Article pubs.acs.org/Langmuir
Bioinspired Multifunctional Au Nanostructures with Switchable Adhesion Xiu Mo, Yunwen Wu, Junhong Zhang, Tao Hang,* and Ming Li* State Key Laboratory of Metal Matrix Composites, School of Material Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Road, Shanghai 200240, China S Supporting Information *
ABSTRACT: Inspired by the self-cleaning of cicada wings, well-aligned Aucoated Ni nanocone arrays (Au@Ni NAs) have been fabricated by a simple and cheap electrodeposition method. After surface modification of nhexadecanethiol, self-cleaning can be realized on this long-lived superhydrophobic surface with extremely low adhesive force. Switchable adhesion is obtained on its complementary porous surface. The porous Au structure is fabricated by a geometric replica of the nanocone arrays. After the same surface modification, it shows superhydrophobicity with high adhesion. The different adhesive behaviors on the two lock-and-key Au structures are ascribed to their different contact modes with a water droplet. Combining the superhydrophobic properties of the two complementary structures, they can be used to transport precious microdroplets without any loss. The bioinspired periodic Au@Ni NAs can also be potentially employed as surface-enhanced Raman scattering (SERS) substrates due to its electromagnetic enhancement effect, especially at the tips of the nanocones. Thus, superhydrophobic, SERS, long-lived, self-cleaning, microtransportation functions are realized on the basis of the two surfaces.
1. INTRODUCTION In nature, great numbers of creatures possess special surface characteristics because of the presence of almost perfect micronanostructures over a wide range.1−3 Plant leaves and insect wings, such as lotus leaves,4,5 rice leaves,6,7 cicada wings,8,9 and butterfly wings,2,10 are self-cleaning, with water rolling off quickly, while some structures, such as gecko feet,11 rose petals,12,13 and peanut leaves,14 show high adhesion to the water droplet. The difference in the adhesive behavior is due to different surface chemical compositions and structures. Controlled adhesion was achieved on the superhydrophobic surfaces by designing a heterogeneous chemical composition on hierarchically structured copper substrate,15 and patterned superhydrophobic TiO2 nanotube array surfaces with high wettability and adhesion contrast were fabricated by a combination method for site-selective alcohol-based ink patterning.16 The nanostructure of the superhydrophobic surface can induce different solid−liquid contact methods, playing an important role in controlling the adhesive strength of water droplets on the surface.17−19 A bioinspired multifunctional TiO2 nanostructure surface with controllable wettability and adhesion for liquid manipulation was realized by simply adjusting the physical structure,20 and a controllable adhesive superhydrophobic surface based on PDMS microwell arrays was produced by adjusting the extent of overlap of the adjacent microwells.21 Inspired by nature, lots of artificial structures mimicking the unique micronanostructures of natural creature surfaces have © XXXX American Chemical Society
been reported. For example, an artificial rose petal surface was prepared from poly(dimethylsiloxane) (PDMS) by a casting technique,22 biomimetic superhydrophobic surfaces on a regular hierarchical polymer substrate were fabricated by heatand-pressure-driven imprinting methods using patterned AAOs as replication templates,23 and lotus-leaf heterohierarchical micro/nanostructure tetragonal arrays were prepared via a primary cell-induced deposition and a galvanic displacement reaction combined with a photolithography technique on Cu foil.24 Most of the methods are complex and time-consuming and could hardly be widely used. Thus, finding a practical way to fabricate the biomimic nanostructures with special functional use remains challenging. The cicada wing has attracted researchers’ eyes due to its unique, evenly distributed nanopillar structure.8,25 It was reported to be used as a model superhydrophobic surface to study its self-cleaning by a self-propelled jumping condensate.9 The uniform nanopillar arrays not only can induce superhydrophobicity but also can trigger other functions.26 For instance, a large-area, high-performance surface-enhanced Raman scattering (SERS) substrate was fabricated by depositing a Au film on a cicada wing.27 SERS spectroscopy has been developed rapidly for its tremendous potentials in chemical and biological sensing Received: July 5, 2015 Revised: August 26, 2015
