Fast magnetic-field-induced formation of one-dimensional structured chain-like materials via sintering of Fe₃O₄/poly(styrene-co-n-butyl acrylate-co-acrylic acid) hybrid microspheres

Abstract

A facile and fast procedure has been developed to prepare one-dimensional (1D) hybrid microchains by sintering of Fe₃O₄/poly(styrene-co-n-butyl acrylate-co-acrylic acid) (Fe₃O₄/P(St-co-nBA-co-AA)) hybrid microspheres. Noteworthily, it is the first time that the 1D structure has been linked and fixed via physical fusion instead of by a chemical method. The crucial process is to sinter the hybrid microspheres at the glass transition temperature (T_g) of P(St-co-nBA-co-AA) particles under a magnetic field. SEM, optical microscopy, EDS, XRD and FTIR have proved the successful fabrication of 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains and the formation mechanism has been discussed. The 1D hybrid microchains have lengths of several hundred micrometers and diameters of 10–15 μm. A TGA measurement has indicated that the large proportion of fused polymer particles in the 1D structures reaches 64.5%. A VSM curve has shown that the saturation magnetization is 18.7 emu g⁻¹, which is enough for the 1D microchains to be controlled and separated by the external magnetic field. Mercury intrusion porosimetry and a SEM image of the fractured 1D microchains have demonstrated that these microchains are porous. Moreover, it has been found that individual microchains can be obtained at a low concentration of Fe₃O₄ particles, otherwise aggregation occurs.

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1. Introduction

Recently, 1D nano- or submicro-iron oxide materials have attracted much attention due to their shape anisotropy and unique properties,^1,2 rendering them with great application value in the fields of optical displays,^3,4 tumor targeting,⁵ magnetic resonance imaging,⁶ catalysis,⁷ high density data storage,⁸ sensors⁹ and microwave absorbing materials.¹⁰ So far, assembly is known to be the most effective method to prepare 1D structures, mainly through dipole-directed assembly,^11,12 magnetic-field-induced assembly^13–16 and template-directed assembly methods.^17,18 Compared to the other two methods, magnetic-field-induced assembly is simpler and cheaper. However, when the magnetic field is removed, the ordered 1D structures can’t be maintained just by the weak dipole interaction. In order to fix the anisotropic structures to endow them with more properties, researchers have tried their best to explore new approaches.^6,19,20 Based on recent research, it has been found that polymers are a good medium with which to connect the separated iron oxide particles permanently. What’s more, precipitation polymerization is one of the effective methods to prepare nanostructures.^21–24 Hence, in recent years, Zhang et al. have prepared a series of polymer-coated 1D Fe₃O₄ peapod-like nano-structures with various functional groups by precipitation polymerization under a magnetic field.^25–28 Nevertheless, precipitation polymerization usually needs several hours for the reaction and causes a waste of organic solvent. Therefore, an efficient and energy-saving method for connecting and fixing 1D structures is desired.

Fe₃O₄/polymer hybrid materials not only possess magnetic properties but also have the good characteristics of polymers, such as viscoelasticity, deformation, molecular chain diffusion and multifunctional groups.^29–31 Sintering of polymer particles is usually defined as the formation of a homogeneous film from the coalescence of solid particles under the action of surface tension.³² In the sintering process, deformation of the particles first occurs, leading to a structure without voids, but the original particles are still distinguishable. Because of the polymer diffusion across particle boundaries, a continuous film is finally formed and the original particles are no longer distinguishable.^33–35 Inspired by the unique properties of polymers, in this work, a kind of special polymer is selected to achieve fast connection and fixing of the 1D structure at the condition of its deformation temperature.

Herein, we report a facile and fast approach for preparation of 1D microchains via sintering of Fe₃O₄/polymer hybrid microspheres for 30 seconds. The P(St-co-nBA-co-AA) particles were synthesized by soap-free emulsion polymerization. The T_g of the particles was controlled by adjusting the ratio of hard monomer St and soft monomer nBA, leading to the successful formation of hybrid microspheres and fixing of the ordered 1D structures. Firstly, an aqueous suspension of Fe₃O₄ particles and P(St-co-nBA-co-AA) particles was added to 1-octanol mixed with Span 80 under stirring to form a water-in-oil emulsion. The emulsion was then sintered at the T_g of the P(St-co-nBA-co-AA) particles, and the polymer deformed and fused to yield smooth hybrid microspheres. After sintering, a magnetic field was applied to align these hybrid microspheres, and the diffusion of polymer between the adjacent microspheres caused the formation of fixed 1D microchains. Due to their porous structure, the various functional groups of polymers and their magnetic properties, these chain-like structures will have potential applications in the fields of recycled catalysis, heavy metal separation, drug release and so on.

