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Fabrication of Low and High Adhesion Hydrophobic Au Surfaces with Micro/ Nano-Biomimetic Structures Wenjie Zhao,†,‡ Liping Wang,*,† and Qunji Xue† State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China, and Graduate School of Chinese Academy of Sciences, Beijing 100039, China ReceiVed: March 7, 2010; ReVised Manuscript ReceiVed: May 17, 2010
Inspired by plants and animals in nature that show obvious superhydrophobic performance with high or low adhesion, developing artificial ways to mimic these surfaces is meaningful and practical for mankind. In this work, we used a simple, efficient, and highly reproducible method for producing large-area positive and negative lotus and rice leaf topography on Au surfaces based on PDMS, and then the as-prepared surfaces were chemically modified with alkanethiol to enhance hydrophobicity. Surface morphologies of Au surfaces with biomimetic micro/nanobinary textures were examined by SEM and 3D noncontact optical profilometry. Hydrophobic properties of surfaces were characterized by the contact angle and sliding angle between a water droplet and the as-prepared Au surfaces, and the smooth Au surface was provided as a comparison. Results show that both positive and negative biomimetic textures (lotus leaf and rice leaf) were successfully generated on Au surfaces, and Au surfaces with biomimetic textures exhibited improved hydrophobic ability after chemical modification. Adhesion properties between water droplet and Au surfaces with positive and negative biomimetic micro/nanotextures showed nearly opposite performance. An understandable model is proposed to interpret the mechanism which causes the different adhesion performance between Au surfaces with positive and negative biomimetic structures. The strong adhesion is attributed to van der Waals and the capillary force interactions between the biomimetic Au surfaces with negative plant topographies and water droplet. 1. Introduction In order to adapt themselves to their surrounding environment and better survival in nature, biospecies exhibit amazing properties such as superhydrophobicity with high or low adhesion, UV protection, gorgeous colors, and self-cleaning ability and so on. Nature often uses topographic patterning to control interfacial interactions, such as superhydrophobic adhesion and release. For example, lotus leaves show self-cleaning function where contaminations can be easily moved away from the surface when the leaves are slightly inclined,1,2 water striders can stand effortlessly on water even if they are being bombarded by raindrops, butterfly wings show multifarious color,3,4 geckos can attach to and easily detach from almost any kinds of surface with varying roughness and orientation, and a water droplet can be pinned on rose petals at any titled angles.5,6 Each example demonstrates that, as well as chemistry and material properties, geometric structure is also critical for optimizing interfacial design.7-9 Of the above properties, superhydrophobicity has received tremendous attention in recent years due to its great advantages in both fundamental research and practical applications. On the other hand, superhydrophobic surfaces with special liquid-solid adhesion have recently aroused extensive interest for their potential applications in sticky tape, transportation of small liquid droplets without loss, selective permeability in a membrane, in situ detection, and operation of wall-climbing robots.5,10-12 However, the simultaneous control of both wet* To whom correspondence should be addressed. Phone: +86 931 4968080. Fax: +86 931 4968163. E-mail:
[email protected]. † Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate School of Chinese Academy of Sciences.
tability and adhesion properties of superhydrophobic surfaces has not been studied in detail. Inspired by the living organisms in nature, taking cues from nature and mimicking biological systems has become one of the most popular and interesting research topics in current nanotechnology.13-15 Understanding comprehensively the roles of surface energy and roughness for natural dewetting surfaces has led to the development of a number of biomimetic superhydrophobic surfaces. Many efforts have been made to obtain artificial surfaces with biomimetic micro/nanobinary textures by a variety of methods, such as the sol-gel method,16 photolithography,17 laser/plasma/chemical etching,18 microcontact printing,19 AFM local anodic oxidation,20 electrochemical deposition,21 and chemical vapor deposition.22 However, due to either the complicated preparation procedures or high cost, practical applications of most of these strategies are still very tough. It is highly desirable to achieve surfaces with complex biomimetic micro/nanotextures under even milder conditions, such as room temperature. Micro/ nanomolding is a low-cost, clean to the environment, simple, convenient, and high precision way to replicate surface structures and can provide a precision on the order of 10 nm.9,23,24 This flexible and highly reproducible technique has been used commonly to fabricate surfaces with micro/nanohierarchical structures. In this work, we described a novel and convenient fabrication method based on polydimethylsiloxane (PDMS) soft elastomer for creating Au surfaces with naturally occurring plant leaves’ surface morphologies, which result in hierarchies from the micrometer scale to nanometer scale. Lotus leaf and rice leaf as two templates were replicated successfully on PDMS initially.
