Multilayer Magnetic Composite Particles with Functional Polymer

Mar 4, 2013 - Charlie Batarseh , Ester Weiss , Raed Abu-Reziq ... Patrick Carney , Anna Laromaine , Emma-Rose Janeček , Anna Roig , Francesco Stellac...
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Multilayer Magnetic Composite Particles with Functional Polymer Brushes as Stabilizers for Gold Nanocolloids and Their Recyclable Catalysis Bin Liu, Dongwei Zhang, Jianchao Wang, Cheng Chen, Xinlin Yang,* and Chenxi Li Key Laboratory of Functional Polymer Materials, the Ministry of Education, Institute of Polymer Chemistry, Nankai University, Tianjin 300071, P. R. China S Supporting Information *

ABSTRACT: The functional poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) brush was grafted onto the alkyl-bromide-modified magnetite/ silica/poly(N,N′-methylenebisacrylamide-co-2-hydroxyethylmethacrylate) (Fe3O4@SiO2@PHEMA) trilayer microspheres via the surface-induced atom transfer radical polymerization. Fe3O4@SiO2@PHEMA trilayer microspheres with surface alkyl bromide groups were prepared by the combination of sol−gel process for the synthesis of Fe3O4@SiO2 core−shell inorganic component and distillation−precipitation polymerization for the formation of PHEMA shell together with the subsequent esterification of the surface hydroxyl group and 2bromoisobutyryl bromide. Furthermore, the gold nanoparticles were facilely loaded into the functional PDMAEMA brushes through the in situ reduction due to their strong coordinate interaction. These PDMAEMA brush-stabilized gold nanocolloids were used as a catalyst with the reduction of 4-nitophenol to 4-aminophenol as a model reaction, which revealed a highly catalytic efficiency and good reusable property.

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in catalysis,14−16 photonic crystal,17,18 biomedicine,19,20 and so on. Surface grafting with a polymer brush was a possible approach for functionalization of the nanomaterials.21 The surface-grafted functional brush not only efficiently improves the solubility of the magnetic particles in different solvents but also endows the particles with a series of functionalities for various applications. In particular, the polymer brush on the functional particles has attracted much attention due to its unique property of the magnetic core together with the tunable functionalities of the polymer brushes. Thus, the magnetic nanoparticles grafting with functional brushes have found applications in a wide range of fields such as a recyclable material for antibacterial,22 selective enrichment of N-linked glycopeptides,23 immobilization of metal nanoparticles,24 controlled drug release,25 and so on. The common methods for grafting polymer brush onto the functional core need the complex surface modification. There are mainly three steps to prepare magnetic nanoparticles grafted with polymer brushes: first, the synthesis of the magnetic nanoparticles through different methods; second, the surface modification of the magnetic nanopartilces to incorporate the reactive or initiative loci; finally, the surface-induced (Supporting Information) polymerization performed by a series of techniques.21 It is an efficient solution for the surface modification of colloids and nanoparticles via the combined polymerization

INTRODUCTION Magnetic particles have attracted much attention in recent years due to their novel physicochemical properties such as the response to the external magnetic field, the production of heat under the alternate magnetic field, the reduction in the relaxation time of the surrounding small molecules such as water, and so on.1 Thus, the magnetic particles exhibited extensive applications for separation,2 catalysis,3,4 MRI contrast agent,5,6 hyperthermia-based therapy,7 drug delivery,8 and so on. However, the traditional magnetic nanoparticles were prone to aggregation and being corroded by harsh environment. Foremost, the magnetic particles always lack an efficient functional group for further modification and application. Thus, it is highly desirable to further tailor the surface functionality of these magnetic nanoparticles, for example, via grafting of hydrophilic and biocompatible components on the surface for the practical application.9,10 It is an efficient method to endow the magnetic nanoparticles with multifunctionality together with the protection of the magnetic particles10−12 via grafting of an outer layer from a reactive functionalization locus for construction of a core−shell structure. Moreover, the core and shell domains may be composed of a variety of different materials including polymers, inorganic solids, metals, and so on.13 In this way, core−shell particles can combine different properties 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, core−shell particles have attracted much attention due to their wide potential application © XXXX American Chemical Society

