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Synthesis of monodisperse magnetic sandwiched gold nanoparticle as an easily recyclable catalyst with a protective polymer shell
Colloids and Surfaces A: Physicochem. Eng. Aspects 466 (2015) 210–218
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of monodisperse magnetic sandwiched gold nanoparticle as an easily recyclable catalyst with a protective polymer shell Wei Zhang a , Bin Liu a , Bei Zhang a , Guomin Bian b , Yonglin Qi b , Xinlin Yang a,∗ , Chenxi Li a a Key Laboratory of Functional Polymer Materials, Ministry of Education, Institute of Polymer Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, PR China b Dynea Ltd. Co., Gaoyao City, Guangdong 526105, PR China
h i g h l i g h t s
g r a p h i c a l
• Magnetic sandwiched gold nanopar-
A magnetic Fe3 O4 @Au/PEGDMA microsphere with a magnetite core, a permeable polymer shell and the sandwiched gold nanoparticles exhibited a stable and readily recoverable catalytic activity.
ticles have been synthesized. • The amino group on Fe3 O4 –NH2 played dual functions during the synthesis. • Fe3 O4 @Au/polyethyleneglycol dimethacrylate was recycled under magnetic field. • Magnetic sandwiched gold nanoparticles had a stable catalytic activity.
a r t i c l e
i n f o
Article history: Received 23 September 2014 Received in revised form 19 November 2014 Accepted 30 November 2014 Available online 9 December 2014 Keywords: Magnetic sandwiched gold nanoparticles Recoverable catalsyst Distillation precipitation polymerization
a b s t r a c t Monodisperse magnetic sandwiched magnetite@gold/poly(ethyleneglycol methacrylate) (Fe3 O4 @Au/PEGDMA) core–shell microspheres were designed and prepared. The whole synthetic procedure mainly involved hydrothermal method for preparation of magnetite core with subsequent modification of (3-aminopropyl)trimethoxysilane (APS) for introduction of the surface amino groups, distillation precipitation polymerization for preparation of the P(EGDMA) shell and in situ reduction of gold precursor HAuCl4 for formation of the sandwiched Au nanoparticles. The thicknesses of the outer polymeric shells were well-controlled via altering the weight ratios of EGDMA monomers to magnetite core during polymerization. The catalytic activity of the sandwiched Fe3 O4 -Au@P(EGDMA) magnetic microsphere was studied by the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AnP) as a model reaction. These magnetic microspheres were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier-transform infrared spectra (FT-IR), thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM). © 2014 Elsevier B.V. All rights reserved.
1. Introduction Noble metallic nanoparticles have been found applications in many fields, such as optical biosensor [1], electronic devices [2],
∗ Corresponding author. Tel.: +86 22 23502023; fax: +86 22 23503510. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.colsurfa.2014.11.055 0927-7757/© 2014 Elsevier B.V. All rights reserved.
a b s t r a c t
biomedical [3], and catalysis [4]. Especially, the gold nanoparticles have received increasing attention due to their unique behavior different from the corresponding single atoms and bulk materials. These gold nanoparticles with large surface area-to-volume ratio and abundant active surface atoms display outstanding catalytic activity and selectivity for many catalytic reactions, including hydrogenation [5], oxidation [6] and C C coupling reaction [7]. However, it is difficult to investigate the recoverable catalytic
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properties of these gold nano-catalytic systems because of their aggregation and tiny size for separation. There are two efficient strategies for stabilization and easy separation of gold nanoparticles to solve these problems. The first one is to directly encapsulate gold nanoparticles within protective shells, such as metal [8], metal oxide [9], silica [10,11] and polymer [12–18]. Unfortunately, the catalytic efficiency of the gold nanoparticles was considerably decreased due to the presence of a dense silica shell reducing to retard reactant molecules reaching active inner core [11]. The efficiencies of the Au@polymer yolk–shell particle [19–21] were better than that of Au@SiO2 core–shell particle [10,11] because polymer shells were swollen and flexible for easy permeation of the reactants. However, the procedure for the synthesis of yolk–shell particle is relatively complex due to its indispensable multi-step operation. The other strategy is to immobilize the gold nanoparticles on appropriate supports, such as metal oxide [22], carbon-based materials [23], molecular sieves [24], mesoporous silica [25], polymer microspheres [26–28]. The gold nanoparticles with good dispersion can be prevented efficiently from aggregation via efficient stabilization of functional groups on surface of the supporters. However, the aggregation and loss of the gold nanoparticles from the support often occurred during the catalytic reaction to lower their catalytic activity. Therefore, a polymer shell-layer was an efficient way for stabilization of the gold nanoparticles to prevent aggregation for silica@Au/polymer systems [29,30]. These sandwiched gold nanoparticles exhibited a highly catalytic efficiency with good stability. As a result, it is still difficult for readily recovery of the sandwiched metallic catalyst due to the tiny size of the silica supporters. It was necessary to perform recovery of the supported catalyst with aid of ultracentrifugation. Therefore, it is still a challenge to design and synthesize metallic nanocolloids with a highly stable and easily recoverable catalytic property. Magnetic nanoparticle has long been used as a promising support for noble metal nanocolloids due to easy separation feature, which makes it convenient to recycle catalyst. The effects of supporters on catalytic reactions have been investigated via direct immobilization of gold nanoparticles onto magnetic nanoparticles [31–36]. On the one hand, magnetite nanoparticles were easily oxidized in air and eroded under acidic condition. On the other hand, loss of partial gold nanoparticles was unavoidable during recycling catalyst due to weak interaction between gold nanoparticles and magnetite supporter. One of the main approaches to overcome these limitations is to protect magnetic nanoparticles with polymers [37,38] or inorganic shell, such as silica [39–41], metal oxide [42] or carbon-based materials [43]. These protective shells not only protected the magnetic nanoparticles, but also immobilized the gold nanoparticles. However, the whole synthetic procedures were tedious and complex. Thus, it is desirable to develop a method for preparation of gold nanoparticles sandwiched between a protective shell and a magnetic core for readily recovery of catalyst with high efficiency. In this paper, monodisperse magnetic sandwiched magnetite@gold/poly(ethyleneglycol methacrylate) (Fe3 O4 @Au/ PEGDMA) microspheres were designed and prepared. These Fe3 O4 @Au/PEGDMA nanoparticles were synthesized via combination of distillation–precipitation polymerization for formation of polymer shell in the presence of the (3aminopropyl)trimethoxysilane (APS)-modified Fe3 O4 core as a template and the subsequent in situ reduction of HAuCl4 with NaBH4 as reductant. Further, the catalytic activity and recycling property of the sandwiched gold nanoparticles were investigated by reduction of 4-nitrophenol to 4-aminophenol with sodium borohydride as a model reaction.
