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Langmuir 2009, 25, 672-675
pH-Amplified Exponential Growth Multilayers: A Facile Method to Develop Hierarchical Micro- and Nanostructured Surfaces Jinhong Fu,† Jian Ji,*,† Liyan Shen,† Alexander Ku¨ller,‡ Axel Rosenhahn,‡ Jiacong Shen,† and Michael Grunze*,‡ Department of Polymer Science and Engineering, Key Laboratory of Macromolecule Synthesis and Functionalization of Minster of Education, Zhejiang UniVersity, 310027 Hangzhou, China, and Institute of Applied Physical Chemistry, UniVersity of Heidelberg, Im Neuenheimer Feld 253, D-69210 Heidelberg, Germany ReceiVed NoVember 6, 2008. ReVised Manuscript ReceiVed December 2, 2008 We report a direct method to amplify the exponential growth of multilayers significantly by the alternating deposition of polyethylenimine (PEI) at high pH and poly(acrylic acid) (PAA) at low pH. The alternating pH switches the degree of ionization of the polyelectrolytes in the multilayers, which enhances the diffusion of PEI into and out of the film and hence increases the deposited mass per cycle. The synergetic action of the pH-tunable charge density and diffusivity of the weak polyelectrolytes provides a new method for the enhanced growth of multilayers with hierarchal microand nanostructured surfaces.
Introduction Biological hierarchical structures provide design criteria for functional micro- and nanostructured surfaces.1,2 Several methods including electrospinning, polymer phase separation, selfassembly, photolithography, and soft lithography can be used to fabricate these surfaces with specific properties.3-6 Most procedures involve complicated multistep processes and require a rigorously controlled environment, typically leading to high manufacturing costs. Layer-by-layer self-assembly (LBL) has been explored recently as a method to construct superhydrophobic coatings containing topographic features on the micro- and nanometer size scales.7-11 This method, which is based on the alternating deposition of oppositely charged polyelectrolytes, provides a simple, versatile, and robust tool for constructing ultrathin films with nanoscale topographical features. However, most layer-by-layer assembly processes are limited with respect to their ability to produce topographic features in the micro- and nanometer size ranges (hierarchical structures) simultaneously. Rubner and co-workers reported that when multilayer films made of weak polyelectrolytes (poly(allylamine hydrochloride) (PAH)/ poly(acrylic acid) (PAA)) are immersed in a low-pH solution the acid group is protonated, causing ionic bond breaking, chain rearrangement, and the formation of nano- and micropores in the thin films.10 However, more than 100 alternating depositions of PAA and PAH are required to create a microporous thin film, * Corresponding authors. E-mail:
[email protected], michael.grunze@ urz.uni-heidelberg.de. † Zhejiang University. ‡ University of Heidelberg.
(1) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (2) Smith, L. A.; Ma, P. X. Colloids Surf., B 2004, 39, 125. (3) Tadanaga, T.; Morinaga, J.; Matsuda, A.; Minami, T. Chem. Mater. 2000, 12, 590. (4) Lau, K.; Bico, J.; Teo, K.; Chhowalla, M.; Amaratunga, G.; Milne, W.; McKinley, G.; Gleason, K. Nano Lett. 2003, 3, 1701. (5) Xie, Q.; Fan, G.; Zhao, N.; Guo, X.; Xu, J.; Dong, J.; Zhang, L.; Zhang, Y.; Han, C. AdV. Mater. 2004, 16, 1830. (6) Han, J.; Lee, D.; Ryu, C.; Cho, K. J. Am. Chem. Soc. 2004, 126, 4796. (7) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (8) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713. (9) Shi, F.; Wang, Z.; Zhang, X. AdV. Mater. 2005, 17, 1005. (10) Zhai, L.; Cebeci, F.; Cohen, R.; Rubner, M. Nano Lett. 2004, 7, 1349. (11) Jisr, R.; Rmaile, H.; Schlenoff, J. Angew. Chem., Int. Ed. 2005, 44, 782.
