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A facile preparation method for single-hole hollow Fe3O4@SiO2 microspheres
Colloids and Surfaces A: Physicochem. Eng. Aspects 453 (2014) 101–108
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A facile preparation method for single-hole hollow Fe3 O4 @SiO2 microspheres Xiaoyi Fu ∗ , Jingjing Liu, Xinhua He School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China
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• Fe3 O4 @SiO2
Schematic representation of single pores hollow Fe3 O4 @SiO2 microspheres with structure prepared by precipitation-phase separation method and the corresponding SEM and TEM images.
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hollow microspheres with single holes in their shells were obtained. A novel preparation method of single hole hollow Fe3 O4 @SiO2 microspheres was reported. Modifying Fe3 O4 NPs with DTMS is essential for encapsulating the Fe3 O4 NPs in the silica matrix. The hole size of the microspheres was depended on PMMA to PS mass ratio. The Ms values of Fe3 O4 @SiO2 microspheres were easy to control.
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Article history: Received 18 November 2013 Received in revised form 19 March 2014 Accepted 22 March 2014 Available online 5 April 2014 Keywords: Single-hole Hollow microspheres Silica Fe3 O4 nanoparticles
a b s t r a c t Hollow Fe3 O4 @SiO2 microspheres with single holes in their shells and Fe3 O4 nanoparticles encapsulated in silica shells were prepared using a precipitation-phase separation method. The synthesis was performed by mixing a tetrahydrofuran (THF)–acetonitrile (MeCN) solution containing polymer, tetraethyl orthosilicate (TEOS) and n-dodecyltrimethoxysilane (DTMS)-modified Fe3 O4 nanoparticles with an aqueous solution containing cetyltrimethyl ammonium bromide (CTAB) before adding ammonia to catalyze the hydrolysis of TEOS and finally removing the polymer by dissolving it in THF. The polymer used was poly(styrene-co-methyl methacrylate) (P(St-co-MMA)) or a mixture of polystyrene (PS) and polymethyl methacrylate (PMMA). The effects of modifying the Fe3 O4 NPs, tailoring the particle size and the hole size and controlling the Fe3 O4 content in the Fe3 O4 @SiO2 hollow microspheres were investigated. The prepared Fe3 O4 @SiO2 single-hole hollow microspheres were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM). We observed that modifying Fe3 O4 NPs with DTMS is essential for encapsulating the Fe3 O4 NPs in a silica matrix. The hole size of the microspheres increased with the PMMA-to-PS mass ratio, and the hole size could be adjusted from 16 nm to 135 nm. The size of the microspheres could be tailored from 240 to 1270 nm by modulating the process parameters. The Fe3 O4 @SiO2 microspheres exhibited superparamagnetic properties, and the corresponding saturation magnetization (Ms) values varied from 10 to 46 emu/g when controlling the modified Fe3 O4 NP content in the microspheres. Based on our results, we propose a possible formation mechanism for the Fe3 O4 @SiO2 hollow microspheres. © 2014 Elsevier B.V. All rights reserved.
∗ Corresponding author. Tel.: +86 20 87111224; fax: +86 20 87111224. E-mail address: [email protected] (X. Fu). http://dx.doi.org/10.1016/j.colsurfa.2014.03.108 0927-7757/© 2014 Elsevier B.V. All rights reserved.
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1. Introduction Studies of hollow microspheres with well-defined shapes and sizes have attracted significant attention because these materials have numerous potential applications, such as controlled drug delivery, the protection of biologically active agents, the removal of pollutants and catalysis [1–3]. In particular, microspheres with a single hole in their shell (single-hole hollow microspheres) are particularly interesting due to their high effective diffusivity for large molecules and nanoparticles, large available surface area and high uptake capacity for drugs and pollutants [4,5]. Fe3 O4 nanoparticles (Fe3 O4 NPs) are among the most important magnetite materials and are an ideal material for biological magnetic applications because they are hydrophilic, biocompatible, nontoxic and chemically stable [6]. Composite Fe3 O4 NPs with a hollow, single-hole structure would offer opportunities for exploring new material properties and broadening their potential applications due to their bifunctionality. Many reports detail the preparation, properties and applications of magnetic hollow microspheres [7–16]; however, the studies of magnetic hollow microspheres with single holes on their surfaces are limited. Indeed, designing and preparing single-hole hollow magnetic microspheres with controllable structures is challenging. Considerable effort has been devoted to synthesizing hollow microspheres with single holes using various chemical and physicochemical methods [4,5,17–29]. These approaches are broadly divided into three categories: (1) the single hole forms after the release of matter from the microsphere interior. If the shell is non-uniform in thickness, the release and diffusion of matter from the interior of the microspheres tends to occur where there is the least resistance, causing the shell to break where it is the thinnest and creating an opening. Many single-hole hollow microspheres are prepared based on this strategy [17–22]. For example, our group has successfully synthesized single-hole hollow silica spheres with controllable particle size using a preparation-phase separation method [17]. Ariga et al. [18] prepared open-mouthed platinum microcapsules with a templated synthesis procedure using polystyrene spheres; the surface-fused crystalline nanoparticles formed a capsule shell, and the subsequent removal of the polystyrene spheres formed the mouth-like openings. Xue et al. [19] prepared polyurethane (PU) hollow microspheres with sizetunable single holes using a facile self-assembly diffusion process. The single holes in the shells of the PU hollow spheres were formed by the outward diffusion of the encapsulated chloroform in methanol, and the hole size could be tuned by adjusting the diffusion time. (2) The formation of single-hole hollow microspheres is based on Ostwald ripening [23–26]. In cases in which this process occurs, microspheres are generally formed via the assembly of small particles. Smaller, less crystalline or dense particles in a colloidal aggregate will be dissolved gradually, while larger, more crystalline or denser particles in the same aggregate grow. As the mass is transported, the void space in the microspheres and the single hole on the surface are generated mainly through Ostwald ripening. Based on this strategy, CaWO4 [23], ZnO [24] and Fe3 O4 [25] hollow microspheres with a single hole on their surface have been prepared. (3) The formation of the hole is attributed to the incomplete shell of the core–shell structure microspheres [2a,7]. For example Guan et al. [4] fabricated shell-crosslinked single-hole hollow polymer nanospheres by precipitation polymerizing acrylamide and ethylene glycol dimethacrylate that were adsorbed onto the surface of carboxyl-capped polystyrene beads; the polystyrene cores were subsequently dissolved. The formation of the hole was attributed to the shrinkage of the polymer shell during polymerization. Yan et al. [27] prepared single-hole hollow PEGDMA nanospheres using a raspberry-like template. In this method, nanometer-scale silica cores were attached to the
surfaces of micrometer-sized silica spheres, and PEGDMA shells were produced on the nano-sized spheres outside the attached area, forming core–shell microspheres; subsequently, the core microspheres were removed, and the holes were attributed to incomplete shell formation. There are only a small number of reported preparations of magnetic hollow microspheres with single holes on their surfaces. Zhou et al. [29] prepared open, hollow Fe3 O4 spheres assembled from nanocrystals generated using a solvothermal method. In this case, the formation yield of the single-hole structure in the Fe3 O4 microspheres reached 80%. Chuang et al. [21] prepared single-hole hollow silica nanocapsules containing magnetic nanoparticles and fluorescent quantum dots by evaporating emulsion droplets via a single-step emulsion-mediated method. Moreover, a single-hole structure is occasionally observed in microspheres prepared using template methods or when assembling small Fe3 O4 NPs [25,30–32]; however, the proportion of single-hole microspheres that is yielded by this method is quite low, and the single-hole structure is not controllable because the formation of the single hole is due to the hollow microspheres breaking during template removal. We report a novel method for synthesizing single-hole hollow Fe3 O4 @SiO2 microspheres using a controllable precipitation-phase separation process. DTMS-modified Fe3 O4 NPs were uniformly encapsulated by a silica shell matrix to afford single-hole Fe3 O4 @SiO2 hollow microspheres with a mean size ranging from 240 to 1270 nm and a mean hole size of 16 to 135 nm, which exhibit excellent superparamagnetic properties. The method is based on the rapid precipitation of DTMS-modified Fe3 O4 NPs, a polymer and TEOS from an aqueous solution. Afterward, the TEOS is hydrolyzed, producing the single-hole structures. Our method is notably simple and convenient and can control the size and structure of the hollow microspheres. 