Inspired smart materials with external stimuli responsive wettability: a review

Abstract

Surfaces equipped with controllable wetting behaviours have received extraordinary attention recently due to their great importance in both fundamental research and practical applications. Through introducing stimuli-sensitive materials whose chemical compositions and/or topological structures can be controlled by external stimuli, different types of smart responsive surfaces that switch reversibly between superhydrophobicity and superhydrophilicity can be effectively fabricated, even though these are two fundamentally opposite wetting states. In this paper, we summarized the most frequently employed methods for the fabrication of surfaces with switchable wettability, focussing on smart materials and recent developments in this field. According to their responsiveness to different external stimuli, smart materials are divided into several groups, including photo-responsive materials, thermally responsive materials, pH-responsive materials, and electricity-responsive materials. Additionally, potential applications of smart materials, such as oil–water separation, biosensors, drug delivery and smart windows are also mentioned. Finally, current challenges for both intelligent surfaces and smart responsive materials and the future prospects for this research field are also mentioned. The purpose of this review is to give a brief and crucial overview of smart surfaces with wettability that is responsive to external stimuli.

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Fei Guo

Mr Fei Guo joined Prof. Guo's Biomimetic Materials of Tribology (BMT) group at the University of Hubei in 2015 for pursuing his PhD degree. His current scientific interests are the design and fabrication of bio-inspired superhydrophobic surfaces with responsive properties and the study of their corresponding applications.

Zhiguang Guo

Professor Zhiguang Guo received his PhD from Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS) in 2007. He then joined Hubei University. From Oct 2007 to Aug 2008, he worked in the University of Namur (FUNDP), Belgium, as a post-doctor. From Sep 2008 to Mar 2011, he worked in the Funds of National Research Science (FNRS), Belgium, as a “Charge de Researcher”. From Feb 2009 to Feb 2010, he worked in the Department of Physics, University of Oxford, UK, as a visiting scholar. He is now a full professor at LICP, financed by the “Top Hundred Talents” program of the CAS. To date, he has published more than 120 papers about the interfaces of materials.

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1. Introduction

Wettability is one of the most important and fundamental properties of a solid surface and is normally governed by the surface chemical composition and surface morphology.^1–3 In general, according to the measured water contact angle (WCA) on a surface, the wetting behaviour of the surface can be divided into 4 different categories. Most common surfaces are hydrophilic or hydrophobic with a WCA in the ranges 10° < θ < 90° or 90° < θ < 150°, respectively. Superhydrophilic surfaces exhibit extreme wetting behaviour with a WCA in the range 0° < θ < 10°, while other surfaces are superhydrophobic with both a WCA in the range 150° < θ < 180° and a sliding angle of less than 10°, since the latter is more important with respect to the lotus effect.⁴ Drawing inspiration from the superhydrophobic phenomenon in nature, which is shown in the lotus leaf, water strider and Namib Desert beetle, numerous artificial superhydrophobic surfaces have been fabricated through creating micro- and nano-hierarchical structures on a hydrophobic substrate or taking advantage of a low-surface-free-energy material to chemically modify its topological structure.^5–7 Accordingly, owing to their applications in many fields,^8–13 the fabrication processes and wetting behaviours of superhydrophobic surfaces have drawn much attention from researchers in the past few decades.

However, as research continues, the wetting behaviour of artificial superhydrophobic surfaces that are relatively unresponsive to external stimuli cannot meet the demands of increasingly complex challenges. Fortunately, over millions of years of evolution, nature has equipped creatures with endless sophisticated surface functions and structures to adapt to their living environments, which provides us with new platforms to overcome functional and performance-related challenges. For instance, natural surfaces and interfaces exhibit stimuli-responsive behaviours, such as the colour changes of echinoderms in response to light,¹⁴ the camouflage behaviour of chameleons in different environments,¹⁵ and heat-shock-responsive behaviour of bacteria,¹⁶ and these have provided researchers with great inspiration for designing numerous intelligent materials. Based on the unique properties of biological systems, many principles concerning the design of these smart materials have been presented and numerous functional surfaces have been fabricated.¹⁷ Among various kinds of intelligent surfaces, functional smart responsive surfaces with switchable wettability have drawn more and more interest due to their outstanding properties in numerous applications, such as controlled drug delivery,¹⁸ cell encapsulation,¹⁹ oil/water separation,²⁰ microfluidic channels,²¹ and sensors.²² Meanwhile, accompanied by continuous developments in science technology and correlated subjects, such as nanotechnology, polymer science and surface chemistry, more and more smart surfaces with switchable wettability for water and/or oil have been developed. Generally, with regard to these smart surfaces, most studies focussed on the static wetting characteristics. As far as the change mechanisms are concerned, the wettability switch of the surface can be achieved by reversible changes in the surface chemical composition and/or the surface morphology of stimuli-sensitive materials in response to corresponding external stimuli such as light irradiation,^23,24 temperature,²⁵ pH,²⁶ solvent,²⁷ counter ions,²⁸ and electrical fields.²⁹ It makes much sense to achieve the perfect control of surface wettability with reversible switches for smart surfaces, particularly those based on stimuli-responsive organic material or functional polymeric material systems, due to both their well-designed structures and reversible property changes, triggered by surrounding stimulation. Thereby, to assist research in this field, it is necessary to conduct a review to show the advances in switchable surfaces inspired by nature, which will enable scientists to rapidly gain an understanding of developments in this field.

To this end, there are 4 parts in this review. First, we will briefly summarize the theory of surface wetting properties, which consists of various theoretical wetting models corresponding to smooth or rough surfaces. In Section 3, some of the most common fabrications of smart surfaces with switchable wettability will be introduced. We will then focus on smart responsive surfaces with switchable wettability triggered by various external stimuli, which are the highlight of this review. Particular emphasis will be placed on surfaces with reversible wettability transformations between two extreme wetting states, based on the amplifying effect of surface roughness and reversible changes in surface chemical composition and/or conformation. Finally, we will provide a glimpse of the applications of smart responsive surfaces with reversible wettability and briefly present our personal view of the remaining challenges and outlook on the future of the field of smart responsive surfaces.

2. Fundamental theories

2.1 Smooth surface

The concept of surface/interface energy or surface tension (γ), which is identical to the tension force per unit length on the surface, was proposed by T. Young in 1804. It is well-known that liquid drops tend to form an angle with a solid surface, without fully spreading out, when they are placed in contact with the solid surface. An apparent contact angle can be measured between the horizontal direction and the tangential line of the liquid surface near the liquid–vapour–solid three-phase contact line. As shown in Fig. 1a, the relationship between the contact angle, θ, made by a drop with a surface and the three surface tensions is given by Young's equation³⁰ as:where γ_SV, γ_SL and γ_LV represent the surface tension at the solid–vapour, solid–liquid, and liquid–vapour interfaces, respectively.

Fig. 1 The wetting state: (a) liquid droplet on a smooth solid; (b) Wenzel state; (c) Cassie–Baxter state; (d) transition state between the Wenzel and the Cassie modes.

A surface with a contact angle less than 10° is conventionally superhydrophilic, while a hydrophilic surface has a CA between 10° and 90°. A hydrophobic surface has a CA between 90° and 150°. Superhydrophobic surfaces with a water contact angle larger than 150° exhibit excellent water-repellent properties. However, this model is only appropriate for smooth surfaces. For rough surfaces, it needs to be reconsidered.

2.2 Rough surface

The Wenzel model³¹ and the Cassie–Baxter model³² are two well-known theories used to explain the effect of roughness on the apparent contact angle of a liquid droplet on a rough solid surface. In the Wenzel state (Fig. 1b), the liquid droplet is in contact with the entire solid surface and completely fills all voids in the rough surface under the liquid droplet. The Young equation was modified by Wenzel as follows:

cos θ_r = r cos θ (2)

where θ_r represents the apparent contact angle and r, the ratio between the actual area of the rough surface and the projected surface area, which is defined as the roughness factor.^33,34 When θ is less than 90°, θ_r will be reduced by an increase in r, but if θ > 90°, θ_r will be increased by an increase in roughness.

However, the Wenzel model is applicable only to a chemically homogeneous wetting regime. In the Cassie–Baxter case, the effect of chemical heterogeneities of the surface on the equilibrium contact angle have been considered. In this wetting state, cavities on the rough surface cannot be penetrated by the liquid drop and these cavities are filled by air. As shown in Fig. 1c, heterogeneous surfaces can be composed of fraction corresponding to the solid–liquid interface and fraction to the liquid–air interface. Therefore, the corresponding equation is given as follows:

cos θ_CB = f_SL(1 + cos θ_r) − 1 (3)

where θ_CB is the apparent contact angle and f_SL is the area fraction of the solid–liquid interface.

As well as the wetting states mentioned above, there is also a transition state between the Wenzel and the Cassie–Baxter cases,³⁵ as shown in Fig. 1d.

It is possible that the Cassie–Baxter state can be transformed into the Wenzel state under some conditions. It is also well-known that surface roughness and chemical composition both play pivotal roles in the wettability of a solid surface.

3. The fabrication of smart surfaces with responsive wettability

Multitudinous fabrication methods have been developed to prepare smart responsive surfaces, and each method provides various degrees of control over the surface roughness/chemical composition and wettability of smart surfaces. Generally, organic materials, especially functional responsive polymers, are good candidates for obtaining smart surfaces due to the potential advantage of their ability to control surface energy and topography.³⁶ The main focus of numerous smart surfaces with responsive wettability preparations based on the these smart materials is to immobilize various stimuli-responsive moieties onto the surfaces. Here, we present only the main approaches recently employed to prepare smart responsive surfaces based on polymers, such as electro-spinning, SI-ATRP, self-assembly and layer-by-layer.

3.1 Electro-spinning method

Electro-spinning is an emerging fabrication technique capable of preparing continuous thin solid polymer micro/nano fibers from several nanometers to several tens of micrometers in diameter, which has been developed in recent years.^37,38 Electrospun nanofibers have become one of the best choices for developing smart surface materials owing to their excellent characteristics, such as high porosity and a large surface-to-volume ratio of electrospun nanofibrous membranes. A great advantage of the electrospun nanofibers is that they can greatly increase the roughness of surfaces when they are assembled in random networks. Therefore, electro-spinning is one of the most powerful and cost-effective ways to prepare smart responsive surfaces. For example, Tanioka and co-workers prepared PNIPAAm fibers with diameters ranging from 0.13 to 3.2 μm by electro-spinning from 5 to 10 wt% PNIPAAm/1,1,1,3,3,3-hexafluoro-2-propanol solutions, and then PNIPAAm fibers were used to structure nanofiber coatings with a large amplitude of thermoresponsive change in the wettability.³⁹ Through using an appropriate polymer concentration (25 wt%) to avoid the breakup of the stable jet, Yuan et al. fabricated very fine bead-free nanofibers with an average diameter of about 700 nm to obtain a CO₂-responsive surface by electro-spinning.⁴⁰ It is easily deduced from these two examples that optimum conditions are required to obtain the desired properties in the surfaces. Electro-spinning is a useful way to structure smart surfaces. However, the structure and properties of electrospun nanofibers can be influenced by many factors, including molecular weight, solvent, solubility of polymers, the solution properties (e.g., viscosity, conductivity, and surface tension), and external environmental conditions (such as humidity, temperature, and air velocity).^41–45 Many factors need to be taken into account to structure smart surfaces with excellent properties using this method. In addition, another problem may occur in that sometimes, nanofibers tend to lock the functionality required in contemporary applications, even though the morphology of electrospun nanofibers is desirable. In order to solve this issue, surface-initiated atom-transfer radical polymerization (SI-ATRP) may prove useful.

