Poly(acrylic acid)-Bearing Photoreactive Azido Groups for Stabilizing

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Langmuir 2009, 25, 2949-2955

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Poly(acrylic acid)-Bearing Photoreactive Azido Groups for Stabilizing Multilayer Films Guanglu Wu,† Feng Shi,‡ Zhiqiang Wang,*,† Zan Liu,† and Xi Zhang*,† Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing 100084, P. R. China, and Key Laboratory for Nanomaterials, Ministry of Education, Beijing UniVersity of Chemical Technology, Beijing 100029, P. R. China ReceiVed December 25, 2008. ReVised Manuscript ReceiVed January 15, 2009 In this article, we have demonstrated a universal method for improving the stability of polyelectrolyte multilayer films by designing a photoreactive polyanion as the building block for layer-by-layer (LbL) self-assembly. By grafting an azido group into poly(acrylic acid), we synthesized a photoreactive polyanion, which can induce the photo-crosslinking between the azido group and polymeric backbone under UV irradiation. Our results show that after photocross-linking, the stability of the polyelectrolyte multilayer is greatly improved. Considering that the polyanionbearing azido group is highly reactive, we have shown that it can be used to stabilize different LbL films. Moreover, by taking advantage of the different stability before and after UV irradiation, a patterned surface can be fabricated, which could be used as a template for selective adsorption.

Introduction The layer-by-layer (LbL) self-assembly technique is a powerful method for fabricating layered nanostructures with tailored composition and architecture.1-3 It has attracted more and more attention for its simplicity, universality, and precise control of thickness in nanoscale. Until now, it has become a general method for fabricating functional multilayer films using almost allaqueous dispersion and functional species as building blocks, such as nanoparticles,4,5 carbon nanotubes,6,7 enzymes,8,9 polymer micelles,10-13 and dendrimers.14,15 Besides electrostatic interaction,16,17 different weak intermolecular interactions have been employed for LbL deposition as well, including hydrogen bond,18-20 charge transfer interaction,21-23 etc.3 However, weakness means unstable. These films are not sufficiently robust * To whom correspondence should be addressed. E-mail: [email protected]. edu.cn. † Tsinghua University. ‡ Beijing University of Chemical Technology. (1) Decher, G.; Schlenoff, J. B. Multilayer Thin FilmssSequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, Germany, 2002. (2) Tang, Z. Y.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203. (3) Zhang, X.; Chen, H.; Zhang, H. Y. Chem. Commun. 2007, 1395. (4) Xiong, H. M.; Chen, M. H.; Zhou, Z.; Zhang, X.; Shen, J. C. AdV. Mater. 1998, 10, 529. (5) Hao, E. C.; Wang, L. Y.; Zhang, J.; Yang, B.; Zhang, X.; Shen, J. C. Chem. Lett. 1999, 5. (6) Correa-Duarte, M. A.; Kosiorek, A.; Kandulski, W.; Giersig, M.; LizMarzan, L. M. Chem. Mater. 2005, 17, 3268. (7) Olek, M.; Ostrander, J.; Jurga, S.; Moehwald, H.; Kotov, N.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, 1889. (8) Lvov, Y.; Moehwald, H. Protein Architectures: Interfacing Molecular Assemblies and Immobilization Biotechnology; Marcel Dekker: New York, 2000. (9) Sun, J. Q.; Sun, Y. P.; Wang, Z. Q.; Sun, C. Q.; Wang, Y.; Zhang, X.; Shen, J. C. Macromol. Chem. Phys. 2001, 202, 111. (10) Lee, G. S.; Lee, Y. J.; Yoon, K. B. J. Am. Chem. Soc. 2001, 123, 9769. (11) Ma, N.; Zhang, H. Y.; Song, B.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2005, 17, 5065. (12) Ma, N.; Wang, Y. P.; Wang, Z. Q.; Zhang, X. Langmuir 2006, 22, 3906. (13) Biggs, S.; Sakai, K.; Addison, T.; Schmid, A.; Armes, S. P.; Vamvakaki, M.; Buetuen, V.; Webber, G. AdV. Mater. 2007, 19, 247. (14) Zhang, H. Y.; Fu, Y.; Wang, D.; Wang, L. Y.; Wang, Z. Q.; Zhang, X. Langmuir 2003, 19, 8497. (15) Huo, F. W.; Xu, H. P.; Zhang, L.; Fu, Y.; Wang, Z. Q.; Zhang, X. Chem. Commun. 2003, 874. (16) Decher, G. Science 1997, 277, 1232. (17) Decher, G.; Hong, J. D. Makromol. Chem., Macromol. Symp. 1991, 46, 321.

