Stimuli-Responsive Wettability of Nonplanar Substrates: pH

Jul 29, 2009 - ... Chemistry, Tsinghua University, Beijing 100084, People's Republic of China ... The relationship between pH and the supporting force...
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Stimuli-Responsive Wettability of Nonplanar Substrates: pH-Controlled Floatation and Supporting Force Xiaoxin Chen,† Jian Gao,† Bo Song,† Mario Smet,‡ and Xi Zhang*,† †

Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China, and ‡Department of Chemistry, University of Leuven, Celestijnenlaan 200F Box 2404, B-3001, Leuven, Belgium Received June 15, 2009. Revised Manuscript Received July 6, 2009

We have prepared pH-responsive planar and nonplanar substrates by chemical modification of a rough gold surface with pH-responsive 2-(11-mercaptoundecanamido)benzoic acid (MUABA). The rough surface exhibits a pH-responsive behavior with an exceptionally large change in contact angle (CA) as a function of the pH, from nearly superhydrophobic (CA of about 144°) to superhydrophilic (CA of near 0°). The relationship between pH and the supporting force of gold threads coated by MUABA has been investigated. The pH-responsive coating on the gold thread can provide bigger supporting forces at low pH than at high pH due to the hydrophobicity in the former case. In addition, the change of surface wettability can influence its floatation as well, therefore providing a new approach for controlling the motion of gold threads on water.

1. Introduction Control of surface wettability of a solid surface has many important applications on industrial processes as well as in our daily life.1 Therefore, it has aroused great interest in various research fields.2-18 Superhydrophobicity and superhydrophilicity are the two extreme examples of surface wettability. Studies on superhydrophobic surfaces in nature, such as lotus leaf19,20 and water strider’s legs,21,22 have revealed that the superhydrophobicity *Corresponding author: e-mail [email protected]; Fax þ86-106279 6283; Tel þ86-10-6279 6283.

(1) Nakajima, A.; Hashimoto, K.; Watanabe, T.; Takai, K.; Yamauchi, G.; Fujishima, A. Langmuir 2000, 16, 7044. (2) Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432. (3) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; Mckinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (4) Feng, L.; Song, Y. L.; Zhai, J.; Liu, B. Q.; Xu, J.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2003, 42, 800. (5) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (6) Soeno, T.; Inokuchi, K.; Shiratori, S. Trans. Mater. Res. Soc. Jpn. 2003, 28, 1207. (7) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (8) Tadanaga, T.; Morinaga, J.; Matsuda, A.; Minami, T. Chem. Mater. 2000, 12, 590. (9) Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Adv. Mater. 1999, 11, 1365. (10) Tsujii, K.; Yamamoto, T.; Onda, T.; Shibuichi, S. Angew. Chem., Int. Ed. 1997, 36, 1011. (11) Shi, F.; Chen, X. X.; Wang, L. Y.; Niu, J.; Yu, J. H.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2005, 17, 6177. (12) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; Zhang, X. Langmuir 2006, 22, 4483. (13) Jiang, Y.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X.; Smet, M.; Dehaen, W. Langmuir 2005, 21, 1986. (14) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y. G.; Wang, Z. Q. J. Mater. Chem. 2008, 18, 621. (15) Zhang, J. L.; Han, Y. C. Langmuir 2008, 24, 796. (16) Ling, X. Y.; Phang, I. Y.; Vancso, G. J.; Huskens, J.; Reinhoudt, D. N. Langmuir 2009, 25, 3260. (17) Zhang, L. B.; Li, Y.; Sun, J. Q.; Shen, J. C. Langmuir 2008, 24, 10851. (18) Bhushan, B.; Jung, Y. C.; Koch, K. Langmuir 2009, 25, 3240. (19) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (20) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, L. Y.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Adv. Mater. 2002, 14, 1857. (21) Gao, X. F.; Jiang, L. Nature 2004, 432, 36. (22) Shi, F.; Niu, J.; Liu, J. L.; Wang, Z. Q.; Liu, F.; Feng, X. Q.; Zhang, X. Adv. Mater. 2007, 19, 2257.

