Facile fabrication of a superhydrophilic–superhydrophobic patterned surface by inkjet printing a sacrificial layer on a superhydrophilic surface

Abstract

Based on the high wettability contrast, superhydrophilic–superhydrophobic patterned surfaces (SSPSs) have been used in a wide variety of applications, such as cell patterning, droplet transport and analyte enrichment. However, the fabricating approaches of SSPSs are commonly complicated and high-cost. Herein, a facile method was developed to fabricate SSPSs by inkjet printing a sacrificial layer on a superhydrophilic surface. The influence of a pinned three phase contact line on the depositing morphology of the inkjet droplet was investigated, and a uniform structure with high resolution was inkjet printed on the superhydrophilic substrate. Moreover, the patterns of the lines and films were directly inkjet printed on the superhydrophilic surface by regulating the inkjet droplet's coalescence. After modifying the surface by fluoro-alkyl silanes and removing the printed water-soluble deposit, the fabricated surface showed high wettability contrast between the printed area and unprinted area. Finally, the fabricated SSPSs were applied to achieve nanoparticle adhesion and droplet transport.

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1. Introduction

Based on the extreme difference in wettability, superhydrophilic–superhydrophobic patterned surfaces (SSPSs) have been used in a wide variety of applications, including cells, hydrogel and microelectrode patterning,^1–5 droplet transport,^6–9 water collection,^10–12 microdroplet self-removal,^13–16 depositing morphology control,^17–19 analyte enrichment,^20–22 oil–water interface interaction,^23–25 and many other important fields. In previous research, many methods were developed to fabricate SSPSs with photomask and plasma treatment or photo-degradation,^26–30 while these approaches are commonly complicated and high-cost. Therefore, it is of vital significance to develop a facile method for fabricating SSPSs.

Compared with various patterning methods, such as lithographic printing,^31,32 nano-imprinting^33,34 and micro-contact printing,^35,36 inkjet printing offers a direct depositing technique using liquid materials.^37–40 By virtue of flexibility, convenience, speediness and low-cost, inkjet printing has attracted great attention and was applied widely in fabricating high-quality patterns.^41–45 Superhydrophilic materials could be directly deposited on a superhydrophobic surface by inkjet printing,^46–50 which needn't photomask and plasma treatment or photo-degradation. However, it is still a challenge to fabricate continuous line with sharp edge as well as uniform film with flat surface on such a low energy surface. Therefore, the printed superhydrophilic area on the superhydrophobic surface is generally nonuniform, which greatly limits its applications.

In this study, a facile fabrication of SSPSs was presented by inkjet printing sacrificial layer on superhydrophilic surface. Three typical depositing morphologies of inkjet droplets were demonstrated on the superhydrophilic surface. The influence of pinned three phase contact line (TCL) on excessive spreading and coffee ring effect was investigated, and uniform structure with high resolution was inkjet printed on the superhydrophilic substrate. Moreover, the depositing morphologies of coalesced inkjet droplets were controlled to fabricate uniform lines and films by regulating droplet distance. After modifying the surface by fluoro-alkyl silanes (FAS) and removing the printed water-soluble deposit, the SSPSs were fabricated. Furtherly, nanoparticle adhesion and droplet transport demonstrated the extreme difference in wettability of the fabricated SSPSs. This study presented a flexible, convenient and low-cost method to fabricate SSPSs and offered an insight of controllable depositing morphologies of microdroplets on a high energy surface. It will be of great potential for the development of material patterning, device fabrication as well as related interface researches.

2. Experimental section

Materials

PAA (Poly Acrylic Acid) with average molecule weight of 1800 (Sigma-Aldrich). Ethanol (Sinopharm Chemical Reagent Co., Ltd.). Perfluorodecyltrimethoxysilane (FAS-17) (Alfa Asia). Deionized water was generated by Mini-Q water purification system. Fluorescent polystyrene particles were synthesized by previously reported method.⁵¹

Preparation of inks

Inkjet printing ink was prepared by dissolving PAA in ethanol. The solution was injected into ink cartridge for use.

Preparation of superhydrophilic surface

The silicon substrate was cleaned with acetone, ethanol and deionized water for 10 min, respectively. Then thin aluminum film was deposited on the silicon substrate via vacuum deposition with a depositing speed of 1–1.5 Å per second. The aluminum film was immersed in 70 °C water bath for 5 min to obtain hierarchical porous nano-structures (Fig. S1†), rinsed with water, and dried in an oven with 80 °C for half hour. The substrate was prepared in our laboratory using reported method.⁵²

Inkjet printing water-soluble PAA deposits

Printing process was performed via Dimatix Materials Printer (FUJIFILM DMP-2800 series, Japan) with a 25 μm nozzle.

