating process. Aliev et al.38 synthesized silica-coated magnetite particles through a slow sol-gel process using sodium silicate as the silica source. The above methods could lead to silicacoated magnetite particles, however, the structure and morphology of resultant composites could not be well controlled because the magnetite nanoparticles were used directly as seeds without any treatment and they can spontaneously aggregate in the liquid reaction systems. In order to synthesize well-dispersed silicacoated magnetite nanoparticles with well-defined structures, surface modification of magnetite nanoparticles is needed to increase their dispersing stability in reaction media during the silica coating process. Philips et al.39 synthesized core-shell silica-coated magnetite particles via sol-gel approach by using surfactant stabilized magnetite nanoparticles as seeds. Through an analogous approach, Lu et al.36 reported the formation of silica on the surface of magnetite nanoparticles stabilized with oleic acid. Although surfactant stabilized magnetic nanoparticles could used to prepare of silica-coated magnetic nanoparticles via sol-gel approach, the obtained composites are usually stained by surfactants and the coating process is difficult to control because surfactant molecules are easy to desorb from magnetic nanoparticles by alcohol dissolution. Therefore, it is of interest and importance to explore an optimized method suitable for reproducible synthesis of silica-coated magnetic nanoparticle in a controllable way. Herein, we report a reliable and facile seeded sol-gel approach to the controlled synthesis of magnetite-silica nanocomposites by using electrostatically stabilized magnetite nanoparticles as seeds. By virtue of the complexation occurring between the iron ions on the magnetic nanoparticles and the citrate groups, highly stable magnetic fluid (MF) containing well-dispersed magnetite nanoparticles can be obtained. Controlling the surface modification degree, magnetite nanoparticles charged with different amounts of citrate groups can be obtained for coating processes with silica through the seeded sol-gel

10.1021/jp900868d CCC: $40.75  2009 American Chemical Society Published on Web 04/14/2009

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approach, and magnetite-silica nanocomposites with morphologies including irregular clusters and core-shell particles can be synthesized. Moreover, silica-magnetite particles with welldefined core-shell structure and controllable silica thickness can be synthesized. 2. Experimental Section 2.1. Materials. Ferric chloride hexahydrate (FeCl3•6H2O) (99%) and ferrous chloride tetrahydrate (FeCl2•4H2O) (99%) were purchased from Alfa Aesar Co., and used as received. Sodium hydroxide (NaOH), trisodium citrate, tetraethyl orthosilicate (TEOS), ethanol, and ammonia aqueous (25 wt%) (Shanghai Chem. Reagent Co.) are all of analytic reagents and were used as supplied. Only distilled water was used. 2.2. Synthesis and Modification of Magnetite Nanoparticles. Magnetic nanoparticles were prepared using simple chemical coprecipitation. Typically, 0.04 mol of FeCl3•6H2O and 0.02 mol of FeCl2•4H2O were dissolved in 150 mL of distilled water in a three-necked bottom (250 mL). The obtained transparent solution was degassed with argon for 1 h. Thereafter, under rapid mechanical stirring (600 rpm), 20 mL of NaOH (10 M) was added into the solution within 30 min using a dropping funnel. After being rapidly stirred for 1.0 h, the resultant black dispersion was heated to 90 °C for 1 h. After cooling to room temperature, the obtained magnetic dispersion was subjected to magnetic separation with a magnet, and the collected magnetic mud was then redispersed in a 200-mL portion of trisodium citrate solution (0.3 M) and heated at 80 °C for 1 h. The magnetic nanoparticles were precipitated with acetone to remove the excessive citrate groups adsorbed on the nanoparticles, collected with a magnet, and vacuum-dried at 30 °C. The obtained powder was redispersed in distilled water (50 mL). Subsequently, the resultant dispersion was treated by dialysis and adjusted to 2.0 wt%. The obtained magnetite dispersion was denoted as magnetic fluid. The magnetic fluid obtained by treating the as-made magnetite with a trisodium citrate solution of 0.1, 0.3, and 0.5 M were denoted as MF-1, MF-2 and MF-3, respectively. 2.3. Coating Magnetite Nanoparticles with Silica. Following the Sto¨ber method,40 with some modifications, the coating magnetite nanoparticles with silica were carried out in a basic ethanol/water mixture at 30 °C by using magnetic fluids as seeds. Magnetic fluid (2.0 g) was first diluted with water (40 mL), ethanol (120 mL), and concentrated aqueous ammonia (3.0 mL, 28 wt%). The resultant dispersion was then homogenized by ultrasonic vibration in water bath. Finally, under continuous mechanical stirring, a certain amount of TEOS diluted in ethanol (20 mL) was dropwise added to this dispersion. After stirring for 12 h, the obtained product was collected by magnetic separation and washed with ethanol for 3×. Using the assynthesized magnetite nanoparticles and the treated nanoparticles (MF-1, MF-2, MF-3) as the seeds, respectively, four kinds of magnetite-silica composites (designated as MS-0, MS-1, MS2, and MS-3, respectively) were obtained through the sol-gel approach. 2.4. Characterization Methods. FT-IR spectra were collected on Nicolet Fourier spectrophotometer using KBr pellets (USA). Rotating Anode X-ray Diffractometer (Rigaku, Japan) using Cu KR radiation was used to study the crystallographic structure of the magnetic iron oxide nanoparticles. Transmission electron microscopy (TEM) images of iron oxide nanoparticles and silica-coated iron oxide nanoparticles were taken with a Hitachi HU-11B microscope (Japan) operated at 120 kV. Scanningelectronmicroscopy(SEM)imagesofthemagnetite-silica nanocomposites were recorded on a Philips XL30 electron