10.1021/jp102052e 2010 American Chemical Society Published on Web 06/11/2010
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Figure 2. Low (a,b) and high (c,d) magnification SEM images of asprepared Au biomimetic surface with lotus leaf topography.
Figure 1. Schematic illustration for fabricating Au surfaces with both positive and negative biological plant surface topographies.
Then Au was sputtered on the PDMS surface, and Au surfaces with positive and negative biomimetic textures were fabricated and presented, followed by chemical modification with hydrophobic alkanethiol to enhance hydrophobic properties of biomimetic Au surfaces. Particularly, Au surfaces with opposite biomimetic structures showed different and interesting adhesion behaviors when a water droplet was placed on them. 2. Experimental Section 2.1. Materials. Templates used in this work were natural plants leaves (rice leaf and lotus leaf), the origins of which are listed as follows. Lotus leaves were picked from Chengdu City (Sichuan Province, China), and rice leaves were picked from Hanzhong City (Shanxi Province, China). PDMS prepolymer (Sylgard 184 silicone elastomer kit, Dow Corning, Midland, MI) was purchased from Kankun Company (Shanghai, China). 1-Decanethiol [TCI-EP, molecular formula, CH3(CH2)9SH] was purchased from Tokyo, 98%. Acetone and anhydrous ethanol were analytical reagents. All reagents were used as received. 2.2. Replication with PDMS. The outline of the fabrication procedures for both positive and negative biomimetic hierarchical Au surfaces comprising various micro- and nanostructures is schematically shown in Figure 1. In order to express clearly, gray and blue colors indicate the same substancesPDMS. Primarily, the PDMS precursors and cross-linking agent were mixed (10:1 weight ratio) and degassed in a desiccator at ambient room temperature for about 3 h to remove any air bubbles in the mixture. Then the PDMS mixture was poured over the biological plant templates. After curing at 70 °C for 10 h, the solidified PDMS mold was peeled off from the template, thus a negative replica of the biological template surface was obtained. Then the same process, but the negative replication was used as the template and a positive replica was produced easily; the complete surface texture of the original template was transferred to the PDMS. Smooth Au surface was obtained by replication of smooth silicon surface P(100) with a root mean square (rms) roughness of 0.2 nm using the same method. Then all of PDMS samples with a smooth surface and positive and negative biomimetic textures were sputter-coated with a thin layer of gold (100 nm) by a JFC-1600 Autofine coater (Japan) and also chemically modified with 1-decanethiol. All Au surfaces were placed in a 5 mmol/L anhydrous ethanol solution of 1-decanethiol for 24 h at room temperature. Then
samples were taken from the 1-decanethiol solution and put in an oven maintained at 70 °C for 3 h. Finally, the samples were washed with sufficient anhydrous ethanol to remove the physically adsorbed molecules and dried under a flow of N2. 2.3. Surface Characterization. The morphologies of these Au surfaces with positive and negative biomimetic textures were observed with a JSM-5600LV scanning electron microscope (SEM) at 20 kV and a MicroXAM 3D noncontact optical profilometer (ADE, USA). The static water contact angles (CAs) and sliding angles (SAs) of the as-prepared Au surfaces with biomimetic textures were measured according to the sessile droplet method using a drop shape analysis system (DSA100, Kruss Company Ltd., Germany) with a computer-controlled liquid dispensing system. Deionized water droplets with volumes of 5 and 10 µL were employed as the source for the CA and SA measurements. The CA was determined by fitting a Young-Laplace curve around the water drop. The SA was obtained by the sessile/captive drop method, which was recorded with a high-speed digital camera and analyzed by using commercial software. The experiment was performed at 20 °C and 30% relative humidity. The mean value was calculated from at least 10 individual measurements. 