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Surface Patterns Induced by Cu2+ Ions on BPEI/PAA Layer-by-Layer Assembly Meiwen Cao, Jinben Wang, and Yilin Wang* Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China ReceiVed NoVember 2, 2006. In Final Form: December 21, 2006 The multilayer films of branched polyethyleneimine (BPEI) and poly(acrylic acid) (PAA) have been fabricated with the layer-by-layer (LbL) method. Two characteristic courses of the film thickness growth are observed, which are the initial exponential-like growth and the following linear growth. The variation of the COOH/COO- ratio indicates that the ionization degree of the polyelectrolyte molecules decreases at the initial stage of the multilayer buildup and then levels off after about eight bilayers. The as-prepared (BPEI/PAA)n films show a relatively smooth surface. However, great morphology changes occur after immersing these films in Cu2+ or Zn2+ solution. In the case of n g7, wavelike surface patterns are induced to form on the films. Both wavelength and fluctuation of these surface patterns show a systematical variation with an increase of the bilayer number. Moreover, thermal treatment can stabilize these patterns and enable the preservation of them after releasing the Cu2+ ions from the LbL films by acidic treatment. Interestingly, only Cu2+ and Zn2+ can induce the formation of such surface patterns, whereas Fe2+, Ca2+, Ag+, and Na+ cannot. This phenomenon may closely relate to the different natures of the metal ions.
* To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn.
the pH or ionic strength of the dipping solutions.18 And, restructuring of the multilayer films after their buildup, which is of great importance from both the experimental and practical aspects, could also be achieved by treating these films with salt solutions or solutions of different pH.19-22 Many multilayer films with different characteristics have been prepared by controlling both the deposition and the following treatment conditions, such as the microporous films,22-24 the multilayer films with controllable bilayer composition and special surface wettability,25 the film with fast ion conduction,26 the multilayer films with different permeabilities for small molecular weight dyes, ions, and even gases,27-30 and the film that can be taken as a solid-state polymer electrolyte.31 Although extensive studies have been dedicated to the construction and utilization of the weak polyelectrolyte LbL films, some fundamental and mechanical knowledge has not been wellunderstood. Most of the previous LbL studies have been focused on the linear polyelectrolytes, while less attention has been paid to the branched ones. Besides, there are still many unknown aspects in the restructuring of the multilayer films in various conditions, e.g., after treatment with different salt solutions.
(1) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831. (3) DeLongchamp, D.; Hammond, P. T. AdV. Mater. 2001, 13, 1455. (4) Decher, G. Science 1997, 277, 1232. (5) Mendelsohn, J. D.; Yang, S. Y.; Hiller, J.; Hochbaum, A. I.; Rubner, M. F. Biomacromolecules 2003, 4, 96. (6) Chluba, J.; Voegel, J.-C.; Decher, G.; Erbacher, P.; Schaaf, P.; Ogier, J. Biomacromolecules 2001, 2, 800. (7) Serizawa, T.; Yamaguchi, M.; Matsuyama, T.; Akashi, M. Biomacromolecules 2000, 1, 306. (8) Fou, A. F.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (9) Wu, A. P.; Lee, J.; Rubner, M. F. Thin Solid Films 1998, 327, 663. (10) Eckle, M.; Decher, G. Nano Lett. 2001, 1, 45. (11) Rubner, M. F.; Stockton, W. B. Macromolecules 1997, 30, 2717. (12) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100. (13) Granick, S.; Sukhishvili, S. A. J. Am. Chem. Soc. 2000, 122, 9550. (14) Cho, J.; Caruso, F. Macromolecules 2003, 36, 2845. (15) Wang, L.; Wang, Z.; Zhang, X.; Shen, J. Macromol. Rapid Commun. 1997, 18, 509. (16) Wang, L.; Fu, Y.; Wang, Z.; Fan, Y.; Zhang, X. Langmuir 1999, 15, 1360. (17) Kotov, N. A. Nanostruct. Mater. 1999, 12, 789.
(18) Multilayer Thin Films; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, Germany, 2003. (19) Sukhorukov, G. B.; Schmitt, J.; Decher, G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 948. (20) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736. (21) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725. (22) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (23) Zhang, H.; Fu, Y.; Wang, D.; Wang, L.; Wang, Z.; Zhang, X. Langmuir 2003, 19, 8497. (24) Fu, Y.; Bai, S.; Cui, S.; Qiu, D.; Wang, Z.; Zhang, X. Macromolecules 2002, 35, 9451. (25) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (26) DeLongchamp, D. M.; Hammond, P. T. Chem. Mater. 2003, 15, 1165. (27) Shi, X.; Caruso, F. Langmuir 2001, 17, 2036. (28) Caruso, F.; Yang, W.; Trau, D.; Renneberg, R. Langmuir 2000, 16, 8932. (29) Antipov, A. A.; Sukhorukov, G. B.; Donath, E.; Mohwald, H. J. Phys. Chem. B 2001, 105, 2281. (30) Kim, B. Y.; Bruening, M. L. Langmuir 2003, 19, 94. (31) Lowman, G. M.; Tokuhisa, H.; Lutkenhaus, J. L.; Hammond, P. T. Langmuir 2004, 20, 9791.
