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Bio-Inspired Hierarchical Macromolecule–Nanoclay

Hydrogels for Robust Underwater Superoleophobicity

By Ling Lin , Mingjie Liu , Li Chen , Peipei Chen , Jie Ma , Dong Han , * and Lei Jiang *

The design of a functional water/solid interface is cen-tral to the development of innovative materials used underwater. [ 1–6 ] In particular, superoleophobic surfaces are attracting increasing attention for their great potential in bio-adhesion, [ 7,8 ] microfl uidic technology, [ 9,10 ] industrial metal cleaning, [ 11,12 ] marine antifouling coating, [ 13–16 ] etc. However, a robust superoleophobic surface is very diffi cult to construct artifi cially because of the complex underwater environment involving chemical and hydrodynamic factors. Here, inspired by the oil-repellent nature of a fi sh’s composite surface, we propose a bionic strategy to achieve robust underwater superoleophobicity by constructing hybrid macromolecule–nanoclay hydrogels with hierarchical surfaces. Also, hybrid hydrogels exhibit excellent mechanical strength in general. Further dynamic oil-adhesion measurements suggest that the strong interaction between rigid nanoclays and fl exible macromolecules at molecular level enhances the mechanical strength of surface structures at micrometer/nanometer scales, and thus supports the stability of trapped water on the hierarchical surface. This feature endows hybrid hydro-gels with a robust superoleophobic surface even if there are loading forces. Our strategy takes into account the synergetic effects of hierarchical structures and mechanical strength in the complex underwater environment, which could offer innovative insights into the design of novel interfacial mate-rials for practical applications.

The robustness of a superoleophobic/superhydrophobic system is an essential issue in applications, but it remains chal-lenging due to the instability of the Cassie state under external forces. [ 17–19 ] In the atmosphere, theoretical research has focused on optimizing the parameters of asperity to enhance the robust-nesss, and some artifi cial superhydrophobic surfaces have been made according to this guide. [ 20,21 ] However, laws of the same cases are more diffi cult to achieve in water, mainly because of

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DOI: 10.1002/adma.201002192

[*] Dr. L. Lin , Dr. L. Chen , Dr. J. Ma , Prof. L. Jiang Beijing National Laboratory for Molecular Sciences (BNLMS)Key Laboratory of Organic SolidsInstitute of ChemistryChinese Academy of SciencesBeijing 100190 (P. R. China) E-mail: jianglei@iccas.ac.cn Dr. M. Liu , Dr. P. Chen , Prof. D. Han National Center for Nanoscience and TechnologyBeijing 100190 (P. R. China)E-mail: dhan@nanoctr.cn Dr. L. Lin , Dr. M. Liu , Dr. L. Chen , Dr. P. Chen Graduate University of Chinese Academy of SciencesBeijing 100049 (P. R. China)

the more complex chemical and hydrodynamic factors. Inter-estingly, the surface of fi sh, an outstanding example in nature, can effectively resist oil pollution underwater. Abundant water trapped in its hydrophilic and micro-/nanostructured scales has proved to be the main mechanism, [ 13 ] but how to maintain the stability of trapped water on a fi sh’s surface in the complex water environment is unclear. We note that the surface of fi sh is composed of hydrophilic fl exible mucus and tough scales, then suggest that a complementary effect of chemistry and mechanics must be introduced in this case.

Drawing inspiration from this, we tried constructing fi sh-scale-like surfaces with micro-/nanostructures from a hydrophilic hydrogel, which has a biophysical similarity to mucus. However, traditional chemically crosslinked hydrogels (T-hydrogels) are limited by their fragility. Recently, novel hydro-gels with extraordinary mechanical strength, such as double-network hydrogels, [ 22 ] topological gels, [ 23 ] composite organic–inorganic hydrogels, [ 24–28 ] and macromolecular microsphere composite hydrogels, [ 29 ] have opened up an area of promising applications in artifi cial muscles, actuators, sensors and so on. [ 22–32 ] Especially, hydrophilic clay, an inorganic material with rigid nanolayered structure, is commonly chosen as a composite component to enhance mechanical strength. [ 24 , 27,28 , 33 ] In our experiment, hybrid poly( N -isopropylacrylamide)–nanoclay (i.e., PNIPAAm–nanoclay) hydrogels (C-hydrogels) were fabricated by a photo-initiated in situ radical polymerization of N -isopro-pylacrylamide, in which clay nanoparticles (synthetic hectorite, [Mg 5.34 Li 0.66 Si 8 O 20 (OH) 4 ]Na 0.66 , molecular weight (M.W.) = 762.24) serve as physical crosslinkers. The photo-initiated polymerization was introduced here for its convenience in molding complex topographies. Through the polymerization, hierarchical structures on the hydrogel surfaces were patterned by a design-guided molding process with micrometer/nanom-eter topographical templates ( Figure 1a ).

