Nanoporous Film by Water-in-Oil Emulsion

Water-in-oil (W/O) emulsion-induced micelles with narrow size distributions of ∼140 nm were prepared by sonicating the polystyrene-b-poly(2-vinylpyr...
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Langmuir 2007, 23, 12817-12820

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Fabrication of an Open Au/Nanoporous Film by Water-in-Oil Emulsion-Induced Block Copolymer Micelles Haeng-Deog Koh, Nam-Goo Kang, and Jae-Suk Lee* Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea ReceiVed September 18, 2007. In Final Form: NoVember 5, 2007 Water-in-oil (W/O) emulsion-induced micelles with narrow size distributions of ∼140 nm were prepared by sonicating the polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer in the toluene/water (50:1 vol %). The ordered nanoporous block copolymer films with the hydrophilic P2VP interior and the PS matrix were distinctly fabricated by casting the resultant solution on substrates, followed by evaporating the organic solvent and water. The porous diameter was estimated to be about 70 nm. Here, we successfully prepared the open nanoporous nanocomposites, the P2VP domain decorated by Au (5 ( 0.4 nm) nanoparticles based on the methodology mentioned. We anticipate that this novelty enhances the specific function of nanoporous films.

Introduction Nano- or microporous materials1-4 with controlled pore sizes and size distributions have become increasingly essential in a variety of applications such as optics,5 gas separation,6 biosensors,7 supports for catalysis,8 and drug release.9 Porous polymeric materials with the hydrophilic pores are especially useful for biological applications and selective transport and separation of biomolecules.10-12 One of the challenging strategies for fabricating the patterned surfaces originates from the self-assembling character of block copolymers. Hillmyer and co-workers suggested a fabrication method for nanoporous films induced by the chemical etching the hydrophilic core segments of core-shell block copolymer micellar films.13,14 However, there is some limitation in choosing the selectively degradable block copolymers. Recently, Hayakawa and co-workers successfully fabricated highly ordered porous block copolymer films, as the amphiphilic character of block copolymers can act as a surfactant to stabilize the inverse emulsion of water droplets in organic solvent.15 The porous films were prepared by casting the polymeric solution under moisture flow on various substrates. One of the limitations of the procedure was that the average porous diameter was only a few micrometers. In this work, we demonstrate the methodology of preparing H2O nanodroplets stabilized by an amphiphilic block copolymer * Corresponding author. Jae-Suk Lee, Prof., Dr. of Engineering. Department of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea, Tel: +82 62 970 2306, Fax: +82 62 970 2304, Email: [email protected]. (1) Lee, J.-S.; Hirao, A.; Nakahama, S. Macromolecules 1988, 21, 274. (2) Lee, J.-S.; Hirao, A.; Nakahama, S. Macromolecules 1989, 22, 2602. (3) Cho, Y.-H.; Cho, G.-J.; Lee, J.-S. AdV. Mater. 2004, 16, 1814. (4) Tung, P.-H.; Kuo, S.-W.; Jeong, K.-U.; Cheng, S. Z. D.; Huang, C.-F.; Chang, F.-C. Macromol. Rapid Commun. 2007, 28, 271. (5) Yu, X.; Lee, Y.-J.; Furstenberg, R.; White, J. O.; Braun, P. V. AdV. Mater. 2007, 19, 1689. (6) Wu, L.; An, D.; Dong, J.; Zhang, Z.; Li, B.-G.; Zhu, S. Macromol. Rapid Commun. 2006, 27, 1949. (7) Bayley, H.; Cremer, P. S. Nature (London) 2001, 413, 226. (8) Handbook of Porous Solids, Schu¨th, F., Sing, K. S. W., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 2002. (9) Berg, M. C.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Biomacromolecules 2006, 7, 357. (10) Kohli, P.; Harrell, C. C.; Cao, Z.; Gasparac, R.; Tan, W.; Martin, C. R. Science 2004, 305, 984. (11) Gasparac, R.; Mitchell, D. T.; Martin, C. R. Electrochim. Acta 2004, 49, 847. (12) Hilder, E. F.; Svec, F.; Fre´chet, M. J. Anal. Chem. 2004, 76, 3887. (13) Rzayev, J.; Hillmyer, M. A. Macromolecules 2005, 38, 3. (14) Mao, H.; Hillmyer, M. A. Macromolecules 2005, 38, 4038.

