Tunable Assembly of Nanoparticles on Patterned Porous Film

Sep 17, 2010 - This article references 46 other publications. 1. Cheng , W.; Wang , J. J.; Jonas , U.; Fytas , G.; Stefanou , N. Nat. Mater. 2006, 5, ...
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Tunable Assembly of Nanoparticles on Patterned Porous Film Bei-Bei Ke, Ling-Shu Wan,* Peng-Cheng Chen, Lu-Yao Zhang, and Zhi-Kang Xu MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Received August 1, 2010. Revised Manuscript Received September 4, 2010 This paper describes an approach to fully selective assembly of nanoparticles on patterned porous surface. Copolymers of polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) synthesized by atom transfer radical polymerization were used to prepare honeycomb-patterned porous films by the breath figure method. The regularity and pore size of the films can be modulated by changing the polymer composition and casting conditions such as concentration and airflow speed. Positively charged films were fabricated directly from the quaternized copolymers or by surface quaternization. X-ray photoelectron spectroscopy and adsorption of negatively charged fluorescein sodium salt confirmed the quaternization. Then assembly of negatively charged silica nanoparticles from its aqueous dispersion was performed. Results indicate that they assemble on the external surface of patterned porous films that without prewetting. For prewetted films, the nanoparticles assemble both on the external surface and in the pores. Poly(acrylic acid) deposited from its aqueous solution can serve as an effective blocking layer, which directs the selective assembly of nanoparticles into the pores, instead of the external surface of the film. It is concluded that the Cassie-Wenzel transition is the key to the selective assembly on the highly porous films. The well-defined selective assembly forms unique hierarchical structures of nanoparticles and greatly enlarges the diversity of structures of nanoparticle aggregates. This general approach also opens a straightforward route to the selective modification of patterned porous films.

Introduction The fabrication of two- or three-dimensional arrays of nanoparticles has received considerable attention due to their potential applications in photonic bandgap materials,1 optoelectronic devices,2 and biochips.3 For these applications precisely controlled and uniformly assembled structures are often preferred. Various methods, including flow-induced packing, spin coating, and electrostatic deposition, have been developed to direct the assembly of nanoparticles on patterned substrates.4-7 Physically or chemically patterned substrates are usually prepared by lithography techniques, such as photolithography,8,9 electron-beam lithography,10 ion-beam lithography,11 and soft lithography.12,13 Although they are proven successful in producing ordered patterns, these top-down approaches suffer from high equipment costs and time-consuming processes.14 As an alternative approach, *Corresponding author. E-mail: [email protected]. Fax: þ86-57187952605. (1) Cheng, W.; Wang, J. J.; Jonas, U.; Fytas, G.; Stefanou, N. Nat. Mater. 2006, 5, 830–836. (2) Kumar, S.; Seo, Y. K.; Kim, G. H. Appl. Phys. Lett. 2009, 94, 153104. (3) Velev, O. D.; Lenhoff, A. M.; Kaler, E. W. Science 2000, 287, 2240–2243. (4) Adamczyk, Z.; Nattich, M.; Barbasz, J. Adv. Colloid Interface Sci. 2009, 147-148, 2–17. (5) Xia, D. Y.; Biswas, A.; Li, D.; Brueck, S. R. J. Adv. Mater. 2004, 16, 1427– 1432. (6) Kim, Y. H.; Park, J.; Yoo, P. J.; Hammond, P. T. Adv. Mater. 2007, 19, 4426–4430. (7) Lee, I.; Zheng, H. P.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 2002, 14, 572–577. (8) Lee, K. J.; Pan, F.; Carroll, G. T.; Turro, N. J.; Koberstein, J. T. Langmuir 2004, 20, 1812–1818. (9) Kruger, C.; Jonas, U. J. Colloid Interface Sci. 2002, 252, 331–338. (10) Jeon, K. J.; Lee, J. M.; Lee, E.; Lee, W. Nanotechnology 2009, 20, 135502. (11) Altun, A. O.; Jeong, J. H.; Rha, J. J.; Kim, K. D.; Lee, E. S. Nanotechnology 2007, 18, 465302. (12) Yao, J. M.; Yan, X.; Lu, G.; Zhang, K.; Chen, X.; Jiang, L.; Yang, B. Adv. Mater. 2004, 16, 81–84. (13) Lalo, H.; Vieu, C. Langmuir 2009, 25, 7752–7758. (14) Gates, B. D.; Xu, Q. B.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171–1196.

