Robust Ion-Permselective Multilayer Films Prepared by Photolysis of

photo-cross-linked PAC-azoBNS/DAR multilayer films were photolyzed in air ..... This work is supported by the National Natural Science Foundation ...
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Langmuir 2006, 22, 7894-7901

Robust Ion-Permselective Multilayer Films Prepared by Photolysis of Polyelectrolyte Multilayers Containing Photo-Cross-Linkable and Photolabile Groups En-Hua Kang, Xiaokong Liu, Junqi Sun,* and Jiacong Shen Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin UniVersity, Changchun, P. R. China 130012 ReceiVed May 3, 2006. In Final Form: July 6, 2006 The azobenzene-containing polyanion PAC-azoBNS was alternately assembled with the polycation diazoresin (DAR) to construct photo-cross-linkable multilayer films of PAC-azoBNS/DAR that contain photolabile groups of azobenzene. Upon mild UV irradiation, the interaction between PAC-azoBNS/DAR multilayers was converted from electrostatic interaction to covalent bonds. Because of the free carboxylic acid groups presented in the film, the photo-cross-linked multilayer film favors the selective permeation of positively charged species. After photolysis of the photo-cross-linked PAC-azoBNS/DAR films with intense UV irradiation, azobenzene groups decompose to produce imine groups, and a photo-cross-linked robust film containing free carboxylic acid and imine groups was fabricated. The resultant film allows the permeation of negatively charged species and meanwhile shows a pH-switchable permselectivity for positively charged species. Because of the covalently cross-linking structure, the photolyzed cross-linked PAC-azoBNS/DAR film shows high reversible switching behavior and has high stability in solution with high ionic strength.

Introduction In the past decade, layer-by-layer (LbL) assembly techniques have emerged as a group of versatile and convenient methods for the construction of layered ultrathin films with precise control of film thickness and composition.1-3 Advanced multilayer film materials with components such as biomacromolecules,4 particles,5 dentritic molecules,6 dyes,7 conductive polymers,8 and so forth have been successfully fabricated. Besides the possible electronic applications, the use of polyelectrolyte multilayer films in sensing and materials separation has attracted much attention. Several research groups have reported applications of polyelectrolyte films as separation membranes, including gas separations,9 * To whom correspondence should be addressed. Phone: 0086-4315168723. Fax: 0086-431-5193421. E-mail: [email protected]. (1) Decher, G. Science 1997, 277, 1232-1237. (2) Hammond, P. T. AdV. Mater. 2004, 16, 1271-1293. (3) Caruso, F. Chem.sEur. J. 2000, 6, 413-419. (4) (a) Sun, Y. P.; Zhang, X.; Sun, C. Q.; Wang, B.; Shen, J. C. Macromol. Chem. Phys. 1996, 197, 147-153. (b) Johansson, J.; Halthur, T.; Herranen, M.; So¨derberg, L.; Elofsson, U,; Hilborn, J. Biomacromolecules 2005, 6, 1353-1359. (c) Thierry, B.; Winnik, F. M.; Merhi, Y.; Tabrizian, M. J. Am. Chem. Soc. 2003, 125, 7494-7495. (5) (a) Gao, M. Y.; Zhang, X.; Yang, B.; Shen, J. C. J. Chem. Soc., Chem. Commun. 1994, 2229-2230. (b) Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 7738-7739. (c) Franzl, T.; Klar, T. A.; Schietinger, S.; Rogach, A. L.; Feldmann, J. Nano Lett. 2004, 4, 1599-1603. (6) (a) Khopade, A. J.; Caruso, F. Nano Lett. 2002, 2, 415-418. (b) Huo, F. W.; Xu, H. P.; Zhang, L.; Fu, Y.; Wang, Z. Q.; Zhang, X. Chem. Commun. 2003, 874-875. (c) Kim, B. Y.; Bruening, M. L. Langmuir 2003, 19, 94-99. (7) (a) Sun, Y. P.; Zhang, X.; Sun, C. Q.; Wang, Z. Q.; Shen, J. C.; Wang, D. J.; Li, T. J. Chem. Commun. 1996, 2379-2380. (b) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224-2231. (c) Linford, M. R.; Auch, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 178-182. (d) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841-5848. (8) (a) Cheung, J. H.; Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2712-2716. (b) Lukkari, J.; Saloma¨ki, M.; A ¨ a¨ritalo, T.; Loikas, K.; Laiho, T.; Kankare, J. Langmuir 2002, 18, 8496-8502. (c) Sun, J. Q.; Cheng, L.; Liu, F.; Dong, S. J.; Wang, Z. Q.; Zhang, X.; Shen, J. C. Colloids Surf., A 2000, 169, 209-217. (9) (a) Leva¨salmi, J.-M.; McCarthy, T. J. Macromolecules 1997, 30, 17521757. (b) Ackern, F. van; Krasemann, L.; Tieke, B. Thin Solid Films 1998, 327329, 762-766. (c) Sullivan, D. M.; Bruening, M. L. Chem. Mater. 2003, 15, 281-287.