104 DOI: 10.1021/la902137b

originates from the interplay of micro/nano-structure and low surface energy coatings. It is clearly seen that this line of research not only stimulates the replication of biosurface structures but also gives us inspiration to design and fabricate surfaces with controlled wettability. The layer-by-layer (LbL) assembly technique has been proven to be a simple and effective way to fabricate various kinds of thin film materials with controlled composition and architecture.14,23-27 Combining the LbL assembly and electrochemical deposition, we have developed a method of preparation of rough gold surfaces that bear gold micro/nano-structures, and by further introducing a coating of low surface energy, it leads to development of a new method of fabrication of superhydrophobic surfaces.5 Because the LbL assembly technique and electrochemical deposition are independent of the size and shape of the substrates, the method can be used for realization of superhydrophobicity not only on planar substrates but also on nonplanar substrates.28 A stimuli-responsive surface, which can change its surface property depending on the external stimuli of the environment, e.g., electric field,29 thermal treatment,30,31 and light irradiation,32 has attracted great interest of scientists. The stimuli response will be essential to control the surface physical properties, such as surface friction and surface wettability. Stimuli-responsive materials are important in a variety of perspectives as versatile and intelligent systems. However, there remains a demand for fabrication of a stimuli-responsive nonplanar substrate. The realization and combination of nonplanar coatings with stimuli-responsive (23) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (24) Decher, G. Science 1997, 277, 1232. (25) Zhang, X.; Shen, J. C. Adv. Mater. 1999, 11, 1139. (26) Zhang, X.; Chen, H.; Zhang, H. Y. Chem. Commun. 2007, 1395. (27) Wang, F.; Ma, N.; Chen, Q. X.; Wang, W. B.; Wang, L. Y. Langmuir 2007, 23, 9540. (28) Shi, F.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2005, 17, 1005. (29) Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (30) Crevoisier, D.; Fabre, P.; Corpart, J.; Leibler, L. Science 1999, 285, 1246. (31) Sun, T. L.; Wang, G. J.; Feng, L.; Liu, B. Q.; Ma, Y. M.; Jiang, L.; Zhu, D. B. Angew. Chem., Int. Ed. 2004, 43, 357. (32) Ichimura, K.; Oh, S.; Nakagawa, M. Science 2002, 288, 1624.

Published on Web 07/29/2009

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Chen et al. Scheme 1. Chemical Structure of 2-(11-Mercaptoundecanamido)benzoic Acid (MUABA)

materials in one system may be important in studying the stimuliresponsive floatation and supporting force of gold threads on a water surface. Therefore, it is highly desirable to develop an effective way for fabricating stimuli-responsive surfaces. It is reported that polyethylene films can show a pH-responsive wettability after being modified with acylated anthranilate moieties through surface reaction.33 Inspired by these works, we designed and synthesized pH-responsive 2-(11-mercaptoundecanamido)benzoic acid (MUABA).13 In this work, we chose a gold thread as a nonplanar substrate and fabricated a rough surface through the LbL assembly technique and electrochemical deposition. Then, a surface with pH-responsive wettability was created by chemisorption of MUABA on the rough surface. The relationships between pH and the floatation and supporting force of the gold threads with the pH-responsive coating will be investigated. It is anticipated that this line of research may represent an interesting model of surface assembly for surface coatings, leading to development of stimuli-responsive motion and floating objects on the water surface.

