Plasma-Driven Reversible Wettability Switching of a Bare

Sep 1, 2009 - We report an approach for fabricating a tunable wettability surface by electroless gold plating on poly-. (dimethylsiloxane) (PDMS). A t...
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Thermal/Plasma-Driven Reversible Wettability Switching of a Bare Gold Film on a Poly(dimethylsiloxane) Surface by Electroless Plating Jian Wu, Hai-Jing Bai, Xian-Bo Zhang, Jing-Juan Xu,* and Hong-Yuan Chen The Key Lab of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China Received June 30, 2009. Revised Manuscript Received August 9, 2009 We report an approach for fabricating a tunable wettability surface by electroless gold plating on poly(dimethylsiloxane) (PDMS). A two-layer structured gold film with a tight layer and a loose layer can be obtained on the surface of a PDMS chip when the PDMS chip is immersed in a gold plating solution at 30 °C for 4 h. Its wettability can be rapidly switched between superhydrophilicity and superhydrophobicity by plasma and heat treatments without any self-assembled monolayer, and the superhydrophobicity can be even changed from the gecko-foot-hair-like character to the lotus-leaf-like character. Benefiting from the various wettabilities of the prepared gold/PDMS composites, protein patterning is successfully achieved on a patterned superhydrophobic/superhydrophilic gold/PDMS composite; a superhydrophobic needle for transferring supersmall water droplets (1 μL) to a superhydrophobic surface is successfully fabricated.

Introduction Surface wettability is a basic property that can be applied in many industrial processes. In recent years, surfaces with special wettabilities, such as superhydrophobic surfaces (contact angle (CA) > 150°) and superhydrophilic surfaces (CA < 10°), has received considerable attention in both fundamental research fields and practical applications such as antifogging, self-cleaning,and lossless transferring of small water droplets.1-8 Additionally, “smart” surfaces, meaning surfaces for which surface wettability can be reversibly tuned, have attracted more and more interest for their wide applications in sensing, manipulation of biomolecules and cells, controllable separation systems, intelligent drug release, and so on.9-14 Specially, the reversible switching between superhydrophobicity and superhydrophilicity is more attractive because it can maintain the advantages of both superhydrophobicity and superhydrophilicity. Different stimuli such as thermal treatment,9 pH,10 redox electrochemistry,11,12 *Corresponding author. E-mail: [email protected]. Tel/Fax: þ86-2583597294. (1) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Adv. Mater. 2002, 14(24), 1857–1860. (2) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377–1380. (3) Su, B.; Li, M.; Shi, Z. Y.; Lu, Q. H. Langmuir 2009, 25(6), 3640–3645. (4) Li, W.; Diao, Y. P.; Wang, S. Y.; Fang, G. P.; Wang, G. C.; Dong, X. J.; Long, S. C.; Qiao, G. J. Langmuir 2009, 25(11), 6076–6080. (5) Zimmermann, J.; Reifler, F. A.; Fortunato, G.; Gerhardt, L. C.; Seeger, S. Adv. Funct. Mater. 2008, 18(22), 3662–3669. (6) Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. Adv. Mater. 2005, 17(16), 1977–1980. (7) Lenz, P. Adv. Mater. 1999, 11, 1531–1534. (8) Srinivasan, S.; Praveen, V. K.; Philip, R.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5750–5754. (9) 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(3), 357–360. (10) Zhu, Y.; Shi, M. H.; Wu, X. D.; Yang, S. R. J. Colloid Interface Sci. 2007, 315(2), 580–587. (11) Xu, L. B.; Chen, Z. W.; Chen, W.; Mulchandani, A.; Yan, Y. S. Macromol. Rapid Commun. 2008, 29(10), 832–838. (12) Lin, Y. H.; Ren, H. W.; Wu, Y. H.; Wu, S. T.; Zhao, Y.; Fang, J. Y.; Lin, H. C. Opt. Express 2008, 16(22), 17591–17598. (13) Sun, W. T.; Zhou, S. Y.; Chen, P.; Peng, L. M. Chem. Commun. 2008, No.5, 603–605. (14) Hou, W. X.; Wang, Q. Langmuir 2009, 25(12), 6875–6879.