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2. Materials and methods 2.1. Materials Ferric chloride (FeCl3 ·6H2 O) and sodium acetate (NaAc) was purchased from Tianjin Guangfu Chemical Engineering Institute. Trisodium citrate (Na3 Cit) was obtained from Tianjin Chemical Reagents I Co. Ethyleneglycol dimethacrylate (EGDMA) and (3-aminopropyl) triethoxysilane (APS) were purchased from Alfa Aesar and used without further purification. 2,2 Azobisisobutyronitrile (AIBN) was provided by Chemical Factory of Nankai University and recrystallized from methanol. Acetonitrile (analytical grade, Tianjin Chemical Reagents II Co.) was dried over calcium hydride and purified by distillation before use. Tetrachloroauric acid trihydrate (HAuCl4 ·3H2 O) was purchased from Shenyang Research Institute of Nonferrous Metal, China. Sodium borohydride (NaBH4 ) and ethylene glycol (EG) were purchased from Tianjin Chemical Reagent II Co. 4-Nitrophenol (4-NP) was purchased from Tianjin Chemical Reagent Factory. The other reagents were of analytical grade and used without any further treatment. 2.2. Synthesis of APS-modified magnetite (Fe3 O4 –NH2 ) nanoparticles Magnetite (Fe3 O4 ) nanoparticles were synthesized by a modified solvothermal method according to the literature [44,45]. The procedure was as following: 3.6 g of FeCl3 ·6H2 O and 0.72 g of Na3 Cit were dissolved in a mixture of ethylene glycol/ethanol (90 mL/10 mL) solution through ultrasound irradiation. 4.8 g of NaAc was then added under vigorous magnetic stirring for 30 min. Finally, the resultant mixture was transferred into a Teflon-lined stainless-steel autoclave (with a capacity of 200 mL). The hydrothermal reaction was carried out under 200 ◦ C for 10 h. After reaction, the autoclave was taken out to cool till room temperature. The resultant solid black product was collected by centrifugation and purified by thoroughly extraction with ethanol and deionized water for three times and then dried in a vacuum oven at 50 ◦ C till constant weight. APS-modified magnetite nanoparticles with surface amino group (Fe3 O4 –NH2 ) were synthesized by a sol–gel process. The details were as following: 0.50 g of Fe3 O4 magnetic nanoparticles were dispersed in a mixture of 160 mL of ethanol, 40 mL of deionized water, and 4 mL of 25 wt% ammonium hydroxide aqueous solution through ultrasound irradiation for 5 min. 1.0 mL of APS was then added under vigorous mechanical stirring for another 48 h. The resultant Fe3 O4 –NH2 nanoparticles were purified by repeating centrifugation, decantation, and resuspension in ethanol for three times. The Fe3 O4 –NH2 nanoparticles were dried in a vacuum oven at 50 ◦ C till constant weight. 2.3. Synthesis of Fe3 O4 –NH2 /PEGDMA core–shell magnetic microspheres Fe3 O4 –NH2 @PEGDMA core–shell magnetic microspheres were synthesized by distillation precipitation polymerization of ethyleneglycol dimethacrylate (EGMDA) in the presence of APS-modified Fe3 O4 nanoparticles as seeds (the weight ratio between the monomer to the magnetite seed was set at 4:1). The details were as following: 0.05 g of Fe3 O4 –NH2 nanoparticle as seed was suspended in 40 mL of acetonitrile. Then, 0.2 mL of EGDMA monomer and 0.004 g of AIBN initiator (2 wt% relative to monomers) were dissolved in above suspension in a 50 mL of flask. The flask attaching with a fractionating condenser and receiver was submerged in a heating mantle. The reaction mixture was heated from ambient temperature till boiling state within 15 min and the
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reaction system was kept under refluxing state for 15 min further. The polymerization was carried out with distilling the solvent out of the reaction system. The reaction was ended after 20 mL of acetonitrile was distilled off the reaction mixture within 70 min. After polymerization, the resultant of Fe3 O4 –NH2 @PEGDMA core–shell magnetic microspheres were purified by repeated centrifugation, decantation, and resuspension in ethanol for three times and then dried in a vacuum oven at 50 ◦ C till constant weight. A series of polymerizations were performed for preparation of Fe3 O4 –NH2 @PEGDMA core–shell magnetic microspheres with different shell thicknesses through altering mass ratios of monomers to the magnetite seeds from 3:1 to 5:1, while the other reaction conditions were maintained the same as those for typical distillation–precipitation polymerizations. 2.4. Synthesis of the sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres The Fe3 O4 –NH2 @PEGDMA core–shell microspheres loaded gold nanoparticles were in situ synthesized by reduction of tetrachloroauric acid trihydrate (HAuCl4 ·3H2 O) with NaBH4 as reductant. The details were as following: 0.05 g of Fe3 O4 –NH2 @PEGDMA core–shell microspheres were dispersed in 2.0 mL of deionized water. 2.0 mL of HAuCl4 aqueous solution (20 mg/mL) was then added into the suspension and incubated for 24 h in dark at room temperature. The resultant microspheres were centrifuged from the solution and washed by water with aid of a magnet for five times till the supernatant was colorless. Finally, the microspheres loaded with gold species (AuCl4 − ) were redispersed in 10 mL of deionized water, and 5 mL of 0.05 M NaBH4 aqueous solution was added dropwise under ice water bathing with strong stirring. The reduction of HAuCl4 was performed at room temperature for 2 h. The final product was purified by washing with deionized water for three times and dried in a vacuum oven till constant weight. 2.5. Catalytic reduction of 4-nitrophenol to 4-aminophenol by NaBH4
2.6. Characterization The morphology of the resultant nanoparticles was determined by transmission electron microscopy (TEM) using a Tecnai G2 20S-TWIN microscope. The samples were dispersed in ethanol and a drop of dispersion was dropped onto the surface of a copper grid coated with a carbon membrane for TEM characterization. All size and size distribution reflect the averages about 100 particles each, which are calculated according to the following formulae: U=
DW Dn
k
k Dn =
nD i=1 i i k n i=1 i
Dw =
n D4 i=1 i i
k
n D3 i=1 i i
where U is the polydispersity index, Dn is the number average diameter, Dw is the weight-average diameter, 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 (FT-IR) spectra were determined on a Bio-Rad FTS 135 FT-IR spectrometer over potassium bromide pellet. Diffusion reflectance spectra were scanned over range of 4000–400 cm−1 . The magnetic properties of Fe3 O4 microspheres, APS-modified Fe3 O4 microspheres, Fe3 O4 –NH2 /PEGDMA core–shell microspheres and the sandwiched Fe3 O4 @Au/PEGDMA microspheres were studied in a 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 K␣ ( = 0.15406 nm) radiation at 40 kV and 100 mA. Thermogravimetric analysis (TGA) data were obtained with a heating rate of 10 K/min using a TA TGA-2950 apparatus. Atomic emission spectrometry was performed on an ICP-9000 (N+M) spectrometer (USA Thermo Jarrell-Ash Corp.). Elementary analysis (EA) was determined on a vario EL CUBE apparatus. UV–vis spectroscopy was performed on a JASCO V570spectrometer. 3. Results and discussion
The catalytic activity of sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres was evaluated by reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AnP) with sodium borohydride (NaBH4 ) as reactant under ambient temperature in aqueous solution as a model reaction. The procedure was as following: 0.10 mL of 4-NP (5 mM, 5 × 10−7 mol), 1.0 mL of freshly NaBH4 (0.20 M, 2 × 10−4 mol) aqueous solution, and 2.0 mL of water were mixed in a colorimetric tube. Then, 0.05 mL of catalyst dispersion (1.0 mg/mL, containing 4.6 × 10−9 mol Au) was introduced into the mixture with gentle shaking. The bright yellow solution faded gradually as catalytic reaction proceeded. The catalytic process was monitored by a UV–vis spectrophotometer with the decrease of peak at 400 nm in UV–vis absorption and a simultaneous increase of the absorption at 310 nm, indicating the formation of 4-AnP. To perform the recoverable catalytic activity of the sandwiched Fe3 O4 -Au@PEGDMA magnetic microspheres within several minutes, the amount of the catalyst was increased by fourfold times comparing to that for the above reduction. The conversion of the 4-NP was traced by UV–vis spectrum with a decreasing adsorption peak at 400 nm. After complete reduction of 4-NP, the catalysts were separated with the aid of a magnet and redispersed in the next cycled reaction system. Meanwhile, the supernatant liquid was taken out by a syringe and detected by UV–visible spectrum. The reduction process was repeated for eight times by the similar method.
The whole procedure for synthesis of sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres was illustrated in Scheme 1. 3.1. Preparation of APS-modified magnetite (Fe3 O4 –NH2 ) nanoparticles APS-modified magnetite nanoparticles (Fe3 O4 –NH2 ) were prepared by a modified solvothermal method [44,45] to obtain magnetite nanoparticles (Fe3 O4 ), which were further functionalized with surface amino groups through saline coupling reaction of (3-aminopropyl)trimethoxysilane (APS). The Fe3 O4 nanoparticles were first prepared by a solvo-thermal method via partial reduction of FeCl3 precursor with ethylene glycol (EG) as solvent, sodium acetate (NaAc) as an alkali source, and trisodium citrate (Na3 Cit) as an electrostatic stabilizer. Particularly, ethanol was introduced to improve dispersion of the solid reagent. The resultant Fe3 O4 nanoparticles were shown by TEM micrograph of Fig. 1a, which exhibited a spherical shape and rough surface with an average diameter of about 180 nm. As shown by inserted image of Fig. 1a, the Fe3 O4 nanoparticles consisted of aggregates of small magnetite particles with a mean size of 3–10 nm. The FT-IR spectrum of Fe3 O4 nanoparticles was shown in Fig. 2a. The strong absorption peak at 594 cm−1 was attributed to the Fe–O group. The XRD pattern of the magnetite was shown in Fig. S1. Five characteristic peaks at
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Scheme 1. The procedure for synthesis of the sandwiched Fe3 O4 @Au/PEGDMA microspheres. (The insets in part a were the HRTEM graph.)
30.1, 35.4, 43.1, 56.9, 62.5◦ were attributed to (2 2 0), (3 1 1), (4 0 0), (5 1 1), (4 4 0) diffraction peaks of magnetite and matched well with the standard JCPS card 74-0748. Then, the surface of the Fe3 O4 nanoparticles was modified with APS through saline coupling reaction. In this work, due to the introduction of Na3 Cit to the magnetite synthetic procedure, the magnetite nanoparticles contained abundant hydroxyl groups and carboxyl groups on the surface. Then, the saline coupling reaction was processed through the condensation reaction between the hydroxyl groups and carboxyl groups of the surface of Fe3 O4 nanoparticles as well as silanol groups of APS. The TEM image of the resultant Fe3 O4 –NH2 nanoparticles was shown in Fig. 1b. The shape and size of the APS-modified Fe3 O4 nanoparticles were near the same as those of the former Fe3 O4 nanoparticles. The corresponding FT-IR spectrum was shown in Fig. 2b. The absorption peak at 2930 cm−1 was attributed to the saturated alkyl groups of the APS. Furthermore, the elemental content of nitrogen on the Fe3 O4 –NH2 nanoparticles was 0.85% determined by elementary analysis (EA), which meant that loading capacity of amino group on Fe3 O4 –NH2 particles was 0.61 mmol/g. Fig. 2. FI-IR spectra of (a) Fe3 O4 nanoparticles; (b) APS-modified Fe3 O4 nanoparticles; (c) Fe3 O4 –NH2 @PEGDMA core–shell microspheres.