Received: November 20, 2012 Revised: March 3, 2013

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Reagent) was distilled in the presence of calcium hydride. Toluene was purchased from Tianjin Chemical Reagent II and purified by distillation in the presence of sodium. 2Dimethylaminoethyl methacrylate (DMAEMA) was purchased from Aldrich and purified through the pass of alkaline alumina column. 2-Bromoisobutyryl bromide was purchased from Aldrich and used as received. All other reagents were of analytical grade and used without any further treatment. Synthesis of Fe3O4 Microspheres. The process for the synthesis of Fe3O4 particles was according to our previous work.35 The details were as follows: 3.6 g of FeCl3·6H2O and 0.72 g of trisodium citrate were dissolved in ethylene glycol/ ethanol (90 mL/10 mL) solution through ultrasonic 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 reaction at 200 °C for 10 h. After that, the autoclave was carefully taken out to cool until room temperature. The asmade black products were thoroughly washed with ethanol and deionized water three times, respectively, and finally vacuumdried. Synthesis of Fe3O4@SiO2 Core−Shell Microspheres. Fe3O4@SiO2 microspheres were prepared through the coating of the silica layer onto the magnetite microspheres by a sol−gel method. The details were as follows: 0.1 g of Fe3O4 seeds was suspended in the mixture of 80 mL of ethanol, 20 mL of deionized water, and 2.5 mL of 25 wt % ammonium hydroxide aqueous solution. Then, 0.3 mL of TEOS was added under the mechanical stirring together with the ultrasonic irradiation at room temperature for 4.5 h. The other reactions for coating the magnetite with different silica shell thicknesses were performed by the addition of different TEOS loadings ranging from 0.1 to 0.5 mL. The resultant Fe3O4@SiO2 microspheres were purified by three cycles of centrifugation, decantation, and resuspension in ethanol with ultrasonic irradiation. The products were dried in a vacuum oven until constant weight. Synthesis of Fe3O4@SiO2@PHEMA Trilayer Microspheres. A typical procedure for the synthesis of Fe3O4@ SiO2@PHEMA trilayer microspheres by DPP was as follows: 0.05 g of Fe3O4@SiO2 core−shell inorganic seeds was suspended in 40 mL of acetonitrile. Then, MBA (0.04 g), HEMA (0.16 mL), and AIBN (0.004 g, 2 wt % relative to all monomers) were dissolved in the above suspension in a 50 mL flask. The flask attached to a fractionating column, Liebig condenser, and receiver was then submerged in a heating mantle. The reaction mixture was heated from ambient temperature until the boiling state within 15 min, and the reaction mixture was kept under refluxing state for a 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@SiO2@PHEMA microspheres were purified by repeated centrifugation, decantation, and resuspension in ethanol three times. The products were dried in a vacuum oven until constant weight. Synthesis of Fe3O4@SiO2@PHEMA-Br Microspheres. 0.15 g of Fe3O4@SiO2@PHEMA microspheres was dispersed in a mixture of 15 mL of dry toluene and 1.5 mL of TEA, and the dispersion was stirred under an ice bath for ∼30 min. Then, a solution of 0.3 mL of 2-bromoisobutyryl bromide in 10 mL of toluene was added dropwise by constant pressure drop funnel, and the mixture was stirred for 2 h at 0 °C and another 20 h at

techniques. For example, tandem reversible addition−fragmentation chain transfer (RAFT) polymerization and click chemistry have been used as an efficient approach for surface modification.26 Furthermore, it is a promising approach to combine the organic and inorganic synthetic techniques for the preparation of various complex and novel hierarchical micro/ nanostructure with desired functionality. For instance, Neoh et al. combined ring-opening polymerization (ROP), reversible RAFT polymerization, and thiol-yne “click” reaction to synthesize the amphiphilic polymer, which was further assembled as a novel micelle with inorganic superparamagnetic iron oxide nanoparticles (SPIONs) as the core and the hydrophilic shell with dicarboxylic groups for the coordination of cisplatin as a controlled anticancer drug delivery system.27 Therefore, it is possible to prepare various composite materials with novel architectures and unique properties via the combination of the inorganic process like sol−gel method or electrostatic assembly and some polymerization techniques, such as emulsion polymerization, precipitation polymerization, together with some controlled polymerization process (ATRP, RAFT, and ROP, etc). Distillation−precipitation polymerization (DPP) is a novel and powerful technique for the synthesis of monodisperse functional polymer microspheres and inorganic/polymer core− shell microspheres.28−34 The combined DPP with some organic (ATRP, thiol−ene chemistry, and alkyne−azide click reaction) and inorganic (sol−gel process, electrostatic interaction) synthetic techniques may provide a new opportunity for designing and fabricating novel inorganic/organic hybrid nanostructures. The combined inorganic methods (solvothermal method and sol−gel process) and different polymerization techniques (DPP and surface-induced atom transfer radical polymerization (SI-ATRP)) were used to prepare a multilayer magnetic/polymer microsphere with a magnetic core, a protective silica shell, and an outer functional polymer brush. As a result, the functional PDMAEMA brush was grafted by an SI-ATRP from the alkyl bromide loci of the modified Fe3O4@ SiO2@PHEMA trilayer microspheres (Fe3O4@SiO2@PHEMABr), which were prepared by the combination of sol−gel process for the synthesis of Fe3O4@SiO2 core−shell inorganic component and DPP for the formation of PHEMA shell together with the subsequent esterification of the surface hydroxyl group and 2-bromoisobutyryl bromide. Furthermore, the gold nanoparticles were facilely loaded into the functional PDMAEMA brushes, and the highly catalytic activity and good reusable property of the stabilized gold nanocolloids were primarily investigated.