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Langmuir applications due to its high sensitivity and rapid response.28−30 The SERS effect is due to the existence of an enormous electromagnetic enhancement in the gap of metal nanostructures, which is called a hot spot.31,32 The surface plasmonic coupling between neighboring nanounits is believed to be the main reason for the enormous electromagnetic enhancement. Compared to the nanoparticle substrates, the ordered nanopillar/nanocone array substrates are more uniform and reproducible, which is more practical.30,33−35 For instance, optical fiber SERS sensors fabricated by the nanoimprint lithography of cicada wing nanostructures offer a potential solution for monitoring low chemical concentrations in remote or in situ sensing scenarios.36 Herein, we report multifunctional Au-coated Ni nanocone arrays (Au@Ni NAs) mimicking the cicada wing by a simple and cheap electrodeposition method. Au, as a desired SERS material and a metal with high stability, is an ideal surface material for causing the substrate to remain stable and functional. Furthermore, we investigate the impact of surface structures on superhydrophobic performance by fabricating their opposite structure based on geometric replicas of the nanocones. We call this opposite structure a porous surface hereafter. The fabrication process is described in Scheme 1. Ni
2. EXPERIMENTAL SECTION 2.1. Materials. Copper plate (cold-rolled alloy C194) is commercially available. All solvents and chemicals were of reagent quality without further purification. Gold plating bath A is a citrate acid system, and gold plating bath B is a noncyanide sulfite system obtained from Beijing Wenyi Technology Co. Ltd. 2.2. Fabrication of Au@Ni NAs. The copper plates were pretreated by electrolytic degreasing and an acid cleaning process and then rinsed with deionized water. To fabricate the Au@Ni NAs, the first step was the preparation of Ni nanocones. Ni NAs were electrodeposited on the pretreated Cu plate at a constant current density of 1 A/dm2 (ASD) for 20 min. The electrolyte was composed of analytically pure NiCl2·6H2O(1 M), H3BO3 (0.5 M), and an ethylenediamine dihydrochloride (1.5 M) crystal modifier, as reported by Hang et al.37 Next, a thin layer of Au film was electrodeposited on the as-prepared Ni NAs in gold plating bath A at a constant density of 0.1 ASD for 5 min. The samples were rinsed with distilled water and dried at room temperature. 2.3. Fabrication of an Opposite Porous Au Surface. On the basis of a geometric replica, the opposite structure of NAs was fabricated. The specific process used to fabricate the porous surface is illustrated as follows. First, a thick layer of Au film (2 μm) was deposited on the prepared Ni NAs in gold plating bath B at a constant current density of 0.5 ASD for 10 min. After being rinsed with deionized water, the prepared specimen was immersed in the 20% HNO3 for 24 h. A porous surface was fabricated after the Cu and Ni corrosion was complete in HNO3. The sample was rinsed and dried in the ambient atmosphere. 2.4. Characterization. The surface morphologies and composition of the as-prepared samples were investigated by scanning electron microscopy (SEM,FEI SIRON 200) equipped with an attached EDS system. Transmission electron microscopy (TEM) images were recorded on JEM-2010 (200 kV). The thickness of the Au film was measured by X-ray fluorescent spectroscopy (XFS, Fisherscope X-ray XUL-XYm). The X-ray diffraction pattern was recorded from 20 to 100°, using a Rigaku Ultima IV X-ray diffractometer with Cu Ka radiation (λ = 0.15418 nm). The water contact angle (CA) and sliding angle (SA) were measured with a 4 μL water droplet at ambient temperature using an optical contact angle meter (Data physics OCA20). Prior to the measurements, the as-prepared samples were immersed in the ethanol solution (0.02 M) of n-hexadecanethiol for 30 min to form a self-assembled monolayer. Then they were cleaned ultrasonically with ethanol for 10 min and dried at room temperature. SERS measurements were carried out with a dispersive Raman microscope (Senterra R200-L, Bruker Optics) with a 523 nm Ar ion laser. An irradiation power of 20 mW was used to excite the samples. The integration time was 5 s. Rhodamine 6G (R6G) dye was used as a Raman probe molecule for the SERS measurements.
