Defense Applications of Nanomaterials - American Chemical Society

Chapter 9

Bioinspired Organic-Inorganic Hybrid Devices 1

1

1

Lawrence L . Brott , Rajesh R. Naik , Sean M . Kirkpatrick , Patrick W. Whitlock , Stephen J . Clarson , and Morley O. Stone 2

2

1

1

Downloaded by UNIV MASSACHUSETTS AMHERST on August 4, 2012 | http://pubs.acs.org Publication Date: January 21, 2005 | doi: 10.1021/bk-2005-0891.ch009

Materials and Manufacturing Directorate, Air Force Research Laboratory, 3005 Ρ Street, Wright-Patterson Air Force Base, OH 45433-7702 Department of Materials Science and Engineering, University of Cincinnati, 497 Rhodes Hall, Cincinnati, OH 45221-0012

2

In order to achieve the high amount of index of refraction mismatch necessary for the fabrication of a photonic bandgap device, a highly ordered hybrid material of organic and inorganic compounds must be developed. Through the identification of peptides from the diatom Cylindrotheca fusiformis, simple silica nanospheres can now be synthesized from silanes under physiological conditions. By incorporating these peptides into a monomer formulation, peptide-rich regions can be created on the polymer surface using a holographic two-photon induced photopolymerization process. After exposing the cured polymer to a silane precursor, silica nanospheres are embedded in the peptide-rich regions resulting in a highly ordered two-dimensional array of silica spheres on the polymer backing. The diffraction efficiency of these devices increases nearly fifty-fold when compared to a polymer hologram without the silica spheres.

132

© 2005 American Chemical Society

In Defense Applications of Nanomaterials; Miziolek, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


133 With the recent growth in interest in the area of nanotechnology, a great deal of interdisciplinary research has focused upon the ability to create nanometer size-scale devices for potential applications in biomedicine, electronics and optics. Numerous cases of nanopatterning and nanostructure are commonly found in nature, with some notable examples appearing in the marine diatoms and sponges (1,2). These simple organisms are able to form nano- and micro-structured components with precise control using proteins. It is humbling to realize that what these single-cell organisms accomplish so elegantly, requires extreme laboratory conditions to duplicate even the simplest of structures (3,4). Kroger et al. identified a set of polycationic peptides (referred to as silaffins) from the diatom Cylindrotheca fusiformis (J). The silaffins, when added to a hydrolyzed silane precursor, catalyze the formation of simple silica nanospheres at neutral pH and ambient temperatures and pressures (6). The R5 peptide, a short 19 amino acid (SSKKSGSYSGSKGSKRRIL) repeat unit of the silaffin protein, is also able to catalyze silica precipitation (5). Here we describe our work to understand the process of biosilification through a study of the reaction conditions by varying the reaction time and pH. This knowledge was then applied to the fabrication of a practical optical device by incorporating the peptide into a polymer hologram to produce a novel hybrid organic/inorganic ordered nanostructure. Continuing work is also presented on potential ways to enhance the properties of this device by modifying the shape of the silica nanostructure from a sphere to a more complex morphology by replacing the static reaction conditions to a more dynamic one. The R5 peptide based on the published sequence was chemically synthesized and used in the research presented here. Using the reaction conditions previously described (5), the R5 peptide (100 fxg/mL)was dissolved in a sodium phosphate-citrate buffer 7.0 pH and added to a 0.1M tetrahydroxysilane solution to form silica spheres with a diameter of400-700 nm within 10 minutes (Figure 1). In the absence of R5 peptide, the tetrahydroxysilane solution remains stable for hours before it slowly converts to a clear amorphous gel. We determined the relevant kinetic parameters (time and pH) in order to maximize the rate of formation of silica spheres. The UV-Vis spectrum obtained using the silica spheres synthesized by R5 peptide has been previously shown experimentally to absorb strongly at around 290 nm. In the kinetic experiments, we monitored the formation of silica spheres by measuring the absorbance at 290 nm From the results in Figure 2, it is evident that the formation of the silica spheres was essentially complete in 10 minutes and the activity of the R5 peptide is maximal at pH 8.0. Little or no silica precipitation is observed at pH below 7. This is consistent with the finding of Kroger et al. requiring lysine modification for activity at acidic pH (5). The ability to use peptides to catalyze the formation of inorganic material offers the possibility of spatially controlling the deposition of the



134

Figure 1. Scanning electron micrographs (SEM) ofspherical biosilica structures obtained by reacting the R5 peptide with tetrahydroxysilane in solution.

A

B

0

2

4

6

8

Time (mins)

10

12

0

2

4

6

8

10

12

pH

Figure 2. The rate ofsilica precipitation using the R5 peptide as a function of (A) time and (B) pH.