2. Experimental Section 2.1. Materials. In this experiment, the following chemicals were used as received: 3-mercaptopropionic acid, poly(diallyldimethylammonium chloride) (PDDA) (Mw =400 000 g mol-1), and poly(4-styrenesulfonate) (PSS) (Mw =70 000 g mol-1) from Acros Organics; HAuCl4, HPtCl4, H2SO4, and H2O2 were analytical-grade reagents and were used as received. The molecule 2-(11-mercaptoundecanamido)benzoic acid (MUABA) shown in Scheme 1 was synthesized as published previously.13 The gold threads with diameter of 0.5 mm were obtained from Grikin Advanced Materials Co., Ltd., Beijing, P. R. China. For pH-dependent contact angle measurements and the measurements on the floating gold threads, aqueous solutions with known pH were used as the test liquid and prepared as follows. The aqueous solution of pH 1.1 and pH 2.0 was prepared from a diluted hydrochloric acidic solution; the buffer of pH 4.0 was prepared from potassium acid phthalate; the buffers of pH 6.8 and pH 7.1 were prepared from mixtures of KH2PO4 and Na2HPO4; the buffer of pH 7.8 was prepared from the buffer of pH 6.8 adjusted with a dilute solution of sodium hydroxide; the buffer of pH 9.2 was prepared from Na2B4O7; the aqueous solution of pH 12.7 was prepared from a dilute solution of sodium hydroxide. 2.2. Preparation of the pH-Responsive Coating. The pHresponsive coatings on gold substrates (flat gold substrates and gold threads) were fabricated as follows. First, the substrates were immersed in an ethanol solution of 3-mercaptopropionic acid (110-3 M) overnight to form a self-assembled monolayer (SAM) bearing negative surface charges. Second, the substrates with negative surface charges were alternately immersed in an aqueous solution of PDDA (1 mg mL-1) and then PSS (1 mg mL-1) for 5 min, rinsed with deionized water, and dried in N2. This dipping procedure was repeated until six bilayers of PDDA/PSS (each bilayer unit consists of one PDDA layer and one PSS layer) were achieved. Third, the substrates modified with the polyelectrolyte multilayer were immersed in a mixture of H2SO4 (0.5 M) and HAuCl4 (2 mg mL-1) for electrochemical deposition of gold micro/nano-structures at -200 mV by chronoamperometry. (33) Wilson, M. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 8718.

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Article The deposition time was 1200 s, followed by rinsing with deionized water and drying in N2. Thus, the rough gold micro/nanostructures surface was achieved as characterized and well described in our groups’ previous publication.28 Fourth, the substrates covered with gold micro/nano-structures were immersed in an ethanol solution of MUABA (110-3 M) overnight, rinsed sequentially with ethanol and deionized water, and dried in N2. The L-shaped (used in the measurement of supporting forces of the water surface) and straight gold threads (used in the investigation of floatation on the water surface) with pHresponsive wettability coatings were all prepared using the above procedure. 2.3. Characterization. Electrochemical depositions were performed using a potentiostat (Autolab PGSTAT12, The Netherlands). A platinum electrode was used as counter electrode and a Ag/AgCl electrode as reference electrode. Water contact angle values were acquired at room temperature using an optical contact angle measuring device (OCA 20, DataPhysics Instruments GmbH, Filderstadt, Germany) by sessile drop and tilting plate measuring method, which is a static contact angle assessment. Halogen lighting with continuous adjustable intensity without hysteresis for homogeneous backlighting was used to image the water droplet, whereas a 0.7-4.5fold magnification CCD-camera video system with a resolution of 768  576 pixels was used to monitor and record the data. Ellipse fitting was selected as the default calculation method. In each measurement, a droplet of 4 μL was dispensed onto the substrates under investigation. A water droplet size of 4 μL was used for tilt angle measurements, if not otherwise indicated. The supporting forces of the water surface on the wettability gold threads were measured with a commercial high-sensitivity microelectromechanical balance (DCAT 21, DataPhysics Instruments GmbH, Filderstadt, Germany). 2.4. Measurement of the Gold Thread Floating. The measurement of the curve of the water surface at different pH was carried out as follows. First, the gold threads were carefully placed on the surface of the water in a quartz cubic box. More water solution was then added into the quartz box to make the water surface as high as the edge of the box to remove the influence of the capillary force between the wall of the box and the water surface. 2.5. Movement of Gold Thread. The gold threads with pHresponsive wettability with Pt aggregates deposited on one of their ends were prepared using the following procedure. First, gold micro/nano-structures on one end of the gold threads were removed carefully by physical friction. Then, the position of the gold threads was carefully adjusted to ensure that the end just contacted the electrolyte surface. Pt aggregates were electrochemically deposited at -200 mV (vs Ag/AgCl) on that open end for 400 s. For movement tests, the hydrogen peroxide aqueous solutions of certain pH were prepared. The hydrogen peroxide aqueous solution of pH 5.8 was mixed from the phosphate buffer (pH=6.8) and 30 wt % H2O2 solution (V:V=1:1). The hydrogen peroxide aqueous solution of pH 8.5 was prepared from the diluted sodium hydroxide solution (pH=12.7) and 30 wt % H2O2 solution (V:V=1:1). The two troughs of 29 cm  2 cm were washed thoroughly with a K2Cr2O7/H2SO4 mixture to ensure that no contamination was present and then were filled with the aqueous hydrogen peroxide solutions of different pH. For direct visualization, refer to the video in Supporting Information that records the movements of the gold thread at pH 5.8 and 8.5. 2.6. Method of the Force Measurement. During the force measurement procedure, the pH-responsive gold thread with diameter of 0.5 mm was folded into an “L” shape with right angle, the short part being about 1 cm and the long part having an exact length of 2.00 cm. During the measurement, the short part was fixed vertically to a high-sensitivity microelectromechanical balance system, and the long part of the L-shape gold thread was kept horizontal to the water surface. The balance was automatically DOI: 10.1021/la902137b