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EXPERIMENTAL SECTION Materials. Ferric chloride (FeCl3·6H2O) was purchased from Tianjin Guangfu Chemical Engineering Institute, and trisodium citrate was obtained from Tianjin Chemical Reagents I. Tetraethyl orthosilicate (Si(OEt)4, TEOS)) was purchased from Aldrich. All of these reagents were used as received. N,N′Methylenebisacrylamide (MBA, chemical grade, Tianjin Bodi Chemical Engineering) was recrystallized from acetone. 2Hydroxyethylmethacrylate (HEMA) was purchased from Tianjin Chemical Reagent II and purified by vacuum distillation. 2,2′-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. Triethylamine (TEA) (Beifang Chemical B

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Scheme 1. Preparation of the Fe3O4@SiO2@PHEMA-g-PDMAEMA Microspheres with the Grafted PDMAEMA Brush

stabilized gold nanocolloid catalyst four-fold compared that for the above typical reduction to increase the reaction rate. After complete reduction of 4-NP, the catalysts were separated by centrifugation (1.2 × 104 rpm for 10 min) and redispersed in a new reaction system. The recovery of the catalyst was further proven by recycling it for five times. Characterization. The morphology of the resultant nanoparticles was determined by transmission electron microscopy (TEM) using a Tecnai 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 distributions reflect the averages about 100 particles each, which are calculated according to the following formulae

room temperature. After the esterification, the resultant products were washed by ethanol five times and dried in a vacuum oven until constant weight. Synthesis of Fe3O4@SiO2@PHEMA-g-PDMAEMA Microspheres with Grafted PDMAEMA Brush. We dispersed 0.13 g Fe3O4@SiO2@PHEMA-Br microspheres, 0.032 g bpy, and 0.8 mL of DMAEMA in H2O/acetone (0.5 mL/1 mL) mixed solvent in a 25 mL dry Schlenk flask. The flask was immersed in the liquid nitrogen for degassing one time. Then, 0.01 g of CuBr was added with further degassing for further three freeze−pump−thaw cycles. The grafting SI-ATRP was performed at 40 °C for 3 h. Then, the product was collected by centrifugation and washed with ethanol five times via repeated resuspension and centrifugation. Finally, the product was dried under the vacuum oven until constant weight. Fe3O4@SiO2@PHEMA-g-PDMAEMA Microspheres as Stabilizer for Loading the Gold Nanoparticles. The details of loading the gold nanoparticles were as following: 0.05 g of Fe3O4@SiO2@PHEMA-g-PDMAEMA multilayer microspheres were dispersed in 10 mL of 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 reaction lasted for 1 h. The final product was purified through washing with water three times and dried under vacuum oven until constant weight. Catalytic Reduction of 4-Nitrophenol to 4-Aminophenol in an Aqueous Medium. A typical experiment for the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AnP) was carried out as follows: 0.1 mL of 4-NP aqueous solution (5 mM, 5 × 10−7 mol), 1.0 mL of NaBH4 (0.2 M, 2 × 10−4 mol) aqueous solution, and 2.0 mL of water were mixed in a colorimetric tube. We introduced 0.05 mL of catalyst dispersion (1.0 mg/mL, 5 × 10−5 g, containing 2.3 × 10−9 mol Au) into the mixture with gentle shaking. The bright-yellow solution faded gradually as the catalytic reaction proceeded. The catalytic activity was determined by a UV−vis spectrophotometer with a decrease at 400 nm in UV−vis absorption and a simultaneous increase in the absorption at 300 nm, indicating the formation of 4-AnP. To determine the catalytic recycling properties of Fe3O4@ SiO2@PHEMA-g-PDMAEMA/Au microspheres within several minutes, we increased the amount of the PDMAEMA brush-

U = Dw /Dn k

Dn =

i=1 k

Dw =

k

∑ niDi /∑ ni i=1 k

∑ niDi4 /∑ niDi3 i=1

i=1

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 as half of the difference between the average diameter of the core−shell 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 4000−400 cm−1. 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@SiO2 microspheres, Fe3O4@SiO2@PHEMA microspheres, and Fe3O4@SiO2@PHEMA-g-PDMAEMA microspheres were studied in the dried state with a vibrating sample magnetometer (9600 VSM, BOJ Electronics, Troy, MI) at room temperature. UV−vis spectroscopy was performed on a JASCO V-570 spectrometer. Atomic emission spectrometry C

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(AES) was performed by the inductive coupled plasma (ICP) mission spectrum on an ICP-9000 (N+M) spectrometer (USA Thermo Jarrell-Ash) to determine the loading capacity of the gold nanoparticles.