In detail, the hierarchical topography of fi sh scales (grass carp, Ctenopharyngodon idella ) was fi rstly replicated on the C-hydrogel (C-25, containing 25 × 10 − 2 mol L − 1 clay in the pre-cursor solution) surface by using dried fi sh scales as a tem-plate (see details in the Experimental section). [ 13 ] As shown in Figure 1 b, micro-/nanostructures were successfully molded on the surface of the fi sh-scale replica. Subsequent underwater oil-resistant testing shows that the surface exhibits superoleopho-bicity with an oil static contact angle (CA) of 156.3 ° ± 1.4 ° (1, 2-dichloroethane (DCE) as the detecting oil). [ 6 ] This indicates that the characteristic of micro-/nanostructures on fi sh scales is vital for oil repellency. According to the parameters from the fi sh-scale replica, a scale-like hierarchical-structured pattern, namely a micrometer array of trapezoid protuberances with nanometer roughness, was created on the C-25 hydrogel sur-face by molding a two-step-etched Si template (see details in the

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Figure 1 . Bio-inspired strategy of constructing hierarchical PNIPAAm–nanoclay hydrogels (C-hydrogels). a) Schematic illustration of the mold method of fabricating C-hydrogels with micro-/nanostructured surface through photo-initiated polymerization. b–d) Optical (left half of each panel; low magni-fi cation) and AFM (lower-right part of each panel; high magnifi cation) images of differently treated C-25 hydrogel surfaces (containing 25 × 10 − 2 mol L − 1 clay in the precursor solution). Top-right part of each panel: Photographs of oil droplets (1, 2-dichloroethane (DCE), 3 μ L) on the different surfaces in water. Substrate (S) = hydrogel, liquid (L) = DCE, medium = water (W). b) Micro-/nanostructured surface of a fi sh-scale replica exhibits superoleopho-bicity. c) Bio-inspired hierarchical surface with micro-protuberances and nanometer-scale roughness (635.3 nm RMS roughness) shows superoleopho-bicity. d) Nonpatterned smooth surface (338.9 nm RMS roughness) shows oleophobicity.

Experimental section). As shown in Figure 1 c, the micrometer protuberances are uniform with a top diameter of 71 ± 4 μ m and a bottom diameter of 161 ± 2 μ m. The average distance between two protuberances is 83 ± 3 μ m. Atomic force micro-scopy (AFM) imaging at high magnifi cation shows nanometer-scale roughness with an average root-mean-squared (RMS) roughness of 635.3 nm. The oil CA reveals that the hierarchical surface exhibits superoleophobicity with a CA of 159.1 ° ± 1.6 ° . As a control, nonpatterned C-25 hydrogel was introduced, exhibiting a relatively smooth surface with 338.9 nm RMS roughness. The smooth surface shows oleophobicity with an oil CA of 147.8 ° ± 1.3 ° (Figure 1 d). These results demonstrate that our molding method has successfully introduced micrometer-array topography and more nanometer-scale roughness to the hydrogel surface. It also proves that the strategy of constructing hierarchical structures indeed leads to an evident enhancement of oleophobicity.

The underwater oil-resistance of hydrogels with different clay content was also systematically studied (see Supporting Infor-mation, Figure S1). The oil CAs of smooth surfaces range from 145.4 ° ± 1.7 ° to 147.8 ° ± 1.3 ° , whereas for hierarchical surfaces, the oil CAs range from 154.2 ° ± 1.4 ° to 159.1 ° ± 1.6 ° . Besides, similar underwater wetting properties were observed when an alkane ( n -decane) was used as the detecting oil. The smooth surface of C-25 hydrogel shows oleophobicity with an oil CA of 144.5 ° ± 1.4 ° , whereas for the hierarchical surface the oil CA is 156.2 ± 1.6 ° , exhibiting superoleophobicity (see Supporting

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Information, Figure S2). The results confi rm that hydrogels with hierarchical surfaces show superoleophobicity to different oils, which shows the potential for different applications.

