pubs.acs.org/Langmuir © 2009 American Chemical Society
Covalently Bonded Layer-by-Layer Assembly of Multifunctional Thin Films Based on Activated Esters Jinhwa Seo,† Philipp Schattling,‡ Thomas Lang,‡ Florian Jochum,‡ Katja Nilles,‡ Patrick Theato,*,‡ and Kookheon Char*,† †
Center for Functional Polymer Thin Films and School of Chemical and Biological Engineering, Seoul National University, Seoul, 151-744, Korea, and ‡Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55099 Mainz, Germany Received July 14, 2009. Revised Manuscript Received September 3, 2009
We demonstrate that chemically stable, multifunctional polymer thin films can be obtained using the layer-by-layer (LbL) deposition based on covalent bonds between adsorbing chains. Poly(pentafluorophenyl-4-vinylbenzoate) (P1) or poly(pentafluorophenylacrylate) (P2) polymers were assembled with poly(allyl amine) (PAAm) to yield LbL multilayer films through amide bond formation by the reaction between activated esters of P1 or P2 and amine groups in PAAm, which was quantitatively monitored by Fourier transform infrared spectroscopy (FT-IR). It was found that the difference in the solubility of P1 and P2 against ethanol, which was used as the solvent for PAAm, during the LbL deposition yields different reaction conversion for the activated esters in either P1 or P2: the reaction conversion of P2 is higher than the conversion with P1. In addition, free (or unreacted) activated esters and amine groups remaining in the PAAm/P1 LbL film were further utilized for the incorporation of multiple functional materials (5-((2-aminoethyl)amino)naphthalene-1-sulfonic acid (EDANS) and Rhodamine B dyes in the present case) by post-treatments in order to further tailor the film properties. It was also demonstrated that the surface functional groups (activated esters) in the LbL films can also be utilized for surface patterning with one functional material, followed by functionalization with a second functional material during the post-treatment throughout the whole film. Finally, the PAAm/P1 and PAAm/P2 LbL films were shown to be quite stable in the extreme pH range, and free-standing films can easily be obtained by the treatment of the films with mild acidic conditions. The versatility of incorporating multiple functional materials into a single multilayer film as well as the excellent physicochemical stability of the covalently bonded multilayer free-standing films proves to be quite useful to design flexible and multifunctional thin film structures for many chemical and biological applications.
Introduction 1,2
Click chemistry as a concept has recently received much attention for the reactions that proceed rapidly with high yields under mild conditions and also are highly selective and reliable. Specifically, 1,3-dipolar as well as Diels-Alder cycloadditions have been well-known and are often used for preparing nanomaterials and multifunctional polymers or engineering surface properties.2 Although those reactions are very popular, the reaction of activated esters with amines, first reported3,4 in 1972, is also one of interesting and useful synthetic methods in the class of click chemistry.5 For example, poly(N-hydroxyl succinimide acrylate) and poly(N-hydroxyl succinimide methacrylate) can react with amine groups to form functionalized polyacrylamide or polymethacrylamide derivatives under mild reaction condition.6-8 One clear advantage of using activated ester polymers over the reaction based on 1,3-dipolar cycloadditions lies in the fact that no additional metal catalyst is required. *Corresponding author. (P.T.) Tel: þ49-6131-3926256; fax: þ49-61313924778; e-mail:
[email protected]. (K.C.) Tel: þ82-2-880-7431; fax: þ82-2-873-1548; e-mail:
[email protected]. (1) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004–2021. (2) Nandivada, H.; Jiang, X.; Lahann, J. Adv. Mater. 2007, 19, 2197–2208. (3) Batz, H.-G.; Franzmann, G.; Ringsdorf, H. Angew. Chem., Int. Ed. 1972, 11, 1103–1104. (4) Ferruti, P.; Bettelli, A.; Fer, A. Polymer 1972, 13, 462–464. (5) Theato, P. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 6677–6687. (6) Cline, G. W.; Hanna, S. B. J. Am. Chem. Soc. 1987, 109, 3087–3091. (7) Bergbreiter, D. E.; Hughes, R.; Besinaiz, J.; Li, C.; Osburn, P. L. J. Am. Chem. Soc. 2003, 125, 8244–8249. (8) Wong, S. Y.; Putnam, D. Bioconjugate Chem. 2007, 18, 970–982.