in an organic solvent, water-in-oil (W/O) inverse emulsion, to yield narrow size distributions compared to previous results.16 The emulsion-induced micelles (EIMs) acted as a precursor to fabricate the ordered nanoporous block copolymer film by casting the polymeric solution followed by evaporating the organic solvent and water nanodroplets. The unique functionality of the nanoporous films was attributed to decorating metallic nanoparticles (NPs) selectively at the arranged hydrophilic block segments of the interiors of the nanopores. Experimental Section Materials. For the micellar study, a polystyrene-block-poly(2vinylpyridine) (PS-b-P2VP) block copolymer with a molecular weight of 72 kg mol-1, a polydispersity index of 1.06, and a P2VP mole fraction of 30% was prepared using living anionic polymerization.17-19 For synthesizing Au NPs, Hydrogen tetrachloroaurate (III) (HAuCl4, 99%) purchased from Aldrich and monohydrated hydrazine (N2H4‚ H2O, 80%) from TCI were used without any treatments. Preparation of the Emulsion-Induced Micelles (EIMs) Using the PS-b-P2VP Block Copolymer, Synthesis of Au NPs, and Formation of an Ordered Au/Nanoporous Film. PS-b-P2VP was dispersed in the mixing solvent of toluene/deionized (DI) water (50:1 vol %) at 5 mg/mL. The resultant polymeric solution was sonicated by a Branson sonifier (W450 Digital) for 10 min at 40 °C followed by stirring for 24 h at room temperature to obtained EIMs. For preparing Au NPs, 0.1-0.5 equiv of HAuCl4 per pyridine unit of the block copolymer was dispersed into the micellar solution and stirred for 24 h at room temperature. The PS-b-P2VP-H+/AuCl4complex was reduced by dispersing 20-60 µL of H2O‚N2H4 solution (30 wt %, in DI water), respectively. The final solutions were stirred for 12 h to complete the reduction and centrifuged for 5 min at 3000 rpm to remove the salts. For the formation of an ordered Au/nanoporous film, the toluene solution of Au/EIMs at a concentration of 5 mg/mL was spin-coated on Si wafer at 1500 rpm. The resultant specimens were annealed for 12 h at 105 °C. Characterizations. To study the morphology, an energy-filtering transmission electron microscopy (TEM) [EF-TEM, EM 912 OMEGA (ZEISS, S-4700)] and field-emission scanning electron microscope (FE-SEM, Hitach S-4700) were used. For the clear measurement of specimens that were coated on carbon-coated copper (15) Hayakawa, T. Langmuir 2005, 21, 10288. (16) Koh, H.-D.; Kang, N.-G.; Lee, J.-S. Langmuir 2007, 23, 11425. (17) Shin, Y.-D.; Han, S.-H.; Samal, S.; Lee, J.-S. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 607. (18) Rahman, M. S.; Samal, S.; Lee, J.-S. Macromolecules 2006, 39, 5009. (19) Hirao, A.; Taunoda, Y.; Matsuo, A.; Sugiyama, K.; Watanabe, T. Macromol. Res. 2006, 14, 272.

10.1021/la702891q CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2007

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Figure 1. Scheme for the formation of water-in-oil (W/O) emulsion-induced micelles (EIMs) in organic solvent, the synthesis of Au NPs using EIMs containing the functional P2VP units that can be coordinated with the metallic precursor as a nanoreactor in the solution, and the fabrication of nanoporous films by casting the polymeric solutions on substrates followed by evaporating the organic solvent and H2O. grids, the P2VP domain of the block copolymer was stained by I2. TEM results were obtained at an acceleration voltage of 120 kV. Specifically, the SEM was measured at a 45°-tilted angle. The particle size analysis was performed using dynamic light scattering at 25 °C (DLS/Malvern Instrument, PCS) and the UV-spectrophotometer (CARY 1E) was used to verify the absorbance.