for use in some rigorous environment, especially for a solution with a high ionic strength, high pH, or low pH. Therefore, improving the stability of a LbL multilayer film is a challenge for the LbL technique. Numerous approaches for enhancing the stability of multilayer films have been developed. One of them is to use step-by-step reaction which forms a covalent bond during the LbL assembly process. Chemical reactions with features like mild reaction conditions, good control, and high yield are required in this regard.24-28 A typical example is the click reaction of azide and alkyne used by Caruso et al. to realize the covalent LbL assembly.29 The coordination bond is also regarded as a strong driving force for gaining robust LbL films.4,30,31 Furthermore, Kunitake et al. introduced a surface sol-gel process for preparation of nanocomposite films which built a stable covalent framework based on the hydrolysis of the chemisorbed alkoxides.32,33 All the methods described above require very well designed building blocks. That is, for click chemistry, azide and alkyne should be introduced to different building blocks, and for the coordination method, ligands should be placed into appropriate positions of the two different building blocks. (18) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509. (19) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (20) Quinn, J. F.; Caruso, F. AdV. Funct. Mater. 2006, 16, 1179. (21) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (22) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1998, 14, 2768. (23) Wang, X. Q.; Naka, K.; Itoh, H.; Uemura, T.; Chujo, Y. Macromolecules 2003, 36, 533. (24) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962. (25) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 4655. (26) Chan, E. W. L.; Lee, D. C.; Ng, M. K.; Wu, G. H.; Lee, K. Y. C.; Yu, L. P. J. Am. Chem. Soc. 2002, 124, 12238. (27) Major, J. S.; Blanchard, G. J. Langmuir 2001, 17, 1163. (28) Zhang, F.; Jia, Z.; Srinivasan, M. P. Langmuir 2005, 21, 3389. (29) Such, G. K.; Quinn, J. F.; Quinn, A.; Tjipto, E.; Caruso, F. J. Am. Chem. Soc. 2006, 128, 9318. (30) Yang, H. C.; Aoki, K.; Hong, H. G.; Sackett, D. D.; Arendt, M. F.; Yau, S. L.; Bell, C. M.; Mallouk, T. E. J. Am. Chem. Soc. 1993, 115, 11855. (31) Byrd, H.; Holloway, C. E.; Pogue, J.; Kircus, S.; Advincula, R. C.; Knoll, W. Langmuir 2000, 16, 10322. (32) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Lett. 1996, 831. (33) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1997, 9, 1296.

10.1021/la804261f CCC: $40.75  2009 American Chemical Society Published on Web 02/06/2009

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Wu et al. Scheme 1. Building Blocks Used in the LbL Fabrication

A more convenient way is to combine the LbL deposition with postchemical reaction. Bruening and co-workers cross-linked the multilayer film of (PAA/PAH)n by a post-thermally induced chemical reaction, and then the film’s stability was significantly improved.34 Our group first proposed and realized the concept of post-photochemical reaction. A photoreactive and watersoluble polycation, diazoresin (DAR), was employed with other building blocks to form a multilayer film in a conventional way. After that, by application of UV irradiation, the film can be cross-linked through the chemical reaction of the diazo group with adjacent groups, leading to the enhancement of the film’s stability.35,36 This method has proved to be a successful way of acquiring a robust film for the applications in chemical sensors, patterned surface, surface imprinting, etc.35-40 However, DAR is far from perfect. First, the diazo group is overactive and will decompose in high-pH environments which means that the LbL deposition is limited only over a narrow pH range. Second, because of the positive charge of the diazo group, it can be used for fabrication of only multilayer films with negatively charged building blocks. Therefore, if there is another system that can be used for the stabilization of positively charged building blocks, it will make the LbL-postphotochemical reaction method more applicable, suitable not only for negatively but also for positively charged building blocks. Furthermore, if the new system can work over a wider pH range, it will have an even broader application. Herein, we design a new photoreactive polyanion by grafting an azido group into poly(acrylic acid), noted PAA-N3. As a polyanion, PAA-N3 has proved to be a good building block for electrostatic LbL assembly for formation of multilayer films. (34) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978. (35) Sun, J. Q.; Wu, T.; Sun, Y. P.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Cao, W. X. Chem. Commun. 1998, 1853. (36) Zhang, X.; Wu, T.; Sun, J. Q.; Shen, J. C. Colloids Surf., A 2002, 198-200, 439. (37) Cao, T. B.; Yang, S. M.; Yang, Y. L.; Huang, C. H.; Cao, W. X. Langmuir 2001, 17, 6034. (38) Cao, T. B.; Wei, L. H.; Yang, S. M.; Zhang, M. F.; Huang, C. H.; Cao, W. X. Langmuir 2002, 18, 750. (39) Shi, F.; Wang, Z. Q.; Zhao, N.; Zhang, X. Langmuir 2005, 21, 1599. (40) Shi, F.; Liu, Z.; Wu, G. L.; Zhang, M.; Wang, Z. Q.; Zhang, X.; Willner, I. AdV. Funct. Mater. 2007, 17, 1821.