3.2 Surface-initiated atom-transfer radical polymerization

Compared with the electro-spinning method, atom-transfer radical polymerization (ATRP) is a novel technique to control the molecular weight and molecular weight distribution of the target grafted polymers. ATRP has been widely used to synthesize different types of topologically structured copolymers, such as block polymers, graft polymers, star polymers and hyperbranched polymers.⁴⁶ It is necessary to obtain smart surfaces by combining functional polymers with substrate surfaces. SI-ATRP provides an appropriate route to surface modification through uniformly grafting functional polymer chains with relatively low polydispersity on a surface.^47,48 Responsive polymer brushes that are fabricated by SI-ATRP have drawn immense attention because of their great potential with respect to responsive surfaces.^49–53 For instance, a thermally responsive poly(N-isopropylacrylamide) (PNIPAM) surface with a nanoscale topography was constructed by Jiang and co-workers through grafting a polymer from silicon nanowire arrays via SI-ATRP.¹³ Adopting the method of SI-ATRP, Huck et al. fabricated well-defined polymer films that change their wettability in response to external stimuli.⁵⁴ However, for this approach, immobilizing an initiator on the rough surface plays an important part in the experimental process. Moreover, this approach is often not readily transferable to the modification of large-surface-area planar or microporous substrates because SI-ATRP usually involves complicated synthetic procedures. So, it is necessary to develop some convenient approaches to smart surface fabrication.

3.3 Layer-by-layer technique and self-assembly methods

Layer-by-layer (LbL) assembly (also called multilayer assembly) is an alternative approach to modifying surface properties. It is considered a versatile approach for controlling the surface roughness or porosity of complex substrates via the LbL assembly of polyelectrolytes or nanoparticles.^55–59 Herein, it provides a simple, versatile and robust tool to construct smart responsive surfaces. For example, based on hydrogen-bonded LbL, films with controllable wettability via pH-induced collapse/solubilization of polymer units within single-component surface-bound hydrogels were reported by Choi and co-workers.⁶⁰ Zhang et al. fabricated a film with tunable wettability by depositing a (PDDA/PSS)₃PDDA multilayer on a commercially available cotton fabric surface by LbL technology, taking advantage of counterion exchange between Cl⁻ and PFO anions during this procedure.⁶¹ This technique enables the inclusion of various polymers and different sized nanoparticles within the resultant functional multilayer thin film. The main advantages of the LbL technique are that it provides good control of a film's structure and chemical composition, and it can be used to assemble LbL films on substrates of any shape in confined environments. However, the intermolecular binding between polymers that assures the stability of LbL films becomes a major obstacle with respect to constructing coatings with the ability to switch their surface wetting behaviour.

Compared to LbL, self-assembly is similarly another method in which components spontaneously form ordered aggregates by noncovalent interactions, such as electrostatic interactions, hydrogen bonds, van der Waals' forces, coordination interactions and solvophobic effects.^62,63 It is considered a general approach for fabricating smart surfaces because the intermolecular forces connect the molecular building blocks in a reversible, controllable way in the self-assembled structures. For example, through fabricating self-assembled monolayers (SAMs) on a gold surface with two dithiooctanoic acid derivatives bearing N,N-disubstituted amide groups, Wang and co-workers reported a film with reversible wettability changes upon alternating treatment with ethanol and cyclohexane.⁶⁴ Huang et al. synthesized smart-responsive cellulose materials with wettability switched reversibly by light stimuli through self-assembling a 7-[(trifluoromethoxyphenylazo)-phenoxy]pentanoic acid (CF₃AZO) monolayer onto TiO₂ ultrathin film, which was pre-coated with nanofibre surfaces of laboratory filter paper.⁶⁵ Compared with the methods mentioned above, this approach is more advantageous in its simplicity and convenience. However, with regard to the smart surfaces fabricated in this way, their mechanical properties limit their applications.

3.4 Other methods

Other fabrication methods have also been reported for the preparation of smart responsive surfaces, such as a replica molding technique,⁶⁶ spin-coating,⁶⁷ electrochemical deposition,⁶⁸ spray deposition,⁶⁹ electrodeposition,⁷⁰ and Langmuir–Blodgett.⁷¹ Combining different methods is also a hot topic with respect to preparing smart responsive surfaces. For example, using regular replica molding and temperature-induced phase-separation micromolding, biostructure-like surfaces with thermally responsive wettability were fabricated by Xu and co-workers.²⁵ Through adopting the LbL method and SI-ATRP, Kim et al. reported a photo-responsive surface with fast and reversibly switchable wettability between hydrophobicity and hydrophilicity, induced by a photothermal effect.⁷² An approach that combined etching and electrodeposition is also reported.⁷³ Although these methods can be applied for the preparation of smart-responsive surfaces, few can be extensively used in industrial production due to their high cost and strict production conditions. In addition, surfaces fabricated by such methods often have poor stability.

4. Smart surfaces with external stimuli responsive wettability

4.1 Single-response smart responsive surfaces

4.1.1 Light response. Light is one of the most significant external stimuli and is easily accessible for triggering stimuli-responsive materials.⁷⁴ It is frequently chosen as a control factor due to its facile operation, low cost, and limited environmental impact.^74,75 The reversible change of surface active molecular properties, such as chemical composition, chemical configuration, and polarity, can be achieved upon exposure to light. This change can cause transformation in surface free energy and lead to reversible changes in wettability. Hence, it is extremely possible to control the surface wettability intelligently through light illumination. In recent years, many surfaces with special wettability have been fabricated through using intelligent light-responsive materials, which have great potential application in many fields.

Several types of photo-sensitive materials have been reported to fabricate light-responsive surfaces. As one group of photo-sensitive materials, inorganic oxides, such as V₂O₅,⁷⁶ TiO₂,^77,78 ZnO,^79,80 SnO₂,⁸¹ Ga₂O₃,⁸² and WO₃,⁸³ have drawn much research attention with respect to the fabrication of smart light-responsive surfaces with reversibly switchable wettability during the past few decades.

ZnO, as one of the most important wide-band-gap semiconductor materials, has been applied in many high technology applications.^84–88 As reported, when the as-prepared ZnO surface is exposed to UV illumination, electron–hole pairs are produced on the surface.^89–93 It is conducive to absorb the water molecules in the air to form high-surface-energy hydroxyl radicals due to the unstable oxygen vacancies, which are produced by the interaction between holes and lattice oxygen. The wetting state of the prepared surfaces can change from hydrophobicity to hydrophilicity because of the appearance of hydroxyl groups. And, when stored in the dark for a period of time, the hydroxyl groups are removed easily by oxygen in the environment.^89–93 As a result, the surfaces recover their original wettability. Based on this, researchers have prepared various kinds of structural ZnO superhydrophobic surfaces with responsive properties through different types of technologies and approaches. Jiang et al. reported aligned ZnO nanorod array-coated mesh films with switchable superhydrophobicity–superhydrophilicity and underwater superoleophobicity at a special oil–water–solid three-phase interface in 2012.⁹⁴ The wettability of these films can be reversibly switched by alternation of ultraviolet (UV) irradiation and dark storage. These had a WCA of ∼155° after dark storage and after UV (a 500 W Hg lamp with a filter centred at 365 ± 10 nm was used as the light source) irradiation for about 0.5 h, had a WCA of about 0° (Fig. 2a). After UV irradiation, the films recovered their wetting state when they were placed in the dark for 7 days. With regard to the oil wettability, as shown in Fig. 2b, the aligned ZnO nanorod array-coated mesh films were superoleophilic in air, while they showed underwater superoleophobic properties (with an oil contact angle of ∼156°) after UV irradiation. However, this had little influence on the surface wettability to different types of oils after light stimulation (Fig. 2c). A rough ZnO layer, which consisted of micro/nanoscale hierarchical structures, was built by Chen et al. via a simple method of direct femtosecond laser ablation.⁹⁵ Such a hierarchical ZnO surface with wettability that can be switched between superhydrophobic and quasi-superhydrophilic states was proven to have good switchable wettability upon alternating UV irradiation and storage in the dark after many cycles.

Fig. 2 Aligned ZnO nanorod array-coated mesh films with photo-responsive surface wettability. (a) The wettability transition between superhydrophobicity and superhydrophilicity on the coated mesh film after dark storage (left) and under UV irradiation (middle) in air; when more water is added, the water droplet will drop (right). (b) The wettability transition of an oil droplet (1,2-dichloroethane) on the mesh film in air (left) and underwater (middle), with a small sliding angle of about 2.6° (right). (c) The contact angles of different oil droplets on the underwater mesh after storage in the dark and under UV irradiation.⁹⁴

As another commonly used inorganic material, TiO₂ has become the focus of biomaterials study because of its biocompatibility and good chemical stability. Moreover, it has a similar light-responsive mechanism to ZnO; the wettability of a TiO₂ nanostructured surface can be switched by controlling the surface chemical composition under UV irradiation.^96–98 It have become a topic research point for TiO₂ nanostructure as electrochemical anodizations since its surface roughness controlled and prominent photocatalytic property can easily adjust surface chemical composition.^99–105 Based on the photodegradation of the low-surface-energy 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PDES) monolayer under UV irradiation and the excellent photocatalytic character of crystallized TiO₂, Lai et al. prepared a large-scale pinecone-like anatase TiO₂ particle film with rapidly responsive wettability between superhydrophobicity and superhydrophilicity via a simple one-step and practical electrochemical anodizing method.¹⁰⁶ This has become a popular research topic due to the various important phenomena and applications of under-oil wettability.^107,108 Chen and co-workers fabricated femtosecond laser-induced rough TiO₂ surfaces with switchable oil wettability between underwater superoleophilicity and underwater superoleophobicity through UV irradiation and dark storage.¹⁰⁹

Inorganic materials usually have the advantages of lower toxicity and greater chemical/thermal stability when they are compared with organic materials. Based on the amplifying effect of roughness, combining certain rough surfaces with low-surface-energy materials allows numerous inorganic material-based surfaces to develop the ability to switch between the two extreme wetting states upon alternating UV irradiation and storage in the dark. However, the relatively long response time (several days or weeks) upon dark storage to realize a change in chemical composition or conformation of the photo-responsive inorganic oxide materials poses a serious obstruction with respect to their practical application. Another limitation is that many inorganic oxide materials only respond to UV.

At the same time, some photo-responsive organic materials have also been applied in smart responsive surfaces with controllable wettability. Typically, the bistable change of light-responsive organic materials leads to a variety of changes in physical and chemical properties. Compared with photo-responsive inorganic materials, photo-responsive organic materials have more advantages with respect to chemical modification and reaction diversity. The organic materials, including photochromic functional groups (such as azobenzene,¹¹⁰ spiropyrane¹¹¹ and diarylethenes^112,113) have an ability to undergo a reversible change of configuration under UV/visible light, which can result in the transition of the surface wettability.

Azobenzene shows reversible isomerization between the trans and the cis state of the azo moiety, accompanied by rapid and significant changes in both geometry and dipole moment when it is irradiated with UV-vis light.¹¹⁴ The change in dipole moment also leads to changes in polarity and the wetting properties of these surfaces. However, the WCA of flat surfaces modified by azobenzene is increased by less than 10° as a result of alternating UV and vis irradiation.¹¹⁵ Therefore, the applications of azobenzene in the light-responsive wettability field are expanded by introducing a functional group with unique wetting properties into the azobenzene. Rühe et al. presented a method to structure surfaces with wettability that can be reversibly adjusted between superhydrophobicity and a Wenzel state, or a Wenzel and a superwetting state, through UV or vis light irradiation via obtaining a silicon surface with nanoscale roughness and attaching a polymer (containing a fluorinated azobenzene moiety) monolayer to it.²³ Zhou et al. synthesized a photo-responsive copolymer containing catechol and azobenzene derivatives and successfully assembled the copolymer on nanoparticles, plate mica, and rough anodized aluminum surfaces for the fabrication of films with switchable wettability.¹¹⁶ Huang et al. synthesized smart responsive cellulose materials with wettability that switched reversibly in response to light stimuli through self-assembling a 7-[(trifluoromethoxyphenylazo)-phenoxy]pentanoic acid (CF₃AZO) monolayer onto TiO₂ ultrathin film, which was pre-coated with nanofibre surfaces of laboratory filter paper.⁶⁵ However, the trans–cis conversion of the azobenzenes is usually far from quantitative and happens in a narrow spectral range. The wettability adjustment of surfaces modified with polymer containing AZO groups is generally limited between two extreme wetting states.