Figure 1. TEM images of magnetite nanoparticles (a) before and (b) after treated with trisodium citrate solution (0.3 M). The insets in parts a and b show the selective area electron diffraction pattern of the magnetite nanoparticles and the magnetic responsiveness of the magnetic fluid to the applied magnet (2000 Oe), respectively.

microscope (Netherlands) operating at 20 kV. A thin gold film was sprayed on the sample before characterization. The ζ potential was determined in a Malvern Instrument (Zetasizer Nano ZS, UK). 3. Results and Discussion Chemical coprecipitation is a simple but effective method for preparation of hydrophilic nanosized magnetite particles. Figure 1a shows the TEM image of the as-made magnetite nanoparticles. The mean diameter determined from the TEM image is about 15 nm, which agrees well with the value calculated from with the Debye-Scherrer formula for the strongest diffraction peak (311) in the X-ray diffraction patterns of the as-made magnetite nanoparticles (Figure S1of the Supporting Information). Selected area electron diffraction (SAED) pattern (Figure 1a, inset) displays spotty diffraction rings, which reveal a polycrystalline feature of the magnetite nanoparticles. As shown in the TEM image, despite of their hydrophilicity, the magnetite particles tend to aggregate, which is due to the ultrasmall size and the requirement to decrease their interface energy. In fact, visible sedimentation of the aqueous suspension of the as-made magnetite nanoparticles can be seen after standing for 2 h. It suggests a poor dispersion stability of the magnetite nanoparticles. Therefore, to obtain a stable dispersion of magnetite nanoparticles for the successful encapsulation with silica in a basic alcohol/water mixture, surface modification of magnetite nanoparticles is needed to avoid the coagulation and sedimentation. A previous report has indicated that the magnetite nanoparticles have an isoelectric point (IEP) of pH ≈ 7, which makes magnetite nanoparticles aggregate rapidly in their reaction media in a wide pH range.36 An effective way to promote their stability is to change the IEP of magnetite nanoparticles by surface modification. In this work, instead of using surfactants, trisodium citrate was employed to treat the magnetite nanoparticles because citric acid has three carboxyl groups and is, therefore, a very powerful ligand for stabilizing magnetite particles.41 The TEM image of magnetite nanoparticles treated by 0.3 M trisodium citrate solution shows that the treated magnetite nanoparticles are well dispersed, suggesting that the nanoparticles were efficiently modified. The obtained colloidal dispersion of magnetite nanoparticles exhibits high stability even in the magnetic field (Figure 1b, inset). The efficient surface modification of magnetite nanoparticles by trisodium citrate is mainly attributed to the complexation (or

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Figure 2. FT-IR spectra of (a) the as-synthesized magnetite nanoparticles and (b) trisodium citrate-treated magnetite nanoparticles (MF2).

Figure 4. TEM images of the magnetic silica nanocomposite particles prepared by the sol-gel process using 0.4 g of TEOS and different magnetic fluid as the seed (a) as-made magnetite without treatment, (b) MF-1 treated by 0.1 M trisodium citrate, (c) MF-2 treated by 0.3 M trisodium citrate, and (d) MF-3 treated by 0.5 M trisodium citrate. Figure 3. the plots of zeta potential of -9- the as-synthesized magnetite nanoparticles, -b- MF-1, -4- MF-2, and -1- MF-3 treated with trisodium citrate vs the pH changes of the dispersion.