Observing inkjet droplet impacting on superhydrophilic surface

Magnified images of inkjet droplets from jetlab R°II print station (MicroFab Technologies, Inc., Plano, Texas, USA) impacting on superhydrophilic surface were captured by a high-speed camera (Vision Research, Inc, Phantom®v12.1) with zoom lens (1.56 μm per pixel) using power source of single hole xenon lamp (XD-300-350W). This inkjet printer was different from the above one for the direct observation.

Hydrophobic modification

The printed substrate was placed in a desiccator with one droplet FAS-17. Then the desiccator was vacuumized for half hour, and left in an 80 °C oven for three hours to make a vapor-phase FAS modifying.

Observing droplet transport on patterned surface

Magnified images of transported droplet was captured by a high-speed camera (Vision Research, Inc, Phantom®v12.1) with zoom lens (1.56 μm per pixel) using power source of single hole xenon lamp (XD-300-350W).

Characterization

Scanning electron microscope (SEM) images were obtained by field emission scanning electron microscope (JSM-7500, Japan). The printed substrate was investigated by an atomic force microscope (AFM) (Icon, Bruker, Germany). Cross-section of printed dot was tested by surface profiler (Kosaka ET4000, Japan). Contact angles were measured by contact angle measurement device (OCA20, DataPhysics, Germany) with droplets of 3 μL. X-ray photoelectron spectroscopy (XPS) was captured by Thermo Scientific ESCALab 250Xi, VG Scientific. Fluorescent optical microscopy images were obtained using a fluorescence microscope (Olympus MX40, Japan).

3. Results and discussion

Characterization of substrate and inkjet printed patterns

Fig. 1a presented the schematic illustration of fabricating SSPSs by inkjet printing sacrificial layer on superhydrophilic surface. Firstly, a hierarchical porous alumina was prepared as the superhydrophilic substrate (Fig. 1b and c). The engineered alumina could be facilely fabricated and used as an inkjet printing surface. Secondly, high resolution patterns were inkjet printed on the superhydrophilic surface (Fig. 1d–g). The printed patterns, including dots, lines and films, covered the substrate and served as a sacrificial layer. The printing ink was the PAA solution with 30 wt%. After modifying the surface hydrophobicity by FAS and removing the printed water-soluble deposit by water, superhydrophilic area and superhydrophobic area were generated on the same surface.

Fig. 1 (a) Schematic illustration of fabricating process by inkjet printing technology. (b) SEM image of hierarchical porous alumina surface. (c) Magnified SEM image of the surface. (d) SEM image of boundary between the printed and unprinted area (e) SEM image of printed dots. (f) SEM image of printed lines. (g) SEM image of printed films.

Controllable depositing morphology of inkjet droplet on superhydrophilic surface

The covering ability of the inkjet printed sacrificial layer was crucial for fabricating SSPSs if use the above method. Once the printed deposits can't form a uniform cover structure, the printed area won't prevent modifying the surface hydrophobicity by FAS. In the actual process of fabrication, inkjet droplet spreads very quickly on the superhydrophilic surface, which greatly influences the printed resolution and covering ability. To obtain an effective depositing morphology of a single inkjet droplet on the superhydrophilic surface, we prepared the water-soluble PAA solution inks with different concentrations in this study. And the different depositing morphologies of inkjet droplets on the superhydrophilic surface were demonstrated with varying ink concentration in Fig. 2a. The depositing morphologies of inkjet droplets on superhydrophilic surface could be defined as three typical structures. As shown in Fig. 2b, there are weak cover structure, ring-like structure and uniform cover structure, respectively. The concentrations of the three droplets were increasing from top to bottom in Fig. 2b. Height profile of inkjet printed pattern with 30% ink concentration verified that the depositing morphology was uniform without coffee ring structure (Fig. S2†). Therefore uniform depositing morphology of inkjet droplet was obtained on the superhydrophilic surface with relative high ink concentration, which could be used as the sacrificial layer in the fabricating process of SSPSs. Meanwhile, the increasing ink concentration improved the ink viscosity and decreased the depositing diameter of inkjet droplet (Fig. 2c).