Compared with azobenzene, spiropyrane has different photochromic mechanisms due to photochemical cleavage of the C–O in its ring in the presence of UV, leading to a reversible switch between a closed nonpolar form and an open polar form.¹¹⁷ Spiropyrane has a broad spectrum and near-quantitative in both directions during photoisomerization process. Spiropyrane-containing copolymers can be used as smart dynamic materials to obtain smart surfaces due to their unique advantages. A smart surface with light-induced tunable wettability and excellent stability was prepared by Zhou and co-workers with a novel photo-responsive fluorinated gradient brush copolymer containing the main-chain gradient structure of [poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate-gradient-2-(trimethylsilyl)ethyl methacrylate) (poly(HFBMA-g-HEMA-TMS))] and the functional side chains of spiropyrane (Sp) and other units.¹¹⁸ As reported, the nonpolar and hydrophobic spiropyrane can switch to polar and hydrophilic merocyanine (Mc) through an isomerization induced by UV light and recover the Sp form when it is exposed to visible light.¹¹⁹ Combining the functional film with surface roughness, the etched silicon surface with controllable wettability was fabricated using a copolymer containing Sp moiety. However, spiropyrane may lose its photo-responsive properties after photo-degradation, thermal relaxation, and side reactions.^120–124

Similarly, diarylethenes are another group of photochromic organic molecules that have been used in many light-responsive surfaces due to their light-switchable wetting properties. However, their isomerisation is normally accompanied by a relatively small change in molecular conformation. In order to synchronously form wider and more rapid WCA changes from hydrophilicity to superhydrophobicity on a surface containing stilbenes, Uchida et al. designed new diarylethenes in 2011.¹²⁵

Furthermore, it is distinctive that a thermally responsive polymer, poly(N-isopropylacrylamide) (PNIPAM) has also been applied to a photo-responsive surface with fast and reversibly switchable wettability. Through depositing SiO₂ nanoparticles, polyallylamine hydrochloride (PAH), poly(styrene sulfonate) (PSS) and gold nanoparticles on a polyethylenimine (PEI)/(poly(styrene sulfonate) (PSS)/PAH)₅-modified silicon wafer via a LBL method one after another, then grafting PNIPAM chains on the gold nanoparticles via SI-ATRP (Fig. 3a), Kim et al. prepared a nanoporous multilayer surface.⁷² As reported, PNIPAM has a lower critical solution temperature (LCST) of about 32 °C in water, and the surface becomes hydrophobic when the temperature is higher than the LCST, while it becomes hydrophilic when the temperature is lower than the LCST.^126,127 At the same time, the gold nanoparticles on the pre-prepared roughened multilayer film can generate thermal energy by photoexcitation under visible light irradiation, which is known as a photothermal effect.^128,129 Therefore, the wettability of the surface can be quickly and reversibly adjusted between hydrophobicity and hydrophilicity under light irradiation (switching from a hydrophilic to a hydrophobic surface within 15 min and switching from a hydrophobic to a hydrophilic surface within 10 min) by simply turning the visible light on (or off) (Fig. 3b).

Fig. 3 (a) Schematic of the fabrication process for a nanoporous multilayer surface. (b) Smart surface with fast and reversibly switchable wettability depending on the presence or absence of light.⁷²

The wettability transition of photo-responsive surfaces based on organic materials can be realized rapidly. However, in many systems, the wettability change is small and the reversible wettability transition between the two extreme wetting states is also difficult to achieve. Also, the biological toxicity and weak stability of many organic materials are also barriers to their application.

4.1.2 Temperature response. Temperature is considered as another promising external stimulus because it can cause changes in chemical composition and/or the roughness of a surface containing thermo-sensitive compounds. So, it is undisputed that thermo-responsive surfaces with reversibly switching wettability have become a hot topic of study. As mentioned above, poly(N-isopropylacrylamide) (PNIPAM) is an outstanding thermally responsive polymer with a low LCST in water. At temperatures below the LCST, polymer chains are extended away from the surface (Fig. 4), the intermolecular hydrogen-bonds between PNIPAM chains and water molecules are dominant and the polar groups are exposed at the surface, which results in the surface having a high surface free energy. On the other hand, at temperatures above the LCST, the polymer chains are compact and collapsed, intramolecular hydrogen-bonds between the polar functional groups are predominant, and the nonpolar polymer backbones are exposed on the surface (Fig. 4), which leads to the surface obtaining a low surface free energy.^130,131 It has been confirmed that the surface free energy has a major impact on the wettability of the surface: surfaces with low free energy are hydrophobic, while surfaces with high free energy are hydrophilic.¹³² Herein, many roughness-enhanced poly(N-isopropylacrylamide) (PNIPAM)-modified surfaces with thermally responsive wettability have been fabricated.

Fig. 4 Temperature effect on the change in conformation of PNIPAM.^131,132

Chen et al. presented a thermally responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel, which was prepared in a simple way through free-radical polymerization in water and could responsively and reversibly control its wettability to oil at a water/solid interface.¹³³ The as-prepared PNIPAM hydrogel surface (PHS) exhibited superoleophobicity with an oil contact angle (OCA) as high as 151.7 ± 1.6° at 23 °C under water; when the environmental temperature was increased to 40 °C above the LCST, the surface showed oleophobicity with an OCA of about 127.0 ± 4.6°. The wetting behavior of the surface returned to its original state after the temperature decreased, indicating that the wettability of the surface could be switched reversibly between superoleophobicity and oleophobicity.

However, the PNIPAM homopolymer is unsuitable for use in coating applications because it becomes amorphous at a wetted surface. A variety of strategies have been studied to generate a surface with thermoresponsive wettability by grafting polymers containing PNIPAM onto different substrate surfaces.¹³⁴ Liu et al. grafted mixed brushes, which were composed of heptadecafluorodecyltrimethoxysilane (HFMS) as the oleophilic component in the aqueous phase and poly(N-isopropylacrylamide) (PNIPAM) as the thermo-responsive component, on a silicon substrate via silane coupling chemistry¹³⁵ and SI-ATRP. The synthesized underwater temperature-responsive surface had a wettability switch between superoleophobicity and superoleophilicity as a result of the cooperation of PNIPAM and HFMS on SiNWAs.¹³⁵ A facile approach, which combines micromolding, LbL assembly of polymer macroinitiators, and surface-initiated polymerization was presented by Advincula and co-workers to fabricate a temperature-responsive coating with the ability of reversible wettability switching between superhydrophobicity and superhydrophilicity.¹³⁶ To introduce roughness on a polymer coating, solvent-assisted micromolding (SAMIM)¹³⁷ was used by Advincula et al. to replicate the lotus-leaf surface morphology on a thin cellulose acetate film, which was coated on glass before depositing polyelectrolyte macroinitiators via LbL assembly and polymerizing PNIPAM via ATRP onto it. The microscale roughness was provided by the cellulose acetate, while the macroinitiator layers and the PNIPAM brushes provided the majority of the nanoscale roughness and the surface energy. Reversible switching between superhydrophobicity and superhydrophilicity was then demonstrated. Wang et al. also reported temperature-responsive surfaces fabricated by regenerated cellulose (RC) nanofibers and a modifier of poly(N-isopropylacrylamide) (PNIPAM) through SI-ATRP.¹³⁸ As shown in Fig. 5, the RC nanofibers were fabricated from a cellulose acetate (CA) solution through an electro-spinning method, followed by hydrolysis in a strong base. Then, the RC nanofibers were surface-functionalized by PNIPAM brushes through SI-ATRP, and thermally responsive wettability was introduced at a water–oil–solid three-phase interface. So, the RC nanofibers can be turned into a smart material by the grafted temperature-sensitive PNIPAM brushes; as a response to different temperatures, the PNIPAM-grafted nanofibrous membrane exhibited wettability switching between superlyophilicity and superlyophobicity at water–oil–solid interface.

Fig. 5 Schematic of the fabrication process for PNIPAM–RC nanofibers and the temperature-responsive transition from superlyophobicity to superlyophilicity at the water–oil–solid interface of the prepared PNIPAAM–RC nanofiber membrane.¹³⁸

It is worth mentioning changes in the mechanical properties of smart surfaces with thermally responsive wettability, achieved by introducing copolymers containing PNIPAM and engineering plastics. For instance, the stability of copolymer films in moist environments can be enhanced by hydrophobic poly(methyl methacrylate) (PMMA) with a high glass transition temperature (T_g), which serves as a physical cross-link to hydrophilic PNIPAM.¹³⁹ Controllable surface wettability of poly(methyl methacrylate) (PMMA) films can be achieved via incorporating the thermally responsive functional monomer of NIPAM. Li and co-workers¹⁴⁰ fabricated a surface with reversibly adjustable wettability between hydrophilicity and hydrophobicity through thermally responsive block copolymers of poly(methyl methacrylate)-block-poly(N-isopropyl-acrylamide) (PMMA-b-PNIPAM), which was successfully synthesized by successive copper(0)-mediated reversible-deactivation radical polymerization technology (RDRP).¹⁴¹ As expected, the volume fraction of PNIPAM in the copolymers also had an influence on the thermally responsive wettability of the block copolymer-modified surfaces. As well as the copolymer of PMMA-b-PNIPAM, many other copolymers, such as poly(N-isopropylacrylamide)-block-polystyrene (PNIPAM-b-PS),¹⁴² poly(N-isopropylacrylamide-co-1-adamantan-1-ylmethylacrylate) (PNIPAM-co-Ada),¹⁴³ and poly-(N-isopropylacrylamide-co-hexafluoroisopropyl acrylate) (P(NIPAM-co-HFIPA))¹⁴⁴ are also used for fabricating smart responsive surfaces.

PNIPAM-based polymers are the most commonly used materials for structuring thermally responsive surfaces. However, the moderate cytotoxicity of PNIPAM at 37 °C has encouraged researchers to find substitutes.¹⁴⁵ Many other types of materials are also applied to thermally responsive surfaces. Zhang and co-workers fabricated a superhydrophobic carbon nanotube (CNT) film via a simple spray-coating method without modification.¹⁴⁶ It was demonstrated that the surface wettability of the CNT film could be reversibly changed between superhydrophobicity and superhydrophilicity by alternating between heating and air storage due to electronic changes in the carbon nanotube film surfaces. A one-step facile spray-deposition process for the fabrication of superhydrophobic and superoleophilic SiO₂ nanoparticle films with reversible wettability was described by Zhang et al.⁶⁹ After the hydrophobic SiO₂ nanoparticles were chemically modified by trimethylchlorosilane (TMCS), reversible superhydrophobicity and hydrophilicity switching of the surface could be easily carried out by adjusting the temperature (Fig. 6a and b). It was interesting to find that the transition mechanism of the surface was different to the examples mentioned above; the transition of the surface from the Cassie to the Wenzel regime due to water vapor condensation on the surface affects the wetting properties of the nanoparticle film (Fig. 6c).

Fig. 6 (a) Images of water droplets on surfaces at 28 °C and −15 °C. (b) Diagram of reversible wettability transition between superhydrophobicity and hydrophilicity of the as-prepared films at different temperatures. (c) Schematic of wettability transition from the Cassie to the Wenzel regime.⁶⁹

Although various temperature-responsive surfaces have been demonstrated in many papers, most of them are fabricated based on several types of commonly used thermo-sensitive materials. The research field needs to be expanded by applying more and more new materials with low-cost and nontoxicity to the fabrication of smart surfaces. On the other hand, it is not difficult to find that many significant wettability transitions need a large change in temperature, which cannot meet the demands of smart devices. Improving the sensitivity of smart surfaces will be an important breakthrough.

4.1.3 pH response. In the past few decades, pH-responsive surfaces with superwettability have attracted a lot of attention because they could be used for many applications, such as drug delivery, biosensors and separation applications.^147–149 As one of the most common external stimuli, it is of great significance to study the surface wettability under different pH conditions.