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utilization of bottom-up molecular self-organization techniques in the fabrication of patterned substrates is quite attractive. The breath figure method, inspired by the foggy arrays of water droplets on cool solid surfaces, is a versatile, convenient, and cost-saving bottom-up process for fabricating honeycomb-patterned films.15-20 The water droplets condensed by solvent evaporation-induced rapid cooling act as templates for ordered arrays of spherical pores with diameters ranging from 50 nm to 20 μm.21-28 The patterned porous films are promising substrates for the assembly of nanoparticles. Honeycomb-nanoparticles hybrid structures were prepared by dropping nanoparticles dispersion onto the self-organized honeycomb-patterned films with a sliding apparatus.29 The nanoparticles are well packed into the pores by capillary force during evaporation. A one-step procedure was also demonstrated for the preparation of nanoparticles-decorated films by casting a mixed solution of polystyrene and CdSe nanoparticles (15) Bunz, U. H. F. Adv. Mater. 2006, 18, 973–989. (16) Connal, L. A.; Vestberg, R.; Gurr, P. A.; Hawker, C. J.; Qiao, G. G. Langmuir 2008, 24, 556–562. (17) Dong, W. Y.; Zhou, Y. F.; Yan, D. Y.; Mai, Y. Y.; He, L.; Jin, C. Y. Langmuir 2009, 25, 173–178. (18) Stenzel, M. H.; Barner-Kowollik, C.; Davis, T. P. J. Polym. Sci. Part A 2006, 44, 2363–2375. (19) Yabu, H.; Shimomura, M. Langmuir 2005, 21, 1709–1711. (20) Widawski, G.; Rawiso, M.; Francois, B. Nature 1994, 369, 387–389. (21) Connal, L. A.; Vestberg, R.; Hawker, C. J.; Qiao, G. G. Adv. Funct. Mater. 2008, 18, 3706–3714. (22) Cui, L.; Han, Y. C. Langmuir 2005, 21, 11085–11091. (23) Li, L.; Zhong, Y. W.; Ma, C. Y.; Li, J.; Chen, C. K.; Zhang, A. J.; Tang, D. L.; Xie, S. Y.; Ma, Z. Chem. Mater. 2009, 21, 4977–4983. (24) Munoz-Bonilla, A.; Ibarboure, E.; Papon, E.; Rodriguez-Hernandez, J. J. Polym. Sci. Part A 2009, 47, 2262–2271. (25) Tian, Y.; Ding, H. Y.; Shi, Y. Q.; Jiao, Q. Z.; Wang, X. L. J. Appl. Polym. Sci. 2006, 100, 1013–1018. (26) Zhang, Y.; Wang, C. Adv. Mater. 2007, 19, 913–916. (27) Zhao, B. H.; Zhang, J.; Wang, X. D.; Li, C. X. J. Mater. Chem. 2006, 16, 509–513. (28) Zhao, H. J.; Shen, Y. M.; Zhang, S. Q.; Zhang, H. M. Langmuir 2009, 25, 11032–11037. (29) Yabu, H.; Inoue, K.; Shimomura, M. Colloids Surf. A 2006, 284, 301–304.

Published on Web 09/17/2010

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under humid airflow.30 The combination of self-assemblies on different length scales leads to the formation of hierarchically structured nanoparticles arrays.31-33 However, these approaches can only achieve nanoparticles arrays decorated inside the pores and still have limitations with respect to the precise spatial control of nanoparticles on the three-dimensional substrates. Poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and its quaternary ammonium salts (quaternized PDMAEMA) are water-soluble polymers and have received increasing interest due to their potential applications in environmental protection, drug delivery, and antimicrobial materials. Block copolymers consisting of a hydrophobic block and a quaternized PDMAEMA block are potential precursors for charged honeycombpatterned films, which can be used as substrates for the assembly of charged species, including polyelectrolytes, inorganic nanoparticles and biomacromolecules. In this work, honeycomb-patterned porous films were prepared from copolymers polystyrene-block-poly(N,N-dimethylaminoethyl methacrylate) (PS-b-PDMAEMA) by the breath figure method and were positively charged by quaternization. Then, we present a facile approach to the manipulation of the assembly of nanoparticles on the positively charged honeycombpatterned porous films by tuning the wettability and electrostatic interaction (Scheme 1). Negatively charged silica nanoparticles were selectively assembled onto the external surface or into the pores of the films. To the best of our knowledge, it is the first report on the electrostatic assembly of nanoparticles as well as the wetting driven selective patterning of nanoparticles on the breath figure arrays. This versatile approach holds great promise for fabrication of hierarchically structured nanoparticles arrays and also opens a new route for selective modification of patterned porous films.