which is time-consuming and most likely to be a challenge in any industrial application.10 For this reason, research efforts have been expended to enhance the buildup of multilayers.12-19 The growth of multilayer films, in which the deposited mass and the thickness increases exponentially rather than linearly with the number of deposition cycles, has been reported by the Strasbourg group.12-17 Picart, Schaaf, and co-workers ascribed the exponential growth to the ability of the polyelectrolyte to diffuse into and out of the film during each deposition step. The diffusivity of the polyelectrolyte was found to correlate with the concentration of uncompensated polycation/polyanion charges within the multilayer. However, the growth of multilayers still exhibited relatively slow growth behavior up to a maximum thickness of 100 nm per bilayer. Rubner and co-workers reported on the effect of pH on the protonation of weak polyelectrolytes. They demonstrated that an alternate deposition of PAH at high pH and PAA at low pH increased the total thickness of the film for a given number of multilayers.18 However, because both commercially available PAA and PAH in their case do not diffuse easily, the multilayered films assembled at different pH values still exhibited typical linear growth behavior up to a maximum thickness of 140 nm for each bilayer.18 Lynn and co-workers19 have synthesized low-molecular-weight PAA, which can diffuse, and they found that this can result in exponential growth when assembled alternately with PAH. However, the thicknesses of the films is still limited to 562 nm for 10 bilayers. Recently, some of us reported the possibility of enhancing the exponential growth of multilayers by adding silver nitrate, which also induced (12) Picart, C.; Lavalle, Ph.; Hubert, P.; Cuisinier, F.; Decher, G.; Schaaf, P.; Voegel, J. C. Langmuir 2001, 17, 7414. (13) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G.; Schaaf, P.; Voegel, J.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531. (14) Boulmedais, F.; Ball, V.; Schwinte, P.; Frisch, B.; Schaaf, P.; Voegel, J. Langmuir 2003, 19, 440. (15) Lavalle, P.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J.; Mesini, P.; Schaaf, P. Macromolecules 2004, 37, 1159. (16) Richert, L.; Lavalle, P.; Payan, E.; Shu, X.; Prestwich, G.; Stoltz, J.; Schaaf, P.; Voegel, J.; Picart, C. Langmuir 2004, 20, 448. (17) Lavalle, P.; Picart, C.; Mutterer, J.; Gergely, C.; Reiss, H.; Voegel, J.; Senger, B.; Schaaf, P. J. Phys. Chem. B 2004, 108, 635. (18) Choi, J.; Rubner, M. Macromolecules 2005, 38, 116. (19) Sun, B.; Jewell, C. M.; Fredin, N. J.; Lynn, D. M. Langmuir 2007, 23, 8452.
10.1021/la803692v CCC: $40.75 2009 American Chemical Society Published on Web 12/16/2008
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topographic features in films exhibiting micro- and nanostructures.20 Here we demonstrate that a combination of the concepts described by the Strasbourg group12-17 and by Rubner and coworkers18 enhances multilayer growth beyond that previously reported. Using the easily diffusing polyethylenimine (PEI) and poly(acrylic acid) (PAA) as polyanions allows us to control the charge density via pH and results in the growth of films as thick as 3.25 µm after only seven deposition cycles. More importantly, this procedure also provides a controllable way to produce hierarchical micro- and nanostructured surfaces simultaneously. Changing the pH allows us to tune the size range of the obtained morphologies. The topography of the multilayer structure can be fixed by thermal cross-linking and can be turned into a superhydrophobic surface by the chemical vapor deposition of (tridecafluoroctyl)-triethoxysilane.
