Facile fabrication of Fe₃O₄@PS/PGMA magnetic Janus particles via organic–inorganic dual phase separation

Abstract

In this study, a facile strategy is developed to fabricate Fe₃O₄@polystyrene/poly (glycidyl methacrylate) (Fe₃O₄@PS/PGMA) magnetic Janus particles (MJPs) by solvent evaporation-induced organic–inorganic dual phase separation. The Fe₃O₄ nanoparticles (MNPs) are selectively dispersed on one side of the particles. The experimental factors on the morphology of Fe₃O₄@PS/PGMA MJPs, including the addition of MNPs, the ratios of polymer precursors, concentrations of surfactant and emulsification speed, are systematically investigated. The particle size, magnetic content, magnetic performance and morphology of prepared MJPs are determined by laser particle size analyzer, vibrating sample magnetometry (VSM), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and microscope. Furthermore, the dual phase separation mechanism of the formation of Fe₃O₄@PS/PGMA magnetic Janus particles is also explored. Further, the method introduced here supports a simple, controlled procedure to prepare large amounts of magnetic Janus particles under mild conditions.

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1. Introduction

During the past few years, research hotspots of particulate materials have been focused on Janus particles¹ that possess asymmetric geometry (in either shape or surface composition). To sum up the existing studies, various morphologies of Janus particles have been fabricated, including Janus hollow spheres,² Janus nanosheets,^3,4 Janus cages⁵ and Janus bowls.⁶ They have played an increasingly important role in numerous applications including solid surfactants,^7,8 sensors,⁹ functional carrier materials¹⁰ and antimicrobial materials¹¹ because of their anisotropy and morphological diversity.

A magnetic Janus particle is one type of functional material that exhibits both magnetic response and anisotropy. These Janus particles have been of great interest due to their potential application in a wide variety of fields, such as electronic paper^12,13 and heterogeneous catalysis.¹⁴ To date, many strategies and techniques to prepare magnetic Janus particles have been reported, including the microfluidic method,^15–17 monomer polymerization,^18–21 flame synthesis^22,23 and physical vapor deposition,²⁴ and so on. Recently, Yabu and co-wokers²⁵ developed a solvent evaporation method denoted as Self-ORganized Precipitation (SORP) to prepare γ-Fe₂O₃@PS/PI particles. γ-Fe₂O₃ nanoparticles were located only on the surface of the PS phase, and the size and size distribution were not controlled. Similarly, Jeong²⁶ obtained magnetic Janus particles by means of a combined solvent evaporation-induced phase separation and microfluidic monodisperse droplet generation device. However, the shortcoming was the difficulty of fabricating MJPs in large scale by microfluidics.

Recently, our group has continued efforts in this direction and proposed a one-step mini-emulsion polymerization to prepare inner asymmetric PS/Fe₃O₄ composite microspheres, in which most of the inorganic particles encapsulated by polymers were on one side of the microspheres.²⁷ We found that the surface property of Fe₃O₄ nanoparticles was the main determining factor. As far as we know, mini-emulsion polymerization imposes restrictions on large-scale production: it hinders the actual commercialization of MJPs. Consequently, we extended a facile method to prepare Janus particles via solvent evaporation.²⁸ The snow-like, acorn-like and hamburg-like Janus particles were prepared by controlling the molecular weights of precursors (PS and PGMA). We also determined, using fluorescence microscopy and EDS energy dispersive spectrometry, the composition of PS/PGMA Janus particles in which the large hemisphere was mainly related to PGMA. and the other hemisphere was primarily PS. Moreover, PS/PGMA Janus particles were modified by amine and carboxyl group to self-assemble Ag⁺ on the surface.²⁹ The asymmetric PS/PGMA@Ag Janus particles have potential applications in drug delivery and as antibacterial materials. However, relevant literature that presents a solvent evaporation-induced phase separation system containing Fe₃O₄ nanoparticles to prepare Janus particles are published relatively infrequently.

Herein, we develop a simple, controlled and mass-produced approach for the fabrication of magnetic Janus particles to promote the possibility of commercial application of MJPs. In this work, we introduced Fe₃O₄ nanoparticles that can be stably dispersed in oil into the phase separation system consisting of a PS and PGMA emulsion. MNPs are capable of embedding into the inside of Janus particles along with the separation of polymer blends. We investigated the effects of the experimental parameters and also explored the dual phase separation mechanism of the formation of MJPs.