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Figure 1. Water contact angle as a function of pH on the rough planar substrate after the modification of MUABA. set to 0.0 g. Then, the water container that was right below the lever was moved upward to the horizontal parts of the gold thread at a rate of 20 μm s-1. The gold thread first contacted and then made the water surface to be curved and next penetrated the water surface until an immersion depth of 3.0 mm was reached. After that, the water container moved downward at a rate of 80 μm s-1. The gold thread emerged out of the water surface and returned to its original position to finish this force measurement cycle. The positions and corresponding supporting forces on the gold threads were collected and plotted by the data analysis software of the balance.

3. Results and Discussion 3.1. Surface with pH-Responsive Wettability. The chemical properties and the surface morphology are very important factors in determining the surface wettability. The gold micro/ nano-structures on the surface can be obtained by the combination of a polyelectrolyte multilayer and electrochemical deposition.5 Next, the surface of gold micro/nano-structures was modified with the pH-responsive MUABA molecule. The pHdependent contact angle measurements were performed on a rough gold coated by MUABA SAMs. When a water droplet with pH = 1.1 was dispensed onto the pH-responsive rough surface, a CA of about 144° was observed, as shown in Figure 1, indicating a nearly superhydrophobic surface. It should be noted that the tilt angle (TA) is also large in this case, and the water droplet does not roll off easily. In contrast, when a water droplet of pH 12.7 was dispensed onto the pH-responsive surface, a CA of about 0° resulted. The water droplet dispensed on the surface spread immediately, and hence a superhydrophilic behavior was observed, as shown in Figure 1. It can be seen clearly that the CA decreases slightly with increasing pH up to approximately pH=7.0 and then decreases sharply between pH 7 and 9. This remarkable change in the wettability originates from the different surface structures of the monolayers of MUABA in different pH environments. MUABA contains a pH-sensitive headgroup: benzoic acid is protonated at low pH, whereas it converts into a polar carboxylate at high pH.34 pKa of MUABA is estimated around 4. However, the contact angles of the surface change sharply at around pH of 7 when MUABA is fully ionized. Since then, the surface becomes hydrophilic. We have repeated the above process on a nonplanar substrate, a gold thread of 0.5 mm diameter. The morphology of the deposited gold micro/nano-structures on the gold thread was similar to that on the planar gold substrate. After modification, the gold thread also shows pH-responsive wettability. As shown in Figure 2, the shape of the water droplet kept well on the gold thread with a large CA, indicating that the gold thread surface is nearly superhydrophobic at low pH. When a water droplet of pH=12.7 contacted (34) Jiang, Y.; Wang, Z. Q.; Xu, H. P.; Chen, H.; Zhang, X.; Smet, M.; Dehaen, W.; Hirano, Y.; Ozaki, Y. Langmuir 2006, 22, 3715.

