Synthesis and application of homogeneous Fe₃O₄ core/Au shell nanoparticles with strong SERS effect

Abstract

Magnetic Fe₃O₄@Au nanoparticles (NPs) were fabricated using a specific strategy for use as a surface-enhanced Raman scattering (SERS) substrate. The Raman enhancement of Fe₃O₄@Au NPs could be verified by the detection of rhodamine and crystal violet. The resonator plasma of gold nanoparticles could significantly improve the SERS activity of Fe₃O₄ magnetic nanoparticles. The results showed that the fabricated Fe₃O₄@Au NPs have superior SERS sensitivity for rhodamine and crystal violet with a detection limit of 10⁻⁷ M. Moreover, the magnetic properties of Fe₃O₄ are conducive for easily separating Fe₃O₄@Au NPs from solution. Due to the convenience of operating and high efficiency technique for separating the product, the Fe₃O₄@Au NPs could be further developed for other applications.

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Introduction

Surface-enhanced Raman scattering (SERS) technology is a highly sensitive technology that is capable of overcoming some of the current disadvantages of conventional Raman spectroscopy.¹ In particular, SERS has been considered to be a very powerful tool for the study of surface phenomena. A single molecular layer adsorbed on the surface of the metal can be detected, providing a wealth of molecular structure information. Up to now, SERS has been widely used in biological,^2–4 chemical,^5–7 and medical fields^8,9 as well as in public security and environmental monitoring.¹⁰ However, the preparation of an active substrate is a prerequisite for obtaining the SERS signal. Thus, fabricating a SERS substrate that has strong ability, high stability, and good reproducibility is still a challenge. Therefore, research on SERS active substrate is highly significant.^11–14

Among the many types of SERS substrates, we focus on the Fe₃O₄ core/Au shell (Fe₃O₄@Au nanoparticle) substrate in this study. Composite nanoparticles that incorporate several components into one single nano-structured system integrate the strengths of individual components.¹⁵ The Fe₃O₄@Au nanoparticles (NPs) are composite nanoparticles with an Au shell indirectly coated onto the Fe₃O₄ core. This core–shell structure with a layer of gold helps to protect the core from oxidation and corrosion, which improves the chemical stability of the nanoparticles. In addition, the core–shell structure has highly complementary advantages for various applications based on the performance of both the Fe₃O₄ core and Au shell. The Fe₃O₄ nanoparticles have high saturation magnetization, low toxicity, and good biocompatibility characteristics.^16,17 As a result, Fe₃O₄ nanoparticles have been used for various biomedical applications, such as wastewater treatment, drug targeting, biological separation, in vivo magnetic resonance imaging, magnetic hyperthermia, and tissue engineering.^18–22 On the other hand, Au nanoparticles have a large specific surface area, high surface energy, and high surface activity, and they show small size effects, surface effects, quantum size effects, biocompatibility and absorption properties, enabling broad applications in the field of catalysis, optics, electricity, and bio-technology.²³ The combined Fe₃O₄@Au nanoparticles not only have external magnetic separation by the core of Fe₃O₄ but can also detect large biological molecules using the Au shell.²⁴ The resonator plasma of gold nanoparticles can significantly improve the surface-enhanced Raman scattering activity of Fe₃O₄ magnetic nanoparticles.²⁵

Currently, researchers have been developing various methods of synthesizing nanomaterials. For instance, Deng et al.²⁶ have reported the synthesis of Fe₃O₄/Au using a special strategy and use Fe₃O₄/Au composites as adsorbent for the extraction of benzo[a]pyrene from aqueous solution. Fang et al.²⁷ have also developed a novel Fe₃O₄@poly(N-isopropylacrylamide-co-acrylamide)@Au composite that can respond to magnetism, temperature, and surface-enhanced Raman scattering. Zhou et al.²⁸ had synthesized 40 nm diameter magnetic nanoparticles utilizing a microwave oven. The resulting magnetic nanoparticles were used to reduce gold precursors as core materials, and Fe₃O₄ core/Au shell nanoparticles, which had a diameter of 50–100 nm, were then synthesized with layer-by-layer growth. Nanoparticles with this core–shell structure could be used for important applications in molecular and biological detection. It has been reported that the bio-modified Fe₃O₄@Au NPs can be used as an effective multimodal contrast probe for targeting and multimodal imaging of cancer cells simultaneously.²⁹

Compared with the above preparation methods, we report the facile synthesis of relatively uniform Fe₃O₄ nanoparticles with a diameter of about 150 nm using the solvothermal method.^30,31 We functionally modified the surface of Fe₃O₄ nanoparticles and synthesize Fe₃O₄@Au nanomaterials with a core–shell structure through the seed growth method.^32,33 We then studied the application of the Fe₃O₄@Au nanoparticles in SERS, and we expect that these nanoparticles will lead to significant SERS developments.