3.2. Preparation of sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres In our previous works, there were two approaches to coat polymer shell on the surface of inorganic core via distillation precipitation polymerization. In the first approach, the inorganic particles were modified by the vinyl group for capturing radical and oligomer to obtain a polymer shell [46]. In the second approach, the hydrogen-bond interaction between hydroxyl groups on the surface of the inorganic particles and the polar groups of functional monomer was utilized for preparation of the core–shell microspheres [47]. Herein, the magnetite nanoparticles with active groups (including amino group, hydroxyl group and carboxyl group) were used as seeds for growth of the polymer shell. These active groups on surface of the magnetite nanoparticles were useful for direct coating of PEGDMA polymer shell via distillation precip-
itation polymerization, which was similar to the mechanism of the synthesis of Fe3 O4 @PEGDMA core–shell microspheres with aid of hydrogen-bonding interaction in our previous work [37]. The TEM images of the resultant Fe3 O4 –NH2 /PEGDMA core–shell microspheres were shown in Fig. 3. PEGDMA shell was uniformly coated onto magnetite nanoparticles core to form a well-defined core–shell structure with a smooth surface. This was confirmed by the presence of a deep contrast magnetite core and a light contrast polymer shell, which was originated from different mass contrasts between inorganic core and polymeric shell domain. In this work, the thicknesses of PEGDMA shell were easily tuned by varying the mass ratios of monomers to magnetite core during polymerization. The TEM images of the resultant Fe3 O4 –NH2 /PEGDMA core–shell microspheres with different shell
Fig. 1. TEM images of (a) Fe3 O4 nanoparticles and (b) APS-modified Fe3 O4 nanoparticles.
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Fig. 3. TEM images of Fe3 O4 -APS@PEGDMA core–shell microspheres with different shell thickness: (a) 6 nm; (b) 24 nm; (c) 40 nm.
Fig. 4. TGA curves: (a) Fe3 O4 –NH2 and (b)–(d) Fe3 O4 –NH2 @PEGDMA core–shell microspheres with different shell thickness: (b) 6 nm; (c) 24 nm; (d) 40 nm.
thicknesses by varying the mass ratios between monomer and magnetite core from 3 to 5 were shown in Fig. 3a–c. The shell thicknesses were increased from 6, 24 to 40 nm with enhancing mass ratios between monomers and magnetite core from 3/1, 4/1 to 5/1 as summarized in Table 1. The PEGDMA shell on the surface of magnetite core nanoparticles was further characterized by FT-IR spectrum as shown in Fig. 2c. The strong absorption peak at 1730 cm−1 was contributed to vibration of carbonyl groups in PEGDMA component. All these results indicated that PEGDMA shell was successfully coated onto the magnetite core during distillation–precipitation polymerization. The mass contents of polymer shell layers on magnetite surfaces were determined by thermogravimetric analysis (TGA) as illustrated in Fig. 4. All these samples exhibited similar weight loss trends. Generally, there were two main weight loss stages Table 1 Reaction conditions, polymer shell thicknesses and polymer contents of Fe3 O4 –NH2 /PEGDMA core–shell microspheres with different shell thickness. Entry
Fe3 O4 –NH2 (g)
EGDMA (mL)
Shell thickness (nm)
Weight loss by TGA (%)
A B C D
0.05 0.05 0.05 0.05
0 0.15 0.20 0.25
0 6 24 40
16.4 31.2 45.1 57.1
when the samples were heated from room temperature to 800 ◦ C. For TGA of APS-modified Fe3 O4 nanoparticles in Fig. 4a, the first mass loss from room temperature to 200 ◦ C was attributed to the evaporation of physically adsorbed water and solvent, while the second weight loss between 200 and 600 ◦ C was due to decomposition of the organic component of APS and trisodium citrate on the magnetite surface. The organic component on Fe3 O4 –NH2 nanoparticles was 16.4%. Fig. 4b–d showed TGA results of the Fe3 O4 –NH2 /PEGDMA core–shell microspheres with different shell thicknesses. The first weight loss stage was similar to the mass loss of Fe3 O4 –NH2 nanoparticles, while the second weight loss stage from 200 to 500 ◦ C was mainly due to decomposition of polymeric components in the shell layers. The mass losses of these core–shell microspheres were increased with increasing feed of EGDMA monomers to the magnetite core, which were consistent with the results from TEM characterizations. These results were summarized in Table 1. Comparing to the mass loss of the initial APS-modified magnetite nanoparticles during the calcination process, the polymer contents in these core–shell microspheres were calculated as 14.8%, 28.7%, 40.7%, when the ratios of the EGDMA monomer to the magnetite core were enhanced from 3/1, 4/1 to 5/1, respectively. In other words, the polymer shell with controllable thickness was successfully encapsulated on the surface of the magnetite core to obtain Fe3 O4 –NH2 /PEGDMA core–shell microspheres. In the present work, the amino groups on the surface of Fe3 O4 –NH2 inorganic template played dual roles for formation of Fe3 O4 @Au/PEGDMA microspheres, i.e., the positions for distillation precipitation polymerization via efficient hydrogen bonding interaction with ester groups of the EGDMA monomer, adsorption and stabilization of gold nanoparticles via efficient chelating with gold atom. During this process, the Fe3 O4 –NH2 /PEGDMA core–shell microspheres with a shell thickness of 24 nm were used as a sample template for loading gold nanoparticles. The gold ion species loaded between the magnetite core and PEGDMA shell can be attributed to the following three reasons: (i) the surface of the obtained magnetite core (Fe3 O4 ) can successfully modified by (3-aminopropyl) triethoxysilane (APS) to form amino groups ( NH2 ); (ii) the gold ion species can permeated through the PEGDMA shell in the aqueous solution [30]; (iii) the gold ion can be easily adsorbed at the interface through coordination together static interactions with between the gold ion and amino groups [46]. Finally, the sandwiched gold nanoparticles were in situ formed via the reduction of the adsorbed gold cations with sodium borohydride as reductant. Typical TEM micrograph of the resultant sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres was shown in Fig. 5. It indicated that these microspheres had a typical core–shell structure with spherical and smooth surface. The
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Fig. 5. TEM images the sandwiched Fe3 O4 @Au/PEGDMA microspheres with magnifications. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
average diameter of Fe3 O4 @Au/PEGDMA particles was 231 nm with a narrow disperse index (U) of 1.07. Because magnetite core and gold nanoparticles had similar contrasts in TEM micrographs, the gold nanoparticles with a diameter of about 5 nm (the red circle in Fig. 5b) were not clearly observed between the surface of the magnetite core and the outer polymer shell. The successful loading of gold nanoparticles was confirmed by X-ray diffraction (XRD) as shown in Fig. S1. Compared with the original XRD curve of the magnetite core, the sandwiched Fe3 O4 @Au/PEGDMA microspheres presented new diffraction peaks at 38.5, 44.5, 64.5, and 77.5◦ assigning to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystal plane of cubic gold matched well with the standard JSPD card 65-2870, which confirmed formation of crystalline gold nanoparticles. This was contributed to amino groups on the magnetite surface to enable Fe3 O4 –NH2 /PEGDMA a strong affinity with gold nanoparticles via coordination effect. Based on the calculation with the Debye–Scherrer equation for the strongest 311 diffraction peak, the grain size of the magnetite particles was about 8.3 nm [44]. However, the Fe3 O4 nanoparticles consisted of aggregates of small magnetite particles, as shown by inserted image of Fig. 1a. The gold nanoparticles were distributed in Fe3 O4 @Au/PEGDMA microsphere as the sandwiched nanocolloids between the Fe3 O4 inner core and PEGDMA shell-layer, as the deep contrast gold nanoparticles were absent from the PEGDMA shell layer. This was consistent with the synthesis of rattle-type microspheres as a support of tiny gold nanoparticles [46], as PEGDMA shell had no functional groups to interact with the gold atoms. The content of gold element in the sandwiched Fe3 O4 @Au/PEGDMA microspheres was 1.81% determined by inductive couple plasma emission spectrometer (ICP), which meant that Au content was 0.092 mmol/g.
3.3. Magnetic properties of the resultant microspheres The magnetic property of magnetite nanoparticles was determined with a shell thickness of 24 nm. The corresponding sandwiched Fe3 O4 -Au@PEGDMA microspheres were studied by a vibrating sample magnetometer (VSM) at room temperature, as shown in Fig. 6. No obvious magnetic hysteresis loops were observed for these three samples from the field-dependent magnetization plots. In other words, the remanence did not exist when the magnetic field was removed, indicating that all resultant microspheres showed a superparamagnetic feature originating
Fig. 6. Hysteresis loops of samples at room temperature: (a) Fe3 O4 nanoparticles; (b) Fe3 O4 –NH2 /PEGDMA microspheres; (c) sandwiched Fe3 O4 @Au/PEGDMA microspheres.
