Continuous Silica Coatings on Glass Fibers via Bioinspired

Whether the heterogeneity of the film indicates incomplete coverage of the fiber, or simply variations in the thickness of the coating, cannot be dedu...
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Langmuir 2007, 23, 6677-6683

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Continuous Silica Coatings on Glass Fibers via Bioinspired Approaches Suvarchala Devi Pogula,†,‡ Siddharth V. Patwardhan,†,‡,§,| Carole C. Perry,§ John W. Gillespie, Jr.,†,|,⊥ Shridhar Yarlagadda,*,| and Kristi L. Kiick*,† Departments of Materials Science and Engineering, 201 DuPont Hall, and of CiVil and EnVironmental Engineering, 301 DuPont Hall, and Center for Composite Materials, 201 Composites Manufacturing Science Laboratory, UniVersity of Delaware, Newark, Delaware 19716, and Biomolecular and Materials Interface Research Group, School of Biomedical and Natural Sciences, Nottingham Trent UniVersity, Clifton Lane, Nottingham NG11 8NS, United Kingdom ReceiVed December 20, 2006. In Final Form: March 23, 2007 Simple methods for producing continuous inorganic coatings on fibers have application in multiple technologies. The application of bioinspired strategies for the formation of particulate inorganic materials has been widely investigated and provides routes to inorganic materials under environmentally benign conditions. In this work, we describe the formation of stable and continuous inorganic coatings on glass fibers via the polymerization of silica in the presence of biopolymers. The formation of both organic and inorganic coatings was investigated via X-ray photoelectron spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray analysis. The simple route to silica coatings presented herein could be interesting for the development of functional hybrid fibrous materials suitable for catalytic and sensor applications, given the homogeneous nature of the silica films and the benign conditions employed for film formation.

Introduction The deposition of uniform silica coatings on materials such as nanoparticles and fibers has been studied for many years to render desired optical, electronic, chemical, or mechanical properties to materials suitable for a range of applications such as nanofiltration, sensing, and scaffolding.1-6 However, most of the current methods employ harsh conditions of pH and/or temperature and thus may not be compatible with biomolecules such as proteins and peptides. However, living organisms, which excel in directing the synthesis of inorganic materials for various functional purposes, typically operate under mild conditions. Biomineralization processes are providing inspiration for the design of new materials and for efficiently organizing materials and biopolymers into predetermined nanostructures. Diatoms and sponges, for example, form beautiful silica structures in ViVo * To whom correspondence should be addressed. (K.L.K.) Phone: (302) 831-0201. Fax: (302) 831-4545. E-mail: [email protected]. (S.Y.) Phone: (302) 831-4941. Fax: (302) 831-8525. E-mail: [email protected]. † Department of Materials Science and Engineering, University of Delaware. ‡ These authors contributed equally to this work. § Nottingham Trent University. | Center for Composite Materials, University of Delaware. ⊥ Department of Civil and Environmental Engineering, University of Delaware. (1) Sakka, S.; Yoko, T. Sol-Gel-Derived Coating Films and Applications. Struct. Bonding 1992, 77, 89-118. (2) Caruso, R. A.; Antonietti, M. Sol-gel nanocoating: An approach to the preparation of structured materials. Chem. Mater. 2001, 13 (10), 3272-3282. (3) Schmidt, H. Considerations about the sol-gel process: From the classical sol-gel route to advanced chemical nanotechnologies. J. Sol-Gel Sci. Technol. 2006, 40 (2-3), 115-130. (4) McEvoy, A. K.; McDonagh, C.; Maccraith, B. D. Optimisation of solgel-derived silica films for optical oxygen sensing. J. Sol-Gel Sci. Technol. 1997, 8 (1-3), 1121-1125. (5) Grant, S. A.; Glass, R. S. Sol-gel-based biosensor for use in stroke treatment. IEEE Trans. Biomed. Eng. 1999, 46 (10), 1207-1211. (6) Tao, S. Q.; Xu, L.; Fanguy, J. C. Optical fiber ammonia sensing probes using reagent immobilized porous silica coating as transducers. Sens. Actuators, B 2006, 115 (1), 158-163.

