Synthesis of hybrid silica nanoparticles grafted with thermoresponsive poly(ethylene glycol) methyl ether methacrylate via AGET-ATRP

Abstract

Thermosensitive SiO₂ nanoparticles were prepared with graft-on poly(ethylene glycol) methyl ether methacrylate (POEGMA, M_n ∼ 300 or 500 g mol⁻¹) via atom transfer radical polymerization with activators generated through electron transfer (AGET ATRP). CuBr₂/PMDETA and ascorbic acid (AA) were employed as the catalyst and reducing agent respectively in the fabrication process. The structure and surface composition of our hybrid materials were analyzed using Fourier Transform Infrared (FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS) in detail. The results of Thermogravimetric Analysis (TGA) show that AGET-ATRP could provide a higher surface grafting density to get more pronounced thermosensitive properties. The morphology of our hybrid SiO₂ nanoparticles was characterized by Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). measurements of turbidity, Dynamic Light Scattering (DLS), and phase transfer were conducted to investigate the thermal responses of the hybrid material. In addition, Pickering emulsions were prepared using such hybrid SiO₂-POEGMA as an emulsifier. Moreover, its fast de-emulsification above a lower critical solution temperature (LCST) may be exploited for the efficient separation and recycling of catalysts in future.

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Introduction

Silica/polymer hybrid nanoparticles have attracted much attention due to their fascinating optical, electronic, magnetic, biological, pharmacological, and catalytic properties.^1–7 Typically, hybrid nanoparticles incorporate polymer shells which are the principal determinants of the chemical properties of nanoparticles and their responsiveness to external stimuli.^8–11

Silica/polymer hybrid nanoparticles can be prepared by efficient controlled polymerization techniques, such as atom transfer radical polymerization (ATRP), which can incorporate organic polymers with a precise degree of substitution, composition, and functionality into both planar surfaces,^12–14 and colloidal/particulate substrates.^15–19 Patten and colleagues^20,21 demonstrated that styrene and methyl methacrylate could be controllably polymerized from silica nanoparticles through surface-initiated ATRP.

However, the initiator system of ATRP is generally toxic and expensive and the catalyst such as CuCl is unstable under air and easily oxidated.^22,23 To overcome these drawbacks, Wang and Matyjaszewski discovered reverse ATRP, in which the initiation step involves the active radical combining a halogen atom with the catalyst to form a dormant halide species and a reduced transition metal activator.^24,25 However, to reach the ATRP equilibrium, a conventional radical initiator such as 2,2-azobisisobutyronitrile (AIBN) is typically added to generate radicals that reduce Cu(II) to Cu(I) and generate the halogen-containing initiator in situ. In reverse ATRP, the amount of catalyst cannot be independently reduced and should be comparable to the radical initiator because the added Cu(II) complex is the only source of transferable atoms.²⁶

Fortunately, a new method to achieve the formation of an active catalyst, a procedure for preparing an “activator generated by electron transfer” for ATRP (AGET ATRP), was developed by Krzysztof Matyjaszewski.²⁷ This method resolved these issues by using electron transfer instead of employing a conventional radical initiator. A reducing agent, typically Sn(EH)₂, ascorbic acid (AA) or phenol, is used to generate the activator through reduction of the Cu(II) complex without the involvement of organic radicals that can initiate new chains.^26,28,29 Therefore, AGET ATRP has the advantages of normal ATRP and reverse ATRP alongside additional benefits such as a high rate of conversion of the monomer in a well controlled manner.^30–32 Zhu and colleagues synthesised a series of versatile magnetic nanoparticles with graft-on functional polymers employing iron-catalyzed AGET ATRP.^33–36

