Combining Hydrogen-Bonding Complexation in Solution and

Oct 4, 2007 - All Publications/Website .... Combining Hydrogen-Bonding Complexation in Solution and .... of Applied Polymer Science 2016 133 (10.1002/...
0 downloads 0 Views 266KB Size
Download PDF
Langmuir 2007, 23, 11631-11636

11631

Combining Hydrogen-Bonding Complexation in Solution and Hydrogen-Bonding-Directed Layer-by-Layer Assembly for the Controlled Loading of a Small Organic Molecule into Multilayer Films Guanghong Zeng, Jian Gao, Senlin Chen, Huan Chen, Zhiqiang Wang,* and Xi Zhang* Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing, 100084, People’s Republic of China ReceiVed July 10, 2007. In Final Form: August 24, 2007 We have combined hydrogen-bonding complexation in solution and layer-by-layer assembly for the controlled loading of a water-insoluble small organic molecule, bis-triazine (DTA), an azobenzene derivative containing multiple hydrogen bond donors and acceptors, into layer-by-layer multilayer films of poly(acrylic acid) and diazo-resin. UVvisible spectroscopy indicates that DTA has been loaded into multilayer films, with the loading amount increasing linearly with the number of layers. The loading amount can be well tuned either by changing the concentration of DTA or the solvent composition at the complexation step. Fourier transform infrared spectroscopy has revealed that both the complexation and layer-by-layer assembly are driven by hydrogen bonding. After photo-cross-linking and immersion in dimethyl sulfoxide to release DTA, the film can serve as an absorbent for DTA. This study provides a new unconventional layer-by-layer assembly that combines hydrogen-bonding complexation in solution and hydrogenbond-driven layer-by-layer assembly at the interface. This method provides a new route to load a variety of waterinsoluble functional organic molecules into layer-by-layer films.

Introduction Since 1991, layer-by-layer (LbL) assembly has emerged as a powerful tool for the fabrication of functional thin films with tailored structure and composition in the nanoscale.1 At the very beginning, this method just allowed for the fabrication of multilayer films of oppositely charged species, such as polyelectrolytes, bolaamphiphiles, multicharged organic molecules, and so forth, driven by electrostatic interactions. Since then, numerous efforts have been made to develop other kinds of LbL assembly methods on the basis of different interactions, such as hydrogen bonding,2 stepwise chemical reaction,3 molecular recognition,4 biorecognition,5 charge-transfer interaction,6 surface sol-gel processes,7 stereocomplex formation,8 and so forth. Even so, there are still many kinds of functional species, mainly hydrophobic species, that cannot be used as building blocks for such conventional LbL assembly. To meet that need, unconventional methods of LbL assembly have been developed.1d The central idea of the unconventional LbL assembly is the formation of supramolecular assemblies via self-assembly, which can be subsequently used for LbL assembly. Typical examples include the formation of a host-guest inclusion complex between homooxacalix[3]arene and fullerene,9 the attachment of charged polynuclear aromatic moieties onto carbon nanotubes based on the combined interactions of hydrophobic effect and electrodonor/ * Corresponding author. E-mail: [email protected]. (1) (a) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321. (b) Decher, G. Science 1997, 277, 1232. (c) Zhang, X.; Shen, J. C. AdV. Mater. 1999, 11, 1139. (d) Zhang, X.; Chen, H.; Zhang, H. Y. Chem. Commun. 2007, 1395. (e) Scho¨nhoff, M. Curr. Opin. Colloid Interface Sci. 2003, 8, 86. (2) (a) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C. Macromol. Rapid Commun. 1997, 18, 509. (b) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (c) Wang, L. Y.; Cui, S. X.; Wang, Z. Q.; Zhang, X. Langmuir 2000, 16, 10490. (3) (a) Lee, H.; Kepley, L. J.; Hong, H. G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (b) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962. (c) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 4655. (d) Chan, E.; Lee, D.; Ng, M.; Wu, G.; Lee, K.; Yu, L. J. Am. Chem. Soc. 2002, 124, 12238. (e) Zhang, F.; Jia, Z.; Srinivasan, M. P. Langmuir 2005, 21, 3389. (f) Such, G. K.; Quinn, J. F.; Quinn, A.; Tjipto, E.; Caruso, F. J. Am. Chem. Soc. 2006, 128, 9318.