pervaporation from water/organic solvent mixture,10 and ion separation.11 The use of polyelectrolyte multilayer films as ionpermselective membranes is due to the net charges contained in the films, which may result in Donnan exclusion of charged species.11a,b There are many reports of ion-permselective membranes prepared by LbL assembly of polyelectrolytes. Tieke and co-workers studied selective ion transport across LbL-assembled multilayers of cationic and anionic polyelectrolytes and found that the films favor the separation of mono- and divalent ions by Donnan exclusion of the divalent ions.11a Bruening and coworkers deposited a variety of polyelectrolyte multilayer films on porous supports. They found that control over charge and composition in the polyelectrolyte multilayer films allows highly selective separation of ions according to charge, size, or hydration energy.11b,12 By photo-cross-linking LbL-assembled multilayer films of benzophenone-modified poly(acrylic acid) (PAA-BP) and poly(allylamine hydrochloride) (PAH-BP), Advincula and co-workers prepared pH-sensitive bipolar ion-permselective ultrathin films.11c Besides permeability and selectivity, stability is another important concern that determines the long-term application of the LbL assembled polyelectrolyte films as permselective membranes. For example, when the separation membranes are used at high temperatures or in solutions with high ionic strength, the film structure, including the configuration of the polymer chains, the net charge, and the charge density are subject to change. These changes will definitely produce an influence on (10) (a) Krasemann, L.; Tieke, B. J. Membr. Sci. 1998, 150, 23-30. (b) MeierHaack, J.; Lenk, W.; Lehmann, D.; Lunkwitz, K. J. Membr. Sci. 2001, 184, 233-243. (c) Tieke, B.; van Ackern, F.; Krasemann, L.; Toutianoush, A. Eur. Phys. J. E 2001, 5, 29-39. (11) (a) Tieke, B.; Krasemann, L. Langmuir 2000, 16, 287-290. (b) Bruening, M. L.; Sullivan, D. M. Chem.sEur. J. 2002, 8, 3833-3837. (c) Park, M.-K.; Deng, S.; Advincula, R. C. J. Am. Chem. Soc. 2004, 126, 13723-13731. (d) Toutianoush, A.; Schnepf, J.; Hashani, A. E.; Tieke, B. AdV. Funct. Mater. 2005, 15, 700-708. (12) (a) Dai, J.; Jensen, A. W.; Mohanty, D. K.; Erndt, J.; Bruening, M. L. Langmuir 2001, 17, 931-937. (b) Dai, J.; Balachandra, A. M.; Lee, L. I.; Bruening, M. L. Macromolecules 2002, 35, 3164-3170. (c) Stair, J. L.; Harris, J. J.; Bruening, M. L. Chem. Mater. 2001, 13, 2641-2648.

10.1021/la0612320 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/05/2006

Preparation of Ion-PermselectiVe Multilayer Films

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Scheme 1. (a) Chemical Structure of DAR and (b) Synthetic Route of the Azobenzene-Containing Polyanion PAC-azoBNS and the Azobenzene Compound MH

the permselectivity of the resultant films. Therefore, robust permselective membranes are highly desired. In our previous work, a facile way to prepare covalently attached multilayer films was developed by the combination of LbL assembly technique with in situ photoreaction.13 Simply, the film preparative procedure is as follows: diazoresin (DAR), which is a diazonium containing polycation, is alternately assembled with a sulfonate- or carboxylate-containing polyanion to produce multilayer films. Upon UV irradiation, diazonium reacts with the sulfonate/carboxylate groups within the layers to form sulfonate/carboxylate ester. Consequently, the ionic interaction between the layers is converted to covalent bonds, and photo-cross-linked multilayer films are fabricated. The photocross-linked film can endure etching in a ternary mixture of H2O-dimethylformamide (DMF)-ZnCl2 (3:5:2, w/w/w) and other types of polar solvents, and is therefore robust.13,14 In this paper, we adopted the concept of photo-cross-linked multilayer films to the preparation of robust ion-permselective multilayer films. We synthesized the azobenzene-containing polyanion PACazoBNS, which contains photo-cross-linkable groups of sulfonate and carboxylate and a photolabile group of azobenzene. Then DAR was alternately assembled with PAC-azoBNS to produce DAR/PAC-azoBNS multilayer films. Upon mild UV light irradiation, the interaction between the layers of DAR/PAC(13) (a) Sun, J. Q.; Wu, T.; Sun, Y. P.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Cao, W. X. Chem. Commun. 1998, 1853-1854. (b) Sun, J. Q.; Wu, T.; Liu, F.; Wang, Z. Q.; Zhang, X.; Shen, J. C. Langmuir 2000, 16, 4620-4624. (c) Sun, J. Q.; Wang, Z. Q.; Wu, L. X.; Zhang, X.; Shen, J. C.; Gao, S.; Chi, L. F.; Fuchs, H. Macromol. Chem. Phys. 2001, 202, 967-973. (14) (a) Chen, J. Y.; Huang, L.; Ying, L. M.; Luo, G. B.; Zhao, X. S.; Cao, W. X. Langmuir 1999, 15, 7208-7212. (b) Cao, T. B.; Yang, S. M.; Yang, Y. L.; Huang, C. H.; Cao, W. X. Langmuir 2001, 17, 6034-6036.

