Facile Method for Synthesis of Fe3O4@Polymer Microspheres and

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Facile Method for Synthesis of Fe3O4@Polymer Microspheres and Their Application As Magnetic Support for Loading Metal Nanoparticles Bin Liu, Wei Zhang, Fengkai Yang, Hailiang Feng, and Xinlin Yang* Key Laboratory of Functional Polymer Materials, The Ministry of Education, Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China

bS Supporting Information ABSTRACT: A general method was developed for the synthesis of Fe3O4@polymer microspheres with well-defined core shell structure and various functional groups and strong magnetization through a facile in situ distillationprecipitation polymerization. In this approach, the as-prepared Fe3O4 microspheres were directly coated by a polymer shell through the hydrogen bond interaction between the Fe3O4 micropsheres and oligomer without any surface modification. Moreover, hydrophilic or hydrophobic monomer or even their comonomers with different functional groups such as carboxyl, hydroxyl, amide, and ester were facilely encapsulated onto the surface of the magnetite microspheres. The thickness of the polymer shell layer was tuned by the feed of monomer amount. As for application, Fe3O4@P(MBAAm-co-MAA) microspheres that contained carboxyl groups were used as a magnetic catalyst support to load a series of metallic nanoparticles such as Ag, Pt, and Au. These strong magnetic microspheres were characterized by transmission electron microscopy, X-ray diffraction, Fourier-transform infrared spectra, thermogravimetric analysis, and vibrating sample magnetometry.

1. INTRODUCTION Coreshell particles are composite materials consisting of a core domain covered by a shell domain. The core serves a functional template and determines the form and size of the composite. The shell layer isolates the core from the hazardous environment, and this behavior may extend the application area because it can protect the core function from being corroded.1 Moreover, the shell layer endows these particles with different functionalities that change the hydrophilicity of the core and make these nanoparticles disperse in some specific solvents. Because of the isolated structure between the core layer and shell layer of the core/shell structure, the different functionalities of the core and shell cannot be reduced or disturbed by each other.2 In some special systems, the shell even increases the functionality of the core.3 The core and shell domains may be composed of a variety of different materials including polymers, inorganic solids, and metals. In this way, coreshell particles can combine different functionalities into one particle such as the mechanical strength, modulus, thermal stability of inorganic component and facile process-abilities, flexibilities, and various functional groups of polymer component. On the basis of these excellent properties, coreshell particles have attracted much attention because of their wide potential application in catalysis,4 drug delivery,5 gene delivery,6 biological imaging and label,7 diagnosis and therapeutic,8,9 and so on. In the functional cores, magnetic particles have attracted much attention because of the magnetic properties that respond to r 2011 American Chemical Society

external magnetic field. In particular, superparamagnetic nanoparticles have been extensively pursued for bioseparation,10 magnetic catalytic carrier,11,12 drug delivery,13 and magnetic resonance imaging (MRI).14,15 In the past few decades, much attention was paid to synthesize magnetic nanoparticles, such as chemical coprecipitation,16 thermal decomposition,1721 and hydrothermal synthesis.22 Although there have been many significant developments in the synthesis of magnetic nanoparticles, maintaining the stability of these particles for a long time without agglomeration or precipitation is an important issue. What’s more, the size polydispersity or hydrophobic property limited their application field. Recently, much effort was devoted to modify the surface property of magnetite. The surfactant exchange method was well studied during the past several years, by which the surface function and hydrophilicity of the magnetic particles were controlled.23,24 However, the magnetite was easily oxidized and eroded in the air or under acidic condition. Coating a functional polymer shell on the magnetite cores could solve the above-mentioned problems, which not only protect the core from damaging environments such as oxygen and acids or bases, but also render the particles stable, watersoluble, desired functional, and biocompatible. In recent years, Received: May 28, 2011 Revised: July 2, 2011 Published: July 05, 2011 15875