acceptor interactions,10 the reaction of 1,4-bis(2,2′:6′,2′′-terpyrid4′-yl)benzene with metal ions to form coordination polyelectrolytes,11 the incorporation of pyrene, azobenzene, and an azobenzene derivative into block copolymer micelles,12 and so forth. These types of supramolecularly modified building blocks are charged and water-soluble, and are thus suitable for normal LbL assembly. Recently, we have taken advantage of an unconventional method of LbL assembly that combines electrostatic complexation in solution and LbL assembly for the incorporation of singlecharged small organic molecules into LbL films.13 Inspired by this concept, herein we attempt to introduce a new method (4) (a) Suzuki, I.; Egawa, Y.; Mizukawa, Y.; Hoshi, T.; Anzai, J. Chem. Commun. 2002, 164. (b) Crespo-Biel, O.; Dordi, B.; Reinhoudt, D. N.; Huskens, J. J. Am. Chem. Soc., 2005, 127, 7594. (5) (a) Hong, J. D.; Lowack, K.; Schmitt, J.; Decher, G. Prog. Colloid Polym. Sci. 1993, 93, 98. (b) Decher, G.; Lehr, B.; Lowack, K.; Lvov, Y.; Schmitt, J. Biosens. Bioelectron. 1994, 9, 677. (c) Bourdillon, C.; Demaille, C.; Moiroux, J.; Save´ant, J. M. J. Am. Chem. Soc. 1994, 116, 10328. (d) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Chem. Soc., Chem. Commun. 1995, 2313. (e) Anzai, J.; Kobayashi, Y.; Nakamura, N.; Nishimura, M.; Hoshi, T. Langmuir 1999, 15, 221. (f) Anzai, J.; Kobayashi, Y. Langmuir, 2000, 16, 2851. (6) (a) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (b) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1998, 14, 2768. (c) Wang, X. Q.; Naka, K.; Itoh, H.; Uemura, T.; Chujo, Y. Macromolecules 2003, 36, 533. (7) (a) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Lett. 1996, 831. (b) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1997, 9, 1296. (c) Ichinose, I.; Kawakami, T.; Kunitake, T. AdV. Mater. 1998, 10, 535. (d) Wang, Q. F.; Zhong, L.; Sun, J. Q.; Shen, J. C. Chem. Mater. 2005, 17, 3563. (e) Kang, E. H.; Jin, P. C.; Yang, Y. Q.; Sun, J. Q.; Shen, J. C. Chem. Commun. 2006, 4332. (8) (a) Serizawa, T.; Hamada, K.; Kitayama, T.; Fujimoto, N.; Hatada, K.; Akashi, M. J. Am. Chem. Soc. 2000, 122, 1891. (b) Serizawa, T.; Hamada, K.; Kitayama, T.; Katsukawa, K.; Hatada, K.; Akashi, M. Langmuir 2000, 16, 7112. (9) Ikeda, A.; Hatano, T.; Shinkai, S.; Akiyama, T.; Yamada, S. J. Am. Chem. Soc. 2001, 123, 4855. (10) (a) Artyukhin, A. B.; Bakajin, O.; Stroeve, P.; Noy, A. Langmuir 2004, 20, 1442. (b) Yan, Y.; Zhang, M.; Gong, K.; Su, L.; Guo, Z.; Mao, L. Chem. Mater. 2005, 17, 3457. (c) Paloniemi, H.; Lukkarinen, M.; A ¨ a¨ritalo, T.; Areva, S.; Leiro, J.; Heinonen, M.; Haapakka, K.; Lukkari, J. Langmuir 2006, 22, 74. (11) Schu¨tte, M.; Kurth, D. G.; Linford, M. R.; Co¨lfen, H.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2891. (12) (a) Ma, N.; Zhang, H. Y.; Song, B.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2005, 17, 5065. (b) Ma, N.; Wang, Y. P.; Wang, Z. Q.; Zhang, X. Langmuir 2006, 22, 3906. (c) Ma, N.; Wang, Y. P.; Wang, B. Y.; Wang, Z. Q.; Zhang, X. Langmuir 2007, 23, 2874.