Upon UV irradiation, the multilayer films can be cross-linked because of the photoreaction between the azido group and polymeric backbone, thus greatly improving the stability of the multilayer film. By taking advantage of the different stabilities before and after UV irradiation, one may prepare a patterned surface. In addition, the azido group is shown to be stable in both low- and high-pH environments, which ensures that PAA-N3 can be applied to a wider range of pH than DAR. Furthermore, PAA-N3 can work with several kinds of positively charged building blocks such as weak polyelectrolyte PAH, strong polyelectrolyte PDDA, and the porphyrin derivative with four positively charged sites. We believe PAA-N3 is a universal system for stabilizing positively charged building blocks, and combined withDARfornegativelychargedones,theLbL-postphotochemical reaction method can be used for fabricating robust multilayer films in applications for different conditions.

Experimental Section Materials. The molecule and polyelectrolyte used in the LbL assembly were 5,10,15,20-tetrakis[4-(trimethylamino)phenyl]21H,23H-porphine tetratosylate (Por), poly(allylamine hydrochloride) (PAH; MW ) 70000), and poly(diallyldimethylammonium chloride) (PDDA; MW ) 400000). Poly(acrylic acid) (PAA; MW ) 450000) and 4-azidoaniline were used in the synthesis of polycation-bearing azido groups (PAA-N3). Branched poly(ethylenimine) (BPEI; MW ) 50000), 50% aqueous solution, and (3-aminopropyl)trimethoxysilane (APTS) were used in the preparation of substrates. All of the reagents described above except APTS were purchased from Sigma-Aldrich. N-Hydroxysuccinimide (NHS) and APTS were purchased from ACROS. N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide (EDC) was purchased from Beijing Chemical Reagent Co. All the reagents were used as received. Quartz slides were purchased from Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences. Some of the building blocks used are listed in Scheme 1. Instruments and Measurements. UV-vis spectra were recorded on a Hitachi U-3010 spectrophotometer using quartz slides as the substrates. FT-IR spectra were recorded on a Bruker IFS-66v/S FTIR spectrometer using CaF2 plates as the substrates. The 1H NMR spectra were recorded on a JEOL ECA-300 apparatus. Atomic force microscopy (AFM) images were taken with a commercial multimode Nanoscope IV atomic force microscope under ambient conditions. AFM was conducted in tapping mode with an optical readout using

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Figure 1. UV-vis spectra (a) and FT-IR spectra (b) for the fabricating process of a PAA-N3/PAH multilayer film in aqueous solution and measured after the deposition of each layer or bilayer. The insets show the layer number dependence of absorbance at 269 nm (a) and transmittance at 2117 cm-1 (b).

Si cantilevers. Silicon slides were employed as the substrates in AFM experiments. The irradiation light source was a high-pressure mercury lamp (RW-UVA œ 05-100) with an optical fiber purchased from Shenzhen Runwing Mechanical&Electrical Co., Ltd., and the intensity was 100 mW/cm2. Synthesis of Polycation-Bearing Azido Groups. PAA-N3 was synthesized by condensation of PAA and 4-azidoaniline according to the literature method.41 By analysis of 1H NMR data, the grafting rate of azido groups was calculated to be 8%. (see Figure S1 of the Supporting Information) Substrate Preparation. Multilayer films were assembled on a variety of substrates, including quartz, silicon, and CaF2 slides. Different agents were used to modify the substrate for the deposition of the first layer of a multilayer film. In the case of quartz and silicon, the substrates were immersed in a fresh Piranha solution [30% H2O2/ 98% H2SO4 (1:3, v/v); CAUTION: Piranha solution is a Very aggressiVe, corrosiVe solution, and appropriate safety precautions should be utilized, including the use of acid-resistant gloVes and adequate shielding] and heated until no bubbles were released. The substrates were rinsed thoroughly with deionized water and dried in an oven for 2 h and then were immersed in 1 × 10-5 M APTS from a toluene solution for 6 h to form a self-assembled monolayer (41) Chen, G. P.; Ito, Y.; Imanishi, Y. Macromolecules 1997, 30, 7001.