2. Experimental

2.1 Materials

Styrene (St, Shanghai Shan Pu Chemical Co. Ltd.), n-butyl acrylate (nBA, Hongyan Reagent Factory Hedong District) and acrylic acid (AA, Tianjin Fuchen Chemical Reagent Factory) were stored in a refrigerator prior to use. Potassium persulfate (KPS, Sinopharm Chemical Reagent Co. Ltd.) was used as initiator. Ferric chloride (FeCl₃·6H₂O, Hongyan Reagent Factory Hedong District), trisodium citrate dehydrate (Tianjin Fuchen Chemical Reagents Factory), sodium polyacrylate (PAAS, Tianjin Kemiou Chemical Reagent Co., Ltd.), urea (Sinopharm Chemical Reagent Co. Ltd.), 1-octanol (Hongyan Reagent Factory Hedong District) and Span 80 (Hongyan Reagent Factory Hedong District) were used as received.

2.2 Preparation of 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains

Magnetic Fe₃O₄ particles were prepared through a hydrothermal method³⁶ with a PAAS mass of 0.8 g, and the P(St-co-nBA-co-AA) particles were synthesized by soap-free emulsion polymerization using a St : nBA ratio of 4 : 1, as in our previous work.^35,37

A typical experimental procedure for the fabrication of 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains is as follows. Span 80 (1 mL) was mixed with 1-octanol (50 mL) in a 100 mL three neck round-bottom flask at an agitation rate of 2000 rpm for 10 min. 15 mg of Fe₃O₄ was dispersed in the colloidal particle suspension (2 mL, 2 wt% P(St-co-nBA-co-AA) particles) to form the water phase, which was then dropped into the oil phase and stirred for another 5 min to obtain a water-in-oil emulsion. Next, the emulsion was sintered at 65 °C for 5 min in the water bath to prepare Fe₃O₄/P(St-co-nBA-co-AA) hybrid microspheres. After sintering, a magnetic field was immediately applied to align the hybrid microspheres and fix them in 30 seconds to obtain the 1D microchains. The products were washed with ethanol several times to remove Span 80.

2.3 Characterization

The morphologies of the samples were characterized and energy dispersive X-ray spectroscopy (EDS) was carried out using a scanning electron microscope (JEOL JSM-6700F). The particle sizes and size distributions of Fe₃O₄ and P(St-co-nBA-co-AA) particles were measured by dynamic light scattering (DLS) using a Beckman Coulter Delsa Nano C (Beckman, Germany) with deionized water as testing medium at 25 °C. Optical microscopy (10XB-PC metallographic microscopes, China) was used to monitor the formation of 1D hybrid microchains. X-ray diffraction (XRD) experiments were carried out using a PHILIPS PW3040/60 power diffractometer with nickel-filtered CuKα radiation at 40 kV and 40 mA. The Raman spectrum of Fe₃O₄ was measured on a Laser Raman spectrometer (InVia Reflex; Renishaw Company, UK). Fourier transform infrared spectra (FTIR) were recorded using a TEN-SOR27 FTIR spectrometer (Bruker). The magnetic contents of the magnetic Fe₃O₄ particles and hybrid microchains were determined using thermogravimetric analysis (TGA, Q50, TA Instruments) from room temperature to 800 °C with a heating rate of 10 °C min⁻¹. The magnetic properties of the magnetic particles and microchains were assessed using a vibrating sample magnetometer (LakeShore 7307). Pore properties of 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains were measured by mercury porosimetry (AUTOPORE IV 9500, Micromeritics, USA).