azoBNS is converted to covalent bonds. The photo-cross-linked films favor the permeation of positively charged species over negatively charged ones because of free carboxylic acids in the films. After photolysis of the photo-cross-linked DAR/PACazoBNS films with intense UV irradiation, azobenzene groups decompose to produce imine groups. The resultant films favor the permeation of negatively charged species and also show a pH-dependent permeation of positively charged species. Experimental Section Materials. Poly(diallyldimethylammonium chloride) (PDDA) aqueous solution with a molecular weight of 100 000-200 000 (Aldrich), sodium poly(styrene sulfonate) (PSS, Aldrich, MW ) 70 000), Ru(NH3)6Cl3 (Alfa Aesar), and K3Fe(CN)6 (Beijing Chemical Reagents Company) were used as received. DAR was synthesized according to a literature procedure, and its chemical structure is shown in Scheme 1a.15 The molecular weight (Mn) of the DAR was ∼2640. Deionized water was used for all the experiments. Synthesis of PAC-azoBNS and the Azobenzene Compound MH. The detailed synthesis procedure of PAC-azoBNS and the azobenzene compound MH is shown in Scheme 1b. Synthesis of N-Methyl-N-hydroxyhexyloxyaniline (MHHA) (1). 6-Bromo-1-hexanol (17.63 g, 0.097 mol), N-methylaniline (10 mL, 0.092 mol), butanol (50 mL), and K2CO3 (13 g, 0.094 mol) were added in a flask and refluxed for 56 h. The precipitate was filtered off. The solution was concentrated under reduced pressure to remove most of the solvent. Finally, the purified product was collected by vacuum distillation at 163-165 °C/3 mm Hg. Yield: 42%. 1H NMR (CDCl3, ppm): δ ) 1.37 (m, 4H), 1.56 (m, 4H), 2.91 (s, 3H), 3.30 (t, 2H), 3.62 (t, 2H), 6.68 (m, 3H), 7.22 (t, 2H). (15) Cao, W. X.; Ye, S. J.; Cao, G. S.; Zhao, C. Macromol. Rapid Commun. 1997, 18, 983-989.

7896 Langmuir, Vol. 22, No. 18, 2006 Synthesis of Poly(acryloyl chloride) (PAC) (2).16 Acryloyl chloride (9 mL), dry 1,4-dioxane (9 mL), and azobisisobutyronitrile (0.3 g) were added into a 50 mL flask. The mixture was reacted at 55 °C under the protection of nitrogen for 14 h. The polymer was precipitated by adding petroleum ether. The product was dried at room temperature under vacuum for 48 h. GPC: Mn ) 104 351 (estimated by using poly(methyl acrylate) prepared from the same batch of PAC and an excess of methanol).17 Synthesis of the Precursor Polymer (PAA-AN) (44%) (3). PAC (0.3 g, 0.0033mol), triethylamine (1 mL, 0.0072 mol), and MHHA (0.55 g, 0.0026 mol) were dissolved in anhydrous DMF (40 mL). The mixture was stirred at 70 °C for 24 h under the protection of nitrogen gas. Then 3 mL of water was added into the mixture and stirred for 10 min. After precipitation in 300 mL of water (pH adjusted by HCl to 3.5-4), the precipitate collected by filtration was washed with water and dried. The crude product was further purified by being dissolved in tetrahydrofuran (10 mL) and dropped into petroleum ether (bp 30-60 °C) (100 mL); the precipitate collected by filtration was washed with petroleum ether and dried. IR (KBr): 3392 (s, OH), 2935 (s, CH), 1730 (s, CdO), 1601, 1506, 1452 (s, benz ring) cm-1. 1H NMR (DMSO-d6, ppm): δ ) 1.24, 1.36-2.40, 2.80, 3.24, 3.95, 6.55, 6.62, 7.11. Synthesis of the Azo Polymer (PAC-azoBNS) (44%) (4). The azo polymer was prepared by a postpolymerization azo coupling reaction.18 A solution of 1.67 mL of hydrochloric acid (5 M) was cooled to 0 °C, and sodium nitrite (0.0168 g, 0.243 mmol) in 0.15 mL of water was slowly added. 2-Nitroaniline-4-sulfonic acid (0.198 g, 0.9 mmol) was dissolved in a solution of sodium hydroxide (0.048 g, 1.2 mmol) in 1.6 mL of water and was then combined with a solution of sodium nitrite (0.072 g, 1.04 mmol) in 0.4 mL of water. This solution was slowly added under stirring to the hydrochloric acid solution containing sodium nitrite while the temperature was kept within 5-10 °C. At the end of the diazotization, a fine precipitate of the diazonium salt was formed. After complete addition, the solution was allowed to stir for another 30 min. A spatula of urea was then added to destroy any excess sodium nitrite. The prepared diazonium solution was then added dropwise into a solution of PAAAN (44%) (0.2 g) in DMF (30 mL) at 0 °C, and stirred for 3.5 h. The PAC-azoBNS was obtained by precipitation of the above solution in plenty of strong acid water and filtration. The crude product was further purified by extraction with CH2Cl2 for 48 h and dried at room temperature under vacuum for 48 h. IR (KBr): 3432 (s, OH), 2929 (s, CH), 1726 (s, CdO), 1603, 1500 (s, benz ring) cm-1. 1H NMR (DMSO-d6, ppm): δ )1.28, 1.40-2.46, 3.02, 3.42, 3.95, 6.82, 7.72, 7.91, 8.03. UV-vis (H2O): 464 nm. Synthesis of the Azo Compound (MH) (5). A solution of 7.3 mL of hydrochloric acid (5 M) was cooled to 0 °C, and sodium nitrate (0.07 g, 1 mmol) in 0.9 mL of water was slowly added. 2-Nitroaniline4-sulfonic acid (1.09 g, 5 mmol) was dissolved in a solution of sodium hydroxide (0.2 g, 5 mmol) in 7.5 mL of water and was then combined with a solution of sodium nitrate (0.36 g, 5.27 mmol) in 2 mL of water. This solution was slowly added under stirring to the hydrochloric acid solution while the temperature was kept within 5-10 °C. At the end of the diazotization, a fine precipitate of the diazonium salt was formed. After complete addition, the solution was allowed to stir for another 30 min, and a spatula of urea was then added to destroy any excess sodium nitrate. A solution of MHHA (1.04 g, 5 mmol) in 3 mL of acetic acid was then added to the above-prepared diazonium solution at 5 °C. After complete addition, the mixture was stirred for another 3 h, and then sodium acetate (5 g) and sodium chloride (8 g) in 50 mL of water were added. The precipitate was filtered off and washed with saturated saltwater. The crude product was dissolved with a small amount of DMF. Then the DMF solution of the crude product was added to plentiful anhydrous ether. The precipitate was filtered off, washed with anhydrous ether (16) Wu, L.; Tuo, X.; Cheng, H.; Chen, Z.; Wang, X. Macromolecules 2001, 34, 8005-8013. (17) Machida, S.; Narita, H.; Kato, K. Angew. Makromol. Chem. 1972, 25, 97-103. (18) Wang, X.; Balasubramanian, S.; Kumar, J.; Tripathy, S. K.; Li, L. Chem. Mater. 1998, 10, 1546-1553.