Introduction The technique of layer-by-layer (LbL) deposition of polyelectrolyte multilayers first developed by Decher et al.1,2 has received much attention in recent years for its applications in many fields, such as electronics,3 biomaterials,4-7 and lightemitting materials.8-10 For the fabrication of LbL films using strong polyelectrolytes, electrostatic attraction has been utilized as the main driving force. Instead, for the layer buildup using weak polyelectrolytes, some weak interactions including hydrogen bonding and hydrophilic/hydrophobic interaction also play important roles. For example, Rubner,11,12 Granick,13 Caruso,14 Zhang,15,16 and their co-workers have constructed the LbL films with weak polyelectrolytes where hydrogen bonding was involved. Kotov17 has confirmed the important role of hydrophobic interaction in multilayer buildup. Because the ionization of a weak polyelectrolyte is very sensitive to its local ionic environment, it is possible to manipulate the molecular organization, composition, surface properties, and chemistry of the multilayer films with suitable adjustments of
10.1021/la063201a CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007
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Figure 1. The molecular structures of BPEI, LPEI, and NaPAA.
In this study, the LbL films have been constructed by branched/ linear polyethyleneimine (BPEI/LPEI) and poly(acrylic acid) (PAA). Then, the effects of Cu2+, Zn2+, Ca2+, and Na+ ions on these polymer films are studied. Interestingly, we find that Cu2+ and Zn2+ can induce the formation of surface patterns on the BPEI/PAA LbL films. Moreover, thermal treatment can stabilize these patterns and enable the preservation of them after releasing the ions by acidic treatment. However, Na+ and Ca2+ treatment cannot induce surface pattern formation. The aim of this work is to further understand the effect of polyelectrolyte architecture on the construction of the LbL films and is expected to probe into the restructuring of the multilayer films induced by different metal ions. Experimental Section Materials. Pure water (18 MΩ cm-1) was obtained from the Milli-Q system and used in all experiments. The solid substrates used were polished silicon wafers (100) (General Research Institute for Nonferrous Metals) and quartz slices. Poly(acrylic acid) sodium salt (NaPAA) (Mw ) 30 000) of 40 wt % solution in water and branched polyethyleneimine (BPEI) (Mw ) 25 000) were obtained from Aldrich. Linear polyethyleneimine (LPEI) (Mw ) 25 000) was purchased from PolySciences. The molecular structures of BPEI, LPEI, and NaPAA used in this study are shown in Figure 1. All materials were used as received without further purification. Substrate Preparation. Before use, the silicon wafers or quartz slices were ultrasonicated in detergent solution and acetone for 30 min, respectively. After rinsing with water, the substrates were treated in a freshly prepared piranha solution (mixture of H2SO4 (98%) and H2O2 (30%) with volume ration V/V ) 7/3) at 80 °C for 1 h. Then, the substrates were rinsed thoroughly with water and dried with a nitrogen stream. After such treatments, a thin oxide layer formed on the substrate surfaces. This left the surfaces perfectly wetted by water. LbL Film Construction. The concentrations of BPEI, LPEI, and PAA solutions for LbL deposition were 1.0 mg/mL. The pH of these solutions was adjusted to 4.7 ( 0.1 with HCl solution. Construction of the multilayer films was carried out at room temperature. The treated silicon or quartz substrates were first exposed to BPEI or LPEI solution for 10 min, followed by rinsing with pure water and drying with a nitrogen flow. Then, they were exposed to PAA solution for 10 min and then rinsed and dried. The cycle was repeated until the needed bilayer number was reached. The resultant multilayer films were denoted as (BPEI/PAA)n or (LPEI/PAA)n (n is the bilayer number). Formation and Stabilization of Surface Patterns. The asprepared (BPEI/PAA)n films were first immersed in different salt solutions for 10 min. Then, they were rinsed with water and dried under a nitrogen flow. After that, some Cu2+-treated films were exposed directly to HCl solution of pH 1.7, while others were first thermally stabilized by heating at 175 °C for 2.5 h and then treated by HCl solution. Film Characterization. The morphology images from atomic force microscopy (AFM) were recorded using a tapping mode (Nanoscope IIIa multimode system, Digital Instruments, Santa Barbara, CA) with silicon cantilever probes in air. All provided AFM morphology images are shown in height mode without any image processing except flattening. Analysis of the AFM images was carried out using the Nanoscope III software, version 5.12r2.