3. Results and Discussion 3.1. Au Surfaces with Positive and Negative Biomimetic Surface Morphologies. Figure 2a-d shows typical scanning electron microscope (SEM) top images of an as-prepared Au surface with positive lotus leaf topography with different magnifications. It is obvious that the micro/nanobinary structures were largely replicated with high resolution from the original lotus leaf. As shown in Figure 2, biomimetic textures on the Au surface were highly uniform and hierarchical in the sense that they had at least two sets of rough structures: many micropapillaes distributed randomly on the surface with diameters ranging from 5 to 9 µm (Figure 2a-c), also there were many nanoscale flower-like structures upon the top of these bumps (Figure 2d). Typical SEM top images with various magnifications of the Au surface with negative lotus leaf topography are depicted in Figure 3. Contrary to the Au surface with positive biomimetic lotus leaf structures, a roughly complementary structure composed of countless micro-orifices with diameters ranging from 6 to 10 µm, whose walls were besprinkled with many nanostructures, was found on the Au surface with negative biomimetic lotus leaf structures. Quantitative data of micropapillaes and micro-orifices on Au surfaces with both positive and negative lotus topography were
Low and High Adhesion Hydrophobic Au Surfaces
Figure 3. SEM images of the Au surface with negative biomimetic lotus topography at low (a,b) and high magnification (c,d).
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Figure 5. (a) SEM images of the Au surface with positive rice leaf morphology; (b-d) high magnification SEM images of (a).
Figure 6. (a) SEM images of the Au surface with negative biomimetic rice leaf morphology; (b-d) high magnification SEM images of (a). Figure 4. Three-dimensional images of the Au surfaces with positive (a) and negative (b) biomimetic lotus topography.
obtained using a 3D noncontact optical profilometer, with a scan size 160 × 160 µm2, and a smooth Au surface with silicon topography was also provided for comparison; all of the images are depicted in Figure 4. As shown in Figure 4a, micropapillaes with a diameter of 5-8 µm and a height of 5-10 µm could be clearly seen on the Au surface. Micro-orifices with a diameter of 5-8 µm and a depth of 5-10 µm were randomly distributed on the Au surface in Figure 4b. Distances between adjacent micropapillaes and micro-orifices were much larger than their diameters. In a word, the Au surfaces with positive and negative lotus leaf topography were successfully fabricated. It also can be seen from Figure 4c that the smooth Au surface was very even compared with Au surfaces with biomimetic textures; the roughness was almost negligible. Figure 5 is the SEM images of the Au surface with positive biomimetic rice leaf textures. It indicated that micro/nanotextures on the rice leaf surface were a one-dimensional ordered structure,21 and papillae and stomata with various sizes were formed on the surface. As seen in Figure 5, the papillaes with diameters of about 2-50 µm were arranged on the surface. The results showed that there were three kinds of micro/nanohierarchical textures, which are shown in Figure 5b-d. Figure 6 shows SEM images of the Au surface with negative biomimetic rice leaf textures. SEM images indicated that micro/ nanobinary structures on the Au surface with negative rice leaf topography were anisotropic, and micro-orifices and stomatas with various sizes were formed on the surface. As seen in Figure 6, the micro-orifices with diameters of about 2-50 µm were
TABLE 1: List of Contact Angle Values of Au Surfaces with Different Biomimetic Textures name smooth Au surface Au surface with positive lotus leaf topography Au surface with positive rice leaf topography Au surface with negative lotus leaf topography Au surface with negative rice leaf topography
before chemical modification (deg)
after chemical modification (deg)
101 120
106 137
132
135
119
136
130
135
arranged on the surface. These results also showed that there were three kinds of micro/nano-orifice textures, which are shown in Figure 6b-d. Three-dimensional images of Au surfaces with positive and negative biomimetic rice leaf topography were not obtained because as-prepared Au surfaces were not smooth enough. 3.2. CA and SA Measurements. Static CA is a primary parameter that provides a convenient means to assess the relative hydrophobicity of a solid surface. The results measured in this work are shown in Table 1. As seen from Table 1, obviously, compared with the smooth Au surface (replication of silicon surface, 106°), Au surfaces with positive biomimetic textures showed much larger CA values, especially after chemical modification with self-assembly of 1-decanethiol under the 10 µL water droplet (replication of lotus leaf, 137°, and rice leaf, 135°). Also, the same change trend happened on Au surfaces with negative biomimetic structures (both replication of lotus leaf, 136°, and rice leaf, 135°). Hydrophobicity of Au surfaces