Fig. 2 (a) SEM images of depositing morphologies of inkjet droplets. (b) The sketch of three typical depositing morphologies of inkjet droplets. (c) The variation trend of viscosity and depositing diameter of inkjet droplets.

To investigate the influence of inkjet droplet's wetting behavior on depositing morphology, the spreading and wetting process of inkjet droplet on the superhydrophilic surface was directly captured by a high-speed camera. As shown in Fig. 3, the spreading and wetting process of one inkjet droplet included three parts: ejecting, spreading and pinning. In this process, ejection of an inkjet droplet provided a kinetic energy (E_k), and the gravity gave a potential energy (E_p). When one droplet impacted on the superhydrophilic surface, a capillary driving energy (E_c) would generate to maintain a stable state: γ_sv − γ_sl = γ_lv cos θ. When the droplet spread on the substrate, the adhesive force between droplet and substrate nearby TCL would produce a resistant energy (E_r). The increasing viscosity of droplet also improved the internal friction and form a high internal friction energy (E_i). The energy ratio r (r = E_k + E_p + E_c/E_r + E_i) determined TCL of droplet to advance or be pinned. In such system, the variation of E_k, E_p and E_c was slight, and E_r and E_i would be improved greatly on the superhydrophilic surface by the increasing ink concentration. Once the energy ratio r ≤ 1, the TCL of droplet would be pinned on the substrate. Therefore, the increasing ink concentration could induce a rapidly pinned TCL and decrease spreading scope of the inkjet droplet. When the inkjet droplet had a pinned TCL, the coffee ring effect of evaporating droplet would happen. The solutes would be carried to the pinned TCL and deposited as a ring-like morphology.^53,54 For the diameter of nozzle was only 25 μm, the time competition between solvent evaporation and solute movement should be considered in the microdroplets.⁵⁵ The solvent of the inkjet droplet would be dried out rapidly with a high ink concentration. As the solvent evaporation rate was much faster than the solute movement rate, the time would be not enough for transferring the solute to the edge of the microdroplet. Thus, the scale of inkjet droplet could be used to inhibit the coffee ring effect. Meanwhile, solvent of inkjet droplet dried out quickly, the increasing cohesive energy of the microdroplet also weakened the coffee ring effect.⁵⁶ Thus the uniform structure with high resolution could be inkjet printed on the superhydrophilic surface as a sacrificial layer for fabricating SSPSs.

Fig. 3 Spreading and wetting process of inkjet droplets on the superhydrophilic surface.

Controllable depositing morphologies of coalesced inkjet droplets on superhydrophilic surface

Single inkjet droplet can be used as building block to fabricate line and film.^57–59 As shown in Fig. 4a, isolated dots, wave line and homogeneous line were fabricated respectively using 30% ink concentration with varying printing distance. When the distance of two inkjet droplets is 40 μm, homogeneous line was generated. The high profile of the homogeneous line pattern is shown in Fig. S3.† Furtherly, films were printed with varying distance of homogeneous lines. As shown in Fig. 4b, the line distance was varied from 40 μm to 60 μm, and high precise square films were prepared directly. When the distance between two homogeneous lines was 30 μm, the depositing morphology was no longer a precise square pattern. The resistance was not large enough to form a square film, the coalescence of inkjet droplets tended to be a bigger droplet. When the printed droplet distance was improved to 70 μm, there would generate a small uncovered pattern with about 10–20 μm wide (Fig. S4†).

Fig. 4 (a) SEM images of line morphology with varying droplet distance. (b) SEM images of film morphology with varying line distance.

Wettability of patterned surface based on inkjet printed sacrificial layer

To study the wettability of the fabricated substrate, the elemental analysis was measured with modifying the FAS. The height profile indicated that the thickness of the printed pattern was about 700 nm. The thickness of the printed pattern's edge increased quickly, and the thickness of the printed pattern's center was uniform. So the printed pattern could prevent modifying the surface hydrophobicity with FAS effectively. The sample was observed by X-ray photoelectron spectroscopy (XPS), it could be seen that F 1s signal located at 689 eV was almost absent before vapor-phase FAS modifying, while the F 1s peak appeared after the modifying (Fig. 5a–c). To furtherly verify the surface modifying effect, the fluorine distribution image of the rinsed dot surface was observed by imaging-XPS. The result showed that the fluorine was modified on the fabricated substrate except the printed area (Fig. S5†). Meanwhile the wettability of the inkjet printing substrate was measured. As shown in Fig. 5d–f, contact angle of the superhydrophilic substrate was 3.9°, contact angle and sliding angle of the modified substrate were 156.1° and 3°.