2. Experimental 2.1. Chemical and materials P(St-co-MMA) (Mw = 100,000–150,000) was purchased from Sigma–Aldrich. PS (Mw = 100,000) and PMMA (Mw = 97,000) were purchased from Acros. TEOS was purchased from Wulian Chemical Reagents Factory. DTMS was purchased from Alfa Aesar. CTAB was obtained from Sanland-chem. Iron(II) sulfate heptahydrate (FeSO4 ·7H2 O) and iron(III) chloride hexahydrate (FeCl3 ·6H2 O) were of analytical grade and were purchased from Damao and Fuchen Chemical Reagents Co. Tianjin. THF was purchased from Jiangsu Tsiangsheng Chemical Reagents Co. MeCN was obtained from Guangdong Chemical Reagent Engineering-technological Research and Developing Center. Ethanol (EtOH) and aqueous ammonia (28 wt%) were acquired from the Guangdong Donghong Chemical Factory. 2.2. Preparation of Fe3 O4 NPs and their modification Fe3 O4 NPs were prepared according to the procedure described by Q. Zhang [33], omitting the aqueous solution of sodium dodecylsulfanate. The surfaces of the Fe3 O4 NPs were modified as follows: 1.0 g of prepared Fe3 O4 NPs was washed with a mixture of ethanol and water (4:1 in volume ratio) twice and suspended in 25 mL of a mixture of EtOH–H2 O before 1.0 mL of DTMS was added. The mixture was refluxed under N2 for 4 h and subsequently cooled to RT. Afterward, the DTMS-modified Fe3 O4 NPs (denoted as m-Fe3 O4 NPs) were washed with ethanol and dried. 2.3. Preparation of single-hole Fe3 O4 @SiO2 hollow microspheres Single-hole silica microspheres were synthesized by precipitating polymer–TEOS–m-Fe3 O4 NPs, hydrolyzing TEOS using catalytic
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ammonia and removing the polymer. In a typical experiment, the m-Fe3 O4 NPs were suspended in THF using ultrasonication for 15 min, followed by centrifugation at 1000 rpm for 5 min to remove any large aggregates. Subsequently, 0.5 mL of the m-Fe3 O4 NPs/THF (1.0 g/L) dispersion and 0.1 mL of TEOS were mixed with P(St-coMMA) in THF (4 mL, 1.25 g/L) before 4 mL of MeCN was added quickly. The organic mixture was stirred for several seconds and subsequently quickly added to 30 mL of ultrapure water at room temperature. After shaking for seconds, an aqueous CTAB solution (0.2 mL, 0.01 mol/L) was added, followed by the addition of 0.5 mL of ammonium hydroxide. After several seconds of stirring, the mixture was allowed to stand under static conditions at room temperature for one day, generating a colloidal solution of m-Fe3 O4 @ silica–P(St-co-MMA) composite microspheres. The composite microspheres were collected by centrifugation and sequentially washed with ultrapure water and EtOH. To produce the single-hole Fe3 O4 @SiO2 hollow microspheres, the m-Fe3 O4 @ silica–P(St-coMMA) composite microspheres were suspended in THF to remove the P(St-co-MMA). During the experiments, the polymer used was P(St-co-MMA) or mixtures of PS and PMMA in various ratios. The synthesis parameters, including the organic solvent type, the volumetric ratio of the two organic solvents, the volumetric ratio of the organic solvent and water, the mass ratio of the polymer with TEOS and the amount of ammonia, were varied to adjust the morphology and size of the obtained silica microspheres systematically. 2.4. Characterization The size and morphology of the prepared microspheres were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using a Nova NanoSEM 430 scanning electron microscope and a Philips Tecnai 10 transmission electron microscope, respectively. EDX measurements were collected on an energy dispersive X-ray spectroscope (EDX, EPMA1600/EDAX). The samples used for the SEM studies were prepared by drying droplets of the microsphere suspensions on copper foil and coating them with gold. The samples for the TEM studies were prepared by drying droplets of the microsphere suspensions on copper grids coated with Formvar film. Magnetic hysteresis curves were measured with a Model 4HF PPMS-9 vibrating sample magnetometer. X-ray diffraction (XRD) patterns were collected with an X’Pert Pro X-ray diffractometer. Infrared measurements
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were recorded with a vector 33 IR spectrometer. Thermogravimetric analysis (TGA) measurements were conducted on a Netsch STA449C analyzer at 10 ◦ C/min under flowing air. 3. Results and discussion 3.1. The preparation of magnetic hollow microspheres Modifying the Fe3 O4 NPs was critical for the formation of hollow single-hole Fe3 O4 @SiO2 microspheres. When the unmodified Fe3 O4 NPs were used to prepare the microspheres, the SEM images revealed that there were numerous Fe3 O4 NPs aggregated outside of the microspheres (Fig. 1). The elemental composition of the microspheres was analyzed using EDX, and the absence of Fe indicated that the Fe3 O4 NPs could not be encapsulated by the microspheres (Fig. 1). This phenomenon may occur for two reasons: the Fe3 O4 NPs may aggregate in the THF–MeCN mixture, and the unmodified Fe3 O4 NPs are hydrophilic. The hydrophilic Fe3 O4 NPs aggregates cannot be encapsulated by the hydrophobic TEOS–polymer composite microspheres. Therefore, stable dispersions of the Fe3 O4 NPs in the organic solvent and a moderate hydrophobicity in the Fe3 O4 NPs are essential for forming the Fe3 O4 @SiO2 microspheres. Consequently, we chose two types of additives for modifying the Fe3 O4 NPs: organic acids (such as oleic acid) and silane coupling agents (DTMS). The precipitation experiments indicated that DTMS-modified Fe3 O4 NPs can be stably dispersed in THF–MeCN (shown in Fig. S1). FTIR spectra of the Fe3 O4 NPs before and after DTMS modification are shown in Fig. 2. The characteristic bands at 2927 cm−1 , 2857 cm−1 (C H stretching of methyl and methylene), 988 cm−1 (Si OH bending) and 1178 cm−1 (Si O stretching) [34] appeared after the modification, confirming that DTMS was successfully grafted to the Fe3 O4 NPs. Afterward, the DTMS-modified Fe3 O4 NPs were used to prepare the hollow magnetic microspheres. 3.2. The structure of Fe3 O4 @SiO2 microspheres SEM and TEM images of the obtained microspheres before and after removing the polymer are shown in Fig. 3. The microspheres exhibited either an opening or a single hole on the surface of the microspheres before polymer removal, and a smaller microsphere was formed in the hole (marked with an arrow in Fig. 3a). After removing the polymer, these smaller microspheres disappeared,
Fig. 1. SEM image of the microspheres prepared using unmodified Fe3 O4 NPs and the EDX spectrum of microspheres (1) and aggregated Fe3 O4 NPs (2).
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Fig. 2. FTIR spectra of the Fe3 O4 NPs before and after DTMS modification.
suggesting that they were composed of P(St-co-MMA). By comparing the TEM images of the microspheres before and after polymer removal, it was observed that the microspheres before polymer removal tended to aggregate into groups of two or three with their
holes pointed in opposite directions, forming clover-like shapes (marked with a red circle in Fig. 3c), whereas without the polymer, the microspheres did not exhibit that behavior. This interesting microsphere self-assembly may be caused by hydrophobic interactions induced by the polymer. Most of the microspheres exhibited a single hole on each surface after THF treatment (shown in Fig. 3b). The outside surface of the microspheres was smooth, whereas the internal surface was quite rough, as shown in the SEM image of a microsphere in Fig. 3e. In the TEM images, the microspheres exhibit eccentric hollow structures with a non-uniform shell thickness. Because the electronic contrast of Fe3 O4 NPs is higher than that of silica, the electronic contrast difference clearly indicates that the m-Fe3 O4 NPs were randomly entrapped in the silica matrix (Fig. 3f). After the THF treatment, the microspheres exhibited diameters ranging from 263 to 605 nm with a mean diameter of 404 ± 65 nm; the diameters of the holes ranged from 119 to 517 nm with a mean diameter of 225 ± 63 nm. The elemental composition of the microspheres was determined using EDXA; the presence of Si, Fe and O indicates that Fe3 O4 @SiO2 microspheres were formed (Fig. 3g, Cu was introduced by the sample holder, whereas Au was introduced by SEM sample preparation). The Fe content of the Fe3 O4 @SiO2 spheres was ∼27.3%, as estimated by EDXA, exceeding the amount of Fe3 O4 (16.9%) used
Fig. 3. SEM (a, b, e) and TEM (c, d, f) images of the microspheres before (a, c) and after (b, d, e, f) P(St-co-MMA) removal. Panels e and f show enlarged images of a single microsphere. Panel g presents the EDX spectrum of the microspheres after P(St-co-MMA) removal.