2. Experimental

2.1. Materials

Ferrous sulfate (FeSO₄·7H₂O) and ferric chloride (FeCl₃·6H₂O) were supplied by Xi'an Miura Fine Chemical Factory; sodium hydroxide (NaOH) was obtained from Xi'an chemical reagent factory; oleic acid (OA) was purchased from Tianj/in Beichen founder chemical reagent factory; and styrene (St) was bought from The morning chemical reagent factory in Tianjin, glycidyl methacrylate (GMA) was obtained from Sartomer Company, azobisisobutyronitrile (AIBN) and dibenzoyl peroxide (BPO) were supplied by Shanghai mountain pure chemical co., Ltd. Polyvinyl Pyrrolidone (PVP) and trichloromethane were purchased from BASF of Germany, sodium alkyl sulfonate (SDBS) was supplied by Tianjin branch close the chemical reagent co., Ltd, and dichloromethane (DCM) and anhydrous ethanol were purchased from Tianjin fu yu chemical co., Ltd. All the above chemical reagents were of analytical grade. Deionized water was ultrapure, produced by an apparatus for pharmaceutical purified water (Aquapro Co. Ltd.).

2.2. Preparation of Fe₃O₄ nanoparticle modified with OA

Fe₃O₄ nanoparticles were prepared by a co-precipitation method and modified with OA, synchronously following these steps: First, 8.2 g FeCl₃·6H₂O and 5.6 g FeSO₄·7H₂O were dissolved in 50 mL water in a 250 mL flask. The mixture was mechanically stirred and warmed to 30 °C. Then, 50 mL of 0.1 mol L⁻¹ NaOH aqueous solution was added to the system under vigorous stirring. Further, 3.0 g OA was added. The system was heated to 90 °C for 60 min. The products were separated from the mixture by an applied magnetic field and washed with water until the pH value was neutral. Finally, oleic acid-modified Fe₃O₄ nanoparticles were dispersed in an appropriate amount of n-heptane and sealed for use.

2.3. Production of polymer precursors of PS and PGMA

A typical procedure for the synthesis of PS precursor by dispersion polymerization was as follows: 4.0 g PVP was dissolved in the mixture of 100 mL absolute ethyl alcohol and 10 mL water in a 250 mL three-neck round-bottomed flask. Subsequently, 0.3 g BPO was dissolved in 20.0 g St, and the mixture was added to the above system. Then, the system was purged with nitrogen for 30 min and heated to 80 °C to start polymerization. The polymerization reaction was kept for 24 h. The obtained products were washed three times with ethyl alcohol and water and dried by vacuum freeze-drying. The PGMA precursor was synthesized in the same way, and the experimental condition are listed in Table 1.

Table 1 The recipe for preparing the PGMA precursor

Reagent	PVP/g	Ethyl alcohol/mL	Water/g	AIBN/g	GMA/g
Amount	3.0	120	10	0.2	25.0

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2.4. Preparation of Fe₃O₄@PS/PGMA magnetic Janus particles

Fe₃O₄@PS/PGMA magnetic Janus particles were produced by the ternary phase separation of PS, PGMA and Fe₃O₄ NPs. Typically, to form an O/W emulsion, 0.5 g SDBS was dissolved in 200 mL water as the aqueous phase in a 500 mL flask. The oil phase, with 40.0 g DCM as the volatile solvent, contained 1.0 g PS, 1.0 g PGMA and 0.2 g Fe₃O₄ nanoparticles. The O/W emulsion was formed by shear stirring (500 rpm) while adding the oil phase into the aqueous phase. The reaction was completed when DCM had completely evaporated. The product was washed with ethyl alcohol and dried by freeze drying. All preparation details are shown in Table 2.