chelating bidenate interaction) between iron ions (Fe3+ or Fe2+) on the magnetite surface and carboxylic groups of citrate. Figure 2 shows the FT-IR spectra of as-synthesized magnetite nanoparticles and modified magnetite nanoparticles. Compared to the untreated magnetite nanoparticles (Figure 2a), two new absorption peaks at 1619 and 1395 cm-1 appear in the FT-IR spectra of citrate-treated magnetite nanoparticles (Figure 2b), which are characteristic of the carboxylate. It verified the complexation between the citrate groups and iron ions on the magnetite nanoparticles surface. The complexation endows magnetite nanoparticles with numerous negative charges and thus greatly enhanced their dispersibility in aqueous solution. Zeta potential measurements (Figure 3) indicate that, after treatment with trisodium citrate, all of the magnetite nanoparticle samples, including MF-1, MF2, and MF-3, were highly negatively charged in the range of pH ) 5-11. Furthermore, from Figure 3, it can be seen that, as the increase of the concentration of trisodium citrate for treatment of the magnetite nanoparticles, the ζ potential of the MF-1, MF-2, MF-3 samples progressively increases from -63.3, to -70.8 and to -79.2 mV at pH 9 in the phosphate buffer solution. It suggests that citrate treatment can continuously impart magnetite nanoparticles with negative charges and thus increase their dispersibility in aqueous solution. The seeded sol-gel approach has been widely employed for synthesis of various nanocomposites. In this study, to investigate the effluence of the surface treatment of the magnetite nanoparticles on the obtained silica-magnetite nanocomposites, the as-synthesized magnetite nanoparticles and the treated nano-

particles (MF-1, MF-2, MF-3) were used as the seeds, respectively, and four kinds of magnetite-silica composites, i.e. MS0, MS-1, MS-2, and MS-3, were obtained through the sol-gel approach, respectively. The magnetic silica MS-0 based on the as-synthesized magnetite nanoparticle as the seed was found to be interconnected rod-like particles with diameter of about 70 nm, and the magnetite nanoparticles were homogeneously embedded in the silica matrix, suggesting the strong affinity of silica toward magnetite nanoparticles (Figure 4a). The formation of the large aggregates of silica-magnetite composites is due to the poor dispersibility of the magnetite nanoparticles in the reaction media. Indeed, it is found that the as-synthesized magnetite could not be homogeneously dispersed in the basic alcohol/H2O solution even under intense ultrasonication vibration, and numerous visible particles suspended in the reaction media throughout the sol-gel coating process. By contrast, by using MF-1 as the seed, well-dispersed silica-magnetite clusters with a mean size of about 150 nm were obtained (Figure 4b), and each composite particle is found to be silica-coated cluster of several magnetite nanoparticles. Despite the irregular shape, the magnetic silica particles could be stably dispersed in ethanol/ water solution, suggesting that citrate-stabilized magnetite nanoparticles can lead to silica-magnetite composite particles with better dispersibility and smaller size. By using MF-2 and MF-3 as the seeds, respectively, magnetic silica composite particles (Figure 4, parts c and d) with typical core-shell structure were obtained, particularly in the case of MS-3. On the basis of the TEM images, it can be seen that, using the higher negatively charged magnetite nanoparticles as the seeds, uniform magnetic silica microspheres consisting of individual magnetite nanoparticle as the core can be obtained. Consequently, higher negative charge helps to endow magnetite nanoparticles with better stability and dispersibility, and thus

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Figure 5. Reaction mechanisms between magnetite nanoparticles and citrate groups and the formation of magnetic silica particles with various structures.

Figure 6. TEM images of core-shell magnetic silica microspheres obtained using MF-3 as the seed and different additional amount of the TEOS (a) 0.9 g, (b) 1.8 g, and (c) 4.5 g.

resulting in silica-magnetite composites with better spherical morphology and regular core-shell structure. On the basis of results mentioned above, we conjecture that the formation of magnetic silica composite particles with various morphologies and structures should be governed by the dispersing status of the magnetic nanoparticles which is related with charge density on their surface, as schematically illustrated in Figure 5. For the case of as-synthesized magnetite and MF-1 treated by low concentration trisodium citrate, the particles are only partially negatively charged and the electrostatic repulsion among the particles is very weak. The magnetite nanoparticles are easy to aggregate to reduce surface energy in the reaction media during the sol-gel process and silica-coated magnetite particles with irregular morphologies were obtained. When the trisodium citrate of higher concentration was used, the magnetite nanoparticles can be charged with more citrate groups, leading to stronger electrostatic repulsion which prevents the nanoparticles from aggregating. As a result, colloidal magnetic silica Fe3O4@SiO2 particles with well-defined core-shell structure were obtained.