Experimental Section Materials. Styrene (St) and N,N-dimethylaminoethyl methacrylate (DMAEMA) were commercially obtained from Sinopharm Chemical Reagent Co. and distilled under reduced pressure before use. 1-Phenylethyl bromide (1-PEBr), N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA), iodomethane, fluorescein sodium salt, and poly(acrylic acid) (PAA, Mw ∼100 000) were used as received from Sigma. CuBr was purified by subsequently washing with acetic acid, ethanol and drying under reduced pressure. Monodispersed silica nanoparticles with a mean diameter of about 150 nm were prepared by hydrolysis of tetraethoxysilane in an alcohol medium in the presence of water and ammonia according to the procedure originally described by St€ ober et al.34 Poly(ethylene terephthalate) (PET) film was kindly provided by Hangzhou Tape Factory and cleaned with acetone for 2 h before use. Water used in all experiments was deionized and ultrafiltrated to 18 MΩ with an ELGA LabWater system. All other reagents were analytical grade and used without further purification. Synthesis of PS-b-PDMAEMA. PS-b-PDMAEMA was synthesized by ATRP using a reported procedure in tetrahydrofuran (THF) solution.35 First, St, 1-PEBr, CuBr, and PMDETA were added into a 25 mL round-bottomed flask with magnetic stirring bar. After degassed by three freeze-pump-thaw cycles, the flask was sealed under reduced pressure and immersed into an (30) Boker, A.; Lin, Y.; Chiapperini, K.; Horowitz, R.; Thompson, M.; Carreon, V.; Xu, T.; Abetz, C.; Skaff, H.; Dinsmore, A. D.; Emrick, T.; Russell, T. P. Nat. Mater. 2004, 3, 302–306. (31) Jiang, X. L.; Zhou, X. F.; Zhang, Y.; Zhang, T. Z.; Guo, Z. R.; Gu, N. Langmuir 2010, 26, 2477–2483. (32) Sun, W.; Ji, J.; Shen, J. C. Langmuir 2008, 24, 11338–11341. (33) Yu, C. L.; Zhai, J.; Li, Z.; Wan, M. X.; Gao, M. Y.; Jiang, L. Thin Solid Films 2008, 516, 5107–5110. (34) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (35) Zhang, X.; Matyjaszewski, K. Macromolecules 1999, 32, 1763–1766.

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Scheme 1. Selective Assembly of Silica Nanoparticles on Positively Charged Honeycomb-Patterned Porous Films

oil bath at 110 °C while stirring. After a prescribed time, the flask was opened and the solution was diluted with THF. The polymer was precipitated by pouring the solution into methanol. For polymerization of the second block, DMAEMA was mixed with PS-Br, CuBr, PMDETA, and THF in a 50 mL round-bottomed flask with magnetic stirring bar. After degassed by three freezepump-thaw cycles, the flask was sealed under reduced pressure and immersed into an oil bath at 50 °C while stirring. After a prescribed time, the reaction mixture was precipitated by pouring the solution into methanol followed by filtration and dried under reduced pressure at 50 °C to obtain the block copolymer. Quaternization of the Block Copolymers. Quaternization of the block copolymers was carried out using iodomethane (MeI). Copolymer (1.0 g) dissolved in THF (20 mL) was mixed with MeI (0.5 mL) at room temperature. After 2 h, the reaction mixture was precipitated by pouring the solution into methanol followed by filtration and dried under reduced pressure.