from small magnetic nanoparticles (smaller than the single magnetic domain) at room temperature. The magnetic properties of the three microspheres were listed in Table 2. The saturation magnetization (Ms ) values of Fe3 O4 nanoparticles, Fe3 O4 –NH2 /P(EGDMA) core–shell microspheres and the corresponding sandwiched Fe3 O4 @Au/PEGDMA microspheres were 44.8, 29.1, and 27.8 emu/g, respectively. These results indicated that the magnetization of these magnetic samples decreased considerably with the addition of polymer shell or gold nanoparticles. Such a decrease was attributed to the decrease in effective mass content of the magnetite component. However, the magnetism of
Table 2 Magnetization of Fe3 O4 , Fe3 O4 –NH2 /PEGDMA microspheres and the corresponding sandwiched Fe3 O4 @Au/PEGDMA microspheres. Entry
Saturation magnetization (emu/g)
Coercive force (Oe)
Fe3 O4 Fe3 O4 –NH2 /P(EGDMA) Fe3 O4 @Au/P(EGDMA)
44.8 29.1 27.8
17.3 18.6 16.5
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these microspheres was still strong enough to be separated and controlled by an external magnetic field. 3.4. Catalytic properties of the sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres The polymer (PEGDMA) shell with good permeability can efficiently protect the gold nanoparticles from dropping or aggregation, and favor the access of the reactant molecules [31,47]. To investigate the catalytic activity of the sandwiched Fe3 O4 @Au/PEGDMA microspheres, reduction of 4-nitrophenol (4NP) to 4-aminophenol (4-AnP) with NaBH4 as reductant under ambient temperature in aqueous solution was used as a model reaction. An aqueous solution of 4-NP with NaBH4 exhibited a light yellow color with an absorption maximum at 400 nm in UV–vis spectra, while the aqueous solution of the corresponding product 4-AnP had a weak peak at 300 nm in UV–vis spectral. Without the addition of catalyst, the reduction reaction did not conduct even after mixing the resultant 4-NP and NaBH4 for 24 h, which was proved by a constant adsorption of UV–vis peak at 400 nm. However, the reduction reaction was quickly performed after addition of a very small amount of catalyst (0.05 mL aqueous solution, 1.0 mg/mL). The reaction was monitored by UV–vis spectroscopy as shown in Fig. 7a. The absorption peak at 400 nm (characteristic absorption peak of 4-NP) was decreased gradually and disappeared completely after 15 min, while a new absorption peak at 300 nm (characteristic absorption peak of 4-AnP) was simultaneously increased with proceeding reaction. It meant that the 4-nitrophonelate was reduced to 4-aminophenol in the presence of sandwiched Fe3 O4 @Au/PEGDMA catalyst. Furthermore, the kinetic of catalytic reaction was studied in this work. As NaBH4 was used in a much higher concentration relative to that of 4nitrophonelate (CNaBH4 /C4-NP = 400), this reaction can be considered as the pseudo-first-order reaction with regarding to the reactant, during which the concentration of NaBH4 was assumed to be constant. The pseudo-first-order rate constant (k) at 293 K was calculated from the slope of a linear correlation between ln(Ct /C0 ) and reaction time as 0.194 min−1 as shown in Fig. 7b. This indicated that the catalyst reactions possessed high reaction rate under the relative low amount of catalyst. The Fe3 O4 –NH2 @PEGDMA microspheres with different shell thickness (6 nm and 40 nm) were incubated in HAuCl4 aqueous solution in the same concentration, and then washed by water. Finally, the adsorbed AuCl4 − was reduced by NaBH4 . The kinetic rate constants (k) were estimated from the slope to be 0.366 and 0.103 min−1 , respectively (Fig. S2). These results indicated that the catalytic activities of the Fe3 O4 –NH2 @PEGDMA microspheres were inversely dependent to the shell thickness, which may be due to the more difficult permeation for the reactants and diffusion of the product between the catalytic media and the sandwiched gold nanoparticles. As shown in Fig. S3, the rate constant (k) of the Fe3 O4 @Au/PEGDMA microspheres with the gold content of 1.01% was about 0.083 min−1 , which was lower than that of the microspheres with the gold content of 1.81% (0.194 min−1 ). This implied that the catalytic reaction rate was proportional to the loading capacity of the gold nanoparticles. In order to study the recycled property of the sandwiched Fe3 O4 –Au@P(EGDMA) microspheres catalyst, we increased the amount of catalyst by five times to catalyze the reduction of 4-NP to 4-AnP within five min. In each cycle, the catalytic reaction was performed completely for 5 min, and then separated by centrifugation for the next cycle of catalysis. Even after recycling for eight times, the conversion of 4-NP was only slightly decreased from 99.3% for the first cycle to 96.8% for the eighth cycle, as illustrated in Fig. 7c. All these results revealed that the sandwiched Fe3 O4 @Au/PEGDMA
Fig. 7. Kinetic of the catalytic reaction: (a) UV–vis spectra of the catalytic reduction of 4-NP to 4AnP developed at different reaction times; (b) plot of ln(Ct /C0 ) versus time; (c) reusability of Fe3 O4 -Au@P(EGDMA) microspheres as catalysts for the reduction of 4-NP with NaBH4 .
microspheres catalyst exhibited high catalytic efficiency and stable activity. In such a manner, the amino groups on the surface of Fe3 O4 –NH2 core can effectively anchor a high amount of metal nanoparticles through in situ reduction, and the polymer shell had a good permeability for the catalysis as well as protected the metal nanoparticles from leakage.
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4. Conclusion The magnetic Fe3 O4 @Au/PEGDMA microspheres with a magnetite core, a permeable polymer shell and the sandwiched gold nanoparticles were easily prepared by a combination of the minor modified solvo-thermal method for magnetite core, distillation–precipitation polymerization for formation of protective shell-layer and in situ reduction of HAuCl4 with NaBH4 as reductant. The amino groups on the surface of Fe3 O4 –NH2 inorganic template played dual roles for formation of Fe3 O4 @Au/PEGDMA microspheres, i.e., the positions for distillation precipitation polymerization and stabilization of the resultant nanoparticles. The magnetic sandwiched gold nanoparticles exhibited a high catalytic activity with reduction of 4-NP to 4-AnP as a model reaction. The formation of a permeable PEGDMA shell with controlled thickness was synthesized by distillation precipitation polymerization in the absence of highly toxic reagent, such as KCN for formation of porous silica shell [29]. Moreover, the Fe3 O4 @Au/PEGDMA with sandwiched gold nanocolloids exhibited a stable and more easily recoverable catalytic activity under an external magnetic field, for which the ultracentrifugation was not necessary any more comparing those in the literatures [29,30]. Acknowledgements This work was supported by the Natural Science Foundation of China (Grant Nos. 21174065 and 21374049), PCSIRT (IRT1257). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa. 2014.11.055. References [1] K. Saha, S.S. Agasti, C. Kim, X.N. Li, V.M. Rotello, Gold nanoparticles in chemical and biological sensing, Chem. Rev. 112 (2012) 2739–2779. [2] D.V. Talapin, J.S. Lee, M.V. Kovalenko, E.V. Shevchenko, Prospects of colloidal nanocrystals for electronic and optoelectronic applications, Chem. Rev. 110 (2010) 389–458. [3] E.C. Dreaden, A.M. Alkilany, X.H. Huang, C.J. Murphy, M.A. El-Sayed, The golden age: gold nanoparticles for biomedicine, Chem. Soc. Rev. 41 (2012) 2740–2749. [4] A.S.K Haschmi, G.J. Hutchings, Gold catalysis, Angew. Chem. Int. Ed. 45 (2006) 7896–7936. [5] Y. Zhu, H.F. Qian, B.A. Drake, R.C. Jin, Atomically precise Au25 (SR)18 nanoparticles as catalysts for the selective hydrogenation of ␣,-unsaturated ketones and aldehydes, Angew. Chem. Int. Ed. 49 (2010) 1295–1298. [6] M.D. Hughes, Y.J. Xu, P. Jenkins, P. Mcmorn, P. Landon, D.I. Enache, A.F. Carley, G.A. Attard, G.J. Hutchings, F. King, E.H. Stitt, P. Johnston, K. Griffin, C.J. Kiely, Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions, Nature 434 (2005) 1132–1135. [7] M.D. Levin, F.D. Toste, Gold-catalyzed allylation of aryl boronic acids: accessing cross-coupling reactivity with gold, Angew. Chem. Int. Ed. 53 (2014) 6211–6215. [8] H.L. Jiang, T. Akita, T. Ishida, M. Haruta, Q. Xu, Synergistic catalysis of Au@Ag core–shell nanoparticles stabilized on metal–organic framework, J. Am. Chem. Soc. 133 (2011) 1304–1306. [9] G.D. Li, Z.Y. Tang, Noble metal nanoparticle@metal oxide core/yolk–shell nanostructures as catalysts: recent progress and perspective, Nanoscale 6 (2014) 3995–4011. [10] P. Reineck, D. Gomez, S.H. Ng, M. Karg, T. Bell, P. Mulvaney, U. Bach, Distance and wavelength dependent quenching of molecular fluorescence by Au@SiO2 core–shell nanoparticles, ACS Nano 7 (2013) 6636–6648. [11] Y. Lu, Y.D. Yin, Z.Y. Li, Y.N. Xia, Synthesis and self-assembly of Au@SiO2 core–shell colloids, Nano Lett. 2 (2002) 785–788. [12] G.Y. Liu, H.F. Ji, X.L. Yang, Y.M. Wang, Synthesis of a Au/silica/polymer trilayer composite and the corresponding hollow polymer microsphere with a movable Au core, Langmuir 24 (2008) 1019–1025. [13] D.X. Li, Q. He, J.B. Li, Smart core/shell nanocomposites: intelligent polymers modified gold nanoparticles, Adv. Colloid Interface Sci. 149 (2009) 28–38. [14] Y. Kang, T.A. Taton, Controlling shell thickness in core–shell gold nanoparticles via surface-templated adsorption of block copolymer surfactants, Macromolecules 38 (2005) 6115–6121.