that are organized over a wide length scale.7-9 The intricate structures are formed by biosilicification under ambient conditions and mediated by proteins. Silaffin proteins isolated from the diatom Cylindrotheca fusiformis, when added to silica precursor solutions in Vitro, rapidly precipitate silica particles under ambient conditions of temperature and at near-neutral pH.10 In chemical approaches, however, the formation of silica via current processes often requires relatively harsh temperatures and pH, which are incompatible with the treatment of more fragile materials or for the inclusion of biomacromolecules into inorganic films. Thus, the demand for benign synthesis methods that minimize adverse environmental effects has increased recently. Accordingly, the study of the biomolecular functionalities involved in biosilica formation has led to the identification of various bioinspired moleculesssynthetic proteins,11-13 polypeptides,14-21 macro(7) Round, F. E.; Crawford, R. M.; Mann, D. G. The Diatoms: Biology & Morphology of the Genera; Cambridge University Press: Cambridge, U.K., 1990. (8) Simpson, T. L.; Volcani, B. E. Silicon and Siliceous Structures in Biological Systems; Springer-Verlag: New York, 1981. (9) Mu¨ller, W. E. G. Silicon Biomineralization; Springer: Berlin, 2003. (10) Sumper, M.; Kroger, N. Silica formation in diatoms: the function of long-chain polyamines and silaffins. J. Mater. Chem. 2004, 14 (14), 2059-2065. (11) Patwardhan, S. V.; Shiba, K.; Raab, C.; Husing, N.; Clarson, S. J. Protein Mediated Bioinspired Mineralization. In Polymer Biocatalysis and Biomaterials, Cheng, H. N., Gross, R. A., Eds.; Oxford University Press: Oxford, U.K., 2005. (12) Wong Po Foo, C.; Patwardhan, S. V.; Belton, D. J.; Kitchel, B.; Anastasiades, D.; Huang, J.; Naik, R. R.; Perry, C. C.; Kaplan, D. L. Novel nanocomposites from spider silk-silica fusion (chimeric) proteins. Proc. Natl. Acad. Sci. U.S.A. 2006, 103 (25), 9428-9433. (13) Patwardhan, S. V.; Shiba, K.; Schroder, H. C.; Muller, W. E. G.; Clarson, S. J.; Perry, C. C. The Role of Bioinspired Peptide and Recombinant Proteins in Silica Polymerisation. The Science and Technology of Silicones; in press. (14) Bellomo, E. G.; Deming, T. J. Monoliths of aligned silica-polypeptide hexagonal platelets. J. Am. Chem. Soc. 2006, 128 (7), 2276-2279. (15) Cha, J. N.; Stucky, G. D.; Morse, D. E.; Deming, T. J. Biomimetic synthesis of ordered silica structures mediated by block copolypeptides. Nature 2000, 403 (6767), 289-292. (16) Coradin, T.; Livage, J. Effect of some amino acids and peptides on silicic acid polymerization. Colloids Surf. B 2001, 21, (4), 329-336. (17) Knecht, M. R.; Wright, D. W. Functional analysis of the biomimetic silica precipitating activity of the R5 peptide from Cylindrotheca fusiformis. Chem. Commun. 2003, No. 24, 3038-3039.