Experimental Section Materials. Poly(acrylic acid) (PAA, Mw ) 100 000), polyethylenimine (PEI, Mw ) 25 000, water-free), and (tridecafluoroctyl)triethoxysilane were obtained from Sigma-Aldrich and Degussa Company (Germany), respectively. FITC-labeled PEI was prepared by adding 5 mg of FITC succinimidyl ester to 80 mL of 0.5% PEI aqueous solutions at 4 °C for 48 h. The free dyes were dialyzed in 0.05 M acetate acid aqueous solutions for 4 weeks. Preparation of Polyelectrolyte Multilayer Thin Films. PEI/ PAA multilayer films were constructed on 3-aminopropyltriethoxysilane (APTES)-coated glass slides using 1 mg mL-1 PEI and 3 mg mL-1 PAA aqueous solutions. The multilayer films were built by first immersing the substrate in the PAA solution for 15 min, followed by rinsing with pure water (with a pH of approximately 5.5) three times; the substrates were then immersed in PEI solution for 15 min, followed again by washing three times with pure water. The adsorption and washing steps were repeated until 15 layers were obtained. The multilayer films were thermally cross-linked at 180 °C for 2 h to preserve the surface morphology and to study their characteristics by SEM and AFM. Throughout the text, PAAm/PEIn is used to mean that multilayer films were constructed using PAA at pH m and PEI at pH n. For example, PAA5/PEI8 means that multilayer films were constructed with PAA at pH 5 and PEI at pH 8. Preparation of Hydrophobic Surfaces. Very low surface energy surfaces were obtained by the chemical vapor deposition of (tridecafluoroctyl)-triethoxysilane. Together with (tridecafluoroctyl)triethoxysilane, the multilayers films were sealed in a chamber and placed in an oven at 130 °C for 2.5 h. Then, the samples were withdrawn from the chamber and placed in an oven at 180 °C for 1.5 h to remove all unreacted silane molecules. Characterization. QCM measurements of dried multilayer films were performed with a KSV QCM-Z500 quartz resonator with both sides coated with Ag (F0 ) 9 MHz). Confocal laser scanning microscopy (CLSM) investigations were carried out with a Zeiss Axiovert 200 microscope equipped with a Bio-Rad Radiance 2100 confocal system using a ×40/1.4 oil-immersion objective and 0.4 µm z-section intervals. The multilayered films for the CLSM investigation were constructed on glass slides as described above, the only difference being that the FITC-labeled PEI replaced nonlabeled PEI in the 16th deposition layer. FITC fluorescence was detected after excitation at 488 nm with a cutoff dichroic mirror (488 nm) and an emission band-pass filter from 505 to 530 nm (green). To image the whole film, consecutive z sections were collected at 0.4 µm intervals. Vertical sections were calculated, and the film thicknesses were determined. A field-emission scanning electron microscope (FESEM; FEI, SiRion100) and an atomic force microscope (SPA400, Seiko Instrument, Inc.) in tapping mode were used to measure the surface morphology and to determine the rms roughness of the multilayer films. Sessile drop contact angles (CA) (20) Ji, J.; Fu, J.; Shen, J. AdV. Mater. 2006, 18, 1441.
Figure 1. (a) QCM frequency shifts of multilayer films assembled at pH PAA7.2/PEI7.2 (9), PAA5/PEI8 (2), and PAA2.85/PEI9 (b). (b) Thickness of the multilayer films prepared at pH PAA2.85/PEI9 as determined by SEM of the film cross sections.
were measured with a Dataphysics OCA20 contact angle system at ambient temperature. A 4 µL water droplet was dispensed onto the substrate at five different positions to obtain an average CA. We also used a tilted plate to determine the sliding angle; a 4 µL deionized water droplet was placed on the sample that was inclined slowly until the water drop started to move. The sliding angle was determined on a scale with a precision of 0.5°. Infrared spectra were taken with a dry-air-purged Bio-Rad model FTS 175c spectrometer. The system was equipped with a liquid-nitrogen-cooled MCT detector, an aluminum wire grid polarizer, and a multiangle reflection unit. Spectra were typically obtained at an angle of incidence of 75°.
Results and Discussion The polyelectrolyte multilayers were constructed by the alternate deposition of PEI at high pH and PAA at low pH. To quantify the effect of pH on the growth rate of the multilayers, the increases in film mass and thickness were monitored by QCM and SEM at different pH values. Figure 1 shows the amplification of film growth with increasing pH difference of the dipping solution as measured by the film mass and its thickness. Films prepared at pH 7.2 for both PEI and PAA grow, as expected, linearly with the number of depositions. A nonlinear increase in mass and thickness is observed when the multilayers are prepared at PEI 8.0/PAA 5.0. The nonlinearity in film growth becomes more pronounced with increasing pH difference. A film of 3.25 µm thickness (as measured by SEM) was obtained after seven deposition cycles with PEI at pH 9 and PAA at pH 2.85. PAA7.2/ PEI7.2 multilayer films remained transparent for up to eight bilayers; PAA5/PEI8 multilayer films became semitransparent at about five bilayers; and PAA2.85/PEI9 multilayer films became nontransparent at about four bilayers when PAA was the outermost layer. The buildup of the multilayers was also monitored by confocal laser scanning microscopy (CLSM) by adding fluorescently labeled PEI (PEI*). Figure 2a shows a vertical section through a (PAA/PEI)8 film containing a labeled PEI-FITC outermost layer. The green fluorescent zone represents a large, continuous band demonstrating the diffusion of PEI molecules into the film. The results are in agreement with previous findings by Hammond and co-workers that the PEI may diffuse in and out within the multilayers.21,22 By assuming that PEI-FITC has diffused through the whole film, the thickness of the film can be estimated to be around 3.8 µm. If we add one deposition cycle to the film, then the thickness of the green fluorescent zone increases from around 3.8 µm to above 6 µm, and the thickness increases to 10 µm when another PAA exposure is added, which suggests that (21) Yoo, P. J.; Nam, K.; Qi, J.; Lee, S.; Park, J.; Belcher, A. M.; Hammond, P. T. Nat. Mater. 2006, 15, 234. (22) Zacharia, N. S.; Modestino, M.; Hammond, P. T. Macromolecules 2007, 40, 9523.
