Facile Method for the Fabrication of Robust Polyelectrolyte Multilayers

Apr 19, 2012 - Multilayers by Post-Photo-Cross-Linking of Azido Groups. Xiaosa Zhang,. †,§ ... University of Chemical Technology, Beijing 10029, Ch...
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Facile Method for the Fabrication of Robust Polyelectrolyte Multilayers by Post-Photo-Cross-Linking of Azido Groups Xiaosa Zhang,†,§ Chao Jiang,†,§ Mengjiao Cheng,† Yong Zhou,† Xiaoqun Zhu,† Jun Nie,† Yajun Zhang,† Qi An,*,‡ and Feng Shi*,† †

State Key Laboratory of Chemical Resource Engineering & State Key Laboratory of Organic Inorganic Composites, Beijing University of Chemical Technology, Beijing 10029, China ‡ MESA Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands S Supporting Information *

ABSTRACT: In this letter, we have developed a facile method to enhance the stability of polyelectrolyte multilayers. We fabricate conventional polyelectrolyte multilayers of PAH/ PAA through electrostatic layer-by-layer (LbL) assembly and then postinfiltrate photosensitive cross-linking agent 4,4′diazostilbene-2,2′-disulfonic acid disodium salt into the LbL films. After cross-linking by UV irradiation, the stability of the photo-cross-linked multilayer is highly improved as evidenced by the lack of dissolution under ultrasonication in saturated SDS aqueous solutions for 10 min. Moreover, by taking advantage of the different stability of the LbL film before and after UV irradiation, a patterned surface can be achieved.

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(EDC) to the LbL process to activate the carboxylic groups.18a Srinivasan and Gao et al. have obtained covalent multilayers using terephthaloyl chloride18b and glutaraldehyde18c as reactive building blocks. Bergbreiter and Crooks et al. have employed the reaction between poly(amidoamine) and Gantrez to form covalent multilayers.19 Caruso et al. used poly(acrylic acid) (PAA) films grafted with either azido groups or alkyne functionality as building blocks to fabricate stable multilayers by click chemistry.20 For the other strategy, Zhang and co-workers first introduced a diazo resin into the LbL assembly and obtained stable polyelectrolyte multilayers through the postphoto-cross-linking of diazonium groups with sulfonic groups21 and carboxylic groups.8c Following this pioneering work, Wu and Cui et al. reported the formation of stable multilayers through a similar postphotoreaction.22 Mayes et al. prepared 2D and 3-D patterns using photo-cross-linkable weak polyelectrolyte poly(acrylic acid-ran-vinylbenzyl acrylate) (PAArVBA) and poly(allylamine hydrochloride) (PAH) by employing a photomasking technique.23 Bruening11 and Rubner8b et al. have stabilized multilayer films via post-thermo-cross-linking of amino and carboxylic groups; Caruso and Picart et al. have cross-linked PAH/PAA multilayer by using EDC as a dehydration reagent.24 A redox reaction also has been used by Niu16a and Xu25 et al. to post-cross-link polyelectrolyte multilayers.

INTRODUCTION The layer-by-layer (LbL) assembly technique has been proven to be a powerful method of constructing nanothin films with tailored composites and structures.1 After being rediscovered by Decher in 1991,2 various functional building blocks have been employed to prepare multilayers by using electrostatic interaction,3 hydrogen bonding,4 and/or coordination bonding5 as driving forces. The constructed multilayers can be used for hollow capsule generation,6 enzyme immobilization,7 surface patterning,8 light-emitting diodes,9 photoelectrochemically active electrodes,10 separation membranes,11 microporous films,12 surface-imprinted multilayers,13 and erasable films14 and also have led to several commercial products. However, because the interlayer driving forces involved in LbL assembly are generally weak supramolecular interactions, the stability of the assembled films should be improved to cater to some laboratory applications under extreme conditions and the durability of the potential commercial products. In general, two methods have been developed to improve the chemical durability by converting the interlayer forces from weak supramolecular interactions to strong covalent bonds. One is to fabricate multilayers using chemical reactions as driving forces;15 the other is post-cross-linking of the asprepared self-assembled films.16 For the first strategy, Li and Martin et al. have fabricated covalent nanotubes with polyeletrolytes in nanopore-containing alumina membranes by using 3,4,9,10-perylenetetracarboxylicdianhydride and glutaraldehyde as not only building blocks but also cross-linking agents.17 Serizawa et al. have obtained covalent multilayers by adding 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide © 2012 American Chemical Society

Received: February 11, 2012 Revised: April 18, 2012 Published: April 19, 2012 7096