dx.doi.org/10.1021/jp204976y | J. Phys. Chem. C 2011, 115, 15875–15884

The Journal of Physical Chemistry C several approaches have been developed to prepare magnetite@ polymer coreshell particles. For instance, polymer can be coated onto a single magnetic nanoparticle through in situ polymerizations such as the surface-induced atomic transfer radical polymerization (ATRP)25,26 and reversible additionfragmentation chain transfer (RAFT) polymerization.27 However, the relatively rigorous experimental condition and a time-resumed polymerization process may limit their practical application. Moreover, the coating of polymer shell onto the magnetic particles may dramatically decrease the magnetization. This behavior made the polymer-coated magnetic particles insensitive to the external magnetic field. Hence, the most concerns in recent years were to increase the magnetization of the polymer-coated magnetic hybrid materials through the high loading of the magnetic nanoparticles. Paquet et al.28 reported the nanobeads with highly loaded superparamagnetic nanoparticles through the emulsification of the presynthesized Fe3O4 nanoparticles to form the magnetite clusters with the size range from 40 to 200 nm and the subsequent seeded-emulsion polymerization on these magnetite clusters. Gu et al.29,30 developed a miniemulsion polymerization method to prepare Fe3O4/polystyrene hybrid latexes. However, the endowing of the surface of these particles with different functionality was complex and difficult. Hence, it urgently needs a facile method for preparation of functional polymer shell-coated magnetic coreshell particles with high magnetization. Metallic nanoparticles were widely studied because of unique electronic, optical, and catalytic properties due to the quantum size effects.31,32 These particles attracted intensive attention for their application in sensor,32 optoelectronic devices,32,33 and catalysis.32 In particular, the big specific surface area of the metal nanoparticles made them highly catalytic active and as attractive tools for catalysis. However, the separation and recovery of the metal nanoparticles in the reaction mixture solution were difficult because of their tiny size. To solve this issue, the utilization of a catalyst support to load the metal nanoparticles was widely studied no matter in the scientific or industrial domain.3436 In particular, the application of magnetic particles as a platform for the metal nanoparticles was fascinating because the paramagnetic nature enable easy and efficient isolation of the catalysts from the reaction solution with the aid of an external magnet.37 In this Article, we described a general and facile method to prepare polymer-coated magnetic coreshell particles. Importantly, the preprepared magnetic microspheres with active groups can be directly encapsulated by a polymer shell with different functional groups without any surface modification through one-pot in situ distillationprecipitation polymerization. The functional polymer shell abundant with carboxyl groups was easily loaded with a series of metallic nanoparticles such as Ag, Pd, and Au nanoparticles. The acquired magnetic polymer hybrids retained the superparamagnetic property of the magnetite template and possessed strong magnetization, thus providing a convenient way for separating these particles from the solution.

2. EXPERIMENTAL SECTION 2.1. Materials. Ferric chloride (FeCl3 3 6H2O) was purchased from Tianjin Guangfu Chemical Engineering Institute. Trisodium citrate was obtained from Tianjin Chemical Reagents I. DVB was acquired from Alfa and purified by washing with NaOH aqueous solution and extracted with saturated NaCl aqueous solution. N,N0 -Methylenebisacrylamide (MBAAm, chemical grade, Tianjin Bodi Chemical Engineering) was recrystallized from

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acetone. Methacrylic acid (MAA), 2-hydroxyethyl methacrylate (HEMA), and ethyleneglycol dimethacrylate (EGDMA) were purchased from Tianjin Chemical Reagent II and purified by vacuum distillation. 2,20 -Azobisisobutyronitrile (AIBN) was provided by Chemical Factory of Nankai University and recrystallized from methanol. Acetonitrile (analytical grade, Tianjin Chemical Reagents II) was dried over calcium hydride and purified by distillation before use. All other reagents were of analytical grade and used without any further treatment. 2.2. Synthesis of Fe3O4 Particles. Magnetite particles were synthesized through a solvothermal method according to the literature with a minor modification.38,39 The details were as follows: 3.6 g of FeCl3 3 6H2O and 0.72 g of trisodium citrate were dissolved in ethylene glycol/ethanol (90 mL/10 mL) solution through ultrasound irradiation; then, 4.8 g of sodium acetate was added under vigorous stirring for 5 min. The resultant mixture was then transferred to a Teflon-lined stainless-steel autoclave (with a capacity of 200 mL) for heating at 200 °C for 10 h. After that, the autoclave was carefully taken out to cool to room temperature. The as-made black products were thoroughly washed with ethanol and deionized water three times, respectively, and finally vacuum-dried. 2.3. Synthesis of Fe3O4@Polymer Microspheres. A typical procedure for synthesis of Fe3O4@PMBAAm coreshell microspheres was as follows: 0.05 g of Fe3O4 inorganic seeds was suspended in 40 mL of acetonitrile. Then, MBAAm (0.10 g) and AIBN (0.002 g, 2 wt % relative to all monomers) were dissolved in the above suspension in a 50 mL flask. The flask attached to a fractionating condenser and receiver was submerged in a heating mantle. The reaction mixture was heated from ambient temperature until the boiling state within 15 min, and the reaction system was kept under refluxing state for further 15 min. The polymerization was further carried out with distilling the solvent out of the reaction system, and the reaction was ended after 20 mL of acetonitrile was distilled off the reaction mixture within 70 min. After the polymerization, the resultant Fe3O4@PMBAAm microspheres were purified by repeated centrifugation, decantation, and resuspension in ethanol for three times. The products were dried in a vacuum oven at room temperature until constant weight. The other distillation precipitation polymerizations to prepare Fe3O4@PEGDMA, Fe3O4@P(EGDMA-co-MAA), and Fe3O4@ P(MBAAm-co-HEMA) coreshell microspheres were very similar to the typical procedure by varying the monomer, whereas the weight ratio between the monomer to the magnetite seed was kept at 4:1. A series of polymerizations was utilized for the preparation of Fe3O4@P(MBAAm-co-MAA) microspheres with different shell thicknesses through changing the weight ratio between monomer to the magnetite seed from 2:1 to 4:1. For all of these polymerizations, the amount of AIBN initiator was maintained at 2 wt % relative to the monomer, and the content of the cross-linker in all of the commoner was 20%. The treatment of these coreshell microspheres was the same as that for the typical procedure. The reproducibility of the polymerizations was confirmed through several duplicate and triplicate experiments. 2.4. Fe3O4@P(MBAAm-co-MAA) As Magnetic Scaffold for Loading Different Metallic Nanoparticles. We incubated 0.1 g Fe3O4@P(MBAAm-co-MAA) microspheres in 1 M NaOH aqueous solution to ionize the carboxyl groups. The residual NaOH was removed by washing with deionized water through centrifugation. Then, the ionized Fe3O4@P(MBAAm-co-MAA) microspheres were dispersed in the solution (30 mL, 0.1 M) of 15876

