J. Phys. Chem. C 2007, 111, 431-434
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Superhydrophobic Patterned Film Fabricated from DNA Assembly and Ag Deposition Linglu Yang, Shuo Bai, Dunshen Zhu, Zhaohui Yang, Maofeng Zhang, Zhifeng Zhang, Erqiang Chen, and Weixiao Cao* The Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China ReceiVed: August 21, 2006; In Final Form: October 18, 2006
In this article we obtained a superhydrophobic patterned Ag film mainly by two steps. The first step is to fabricate a DNA film pattern through poly(dimethyldiallylammonium chloride) alternate deposition with DNA on a latent imaging film formed by the selective UV exposure of a photosensitive diazoresin/poly(acrylic acid) self-assembly (SA) film. The second step is to build up the patterned Ag film on a DNA film pattern with Ag electroless deposition. After surface modification with n-dodecanethiol the patterned Ag film, which exhibits obvious microstructures and nanostructures, will possess superhydrophobic properties and the contact angle can reach ∼162°.
Introduction
Experimental Section
Superhydrophobic surfaces with a contact angle (CA) higher than 150° are so attractive due to their important applications in daily life and industrial processes including protective coatings, microfluidic devices, and no-contaminant surfaces.1-3 Numerous methods have been developed to fabricate this kind of surface, including lithographic patterning,4,5 chemical vapor deposition,6 reconformation of polymers,7 plasma fluorination,8 electrodeposition,9,10 and galvanic displacement reaction.11,12 The structure of superhydrophobic surfaces can be regular or irregular depending on the different applications. Irregularly structured surfaces possess the advantages of simplicity and low cost and may be easier to apply practically. Regular structured surfaces provide useful models for quantitatively evaluating the relationship between contact angle and surface structure.13,14 However, the existing techniques to fabricate regular structured surfaces4,5,15,16 are complicated, expensive, or time-consuming. Therefore, developing a new method with low-cost and convenient operation is necessary.
Materials. Double-stranded DNA (herring sperm, Sigma), PAA (MW ) 100 000, Aldrich), PDDA (MW ) 200 000350 000, Aldrich) are commercially available and were used as received. DR, a photosensitive polyelectrolyte, was synthesized according to the literature.18 Silver nitrate, ammonia, SnCl2, formaldehyde, and potassium sodium tartrate are all analytic purity agents. Latent Image of the DR/PAA SA Film. Si wafer used as substrate was washed with ethanol and acetone alternately for 5 min in ultrasonic conditions. It then was immersed in an aqueous solution of DR (2 mg/mL) for 5 min, washed with deionized (DI) water thoroughly, and then immersed in PAA aqueous solution (2 mg/mL, pH ) 3) for 5 min, washed, and air-dried to complete a fabrication cycle. The processes were repeated four times to achieve a (DR/PAA)4 film on Si wafer. This photosensitive (DR/PAA)4 film was exposed upon UV light with a 300 W medium-pressure Hg lamp at a distance of ∼20 cm for 20 s through a photomask (∼5 µm resolution) to form a latent image, which can induce the deposition of PDDA and DNA. DNA/PDDA Film Pattern. The latent imaging (DR/PAA)4 film was immersed in an aqueous solution of PDDA (2 mg/ mL) for 5 min, washed with water thoroughly, and then was immersed in DNA aqueous solution (0.8 mg/mL, pH ) 4.6) for 5 min, washed, and air-dried to complete a fabrication cycle. The processes were repeated five times to achieve a (DNA/ PDDA)5 film pattern on the (DR/PAA)4 latent imaging film. Ag Electroless Deposition. The DNA film pattern was immersed in SnCl2/HCl aqueous solution (SnCl2‚2H2O 0.25 g; HCl 1 mL in 14 mL of water) for 15 min, rinsed with water, and then immersed in a fresh Ag electroless deposition solution (AgNO3 0.2 g, NH3‚H2O 0.5 mL, potassium sodium tartrate 1.0 g, and H2O 20 mL) for different times (1∼10 min) to allow Ag growth and form a patterned Ag film. Surface Modification of Patterned Ag Film. The patterned Ag film was immersed in an ethanol solution of n-dodecanethiol (1 × 10-3 M) for 24 h. After thorough washing with ethanol, a low-surface-energy monolayer of n-dodecanethiol was formed on the patterned Ag film.