Preparation and Quaternization of Honeycomb-Patterned Films. The porous films were cast by the breath figure

method. In a typical experiment, an aliquot of 100 μL of carbon disulfide solution of the block copolymer was drop cast onto a PET substrate placed under a 1 L/min humid airflow. The humidity of the airflow was maintained to be above 80% by bubbling through distilled water and was measured by a hygrothermograph (DT-321S, CEM Corporation). After solidification, the film was dried at room temperature. The films fabricated from the unquaternized polymers were immersed in iodomethane/ methanol solution (1% v/v) at room temperature for further quaternization. After 2 h, the film was rinsed with methanol and dried at room temperature under reduce pressure. Adsorption of Fluorescein Salt. A piece of honeycombpatterned porous film (2  4 cm2) was immersed in 10 mL fluorescein sodium salt aqueous solution (0.1 mg/mL, pH 7.0) and incubated at 25 °C for 24 h. Then the films were washed with water six times. After being dried under reduced pressure at room temperature, fluorescence images of the films were recorded. Assembly of Silica Nanoparticles. The assembly of nanoparticles on the external surface of the honeycomb-patterned porous films was accomplished by direct dropping the dispersion of silica nanoparticles (0.5 wt %, pH 8.0) onto the films. After 10 min, the sample was rinsed several times with water and then dried in air. The assembly of nanoparticles on both of the external surface and in the pores of the films was conducted by prewetting the films with ethanol. For the third strategy, the film was first immersed into a PAA aqueous solution (1 mg/mL, pH 5.0) for 10 min to form a blocking layer. Then the film was rinsed with water and prewetted with ethanol. Nanoparticles arrays inside the pores were prepared by dropping a nanoparticles suspension on the PAA-blocking film. Characterization. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker (Advance DMX500) DOI: 10.1021/la1030608

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[M]/[I]

Mn,GPCa

Mw/Mna

ratiob

Mn,NMR

Mn,GPC-Qc

Mw/Mn-Qc

PS247-Br 300:1 25700 1.15 5:1 23600 1.22 247:2 26000 20900 1.20 PS247-b-PDMAEMA2 25:1 23400 1.24 247:14 27900 18500 1.21 PS247-b-PDMAEMA14 50:1 22100 1.27 247:35 31200 18200 1.25 PS247-b-PDMAEMA35 a GPC results with polystyrene as calibration standard. b Ratios of St to DMAEMA in the copolymers calculated from 1H NMR spectra. c GPC results for corresponding quaternized copolymers with polystyrene as calibration standard.

NMR instrument. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet FTIR/Nexus470 spectrometer. All spectra were taken by 32 scans at a nominal resolution of 1 cm-1. Molecular weight and its distribution were determined on a Waters gel permeation chromatograph (GPC) system at 25 °C, which consists of a Waters 510 HPLC pump, three Waters Ultrastyragel columns (500, 103, and 105 A˚), and a Waters 410 DRI detector. THF was used as the eluent at a flow rate of 1.0 mL/ min, and the calibration of the molecular weights was based on polystyrene standards. Field emission scanning electron microscope (FESEM, Sirion100, FEI) was used to observe the surface morphology of films after being sputtered with gold using ion sputter JFC-1100 and the energy dispersive X-ray spectroscopy (EDX) was used to map the Si element at the film surface. X-ray photoelectron spectroscopy (XPS) were recorded on a PHI-5000C ESCA system (PerkinElmer) with Al KR excitation radiation (1486.6 eV). The pressure in the analysis chamber was maintained at about 10-6 Pa during measurement. All survey and core-level spectra were referenced to the C1s hydrocarbon peak at 284.7 eV to compensate for the surface charging effect. The water contact angle was analyzed by a DropMeter A-200 contact angle system (MAIST Vision Inspection & Measurement Ltd. Co.) at room temperature. The fluorescence images were recorded by an inverted fluorescence microscope (Nikon TE2000).