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molecules,22-25 and small functional molecules16,26-30sthat are not only able to form silica under mild conditions, but also able to affect a range of properties of silica formation including reaction kinetics, particle aggregation, and morphologies. Electrostatic interactions between bioinspired molecules and silica species have been proposed to be an important factor in facilitating the synthesis of silica under ambient conditions.31,32 These investigations have opened doors to the controlled synthesis of a wide range of inorganic functional materials and composites that also include materials other than silica, such as germania and titania.11,33-38 The information gained from bioinspired silica investigations has been recently used to develop silica films, patterned surfaces, and surface coatings via soft matter routes (see below). A variety of techniques have been used to render appropriate functionalities to surfaces suitable for silica formation. For example, Brott et al. prepared holograms with patterned silica particles using twophoton-induced photopolymerization. R5 peptide (derived from silaffin proteins of diatom C. fusiformis), when mixed in a (18) Naik, R. R.; Brott, L. L.; Clarson, S. J.; Stone, M. O. Silica-precipitating peptides isolated from a combinatirial phage display library. J. Nanosci. Nanotechnol. 2002, 2, 95. (19) Patwardhan, S. V.; Maheshwari, R.; Mukherjee, N.; Kiick, K. L.; Clarson, S. J. Conformation and Assembly of Polypeptide Scaffolds in Templating the Synthesis of Silica: An Example of a Polylysine Macromolecular “Switch”. Biomacromolecules 2006, 7 (2), 491-7. (20) Tomczak, M. M.; Lawrence, C.; Drummy, L. F.; Sowards, L. A.; Glawe, D. C.; Stone, M. O.; Perry, C. C.; Pochan, D. J.; Deming, T. J.; Naik, R. R. Polypeptide-Templated Synthesis of Hexagonal Silica Platelets. J. Am. Chem. Soc. 2005, 127 (36), 12577. (21) Patwardhan, S. V.; Mukherjee, N.; Steinitz-Kannan, M.; Clarson, S. J. Bioinspired Synthesis of New Silica Structures. Chem. Commun. 2003, No. 10, 1122-1123. (22) Patwardhan, S. V.; Clarson, S. J. Silicification and Biosilicification Part 3. Silicon Chem. 2002, 1 (3), 207-214. (23) Menzel, H.; Horstmann, S.; Behrens, P.; Barnreuther, B.; Krueger, I.; Jahns, M. Chemical properties of polyamines with relevance to the biomineralization of silica. Chem. Commun. 2003, (24), 2994-2995. (24) Mizutani, T.; Nagase, H.; Fujiwara, N.; Ogoshi, H. Silicic acid polymerization catalyzed by amines and polyamines. Bull. Chem. Soc. Jpn. 1998, 71, 2017-2022. (25) Jin, R.-H.; Yuan, J.-J. Synthesis of poly(ethyleneimine)s-silica hybrid particles with complex shapes and hierarchical structures. Chem. Commun. 2005, 1399-1401. (26) Belton, D.; Paine, G.; Patwardhan, S. V.; Perry, C. C. Towards an understanding of (bio)silicification: the role of amino acids and lysine oligomers in silicification. J. Mater. Chem. 2004, 14 (14), 2231-2241. (27) Belton, D.; Patwardhan, S. V.; Perry, C. C. Putrescine homologues control silica morphogenesis by electrostatic interactions and the hydrophobic effect. Chem. Commun. 2005, No. 27, 3475-7. (28) Belton, D.; Patwardhan, S. V.; Perry, C. C. Spermine, spermidine and their analogues generate tailored silicas. J. Mater. Chem. 2005, 15 (43), 46294638. (29) Knecht, M. R.; Wright, D. W. Amine-Terminated Dendrimers as Biomimetic Templates for Silica Nanosphere Formation. Langmuir 2004, 20 (11), 4728-4732. (30) Roth, K. M.; Zhou, Y.; Yang, W.; Morse, D. E. Bifunctional Small Molecules Are Biomimetic Catalysts for Silica Synthesis at Neutral pH. J. Am. Chem. Soc. 2005, 127, 325-330. (31) Patwardhan, S. V.; Clarson, S. J.; Perry, C. C. On the role(s) of additives in bioinspired silicification. Chem. Commun. 2005, No. 9, 1113-1121. (32) Lopez, P. J.; Gautier, C.; Livage, J.; Coradin, T. Mimicking biogenic silica nanostructures formation. Curr. Nanosci. 2005, 1 (1), 73-83. (33) Sumerel, J. L.; Yang, W. J.; Kisailus, D.; Weaver, J. C.; Choi, J. H.; Morse, D. E. Biocatalytically templated synthesis of titanium dioxide. Chem. Mater. 2003, 15, 4804-4809. (34) Patwardhan, S. V.; Clarson, S. J. Bioinspired mineralisation: macromolecule mediated synthesis of amorphous germania structures. Polymer 2005, 46 (12), 4474-4479. (35) Kisailus, D.; Choi, J. H.; Weaver, J. C.; Yang, W. J.; Morse, D. E. Enzymatic synthesis and nanostructural control of gallium oxide at low temperature. AdV. Mater. 2005, 17 (3), 314. (36) Regan, M. R.; Banerjee, I. A. Preparation of Au-Pd bmetallic nanoparticles in porous germania nanospheres: A study of their morphology and catalytic activity. Scr. Mater. 2006, 54 (5), 909-914. (37) Banerjee, I. A.; Regan, M. R. Preparation of gold nanoparticle templated germania nanoshells. Mater. Lett. 2006, 60 (7), 915-918. (38) Kro¨ger, N.; Dickerson, M. B.; Ahmad, G.; Cai, Y.; Haluska, M. S.; Sandhage, K. H.; Poulsen, N.; Sheppard, V. C. Bioenabled Synthesis of Rutile (TiO2) at Ambient Temperature and Neutral pH. Angew. Chem., Int. Ed. 2006, 43 (43), 7239-7243.

Pogula et al. Scheme 1. Schematic Representation of Silica Deposition. (a) Bare fibers, (b) fibers functionalized with amines via treatment with polyamine solution, (c) accumulation of silica particles on surface of polyamine-treated fibers, and (d) formation of continuous silica filma

a

Note that the scheme is not to scale.