3. Results and discussion

To ensure the successful fabrication of 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains, two vital factors should be controlled, that is, the hydrophilicity of both Fe₃O₄ and P(St-co-nBA-co-AA) particles and the T_g of P(St-co-nBA-co-AA) particles. Hydrophilicity facilitates the dispersion of the Fe₃O₄ and P(St-co-nBA-co-AA) particles in water, and hence the formation of the water-in-oil emulsion to obtain the Fe₃O₄/P(St-co-nBA-co-AA) hybrid microspheres. An appropriate T_g was designed to convert the hybrid microspheres to 1D structures via magnetic-field-induced assembly. Therefore, the Fe₃O₄ particles were prepared by a hydrothermal method and PAAS was selected as stabilizer to endow the magnetic particles with excellent hydrophilicity, while the polymer particles were well-designed with an appropriate T_g by adjusting the ratio of St (T_g ≈ 100 °C) and nBA (T_g ≈ −50 °C).^38,39 As we know, the chain segments start to move at temperatures above T_g, which will result in the deformation and fusion of polymer particles after sintering at the temperature of T_g.^40–43 Therefore, the T_g of the polymer particles was crucial for the connection of Fe₃O₄/P(St-co-nBA-co-AA) hybrid microspheres to form 1D microchain structures. The morphologies and particle size distributions of Fe₃O₄ and P(St-co-nBA-co-AA) particles are presented in Fig. 1. As can be seen from Fig. 1a and c, the Fe₃O₄ particles were relatively monodisperse in a spherical shape and the average diameter was 337 nm, given by DLS. Besides, the rough surfaces of the Fe₃O₄ particles could be clearly observed, indicating that the Fe₃O₄ particles were assembled from many primary crystals.⁴⁴ In Fig. 1b, the P(St-co-nBA-co-AA) particles were in a hexagonal close packed arrangement, demonstrating a very narrow particle size distribution. Measured by DLS, the average particle size of the P(St-co-nBA-co-AA) particles was 280 nm, presented in Fig. 1d, and the T_g of these designed polymer particles was 66 °C, given by DSC.³⁷

Fig. 1 SEM images of (a) Fe₃O₄ particles and (b) P(St-co-nBA-co-AA) particles; the corresponding particle size distribution is shown in (c) and (d).

Fig. 2 gives a schematic illustration of the formation process of the 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains. Firstly, the aqueous suspension of magnetic particles and polymer particles was added to 1-octanol which was premixed with Span 80 (Fig. 2A), followed by vigorous stirring to form a water-in-oil emulsion (Fig. 2B). Secondly, the obtained emulsion was sintered at a temperature around the T_g of the P(St-co-nBA-co-AA) particles. Because of the fusion of polymer particles, spherical Fe₃O₄/P(St-co-nBA-co-AA) hybrid microspheres with smooth surfaces were obtained (Fig. 2C). At the same time, a significant amount of water was encapsulated in the hybrid microspheres. Finally, a plane magnet was immediately applied to one side of the sintered suspension to align the hybrid microspheres, and the diffusion of polymer across the adjacent microspheres rapidly connected them, giving the 1D structured hybrid materials (Fig. 2D).

Fig. 2 Schematic illustration of the formation of 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microstructures.

Fig. 3a and b show the corresponding optical microscope images of Fig. 2C and D. Fig. 3a is the hybrid microspheres after sintering of the water-in oil emulsion at 65 °C for 5 min. It was clear that the hybrid microspheres had a smooth surface and a good spherical shape with diameters of 10–15 μm and the polymer particles were not distinguished, indicating that the polymer particles underwent a process of film formation. Fig. 3b and c depict the successful formation of 1D microchain structures with a length of several hundred microns. Hitherto, much research has focused on the investigation of 1D nano- and submicro-structures, while 1D micro-structures with larger sizes have seldomly been reported. From Fig. 3c, it is confirmed that 1D microchains were formed by the connection of hybrid Fe₃O₄/P(St-co-nBA-co-AA) microspheres. Obviously, these 1D structures were permanently fixed and couldn’t be separated into single microspheres anymore. Fig. 3d presents a scanning electron microscopy (SEM) image of the 1D chain-like structures, the sizes of which were almost in accordance with those in the optical microscope images. Fig. 3e gives the EDS spectrum of the 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains selected in a little area from Fig. 3d. As can be seen, the peaks of four elements were clearly observed. In particular, the C Kα peak (0.277 keV) was much stronger than those of any other element, indicating that the hybrid microchains contained a large proportion of polymer. In this way, 1D structures could be facilely prepared by the connection of fused polymer.