Results and Discussion Synthesis and Characterization of PS-b-PDMAEMA. Honeycomb-patterned porous films have been prepared from a variety of polymers by the breath figure method. In an empirical fashion it has been shown that amphiphilic block copolymers, especially with a polystyrene block, can be easily cast into regular porous arrays. In this work, an amphiphilic block copolymer, PSb-PDMAEMA was synthesized by ATRP for the film preparation. The number-average molecular weight and molecular weight distribution of PS-Br measured by GPC are 25700 and 1.15, respectively (Table 1). The block copolymerization was carried out in THF at 50 °C and about 50% yield was normally obtained after a polymerization time of 24 h. Block copolymers with different lengths of PDMAEMA block were synthesized by changing the monomer-to-initiator ratios in feed. GPC analysis of the block copolymers confirms that the molecular weight distribution is relatively narrow (Mw/Mn = 1.22-1.27). However, the molecular weights of the block copolymers measured by GPC are slightly smaller than that of the macroinitiator, and decrease with the monomer-to-initiator ratio in feed (Figure S1). This is likely due to the adsorption of PDMAEMA block onto the GPC column leading to an increase in retention time and hence lower detected molecular weights.35-37 Figure 1a shows a typical 1H NMR spectrum of PS-bPDMAEMA in CDCl3. Peaks at 2.55 and 4.05 ppm are assigned to the protons of -OCH2CH2N< in the PDMAEMA block. The St/DMAEMA ratio in the copolymer can be calculated from the (36) Zhang, X.; Xia, J. H.; Matyjaszewski, K. Macromolecules 1998, 31, 5167– 5169. (37) Creutz, S.; Teyssie, P.; Jerome, R. Macromolecules 1997, 30, 6–9.

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Figure 1. 1H NMR spectra of PS247-b-PDMAEMA35 (a) before and (b) after quaternization (CDCl3).

integral of these peaks and that of the aromatic protons of PS at 6.4-7.2 ppm. Since the molecular weight of PS block was accurately measured by GPC, from the St/DMAEMA ratio we can deduce the chain length of the PDMAEMA block (Table 1, Figure S2). It is clear that Mn,NMR increases with the monomerto-initiator ratio in feed. Quaternization of the block copolymers was carried out using iodomethane (MeI) at room temperature. The 1H NMR spectrum for a typical quaternized copolymer is present in Figure 1b. Compared with that unquaternized, the peaks arising from the PDMAEMA block at 2.3-4.1 ppm are almost absent, which may be due to the solubility difference between the blocks. The PDMAEMA block is soluble in organic solvents such as THF, CH3Cl, and CH2Cl2. After quaternization, this block cannot dissolve in these solvents. It was reported that the peaks of PDMAEMA shifted downfield to 3.5-4.3 ppm in D2O after quaternization.38 However, in CDCl3, the quaternized amphiphilic blocks probably form cores of micelles, and thus the signals were shielded. The molecular weight and the distribution of quaternized copolymers were also measured by GPC (Table 1, Figure S1b). The measured molecular weights are smaller after quaternization and also decrease with the monomer-to-initiator ratio in feed. This is because of the adsorption onto the GPC column and the poor solubility of quaternized block in THF, leading to smaller hydrodynamic volume. Preparation of Honeycomb-Patterned Porous Films and the Pore Size Modulation. The synthesized copolymers before and after quaternization were cast from CS2 under a humid air flow to form porous films. To obtain reproducible and reliable results, each polymer was cast up to 20 times under the same condition. The obtained films show different pore sizes and qualities depending on the polymer used. As reported previously, (38) Butun, V.; Armes, S. P.; Billingham, N. C. Macromolecules 2001, 34, 1148– 1160.

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Figure 2. FESEM images of the porous films prepared from 3 mg/mL CS2 solution of different polymers under 1 L/min airflow: (a) PS247-bPDMAEMA2, (b) PS247-b-PDMAEMA14, (c) PS247-b-PDMAEMA35, (d) quaternized PS247-b-PDMAEMA2, (e) quaternized PS247-bPDMAEMA14, and (f) quaternized PS247-b-PDMAEMA35.