photopolymerization formulation, phase separated into peptiderich domains upon polymerization, and upon incubation with a silica precursor solution resulted in the formation of patterned silica particles which exhibited a high diffraction efficiency when compared to a polymeric hologram.39 In another example, Coffmann et al. used polylysine-functionalized substrates to produce patterned silica surfaces.40 Polylysine patterning was achieved either by reagent jetting or by conventional photolithography, and it was found that silica formation was observed only in the areas where polylysine was present. Glawe et al. have used electric fields to deposit polylysine on indium tin oxide (ITO) surfaces, which when treated with a silane solution, precipitated silica on the ITO.41 In another study, Tahir et al. demonstrated that the silicatein protein (responsible for biosilica formation in sponges), when immobilized on surfaces, was able to direct the formation of silica, titania, and zirconia.42-44 In addition, surface-grafted polymers containing tertiary amine moieties synthesized in situ by surface-initiated polymerization have been shown to initiate formation of silica films and patterned silica surfaces under mild conditions.45,46 However, the deposition of uniform silica coatings on fibrous materials has not been demonstrated under ambient conditions, despite its potential importance in applications such as nanofiltration, sensing, and scaffolding. Here we report the controlled formation of silica coatings on E-glass fibers using biomolecular and bioinspired templates under mild conditions at room temperature. The mild reaction conditions allow compatibility with biological polymers. Poly-L-lysine (PLL), poly(L-lysine-tyrosine (1:1)) (PLT), and poly(allylamine hydrochloride) (PAH) were deposited on glass fibers via soaking the fibers in respective solutions of macromolecules, and these coatings were used to initiate silica formation (Scheme 1). The (39) Brott, L. L.; Pikas, D. J.; Naik, R. R.; Kirkpatrick, S. M.; Tomlin, D. W.; Whitlock, P. W.; Clarson, S. J.; Stone, M. O. Ultrafast holographic nanopatterning of biocatalytically formed silica. Nature 2001, 413, 291. (40) Coffman, E. A.; Melechko, A. V.; Allison, D. P.; Simpson, M. L.; Doktycz, M. J. Surface Patterning of Silica Nanostructures Using Bio-Inspired Templates and Directed Synthesis. Langmuir 2004, 20 (20), 8431-8436. (41) Glawe, D. D.; Rodriguez, F.; Stone, M. O.; Naik, R. R. Polypeptidemediated silica growth on indium tin oxide surfaces. Langmuir 2005, 21, (2), 717-720. (42) Tahir, M. N.; Eberhardt, M.; Therese, H. A.; Kolb, U.; Theato, P.; Muller, W. E. G.; Schroder, H. C.; Tremel, W. From single molecules to nanoscopically structured functional materials: Au nanocrystal growth on TiO2 nanowires controlled by surface-bound silicatein. Angew. Chem., Int. Ed. 2006, 45 (29), 4803-4809. (43) Tahir, M. N.; Theato, P.; Muller, W. E. G.; Schroder, H. C.; Borejko, A.; Faiss, S.; Janshoff, A.; Huth, J.; Tremel, W. Formation of layered titania and zirconia catalysed by surface-bound silicatein. Chem. Commun. 2005, No. 44, 5533-5535. (44) Tahir, M. N.; Theato, P.; Muller, W. E. G.; Schroder, H. C.; Janshoff, A.; Zhang, J.; Huth, J.; Tremel, W. Monitoring the formation of biosilica catalysed by histidine-tagged silicatein. Chem. Commun. 2004, No. 24, 2848-2849. (45) Kim, D. J.; Lee, K. B.; Chi, Y. S.; Kim, W. J.; Paik, H. J.; Choi, I. S. Biomimetic formation of silica thin films by surface-initiated polymerization of 2-(dimethylamino) ethyl methacrylate and silicic acid. Langmuir 2004, 20 (19), 7904-7906.

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formation of both organic and inorganic coatings was investigated via X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray analysis (EDXA). The simple route to silica coatings presented herein could be interesting for the development of functional hybrid fibrous materials suitable for catalytic and sensor applications, given the homogeneous nature of the silica films and the benign conditions employed for film formation. Experimental Section Reagents. Tetramethyl orthosilicate (TMOS), PAH (MW 1500030000), PLL (MW(MALLS) 94600) and PLT (MW(LALLS) 72000) were purchased from Sigma-Aldrich (Minneapolis, MN). Acetone was purchased from Fisher Scientific Co. (Pittsburgh, PA). E-glass fibers with diameters of approximately 14 µm were obtained from Owens Corning Science and Technology Center (Granville, OH). Silica Synthesis. E-glass fibers were sonicated during washing with various solvents such as DCM, hexane, and acetone and heattreated at 500 °C prior to use to remove any residual organic matter. The fibers were dipped in 5-50 mg/mL PAH, PLL, or PLT in 0.1 M phosphate buffer (pH 7 or 7.5) for 1-24 h and washed with copious amounts of water. In a typical silica coating experiment, 1 M TMOS solution was hydrolyzed in 1 mM HCl for 15 min and was added to the buffer. The volume ratio of buffer and TMOS solution was 9:1. This mixture was added to polymer-coated E-glass fibers incubated in a covered Petri dish. After a reaction time of up to 1 h the fibers were removed, washed copiously with deionized water, and dried. Electron Microscopy. Fibers were mounted on aluminum stubs via a double-sided adhesive carbon tape and coated with palladium/ gold. The fibers were imaged using a JEOL JSM 7400F FE scanning