dx.doi.org/10.1021/jp204976y |J. Phys. Chem. C 2011, 115, 15875–15884

The Journal of Physical Chemistry C

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AgNO3 or PdCl2 for exchanging the sodium cation between noble metallic ions and sodium ions. After 1 h, the microspheres were harvested with the aid of the magnet and washed with deionized water four times. Then, the microspheres were redispersed in 20 mL of deionized water, and 1 mL of 0.05 M NaBH4 aqueous solution was added dropwise under the ice water bath with shaking. The final product was purified through washing with water three times and dried under vacuum oven until constant weight. The steps for loading the gold nanoparticles were according to our previous work.40 The details were as follows: 0.05 g Fe3O4@P(MBAAm-co-MAA) coreshell microspheres was dispersed in 10 mL deionized water and 0.25 mL of 4 mM HAuCl4 aqueous solution was added; then, 1 mL of 0.05 M NaBH4 aqueous solution was added dropwise under the ice water bath with shaking. The treatment of the product was the same as that for loading Ag and Pd nanoparticles. 2.5. Characterization. The morphology of the resultant nanoparticles was determined by transmission electron microscopy (TEM) using a Technai G2 20-S-TWIN microscope. The samples for TEM characterization were dispersed in ethanol, and a drop of the dispersion was dropped onto the surface of a copper grid coated with a carbon membrane and then dried under vacuum state at room temperature. All of the size and size distribution reflect the averages of about 100 particles each, which are calculated according to the following formula U ¼ Dw =Dn

Dn ¼

k

k

∑ niDi= i∑¼ 1 ni i¼1

Dw ¼

k

Figure 1. TEM images of (A) Fe3O4 microspheres, (B) Fe3O4@ P(MBAAm) coreshell microspheres, (C) Fe3O4@P(EGDMA) core shell microspheres, and (D) Fe3O4P(DVB) composites. (The insets in parts A and B were the HRTEM graphs.)

k

∑ niD4i = i∑¼ 1 niD3i i¼1

where U is the polydispersity index, Dn is the number-average diameter, Dw is the weight-average diameter, and Di is the diameter of the determined microspheres. The thickness of the shell layer is calculated to be half of the difference between the average diameter of the coreshell particles and that of the cores. Fourier transform infrared spectra were determined on a BioRad FTS 135 FT-IR spectrometer over potassium bromide pellet, and the diffusion reflectance spectra were scanned over the range of 4000400 cm1. Thermogravimetric analysis (TGA) data were obtained with a heating rate of 10 K/min using a TA TGA-2950 apparatus. The magnetic properties of Fe3O4 microspheres, Fe3O4@ P(MBAAm-co-MAA) microspheres, and Fe3O4@P(MBAAmco-MAA)@Ag microspheres were studied in the dried state with a vibrating sample magnetometer (9600 VSM, BOJ Electronics, Troy, MI) at room temperature. The crystalline structure of the samples was analyzed on a D/max 2500 V X-ray diffractometer using Cu KR (λ = 0.15406 nm) radiation at 40 kV and 100 mA. The crystal size of Fe3O4 was estimated by applying the Scherrer equation (j = kλ/β cos θ), where j is the crystal size, λ is the wavelength of the X-ray irradiation, k is usually taken to be 0.89 here, β is the peak width at half-maximum height of the (311) peak of magnetite after subtracting the instrumental line broadening, and θ is the diffraction angle.

3. RESULTS AND DISCUSSION Magnetite microspheres were prepared through a solvothermal method according to the literature38,39 by partial reduction of FeCl3 with EG as solvent, sodium acetate as an alkali source, and trisodium citrate (Na3Cit) as an electrostatic stabilizer at 200 °C. Additionally, some ethanol was introduced to improve the solubility

Figure 2. XRD spectrum of Fe3O4 microspheres.

of the solid reagents. The acquired magnetite microspheres were shown in the TEM image of Figure 1A. The magnetite had a spherical shape with a rough surface and relative uniform size of ∼180 nm. With the clear observation of the magnetite microspheres through the high-resolution TEM (HRTEM) as shown in inset of Figure 1A, the microsphere was an aggregate of small magnetite particles with the size range of 510 nm (the red arrow in Figure 1A). The XRD pattern of the product was shown in Figure 2. Six characteristic peaks for magnetite marked by their indices (220) (311) (400) (422) (511) (440), matched well with the standard JCPDS data (74-748). No obvious XRD peaks arising from impurities were found, meaning that magnetite microspheres were successfully synthesized with high purity. 3.1. Preparation of Fe3O4@P(MBAAm) CoreShell Microspheres. In our previous work, the hydroxyl groups on silica 15877