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UV,13,14 and so on have been used in the field of switchable surface wettabilities. Gold is one of the commonly used substrate materials in fabricating special wettability surfaces because of its easy preparation of micro-nano hierarchical porous structure and easy introduction of stimuli-responsive materials on its surface via selfassembly technique. For example, various stimuli-responsive materials such as dendron thiols,15 R-cyclodextrin/azobenzene inclusion complex,16 malachite-green-derivative,17 and so forth were successfully introduced to rough gold surfaces; the wettabilities could be tuned from superhydrophilicity to superhydrophobicity by pH or light irradiation. However, pure gold substrate with reversible response to external stimuli has not yet been reported. In this work, a gold film with two-layer structure is successfully constructed on poly(dimethylsiloxane) (PDMS), which is known in many fields due to its fascinating properties such as low cost, optical transparency, nontoxicity, and film forming ability.18-20 It is also demonstrated that the wettability of the gold film could be easily tuned between superhydrophilicity and superhydrophobicity by the alternation of heat and plasma treatment for at least 40 cycles. It is interesting that the gecko-foot-hair-like and lotusleaf-like surface wettabilities could be also achieved by simple heat treatment, which enriches the potential applications of the fabricated gold/PDMS material greatly. On the basis of the above properties, protein patterning only induced by the special wettabilities is successfully prepared; a superhydrophobic needle for (15) Jiang, Y. G.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X. Langmuir 2005, 21(5), 1986–1990. (16) Wan, P. B.; Jiang, Y. G.; Wang, Y. P.; Wang, Z. Q.; Zhang, X. Chem. Commun. 2008, No.44, 5710–5712. (17) Jiang, Y. G.; Wan, P. B.; Smet, M.; Wang, Z. Q.; Zhang, X. Adv. Mater. 2008, 20(10), 1972–1977. (18) Effenhauser, C. S.; Bruin, G. J. M.; Paulus, A.; Ehrat, M. Anal. Chem. 1997, 69(17), 3451–3457. (19) Hofmann, O.; Wang, X. H.; deMello, J. C.; Bradley, D. D. C.; deMello, A. Lab Chip 2005, 5(8), 863–868. (20) Khorasani, M. T.; Mirzadeh, H. J. Appl. Polym. Sci. 2004, 91(3), 2042– 2047.

Published on Web 09/01/2009

DOI: 10.1021/la902332q

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transferring supersmall water droplets (1 μL) to a superhydrophobic surface is fabricated.

Scheme 1. The Experimental Procedure Scheme of the Electroless Gold Plating on PDMSa

Experimental Section Reagents. Sylgard 184 (including PDMS monomer and curing agent) was obtained from Dow Corning (Midland, MI). Hydrogen tetrachloroaurate hydrate (HAuCl4 3 4H2O) was from Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). Glucose was from Shanghai Bio Life Science & Technology Co., Ltd. (Shanghai, China). Potassium hydrogen carbonate (KHCO3) was purchased from Wenzhou Chemical Material Factory (Wenzhou, China). The human serum albumin marked with fluorescein isothiocyante (FITC-HSA) was from Sigma (St. Louis, MO) and was dissolved in 10 mM phosphate-buffered saline (PBS, pH 7.4). All aqueous solutions were prepared in 18 MΩ/cm Millipore purified water (Millipore Inc., Bedford, MA). Other chemicals were of analytical reagent grade. Fabrication of PDMS Slices and Molds. All PDMS slices in this paper were prepared by mixing the monomer and the curing agent in a proportion of 10:1 and cured at 80 °C for 90 min. The circular holes array in PDMS molds was prepared by a puncher, and the square aperture was fabricated by cutting PDMS with a blade. Square aperture molds for fabricating gold film on PDMS were 5 cm in length and 3.5 cm in width. Molds used in preparing protein patterning were 2  8 hole arrays with 1.5 mm in diameter for each hole. Fabrication of Gold Film on PDMS. The plating solution containing 12 mM HAuCl4 3 4H2O, 0.5 M KHCO3, and 25 mM glucose (pH 9.3) was injected into the square aperture PDMS mold, which jointed with a base PDMS slice, and was kept at 30 °C for 4 h to form a gold film on the base PDMS slice as shown in Scheme 1. All the gold films in the paper without specification were prepared on base PDMS substrate just as mentioned above. Characterization of the Gold Film. Electrochemical Performance. The electrochemical performance of the gold film was assessed by cyclic voltammetry (CV) on CHI 660b (Chenhua, Shanghai, China) in 0.5 M H2SO4 with the gold film as the working electrode, a platinum wire as the auxiliary electrode, and an Ag/ AgCl electrode as the reference electrode at a scan rate of 0.1 V/s.