Results and discussion

Scheme 1 shows the procedure for synthesizing Fe₃O₄ core/Au shell nanoparticles. The synthesis of Fe₃O₄ core/Au shell nanoparticles includes three steps. First, Fe₃O₄ NPs and Au NPs are synthesized. Second, Fe₃O₄@SiO₂ NPs are prepared, and the surface is amino-functionalized. Third, the surface of amino-functionalized Fe₃O₄@SiO₂ NPs adsorbs a member of Au NPs by electrostatic interaction. The Fe₃O₄@Au NPs are formed by reducing HAuCl₄ on Au-loaded Fe₃O₄ NPs.

Scheme 1 The procedure of synthesizing Fe₃O₄@Au NPs.

The size and morphology of Fe₃O₄ NPs were examined by transmission electron microscopy (TEM) and field-emission scanning electron microscopy (SEM). As shown in Fig. 1a and b, the nanoparticles were spherical in structure and were relatively uniformly sized with an average diameter of 150 nm, and the surface of the particles is rough. As shown in Fig. 2, Dynamic Light Scattering (DLS) was used to measure the hydrodynamic diameter and size distribution of Fe₃O₄ NPs. The mean hydrodynamic diameter determined by number was 175.1 nm with polydispersity index values of 0.142 at 25 °C, which was slightly larger than that of TEM results. In particular, two adjacent planes on the Fe₃O₄ NP surface have a lattice separation of 0.48 nm (Fig. 1c), which corresponds to the X-ray diffraction (XRD) results (Fig. 3a) showing the spacing between two (1 1 1) planes in spinel-structured Fe₃O₄. These results indicate that the Fe₃O₄ NPs are highly pure and show good crystallinity.

Fig. 1 (b) SEM image of Fe₃O₄ NPs. TEM images for the same batch of Fe₃O₄ NPs, Fe₃O₄@SiO₂ NPs, Au NPs and Fe₃O₄@SiO₂@Au NPs. (a) TEM image of Fe₃O₄ NPs. (c) Lattice image of Fe₃O₄ NPs. (d) Fe₃O₄@SiO₂ NPs. (e) Au NPs. (f) Fe₃O₄@Au NPs.

Fig. 2 Hydrodynamic diameter distribution of Fe₃O₄ NPs dispersion in ethanol solution measured by DLS.

Fig. 3 XRD spectra for (a) Fe₃O₄ NPs, (b) Fe₃O₄@SiO₂ NPs and (c) Fe₃O₄@SiO₂@Au NPs.

Two regions with different electron densities could be distinguished, verifying the formation of Fe₃O₄@SiO₂ NPs (Fig. 1d). The relatively translucent region surrounding these cores was a silica coating shell with a thickness of about 5 nm. The thickness of silica layer could be controlled by adjusting the amount of tetraethyl orthosilicate (TEOS).

The Au NPs were prepared by reducing HAuCl₄ with sodium borohydride using sodium citrate as the stabilizer. The TEM image of the Au NPs (Fig. 1e) showed that the size of Au seeds was about 5 nm. Because the Au NPs were coated by citrate that has three negatively charged carboxyl groups, the Au NPs can be easily adsorbed on amino-functionalized Fe₃O₄@SiO₂ NPs to form Au-loaded Fe₃O₄ NPs. Then, we use the seed growth method to synthesize the gold shell. From the TEM image (Fig. 1f), the surface of the Au-loaded Fe₃O₄ NPs is clearly coated with a dense layer of Au NPs. The XRD results of the Fe₃O₄ NPs and Au NPs in Fig. 3 also reveal the structure of Fe₃O₄@Au NPs. The thickness of the gold shell can be controlled by altering the amount of HAuCl₄.