10.1021/la702054d CCC: $37.00 © 2007 American Chemical Society Published on Web 10/04/2007

11632 Langmuir, Vol. 23, No. 23, 2007

Zeng et al.

Figure 1. Schematic illustration of the fabrication of an LbL film: (a) formation of hydrogen-bonding PAA-DTA complexes; (b) LbL assembly of PAA-DTA and DAR.

Figure 2. UV-vis spectra following the LbL assembly of a (PAADTA/DAR)5PAA-DTA multilayer film. Inset: absorbance at 380 nm versus the number of layers deposited.

combining hydrogen-bonding complexation and LbL assembly to load a water-insoluble small organic molecule into multilayer films. As shown in Figure 1, first, a small organic molecule, bis-triazine (DTA) is mixed with poly(acrylic acid) (PAA) in methanol to form a hydrogen-bonding complex (PAA-DTA); second, LbL assembly is performed between the methanol solutions of PAA-DTA and diazo-resin (DAR), driven by hydrogen-bonding. In this way, DTA may be loaded into the LbL film in a convenient and well-controlled manner. Since DAR is a photoreactive polycation, we can irradiate the film with UV light, forming a covalently attached multilayer film. Then we will investigate whether DTA could be released and subsequently reloaded into the film once again. By applying the method to a series of molecules, we also attempt to determine what kind of water-insoluble molecules are suitable for this unconventional LbL method.

and drying with nitrogen stream. The substrates were then immersed in a DAR methanol solution (0.5 g/L) for 10 min, followed by the same rinsing and drying cycle. By repeating the above process, multilayer films were prepared. Unless otherwise denoted, all films were terminated with a PAA-DTA layer on five bilayers, that is, the film structure can be denoted as (PAA-DTA/DAR)5PAA-DTA. LbL assembly on gold electrodes was performed in the same way, except that the assembly started from DAR. Photo-Cross-Linking. The (PAA-DTA/DAR)5PAA-DTA film was irradiated to form the cross-linked LbL films in air 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. Release and Reloading of DTA. For the release of DTA, the cross-linked film was immersed in DMSO, followed by rinsing with methanol and drying with nitrogen stream. For the reloading of DTA, the film was immersed in a methanol solution of DTA, followed by the same rinsing and drying process. Film Characterization. UV-vis spectra of films on quartz slides were recorded with a Hitachi UV-vis 3010 spectrophotometer. Fourier transform infrared (FT-IR) spectra of films on CaF2 plates were collected on a Bruker IFS 66V instrument equipped with a DTGS detector. Films were evacuated for 30 min before measurement. Spectra (4000-800 cm-1) were collected from 256 scans at 4 cm-1 resolution. Atomic force microscopy (AFM) images were obtained on a Multimode Nanoscope IV, Veeco Company. Imaging was performed in tapping mode in air using silicon cantilevers (200300 kHz). Cyclic voltammetry was performed on an Autolab PGSTAT 12 in a three-electrode electrochemical cell at a scan rate of 100 mV/s. A platinum wire was used as the counter-electrode, and Ag/AgCl (3 M KCl) was used as the reference electrode. A 0.1 M KCl aqueous solution containing 2 mM Fe(CN)63- was used as the electrolyte solution.