Measurement and Calculation of Water Contact Angles (CAs) and Sliding Angles (SAs). An OCA 20 CA goniometer (Dataphysics, Germany) was used for the determination of CAs and SAs. All the CAs and SAs were measured with 5.0 μL water droplet. Each datum is an average of five readings. Circle fitting mode and Laplace-Yang fitting mode were used for the calculation of CA at CA < 150° and CA > 150°, respectively. X-ray Photoelectron Spectroscopy (XPS) Spectra. The chemical compositions of the as-prepared gold films treated under different conditions were investigated using XPS, which was conducted on an ESCALAB 250 spectrometer (Thermo, USA), and the C 1s line (284.6 eV) was used for bending energy calibration. Scanning Electron Microscopy (SEM) Images. The morphologies of the gold films were examined by SEM. SEM images were obtained using an FEI Sirion 200 field-emission scanning electron microscope. Thermogravimetry/Mass Spectrometry (TG/MS). TG/MS was used to analyze the content of the gold films, which was performed on a thermal-mass spectrometric analyzer STA409PCQMS403 (Netzsch, Germany). First the freshly prepared gold/ PDMS slices were treated under different conditions, then these gold films were scraped off and used as samples in the TG/MS analysis. Thermal behaviors of these samples were studied in an argon atmosphere from ambient temperature to 300 °C at a heating rate of 20 °C/min. Fabrication of the Superhydrophobic Needle. The mixture of PDMS monomer and the curing agent in the proportion of 10:1 was smeared on a steel needle (an accessory of the OCA 20 CA 1192 DOI: 10.1021/la902332q

a (1) PDMS framework; (2) PDMS base chip; (3) Injector filled with the electroless gold plating solution.

goniometer), and cured at 80 °C for 90 min. Then, the needle was dipped into the gold plating solution for 4 h at 30 °C to prepare gold film on PDMS. After that, the needle was heated at 120 °C for 6 h to obtain a superhydrophobic surface. Protein Patterning. The superhydrophobic gold film (CA = 163°) was covered with the mold mentioned above, then treated in plasma cleaner for 10 s. The treatment time should be strictly controlled to avoid the overetched case. After removing the mold, 3 μL of FITC-HSA solutions with different concentrations were injected into the superhydrophilicity circles carefully. After incubation for 6 h, the substrate was rinsed with 10 mM PBS solution (pH 7.4) thoroughly and then observed under an inverted fluorescence microscope (Leica, Germany) equipped with a DP71 CCD camera (Olympus, Japan). Then, the gold film was treated in the plasma cleaner for 5 min, and the same region was recorded by the camera again. The program of ImageJ21 was used for the fluorescence intensity measurement of the obtained pictures.

Results and Discussions Characterization of Gold Film on PDMS. Recently, an in situ synthesis method was used to prepare PDMS/gold nanoparticle composite film by our group.22 On the basis of this approach, both PDMS chips, named the cover chip and the base chip, were coated with a gold film simultaneously via electroless plating with the help of a sandwich-typed framework, during which the gold nanoparticles in situ synthesized on the PDMS surface served as seeds. Interestingly, the morphologies of the gold films on both chips were notably different from each other. A one-layer structure is presented on the cover chip which has good conductivity and has been successfully applied for cell patterning,23 while a two-layer structured film comprised of a tight layer and a loose layer is obtained on the base chip. It has been mentioned that the gold film on the base PDMS substrate is thicker and rougher than that on the cover PDMS substrate because of the gravity effect.23 Figure 1 illustrates the morphology and the microstructure of the gold film on the base PDMS substrate. It could be observed that the gold film was composed of many microscale gold clusters randomly distributed on the PDMS surface (Figure 1a). The gold clusters present a three-dimensional porous microstructure. With the software ImageJ,21 the pore space is roughly measured to be 30%. The cross-section SEM graph (Figure 1b) and an amplification graph of the gold film (Figure 1c) clearly show a two-layer structure with a tight layer (about 100 nm thick) and a loose layer (about 3-5 μm thick). Benefiting from this tight layer, the two-layer structured gold film also has good conductivity. The conductivity of the gold/PDMS base chip is demonstrated by CV in 0.5 M H2SO4. Typical (21) Rasband, W.S.; ImageJ; http://rsb.info.nih.gov/ij/ (accessed 1997-2007). (22) Zhang, Q.; Xu, J. J.; Liu, Y.; Chen, H. Y. Lab Chip 2008, 8(2), 352–357. (23) Bai, H. J.; Shao, M.-L.; Gou, H. L.; Xu, J. J.; Chen, H. Y. Langmuir DOI:10.1021/la900944c.