The crystallinity and phase purity of the Fe₃O₄NPs, Fe₃O₄@SiO₂ NPs, and Fe₃O₄@SiO₂@Au NPs were determined by XRD (Fig. 3). The patterns are adopted through comparison with standard Fe₃O₄ and Au patterns. Based on the diffraction peaks in Fig. 3a, the detected product is Fe₃O₄. Moreover, the XRD data reveals a broad peak from θ = 17.600 to 29.050 in Fig. 3b, and this typical amorphous silica broad peak could be used to prove the existence of silica.³⁴ In Fig. 3c, the peaks of Fe₃O₄ are unchanged, and four new peaks appear, which could be indexed to the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of Au in the cubic phase, confirming the crystallinity of the Fe₃O₄@Au NPs. The XRD analyses are well matched with the TEM results discussed above.

The magnetic properties of Fe₃O₄ NPs and Fe₃O₄@Au NPs were characterized using a vibrating sample magnetometer (VSM) at room temperature. The results showed that the saturation magnetization (M_s) value of Fe₃O₄ NPs was about 107.5 emu g⁻¹ (Fig. 4). After being wrapped with silane layers and a gold shell, the M_s value of Fe₃O₄@Au NPs decreased to 48.7 emu g⁻¹. The reduction of saturation magnetization value can be explained by considering the silane layers and the Au shell, which has a diamagnetic contribution and increases the distance between magnetic particles.³⁵ The inset pictures demonstrate that magnetic microspheres could be attracted to the magnet in a few minutes with the transparent solution. These results reveal that the multifunctional nanocomposite still has strong magnetization and can be used for magnetic separation.

Fig. 4 Room temperature magnetic hysteresis curves for Fe₃O₄ NPs and Fe₃O₄@Au NPs. The inset photographs shows magnetic microspheres dispersed in solution (left) and separated from water solution with a magnet (right).

As shown in Fig. 5, for Au NPs with diameter of about 5 nm, a plasmon resonance band emerges near 512 nm.^36–38 The size of the nanoparticles and the interparticle distance affect the exact absorption position. After Fe₃O₄ NPs are wrapped with Au NPs, the absorption band was broadened and experienced a red shift. This shift can be explained through indexing strong interactions between particles and the coupling of the surface plasmons of neighboring Au NPs deposited on Fe₃O₄ NPs. The analysis shows that the Fe₃O₄@Au NPs had inherited the plasmon absorption properties of Au NPs.

Fig. 5 UV-vis absorption spectra of Fe₃O₄ NPs, Au NPs and Fe₃O₄@Au NPs.

The Fe₃O₄@Au NPs have a core of Fe₃O₄ with magnetic properties and an Au shell with well-known SERS activity. Thus, the Fe₃O₄@Au NPs can be easily gathered with a magnet and display SERS activity. As shown in Scheme 2, we use the magnetic properties of Fe₃O₄@Au NPs to prepare a SERS substrate, which can be easily collected using a magnet and exhibits SERS activity.

Scheme 2 The schematic of the preparation of SERS substrate. The inset photographs show (a) the rhodamine solution, (b) Fe₃O₄ NPs dispersed in the solution of rhodamine and (c) separated from water solution with a magnet, (d) SERS substrate on the slide.

We also found that the size and amount of Au NPs increased with the increase in the mass ratio of HAuCl₄ and Au-loaded Fe₃O₄, as shown in Fig. 6a–c. Moreover, as shown in Fig. 6d, when the mass ratio reaches a certain value, the Au NPs start to aggregate. As shown in Fig. 6e, at first, the horizontal and vertical coordinates are proportional. When the mass ratio is 1 : 3.3, the limit magnitude of rhodamine is 10⁻⁷ M. If we continue to increase the mass ratio, the limit magnitude of rhodamine would start to decrease. When the mass ratio reached a certain amount, the vertical axis would not change. This analysis shows that the loading of Au NPs has an important influence on the Raman enhancement.

Fig. 6 TEM images for the same batch of Fe₃O₄@SiO₂@Au NPs with different mass ratio of HAuCl₄ and Au-load Fe₃O₄: (a) 1 : 5.6, (b) 1 : 4.2, (c) 1 : 3.3, (d) 1 : 2.8; (e) the relationship of m_HAuCl4 : m_{Au-load Fe3O4} and limit magnitude of SERS.