To date, perhaps the most extensively studied thermosensitive hybrid nanoparticles have been coated with poly(N-isopropylacrylamide) (PNIPAM) brushes, which exhibit LCST at ∼32 °C, close to body temperature.^37–40 Wang and colleagues⁴¹ synthesised SiO₂ nanoparticles grafted with PNIPAM shells by surface-initiated ATRP in aqueous solution, using CuCl/bpy as a catalytic system. Despite its widespread popularity in materials science, PNIPAM has narrow and low LCST, which limits its practical applications. So, the pursuit of tunable LCST is one of major objectives of thermosensitive hybrid nanoparticles.^42–47 Thermosensitive poly(ethylene glycol) methyl ether methacrylate was recently proposed as an attractive alternative to PNIPAM.^46,48–52 In fact, the balance between hydrophobic and hydrophilic moieties in the molecular structure of the amphiphilicity polymers is the key-parameter that determines LCST.⁵³ The side-chains (i.e., EO unit) of poly(ethylene glycol) methyl ether methacrylate are hydrophilic group by forming stabilizing H-bonds with water.^54,55 The introduction of hydrophilic group, such as poly(ethylene glycol, EO unit) moieties, into the temperature-responsive polymers might raise the LCST, whereas that of a hydrophobic group might lower the LCST.^49,51,56 Therefore, the LCST can be finely tuned through modification of the length of the oligo(ethylene glycol) side chain.^43,53,57 In addition, POEGMA is well-known to be an uncharged, nontoxic, protein-resistant, and biocompatible polymer and has been widely applied in biomedical applications.^58,59

The efficient segregation and recycling of catalysts is one of major obstacles for sustainable and green chemistry.^60–65 Pickering emulsions using nanoparticles as emulsifiers are emerging as an attractive platform for designing efficient catalytic systems by providing a large oil–water reaction interface.^66–72 Yang⁶⁷ successfully developed a novel method for in situ separation and recycling of submicrometer-sized solid catalysts by tuning the pH value based on Pickering-emulsion inversion.

In this paper we describe the synthesis of thermosensitive poly(ethylene glycol) methyl ether methacrylate (POEGMA, M_n ∼ 300 or 500 g mol⁻¹) brushes on silica nanoparticles using AGET ATRP and study the thermal properties of this hybrid material through measurements of turbidity, phase transfer, and dynamic light scattering (DLS). Interestingly, the LCST can be finely tuned through modification of the length of the oligo(ethylene oxide) side chain. Then, Pickering emulsions prepared using hybrid SiO₂-POEGMA were investigated for the efficient separation and recycling of catalysts in future. The general scheme for the preparation of bare SiO₂ nanoparticles (SiO₂), amino-functionalized (SiO₂-NH₂), 2-bromoisobutyrate-functionalized (SiO₂-Br), and thermoresponsive POEGMA grafted SiO₂ nanoparticles (SiO₂-POEGMA) are shown in Scheme 1.

Scheme 1 Schematic illustration of the synthesis of hybrid silica nanoparticles coated with thermoresponsive poly(ethylene glycol) methyl ether methacrylate (POEGMA) brushes via AGET-ATRP. The densely grafted POEGMA brushes exhibit thermosensitive swelling/collapse phase transitions.

Experimental

Materials

Tetraethoxysilane (TEOS), CuBr₂, CH₂Cl₂, 2-bromoisobutyryl bromide, 3-aminopropyltriethoxysilane (APTES), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA), and ascorbic acid (AA) were obtained from Aladdin Reagent Co. Poly(ethylene glycol) methyl ether methacrylate (POEGMA₃₀₀ and POEGMA₅₀₀) were purchased from Sigma-Aldrich and used without further purification. Absolute alcohol and ammonia were of analytical grade from Beijing Chemical works. Toluene, mineral oil, ethyl acetate and triethylamine were purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd. Water used in the experiments was triply distilled by a quartz water purification system. Its conductivity was lower than 1.8 μS cm⁻¹, as measured by a DDSJ-308A type conductivity instrument in our laboratory.

Synthesis of SiO₂ and SiO₂-NH₂

Nano-SiO₂ particles were prepared in absolute ethanol according to the Stöber method.^73,74 TEOS (9 mL) and ethanol (300 mL) were added into a three-necked round-bottomed flask at ambient temperature, and ammonium hydroxide (25% (w/w) in water, 30 mL) then added to the system through a dropping funnel while the flask was stirred vigorously. The solution was incubated for eight hours to obtain a bare nano-SiO₂ particle suspension (solution A). APTES (9 mL), ethanol (15 mL) and ammonium hydroxide (25% (w/w) in water, 3 mL) were added into a round-bottomed flask, and then injected dropwise into solution A while the flask was stirred vigorously. The reaction mixture was heated to reflux temperature (78 °C) for 10 h. The solution was then cooled to room temperature and a powdery white product obtained. The powder was filtered and washed with ethanol. The sample was then dried in a vacuum oven at 60 °C for 12 h.