The Journal of Physical Chemistry C microspheres or hematite ellipsoids played an important role for coating PMBAAm shells onto these particles through distillationprecipitation polymerization to form the coreshell particles.41,42 The synergetic hydrogen bond interaction between the hydroxyl groups of silica or hematite particles and the amide group of MBAAm component was strong enough for silica or hematite to capture the oligomer during the polymerization process to prepare silica@PMBAAm coreshell microspheres or hematite@PMBAAm coreshell ellipsoids. That was to say, the silica or hematite particles could be directly coated with PMBAAm shells without any surface modification by distillationprecipitation polymerization. In this work, because the surface of the magnetite was abundant with hydroxyl groups and had similar chemical composition with hematite, the distillationprecipitation polymerization was performed to afford magnetite@PMBAAm core shell microspheres in the presence of the magnetite microspheres as seeds without any stabilizer and surfactant. The previous results indicated that acetonitrile met the solvency conditions required for the formation of monodisperse PMBAAm microspheres with regular shape by distillationprecipitation polymerization; that is, it dissolved the MBAAm monomer but precipitated the forming PMBAAm.43 The essential role of the hydrogen bond for the formation of monodisperse polymer microspheres was confirmed by our previous work in the case of polymerization of the hydrophilic monomers involving the hydrogen bond interaction, such as MBAAm, N-isopropylacrylamide (NIPAM),43 and acrylic acid.44 The corresponding TEM image of Fe3O4@PMBAAm core shell microspheres is shown in Figure 1B. The well-defined coreshell structure was distinguished by a deep contrast core and a light contrast shell due to the different mass contrast between the core and shell layer. The resultant Fe3O4@PMBAAm microsphere had a cauliflower-like morphology and a rougher surface compared with the magnetite template, which may be originated from the intrinsic character and high reactivity of the MBAAm monomer. This result was consistent with our previous works for preparing PMBAAm microspheres43 and silica@PMBAAm microspheres.41 There were not any secondary particles in the TEM image, suggesting that the hydrogen bond interaction between the hydroxyl groups on the surface of magnetite microspheres and the amide group of MBAAm component was efficient for capturing the newly formed oligomers during the process of the polymerization to form coreshell microspheres. The size of the Fe3O4@PMBAAm microspheres from the statistic of the particles in TEM image was ∼248 nm, and the shell thickness calculated from the difference of the radii of the Fe3O4@ PMBAAm microspheres and the radii of the magnetite was ∼34 nm. The FT-IR spectrum of the Fe3O4@PMBAAm microspheres is shown in Figure 3a. The strong absorption peak at 1664 cm1 was assigned to the stretching vibration of the carbonyl group and the wide absorption peak ranging from 3500 to 3100 cm1 centered at 3300 cm1 was attributed to the stretching vibration of NH group. This result further proved that the PMBAAm shell was successfully coated onto the magnetite microspheres through distillation precipitation polymerization by hydrogen bond interaction. 3.2. Preparation of Fe3O4@PEGDMA CoreShell Microspheres. EGDMA was a hydrophobic monomer that contained two ester groups and two vinyl groups and was widely used as a cross-linker in the polymer chemistry, especially in the polymer colloid chemistry. In our previous work, it is difficult to implement directly the polymerization of sole EGDMA monomer with the unmodified hematite as seeds for preparation of coreshell composites containing hematite and polymer due to the hydrophobic

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Figure 3. FT-IR spectra of (a) Fe3O4 microspheres, (b) Fe3O4@PMBAAm coreshell microspheres, (c) Fe3O4@PEGDMA coreshell microspheres, (d) Fe3O4@P(MBAAm-co-MAA) coreshell microspheres, and (e) Fe3O4@P(MBAAm-co-HEMA) coreshell microspheres.

nature of EGDMA component lacking the efficient interaction with the hydrophilic hematite particles.42 To solve this problem for performing polymerization on the surface of hematite ellipsoids to form coreshell structure, surface modification of the hematite with vinyl group was necessary.41 However, in the present work, this hydrophobic monomer could be used to perform polymerization on the surface of hydrophilic magnetite without any surface modification. As shown in Figure 1C, the TEM image of magnetite@PEGDMA revealed a well-defined coreshell structure with a black core and a gray shell of ∼11 nm without the appearance of any secondary polymer particles. Furthermore, the surface of magnetite@PEGDMA was smoother compared with that of the initial magnetite microspheres. These results suggested that the PEGDMA was successfully encapsulated onto the magnetite microspheres. In other words, the magnetite@PEGDMA coreshell microspheres were successfully prepared through distillationprecipitation polymerization using the magnetite microspheres without any surface modification. The surface encapsulation of the PEGDMA component leading to magnetite@PEGDMA coreshell composite particles was further proven by FT-IR spectrum, as shown in Figure 3c, which had a strong absorption peak at 1734 cm1 contributing to the vibration of the carbonyl groups in ester. It meant that PEGDMA shell was successfully coated onto the magnetite core during distillationprecipitation polymerization. On the basis of the difference between the results from this work and our previous work,42 the intrinsic surfaces of hematite ellipsoids obtained through homogeneous hydrolysis from a solution of iron salt (FeCl3) and phosphate anions (NaH2PO4) were abundant with hydroxyl groups, but the hydrogen bond interaction between hydroxyl groups of hematite and ester groups of EGDMA monomer or oligomer was not strong enough to capture the newly formed polymer onto the hematite surface. However, in the present work, because of the introduction of Na3Cit to the magnetite synthetic system, it made the magnetite microspheres maintain some carboxyl groups on their surface. The synergetic hydrogen bond interaction between carboxyl groups and ester groups as well as hydroxyl groups and ester groups was 15878