Scheme 1. Fabrication of Au-Coated Ni Nanocone Arraysa
a Schematic illustration of the fabrication of Au@Ni NAs (a−c) and their opposite porous surfaces (a, b, d, e). (f) Photograph of a cicada and atomic force microscope (AFM) image of the nanostructures on the cicada wing.9
3. RESULTS AND DISCUSSION 3.1. Morphology and Crystal Analysis. The biomimetic Au@Ni NAs and their opposite porous surface were fabricated by the electrodeposition method and geometric replica. They are complementary lock-and-key surfaces. Figure 1 shows the SEM images of the Au@Ni NAs and the porous Au surface. Large-area uniform arrays of vertically aligned Au@Ni NAs on the flat Cu plate are shown in Figure 1a. The average root diameter of the nanocones is estimated to be about 300 nm, and the height is about 800 nm, respectively. The Au@Ni nanocones are grown perpendicularly to the Cu substrate densely and evenly, with a density of about 1.6 × 109 cones/ cm2. Steps on each nanocone surface can be seen clearly, according to the high-magnification SEM image as shown in Figure 1b. Figure 2a−e illustrates the morphology change with the increase in the deposition time of Au. We can see that as the deposition time of Au increased from 5 to 60 min, the structure transformed from nanocone to pinelike due to the
nanocones were electrodeposited onto the Cu substrate first, followed by the deposition of a thin layer of Au film to prepare the Au@Ni NAs. Scheme 1f illustrates the AFM image of the nanostructures on the cicada wing,9 from which we can see that well-alligned and evenly distributed nanopillars are grown on the cicada surface. Opposite porous structure was obtained via a geometric replica of the as-preapred Ni nanocones. Switchable adhesion of the two Au nanostructures has been investigated when the surface chemical composition remained constant. The low-adhesion nanocone surface is self-cleaning while the highadhesion porous surface can be used as a mechanical hand to transport precious microliquids. Additionally, the ordered Au@ Ni NAs surface can enhance Raman scattering and has the potential to be used as a large-area, low-cost SERS substrate. B
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Figure 1. (a, b) Oblique-view SEM images of the Au@Ni NAs and (c, d) top-view SEM images of the opposite porous surface with different magnifications.
laminar growth of Au in different directions.38,39 At a time of 5 min, the structure is mostly mimicking the cicada wing, as illustrated in Figure 1a,b. Hereafter, the Au@Ni NAs refer to the structure with a Au deposition time of 5 min. Figure S1 is the spectrum of EDS of the Au@Ni NAs surface. The strong peak of Ni comes from the Ni nanocones beneath the Au film. The peak of Au can be observed but is relatively weak because the Au coating is quite thin, with tens of nanometers. Figure 3a−c shows the growth process of Au on the Ni nanocone surface. Figure 3a is the TEM image of the Ni nanocone. In correspondence with the SEM result, Ni nanocones have sharp tips and steps on the surface. After depositing Au for 30 s on the Ni nanocones, Au nanospheres with an average diameter of 20 nm grew on the Ni surface due to the heterogeneous nucleation. The Au nanospheres were arranged one by one and densely covered the Ni surface. With the Au deposition time increased to 60 s, no Au nanospheres could be observed. Instead, a compact Au film covered the surface of the Ni nanocones after depositing Au for 60 s. Comparing the TEM image of the Au coating for 60 s to that of the Ni nanocone (Figure 3a,c), it is not hard to observe that the tips became less sharp due to the coverage of the Au film. In the HRTEM image of the Au-coating nanocone for 60 s, the crystalline plane in Figure 3d is calculated to be 2.35 Å, which corresponds to the (111) reflection of the Au face-centered cubic (fcc) structure. Opposite cicada wing structure, namely, the porous surface, was fabricated by geometric replicas of the nanocone arrays. Typical SEM images with different magnifications of the porous surface are depicted in Figure 1c,d. Figure 1c illustrates the lowmagnification SEM image of the porous surface, demonstrating the evenly and densely distributed pores. Contrary to the
nanocone array, the porous surface is a complementary structure composed of numerous inverted conical holes with a diameter of about 300 nm and an estimated depth of 800 nm according to the high-magnification SEM image in Figure 1d. There are sharp steps on the inner surface of those holes, corresponding to the steps on the surface of the Ni nanocones, which indicates that the Au film fully fits the Ni surface. From the X-ray diffraction (XRD) patterns of the porous surfaces as shown in Figure S2, we can determine that the diffraction peaks correspond to (111), (200), (220), (311), and (222) diffraction peaks of metallic Au, demonstrating that the porous structure is composed of pure Au with face-centered cubic (fcc) structures. 3.2. Tunable Superhydrophobicity on the Lock and Key Surfaces. The wettability of the above prepared samples has been explored. A number of chemical surface treatments have been employed to lower the surface energy, such as forming silane-based fluorinated and synthetic polymer lowenergy surfaces.5,40,41 In our study, environmentally friendly nhexadecanethiol was used to modify the prepared surface. After n-hexadecanethiol modification, the contact angle grew from 68 to 165° (Figure S3) for the Au@Ni NAs and showed excellent superhydrophobicity compared to those structures with different Au deposition times (Figure 2f).The sliding angle of the Au@Ni NAs was 4°, showing extremely low adhesion to water droplets. As shown in Figure S4, when a 4 μL water droplet was placed on the Au@Ni NAs, the water droplet rolled off quickly due to the low adhesion. Figure 4c is the sliding moment of the water droplet. After storage in air at room temperature for 1 year, the biomimic structure still shows excellent superhydrophobicity with a contact angle larger than 160°, as shown in Figure S5.This may be attributed to the intrinsic nature of Au, which is stable and antioxidized.42,43 C
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Figure 2. SEM images of Au deposited on Ni nanocones for (a) 0, (b) 5, (c) 10, (d) 30, and (e) 60 min. (f) Changes in CA and SA corresponding to the roughness factor (Au deposition time).