1830 DOI: 10.1021/la902574z
One simple and elegant technique to assemble polymers based on activated esters is layer-by-layer (LbL) deposition. LbL assembly9,10 is a well-known fabrication method to prepare versatile thin films with highly tunable properties for diverse applications such as chemical sensors,11 drug delivery vehicles,12-14 and solid electrolyte membranes.15,16 Important fundamental and technological issues for LbL deposition have been well documented in several review articles.17-19 Most commonly, noncovalent intermolecular interactions, such as Coloumbic interactions1,2 and hydrogen-bonding interactions,20-23 have been employed for LbL deposition. Only very recently, although the concept has (9) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210-211, 831– 835. (10) Decher, G. Science 1997, 277, 1232–1237. (11) Lvov, Y. M.; Lu, Z.; Schenkman, J. B.; Zu, X.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073–4080. (12) Wood, K. C.; Chuang, H. F.; Batten, R. D.; Lynn, D. M.; Hammond, P. T. Proc. Natl. Acad. Sci. 2006, 103, 10207–10212. (13) Kim, B.-S.; Park, S. W.; Hammond, P. T. ACS Nano 2008, 2, 386–392. (14) Berg, M. C.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Biomacromolecules 2006, 7, 357–364. (15) Farhat, T. R.; Hammond, P. T. Adv. Funct. Mater. 2005, 15, 945–954. (16) Jiang, S. P.; Liu, Z.; Tian, Z. Q. Adv. Mater. 2006, 18, 1068–1072. (17) Hammond, P. T. Adv. Mater. 2004, 16, 1271–1293. (18) Tang, Z.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. Adv. Mater. 2006, 18, 3203–3224. (19) Wang, Y.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2008, 20, 848–858. (20) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717–2725. (21) Wang, L.; Wang, Z.; Zhang, X.; Shen, J.; Chi, L.; Fuchs, H. Macromol. Rapid Commun. 1997, 18, 509–514. (22) Kharlampieva, E.; Sukhishvili, S. A. Polym. Rev. 2006, 46, 377–395. (23) Seo, J.; Lutkenhaus, J. L.; Kim, J.; Hammond, P. T.; Char, K. Langmuir 2008, 24, 7995–8000.
Published on Web 09/18/2009
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already been theoretically mentioned in the patent of Decher et al., have LbL films based on the covalent bonding between adsorbing chains been realized.24-27 It is obvious that covalently bonded LbL films have high mechanical and chemical stability even under harsh environment, whereas noncovalently bonded LbL films typically require post-treatment processes such as cross-linking with the addition of chemical moieties (e.g., 1-ethyl-3,3-dimethylaminopropylcarbodiimide hydrochloride (EDC))28 and thermal treatment29-31 to secure the physicochemical stability of the films. Although several studies have already been reported on covalently bonded LbL systems, only a few studies32,33 have briefly touched upon the qualitative verification of the reaction of activated ester polymers based on pentafluorophenyl (PFP) ester groups in LbL films. On the other hand, we have recently reported successful preparation of covalently bonded LbL films based on gold nanoparticles34 and carbon nanotubes35 modified with PFP ester groups reacting with poly(allyl amine) (PAAm). In the present study, we demonstrate that multifunctional polymer thin films with superb physicochemical stability can be obtained by LbL deposition based on the covalent bond formation between the adsorbing pair. The incorporation of multiple functions within a single covalently bonded multilayer freestanding film, exploiting unreacted moieties within the film, was also investigated. We introduce two different activated ester polymers based on PFP ester groups (i.e, poly(pentafluorophenyl-4-vinylbenzoate) (P1) and poly(pentafluorophenylacrylate) (P2)), reacting with primary amines in PAAm during the LbL deposition, to produce covalently bonded multilayer films. The reaction conversion of activated esters in both P1 and P2 was investigated in detail based on Fourier transform infrared (FT-IR) analysis, and the difference in the reactivity between P1 and P2 was analyzed in terms of solubility difference against the solvent for PAAm. Furthermore, simultaneous incorporation of different functional molecules into a single LbL film by posttreatments as well as the surface patterning with functional groups, taking full advantage of remaining reaction sites, buried inside or residing at the surface of a covalently bonded LbL film, was also investigated. Finally, the film stability in different environmental conditions as well as the facile preparation of free-standing films off substrates is addressed to promise great potential as flexible and multifunctional film platforms for many chemical and biological applications.