Experimental Section Materials. 4-(phenyldiazenyl)aniline was purchased from TGI. 4,4′-(diazene-1,2-diyl)dianiline was purchased from Aldrich. N2(4-(phenyldiazenyl)phenyl)-1,3,5-triazine-2,4,6-triamine and N2,N2′(4,4′-(diazene-1,2-diyl)bis(4,1-phenylene))bis(1,3,5-triazine-2,4,6triamine) were synthesized according to the literature.14 DAR was kindly provided by Weixiao Cao (Peking University). The number average molecular weight was ca. 2640. PAA (Mw ) 2000) and poly(ethyleneimine) (PEI, Mw ) 50 000) were purchased from Aldrich and used as received. Methanol and dimethyl sulfoxide (DMSO) were analytical-grade products from Beijing Chemical Reagent Company. Quartz slides were purchased from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences. Substrate Preparation. Quartz slides were treated in a hot piranha solution (mixture of 98% H2SO4 and 30% H2O2, v/v ) 7:3) for 30 min (CAUTION: piranha solution is extremely corrosiVe), thoroughly washed with pure water, dried in an oven, and then immersed in a dilute toluene solution of (4-aminobutyl)-dimethylmethoxysilane for 10 h. CaF2 plates were immersed in 1 mg/mL PEI aqueous solution for 4 h to adsorb a PEI layer. Both the above treatments modified the substrates with NH2-modified surfaces. Gold electrodes were polished with Al2O3 powders, washed with deionized water, and immersed in a 2 mM ethanol solution of 3-mercapto-1-propane carboxylic acid for 8 h. LbL Assembly of Multilayer Films. NH2-modified substrates were first immersed in a PAA-DTA methanol solution (PAA, 0.5 g/L; DTA, 0.06 mM) for 10 min, followed by rinsing with methanol (13) (a) Chen, H.; Zeng, G. H.; Wang, Z. Q.; Zhang, X.; Peng, M. L.; Wu, L. Z.; Tung, C. H. Chem. Mater. 2005, 17, 6679. (b) Chen, H.; Zeng, G. H.; Wang, Z. Q.; Zhang, X. Macromolecules 2007, 40, 653. (14) (a) Gao, J.; He, Y. N.; Xu, H. P.; Song, B.; Zhang, X.; Wang, Z. Q.; Wang, X. G. Chem. Mater. 2007, 19, 14. (b) Gao, J.; He, Y. N.; Liu, F.; Zhang, X.; Wang, Z. Q.; Wang, X. G. Chem. Mater. 2007, 19, 3877.

Results and Discussion LbL Assembly of PAA-DTA and DAR. The process of LbL assembly between PAA-DTA and DAR is schematically illustrated in Figure 1. UV-vis spectroscopy has been used to follow the process of LbL assembly. As the UV-vis spectra of DTA and DAR are quite similar, both the adsorption of DTA and DAR has been followed by the absorbance at 380 nm. Study on the time-dependent growth of a single layer of PAA-DTA or DAR indicates that the adsorption amount reaches a plateau in 2 min. However, in order to achieve adsorption equilibrium, the dipping time has been set to 10 min. Figure 2 shows the UV-vis absorbance of a multilayer film with an increasing number of layers. There is a uniform increment of the absorbance at 380 nm upon the deposition of each layer of PAA-DTA or DAR. The linear deposition pattern of PAA-DTA can be clearly seen in the inset, in which the absorbance at 380 nm of films with PAA-DTA as the outermost layer is plotted against the number

Loading DTA into Multilayer PAA-DAR Films

Figure 3. The loading amount of DTA versus different concentrations of DTA in PAA-DTA solutions, indicated by the absorbance at 380 nm. The concentrations of PAA and DAR were both fixed to 0.5 mg/mL.