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Synthesis of monodisperse magnetic sandwiched gold nanoparticle as an easily recyclable catalyst with a protective polymer shell Wei Zhang a , Bin Liu a , Bei Zhang a , Guomin Bian b , Yonglin Qi b , Xinlin Yang a,∗ , Chenxi Li a a Key Laboratory of Functional Polymer Materials, Ministry of Education, Institute of Polymer Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, PR China b Dynea Ltd. Co., Gaoyao City, Guangdong 526105, PR China
h i g h l i g h t s
g r a p h i c a l
• Magnetic sandwiched gold nanopar-
A magnetic Fe3 O4 @Au/PEGDMA microsphere with a magnetite core, a permeable polymer shell and the sandwiched gold nanoparticles exhibited a stable and readily recoverable catalytic activity.
ticles have been synthesized. • The amino group on Fe3 O4 –NH2 played dual functions during the synthesis. • Fe3 O4 @Au/polyethyleneglycol dimethacrylate was recycled under magnetic field. • Magnetic sandwiched gold nanoparticles had a stable catalytic activity.
a r t i c l e
i n f o
Article history: Received 23 September 2014 Received in revised form 19 November 2014 Accepted 30 November 2014 Available online 9 December 2014 Keywords: Magnetic sandwiched gold nanoparticles Recoverable catalsyst Distillation precipitation polymerization
a b s t r a c t Monodisperse magnetic sandwiched magnetite@gold/poly(ethyleneglycol methacrylate) (Fe3 O4 @Au/PEGDMA) core–shell microspheres were designed and prepared. The whole synthetic procedure mainly involved hydrothermal method for preparation of magnetite core with subsequent modification of (3-aminopropyl)trimethoxysilane (APS) for introduction of the surface amino groups, distillation precipitation polymerization for preparation of the P(EGDMA) shell and in situ reduction of gold precursor HAuCl4 for formation of the sandwiched Au nanoparticles. The thicknesses of the outer polymeric shells were well-controlled via altering the weight ratios of EGDMA monomers to magnetite core during polymerization. The catalytic activity of the sandwiched Fe3 O4 -Au@P(EGDMA) magnetic microsphere was studied by the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AnP) as a model reaction. These magnetic microspheres were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier-transform infrared spectra (FT-IR), thermogravimetric analysis (TGA) and vibrating sample magnetometry (VSM). © 2014 Elsevier B.V. All rights reserved.
1. Introduction Noble metallic nanoparticles have been found applications in many fields, such as optical biosensor [1], electronic devices [2],
∗ Corresponding author. Tel.: +86 22 23502023; fax: +86 22 23503510. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.colsurfa.2014.11.055 0927-7757/© 2014 Elsevier B.V. All rights reserved.
a b s t r a c t
biomedical [3], and catalysis [4]. Especially, the gold nanoparticles have received increasing attention due to their unique behavior different from the corresponding single atoms and bulk materials. These gold nanoparticles with large surface area-to-volume ratio and abundant active surface atoms display outstanding catalytic activity and selectivity for many catalytic reactions, including hydrogenation [5], oxidation [6] and C C coupling reaction [7]. However, it is difficult to investigate the recoverable catalytic
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properties of these gold nano-catalytic systems because of their aggregation and tiny size for separation. There are two efficient strategies for stabilization and easy separation of gold nanoparticles to solve these problems. The first one is to directly encapsulate gold nanoparticles within protective shells, such as metal [8], metal oxide [9], silica [10,11] and polymer [12–18]. Unfortunately, the catalytic efficiency of the gold nanoparticles was considerably decreased due to the presence of a dense silica shell reducing to retard reactant molecules reaching active inner core [11]. The efficiencies of the Au@polymer yolk–shell particle [19–21] were better than that of Au@SiO2 core–shell particle [10,11] because polymer shells were swollen and flexible for easy permeation of the reactants. However, the procedure for the synthesis of yolk–shell particle is relatively complex due to its indispensable multi-step operation. The other strategy is to immobilize the gold nanoparticles on appropriate supports, such as metal oxide [22], carbon-based materials [23], molecular sieves [24], mesoporous silica [25], polymer microspheres [26–28]. The gold nanoparticles with good dispersion can be prevented efficiently from aggregation via efficient stabilization of functional groups on surface of the supporters. However, the aggregation and loss of the gold nanoparticles from the support often occurred during the catalytic reaction to lower their catalytic activity. Therefore, a polymer shell-layer was an efficient way for stabilization of the gold nanoparticles to prevent aggregation for silica@Au/polymer systems [29,30]. These sandwiched gold nanoparticles exhibited a highly catalytic efficiency with good stability. As a result, it is still difficult for readily recovery of the sandwiched metallic catalyst due to the tiny size of the silica supporters. It was necessary to perform recovery of the supported catalyst with aid of ultracentrifugation. Therefore, it is still a challenge to design and synthesize metallic nanocolloids with a highly stable and easily recoverable catalytic property. Magnetic nanoparticle has long been used as a promising support for noble metal nanocolloids due to easy separation feature, which makes it convenient to recycle catalyst. The effects of supporters on catalytic reactions have been investigated via direct immobilization of gold nanoparticles onto magnetic nanoparticles [31–36]. On the one hand, magnetite nanoparticles were easily oxidized in air and eroded under acidic condition. On the other hand, loss of partial gold nanoparticles was unavoidable during recycling catalyst due to weak interaction between gold nanoparticles and magnetite supporter. One of the main approaches to overcome these limitations is to protect magnetic nanoparticles with polymers [37,38] or inorganic shell, such as silica [39–41], metal oxide [42] or carbon-based materials [43]. These protective shells not only protected the magnetic nanoparticles, but also immobilized the gold nanoparticles. However, the whole synthetic procedures were tedious and complex. Thus, it is desirable to develop a method for preparation of gold nanoparticles sandwiched between a protective shell and a magnetic core for readily recovery of catalyst with high efficiency. In this paper, monodisperse magnetic sandwiched magnetite@gold/poly(ethyleneglycol methacrylate) (Fe3 O4 @Au/ PEGDMA) microspheres were designed and prepared. These Fe3 O4 @Au/PEGDMA nanoparticles were synthesized via combination of distillation–precipitation polymerization for formation of polymer shell in the presence of the (3aminopropyl)trimethoxysilane (APS)-modified Fe3 O4 core as a template and the subsequent in situ reduction of HAuCl4 with NaBH4 as reductant. Further, the catalytic activity and recycling property of the sandwiched gold nanoparticles were investigated by reduction of 4-nitrophenol to 4-aminophenol with sodium borohydride as a model reaction.