Kang et al. and dried under vacuum. Yield: 1.34 g (62%). IR (KBr): 3446 (s, OH), 2929 (s, CH), 1606 (s, benz ring) 1521 (s, NO2), 1382 (m, NdN), 1146 (s, Ar-N), 1225, 1198, 1041 (s, SO3-) cm-1. 1H NMR (D2O, ppm): δ ) 0.96 (m, 4H), 1.24 (m, 4H), 2.63 (s, 3H), 2.95 (t, 2H), 3.32 (t, 2H), 6.24 (d, 2H), 7.35 (t, 3H), 7.86 (d, 1H), 8.13 (s, 1H). UV-vis (H2O): 496 nm. The graft ratio of PAA-AN was found to be 44%, calculated from the 1H NMR spectrum. About 100% of the aniline groups were azo functionalized, as estimated from the 1H NMR spectrum Therefore, the graft ratio of PAC-azoBNS was also 44%. Film Preparation. Quartz wafers were immersed in piranha solution (1:3 mixture of 30% H2O2 and 98% H2SO4) and heated until no bubbles were released. Caution: Piranha solution reacts Violently with organic material and should be handled carefully. The cleaned quartz wafer was immersed in PDDA aqueous solution (1.0 mg/mL) for 20 min to obtain a cationic ammonium-terminated surface and was then ready for PAC-azoBNS/DAR multilayer deposition. Indium tin oxide (ITO) glass was first cleaned by sonication in chloroform, acetone, and deionized water. Then ITO glass was immersed in 30% H2O2 solution and heated until no bubbles were released. The abovetreated ITO glass can adsorb a layer of DAR after immersion in 1.0 mg/mL aqueous DAR solution for 5 min. Ag-coated quartz crystal microbalance (QCM) resonators were sonicated in ethanol and water and were dried by nitrogen gas. Prior to DAR/PAC-azoBNS multilayer film deposition on QCM resonators, a precursor film of (PDDA/ PSS)*2.5 (PDDA as the outmost layer) was deposited by alternately immersing the resonator in aqueous PDDA (1.0 mg/mL) and PSS (1.0 mg/mL) solution for 20 min each with intermediate water rinsing. A typical procedure for PAC-azoBNS/DAR multilayer film preparation and the subsequent photo-cross-linking and photolysis is schematically shown in Scheme 2 and described as follows: the positively charged substrate was alternately immersed in an aqueous solution of PAC-azoBNS (0.5 mg/mL) and DAR (1.0 mg/mL) for 5 min, with intermediate water rinsing and N2 drying. Multilayer films of (PAC-azoBNS/DAR)*n (note: n refers to the number of deposition cycles) can be fabricated by repeating these steps in a cyclic fashion. Photo-cross-linking of the PAC-azoBNS/DAR multilayer films was conducted by irradiating the films with a 16 W UV lamp at 365 nm wavelength at a distance of 5 cm from the source. The photo-cross-linked PAC-azoBNS/DAR multilayer films were photolyzed in air with a medium-pressure mercury arc lamp (250 W, 360 nm) at a distance of 5 cm from the source. After photolysis, the films were sonicated in water for 1 min to remove the photolyzed fragments embedded in the films. Characterization. UV-vis absorption spectra were recorded with a Shimadzu UV-3100 spectrophotometer. QCM measurements were taken with a KSV QCM-Z500 using quartz resonators with both sides coated with Ag (F0 ) 9 MHz). Fourier transform infrared (FT-IR) spectra were collected on a Bruker IFS 66V instrument.1H NMR spectra were collected on a Bruker Avance-500 500 MHz NMR spectrometer. Cyclic voltammetry was carried out on a BAS100 (Bioanalytical Systems) electrochemistry work station. The electrochemical cell consisted of three electrodes, where an ITO glass deposited with DAR/PAC-azoBNS before and after photocross-linking/photolysis acted as the working electrode, a platinum wire served as the counter electrode, and a Ag/AgCl electrode served as the reference electrode. Aqueous solutions of 0.1M KCl containing 1 mM of either Ru(NH3)63+ or Fe(CN)63- at different pH values adjusted by HCl or NaOH were used as electrolyte solutions.