of layers. These results indicate that, using our method, an equal amount of DTA in each PAA-DTA layer is incorporated into the multilayer films. Methods of Tuning the Loading Amount: Concentration of DTA. One of the advantages of this method involving a twostep self-assembly is that each step of the self-assembly can be controlled. The loading amount of DTA can be adjusted by several parameters, such as, concentration of DTA, solvent composition, temperature, and so forth. Figure 3 displays the loading amount of DTA versus the different concentrations of DTA in the PAADTA complex solution. As expected, the loading amount of DTA increases with the concentration of DTA, reaching a maximum absorbance of 0.22 at 0.06 mM. Further increase of DTA concentration to 0.10 mM does not obtain a higher loading amount. It should be noted that an even higher concentration is not available because of precipitation. To confirm the reproducibility of the DTA loading amount, we have fabricated seven parallel multilayer films, using the same dipping solutions and under the same temperature. The loading amount of DTA shows good reproducibility, with a maximum deviation from the average value of less than 3%. This result is consistent with reported results15 and demonstrates the feasibility of our methods for tuning the loading amount of DTA. Methods of Tuning the Loading Amount: Solvent Composition. Hydrogen bonds in solution are intensely influenced by solvent composition, which provides the possibility to control hydrogen-bonding-directed LbL assembly.16 We have used a mixture of methanol and DMSO as the mixed solvent of PAADTA complexes to study the influence of solvent composition on the DTA loading amount. As shown in Figure 4, as the percentage of DMSO is gradually increased from 0 to 100%, the loading amounts initially undergo a small decrease from 0 to 2.5%, followed by a dramatic decrease from 10 to 20%. DTA cannot be loaded into the film in a solvent containing DMSO higher than 20%. This result can be rationalized by weak hydrogen bonding between DTA and PAA in high-polarity solvents. Hydrogen Bond Formation Confirmed by FT-IR Spectroscopy. FT-IR spectroscopy has been employed to investigate the hydrogen bonds involved in PAA-DTA complexes and those between PAA and DAR during the multilayer fabrication. As it is hard to study hydrogen bonding in methanol solution by FTIR spectroscopy because the solvent interacts intensely with the solute, we resort to the FT-IR spectra of cast films to study the (15) Decher, G.; Schlenoff, J. B. Multilayer Thin Films; Wiley-VCH Verlag GmbH & Co.: Weinheim, Germany, 2003; p 6. (16) (a) Zhang, H. Y.; Wang, Z. Q.; Zhang, Y. Q.; Zhang, X. Langmuir 2004, 20, 9366. (b) Chen, Q. X.; Ma, N.; Qian, H. J.; Wang, L. Y.; Lu, Z. Y. Polymer 2007, 48, 2659.

Langmuir, Vol. 23, No. 23, 2007 11633

Figure 4. The loading amount of DTA versus different solvent compositions (DMSO and methanol) of the PAA-DTA solution, indicated by the absorbance at 380 nm.

Figure 5. (a) FT-IR spectra of an LbL film of PAA-DTA/DAR and cast films of a PAA-DTA complex, PAA, and DTA on CaF2 plates. (b) Second-derivative plots of FT-IR spectra of LbL films of (PAA-DTA/DAR)5PAA-DTA and cast films of a PAA-DTA complex and PAA.

hydrogen bonding in PAA-DTA complexes. Figure 5a shows the FT-IR spectra of films cast directly on CaF2 plates from methanol solutions of PAA, DTA, and PAA-DTA complexes. We mainly focus on the carboxylic acid region ranging from 1700-1750 cm-1 as it reflects the hydrogen-bonding state of PAA. The states of carboxylic acid groups in PAA and proposed models of hydrogen bonding are illustrated in Figure 6. The hydrogen-bonding modes between PAA and DTA are proposed

11634 Langmuir, Vol. 23, No. 23, 2007

Zeng et al.

Figure 7. FT-IR spectra of a cast film of PAA-DTA recorded at 40, 70, 110, and 150 °C.