Synthesis of SiO₂-Br

SiO₂-NH₂ (2.50 g) was dispersed in 150 mL of anhydrous toluene by ultrasonication. After cooling to 0 °C, triethylamine (4 mL) and 2-bromoisobutyryl bromide (3 mL) were added dropwise. The solution was stirred at 0 °C for 6 h and then at room temperature for 12 h. The nanoparticles were purified and isolated following similar procedures to those described for the synthesis of SiO₂-NH₂. The final product was dried in a vacuum oven at 60 °C for 12 h.

Synthesis of SiO₂-POEGMA via AGET-ATRP

SiO₂-Br (1.50 g) was dispersed in 10 mL of CH₂Cl₂ via ultrasonication. Then, CuBr₂ (0.1671 g) and PMDETA (0.7793 g) were added to a stirred round-bottomed flask at room temperature to form a homogeneous solution. POEGMA₅₀₀ (3.0 g) was then added and, once the solution became well mixed and had returned to room temperature, 30 mL of deionized water was added. The CH₂Cl₂ was slowly evaporated from the mixture through purging with nitrogen for 30 min. Ascorbic acid (0.04 mmol) was injected dropwise into the deoxygenated aqueous solution at 70 °C. The SiO₂-POEGMA₅₀₀ nanoparticles were recovered by filtration, washed with ethanol several times and then extracted 12 h using a Soxhlet extractor. The collected sample was dried in a vacuum oven at 60 °C for 12 h. The SiO₂-POEGMA₃₀₀ nanoparticles were prepared by similar steps.

Characterization

Fourier transform infrared spectroscopy (FTIR) spectra were obtained using a Bruker V70 instrument by incorporating hybrid particles into KBr pellets.

X-ray photoelectron spectroscopy (XPS) was recorded with a PHI quantum ESCA micro-probe system (THERMO VG ESCALAB250), using the Al Kα line as the radiation source at an energy of 1486.6 eV, 16 mA × 12 kV. The fitting XPS curves were analyzed with multipeak 6.0 A software.

Thermogravimetric analysis (TGA) was carried out using a thermal analyzer (HTC-3) under a nitrogen flow (10 mg of the sample was heated in steps of 10 °C min⁻¹ in an Al₂O₃ pan for analysis).

Weight- and number-average molecular weights, M_w and M_n, respectively, were determined using a Waters-greeze gel permeation chromatography (GPC) with polystyrene as the standard for molecular weight calibration. GPC analyses were run in THF at 20 °C with an RI-detector.

¹H NMR spectra were obtained in CDCl₃ at room temperature using a Bruker DRX-300 instrument and tetramethylsilane as an internal standard.

The morphologies of pure silica and the polymer grafted silica nanoparticles were characterized with a JEM-1100 TEM (JEOL, Tokyo, Japan) at 100 kV. They were dispersed in ethanol in an ultrasonic bath for 15 min, deposited on a copper grid, and the grid then dried at room temperature.

Scanning electron microscopy (SEM) images were acquired on a LEO 1525 scanning electron microscope. The SEM samples were prepared by adding a few drops of a dilute suspension onto a clean silicon wafer and allowing the solvent to evaporate in air.

DLS measurements were conducted with a Zeta Plus Particle Size Analysis instrument (Brookhaven, USA) at a scattering angle of 90°. The light source was a Spectra-physics 127 helium neon laser (633 nm, power 35 mW). The sample was held at each temperature for 15 min to ensure that it had reached equilibrium. The optical transmittance of aqueous solutions of hybrid nanoparticles was measured at a wavelength of 560 nm on a Unico UV/vis 2802PCS spectrophotometer using a thermostatically controlled bath.

Emulsion droplets were observed by microscope (Leica DMI 6000) at 10 × 10 magnification.