Weak polybases and weak polyacids are the most commonly used types of pH-responsive polymers for obtaining intelligent surfaces with switchable wetting properties. Weak polybases transform into polyelectrolytes at low pH with electrostatic repulsion forces between the molecular chains, and they transform into neutral polymers at high pH. A momentum along with the hydrophobic interaction is given to govern hydrophobic/hydrophilic characteristics of surfaces.¹⁵⁰ At the same time, weak polyacids have a pH-responsive carboxyl group, consisting of a carbonyl and a hydroxyl group. The carboxyl groups can be protonated (–COOH), and hydrophobic interactions are predominant at low pH, while carboxyl groups dissociate into carboxylate anions (–COO⁻) with relatively hydrophilic properties at high pH. By combining the pH-responsive polymers with rough substrate materials, various smart pH-responsive surfaces with controllable wettability have been fabricated. Jiang and co-workers reported a novel pH-responsive surface with reversibly switchable wettability between high hydrophilicity at low pH and high hydrophobicity at high pH (Fig. 7).¹⁵¹

Fig. 7 Reversible conformational changes of PDMAEMA under different pH conditions. Amine groups are protonated and the polymer chains are extended (left) due to electrostatic repulsion forces at low pH, resulting in a smaller WCA. At high pH, the ionizable groups are deprotonated and the electrostatic repulsion force simultaneously disappears within the polymer chains, causing aggregation of the polymer chains (right) and inducing a larger WCA.¹⁵²

Through fabricating a weak polybase of poly(N,N′-dimethyl-aminoethyl methacrylate) (PDMAEMA) thin film on a roughly etched Si substrate via a typical SI-ATRP, the as-prepared surface achieved pH-responsive wettability due to the competition between protonation at low pH and deprotonation at high pH.¹⁵² Combining hierarchical nanostructured fibrous webs and a polymer, poly[2-(diisopropylamino)ethylmethacrylate-co-3-(trimethoxysilyl)propylmethacrylate] ([poly(DP-AEMA-co-TSPM)]) together, Cho and co-workers developed smart electrospun fabrics that undergo reversible superhydrophobic/superhydrophilic transitions as a result of pH changes.¹⁵³ In the copolymer, the PDMAEMA part played a crucial role in the conformational transition through its response to pH stimulation in different pH surroundings, while TSPM was used as a sol–gel precursor to hold the polymer on the nanostructured substrates and provide intramolecular cross-linking among the polymer chains. Wu et al. prepared a novel V-shaped polymer brush-functionalized surface with reversible controllable two-way responsive wettability by grafting ABC-type block copolymers, consisting of tert-butylmethacrylate (tBMA), 2-hydroxyethylmethacrylate (HEMA), and 2-(diisopropylamino)ethyl methacrylate (DPAEMA), onto acyl chloride-functionalized SiO₂ film for the first time.¹⁵⁴

Polymers containing PDPAEMA have been frequently used during the past few years. On the other hand, the weak polyacids of carboxyl-terminated polymers are also considered as suitable candidates to achieve surfaces with pH-responsive wettability.^155,156 Jiang and co-workers reported poly(styrene-methyl methacrylate-acrylic acid) (poly(St-MMAAA)) colloidal crystal films with hydrogen-bonding-driven wettability, which can be easily controlled between superhydrophobicity and superhydrophilicity by pH at constant temperature, due to hydrogen-bonding interactions between carboxyl groups and sodium dodecylbenzenesulfonate (SDBS), as early as 2006.¹⁵⁷ In the past few years, based on hydrogen-bonded LbL films,¹⁵⁸ surfaces with controllable wettability, due to the collapse/solubilization of polymer units within single-component surface-bound hydrogels, induced by pH were prepared through selective chemical cross-linking of a polycarboxylic acid component, followed by the release of neutral hydrogen-bonded chains.⁵⁹ It was shown that LbL hydrogel coatings constructed of PaAAs (PEAA_LbL, PPAA_LbL, and PBAA_LbL) on micropatterned substrates led to highly functional coatings with large-amplitude surface wettability transitions from hydrophobicity at acidic pH to superhydrophilicity at basic pH values and that the polyacid hydrophobicity increased when the transition pH value increased from 6.2 to 8.4. Meanwhile, polymers comprising pH-responsive block copolymers, poly(2-vinylpyridine) (P2VP) and oleophilic/hydrophobic polydimethylsiloxane blocks, were used by Wang et al. to prepare surfaces with switchable superoleophilicity and superoleophobicity in aqueous media.²⁶ The as-prepared surfaces provided switchable wettability by oil as a result of the oleophilic PDMS block on the surface, based on the protonation and deprotonation of the P2VP block on the grafted block copolymer upon pH changes in the aqueous media.

As well as fabricating smart surfaces through transforming the surface morphology by introducing pH-responsive polymers, smart surfaces can be structured with pH-responsive wettability by delicate design of the surface chemistry. In this way, both hydrophilic and oleophilic/hydrophobic characteristics of low surface energy materials can be incorporated in the surface, with either characteristic becoming dominant over the other as a result of pH changes. With the different chemical compositions being released to the top surfaces, pH-responsive wettability of the surface is obtained. Smart fabrics with both pH-responsive water wettability in air and pH-responsive oil wettability underwater were successfully fabricated by Guo et al. via in situ growth of Ag nanocrystals on the fabric surface, followed by surface modification with a mixture of methyl-terminated thiol (HS(CH₂)₉CH₃) and carboxyl-terminated thiol (HS(CH₂)₁₀COOH).¹⁵⁹ Using the same modifier, through an electrochemical deposition strategy, followed by an Au sputter-coating process, Guo's group also fabricated a superhydrophobic copper mesh film (CMF) with pH-responsive properties.⁶⁴ Shi and co-workers prepared a smart surface with a pH-responsive transformation from superhydrophobicity to superhydrophilicity through a combined approach consisting of the electroless deposition of gold and subsequent modification with a SAM of HS(CH₂)₁₁CH₃, HS(CH₂)₁₁NH₂ and HS(CH₂)₁₀COOH, which resulted in the presence of both alkyl and amino groups on the gold surface.¹⁶⁰ According to correlative theories, under acidic conditions, the amino groups would be mostly protonated and covered by hydrate layers,¹⁶¹ resulting in the entire surface showing superhydrophilicity. Correspondingly, the amino groups would be deprotonated when the pH value was changed from 1 to 13, making the methyl groups dominant and resulting in a superhydrophobic surface.

For pH-responsive surfaces, a rapid responsive wettability transformation is of great significance in potential applications. However, many applications, especially biotechnological applications, require higher pH sensitivity. However, a drawback for many smart surfaces is that a drastic pH change is needed to achieve a large variation in wettability. Simultaneously, the biotoxicity of many polymers is still an obstacle that limits their application. Further research and development is essential in order to overcome these drawbacks.

4.1.4 Electrical potential as external stimulus. Electrowetting is the re-arrangement of charges and dipoles when a potential is applied between a liquid and a solid, resulting in a reduction in the interfacial energy, leading to an increase in surface wettability.¹⁶² Electrowetting has been successfully applied to surface wettability control without changes in the surface composition and structure. Electrowetting is a promising technique for adjusting the water wetting behavior on a surface due to its simplicity, efficiency, and fast response.^163–166 Lahann et al. fabricated a surface with a wettability transition, which was caused by a SAM conformational reorientation under an electrical potential, by depositing a (16-mercapto)-hexadecanoic acid (MHA) SAM on a gold surface.¹⁶⁷ Also, a strategy applied to achieve effective reversible electrowetting by liquid droplets in oil and in air on superhydrophobic silicon nanowires was described by Nicolas.¹⁶⁸

With the development of the correlation technique in recent years, many other kinds of intelligent electrically-responsive surfaces have been synthesized. Through adopting plasma/ion-aided deterministic nanofabrication without low-surface-energy coatings, Han synthesized superhydrophobic carbon nanotube (CNT)-based nanocomposites with electrowetting-controlled wettability.¹⁶⁹ The control and monitoring of the transition between the Cassie state and the Wenzel state was achieved by applying a potential between the water droplet and the solid surface. Zhao et al. reported a new tilt-aligned, sodium stearate-treated conducting MnO₂ nanotube array (NTA) superhydrophobic surface with both a large CA and electrically-adjustable adhesion.¹⁷⁰ Meng et al. fabricated a conductive polymer porous film with tunable wettability through a chloroform solution of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyricacid-methyl-ester (PCBM) via the freeze drying method.¹⁷¹

As well as the electrowetting mechanism, other types of electric treatment can also be applied to obtain electrically-responsive surfaces. It is easy to prepare the typical electrochemically responsive materials of π-conjugated polymers.^172,173 The wetting conversion properties of the π-conjugated polymers can be achieved through a process of doping (oxidization) and dedoping (reduction) by applying a voltage in an electrolyte solution. Therefore, it is possible to introduce π-conjugated polymers into surfaces with electrically-responsive wettability. Polypyrrole (PPy) is a material widely applied to electrically-responsive surfaces because of its mechanical and electrical properties.¹⁷⁴ For instance, a simple electrochemical process was used to fabricate superhydrophobic conducting polypyrrole (PPy) film by Xu and co-workers, and the as-prepared film exhibited reversible wettability switching between superhydrophilicity and superhydrophobicity by simply adjusting the electrical potential.¹⁷⁵ Through directly electrodepositing poly(3,4-ethylenedioxythiophene) (PEDOT) on an indium tin oxide (ITO)-coated glass electrode and further electrodepositing hydrophobic poly(3-hexylthiophene) on the surface of the porous PEDOT, Lu et al. fabricated a highly porous structured double-layer polythiophene film.⁷⁰ In an electrolyte solution containing ClO₄⁻, the as-prepared PEDOT–P(3-MTH) could be reversibly doped and dedoped under oxidation and reduction potentials, respectively (Fig. 8a). At the same time, this film exhibited superhydrophobicity with a high static WCA of 162.4° ± 2° and a low sliding angle of 3.0° ± 1° due to the dedoping (reduction) of the PEDOT–P(3-MTH) at a potential of −0.2 V, while it was easy to obtain a superhydrophilic PEDOT–P(3-MTH) film by doping (oxidation) the double-layer film at +1.2 V (Fig. 8b). The film still maintained great ability of reversible wettability transition between superhydrophobicity and superhydrophilicity after many cycles (Fig. 8c).

Fig. 8 (a) The doping (oxidation) and dedoping (reduction) of PEDOT–P(3-MTH) in LiClO₄/acetonitrile solution at different potentials. (b) The wettability switching of the PEDOT–P(3-MTH) film between superhydrophobicity (WCA about 162.4° ± 2°) and superhydrophilicity (WCA about 0°) upon the alternate application of potentials of −0.2 (reduction) and +1.2 V (oxidation), respectively. (c) Cycling switchable wettability transition of the PEDOT–P(3-MTH) film between superhydrophobicity and superhydrophilicity.⁷⁰

The wetting state of smart surfaces with electrowetting properties can transform rapidly upon potential stimuli without changes of surface composition and structure. However, in general, the WCA change is not large enough. As for the electrochemical responsive surfaces, to achieve the wetting transformation, a suitable chemical environment is required. Such limitations need to be overcome before these smart surfaces are ready for real-life application.

4.1.5 Stress response. As mentioned before, the surface wettability of a solid substrate is mainly governed by chemical composition and surface structures. Many smart surfaces with wettability that is reversibly controlled by thermal treatment, light illumination, and pH have been reported. However, the surface wettability can also be tuned by manipulating the geometric structures of the solid surface through exerting stress, even though there are few reports about this.

A film made from PTFE with reversible wettability upon stress stimulation was reported as early as 2004.¹⁷⁶ Han fabricated an elastic polyamide film with a reversible wettability transition between superhydrophobicity and superhydrophilicity stimulated by extending and unloading.¹⁷⁷ Synthesizing a stretchable hydrogel from a mixture of alginate, acrylamide, ammonium persulfate, N,N′-methylenebisacrylamide and then coating silanized particles into the prepared stretchable hydrogel, Soh and co-workers fabricated smart surfaces that varied reversibly from superhydrophobicity to superhydrophilicity under the influence of stress.¹⁷⁸ An SEM image of the glass particles on the surface is shown in Fig. 9d. As for the wetting behavior of the surfaces, it was found that the water spread quickly across the surface and penetrated into the material in the absence of silanization, while the surface showed superhydrophobicity after silanization. It was also interesting to find that the surface was superhydrophilic when the material was stretched to 600% (Fig. 9a). The integrity of the material and the influence of different extensions on the contact angle were also tested (Fig. 9b and c).

Fig. 9 The wettability control of a stretch-responsive composite material. (a) The surface exhibits a negligible contact angle when the glass particles are not silanized and becomes superhydrophobic after silanization. After silanization, the surface becomes superhydrophilic when the material is stretched. (b) The transition between superhydrophobicity and superhydrophilicity is reversible. The material shows great stability with a contact angle maintained ≈0° when stretched and CA > 150° when released even when it is stretched and released 20 times. (c) The contact angle can be adjusted by stretching the material to different extents; (d) an SEM image of the glass particles on the surface.¹⁷⁸

Stress-responsive surfaces provide a characteristic way to adjust the surface wettability. These may be good candidates for engineering materials, but some aspects still need to be taken into consideration, including the cost, the mechanical properties and the fabrication process.