DNA films have various applications in gene delivery, microelectronics, biosensor devices, diagnostics, chiral switches, etc.,17 and arouse a lot of interest. However, as far as we know there is no report on the formation of a DNA film pattern by means of DNA deposition on a latent imaging SA film. In addition, fabrication of patterned superhydrophobic metal films based on a DNA film pattern has been not reported. In this article, we report a novel method to achieve them. First we built up a latent imaging film by the selective exposure of a photosensitive diazoresin (DR)/poly(acrylic acid) (PAA) SA film. Then DNA and poly (dimethyldiallylammonium chloride) (PDDA) were deposited layer-by-layer on the latent imaging film to construct (DNA/PDDA)5 film pattern (briefly, DNA film pattern). The patterned Ag/DNA film (briefly, patterned Ag film) then was built up with Ag electroless deposition. After surface modification with n-dodecanethiol, a patterned Ag film with superhydrophobic properties was obtained. The water contact angle of this surface is higher than 150°. The main preparation processes are illustrated in Scheme 1.
10.1021/jp065407q CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006
432 J. Phys. Chem. C, Vol. 111, No. 1, 2007
Yang et al.
SCHEME 1: Schematic Representation of the Formation of Ag Film Pattern
Characterization. UV-visible spectra were recorded on a Shimadzu-2100 spectrometer. Scanning electron microscopy (SEM) micrographs were taken on a field-emission environmental scanning electron microscope (FEI, Quanta 200) at 15 kV. Atomic force Mmcroscope (AFM) images were achieved on Nanoscope-IIIa instrument (DI Company) in tapping mode. The contact angle was measured on an OCA20 instrument (Dataphysics Instrument Gmbh, Filderstadt, Germany). Results and Discussion The basic idea is that DNA deposition on the unexposed (DR/ PAA)4 film is much faster than that on the exposed one. It can be proved by the experimental data, that is, the absorbance of DNA film on the unexposed film reaches 1.3 (Figure 1a) but it only reaches 0.36 on the exposed film (Figure 1b) after five fabrication cycles. This result should be ascribed to the more -COOH groups existing in the unexposed film, which is favorable for the PDDA and DNA deposition driven by coulombic interaction. The -COOH groups of the exposed film had been almost reacted with -N2+ groups under UV irradiation.19
Figure 1. UV-Vis spectra of DNA/PDDA SA film fabricated on the unexposed (a) and exposed (b) (DR/PAA)4 film. Five fabrication cycles of DNA and PDDA are shown (bottom to top, 1-5).
The thickness of DNA film on the unexposed and exposed (DR/PAA)4 film was measured to be 210 and 85 nm, respectively, with a scratch method by AFM.20 The obvious difference of the thickness provides a way to fabricate DNA film pattern on the latent imaging (DR/PAA)4 film. Figure 2a shows the AFM image of a DNA film pattern fabricated on it. The separated circles ∼5 µm in diameter represent the concave areas of the pattern and correspond to the exposed areas on which thinner DNA films were coated. They are uniform and consistent with the photomask. The continuous area with light color represents the unexposed region of the latent image on which thicker DNA film was coated. From Figure 2b the depth of the concavities can be estimated to be ∼100 nm. DNA film pattern has uncompensated negative charge due to the existence of DNA component. The positive ions (Sn2+), therefore, can penetrate into the film and further induce the Ag electroless deposition. Figure 3a shows the SEM image of the DNA film pattern. We use the red dotted lines to represent the part of thicker DNA films built up on the (DR/PAA)4 film. After Ag deposition for 4 min, the Ag film with a starfishlike pattern (brighter areas of Figure 3b) was formed. With the deposition time increasing to 10 min, the starfishlike pattern can further develop to a continuous crosslike one as shown in Figure 3c, which is almost the same as the pattern of red dotted lines in Figure 3a. Therefore, we consider that both starfishlike and crosslike patterns are formed in a thicker DNA film because it can combine more Sn2+ ions to make faster Ag growth than in the thinner one. Figure 4a,b shows the planar and three-dimentional (3D) AFM images of the patterned Ag film, respectively. From them we can see that the Ag film surface is very rough and the starfishlike pattern is actually the thicker Ag film formed in thicker DNA film. The average height of the starfishlike pattern in Figure 4b (4 min deposition) was determined to be ∼150200 nm from the sectional analysis (Figure 4c). The starfishlike pattern will develop rapidly with increasing deposition time and can turn into the crosslike pattern with ∼1 µm in height in 10 min (not shown).