Results and Discussion Photolyzed Mechanism of PAC-azoBNS. When azobenzene is connected with electron-withdrawing groups such as -NO2 or -CN, its photostability usually decreases.19 FT-IR and 1H NMR spectroscopies were used to study the photolysis of the azobenzene side chains in the PAC-azoBNS. To make the FT-IR (19) (a) Peters, A. T. J. Soc. Dyers Colour. 1988, 104, 344-348. (b) Seu, G. Am. Dyest. Rep. 1985, 74, 29-30. (c) Mallet, V. N. J. Soc. Dyers Colour. 1974, 90, 4-7.

Preparation of Ion-PermselectiVe Multilayer Films

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Scheme 2. Schematic Illustration of the LbL Deposition Process of DAR/PAC-azoBNS Multilayer Films and the Subsequent Photo-Cross-linking and Photolysisa

Figure 1. FT-IR spectra of the azobenzene compound MH before (a) and after (b) photolysis with strong UV irradiation.

Figure 2. 1H NMR spectra of the azobenzene compound MH before (a) and after (b) photolysis with strong UV irradiation. The inset shows an enlarged image of spectrum b in the range of 2.353.2 ppm.

a Multilayer films can be produced by repeating steps 1 and 2 in a cyclic fashion.

and 1H NMR spectra simple for analysis, MH, which has the same structure as that of the side chain of PAC-azoBNS, was used as a model compound to study the photolysis of PACazoBNS. MH was mixed with KBr and pressed to a pellet for FT-IR measurements. Figure 1 shows FT-IR spectra of the MH before and after photolysis with intense UV irradiation. The most obvious changes are the bands that appear at 1606, 1382, and 1146 cm-1 for MH after photolysis. The characteristic peak at 1606 cm-1 is assigned to the benzene ring CdC stretching vibration.11c,20 After photolysis, its intensity is evidently decreased. This demonstrates that the benzene ring structure of MH was destroyed. (20) McCaig, M. S.; Paul, D. R. Polymer 1999, 40, 7209-7225.

The strong peak at 1382 cm-1 is assigned to the stretching vibration peak of azo groups (-NdN-) in MH.21 After photolysis, its peak disappeared completely, indicating the complete decomposition of azo groups. The peak at 1146 cm-1 is assigned to the stretching vibration of -Ar-N- in MH.21After photolysis, its peak also disappeared. The peak at 1041 cm-1 is assigned to the symmetric stretching vibration of SO3- in MH. The peaks at 1198 and 1227 cm-1 are both assigned to the asymmetric stretching vibration of the SO3- in MH.22 After photolysis by 250W UV light, these three peaks are present, although their intensities change. Therefore, the formation of SO3- anions is evident. From FT-IR spectra, one can speculate that 6-(N-methyl)amino-1-hexanol was produced after the photolysis of benzene rings and azo groups. Figure 2 shows 1H NMR spectra of the MH dissolved in D2O before and after photolysis. After photolysis, the color of the solution changed from red to colorless. The decolorization demonstrated that azo dye structure was destroyed. For MH before photolysis, the chemical shifts of azobenzene moieties appear at about 8.13 (Hh), 7.86 (Hi), 7.35 (Hg), and 6.24 ppm (Hf), which are attributed to the protons at the benzene rings (Figure 2a). After photolysis, the resonance corresponding to protons at the benzene rings almost disappears (Figure 2b), which indicates that the azobenzene structure was destroyed. After photolysis, the chemical shift of protons at flexible alkyl chain moieties of MH appear at about 3.12 (Ha′), 2.65 (Hd′), 2.63 (He′), 2.51 (Hb′), and 2.43 ppm (Hc′) (inset of Figure 2). These chemical shifts coincide well with the protons in 6-(N-methyl)amino-1-hexanol. Therefore, 1H NMR supports the formation of 6-(N-methyl)amino-1-hexanol after photolysis of MH with intense UV irradiation. (21) Biswas, N.; Umapathy, S. J. Phys. Chem. A 2000, 104, 2734-2745. (22) (a) Fan, X.; Bazuin, C. G. Macromolecules 1995, 28, 8209-8215. (b) Priimagi, A.; Cattaneo, S.; Ras, R. H. A.; Valkama, S.; Ikkala, O.; Kauranen, M. Chem. Mater. 2005, 17, 5798-5802.

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Figure 3. (a) UV-vis absorption spectra of PAC-azoBNS/DAR multilayer films with different deposition cycles. The number of deposition cycles is 1-8 from the bottom to the top. The inset shows the absorbance at 383 and 481 nm vs the number of deposition cycles. (b) QCM frequency decrease (-∆F) of alternative deposition of PAC-azoBNS (0) and DAR (b).