Figure 2. (A) Thickness growth of the (BPEI/PAA)12 multilayer film with the increase of layer number. The odd numbers (1, 3, 5, ...) relate to the BPEI layers, and the even numbers (2, 4, 6, ...) relate to the PAA layers. (B) Film thickness growth profile of the 1-6 BPEI/PAA bilayers shown as log thickness vs layer number. Two methods were used to carry out the thickness measurements. For the first eight layers, after every layer deposition, a sharp and clean needle was used to make several scratches in the film. Moderate force was applied to make sure that the silicon surface was not damaged. The film was then monitored by AFM. The film thickness values, i.e., the vertical distance from the multilayer film plane to the scratched valley bottom, were obtained from the section analysis. However, for the films beyond the eighth layer, the thickness values were detected using XP-2 Stylus Profiler (XPSP) after scratching. The reason we use two methods to carry out the thickness measurements is that the XPSP cannot give the exact values when the thickness is lower than 10 nm, while scratching of a thick film may produce dog-eared boundaries which will bring uncertainties into the thickness measurements. In order to compare these two methods, some overlapped measurements were conducted, and they were in good accordance with each other. The thickness values reported are averages of at least three measurements made on different areas of the same sample. UV-vis spectra were recorded using a JASCO UV-530 plus spectrophotometer. In the UV-vis measurements, quartz slices were used to carry out the LbL deposition. Transmission infrared (IR) spectra of the LbL films deposited on the silicon substrates were obtained by a JASCO FT/IR-600 Plus spectrophotometer. The background spectrum was from a bare treated silicon substrate. All spectra were recorded at a resolution of 4 cm-1.
Results and Discussion Construction and Characterization of (BPEI/PAA)n LbL Films. Thickness growth of the BPEI/PAA multilayer film with increasing the layer number is shown in Figure 2. The film thickness of the first through sixth bilayer shows an exponential-
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Figure 3. IR spectra of (A) (BPEI/PAA)n (n ) 1, 2, ..., 11 from bottom to top) and (C) (LPEI/PAA)n (n ) 1, 2, ..., 9 from bottom to top) films measured after each deposition cycle. IR-determined COOH/COO- absorbance ratio as a function of the bilayer number for (B) (BPEI/PAA)n and (D) (LPEI/PAA)n films, respectively.
like growth, as shown in Figure 2B. This result is similar to the results by Ji et al.,32 where an exponential growth of the PEI/ PAA multilayer films during the initial several bilayers was reported. However, the 7th through 12th bilayer show a linear dependence of film thickness on the number of deposited layers. At this stage, the bilayer thickness is close to a constant value averaged to be about 48 nm. This value is much larger than the bilayer thickness of the strong polyelectrolytes previously reported,33-35 which is usually smaller than 10 nm. Transmission IR is also used to follow the LbL process of the (BPEI/PAA)n film. Figure 3A,B presents the IR spectra of the (BPEI/PAA)n (n ) 1, 2, ..., 11) films and the plot of the IRdetermined COOH/COO- absorbance ratio as a function of the bilayer number. The COOH/COO- absorbance ratios are derived from the intensity values of the peaks at ∼1715 cm-1 for COOH and ∼1558 cm-1 for COO-. As pointed out previously,22,36-38 the COOH/COO- absorbance ratio can be used to determine the ionization degree of the PAA molecules. The plot in Figure 3B comprises three sections that exhibit different profiles. The first stage (I) of the first through fourth bilayer shows that the COOH/COO- absorbance ratio is small and only increases slightly. Instead, for the second stage (II) of 4-7 bilayers, the COOH/COO- absorbance ratio increases sharply to reach a high value. Then, in the third stage (III), the COOH/COO- absorbance ratio becomes almost constant at the high value. This variation of COOH/COO- absorbance ratio indicates that more and more functional groups of PAA are present (32) Ji, J.; Fu, J. H.; Shen, J. C. AdV. Mater. 2006, 18, 1441. (33) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (34) Sui, Z.; Salloum, D.; Schlenoff, J. B. Langmuir 2003, 19, 2491. (35) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249. (36) Xie, A. F.; Granick, S. Macromolecules 2002, 35, 1805. (37) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354. (38) Mu¨ller, M.; Rieser, T.; Lunkwitz, K.; Berwald, S.; Meier-Haack, J.; Jehnichen, D. Macromol. Rapid Commun. 1998, 19, 333.
