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Biomimetic Formation of Silica Thin Films by Surface-Initiated Polymerization of 2-(Dimethylamino)ethyl Methacrylate and Silicic Acid Dong Jin Kim,† Kyung-Bok Lee,† Young Shik Chi,† Wan-Joong Kim,† Hyun-jong Paik,‡ and Insung S. Choi*,† Department of Chemistry and School of Molecular Science (BK21), Center for Molecular Design and Synthesis, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea, and Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Korea Received June 1, 2004. In Final Form: July 27, 2004 Biosilicification in diatoms is achieved by specific interactions between silaffins, composed of polypeptides and long-chain polyamines, and silicic acid derivatives. The polycondensation of silicic acids is reported to be catalyzed by the long-chain polyamines that mainly contain tertiary N-methylpropyleneimine moieties. In this report, we utilized a tertiary amine-containing polymer, poly(2-(dimethylamino)ethyl methacrylate) (poly(DMAEMA)), as a surface-grafted, biomimetic counterpart of the long-chain polyamines in silaffins and demonstrated that the surface-initiated polycondensation of silicic acids, leading to the formation of silica thin films, proceeded smoothly on surfaces presenting poly(DMAEMA), where poly(DMAEMA) was grown from gold surfaces by surface-initiated, atom transfer radical polymerization. The formed silica film was characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy.
Biosilicification, found in diatoms1 and glass sponges,2 has been of interest in nanotechnology and materials sciences3 because biosilicification (silica biomineralization) occurs under ambient conditions at slightly acidic pH values and the formed silica structures are precisely controlled at the nanometer scale. For example, the biosilicification in diatoms is achieved by specific interactions between silicic acid derivatives and biopolymers, such as cationic polypeptides named silaffins containing long-chain polyamines: the self-assembled structure of the peptide part of the silaffins is thought to act as a template for the in vivo polycondensation of silicic acid derivatives catalyzed by the long-chain polyamines.4 With the aim of fabricating custom-tailored nanostructures based on the silica biomineralization, silica nanospheres have been produced in vitro with various polyamines, such as poly-L-lysine,5 poly(allylamine hydrochloride),6 amine-terminated dendrimers (PPI and * To whom correspondence may be addressed. E-mail: ischoi@ kaist.ac.kr. † Korea Advanced Institute of Science and Technology. ‡ Pusan National University. (1) Round, F. E.; Crawford, R. M.; Mann, D. G. The Diatoms: Biology & Morphology of the Genera; Cambridge University Press: Cambridge, U.K., 1990. (2) Sundar, V. C.; Yablon, A. D.; Grazul, J. L.; Ilan, M.; Aizenberg, J. Nature 2003, 424, 899. (3) (a) Sumper, M. Angew. Chem., Int. Ed. 2004, 43, 2251. (b) Brott, L. L.; Naik, R. R.; Pikas, D. J.; Kirkpatrick, S. M.; Tomlin, D. W.; Whitlock, P. W.; Clarson, S. J.; Stone, M. O. Nature 2001, 413, 291. (4) (a) Sumper, M.; Lerenz, S.; Brunner, E. Angew. Chem., Int. Ed. 2003, 42, 5192. (b) Knecht, M. R.; Wright, D. W. Chem. Commun. 2003, 3038. (c) Kro¨ger, N.; Lorenz, S.; Brunner, E. Sumper, M. Science 2002, 298, 584. (d) Sumper, M. Science 2002, 295, 2430. (e) Noll, F.; Sumper, M.; Hampp, N. Nano Lett. 2002, 2, 91. (f) Kro¨ger, N.; Deutzmann, R.; Sumper, M. J. Biol. Chem. 2001, 276, 26066. (g) Kro¨ger, N.; Deutzmann, R.; Bergsdorf, C.; Sumper, M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 14133. (h) Kro¨ger, N.; Deutzmann, R.; Sumper, M. Science 1999, 286, 1129. (5) Patwardhan, S. V.; Mukherjee, N.; Clarson, S. J. J. Inorg. Organomet. Polym. 2001, 11, 193. (6) (a) Brunner, E.; Lutz, K.; Sumper, M. Phys. Chem. Chem. Phys. 2004, 6, 854. (b) Patwardhan, S. V.; Clarson, S. J. Mater. Sci. Eng. 2003, 23, 495.