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Among the above cross-linking methods, the formation of stable LbL films using phenyl azido groups as photo-crosslinker is the most broadly applicable because a phenyl azido group can easily photolyze upon UV irradiation to generate a highly reactive nitrene intermediate that reacts with almost all kinds of organic matter to form a covalent bond.22a However, it is a toilsome process because the synthesis of special compounds grafted with phenyl azido groups is still needed in this strategy. One possible approach to solving this problem is applying postinfiltrating difunctional molecules as a crosslinking agent to stabilize LbL films, which is a facile strategy and does not need further synthesis of specified building blocks. Until now, only a few reports of this strategy have appeared in which the bridging of amino groups in the building blocks was accomplished using glutaraldehyde16b or epichlorohydrin16c as a cross-linking agent to obtain stabilized multilayers. Because these reactions can occur only between amino groups, a more general and facile method is desirable. Herein, we report the combination of the facile postinfiltration of multifunctional molecules with the universal photoreaction between azido groups and organic compounds26 to develop a novel method to enhance the stability of LbL films. To the best of our knowledge, this is the first demonstration of the postinfiltration of photoreactive molecules with two azido groups into the LbL films and the stabilization of a multilayer by postphotoirradiation. In this report, we have employed 4,4′-diazostilbene-2,2′disulfonic acid disodium salt (DAS) as the photosensitive crosslinking agent by using it to postinfiltrate conventional LbL multilayers and stabilized the films by UV irradiation to initiate interlayer and intralayer photo-cross-linking with DAS. Moreover, by taking advantage of the different stability of the film before and after UV irradiation, a patterned surface that can be used as a template for selective adsorption can be achieved.

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Scheme 1. Illustration of the Stabilization of the PAH/PAA Multilayer and Its Surface Patterning and the Building Blocks Used in the Experiment

To stabilize the as-prepared multilayers, the substrates were immersed in an aqueous solution of DAS (5 mg/mL, pH 3.8). The absorption kinetics of DAS infiltrating the multilayer was studied by UV−visible spectroscopy as shown in Figure 1a. We can clearly observe that upon increasing the immersion time the absorption values at 340 nm (the peak for DAS) increase correspondingly. This indicates that the content of absorbed DAS within the multilayer increases with the absorption time. The maximum loading amount of DAS molecules in a (PAH/ PAA)7PAH film is calculated to be ∼0.77 μg/cm2/layer. From the inset of Figure 1a, the absorption value of DAS reaches an almost constant value after immersion for 20 min. The driving force for the infiltration of DAS is the electrostatic interaction between the protonated amino groups in PAH and the sulfonate groups in DAS. Because the LbL assembly is carried out at a pH value of 9 for PAH and a pH value of 6 for PAA, the amino groups in PAH are partially protonated and the carboxyl groups in PAA are almost completely deprotonated (Scheme 2a). After immersion in DAS at a pH value of 3.8, the low-pH treatment protonates many of the carboxylate ions in the multilayers, thus breaking numerous electrostatic interactions. This phenomenon leads to additional binding sites bearing positive charges and further drives DAS absorption, as illustrated in Scheme 2b. Because the external layer is PAH, there are two kinds of absorption sites in the multilayer, namely, inside the multilayer and on the outermost layer, but the latter is expected to provide little additional stabilization of the film. To confirm that DAS molecules do infiltrate the multilayer, we carried out a control experiment with a (PAH/PAA)8 multilayer film that has negative charges on the absorption outer layer and could prevent the absorption of DAS. As shown in Figure 1b, a similar absorption curve could be obtained, but the time required to reach a constant absorbance value is much longer than that of the multilayer with positive charges on the external layer. In this situation, the deprotonated carboxyl groups on the surface repel the negatively charged DAS and make the absorption process slower. All of the experiments

RESULTS AND DISCUSSION

Because it is well known that a PAH/PAA multilayer film can be disassembled by immersion in a saturated aqueous solution of sodium dodecyl sulfate (SDS) or an aqueous solution with a pH value below 1.4 or above 13.5,22a we selected PAA and PAH as a model system for the research of multilayer stability. The experimental procedure for the photostabilization of the LbL assembly and subsequent surface patterning is illustrated in Scheme 1. The substrate (quartz for UV−visible spectrum characterization and a silicon wafer for AFM) was first cleaned in piranha solution (7:3 v/v H2SO4/H2O2) and then modified with a self-assembled monolayer of sulfonate groups as reported previously.8c Afterward, the substrate was alternately immersed in aqueous solutions of PAH (1 mg/mL, pH 9) and PAA (1 mg/mL, pH 6) for 20 min to obtain (PAH/PAA)7PAH multilayers in which the driving forces are the coordination of electrostatic interaction and hydrogen bonding. The stepwise assembly of the PAH/PAA multilayer was characterized by UV−visible absorption spectra, and the results are shown in Figure S1. Plotting the UV−visible absorption versus the number of bilayers presents typical exponential growth. The stability of the PAH/PAA multilayer was then tested by ultrasonication in a saturated aqueous solution of SDS. The results showed that the untreated PAH/PAA multilayer and the same film after UV irradiation in the absence of DAS for 150 s could both be completely dissolved by ultrasonicatation for 10 min, as indicated by an absorption spectrum similar to that of a blank quartz substrate (Figure S2). 7097

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Figure 1. UV−visible spectra and absorption kinetics of DAS in (a) (PAH/PAA)7PAH multilayers and (b) (PAH/PAA)8 multilayers.