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Scheme 1. Process during Synthesis of Fe3O4@P(EGDMA) Core-Shell Microspheres

strong enough to make the magnetite capture EGDMA monomer or oligomer onto its surface. In other words, hydrogen bond interaction played a key role as the driving force during the encapsulation of PEGDMA on magnetite cores with initial incorporation of the reactive vinyl groups or radical. The process during the synthesis of Fe3O4@P(EGDMA) coreshell microspheres was presented in Scheme 1. In case I, the adsorbed vinyl groups on the surface of magnetite microspheres via the hydrogen bond interaction between the ester groups of EGDMA monomer and the carboxyl groups of Na3Cit as well as hydroxyl groups of magnetite microspheres captured the newly formed oligomer radicals for the encapsulation of PEGDMA for the growth of magnetite/ PEGDMA composite particles. In case II, the adsorbed radicals via the hydrogen bond interaction, which was formed by the reaction between the initiator and EGDMA monomers in the solution, reacted with the EGDMA monomers from the solution to enlarge the PEGDMA shell layer. The hydrogen bond capture mechanism for the growth of PEGDMA microspheres has been investigated in detail as our previous work for preparation of PMAA microspheres.44 To understand well the mechanism for formation of magnetite@PEGDMA microspheres during the process of distillation precipitation polymerization, we synthesized monodispersed silica microspheres with the size of ∼200 nm (Figure S1 of the Supporting Information), as seeds to perform polymerization of EGDMA monomer on their surface. In such a case, only the secondary-initiated PEGDMA domains around the silica seeds were formed in the absence of any well-defined coreshell structures, as shown by TEM micrograph in Figure S2 of the Supporting Information. This meant that the hydrogen bond interaction between the hydroxyl groups on the surface of silica and the EGDMA monomers or oligomers radical was too weak to capture the newly formed PEGDMA as a typical coreshell structure. This result was similar to the polymerization using the unmodified hematite ellipsoids as seeds.42 However, after the surface modification of the silica particles with amine group through condensation of APS, the silica can be well-coated with a PEGDMA shell via the hydrogen bond interaction between the amino group of APS-modified silica and the ester group of EGDMA (Figure S3 of the Supporting Information). This result was very similar to the preparation of Fe3O4@PEGDMA microspheres in the particle seeds (APS-modified silica or unmodified

magnetite) as a hydrogen donor and the EGDMA monomer as a hydrogen acceptor for hydrogen bond interaction between them. Furthermore, to prove that the hydrogen bond interaction was necessary for the preparation of coreshell microspheres, polymerization of DVB was performed in the presence of magnetite microspheres as seeds. The resultant TEM image was shown in Figure 1D. Some of the magnetite microspheres were bare without polydivinylbenzene (PDVB) on the surface, and some of the magnetite microspheres were partially connected to the polymer, whereas the other part of the surface was uncovered (the red arrow in Figure 1D). Furthermore, the polymerization of DVB was performed to form the secondary particles (the blue arrow in Figure 1D). The PDVB could not be efficiently coated onto the magnetite microspheres because of the lack of the interaction between the PDVB and magnetite microspheres. 3.3. Preparation of Magnetite/Polymer CoreShell Composites with Various Polarity and Functional Groups on the Polymer Shell Layer through H-Bond Interaction. An important concern for the present work is to afford the particle functionality and monodispersity in the absence of secondaryinitiated particles through the second-stage distillation precipitation polymerization without any surface modification of the magnetite template. The polarity of the polymer microspheres is an important factor for many applications. Furthermore, microspheres with various functional groups have wide utilization in many fields, such as solid carriers for the immobilization of biological substances including enzymes, antibodies, and so on.45 To confirm the present synthetic route as a general strategy, we prepared the other coreshell magnetite/polymer composites with different polarities and functional groups on the outer polymer shell layer by distillation polymerization in the presence of magnetite microspheres as seeds without any surface modification. 3.3.1. Preparation of Fe3O4@P(MBAAm-co-MAA) CoreShell Microspheres. Fe3O4@P(MBAAm-co-MAA) coreshell composite was prepared by distillationprecipitation polymerization of MBAAm as cross-linker and MAA as functional monomer in neat acetonitrile with AIBN as initiator without any surfactant and stabilizer. The magnetite particles with active groups were used as seeds for the growth of the shell layer. The TEM images of the resultant Fe3O4@P(MBAAm-co-MAA) coreshell composite materials with different shell thickness were shown in Figure 4, which were afforded by controlling the amount of 15879



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Figure 4. TEM images of Fe3O4@P(MBAAm-co-MAA) coreshell microspheres with different shell thickness.