Figure 4a. Liquids are assumed to contact only the sharp tips of the naonocones, and air pockets are trapped beneath the liquid, which gives a composite state. In this state, the air parts of the surface can be considered to be completely nonwetting.44,45 The contact angle of the modified Au@Ni NAs can be described in terms of the Cassie equation
For the porous structure, after some surface modification, the contact angle grew from 56 to 154° (Figure S6). Interestingly, in contrast to the rolling behavior of a water droplet on the Au surfaces with nanocone topography, the behavior of a water droplet on the porous surface is totally different: the water droplet is pinned firmly on the porous surface and can resist against the gravitational force when the surface is upside down (180°) as shown in Figure 4f, indicating that strong adhesion between the surface and the water droplet is generated. The switchable adhesive performances were achieved by tailoring the nanostructures of the Au surface while keeping the surface chemical composition constant. The Cassie state is established to explain the wettability of the Au@Ni NAs, and the schematic diagram is illustrated in
cos θc = f1 cos θ − f2
(1)
where θc and θ are the water CAs on rough and flat Au surfaces, respectively. f1 and f 2 are the area fractions of the solid and vapor on the surface (i.e., f1 + f 2 = 1). The water contact angle θc on the Au@Ni NAs after surface modification was 165°, and the contact angle θ on the modified flat Au substrate was 110° (Figure S7). Using these values, we calculated f1 and f 2 to be D
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Figure 3. TEM images of (a) Ni nanocone, (b) Ni nanocone-deposited Au for 30 s, and (c) Ni nanocone-deposited Au for 60 s. (d) HRTEM image of the Au coating for 60 s.
smaller area fraction of the vapor on the surface will lead to a decrease in the contact angle. Therefore, with the increase in Au deposition time, the air volume beneath the water droplet decreases and leads to a decrease in the contact angle. On the other hand, according to the SEM images of the Ni nanocone with a Au coating for various durations, antennas appear on the surface of the nanocone first and then grow into flakes when the deposition time increases to 60 min, also contributing to the roughness increase. The top of the nanocone, where the structure and the water are in contact, changes from point to area. This change will induce a greater van der Waals force between water and the surface, thus causing an increase in the sliding angle. The low-adhesive superhydrophobic surface could be switched to a high-adhesive state by constructing a complementary structure of the nanocone surface. When a water droplet contacts the solid surface, sealed air pockets could be formed on the porous surface, while open air pockets could be formed on the nanocone surface. As the droplet gradually retracted from the porous surface, the air−liquid contact line on each nanopore would change from concave to convex as shown in Figure 4g and the volume of the sealed air would increase. According to Boyle’s law, the expansion of air will lead to a decrease in pressure, which will result in the formation of a negative pressure (ΔP).19 Water will impregnate the nanopores
0.052 and 0.948, respectively. These values reveal that there is a very large fraction of air. Air is an effective hydrophobic medium. In general, a higher fraction of liquid−air contact area occupying the apparent surface area leads to a stronger surface water repellence.46 In addition, the Au@Ni NAs, which apply to the Cassie mode, only with its tips in contact with the water, generate a typical discrete point contact three-phase (solid− air−liquid) line, which is energetically advantageous to driving a droplet off a superhydrophobic surface, showing lower surface adhesion. The effect of the roughness of Ni nanocone substrates with the Au coating for various durations on wettability and adhesion is also discussed.The roughness factor is calculated according to the parameters of their SEM image along with the thickness of the Au layer measured by XFS. The structures are assumed to be circular cones. The roughness factor r, defined as the ratio of the surface area and projected area, equals L/R, where L is approximately the height of the nanocone and R is the radius of the bottom.47 The relationship of the contact angle and sliding angle versus the roughness factor is described is Figure 2f. The roughness factor increases along with the Au deposition time because of the laminar growth of Au. The nanocones grow larger and thicker, and the gaps between each unit decrease. When a water droplet is placed on the surface, the air in the gap will generate air pockets to support the droplet. According to the Cassie equation, a E