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Figure 2. Vertical sections through different PEI/PAA multilayer films containing PEI-FITC obtained from confocal laser scanning microscopy. For all of the multilayer films, PEI-FITC was assembled in 16 layers. PEI* means PEI labeled by FITC (fluorescein isotiocyanate). The glass substrate is indicated by the red line. Scheme 1. Schematic Illustration of the pH-Amplified Growth of PAA/PEI Multilayer Films by Alternating Deposition of PEI at High pH and PAA at Low pHa
a (a) pH-amplified exponential growth of PAA/PEI starts with a negatively charged, PAA-terminated film. (b, c) The film is then immersed in high-pH PEI solution, which deprotonates the multilayer film and thus renders the charge negative. This drives PEI diffusion into the film, and (d) after rinsing, uncompensated for polycations remain in the film. (e) The multilayer films are then immersed in PAA solution with low pH, which protonates the multilayer. This causes uncompensated for PEI to diffuse out of the film and to react with PAA at the film/solution interface. (f) Finally, a negatively charged PAA-terminated surface is found. The alternating exposure of the complete film structure at high pH (in PEI solution) and low pH (in PAA solution) enhances the concentration of uncompensated for polycations/ polyanions within the multilayer, which amplifies the growth per deposition cycle.
PEI-FITC can diffuse out of the film. Thus, the exponential growth mechanism of the multilayer films relies on the diffusion of PEI molecules into and out of the multilayer films during each deposition cycle. Both polyethylenimine (PEI) and poly(acrylic acid) (PAA) are weak polyelectrolytes whose charge density can be changed by the pH of the solution. The alternating exposure of the complete film structure to high pH (in PEI solution) and low pH (in PAA solution) enhances the concentration of uncompensated for polycations/polyanions within the multilayer (Scheme 1). Hence, the PEI in the high-pH dipping solution not only will compensate for the countercharges in the surface layer but also will diffuse into the film to compensate for the bulk charges. When the multilayer is further brought in contact with PAA at low pH, the protonated PEI with uncompensated bulk
charges will diffuse out to the interface to be neutralized. Because the pH will affect the charge density in the complete film structure, the amount of uncompensated for polyelectrolyte is controlled by the concentration of uncompensated for bulk charges, which is proportional to the total thickness of the film. The synergetic action of the pH-tunable charge density and diffusivity of the weak polyelectrolytes causes accelerated multilayer growth. IRAS-FTIR was used to measure the degree of ionization of carboxyl groups of PAA molecules in the multilayer films. Two adsorption bands of the carboxylic acid functional groups of PAA were considered: ν ) 1565-1542 cm-1 (asymmetric stretching band of COO-) and ν )1710-1700 cm-1 (CdO stretching of COOH). Deconvolution of the vibrational bands was carried out with Microcal Origin software assuming a Gaussian band shape. The degree of ionization of PAA (R) at a given pH was calculated from R ) [A(COO-)]/[A(COOH) + A(COO-)] × 100(%), where A is the area under the bands.17 For the PAA/PEI multilayer films constructed at pH 7.2, the degree of ionization of carboxyl groups of PAA in the multilayers remains constant with the number of adsorption steps. However, when the multilayer films are constructed with PAA at pH 5 and PEI at pH 8, the degree of ionization of carboxyl groups of PAA alternates between 73% (PAA as the outermost layer) and 88% (PEI as the outermost layer). Upon increasing the pH difference of the dipping solutions, the effect of adsorption steps on the ionization of the carboxylic acid residues becomes more pronounced (e.g., for PAA2.85/PEI9, it alternates between 35 and 91%). To test the hypothesis that the amount of polyelectrolyte diffusion into and out of the film and hence the resulting incremental film thickness increase are proportional to the concentration of uncompensated for charge in the film, we fitted the QCM data for film growth to the expression y ) a exp(bx) + c, where y is the measured frequency shift after x deposition cycles, a is the point at which steep growth is initiated, and b is the rate of film growth.15,16 The values of exponent b were plotted against the difference in the degree of ionization of the multilayers assembled at different pH value (Figure 3). The linear relationship supports our model because it demonstrates that the growth rate (or incremental thickness increase) of the PAA/PEI multilayers is proportional to the degree of ionization of the bulk polyelectrolyte.