Figure 2. AFM image (30 µm × 30 µm) of the DNA film pattern grown on the latent image of (DR/PAA)4 film. (a) Planar image; (b) sectional analysis of image a.
Superhydrophobic Patterned Film
Figure 3. SEM images of DNA film pattern (a); after Ag electroless deposition for 4 min (b); and after Ag electroless deposition for 10 min (c).
Figure 4. AFM images (30 µm × 30 µm) of patterned Ag film deposited for 4 min. (a) Planar image; (b) 3D image; (c) sectional analysis of image a.
It is recognized that the surface structure and low-surfaceenergy coating are two necessary factors to determine the surface wetting properties.21 Therefore, we carried out a surface modification of the patterned Ag film (electroless depositiontime: 10 min) to achieve superhydrophobic properties. The lowsurface-energy coating was obtained after modifying an Ag film with n-dodecanethiol. From Figure 5a we can observe that the surface morphology of the patterned Ag film remains perfect after surface modification and there are regular structures in micrometer scale. In addition, from the enlarged images (Figure 5b,c) many small outshoots in nanometer scale on the crosslike
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Figure 5. SEM images of patterned Ag film after surface modification (a) and amplified images of a (b, c).
Figure 6. Shape of a water droplet on the patterned Ag film (a) before and (b) after surface modification with n-dodecanethiol in the same condition.
pattern can be observed. Therefore we predict that the patterned Ag films with obvious microstructures and nanostructures may have hydrophobic characters after surface modification. We carried out the contact-angle (CA) measurement22 of the patterned Ag film before and after surface modification. Figure 6a shows that the CA of the patterned Ag film without modification is only about 90° and does not possess a selfcleaning property. After modification, the surface property of the patterned Ag film increases obviously. The water drop with 4 µL volume can keep its spherical shape completely on it, and the contact angle reaches about 162° (Figure 6b). A little shape deformation of the water drop can be ascribed to the weight of itself. This phenomenon also proves that the low-surface-energy
434 J. Phys. Chem. C, Vol. 111, No. 1, 2007 coating is also an important factor for superhydrophobic properties of the Ag film pattern. In summary, the DNA film pattern was fabricated by means of DNA deposition on the latent image of the DR/PAA selfassembly film. Then the patterned Ag film was achieved with Ag electroless deposition. With increasing deposition time, the starfishlike pattern can be further developed to the crosslike one. After treatment with n-dodecanethiol, the modified patterned Ag film exhibits superhydrophobic properties with a contact angle of 162°. This kind of surface may be important in practical applications and theoretical research. Acknowledgment. We are grateful to the NSFC for financial support of this work (Grant 90406018). Supporting Information Available: EDX graph of DNA/ Ag film pattern (Ag deposition time 10 min). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Alessandrini, G.; Aglietto, M.; Castelvetro, V.; Ciardelli, F.; Peruzzi, R.; Toniolo, L. J. Appl. Polym. Sci. 2000, 76, 962. (2) Lam, P.; Wynne, K. J. K.; Wnek, G. E. Langmuir 2002, 18, 948. (3) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31. (4) Shiu, J. Y.; Kuo, C. W.; Chen, P.; Mou, C. Y. Chem. Mater. 2004, 16, 561. (5) O ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (6) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, C. A. J.; Milne, W. I.; Mckinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701.
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