PAMAM),7 and others.8 From the reported studies, we reasoned that the surface-initiated silicification would occur from the amine-presenting polymeric surfaces, leading to the formation of silica thin films, and in this paper we report on the formation of silica thin films by the sequential surface-initiated polymerization of an amine-containing methacrylate (2-(dimethylamino)ethyl methacrylate, DMAEMA) and silicic acids. Silaffins are posttranslationally modified peptides where many of the lysines are modified to -N-dimethyllysine or oligo-N-methylpropyleneimine-linked lysine.4c,f The long-chain polyamines and other amines found in the silaffins are, therefore, mostly methylated tertiary amines, and therefore we chose DMAEMA as a building block because DMAEMA contains the tertiary dimethylamino group and is polymerized by atom transfer radical polymerization in the controlled way. The surface coated with poly(DMAEMA) was generated by a combination of the formation of self-assembled monolayers (SAMs) terminating in a polymerization initiator, 2-bromo-2methyl propionyl group,9 and the surface-initiated, atom transfer radical polymerization of DMAEMA (Figure 1).10 After the formation of the SAMs on a gold substrate, the substrate was placed in a 10-mL aqueous solution of DMAEMA (10 mmol), CuBr (0.1 mmol), and 2,2′-bipyridine (7) Knecht, M. R.; Wright, D. W. Langmuir 2004, 20, 4728. (8) Patwardhan, S. V.; Clarson, S. J. Silicon Chem. 2002, 1, 207. (9) The polymerization initiator, (BrC(CH3)2COO(CH2)11S)2, was synthesized by following the reported procedure (Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597). The SAMs presenting the polymerization initiator were formed by immersing a freshly prepared, gold-coated (with a titanium adhesion layer of 50 Å and thermally evaporated gold layer of 1000 Å) silicon wafer in a 1 mM ethanolic solution of (BrC(CH3)2COO(CH2)11S)2 for 24 h at room temperature. After the formation of the SAMs, the gold substrate was rinsed with ethanol several times and dried under a stream of argon. The formation of the SAMs was confirmed by polarized infrared external reflectance spectroscopy (PIERS): 1737 cm-1 (CdO), 1465 cm-1 (-CH2-), and 1169 cm-1 (C-O). (10) Lee, S. B.; Koepsel, R. R.; Morley, S. W.; Matyjaszewski, K.; Sun, Y.; Russell, A. J. Biomacromolecules 2004, 5, 877.
10.1021/la048657b CCC: $27.50 © 2004 American Chemical Society Published on Web 08/10/2004
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Figure 1. Schematic representation of the procedure for the formation of silica thin films by surface-initiated, atom-transfer radical polymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and subsequent polycondensation of silicic acids.
(0.2 mmol), and the mixture was stirred at room temperature for 4 h. Monosilicic acid was formed by stirring a 0.1 mM HCl solution of tetramethyl orthosilicate (TMOS) (100 mM) at room temperature for 10 min, and the resulting solution of monosilicic acid (2 mL) was added to a 100 mM aqueous phosphate buffer (pH 5.5) containing the poly(DMAEMA)presenting gold substrate (2 mL) (Caution: We used polypropylene containers for the reactions. Glassware was avoided because there was a possibility that monosilic acid condensed onto the surface of glass.) After 1 h of stirring, the substrate was taken, washed with deionized water and ethanol, and dried under a stream of argon. The silica thin film was characterized by FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM).11 Figure 2 shows the IR and XPS spectra of the surfaces presenting the polymeric thin film of poly(DMAEMA) and the silica thin film, respectively. The IR spectrum of the poly(DMAEMA) film (140 nm in thickness by ellipsometric measurement) showed characteristic peaks at 1732 cm-1 (CdO stretching), 1461 cm-1 (-CH2bending), and 1155 cm-1(C-N stretching). After the silicification, new peaks appeared at 1228, 963, and 802 cm-1 in the IR spectrum. The peaks at 1228, 963, and 802 cm-1 were assigned as Si-O-Si asymmetric stretching, Si-O- stretching, and Si-O-Si symmetric stretching, respectively.12 The XPS spectrum also confirmed the silica formation: the Si peaks were observed at 153 eV (Si 2s) (11) PIERS spectra were recorded on a Thermo Nicolet Nexus FT-IR spectrometer in a SAGA mode. An ellipsometer (Gaertner L116s) equipped with a He-Ne laser (632.8 nm) was used to determine the thickness of the films. XPS study was performed with a VG-Scientific ESCA-LAB 250 spectrometer with monochromatized Al KR X-ray source. Tapping-mode atomic force microscopy (AFM) images were obtained on a MultiMode SPM (Digital Instruments), and scanning electron microscope (SEM) images with an XL 30SFEG (Philips). (12) (a) de Man A. J. M.; van Santen, R. A. Zeolites 1992, 12, 269. (b) Flanigen, E. M.; Khatami, H.; Szymanski, H. A. Adv. Chem. Ser. 1971, 101, 201.