Fig. 5 The wettability of the fabricated substrate. (a) SEM image of the printed substrate; XPS spectra of the printed substrate (b) before and (c) after modifying the FAS. Wetting contact angles of (d) the superhydrophilic substrate and (e) the substrate after modifying the FAS; (f) sliding contact angle of the substrate after modifying the surface by FAS (the size of droplet is 3 μL for contact angle measurement).

Applications of the fabricated surface in nanoparticle adhesion and droplet transport

By virtue of the wettability contrast of the SSPSs, patterned functional material can be realized easily on the surface. As shown in Fig. 6a, the fabricated surface was raised from a fluorescent polystyrene nanoparticle aqueous solution. The nanoparticles were directly adhered on the surface and formed microarray (Fig. 6b and e). Meanwhile, the area adhered nanoparticles matched perfectly with the printed area on the surface (Fig. 6c, d, f and g).

Fig. 6 (a) Schematic illustrations of fabricating fluorescent microarray with patterned surface. (b) Fluorescent image of polystyrene particles adhered on printed dot surface. (c) SEM image of patterned dots with adhered polystyrene particles. (d) SEM image of boundary between adhered polystyrene particles and the substrate. (e) Fluorescent image of polystyrene particles adhered on printed line surface. (f) SEM image of patterned lines with adhered polystyrene particles. (g) SEM image of boundary between adhered polystyrene particles and substrate.

Surface tension driven transport of liquid on planar substrate offers an effective tool for open analysis systems. Large wettability contrast and gradient shape can transport a certain volume of liquid, because there will generate a Laplace pressure to provide a propelling force.^60,61 A cone-shape pattern with wide and length ratio 1 : 10 was inkjet printed as a sacrificial layer for fabricating SSPSs by the above method (Fig. 7a). As shown in Fig. 7b, the fabricated surface could transport one microliter droplet to one centimeter length in 0.5 second, and the whole droplet was transported to the destination in 2.4 seconds. The result further indicated a high application performance of the fabricated SSPSs.

Fig. 7 (a) Optical photograph of the inkjet printed cone-shape deposit. (b) Captured images of droplet transport on the gradient wettability surface at different times.

4. Conclusions

In this work, the relationship between energy state and pinned TCL of inkjet droplet was investigated on the superhydrophilic surface. With a rapidly pinned TCL, the coffee ring effect of the inkjet printed microdroplet was inhibited on the superhydrophilic surface. Meanwhile, the resolution of the inkjet printed pattern was improved greatly. Moreover, the patterns of lines and films were inkjet printed on the superhydrophilic surface by regulating inkjet droplet's coalescence. After modifying the surface by FAS and removing the printed water-soluble deposit, the SSPSs were fabricated. Based on the great wettability contrast between the printed area and the unprinted area, nanoparticle adhesion and droplet transport were achieved on the surface. This simple SSPSs fabricating method will provide a promising avenue for applications in liquid patterning, optoelectronic device, biological analysis, microreactor and related fields.

Acknowledgements

We gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 51173190, 51473173, 51573193, 21203209, 21121001, 21303220), National Basic Research Program of China (973 Program) (No. 2013CB933004), and "Strategic Priority Research Program" of the Chinese Academy of Sciences (No. XDA09020000).