Results and discussion

FTIR spectra

Infrared spectroscopy was employed to verify that molecules had been successfully attached to the particle surface.⁷⁵ The FTIR spectra of bare SiO₂, SiO₂-Br and SiO₂-POEGMA₅₀₀ are shown in Fig. 1. For bare SiO₂ (Fig. 1A), absorption bands characteristic of tetrahedral silicate structures occur at 1100 cm⁻¹ (Si–O stretching) and 465 cm⁻¹ (Si–O bending). Si–OH bending can be observed at 945 cm⁻¹ and Si–O–Si bending at 800 cm⁻¹. After amidation, a doublet appears in the vicinity of 1380 cm⁻¹, characteristic absorptions due to the deformation of the two isopropyl methyl groups in 2-bromoisobutyrate residues.⁷⁶ (Fig. 1B and insert diagram.) For SiO₂-POEGMA₅₀₀, the absorption band at 2980 cm⁻¹ is ascribed to the –C–H antisymmetric stretching vibration, and the absorption band at 2850 cm⁻¹ is the –CH₂ stretching vibration. The absorption band at 1730 cm⁻¹ belongs to –C=O stretching (Fig. 1C). The presence of these bands confirm that the SiO₂-POEGMA₅₀₀ has been prepared successfully. Similar FTIR spectra were noticed for SiO₂-POEGMA₃₀₀ (data not shown).

Fig. 1 FTIR spectra of (A) bare SiO₂, (B) SiO₂-Br, and (C) SiO₂-POEGMA₅₀₀.

Surface composition analysis by XPS

XPS was used to determine the composition of samples and identify the valence states of various species.⁷⁷ As shown in Fig. 2A, the POEGMA₅₀₀'s C1s spectra could be fitted with three peak components attributable to: hydrocarbon (C_xH_y: 284.71 eV), carbon adjacent to oxygen (C–O: 286.02 eV), and carbonyl (O–C=O: 288.64 eV). For O1s spectra of bare SiO₂ (Fig. 2B), there is only one peak at 532.67 eV (O atom in Si–O bond), while a further peak with a binding energy of 533.66 eV (O atom in C=O bond) appears in O1s spectra of SiO₂-POEGMA₅₀₀ (Fig. 2C). From a wide scan of the bare SiO₂ and SiO₂-POEGMA₅₀₀ (Fig. S1 in the ESI†), we could see that the carbon content of SiO₂-POEGMA₅₀₀ increased and silicon content decreased in comparison to bare SiO₂. The specific elemental composition of bare SiO₂ and SiO₂-POEGMA₅₀₀ is shown in Tables S1 and S2 of ESI† respectively. The above results can be considered clear evidence of our success in preparing SiO₂-POEGMA₅₀₀. Similar XPS spectra were got for SiO₂-POEGMA₃₀₀ (data not shown).

Fig. 2 XPS spectra of (A) C1s of SiO₂-POEGMA₅₀₀, (B) O1s of the bare SiO₂, (C) O1s of SiO₂-POEGMA₅₀₀.

Thermogravimetric analysis

Thermogravimetric analysis (TGA), which was performed under a nitrogen flow at a heating rate of 10 °C min⁻¹, indicated that the weight retention at 420 °C was 93.5% for bare SiO₂ and 92.2% for SiO₂-NH₂ nanoparticles (Fig. 3A and B). The weight loss of bare particles might be due to the associated water loss and continued condensation reaction.⁷⁸ The mass retention of SiO₂-POEGMA₅₀₀ was far less than of SiO₂-Br (Fig. 3C and D), showing that POEGMA₅₀₀ was grafted onto SiO₂ by high-density.⁷⁹ We got alike mass retention for SiO₂-POEGMA₃₀₀ (data not shown). The molecular weights (M_n = 17 234 and M_w = 38 237) of the polymer POEGMA₅₀₀ resulting from AGET ATRP conducted by GPC further verified that POEGMA₅₀₀ could be well polymerized by AGET ATRP.⁸⁰ A typical ¹H NMR spectrum of the polymer POEGMA₅₀₀ was shown in Fig. S2 in the ESI.†

Fig. 3 Thermogravimetric analysis (TGA) of (A) bare SiO₂, (B) SiO₂-NH₂, (C) SiO₂-Br, and (D) SiO₂-POEGMA₅₀₀.

Morphology analysis

The morphology (size, shape and distribution) of the nanoparticles was observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). TEM images of bare SiO₂, SiO₂-NH₂, SiO₂-Br and SiO₂-POEGMA₅₀₀ composites are showed in Fig. 4. As can be seen from the images, bare SiO₂ exhibits an average external diameter about 327 nm and nothing can be seen around the nanoparticles (Fig. 4A). After grafted with POEGMA₅₀₀, the observed diameter of SiO₂-POEGMA₅₀₀ became larger and clear core/shell structures were formed (Fig. 4D). This implied that the polymer chains had been linked to SiO₂.