To evaluate the sensitivity of the SERS substrate, the Raman responses of two probes, rhodamine and crystal violet, were acquired on a Renishaw Laser Raman Spectrometer. With the Fe₃O₄@Au NPs obtained with the best mass ratio used as the SERS substrate, Fig. 7 depicted SERS spectra for different concentrations of rhodamine solution and crystal violet solution. As shown in Fig. 7A, when the concentration of rhodamine is 10⁻⁸ M, the spectrum is poorly resolved. Moreover, when the concentration of rhodamine is 10⁻⁷ M, the signal of the spectrum is weak. However, from the upper left figure, we can observe many peaks. The observed peaks are consistent with those in the Raman spectrum of rhodamine on roughened gold.³⁹ Similar detection of crystal violet also confirmed the same (Fig. 7B). The limit of detection of crystal violet could also reach 10⁻⁷ M. With the increasing concentration of rhodamine and crystal violet, the intensity of the their peaks increases respectively. Compared with rhodamine, the amplification effect of crystal violet detection is totally different when Fe₃O₄@Au NPs were utilized as the SERS substrates. As rhodamine and crystal violet are in different coordinations, the Raman signal is very sensitive to these ligands under the existence of Au particles.⁴⁰ These results demonstrate that the fabricated Fe₃O₄@Au NPs can be utilized as an effective probe for rhodamine and crystal violet.

Fig. 7 SERS spectra of rhodamine (A) and crystal violet (B) at different concentrations (mol L⁻¹): intensity of spectra (c–e in A) is magnified 3 times, 8 times, and 8 times. (Inset of A is SERS spectra of 10⁻⁷ M rhodamine.)

Experimental

Preparation of Fe₃O₄ NPs

In this paper, chemicals were all analytical purity. The Fe₃O₄ NPs were prepared by solvothermal method described in ref. 31. In brief, 1.35 g FeCl₃·6H₂O was dissolved in 40 mL of ethylene glycol to form a clear solution. Then 3.6 g of sodium acetate and 1.0 g of polyethylene glycol were added to the upper solution. The mixture was being stirred vigorously for 30 minutes with a magnetic stirrer. Finally, the solution was sealed after transferred into a Teflon-lined autoclave of 50 mL capacity. The autoclave was put in an electric oven thermostat blast and being heated at 200 °C for 7 h. After that, the autoclave was cooled to room temperature. The solid products were recovered by a magnet and washed with anhydrous ethanol and ultrapure water for several times.

Synthesis of amino modified Fe₃O₄@SiO₂ NPs

The Fe₃O₄ NPs were pretreated with 50 mL of 0.1 mol L⁻¹ of hydrochloric acid under ultrasonic treatment for 10 min. The products were washed with water for several times after gathered by a magnetic. The Fe₃O₄@SiO₂ NPs were prepared through a Stöber process.²⁶ Typically, 0.5 g of pretreated Fe₃O₄ NPs was dispersed in the mixture of 120 mL of ethanol and 30 mL of ultrapure water under ultrasonic treatment for 5 min. Then 2 mL of ammonia solution and 0.2 mL of TEOS were dropped in the mixture. The reaction was conducted at room temperature under mechanical agitation for 6 h. Then 3 mL of N-[3-(trimethoxysilyl)propyl]ethylenediamine was dropped into the upper solution and the mixture was stirred for 12 h. The resulting materials were gathered using the magnetic and washed several times with anhydrous ethanol and ultrapure water.

Preparation of Au NPs and prepared the reduction of gold salt solution

Au NPs was prepared by the method as reported in ref. 33. In a typical experiment, 1 mL of 1% mass fraction (1 wt%) of HAuCl₄·4H₂O was dissolved in 99 mL of ultrapure water. The mixture was mixed with magnetic stirring for 15 min. Then 1 mL of 1% mass fraction (1 wt%) of Na₃Cit was added into the solution at room temperate. At last, the mixture of 1 mL of 0.075% mass fraction of NaBH₄ and 1 mL of 1% mass fraction of Na₃Cit were dropped into the upper solution with stirring throughout. The reaction was continued for 30 min. The color of the solution changed gradually from pink to red, indicating the formation of particles.

The preparation of reduction of gold salt solution was performed. In brief, 0.05 g of sodium carbonate (Na₂CO₃) was dissolved in 200 mL of ultrapure water. After vigorous magnetic stirring for 10 min, 3 mL of 1 wt% of HAuCl₄·4H₂O was added in the solution. Then, the mixture was stirred for 12 h without light.