Langmuir, Vol. 23, No. 12, 2007 6679 electron microscope at 3 kV. EDXA was used to obtain information on fiber composition. Attenuated Total Reflection Fourier Transform Infrared Spectroscopy. Attenuated total reflection Fourier transform infrared spectroscopy was carried out on samples per previously established methods (1024 scans, 4 cm-1 resolution, diamond crystal).19 The analysis was carried out twice on identical samples on different days to confirm the data consistency. X-ray Photoelectron Spectroscopy. XPS analysis was carried out on dried samples using a VG Scientific 220i-XL imaging multitechnique surface analysis system. Samples for XPS analysis were prepared by mounting the fibers on double-sided carbon tape. The data were calibrated by adjusting all the peaks with reference to the hydrocarbon carbon peak located at a binding energy of 284.6 eV.

Results and Discussion Figure 1a shows a typical XPS survey scan of E-glass fibers before and after treatment with PAH. The dominant peaks observed for untreated fibers can be assigned to silicon, calcium, and oxygen, with traces of other metal oxidessall known to be components of glass fibers.47 The XPS spectrum of fibers after PAH treatment is similar to that of untreated fibers except for the additional nitrogen peak centered at a binding energy of 400 eV, indicating the presence of PAH on the fibers after treatment and stringent washing. A high-resolution spectrum of the nitrogen peak is shown in Figure 1b; deconvolution of the peak resulted in two peaks centered at ∼399 and ∼401 eV (at a ratio of ∼1:2), which arise from uncharged and charged amines, respectively. The relative areas of the two peaks, which roughly remained the same for the range of coating conditions studied, suggest that the

Figure 1. (a) XPS of fibers before and after PAH treatment. (b) High-resolution nitrogen spectrum for PAH-treated fibers. SEM images of (c) bare fibers (untreated) and (d) fibers after PAH treatment. Scale bars ) 5 µm for (c) and 500 nm for the inset in (c) and for (d).

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Figure 2. SEM images of treated fibers: (a) bare fibers after incubation with TMOS for 1 h; (b-d) PAH-treated fibers after 15, 40, and 60 min of incubation in hydrolyzed TMOS solution. The fiber surface appears to be visible under the particles in (b), but then is obscured by more complete silica coating in (c) and (d). Scale bars ) 5 µm (a, d) and 1 µm for the insets in (a) and (d) and for (b) and (c).

number of charged sites is twice that of the uncharged sites; the charged sites facilitate the subsequent precipitation of silica on the fiber surfaces. SEM images obtained from fibers before and after PAH treatment are presented in parts c and d, respectively, of Figure 1, wherein the PAH-treated fiber (Figure 1d) clearly shows the presence of a heterogeneous film in contrast with the featureless (“smooth”) surface of the untreated fibers. Whether the heterogeneity of the film indicates incomplete coverage of the fiber, or simply variations in the thickness of the coating, cannot be deduced from the XPS and SEM data. Nonetheless, the immobilized PAH film was investigated for its utility in mediating formation of silica coatings on these fibers. Both coated and uncoated fibers were treated for up to 1 h with a prehydrolyzed solution of TMOS. After extensive washing, these treated fibers were analyzed via SEM; representative micrographs are shown in Figure 2. Very little coating was observed on unmodified fibers that were treated with TMOS for 15 min to 1 h. Figure 2a shows untreated fibers after 1 h of incubation in TMOS; identical results were obtained at earlier time points. On the other hand, formation, over sufficient time, of a continuous thick silica coating on the PAH-treated fibers was observed (Figure 2b-d). To investigate the intermediate stages of coating formation, SEM images were collected after incubation times of 15, 40, and 60 min, as shown in Figure 2b-d. The main images are shown at dissimilar magnifications here and in later figures to highlight the salient features of each stage of film formation. Higher magnification images of uncoated