Fig. 3 Optical microscope images of (a) Fe₃O₄/P(St-co-nBA-co-AA) hybrid microspheres, (b) 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains and (c) the amplified image of (b); (d) SEM image and (e) EDS spectrum of the 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains. The 1D hybrid microchains were made with 15 mg of Fe₃O₄ particles.

The crystalline structures of Fe₃O₄ particles and 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains were characterized by powder XRD. As presented in Fig. 4a, six main diffraction peaks (2θ = 30.1, 35.5, 43.1, 53.4, 57.1 and 62.8°) corresponded to the (220), (311), (400), (422), (511) and (440) crystal planes, respectively, in the face-centered cubic structure of inverse spinel Fe₃O₄ particles. The positions and relative intensities of all the observed peaks were in accordance with the standard card (JCPDS 75-1609). Compared with Fig. 4a, a broad peak appeared at around 2θ = 20° in Fig. 4b, which belonged to the amorphous P(St-co-nBA-co-AA).

Fig. 4 XRD patterns of (a) Fe₃O₄ particles and (b) 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains. The 1D hybrid microchains were made with 15 mg of Fe₃O₄ particles.

Because magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) have almost identical XRD patterns, Raman spectroscopy was used to distinguish the different structural phases of iron oxides. From Fig. 5, the main characteristic peak at 668 cm⁻¹ was observed, which is attributed to the A_1g vibrational mode of magnetite, whereas the characteristic peaks of maghemite at around 720, 500, and 350 cm⁻¹ didn’t appear.^45–47 This further proved that the iron oxide synthesized in our work was Fe₃O₄.

Fig. 5 Raman spectrum of Fe₃O₄ particles.

The FTIR spectra of Fe₃O₄ particles and 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains are shown in Fig. 6. The strong absorption bands at 584 and 3440 cm⁻¹ arise from vibrations of Fe–O bonds and the surface hydroxyl groups of Fe₃O₄ particles, respectively. The peaks located at 1631 and 1408 cm⁻¹ are identified as the asymmetric and symmetric stretching vibrations of COO⁻ in PAAS, respectively, indicating that PAAS was anchored on the surface of the Fe₃O₄ particles. Meanwhile, the peaks at 2920 and 2852 cm⁻¹, due to the stretching vibrations of saturated C–H bonds, further confirmed the presence of PAAS. Compared with Fig. 6a, some new peaks appeared in Fig. 6b. The three peaks at 1601, 1493 and 1450 cm⁻¹ are ascribed to the C=C stretching vibration of the benzene ring. In addition, the bands at around 3082, 3059 and 3024 cm⁻¹ are the stretching vibrations of C–H bonds in benzene rings and the strong band at 698 cm⁻¹ is considered as the C–H out-of-plane bending vibration of benzene rings. All of the above characteristic bands proved the presence of PSt. The big absorption at 1726 cm⁻¹ corresponds to the C=O stretching vibration. The band at around 905 cm⁻¹ is the –OH bending vibration of –COOH. Besides, the bands of the asymmetric and symmetric stretching vibrations of C–O–C are located at 1155 and 1066 cm⁻¹, respectively. What’s more, the band at 756 cm⁻¹ is attributed to the CH₂ in-plane rocking vibration of nBA. The FTIR spectra confirmed that P(St-co-nBA-co-AA) was composited with magnetic Fe₃O₄ particles successfully.

Fig. 6 FTIR spectra of (a) Fe₃O₄ particles and (b) 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains. The 1D hybrid microchains were made with 15 mg of Fe₃O₄ particles.

From the TGA experiments shown in Fig. 7, it is known that the Fe₃O₄ particles have a weight loss of 1.9%, while the residual content of the 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains reached 35.5%. It was clear that the little weight loss of the Fe₃O₄ particles was due to the adsorbed water and stabilizer. However, P(St-co-nBA-co-AA) mostly decomposed in the temperature range from 300 to 400 °C, leading to the large weight loss of the 1D hybrid microchains. As we know, the amounts of P(St-co-nBA-co-AA) particles and Fe₃O₄ particles added were 40 and 15 mg, respectively, in the preparation of the 1D hybrid microchains. That is, the weight fraction of P(St-co-nBA-co-AA) particles in the total particles was 72.7%. Therefore, the weight loss of 64.5% in the 1D hybrid microchains proved that just a few of the P(St-co-nBA-co-AA) particles were not composited with the magnetic particles.