of silicane terminated with NH2 functional groups at the exposed surface. Then, the substrates were subjected to ultrasonic agitation in toluene for a short period of time to remove the physically adsorbed APTS. In the case of CaF2, the substrates were immersed in a 0.5 mg/mL BPEI solution for 2 h to yield a NH2 tailoring surface. LbL Assembly of Multilayer Films. In our work, different multilayer films, (PAA-N3/PAH)n, (PAA-N3/PDDA)n, and (PAAN3/Por)n, were fabricated on different substrates such as quartz, silicon, and CaF2. In all cases, n denotes the number of bilayers. One polyanion and one polycation were defined as one bilayer. Given that the procedure of preparation was similar, herein the fabrication of (PAA-N3/PAH)n was employed as a typical example for introducing the preparing procedure, described as follows. An NH2modified substrate was first immersed in a PAA-N3 aqueous solution (0.5 mg/mL) for 10 min, followed by rinsing with deionized water and drying with a nitrogen stream. The substrates were then immersed in a PAH aqueous solution (1.0 mg/mL) for an additional 10 min, followed by the same rinsing and drying cycle. By repeating the process described above, we prepared a (PAA-N3/PAH)n multilayer film. The concentrations of PAA-N3, PAH, PDDA, and Por aqueous solutions here were 0.5, 1.0, 1.0, and 0.5 mg/mL, respectively. The pH of the aqueous solution was adjusted with HCl and NaOH. It

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Figure 2. UV-vis spectra of PAA-N3/PDDA (a) and PAA-N3/Por (b) multilayered films fabricated from aqueous solutions and measured after the deposition of each layer. The insets show the layer number dependence of the absorbance of the azido group at 272 (a) or 273 nm (b) and that characteristic of the Soret band of the porphyrin at 423 nm.

should be pointed out that the fabrication of multilayer film was carried out in a dark room because of the photosensitivity of PAAN3. Photo-Cross-Linking of Multilayer Films. The multilayer films prepared were irradiated to form the cross-linked structure using a high-pressure mercury lamp with an optical fiber at a distance of 10 cm. The central wavelength of the lamp was 365 nm, and the intensity was 100 mW/cm2. A timer was used to control the irradiating time which was set to 10 min to ensure a complete cross-linking in all cases except in the kinetic analysis.

Results and Discussion Fabrication of Multilayer Films via Electrostatic Interaction. We wondered whether PAA-N3 could be applied for LbL assembly. Given that PAA-N3 is negatively charged, we employed typical positively charged building blocks, weak polyelectrolyte PAH, strong polyelectrolyte PDDA, and a porphyrin derivative with four positively charged sites, Por, to fabricate the multilayer films. In PAA-N3, the ungrafted carboxylate groups should provide the driving force for LbL assembly, and the grafted azido group should be responsible for the postphotochemical

reaction. In all cases, the pH of weak polyelectrolyte PAH (pKa ) 8.5) was set at 7.5 to ensure that all the amido was protonated as NH3+ to offer enough electrostatic force. For the PAA-N3 aqueous solution prepared with deionzied water, the pH was adjusted to 4.5 to make sure that all the carboxylate groups were ionic. The PAA-N3/PAH multilayer film was fabricated by alternating deposition of polyanion PAA-N3 and polycation PAH. The stepwise assembly of the multilayer film was characterized by UV-vis absorption spectroscopy and FT-IR spectroscopy. Figure 1a shows the UV-vis absorption spectra of the PAA-N3/PAH film assembled on a quartz slide from one to eight bilayers. The absorbance at 269 nm is attributed to the π-π* transition of the azido group.42 A linear increase in absorbance at 269 nm with the number of bilayers is displayed in the inset of Figure 1a, indicating that PAA-N3 can be used as a negatively charged building block for an electrostatic LbL assembly and the amount of polyanion deposition in each cycle is almost equal. The FT-IR (42) Chen, G. P.; Ito, Y.; Imanishi, Y. Macromolecules 1998, 31, 4379.