Experimental Section 36
Materials. P1 (Mn=19 600 g/mol, PDI=1.2), P237 (Mn=14 600 g/mol, PDI=1.4), and poly(allylamine hydrochloride) labeled (24) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 4655–4661. (25) Such, G. K.; Quinn, J. F.; Quinn, A.; Tjipto, E.; Caruso, F. J. Am. Chem. Soc. 2006, 128, 9318–9319. (26) Bergbreiter, D. E.; Liao, K.-S. Soft Matter 2009, 5, 23–28. (27) Buck, M. E.; Zhang, J.; Lynn, D. M. Adv. Mater. 2007, 19, 3951–3955. (28) Engler, A. J.; Richert, L.; Wong, J. Y.; Picart, C.; Discher, D. E. Surf. Sci. 2004, 570, 142–154. (29) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978–1979. (30) Dai, J. H.; Sullivan, D. M.; Bruening, M. L. Ind. Eng. Chem. Res. 2000, 39, 3528–3535. (31) Lutkenhaus, J. L.; Hrabak, K. D.; McEnnis, K.; Hammond, P. T. J. Am. Chem. Soc. 2005, 127, 17228–17234. (32) Liang, Z. Q.; Wang, Q. Langmuir 2004, 20, 9600–9606. (33) Liang, Z. Q.; Dzienis, K. L.; Xu, J.; Wang, Q. Adv. Funct. Mater. 2006, 16, 542–548. (34) Roth, P. J.; Theato, P. Chem. Mater. 2008, 20, 1614–1621. (35) Park, H. J.; Kim, J.; Chang, J. Y.; Theato, P. Langmuir 2008, 24, 10467– 10473. (36) Nilles, K.; Theato, P. Eur. Polym. J. 2007, 43, 2901–2912. (37) Eberhardt, M.; Mruk, R.; Zentel, R.; Theato, P. Eur. Polym. J. 2005, 41, 1569–1575.
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with Rhodamine B isothiocyanate (PAH-rho)38 were all synthesized in the laboratory. Details on the synthesis and characterization are described in the references given above.36-38 PAAm (Mw = 17 000 g/mol), methylamine (40 wt % in H2O), and 5-((2-aminoethyl)amino)naphthalene-1-sulfonic acid (EDANS) were purchased from Aldrich, and Rhodamine B was purchased from Fluka. All chemicals were used as received. Silicon wafer and quartz were used as substrates for the LbL deposition. Prior to the LbL deposition, all substrates were cleaned under piranha treatment (70% H2SO4 and 30% H2O2) (Caution! Piranha solution is highly corrosive) for 10 min and washed thoroughly with Milli-Q water, and dried under nitrogen stream. LbL Assembly of Multilayer Films. All polymer solutions used for the LbL deposition were prepared by dissolving the activated ester polymers P1 and P2 in tetrahydrofuran (THF) and PAAm in ethanol, respectively. The concentration of all the polymer solutions prepared was fixed at 0.01 M based on the repeat units. The LbL-assembled multilayer films were constructed using a dipping robot (Carl Zeiss HMS70 slide stainer). Substrates were first exposed to the PAAm solution for 10 min and then rinsed in two baths of ethanol for 2 and 1 min, respectively. The PAAm-coated substrates were then dipped into P1 or P2 solution following the same deposition procedure in THF. This cycle comprised one pair layer (i.e., one bilayer) and was repeated for a desired number ‘n’ of cycles (or bilayers) to obtain (PAAm/P1)n or (PAAm/P2)n multilayer films. The topmost surface of assembled multilayer films is the activated ester polymer P1 or P2 layer. Post-treatments of LbL Films. Methyl amine 40% aqueous solution, EDANS, and Rhodamine B dissolved in dimethyl sulfoxide (DMSO) with 0.01 M concentration were prepared and used for the post-treatments of LbL films. (PAAm/P1)60 or (PAAm/P2)60 LbL films were dipped into the solutions containing