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during the preparation. For the silica hollow microspheres with a single hole on the surface, we observed that only some of the TEOS participated in shell formation [17], whereas a large proportion of the TEOS was converted to silica nanoparticles measuring 10–50 nm in diameter. These particles were discarded during the centrifugation and washing steps. Therefore, it is reasonable that the Fe content of the microspheres estimated during the EDX analysis exceeds the theoretical values. To estimate the polymer content of the microspheres before the THF treatment, TGA measurements were collected; the TGA curves of the microspheres are shown in figure S2. The weight loss below 200 ◦ C can be ascribed to the desorption of physically bound organic solvent and water molecules [35], and the weight loss between 200 and 280 ◦ C can be attributed to dehydroxylation and the removal of low-molecular-weight organic material [36]. The sharp decrease in weight from 280 to 450 ◦ C may have been caused by P(St-co-MMA) decomposition [35], whereas the residual weight can be ascribed to the silica and Fe3 O4 NPs. The weight loss from the polymer and the residual combined weight of the silica and Fe3 O4 NPs were 18.1% and 78%, respectively, generating a 0.23 weight ratio of P(Stco-MMA)/SiO2 + Fe3 O4 for the microspheres prepared using a 0.05 mass ratio of polymer to TEOS. The XRD spectrum of the microspheres is shown in Fig. S3. The broad diffraction peak at 2Â = 20◦ to 30◦ is attributed to amorphous silica. The sharp, major diffraction peaks at 2Â = 30.07, 35.48, 43.04, 56.98 and 62.48 can be assigned to the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) reflections, respectively; these reflections can be
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indexed to the spinel structure of pure magnetite (Fe3 O4 ) (JCPDS file 19-0629). 3.3. The tailoring of microsphere and opening size The size of the opening in the microspheres could be controlled using different mixtures of PS and PMMA, as shown in Fig. 4. The microspheres prepared using PS only exhibited very small openings (16 ± 2 nm), whereas the microspheres prepared using PMMA only showed large openings (135 ± 36 nm). The opening size of the microspheres increased with the PMMA content in the PSt-PMMA mixture; the mean opening diameters of the microspheres prepared using PMMA–PS mixtures with mass ratios of 0, 0.25, 1 and 4 were 16 ± 2, 59 ± 14, 84 ± 14 and 135 ± 36 nm, respectively. The ratio of PS and PMMA also affected the size of the microspheres; however, this effect was irregular and was observed over a small range of sizes. A similar phenomenon could be observed when mixtures of P(St-co-MMA)-PMMA and P(St-co-MMA)-PS were used (Fig. S4). The opening size in the microspheres decreased as the PS content in the PSt-P(St-co-MMA) mixture increased. When the amount of PS was large enough (PS/P(St-co-MMA) = 4), the microspheres did not contain openings on their surfaces (Fig. S4a). When PMMA was mixed with P(St-co-MMA), the opening size of the microspheres increased dramatically. However, the dependence of the opening size on the St/MMA ratio was irregular (Table S1). Therefore, the opening size of the microspheres depended either on the ratio of the two polymers or on the types of polymer used.
Fig. 4. SEM images of the microspheres prepared with various PMMA-to-PS mass ratios (a: pure PS; b: 0.25, c: 1, d: 4 and e: pure PMMA) and the dependence of the size and opening size on the PMMA-to-PS mass ratio.
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Fig. 5. SEM images of microspheres of various sizes (preparation: a: mixture polymer of PS (0.62 g/L) and PMMA (1.25 g/L), polymer/TEOS = 0.054 wt/wt, Fe3 O4 NPs (1 g/L); b: P(St-co-MMA) (3 g/L), polymer/TEOS =0.12 wt/wt, Fe3 O4 NPs (1 g/L); c: P(St-co-MMA), (2 g/L), polymer/TEOS = 0.086 wt/wt, Fe3 O4 NPs (1 g/L); d: P(St-co-MMA) (3 g/L), polymer/TEOS = 0.11 wt/wt, Fe3 O4 NPs (0.2 g/L)).
Microspheres of various sizes could be obtained by adjusting the process parameters employed, such as the type and amount of organic solvent and the ratio of organic and aqueous solutions. By modulating these parameters, we could obtain microspheres ranging in size from 200 nm to 1.5 m. Fig. 5 shows SEM images of the microspheres produced: (a) 232 ± 44 nm; (b) 517 ± 138 nm; (c) 1287 ± 1379 nm; (d) 1796 ± 512 nm.
3.4. The effect of the DTS-Fe3 O4 content The Fe3 O4 content in the microspheres could be modulated by changing the amount of m-Fe3 O4 NPs initially added. TEM images of the microspheres prepared using different amounts of m-Fe3 O4 NPs are shown in Fig. 6. The high electronic contrast indicates that the amount of m-Fe3 O4 NPs produced increased as the amount of
Fig. 6. TEM images of the microspheres with various Fe3 O4 NPs contents (Fe/Si ratio of 0.23 (a), 0.65 (b), 1.10 (c) and 1.61 (d)).
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Fig. 7. The magnetic hysteresis curves for the Fe3 O4 @SiO2 spheres with varied Fe3 O4 contents and bare Fe3 O4 NPs. The Fe/Si ratios of 0.23 (1#), 0.65 (2#), 1.10 (3#) and 1.61 (4#) were estimated using EDX analysis. The inset photographs depict the Fe3 O4 @SiO2 spheres separated from the solution using an external magnetic field.