Figure 3. FESEM images of the porous films from PS247-b-PDMAEMA14 solutions in CS2 at different concentration under 1 L/min airflow: (a) 0.5, (b) 1, (c) 3, (d) 5, and (e) 10 mg/mL and (f) correlation between pore size and solution concentration.

homopolymer of PS is unable to routinely form regular pores,39,40 which is also confirmed by our results. Figure 2 shows the FESEM images of films prepared from PS-b-PDMAEMA copolymers with different PDMAEMA block lengths. Regular honeycomb-patterned porous films are obtained from unquaternized copolymers with relatively short PDMAEMA blocks (Figure 2a, b). In the case of film prepared from the copolymer with a longer PDMAEMA block (PS247-b-PDMAEMA35), the pore size distribution is wider. Moreover, an increase in the pore size with PDMAEMA block length can be observed. The quaternized copolymers lead to similar results with the corresponding unquaternized copolymers (Figure 3e-g). It can be concluded that the regularity and pore size of the films depend on the length of hydrophilic block. Although various mechanisms for the breath figure method have been proposed, the water templating mechanism is generally used to explain the formation of this porous morphology. The crucial point for the formation of a regular honeycomb-patterned porous film is preventing the coalescence of water droplets. This can be achieved by interfacial-active compounds, e.g., amphiphilic copolymers. The copolymers tend

to aggregate at the organic-water interface and to stabilize the water droplets. Copolymers with longer hydrophilic block can stabilize bigger water droplets and hence the pores are larger. When the hydrophilic segment is too long, the polymer cannot keep water droplets from coalescence. As a result, the regularity of the porous films decreases.41 After quaternization, the hydrophilicity of the PDMAEMA segment increases. It is reasonable that the films of quaternized copolymers have larger pore sizes compared with that of the corresponding unquaternized copolymers. Casting conditions such as concentration and airflow speed have great effects on the morphology and the pore size of the cast films. As shown in Figure 3, the pore diameter of PS247-bPDMAEMA14 film decreases obviously with the concentration. With increasing polymer concentration, more polymers precipitate at the organic-water interface. As a result, the water droplets are stabilized in a short time with smaller size. Therefore, there is an optimal concentration range for honeycomb-patterned porous films. When the concentration is too low or too high, irregular patterns are formed. Figure 4 shows films prepared from

(39) Peng, J.; Han, Y. C.; Yang, Y. M.; Li, B. Y. Polymer 2004, 45, 447–452. (40) Widawski, G.; Rawiso, M.; Francois, B. Nature 1994, 369, 387–389.

(41) Wong, K. H.; Davis, T. P.; Bamer-Kowollik, C.; Stenzel, M. H. Polymer 2007, 48, 4950–4965.

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Figure 4. FESEM images of the porous films from 3 mg/mL PS247-b-PDMAEMA14 solution in CS2 with different airflow speeds: (a) 0.5, (b) 1, (c) 2, (d) 4, and (e) 8 L/min and (f) correlation between pore size, relative humidity, and airflow speed.

Figure 5. FESEM images of the porous films of PS247-b-PDMAEMA14 (a) before and (b) after quaternization in iodomethane/ methanol solution. Insets show water droplet profiles on the films at room temperature.

PS247-b-PDMAEMA14 at various airflow speeds. At zero airflow speed, no comparable pores were generated on the film (data not shown). When the airflow speed was raised to 0.5-4 L/min, highly ordered pores are formed. Airflow facilitates the evaporation of solvent and decreases the pore size. Oppositely, the air flow also results in additional moisture and then increases the pore size (Figure 4f). Therefore, the pore size of the films generated at 0.5-4 L/min does not change almost. An 8 L/min airflow devastated the film, which may be due to the disturbance of water droplets arrangement by the strong airflow. Characterization of the Quaternized Films. The copolymer employed has a pKa of 7.5;42 thus, below this pH value the surface is positively charged. However, to create a more stable positively charged surface, quaternization is preferred in this work. Moreover, water cannot wet the pores whereas the solvent (methanol) for the quaternization can, which ensures the next tunable assembly. Positively charged films were fabricated by two strategies. One is direct casting of the quaternized copolymers, which has been discussed in the former section (Figure 2d-f); the other is by the surface quaternization of the PS-b-PDMAEMA films using iodomethane. The quaternization process at solid-liquid interface is highly active in mild condition. As shown in Figure 5, there is no significant change in the topography before and after the quaternization in iodomethane solution. Figure 6 shows the XPS N1s core-level spectra of the films. The PS-b-PDMAEMA film exhibits predominantly one N1s peak component at the binding energy of 399.1 eV. The N1s spectrum of quaternized films can be (42) Su, Y. L.; Li, C. J. Colloid Interface Sci. 2007, 316, 344–349.