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2. Materials and methods 2.1. Materials Ferric chloride (FeCl3 ·6H2 O) and sodium acetate (NaAc) was purchased from Tianjin Guangfu Chemical Engineering Institute. Trisodium citrate (Na3 Cit) was obtained from Tianjin Chemical Reagents I Co. Ethyleneglycol dimethacrylate (EGDMA) and (3-aminopropyl) triethoxysilane (APS) were purchased from Alfa Aesar and used without further purification. 2,2 Azobisisobutyronitrile (AIBN) was provided by Chemical Factory of Nankai University and recrystallized from methanol. Acetonitrile (analytical grade, Tianjin Chemical Reagents II Co.) was dried over calcium hydride and purified by distillation before use. Tetrachloroauric acid trihydrate (HAuCl4 ·3H2 O) was purchased from Shenyang Research Institute of Nonferrous Metal, China. Sodium borohydride (NaBH4 ) and ethylene glycol (EG) were purchased from Tianjin Chemical Reagent II Co. 4-Nitrophenol (4-NP) was purchased from Tianjin Chemical Reagent Factory. The other reagents were of analytical grade and used without any further treatment. 2.2. Synthesis of APS-modified magnetite (Fe3 O4 –NH2 ) nanoparticles Magnetite (Fe3 O4 ) nanoparticles were synthesized by a modified solvothermal method according to the literature [44,45]. The procedure was as following: 3.6 g of FeCl3 ·6H2 O and 0.72 g of Na3 Cit were dissolved in a mixture of ethylene glycol/ethanol (90 mL/10 mL) solution through ultrasound irradiation. 4.8 g of NaAc was then added under vigorous magnetic stirring for 30 min. Finally, the resultant mixture was transferred into a Teflon-lined stainless-steel autoclave (with a capacity of 200 mL). The hydrothermal reaction was carried out under 200 ◦ C for 10 h. After reaction, the autoclave was taken out to cool till room temperature. The resultant solid black product was collected by centrifugation and purified by thoroughly extraction with ethanol and deionized water for three times and then dried in a vacuum oven at 50 ◦ C till constant weight. APS-modified magnetite nanoparticles with surface amino group (Fe3 O4 –NH2 ) were synthesized by a sol–gel process. The details were as following: 0.50 g of Fe3 O4 magnetic nanoparticles were dispersed in a mixture of 160 mL of ethanol, 40 mL of deionized water, and 4 mL of 25 wt% ammonium hydroxide aqueous solution through ultrasound irradiation for 5 min. 1.0 mL of APS was then added under vigorous mechanical stirring for another 48 h. The resultant Fe3 O4 –NH2 nanoparticles were purified by repeating centrifugation, decantation, and resuspension in ethanol for three times. The Fe3 O4 –NH2 nanoparticles were dried in a vacuum oven at 50 ◦ C till constant weight. 2.3. Synthesis of Fe3 O4 –NH2 /PEGDMA core–shell magnetic microspheres Fe3 O4 –NH2 @PEGDMA core–shell magnetic microspheres were synthesized by distillation precipitation polymerization of ethyleneglycol dimethacrylate (EGMDA) in the presence of APS-modified Fe3 O4 nanoparticles as seeds (the weight ratio between the monomer to the magnetite seed was set at 4:1). The details were as following: 0.05 g of Fe3 O4 –NH2 nanoparticle as seed was suspended in 40 mL of acetonitrile. Then, 0.2 mL of EGDMA monomer and 0.004 g of AIBN initiator (2 wt% relative to monomers) were dissolved in above suspension in a 50 mL of flask. The flask attaching with a fractionating condenser and receiver was submerged in a heating mantle. The reaction mixture was heated from ambient temperature till boiling state within 15 min and the
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reaction system was kept under refluxing state for 15 min further. The polymerization was carried out with distilling the solvent out of the reaction system. The reaction was ended after 20 mL of acetonitrile was distilled off the reaction mixture within 70 min. After polymerization, the resultant of Fe3 O4 –NH2 @PEGDMA core–shell magnetic microspheres were purified by repeated centrifugation, decantation, and resuspension in ethanol for three times and then dried in a vacuum oven at 50 ◦ C till constant weight. A series of polymerizations were performed for preparation of Fe3 O4 –NH2 @PEGDMA core–shell magnetic microspheres with different shell thicknesses through altering mass ratios of monomers to the magnetite seeds from 3:1 to 5:1, while the other reaction conditions were maintained the same as those for typical distillation–precipitation polymerizations. 2.4. Synthesis of the sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres The Fe3 O4 –NH2 @PEGDMA core–shell microspheres loaded gold nanoparticles were in situ synthesized by reduction of tetrachloroauric acid trihydrate (HAuCl4 ·3H2 O) with NaBH4 as reductant. The details were as following: 0.05 g of Fe3 O4 –NH2 @PEGDMA core–shell microspheres were dispersed in 2.0 mL of deionized water. 2.0 mL of HAuCl4 aqueous solution (20 mg/mL) was then added into the suspension and incubated for 24 h in dark at room temperature. The resultant microspheres were centrifuged from the solution and washed by water with aid of a magnet for five times till the supernatant was colorless. Finally, the microspheres loaded with gold species (AuCl4 − ) were redispersed in 10 mL of deionized water, and 5 mL of 0.05 M NaBH4 aqueous solution was added dropwise under ice water bathing with strong stirring. The reduction of HAuCl4 was performed at room temperature for 2 h. The final product was purified by washing with deionized water for three times and dried in a vacuum oven till constant weight. 2.5. Catalytic reduction of 4-nitrophenol to 4-aminophenol by NaBH4
2.6. Characterization The morphology of the resultant nanoparticles was determined by transmission electron microscopy (TEM) using a Tecnai G2 20S-TWIN microscope. The samples were dispersed in ethanol and a drop of dispersion was dropped onto the surface of a copper grid coated with a carbon membrane for TEM characterization. All size and size distribution reflect the averages about 100 particles each, which are calculated according to the following formulae: U=
DW Dn
k
k Dn =
nD i=1 i i k n i=1 i
Dw =
n D4 i=1 i i
k
n D3 i=1 i i
where U is the polydispersity index, Dn is the number average diameter, Dw is the weight-average diameter, 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 (FT-IR) spectra were determined on a Bio-Rad FTS 135 FT-IR spectrometer over potassium bromide pellet. Diffusion reflectance spectra were scanned over range of 4000–400 cm−1 . The magnetic properties of Fe3 O4 microspheres, APS-modified Fe3 O4 microspheres, Fe3 O4 –NH2 /PEGDMA core–shell microspheres and the sandwiched Fe3 O4 @Au/PEGDMA microspheres were studied in a 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 K␣ ( = 0.15406 nm) radiation at 40 kV and 100 mA. Thermogravimetric analysis (TGA) data were obtained with a heating rate of 10 K/min using a TA TGA-2950 apparatus. Atomic emission spectrometry was performed on an ICP-9000 (N+M) spectrometer (USA Thermo Jarrell-Ash Corp.). Elementary analysis (EA) was determined on a vario EL CUBE apparatus. UV–vis spectroscopy was performed on a JASCO V570spectrometer. 3. Results and discussion
The catalytic activity of sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres was evaluated by reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AnP) with sodium borohydride (NaBH4 ) as reactant under ambient temperature in aqueous solution as a model reaction. The procedure was as following: 0.10 mL of 4-NP (5 mM, 5 × 10−7 mol), 1.0 mL of freshly NaBH4 (0.20 M, 2 × 10−4 mol) aqueous solution, and 2.0 mL of water were mixed in a colorimetric tube. Then, 0.05 mL of catalyst dispersion (1.0 mg/mL, containing 4.6 × 10−9 mol Au) was introduced into the mixture with gentle shaking. The bright yellow solution faded gradually as catalytic reaction proceeded. The catalytic process was monitored by a UV–vis spectrophotometer with the decrease of peak at 400 nm in UV–vis absorption and a simultaneous increase of the absorption at 310 nm, indicating the formation of 4-AnP. To perform the recoverable catalytic activity of the sandwiched Fe3 O4 -Au@PEGDMA magnetic microspheres within several minutes, the amount of the catalyst was increased by fourfold times comparing to that for the above reduction. The conversion of the 4-NP was traced by UV–vis spectrum with a decreasing adsorption peak at 400 nm. After complete reduction of 4-NP, the catalysts were separated with the aid of a magnet and redispersed in the next cycled reaction system. Meanwhile, the supernatant liquid was taken out by a syringe and detected by UV–visible spectrum. The reduction process was repeated for eight times by the similar method.
The whole procedure for synthesis of sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres was illustrated in Scheme 1. 3.1. Preparation of APS-modified magnetite (Fe3 O4 –NH2 ) nanoparticles APS-modified magnetite nanoparticles (Fe3 O4 –NH2 ) were prepared by a modified solvothermal method [44,45] to obtain magnetite nanoparticles (Fe3 O4 ), which were further functionalized with surface amino groups through saline coupling reaction of (3-aminopropyl)trimethoxysilane (APS). The Fe3 O4 nanoparticles were first prepared by a solvo-thermal method via partial reduction of FeCl3 precursor with ethylene glycol (EG) as solvent, sodium acetate (NaAc) as an alkali source, and trisodium citrate (Na3 Cit) as an electrostatic stabilizer. Particularly, ethanol was introduced to improve dispersion of the solid reagent. The resultant Fe3 O4 nanoparticles were shown by TEM micrograph of Fig. 1a, which exhibited a spherical shape and rough surface with an average diameter of about 180 nm. As shown by inserted image of Fig. 1a, the Fe3 O4 nanoparticles consisted of aggregates of small magnetite particles with a mean size of 3–10 nm. The FT-IR spectrum of Fe3 O4 nanoparticles was shown in Fig. 2a. The strong absorption peak at 594 cm−1 was attributed to the Fe–O group. The XRD pattern of the magnetite was shown in Fig. S1. Five characteristic peaks at
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Scheme 1. The procedure for synthesis of the sandwiched Fe3 O4 @Au/PEGDMA microspheres. (The insets in part a were the HRTEM graph.)
30.1, 35.4, 43.1, 56.9, 62.5◦ were attributed to (2 2 0), (3 1 1), (4 0 0), (5 1 1), (4 4 0) diffraction peaks of magnetite and matched well with the standard JCPS card 74-0748. Then, the surface of the Fe3 O4 nanoparticles was modified with APS through saline coupling reaction. In this work, due to the introduction of Na3 Cit to the magnetite synthetic procedure, the magnetite nanoparticles contained abundant hydroxyl groups and carboxyl groups on the surface. Then, the saline coupling reaction was processed through the condensation reaction between the hydroxyl groups and carboxyl groups of the surface of Fe3 O4 nanoparticles as well as silanol groups of APS. The TEM image of the resultant Fe3 O4 –NH2 nanoparticles was shown in Fig. 1b. The shape and size of the APS-modified Fe3 O4 nanoparticles were near the same as those of the former Fe3 O4 nanoparticles. The corresponding FT-IR spectrum was shown in Fig. 2b. The absorption peak at 2930 cm−1 was attributed to the saturated alkyl groups of the APS. Furthermore, the elemental content of nitrogen on the Fe3 O4 –NH2 nanoparticles was 0.85% determined by elementary analysis (EA), which meant that loading capacity of amino group on Fe3 O4 –NH2 particles was 0.61 mmol/g. Fig. 2. FI-IR spectra of (a) Fe3 O4 nanoparticles; (b) APS-modified Fe3 O4 nanoparticles; (c) Fe3 O4 –NH2 @PEGDMA core–shell microspheres.