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Figure 1. SEM images of the top view (a), the cross-sectional view (b), and an amplification picture (c) of the gold film on PDMS. Table 1. The Water CAs and the SAs of the Gold Filmsa at Different Temperatures treatment time

treatment temperature (°C)

1h

2h

4h

6h

72 h

CA(°)/ SA(°)

CA(°)/ SA(°)

CA(°)/ SA(°)

CA(°)/ SA(°)

CA(°)/ SA(°)

4 25 80 120

0/0/0/0/0/0/0/13.5/29.2/75.5/130.2/145.3/180 151.1/180 155.3/180 163.4/180 152.1/53.1 158.5/27.2 166.2/6.7 169.3/3.5 -b b a All the gold films were washed with 18 MΩ/cm Millipore water three times and then dried with nitrogen before measurements. The SA of the gold film was too little to keep the water drop on it.

oxidative and reductive peaks corresponding to the electrochemical behavior of Au could be observed clearly (see Supporting Information, Figure S1). Wettability of the Two-Layer Structured Gold Film. The rough gold surface with porous microstructure on the PDMS suggests that the wettability of gold film might be special according to our previous reports.24 The CAs and SAs of water droplets on the gold films are shown in Table 1. At room temperature of 25 °C, the CA of freshly prepared gold film on PDMS is 0°, so the gold film represents superhydrophilic property. With the increase of storage time, CA of the surface gradually increased. After 3 days, it could reach around 75°. We further investigated the influences of temperature on the wettability of the gold film. It was found that the gold film could maintain superhydrophilicity for at least 72 h at the temperature of 4 °C. But when the gold film is treated at 80 or 120 °C, its wettability would turn into hydrophobicity in the very beginning. As time goes on, it could change to be superhydrophobic (CA > 150°). It is interesting that the SAs of the gold films simultaneously varied along with the change of CAs with different change rates at different temperatures. At 80 °C, heat treatment time has no obvious effect on the SA, which could remain at 180° for at least 7 days (Table S1). This surface with high CA and SA is known as gecko-foot-hair-like character.6 However, SAs decreased with the increase of treatment time at 120 °C. Finally, the superhydrophobic surface showed a tiny SA, which is called a lotus-leaf-like surface characterized by very low adherence force to water. The wettability of the gold film was very stable while preserved at room temperature after the film had become superhydrophobic. For example, CAs and SAs of the gold films treated at 80 or 120 °C for 4 h were measured every day up to 1 week at room temperature. As shown in the Supporting Information Table S1, the average CAs of the films are 152.4 ( 1.7° (80 °C) and 165.9 ( 1.8° (120 °C), respectively. The SA of the gold film treated at 80 °C remained at 180°, and that of the gold film treated at 120 °C was 6.5 ( 0.2°. Thus, by simply (24) Zhang, H.; Xu, J. J.; Chen, H. Y. J. Phys. Chem. C 2008, 112(36), 13886– 13892.