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resolved into C-N (399.1 eV) and C-Nþ (401.9 eV) components.43 The degree of quaternization can be calculated by comparing the area of the peak of C-Nþ species to that of the total nitrogen species. The quaternization ratio for the film fabricated by the second strategy is about 60% and that from the quaternized polymer is as high as 80%. The quaternization of the honeycomb-patterned porous films is further confirmed by the adsorption of fluorescein salt. This dye is one of the most common fluorophore and is extensively employed in many fields. It is negatively charged in water and can easily adsorb onto the positively charged surface by electrostatic force. The control film prepared from polystyrene does not show any adsorption of fluorescein salt (Figure 7a). Unquaternized copolymer film is also partially positively charged at the present pH value (7.0) since this pH is slightly lower than the pKa (7.5). Therefore, a small amount of fluorescein salt adsorbs on the unquaternized film and emits weak fluorescence (Figure 7b). The honeycomb pattern of strong fluorescence emission can be observed from the quaternized films (Figure 7c,d), indicating the successful quaternization via both of the strategies. Tunable Assembly of Silica Nanoparticles. Monodispersed silica nanoparticles with mean diameter of 150 nm were prepared according to the St€ober method34 and chosen as the representative negatively charged nanoparticles. The assembly of nanoparticles was accomplished by applying a droplet of aqueous dispersion of the particles onto the positively charged films. It is to be noted that the films prepared by the two methods do not show significant difference in directing the assembly of nanoparticles. Therefore, films quaternized after casting were chosen as the representative template. The assembly was performed for 10 min followed by a standard washing step. Silica particle arrays assembled were observed by FESEM (Figure 8a). And, part of the top layer of the film was removed with an adhesive tape to expose the pores (Figure 8b). It is obvious that the nanoparticles are selectively adsorbed on the external surface of the film. The EDX map of silicon provides further confirmation of the selective assembly (Figure 8c). For comparison, the assembly of silica nanoparticles on a neutral substrate was studied, which was prepared from polystyrene-block-poly(2-hydroxyethyl methacrylate) (PSb-PHEMA).44 No nanoparticles are assembled on this neutral patterned porous film (data not shown). Thus, the assembly of (43) Yang, Y. F.; Wan, L. S.; Xu, Z. K. J. Membr. Sci. 2009, 326, 372–381. (44) Ke, B. B.; Wan, L. S.; Xu, Z. K. Langmuir 2010, 26, 8946–8952.

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Figure 6. XPS N1s core-level spectra of the porous films of (a) PS247-b-PDMAEMA14, (b) PS247-b-PDMAEMA14 quaternized after casting, and (c) PS247-b-PDMAEMA14 quaternized before casting.

Figure 7. Fluorescence images of the porous films after assembly

of fluorescein sodium salt. (a) PS247-Br, (b) PS247-b-PDMAEMA14, (c) PS247-b-PDMAEMA14 quaternized after casting, and (d) PS247-b-PDMAEMA14 quaternized before casting. The scale bars are 20 μm.

silica nanoparticles in our cases is primarily driven by electrostatic attraction between the negatively charged nanoparticles and the positively charged film. The selective assembly of nanoparticles on the external surface of the porous film relies on the wettability. For highly porous films that entrap air in the pores and prevent the pores being wetted by water, the contact angles (θM) can be theoretically calculated using Cassie and Baxter’s law:45 cos θM ¼ ð1 - fpore Þ cos θpolymer - fpore where fpore is the area fraction of pores and θpolymer is the water contact angle of a polymer in the form of a thin and smooth film. The fpore can be estimated from the FESEM images (e.g., fpore of sample shown in Figure 5b is about 0.53). It was reported that for honeycomb-patterned films prepared from polystyrene-based amphiphilic block copolymers, the external surface is mainly enriched with polystyrene.41,44,46 Considering that θPS is 89°, the calculated θM of PS-b-PDMAEMA porous film is about 121°, which is very close to the experimental result (120°, Figure 5b). That means the highly porous film cannot be wetted by water, forming a Cassie state. (45) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 0546–0550. (46) Ting, S. R. S.; Min, E. H.; Escale, P.; Save, M.; Billon, L.; Stenzel, M. H. Macromolecules 2009, 42, 9422–9434.