Preparation of DAR/PAC-azoBNS Multilayer Films and Their Photo-Cross-Linking and Photolysis. UV-vis spectroscopy and QCM measurements were employed to monitor the fabrication process of PAC-azoBNS/DAR multilayer films. UVvis absorption spectra of different deposition cycles of PACazoBNS/DAR multilayer films are shown in Figure 3a. All spectra have two absorption peaks. The one at 383 nm is attributed to the π-π* transition of the diazonium group of DAR,13,14,23 while the other one at 481 nm is attributed to the π-π* transition of trans-azobenzene of PAC-azoBNS.24 The absorbance at 383 and 481 nm increases linearly with the increase in the number of deposition cycles, indicating a satisfactory deposition process with an almost equal amount of DAR and PAC-azoBNS deposited in each deposition cycle. As shown in Figure 3b, the QCM frequency regularly decreases because of the successive deposition of PAC-azoBNS/DAR multilayers on the resonator. The frequency decreases for the deposition of one layer of PAC-azoBNS and DAR were 43.2 ( 4.2 and 51.3 ( 9.5 Hz, respectively. Under mild UV irradiation, diazonium group in DAR decomposes to produce cationic phenyl groups in the DAR chains. The cationic phenyl groups react with nucleophile groups such as carboxylate and sulfonate to form carboxylate and sulfonate esters.13,14 A (DAR/PAC-azoBNS)*8 film was irradiated with a 16 W UV lamp at 365 nm at a distance of 5 cm for different interval times. Figure 4a shows the changes in the absorption spectra of the film with different interval times of irradiation. Upon UV irradiation, the diazonium groups decomposed with a dramatic decrease in the absorbance at 383 nm and a concomitant increase in the absorbance in the vicinity of 296 nm. At the same time, an isosbestic point at 338 nm appears, which indicates that (23) Patai, S. The Chemistry of Diazonium and Diazo Groups; John Wiley & Sons: New York, 1978; p 351. (24) (a) Lee, S.-H.; Balasubramanian, S.; Kim, D. Y.; Viswanathan, N. K.; Bian, S.; Kumar, J.; Tripathy, S. K. Macromolecules 2000, 33, 6534-6540. (b) Dante, S.; Advincula, R.; Frank, C. W.; Stroeve, P. Langmuir 1999, 15, 193-201.

Kang et al.

Figure 4. (a) UV-vis absorption spectral change of an as-prepared (DAR/PAC-azoBNS)*8 film with different time intervals of photocross-linking with mild UV irradiation. The irradiation time from the top to the bottom is 0, 1, 2.5, 6, 10, and 20 min. (b) UV-vis absorption spectral change of a photo-cross-linked (DAR/PACazoBNS)*8 film with different time intervals of photolysis with strong UV irradiation. The irradiation time from the top to the bottom is 0, 3, 8, 13, and 20 min.

only two spectroscopically active species are present.25 The decomposition of the diazonium groups proceeded completely within 20 min. On the basis of our previous work, the interaction of the film between the layers converted from electrostatic interaction to covalent bonds after UV irradiation. Therefore, a robust multilayer film was obtained. It is noticed that, during the photoreaction, the absorption of the azobenzene at 481 nm does not change after calibrating the absorption decrease caused by the decomposition of diazonium groups. Therefore, azobenzene groups are stable under the UV irradiation of 365 nm when a 16 W UV lamp is used. The photo-cross-linked PAC-azoBNS/DAR film was subjected to 360 nm intense UV irradiation generated from a 250 W UV lamp at a distance of 5 cm. The changes in the UV-vis absorption spectra of a photo-cross-linked (PAC-azoBNS/DAR)*8 film were recorded and are shown in Figure 4b. Upon intense UV irradiation, the absorbance at 481 nm decreases, indicating the photolysis of the azo groups. The photolysis of the azo groups proceeded completely after 20 min of irradiation since no absorbance decrease could be found with further extending the irradiation time. The photolysis of the azo group produces imine groups, as demonstrated by the photolysis of azobenzene-containing MH molecules. The photo-cross-linking and photolysis of the PAC-azoBNS/ DAR multilayer films was further characterized by QCM measurements. As shown in Figure 5, a (PAC-azoBNS/DAR)*8 film deposited on a 9 MHz Ag-coated resonator leads to a frequency decrease of ∼757 Hz. After photo-cross-linking the film with a 16 W UV lamp at a distance of 5 cm for 20 min, the film was water rinsed for 1 min and N2 dried. A frequency increase of 11 Hz was detected. The frequency increase, which corresponds (25) McBryde, W. A. E. Talanta 1974, 21, 979-1004.

Preparation of Ion-PermselectiVe Multilayer Films

Figure 5. QCM frequency decrease (-∆F) of a (PAC-azoBNS/ DAR)*8 film with different time interval of UV irradiation: b represents the total frequency decrease of a (PAC-azoBNS/DAR)*8 film; 2 and [ represent the frequency decrease of the film during photocross-linking and the photolysis process, respectively.

Figure 6. CVs of an ITO electrode in an aqueous 0.1 M KCl electrolyte solution containing 1 mM Fe(CN)63- (pH ) 6.5). (a) Bare ITO electrode; (b) ITO electrode deposited with a (DAR/PAC-azoBNS)*2.5 film (DAR as the outmost layer); (c) ITO electrode in b after photo-cross-linking; (d) ITO electrode in c after photolysis. The scan rate is 50 mV/s.