Figure 2. (a) IR spectra of the poly(DMAEMA) film and the silica film. (b) XPS spectra of the poly(DMAEMA) film and the silica film. The IR and XPS spectra of the poly(DMAEMA) film are presented in gray and those of the silica film in black.
and 103 eV (Si 2p), in addition to the peaks of O 1s (at 531 eV), N 1s (at 402 eV), and C 1s (at 287 eV). The peak corresponding to P 2p was also observed at 134 eV with a weak intensity in the high-resolution XPS spectrum. The silica formation was reported to be triggered by phosphate or other polyvalent anions.4a To investigate the role of phosphate anions in the silica formation in our system, the control experiment was performed: we did not observe any silica formation without phosphate anions, which implies that phosphates (or other anions) are also involved in the biosilicification in our system. We investigated the surface morphology of the films by AFM. The poly(DMAEMA) film was remarkably uniform, and the root-mean-square (rms) of the 140-nm-thick film was 0.6 nm (Figure 3a). After the silicification, the film became rougher (rms ) 6.4 nm) but was still relatively uniform, which implies that the silicification occurred homogeneously on the surface (Figure 3b). Because it was not feasible to measure the thickness of the silica film by ellipsometry, the thickness was estimated by the SEM micrographs of the cross section of the substrates. The thickness increased to 725 nm from the 140-nm-thick poly(DMAEMA) film (Figures 3c,d). In the cross-sectional SEM micrograph of the silica film, we did not observe two distinguishable layers of poly(DMAEMA) and silica: the SEM data are in agreement with the previous reports
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Figure 3. (a and b) AFM micrographs of the poly(DMAEMA) film and the silica film. (c and d) Cross-sectional SEM micrographs of the poly(DMAEMA) film and the silica film. The scale bar is 200 nm.
that the formed silica is a composite of polyamines and SiO2 (and phosphates).7 Unlike a reported failure of the formation of silica from surfaces where poly(allylamine hydrochloride) and poly(acrylic acid) were deposited onto the surfaces by layer-by-layer self-assembly,8 the data clearly confirmed the successful formation of silica composite layers from the surface-grown, tertiary aminecontaining polymers. In summary, we demonstrated the biomimetic silica formation on the tertiary amine-presenting solid surface.
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The polyamines utilized so far for the biomimetic formation of silica nanoparticles contain primary or secondary amines, but the polymer used in this study contains tertiary amines, the functionality of which is found in the silaffins. The role of polyamines in the silicification is still not clear but is reported to depend on the amine functionality: in the case of primary or secondary aminecontaining polymers, the amine groups act as a general acid-base catalyst where the deprotonated amine groups (“base”) abstract a proton from silicic acid, leading to the formation of the reactive silanolate, and the protonated amine groups (“acid”) facilitate the release of water by the protonation of silicic acid.13 In the case of tertiary amine-containing polymers, the primary interaction is between the polymer and poly(silicic acid) (not monosilicic acid) through ionic interactions and hydrogen bonding, and the interaction makes the local density of poly(silicic acid)s increased and poly(silicic acid)s polycondensate into silica.4a We presume that in our system the high surfacelocalized density of tertiary amines facilitated the interaction between the amines and poly(silicic acid)s and the silica was therefore formed only on the surface. Acknowledgment. This work was supported by the National R&D Project for Nano Science and Technology. I.S.C. thanks the Basic Research Program of Korea Science and Engineering Foundation (R08-2003-000-10533-0) for providing the fund for the purchase of the AFM used in this study. We thank Dr. Won of the Korea Basic Science Institute for XPS analysis. LA048657B (13) Mizutani, T.; Nagase, H.; Fujiwara, N.; Ogoshi, H. Bull. Chem. Soc. Jpn. 1998, 71, 2017.