Results and Discussion In comparison to amphiphilic organic molecules20 with low molecular weights such as surfactants or lipids, amphiphilic diblock copolymers present diverse ways of producing different shapes of nanostructures or functionalities depending on their chemical structures or compositions.21-24 Our strategy for the fabrication of self-organized ordered nanoporous films originates from the amphiphilic character of block copolymers, which allows them to be used as a surfactant to stabilize H2O droplets in organic solvents, resulting in W/O EIMs. When the PS-b-P2VP was dispersed in the organic solvent, the micellar structure of a P2VP core and a PS corona is generally formed due to the different solubility of the solvent. The existence of H2O molecules in the solution can induce a different way to self-organize a block copolymer in solution as shown in Figure 1.25 When a sufficient amount of H2O coexists with toluene, the amphiphilic block copolymer can play an important role in stabilizing the mixing system.26,27 The sonication and continuous stirring of the resultant (20) Koh, H.-D.; Lee, J.-S. Macromol. Rapid Commun. 2007, 28, 315. (21) Fo¨rster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195. (22) Lopes, W. A.; Jaeger, H. M. Nature (London) 2001, 414, 735. (23) Jeneche, S. A.; Chen, X. L. Science 1999, 283, 372. (24) Yoon, B.-K.; Hwang, W.; Park, Y.-J.; Hwang, J.; Park, C.; Chang, J. Macromol. Res. 2005, 13, 435. (25) Hayagawa, T.; Horiuchi, S. Angew. Chem., Int. Ed. 2003, 42, 2285.

Figure 2. (a) SEM image at 45°-tilted angle of the open nanoporous block copolymer film after casting the polymeric solution on substrates followed by evaporating toluene and H2O. (b) Particle size distribution (Dh ) 140 nm) of EIMs dispersed in toluene by DLS.

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Figure 4. (a) UV-vis absorption spectra of Au/EIMs in the toluene solution that was prepared by different incorporation ratios of HAuCl4 after reduction: 0.1 equiv, 0.3 equiv, and 0.5 equiv. TEM images of Au/nanoporous films from (b) 0.1 equiv and (c) 0.5 equiv.

Figure 3. TEM image of an Au/nanoporous block copolymer film: (a) 3 days after reduction (inset: SEM image at 45°-tilted angle) and (b) 2 months after reduction. Au NPs synthesized by incorporating 0.3 equiv of HAuCl4 are embedded in the hydrophilic P2VP segments of the interiors of the nanopores in both (a) and (b) after casting the polymeric solutions on substrates followed by evaporating toluene and H2O. The specimens were prepared by casting the polymeric solution of EIMs followed by evaporating toluene and H2O.

solution results in emulsification, indicating that the block copolymer works as an emulsifier/surfactant to stabilize H2O droplets. As a result, W/O EIMs can be obtained. On the basis of previous results of an ordered core-shell micellar monolayer film from the toluene solution of PS-b-P2VP, we can design the fabrication of a nanoporous film by coating the EIM solution and evaporating toluene and H2O followed by an annealing process as shown in the schematic illustration. The nanoporous PS-b-P2VP film formed by coating and annealing the toluene solution of EIMs is clearly shown in the SEM image in Figure 2a. The average pore size and the thickness of PS-b-P2VP layer were estimated by SEM to be 70 ( 8 nm and 35 ( 5 nm, respectively. Note that the film allowed only a single layer of open nanopores and the open pores showed a narrow size distribution. The structure of the arranged hydrophilic P2VP segments remained on the interiors of the nanopores. On the whole, the results verified by DLS (Figure 2b) in solution were consistent with the SEM result. The average particle size of EIMs dispersed in toluene, Dh ) 140 nm, was confirmed by DLS. The overall particle size distribution in DLS also showed the narrow size distribution. (26) Mirjam, W.; Rothe, R.; Landfester, K.; Antonietti, M. Chem. Mater. 2001, 13, 4681. (27) Capek, I. AdV. Colloid Interface Sci. 2004, 110, 49.