Table 2 Components for the production of magnetic Janus particles

Simple	SDBS/g	PS/g	PGMA/g	Fe3O4/g	Stirring rate/rpm
1	0.5	1.0	1.0	0.3	500
2	0.5	1.0	1.0	0.15	500
3	0.5	1.0	1.0	0.1	500
4	0.5	1.2	0.8	0.2	500
5	0.5	1.0	1.0	0.2	500
6	0.5	0.8	1.2	0.2	500
7	0.3	1.0	1.0	0.2	500
8	0.4	1.0	1.0	0.2	500
9	0.6	1.0	1.0	0.2	500
10	0.5	1.0	1.0	0.2	600
11	0.5	1.0	1.0	0.2	700
12	0.5	1.0	1.0	0.2	800

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2.5. Characterization

The microstructure of particles could be easily observed by metallographic microscope (DMM 300C). A suitable amount of the sample under ultrasonic dispersion was put on the slide to observe its morphology on the objective table. Average diameter and particle size distribution of magnetic Janus particles were determined by an LS13320 Laser Particle Size Analyzer (Beckman Coulter). The sample was dispersed in purified water. Fourier transform infrared (FTIR) spectra were acquired on a TENSOR27 FTIR spectrometer (Bruker). A sample of the powder on the tabulating machine was pressed into sheets. The morphology of particles was precisely observed by scanning electron microscope (SEM, AMERY-1000B). The magnetic properties of magnetic particles were assessed with a vibrating sample magnetometer (VSM, LakeShore 7307).

Furthermore, the magnetic contents were determined by EDTA titration and thermogravimetric analysis (TGA, Q50, TA instruments). Operation of the EDTA titration method proceeded as follows: Fe₃O₄@PS/PGMA MJPs were put into acetone to dissolve the polymers. Then, the residual Fe₃O₄ nanoparticles were separated into Fe⁺³ and Fe⁺² through the addition of hydrochloric acid. Finally, the amount of Fe⁺³ and Fe⁺² was obtained by EDTA titration. Thermal properties were also tested with TGA. The magnetic content was found according to the weight percentage of the residue after thermal analysis from room temperature to 800 °C with a heating rate of 20°C min⁻¹ under a nitrogen atmosphere.

3. Results and discussion

3.1. Characterization of Fe₃O₄@PS/PGMA magnetic Janus particles

The FTIR spectra of Fe₃O₄ nanoparticles modified by OA (A), PS precursor (B), PGMA precursor (C) and prepared Fe₃O₄@PS/PGMA magnetic Janus particles (D), were first determined to identify the chemical constituents of obtained composite particles, as shown in Fig. 1. In Fig. 1A, the peaks at 3444 cm⁻¹ and 586 cm⁻¹ were attributed to the stretching vibration of hydroxy and Fe–O bond of Fe₃O₄ nanoparticles, respectively. The absorption peaks at 2920 cm⁻¹ and 2886 cm⁻¹ referred to the saturated carbon–hydrogen bond (CH₃ and CH₂) of OA on the surface of Fe₃O₄ nanoparticles. The stretching vibration caused by a carbonyl group (C=O) was at 1728 cm⁻¹ in Fig. 1B, while 1150 cm⁻¹ was assigned to stretching vibration of O–C–O. The absorption peak at 906 cm⁻¹ belonged to the absorption peak of the epoxy group. Therefore, the results confirmed the presence of the PGMA precursor. Moreover, the FTIR spectra of PS are shown in Fig. 1C. The absorption bands at 1452 cm⁻¹, 1490 cm⁻¹ and 1599 cm⁻¹ referred to the stretch vibrations of the benzene ring skeleton. Peaks at 755 cm⁻¹ and 697 cm⁻¹ were attributed to singly substituted benzene. The results of FTIR spectra in Fig. 1D indicated that the obtained composite particles contained modified Fe₃O₄ nanoparticles and polymer blends of PS and PGMA. Based on the above analyses, the preparation of Fe₃O₄@PS/PGMA magnetic Janus particles were confirmed.

Fig. 1 Infrared spectra of Fe₃O₄ nanoparticles (A), PGMA precursor (B), PS precursor (C) and Fe₃O₄@PS/PGMA magnetic Janus particles (D).

A typical method for preparing magnetic Janus particles by phase separation is shown in Table 2-sample 5. The oil phase in this reaction system, which consists of the same ratios of PS/PGMA and MNPs/polymer precursors, was transferred to a 0.3% w/w SDBS aqueous solution. After the evaporation of DCM, ternary phase separation of PS, PGMA and Fe₃O₄ NPs could be achieved under vigorous stirring. The SEM image and particle size distribution are shown in Fig. 2. In Fig. 2A, particles having one side that was relatively smooth and the other side having small protuberances showed obvious double-hemisphere morphology. Particle size measurement results showed that the particle size of Fe₃O₄@PS/PGMA MJPs was mainly concentrated in 18–30 μm (>86%) range, and the average particle size was 23 μm.