Table 1. Reaction Conditions, Sizes, Size Distributions, Polymer Shell Thicknesses, and Organic Contents of Fe3O4 Microspheres and Fe3O4@P(MBAAm-co-MAA) Core-Shell Microspheres with Different Shell Thicknesses MBAAm (g)

MAA (mL)

Dn (nm)

Dw (nm)

U

entry

Fe3O4 (g)

shell thickness (nm)

weight loss by TGA (%)

A

0.05

0

0

180

185

1.03

0

10.2

B

0.05

0.02

0.08

186

192

1.04

3

24.6

C

0.05

0.025

0.1

190

198

1.04

5

25.5

D

0.05

0.03

0.12

211

216

1.02

15.5

39.8

E

0.05

0.035

0.14

216

227

1.05

18

40.2

F

0.05

0.04

0.16

229

232

1.01

24.5

41.8

monomers’ (MBAAm and MAA) feed during the second-stage polymerization. As shown in the TEM graphs of Figure 4, P(MBAAm-coMAA) was uniformly coated onto the magnetite core to form a well-defined coreshell structure due to the dark contrast core and light contrast shell resulting from the different mass contrast between the core and shell domain. The coreshell composites had spherical shape with smooth surface without the appearance of any secondary particles. The P(MBAAm-co-MAA) shell on the Fe3O4 microspheres was further characterized by the FT-IR spectrum, as shown in Figure 3d. The absorption peak at 1663 cm1 was assigned to the vibration of the carbonyl groups of the amide groups in the MBAAm component. The strong absorption peak at 1709 cm1 was due to the vibration of the carbonyl groups of the carboxyl groups in the MAA component. The wide absorption peak between 3400 and 2500 cm1 centered at 3100 cm1 contributed to OH groups in the carboxyl groups. These results demonstrated that the P(MBAAm-coMAA) was coated onto the magnetite microspheres. A series of experiments was carried out for the synthesis of magnetite/P(MBAAm-co-MAA) core/shell composite microspheres with different shell thicknesses by varying the ratios of MBAAm together with MAA comonomers to magnetite seeds, whereas the amount of solvent, magnetite core, the degree of cross-linker, the ratio of whole weight of MBAAm and MAA comonomer to initiator AIBN as well as the reaction time were

maintained constant for the polymerizations. The TEM micrographs of the magnetite/P(MBAAm-co-MAA) coreshell microspheres with different polymer shell thicknesses by varying the mass ratios between monomers and magnetite seed from 2 to 4 are illustrated in Figure 4AE. The size, size distribution, and the shell thickness of the resultant magnetite/P(MBAAm-coMAA) composite materials from the TEM images with different experimental conditions for the distillationprecipitation polymerization of MBAAm and MAA with magnetite particles as seeds were summarized in Table 1. The size of the coreshell composite materials increased significantly with increasing comonomer loading in the polymerization system. The maximum diameter of 229 nm was obtained when the ratio of MBAAm and MAA comonomers to Fe3O4 (in mass) was 4/1. This meant that the thickness of the shell layer was in the range of 324.5 nm with the monomer feed ranging from 2/1 to 4/1 (in mass ratio to magnetite particles). The monodisperse coreshell composites were obtained with the polydispersity index (U) in the range of 1.01 to 1.05. It meant that the Fe3O4@P(MBAAm-co-MAA) coreshell microspheres retained the monodispersity of the magnetite template (1.03) after the polymerization. The mass content of the shell layer on the magnetite surface was studied by TGA, as shown in Figure 5. All magnetite/ P(MBAAm-co-MAA) microspheres exhibited the similar weight loss trends. In general, there were two weight loss stages when the sample was heated from room temperature to 800 °C. 15880


The Journal of Physical Chemistry C The first weight loss until 200 °C was due to the evaporation of the physical adsorbed water or solvent, and the second major weight loss from 300 to 500 °C was due to the decomposition of the polymer component in the shell layer of the magnetite/ P(MBAAm-co-MAA) microspheres. After the temperature was elevated to 800 °C, the whole weight loss for these microspheres was increased with the increase in the feed of monomers to the magnetite seeds. The resultant weight loss of these coreshell microspheres during the calcination process is summarized in Table 1. The weight loss of these microspheres during the calcination ranged from 24.6 to 41.8%. Because the weight loss of the initial magnetite microspheres during the calcination process was 10.2% because of the trisodium citrate as an electrostatic stabilizer, the polymer contents in these coreshell microspheres were roughly calculated to be 14.4, 15.3, 28.6, 30, and 31.6% when the ratio of the monomer to the magnetite ranged from 2/1 to 4/1. This result was consistent with the increase in the shell thickness from TEM images. In other words, P(MBAAm-co-MAA) was

Figure 5. TGA curves of (a) Fe3O4 microspheres and (bf) Fe3O4@ P(MBAAm-co-MAA) coreshell microspheres with different shell thickness.