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heating the prepared gold film at different temperatures for some time, we could obtain a stable superhydrophobic gold surface with different properties. Conventionally, superhydrophobic surfaces have been produced mainly in two ways. One is to create a rough structure on a hydrophoic surface, and the other is to modify a rough surface by materials with low surface free energy.1 Generally, the surface’s microstructures need to be carefully designed to obtain superhydrophobicity, especially for a gecko-foot-hairlike superhydrophobic surface.6 Interestingly, the wettability of the gold film provided here could change from superhydrophilic to superhydrophobic without any modification. Also, we could simply prepare the superhydrophobic surface with a different adherence force to water by heat treatment at a different temperature. Plasma treatment is one of the commonly used surface modification methods. It was found that superhydrophobic gold film with gecko-foot-hair-like or lotus-leaf-like character changed to a superhydrophilic one after air plasma treatment for 50 s and then resumed its original state gradually at room temperature; it presented a more rapid recovery speed at higher temperature (Table 2). Using heat and plasma treatment, a reversible switching could be realized between superhydrophilicity and superhydrophobicity rapidly on the gold film. The CA of the superhydrophobic gold film (CA = 162.2°) changed to 0° after treated in the plasma cleaner for 50 s. After being heated at 120 °C for 10 min, it recovered to 159.3°. The rapid switching between superhydrophilicity and superhydrophobicity showed excellent reversibility for at least 40 cycles as seen in Figure 2. It is well-known that the surface compositions and the microstructure are two main factors that determine the surface wettability. First, we investigated the effect of microstructure on the wettability. The same treatments have been applied to other two kinds of gold films. One is a commercial gold foil (CA = 65.0°, denoted as film a), which is composed of a silicon layer, a Ti layer (∼70 nm in thickness), and a Au layer (∼160 nm in thickness); the other is a gold film (CA = 110.6°, denoted as film b), which is deposited on a cover PDMS chip while using a sandwich-typed framework. The same change trends were DOI: 10.1021/la902332q

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treatment temperature (° C)

0 min

10 min

20 min

60 min

240 min

360 min

CA(°)/ SA(°)

CA(°)/ SA(°)

CA(°)/ SA(°)

CA(°)/ SA(°)

CA(°)/ SA(°)

CA(°)/ SA(°)

0/0/18.5/40.2/158.2/180 162.1/180 film 1a 0/7.1/22.8/60.9/164.3/7.2 165.3/6.7 film 2b 80 film 1 0/68.5/90.2/162.1/180 165.6/180 168.2/180 film 2 0/70.5/92.3/164.1/8.5 167.1/5.5 169.1/3.5 120 film 1 0/160/51.0 162.3/37 164.8/7.0 167.3/4.1 169.1/3.5 film 2 0/164.3/7.2 167.5/5.9 168.0/3.9 170.0/3.3 173.5/1.9 a Film 1 is a superhydrophobic gold film with gecko-foot-hair-like character (CA/SA = 163.4°/180°). b Film 2 is a superhydrophobic gold film with lotus-leaf-like character (CA/SA = 166.2°/6.7°).

25

Figure 2. Reversible switching between superhydrophilicity and superhydrophobicity by alternate plasma and heat treatment.

observed: the CA of films a and b decreased to 15° after plasma treatment for 50 s and could gradually change back to 70.0° and 112.2°, respectively. The recovery speed increased with the temperature increase, and the switching also showed reversibility. However, these two flat films did not present superhydrophobicity no matter how long the heat treatment time or how high the temperature. This phenomenon is in good agreement with those observed on self-assembled monolayer-coated gold surfaces, in which the changes of CAs on flat surfaces could be enhanced greatly by using a rough surface.15-17 Different from those stimuli-responsive material assembled gold surfaces, in this case, there is no additive on the gold surface. In order to investigate the source of the wettability change, here, SEM, XPS, and TG/MS techniques were applied to investigate the changes of the surface compositions and the microstructure of gold film after experiencing plasma and heat treatment. The morphology of the gold film was almost the same before and after experiencing plasma and 80 °C heat treatment (Figure 3a-c). This means the wettability change may result from the surface compositions. As the gold film deposited on PDMS surface was exposed in air, (25) Kim, J.; Chaudhury, M. K.; Owen, M. J. J. Colloid Interface Sci. 2006, 293 (2), 364–375.