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with lotus leaf structure was more affected by chemical modification, considering that surface topography is the main reason for this contrast. It is well-known that the CA of a solid surface depends on several factors, such as surface topography,25 surface roughness,2,24,26 and surface chemistry.2,27 Generally, larger roughness and lower surface energy lead to higher CAs and more hydrophobic ability. This observation can be expressed theoretically by eq 1, first proposed by Wenzel to describe the CA for a liquid droplet on a rough solid surface, which explains how surface roughness increases wettability.2,28,29
cos θr ) r cos θ
(1)
where θ is the equilibrium CA on a smooth surface, θr is the apparent CA on a rough surface made of the same material, and r is the roughness factor. It is responsible for the increase in the CA for a hydrophobic surface (θ > 90°) and the decrease in CA for a hydrophilic surface (θ < 90°). Also, according to the Cassie and Baxter model, with the increasing surface roughness combination of micro- and nanometer scale roughness (these micro- and nanometer scale structures were demonstrated by SEM), air would be trapped in the cavities of a rough surface, resulting in a composite solid-air-liquid interface, as opposed to the homogeneous solid-liquid interface.2,24,30 When surface energy was decreased by chemical modification, 1-decanethiol SAMs with terminal groups (-CH3), which are hydrophobic, would lead to a higher CA.22,31 For some practical applications, SA is a parameter even more useful and important than CA. SAs of the as-prepared Au surfaces with both positive and negative biomimetic structures were also elevated using the same instrument. Generally, a surface with a water CA higher than 150° and SA lower than 10° was considered as a superhydrophobic surface.32,33 The SA of the Au surface with positive lotus topography after chemical modification was 7°, which was less than 10°, and showed extremely low adhesion to water droplets. Au surface with positive rice leaf topography was anisotropic; the arrangement and orientation of the micro- and nanostructures may result in directional liquid-solid adhesion, so different SAs were found when the water droplet rolled off from a different direction. The transverse SA of Au surface with positive rice leaf topography after chemical modification was 24°, but the longitudinal SA had two different values of 14 and 24°. The results also confirmed that the topography of the rice leaf is anisotropic.21 3.3. Adhesion Performance. More interestingly, in contrast to the rolling behavior of a water droplet on the Au surface with positive biomimetic micro/nanostructures, the behavior of a water droplet on the Au surfaces with negative biomimetic micro/nanostructures is totally different: a strong adhesion between the surface and the water droplet was generated, and the water droplet does not slide on the surface even when the surface is tilted vertically (Figure 7a-d) or turned upside down (Figure 7e-h). Shapes of water droplets with different volumes adhered to the as-prepared Au surfaces with negative biomimetic structures under different tilt angles are shown in Figure 7. From Figure 7, it is revealed that the water droplet always maintains a spherical shape without any noticeable distortion as the surface with a tilted angle of 180°. With the surface tilted at an angle of 90°, the water droplet shape changes from spherical to elliptical. It is believed that the high adhesion was caused by the combination of capillary forces between the micro-orifices and the water and weak but universal van der Waal’s forces
Figure 7. Shapes of water droplets on the as-prepared Au surfaces with negative biomimetic structures under tilt angles of 90 and 180°: (a,b,e,f) Au surface with negative biomimetic lotus leaf topography; (c,d,g,h) Au surface with negative biomimetic rice leaf topography.