in the un-ionized COOH form with the increase of the deposited layers. We should note that COOH groups can form different forms of hydrogen bonding that show different COOH peaks among themselves.39 With the increase of the layer number, a change in hydrogen bonding or in the molecular environment causes the shift of the COOH peak and broadens its band. This makes it difficult to get the exact peak values. Thus, only the COOH/COO- absorbance ratios for the first 11 bilayers are given here. For comparison, IR results of the LPEI/PAA multilayer films are also shown in Figure 3C,D. As presented in Figure 3D, the COOH/COO- absorbance ratio for (LPEI/PAA) multilayer films shows only two stages, a rapidly increasing region up to four bilayers and the plateau region for the following bilayers. The difference between BPEI/PAA and LPEI/PAA films in the variation of the COOH/COO- absorbance ratio indicates that the difference in the architecture of BPEI and LPEI significantly affects the LbL process. Previously, Mu¨ller et al.38 studied the multilayer buildup of the LPEI/PAA films using in situ attenuated total reflection (ATR)-FTIR spectroscopy. They found that both COO- and COOH absorbance bands show the following characteristics: (1) showing a continuous increase after each PAA adsorption step due to the subsequent layer deposition, (2) reaching a plateau region after about 10 cycles because the film thickness had exceeded the sampling depth of the evanescent field, and (3) showing fluctuations after each PAA or LPEI addition due to the protonation and deprotonation of COOH groups. In the present study, we find the similar protonation of COO- groups after each PAA adsorption and deprotonation of COOH groups after each LPEI adsorption using transmission IR. However, both COOH and COO- bands show an all-the-way increase in intensity, as shown in Figure 3C, and no plateau region as found by Mu¨ller (39) Dong, J.; Ozaki, Y.; Nakashima, K. Macromolecules 1997, 30, 1111.
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Figure 4. Representative AFM height images (30 × 30 µm2) and the corresponding section analysis of the as-prepared (BPEI/PAA)n films: (A) 8 bilayer, (B) 12 bilayer.
et al. is reached even after 9 bilayers. This difference should come from the different techniques. Transmission FTIR enables all of the deposited layers to be monitored, thus presents an all-the-way growth for the peak intensity. But for ATR-FTIR, only several layers on the top could be detected, so the peak intensity would keep a constant value after the adsorption situation becomes similar and the film thickness goes beyond the depth that the ATR-FTIR can reach. In the present work, by recording the COOH/COO- absorbance ratio after every BPEI/PAA or LPEI/PAA deposition cycle, further information has been provided, which helps us to understand the PEI/PAA LbL process. The layer deposition driven by different forces would give quite different layer thickness values. For the deposition of strong polyelectrolytes mainly driven by strong electrostatic interaction, the layer thickness is normally small because the polyelectrolyte molecules will adopt very straight morphology. The electrostatic repulsion among the same polyelectrolyte will prevent the further adsorption of the polyelectrolyte after the charged film surface is overcompensated by the oppositely charged polyelectrolyte molecules.35,40 In contrast, for the deposition of weak polyelectrolytes driven by some secondary interactions such as hydrophobic interaction and hydrogen bonding, more polyelectrolytes may be adsorbed, and these molecules can adopt a more coiled morphology with tails and loops that usually enlarge the layer thickness. Besides, the interpenetration of the polyelectrolytes between different layers may also play an important role in the LbL process and results in the layer thickness not being directly related to the adsorption amount. In the present system, the pKa values of the primary amine in BPEI is around 9, for the secondary amine around 8, and for the tertiary amine around 6-7.41 The pKa value of PAA in aqueous solution is in the range 4.55.5.22,42 The chains of BPEI and PAA should be partially ionized at pH 4.7. Moreover, for BPEI, the hydrocarbon regions of the backbone introduce hydrophobic regions to BPEI. Hence, the hydrophobic interaction among the polymer backbones may prefer BPEI adsorption. For PAA, the COOH-COOH hydrogen (40) Caruso, F.; Lichtenfeld, H.; Donath, E.; Mohwald, H. Macromolecules 1999, 32, 2317. (41) Wang, D.; Narang, A. S.; Kotb, M.; Gaber, O.; Miller, D. D.; Kim, S. W.; Mahato, R. I. Biomacromolecules 2002, 3, 1197. (42) Chung, A. J.; Rubner, M. F. Langmuir 2002, 18, 1176.