Fig. 7 TGA curves of (a) Fe₃O₄ particles and (b) 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains. The 1D hybrid microchains were made with 15 mg of Fe₃O₄ particles.

Magnetic response is an important property that is concerned in practical applications. Magnetization curves of the Fe₃O₄ particles and the 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains are shown in Fig. 8. Obviously, both of the samples exhibited superparamagnetism. The saturation magnetization of the Fe₃O₄ particles was 72.2 emu g⁻¹, indicating that the Fe₃O₄ particles synthesized by a hydrothermal method had an excellent magnetic response. Because of the large proportion of polymer in the hybrid microchains, the saturation magnetization decreased to 18.7 emu g⁻¹. This could be explained by the reason that amorphous P(St-co-nBA-co-AA) in the chain-like structures provided a smaller magnetic torque per unit mass as opposed to that of the Fe₃O₄ particles.^48,49 Even so, the hybrid microchains could easily be collected and separated by an external magnet. The inserted image shows that hybrid microchains dispersed in 1-octanol could be attracted towards the magnet within 30 seconds, forming a clear solution.

Fig. 8 Magnetization curves of (a) Fe₃O₄ particles and (b) 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains, measured at room temperature. The 1D hybrid microchains were made with 15 mg of Fe₃O₄ particles.

Fig. 9a presents the pore size distribution of the 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains, measured by mercury intrusion porosimetry. It was clearly observed that two kinds of pores existed in the 1D structures. The pores with sizes ranging from 10 to 80 nm belonged to the porosity of the constituent polymer and the sites where the surfactant Span 80 was located. However, we also observed that a strong peak was located at a pore size diameter of 9 μm. In order to find out the inner pore structure, a SEM image of the fractured 1D microchains is given in Fig. 9b. Many smaller pores were observed in the surface, while the sizes of the inner pores were mainly in the range of 2–6 μm, which was not in accordance with the pore size distribution. For the pores of 2–6 μm in diameter, they were predicted and derived from the encapsulated water in the separate hybrid microspheres, seen from Fig. 2C. Because the microchain structures were prone to pack together, the pores which were in the diameter range of 6–12 μm may be the voids between adjacent microchains. Another probable reason for the appearance of large pores is that the materials were broken when they were analyzed under high pressure in mercury porosimetry. Anyway, this porous structure may find its application in the field of adsorption and separation.

Fig. 9 (a) Pore size distribution of the 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains, measured by mercury intrusion porosimetry and (b) SEM image of the fractured 1D microchains. The 1D hybrid microchains were made with 15 mg of Fe₃O₄ particles.

To investigate the effect of the amount of Fe₃O₄ particles on the formation of the 1D Fe₃O₄/P(St-co-nBA-co-AA) hybrid microchains, three other experiments were carried out. Fig. 10 depicts the optical microscope images of 1D hybrid microchains prepared using different amounts of Fe₃O₄ particles. As can be seen, P(St-co-nBA-co-AA) particles could still connect the hybrid microspheres, though the weight fraction of polymer decreased from 72.7% to 30.8%. However, it was clearly observed that the 1D hybrid structures were inclined to aggregate with the increasing amount of magnetic particles. More Fe₃O₄ particles resulted in a higher magnetic response of the hybrid microspheres, and hence these hybrid microspheres were easily attracted to one side of the magnet to form the aggregated microchains. Moreover, the results showed that individual 1D fixed hybrid microchains could be obtained when the added Fe₃O₄ particles were in a low concentration, such as the 15 and 30 mg used in this paper.

Fig. 10 Optical microscope images of 1D hybrid microchains prepared using different amounts of Fe₃O₄ particles: (a) 30 mg, (b) 60 mg and (c) 90 mg.