between the nanostructures with water.34,35 The addition of two kinds of forces can sustain the weight of the water droplet, keeping the water droplet suspended on the surface even if the surface is turned upside down completely. Chemical modification with 1-decanethiol SAMs, which has terminal groups (-CH3) and is hydrophobic, would lead to reduction of adhesion force between the as-prepared surfaces and the water droplet. The results obtained in this work show that the adhesion force between the as-prepared Au surface with negative lotus leaf topography and water droplet showed nearly no obvious difference before and after modification; both of them exhibited ultrahigh adhesive force with water; no matter the water droplet with a volume of 5 or 10 µL, the water droplet does not slide on the surface even when the surface is tilted vertically (90°) or turned upside down (180°). Similar to wings of the butterfly, which show directional adhesion, a droplet easily rolls off the surface of wings along one direction but is pinned tightly against rolling in the opposite direction. Au surface with negative biomimetic rice leaf topography showed different adhesion performance after chemical modification, especially in the longitudinal direction. When the volume of the water droplet was 5 µL, both the transverse and longitudinal directions showed stable adhesion, and even when the surface was tilted 90° or 180°, the water droplet did not slide on the surface. When the volume of water droplet increased to 10 µL, only the transverse direction showed high adhesion (SA in this direction could reach as high as 180°) and the longitudinal direction with two larger SAs of 84° and 65°. So the water droplet which rolled on the Au surface with negative leaf topography had special orientation. Detail measurements showed that anisotropic micropatterns with anisotropic SAs and stronger water adhesion and drag resistance orient more in the perpendicular direction than in the parallel. These results have demonstrated that a smart directional adhesive can be fabricated by a simple micro- or nanofabrication technique, and the adhesion strength can be tunable by changing the surface chemical composition. Besides isotropic adhesion on natural superhydrophobic surfaces, such as lotus leaves, anisotropic adhesion of rice leaves have offered a robust model to regulate fluid. It is generally recognized that anisotropic liquid-solid adhesion is ascribed to the arrangement and orientation of micro- and nanostructures. Inspired by this interesting phenomenon, people can design micro/nanoelectronic devices that have special applications. What is the main factor that causes the different adhesion performance between Au surfaces with positive and negative biomimetic topography? Experimental results reveal that special designs of micro/nanostructures bring special adhesion properties. On the basis of the results above, a supposed schematic
Low and High Adhesion Hydrophobic Au Surfaces
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Figure 8. Action model between Au surfaces with both positive and negative biomimetic structures and water droplet.
model is shown in Figure 8. Although the capillary force produced by each micro-orifice can be miniscule,34-36 the summation of numberless micro-orifices will create significant adhesion, which is sufficient to sustain the weight of the water droplet even when the surface is turned upside down. As shown in Figure 8, the main difference between Au surfaces with positive and negative biomimetic textures when they are turn upside down (with a tilted angle of 180°) is the radius and numbers of micro-orifices which influence capillary force significantly. According to the capillary mechanism, it is wellestablished that the greater the number and the smaller the radius of micro-orifices, the greater the capillary force. It is obvious that the number of micro-orifices on the Au surface with negative biomimetic textures is much greater than the positive one, and the radius of micro-orifices on the Au surface with negative biomimetic textures is much smaller than the positive one. It is also assumed that Au surfaces with positive and negative structures can induce two kinds of trapped air;5,11 there are air pockets in the open state (continuous with the atmosphere), as well as sealed air pockets trapped in the micro/ nanopores. When a water droplet contacts the solid surface, sealed air pockets could be formed on the Au surfaces with negative biomimetic structures, while only open air pockets could be formed on the Au surfaces with positive biomimetic structures. In this case, the negative pressure induced by the volume change of sealed air as the surfaces changes with different tilted angles can produce a normal adhesive force. As a result, the surface topography plays a dominant role in enhancing adhesion behavior on the Au surfaces with negative biomimetic structures, while the Au surfaces with positive biomimetic structures showed extremely low adhesion to water. Therefore, a combination of the above two factors is the primary factor responsible for the water droplet showing high adhesion to the Au surface with negative biomimetic textures but shows a very small adhesion to the Au surface with positive biomimetic textures. The discussion of the results presented in this paper will contribute to a better understanding and more accurate identification of the relevant factors responsible for hydrophobic interfaces with special adhesion, such as low adhesion, high adhesion, and directional adhesion, and will guide us to design and control interfacial adhesion, which is a topic of considerable technological importance.