3.2. Preparation of sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres In our previous works, there were two approaches to coat polymer shell on the surface of inorganic core via distillation precipitation polymerization. In the first approach, the inorganic particles were modified by the vinyl group for capturing radical and oligomer to obtain a polymer shell [46]. In the second approach, the hydrogen-bond interaction between hydroxyl groups on the surface of the inorganic particles and the polar groups of functional monomer was utilized for preparation of the core–shell microspheres [47]. Herein, the magnetite nanoparticles with active groups (including amino group, hydroxyl group and carboxyl group) were used as seeds for growth of the polymer shell. These active groups on surface of the magnetite nanoparticles were useful for direct coating of PEGDMA polymer shell via distillation precip-
itation polymerization, which was similar to the mechanism of the synthesis of Fe3 O4 @PEGDMA core–shell microspheres with aid of hydrogen-bonding interaction in our previous work [37]. The TEM images of the resultant Fe3 O4 –NH2 /PEGDMA core–shell microspheres were shown in Fig. 3. PEGDMA shell was uniformly coated onto magnetite nanoparticles core to form a well-defined core–shell structure with a smooth surface. This was confirmed by the presence of a deep contrast magnetite core and a light contrast polymer shell, which was originated from different mass contrasts between inorganic core and polymeric shell domain. In this work, the thicknesses of PEGDMA shell were easily tuned by varying the mass ratios of monomers to magnetite core during polymerization. The TEM images of the resultant Fe3 O4 –NH2 /PEGDMA core–shell microspheres with different shell
Fig. 1. TEM images of (a) Fe3 O4 nanoparticles and (b) APS-modified Fe3 O4 nanoparticles.
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Fig. 3. TEM images of Fe3 O4 -APS@PEGDMA core–shell microspheres with different shell thickness: (a) 6 nm; (b) 24 nm; (c) 40 nm.
Fig. 4. TGA curves: (a) Fe3 O4 –NH2 and (b)–(d) Fe3 O4 –NH2 @PEGDMA core–shell microspheres with different shell thickness: (b) 6 nm; (c) 24 nm; (d) 40 nm.
thicknesses by varying the mass ratios between monomer and magnetite core from 3 to 5 were shown in Fig. 3a–c. The shell thicknesses were increased from 6, 24 to 40 nm with enhancing mass ratios between monomers and magnetite core from 3/1, 4/1 to 5/1 as summarized in Table 1. The PEGDMA shell on the surface of magnetite core nanoparticles was further characterized by FT-IR spectrum as shown in Fig. 2c. The strong absorption peak at 1730 cm−1 was contributed to vibration of carbonyl groups in PEGDMA component. All these results indicated that PEGDMA shell was successfully coated onto the magnetite core during distillation–precipitation polymerization. The mass contents of polymer shell layers on magnetite surfaces were determined by thermogravimetric analysis (TGA) as illustrated in Fig. 4. All these samples exhibited similar weight loss trends. Generally, there were two main weight loss stages Table 1 Reaction conditions, polymer shell thicknesses and polymer contents of Fe3 O4 –NH2 /PEGDMA core–shell microspheres with different shell thickness. Entry
Fe3 O4 –NH2 (g)
EGDMA (mL)
Shell thickness (nm)
Weight loss by TGA (%)
A B C D
0.05 0.05 0.05 0.05
0 0.15 0.20 0.25
0 6 24 40
16.4 31.2 45.1 57.1
when the samples were heated from room temperature to 800 ◦ C. For TGA of APS-modified Fe3 O4 nanoparticles in Fig. 4a, the first mass loss from room temperature to 200 ◦ C was attributed to the evaporation of physically adsorbed water and solvent, while the second weight loss between 200 and 600 ◦ C was due to decomposition of the organic component of APS and trisodium citrate on the magnetite surface. The organic component on Fe3 O4 –NH2 nanoparticles was 16.4%. Fig. 4b–d showed TGA results of the Fe3 O4 –NH2 /PEGDMA core–shell microspheres with different shell thicknesses. The first weight loss stage was similar to the mass loss of Fe3 O4 –NH2 nanoparticles, while the second weight loss stage from 200 to 500 ◦ C was mainly due to decomposition of polymeric components in the shell layers. The mass losses of these core–shell microspheres were increased with increasing feed of EGDMA monomers to the magnetite core, which were consistent with the results from TEM characterizations. These results were summarized in Table 1. Comparing to the mass loss of the initial APS-modified magnetite nanoparticles during the calcination process, the polymer contents in these core–shell microspheres were calculated as 14.8%, 28.7%, 40.7%, when the ratios of the EGDMA monomer to the magnetite core were enhanced from 3/1, 4/1 to 5/1, respectively. In other words, the polymer shell with controllable thickness was successfully encapsulated on the surface of the magnetite core to obtain Fe3 O4 –NH2 /PEGDMA core–shell microspheres. In the present work, the amino groups on the surface of Fe3 O4 –NH2 inorganic template played dual roles for formation of Fe3 O4 @Au/PEGDMA microspheres, i.e., the positions for distillation precipitation polymerization via efficient hydrogen bonding interaction with ester groups of the EGDMA monomer, adsorption and stabilization of gold nanoparticles via efficient chelating with gold atom. During this process, the Fe3 O4 –NH2 /PEGDMA core–shell microspheres with a shell thickness of 24 nm were used as a sample template for loading gold nanoparticles. The gold ion species loaded between the magnetite core and PEGDMA shell can be attributed to the following three reasons: (i) the surface of the obtained magnetite core (Fe3 O4 ) can successfully modified by (3-aminopropyl) triethoxysilane (APS) to form amino groups ( NH2 ); (ii) the gold ion species can permeated through the PEGDMA shell in the aqueous solution [30]; (iii) the gold ion can be easily adsorbed at the interface through coordination together static interactions with between the gold ion and amino groups [46]. Finally, the sandwiched gold nanoparticles were in situ formed via the reduction of the adsorbed gold cations with sodium borohydride as reductant. Typical TEM micrograph of the resultant sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres was shown in Fig. 5. It indicated that these microspheres had a typical core–shell structure with spherical and smooth surface. The
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Fig. 5. TEM images the sandwiched Fe3 O4 @Au/PEGDMA microspheres with magnifications. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)
average diameter of Fe3 O4 @Au/PEGDMA particles was 231 nm with a narrow disperse index (U) of 1.07. Because magnetite core and gold nanoparticles had similar contrasts in TEM micrographs, the gold nanoparticles with a diameter of about 5 nm (the red circle in Fig. 5b) were not clearly observed between the surface of the magnetite core and the outer polymer shell. The successful loading of gold nanoparticles was confirmed by X-ray diffraction (XRD) as shown in Fig. S1. Compared with the original XRD curve of the magnetite core, the sandwiched Fe3 O4 @Au/PEGDMA microspheres presented new diffraction peaks at 38.5, 44.5, 64.5, and 77.5◦ assigning to (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystal plane of cubic gold matched well with the standard JSPD card 65-2870, which confirmed formation of crystalline gold nanoparticles. This was contributed to amino groups on the magnetite surface to enable Fe3 O4 –NH2 /PEGDMA a strong affinity with gold nanoparticles via coordination effect. Based on the calculation with the Debye–Scherrer equation for the strongest 311 diffraction peak, the grain size of the magnetite particles was about 8.3 nm [44]. However, the Fe3 O4 nanoparticles consisted of aggregates of small magnetite particles, as shown by inserted image of Fig. 1a. The gold nanoparticles were distributed in Fe3 O4 @Au/PEGDMA microsphere as the sandwiched nanocolloids between the Fe3 O4 inner core and PEGDMA shell-layer, as the deep contrast gold nanoparticles were absent from the PEGDMA shell layer. This was consistent with the synthesis of rattle-type microspheres as a support of tiny gold nanoparticles [46], as PEGDMA shell had no functional groups to interact with the gold atoms. The content of gold element in the sandwiched Fe3 O4 @Au/PEGDMA microspheres was 1.81% determined by inductive couple plasma emission spectrometer (ICP), which meant that Au content was 0.092 mmol/g.
3.3. Magnetic properties of the resultant microspheres The magnetic property of magnetite nanoparticles was determined with a shell thickness of 24 nm. The corresponding sandwiched Fe3 O4 -Au@PEGDMA microspheres were studied by a vibrating sample magnetometer (VSM) at room temperature, as shown in Fig. 6. No obvious magnetic hysteresis loops were observed for these three samples from the field-dependent magnetization plots. In other words, the remanence did not exist when the magnetic field was removed, indicating that all resultant microspheres showed a superparamagnetic feature originating
Fig. 6. Hysteresis loops of samples at room temperature: (a) Fe3 O4 nanoparticles; (b) Fe3 O4 –NH2 /PEGDMA microspheres; (c) sandwiched Fe3 O4 @Au/PEGDMA microspheres.