Mechanical measurements were then carried out since excel-lent mechanical properties are prerequisites for practical appli-cations. Figure 2a shows photographs of as-prepared PNIPAAm hydrogels during compression testing. The T-hydrogel (chemi-cally crosslinked by 2 mol% N , N ′ -methylenebis(acrylamide) (BIS), no clay content) easily fractures under a low deforma-tion. In contrast, C-hydrogels can endure high stress and quickly recover their original shape after the loading force is released. Detailed analyses given by the corresponding stress–strain curves are presented in Figure 2 b. The brittle T-hydrogel (crosslinked by 2 mol% BIS) breaks at about 47% under a 33 kPa stress (Figure 2 b, inset). Although strain at fracture increases with decreasing crosslinking density, the stress and modulus of the T-hydrogel are still low (see Supporting Infor-mation, Figure S3). In contrast, C-hydrogels show high com-pressive strength with increase of clay content ( C clay [mol L − 1 ]). In particular, C-25 reaches almost 2.5 MPa at 75% strain (C-5 n : hydrogel containing 5 n × 10 − 2 mol L − 1 clay in the precursor solution). The compressive modulus, calculated from the slope of the initial linear area (10–20%), also increases monotonically with increasing C clay (Figure 2 c). Further tensile measurements of the C-hydrogels show their striking tensile properties, while the T-hydrogel is too fragile to be applied in our experiments (see Supporting Information, Figure S4). It is remarkable that

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Figure 2 . Compressive properties of as-prepared traditional PNIPAAm hydrogel (T-hydrogel) and hybrid PNIPAAm–nanoclay hydrogel (C-hydrogel). a) Photographs of T-hydrogel (left) and C-hydrogel (right) during a compression test. T-hydrogel (crosslinked by 2 mol% BIS) fractures under a low deformation, while C-hydrogel recovers its original shape after the high load has been released. b) Stress–stain curves of C-hydrogels and T-hydrogel (inset). With the increase of clay content ( C clay ), compressive strength is greatly enhanced (C-5 n : hydrogel containing 5 n × 10 − 2 mol L − 1 clay in the precursor solution). c) The compressive modulus, calculated from the slope of initial linear area (10–20%), increases monotonically with increasing C clay .

the tensile strength rises with increasing C clay , without sacri-fi cing most of the extensibility. All mechanical testing results indicate that C-hydrogels possess excellent mechanical proper-ties for applications.

To better understand the oil-resistant ability of C-hydrogels in a complex underwater environment, the underwater oil-adhesion force was dynamically measured using our estab-lished technique. [ 6 ] It is common knowledge that water pres-sure, including the fl ow pressure and hydrostatic pressure, can provide more opportunity for greasiness touching the sur-face than atmosphere pressure. Therefore, in the underwater measurement process, an oil droplet (3 μ L, DCE) was squeezed against the surface with a constant preload and then allowed to relax. The adhesion force between oil and surface was recorded by a high sensitivity micro-electromechanical balance system ( Figure 3a ). Firstly, when an oil droplet slightly contacted the surface with no preload, the adhesion forces were in the range from 4.9 ± 0.9 μ N to 0.9 ± 0.5 μ N on hierarchical surfaces and from 15.8 ± 1.7 μ N to 7.5 ± 1.8 μ N on smooth surfaces, fol-lowing the increase of clay content (Figure 3 b). It proves that micro-/nanostructures also contribute to the decrease of oil-adhesion in a dynamic adhesion process. Secondly, when different preloads were used on hierarchical surfaces, more striking results were found (Figure 3 c). As the preload increased, the oil-adhesion with low-clay-content hydrogels increased

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remarkably. For example, the adhesion forces on the T-hydrogel surface increased from 4.9 ± 0.9 μ N to 13.48 ± 1.1 μ N, when the preloads changed from 0 to ca. 20 μ N. In contrast. a C-hydrogel with high clay content (C-25) retained excellent ultralow affi nity to an oil droplet, the adhesion forces ranging from 0.9 ± 0.5 μ N to 1.3 ± 0.6 μ N. Similar adhe-sion properties were observed when n -decane was used as the detecting oil (see Supporting Information, Figure S5). Note that the hybrid of macromolecules and nanoclays enhances the robustness of superoleophobicity.

A synergetic model is proposed to explain how to achieve robust low-adhesion supe-roleophobicity on the hierarchical surface of hybrid macromolecule–nanoclay hydrogels ( Scheme 1 ). It is generally admitted that PNIPAAm hydrogel (T-hydrogel) possesses hydrophilic macromolecule chains that can trap water molecules on the hydrogel surface. [ 6 , 8 , 34–36 ] The trapped water has been proved to be the main reason for the resist-ance to oil-adhesion in water; modeled as Scheme 1 a. [ 6 , 8 ] The hierarchical micro-/nanostructures can further decrease the oil-adhesion and achieve superoleophobicity. This effect is caused by the high content of water trapped in micro-/nanostructures reducing the contact area between the oil and the surface (Scheme 1 b). [ 13 ] In fact, this superoleophobicity is unstable. For example, the surface shows high oil-adhesion when

there is a loading force. The phenomenon is attributed to the instability of trapped water on the surface, which is likely to be related to the poor mechanical strength of the T-hydrogel surface. This deduction is supported by the small value of the atomic force microscopy (AFM) force curve slope on the T-hydrogel surface (see Supporting Information, Figure S6). [ 37 ] Therefore, when a force is applied, the T-hydrogel surface with low mechanical strength hardly retains the trapped water. Then the oil–hydrogel contact area increases greatly, leading to high oil-adhesion (Scheme 1 c).