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the Au@Ni NAs which were contaminated deliberately by chalk dust, the droplet rolled on the surface quickly and became larger because of the dust wrapped inside. The chalk dusts could be removed by the roll of water, which demonstrated the self-cleaning of the Au@Ni NAs. On the other hand, with high adhesive force, the porous Au surface could be used as a mechanical hand to transfer microdroplets. As illustrated in Figure 5b, when a 4 μL water droplet was placed on a low-adhesive superhydrophobic surface, the porous film got close to it slowly and would attract the water droplet and catch it. After lifting up the porous surface, the water droplet would pin on the surface. To complete the transport process, the water droplet could be detached to another surface without loss. Because of the intrinsic characteristic of gold, the surface is anticorrosive and could be especially used to transport precious but corrosive droplets. Experiments have demonstrated that the two Au surfaces, after surface modification with n-hexadecanethiol, are not only liquid-repellent but also anticorrosive. The contact angles of a 4 μL water droplet on the modified Au surfaces after immersion in solutions with different pH values for 1 day are depicted in Figure 6. For both surfaces, the contact angles are larger than 150° and change little when the pH is in the range between 1 and 14, which means corrosive solutions have nearly no effect on the wettability of the Au@Ni NAs and the porous Au surface. 3.4. Au@Ni Cicada Wing Structure Employed as a SERS Substrate. As previously mentioned, Au@Ni NAs mimicking a cicada wing is homogeneously distributed, and the morphology of the nanocone after the Au coating does not show any obvious change considering that the thickness of the Au nanolayer is only about 50 nm. Both the TEM image and the EDS result indicate the coverage of the Au layer on the Ni nanocone. The hot spots between each Au@Ni nanocone can induce localized surface plasmon resonance (LSPR), enhancing the electromagnetic field eventually. To evaluate the Ramanenhancing capability of the Au@Ni NAs, an ethanol solution of R6G with different concentrations is served as probe molecules. When the concentration of R6G is 10−6 M, the spectral feature characteristics of R6G can be identified clearly in Figure 7.
Figure 4. (a) Schematic diagram of the contact state of Au@Ni NAs and a water droplet. (b) Contact angle of a 4 μL water drop on the Au@Ni NAs after surface modification. (c) Sliding moment of a water droplet. (d) Schematic diagram of the contact state of the porous surface and water droplet. (e) Contact angle of a 4 μL water drop on the porous Au surface after surface modification. (f) Water droplet pinned firmly on the porous surface at a tilting angle of 180°. (g) Change in the three-phase contact line when the water droplet is retracted from the porous surface.
to some extent and form the Cassie impregnating state, as shown in Figure 4d.3,48 With the water penetrating the nanopores, the contact area between the water and the porous surface becomes larger, which will lead to a larger van der Waals force. The open air pockets formed in the nanocone surface cannot form a closed system to generate negative pressure, and their contribution to the adhesive force may be negligible. 3.3. Applications for Self-Cleaning and Microdroplet Transfer. The superhydrophobic nanostructure Au surfaces with switchable adhesion showed potential technical applications. Similar to the natural cicada wing, the Au@Ni NAs with extremely low adhesion could be used as a self-cleaning surface. As shown in Figure 5a, when a water droplet was dropped on
Figure 5. (a) Self-cleaning performance of the fabricated Au@Ni NAs with contaminated chalk dust. (b) Transfer of a water droplet by the porous Au surface. F
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on SERS and Raman spectra at concentrations of 10−6 and 10−1 M, respectively (Figure S8). An enhancement factor of 5.67 × 105 has been achieved using the Au@Ni NAs substrate. In order to assess the contribution of the geometry of nanocone to the E-field enhancement, a finite-difference time-domain (FDTD) method was employed to calculate the local EM field, as shown in the inset of Figure 7. FDTD simulation shows that the largest enhancement of the E-field intensity appears mostly in the bottom of the valley and at the tip position. Sharp tips will induce an E-field enhancement, and a tens-ofnanometer gap will form in the bottom of the valley, which is conducive to the formation of hot spots. The simulation is in good agreement with the experimental data. The SERS ability of Au@Ni with other durations and the porous Au substrate is not as good as Au@Ni for 5 min because of lacking sharp tips (Figure S9) .