nanoparticles initially added increased. For the microspheres prepared using m-Fe3 O4 NPs with Fe3 O4 /TEOS molar ratios of 0.0087, 0.044, 0.087 and 0.13 molar ratios; the corresponding Fe/Si ratio values estimated via EDX analysis were 0.231, 0.635, 1.10 and 1.61, respectively. The morphology and size of the microspheres were also affected by the amount of m-Fe3 O4 NPs initially added. Microspheres with smooth surfaces and regular shapes were produced when a low amount of m-Fe3 O4 NPs (mol ratio of Fe3 O4 /TEOS of 0.0087) was used. When a high amount of m-Fe3 O4 NPs (mol ratio of Fe3 O4 /TEOS of 0.087) was added, the shape of the microspheres became irregular, and the size of the cavity decreased. However, when excess m-Fe3 O4 NPs were used (mol ratio of Fe3 O4 /TEOS of 0.13), the obtained microspheres exhibited many pores on their surfaces. Therefore, to obtain hollow microspheres with a single hole, the molar ratio of Fe3 O4 /TEOS should remain below 0.087. The room-temperature magnetization hysteresis curve of the Fe3 O4 @SiO2 microspheres (Fig. 7) exhibits negligible hysteresis, indicating that the particles are superparamagnetic. The magnetization curve of the Fe3 O4 @SiO2 microspheres is similar to that of the bare Fe3 O4 NPs, indicating that the encapsulated m-Fe3 O4 NPS can preserve their superparamagnetic properties. The saturation magnetization (Ms) values of the bare Fe3 O4 and the Fe3 O4 @SiO2 microspheres prepared using m-Fe3 O4 NPs generated with Fe3 O4 -to-TEOS molar ratios of 0.0087, 0.0044, 0.087 and 0.13 were 52.2 and 9.5, 25.0, 41.8 and 44.6 emu/g, respectively. The prepared Fe3 O4 @SiO2 microspheres dispersed in an aqueous solution could be separated easily using a magnet (inset photographs), demonstrating their strong magnetic response. 3.5. The microsphere formation mechanism Our group prepared hollow silica microspheres with a single opening using a precipitation-phase separation method [5a], and we observed that the TEOS and P(St-co-MMA) microphases separated from each other, forming a hemispherical structure before ammonia was added. After adding the ammonia to catalyze the hydrolysis and condensation of the TEOS, the composite microspheres transformed into silica–P(St-co-MMA) composite microspheres with a single hole in their shells and P(St-co-MMA) located randomly in their cavities. The opening formed through the diffusion of ammonia, solvent and resultant molecules at the point of least resistance, breaking the silica shell where it was the
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thinnest. The hemispherical shape of the TEOS and P(ST-co-MMA) is essential for the formation of hollow microspheres with a single hole. In this work, the difference between the preparations generating Fe3 O4 @SiO2 microspheres and those producing silica hollow microspheres with a hole was the addition of the m-Fe3 O4 NPs; the resulting microspheres also exhibited a hollow structure with a hole when the added Fe3 O4 NPs were encapsulated in the silica matrix. The mechanism of Fe3 O4 @SiO2 microsphere formation is similar to that of hollow silica microspheres. Introducing m-Fe3 O4 NPs does not alter the microphase separation of TEOS and the polymer, and the m-Fe3 O4 NPs tend to disperse into the TEOS microphase. After the hydrolysis and condensation of the TEOS, hemispherical TEOS–m-Fe3 O4 NPs-polymer transformed into silica–m-Fe3 O4 NPs-polymer composite microspheres with a hole in the shell and m-Fe3 O4 NPs encapsulated into the silica matrix. The hydrophilic Fe OH on the surface of the m-Fe3 O4 NPs might explain why hydrophobic m-Fe3 O4 NPs could be encapsulated into the hydrophilic silica matrix? Grafting the DTMS to the surface of the Fe3 O4 NPs occurs through reactions between the Si OMe groups and the Fe OH groups [37]. The grafting efficiency is quite low [38,39]; therefore, few Fe OH groups participated in the grafting reaction. The residue Fe OH on the surface of m-Fe3 O4 NPs could react with the TEOS or the Si OH produced during the hydrolysis of TEOS, fixing the nanoparticles to the silica matrix. The m-Fe3 O4 NPs tended to disperse into the TEOS microphase of the hemispherical structure of the TEOS polymer because the m-Fe3 O4 NPs embedded in the silica matrix and the Fe3 O4 content increased with increased amounts of m-Fe3 O4 NPs added. However, this explanation does not indicate that the m-Fe3 O4 NPs could not completely disperse in the polymer microphase. The m-Fe3 O4 NPs might disperse in the polymer microphase. When the silica/mFe3 O4 NPs-polymer composite microspheres that were prepared using a high m-Fe3 O4 NPs content were treated with THF to remove the polymer, the polymer/THF solution turned yellow, indicating that m-Fe3 O4 NPs were released into solution and proving that some m-Fe3 O4 NPs could also disperse in the polymer microphase. We observed that opening size in the microspheres increased with the MMA/St ratio. This phenomenon is related to the microphase separation of the TEOS and the polymer. When an aqueous CTAB solution was added to the mixed organic solution containing the polymer, TEOS and m-Fe3 O4 NPs, the hydrophobic TEOS molecules and the m-Fe3 O4 NPs swelled inside the polymer matrix, producing the polymer–TEOS–m-Fe3 O4 NPs composite microspheres. Subsequently, the microspheres underwent structural adjustments to become stable. Based on the classical two-component polymer model [40,41], the final morphology of the composite microspheres, which are thermodynamically stable and in a completely phase-separated state, are broken down into three types: the core–shell, the hemispherical structure and the individual particles. PS and TEOS tend to form microspheres with an asymmetrical PS core/TEOS shell structure that transform into SiO2 shell/PS core structures with non-uniform shell thicknesses [42]. When the PS was removed from the microspheres, the spheres broke at the thinnest point, forming an opening. PS is a highly hydrophobic polymer; however, PMMA is more hydrophilic. When P(St-co-MMA) was used instead of PS or PMMA was mixed with PS, the core–shell structure became a hemispherical structure with TEOS- and polymer-rich regions. Incorporating PMMA gave rise to greater deviations in the polymer core from the center position. It was clearly observed that the hemispherical structure of the TEOS–polymer became a SiO2 –polymer structure with a larger opening than that of the core–shell structure after catalytic ammonia treatment. Therefore, the size of the opening increased when more MMA was added to the mixture.
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4. Conclusions Fe3 O4 @SiO2 hollow microspheres with a single hole in their shells were prepared using a precipitation-phase separation method. Modifying the Fe3 O4 NPs with DTMS was essential for encapsulating the Fe3 O4 NPs that were dispersed randomly into the silica shell matrix. The Fe3 O4 nanoparticle content of the microspheres was controlled by adjusting the ratio of Fe3 O4 nanoparticles to TEOS. The hole sizes in the microspheres were controlled by adjusting the polymer content, specifically increasing the PMMA-to-PS mass ratio. The Fe3 O4 @SiO2 microspheres exhibited superparamagnetic properties, and the saturation magnetization (Ms) value of the microspheres varied from 10 to 45 emu/g. Thus, we propose a possible formation mechanism for the Fe3 O4 @SiO2 hollow microspheres. Supporting information available Photographs of pure Fe3 O4 NPs and m-Fe3 O4 NPs stably dispersed in a mixture of THF–MeCN, TGA curves of P(Stco-MMA)–SiO2 –m-Fe3 O4 NPs composite microspheres, an XRD pattern for the Fe3 O4 @SiO2 microspheres and SEM images of microspheres prepared using various P(St-co-MMA)-to-PMMA (PS) ratios are available. Acknowledgments This work was supported by grants from NSFC (project no. 21103052), the China Postdoctoral Science Foundation (200801004), the Specialized Research Fund for the Doctoral Program of Higher Education in China (20114433110001) and the Scientific and Technological Planning Project of Guangdong Province (No. 2007A010500012). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.colsurfa.2014.03.108. References [1] X. Lou, L. Archer, Z. Yang, Hollow micro-/nanostructures: synthesis and applications, Adv. Mater. 20 (2008) 3987–4019. [2] Y. Zhao, L. Jiang, Hollow micro/nanomaterials with multilevel interior structures, Adv. Mater. 21 (2009) 3621–3638. [3] K. Cheng, S. Sun, Recent advances in syntheses and therapeutic applications of multifunctional porous hollow nanoparticles, Nano Today 5 (2010) 183–196. [4] G. Guan, Z. Zhang, Z. Wang, B. Liu, D. Gao, C. Xie, Single-hole hollow polymer microspheres toward specific high-capacity uptake of target species, Adv. Mater. 19 (2007) 2370–2374. [5] S. Luo, J. Jiang, S. Liour, S. Gao, J. Ying, H. Yu, Magnetic PEDOT hollow capsules with single holes, Chem. Commun. (2009) 2664–2666. [6] R. Hao, R. Xing, Z. Xu, Y. Hou, S. Gao, S. Sun, Functionalization, and biomedical applications of multifunctional magnetic nanoparticles, Adv. Mater. 22 (2010) 2729–2742. [7] W. Zhao, H. Chen, Y. Li, L. Li, M. Lang, J. Shi, Uniform rattle-type hollow magnetic mesoporous spheres as drug delivery carriers and their sustained-release property, Adv. Funct. Mater. 18 (2008) 2780–2788. [8] S. Peng, S. Sun, Synthesis and characterization of monodisperse hollow Fe3 O4 nanoparticles, Angew. Chem. Int. Ed. 46 (2007) 4155–4158. [9] Y. Ding, Y. Hu, X. Jiang, L. Zhang, C. Yang, Polymer–monomer pairs as a reaction system for the synthesis of magnetic Fe3 O4 –polymer hybrid hollow nanospheres, Angew. Chem. Int. Ed. 43 (2004) 6369–6372. [10] F. Lan, H. Hu, W. Jiang, K. Liu, X. Zeng, Y. Wu, Z. Gu, Synthesis of superparamagnetic Fe3 O4 /PMMA/SiO2 nanorattles with periodic mesoporous shell for lysozyme adsorption, Nanoscale 4 (2012) 2264–2267. [11] J. Zhou, W. Wu, D. Caruntu, M.H. Yu, A. Martin, J.F. Chen, C.J. O’Connor, W.L. Zhou, Synthesis of porous magnetic hollow silica nanospheres for nanomedicine application, J. Phys. Chem. C 111 (2007) 17473–17477. [12] W. Wu, D. Caruntu, A. Martin, M. Yu, C. Connora, W. Zhou, Synthesis of magnetic hollow silica using polystyrene bead as a template, J. Magn. Magn. Mater. 311 (2007) 578–582.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
A facile preparation method for single-hole hollow Fe3 O4 @SiO2 microspheres Xiaoyi Fu ∗ , Jingjing Liu, Xinhua He School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China
h i g h l i g h t s
g r a p h i c a l
• Fe3 O4 @SiO2
Schematic representation of single pores hollow Fe3 O4 @SiO2 microspheres with structure prepared by precipitation-phase separation method and the corresponding SEM and TEM images.