bonding increases its adsorption as well. Additionally, as demonstrated elsewhere,18 the substrate would also affect the adsorption of the polyelectrolytes for the initial several layers. Here, the substrate of silica is negatively charged (Si-O-).43 Then, upon deposition of the first BPEI layer, more positive charges may be induced on BPEI chain, which would cause the increase of its charge density. This inducement may be transferred to the sequentially adsorbed layers due to the interaction between the oppositely charged polyions of BPEI and PAA. The above knowledge together with the present experimental results indicate that electrostatic interaction, hydrophobic interaction, and hydrogen bonding play important roles in the construction of the (BPEI/PAA)n films; however, their roles experience an evolving course during the LbL process. At the beginning of the LbL process, a lower COOH/COO- ratio is present due to the substrate effect. Electrostatic interaction is the main driving force in this stage. The small layer thickness is a result of the straight morphology the polyelectrolyte molecules adopt. With the increase of deposited layers, the substrate effect is gradually screened. The ionization degree of the polyelectrolyte chains gets smaller. Hence, electrostatic interaction is weakened, whereas hydrophobic interaction and hydrogen bonding mentioned above are enhanced. Accompanying this, the adsorption amount increases and the polyelectrolyte molecules adopt more coiled morphology with tails and loops, causing a great increase of layer thickness. After about 12 layers, the substrate effect is fully screened, and the charge densities of the polyelectrolyte molecules keep relatively lower values. The larger contribution of hydrophobic interaction and hydrogen bonding generates the larger layer thickness. Formation of Ion-Induced Surface Patterns on the (BPEI/ PAA)n Films. Figure 4 presents several representative AFM height images of the as-prepared (BPEI/PAA)n films. As seen, all the films show relatively smooth surfaces. However, the morphologies of the film surfaces change greatly after treatment with Cu2+ solution. Figure 5 shows a series of morphology images of the (BPEI/PAA)n films treated with 5.0 mM Cu2+ solution for 10 min. As seen, in the case of n e 6, some protuberant structures emerge on the film surfaces (Figure (43) Liu, J. -F.; Min, G.; Ducker, W. A. Langmuir 2001, 17, 4895.
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Figure 5. Representative AFM height images (30 × 30 µm2) of the (BPEI/PAA)n (n ) 4, 6, 7, 8, 9, 10, 12, 14, and 16) films after treatment with 10 mM Cu2+ solution. Inset images are the corresponding FFT images and height scale bars.
Figure 6. The 2D PSD analysis of the morphology image of Figure 5(10).
5(4),(6)) and cause these surfaces to be rougher than the untreated films. Interestingly, for the (BPEI/PAA)n films with n g 7, the surfaces become more textured, and some regular wavelike surface patterns appear after Cu2+ treatment (Figure 5(7)-(16)). 2D fast Fourier transform (FFT) analysis of these surface patterns is shown as the inset images in Figure 5. The results show some isotropic diffusive rings. The well-defined rings indicate the existence of the characteristic average wavelengths, while their radial symmetry reveals that the surface patterns have no preferential orientation. Average wavelength and fluctuation of these patterns can be obtained from the isotropic power spectral density (PSD) analysis and section analysis. One example for the PSD plot corresponding to the image of Figure 5(10) is shown in Figure 6. The peak in the PSD plot is indicative of the wavelength value. The wavelength
is ∼2.0 µm/cycle for the surface pattern observed on the (BPEI/ PAA)10 film. Average wavelengths of these surface patterns and the average heights of the waves vary accordingly with the increase of the bilayer number, as shown in Figure 7. As seen from Figure 7A, the wavelength value is ∼0.7 µm for the surface pattern on the (BPEI/PAA)7 film. Then, the wavelength of the patterns increases nearly linearly with the increase of the bilayer number up to 12. Beyond 12 bilayers, the wavelength of these patterns reaches a nearly constant value of around 3 µm. Meanwhile, the average heights of the waves show quite similar changing profiles to the wavelength, as presented in Figure 7B. The heights vary from ∼10 nm for the (BPEI/PAA)7 film to ∼20 nm for the (BPEI/ PAA)8 film, then reach a constant value of 280 nm beyond 12 bilayers after a nearly linear increase. In order to know the effect of Cu2+ concentration on the surface pattern, the (BPEI/PAA)8 and (BPEI/PAA)10 films were also treated with 10.0 mM Cu(NO3)2 solution. The average wavelength and heights of the surface patterns treated with 5.0 and 10.0 mM Cu(NO3)2 are all listed in Table 1. As seen, the wavelength values are comparable for the two concentrations, while the height value shows a very slight increase with increasing Cu2+ concentration. The variation of the film thickness before and after Cu2+ treatment was also monitored. In comparison with the corresponding untreated BPEI/PAA films, the thickness values for the 6, 8, 10, and 12 bilayered BPEI/PAA films after Cu2+ treatment increase ∼6, ∼9, ∼16, and ∼22 nm, respectively. Lowman et