Fig. 4 TEM images of (A) bare SiO₂ (327 nm), (B) SiO₂-NH₂ (337 nm), (C) SiO₂-Br (345 nm), and (D) SiO₂-POEGMA₅₀₀ (357 nm).

SEM images of the colloidal silica particles before and after the surface initiated polymerization of POEGMA₅₀₀ were depicted in Fig. 5. The bare SiO₂ morphology was spherical and monodisperse, as expected (Fig. 5A). As can be seen in Fig. 5B, each silica particle is coated with a relatively thick outer layer of POEGMA₅₀₀ chains compared with bare SiO₂, consistent with the observations of C. Perruchot.⁸¹ Similar TEM and SEM images of SiO₂-POEGMA₃₀₀ were noticed (data not shown).

Fig. 5 SEM images of (A) bare SiO₂, (B) SiO₂-POEGMA₅₀₀.

Turbidity measurements

Fig. 6 shows the temperature dependence of optical transmittance at 560 nm obtained for aqueous solutions of SiO₂-POEGMA and bare SiO₂. For SiO₂-POEGMA₅₀₀ (Fig. 6B), no appreciable changes in the optical transmittance were observed within the range 25–85 °C. In this temperature range, the outer zone of POEGMA brushes remained hydrophilic and aggregation between particles did not occur. Above 85 °C, however, transmittance increased abruptly from 2% to 70%, implying that the hydrogen bonds had broken and that aggregation of hybrid nanoparticles occurred in the temperature range 85–94 °C. For SiO₂-POEGMA₃₀₀ (Fig. 6C), transmittance increased abruptly in the temperature range 58–68 °C. Liu⁸ predicted that this type of collision will contribute to the aggregation between different hybrid nanoparticles since the PNIPAM corona becomes “sticky” above the critical phase transition temperature. However, no noticeable variations were recognized for bare SiO₂ at all temperatures trialed. Illustrative images of bare SiO₂ and SiO₂-POEGMA₅₀₀ before and after heating are shown in Fig. 7.

Fig. 6 Temperature-dependent optical transmittance at 560 nm obtained for 5.0 × 10⁻² g mL⁻¹ aqueous solution of SiO₂-POEGMA₃₀₀ (red line), SiO₂-POEGMA₅₀₀ (blue line), bare SiO₂ (black line).

Fig. 7 Illustrative images of bare SiO₂ and SiO₂-POEGMA₅₀₀ before and after heating: bare SiO₂: (A) 25 °C, (B) 90 °C; SiO₂-POEGMA₅₀₀: (C) 25 °C, (D) 90 °C.

DLS analysis

The temperature-dependent size and size distribution of SiO₂-POEGMA₃₀₀ and SiO₂-POEGMA₅₀₀ in aqueous solutions were measured by dynamic light scattering (DLS). Aqueous dispersions of SiO₂-POEGMA₅₀₀ at a concentration of 0.005 g mL⁻¹ were gradually heated from 25 °C to 95 °C. As shown in Fig. 8A, when the temperature dropped below 75 °C, no significant changes in the average hydrodynamic diameter were observed with increasing temperature. The average hydrodynamic size decreased over a broad temperature range from 75 °C (366.5 nm) to 95 °C (183.2 nm), a much wider range than the LCST of free POEGMA in water (90 °C), consistent with the observations by Kizhakkedathu and colleagues of PNIPAm brushes on PS latex particles.⁸² While, as shown in Fig. 8B the average hydrodynamic size decreased from 60 °C (352.9 nm) to 67 °C (219.8 nm) for SiO₂-POEGMA₃₀₀ (LCST ∼ 65 °C).

Fig. 8 Average hydrodynamic diameters of the SiO₂-POEGMA₃₀₀ and SiO₂-POEGMA₅₀₀ in the different temperature measured by dynamic light scattering.

Owing to the relatively high graft densities (see Fig. 3D) of POEGMA chains on the nanoparticles, the interactions between neighboring polymer chains may play a role in the transition. It was predicted by Zhulina and colleagues that spherical polymer brushes should undergo a transition in which the collapse of polymer brushes would begin in the outermost layers.⁸³ Inter-chain interactions make the collapse of the grafted polymers weaker than that of free polymer chains in a dilute solution as the inter-chain contacts reduce water–monomer interactions. Zhao and colleagues⁸⁴ proposed that the broader transitions of polymer brushes on the nanoparticles might be also related to different degrees of freedom of segments along the grafted polymer chains.