Figure 6. (a) Possible forms of carboxylic acid groups in PAA and PAA-DTA complexes. (b) Proposed model of hydrogen-bonding between PAA and DTA. (c) Proposed model of hydrogen-bonding between PAA and DAR.

according to the literature.17 As shown in Figure 5a, for PAA, the absorption band at 1708 cm-1 and a small shoulder peak at 1741 cm-1 are assigned to the CdO stretching of carboxylic groups in the associated state of cyclic dimers and in the freemonomer state, respectively,18a which indicates that carboxylic groups are mainly involved in intramolecular hydrogen bonding. For PAA-DTA complexes, a broadened band suggests that carboxylic acid groups are involved in several different types of hydrogen bonding, which is consistent with the models proposed in Figure 6. The broadened band is centered at 1718 cm-1, indicating that parts of the carbonyl groups in PAA are freed from the associated state of cyclic dimers to form intermolecular hydrogen bonds with DTA.18 To distinguish the different types of hydrogen bonds, we have performed second-derivative analysis of the spectra. As shown in Figure 5b, the two bands at 1707 and 1741 cm-1 are separated from each other clearly in the second-derivative plot of the PAA spectrum. However, for the PAA-DTA complex, only one peak at 1728 cm-1 appears, which means that the several different types of hydrogen bonds are not distinguishable in the FT-IR spectrum. To further confirm the hydrogen bonding in the PAA-DTA complex, we have explored the temperature-dependent FI-IR spectra. As shown in Figure 7, as temperature increases from 40 to 150 °C, the absorbance at 1725 cm-1 decreases and that at 1740 cm-1 increases, indicating that some carboxylic groups are dissociated from hydrogen-bonding, forming free monomers. (17) Zhang, X. L.; Chen, X. M. Cryst. Growth Des. 2005, 5, 617. (18) (a) Dong, J.; Ozaki, Y.; Nakashima, K. Macromolecules 1997, 30, 1111. (b) Dong, J.; Ozaki, Y. Macromolecules 1997, 30, 286. (c) Lee, J. Y.; Painter, P. C.; Coleman, M. M. Macromolecules 1988, 21, 954. (d) Lee, J. Y.; Painter, P. C.; Coleman, M. M. Macromolecules 1988, 21, 346. (e) Nishi, S.; Kotaka, T. Macromolecules 1985, 18, 1519.

Figure 8. UV-vis spectra of a (PAA-DTA/DAR)5PAA-DTA film irradiated by UV light for different times.

The FT-IR spectra of an LbL film of a (PAA-DTA/ DAR)5PAA-DTA film are shown in Figure 5a,b. The stretching bands of the carboxylic groups are similar to those of the PAADTA complex. This is reasonable because PAA in the LbL film is also complexed with DTA. Furthermore, it forms hydrogen bonds with DAR as a driving force for multilayer buildup.19 There is also a possibility that electrostatic interaction contributes. However, as the LbL assembly is carried out in methanol, and the organic environment can inhibit ionization of PAA and DAR, electrostatic interaction cannot be the main driving force for the multilayer buildup. Photo-Cross-Linking. DAR can serve as a photoreactive crosslinker to stabilize the LbL films.20 A (PAA-DTA/DAR)5PAADTA film has been irradiated under UV light to photo-cross-link the film. As shown in Figure 8, the absorbance around 380 nm decreases, and that around 300 nm increases under irradiation. The photo-cross-linking is finished in 30 s. However, there remains a possibility that the azobenzene moiety in DTA undergoes trans to cis isomerization and causes the decrease of absorbance at 380 nm. The latter possibility can be excluded by the fact that the absorbance at 380 nm remains unchanged even 1 day after irradiation, which is long enough for the cis to trans isomerization if there is a trans to cis isomerization previously. (19) (a) Chen, J. Y.; Cao, W. X. Chem. Commun. 1999, 1711. (b) Cao, T. B.; Chen, J. Y.; Yang, C. H.; Cao, W. X. New J. Chem. 2001, 25, 305. (20) (a) Sun, J. Q.; Wu, T.; Sun, Y. P.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Cao, W. X. Chem. Commun. 1998, 1853. (b) 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, 961. (c) Sun, J. Q.; Wu, T.; Liu, F.; Wang, Z. Q.; Zhang, X.; Shen, J. C. Langmuir 2000, 16, 4620.