In conclusion, artificial Au surfaces with a large area of both positive and negative biomimetic textures have been successfully fabricated via a simple yet robust method, using natural lotus leaf and rice leaf as replication templates. By measuring the CA and SA, it is clearly observed that hydrophobicity of the Au surface is influenced strongly by biomimetic textures and chemical modification. Both biomimetic micro/nanoscale binary structures and chemical modification by low-energy materials enhance surface hydrophobicity largely. Interestingly, Au surfaces with negative biomimetic structures show great adhesion to the water droplet, but the ones with positive biomimetic structures only have a little SA. Especially, Au surfaces with both of positive and negative rice leaf topography show anisotropic SA performances. After chemical modification with 1-decanethiol, adhesion between the water droplet and the Au surface with negative rice leaf topography shows obvious reduction, but adhesion between the water droplet and the Au surface with negative lotus leaf topography shows no obvious reduction. So reversible switching of solid-liquid adhesion on hydrophobic surfaces can be achieved with the precise coordination of surface chemistry response and surface roughness. These nearly superhydrophobic surfaces with controllable liquid-solid adhesion can be used for the construction of future generation smart devices. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (NSFC 20773148) and the 863 program of Chinese Ministry of Science and Technology (No. 2009AA03Z105). References and Notes (1) Cao, L. L.; Hu, H. H.; Gao, D. Langmuir 2007, 23, 4310–4314. (2) Bhushan, B.; Jung, Y. C. Nanotechnology 2006, 17, 2758–2772. (3) Gao, X. F.; Jiang, L. Nature 2004, 432, 36–36. (4) Kerte´sz, K.; Ba´lint, Z.; Ve´rtesy, Z.; Ma´rk, G. I.; Lousse, V.; Biro´, J.-P.; Vigneron, L. P. Curr. Appl. Phys. 2006, 6, 252–258. (5) Liu, M. J.; Zheng, Y. M.; Zhai, J.; Jiang, L. Acc. Chem. Res. 2010, 43, 368–377. (6) Murphy, M. P.; Kim, S.; Sitti, M. ACS Appl. Mater. Interfaces 2009, 1, 849–855. (7) Guo, Z. G.; Zhou, F.; Hao, J. C.; Liu, W. M. J. Am. Chem. Soc. 2005, 127, 15670–15671. (8) Lee, Y.; Park, S. H.; Kim, K. B.; Lee, J. K. AdV. Mater. 2007, 19, 2330–2335. (9) Bhushan, B.; Koch, K.; Jung, Y. C. Ultramicroscopy 2009, 109, 1029–1034. (10) Crosby, A. J.; Hageman, M.; Duncan, A. Langmuir 2005, 21, 11738–11743. (11) Gao, X. F.; Yao, X.; Jiang, L. Langmuir 2007, 23, 4886–4891. (12) Lai, Y. K.; Lin, C. J.; Huang, J. Y.; Zhuang, H. F.; Sun, L.; Nguyen, T. Langmuir 2008, 24, 3867–3873. (13) Min, W. L.; Jiang, B.; Jiang, P. AdV. Mater. 2008, 20, 3914–3918. (14) Nosonovsky, M.; Bhushan, B. Nano Lett. 2007, 7, 2633–2637. (15) Feng, X. J.; Jiang, L. AdV. Mater. 2006, 18, 3063–3078. (16) Wal, P. V.; Steiner, U. Soft Matter 2007, 3, 426–429. (17) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. AdV. Mater. 2004, 16, 1929–1932. (18) Etsion, I. Tribol. Lett. 2004, 17, 733–737. (19) Zheng, Z. J.; Azzaroni, O.; Zhou, F.; Huck, W. T. S. J. Am. Chem. Soc. 2006, 128, 7730–7731. (20) Mo, Y. F.; Wang, Y.; Bai, M. W. Physica E 2008, 41, 146–149. (21) Wang, Y.; Mo, Y. F.; Zhu, M.; Bai, M. W. Surf. Coat. Technol. 2008, 203, 137–141. (22) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratung, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701–1705. (23) Zhao, W. J.; Wang, L. P.; Xue, Q. J. ACS Appl. Mater. Interfaces 2010, 2, 788–794. (24) Liu, X. J.; Wu, W. C.; Wang, X. L.; Luo, Z. Z.; Liang, Y. M.; Zhou, F. Soft Matter 2009, 5, 3097–3105.
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