Preparation of Au-seeded Fe₃O₄ nanoparticles

All the amine functionalized Fe₃O₄@SiO₂ NPs were scattered into 200 mL of Au NPs solution. The reaction was carried out with mechanical stirring for 3 h. The products were obtained using the magnetic and washed several times with water. The supernatant was clarified and showed a good adsorption performance.

Preparation of Fe₃O₄ core/Au shell (Fe₃O₄@Au)

The resulting products were dispersed into 40 mL of reduction of gold salt solution under mechanical stirring. Then 0.1 mL of formaldehyde was added into the mixture and the reaction was continued for 10 min. After that, 40 mL reduction of gold salt solution and 0.1 mL of formaldehyde were added into the upper mixture (repeated several times). The final products were washed and dispersed in water.

Characterization

Morphology of NPs was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). TEM micrograph of sample was taken using a Tecnai G2F20 electron microscope (FEI, USA) with a maximum accelerated voltage of 200 kV. Additionally, the average size and the size distribution of Fe₃O₄ NPs in ethanol solution were measured by dynamic light scattering (Zetasizer 3000HSA, Malvern, UK). UV-vis (ultraviolet-visible) spectra were recorded on a Lambda 25 UV-vis spectrophotometer. Raman scattering was performed on a Renishaw InVia Reflex confocal microscopy Raman Spectrometer with a CCD detector, using a 532 nm laser source. The laser power was 30 mW. SERS spectra were collected at 50× objective with an exposure time of 3 s and 10 accumulations. The spot size of the laser was ∼1 μm in diameter. The crystalline structures were recorded by X-ray diffraction (XRD) analysis on D/MAX-2500 diffractometer (Rigaku, Japan) using Cu Kα radiation as the X-ray source in the 2theta range of 10–85°. Magnetic measurement was carried out using a PPMS-9 vibrating sample magnetometer (Quantum Design, USA) at room temperature.

Conclusions

In summary, we successfully obtained Fe₃O₄ NPs. We then fabricated Fe₃O₄@Au NPs using absorbed Au NPs on the surface of amino-functionalized Fe₃O₄@SiO₂ NPs, followed by the seed growth method to reduce HAuCl₄. The Fe₃O₄@SiO₂ NPs possess the advantages of both Fe₃O₄ and Au. The resulting NPs have superior SERS sensitivity for dyeware. The enhanced SERS signal could be a result of the increase in Au hot spots. Thus, the Fe₃O₄@Au NP substrate could be expected to have great potentials in detecting food additives⁴¹ and heavy metals.⁴²

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 11272232).