The PS-b-P2VP can be used as a practicable nanoreactor to synthesize metallic or semiconductor NPs as well, because it contains the functional pyridine units that can be coordinated with most of the metallic precursors.28 It offers us the possibility of fabricating the nanoporous block copolymer films containing NPs selectively in the P2VP domain after casting process, thus expanding the functionality of open nanopore structure to optical, electronic, and biological fields.29 In our work, the synthesis of Au NPs was attempted using EIMs containing the P2VP segments in solution. When HAuCl4 was dispersed in the solution of EIMs, a complex of PS-b-P2VP-H+/AuCl4- was formed. The reduction of the coordinated complex by N2H4‚H2O resulted in the formation of Au NPs in the P2VP domain. After reduction, the solution of EIMs embedded with Au NPs showed a color change from yellowish to deep purple. Figure 3a shows the TEM image (inset: a long-range ordered SEM image) of the open nanoporous block copolymer monolayer film containing Au NPs prepared by incorporating 0.3 equiv of HAuCl4. The Au NPs, P2VP/PS, and nanoporous domains were assigned in the TEM image. This was well-matched with the self-organized EIM structure, H2O nanodroplets stabilized by the hydrophilic P2VP inner-shell and hydrophobic PS outershell, in toluene (Figure 1). The open nanopores with 85 ( 3 nm are clearly observed in the TEM image. The arranged hydrophilic P2VP block segments of the interiors of the nanopores were decorated with Au NPS with an average size of 5 ( 0.4 nm. As a result, the adoption of EIM structures containing the P2VP inner shell in toluene as templates resulted in the formation of well-defined Au nanonecklaces30 after the casting process on substrates. Due to the open nanopores with narrow size distribution, the Au nanonecklaces were uniform in size. We (28) Mo¨ssmer, S.; Spatz, J. P.; Mo¨ller, M. Macromolecules 2000, 33, 4791. (29) Yuranov, I.; Kiwi-Minsker, L.; Buffat, P.; Renken, A. Chem. Mater. 2004, 16, 760. (30) Y James, Q.; Worden, J. G.; Trullinger, J.; Huo, Q. J. Am. Chem. Soc. 2005, 127, 8008.

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anticipated that this would intensify the function of open nanoporous monolayer films. It is well-known that Au NPs dispersed in the solution are easily aggregated due to the strong particle-particle interaction. This has been considered as one of the drawbacks of Au NPs using block copolymers as a nanoreactor in solution. In our study, the solution of Au/EIMs dispersed in toluene showed long-term stability. Figure 3b shows the TEM image of an Au/nanoporous block copolymer (from 0.3 equiv of HAuCl4) fabricated by casting the polymeric solution 2 months after the Au NPs were synthesized in the solution of EIMs. Although some of the isolated Au NPs were observed and the number of embedded Au NPs decreased due to the isolated Au NPs as observed, the well-organized nanoporous structure after casting was still fabricated. Figure 4 shows the UV-vis absorption spectra of the toluene solutions of Au/EIMs as a function of different incorporated HAuCl4. The maximum wavelengths of absorption bands indicating the surface plasmon resonance of synthesized Au NPs were observed at ∼540 nm irrespective of the incorporation ratio of HAuCl4. However, the intensity of absorption bands increased, because the number of synthesized Au NPs in the P2VP interior of nanoporous films increased proportionally to the incorporation ratio as shown in Figure 4b,c.

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Conclusion In summary, a novel fabrication method for the ordered nanoporous block copolymer film was demonstrated using the amphiphilic block copolymers. This methodology originated from the self-organization behavior of block copolymers in solutions or during casting on substrates. Block copolymers successfully acted as a surfactant to stabilize the H2O segments, resulting in W/O EIMs with the long-term stability in an organic solvent. The nanoporous films were fabricated by casting the solution of EIMs followed by evaporating the organic solvent and H2O segments. The average porous diameter in films was estimated to be 70 nm by TEM. Au NPs with 5 ( 0.4 nm were selectively decorated at the hydrophilic block segments of the interiors of the nanopores for widening the application fields such as biological and electronic devices. Acknowledgment. This work was supported by the Korean Science and Engineering Foundation (R01-2004-000-10143-0) and the Program for Integrated Molecular Systems, GIST. We thank the Korea Basic Science Institute (KBSI) for EF-TEM analysis. LA702891Q