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Figure 2. Water droplet contacting gold threads with the pHresponsive coatings: (a) pH = 1.1, (b) pH = 9.2.

the gold thread surface, the water droplet spreads immediately. Therefore, we have confirmed that modification of the gold threads with gold micro/nano-structures and MUABA results in pH-responsive wettability. The coatings show near superhydrophobicity at low pH and superhydrophilicity at high pH. 3.2. Floatation and Motion of MUABA-Modified Gold Threads. Gold threads with superhydrophobic surface have less area contacting water than their unmodified counterparts.28 It would be of interest to study the contacting condition of the gold threads with pH-responsive coating. After the pH-responsive gold thread was obtained, its ability to support its weight on water surfaces with different pH was investigated by measurement of the curvature force. After carefully putting the gold thread on the water surface, it was observed that the gold threads floated on the surface from pH=1.1 to pH=7.1, whereas the gold thread sank when the pH of the water was higher than pH=7.1. The crosssectional views of the floating gold thread are shown in Figure 3. The floating gold thread on the water is supported by a combination of two main forces: buoyancy force (Fb) which can be deduced by integrating the hydrostatic pressure over the thread area in contact with the water and the curvature force (Fc) which is deduced by estimating the vertical component of the surface tension along the contact perimeter (σ).35 The contacting conditions of the gold thread with water were different at different pH. When the pH was 1.1 (Figure 3a), the gold thread had a small area contacting the water. A bigger contacting area could be clearly seen when the gold thread floated on a pH= 7.1 water surface (Figure 3b). The depth between gold thread and water surface was farther at pH 7.1 than at pH 1.1. Therefore, the Fb of the gold threads is bigger at pH 7.1 than at pH 1.1. It can be found that the contacting area increases with increasing pH of the water. The MUABA-modified gold thread was supported by a bigger curvature force at pH 1.1 than at pH 7.1. (35) Hu, D. L.; Chan, B.; Bush, J. W. M. Nature 2003, 424, 663.

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Figure 3. Curve of water surface at which MUABA modified gold thread floated: (a) pH=1.1, (b) pH=7.1.

It is well-known that a superhydrophobic coating can be wrapped by a thin layer of air,36 reducing the fluidic drag during movement on the water surface.37-39 This explains for instance why nature selected the superhydrophobic coating for the water strider as it contributes to their locomotion on a water surface.40 Therefore, it would be of interest to further study the change of the motion of the pH-responsive gold threads on water of different pH values. First, the gold micro/nano-structures on one end of the pH-responsive coated gold threads were removed by physical friction. Then Pt aggregates were electrochemically deposited on one end of the gold thread in order to obtain threads that can be propelled by electrocatalytic decomposition of hydrogen peroxide.41-43 The MUABA-modified gold threads were carefully placed on the surface of aqueous hydrogen peroxide solutions of pH 5.8 and pH 8.5. Interestingly, the gold thread could move normally in virtue of the fuel-driven nanomotors and reached the other end of the trough at pH 5.8 (Figure 4). However, when the pH value changed to 8.5, the gold thread floated on the water surface at the beginning but then sank in the water and could not move. At low pH value, the pH-responsive gold threads exhibit nearly superhydrophobicity; thus, the gold threads can float and move on the water surface efficiently, propelled by decomposition of hydrogen peroxide. In contrast, when the pH value of water is high, the corresponding CA of the pH-responsive coating is low, indicating that the coatings exhibit superhydrophilic behavior. Under these conditions, the prepared gold threads cannot float. 3.3. Supporting Force Measurement at Different pH. To better understand the effect of the pH-responsive coating on the floating gold thread, we tried to obtain some detailed information on the supporting force (Fs) exerted on the pH-responsive gold thread. During the whole measurement cycle, the positions and corresponding supporting forces of the water surface were plotted automatically as a force curve by the data collecting system of the balance (Figure 5). When the gold thread contacted the water, an upward force appeared and increased as the immersion proceeded. Once the gold thread penetrated into the water surface, the force jumped to around zero. The critical point when the gold (36) Wagterveld, R. M.; Berendsen, C. W. J.; Bouaidat, S.; Jonsmann, J. Langmuir 2006, 22, 10904. (37) Choi, C. H.; Kim, C. J. Phys. Rev. Lett. 2006, 96, 066001. (38) Cottin-Bizonne, C.; Barrat, J. L.; Bocquet, L.; Charlaix, E. Nat. Mater. 2003, 2, 237. (39) Ou, J.; Perot, B.; Rothstein, J. P. Phys. Fluids 2004, 16, 4635. (40) Dickinson, M. Nature 2003, 424, 621. (41) Ismagilov, R. F.; Schwartz, A.; Bowden, N.; Whitesides, G. M. Angew. Chem., Int. Ed. 2002, 41, 652. (42) Paxton, W. F.; Kistler, K. C.; Olmeda, C. C.; Sen, A.; Angelo, S. K.; Cao, Y.; Mallouk, T. E. J. Am. Chem. Soc. 2004, 126, 13424. (43) Kline, T. R.; Paxton, W. F.; Mallouk, T. E.; Sen, A. Angew. Chem., Int. Ed. 2005, 44, 744.