Loading DTA into Multilayer PAA-DAR Films

Langmuir, Vol. 23, No. 23, 2007 11635

Figure 9. UV-vis spectra of a (PAA-DTA/DAR)5PAA-DTA film after photo-cross-linking (solid line), immersed in DMSO for 2 min (dashed line), and immersed in 0.06 mM DTA solution for 10 min (dash-dotted line).

Figure 11. Cyclic voltammograms of a gold electrode coated with a (DAR/PAA-DTA)2 film after DMSO treatment in Fe(CN)63solutions with different pH values.

Figure 10. Cyclic voltammograms of a gold electrode coated with a (DAR/PAA-DTA)2 film before (solid line) and after (dashed line) DMSO treatment by using Fe(CN)63- as a redox probe.

Figure 12. UV-vis absorbance increase at 380 nm for (PAADTA/DAR)5PAA-DTA films (circles) and (PAA/DAR)5PAA films (squares) immersed in DTA methanol solutions of different concentrations for 10 min.

Another piece of evidence obtained from FT-IR measurement is that the absorption bands of the diazo group in DAR disappear entirely after UV irradiation (not presented). Release and Reloading of DTA. DMSO is a well-known hydrogen bond breaker. Figure 9 shows that the immersion of a cross-linked LbL film into DMSO for 2 min can cause a total release of DTA, evidenced by the disappearance of the absorption band at 380 nm. This fact can be rationalized by the dissociation of hydrogen bonds and good solubility of DTA in DMSO. The loading amount is calculated simply from the decrease of the absorbance at 380 nm before and after DMSO treatment. To exclude the possibility that the multilayer film may dissolve upon DMSO treatment, we treated a photo-cross-linked (PAA/ DAR)5PAA film with DMSO and nearly no change of the UVvis spectrum was observed after 40 min of immersion. Cyclic voltammetry was performed as another support for the release of DTA from the film. Figure 10 shows the cyclic voltammograms of a gold electrode coated by a (DAR/PAADTA)2 film obtained in the presence of Fe(CN)63- as a electrochemical probe. Initially, no electrical response is observed, implying no permeability for the probe. After DMSO treatment, a significant redox signal is observed. The increase in electrical response is a result of enhanced ion-permeability for Fe(CN)63-. Previous reports21 on cyclic voltammetry study on LbL-coated electrodes have demonstrated that enhanced ion-permeability is mainly due to two factors: (1) generation of binding sites in the film and (2) increased space for accommodating ions. We have (21) (a) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978. (b) Dai, J. H.; Jensen, A. W.; Mohanty, D. K.; Erndt, J.; Bruening, M. L. Langmuir 2001, 17, 931. (c) Park, M. K.; Deng, S. X.; Advincula, R. C. J. Am. Chem. Soc. 2004, 126, 13723.

also investigated the effect of pH on cyclic voltammograms and confirmed that redox current decreases remarkably with increasing pH (Figure 11), indicating that, after the release of DTA, the ionized carboxylic groups do not serve as a binding site but repel ferricyanide ions instead. Therefore, we believe the enhanced permeability is a result of increased space for accommodating ferricyanide ions after the release of DTA. The different ionpermeability of LbL films before and after DMSO treatment is an indicator of the removal of DTA. Interestingly, DTA can be reloaded upon further immersion of the film into a methanol solution of DTA. The reloading amount is about half the amount previously loaded (Figure 9). A possible reason for the reduced loading amount is that some of the binding sites previously occupied by DTA may be either broken by DMSO treatment or not accessible after photo-crosslinking the film. To further investigate the reloading behavior, we used a series of DTA methanol solutions of different concentrations for DTA loading into (PAA/DAR)5PAA films and (PAA-DTA/DAR)5PAA-DTA films treated with DMSO. As shown in Figure 12, the loading of DTA shows concentration dependence for both types of films. However, the latter shows great advantages over the former in terms of loading capacity. This can be explained as follows: for a DTA-loaded film, the release of DTA leaves many binding sites, both on the surface and in the interior of the multilayer film; however, for a (PAA/ DAR)5PAA film, DTA is hard to permeate into the interior of the compact film, which is supported by the poor ion-permeability of (PAA/DAR)5PAA films revealed by cyclic voltammetry. Thus we have demonstrated that DTA can serve as a template to

11636 Langmuir, Vol. 23, No. 23, 2007

Zeng et al.