and fully coated fibers are sometimes featureless, and the continuity and homogeneity of the coatings over larger length scales, which is of most interest in these studies, is highlighted via visualization of the fiber rather than just the surface. On the basis of the progression of morphology during film formation, silica growth is suggested to proceed on the surface of the fibers by adsorption of nuclei and small particles from solution on fibers due to electrostatic attractions between charged amines on the fiber surface and anionic silica species, followed by particle growth (Figure 2b). The effect of the presence of polycations on silica formation has been extensively studied, and it has been well-established that electrostatic interaction between organic polycations and anionic silica species is one of the main factors controlling silicification.26-28,31,32,48 The nucleated coating, mainly particulate in nature, slowly transforms into a relatively thick and somewhat rough coating via coalescence of the particles (Figure 2c) into the coating (Figure 2d), which likely occurs via particle fusion via traditional sol-gel mechanisms. Timedependent particle growth and coalescence in the presence of organic additives has been reported previously for bioinspired silicification and supports our observations described above.49,50 (46) Kim, D. J.; Lee, K. B.; Lee, T. G.; Shon, H. K.; Kim, W. J.; Paik, H. J.; Choi, I. S. Biomimetic micropatterning of silica by surface-initiated polymerization and microcontact printing. Small 2005, 1 (10), 992-996.. (47) Gupta, P. K. In Fibre Reinforcements for Composite Materials; Bunsell, A. R., Ed.; Elsevier: Amsterdam, 1988; p 19 (48) Jin, R.-H.; Yuan, J.-J. Multiply shaped silica mediated by aggregates of linear poly(ethyleneimine). AdV. Mater. 2005, 17 (7), 885.

Bioinspired Silica Coatings

As evident in the SEM data of silica films formed, a slight extent of cracking was observed in the thicker coatings. Estimation of the thickness of the cracks via inspection of these and other SEM data (not shown) suggests average coating thicknesses (under these film formation conditions) on the order of 1-2 µm, although both thicker and thinner layers are possible as a function of the deposition conditions and time. That the coatings appear to increase in thickness with time was assessed via comparisons of particle diameters and coating thicknesses in the SEM images; indeed, even thicker coatings than those shown can be formed with increased deposition time, but often with resulting undesirable fusion of fibers. The slight cracking observed in these films (Figure 2d) is therefore consistent with previous reports in which films of thickness greater than 1 µm develop cracks as a result of the internal stress in the plane of the substrate.1,3,51 If desired for a specific application, the cracking could be prevented by controlled drying or using drying control additives as established previously.51,52 The initial particle formation nucleated by the polymeric coating is consistent with many previous literature reports of such formation in solution,12,14,18-20,26-32,53 although the formation of such continuous silica coatings on uncurved or curved surfaces has not been previously reported under mild processing conditions. Elemental analysis via EDXA was also performed on the fibers before and after the formation of the silica coating (data not shown). The peaks centered at ∼0.5 and ∼1.75 keV confirmed the presence of oxygen and silicon, respectively. However, owing to the similarity in the composition of the inorganic coating and that of the fibers themselves, the EDXA analysis cannot distinguish the chemical composition of the silica films from the composition of the native fibers. Therefore, to further investigate the chemical nature of the silica coating formed upon TMOS treatment, ATR-FTIR analysis of the samples was performed. The control sample in Figure 3a (trace 1) shows two important peaks: one centered at ∼1400 cm-1 and the other between 1200 and 800 cm-1. The latter peak primarily arises from Si-O and Si-OH modes and can be deconvoluted into separate peaks. It may also consist of traces of other oxide peaks (e.g., CaO, Al2O3) that are present in E-glass fibers. The other peak centered at 1400 cm-1 can be attributed to CaO (and potentially other oxides present in glass fibers); these data are consistent with previous literature reports.54,55 Data from fibers that were incubated with TMOS but were not treated with PAH showed peaks identical to those of the untreated fibers (Figure 3a, trace 2), supporting the SEM data and indicating that little additional surface coating was formed on uncoated fibers upon the TMOS treatment. However, fibers that were coated with PAH prior to TMOS incubation produced a drastically different spectrum (Figure 3a, trace 3). The peaks in the characteristic region for silicate fibers (between 800 and 1200 cm-1) were changed significantly. In addition, the peak centered at 1400 cm-1 disappeared, and peaks at ∼1500-1660 and ∼3000 cm-1 (broad peak) were clearly observed (Figure 3a,b). The (49) Patwardhan, S. V.; Mukherjee, N.; Clarson, S. J. Effect of Process Parameters on Polymer Mediated Synthesis of Silica at Neutral pH. Silicon Chem. 2002, 1 (1), 47. (50) Kroger, N.; Lorenz, S.; Brunner, E.; Sumper, M. Self-assembly of highly phosphorylated silaffins and their function in biosilica morphogenesis. Science 2002, 298 (5593), 584-586. (51) Brinker, C. J.; Hurd, A. J.; Schunk, P. R.; Frye, G. C.; Ashley, C. S. Review of Sol-Gel Thin-Film Formation. J. Non-Cryst. Solids 1992, 147, 424436. (52) Hench, L. L.; West, J. K. The Sol-Gel Process. Chem. ReV. 1990, 90 (1), 33-72. (53) Knecht, M. R.; Wright, D. W. Dendrimer-Mediated Formation of Multicomponent Nanospheres. Chem. Mater. 2004, 16 (24), 4890-4895. (54) Arora, P. S.; Matisons, J. G.; Provata, A.; Smart, R. C. Aminohydroxysiloxanes on E-Glass Fibers. Langmuir 1995, 11, 2009-2017-