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Figure 3. Growth rate (or incremental thickness increase) expressed by exponent b extracted from the QCM data vs the difference in ionization of carboxyl groups ∆R at different pH values.
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constructed at PAA 2.85/PEI 9 exhibit distinct features on the micro- and nanometer length scales (i.e., vermiculate patterns of 500-1500 nm and micropores on the order of 10 µm (Figure 4c)). A magnification of the vermiculate patterns shows nanowrinkles of 100-200 nm (Figure 4d). The rms roughnesses of the multilayers prepared at 7.2/7.2, 8.0/5.0, and 2.85/9 are 23, 144, and 1329, respectively. For the rough surface of the PAA2.85/PEI9 multilayer, the water contact angle is very close to zero before cross-linking. However, the water contact angle of the multilayer after heat cross-linking treatment is about 110° because of the formation of hydrophobic amide groups and the surface roughness. The rough surfaces were further modified by the chemical vapor deposition of (tridecafluoroctyl)-triethoxysilane to render them more hydrophobic. The thickness of the (tridecafluoroctyl)triethoxysilane layer after cross-linking is about 2.5 nm according to the XPS measurement. The contact angle reaches 168° on the multilayer films constructed at 2.85/9, and the sliding angle becomes as small as 8°.
Conclusions
Figure 4. SEM images of 7.5 multilayers of PEI/PAA after heating cross-links constructed at different pH values: (A) PAA7.2/PEI7.2, (B) PAA5.0/PEI8.0, and (C) PAA2.85/PEI9.0. Image D is an enlargement of image C.
The multilayered films were thermally cross-linked (by heatinduced amide formation of the carboxylate-ammonium complexes23) to preserve the surface morphological features. Both scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to examine the topography of the multilayered films constructed at different pH pairs. Not shown here are the SEM data before thermal treatment, but they closely resemble those recorded after thermal cross-linking. A progressive increase in the roughness of the films is observed with increasing pH difference of the dipping solutions. Films prepared at pH 7.2 are flat and featureless with a slight granular structure (Figure 4a). A more globular structure emerges in the multilayer films prepared at PEI 8.0/PAA 5.0 (Figure 4b). The multilayer films (23) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978.
We demonstrated an easy, efficient method to amplify the growth of polyelectrolyte multilayers via the alternating deposition of PEI at high and PAA at low pH. The alternating pH switches the degree of ionization of the polyelectrolytes in the multilayers, which enhances their diffusion into and out of the film and hence increases the deposited mass per cycle. The synergetic action of the pH-dependent tunable charge density and diffusivity of the weak polyelectrolytes provides a new method for the enhanced growth of multilayers with hierarchal micro- and nanostructured surfaces. Therefore, surfaces exhibiting tunable topographic features in both the micro and nanometer size ranges can be achieved in only seven alternate deposition cycles of PEI and PAA. The topography of the multilayer structure can be fixed by thermal cross-linking and can be turned into a superhydrophobic surface by the chemical vapor deposition of (tridecafluoroctyl)-triethoxysilane. Compared to the previously described methods involving the addition of silver ions to enhance film growth,20 the described pH-modulation approach is simple and versatile. The strategy of LBL assembly at alternating pH, which deposits a diffusible weak polycation at high pH and a weak polyanion at low pH, provides a new route to the rapid, easy fabrication of multilayers with hierarchical topographic features. Acknowledgment. The work was funded by the EC Framework 6 Integrated Project AMBIO (Advanced Nanostructured Surfaces for the Control of Biofouling), the Natural Science Foundation of China (NSFC- 20774082, 50830106), Zhejiang Provincial Natural Science Foundation of China Y4080250, and the 863 National High-Tech R&D Program (2006AA03Z329, 2006AA03Z444). J.J. acknowledges support from NCET-050527. M.G. and A.R. acknowledge support from the Fonds der Chemischen Industrie. LA803692V