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quantitatively encapsulated on the surface of magnetite core with the aid of hydrogen bond interaction between the hydroxyl group and carboxyl group on the surface of magnetite core and the amide group of MBAAm and carboxyl group of MAA component. 3.3.2. Preparation of Fe3O4@P(EGDMA-co-MAA) CoreShell Microspheres. In this work, the hydrophobic monomer EGDMA (cross-linker) and hydrophilic monomer MAA were copolymerized to afford Fe3O4@P(EGDMA-co-MAA) coreshell microspheres. The resultant product was is in Figure 6A. The Fe3O4@ P(EGDMA-co-MAA) microspheres had similar coreshell structure to those of the above-mentioned Fe3O4@P(MBAAm) coreshell microspheres, Fe3O4@P(EGDMA) coreshell microspheres, and Fe3O4@P(MBAAm-co-MAA) coreshell microspheres. There was a little adhesion between the Fe3O4@ P(EGDMA-co-MAA) microspheres, which may be due to the strong hydrogen bond interaction, as indicated in our previous work.46 3.3.3. Preparation of Fe3O4@P(MBAAm-co-HEMA) Core Shell Microspheres. The Fe3O4@P(MBAAm-co-HEMA) core shell microspheres were further prepared to afford the hydroxyl groups onto the surface of the second polymer layer. The TEM image of the sample was presented in Figure 4. The Fe3O4@ P(MBAAm-co-HEMA) microspheres had a dark contrast core and light contrast shell with a smooth surface. The size of the Fe3O4@P(MBAAm-co-HEMA) microspheres was 212 nm. The shell thickness was calculated to be 16 nm. The polymer component was studied by FT-IR, as shown in Figure 2e. The absorption peaks at 1728 and 1159 cm1 were due to the vibration of the carbonyl groups and CO groups of ester groups in the HEMA component. The absorption peaks at 1667 and 1392 cm1 contributed to the carbonyl groups and CN groups of amide groups in the MBA component. These results suggested that the polymer component was surly P(MBAAm-co-HEMA) and meant that the hydroxyl groups were be encapsulated onto the surface of the magnetite through the polymerization. 3.4. Loading of Metallic Nanoparticles Using Fe3O4@ P(MBAAm-co-MAA) Microspheres as Support. Because the magnetite microspheres could be uniformly coated with polymer, the Fe3O4@P(MBAAm-co-MAA) microspheres with functional carboxyl groups on the shell layer were used as a support or

Figure 6. TEM images of (A) Fe3O4@P(EGDMA-co-MAA) coreshell microspheres and (B) Fe3O4@P(MBAAm-co-HEMA) coreshell microspheres. 15881



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Scheme 2. Process for Fe3O4@P(MBAAm-co-MAA) Microspheres to Load Metallic Nanoparticles

Figure 7. TEM images of (A,D) Fe3O4@P(MBAAm-co-MAA)/Ag microspheres, (B,E) Fe3O4@P((MBAAm-co-MAA)/Pd microspheres, and (C,F) Fe3O4@P((MBAAm-co-MAA)/Au microspheres.

carrier to load a series of metallic nanoparticles. The process for loading of metallic nanoparticles in the polymer shell of the Fe3O4@P(MBAAm-co-MAA) microspheres is illustrated in Scheme 2. Because the carboxyl groups were a weak acid, it can be ionized in their aqueous solution with the aid of the aqueous alkali. The ionized magnetic microspheres can exchange their sodium cations with noble metallic ions because these noble metallic ions have strong complex interaction with the COO negative ions on the microspheres and the stronger Coulombic force between the noble metallic ions and COO negative ions.47 The noble metallic ions loaded in the polymer shell of the magnetic coreshell microspheres were reduced to the metallic nanoparticles in situ. The corresponding TEM images of Fe3O4@P(MBAAm-coMAA) microspheres loaded with Ag and Pd nanoparticles are presented in the Figure 9. The Fe3O4@P(MBAAm-co-MAA)/Ag and Fe3O4@P(MBAAm-co-MAA)/Pd microspheres were maintained the original shape of the Fe3O4@P(MBAAm-co-MAA) microspheres. The metallic nanoparticles were placed in the shell layer of the Fe3O4@P(MBAAm-co-MAA) microspheres by observing the gray shell layer and some darker dots homogeneously deposited in the polymer matrix. The in situ reduction of inorganic

precursor salts was a facile approach for the preparation of the magnetic/polymer microspheres-loaded metallic colloids, during which the presynthesis and immobilization of the metallic nanoparticles on the support were no longer necessary. Importantly, the size of these nanoparticles was very uniform and dispersed in the polymer without aggregation. The XRD patterns are shown in Figure 8. Compared with the original XRD curves of the magnetite, Fe3O4@P(MBA-MAA)@Ag revealed the new peaks at 38.2, 44.4, 64.6, 77.6, and 81.8° corresponding to the (111), (200), (220), (311), and (222) crystal planes of cubic silver and matched well with standard PDF card 87-720; however, the Fe3O4@P(MBA-MAA)@Pd microspheres presented the new peaks of 40.1, 46.7, 68.2, 82.2, and 86.7° corresponding to the (111), (200), (220), (311), and (222) crystal planes of cubic Palladium, which matched well with standard PDF card 65-6174. There were not any other peaks in the XRD curves except those that respond to magnetite and the corresponding metallic nanoparticles, which revealed the high purity of the metallic loaded magnetic particles. The COOH groups were coordinated with the gold particles in a medium strength. The efficient preparation of the P(EGDMAco-MAA)/Au composite with the reduction of the HAuCl4 in the 15882