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RESULTS AND DICUSSION The functional PDMAEMA brush was grafted onto the PHEMA-coated magnetic silica (Fe3O4@SiO2@PHEMA-Br) microspheres through the SI-ATRP of DMAEMA. During this process, the anchored PHEMA-Br initiator for SI-ATRP was afforded via the combination of sol−gel process and DPP in the presence of Fe3O4 microspheres as cores to synthesize Fe3O4@ SiO2@PHEMA trilayer microspheres together with the subsequent esterification of the surface hydroxyl groups with 2-bromoisobutyryl bromide with the aid of TEA. Furthermore, the gold nanoparticles were loaded into the functional PDMAEMA brush by in situ reduction of HAuCl4 with NaBH4 as reductant via the coordination of amino group to gold atom in the presence of Fe3O4@SiO2@PHEMA-gPDMAEMA microsphere as stabilizer. The whole process is illustrated in Scheme 1. Synthesis of Fe3O4@SiO2@PHEMA Trilayer Composite Microspheres. The Fe3O4 microspheres were first prepared through a solvothermal method according to the previous work.35 The corresponding TEM image is shown in Figure 1a,

Figure 2. FT-IR spectra: (a) Fe3O4@SiO2 microspheres; (b) Fe3O4@ SiO2@PHEMA trilayer microspheres; (c) Fe3O4@SiO2@PHEMA-Br trilayer microspheres; and (d) multilayer Fe3O4@SiO2@PHEMA-gPDMAEMA with the grafted PDAEMA brush.

core−shell microspheres exhibited a strong absorption peak at 1100 cm−1 attributed to the asymmetrical vibration of Si−O−Si group in the silica outer layer. On the basis of the above results, the silica was well-coated onto the magnetite microspheres to form the core−shell microspheres having different silica shell thicknesses with the aid of ultrasonication. The ultrasonic irradiation was used for coating the silica layer, which was efficient not only to reduce the reaction time (the sol−gel reaction was completed in 5 h) but also to get well-dispersed core−shell particles compared with those prepared with the mechanic stirring to result in an irregular shape, as shown in Figure S3 in the Supporting Information. In our previous work, there were two approaches for preparation of inorganic/polymer core−shell microspheres. In the first path, the as-prepared inorganic particles were modified with the vinyl group via the silica coupling reaction for capturing the radical in the reaction system to coat an outer layer to form the hybrid core−shell structure.30,36,37 In the second approach, the hydrogen-bonding interaction between the intrinsic hydroxyl groups on the surface of the inorganic particles and the functional monomer was used for the preparation of the core−shell composite microspheres.35,38,39 For example, the hydroxyl groups on silica microspheres,38 magnetite microspheres,35 or hematite ellipsoids39 played an important role for coating PMBA shells onto these particles through DPP to afford the core−shell particles. The synergetic hydrogen-bond interaction between the hydroxyl groups of silica particles and the amide group of MBA component was strong enough for silica to capture the oligomers during the polymerization process to prepare SiO2@PMBA core−shell microspheres.38 It meant that the polarity of the MBA microspheres was strong enough for coating the polymer layer. In the later cases, it was not necessary to modify the surface of inorganic templates during the further-stage DPP, affording the inorganic/polymer core−shell microspheres. However, the polarity of the monomer was an essential issue for the coating using the hydrogen-bonding interaction. Thus, the polymethacrylic acid (PMAA) layer cannot be encapsulated onto the silica particles even copolymerization with the MBA as cross-linker lower than 50%.40 The PHEMA shells with MBA as cross-linker (20%) were coated onto the Fe3O4@SiO2 core−shell particles via the in situ

Figure 1. TEM images: (a) Fe3O4 microspheres and (b) Fe3O4@SiO2 core−shell microspheres (scale bars = 200 nm).

which exhibited a spherical shape with the average diameter of 180 nm with the polydispersity of 1.012 (Figure S1a in the Supporting Information). Fe3O4@SiO2 microspheres were prepared by a sol−gel process via the controlled hydrolysis of various TEOS loadings with presence of magnetite particles as seeds to coat the silica shell-layer with different shell thicknesses under ultrasonic irradiation, which were clearly observed in the TEM micrographs, as shown in Figure 1b via the controlled hydrolysis of 0.30 mL of TEOS. The results indicated that these particles had spherical shape and smooth surface with a well-defined core−shell structure in the presence of a deep contrast core and a light contrast shell, which arose from the different mass contrasts between the magnetite core and silica shell. As shown in Figure S1 in the Supporting Information, the shell thickness was increased from 10 (Figure S2a in the Supporting Information), to 25 (Figure 1b), to 40 nm (Figure S2b in the Supporting Information) as the loadings of the TEOS for the controlled hydrolysis were increased from 0.10, to 0.30, to 0.50 mL, respectively. In other words, the silica shell thickness of the Fe3O4@SiO2 microspheres can be facilely and well-tuned in the range of 10−40 nm by increasing the amount of the TEOS from 0.10 to 0.50 mL. The encapsulation of silica on the magnetite microspheres was further investigated by FT-IR spectrum, as shown in Figure 2a. The Fe3O4@SiO2 D