Fig. 2 SEM image of Fe₃O₄@PS/PGMA magnetic Janus particles (A) and particle size distribution (B).

To analyze the magnetic property of Fe₃O₄@PS/PGMA magnetic Janus particles, the magnetic response curve was determined at room temperature, as shown in Fig. 3. It could be clearly observed that the maximum saturation magnetization of magnetic Janus particles was 3.47 emu g⁻¹. Further, we could judge that the prepared magnetic Janus particles possessed superparamagnetism by the coincidence of the hysteresis loop, no retentivity and no coercivity.³⁰

Fig. 3 The magnetization curves of Fe₃O₄@PS/PGMA magnetic Janus particles.

3.2. Influence factors of Fe₃O₄@PS/PGMA magnetic Janus particles

Amount of Fe₃O₄ nanoparticles. We first demonstrated that the amount of Fe₃O₄ nanoparticles mainly affected the magnetic response of the magnetic Janus particles. Increasing the embedded quantity of Fe₃O₄ nanoparticles enhanced the magnetic responsibility of MJPs. Table 3 provides the amount of Fe₃O₄ nanoparticle in magnetic Janus particles determined by EDTA titration. When the mass ratios of Fe₃O₄ nanoparticles and polymer precursors were 3 : 20, 1.5 : 20 and 1 : 20, the magnetic content of Fe₃O₄@PS/PGMA MJPs was 4.47%, 1.98% and 1.89%, respectively (Table 3). The magnetic contents were 8.9%, 5.4% and 3.6%, respectively, as shown in Fig. S2.† It turned out that with augmentation of the dosage of Fe₃O₄ nanoparticles, the magnetic content of magnetic Janus particles increased. Note that Fe₃O₄@PS/PGMA MJPs were not obtained when the amount of Fe₃O₄ nanoparticles added exceeded 3 : 20. Consequently, Fe₃O₄ nanoparticles had a profound influence on the stability of the magnetic Janus particles. One possible reason may be that the stability of the magnetic emulsion formed by the emulsification of the oil and water phase deteriorated in the presence of excessive amounts of Fe₃O₄ nanoparticles.

Table 3 The amount of Fe₃O₄ nanoparticle in Fe₃O₄@PS/PGMA MJPs

Sample	3 : 20^a	1.5 : 20	1 : 20
a Mass ratios of Fe3O4 nanoparticles and polymer precursors.
Amount	4.47%	1.98%	1.89%

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The ratio of PS and PGMA. The morphology of the Janus particles was largely dependent on the ratio of two polymer precursors, which could cause different degrees of phase separation. Therefore, the influence of the ratio of polymer precursors was investigated (Fig. 4). The size of the two hemispheres, which could be changed by various ratios of polymer precursors, was basically the same when the ratio of PS and PGMA was 1 : 1 (Fig. 5B). To confirm the components of the two hemispheres, we decreased the proportion of PGMA (Fig. 4A) and increased the amount of PS (Fig. 4C and D). The results showed that with increasing dosage of PGMA, magnetic Janus particles experienced a change from a snowman-like shape to a dumbbell-like shape and finally again to a snowman-like shape. The surface of a rough hemisphere continued to expand at the same time. It was found from the results that the component of the bigger rough hemisphere was mainly PGMA, and the smaller smooth hemisphere was mainly PS.

Fig. 4 SEM images of Fe₃O₄@PS/PGMA magnetic Janus particles, the ratios of PS and PGMA were 3 : 2 (A); 1 : 1 (B); 2 : 3 (C); 1 : 2 (D).

Fig. 5 SEM images of Fe₃O₄@PS/PGMA magnetic Janus particles prepared using different stirring speeds: 500 rpm (A); 600 rpm (B); 700 rpm (C); and 800 rpm (D).