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Figure 8. UV-vis spectra of the (BPEI/PAA)10 films: (1) asprepared without Cu2+ treatment, and (2) after Cu2+ treatment. Table 2. Results of Treating the (BPEI/PAA)10 Film Using Various Salt Solutions results salt solutions Cu(NO3)2 (5.0 mM) CuCl2 (5.0 mM) Zn(NO3)2 (5.0 mM) CaCl2 (5.0 mM, 30.0 mM) NaNO3 (10.0 mM, 60.0 mM) NaCl (10.0 mM, 60.0 mM) Fe(NO3)2 (5.0 mM) AgNO3 (10.0 mM)
Figure 7. Variation of the wavelength and the average heights of the waves on the Cu2+-treated (BPEI/PAA)n films with the increase of the bilayer number: (a) variation of wavelength, and (b) variation of the wave height. Table 1. Average Wavelength and Height of the Surface Patterns on (BPEI/PAA)8 and (BPEI/PAA)10 Films Induced by Either 5.0 or 10.0 mM Cu2+ Solution wavelength (µm)
(BPEI/PAA)8 (BPEI/PAA)10
height (nm)
5.0 mM Cu2+
10.0 mM Cu2+
5.0 mM Cu2+
10.0 mM Cu2+
∼0.8 ∼2.0
∼0.9 ∼2.0
18 ( 5 150 ( 20
20 ( 9 160 ( 35
al.31 have described a similar swelling behavior and a surface roughness increase of LPEI/PAA multilayer films induced by reorganization of ionic interactions between LPEI and PAA after immersing the LPEI/PAA multilayer films into OEGDA solutions. In the present system, Cu2+ seems to have the similar effect as OEGDA in causing the film reorganization to elevate local areas on the surface. For the films with the bilayer number lower than 6, no obvious change was observed in the film thickness after Cu2+ treatment due to the very thin original films. Additionally, UV-vis is used to monitor the interaction of Cu2+ ions with the multilayer films. As presented in Figure 8, the as-prepared (BPEI/PAA)10 film shows no absorbance band in the whole wavelength range 200-800 nm. But after treatment with Cu2+ solution, a band at 265.5 nm appears. Both the amine groups of BPEI and carboxylate groups of PAA can coordinate with Cu2+ ions. They own different absorption bands in the UVvis spectra, which are a strong band at 285 nm for the Cu2+amine complex44 and a band at 260 nm for Cu2+-carboxylate complex.45,46 Here, the band at 265.5 nm should be due to the (44) Ungaro, F.; De Rosa, G.; Miro, A.; Quaglia, F. J. Pharm. Biomed. Anal. 2003, 31, 143. (45) Schuetz, P.; Caruso, F. AdV. Funct. Mater. 2003, 13, 929. (46) Schuetz, P.; Caruso, F. Chem. Mater. 2004, 16, 3066.
wavelength (µm)
height (nm)
∼2.0 150 ( 20 ∼1.9 145 ( 25 ∼0.8 11 ( 3 no surface pattern formation no surface pattern formation no surface pattern formation no surface pattern formation no surface pattern formation
overlapping of the bands at 285 and 260 nm. This confirms the incorporation of Cu2+ ions into the (BPEI/PAA)10 film. In order to know if other metal ions could have the same function as Cu2+ and if the nature of the counteranions has an effect on the film reorganization, the (BPEI/PAA)10 films are treated with different salt solutions. The results are listed in Table 2. As shown, Cu2+ and Zn2+ ions can induce the formation of the surface patterns, while Fe2+, Ca2+, Ag+, and Na+ cannot. Moreover, we also note that, no matter whether the counteranion is NO3- or Cl-, Cu2+ can induce the formation of surface patterns, while Na+ cannot. These results indicate that it is the metal ions but not the counteranions that play the key role in the surface pattern formation. As is well-known, both Cu2+ and Zn2+ are octahedral metal ions. They are not only able to form complexes with anions through their double positive charges, but are also able to accept electrons using their unoccupied orbitals. Thus, Cu2+ and Zn2+ coordinate strongly with -COO- groups of PAA as well as form chelates with amine groups of BPEI through electron transfer. However, Ca2+ and Na+ ions are only positively charged but have no unoccupied orbitals. They can only interact with -COO- groups of PAA through electrostatic attraction. Thus, when the LbL films are immersed in the solutions of Cu2+, Zn2+, Ca2+, and Na+, different kinds of molecular rearrangements take place in the films. This is the possible reason for the special selectivity of Cu2+ and Zn2+ ions in the inducement to the surface patterns in the present system. However, Fe2+ and Ag+, which also have unoccupied orbitals, cannot produce surface patterns on the multilayer films. Even in the cases of surface pattern formation by Cu2+ and Zn2+, the resultant patterns are not completely the same. For the BPEI/PAA films with the same thickness, Zn2+ induces a surface pattern with a much smaller wavelength and height than Cu2+. These results suggest us that the surface pattern formation induced by metal ions is a very complex process. Besides the charge and the occupied orbitals, other aspects, such as the size of the metal ions and affinity of the metal ions to the films would also affect the surface pattern formation.