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Figure 3. ATR-FTIR characterization of fibers: (a) fibers before incubation with TMOS (1), after TMOS treatment without PAH coating (2), and after TMOS treatment with PAH coatings (3); (b) PAH-coated fibers after incubation with TMOS (trace 3 in (a) replotted in (b)). For reference, the spectra of PAH and silica are presented.

peaks at ∼1500-1660 and ∼3000 cm-1 (marked by an asterisk in Figure 3b) arise from the PAH coating, as clearly indicated by comparison with the spectrum obtained from pure PAH (Figure 3b). This suggests the presence of a PAH coating on the fiber surface, in agreement with the XPS and SEM data obtained (Figure 1). The disappearance of the peak at 1400 cm-1 also confirms the presence of some coating on the fibers, as this peak arises from CaO and other oxides, which are not present in either the PAH or the subsequently formed silica film. Although the new peaks observed in the region 800-1200 cm-1 for PAH- and TMOS-treated fibers are unusual, it is believed that they arise from the freshly formed silica derived from TMOS. Indeed, in studies of synthetically prepared silica, the appearance of freshly formed silica is indicated by the presence of a Si-OH peak centered at ∼950 cm-1 (marked by an arrow in Figure 3b), arising from a less condensed silica phase with a fraction of Si-OH greater than that of routinely formed silicas prepared by thermal treatment. It can therefore be concluded from the ATR-FTIR analysis of the surface of the samples that a PAH coating followed by treatment with TMOS is necessary for the generation of silica on the surface of the fiber. In contrast, no such silica coating was detected on the fiber surface in the absence of PAH treatment (Figure 3a). We then investigated the ability of additional polypeptides, PLL and PLT, to coat glass fibers and mediate formation of silica films when treated with TMOS, owing to the interesting conformational and adhesion properties possible with these polymers. Polylysine has been previously shown to facilitate rapid silica formation in Vitro under ambient pH and temper-

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Figure 4. SEM and IR analysis of PLL-treated fibers: (a) PLL-coated fibers before treatment with TMOS; (b, c) PLL-coated fibers after treatment with TMOS for 15 min (b, fiber surface still visible) and 60 min (c, complete coating). (d) ATR-FTIR characterization of fibers before incubation with TMOS (1), after TMOS treatment without PLL coating (2), and after TMOS treatment with PLL coatings (3). The inset in (d) shows peaks in the amide region (marked by an arrow). Scale bars ) 5 µm (a, c) and 1 µm (b).

ature.24,26,56,57 Furthermore, owing to its conformation and assembly characteristics, PLL is capable of controlling silica morphologies from spherical particles to sheets.19,21 Poly(lysinetyrosine), in addition to its ability to nucleate silica formation on the basis of its cationic charge, also possesses the potential for oxidation of tyrosine to 3,4-dihydroxy-L-phenylalanine via the enzymatic action of tyrosinase. This can consequently improve the adhesion and film integrity/mechanical properties via the adhesive and cross-linking activities of DOPA-modified polymers as demonstrated in the literature.58-60 Figures 4a and 5a show the SEM images of glass fibers coated with PLL and PLT, respectively. Both homogeneous and heterogeneous areas were observed in these experiments; the heterogeneity is consistent with our observations of the PAH-coated fibers, although the origins of the apparent particulate nature of sections of the coatings are unknown. The presence of the polypeptide coatings was confirmed as above via ATR-FTIR analysis (data not shown). Silica coatings can be deposited on PLL- and PLT-coated fibers (55) Martinez-Richa, A. Studies of structure and molecular motion in an epoxy/E-Glass composite system. Ph.D. Dissertation, Case Western Reserve University, 1994. (56) Patwardhan, S. V.; Mukherjee, N.; Clarson, S. J. The use of poly-Llysine to form novel silica morphologies and the role of polypeptides in biosilicification. J. Inorg. Organomet. Polym. 2001, 11 (3), 193-198. (57) C oradin, T.; Durupthy, O.; Livage, J. Interactions of amino-containing peptides with sodium silicate and colloidal silica: A biomimetic approach of silicification. Langmuir 2002, 18, 2331-2336. (58) Yu, M. E.; Deming, T. J. Synthetic polypeptide mimics of marine adhesives. Macromolecules 1998, 31 (15), 4739-4745. (59) Waite, J. H. Adhesion a la Moule. Integr. Comp. Biol. 2002, 42 (6),11721180.