To determine the distribution of Fe₃O₄ nanoparticles in Fe₃O₄@PS/PGMA magnetic Janus particles, two types of MJPs prepared under conditions where the proportions of PS and PGMA were 3 : 2 and 2 : 3 and were observed by optical microscopy (Fig. S3†). It could be seen that Fe₃O₄ nanoparticles were concentrated on one side. When PS : PGMA = 3 : 2, Fe₃O₄ nanoparticles distributed on the bigger side; when PS : PGMA = 2 : 3, Fe₃O₄ nanoparticles were distributed on the smaller side. Therefore, combining the above analyses, we ascertained that Fe₃O₄ nanoparticles distributed on the PS side. The primary cause was that the compatibility of oleic acid-modified Fe₃O₄ nanoparticles with hydrophobic PS being relatively stronger than that with PGMAs.

The surfactant concentration. The morphology of magnetic Janus particles prepared by phase separation was in a state of minimum interfacial free energy of a surface composed of different components.³¹ Therefore, the concentration of surfactant indirectly influenced the morphology of magnetic Janus particles. MJPs produced with altered concentrations of SDBS are shown in Fig. S4.† When the proportion of SDBS was 0.15%, Janus particles were seldom formed because the emulsifying agent was not enough to stabilize the oil–water interface. With the increasing dosage of SDBS, the morphology of particles tended to be in good condition and gradually unfolded into a snowman-like shape. When 0.25% SDBS was used, the optimal situation was achieved: a dumbbell-like shape with hemispheres of the same size appeared. With further increases in the amount of SDBS, the hydrophilic PGMA tended towards an aqueous solution, owing to the decrease of surface tension. The Janus particles gradually separated into two single particles. Their characteristic double-faced morphology continued to weaken. Fig. S5† showed the size distribution curve of Fe₃O₄@PS/PGMA magnetic Janus particles prepared with different concentrations of SDBS. The results showed that the particle size of magnetic Janus particles first declined and then remained the same. The size distribution of particle prepared with 0.15% SDBS was the most expansive, which was basically identical with the results observed by optical microscopy. The potential reason was that the amount of SDBS was insufficient to stabilize particles when its minimal dosage was 0.15%. In order to provide a minimum surface free energy, it resulted in a large particle size and broad size distribution. When the dosage of SDBS was more than 0.30%, the surface free energy of the solution was greatly reduced because of the high concentration of surfactant and allowed the two hemispheres to steadily exist alone.

Emulsification speed. In the process of phase separation, the solvent evaporation rate controlled the morphology of Janus particles to a larger degree. The solvent evaporation rate was associated with the stirring speed, the dosage of solvent, evaporation temperature and other factors. Furthermore, the stirring rate affected the size of emulsion droplets generated from the hybrid emulsion and eventually formed Janus particles. Under different stirring speeds, the morphologies and size distribution of Fe₃O₄@PS/PGMA magnetic Janus particles are shown in Fig. 5 and S6,† respectively. As shown in Fig. 5, particle size decreased with the increasing stirring rate, which showed the same results as in Fig. S5.† We obtained perfect double-hemisphere morphology of magnetic Janus particles with no existing impurities under stirring speeds of 500 and 600 rpm. With increased stirring speed, the double-hemisphere morphology weakened. When the emulsification speed reached 800 rpm, the particle morphology significantly deteriorated, and impurities appeared on their surfaces. That was caused by small oil droplets that were generated at the initial emulsification under high stirring speed. Those tiny oil droplets collided with larger diameter droplets or particles in the subsequent high-speed stirring solvent evaporation process and became aggregated. In addition, the higher speed led to accelerated evaporation and caused NH₃-shaped particles, as shown by the marking in Fig. 5C.

3.3. Formation mechanism of Fe₃O₄@PS/PGMA magnetic Janus particles

Based on the above information, the amount of methylene chloride directly affected the solvent evaporation rate, namely, the time required for polymer phase separation. Hence, we investigated the influence of methylene chloride dosage to study the process of magnetic Janus particle formation. The SEM images of MJPs prepared with different dosages of DCM are shown in Fig. S7.† We observed that MJPs underwent a change of morphology from three-hemispheric to double-hemispheric, and finally to polymictic double-hemispheric. When the dosage of methylene chloride was less than 40 mL, the rate of polymer phase separation became significantly faster, which weakened the degree of phase separation. It finally resulted in the appearance of three-hemispheric particle morphology, as shown in Fig. S7A.† The principal reason was possibly that the separation speed of the two polymer precursors increased at the moment, and at the same time, a polymer component separated from another polymer, forming hemispheres through both sides. Increasing the amount of DCM to 40–50 mL, the time of phase separation was suitable, and the degree of separation was also apt to form the preferred Janus morphology. However, the time became comparatively long with excessive dosage of methylene chloride. In this circumstance, an opportunity was provided for the aggregation and coalescence of oil droplets and particles to make the morphology of prepared MJPs deteriorate (Fig. S7D†).