4.1.6 Solvent response. The solvent-responsive wettability of smart surfaces is the wettability that is sensitive to the properties of the surrounding media.^179–182 Various responsive polymers have been reported; compared to other external stimuli-responsive polymers, solvent-responsive polymers have a common mechanism, in which the interfacial free energy is driven by configurational changes in polymer chains, which are governed by the interactions between the polymer chains and the solvent. Accordingly, switchable wettability is gained owing to the change in interfacial free energy upon solvent treatment. Sun and co-workers reported an unusual water-induced superhydrophobicity on a smart copolymer surface of PNIPAM-co-Cy & AA, containing double amino acid units.¹⁸³ It was interesting to find that the reversible wettability switching between superhydrophobicity and high hydrophilicity could be realized through alternating treatments by water and methanol–alkali solution, by the amplification effect of the structured substrate, while the CA showed a small change in response to the solvent on a flat substrate. However, the repeatability of the surface wettability reversibility switch was poor. More interestingly, when methanol replaced the methanol–alkali mixture, due to the extra contribution of the electrostatic interactions influencing the conversion of polymer chains, the CA change was significantly improved and the film showed good reversibility. Through fabricating SAMs on a gold surface with two dithiooctanoic acid derivatives bearing N,N-disubstituted amide groups, Wang reported a film with a reversible wettability change upon alternating treatment with ethanol and cyclohexane.⁶³ As reported, solvent polarity had an important influence on the relative populations of the two isomers for a given amide.¹⁸⁴ Reversible changes in the wettability of the as-prepared film, based on conformational changes in the amide groups in different solvents, can be anticipated (Fig. 10).

Fig. 10 Schematic of two amide stereoisomers involved in a reversible change upon alternating treatment with ethanol and cyclohexane.⁶³

Wettability switching of solvent-responsive surfaces requires a suitable solvent environment. For the moment, many solvent-responsive surfaces can only respond to a few solvents, which limits their applications. It may be necessary to synthesize multifunctional solvent-responsive materials to extend this field.

4.1.7 Ion response. Based on the reversible process of ion exchange between cationic or anionic electrolytes and their complexes, ion-pairing interactions have the ability to induce a transition between superhydrophobicity and superhydrophilicity.¹⁸⁵ Up to now, many ion-responsive surfaces have been fabricated. Through grafting a polyelectrolyte with quaternary ammonium groups of poly[2-(methacryloyloxy)ethyl trimethyl-ammonium chloride] (PMETAC) brushes onto a rough gold surface via SI-ATRP, Cho et al. fabricated a ion-responsive surface with a wettability transition between superhydrophobicity and superhydrophilicity.¹⁸⁶ Zhang et al. fabricated a film with tunable wettability through depositing a (PDDA/PSS)₃PDDA multilayer on a commercially available cotton fabric surface using LbL technology and taking advantage of counterion exchange between Cl⁻ and PFO anions.⁶¹ As illustrated in Fig. 11, the polyelectrolyte showed superhydrophilicity and superoleophilicity with CAs of 0° when it coordinated with Cl⁻ counterions. The CAs with water and hexadecane increased to 151 ± 3° and 140 ± 4°, respectively, due to the reduction in the surface energy when the Cl⁻ counterions were replaced with PFO anions. According to the results of these experiments, switchable wettability of the as-prepared fabric could be retained for at least 4 months under atmospheric conditions, with no contact angle change for water and hexadecane. Zhang et al. have also successfully manufactured a poly[2-(methacryloyloxy)ethyl trimethyl-ammonium chloride-co-trifluoromethyl methacrylate] (poly-(METAC-co-TMA))-tethered transparent surface with switchable wettability between superhydrophobicity and superhydrophilicity via counterion exchange between Cl⁻ and PFO⁻.¹⁸⁷

Fig. 11 Images of water and hexadecane droplets on polyelectrolyte deposited surfaces when they are coordinated with PFO anions and Cl⁻, respectively.⁶¹

In addition, the chelation between the ion and the polymer also provides a platform for ion-responsive surfaces. A novel Hg²⁺-responsive oil/water separation mesh coated with poly(acrylic acid) hydrogel was manufactured by Feng et al. by obtaining a polydopamine-coated mesh (PDA mesh), on which was grafted linear polyacrylic acid (LPAA) (Fig. 12a–c).¹⁸⁸ Based on the superhydrophilic poly(acrylic acid) hydrogel coating and wettability switching due to the chelation between Hg²⁺ and PAA, the novel mesh had the ability to separate oil/water. Without Hg²⁺, the novel mesh was hydrophilic and oleophobic, resulting in water permeating through the mesh when oil was blocked (Fig. 12d′). On the contrary, the wettability of the prepared mesh changed to oleophilic and hydrophobic, resulting in oil permeating through the mesh when water was blocked in the presence of Hg²⁺ (Fig. 12d).

Fig. 12 Illustration of the preparation and wettability transition of mussel-inspired hydrogel coated mesh: (a) stainless steel mesh. (b) Coating polydopamine on the mesh (PDA mesh). (c) Grafting linear polyacrylic acid onto the PDA mesh (LPAA–PDA mesh). (d) LPAA–PDA mesh with oleophilic and hydrophobic properties in the presence of Hg²⁺. (d′) LPAA–PDA mesh with hydrophilic and oleophobic properties in the absence of Hg²⁺.¹⁸⁸

Ion-responsive surfaces open up a new approach to obtaining wettability switching. However, due to many limitations, such as the chemical environment, complex fabrication and expensive cost, as with most smart surfaces based on responsive polymers, many surfaces have difficulty in meeting the demands of industrial production.

4.1.8 Other responses. As well as the smart surfaces with stimuli-responsive wettability mentioned above, there are other responsive surfaces, such as gas-responsive surfaces, and magnetism-responsive surfaces. Jiang's group reported an ammonia-responsive surface wettability switch on indium hydroxide (In(OH)₃) films with a micro-nano structure¹⁸⁹ and polyaniline-coated fabric¹⁹⁰ in 2008. In recent years, this has been considered as a “green” trigger for artificial smart systems due to nontoxicity and easy removal of CO₂.¹⁹¹ Many CO₂-responsive polymers containing amidine/amine groups, such as poly(N-amidino)dodecyl acrylamide (PAD),^192–195 poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA),¹⁹⁶ and poly(N,N-diethylaminoethyl methacrylate) (PDEAEMA)¹⁹⁷ have been synthesized. Smart surfaces with CO₂-responsive wettability have attracted significant research attention. A CO₂-responsive surface with a SAM of N-(2-aminoethyl)-5-(1,2-dithiolan-3-yl)pentanamide amidine (NADPA) was constructed by Wang and co-workers through a molecular self-assembly approach.¹⁹⁸ The wettability of the NADPA SAM showed reversible switching due to reversion of the amidine conjugation systems, which was caused by reversible protonation/deprotonation in the presence/absence of CO₂. Through introducing polymethylmethacrylate (PMMA) into poly(N,N-dimethylaminoethyl methacrylate), PMMA-co-PDEAEMA was successfully synthesized to effectively improve the stability of the nanofibers, and Yuan et al. fabricated a CO₂-responsive electrospun nanofibrous membrane with switchable oil/water wettability through the prepared copolymer (Fig. 13).⁴⁰

Fig. 13 (a) Illustration of the fabrication process for PMMA-co-PDEAEMA nanofibers. (b) SEM image of a typical porous membrane with a diameter of about 700 nm. Scale bars of ∼5 mm. (c) Illustration of the switchable oil/water wettability transition between hydrophobicity/oleophilicity and hydrophilicity/oleophobicity using CO₂ as the trigger.⁴⁰

Filling elastomeric materials with magnetic nanoparticles and shaping different micro geometries through molding methods are methods of structuring smart surfaces that are responsive to magnetic fields.^199–202 The elastomeric materials respond to magnetic fields due to the magnetic nanoparticles. At the same time, the ability to undergo a wettability transition is acquired with a change of surface topological structure. This provides a new platform for achieving remote control of a solid surface wettability by applying magnetic stimulation. Adopting an approach of template-assisted electrodeposition and 30–60 μm track-etch polycarbonate (PC) membranes with uniform cylindrical pores as sacrificial templates to produce arrays of high-aspect-ratio Ni wires topped with micrometer-sized hemispherical caps, Minko and co-workers fabricated a microstructured surface with a wettability transition from superomniphobicity to an omniphilic wetting state under an external magnetic field.²⁰³ The as-prepared surface showed superomniphobicity after modification with a SAM of 1H,1H,2H,2H-perfluorodecanethiol without a magnetic field, while the transition from the nonwetting Cassie–Baxter state to the wetting Wenzel state was achieved when an external magnetic field was used to bend Ni micronails. Jiang et al. reported superparamagnetic Fe₃O₄ nanoparticles in a microdroplet with reversible switching between the Cassie state and the Wenzel state on a robust superhydrophobic surface by the application of the magnetic field.²⁰⁴ Although magnetic response surfaces possess the advantage of the remote control of wetting behavior, the small change in wettability is still a barrier to practical application.

Furthermore, inspired by the mussel, Woisel and co-workers obtained surfaces with switchable wettability based on the thermal reversibility of the Diels–Alder reaction through grafting catechol-based biomimetic anchors integrating either a furan or a maleimide moiety onto titanium surfaces and then employing the Diels–Alder cycloaddition reaction to chemically modify TiO₂ surfaces.²⁰⁵ Wang and co-workers presented a enthalpy-driven smart surface with switchable wettability between superhydrophilicity and superhydrophobicity by immobilizing i-motif DNA strands with a fluoride-containing hydrophobic group onto a gold surface through Au–S bonds.²⁰⁶ Taking advantage of hydrogen-bonding interactions between thiourea and 3-arylamido phenylboronic acid (PBA) units in a polymer, Sun et al. demonstrated a smart polymer surface with wettability switching between superhydrophobicity and superhydrophilicity on exposure to sugar solutions.²⁰⁷

4.2 Dual-responsive and multiple-responsive surfaces

As exhibited in the above section, through combining responsive materials with surface roughness, numerous intelligent surfaces with switchable wettability, especially between superhydrophilicity and superhydrophobicity, have been fabricated successfully. However, most of these surfaces are responsive to only one type of external stimulus, which limits their applications under complex practical conditions. Therefore dual-responsive surfaces and multiple-responsive surfaces need to be researched to meet the demands both of fundamental research and industry applications.