to a mass loss of the film, is mainly caused by the release of N2 during the decomposition of diazonium groups. The frequency does not increase with another 10 min UV irradiation, confirming that 20 min is long enough for the completion of the photocross-linking reaction in a (PAC-azoBNS/DAR)*8 film. After photolyzing the photo-cross-linked (PAC-azoBNS/DAR)*8 film with a 250 W UV lamp at a distance of 5 cm for 20 min, the film was sonicated in water for 1 min to remove the photolyzed fragments. A frequency increase of ∼105 Hz was obtained when compared with the photo-cross-linked film. The frequency increase is attributed to the decomposition of the azobenzene group under strong UV light irradiation. Another 5 min UV irradiation does not change the frequency, indicating the complete decomposition of the azobenzene groups with 20 min irradiation. After photo-cross-linking and photolysis, a total frequency increase of 116 Hz was obtained, which corresponds to a mass loss of ∼15% of the as-prepared (PAC-azoBNS/DAR)*8 film. Most importantly, the QCM measurements confirm that after photolysis with strong UV irradiation, the resultant film is still adhered to the substrate. Permeable Studies of the Multilayer Films. The ionic permeability of the PAC-azoBNS/DAR films was investigated by use of anionic Fe(CN)63- and cationic Ru(NH3)63+ as redox probe molecules. Figure 6 shows cyclic voltammograms (CVs) of an ITO electrode deposited with a (DAR/PAC-azoBNS)*2.5 film (2.5 bilayers, DAR as the outmost layer) before and after photo-cross-linking and photolysis in an aqueous 0.1 M KCl electrolyte solution containing 1 mM Fe(CN)63- (pH ) 6.5). The deposition of a (DAR/PAC-azoBNS)*2.5 film on an ITO

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Figure 7. CVs of an ITO electrode in an aqueous 0.1 M KCl electrolyte solution containing 1 mM Ru(NH3)63+ (pH ) 5.6). (a) Bare ITO electrode; (b) ITO electrode deposited with a (DAR/PAC-azoBNS)* 5 film (PAC-azoBNS as the outmost layer); (c) ITO electrode in b after photo-cross-linking; (d) ITO electrode in c after photolysis. The scan rate is 50 mV/s.

electrode completely blocks the permeation of Fe(CN)63- to the electrode surface. The block of the permeation of Fe(CN)63does not change after the (DAR/PAC-azoBNS)*2.5 film is photocross-linked because cross-linking usually leads to a more compact film. After photolysis of the cross-linked (DAR/PAC-azoBNS)*2.5 film, the peak current of Fe(CN)62-/3- obviously increases compared to the as-prepared and photo-cross-linked films, indicating an improved permeability of the films after photolysis. The improved permeability originates on one hand from the possible pores created within the film due to the decomposition of azobenzene groups, and, on the other hand, from imine groups, which can partly neutralize the negatively charged carboxylate groups. Figure 7 shows CVs of an ITO electrode deposited with a (DAR/PAC-azoBNS)*5 film (5 bilayers, PAC-azoBNS as the outmost layer) in an aqueous 0.1 M KCl electrolyte solution containing 1 mM Ru(NH3)63+ (pH ) 5.6). The deposition of a five-bilayer DAR/PAC-azoBNS film on an ITO electrode obviously decreases its permeability to Ru(NH3)63+, but not completely. After photo-cross-linking the film, a further decrease in the permeation toward Ru(NH3)63+ is observed, indicating a more compact film produced. Photolysis leads to a complete block of the permeation toward Ru(NH3)63+, although pores might be produced. The complete block of Ru(NH3)63+ in a photolyzed (DAR/PAC-azoBNS)*5 film in a pH 5.6 aqueous solution is mainly due to the imine groups produced in the film. In pH 5.6 aqueous solution, most of the imine groups are protonated, and therefore are positively charged. The complete block of Ru(NH3)63+ demonstrates that the number of positively charged groups of imine prevails over that of negatively charged carboxylate, and the resultant films are positively charged. Since the diameters of Fe(CN)63- and Ru(NH3)63+ are similar to each other (6.0 Å for Fe(CN)63- and 6.2 Å for Ru(NH3)63+),11c the differences in the transport properties for these two probe molecules to DAR/PAC-azoBNS films will be the reflection of the net charge presented in the film. From the fact that a (DAR/ PAC-azoBNS)*2.5 film completely blocks the permeation of Fe(CN)63- while a (DAR/PAC-azoBNS)*5 film allows the permeation of Ru(NH3)63+, one can conclude that the as-prepared DAR/PAC-azoBNS contains net negative charges within the film.26 This conclusion is reasonable because PAC-azoBNS has a longer flexible chain than that of DAR and contains pHdependent ionizable carboxylic acid groups. PAC-azoBNS adopts (26) DAR contains imine groups. These imine groups are believed to either form inter-hydrogen bonds with each other or complex with carboxylate and sulfonate groups of PAC-azoBNS in DAR/PAC-azoBNS multilayer films.

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Figure 8. CVs of an ITO electrode deposited with a photo-crosslinked (DAR/PAC-azoBNS)*5.5 film (DAR as the outmost layer) after photolysis in a 0.1 M KCl electrolyte solution containing 1 mM of either Fe(CN)63- or Ru(NH3)63+ at pH 3.0 and 8.5. The scan rate is 50 mV/s.