different dyes for 16 h, followed by rinsing with pure solvent for 4 h. The post treatment was performed with a single step or two sequential steps (e.g., LbL films were treated with EDANS only or the EDANS treatment followed by the treatment with Rhodamine B). For surface patterning on the LbL films, a polydimethylsiloxane (PDMS) stamp was spun-cast with 0.01 M PAH-rho aqueous solution at 2000 rpm for 10 s, followed by rinsing with water. The PAH-rho-coated PDMS mold was then stamped at the top surface of the (PAAm/P1)60 or (PAAm/P2)60 LbL film for 5 min, and the stamped multilayer surface was then rinsed with water and dried under nitrogen stream. Free-standing films were obtained by dipping the LbL films into 0.1 M HCl solution for several hours (typically, between 3 and 12 h). Film Characterization. Functional groups such as reactive esters and amine groups residing in the LbL films were monitored by a Perkin-Elmer Lambda 35 UV-vis spectrometer and an FTIR-200 spectrometer (JASCO Corporation). The film thickness and surface morphology were obtained by ellipsometry (Gaertner Scientific Corp.) and atomic force microscopy (AFM; Nanoscope IIIa, Digital Instruments), respectively. The optical and fluorescence images were captured using a Zeiss Axio Imager M1m (Carl Zeiss, Inc.).
Results and Discussion The covalently bonded polymer thin films were prepared by the reaction between activated ester polymers (i.e., P1 or P2) and PAAm during LbL deposition. The chemical structures for P1 and P2 as well as the reaction schemes for each of the reactive ester polymers (P1 and P2) with PAAm are shown in Scheme 1. The activated ester groups in P1 and P2 are known to be highly reactive toward the primary amine groups present in PAAm; the amide bonds between the pairing polymer chains are thus readily formed in ambient condition, and those bond formations (38) Ibarz, G. D., L.; Donath, E.; Mohwald, H. Adv. Mater. 2001, 13, 1324– 1327.
DOI: 10.1021/la902574z
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Scheme 1. Chemical Structure and Reaction Schematics of (a) P1 and (b) P2 reacting with PAAm
effectively lead to the irreversible adsorption of one polymer layer as well as cross-linking within the forming polymer film. We expect that film properties for PAAm/P1 and PAAm/P2 LbL films would be different because the reactivity of activated esters from either P1 or P2 is slightly different with respect to the amine groups in PAAm.36 The film growth in multilayers, surface morphology, and conversion of reactive groups for PAAm/P1 and PAAm/P2 LbL films were investigated and discussed in detail in the following. Characterization of Covalently Bonded LbL Films. The most evident chemical difference between P1 and P2 polymers lies in the existence of phenyl structure between the polymer backbone and the activated ester group, as shown in P1 of Scheme 1. Thus, this difference in chemical structure would serve as a good indicator to distinguish between PAAm/P1 and PAAm/P2 LbL films when monitoring with UV-vis spectroscopy. The typical UV absorption of phenyl groups is shown at 246 nm in the case of PAAm/P1 LbL films (Figure 1a), whereas no such absorption peak was observed in the case of the PAAm/P2 LbL films except the broad absorption at