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Supporting Information. Thus, the thickness of the outer PHEMA polymer shell was ∼20 nm with a uniform structure. As a comparison, the contrast experiment was performed by using the MPS-modified Fe3O4@SiO2 core−shell microspheres for the further stage DPP to get a polymer layer. However, only a very thin layer was formed after the polymerization, as shown in Figure S4 in the Supporting Information, which indicated that the coating of PHEMA shell-layer exhibited a low efficiency via a vinyl-capture DPP process. The FT-IR spectrum of the Fe3O4@SiO2@PHEMA trilayer microspheres is shown in Figure 2b. The strong absorption peak at 1727 cm−1 was assigned to the stretching vibration of the carbonyl unit of ester group of PHEMA component, and the peak at 1650 cm−1 was contributed to the carbonyl group of amide in MBA cross-linker, further confirming the successful formation of Fe3O4@SiO2@PHEMA composite microspheres. On the basis of the above results, the hydrogen-bond interaction-capture was a very efficient method to encapsulate the Fe3O4@SiO2 inorganic templates with a PHEMA shell, which was even more efficient than vinyl group-capture for such a coating process via the DPP. Thus, the functional PHEMA layer with reactive hydroxyl group can be facilely and efficiently coated onto the Fe3O4@SiO2 inorganic seeds, which made it possible to be further modified by 2-bromoisobutyryl bromide for SI-ATRP. Synthesis of Fe3O4@SiO2@PHEMA-g-PDMAEMA Microspheres-Grafted PDMAEMA Functional Brush. Because there were abundant functional hydroxyl groups on the PHEMA shell layer, these groups were facilely reacted with 2bromoisobutyryl bromide in presence of TEA to get alkyl bromide as the further ATRP initiated loci. The corresponding Fe3O4@SiO2@PHEMA-Br microspheres had similar functional groups as that of the Fe3O4@SiO2@PHEMA microspheres. Thus, the FT-IR spectra in Figure 2b,c had similar absorption peaks for both the modified and unmodified Fe3O4@SiO2@ PHEMA trilayer microspheres. Because of the incorporation of the alkyl bromide on the surface, the Fe3O4@SiO2@PHEMA-Br microspheres were used as efficient loci for further grafting polymer brush via a SIATRP technique. Here the PDMAEMA brushes were grafted from the surface alkyl bromide loci of Fe3O4@SiO2@PHEMABr microspheres by SI-ATRP of DMAEMA monomer in the presence of the CuBr/bpy catalyst to afford Fe3O4@SiO2@ PHEMA-g-PDMAEMA. The TEM image of the resultant Fe3O4@SiO2@PHEMA-g-PDMAEMA multilayer microspheres is shown in Figure 3c, which also exhibited a trilayer structure with a deepest contrast magnetite core, a gray sandwiched silica midlayer, and a lightest contrast polymer shell. Because the PDMAEMA had the similar mass contrast with PHEMA due to their composition, it was very difficult to distinguish the grafted PDMAEMA brush from the PHEMA component via the simple contrast from TEM observation (general Figure 3c). Anyhow, the hairy-like structure was clearly observed in the outer polymer shell of Fe3O4@SiO2@PHEMA-g-PDMAEMA (inserted TEM micrograph in Figure 3c with a higher magnification) compared with that of the Fe3O4@SiO2@ PHEMA core−shell microspheres (Figure 3b), which may be resulted from the loose structure of the PDMEMA brush in the absence of any cross-linker compared with the inner PHEMA component (general Figure 3c). The FT-IR spectrum of the Fe3O4@SiO2@PHEMA-g-PDMAEMA microsphere is shown in Figure 2d. The new absorption peaks at 2820 and 2770 cm−1 contributed to the C−H stretching of the-N(CH3)2 groups

DPP of MBA and HEMA through the efficient synergic hydrogen-bond interaction between the amide group of MBA as well as the ester group of the polar HEMA functional monomer and the hydroxyl group on the surface of inner core without any surface modification of inorganic templates. The morphology and structure of the Fe3O4@SiO2@PHEMA microspheres were studied by SEM and TEM, as shown in Figure 3a,b. The SEM image in Figure 3a exhibited the well-

Figure 3. SEM image: (a) Fe3O4@SiO2@PHEMA trilayer microspheres (scale bar = 1 μm). TEM image: (b) Fe3O4@SiO2@PHEMA trilayer microspheres, (c) Fe3O4@SiO2@P(HEMA-g-DMAEMA) microspheres, and (d) Fe3O4@SiO2@P(HEMA-g-DMAEMA)/Au microspheres (scale bars = 500 nm for panels c and d). The inserts of panels c and d were the magnified figures with scale bars = 100 nm.

dispersed Fe3O4@SiO2@PHEMA microspheres with spherical shape and narrow size distribution. The diameter of the Fe3O4@SiO2@PHEMA microspheres from the SEM was ∼270 nm with a polydispersity index of 1.02 (Figure S1b in the Supporting Information). The TEM micrograph of the resultant Fe3O4@SiO2@PHEMA trilayer microspheres is shown in Figure 3b, which exhibited a typical trilayer structure with a deepest contrast inner magnetite core, a lightest contrast PHEMA outer shell, and a modest contrast sandwiched silica layer. The resultant Fe3O4@SiO2@PHEMA trilayer microspheres retained the spherical shape with slight aggregation, as observed in Figure 3b, which may be originated from the interparticle hydrogen-bonding interaction between the amide groups. This was very similar to the results of P(MBA-coHEMA) microspheres by DPP in our previous work.41 The polymer shell was homogenously coated onto the Fe3O4@SiO2 core−shell microspheres in the absence of any secondaryinitiated particles. It meant that the PHEMA shell can be efficiently coated onto the Fe3O4@SiO2 core−shell microspheres with the aid of the efficient hydrogen-bonding interaction between the amide group of MBA cross-linker as well as the ester group of HEMA and the silanol group on the surface of inorganic template. The diameter of the Fe3O4@ SiO2@PHEMA from the TEM images was also ∼270 nm, which was consistent with the results from the SEM images (Figure 3a). The polydispersity of the Fe3O4@SiO2@PHEMA trilayer microspheres was 1.02, and the corresponding diagram of the size distribution is shown in Figure S1b in the E