Poly(acrylic acid)-Bearing PhotoreactiVe Azido Groups

Figure 3. UV-vis spectra record the absorbance change of the (PAAN3/PAH)8 film during UV irradiation. By the approximation of elementary reaction, the reaction rate constant can be calculated from the slope of the fitting line.

spectra in Figure 1b of the PAA-N3/PAH film assembled on a CaF2 slide show that the absorbance at 2117 cm-1, attributed to the asymmetric stretching vibration of NdNdN, exhibits a linear increase with the number of bilayers as shown in the inset. This FT-IR result is consistent well with that of UV-vis spectroscopy. In addition to weak polyelectrolyte PAH, we have demonstrated that strong polyelectrolyte PDDA and oligo-charged molecule Por can form a multilayer film with PAA-N3, indicating a wide range of applicability of PAA-N3 as a building block for LbL assembly. Figure 2 a shows the UV-vis absorption spectra of the PAA-N3/PDDA film assembled on a quartz slide. There is a linear increase in the absorbance of azido groups at 272 nm versus the number of bilayers except of the first three bilayers. As described by J. Schlenoff, the cause of the deviation for the first three bilayers should be attributed to the impenetrable substrate-multilayer interface.43 Figure 2b shows the UV-vis absorption spectra of PAA-N3/Por multilayer films assembled on a quartz slide. The absorbance at 273 nm of azido groups and that at 423 nm, the characteristic of Soret band of the porphyrin,44,45 were chosen to monitor the stepwise assembly. Again, the absorbance of azido groups at 273 nm shows a linear increase with the number of bilayers. However, there is a zigzag pattern when using the Soret band of Por at 423 nm as an indicator. It means that adsorption and desorption of Por occur during LbL assembly. This phenomenon is quite usual for the LbL assembly of oligo-charged porphyrin, and it is attributed to the competitive association of the positively charged PAA-N3 in the deposited film versus that in solution, which has also been observed when preparing diazo-resins or a Por multilayer film.40 Photo-Cross-Linking of Multilayer Films. We have employed UV-vis and FT-IR spectroscopy to investigate the photocross-linking of the (PAA-N3/PAH)8 film during UV irradiation. As shown in Figure 3, a dramatic decrease in the absorbance at 269 nm with UV irradiation indicates that the azido groups are decomposed. In the mean time, there are some concomitant increases in absorbance in the vicinity of 238 and 356 nm, giving two isosbestic points. Similarly, the FT-IR absorbance of azido groups at 2117 cm-1 (νasN3) deceases gradually with irradiation (43) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (44) Sun, J. Q.; Wu, T.; Zou, B.; Zhang, X.; Shen, J. C. Langmuir 2001, 17, 4035. (45) Sun, J. Q.; Wang, Z. Q.; Sun, Y. P.; Zhang, X.; Shen, J. C. Chem. Commun. 1999, 693.

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time and disappears totally after UV irradiation of 90 s. These data provide enough evidence that the photoinduced reaction is very fast. In addition, the isosbestic point suggests that it is an elementary reaction. The AFM observation suggests that the LbL assembly produces compact films with a smooth surface. Taking the eight bilayers of PAA-N3/PAH, for example, we find the surface roughness is ∼1.1 nm in a 3 µm × 3 µm area. No obvious difference, scaled by the roughness analysis, is found before or after UV irradiation. This indicates that the change in roughness yielded by the photoreaction is too little to be detected by the existing characterizing technique with limited resolution. Kinetics of Photoreaction between the Layers. Under UV irradiation, the azido group can be decomposed into highly reactive nitrene,46,47 which can spontaneously form intra- and intermolecular covalent bonds with adjacent groups to produce the cross-linking, as shown in Scheme 2. We assume that the photoreaction in this system can be modeled approximately as a first-order reaction. The process of the photoreaction can be divided into two steps. First, upon UV irradiation, the azido groups are converted into nitrene and release N2. Second, the nitrenes immediately undergo further reaction with adjacent groups to give cross-linking.46-49 Considering that the nitrene is highly reactive, it can react with the C-H bond to form a cross-linking bridge of aminobenzene by insertion. It can also couple with an other nitrene to form a cross-linking bridge of azobenzene.50-52 However, it is likely that, in this work, the contribution to cross-linking mainly comes from the insertion because of the low density of nitrenes compared with that of C-H bonds. Therefore, in Scheme 2, we mainly take the CH group as example to illuminate the process of cross-linking. PAAN3 and PAH were deposited so compactly that the nitrenes surrounded by polymer chains could insert into C-H bonds quickly, so the second step of the photoreaction should be much faster than the first one. Therefore, the first step is the rate control step. The number of azido groups in the films is assumed to be proportional to its relative absorbance (Beer-Lambert law) at any time of decomposition, and then the reaction rate constant k can be calculated according to eq 1:

ln[(A0 - Amin) ⁄ (At - Amin)] ) kt

(1)

where A0 - Amin and At - Amin represent the relative absorbance at 269 nm of the films at time zero and time t, respectively. Amin is the minimum invariable absorbance after UV irradiation for 90 s. A0 and At are the absolute absorbances at 269 nm at time zero and time t, respectively. As shown in the inset of Figure 3, a linear relation can be obtained between ln[(A0 - Amin)/(At - Amin)] and t with a straight fitting line through the origin. The value of k calculated from the slope of the fitting line is 0.047 s-1. Film Stability Analysis. Having prepared covalently attached films, we are eager to determine the stability difference of the film before and after UV irradiation. The stability was investigated (46) Poe, R.; Schnapp, K.; Young, M. J. T.; Grayzar, J.; Platz, M. S. J. Am. Chem. Soc. 1992, 114, 5054. (47) Schnapp, K. A.; Poe, R.; Leyva, E.; Soundararajan, N.; Platz, M. S. Bioconjugate Chem. 1993, 4, 172. (48) Braese, S.; Gil, C.; Knepper, K.; Zimmermann, V. Angew. Chem., Int. Ed. 2005, 44, 5188. (49) Khong, S. H.; Sivaramakrishnan, S.; Png, R. Q.; Wong, L. Y.; Chia, P. J.; Chua, L. L.; Ho, P. K. H. AdV. Funct. Mater. 2007, 17, 2490. (50) Hayashi, A.; Goto, Y.; Nakayam, M.; Sato, H.; Watansbe, T.; Miyata, S. Macromolecules 1992, 25, 5094. (51) Airinei, A.; Barboiu, V.; Rusu, E.; Timpu, D. J. Photochem. Photobiol., A 2004, 162, 579. (52) Shi, F.; Niu, J.; Wang, Z. Q.; Smet, M.; Dahaen, W.; Qiu, Y.; Zhang, X. Langmuir 2007, 23, 1253.

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Figure 4. UV-vis spectra of un-cross-linked (A) and cross-linked (B) films before (a) and after (b) immersion in a NaOH solution at pH 13.5. Scheme 2. Possible Scheme of Photoinduced Cross-Linking in Multilayer Films

by immersing the un-cross-linked and cross-linked (PAA-N3/ PAH)8 film in sodium hydroxide (NaOH) with a high pH of 13.5. Then, UV-vis spectroscopy was exploited to record the spectral changes before and after the solvent etching. As shown in Figure 4A, after immersion for 10 min, the absorbance of the un-crosslinked film decreased dramatically at all wavelength scales, especially at 269 nm of azido groups. An additional experiment was conducted to provide evidence that the azido groups were stable in a NaOH solution with a high pH (see Figure S2 of the Supporting Information). So we believe that the spectral decrease was caused by the dissociation of the multilayer film under the influence of etching solvent. In comparison, we also etched the cross-linked film by immersing it in the same solvent for 10 min. However, the cross-linked film displays almost no change in the spectra before and after immersion in NaOH, as shown in Figure 4B, indicative of the enhanced stability after photo-cross-linking. This observation is also true for the cross-linked (PAA-N3/ PDDA)n and (PAA-N3/Por)n films, which means that this is a universal and effective method for enhancing the stability of multilayer films (see Figure S3 of the Supporting Information). NaOH was chosen as the etching solvent for the following reason. First, the NaOH solvent with a pH of 13.5 supplied a high-ionic strength environment, in which the attractive ionic interaction between each layer could be greatly weakened, leading to dissociation. This was because the electrostatic force was the main driving force for making layers assemble together. Second, a PAH with a pKa of 8.5 was deionized at pH 13.5, leaving a large amount of uncoupled ionized carboxylate groups struggling in the film, greatly strengthening the repulsion force in the film. Both high ionic strength and deionization effects were responsible for the dissociation of the electrostatic assembled film. Overall, in such a solvent, the ordinary LbL-assembled film is not adequately stable. We can conclude that this photoreaction has offered an adequately stable structure by introducing covalent bonds into