In order to further investigate the phase separation mechanism with the introduction of magnetic particles, we produced Fe₃O₄@PS/PGMA magnetic Janus particles under the condition in which the ratio of PS and PGMA was 1 : 1, the amount of DCM was 40 mL and the stirring speed was 700 rpm; The whole process of phase separation was investigated by a tracking study.

Optical images of particles at each reaction time point are shown in Fig. 6. The formation of Janus particles experienced a procedure of emulsification (Fig. 6A), solvent evaporation induced internal phase separation (Fig. 6B and C) and two phases completely separated to form Janus particles (Fig. 6D). That is, the oil phase was emulsified when poured into the water phase. Hydrophobic Fe₃O₄ NPs were clearly embedded in the emulsion droplets, as shown in Fig. 6A. At 36 min with evaporation of the solvent, emulsion droplets started to solidify, and internal phase separation began (Fig. 6B). Black moieties represent droplets located with Fe₃O₄ NPs; white areas of the droplets were relatively clear polymer blends, mainly PGMA, as mentioned above. We can draw the conclusion that PS and Fe₃O₄ NPs simultaneously separated from PGMA. Internal phase separation was basically completed at 1 h (Fig. 6C). Nevertheless, it was an intermediate state with different particle morphologies, with three-hemisphere and core–shell structures. Fe₃O₄ NPs were mainly distributed on the interface of the two phases due to the increased viscosity of the system. Subsequently, the system completed phase separation at 2 h (Fig. 6D) and formed magnetic Janus particles. Composite particles of three-hemispheric structure provided a possibility for a part of the formation of three-hemisphere under 30 mL DCM. Scheme 1 showed the formation process of Fe₃O₄@PS/PGMA magnetic Janus particles. From the above analysis, the formation of magnetic Janus particles was a process of two kinds of phase separation proceeding synchronously, including (1) separation of PS and PGMA,³² and (2) separation of PGMA and modified Fe₃O₄ nanoparticles. The dual phase separation occurred simultaneously, producing Fe₃O₄@PS/PGMA magnetic Janus particles.

Fig. 6 Optical microscopy images of Fe₃O₄@PS/PGMA magnetic Janus particles at different phase separation times: 7 min (A); 36 min (B); 1 h (C); 2 h (D).

Scheme 1 Schematic illustration of the formation of Fe₃O₄@PS/PGMA magnetic Janus particles.

4. Conclusions

In summary, we prepared Fe₃O₄@PS/PGMA magnetic Janus particles by introducing Fe₃O₄ nanoparticles into a simple phase separation system. Under high-speed stirring, the strong immiscibility among PGMA, PS and OA-coated Fe₃O₄ nanoparticles was the driving force for the formation of MJPs. Because the compatibility of OA-modified Fe₃O₄ nanoparticles with hydrophobic PS was stronger than that with PGMAs, dual phase separation existed simultaneously during the entire preparation process: (i) PGMA and PS; (ii) PGMA and OA-modified Fe₃O₄ nanoparticles. The morphology and size of Fe₃O₄@PS/PGMA MJPs can be easily tailored by adjusting the ratio of polymer precursors, the amount of Fe₃O₄ nanoparticles, the surfactant concentration and the emulsifying speed. In addition, in the process of preparation of MJPs, we also obtained mushroom-like and hat-like magnetic Janus particles. These observations further stimulates our research interest and lays a certain foundation to explore the controlled phase separation for preparing asymmetric particles with controllable morphology.

Acknowledgements

The authors are grateful for the financial support provided by the National High-Tech Research and Development Program of China (863 Program) (no. 2012AA02A404), the National Natural Science Foundation of China (no. 51173147), and the problem plan of Xi'an (no. CX12164).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03140g

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