Compared with T-hydrogels, high-clay-content C-hydrogels display excellent mechanical strength, which results from the interaction between clay nanoparticles and PNIPAAm chains at molecular level. [ 24 , 28 ] On the C-hydrogel surface, as shown in the energy-dispersive spectrometry (EDS) and transmission electron microscopy (TEM) images, a larger proportion of exfo-liated nanoclays is found with an increase of clay content. These rigid nanoclays are dispersed homogeneously throughout the polymer matrix, which would improve the surface mechanical strength (see Supporting Information, Figures S7 and S8). This deduction is proved by the AFM force curve with steep slope on the C-hydrogel surface (see Supporting Information, Figure S6). [ 37 ] Therefore, in the dynamic oil-adhesion process, when there is no preload, the smooth C-hydrogel surface shows oleophobicity by water molecules trapped in fl exible macro-molecule chains (Scheme 1 d). When hierarchical structures

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Figure 3 . Dynamic underwater oil-adhesion measurements on hydrogel surfaces with different preloads (3 μ L DCE droplet as the detecting oil). a) Photographs of the measurement process. An oil droplet is squeezed against the surface with a constant preload and then allowed to relax. The adhesion force is recorded by a high-sensitivity balance system. b) Sta-tistical histogram of the oil-adhesion forces on smooth and hierarchical surfaces for different clay contents. A large reduction of oil adhesion is observed on hierarchical surfaces when there is no preload. c) Statis-tical histogram of the oil-adhesion forces on hierarchical surfaces with increasing C clay , with different preloads. Remarkably, high-clay-content hydrogels exhibit excellent ultralow affi nity to the oil droplet, achieving robust superoleophobicity.

Scheme 1 . Schematic illustration of underwater oil-adhesion mecha-nism on different surfaces of T-hydrogel (a,b,c) and C-hydrogel (d,e,f). a,d) Smooth surfaces of T-hydrogel (a) and C-hydrogel (d). Hydrophilic PNIPAAm chains (represented by black lines) can trap water molecules to resist oil-adhesion. b,e) Micro-/nanostructured surface of T-hydrogel (b) and C-hydrogel (e). Hierarchical structures enhance the water trap-ping effect and reduce the oil–hydrogel contact area, leading to low oil-adhesion. c) Micro-/nanostructured surface of T-hydrogel under preload. The trapped water is hardly retained by the surface with low mechanical strength, leading to an increase of oil-adhesion. f) Micro-/nanostructured surface of C-hydrogel under preload. The strong interaction between the rigid nanoclays (represented by red ellipses) and fl exible chains enhances the mechanical strength of hierarchical structures and supports the sta-bility of trapped water on the surface, achieving robust oil-repellency.

are constructed, trapped water increases by the structure effect and the surface exhibits superoleophobicity (Scheme 1 e). More importantly, when there is a loading force, the strong interac-tion between the rigid nanoclays and fl exible chains supports the stability of trapped water, owing to the enhanced mechan-ical strength of micro-/nanostructures on the C-hydrogel sur-face. As a result, the oil–hydrogel contact area remains small, and robust oil-repellency is achieved by this synergetic effect of hierarchical structures and mechanical strength (Scheme 1 f).

In conclusion, inspired by the functional surface of fi sh, we have developed a strategy to achieve robust underwater supero-leophobicity by constructing a synergetic surface of hierarchical macromolecule–nanoclay hydrogels. The interaction of rigid nanoclays and fl exible macromolecules supports the stability of trapped water on the micro-/nanostructured surface and

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contributes to robust low-adhesion superoleophobicity. It is also noticeable that the biocompatible hybrid hydrogels exhibit high compressive and tensile strength, which is crucial for applica-tions. The strategy consists of combining effects from the mac-romolecule-nanolayer and micrometer/nanometer topographic surface, and fi nally from the macroscale. Moreover, the dynamic and mechanical effects regarding the underwater environment are emphasized. These fi ndings bring a new concept to the fab-rication of functional materials in broad fi elds underwater, such as microfl uidics, biotechnologies, and antifouling coatings.