lamp with an intensity of 2.5 mW cm−2 at a distance of 20 cm, and the kinetics of photoreaction is shown in Figure 2a. A gradual decrease in the absorbance at 340 nm with UV irradiation is observed; simultaneously, there are concomitant increases in absorbance in the vicinities of 270 and 415 nm, giving two isosbestic points. The decrease suggests that under UV irradiation the azido groups in DAS can be decomposed into highly reactive nitrene intermediates with released nitrogen as illustrated in Scheme 2c. The nitrene can insert into the related C−H or C−C bonds and form interlayer and intralayer photo-cross-links (Scheme 2d), which can be modeled as an approximately first-order reaction. For peak absorbance (A) at 340 nm, a linear fit of ln{(A0 − Amin)/(At − Amin)} versus time with a rate constant (k) of 0.048 s−1 is shown in the inset of Figure 2a, where Amin is the minimum invariable absorbance after UV irradiation for 150 s and A0 and At represent the absorbance at time zero and time t, respectively. Atomic force microscopy (AFM) images of the films before and after UV irradiation are provided in Figure S6. After UV irradiation, the as-prepared multilayer should be cross-linked with DAS, leading to improved stability. To investigate the stability difference of the films before and after UV irradiation, we treated both of the multilayers with

Scheme 2. Possible Mechanism of Photoinduced CrossLinking in Multilayers

involved were carried out by immersing the substrates modified with (PAH/PAA)7PAH multilayers in DAS for 4 h, unless otherwise stated. In an attempt to induce photo-cross-linking, the multilayers with DAS were irradiated with a 400 W high-pressure mercury

Figure 2. (a) Kinetic decomposition of DAS. (b) UV−visible spectra of (PAH/PAA)7PAH multilayers before (line 1) and after (line 3) UV irradiation. Ultrasonication in SDS of un-cross-linked (line 2) and cross-linked (line 4) films. (c) AFM image and sectional analysis of the patterned multilayer. 7098

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Author Contributions

ultrasonication in saturated SDS aqueous solutions for 10 min and evaluated the films by UV−vis spectroscopy (Figure 2b). It was found that the un-cross-linked films with an absorbance at 340 nm (Figure 2b, line 1) could be completely dissolved when sonicated in SDS for 10 min, on the basis of an absorption spectrum similar to that of a blank quartz substrate (Figure 2b, line 2). In contrast, after UV irradiation, the UV−visible spectra of the cross-linked films display almost no changes before (Figure 2b, line 3) and after ultrasonication (Figure 2b, line 4) in SDS aqueous solutions, which implies that the stability of the films is greatly improved. In principle, this approach to increasing the stability of the multilayers could be extended to any LbL system with weak polyelectrolytes as building blocks, such as PAH/poly(4-stylene sulfonate). The different solubilities of the multilayers in SDS solution before and after UV irradiation provide an opportunity to create patterned surfaces by photolithography. The patterned multilayers were achieved by forming (PAH/PAA)7PAH multilayers on silicon wafers, infiltrating the films with DAS, selectively exposing the films to UV through a photomask for 10 s, and ultrasonicating in SDS aqueous solutions for 10 min. The resulting patterned surface was characterized by AFM as shown in Figure 2c. As seen clearly, the well-patterned strips are 3 μm wide and 3 μm apart with a depth of 30 nm, which demonstrates the film thickness of the (PAH/PAA)7PAH multilayers.

§

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (50903005), the Beijing Nova Program of China (2009B011), the Program for New Century Excellent Talents in University (NCET-10-0211), Fundamental Research Funds for the Central Universities (ZZ1104), the Fok Ying Tung Education Foundation (131013), and the Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201213).

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CONCLUSIONS We have developed a facile method of stabilizing LbL multilayers of weak polyelectrolytes. Using PAH/PAA multilayer films as a model system, we postinfiltrated DAS as a photosensitive cross-linking agent and then exposed the films to UV irradiation to initiate interlayer and intralayer photocross-linking. The experimental data show that the durability of the multilayers is greatly improved, resulting in films that remain stable under ultrasonication in an SDS solution for 10 min. Moreover, a patterned surface was prepared by taking advantage of the different stability of the film before and after UV irradiation. Considering that the azido groups can easily form chemical bonds with almost any kind of organic surface under UV irradiation, we believe that by proper post-treatment this method could be extended to other LbL systems, which is especially suitable for biomacromolecules with weak charges. Moreover, this method is promising for laboratory applications of LbL films used under extreme conditions and could promote their potential commercial applications.

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ASSOCIATED CONTENT

* Supporting Information S

Information concerning the modification of sulfonic groups on the substrate. PAH/PAA multilayer formation and its AFM images. Possible dissolution mechanism for the PAH/PAA multilayer. Controlled experiment of UV irradiation for the PAH/PAA multilayer without DAS. Measurement of the loaded amount of DAS and its release from the multilayer. Influence of the DAS absorption stage on the stability of the multilayer. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. 7099

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