uniformly regardless of the nature (heterogeneous or not) of the polypeptide film. When the polypeptide-coated samples were incubated with prehydrolyzed TMOS for 15 and 60 min, the formation of first particles and then continuous silica coatings with rough surfaces was observed (Figures 4b,c and 5b,c), with morphologies extremely similar to those mediated by PAH coatings (Figure 2b-d). Regardless of variations in the heterogeneity of the polyamine coatings (Figures 4a and 5a), similar silica coatings were observed, indicating that a specific polymer film morphology is not required for uniform coating by silica at the deposition times investigated. The samples before and after polyamine and/or TMOS treatments were studied using ATR-FTIR, as above for PAH-treated fibers; the data for PLLand PLT-coated fibers are presented in Figures 4d and 5d, respectively, and can be identically interpreted. Consistent with the data shown above for PAH-treated fibers (Figure 3), freshly formed silica coatings were only observed when either PLL or PLT coatings were present on the fibers prior to incubation with TMOS (Figures 4d and 5d). Via the use of a combination of XPS, SEM, and ATR-FTIR, we have therefore established a general method employing soaking of fibers in various polyamine solutions for generation of a polyamine coating that, in the presence of a silica precursor, can facilitate the deposition of silica films. The formation of silica coatings on the PAH-, PLL-, and PLT-coated fibers is consistent with the fact that amine-containing polymers are known

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Figure 5. SEM and IR analysis of PLT-treated fibers: (a) PLT-coated fibers before treatment with TMOS; (b, c) PLT-coated fibers after treatment with TMOS for 15 min (b, fiber surface still likely visible) and 60 min (c, complete coating). (d) ATR-FTIR characterization of fibers before incubation with TMOS (1), after TMOS treatment without PLT coating (2), and after TMOS treatment with PLT coatings (3). The inset in (d) shows peaks in the amide region (marked by an arrow). Scale bars ) 5 µm (a, c) and 1 µm (b).

to strongly physisorb to silicon and glass surfaces,61,62 while any unbound polymer is removed readily with water. Therefore, modification of fibers with a variety of polyamines provides a general strategy for equipping the fiber surface with chemical functionality that can facilitate silica formation on the surface of fibers, first as particles and then as a continuous film, in a manner that appears to be independent of the identity of the polymer initially deposited. A more detailed analysis that would correlate polymer identity and polymer film thickness/homogeneity with silica chemical structure and thickness may provide interesting insights and strategies for controlling film formation, although such detail is outside the scope of this initial study and will be investigated in the future.

Conclusions Taken together, these studies illustrate that continuous silica films can be easily deposited on E-glass fibers simply coated with various amine-containing polymers and polypeptides. The surface modification and the formation of silica coatings were systematically investigated via X-ray photoelectron spectroscopy, (60) Lee, H.; Scherer, N. F.; Messersmith, P. B. Single-molecule mechanics of mussel adhesion. Proc. Natl. Acad. Sci. U.S.A. 2006, 103 (35), 12999-13003. (61) Decher, G. Fuzzy nanoassemblies: Toward layered polymeric multicomposites. Science 1997, 277 (5330), 1232-1237. (62) Shiratori, S. S.; Rubner, M. F. pH-dependent thickness behavior of sequentially adsorbed layers of weak polyelectrolytes. Macromolecules 2000, 33(11), 4213-4219.

scanning electron microscopy, and attenuated total reflectance Fourier transform infrared spectroscopy. The formation of the silica coatings is suggested to proceed on the surface of the fibers via adsorption of nuclei and small particles from solution, due to electrostatic attractions between the cationic coated surface and anionic silica species, followed by particle growth into a continuous film. The coatings adhered well to the curved fibers during sample handling, as indicated by their continuity and homogeneity across large fiber areas and in multiple fiber samples, with some cracking observed in thicker films. The homogeneous coating of fibrous and nanofibrous samples under these environmentally benign conditions offers multiple opportunities for the production of biofunctional fibrous materials with applications in membrane, optical, and sensor applications. Acknowledgment. X-ray photoelectron spectroscopy spectra were collected at the Surface Analysis Facility and microscopy data at the Keck Microscopy facility, both at the University of Delaware. The project was supported in part by grants from the U.S. Army Research Laboratory (CMR-20), the European Union, and the U.S. Air Force Office of Scientific Research. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing official policies, either expressed or implied, of the Army Research Laboratory of the U.S. Government. LA063685A