Notes and references

1. H. Z. Li, Q. Yang, G. N. Li, M. Z. Li, S. T. Wang and Y. L. Song, ACS Appl. Mater. Interfaces, 2015, 7, 9060.
2. F. L. Geyer, E. Ueda, U. Liebel, N. Grau and P. A. Levkin, Angew. Chem., Int. Ed., 2011, 50, 8424.
3. A. N. Efremova, E. Stanganello, A. Welle, S. Scholpp and P. A. Levkin, Biomaterials, 2013, 34, 1757.
4. S. Sugaya, S. Kakegawa, S. Fukushima, M. Yamada and M. Seki, Langmuir, 2012, 28, 14073.
5. D. Y. Ji, L. Jiang, Y. L. Guo, H. L. Dong, J. P. Wang, H. J. Chen, Q. Meng, X. L. Fu, G. F. Tian, D. Z. Wu, G. Yu, Y. Q. Liu and W. P. Hu, Adv. Funct. Mater., 2014, 24, 3783.
6. S. W. Hu, B. Y. Xu, W. K. Ye, X. H. Xia, H. Y. Chen and J. J. Xu, ACS Appl. Mater. Interfaces, 2015, 7, 935.
7. J. Seo, J. Lee, J. Lee and T. Lee, ACS Appl. Mater. Interfaces, 2011, 3, 4722.
8. A. Ghosh, R. Ganguly, T. M. Schutziusac and C. M. Megaridis, Lab Chip, 2014, 14, 1538.
9. S. Y. Xing, J. Jiang and T. R. Pan, Lab Chip, 2013, 13, 1937.
10. A. R. Parker and C. R. Lawrence, Nature, 2001, 414, 33.
11. R. P. Garrod, L. G. Harris, W. C. E. Schofield, J. McGettrick, L. J. Ward, D. O. H. Teare and J. P. S. Badyal, Langmuir, 2007, 23, 689.
12. A. Lee, M. W. Moon, H. Lim, W. D. Kim and H. Y. Kim, Langmuir, 2012, 28, 10183.
13. K. K. Varanasi, M. Hsu, N. Bhate, W. S. Yang and T. Deng, Appl. Phys. Lett., 2009, 95, 094101.
14. L. Mishchenko, M. Khan, J. Aizenberg and B. D. Hatton, Adv. Funct. Mater., 2013, 23, 4577.
15. M. He, Q. L. Zhang, X. P. Zeng, D. P. Cui, J. Chen, H. L. Li, J. J. Wang and Y. L. Song, Adv. Mater., 2013, 25, 2291.
16. Y. M. Hou, M. Yu, X. M. Chen, Z. K. Wang and S. H. Yao, ACS Nano, 2015, 9, 71.
17. H. Sirringhaus, T. Kawase, R. H. Friend, T. Shimoda, M. Wu, W. Inbasekaran and E. P. Woo, Science, 2000, 290, 2123.
18. P. Q. M. Nguyen, L. P. Yeo, B. K. Lok and Y. C. Lam, ACS Appl. Mater. Interfaces, 2014, 6, 4011.
19. L. Wu, Z. C. Dong, M. X. Kuang, Y. N. Li, F. Y. Li, L. Jiang and Y. L. Song, Adv. Funct. Mater., 2015, 25, 2237.
20. J. Hou, M. Z. Li, Q. Yang, Y. L. Song and L. Jiang, Angew. Chem., Int. Ed., 2014, 53, 5791.
21. J. Hou, H. C. Zhang, Q. Yang, M. Z. Li, L. Jiang and Y. L. Song, Small, 2015, 11, 2738.
22. L. P. Xu, Y. Chen, G. Yang, W. Shi, B. Dai, G. Li, Y. Cao, Y. Wen, X. Zhang and S. Wang, Adv. Mater., 2015, 27, 6878.
23. S. Nishimoto, M. Becchaku, Y. Kameshima, Y. Shirosaki, S. Hayakawa, A. Osaka and M. Miyake, Thin Solid Films, 2014, 558, 221.
24. E. Ueda and P. A. Levkin, Adv. Healthcare Mater., 2013, 2, 1425.
25. V. Jokinen, R. Kostiainen and T. Sikanen, Adv. Mater., 2012, 24, 6240.
26. E. Ueda and P. A. Levkin, Adv. Mater., 2013, 25, 1368.
27. L. Zhang, N. Zhao and J. Xu, J. Adhes. Sci. Technol., 2014, 28, 769.
28. Y. Huang, F. Y. Li, M. Qin, L. Jiang and Y. L. Song, Angew. Chem., Int. Ed., 2013, 52, 7296.
29. K. Tadanaga, J. Morinaga, A. Matsuda and T. Minami, Chem. Mater., 2000, 12, 590.
30. Z. Z. Gu, A. Fujishima and O. Sato, Angew. Chem., Int. Ed., 2002, 41, 2068.
31. T. Ito and S. Okazaki, Nature, 2000, 406, 1027.
32. A. M. Hung, C. M. Micheel, L. D. Bozano, L. W. Osterbur, G. M. Wallraff and J. N. Cha, Nat. Nanotechnol., 2010, 5, 121.