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together with a strong absorption peak at 1727 cm−1 arising from the CO stretch of the ester group of PDMAEMA, whereas the absorption peak at 2937 cm−1 assigned to the C− H symmetric and asymmetric stretching of methyl and methylene groups in the PHEMA component was similar to those of Fe3O4@SiO2@PHEMA (Figure 2b) and Fe3O4@ SiO2@PHEMA-Br microspheres (Figure 2c). TGA measurements were determined for Fe3O4@SiO2 microspheres, Fe3 O4@SiO2@PHEMA core−shell microspheres, Fe3O4@SiO2@PHEMA-Br trilayer microspheres, and Fe3O4@SiO2@PHEMA-g-PDMAEMA microspheres with the grafted PDAEMA brush for further demonstration of the whole polymerization process as well as the surface modification. As shown in Figure 4a, a weight loss of 13.7% for the Fe3O4@SiO2

SiO2@PHEMA with 2-bromoisobutyryl bromide in the presence of TEA. Such a successful modification provided the Fe3O4@SiO2@PHEMA-Br microsphere with enough loci (4.77 × 10−3 mmol/g alkyl bromide) for the further SI-ATRP of DMEMA. For the resultant Fe 3 O 4 @SiO 2 @PHEMA-gPDMAEMA multilayer microspheres with the grafted PDMAEMA brushes, the weight loss was as high as 89.1% (Figure 4d) with two distinct weight loss stages between 200 and 500 °C. Furthermore, these two distinct weight loss stages of Fe3O4@ SiO2@PHEMA-g-PDMAEMA microspheres between 200−500 o C can be observed more clearly by derivative thermogravimetry (DTG), as shown in Figure S5 of the Supporting Information. All of these results demonstrated that the PDMAEMA brush was successfully grafted onto the Fe3O4@SiO2@PHEMA-Br microspheres via a SI-ATRP technique, in which the mass ratio of PDMEMA brush was estimated as high as 36.3%. The two distinct weight loss ranges may be attributed to the hierarchical structure of the cross-linked PHEMA component and hairy-like PDMAEMA brush. As a result, the polymeric components in the Fe3O4@SiO2@PHEMA, Fe3O4@SiO2@PHEMA-Br, and Fe3O4 @SiO2@PHEMA-g-PDMAEMA microspheres from TGA characterizations are summarized in Table 1. Table 1. Magnetization and Weight Losses: (A) Fe3O4@ SiO2, (B) Fe3O4@SiO2@PHEMA, (C) Fe3O4@SiO2@ PHEMA−Br, and (D) Fe3O4@SiO2@PHEMA-g-PDMAEMA Microspheres

Figure 4. TGA curves: (a) Fe3O4@SiO2 microspheres, (b) Fe3O4@ SiO2@PHEMA core−shell microspheres, (c) Fe3O4@SiO2@PHEMABr trilayer microspheres, and (d) Fe3O4@SiO2@PHEMA-g-PDMAEMA microspheres with the grafted PDMAEMA brush.

microsphere was assigned to the physically adsorbed water on the silica surface. The weight loss of the Fe3O4@SiO2@ PHEMA core−shell microspheres and the Fe3O4@SiO2@ PHEMA-Br core−shell microspheres were 45.7 (Figure 4b) and 52.8% (Figure 4c), respectively. These two microspheres both exhibited a two-stage weight-loss process. The first weight loss until 200 °C was also due to the evaporation of the physically adsorbed water or solvent, and the second major weight loss from 200 to 600 °C was due to the decomposition of the polymer component in the shell layer of the corresponding microspheres. The TGA curve of the Fe3O4@ SiO2@PHEMA (Figure 4b) microspheres exhibited a remarkably higher weight loss compared with that of the Fe3O4@SiO2 microspheres (Figure 4a), which implied that ∼32.0% of PHEMA (mass ratio of the whole trilayer composite inorganic/ polymer microsphere) was efficiently coated onto the inorganic magnetic/silica core−shell particles via DPP. According to the previous work,42 the weight loss of PHEMA (the density of neat PHEMA component as 1.2 g/cm3) was used for the calculation of the shell thickness of the PHEMA around 18 nm, which was consistent with the result (20 nm) from the TEM determination, as discussed above. The considerable difference (7.1%) between the mass loss of Fe3O4@SiO2@PHEMA (45.7%) and that of Fe3O4@SiO2@ PHEMA-Br microsphere (52.8%) revealed that the loading capacity of alkyl bromide was ∼4.77 × 10−3 mmol/g via the esterification of the surface hydroxyl groups of the Fe3O4@

entry

weight loss by TGA (%)

saturation magnetization (emu/g)

coercive force (Oe)