4. Conclusion

In summary, we developed a facile and fast method to convert the hybrid Fe₃O₄/polymer microspheres to 1D structures by sintering under a magnetic field. The successful formation of 1D microchains lies in the unique characteristics of polymers, in that the P(St-co-nBA-co-AA) particles can deform and fuse to form a film, connecting independent microspheres together at the temperature of their T_g. These microchains possess not only the properties of polymers but also the excellent characteristics of magnetic materials. The saturation magnetization is 18.7 emu g⁻¹, which makes it easy for them to be collected and separated by a magnet in 30 seconds. Because the hybrid microspheres are prepared from a water-in-oil emulsion, a large amount of water is encapsulated, leading to the porous structure of the microchains. This paper provides a new approach to prepare organic/inorganic materials.

Acknowledgements

This work was supported by National Natural Science Foundation of China (no. 51173146, 51433008), the Doctorate Foundation of Northwestern Polytechnical University (CX201515) and Basic Research Fund of Northwestern Polytechnical University (3102014JCQ01094, 3102014ZD).

Notes and references

1. B. Geng, J. Ma, X. Liu, Q. Du, M. Kong and L. Zhang, Appl. Phys. Lett., 2007, 90, 043120–043123.
2. Z. Nie, A. Petukhova and E. Kumacheva, Nat. Nanotechnol., 2009, 5, 15–25.
3. Y. Hu, L. He and Y. Yin, Angew. Chem., Int. Ed., 2011, 50, 3747–3750.
4. M. Wang, L. He, Y. Hu and Y. Yin, J. Mater. Chem. C, 2013, 1, 6151–6156.
5. J. H. Park, G. von Maltzahn, L. Zhang, A. M. Derfus, D. Simberg, T. J. Harris, E. Ruoslahti, S. N. Bhatia and M. J. Sailor, Small, 2009, 5, 694–700.
6. J. H. Park, G. von Maltzahn, L. Zhang, M. P. Schwartz, E. Ruoslahti, S. N. Bhatia and M. J. Sailor, Adv. Mater., 2008, 20, 1630–1635.
7. Y. Liu, L. Zhou, Y. Hu, C. Guo, H. Qian, F. Zhang and X. W. D. Lou, J. Mater. Chem., 2011, 21, 18359–18364.
8. P. Sivakumar, R. Ramesh, A. Ramanand, S. Ponnusamy and C. Muthamizhchelvan, J. Alloys Compd., 2013, 563, 6–11.
9. R. Xuan, Q. Wu, Y. Yin and J. Ge, J. Mater. Chem., 2011, 21, 3672–3676.
10. Y. Chen, P. Gao, C. Zhu, R. Wang, L. Wang, M. Cao and X. Fang, J. Appl. Phys., 2009, 106, 054303.
11. F. Zhang and C.-C. Wang, J. Phys. Chem. C, 2008, 112, 15151–15156.
12. Y. Xiong, J. Ye, X. Gu and Q. W. Chen, J. Phys. Chem. C, 2007, 111, 6998–7003.
13. H. Wang, Q. W. Chen, L. X. Sun, H. P. Qi, X. Yang, S. Zhou and J. Xiong, Langmuir, 2009, 25, 7135–7139.
14. H. Niu, Q. Chen, H. Zhu, Y. Lin and X. Zhang, J. Mater. Chem., 2003, 13, 1803–1805.
15. R. Sheparovych, Y. Sahoo, M. Motornov, S. Wang, H. Luo, P. N. Prasad, I. Sokolov and S. Minko, Chem. Mater., 2006, 18, 591–593.
16. M. Mashkour, T. Kimura, F. Kimura, M. Mashkour and M. Tajvidi, RSC Adv., 2014, 4, 52542–52549.
17. C. Bae, H. Yoo, S. Kim, K. Lee, J. Kim, M. M. Sung and H. Shin, Chem. Mater., 2008, 20, 756–767.