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Figure 10. UV-vis spectra of the (BPEI/PAA)10 film: (1) after Cu2+ treatment and thermal stabilization; (2) after Cu2+ treatment, thermal stabilization, and HCl treatment.
Figure 9. AFM morphology images of the (BPEI/PAA)10 films: (A) as-prepared, (B) after Cu2+ treatment, (C) after Cu2+ and HCl treatment, and (D) after Cu2+ treatment, thermal stabilization, and HCl treatment. The inset images in B and D are FFT images. The top lines are section analyses of the corresponding images.
To know whether Cu2+ ions can induce similar surface patterns on LPEI/PAA multilayer films, we have done the parallel experiments of constructing the LbL films using LPEI and PAA and then treating them with Cu2+ solution. For the (LPEI/PAA)10 and (LPEI/PAA)12 films, the thickness values are 8 ( 0.5 nm and 10 ( 0.8 nm, respectively. These thickness values are much smaller than those of the (BPEI/PAA)10 and (BPEI/PAA)12 films. Previously, Hammond and co-workers47 reported similar results. In the present system, BPEI has three kinds of amine groups, primary, secondary, and tertiary amine groups, while LPEI has only secondary amine groups. Thus, the linear and branched PEI molecules would have a different amount of positive charges and unprotonated amine groups at the same pH value. This difference would affect the balance between electrostatic interaction and hydrophobic interaction and result in different deposition behaviors of polyelectrolyte molecules during LbL construction. No surface patterns are obtained on the surfaces of (LPEI/PAA)10 and (LPEI/PAA)12 films after treating with 5.0 mM Cu2+ solution for 10 min. The results clearly indicate that the molecular nature of PEI has a great influence on the characteristics of (PEI/PAA)n films. The different phenomena of the (BPEI/PAA)n and (LPEI/ PAA)n films while experiencing Cu2+ treatment may also be explained by the presence of different amine groups on the BPEI and LPEI chains that would have different interaction behaviors with Cu2+ ions. Another possible reason is that the thicknesses of (LPEI/PAA)10 and (LPEI/PAA)12 films are too thin to produce surface patterns after Cu2+ treatment. (47) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206.
Here, we propose a possible mechanism for the formation of the surface patterns induced by Cu2+ ions. As stated above, the integrity of the BPEI/PAA multilayer films is maintained by electrostatic interaction, hydrophobic interaction, and hydrogen bonding. Once the films are immersed in the Cu2+ solutions, Cu2+ ions would diffuse into the films and form complexes with both amine groups of BPEI and carboxylate groups of PAA. The original interchain balance forces would be weakened or even broken. Then, the rearrangement of the polyelectrolyte molecules would occur due to the breaking of the balance of forces within the film. This is quite like an annealing process. The molecular rearrangement would cause the change of strain that may further regulate the restructuring of the film. During the restructuring process, a local swelling of the film may happen and cause the elevation of the local surface. Accompanied by the local swelling, some local surface would be lowered, that is to say, the joint effect of both the change of the internal strain and the swelling of the local film may lead to the formation of the surface patterns. As to the variation of the wavelength and the wave heights of the surface patterns, it may be affected by the diffusing rate of Cu2+ ions into the (BPEI/PAA)n films. With a limited diffusion rate, Cu2+ ions can only diffuse into a certain depth of the films within a fixed time. When the thickness of the (BPEI/PAA)n film is smaller than the diffusion depth of Cu2+ ions in the film (n e 12), Cu2+ ions can penetrate the whole film, and their actual diffusion depth is the film thickness. While n is smaller than 7, the thin film is not enough to form an observable surface pattern via Cu2+-induced rearrangement. Beyond 7 bilayers, the actual diffusion depth of Cu2+ shows a linear increase; accordingly, both the wavelength and the fluctuation increase nearly linearly, in the surface patterns after Cu2+ treatment. However, when the (BPEI/PAA)n film thickness is larger than the diffusing depth that Cu2+ ions could reach (n > 12), Cu2+ ions cannot penetrate the whole film. Thus, no further increase of the wavelength and the fluctuation would be found with the further increase of film thickness. Stabilization of Cu2+-Induced Surface Patterns on the (BPEI/PAA)n Films. Figure 9 shows the typical 2D AFM morphology images of the (BPEI/PAA)10 films with different treatments. The as-prepared (BPEI/PAA)10 film before Cu2+ treatment shows a homogeneously smooth surface (Figure 9A). The section analysis reveals that the surface fluctuation is less than 5.0 nm. After being treated with 5.0 mM Cu2+ solution, the morphology is transformed to a regular wavelike pattern (Figure 9B). The average wavelength of the pattern is ∼2.0 µm from the PSD analysis, and wave heights show an elevated value of 150 ( 20 nm from the section analysis. Stability of the surface pattern in acidic condition is studied before and after thermal stabilization,
Surface Patterns in LbL Assembly
Figure 11. Representative IR spectra of a (PEI/PAA)10 multilayer film deposited on the silica substrate: (a) as prepared, and (b) after thermal stabilization and HCl treatment.