Dual-responsive surfaces that respond to temperature and pH, light and pH, and other external stimuli at the same time, were recently reported. It makes sense to fabricate dual-responsive smart surfaces through combining roughness and dual-responsive materials. Polymers of poly(N-isopropyl acrylamide-co-acrylic acid) [P(NIPAM-co-AAc)], which contain a thermo-sensitive component (PNIPAM) and a pH-sensitive component (PAAc), have been widely used to obtain thermo and pH dual-responsive surfaces. Jiang's group reported a dual-responsive surface with switchable wettability between superhydrophilicity and superhydrophobicity through grafting a copolymer onto an etched silicon substrate.²⁰⁸ PDMAEMA is one of the most promising stimulus-responsive polymers with a LCST ranging from 30 to 50 °C at different values of pH, molecular weight, and salt concentration.¹³¹ It has been extensively researched due to this thermo and pH dual-responsive characteristic. Feng's group presented thermo and pH dual-responsive materials through coating PDMAEMA hydrogel, which was fabricated by photo-initiated free-radical polymerization of dimethylamino ethyl methacrylate (DMAEMA) on stainless steel mesh.²⁰⁹

Through synthesizing an ABC-type triblock copolymer consisting of 2-(diisopropylamino)ethyl methacrylate (DPAEMA), 2-hydroxyethyl methacrylate (HEMA), and (4-(2-methylacryloyloxy)ethyloxy-4′-trifluoromethoxy)azobenzene (MAAZO) by a reversible addition–fragmentation chain transfer (RAFT) process followed by grafting of the copolymers onto SiO₂ films through the reactions between the hydroxyl groups of the PHEMA middle segments and the acyl chloride groups of the SiO₂ film surfaces to form specific V-shaped polymer brushes, leaving highly free PDPAEMA and PMAAZO chains, Wu and co-workers synthesized V-shaped polymer brush-functionalized films with reversible dual-stimulus responsive wettability triggered by the joint action of pH and UV light irradiation.²¹⁰ Through grafting dimethylaminoethyl methacrylate (DMAEMA) and 2-methyl-4-phenylazo acrylate (MPA-AZO) on a substrate via a two-stage photo polymerization, dual reversible surfaces with pH and light-responsive wettability were prepared.²¹¹ A temperature and UV light-responsive surface with switchable wettability between hydrophobicity and superhydrophobicity has been fabricated successfully by grafting a block copolymer of poly(7-(6-(acryloyloxy)hexyloxy)coumarin)-b-poly(N-isopropylacryl amide), generated via combining reversible addition–fragmentation chain transfer polymerization (RAFT) onto an SiO₂ surface modified by toluene diisocyanate (TDI).²¹² Moreover, a superhydrophobic aligned-ZnO-nanorod array surface, which exhibited a patterned wettability transition from the Cassie to the Wenzel state via a photoelectric cooperative wetting process was also reported.²¹³

In addition, considering more complex environments, for example the human body, multi-responsive smart surfaces urgently need to be explored. Through introducing a pH/glucose-sensitive component, acrylamidophenylboronic acid (PBA), into PNIPAAm and grafting the copolymer, p(NIPAAM-co-PBA), onto a rough silicon substrate, a surface that exhibited switchable wettability between superhydrophilicity and superhydrophobicity in response to pH, glucose, and temperature was obtained.²¹⁴ By mixing poly(styrene-n-butyl acrylate-acrylic acid) (P(S-BA-AA)) with TiO₂ nanoparticles in tetrahydrofuran (THF), simply casting onto glass substrates and drying at room temperature, Wu et al. presented intelligent films that exhibited reversibly tunable wettability with a very fast response to UV light, heat and pH.²¹⁵

Comparing with the singly-responsive smart surfaces, there is no doubt that dual-response and multiple-response smart surfaces have more advantages, including functions, advanced principles and suitability for complex practical conditions. However, dual- or multiple-responsive smart surfaces are difficult to fabricate because of the interference between different stimuli. Meanwhile, more complex fabrication processes are inevitably associated with dual-responsive and multiple-responsive surfaces. Even so, more and more functional polymers are being introduced into surface science and bring endless possibilities for expanding this field. Much work will be required to achieve the extensive application of these materials.

5. Conclusions and outlook

In this review, we concentrated on the recent development of various smart responsive surfaces, including photo-responsive surfaces, thermo-responsive surfaces, pH-responsive surfaces, and solvent-responsive surfaces. Moreover, the fabrication of surfaces with smart responsive wettability has also been introduced. Through introducing smart responsive materials into superhydrophobic surfaces, these intelligent surfaces not only inherit the core properties of water repellency but also expand their capabilities owing to their peculiar responsive wetting behaviour, which generates more practical applications in both daily life and industry in even more fields.

Oil–water separation has always been considered as an important issue and a key challenge because of frequently occurring leakage of organic pollutants into water and oily wastewater from industries. As one type of material that can achieve oil–water separation, smart controllable separation systems have been widely researched.²¹⁶ As mentioned above, based on wetting transitions upon alternation of stimuli, various smart systems, including photo-responsive surfaces, thermo-responsive systems, and CO₂-responsive surfaces, have been created for this purpose. Compared with other types of materials, smart controllable separation systems have the advantages of being flexible enough to separate water and oil due to their responsive wettability achieved by controlling various conditions. In addition, applications of smart materials with responsive wettability include: (1) functional membranes, (2) biomaterials (mainly due to the switchable wettability, which can be used in medical fields), (3) controllable transportation of microfluids, (4) intelligent coating (these coatings can be applied to buildings, smart windows, and precision instruments), and (5) sensors.

However, some disadvantages and challenges with respect to industrial production and practical applications need to be overcome. Firstly, fabrication technologies and materials pose obstacles to large-scale application. Many fabrications can only be carried out in laboratories, which limits the potential for industrialization. Many subtle surface changes are required to design smart materials with stimuli-responsive properties, thus synthetic routes tend to be complex. Secondly, the long-term durability of nanostructures is important to allow such smart surfaces to resist shear, liquid flow and mechanical forces in practical applications. However, the mechanical stability of the smart responsive surfaces is worth investigating, even though many surfaces maintain good wettability transitions after many cycling experiments under relatively stable experimental conditions. The mechanical stability of these materials has not been researched under real industrial conditions. It is of particular concern that there are many surfaces that cannot be put into use because of their insufficient mechanical stability. Thirdly, the wettability change of many surfaces is too small upon environmental stimulation, and it takes a long time to achieve wettability switching. Fourthly, their high cost remains a challenge that needs to be overcome. Even when suitable smart surfaces are prepared, they still cannot be applied on a large scale due to their high cost.

To overcome the problems mentioned above, the following points need to be taken into account: first and foremost, it is important to pay attention to fundamental research on smart surfaces with responsive wettability, especially the structural design and synthesis of responsive materials with a high performance. Secondly, simplified preparation methods are necessary to meet the demands of industrialization. The durability and mechanical strength of smart surfaces under different conditions need to be taken into consideration. Mass production is in urgent need of common, inexpensive and environmentally non-toxic materials. Many researchers concentrate on the static wetting characteristics; however, contact angle hysteresis may restrict the application of intelligent surfaces when the surface is superhydrophobic. There is therefore a need to study and establish a relationship between contact angle hysteresis and the structure and chemical composition of surfaces.

All in all, the ultimate goal for artificial smart surfaces with responsive wettability is to achieve production with a great performance and long service life by combining novel structures and excellent properties. It can be expected that, with further development, smart surfaces with responsive wettability would possess properties such as great stability, high sensitivity to external stimuli, fast-response ability, large area, biological activity and biocompatibility, to meet the requirements of multidisciplinary applications. We believe there is a bright future in the theory, synthesis, and industrialization of smart surfaces with external stimuli-responsive wettability as more and more attention is focused on this area.

Acknowledgements

This study is supported by the National Nature Science Foundation of China (No. 51522510 and 51405477), the Co-joint Project of Chinese Academy of Sciences and the “Top Hundred Talents” Program of Chinese Academy of Sciences and the National 973 Project (2013CB632300) for financial support.