Figure 14. A series of molecules tested by this unconventional LbL method.

Figure 13. (a) AFM image of a (PAA-DTA/DAR)5PAA-DTA film (2 × 2 µm). (b) AFM image of a (PAA/DAR)5PAA film (2 × 2 µm).

fabricate multilayer films with enhanced loading capacity of DTA. This is conceptually similar to molecular imprinting.22 The Influence of DTA Loading on the Surface Morphology. AFM imaging was performed to investigate the influence of the loading of DTA on the surface morphology of LbL films. Figure 13 shows AFM images of a DTA-loaded multilayer film and a normal (PAA/DAR)5PAA film. The surface of the DTA loaded multilayer film is much rougher than that of a normal multilayer film. To compare the surface roughness quantitatively, we calculated the root mean square (rms) of the height values of each AFM image, and the rms values are 6.7 and 4.2 nm in an area of 2 × 2 µm for the DTA-loaded film and the normal multilayer film, respectively. The increase of surface roughness can be explained as follows: the binding of DTA to PAA could make it unable to form a compact LbL film with DAR. Instead, loopy chains of PAA could be adsorbed, resulting in a rougher surface. Additionally, according to our previous reports,13a it is possible that the loopy conformation of PAA chains also contributes to the increased loading capacity and enhanced ionpermeability. (22) (a) Wulff, G.; Sarhan, A. Angew. Chem., Int. Ed. Engl. 1972, 11, 341. (b) Wulff, G. Chem. ReV. 2002, 102, 1. (c) Vlatakis, G.; Andersson, L. I.; Mueller, R.; Mosbach, K. Nature 1993, 361, 645. (d) Haupt, K.; Mosbach, K. Chem. ReV. 2000, 100, 2495. (e) Shi, F.; Liu, Z.; Wu, G. L.; Zhang, M.; Chen, H.; Wang, Z. Q.; Zhang, X.; Willner, I. AdV. Funct. Mater. 2007, 17, 1821.

Molecular Candidates Suitable for this Unconventional LbL Method. We applied our method to a series of structurally related molecules with an increasing number of hydrogen bond donors and acceptors to find out the structural demand of the method, shown in Figure 14. With the exception of DTA, we failed in loading the molecules into multilayer films by using our method. Interestingly, we noticed that a floccule formed when a high concentration of DTA was added to the PAA solution. However, this does not happen for the other three molecules, in accordance with the fact that they are not able to be loaded into a multilayer film. Because floccule formation is the result of strong interactions between DTA and PAA, we believe that the reason the other three molecules cannot be loaded into multilayer films is that their hydrogen-bonding interactions with PAA are not strong enough. Therefore, we can draw the conclusion that molecules that can form multiple and strong hydrogen bonds with PAA are suitable for our method.

Conclusion In conclusion, we have presented a method combining hydrogen-bonding complexation and LbL assembly to load a water-insoluble small organic molecule into multilayer films in a well-controlled manner. The loading amount can be well tuned by changing either the concentration of DTA or the solvent composition at the complexation step. The method has shown great advantages over synthetic methods such as grafting in terms of simplicity as well as tunability. By performing photo-crosslinking, a robust multilayer film can be prepared, which can be used as a good absorbent for DTA after the release of DTA. We anticipate that the method can be used to load more functional small organic molecules into multilayer films, extending the application of LbL assembly. Acknowledgment. We thank the National Natural Science Foundation of China (20334010, 20473045, 50573042, 20574040) and the National Basic Research Program (2007CB808002) for financial support. LA702054D