Figure 8. XRD spectra of (a) Fe3O4@P(MBAAm-co-MAA)/Ag microspheres, (b) Fe3O4@P((MBAAm-co-MAA)/Pd microspheres, and (c) Fe3O4@P((MBAAm-co-MAA)/Au microspheres. (* represents the diffraction peaks of magnetite.)

presence of the P(EGDMA-co-MAA) microspheres as the scaffold and the NaBH4 as the reductant was due to the coordination interaction between the Au and carboxyl groups of the P(EGDMA-coMAA) microspheres.40 As the intrinsic carboxyl groups in the polymer shell of our magnetite@P(MBAAm-co-MAA) microspheres, these microspheres may be used as a platform for the gold nanoparticles. In this Article, the Au nanoparticles were loaded onto the surface of the magnetite@P(MBAAm-co-MAA) microspheres through in situ reduction of the HAuCl4 aqueous solution. The corresponding magnetite@P(MBAAm-co-MAA)/ Au hybrid microspheres are shown in Figure 7. The Au nanoparticles with irregular shape and nonuniform size may be due to the relatively weak interaction between the Au nanoparticles and the carboxyl groups. The XRD spectrum shown in Figure 8c presents the new peaks at 38.1, 44.4, 64.6, and 77.6° assigned to (111), (200), (220), and (311) crystal planes of cubic gold matching well with the standard PDF card 65-2870. In the literature,40,48,49 the loading of metal nanoparticles was usually aimed at one special particle. In this Article, we afforded a general and facile way to load a series of metal nanoparticles. Moreover, as for Ag and Pd nanoparticles, the loading amount was very high, and these nanoparticles were uniformly dispersed in the polymer matrix. 3.5. Magnetic Properties of Corresponding Microspheres. The magnetic properties of magnetite, Fe3O4@P(MBAAm-coMAA) and Fe3O4@PMBAAm-co-MAA)/Ag hybrid materials were studied by a vibrating sample magnetometer (VSM) at room temperature. Figure 9 shows the magnetization curves of magnetite microspheres, Fe3O4@P(MBAAm-co-MAA) core shell microspheres with polymer shell thickness of 24.5 nm, and the corresponding Fe3O4@PMBAAm-co-MAA)/Ag hybrid microspheres. No obvious magnetic hysteresis loops (Hc < ∼20 Oe) were observed for all samples from the field-dependent magnetization plots in Figure 9. In other words, the remanence did not exist when the magnetic field was removed, indicating that all resultant microspheres showed a superparamagnetic feature originating from the magnetite cores at room temperature. The magnetic properties of the three microspheres are listed in Table 2. The

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Figure 9. Hysterisis loops of samples at room temperature: (a) Fe3O4 microspheres, (b) Fe3O4@P(MBAAm-co-MAA) microspheres, and (c) Fe3O4@P(MBAAm-co-MAA)/Ag microspheres. The inset was the magnified scale of the plot.

Table 2. Magnetization of Fe3O4, Fe3O4@P(MBAAm-coMAA), and Fe3O4@P(MBAAm-co-MAA)/Ag Microspheres saturation entry

coercive

magnetization (emug1) force (Oe)

Fe3O4

53.0

18.6

Fe3O4@P(MBAAm-co-MAA)a Fe3O4@P(MBAAm-co-MAA)/Agb

32.6 22.3

13.6 20.8

a Fe3O4@P(MBAAm-co-MAA) microspheres with the shell thickness of 24.5 nm (entry F in Table 1). b Hybrid microspheres using Fe3O4@P(MBAAm-co-MAA) microspheres with the shell thickness of 24.5 nm (entry F in Table 1) as support.

saturation magnetization (Ms) values for Fe3O4 microspheres, Fe3O4@P(MBAAm-co-MAA) coreshell microspheres, Fe3O4@ P(MBAAm-co-MAA)/Ag hybrid microspheres were 53.02, 32.55, and 22.28 emu/g, as summarized in Table 2, respectively. These results indicated that the magnetization of these magnetic particles decreased considerably with the addition of a polymer layer or inorganic silver nanoparticles because of the decrease in the effective mass of magnetite core in these cases. However, the magnetism of these coreshell microspheres was still strong enough to be separated and controlled by an external magnetic field.

4. CONCLUSIONS In this Article, a facile and general method was afforded to prepare magnetite/polymer coreshell microspheres. The hydrogen bond interaction between the magnetite and a series of functional monomers was strong enough for coating polymer shells onto the magnetite microspheres. Various functional shells such as carboxyl, hydroxyl, amide, and ester were provided via the tune of the monomer during the polymerization. The P(MBAAm-co-MAA) shell-coated magnetic microspheres were loaded with a series of metal nanoparticles like Au, Ag, and Pd. Because of their high saturation magnetization values and easy preparation, these microspheres may be promising materials for catalysis. Further work is in progress. 15883



’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed experimental procedures of the synthesis and TEM micrographs for SiO2, SiO2/ PEGDMA, and APS-modified SiO2/PEGDMA. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +86-22-23502023; Fax: +86-22-23503510. E-mail: [email protected].

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Facile Method for Synthesis of Fe3O4@Polymer Microspheres and

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