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the adsorption of gas components (e.g., organic molecules) in the air and the diffusion of silicone oils25 in PDMS should be considered. Table S2 is the atom percentage of Au, Si, C, and O in the gold/ PDMS films treated under different conditions. The possible sources of Si are PDMS that has not been covered by gold film, and silicone oils, which diffuse to the surface of the gold.25 The element C is from the adsorption of organic molecules in the air and PDMS. If silicone oils diffuse to the gold surface from PDMS and affect the wettability of the gold film, the content of Si should increase while the gold film changes from superhydrophilic to superhydrophobic by heat treatment. However, such increase has not been observed. The atom ratio of Au/Si is not relevant to the CA variation especially after plasma treatment. For example, the atom ratio of Au/Si changed from 0.94 to 0.96 before and after plasma treatment. It remained at 0.96 after the film was heated at 120 °C for 10 min. However, the CA of the gold film changed from 167.6° to 0° then back to 163 o. This means that the change of wettability for gold films does not result from the diffusion of silicone oils. From Table S2 we can also see that heat treatment does not change the atom ratio of Au/C (ca. 0.33), while, after plasma treatment, it changes from 0.33 to 0.87, which means that the adsorbed gas components on the gold film are removed. Another 10 min heat treatment at 120 °C does not result in an increase in the content of the C element. These results indicate that Langmuir 2010, 26(2), 1191–1198

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Figure 3. SEM images of the gold film at different treatment conditions: (a) the film treated at 80 °C for 3 days, (b) the film in a further treated by plasma for 50 s, (c) the film in b further heated at 80 °C for 60 min, (d) the film treated at 120 °C for 4 h, (e) the film in d further treated by plasma for 50 s, (f) the film in e further heated at 120 °C for 10 min, (g) the film after 40 heat and plasma treatment cycles, (h) the amplification picture of the film in g. Inset in the top-right corner in each SEM image is the CA of the corresponding film; inset in the bottom-right corner is the SA of the corresponding film.

Figure 4. XPS spectra of Au 4f7/2-5/2 (a) and O 1s (b) of the gold film on PDMS. Curve 1: the freshly prepared gold film; curve 2: the freshly prepared gold film heated at 120 °C for 4 h; curve 3: the freshly prepared gold film heated at 120 °C for 4 h then treated by plasma for 50 s.

the adsorption of organic molecules will not affect the rapid switch of wettability of the gold film. Thus the change of wettability should come from the character of the bare gold film itself. The XPS spectra of Au 4f7/2 and O 1s Langmuir 2010, 26(2), 1191–1198

on the gold surface before and after plasma treatment are shown in Figure 4. After plasma treatment, the Au 4f7/2 binding energy shifted from 83.95 to 84.31 eV (Figure 4a, curve 1 and 3), which indicated that parts of the gold were at a relatively high valence DOI: 10.1021/la902332q

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state. The O 1s binding energy shifted from 532.56 to 533.75 eV (Figure 4b, curve 1 and 3), and the amount of oxygen element increased largely with the atom ratio of gold and oxygen changing from 0.45 to 0.14. These changes corresponded to part of the gold film being in an oxidized state.

Figure 5. Variation of the mass signal of m/z = 18 (H2O) with temperature for the samples of gold film for 1 day storage at 4 °C without any treatment (curve 1), heated at 80 °C for 4 h (curve 2), heated at 120 °C for 4 h (curve 3), heated at 120 °C for 4 h then treated by plasma for 50 s (curve 4), heated at 120 °C for 4 h, treated by plasma for 50 s, then heated at 120 °C for 10 min (curve 5), and for 1 week storage at room temperature without any treatment (curve 6). The CA/SA of these gold films has been marked on the figure.

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Compared with the plasma treatment, heat treatment did not have an obvious influence on Au and O elements on the gold surface (Curve 2 in Figure 4). This suggested that the changes of Au and O binding energy were caused by plasma rather than heat treatment. Besides SEM and XPS, six types of gold films, which were the typical states of the films, were used in TG/MS analysis (Figure 5). A mass signal of m/z = 18 appeared at a temperature of ca. 100 °C at a gold film for 1 day storage at 4 °C without any treatment (curve 1). The major elements of the gold/PDMS composite were gold, silicon, hydrogen, oxygen, and carbon, thus the mass signal of m/z = 18 in the TG/MS analysis was from H2O. After the gold films were heated at 80 and 120 °C for 4 h, this mass signal disappeared (curves 2 and 3). Further treated by plasma for 50 s, the mass signal appeared again (curve 4). After the film was heated at 120 °C for 10 min, this mass signal disappeared again (curve 5). If the freshly prepared gold film was stored at room temperature (ca. 30 °C) for 4 days, this mass signal did not appear (curve 6). From these curves, we can see that the mass signal did not appear in all the superhydrophobic gold films (curves 2, 3, 5, and 6) but appeared in all the superhydrophilic gold film (curves 1 and 4). This phenomenon means that a lot of water exists in the superhydrophilic gold films, and plasma treatment can introduce H2O into the microstructured gold film. Combined with the results of XPS, we could conclude that the composition of the freshly prepared superhydrophilic surface was Au/H2O, and that for the superhydrophilic surface after air plasma treatment was AuOx/H2O (The subscript x represents the uncertain oxidation degree of the gold) after air plasma treatment. The origin of the moisture in the gold film after plasma treatment may be explained by the decomposition of -OH radicals,