• • • •
hollow microspheres with single holes in their shells were obtained. A novel preparation method of single hole hollow Fe3 O4 @SiO2 microspheres was reported. Modifying Fe3 O4 NPs with DTMS is essential for encapsulating the Fe3 O4 NPs in the silica matrix. The hole size of the microspheres was depended on PMMA to PS mass ratio. The Ms values of Fe3 O4 @SiO2 microspheres were easy to control.
a r t i c l e
i n f o
Article history: Received 18 November 2013 Received in revised form 19 March 2014 Accepted 22 March 2014 Available online 5 April 2014 Keywords: Single-hole Hollow microspheres Silica Fe3 O4 nanoparticles
a b s t r a c t Hollow Fe3 O4 @SiO2 microspheres with single holes in their shells and Fe3 O4 nanoparticles encapsulated in silica shells were prepared using a precipitation-phase separation method. The synthesis was performed by mixing a tetrahydrofuran (THF)–acetonitrile (MeCN) solution containing polymer, tetraethyl orthosilicate (TEOS) and n-dodecyltrimethoxysilane (DTMS)-modified Fe3 O4 nanoparticles with an aqueous solution containing cetyltrimethyl ammonium bromide (CTAB) before adding ammonia to catalyze the hydrolysis of TEOS and finally removing the polymer by dissolving it in THF. The polymer used was poly(styrene-co-methyl methacrylate) (P(St-co-MMA)) or a mixture of polystyrene (PS) and polymethyl methacrylate (PMMA). The effects of modifying the Fe3 O4 NPs, tailoring the particle size and the hole size and controlling the Fe3 O4 content in the Fe3 O4 @SiO2 hollow microspheres were investigated. The prepared Fe3 O4 @SiO2 single-hole hollow microspheres were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM). We observed that modifying Fe3 O4 NPs with DTMS is essential for encapsulating the Fe3 O4 NPs in a silica matrix. The hole size of the microspheres increased with the PMMA-to-PS mass ratio, and the hole size could be adjusted from 16 nm to 135 nm. The size of the microspheres could be tailored from 240 to 1270 nm by modulating the process parameters. The Fe3 O4 @SiO2 microspheres exhibited superparamagnetic properties, and the corresponding saturation magnetization (Ms) values varied from 10 to 46 emu/g when controlling the modified Fe3 O4 NP content in the microspheres. Based on our results, we propose a possible formation mechanism for the Fe3 O4 @SiO2 hollow microspheres. © 2014 Elsevier B.V. All rights reserved.
∗ Corresponding author. Tel.: +86 20 87111224; fax: +86 20 87111224. E-mail address: [email protected] (X. Fu). http://dx.doi.org/10.1016/j.colsurfa.2014.03.108 0927-7757/© 2014 Elsevier B.V. All rights reserved.
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1. Introduction Studies of hollow microspheres with well-defined shapes and sizes have attracted significant attention because these materials have numerous potential applications, such as controlled drug delivery, the protection of biologically active agents, the removal of pollutants and catalysis [1–3]. In particular, microspheres with a single hole in their shell (single-hole hollow microspheres) are particularly interesting due to their high effective diffusivity for large molecules and nanoparticles, large available surface area and high uptake capacity for drugs and pollutants [4,5]. Fe3 O4 nanoparticles (Fe3 O4 NPs) are among the most important magnetite materials and are an ideal material for biological magnetic applications because they are hydrophilic, biocompatible, nontoxic and chemically stable [6]. Composite Fe3 O4 NPs with a hollow, single-hole structure would offer opportunities for exploring new material properties and broadening their potential applications due to their bifunctionality. Many reports detail the preparation, properties and applications of magnetic hollow microspheres [7–16]; however, the studies of magnetic hollow microspheres with single holes on their surfaces are limited. Indeed, designing and preparing single-hole hollow magnetic microspheres with controllable structures is challenging. Considerable effort has been devoted to synthesizing hollow microspheres with single holes using various chemical and physicochemical methods [4,5,17–29]. These approaches are broadly divided into three categories: (1) the single hole forms after the release of matter from the microsphere interior. If the shell is non-uniform in thickness, the release and diffusion of matter from the interior of the microspheres tends to occur where there is the least resistance, causing the shell to break where it is the thinnest and creating an opening. Many single-hole hollow microspheres are prepared based on this strategy [17–22]. For example, our group has successfully synthesized single-hole hollow silica spheres with controllable particle size using a preparation-phase separation method [17]. Ariga et al. [18] prepared open-mouthed platinum microcapsules with a templated synthesis procedure using polystyrene spheres; the surface-fused crystalline nanoparticles formed a capsule shell, and the subsequent removal of the polystyrene spheres formed the mouth-like openings. Xue et al. [19] prepared polyurethane (PU) hollow microspheres with sizetunable single holes using a facile self-assembly diffusion process. The single holes in the shells of the PU hollow spheres were formed by the outward diffusion of the encapsulated chloroform in methanol, and the hole size could be tuned by adjusting the diffusion time. (2) The formation of single-hole hollow microspheres is based on Ostwald ripening [23–26]. In cases in which this process occurs, microspheres are generally formed via the assembly of small particles. Smaller, less crystalline or dense particles in a colloidal aggregate will be dissolved gradually, while larger, more crystalline or denser particles in the same aggregate grow. As the mass is transported, the void space in the microspheres and the single hole on the surface are generated mainly through Ostwald ripening. Based on this strategy, CaWO4 [23], ZnO [24] and Fe3 O4 [25] hollow microspheres with a single hole on their surface have been prepared. (3) The formation of the hole is attributed to the incomplete shell of the core–shell structure microspheres [2a,7]. For example Guan et al. [4] fabricated shell-crosslinked single-hole hollow polymer nanospheres by precipitation polymerizing acrylamide and ethylene glycol dimethacrylate that were adsorbed onto the surface of carboxyl-capped polystyrene beads; the polystyrene cores were subsequently dissolved. The formation of the hole was attributed to the shrinkage of the polymer shell during polymerization. Yan et al. [27] prepared single-hole hollow PEGDMA nanospheres using a raspberry-like template. In this method, nanometer-scale silica cores were attached to the
surfaces of micrometer-sized silica spheres, and PEGDMA shells were produced on the nano-sized spheres outside the attached area, forming core–shell microspheres; subsequently, the core microspheres were removed, and the holes were attributed to incomplete shell formation. There are only a small number of reported preparations of magnetic hollow microspheres with single holes on their surfaces. Zhou et al. [29] prepared open, hollow Fe3 O4 spheres assembled from nanocrystals generated using a solvothermal method. In this case, the formation yield of the single-hole structure in the Fe3 O4 microspheres reached 80%. Chuang et al. [21] prepared single-hole hollow silica nanocapsules containing magnetic nanoparticles and fluorescent quantum dots by evaporating emulsion droplets via a single-step emulsion-mediated method. Moreover, a single-hole structure is occasionally observed in microspheres prepared using template methods or when assembling small Fe3 O4 NPs [25,30–32]; however, the proportion of single-hole microspheres that is yielded by this method is quite low, and the single-hole structure is not controllable because the formation of the single hole is due to the hollow microspheres breaking during template removal. We report a novel method for synthesizing single-hole hollow Fe3 O4 @SiO2 microspheres using a controllable precipitation-phase separation process. DTMS-modified Fe3 O4 NPs were uniformly encapsulated by a silica shell matrix to afford single-hole Fe3 O4 @SiO2 hollow microspheres with a mean size ranging from 240 to 1270 nm and a mean hole size of 16 to 135 nm, which exhibit excellent superparamagnetic properties. The method is based on the rapid precipitation of DTMS-modified Fe3 O4 NPs, a polymer and TEOS from an aqueous solution. Afterward, the TEOS is hydrolyzed, producing the single-hole structures. Our method is notably simple and convenient and can control the size and structure of the hollow microspheres. 