18. H. W. Liang, S. Liu and S. H. Yu, Adv. Mater., 2010, 22, 3925–3937.
19. H. D. Tran, D. Li and R. B. Kaner, Adv. Mater., 2009, 21, 1487–1499.
20. S. A. Corr, S. J. Byrne, R. Tekoriute, C. J. Meledandri, D. F. Brougham, M. Lynch, C. Kerskens, L. O’Dwyer and Y. K. Gun’ko, J. Am. Chem. Soc., 2008, 130, 4214–4215.
21. G. L. Li, H. Möhwald and D. G. Shchukin, Chem. Soc. Rev., 2013, 42, 3628–3646.
22. G. L. Li, Z. Zheng, H. Möhwald and D. G. Shchukin, ACS Nano, 2013, 7, 2470–2478.
23. G. L. Li, L. Q. Xu, K. Neoh and E. Kang, Macromolecules, 2011, 44, 2365–2370.
24. G. Li, Y. Song, X. Yang and W. Huang, J. Appl. Polym. Sci., 2007, 104, 1350–1357.
25. M. Ma, Q. Zhang, J. Dou, H. Zhang, D. Yin, W. Geng and Y. Zhou, J. Colloid Interface Sci., 2012, 374, 339–344.
26. M. Ma, Q. Zhang, J. Dou, H. Zhang, W. Geng, D. Yin and S. Chen, Colloid Polym. Sci., 2012, 290, 1207–1213.
27. M. Ma, Q. Zhang, H. Zhang, T. Xin, B. Zhang and X. Fan, Sci. Adv. Mater., 2013, 5, 623–629.
28. C. Li, J. Tan, X. Fan, B. Zhang, H. Zhang and Q. Zhang, Ceram. Int., 2015, 41, 3860–3868.
29. B. Liu, W. Zhang, F. Yang, H. Feng and X. Yang, J. Phys. Chem. C, 2011, 115, 15875–15884.
30. S. Xuan, Y. X. J. Wang, J. C. Yu and K. C. F. Leung, Langmuir, 2009, 25, 11835–11843.
31. H. Yang, B. Yuan, Y. Lu and R. Cheng, Sci. China, Ser. B: Chem., 2009, 52, 249–256.
32. C. T. Bellehumeur, M. Kontopoulou and J. Vlachopoulos, Rheol. Acta, 1998, 37, 270–278.
33. A. F. Routh and W. B. Russel, Langmuir, 1999, 15, 7762–7773.
34. H. N. Yow and A. F. Routh, Langmuir, 2008, 25, 159–166.
35. C. Li, J. Tan, Y. Liu, B. Zhang, X. Fan and Q. Zhang, Colloid Polym. Sci., 2015, 293, 993–1001.
36. Y. Liu, C. Li, H. Zhang, X. Fan, Y. Liu and Q. Zhang, Chem. Eng. J., 2015, 259, 779–786.
37. C. Li, B. Zhang, J. Tan, X. Fan, Y. Liu, H. Zhang and Q. Zhang, J. Mater. Sci., 2014, 49, 2653–2661.
38. W. Wang and Q. Zhang, J. Colloid Interface Sci., 2012, 374, 54–60.
39. Y. Luo, X. Wang, Y. Zhu, B. G. Li and S. Zhu, Macromolecules, 2010, 43, 7472–7481.
40. S. Mazur, R. Beckerbauer and J. Buckholz, Langmuir, 1997, 13, 4287–4294.
41. W. Wohlleben, F. W. Bartels, M. Boyle and R. J. Leyrer, Langmuir, 2008, 24, 5627–5635.
42. Y. Yin, Y. Lu and Y. Xia, J. Am. Chem. Soc., 2001, 123, 771–772.
43. Y. Yin and Y. Xia, Adv. Mater., 2001, 13, 267–271.
44. M. Zhu and G. Diao, J. Phys. Chem. C, 2011, 115, 18923–18934.
45. T. Daou, G. Pourroy, S. Begin-Colin, J. Greneche, C. Ulhaq-Bouillet, P. Legare, P. Bernhardt, C. Leuvrey and G. Rogez, Chem. Mater., 2006, 18, 4399–4404.
46. X. Sun, C. Zheng, F. Zhang, Y. Yang, G. Wu, A. Yu and N. Guan, J. Phys. Chem. C, 2009, 113, 16002–16008.
47. T. Muraliganth, A. V. Murugan and A. Manthiram, Chem. Commun., 2009, 7360–7362.
48. K. Tamura and H. Endo, Phys. Lett. A, 1969, 29, 52–53.
49. Z. Zhang, H. Duan, S. Li and Y. Lin, Langmuir, 2010, 26, 6676–6680.

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