Figure 6. Water droplet transfer onto a superhydrophobic gold film (CA is 166.2°) by using a commercial needle (a) and the homemade superhydrophobic needle (b,c) with water droplets of 5 μL (a,b) and 1 μL (c). Arrows show the direction of the gold film movement. 1196 DOI: 10.1021/la902332q

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Figure 7. (a) Photograph of the protein array on patterned superhydrophobic/superhydrophilic gold film with different concentrations of FITC-HSA marked in the picture (unit: μM). (b) Fluorescent photo of the marked part in panel a after the film was held for 6 h and washed by 10 mM PBS. (c) The fluorescent intensity of the immobilized FITC-HSA before (curve 1) and after (curve 2) plasma treatment for 5 min (d) XPS spectra of the gold surface after plasma treatment for 50 s (curve 1), immersion in HSA for 6 h and wash by PBS (curve 2), plasma treatment for 5 min (curve 3), and immersion in HSA for 6 h and wash by PBS again (curve 4).

which formed on the gold surface by the ozone generated in plasma treatment.26 These changes were presented as follows:

It could be suggested that the gain and loss of water controlled by plasma and heat treatment triggered the switching of gold film between hydrophilicity and hydrophobicity, and the nanostructure of the gold film can enhance this wettability change and result in the switching between superhydrophilicity and superhydrophobicity. It could also be explained by the Wenzel equation (eq 1)27 and the Cassie equation (eq 2):28 cos θr ¼ r cos θ

ð1Þ

cos θr ¼ f1 cos θ -f2

ð2Þ

where θ is the CA on a flat solid surface. θr is the apparent CA on the rough surface. r is the roughness factor of the rough surface. f1 and f2 stand for the area fractions of the solid and air of the composite surface, respectively (i.e., f1 þ f2 = 1). (26) Kim, J. W.; Yang, K. Y.; Hong, S. H.; Lee, H. Appl. Surf. Sci. 2008, 254(17), 5607–5611. (27) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (28) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.

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While the porous structure of gold film was filled with water after electroless plating or after plasma treatment, it was according to Wenzel assumption that the liquid filled up the grooves on the rough surfaces. According to eq 1, with the increase of roughness, the hydrophilic gold surface (θ = 65°, for a flat gold surface) will be more hydrophilic. After storing or heating the gold film for enough time, water in the loose layer will be evaporated, and air will occupy the space, and the surface will fit the Cassie model gradually because of the hierarchical micro/ nanostructures.29 According to eq 2, we will get a larger θr. At suitable conditions, θr would be even larger than 150° without any surface modification. Water has an accelerated evaporation at a higher treatment temperature, which induced the rapid increase of θr. In the alternate treatment of plasma (50 s) and heat (120 °C, 10 min), the morphology of the loose layer, which could be looked at as a container of water or air, did not show obvious change (Figure 3d-f) even after 40 heat and plasma treatment cycles (Figure 3g,h), so the switching should be reversible and could last for a lot of cycles. Another characteristic of the gold film was the change of SA. The morphological changes of the nanostructures at 80 and 120 °C were considered to be responsible for the character. The Wenzel and Cassie models can also be used to explain this SA change on the gold film. Because the flat gold surface is hydrophilic, the water droplet that is put on the rough gold surface will wet the protruded gold surface. The droplet cannot wet all the loose layer once air has occupied the space because of the hierarchical micrometerano structures.29 Thus the water droplet (29) Herminghaus, S. Europhys. Lett. 2000, 52, 165.