A B C D

13.7 45.7 52.8 89.1

28.1 17.8

19.1 19.2

3.5

16.8

The magnetic properties of Fe3O4@SiO2 microspheres, Fe3O4@SiO2@PHEMA trilayer microspheres, and Fe3O4@ SiO2@PHEMA-g-PDMAEMA microspheres with the grafted PDMAEMA brush were studied by a vibrating sample magnetometer (VSM) at room temperature, as shown in Figure 5. No obvious magnetic hysteresis loops were observed for all samples. In other words, the remanence did not exist

Figure 5. Hysterisis loops of samples at room temperature: (a) Fe3O4@SiO2 microspheres, (b) Fe3O4@SiO2@PHEMA core−shell microspheres, and (c) Fe3O4@SiO2@PHEMA-g-PDMAEMA multilayer microspheres. F

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when the magnetic field was removed, indicating that all the resultant microspheres showed superparamagetic feature originating from the magnetite inner cores at room temperature. The saturation magnetization (Ms) values for Fe3O4@ SiO2 microspheres, Fe3O4@SiO2@PHEMA core−shell microspheres, and Fe3O4@SiO2@PHEMA-g-PDMAEMA multilayer microspheres were 28.1, 17.8, and 3.5 emu/g, respectively (Table 1). Compared with the Ms values of Fe3O4@SiO2 microspheres and Fe3O4@SiO2@PHEMA-g-PDMAEMA multilayer microspheres, the total amount of the PHEMA-gPDMAEMA polymeric components was calculated as around 87.5%, which was consistent with the results from TGA (totally 89.1 mass % loss). With the successive coating of the PHEMA layer and grafting PDMAEMA brush onto the Fe3O4/silica particles, the saturation magnetization value was remarkably reduced due to the decrease in the efficient mass content of the magnetite component. Anyhow, the magnetism of these core− shell microspheres was still strong enough for the Fe3O4@ SiO2@PHEMA-g-PDMAEMA multilayer microspheres to be separated and controlled by external magnetic field during their application. Preparation of Fe3O4@SiO2@PHEMA-g-PDMAEMAStabilized Gold Nanocolloids and Their Catalytic Properties. Because the outer PDMAEMA brush in Fe3O4@ SiO2@PHEMA-g-PDMAEMA microspheres contained abundant tertiary amino groups as a good chelator for metallic nanoparticles, the gold nanocolloids were deposited onto the Fe3O4@SiO2@PHEMA-g-PDMAEMA microspheres by the in situ reduction of HAuCl4 with the NaBH4 as reductant with the stabilization of Au nanocolloids from the efficient coordination effect. The corresponding Au-loaded magnetic composite (Fe3O4@SiO2@PHEMA-g-PDMAEMA@Au) was investigated by TEM as shown in Figure 3d, which indicated that the tinysized gold nanocolloids were well-dispersed in the outer hairlike PDMAEMA brush with the presence of many deeper contrast dots without any aggregation. It was observed more clearly from the inserted TEM micrograph in Figure 3d with a higher magnification. These results proved the efficient stabilization of the PDMAEMA brush for the resultant gold metallic nanocolloids with the aid of the coordination between the tertiary amino groups with gold atoms. Thus, the combined inorganic−organic technique was efficient for the preparation of the magnetic composite microspheres with functional brushes for loading of the gold nanoparticles. The size of the gold nanoparticles was ∼3.7 nm with the polydispersity index of 1.07, and the corresponding size distribution is exhibited in Figure S6 in the Supporting Information. The mechanisms for the stabilization of the functional groups to the gold nanocolloids via the efficient coordination have been wellinterpreted by XPS characterizations in our previous papers.43,44 The gold content in the Fe3O4@SiO2@PHEMAg-PDMAEMA@Au) was 0.91% determined by ICP. It is well known that the gold nanoparticles can be used for a wide range of catalytic reactions. The catalytic reduction of 4NP to 4-AnP with NaBH4 as the reductant was widely used for testing the activation of the catalyst,45 and the catalytic mechanism in the supported gold nanoparticles was wellstudied in the literatures.46−48 In the present work, the catalytic properties of Fe3O4@SiO2@PHEMA-g-PDMAEMA/Au microspheres were investigated via the reduction of 4-NP to 4-AnP with NaBH4 as the reductant under ambient temperature in aqueous solution as a model reaction. However, the reduction process did not proceed even after 24 h without the addition of

the catalyst, which was proved by the constant UV−vis absorption peak at 399 nm. After the introduction of the catalyst into the reaction system, the reduction was performed quickly, as shown in Figure 6a. The absorption peak at 399 nm,

Figure 6. (a) Reduction of 4-NP in aqueous solution recorded by UV−vis spectroscopy every 3 min using the Fe3O4@SiO2@PHEMA-gPDMAEMA/Au microspheres as a catalyst. (b) Plot of In(Ct/C0) versus time. (c) Reusability of the Fe 3O4@SiO2@PHEMA-gPDMAEMA/Au as a catalyst for the reduction of 4-NP with NaBH4.