Fig. 9 shows the particle size distribution of SiO₂-POEGMA₅₀₀ at 55 °C and 92 °C. It can be seen that the distribution of the particle size was monodisperse, in agreement with the results of TEM and SEM.

Fig. 9 The particle size distribution of the SiO₂-POEGMA₅₀₀ measured by dynamic light scattering at 55 °C and 92 °C, respectively.

Phase transfer triggered by temperature

Bare SiO₂ (0.05 g) and SiO₂-POEGMA₅₀₀ (0.05 g) were dispersed in 4 mL of deionized water by ultrasonication, respectively. Ethyl acetate (4 mL) was added into the bottle and emulsified for 2 min. Illustrative images showing phase transfer before and after heating are recorded in Fig. 10. Bare SiO₂ precipitated at all temperatures trialed (Fig. 10A and B). SiO₂-POEGMA₅₀₀ remained in the aqueous layer at 25 °C, however, they then transferred into the oil phase above 90 °C (Fig. 10C and D), which is similar to the observations of Zhao and colleagues.⁸⁵ For SiO₂-POEGMA₃₀₀, the phase transfer temperature was about 65 °C. A similar conclusion was got when the oil phase was changed to toluene (Fig. 10E and F) or mineral oil (Fig. 10G and H).

Fig. 10 Phase transfer triggered by temperature of bare SiO₂ and SiO₂-POEGMA₅₀₀.

Pickering-emulsion preparation

SiO₂-POEGMA₅₀₀ (0.2 g) was dispersed in 4 mL of deionized water using a vortex mixer (Fisher Scientific Analogue Vortex Mixer, 120 V at 3000 rpm) for 2 min. Toluene (4 mL) was added into the bottle and emulsified for 2 min and emulsions were formed. As can be seen in Fig. 11A, SiO₂-POEGMA₅₀₀ was well distributed in the upper layer with good stability against sedimentation (at least two weeks). Microscopic images confirmed that the upper layer was a Pickering emulsion phase; spherical droplets can clearly be observed in Fig. 11C. Interestingly, de-emulsification was observed when the Pickering emulsion was heated above 90 °C (Fig. 11B) and emulsion droplets were damaged (Fig. 11D). Intriguingly, a Pickering emulsion could be rapidly reformed by cooling down the sample to allow re-emulsification, and emulsion droplets were still observed (Fig. S3 ESI†). Similar phenomenon was observed for Pickering-emulsion prepared by SiO₂-POEGMA₃₀₀. This emulsification–demulsification inversion related to the temperature-responsive surface chemistry could in future be exploited for separation and recycling of catalysts.^67,86,87

Fig. 11 Photographs and light micrographs of the emulsification–demulsification inversion of the Pickering emulsion.

Conclusions

Narrow-disperse thermoresponsive hybrid silica nanoparticles grafted with POEGMA (M_n ∼ 300 or 500 g mol⁻¹) brushes were synthesized by AGET ATRP from SiO₂-Br nanoparticles. The experimental results confirmed that the SiO₂-POEGMA was prepared successfully. Optical transmittance measurement, dynamic light scatter (DLS) and phase transfer indicated that thermo-induced phase transitions of SiO₂-POEGMA₃₀₀ were around 65 °C, while thermo-induced phase transitions of SiO₂-POEGMA₅₀₀ was around 90 °C. Pickering emulsions prepared by hybrid SiO₂-POEGMA possessed the ability to undergo emulsification–demulsification inversion in response to temperature, which may have potential applications in biomaterials, sensors, drug delivery, stimuli-responsive materials, and catalyst separation and recycling.

Acknowledgements

We gratefully acknowledge financial support from the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (Grant no. 2014BAE03B03), Natural Science Found of Shanxi Province (Grant no. 2014011014-1), China National Petroleum & gas Corporation science and technology development project “Nano intelligent chemical flooding agent” (2011A-1001), Shanxi Scholarship Council of China (no. 2013-151), Shanxi Natural Science Foundation (no. 2012011046-3) and Shanxi Province Graduate Outstanding Innovation Project (no. 0113127).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra17013j

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