from small magnetic nanoparticles (smaller than the single magnetic domain) at room temperature. The magnetic properties of the three microspheres were listed in Table 2. The saturation magnetization (Ms ) values of Fe3 O4 nanoparticles, Fe3 O4 –NH2 /P(EGDMA) core–shell microspheres and the corresponding sandwiched Fe3 O4 @Au/PEGDMA microspheres were 44.8, 29.1, and 27.8 emu/g, respectively. These results indicated that the magnetization of these magnetic samples decreased considerably with the addition of polymer shell or gold nanoparticles. Such a decrease was attributed to the decrease in effective mass content of the magnetite component. However, the magnetism of
Table 2 Magnetization of Fe3 O4 , Fe3 O4 –NH2 /PEGDMA microspheres and the corresponding sandwiched Fe3 O4 @Au/PEGDMA microspheres. Entry
Saturation magnetization (emu/g)
Coercive force (Oe)
Fe3 O4 Fe3 O4 –NH2 /P(EGDMA) Fe3 O4 @Au/P(EGDMA)
44.8 29.1 27.8
17.3 18.6 16.5
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these microspheres was still strong enough to be separated and controlled by an external magnetic field. 3.4. Catalytic properties of the sandwiched Fe3 O4 @Au/PEGDMA magnetic microspheres The polymer (PEGDMA) shell with good permeability can efficiently protect the gold nanoparticles from dropping or aggregation, and favor the access of the reactant molecules [31,47]. To investigate the catalytic activity of the sandwiched Fe3 O4 @Au/PEGDMA microspheres, reduction of 4-nitrophenol (4NP) to 4-aminophenol (4-AnP) with NaBH4 as reductant under ambient temperature in aqueous solution was used as a model reaction. An aqueous solution of 4-NP with NaBH4 exhibited a light yellow color with an absorption maximum at 400 nm in UV–vis spectra, while the aqueous solution of the corresponding product 4-AnP had a weak peak at 300 nm in UV–vis spectral. Without the addition of catalyst, the reduction reaction did not conduct even after mixing the resultant 4-NP and NaBH4 for 24 h, which was proved by a constant adsorption of UV–vis peak at 400 nm. However, the reduction reaction was quickly performed after addition of a very small amount of catalyst (0.05 mL aqueous solution, 1.0 mg/mL). The reaction was monitored by UV–vis spectroscopy as shown in Fig. 7a. The absorption peak at 400 nm (characteristic absorption peak of 4-NP) was decreased gradually and disappeared completely after 15 min, while a new absorption peak at 300 nm (characteristic absorption peak of 4-AnP) was simultaneously increased with proceeding reaction. It meant that the 4-nitrophonelate was reduced to 4-aminophenol in the presence of sandwiched Fe3 O4 @Au/PEGDMA catalyst. Furthermore, the kinetic of catalytic reaction was studied in this work. As NaBH4 was used in a much higher concentration relative to that of 4nitrophonelate (CNaBH4 /C4-NP = 400), this reaction can be considered as the pseudo-first-order reaction with regarding to the reactant, during which the concentration of NaBH4 was assumed to be constant. The pseudo-first-order rate constant (k) at 293 K was calculated from the slope of a linear correlation between ln(Ct /C0 ) and reaction time as 0.194 min−1 as shown in Fig. 7b. This indicated that the catalyst reactions possessed high reaction rate under the relative low amount of catalyst. The Fe3 O4 –NH2 @PEGDMA microspheres with different shell thickness (6 nm and 40 nm) were incubated in HAuCl4 aqueous solution in the same concentration, and then washed by water. Finally, the adsorbed AuCl4 − was reduced by NaBH4 . The kinetic rate constants (k) were estimated from the slope to be 0.366 and 0.103 min−1 , respectively (Fig. S2). These results indicated that the catalytic activities of the Fe3 O4 –NH2 @PEGDMA microspheres were inversely dependent to the shell thickness, which may be due to the more difficult permeation for the reactants and diffusion of the product between the catalytic media and the sandwiched gold nanoparticles. As shown in Fig. S3, the rate constant (k) of the Fe3 O4 @Au/PEGDMA microspheres with the gold content of 1.01% was about 0.083 min−1 , which was lower than that of the microspheres with the gold content of 1.81% (0.194 min−1 ). This implied that the catalytic reaction rate was proportional to the loading capacity of the gold nanoparticles. In order to study the recycled property of the sandwiched Fe3 O4 –Au@P(EGDMA) microspheres catalyst, we increased the amount of catalyst by five times to catalyze the reduction of 4-NP to 4-AnP within five min. In each cycle, the catalytic reaction was performed completely for 5 min, and then separated by centrifugation for the next cycle of catalysis. Even after recycling for eight times, the conversion of 4-NP was only slightly decreased from 99.3% for the first cycle to 96.8% for the eighth cycle, as illustrated in Fig. 7c. All these results revealed that the sandwiched Fe3 O4 @Au/PEGDMA
Fig. 7. Kinetic of the catalytic reaction: (a) UV–vis spectra of the catalytic reduction of 4-NP to 4AnP developed at different reaction times; (b) plot of ln(Ct /C0 ) versus time; (c) reusability of Fe3 O4 -Au@P(EGDMA) microspheres as catalysts for the reduction of 4-NP with NaBH4 .
microspheres catalyst exhibited high catalytic efficiency and stable activity. In such a manner, the amino groups on the surface of Fe3 O4 –NH2 core can effectively anchor a high amount of metal nanoparticles through in situ reduction, and the polymer shell had a good permeability for the catalysis as well as protected the metal nanoparticles from leakage.
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4. Conclusion The magnetic Fe3 O4 @Au/PEGDMA microspheres with a magnetite core, a permeable polymer shell and the sandwiched gold nanoparticles were easily prepared by a combination of the minor modified solvo-thermal method for magnetite core, distillation–precipitation polymerization for formation of protective shell-layer and in situ reduction of HAuCl4 with NaBH4 as reductant. The amino groups on the surface of Fe3 O4 –NH2 inorganic template played dual roles for formation of Fe3 O4 @Au/PEGDMA microspheres, i.e., the positions for distillation precipitation polymerization and stabilization of the resultant nanoparticles. The magnetic sandwiched gold nanoparticles exhibited a high catalytic activity with reduction of 4-NP to 4-AnP as a model reaction. The formation of a permeable PEGDMA shell with controlled thickness was synthesized by distillation precipitation polymerization in the absence of highly toxic reagent, such as KCN for formation of porous silica shell [29]. Moreover, the Fe3 O4 @Au/PEGDMA with sandwiched gold nanocolloids exhibited a stable and more easily recoverable catalytic activity under an external magnetic field, for which the ultracentrifugation was not necessary any more comparing those in the literatures [29,30]. Acknowledgements This work was supported by the Natural Science Foundation of China (Grant Nos. 21174065 and 21374049), PCSIRT (IRT1257). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa. 2014.11.055. References [1] K. Saha, S.S. Agasti, C. Kim, X.N. Li, V.M. Rotello, Gold nanoparticles in chemical and biological sensing, Chem. Rev. 112 (2012) 2739–2779. [2] D.V. Talapin, J.S. Lee, M.V. Kovalenko, E.V. Shevchenko, Prospects of colloidal nanocrystals for electronic and optoelectronic applications, Chem. Rev. 110 (2010) 389–458. [3] E.C. Dreaden, A.M. Alkilany, X.H. Huang, C.J. Murphy, M.A. El-Sayed, The golden age: gold nanoparticles for biomedicine, Chem. Soc. Rev. 41 (2012) 2740–2749. [4] A.S.K Haschmi, G.J. Hutchings, Gold catalysis, Angew. Chem. Int. Ed. 45 (2006) 7896–7936. [5] Y. Zhu, H.F. Qian, B.A. Drake, R.C. Jin, Atomically precise Au25 (SR)18 nanoparticles as catalysts for the selective hydrogenation of ␣,-unsaturated ketones and aldehydes, Angew. Chem. Int. Ed. 49 (2010) 1295–1298. [6] M.D. Hughes, Y.J. Xu, P. Jenkins, P. Mcmorn, P. Landon, D.I. Enache, A.F. Carley, G.A. Attard, G.J. Hutchings, F. King, E.H. Stitt, P. Johnston, K. Griffin, C.J. Kiely, Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions, Nature 434 (2005) 1132–1135. [7] M.D. Levin, F.D. Toste, Gold-catalyzed allylation of aryl boronic acids: accessing cross-coupling reactivity with gold, Angew. Chem. Int. Ed. 53 (2014) 6211–6215. [8] H.L. Jiang, T. Akita, T. Ishida, M. Haruta, Q. Xu, Synergistic catalysis of Au@Ag core–shell nanoparticles stabilized on metal–organic framework, J. Am. Chem. Soc. 133 (2011) 1304–1306. [9] G.D. Li, Z.Y. Tang, Noble metal nanoparticle@metal oxide core/yolk–shell nanostructures as catalysts: recent progress and perspective, Nanoscale 6 (2014) 3995–4011. [10] P. Reineck, D. Gomez, S.H. Ng, M. Karg, T. Bell, P. Mulvaney, U. Bach, Distance and wavelength dependent quenching of molecular fluorescence by Au@SiO2 core–shell nanoparticles, ACS Nano 7 (2013) 6636–6648. [11] Y. Lu, Y.D. Yin, Z.Y. Li, Y.N. Xia, Synthesis and self-assembly of Au@SiO2 core–shell colloids, Nano Lett. 2 (2002) 785–788. [12] G.Y. Liu, H.F. Ji, X.L. Yang, Y.M. Wang, Synthesis of a Au/silica/polymer trilayer composite and the corresponding hollow polymer microsphere with a movable Au core, Langmuir 24 (2008) 1019–1025. [13] D.X. Li, Q. He, J.B. Li, Smart core/shell nanocomposites: intelligent polymers modified gold nanoparticles, Adv. Colloid Interface Sci. 149 (2009) 28–38. [14] Y. Kang, T.A. Taton, Controlling shell thickness in core–shell gold nanoparticles via surface-templated adsorption of block copolymer surfactants, Macromolecules 38 (2005) 6115–6121.
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