as shown in Figure 9C,D. After the Cu2+-treated film (Figure 9B) is directly immersed into HCl solution of pH 1.7, the surface pattern disappears and some randomly distributed micro- and nanopores appear (Figure 9C). The heights of the aggregates are in the range 50-100 nm. The reason for the formation of the pores may be that the PAA molecules tend to be zero-charged at pH 1.7 and the electrostatic interaction between BPEI and PAA is destroyed. Redissolution of the polyelectrolytes may happen and leads to the formation of pores. Interestingly, if the Cu2+-treated film is treated first by thermal stabilization, and then immersed in the same HCl solution, the surface pattern remains unchanged (Figure 9D). The UV-vis spectrum of Figure 10(1) shows that the position of the absorbance band at 265.5 nm remains unchanged after thermal treatment. However, the band at 265.5 nm disappears after the thermally treated film is put into HCl solution of pH 1.7 (Figure 10(2)). This means that the complexed Cu2+ is released from the films.46 We suggest that HCl treatment protonates the functional groups of the polyelectrolytes and destroys the Cu2+amine and Cu2+-carboxylate complexes. This result reveals that the thermal treatment can stabilize the patterned LbL films. Previously, Bruening and co-workers48 have used PAH and PAA solutions containing Cu2+ ions to construct the multilayer films. By treating these films using acid solution to remove the Cu2+ ions, free carboxylate groups could be obtained and the charge density could be controlled within the films. In the present (48) Balachandra, A. M.; Dai, J.; Bruening, M. L. Macromolecules 2002, 35, 3171. (49) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978.
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study, the surface patterns remain unchanged after releasing the Cu2+ ions from the thermal-stabilized films. These patterns may be further used as templates to produce other structures due to the presence of free carboxylate groups in the films. The stabilization mechanism is further studied by the IR measurements. Figure 11 shows the representative IR spectra of the as-prepared (BPEI/PAA)10 film without any treatments and the LbL film after thermal stabilization and HCl treatment. For the as-prepared film, the characteristic peak at ∼1708 cm-1 can be assigned to the -COOH carbonyl stretch and the peaks at ∼1560 and ∼1405 cm-1 ascribed to the -COO- asymmetric and symmetric stretches, respectively.39,49 After the film is thermal-stabilized and HCl-treated, the -COO- peaks become very weak, and the amide I peak12 appears at ∼1669 cm-1 (spectrum b). This suggests the formation of amide bonds between the amine groups of BPEI and the acid groups of PAA after the thermal treatment. Some protonated acid remains, but its peak shifts to a higher wavenumber of 1723 cm-1, indicating a change in hydrogen bonding or in environment.39,49 The amide bonds help to stabilize the LbL film and maintain its integrity. Thus, upon later acidic treatment, the surface pattern is stable and the polyelectrolyte molecules cannot be dissolved. Moreover, the stabilization effect of the thermal treatment can also be reflected by the long-term maintenance of the surface patterns. The wavelike surface patterns were distorted in about 1 month due to aging. However, after thermal stabilization, the stability can be greatly improved, and the surface patterns were preserved for at least 5-6 months.
Conclusions The (BPEI/PAA)n LbL films have been constructed, and the surface patterns on the (BPEI/PAA)n films have been generated by Cu2+ and Zn2+ ions in this work. The wavelength and fluctuation of the surface patterns can be easily adjusted by varying the layer number. After thermal stabilization, the Cu2+ ions can be released from these films by acidic treatment while leaving the surface patterns unchanged. However, Ca2+ and Na+ ions cannot induce the surface pattern in the films. The possible mechanism for the different effects of different ions on the morphologies of the (BPEI/PAA)n films has also been investigated. This study may introduce a novel approach to use ions to generate surface patterns on the LbL polymer films. Acknowledgment. We are grateful for financial support from the National Science Foundation of China (20233010, 20473101) and National Basic Research Program of China (2005CB221300). LA063201A