DOI: 10.1021/la902332q

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will sit on the air and the protruded gold surface to form a large CA. Different from the complete Cassie model, here, the protruded gold, which in close contact with the water drop, is soaked in the water droplet, which enhances the van der Waals’ force between the gold film and the water droplet. As Figure 3 shows, the size of the nanostructures heated at 80 °C (Figure 3a) was about 50 nm, which could provide a sufficiently large surface area in close contact with the water drop for adhesion through van der Waals’ forces.6 However, the size of the nanostructures changed to about 200 nm after being heated at 120 °C, which might result from the thermal annealing process of the gold (Figure 3d). In this case, the protruded gold in contact with the water droplet decreased, then the van der Waals’ forces of the surface was much smaller than those of the film being heated at 80 °C, so the SA of the gold film decreased. Applications. The various wettabilities of the gold film make the film be applied in many fields. Combined with the advantageous of PDMS, some applications have been performed. First, a lotus-leaf-like superhydrophobic surface on CA measurement needle was built, which benefits from the film-forming ability of PDMS and the special wettability of gold. As shown in Figure 6a, it was very difficult to put trace water (5 μL) onto a superhydrophobic surface (CA = 166.2°) by a commercial CA measurement needle. The surface free energy of the needle (CA is less than 80°30) is higher than that of the superhydrophobic film, so the water droplet adhered to the needle rather than the superhydrophobic film. However, the water droplet (5 or 1 μL) could be easily transferred to a superhydrophobic film by the selfmade superhydrophobic needle (Figure 6b,c). It was considered to be the result of the lower adherence force of the superhydrophobic needle to water. The needle with the superhydrophobic gold/PDMS composite was demonstrated to be used in water CA measurement for superhydrophobic surfaces. A protein pattern could be prepared easily on the gold film with regional wettabilities produced by air plasma etching the surface through a mask. Figure 7a shows the photograph of the protein array on patterned superhydrophobicity/superhydrophilicity gold film with different concentrations of FITC-HSA. As we can see, the protein solution can be restricted in the superhydrophilic area and can be adsorbed on the superhydrophilic gold surface (Figure 7b). The adsorbed protein can be cleaned by plasma treatment for 5 min. The quantitative analysis results of the fluorescence intensity of the FITC-HSA array in Figure 7a (30) Bargir, S.; Dunn, S.; Jefferson, B.; Macadam, J.; Parsons, S. Appl. Surf. Sci. 2009, 255(9), 4873–4879.

1198 DOI: 10.1021/la902332q

Wu et al.

before and after plasma treatment are shown in Figure 7c. The XPS experimental evidence has been given to further confirm whether protein can be completely eliminated by plasma and whether the film can be used again. As shown in Figure 7d, the peak of the 1s core-level of nitrogen, which was from the protein, completely disappeared after the film was treated with plasma, which suggested that the protein could be completely eliminated by plasma treatment. And the film could be used for protein adsorption repeatedly as the peak of N 1s reappeared after the film immerged in FITC-HSA solution for 6 h again. These experimental results suggest that this patterned superhydrophobic/superhydrophilic gold/PDMS composite can be used in barrierless immobilization and analysis of biological molecules, even cells. Additionally, the substrate could be simply cleaned by plasma treatment and then put into service again after heat treatment.

Conclusions In this paper, we have built a gold film with tunable wettability on PDMS by very simple electroless gold plating. A reversible switch between superhydrophilicity and superhydrophobicity rapidly could be realized on the gold film by the alternate process of plasma and heat treatment. Additionally, the superhydrophobicity of the gold film can change from gecko-foot-hair-like character to lotus-leaf-like character. On the basis of these properties, a self-made needle used in water CA measurement for the superhydrophobic surface and a patterned substrate with special wettabilities for inducing protein patterning were achieved. This approach to prepare gold surface with tunable wettability on PDMS shows its advantages in simple-operation, low-cost, and promising applications in many fields such as labon-chip and barrierless biomolecule patterning and analysis. Acknowledgment. This research was financially supported by the NSFC (Grant No. 20890021), the NSFC for Creative Research Groups (Grant No. 20821063), and the 973 Program (2007CB936404). Supporting Information Available: Cyclic voltammogram of the two-layer structured gold film in 0.5 M H2SO4 with a platinum wire as the auxiliary electrode and an Ag/AgCl electrode as the reference electrode. Stability of the superhydrophobic gold film. Atom percentage of Au, Si, C, O in different films. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(2), 1191–1198