2. Experimental 2.1. Chemical and materials P(St-co-MMA) (Mw = 100,000–150,000) was purchased from Sigma–Aldrich. PS (Mw = 100,000) and PMMA (Mw = 97,000) were purchased from Acros. TEOS was purchased from Wulian Chemical Reagents Factory. DTMS was purchased from Alfa Aesar. CTAB was obtained from Sanland-chem. Iron(II) sulfate heptahydrate (FeSO4 ·7H2 O) and iron(III) chloride hexahydrate (FeCl3 ·6H2 O) were of analytical grade and were purchased from Damao and Fuchen Chemical Reagents Co. Tianjin. THF was purchased from Jiangsu Tsiangsheng Chemical Reagents Co. MeCN was obtained from Guangdong Chemical Reagent Engineering-technological Research and Developing Center. Ethanol (EtOH) and aqueous ammonia (28 wt%) were acquired from the Guangdong Donghong Chemical Factory. 2.2. Preparation of Fe3 O4 NPs and their modification Fe3 O4 NPs were prepared according to the procedure described by Q. Zhang [33], omitting the aqueous solution of sodium dodecylsulfanate. The surfaces of the Fe3 O4 NPs were modified as follows: 1.0 g of prepared Fe3 O4 NPs was washed with a mixture of ethanol and water (4:1 in volume ratio) twice and suspended in 25 mL of a mixture of EtOH–H2 O before 1.0 mL of DTMS was added. The mixture was refluxed under N2 for 4 h and subsequently cooled to RT. Afterward, the DTMS-modified Fe3 O4 NPs (denoted as m-Fe3 O4 NPs) were washed with ethanol and dried. 2.3. Preparation of single-hole Fe3 O4 @SiO2 hollow microspheres Single-hole silica microspheres were synthesized by precipitating polymer–TEOS–m-Fe3 O4 NPs, hydrolyzing TEOS using catalytic
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ammonia and removing the polymer. In a typical experiment, the m-Fe3 O4 NPs were suspended in THF using ultrasonication for 15 min, followed by centrifugation at 1000 rpm for 5 min to remove any large aggregates. Subsequently, 0.5 mL of the m-Fe3 O4 NPs/THF (1.0 g/L) dispersion and 0.1 mL of TEOS were mixed with P(St-coMMA) in THF (4 mL, 1.25 g/L) before 4 mL of MeCN was added quickly. The organic mixture was stirred for several seconds and subsequently quickly added to 30 mL of ultrapure water at room temperature. After shaking for seconds, an aqueous CTAB solution (0.2 mL, 0.01 mol/L) was added, followed by the addition of 0.5 mL of ammonium hydroxide. After several seconds of stirring, the mixture was allowed to stand under static conditions at room temperature for one day, generating a colloidal solution of m-Fe3 O4 @ silica–P(St-co-MMA) composite microspheres. The composite microspheres were collected by centrifugation and sequentially washed with ultrapure water and EtOH. To produce the single-hole Fe3 O4 @SiO2 hollow microspheres, the m-Fe3 O4 @ silica–P(St-coMMA) composite microspheres were suspended in THF to remove the P(St-co-MMA). During the experiments, the polymer used was P(St-co-MMA) or mixtures of PS and PMMA in various ratios. The synthesis parameters, including the organic solvent type, the volumetric ratio of the two organic solvents, the volumetric ratio of the organic solvent and water, the mass ratio of the polymer with TEOS and the amount of ammonia, were varied to adjust the morphology and size of the obtained silica microspheres systematically. 2.4. Characterization The size and morphology of the prepared microspheres were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) using a Nova NanoSEM 430 scanning electron microscope and a Philips Tecnai 10 transmission electron microscope, respectively. EDX measurements were collected on an energy dispersive X-ray spectroscope (EDX, EPMA1600/EDAX). The samples used for the SEM studies were prepared by drying droplets of the microsphere suspensions on copper foil and coating them with gold. The samples for the TEM studies were prepared by drying droplets of the microsphere suspensions on copper grids coated with Formvar film. Magnetic hysteresis curves were measured with a Model 4HF PPMS-9 vibrating sample magnetometer. X-ray diffraction (XRD) patterns were collected with an X’Pert Pro X-ray diffractometer. Infrared measurements
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were recorded with a vector 33 IR spectrometer. Thermogravimetric analysis (TGA) measurements were conducted on a Netsch STA449C analyzer at 10 ◦ C/min under flowing air. 3. Results and discussion 3.1. The preparation of magnetic hollow microspheres Modifying the Fe3 O4 NPs was critical for the formation of hollow single-hole Fe3 O4 @SiO2 microspheres. When the unmodified Fe3 O4 NPs were used to prepare the microspheres, the SEM images revealed that there were numerous Fe3 O4 NPs aggregated outside of the microspheres (Fig. 1). The elemental composition of the microspheres was analyzed using EDX, and the absence of Fe indicated that the Fe3 O4 NPs could not be encapsulated by the microspheres (Fig. 1). This phenomenon may occur for two reasons: the Fe3 O4 NPs may aggregate in the THF–MeCN mixture, and the unmodified Fe3 O4 NPs are hydrophilic. The hydrophilic Fe3 O4 NPs aggregates cannot be encapsulated by the hydrophobic TEOS–polymer composite microspheres. Therefore, stable dispersions of the Fe3 O4 NPs in the organic solvent and a moderate hydrophobicity in the Fe3 O4 NPs are essential for forming the Fe3 O4 @SiO2 microspheres. Consequently, we chose two types of additives for modifying the Fe3 O4 NPs: organic acids (such as oleic acid) and silane coupling agents (DTMS). The precipitation experiments indicated that DTMS-modified Fe3 O4 NPs can be stably dispersed in THF–MeCN (shown in Fig. S1). FTIR spectra of the Fe3 O4 NPs before and after DTMS modification are shown in Fig. 2. The characteristic bands at 2927 cm−1 , 2857 cm−1 (C H stretching of methyl and methylene), 988 cm−1 (Si OH bending) and 1178 cm−1 (Si O stretching) [34] appeared after the modification, confirming that DTMS was successfully grafted to the Fe3 O4 NPs. Afterward, the DTMS-modified Fe3 O4 NPs were used to prepare the hollow magnetic microspheres. 3.2. The structure of Fe3 O4 @SiO2 microspheres SEM and TEM images of the obtained microspheres before and after removing the polymer are shown in Fig. 3. The microspheres exhibited either an opening or a single hole on the surface of the microspheres before polymer removal, and a smaller microsphere was formed in the hole (marked with an arrow in Fig. 3a). After removing the polymer, these smaller microspheres disappeared,
Fig. 1. SEM image of the microspheres prepared using unmodified Fe3 O4 NPs and the EDX spectrum of microspheres (1) and aggregated Fe3 O4 NPs (2).
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Fig. 2. FTIR spectra of the Fe3 O4 NPs before and after DTMS modification.
suggesting that they were composed of P(St-co-MMA). By comparing the TEM images of the microspheres before and after polymer removal, it was observed that the microspheres before polymer removal tended to aggregate into groups of two or three with their
holes pointed in opposite directions, forming clover-like shapes (marked with a red circle in Fig. 3c), whereas without the polymer, the microspheres did not exhibit that behavior. This interesting microsphere self-assembly may be caused by hydrophobic interactions induced by the polymer. Most of the microspheres exhibited a single hole on each surface after THF treatment (shown in Fig. 3b). The outside surface of the microspheres was smooth, whereas the internal surface was quite rough, as shown in the SEM image of a microsphere in Fig. 3e. In the TEM images, the microspheres exhibit eccentric hollow structures with a non-uniform shell thickness. Because the electronic contrast of Fe3 O4 NPs is higher than that of silica, the electronic contrast difference clearly indicates that the m-Fe3 O4 NPs were randomly entrapped in the silica matrix (Fig. 3f). After the THF treatment, the microspheres exhibited diameters ranging from 263 to 605 nm with a mean diameter of 404 ± 65 nm; the diameters of the holes ranged from 119 to 517 nm with a mean diameter of 225 ± 63 nm. The elemental composition of the microspheres was determined using EDXA; the presence of Si, Fe and O indicates that Fe3 O4 @SiO2 microspheres were formed (Fig. 3g, Cu was introduced by the sample holder, whereas Au was introduced by SEM sample preparation). The Fe content of the Fe3 O4 @SiO2 spheres was ∼27.3%, as estimated by EDXA, exceeding the amount of Fe3 O4 (16.9%) used
Fig. 3. SEM (a, b, e) and TEM (c, d, f) images of the microspheres before (a, c) and after (b, d, e, f) P(St-co-MMA) removal. Panels e and f show enlarged images of a single microsphere. Panel g presents the EDX spectrum of the microspheres after P(St-co-MMA) removal.