which is the characteristic absorption peak attributed to 4nitrophonelate (complex of 4-NP and NaBH 4),49 was decreased gradually and disappeared completely after 15 min. The new adsorption peak at 300 nm was simultaneously increased with proceeding of the reaction. It meant that the 4NP was reduced to 4-AnP in the presence of Fe3O4@SiO2@ PHEMA-g-DMAEMA/Au catalyst. Furthermore, the kinetic process was studied in this work. Because of the much higher concentration of the NaBH4 relative to that of the 4-NP G

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(CNaBH4/C4‑NP = 400) this reaction can be considered as the pseudo-first-order reaction with regard to the 4-NP, during which the concentration of the NaBH4 was assumed to be constant. A linear relationship between ln(Ct/C0) and reaction time was obtained in the reduction catalyzed by Fe3O4@SiO2@ PHEMA-g-DMAEMA/Au microspheres (Figure 6b), which was consistent with the first-order kinetics. The rate contrast k was calculated to be 0.270 min−1, which was somewhat higher than those ranging from 0.029 to 0.096 and 0.150 min−1 by increasing the porosity of the silica shell for the hollow mesoporous silica particles loaded with small gold nanoparticles.50 It should be noticed that a much higher neat gold catalyst (2.35 × 10−8 mol) for the latter case was used than that in the present work (2.3 × 10−9 mol). Furthermore, the catalytic rate constant of this catalyst (4.5 × 10−3 s−1) was about half of the reported catalyst of Au nanoparticlesdeposited PMMA particles ((7.2 to 7.9) × 10−3 s−1), which was considered to be that of the highest catalytic activity among polymer-supported Au nanoparticles since 2009.51 Here the molar ratio of the catalyst (Au) to reactant (4-NP) (1/224) was much smaller than that (1/15) in the literature.51 These results indicated that Fe3O4@SiO2@PHEMA-gDMAEMA/Au behaved with high catalytic activity toward this reduction reaction. To investigate the recycled property of the Fe3O4@SiO2@ PHEMA-g-PDMAEMA/Au, we used a larger amount of catalyst (four times the above typical case) to catalyze the reduction of 4-NP to 4-An to increase the reaction rate. In each cycle, the catalytic reduction was performed for 5 min; then, the catalyst was separated by centrifugation for the next cycle of catalysis. In the present work, the catalytic reaction was performed in a given time to investigate whether the conversion of the reaction was decreased to reveal the recovery and reusability of the catalyst, which was similar to the other papers.50,52,53 Even after recycling for the sixth time, the conversion of 4NP was only slightly decreased from 99.0% for the first cycle to 96.7%, as illustrated in Figure 6c. In our previous work, the catalytic activity retained only ∼1/3 after recycling the poly(DVB-co-AA)/Au catalyst only for four times with the stabilization of the surface carboxyl to gold nanocolloids.43,44 Therefore, the catalytic activity was well retained even after at least six cycles of Fe3O4@SiO2@PHEMA-g-PDMAEMA/Au, suggesting the excellent stabilization ability of the grafted PDMAEMA brush to gold nanocolloids as a catalyst.

Article

ASSOCIATED CONTENT

S Supporting Information *

Size distribution: Fe3O4 micropsheres and Fe3O4@SiO2@ PHEMA microspheres. TEM images of Fe3O4@SiO2 core−shell microspheres with various shell thicknesses via addition of different TEOS loadings: 0.10 and 0.50 mL. TEM image of Fe3O4@SiO2 core−shell composite via the controlled hydrolysis of TEOS in the presence of magnetite template under mechanic stirring. TEM image of Fe3O4@SiO2@P(HEMA) microspheres from the vinyl-modified Fe3O4@SiO2 seed via a vinyl-capture distillation precipitation polymerization. DTG of Fe3O4@SiO2@PHEMA-g-PDMAEMA microspheres with the grafted PDMAEMA brush. Size distribution of the gold nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the NSFC (21174065) and financial support from Tianjin Science Technology Research Funds of China (11JCYBJC02100).

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REFERENCES

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CONCLUSIONS Multifunctional Fe3O4@SiO2@PHEMA-g-PDMAEMA microspheres with magnetic core and polymer brushes were prepared via a successive process, which combined the solvothermal method and sol−gel technique for the synthesis of inorganic core with DPP and ATRP for the preparation of the outer polymer shell. The gold nanoparticles were facilely loaded onto the multifunctional microspheres by the in situ reduction of HAuCl4 with NaBH4 as reductant via the efficient stabilization of the PDMAEMA brush to the gold metallic nanocolloids. The Fe3O4@SiO2@PHEMA-g-PDMAEMA/Au had a stable catalytic activity due to the efficient stabilization of the grafted PDMAEMA brush to the gold metallic nanocolloids during their recycling with a facile recovery (at least six times). The combined inorganic−organic process may be used for the synthesis of other functional materials with unique structures and novel properties. H

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