Through the seeded sol-gel approach, the thickness of silica shell of the Fe3O4@SiO2 particles can be conveniently adjusted by controlling the addition amount of silica source (TEOS). Using MF-3 as the seed, magnetic silica particles with uniform silica shell of different thicknesses were synthesized (Figure 6). As shown in the TEM images, as the TEOS amount increases from 0.9 to 1.8 and to 4.5 g, the silica thickness dramatically increase from ∼24.5 to ∼36.0 and to ∼56.0 nm, respectively. Meanwhile, upon the increase of the particle sizes, their morphologies become more regular in spherical shape due to the requirement of minimum of the surface energy, as shown in their SEM images (Figure 7, parts a, b, and c). The plot shown in Figure 7d indicates that diameter of the magnetic silica particles rapidly increases at first and the increased amplitude of diameters decrease as the additional amount of TEOS amount increases. It is due to the fact that, as the diameter increases, the equivalent amount of TEOS can produce thinner silica layers on the larger particles than the smaller ones. With the increase of the silica thickness, the aqueous dispersion of the magnetic

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Figure 7. SEM images of core-shell magnetic silica microspheres obtained using MF-3 as the seed and different addition amount of the TEOS (a) 0.9 g, (b) 1.8 g, (c) 4.5 g, and (d) plot of the diameter of magnetic silica microspheres using MF-3 as the seed vs the addition amount of TEOS. The inset in part d shows photographic images of the aqueous dispersion of the core-shell magnetic silica composite particles obtained using MF-3 as the seeds and different additional amounts of TEOS. For comparison, MF-3 aqueous dispersion and colloidal silica dispersion are also displayed.

silica particles become pale due to the increase in the light scattering of the colloidal dispersion (Figure 7d, inset). The magnetic silica particles obtained using MF-3 as the seeds and 1.8 g of TEOS exhibit similar X-ray diffraction patterns with that of the as-made magnetite nanoparticles (see Supporting Information, Figure S2), indicating that the magnetite in the magnetic silica particles was well retained. Compared to the as-made magnetite nanoparticles, the slight broadening in the diffraction peaks and the minor decrease in the diffraction intensity are probably caused by the existence of the amorphous silica. The magnetic characterization results (see Supporting Information, Figure S3) indicate that both the as-made magnetite nanoparticles and the magnetic silica composite particles have superparamagnetic properties at 300 K, i.e., the sample shows no remanence when the applied magnetic field is removed. It can be calculated that the as-made magnetite nanoparticles have magnetization saturation (Ms) of about 61 emu/g, and when the TEOS amount increases, the Ms values of the resultant magnetic silica particles rapidly decrease from 15 to 3.0 emu/ g, due to the introduction of nonmagnetic silica coating on the magnetite nanoparticles. 4. Conclusions Wehavedemonstratedthecontrolledsynthesisofmagnetite-silica nanocomposites via a seeded sol-gel approach using electrostatically stabilized magnetite nanoparticles as seeds. By virtue of the complexation occurring between the iron ions on the magnetic nanoparticles and the citrate groups, highly stable magnetic fluid (MF) containing well-dispersed magnetite nanoparticles can be obtained. Controlling the surface modification degree, magnetite nanoparticles charged with different amounts

of citrate groups can be obtained for coating processes with silica through the seeded sol-gel approach, and magnetite-silica nanocomposites with morphologies including irregular clusters, core-shellparticlescanbesynthesized.Moreover,silica-magnetite particles with well-defined core-shell structures and controllable silica thicknesses can be synthesized. The method can be extended to synthesize other functional nanocomposites, such as those with cores of cobalt oxides, nickel oxides, gold and silver nanoparticles, and so forth. Acknowledgment. This work was supported by the NSF of China (50873029) and Shanghai Science and Technology Community (08431902300). D.Y. thanks the China Postdoctoral Science Foundation for financial support (20080440569). Supporting Information Available: XRD patterns and hysteresis loops. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ha¨feli, U.; Schu¨tt, W.; Teller, J.; Zborowski, M. Scientific and Clinical Applications of Magnetic Carriers; Plenum Press: New York, 1997. (2) McCloskey, K. E.; Chalmers, J. J.; Zborowski, M. Anal. Chem. 2003, 75, 6868. (3) Haik, Y.; Pai, V.; Chen, C. J. J. Magn. Magn. Mater. 1999, 194, 254. (4) Weizmann, Y.; Patolsky, F.; Katz, E.; Willner, I. J. Am. Chem. Soc. 2003, 125, 3452. (5) Hirsch, R.; Katz, E.; Willner, I. J. Am. Chem. Soc. 2000, 122, 12053. (6) De Palma, R.; Liu, C. X.; Barbagini, F.; Reekmans, G.; Bonroy, K.; Laureyn, W.; Borghs, G.; Maes, G. J. Phys. Chem. C 2007, 111, 12227. (7) Katz, E.; Willner, I. J. Am. Chem. Soc. 2002, 124, 10290. (8) Wang, J.; Kawde, A. N. Electrochem. Commun. 2002, 4, 349.

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