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during the preparation. For the silica hollow microspheres with a single hole on the surface, we observed that only some of the TEOS participated in shell formation [17], whereas a large proportion of the TEOS was converted to silica nanoparticles measuring 10–50 nm in diameter. These particles were discarded during the centrifugation and washing steps. Therefore, it is reasonable that the Fe content of the microspheres estimated during the EDX analysis exceeds the theoretical values. To estimate the polymer content of the microspheres before the THF treatment, TGA measurements were collected; the TGA curves of the microspheres are shown in figure S2. The weight loss below 200 ◦ C can be ascribed to the desorption of physically bound organic solvent and water molecules [35], and the weight loss between 200 and 280 ◦ C can be attributed to dehydroxylation and the removal of low-molecular-weight organic material [36]. The sharp decrease in weight from 280 to 450 ◦ C may have been caused by P(St-co-MMA) decomposition [35], whereas the residual weight can be ascribed to the silica and Fe3 O4 NPs. The weight loss from the polymer and the residual combined weight of the silica and Fe3 O4 NPs were 18.1% and 78%, respectively, generating a 0.23 weight ratio of P(Stco-MMA)/SiO2 + Fe3 O4 for the microspheres prepared using a 0.05 mass ratio of polymer to TEOS. The XRD spectrum of the microspheres is shown in Fig. S3. The broad diffraction peak at 2Â = 20◦ to 30◦ is attributed to amorphous silica. The sharp, major diffraction peaks at 2Â = 30.07, 35.48, 43.04, 56.98 and 62.48 can be assigned to the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) reflections, respectively; these reflections can be
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indexed to the spinel structure of pure magnetite (Fe3 O4 ) (JCPDS file 19-0629). 3.3. The tailoring of microsphere and opening size The size of the opening in the microspheres could be controlled using different mixtures of PS and PMMA, as shown in Fig. 4. The microspheres prepared using PS only exhibited very small openings (16 ± 2 nm), whereas the microspheres prepared using PMMA only showed large openings (135 ± 36 nm). The opening size of the microspheres increased with the PMMA content in the PSt-PMMA mixture; the mean opening diameters of the microspheres prepared using PMMA–PS mixtures with mass ratios of 0, 0.25, 1 and 4 were 16 ± 2, 59 ± 14, 84 ± 14 and 135 ± 36 nm, respectively. The ratio of PS and PMMA also affected the size of the microspheres; however, this effect was irregular and was observed over a small range of sizes. A similar phenomenon could be observed when mixtures of P(St-co-MMA)-PMMA and P(St-co-MMA)-PS were used (Fig. S4). The opening size in the microspheres decreased as the PS content in the PSt-P(St-co-MMA) mixture increased. When the amount of PS was large enough (PS/P(St-co-MMA) = 4), the microspheres did not contain openings on their surfaces (Fig. S4a). When PMMA was mixed with P(St-co-MMA), the opening size of the microspheres increased dramatically. However, the dependence of the opening size on the St/MMA ratio was irregular (Table S1). Therefore, the opening size of the microspheres depended either on the ratio of the two polymers or on the types of polymer used.
Fig. 4. SEM images of the microspheres prepared with various PMMA-to-PS mass ratios (a: pure PS; b: 0.25, c: 1, d: 4 and e: pure PMMA) and the dependence of the size and opening size on the PMMA-to-PS mass ratio.
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Fig. 5. SEM images of microspheres of various sizes (preparation: a: mixture polymer of PS (0.62 g/L) and PMMA (1.25 g/L), polymer/TEOS = 0.054 wt/wt, Fe3 O4 NPs (1 g/L); b: P(St-co-MMA) (3 g/L), polymer/TEOS =0.12 wt/wt, Fe3 O4 NPs (1 g/L); c: P(St-co-MMA), (2 g/L), polymer/TEOS = 0.086 wt/wt, Fe3 O4 NPs (1 g/L); d: P(St-co-MMA) (3 g/L), polymer/TEOS = 0.11 wt/wt, Fe3 O4 NPs (0.2 g/L)).
Microspheres of various sizes could be obtained by adjusting the process parameters employed, such as the type and amount of organic solvent and the ratio of organic and aqueous solutions. By modulating these parameters, we could obtain microspheres ranging in size from 200 nm to 1.5 m. Fig. 5 shows SEM images of the microspheres produced: (a) 232 ± 44 nm; (b) 517 ± 138 nm; (c) 1287 ± 1379 nm; (d) 1796 ± 512 nm.
3.4. The effect of the DTS-Fe3 O4 content The Fe3 O4 content in the microspheres could be modulated by changing the amount of m-Fe3 O4 NPs initially added. TEM images of the microspheres prepared using different amounts of m-Fe3 O4 NPs are shown in Fig. 6. The high electronic contrast indicates that the amount of m-Fe3 O4 NPs produced increased as the amount of
Fig. 6. TEM images of the microspheres with various Fe3 O4 NPs contents (Fe/Si ratio of 0.23 (a), 0.65 (b), 1.10 (c) and 1.61 (d)).
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Fig. 7. The magnetic hysteresis curves for the Fe3 O4 @SiO2 spheres with varied Fe3 O4 contents and bare Fe3 O4 NPs. The Fe/Si ratios of 0.23 (1#), 0.65 (2#), 1.10 (3#) and 1.61 (4#) were estimated using EDX analysis. The inset photographs depict the Fe3 O4 @SiO2 spheres separated from the solution using an external magnetic field.