Experimental Section Fabrication of Hierarchical Hybrid PNIPAAm–Nanoclay

Hydrogels : NIPAAm monomer (Aldrich) and inorganic clay (Laponite XLG, [Mg 5.34 Li 0.66 Si 8 O 20 (OH) 4 ]Na 0.66 , M.W. = 762.24, layer size = 20–30 nm in diameter and 1 nm in thickness, Rockwood Ltd., UK) were used after purifi cation. NIPAAm (1 mol L − 1 in H 2 O), clay (5 n × 10 − 2 mol L − 1 in H 2 O, n = 1–5, C clay [mol L − 1 ] was calculated based on the molecular weight) and initiator (2,2 ′ -diethoxyacetophenone, DEOP, 2 mol% in H 2 O) were mixed by two-step stirring to form a transparent solution. [ 24 ] T-hydrogels were prepared using NIPAAm (1 mol L − 1 in H 2 O), crosslinker BIS (0.5 mol%, 1 mol%, and 2 mol% in H 2 O) and initiator (DEOP, 2 mol% in H 2 O).

Fish-Scale Replica Made of C-Hydrogel : The original template was dried fi sh scales after ethanol gradient elution. Then polydimethylsiloxane (PDMS) precursor liquid (Sylgard 184, Dow Corning, Midland, MI) mixed with the curing agent (10:1 by weight) was poured onto the fi sh scales. After this had been cured at 70 ° C for about 2 h. a PDMS replica was obtained. [ 13 ] Then the mixture of C-hydrogel precursor was poured onto the PDMS replica. Through a photo-initiated in situ radical polymerization ( λ = 365 nm, 40 min), a fi sh-scale replica made of C-hydrogel was obtained.

Scale-like Hierarchically Patterned C-Hydrogel : The Si template with micrometer/nanometer topography was generated by a two-step etching process. The traditional photolithography and subsequent wet chemical etching resulted in micro-protuberances. The second-step etching

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using HNA etchant [HF(40 wt%)/HNO 3 (80 wt%) = 12:1(v:v)] for 20 s

induced nanometer-scale roughness on the Si surface. Then the mixture of C-hydrogel precursor was poured onto the micrometer/nanometer topographic Si template. Through a photo-initiated in situ radical polymerization ( λ = 365 nm, 40 min), hierarchically structured surfaces on C-hydrogels were obtained.

Instruments and Characterization : Atomic force microscopy: AFM images were acquired using an Agilent 5500 AFM (Agilent Corp.) in MAC mode in water. The force curves were acquired in Contact mode in water.

TEM images were obtained using a JEOL JEM-2010 instrument at 200 kV for dried C-hydrogels. Ultrathin fi lms of C-hydrogels were prepared for TEM observation by cutting the dried gels embedded in epoxy resin using an ultra-microtome (Leica EM UC7).

Element analysis of the C-hydrogel surfaces was obtained by EDS using a fi eld-emission scanning electron microscopy (HITACHI-S4800, Japan) and its EDS component.

Compression measurements, using a Model 3365 Table Mounted Materials Testing System (Instron Co. USA), were obtained under the following conditions: size, 15 mm × 15 mm × 10 mm; compression speed, 10 mm min − 1 ; compression distance, 7.5 mm; T = 25 ° C. Tensile measurements were performed under the following conditions: size, 2 mm × 10 mm × 50 mm; gauge length, 20 mm; speed, 50 mm min − 1 ; T , 25 ° C.

Oil static contact angles were measured on an OCA20 machine (DataPhysics, Germany) underwater at ambient temperature. As detecting oils, either 1,2-dichloroethane (DCE, 3 μ L) or n -decane (3 μ L) was used.

The oil-adhesion forces were measured using a high-sensitivity micro-electromechanical balance system (DataPhysics DCAT 11, Germany) underwater. An oil drop (3 μ L) was suspended with a metal ring and controlled to squeeze the hydrogel surface at a constant speed of 0.005 mm s − 1 and then relax. The forces were recorded during the whole time. As detecting oils, 1, 2-dichloroethane (DCE) and n- decane were used.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors are grateful for fi nancial support from the National Research Fund for Fundamental Key Projects (2010CB934700, 2009CB930404, 2007CB936403) and the National Natural Science Foundation (20974113, 20601005). The Chinese Academy of Sciences is gratefully acknowledged. The authors also thank Dr. Yong Zhao, Hongyan Chen, Changqing Ye, Hao Bai, and Ye Tian for technical support and helpful discussion.

Received: June 15, 2010 Published online: August 31, 2010

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