Figure 6. Contact angles of a water droplet on the two surfaces after immersing in solutions with different pH values for 1 day.
4. CONCLUSIONS Cicada-wing-like Au@Ni NAs were fabricated by a cheap and efficient electrodeposition method. After surface modification by n-hexadecanethiol, superhydrophobicity with low adhesion and self-cleaning were realized on the Au@Ni NAs surface. Using the nanocone array as a template, the porous Au structure was obtained to form a lock and key relation with the nanocone surface. Superhydrophobicity with extremely high adhesion was obtained on the porous Au structure due to different contact modes. Combining the two superhydrophobic properties, a porous Au surface and a Au@Ni NA surface can be used to transfer precious microdroplets without any loss. With well-aligned nanocone structure, the Au@Ni NAs can be potentially employed as a SERS substrate due to the enhancement of the electromagnetic field, which is in correspondence with the FDTD stimulation result. The Au@ Ni NAs retain a contact angle that is larger than 160° after 1 year of storage. Thus, superhydrophobic, SERS, long-lived, selfcleaning, and microtransportation functions were realized on the basis of the well-designed Au nanoarrays.
Figure 7. Collection of spectra of R6G with different concentrations absorbed on the Au@Ni NAs substrate. The inset is the simulated EM-field distribution map of the Au@Ni NAs structure.
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Sharp peaks at 611 and 773 cm−1 are associated with C−C−C ring in-plane and out-of-plane bending vibrations, while the bands at 1184, 1360, 1504, and 1647 cm−1 are assigned to symmetric modes of in-plane C−C stretching vibrations.49,50 The spectra of R6G decrease gradually with the decrease in R6G concentration. Characteristic peaks still can be distinctly observed when the concentration is reduced to 10−8 M. It is believed that the high SERS activity of this substrate contributes to the detection sensitivity and also that the large surface area of nanocones will lead to a better enrichment of the analyte molecules. Simple calculations reveal that about five times more analyte molecules are absorbed on the nanocone surface than on a flat surface, with the same area of the projected spot of the incident laser. The Raman enhancement factor (EF) is calculated to characterize the SERS enhancement of analyte molecules adsorbed on the SERS substrate.51 EF is defined as EF= (ISERS/IRaman) × (CRaman/CSERS), where ISERS and IRaman are the signals recorded on SERS and the normal Raman substrate at their respective concentrations, whereas CSERS and CRaman are the corresponding concentrations measured using a Au@Ni NAs substrate and a flat copper substrate, respectively. EF is calculated on the basis of the intensities of the 611 cm−1 peak
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02472. EDS spectrum taken from the Au@Ni NAs, XRD spectrum of a porous Au surface, contact angles of water drops on the Au@Ni NAs and a flat Au surface, behavior of a water droplet rolling off the Au@Ni NAs, and Raman spectrum of R6G on a flat copper substrate and on different substrates (PDF)
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AUTHOR INFORMATION
Corresponding Authors
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[email protected]. Fax: +86-21-3420-2748. Tel: +86-21-3420-2748. *E-mail:
[email protected]. Fax: +86-21-3420-2748. Tel: +86-21-3420-2748. Notes
The authors declare no competing financial interest. G
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ACKNOWLEDGMENTS This work is sponsored by the National Natural Science Foundation of China (no. 21303100) and the Shanghai Natural Science Foundation (no. 13ZR1420400).
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DOI: 10.1021/acs.langmuir.5b02472 Langmuir XXXX, XXX, XXX−XXX