nanoparticles initially added increased. For the microspheres prepared using m-Fe3 O4 NPs with Fe3 O4 /TEOS molar ratios of 0.0087, 0.044, 0.087 and 0.13 molar ratios; the corresponding Fe/Si ratio values estimated via EDX analysis were 0.231, 0.635, 1.10 and 1.61, respectively. The morphology and size of the microspheres were also affected by the amount of m-Fe3 O4 NPs initially added. Microspheres with smooth surfaces and regular shapes were produced when a low amount of m-Fe3 O4 NPs (mol ratio of Fe3 O4 /TEOS of 0.0087) was used. When a high amount of m-Fe3 O4 NPs (mol ratio of Fe3 O4 /TEOS of 0.087) was added, the shape of the microspheres became irregular, and the size of the cavity decreased. However, when excess m-Fe3 O4 NPs were used (mol ratio of Fe3 O4 /TEOS of 0.13), the obtained microspheres exhibited many pores on their surfaces. Therefore, to obtain hollow microspheres with a single hole, the molar ratio of Fe3 O4 /TEOS should remain below 0.087. The room-temperature magnetization hysteresis curve of the Fe3 O4 @SiO2 microspheres (Fig. 7) exhibits negligible hysteresis, indicating that the particles are superparamagnetic. The magnetization curve of the Fe3 O4 @SiO2 microspheres is similar to that of the bare Fe3 O4 NPs, indicating that the encapsulated m-Fe3 O4 NPS can preserve their superparamagnetic properties. The saturation magnetization (Ms) values of the bare Fe3 O4 and the Fe3 O4 @SiO2 microspheres prepared using m-Fe3 O4 NPs generated with Fe3 O4 -to-TEOS molar ratios of 0.0087, 0.0044, 0.087 and 0.13 were 52.2 and 9.5, 25.0, 41.8 and 44.6 emu/g, respectively. The prepared Fe3 O4 @SiO2 microspheres dispersed in an aqueous solution could be separated easily using a magnet (inset photographs), demonstrating their strong magnetic response. 3.5. The microsphere formation mechanism Our group prepared hollow silica microspheres with a single opening using a precipitation-phase separation method [5a], and we observed that the TEOS and P(St-co-MMA) microphases separated from each other, forming a hemispherical structure before ammonia was added. After adding the ammonia to catalyze the hydrolysis and condensation of the TEOS, the composite microspheres transformed into silica–P(St-co-MMA) composite microspheres with a single hole in their shells and P(St-co-MMA) located randomly in their cavities. The opening formed through the diffusion of ammonia, solvent and resultant molecules at the point of least resistance, breaking the silica shell where it was the
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thinnest. The hemispherical shape of the TEOS and P(ST-co-MMA) is essential for the formation of hollow microspheres with a single hole. In this work, the difference between the preparations generating Fe3 O4 @SiO2 microspheres and those producing silica hollow microspheres with a hole was the addition of the m-Fe3 O4 NPs; the resulting microspheres also exhibited a hollow structure with a hole when the added Fe3 O4 NPs were encapsulated in the silica matrix. The mechanism of Fe3 O4 @SiO2 microsphere formation is similar to that of hollow silica microspheres. Introducing m-Fe3 O4 NPs does not alter the microphase separation of TEOS and the polymer, and the m-Fe3 O4 NPs tend to disperse into the TEOS microphase. After the hydrolysis and condensation of the TEOS, hemispherical TEOS–m-Fe3 O4 NPs-polymer transformed into silica–m-Fe3 O4 NPs-polymer composite microspheres with a hole in the shell and m-Fe3 O4 NPs encapsulated into the silica matrix. The hydrophilic Fe OH on the surface of the m-Fe3 O4 NPs might explain why hydrophobic m-Fe3 O4 NPs could be encapsulated into the hydrophilic silica matrix? Grafting the DTMS to the surface of the Fe3 O4 NPs occurs through reactions between the Si OMe groups and the Fe OH groups [37]. The grafting efficiency is quite low [38,39]; therefore, few Fe OH groups participated in the grafting reaction. The residue Fe OH on the surface of m-Fe3 O4 NPs could react with the TEOS or the Si OH produced during the hydrolysis of TEOS, fixing the nanoparticles to the silica matrix. The m-Fe3 O4 NPs tended to disperse into the TEOS microphase of the hemispherical structure of the TEOS polymer because the m-Fe3 O4 NPs embedded in the silica matrix and the Fe3 O4 content increased with increased amounts of m-Fe3 O4 NPs added. However, this explanation does not indicate that the m-Fe3 O4 NPs could not completely disperse in the polymer microphase. The m-Fe3 O4 NPs might disperse in the polymer microphase. When the silica/mFe3 O4 NPs-polymer composite microspheres that were prepared using a high m-Fe3 O4 NPs content were treated with THF to remove the polymer, the polymer/THF solution turned yellow, indicating that m-Fe3 O4 NPs were released into solution and proving that some m-Fe3 O4 NPs could also disperse in the polymer microphase. We observed that opening size in the microspheres increased with the MMA/St ratio. This phenomenon is related to the microphase separation of the TEOS and the polymer. When an aqueous CTAB solution was added to the mixed organic solution containing the polymer, TEOS and m-Fe3 O4 NPs, the hydrophobic TEOS molecules and the m-Fe3 O4 NPs swelled inside the polymer matrix, producing the polymer–TEOS–m-Fe3 O4 NPs composite microspheres. Subsequently, the microspheres underwent structural adjustments to become stable. Based on the classical two-component polymer model [40,41], the final morphology of the composite microspheres, which are thermodynamically stable and in a completely phase-separated state, are broken down into three types: the core–shell, the hemispherical structure and the individual particles. PS and TEOS tend to form microspheres with an asymmetrical PS core/TEOS shell structure that transform into SiO2 shell/PS core structures with non-uniform shell thicknesses [42]. When the PS was removed from the microspheres, the spheres broke at the thinnest point, forming an opening. PS is a highly hydrophobic polymer; however, PMMA is more hydrophilic. When P(St-co-MMA) was used instead of PS or PMMA was mixed with PS, the core–shell structure became a hemispherical structure with TEOS- and polymer-rich regions. Incorporating PMMA gave rise to greater deviations in the polymer core from the center position. It was clearly observed that the hemispherical structure of the TEOS–polymer became a SiO2 –polymer structure with a larger opening than that of the core–shell structure after catalytic ammonia treatment. Therefore, the size of the opening increased when more MMA was added to the mixture.
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4. Conclusions Fe3 O4 @SiO2 hollow microspheres with a single hole in their shells were prepared using a precipitation-phase separation method. Modifying the Fe3 O4 NPs with DTMS was essential for encapsulating the Fe3 O4 NPs that were dispersed randomly into the silica shell matrix. The Fe3 O4 nanoparticle content of the microspheres was controlled by adjusting the ratio of Fe3 O4 nanoparticles to TEOS. The hole sizes in the microspheres were controlled by adjusting the polymer content, specifically increasing the PMMA-to-PS mass ratio. The Fe3 O4 @SiO2 microspheres exhibited superparamagnetic properties, and the saturation magnetization (Ms) value of the microspheres varied from 10 to 45 emu/g. Thus, we propose a possible formation mechanism for the Fe3 O4 @SiO2 hollow microspheres. Supporting information available Photographs of pure Fe3 O4 NPs and m-Fe3 O4 NPs stably dispersed in a mixture of THF–MeCN, TGA curves of P(Stco-MMA)–SiO2 –m-Fe3 O4 NPs composite microspheres, an XRD pattern for the Fe3 O4 @SiO2 microspheres and SEM images of microspheres prepared using various P(St-co-MMA)-to-PMMA (PS) ratios are available. Acknowledgments This work was supported by grants from NSFC (project no. 21103052), the China Postdoctoral Science Foundation (200801004), the Specialized Research Fund for the Doctoral Program of Higher Education in China (20114433110001) and the Scientific and Technological Planning Project of Guangdong Province (No. 2007A010500012). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.colsurfa.2014.03.108. References [1] X. Lou, L. Archer, Z. Yang, Hollow micro-/nanostructures: synthesis and applications, Adv. Mater. 20 (2008) 3987–4019. [2] Y. Zhao, L. Jiang, Hollow micro/nanomaterials with multilevel interior structures, Adv. Mater. 21 (2009) 3621–3638. [3] K. Cheng, S. Sun, Recent advances in syntheses and therapeutic applications of multifunctional porous hollow nanoparticles, Nano Today 5 (2010) 183–196. [4] G. Guan, Z. Zhang, Z. Wang, B. Liu, D. Gao, C. Xie, Single-hole hollow polymer microspheres toward specific high-capacity uptake of target species, Adv. Mater. 19 (2007) 2370–2374. [5] S. Luo, J. Jiang, S. Liour, S. Gao, J. Ying, H. Yu, Magnetic PEDOT hollow capsules with single holes, Chem. Commun. (2009) 2664–2666. [6] R. Hao, R. Xing, Z. Xu, Y. Hou, S. Gao, S. Sun, Functionalization, and biomedical applications of multifunctional magnetic nanoparticles, Adv. Mater. 22 (2010) 2729–2742. [7] W. Zhao, H. Chen, Y. Li, L. Li, M. Lang, J. Shi, Uniform rattle-type hollow magnetic mesoporous spheres as drug delivery carriers and their sustained-release property, Adv. Funct. Mater. 18 (2008) 2780–2788. [8] S. Peng, S. Sun, Synthesis and characterization of monodisperse hollow Fe3 O4 nanoparticles, Angew. Chem. Int. Ed. 46 (2007) 4155–4158. [9] Y. Ding, Y. Hu, X. Jiang, L. Zhang, C. Yang, Polymer–monomer pairs as a reaction system for the synthesis of magnetic Fe3 O4 –polymer hybrid hollow nanospheres, Angew. Chem. Int. Ed. 43 (2004) 6369–6372. [10] F. Lan, H. Hu, W. Jiang, K. Liu, X. Zeng, Y. Wu, Z. Gu, Synthesis of superparamagnetic Fe3 O4 /PMMA/SiO2 nanorattles with periodic mesoporous shell for lysozyme adsorption, Nanoscale 4 (2012) 2264–2267. [11] J. Zhou, W. Wu, D. Caruntu, M.H. Yu, A. Martin, J.F. Chen, C.J. O’Connor, W.L. Zhou, Synthesis of porous magnetic hollow silica nanospheres for nanomedicine application, J. Phys. Chem. C 111 (2007) 17473–17477. [12] W. Wu, D. Caruntu, A. Martin, M. Yu, C. Connora, W. Zhou, Synthesis of magnetic hollow silica using polystyrene bead as a template, J. Magn. Magn. Mater. 311 (2007) 578–582.
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