References

1. J. Popp and T. Mayerhöfer, Anal. Bioanal. Chem., 2009, 394, 1717–1718.
2. T. Vo-Dinh, K. Houck and D. L. Stokes, Anal. Chem., 1994, 66, 3379–3383.
3. T. Vo-Dinh, L. R. Allain and D. L. Stokes, J. Raman Spectrosc., 2002, 33, 511–516.
4. F. Shao, Z. Lu, C. Liu, H. Han, K. Chen, W. Li, Q. He, H. Peng and J. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 6281–6289.
5. A. Tao, F. Kim, C. Hess, J. Goldberger, Y. Sun and P. Yang, Nano Lett., 2003, 3, 1229–1233.
6. S. Schlücker, Angew. Chem., Int. Ed., 2014, 53, 4756–4795.
7. M. Fan, G. F. Andrade and A. G. Brolo, Anal. Chim. Acta, 2011, 693, 7–25.
8. N. R. Isola, D. L. Stokes and T. Vo-Dinh, Anal. Chem., 1998, 70, 1352–1356.
9. L. R. Allain and T. Vo-Dinh, Anal. Chim. Acta, 2002, 469, 149–154.
10. R. A. Halvorson and P. J. Vikesland, Environ. Sci. Technol., 2010, 44, 7749–7755.
11. R. G. Freeman, M. B. Hommer, K. C. Grabar, M. A. Jackson and M. J. Natan, J. Phys. Chem., 1996, 100, 718–724.
12. Y. C. Liu and S. J. Yang, Electrochim. Acta, 2007, 52, 1925–1931.
13. K. Kim, K. L. Kim and S. J. Lee, Chem. Phys. Lett., 2005, 403, 77–82.
14. L. Rivas, S. Sanchez-Cortes, J. V. Garcia-Ramos and G. Morcillo, Langmuir, 2000, 16, 9722–9728.
15. Q. Xiang, J. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575–6578.
16. I. Y. Goon, L. M. Lai, M. Lim, P. Munroe, J. J. Gooding and R. Amal, Chem. Mater., 2009, 21, 673–681.
17. R. Prucek, J. Tuček, M. Kilianová, A. Panáček, L. Kvítek, J. Filip and R. Zbořil, Biomaterials, 2011, 32, 4704–4713.
18. D. Zhang, D. Xu, Y. Ni, C. Lu and Z. Xu, Mater. Lett., 2014, 123, 116–119.
19. P. J. Chen, S. H. Hu, C. S. Hsiao, Y. Y. Chen, D. M. Liu and S. Y. Chen, J. Mater. Chem., 2011, 21, 2535–2543.
20. S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. Vander Elst and R. N. Muller, Chem. Rev., 2008, 108, 2064–2110.
21. L. Nie, Z. Ou, S. Yang and D. Xing, Med. Phys., 2010, 37, 4193–4200.
22. M. D. Krebs, R. M. Erb, B. B. Yellen, B. Samanta, A. Bajaj, V. M. Rotello and E. Alsberg, Nano Lett., 2009, 9, 1812–1817.
23. M. C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346.
24. H. Y. Park, M. J. Schadt, L. Wang, I. I. S. Lim, P. N. Njoki, S. H. Kim, M. Y. Jang, J. Luo and C. J. Zhong, Langmuir, 2007, 23, 9050–9056.
25. L. Sun, Y. Song, L. Wang, C. Guo, Y. Sun, Z. Liu and Z. Li, J. Phys. Chem. C, 2008, 112, 1415–1422.
26. X. Deng, K. Lin, X. Chen, Q. Guo and P. Yao, Chem. Eng. J., 2013, 225, 656–663.
27. J. H. Fang, C. N. Wang, M. Cao, J. Z. Shi and Y. L. Jin, Mater. Lett., 2013, 96, 89–92.
28. X. Zhou, W. Xu, Y. Wang, Q. Kuang, Y. Shi, L. Zhong and Q. Zhang, J. Phys. Chem. C, 2010, 114, 19607–19613.
29. T. Zhou, B. Wu and D. Xing, J. Mater. Chem., 2012, 22, 470–477.
30. A. Yan, X. Liu, G. Qiu, H. Wu, R. Yi, N. Zhang and J. Xu, J. Alloys Compd., 2008, 458, 487–491.
31. H. Deng, X. Li, Q. Peng, X. Wang, J. Chen and Y. Li, Angew. Chem., 2005, 117, 2842–2845.
32. Y. T. Lim, O. O. Park and H. T. Jung, J. Colloid Interface Sci., 2003, 263, 449–453.
33. J. H. Kim, B. W. Lavin, B. W. Boote and J. A. Pham, J. Nanopart. Res., 2012, 14, 1–10.
34. J. Zheng and X. H. Chen, Chin. Sci. Bull., 2008, 6, 006.
35. S. Guo, S. Dong and E. Wang, Chem.–Eur. J., 2009, 15, 2416–2424.
36. F. Hache, D. Ricard, C. Flytzanis and U. Kreibig, Appl. Phys. A: Solids Surf., 1988, 47, 347–357.
37. Z. Wang, L. Wu, B. Shen and Z. Jiang, Talanta, 2013, 114, 124–130.
38. Z. Wang, L. Wu, W. Cai and Z. Jiang, J. Mater. Chem., 2012, 22, 3632–3636.
39. Z. Wang, L. Wu, F. Wang, Z. Jiang and B. Shen, J. Mater. Chem. A, 2013, 1, 9746–9751.
40. K. Zhang, T. Zeng, X. Tan, W. Wu, Y. Tang and H. Zhang, Appl. Surf. Sci., 2015, 347, 569–573.
41. N. N. Yazgan, H. Boyac and A. Topcu, Anal. Bioanal. Chem., 2012, 403, 2009–2017.
42. J. Du and C. Jing, J. Colloid Interface Sci., 2011, 358, 54–61.

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