Notes and references

1. M. Calliea and D. Quéré, Soft Matter, 2005, 1, 55.
2. H. Zhu and Z. Guo, J. Bionic Eng., 2016, 13, 1.
3. G. D. Crevoisier, P. Fabre, J. M. Corpart and L. Leibler, Science, 1999, 285, 1246.
4. A. Marmur, Soft Matter, 2012, 8, 6867.
5. Z. G. Guo, W. M. Liu and B. L. Su, J. Colloid Interface Sci., 2011, 353, 335.
6. R. Fürstner, W. Barthlott, C. Neinhuis and P. Walzel, Langmuir, 2005, 21, 956.
7. L. Introzzi, J. M. Fuentes-Alventosa, C. a. Cozzolino, S. Trabattoni, S. Tavazzi, C. L. Bianchi, A. Schiraldi, L. Piergiovanni and S. Farris, ACS Appl. Mater. Interfaces, 2012, 4, 3692.
8. X. Li and J. He, ACS Appl. Mater. Interfaces, 2013, 5, 5282.
9. K. Ichimura, Science, 2000, 288, 1624.
10. H. Zhu, Z. G. Guo and W. M. Liu, Chem. Commun., 2016, 52, 3863.
11. D. Baigl, Lab Chip, 2012, 12, 3637.
12. Y. H. Yu, Y. Y. Lin, C. H. Lin, C. C. Chan and Y. C. Huang, Polym. Chem., 2014, 5, 535.
13. G. Y. Wang, Z. G. Guo and W. M. Liu, Journal of Bionic Engineering, 2014, 11, 325.
14. J. Aizenberg, A. Tkachenko, S. Weiner, L. Addadi and G. Hendler, Nature, 2001, 412, 819.
15. M. Stevens and S. Merilatita, Philos. Trans. R. Soc. London, Ser. B, 2009, 364, 423.
16. Stress Responsive in Microbiology, ed. J. M. Requena, Calster Academic press, Norfolk, UK, 2012.
17. F. Xia and L. Jiang, Adv. Mater., 2008, 20, 2842.
18. Y.-J. Kim, M. Ebara and T. Aoyagi, Adv. Funct. Mater., 2013, 23, 5753.
19. Y.-J. Kim, M. Ebara and T. Aoyagi, Angew. Chem., Int. Ed., 2012, 51, 10537.
20. A. K. Kota, G. Kwon, W. Choi, J. M. Mabry and A. Tuteja, Nat. Commun., 2012, 3, 1025.
21. E. Ueda and P. A. Levkin, Adv. Mater., 2013, 25, 1234.
22. J. Chapman and F. Regan, Adv. Eng. Mater., 2012, 14, 175.
23. J. Groten, C. Bunte and J. Rühe, Langmuir, 2012, 28, 15038.
24. S. Abrakhi, S. Péralta, O. Fichet, D. Teyssié and S. Cantin, Langmuir, 2013, 29, 9499.
25. J. Gao, Y. L. Liu, H. P. Xu, Z. Wang and X. Zhang, Langmuir, 2010, 26, 9673.
26. L. B. Zhang, Z. H. Zhang and P. Wang, NPG Asia Mater., 2012, 4, 8.
27. E. Stratakis, A. Mateescu, M. Barberoglou, M. Vamvakaki, C. Fotakisae and S. H. Anastasiadis, Chem. Commun., 2010, 46, 4136.
28. C. Jiang, Q. Wang and T. Wang, New J. Chem., 2012, 36, 1641.
29. G. Kwon, A. K. Kota, Y. X. Li, A. Sohani, J. M. Mabry and A. Tuteja, Adv. Mater., 2012, 24, 3666.
30. A. W. Adamson and A. P. Gast, Physical Chemistry of Surfaces, John Wiley-Interscience, New York, 6th edn, 1997.
31. R. N. Wenzel, Ind. Eng. Chem., 1936, 28, 988.
32. A. B. D. Cassie and S. Baxter, Trans. Faraday Soc., 1944, 40, 546.
33. M. Nosonovsky, Langmuir, 2007, 23, 3157.
34. P. F. Hao, C. J. Lv, Z. H. Yao and F. He, Europhys. Lett., 2010, 90, 66003.
35. A. Marmur, Langmuir, 2003, 19, 8343.
36. B. W. Xin and J. C. Hao, Chem. Soc. Rev., 2010, 39, 769.
37. D. Li and Y. Xia, Adv. Mater., 2004, 16, 1151.
38. X. Lu, C. Wang and Y. Wei, Small, 2009, 5, 2349.
39. Y. Konosu, H. Matsumoto, K. Tsuboi, M. Minagawa and A. Tanioka, Langmuir, 2011, 27, 14716.
40. H. L. Che, M. Huo, L. Peng, T. Fang, N. Liu, L. Feng, Y. Wei and J. Y. Yuan, Angew. Chem., 2015, 127, 9062.
41. Z. M. Huang, Y. Z. Zhang, M. Kotaki and S. Ramakrishna, Compos. Sci. Technol., 2003, 63, 2223.
42. S. Megelski, J. S. Stephens, D. Bruce Chase and J. F. Rabolt, Macromolecules, 2002, 35, 8456.
43. K. H. Lee, H. Y. Kim, M. S. Khil, Y. M. Ra and D. R. Lee, Polymer, 2003, 44, 1287.
44. L. Moroni, R. Licht, J. de Boer, J. R. de Wijn and C. A. van Blitterswijk, Biomaterials, 2006, 27, 4911.
45. J. Doshi and D. H. Reneker, J. Electrost., 1995, 35, 151.
46. M. Ouchi, T. Terashima and M. Sawamoto, Chem. Rev., 2009, 109, 4963.
47. W. Chen, L. Qu, D. Chang, L. Dai, S. Ganguli and A. Roy, Chem. Commun., 2008, 163.
48. Q. Fu, G. V. R. Rao, S. B. Basame, D. J. Keller, K. Artyushkova, J. E. Fulghum and G. P. López, J. Am. Chem. Soc., 2004, 126, 8904.
49. R. Barbey, L. Lavanant, D. Paripovic, N. Schüwer, C. Sugnaux, S. Tugulu and H.-A. Klok, Chem. Rev., 2009, 109, 5437.
50. P. Jain, G. L. Baker and M. L. Bruening, Annu. Rev. Anal. Chem., 2009, 2, 387.
51. K. Matyjaszewski and N. Tsarevsky, Nat. Chem., 2009, 1, 276.
52. Q. Chen, E. S. Kooij, X. Sui, C. J. Padberg, M. A. Hempenius, P. M. Schon and G. J. Vancso, Soft Mater., 2014, 10, 3134.
53. M. Koenig, D. Magerl, M. Philipp, K. J. Eichhorn, M. Muller, P. Muller-Buschaum, M. Stamm and P. Uhlmann, RSC Adv., 2014, 4, 17579.
54. A. A. Brown, O. Azzaroni, L. M. Fidalgo and W. T. S. Huck, Soft Matter, 2009, 5, 2738.
55. J. H. Rouse and G. S. Ferguson, J. Am. Chem. Soc., 2003, 125, 15529.
56. T. Soeno, K. Inokuchi and S. Shiratori, Appl. Surf. Sci., 2004, 237, 539.
57. R. M. Jisr, H. H. Rmaile and J. B. Schlenoff, Angew. Chem., Int. Ed., 2005, 44, 782.
58. L. Zhai, F. Ç. Cebeci, R. E. Cohen and M. F. Rubner, Nano Lett., 2004, 4, 1349.
59. J. T. Han, S. Kim and A. Karim, Langmuir, 2007, 23, 2608.
60. Y. M. Lu, M. A. Sarshar, K. Du, T. Chou, C. H. Choi and S. A. Sukhishvili, ACS Appl. Mater. Interfaces, 2013, 5, 12617.
61. J. Yang, Z. Z. Zhang, X. H. Men, X. H. Xu, X. T. Zhu and X. Y. Zhou, Langmuir, 2011, 27, 7357.
62. G. M. Whitesides and M. Boncheva, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4769.
63. I. W. Hamley, Angew. Chem., Int. Ed., 2003, 42, 1692.
64. F. M. Liu, J. Pang, C. Y. Wang and L. Y. Wang, Langmuir, 2013, 29, 13003.
65. C. F. Jin, R. S. Yan and J. G. Huang, J. Mater. Chem., 2011, 21, 17519.
66. Z. L. Wu, R. B. Wei, A. Buguin, J. Taulemesse, N. L. Moigne, A. Bergeret, X. G. Wang and P. Keller, ACS Appl. Mater. Interfaces, 2013, 5, 7485.
67. C. Li, R. W. Guo, X. Jiang, S. X. Hu, L. Li, X. Y. Cao, H. Yang, Y. L. Song, Y. M. Ma and L. Jiang, Adv. Mater., 2009, 21, 4254.
68. B. Wang and Z. G. Guo, Chem. Commun., 2013, 49, 9416.
69. X. Zhang, Y. G. Guo, P. Y. Zhang, Z. S. Wu and Z. J. Zhang, ACS Appl. Mater. Interfaces, 2012, 4, 1742.
70. L. Y. Xu, Q. Ye, X. M. Lu and Q. H. Lu, ACS Appl. Mater. Interfaces, 2014, 6, 14736.
71. C. Feng, Y. Zhang, J. Jin, Y. Song, L. Xie, G. Qu, L. Jiang and D. Zhu, Langmuir, 2001, 17, 4593.
72. J. Byun, J. Shin, S. Kwon, S. Jang and J. K. Kim, Chem. Commun., 2012, 48, 9278.
73. G. Y. Wang and T. Y. Zhang, ACS Appl. Mater. Interfaces, 2012, 4, 273.
74. S. T. Wang, Y. L. Song and L. Jiang, J. Photochem. Photobiol., C, 2007, 8, 18.
75. S. H. Anastasiadis, M. I. Lygeraki, A. Athanassiou, M. Farsari and D. Pisignano, J. Adhes. Sci. Technol., 2008, 22, 1853.
76. H. S. Lim, D. Kwak, D. Y. Lee, S. G. Lee and K. Cho, J. Am. Chem. Soc., 2007, 129, 4128.
77. X. J. Feng, J. Zhai and L. Jiang, Angew. Chem., Int. Ed., 2005, 44, 5115.
78. X. T. Zhang, M. Jin, Z. Y. Liu, D. A. Tryk, S. Nishimoto, T. Murakami and A. Fujishima, J. Phys. Chem. C, 2007, 111, 14521.
79. X. J. Feng, L. Feng, M. H. Jin, J. Zhai, L. Jiang and D. B. Zhu, J. Am. Chem. Soc., 2004, 126, 62.
80. M. Guo, P. Diao and S. M. Cai, Thin Solid Films, 2007, 515, 7162.
81. W. Zhu, X. Feng, L. Feng and L. Jiang, Chem. Commun., 2006, 26, 2753.
82. L. Y. Gao, M. J. Zheng, M. Zhong, M. Li and L. Ma, Appl. Phys. Lett., 2007, 91, 013101.
83. S. Wang, X. Feng, J. Yao and L. Jiang, Angew. Chem., Int. Ed., 2006, 45, 1264.
84. Z. K. Tang, G. K. L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma and Y. Segawa, Appl. Phys. Lett., 1998, 72, 3270.
85. Z. W. Pan, Z. R. Dai and Z. L. Wang, Science, 2001, 291, 1947.
86. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, R. Weber, R. Russo and P. Yang, Science, 2001, 292, 1897.
87. Z. R. Tian, J. A. Voigt, J. Liu, B. Mckenzie and M. J. Mcdermott, J. Am. Chem. Soc., 2002, 124, 12954.
88. L. E. Greene, M. Law, J. Goldberger, F. Kim, J. C. Johnson, Y. Zhang, R. J. Saykally and P. Yang, Angew. Chem., Int. Ed., 2003, 42, 3031.
89. K. Liu, M. Cao, A. Fujishima and L. Jiang, Chem. Rev., 2014, 114, 1044.
90. H. Wang and Z. Guo, Appl. Phys. Lett., 2014, 104, 183703.
91. D. Wang, Y. Liu, X. Liu, F. Zhou, W. Liu and Q. Xue, Chem. Commun., 2009, 7018.
92. Y. Li, G. Duan, G. Liu and W. Cai, Chem. Soc. Rev., 2013, 42, 3614.
93. Y. Li, W. Cai, G. Duan, B. Cao, F. Sun and F. Lu, J. Colloid Interface Sci., 2005, 287, 634.
94. D. L. Tian, X. F. Zhang, Y. Tian, Y. Wu, X. Wang, J. Zhai and L. Jiang, J. Mater. Chem., 2012, 22, 19652.
95. J. L. Yong, F. Chen, Q. Yang, Y. Fang, J. L. Huo and X. Hou, Chem. Commun., 2015, 51, 9813.
96. R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi and T. Watanabe, Nature, 1997, 388, 431.
97. E. Balaur, J. M. Macak, L. Taveira and P. Schmuki, Electrochem. Commun., 2005, 7, 1066.
98. Y. K. Lai, Y. X. Huang, H. Wang, J. Y. Huang, Z. Chen and C. J. Lin, Colloids Surf., B, 2010, 76, 117.
99. T. Darmanin, E. Taffin de Givenchy, S. Amigoni and F. Guittard, Adv. Mater., 2013, 25, 1378.
100. A. Fujishima, X. T. Zhang and D. A. Tryk, Surf. Sci. Rep., 2008, 63, 515.
101. L. D. Sun, X. Y. Wang, M. L. Li, S. Zhang and Q. Wang, Langmuir, 2014, 30, 2835.
102. D. Gong, C. A. Grimes, O. K. Varghese, W. C. Hu, R. S. Singh, Z. Chen and E. C. Dickey, J. Mater. Res., 2001, 16, 3331.
103. Y. K. Lai, L. Sun, Y. C. Chen, H. F. Zhuang, C. J. Lin and J. W. Chin, J. Electrochem. Soc., 2006, 153, 123.
104. Y. K. Lai, J. Y. Huang, H. F. Zhuang, V. P. Subramaniam, Y. X. Tang, D. G. Gong, L. Sundar, L. Sun, Z. Chen and C. J. Lin, J. Hazard. Mater., 2010, 184, 855.
105. I. Paramasivam, H. Jha, N. Liu and P. Schmuki, Small, 2012, 8, 3073.
106. J. Y. Huang, Y. K. Lai, L. Wang, S. H. Li, M. Z. Ge, K. Q. Zhang, H. Fuchs and L. F. Chi, J. Mater. Chem. A, 2014, 2, 18531.
107. Z. Xue, M. Liu and L. Jiang, J. Polym. Sci., Part B: Polym. Phys., 2012, 50, 1209.
108. Z. Xue, Y. Cao, N. Liu, L. Feng and L. Jiang, J. Mater. Chem. A, 2014, 2, 2445.
109. J. L. Yong, F. Chen, Q. Yang, U. Farooq and X. Hou, J. Mater. Chem. A, 2015, 3, 10703.