Langmuir 2010, 26(20), 15982–15988

Figure 8. (a) FESEM images of silica nanoparticles assembled on the positively charged films (quaternized after casting) without prewetting, (b) part of the top surface was removed, (c) silicon EDX map of the nanoparticle arrays, and (d) silica nanoparticles assembled on the film after prewetting with ethanol. Insets show water droplet profiles and contact angles.

After assembly of silica nanoparticles, the water contact angle decreases from 120° to 92° (Figure 8a). Considering that the contact angle of a smooth film decorated by silica nanoparticles is measured to be about 30°, the calculated θM is 97°, which is also close to the experimental value. In other words, aqueous droplet placed on the film assembled with silica nanoparticles is still in a Cassie state. Moreover, the sessile drop remains stable for at least 10 min, indicating water is prevented from infiltration into the pores during the assembly procedure. This nonwettable porous surface further ensures the selective assembly on the external surface. Prewetting, which induces a Cassie-Wenzel transition and hence allows infiltration of aqueous suspension of nanoparticles into the pores, can realize the assembly of silica nanoparticles inside the pores. Experimentally, white honeycomb-patterned porous films turn into semitransparent after being completely wetted with ethanol. After prewetting, the assembly of silica nanoparticles was conducted in the same manner with the former strategy. Adsorption of nanoparticles was observed both on the external surface and in the pores (Figure 8d). After drying, the film decorated with silica nanoparticles turns into white again and the contact angle value is about 95°. The assembly of nanoparticles on the prewetted neutral substrate (PS-b-PHEMA film) was also studied and the particles did not assemble on the film. DOI: 10.1021/la1030608

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Ke et al.

without PAA blocking layer (Figure 8d). This is because that the electrostatic repulsion of the negatively charged PAA layer partially counteracts the electrostatic attraction.

Conclusions

Figure 9. FESEM images of (a) silica nanoparticles assembled on the PAA-blocked film after prewetting with ethanol, inset shows water droplet profile and contact angle, (b) part of the top surface was removed.

The next objective is to assemble nanoparticles selectively in the pores and hence form separate nanoparticles aggregates. A facile approach is introduced for the fabrication of oppositely charged patterned porous films using a nonwetting solvent. The positively charged film was directly immersed in a PAA aqueous solution. This aqueous solution cannot infiltrate into the pores as discussed above and hence negatively charged PAA was specifically adsorbed on the external surface. The resultant film provides welldefined charged patterns to guide the assembly of silica nanoparticles. The contact angle value of the PAA-adsorbed film is 110°, indicating the pores of this film still can not be wetted without prewetting. Figure 9 shows the FESEM images of silica nanoparticles assembled on the prewetted films with PAA blocking layer. Obviously, the external surface is quite clear, and the nanoparticles are selectively assembled in the pores and form separate aggregates. It is to be noted that the amount of the adsorbed nanoparticles in each pore is smaller than that in films

15988 DOI: 10.1021/la1030608

We have developed a facile approach to selective assembly of nanoparticles on patterned porous surfaces. Amphiphilic block copolymer PS-b-PDMAEMA was synthesized by ATRP and used for the preparation of honeycomb-patterned porous films by the breath figure method. The regularity of the film is mainly determined by the composition of the copolymers. The pore size of the films can be modulated in the range of 1-5 μm by changing the concentration and airflow speed. Positively charged films were fabricated by direct casting of the quaternized copolymer or by surface quaternization. Silica nanoparticles can selectively assemble on the external surface or across all surfaces of the film by simple modulation of the wettability. PAA adsorbed on the external surface of the film can serve as an effective blocking layer to direct the selective assembly of nanoparticles into the pores. The Cassie-Wenzel transition is proved to be the key to the selective assembly on the highly porous films. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50803053) and the National Natural Science Foundation of China for Distinguished Young Scholars (50625309) Supporting Information Available: GPC, NMR, and FTIR results of the copolymer. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(20), 15982–15988