33. S. H. Ahn and L. J. Guo, Adv. Mater., 2008, 20, 2044.
34. L. J. Guo, Adv. Mater., 2007, 19, 495.
35. X. Yan, J. M. Yao, G. A. Lu, X. Chen, K. Zhang and B. Yang, J. Am. Chem. Soc., 2004, 126, 10510.
36. H. Xu, X. Y. Ling, J. van Bennekom, X. Duan, M. J. W. Ludden, D. N. Reinhoudt, M. Wessling, R. G. H. Lammertink and J. Huskens, J. Am. Chem. Soc., 2009, 131, 797.
37. D. L. Tian, Y. L. Song and L. Jiang, Chem. Soc. Rev., 2013, 42, 5184.
38. M. Singh, H. M. Haverinen, P. Dhagat and G. E. Jabbour, Adv. Mater., 2010, 22, 673.
39. A. Shimoni, S. Azoubel and S. Magdassi, Nanoscale, 2014, 6, 11084.
40. M. Abulikemu, E. H. Da'as, H. Haverinen, D. Cha, M. A. Malik and G. E. Jabbour, Angew. Chem., Int. Ed., 2014, 53, 420.
41. M. X. Kuang, L. B. Wang and Y. L. Song, Adv. Mater., 2014, 26, 6950.
42. S. P. Chen, H. L. Chiu, P. H. Wang and Y. C. Liao, ECS J. Solid State Sci. Technol., 2015, 4, 3026.
43. J. F. Dijksman, P. C. Duineveld, M. J. J. Hack, A. Pierik, J. Rensen, J. E. Rubingh, I. Schram and M. M. Vernhout, J. Mater. Chem., 2007, 17, 511.
44. J. Z. Sun, M. X. Kuang and Y. L. Song, Progr. Chem., 2015, 27, 929.
45. J. Z. Sun, B. Bao, M. He, H. H. Zhou and Y. L. Song, ACS Appl. Mater. Interfaces, 2015, 7, 28086.
46. S. Nishimoto, A. Kubob, K. Noharac, X. T. Zhang, N. Taneichi, T. Okuib, Z. Y. Liu, K. Nakata, H. Sakai, T. Murakamia, M. Abec, T. Komineb and A. Fujishima, Appl. Surf. Sci., 2009, 255, 6221.
47. J. S. Li, E. Ueda, A. Nallapaneni, L. X. Li and P. A. Levkin, Langmuir, 2012, 28, 8286.
48. W. Z. Shen, M. Z. Li, C. Q. Ye, L. Jiang and Y. L. Song, Lab Chip, 2012, 12, 3089.
49. M. Elsharkawy, T. M. Schutzius and C. M. Megaridis, Lab Chip, 2014, 14, 1168.
50. L. B. Zhang, J. B. Wu, M. N. Hedhili, X. L. Yang and P. Wang, J. Mater. Chem. A, 2015, 3, 2844.
51. B. Bao, F. Y. Li, H. Li, L. F. Chen, C. Q. Ye, J. M. Zhou, J. X. Wang, Y. L. Song and L. Jiang, J. Mater. Chem. C, 2013, 1, 3802.
52. Q. L. Zhang, M. He, J. Chen, J. J. Wang, Y. L. Song and L. Jiang, Chem. Commun., 2013, 49, 4516.
53. R. D. Deegan, O. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel and T. A. Witten, Nature, 1997, 389, 827.
54. M. Layani, M. Gruchko, O. Milo, I. Balberg, D. Azulay and S. Magdassi, ACS Nano, 2009, 3, 3537.
55. X. Y. Shen, C. M. Ho and T. S. Wong, J. Phys. Chem. B, 2010, 114, 5269.
56. L. Y. Cui, Y. F. Li, J. X. Wang, E. T. Tian, X. Y. Zhang, Y. Z. Zhang, Y. L. Song and L. Jiang, J. Mater. Chem., 2009, 19, 5499.
57. M. J. Liu, J. X. Wang, M. He, L. B. Wang, F. Y. Li, L. Jiang and Y. L. Song, ACS Appl. Mater. Interfaces, 2014, 6, 13344.
58. E. Tekin, B. J. de Gans and U. S. Schubert, J. Mater. Chem., 2004, 14, 2627.
59. D. Soltman and V. Subramanian, Langmuir, 2008, 24, 2224.
60. J. L. Zhang and Y. C. Han, Langmuir, 2007, 23, 6136.
61. S. Daniel, M. K. Chaudhury and J. C. Chen, Science, 2001, 291, 633.

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Footnote

† Electronic supplementary information (ESI) available: See the details of Fig. S1–S6. See DOI: 10.1039/c6ra02170k

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