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Figure 4. Digital images of the pH-responsive coating gold thread with Pt aggregates on one end (left) was simultaneously put onto the surface of 30% H2O2 solution with different pH in trough. (The red dashed circle was the position of the gold thread.) The pH is 5.8: (a) the initial position, (b) the final position. The pH is 8.5: (c) the initial position, (d) the final position. (e) The position of the pHresponsive coating gold thread versus time of movement at pH= 5.8 and pH=8.5.

Figure 5. Typical supporting force curves of MUABA modified gold thread during the supporting force measurement. (a) pH=1.1, (b) pH=2.0, (c) pH=4.0, (d) pH=6.8, (e) pH=7.1, (f) pH=7.8.

thread is about to penetrate the water corresponds to the maximal upward force under these conditions. When the pH-responsive gold thread contacted with the pH=1.1 water surface, the water DOI: 10.1021/la902137b

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pH-responsive gold thread becomes less, and the supporting force also decreases. The maximum upward forces per centimeter of the gold threads are plotted versus the pH value in Figure 6. It can be seen clearly that the Fs decreases from 129 mg cm-1 at pH 1.1 to 105 mg cm-1 at pH 7, and then it decreases sharply between pH 7 and pH 9, from 105 mg cm-1 at pH 7 and 62 mg cm-1 at pH 7.8 to 0 mg cm-1 at pH 9. Comparing Figure 6 with Figure 1, the effect of pH on the supporting force is similar to its effect on CA. This correlation points to the fact that the effect of pH on the supporting force originated from changes of surface wettability. Figure 6. Maximal supporting force per unit length of gold thread versus pH.

surface was curved as deep as 2.20 mm, as shown in Figure 5a, and then immersed in the water, which means that the supporting force per unit length of gold thread provided by the pH-responsive coatings is 129 mg cm-1. The depth of the curving water surface decreased from 2.20 mm at pH=1.1 to 1.87 mm at pH= 2.0 (Figure 5b), the corresponding supporting force per unit length being 127 mg cm-1. When the pH of the water increased to 4.0, 6.8, and 7.1, the depth of the curving water further decreased from 2.20 mm to 1.83, 1.67, and 1.62 mm, respectively, as shown in Figure 5c,d,e, the corresponding supporting force per unit length being 117, 108, and 105 mg cm-1, respectively. Further increasing the pH of the water to 7.8 resulted in significant decrease of the depth of the curving water surface to only 1.03 mm, the corresponding supporting force per unit length being 62 mg cm-1. When the pH of the water is larger than 9.2, the supporting force provided by the pH-responsive coatings was zero, meaning that when the gold thread contacted the water surface, it immersed in the water immediately. The depth of the curving water surface was the biggest and the supporting force maximal when the pH was 1.1. With increasing the pH of the water, the depth of the water surface that is curved by the

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4. Conclusions pH-responsive surfaces were achieved by modifying a rough gold coating with MUABA. Interestingly, the surface wettabilities can be manipulated from near superhydrophobicity to superhydrophilicity by adjusting the pH. The relationships between pH and supporting force of the MUABA-modified gold threads have been established. The relationship between the wettability and the supporting force on the water surface may be helpful in the design and development of novel devices. It is anticipated that this research may open a route to the application of new stimuli-responsive materials. Acknowledgment. We thank the National Basic Research program of China (2007CB808000), the National Natural Science Foundation of China (NSFC, 50573042), joint project supported by NSFC-DFG (TRR61), the Postdoctoral Science Foundation of China (20080440360), and bilateral grant of Flemish government (BIL07/04) for financial support. Supporting Information Available: Movements of the gold thread at pH 5.8 and 8.5. This material is available free of charge via the Internet at http://pubs.acs.org.

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