a multilayer film and making the whole film one united robust three-dimensional network. However, to prevent the whole threedimensional network from being peeled from the bottom, the first layer modification of the substrate is very important. In our case, we have employed APTS to modify the silicon and quartz substrates. On the one hand, APTS can modify the substrate by chemisorption; on the other hand, the protonated alkyl amine end groups can assemble with PAA-N3 as well as react with nitrenes upon irradiation. Therefore, after UV irradiation, the whole film can stick to the substrate. From another aspect, one may change the modification of the substrate to peel off the whole cross-linked film if required. Surface Patterning. The different stabilities of multilayer films before and after UV irradiation provide a chance to prepare a patterned surface. The procedures of pattern formation are described as follows. A silicon wafer modified with alkyl amines was chosen as the substrate. Thus, PAA-N3 was deposited as the first layer to ensure that the prepared multilayer film was covalently attached to the substrate. Then, a multilayer (PAAN3/PAH)n film was fabricated by LbL self-assembly. The film was exposed to UV irradiation for 4 min to produce cross-linked and non-cross-linked areas by placing a mask between the source of UV irradiation and the film. The cross-linked areas were sufficiently stable to stick to the substrate while the un-crosslinked parts were peeled from the substrate when simultaneously immersed into the etching solvent, NaOH, at pH 13.5. A smooth patterned surface was obtained when the sample was finally immersed in pure water for 10 min. AFM was employed to show the morphology of the surface patterning. Figure 5a shows the AFM image of the patterned structures produced by etching a (PAA-N3/PAH)8 film (pretreated by photolithography) in NaOH. A striped photomask was used here. As seen clearly, the well-patterned strips are 2.5 µm wide and 5.0 µm apart. Additionally, the strips are ∼45 nm high for an eight-bilayers film as shown by AFM sectional analysis in

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Figure 5. (a) AFM image of the surface patterning prepared from a (PAA-N3/PAH)8 film. (b) AFM sectional analysis for the height of strips.

Figure 5b. Via careful observation, one can find from the sectional analysis that the height on the edge of each strip is greater than the central part. It may be attributed to the energy distribution of UV light which follows the Gauss distribution. The central part of strip received more energy than the edge, leading to a higher degree of cross-linking, and was shrinking compactly. Another possibility is related to the polymer chains on the border of the strips. In such a continuous film, some polymer chains attributed to un-cross-linked areas are consequently entangled with the ones attributed to cross-linked areas. At the last preparation step, the sample was immersed in pure water for some time due to the swollen effect of PAA/PAH in a high-pH environment. Without this step, there would be many folds on the surfaces of strips. As mentioned earlier, a great number of uncoupled ionized carboxylate groups existed in the film under high-pH conditions, which greatly enhanced the repulsion between each layer. For the un-cross-linked one, the film is dissociated, while for the cross-linked one, it is swollen, thus resulting in strips full of folds. Therefore, reducing the pH by immersion in pure water can make the folds disappear (see Figure S4 of the Supporting Information). Conclusion. In summary, we have synthesized a photoreactive polyanion, PAA-N3, by grafting an azido group onto poly(acrylic

acid) and provided an effective way to improve the stability of multilayer films by postphotoreaction. We found that this polyanion can form multilayer films with various positively charged building blocks, including weak or strong polycations and small molecules with multiple positively charged sites. One of the advantages of PAA-N3 is the wide range of pH tolerance. Moreover, besides electrostatic LbL assembly, this polyanion could also be employed for multilayer fabrication on the basis of hydrogen bonding in organic solvent. Therefore, PAA-N3 is a versatile building block for fabricating stable and covalently attached multilayer films, and its application in the fields of devices and sensors is greatly anticipated. Acknowledgment. This work was financially supported by the National Basic Research Program (2007CB808000 and 2005CB724400), NSFC (50573042), and a NSFC-DFG joint grant (TRR61). Supporting Information Available: 1H NMR spectra of PAAN3, stability of PAA-N3 at high pH, stability test for (PAA-N3/PDDA)n and (PAA-N3/Por)n films after photo-cross-linking, and swollen phenomena of the surface patterning. This material is available free of charge via the Internet at http://pubs.acs.org. LA804261F