a coil configuration in solution. There are unpaired carboxylic acid groups in alternately assembled DAR/PAC-azoBNS film. Therefore, net negative charges can be induced in solution with suitable pH. The net negative charges favor the permeation of Ru(NH3)63+ but prevent the permeation of Fe(CN)63-. At present, a photo-cross-linked DAR/PAC-azoBNS film with both free carboxylic acid and imine groups is fabricated. Because of the carboxylic acid and imine groups, the permeability of the film must be pH-dependent. The pH-dependent permeability of DAR/PAC-azoBNS film toward Fe(CN)63- and Ru(NH3)63+ was further investigated. Figure 8 shows the CVs of an ITO electrode deposited with a (DAR/PAC-azoBNS)*5.5 film (5.5 bilayers, DAR as the outmost layer) in an aqueous 0.1 M KCl electrolyte solution containing 1 mM of either Fe(CN)63- or Ru(NH3)63+ at pH 3 and 8.5. The magnitude of the CVs was significantly affected by the pH of the electrolyte solutions. The (DAR/PACazoBNS)*5.5 film is permeable toward Fe(CN)63- at pH values of 3.0 and 8.5. From pH 3-8.5, an ∼16% reduction in peak current for the anionic Fe(CN)63- probe molecule was found. The reduction in peak current is due to the deprotonation of the -NRH2+ and COOH groups in the film, which reduces the net positive charge in the film and therefore decreases the peak current of Fe(CN)63- molecules. In contrast, the (DAR/PAC-azoBNS)*5.5 film is permeable toward Ru(NH3)63+ at pH 8.5 but impermeable toward Ru(NH3)63+ at pH 3.0, demonstrating a pHswitchable permselectivity for cationic probe molecules. The permeation toward Ru(NH3)63+ at pH 8.5 is due to the deprotonation of the -NRH2+ and COOH groups at high pH, which makes the film contain net negative charges and therefore favor the permeation of Ru(NH3)63+. At pH 3.0, the -NRH and COOH groups are protonated, and the resultant film contains net positive charges, which blocks the permeation of Ru(NH3)63+ molecules. The voltammetric responses for an anionic probe molecule of Fe(CN)63- at pH 3 is evidently greater than the voltammetric peak current for a cationic probe of Ru(NH3)63+. It indicates that the multilayer films is “on” for Fe(CN)63- and “off” for Ru(NH3)63+ at pH 3. The Reversible Switching Behavior and Stability of the Photolyzed Cross-linked DAR/PAC-azoBNS Films. The reversible switching behavior and stability of the ion-permselective membrane is a prerequisite for its long-term application. The covalently cross-linked structure makes the film robust in solution with high polarity and ionic strength, which is believed to guarantee the reversible switching behavior and the stability of the resultant film as permselective membranes. Figure 9a shows a successive redox switching process of an ITO electrode deposited with a (DRA/PAC-azoBNS)*5.5 film in a 0.1 M KCl

Figure 9. (a) Reversible switching behavior of an ITO electrode deposited with a photo-cross-linked (DAR/PAC-azoBNS)*5.5 film (DAR as the outmost layer) after photolysis in a 0.1 M KCl electrolyte solution containing 1 mM Ru(NH3)63+ when the pH of the electrolyte is switched between 3.0 and 8.5. (b) The anodic peak currents of the same electrode in panel a in 0.1 M KCl solution containing 1 mM Ru(NH3)63+ at pH 3 and 8.5 after immersing the electrode in aqueous NaCl solution with different concentrations for 24 h followed by a 1 h immersion in deionized water. All the anodic peaks were normalized with the first anodic peak current measured in pH 3 electrolyte solution.

electrolyte solution containing 1 mM Ru(NH3)63+ when its pH is switched between 3 and 8.5. The figure was obtained by plotting the anodic peak currents, which were normalized with the first anodic peak current measured in pH 3 solution, against the pH value of the electrolyte solution. It can be seen that the switching behavior was fully reversible by repeatedly switching the pH of the solution between 3 and 8.5. The film is “off” for Ru(NH3)63+ at pH 3 and “on” for Ru(NH3)63+ at pH 8.5. The stability of the ion-permselective films was further investigated. The (DAR/ PAC-azoBNS)*5.5-deposited ITO electrode was first immersed in aqueous NaCl solution with different concentrations for 24 h then in deionized water for 2 h. The anodic peak currents of the electrode in 0.1 M KCl solution containing 1 mM Ru(NH3)63+ at pH 3 and 8.5 were measured. Figure 9b shows the normalized anodic peak currents after immersing the film in NaCl solution with different concentrations. Even when the concentration of NaCl solution is as high as 1.0 M, the photolyzed cross-linked film is still permselective toward Ru(NH3)63+ at pH 3 and 8.5. Therefore, the photolyzed cross-linked multilayer films have very high stability in high ionic strength solution as ionpermselective membranes.

Conclusions In the present study, we show that PAC-azoBNS/DAR multilayer films containing photo-cross-linkable and photolabile groups can be fabricated by a LbL assembly process. Robust ionpermselective membranes can be fabricated by photo-crosslinking PAC-azoBNS/DAR multilayer films followed by photolyzing the photolabile azobenzene groups. The resultant films contain free imine and carboxylic groups and show pH-

Preparation of Ion-PermselectiVe Multilayer Films

dependent permeation properties toward anionic and cationic molecular species. The covalently cross-linking structure of the film guarantees its reversible switching behavior and high stability in solution with high ionic strength. Its simplicity in film preparation, easiness in depositing the film on substrates with complicated morphologies, and high stability will make the film potentially useful in areas such as membrane separation, ion selective sensors, and so on.

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Acknowledgment. This work is supported by the National Natural Science Foundation of China (NSFC Grant 20304004), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (FANEDD Grant 200323), and the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT Grant IRT0422). LA0612320