110. N. Delorme, J. F. Bardeau, A. Bulou and F. Poncin-Epaillard, Langmuir, 2005, 21, 12278.
111. R. Rosario, D. Gust, M. Hayes, F. Jahnke, J. Springer and A. A. Garcia, Langmuir, 2002, 18, 8062.
112. K. Matsuda and M. Irie, J. Photochem. Photobiol., C, 2004, 5, 169.
113. T. Kudernac, S. J. van der Molen, B. J. van Wees and B. L. Feringa, Chem. Commun., 2006, 3597.
114. C. Radüge, G. Papastavrou, D. G. Kurth and H. Motschmann, Eur. Phys. J. E: Soft Matter Biol. Phys., 2003, 10, 103.
115. L. M. Siewierski, W. J. Brittain, S. Petrash and M. D. Foster, Langmuir, 1996, 12, 5838.
116. Y. Wu, Z. L. Liu, Y. M. Liang, X. W. Pei, F. Zhou and Q. J. Xue, Soft Matter, 2014, 10, 5318.
117. B. C. Bunker, B. I. Kim, J. E. Houston, R. Rosario, A. A. Garcia and M. Hayes, Nano Lett., 2003, 3, 1723.
118. Y. N. Zhou, J. J. Li, Q. Zhang and Z. H. Luo, Langmuir, 2014, 30, 12236.
119. S. Samanta and J. Locklin, Langmuir, 2008, 24, 9558.
120. R. Klajn, Chem. Soc. Rev., 2014, 43, 148.
121. A. V. Lyubimov, N. L. Zaichenko and V. S. Marevtsev, J. Photochem. Photobiol., A, 1999, 120, 55.
122. V. Malatesta, J. Hobley and C. Salemi-Delvaux, Mol. Cryst. Liq. Cryst., 2000, 344, 69.
123. G. Such, R. A. Evans, L. H. Yee and T. P. Davis, J. Macromol. Sci., Polym. Rev., 2003, 43, 547.
124. H. Schenderlein, A. Voss, R. W. Stark and M. Biesalski, Langmuir, 2013, 29, 4525.
125. A. Uyama, S. Yamazoe, S. Shigematsu, M. Morimoto, S. Yokojima, H. Mayama, Y. Kojima, S. Nakamura and K. Uchida, Langmuir, 2011, 27, 6395.
126. S. Lin, K. Chen and R. Liang, Polymer, 1999, 40, 2619.
127. T. Sun, W. Song and L. Jiang, Chem. Commun., 2005, 1723.
128. A. O. Govorov and H. H. Richardson, Nano Today, 2007, 2, 30.
129. F. Y. Cheng, C. H. Su, P. C. Wu and C. S. Yeh, Chem. Commun., 2010, 46, 3167.
130. X. Feng and L. Jiang, Adv. Mater., 2006, 18, 3063.
131. T. Sun, G. Wang, L. Feng, B. Liu, Y. Ma, L. Jiang and D. Zhu, Angew. Chem., Int. Ed., 2004, 43, 357.
132. D. K. Owens and R. C. Wendt, J. Appl. Polym. Sci., 1969, 13, 1741.
133. L. Chen, M. J. Liu, L. Lin, T. Zhang, J. Ma, Y. L. Song and L. Jiang, Soft Matter, 2010, 6, 2708.
134. G. T. Hermanson, Bioconjugate Techniques, Academic Press, Oxford, UK, 2008, p. 562.
135. H. L. Liu, X. Q. Zhang, S. T. Wang and L. Jiang, Small, 2014, 3, 190.
136. A. Leon and R. C. Advincula, ACS Appl. Mater. Interfaces, 2014, 6, 22666.
137. E. Kim, Y. Xia, X. M. Zhao and G. M. Whitesides, Adv. Mater., 1997, 9, 651.
138. Y. F. Wang, C. L. Lai, H. W. Hu, Y. Liu, B. Fei and J. H. Xin, RSC Adv., 2015, 5, 51078.
139. B. Xue, L. Gao, Y. Hou, Z. Liu and L. Jiang, Adv. Mater., 2013, 25, 273.
140. J. J. Li, Y. N. Zhou and Z. H. Luo, Ind. Eng. Chem. Res., 2014, 53, 18112.
141. W. A. Braunecker and K. Matyjaszewski, Prog. Polym. Sci., 2007, 32, 93.
142. Q. Yu, Y. X. Zhang, H. Chen, F. Zhou, Z. Q. Wu, H. Huang and J. L. Brash, Langmuir, 2010, 26, 8582.
143. X. J. Shi, G. J. Chen, Y. W. Wang, L. Yuan, Q. Zhang, D. M. Haddleton and H. Chen, Langmuir, 2013, 29, 14188.
144. J. M. Yang, M. Hida, S. F. Mao, H. L. Zeng, H. Nakajima and K. Uchiyama, Chem. Commun., 2014, 50, 10265.
145. H. Vihola, A. Laukkanen, L. Valtola, H. Tehnhu and J. Hirnoven, Biomaterials, 2005, 26, 3055.
146. J. Yang, Z. Z. Zhang, X. H. Men, X. H. Xu and X. T. Zhu, Carbon, 2011, 49, 19.
147. D. S. Bai, B. M. Habersberger and G. K. Jennings, J. Am. Chem. Soc., 2005, 127, 16486.
148. Y. Liu, M. Zhao, D. E. Bergbreiter and R. M. Crooks, J. Am. Chem. Soc., 1997, 119, 8720.
149. Z. Hou, N. L. Abbott and P. Stroeve, Langmuir, 2000, 16, 2401.
150. E. S. Gil and S. A. Hudson, Prog. Polym. Sci., 2004, 29, 1173.
151. Q. L. Zhang, F. Xia, T. L. Sun, W. L. Song, T. Y. Zhao, M. C. Liu and L. Jiang, Chem. Commun., 2008, 1199.
152. S. Y. Liu and S. P. Armes, Angew. Chem., Int. Ed., 2002, 41, 1413.
153. C. H. Lee, S. K. Kang, J. A. Lim, H. S. Lim and J. H. Cho, Soft Matter, 2012, 8, 10238.
154. W. Sun, S. X. Zhou, B. You and L. M. Wu, Macromolecules, 2013, 46, 7018.
155. N. Faucheux, R. Schweiss, K. Lutzow, C. Werner and T. Groth, Biomaterials, 2004, 25, 2721.
156. M. Guix, J. Orozco, M. García, W. Gao, S. Sattayasamitsathit, A. Merkoçi, A. Escarpa and J. Wang, ACS Nano, 2012, 6, 4445.
157. J. X. Wang, J. P. Hu, Y. Q. Wen, Y. L. Song and L. Jiang, Chem. Mater., 2006, 18, 4984.
158. E. Kharlampieva and S. A. Sukhishvili, Polym. Rev., 2006, 46, 377.
159. B. Wang, Z. G. Guo and W. M. Liu, RSC Adv., 2014, 4, 14684.
160. M. J. Cheng, Q. Liu, G. N. Ju, Y. J. Zhang, L. Jiang and F. Shi, Adv. Mater., 2014, 26, 306.
161. X. X. Chen, R. Ferrigno, J. Yang and G. Whitesides, Langmuir, 2002, 18, 7009.
162. F. Mugele and J.-C. Baret, J. Phys.: Condens. Matter, 2005, 17, 705.
163. C. Bodre and T. Pauporte, Adv. Mater., 2009, 21, 697.
164. B. Kakade, R. Mehta, A. Durge, S. Kulkarni and V. Pillai, Nano Lett., 2008, 8, 2693.
165. J. Y. Chen, A. Kutana, C. P. Collier and K. P. Giapis, Science, 2005, 310, 1480.
166. T. N. Krupenkin, J. A. Taylor, E. N. Wang, P. Kolodner, M. Hodes and T. R. Salamon, Langmuir, 2007, 23, 9128.
167. J. Lahann, S. Mitragotri, T. N. Tran, H. Kaido, J. Sundaram, I. S. Choi, S. Hoffer, G. A. Somorjai and R. Langer, Science, 2003, 299, 371.
168. N. Verplanck, E. Galopin, J. C. Camart and V. Thomy, Nano Lett., 2007, 7, 3.
169. Z. J. Han, B. Tay, C. Tan, M. Shakerzadeh and K. Ostrikov, ACS Nano, 2009, 3, 3031.
170. X. D. Zhao, H. M. Fan, J. Luo, J. Ding, X. Y. Liu, B. S. Zou and Y. P. Feng, Adv. Funct. Mater., 2011, 21, 184.
171. Y. Q. Teng, Y. Q. Zhang, L. P. Heng, X. F. Meng, Q. W. Yang and L. Jiang, Materials, 2015, 8, 1817.
172. R. B. Pernites, R. R. Ponnapati and R. C. Advincula, Adv. Mater., 2011, 23, 3207.
173. T. Darmanin, E. T. de Givenchy, S. Amigoni and F. Guittard, Adv. Mater., 2013, 25, 1378.
174. D. Mecerreyes, V. Alvaro, I. Cantero, M. Bengoetxea, P. A. Calvo, H. Grande, J. Rodriguez and J. A. Pomposo, Adv. Mater., 2002, 14, 749.
175. L. Xu, W. Chen, A. Mulchandani and Y. Yan, Angew. Chem., Int. Ed., 2005, 44, 6009.
176. J. Zhang, J. Li and Y. Han, Macromol. Rapid Commun., 2004, 25, 1105.
177. J. L. Zhang, X. Y. Lu, W. H. Huang and Y. C. Han, Macromol. Rapid Commun., 2005, 26, 477.
178. X. Huang, Y. J. Sun and S. Soh, Adv. Mater., 2015, 27, 4062.
179. Y. Liu, L. Mu, B. H. Liu and J. L. Kong, Chem.–Eur. J., 2005, 11, 2622.
180. S. Minko, M. Mueller, M. Motornov, M. Nitschke, K. Grundke and M. Stamm, J. Am. Chem. Soc., 2003, 125, 3896.
181. M. Motornov, S. Minko, K. J. Eichhorn, M. Nitschke, F. Simon and M. Stamm, Langmuir, 2003, 19, 8077.
182. X. Y. Song, M. W. Cao, Y. C. Han, Y. L. Wang and J. C. T. Kwak, Langmuir, 2007, 23, 4279.
183. X. Wang, G. G. Qing, L. Jiang, H. Fuchs and T. L. Sun, Chem. Commun., 2009, 2658.
184. I. Okamoto, M. Nabeta, M. Yamamoto, M. Mikami, T. Takeya and O. Tamura, Tetrahedron Lett., 2006, 47, 7143.
185. S. Toshikatsu, Ion Exchange Membranes: Preparation, Characterization, Modification and Application, RSC, Cambridge, 2004.
186. H. S. Lim, S. G. Lee, D. H. Lee, D. Y. Lee, S. Lee and K. Cho, Adv. Mater., 2008, 20, 4438.
187. Z. Hua, J. Yang, T. Wang, G. M. Liu and G. Z. Zhang, Langmuir, 2013, 29, 10307.
188. L. X. Xu, N. Liu, Y. Z. Cao, F. Lu, Y. N. Chen, X. Y. Zhang, L. Feng and Y. Wei, ACS Appl. Mater. Interfaces, 2014, 6, 13324.
189. W. Q. Zhu, J. Zhai, Z. W. Sun and L. Jiang, J. Phys. Chem. C, 2008, 112, 8339.
190. Y. Zhu, J. M. Li, H. Y. He, M. X. Wan and L. Jiang, Macromol. Rapid Commun., 2007, 28, 2230.
191. S. J. Lin and P. Theato, Macromol. Rapid Commun., 2013, 34, 1118.
192. Q. Yan, R. Zhou, C. Fu, H. Zhang, Y. Yin and J. Yuan, Angew. Chem., Int. Ed., 2011, 50, 4923 (Angew. Chem., 2011, 123, 5025).
193. Q. Yan, J. Wang, Y. Yin and J. Yuan, Angew. Chem., Int. Ed., 2013, 52, 5070.
194. Q. Yan and J. Yuan, Chemical Journal of Chinese Universities, 2012, 33, 1877.
195. A. Feng, Q. Yan and J. Yuan, Prog. Chem., 2012, 24, 1995.
196. D. Han, X. Tong, O. Boissière and Y. Zhao, ACS Macro Lett., 2011, 1, 57.
197. Q. Yan and Y. Zhao, J. Am. Chem. Soc., 2013, 135, 16300.
198. N. Li, L. Thia and X. Wang, Chem. Commun., 2014, 50, 4003.
199. J. Kim, S. E. Chung, S. E. Choi, H. Lee, J. Kim and S. Kwon, Nat. Mater., 2011, 10, 747.
200. P. J. Glazer, J. Leuven, H. An, S. G. Lemay and E. Mendes, Adv. Funct. Mater., 2013, 23, 2964.
201. F. Khademolhosseini and M. Chiao, J. Microelectromech. Syst., 2013, 22, 131.
202. F. Fahrni, M. W. J. Prins and L. J. van IJzendoorn, Lab Chip, 2009, 9, 3413.
203. A. Grigoryev, I. Tokarev, K. G. Kornev, I. Luzinov and S. Minko, J. Am. Chem. Soc., 2012, 134, 12916.
204. Z. Cheng, H. Lai, N. Q. Zhang, K. N. Sun and L. Jiang, J. Phys. Chem. C, 2012, 116, 18796.
205. W. Laure, P. Woisel and J. Lyskawa, Chem. Mater., 2014, 26, 3771.
206. S. Wang, H. Liu, D. Liu, X. Ma, X. Fang and L. Jiang, Angew. Chem., Int. Ed., 2007, 46, 3915.
207. G. Y. Qing, X. Wang, L. Jiang, H. Fuchs and T. L. Sun, Soft Matter, 2009, 5, 2759.
208. F. Xia, L. Feng, S. T. Wang, T. L. Sun, W. L. Song, W. H. Jiang and L. Jiang, Adv. Mater., 2006, 18, 432.
209. Y. Z. Cao, N. Liu, C. K. Fu, K. Li, L. Tao, L. Feng and Y. Wei, ACS Appl. Mater. Interfaces, 2014, 6, 2026.
210. W. Sun, S. X. Zhou, B. You and L. M. Wu, J. Mater. Chem. A, 2013, 1, 10646.
211. S. P. Wu, X. Q. Zhu, J. L. Yang and J. Nie, Chem. Commun., 2015, 51, 5649.
212. M. S. Han, X. Y. Zhang, L. Li, C. Peng, L. Bao, E. C. Ou, Y. Q. Xiong and W. J. Xu, eXPRESS Polym. Lett., 2014, 8, 528.
213. D. L. Tian, J. Zhai, Y. L. Song and L. Jiang, Adv. Funct. Mater., 2011, 21, 4519.
214. F. Xia, H. Ge, Y. Hou, T. L. Sun, L. Chen, G. Z. Zhang and L. Jiang, Adv. Mater., 2007, 19, 2520.
215. W. Sun, S. X. Zhou, B. You and L. M. Wu, J. Mater. Chem. A, 2013, 1, 3146.
216. B. Wang, W. X. Liang, Z. G. Guo and W. M. Liu, Chem. Soc. Rev., 2015, 44, 336.

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