135 inorganic material. In the recent past, a number of techniques have been exploited for the deposition and patterning of peptides (7,5). Using a polymer hologram created by a holographic two-photon induced polymerization (HTPIP) process to spatially control the deposition within a polymer matrix, we were able to create a hybrid organic/inorganic ordered nanostructure (9,10). The advantage of using a two-photon polymerization process to cure a peptidecontaining monomer matrix is that the ultrafast infrared laser used in this process typically does not alter the biological activity of the incorporated protein. It was also expected that during the polymerization process, the peptide would be segregated into regions of low crosslinking density. The approach of using ultraviolet lasers to phase separate small liquid crystal molecules in a polymerbased hologram has been used extensively (11) and it was reasoned mat this technique was also applicable to the H-TPIP process as well. Furthermore, by exposing the peptide-containing hologram to the liquid silicic acid, it was theorized that silica would form in the holographic nanopattern. A water miscible monomer formulation was created by combining two poly(ethylene glycol)-based tri- and penta-acrylates with triethanol amine and isopropylthioxanthone as initiators (12,13). The peptide was added to this formulation, which was then spin coated onto a glass slide. The sample was cured under nitrogen in a two-beam transmission holographic arrangement using a 790 nm titanium-sapphire laser. Because certain areas of the sample cure more rapidly than others, the smaller molecules (namely water and the peptide) phase separate from the areas of higher crosslink density and migrate into areas of lower density. As a result, peptide rich domains are created in the polymer sample with the periodicity of the hologram. After the curing process, the sample was briefly rinsed with water to remove any uncured monomer. Freshly prepared hydrolyzed silane was applied to the hologram and allowed to react with the R5 peptide embedded in the hologram for 10 minutes before being rinsed with water to remove any unreacted silane. A study of this hologram by scanning electron microscopy revealed that silica spheres formed a regular two-dimensional array with the periodicity of the hologram (see Figure 3). A study of the size distribution of the silica spheres reveals that the average nanosphere diameter is 452 nm (± 81 nm) and the periodicity of the hologram is 1.60 um. As a result, this photonic device exhibits a nearly fifty-fold increase in diffraction efficiency over a comparable polymer hologram without silica. The untreated grating exhibited a diffraction efficiency of approximately 0.02%, while the grating with the silica spheres showed an efficiency of approximately 0.95%. This large increase can be attributed to the fact that the spheres form an almost continuous line of silica in the hologram, achieving a high fill factor. While these results are promising, further improvements upon this photonic device would include the replacement of the individual silica spheres with a long, continuous region of silica. Work has begun to realize this goal



136

Figure 3. Two-dimensional array of ordered silica nanospheres formed by reacting the R5-embedded hologram to the silane.

Figure 4. Scanning electron micrographs offibrillar arch-shaped morphologies obtained by (A,B) applying a mechanical shear or (C) bubbling nitrogen gas through a solution of the R5 peptide and tetrahydroxysilane. Scale bar equal Ifjm.



137 through the careful manipulation of the physical reaction environment, in particular the microfluidic environment, during the R5/silicic acid reaction. Preliminary results indicate that this can be achieved by either bubbling nitrogen gas through the reaction vessel or applying mechanical shear to the nucleating sites. Silica produced in the presence of bubbling nitrogen results in the formation of arched, elongated structures that resembles rods (see Figure 4c). However, biosilification in the presence of a shear flow in a linear environment results in fibrillar-like structures present throughout the sample with a complete absence of the spherical structures (see Figure 4a). These preliminary results are encouraging and research continues on applying these new post-processing techniques to create rod-shaped silica nanostructures on the surface of the peptide-containing hologram. Such materials will have considerable potential for use in photonic devices since these techniques combine the ease of processability of an organic polymer with the improved mechanical and optical properties of an inorganic to produce a hybrid system with properties not possible from each structure alone. Additionally, the holographic phase separation technique is universally applicable for any catalyst or binding agent that can be incorporated into a polymer. For example, as different catalysts are identified, a wide variety of unique hybrid structures will be possible with differing shapes and mechanical properties. Consequently, the promise of this technique is that it allows a simple yet general and easily modifiable method for nanopatterning.

Acknowledgements This research was supported by the Air Force Office of Scientific Research.

References 1. Round, R.E.; Crawford, R.M.; Mann, D.G. The Diatoms: Biology and Morphology of the Genera; Cambridge University Press: Cambridge, 1990. 2. Volcani, B.E. "Cell Wall Formation in Diatoms: Morphogenesis and Biochemistry," in Silicon and Siliceous Structures in Biologic Systems; Springer-Verlag: New York, 1981. 3. Parkinson, J.; Gordon, R. Trends in Biotechnology, 1999, 17, 190-196. 4. Morse, D.E. Trends in Biotechnology, 1999, 17, 230-232. 5. Kröger, N.; Deutzmann, R.; Sumper, M . Science, 1999, 286, 1129-1132.


138 6. 7.

8.


9. 10. 11. 12. 13.

Cha, J.N.; Stucky, G.D.; Morse, D.E.; Deming, T.J. Nature, 2000, 403, 289292. Berggren, K.K.; Bard, A.; Wilbur, J.L.; Gallaspy, J.D.; Helg, A.G.; McClelland, J.J.; Rolston, S.L.; Phillips, W.D.; Prentiss, M . ; Whitesides, G. Science, 2000, 260, 1255-1257 Wilson, D.L.; Martin, R.; Hong, S.; Cronin-Golomb, M . ; Mirkin, C.A.; Kaplan, D.L. Proc. Natl. Acad. Sci. USA, 2001, 98, 13660-13664. 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, 291-293. Kirkpatrick, S.M.; Baur, J.W.; Clark, C.M.; Denny, L.R.; Tomlin, D.W.; Reinhardt, B.A.; Kannan, R.; Stone, M.O. Appl. Phys., 1999, 69, 461-464. Bunning, T.J.; Natarajan, L.V.; Tondiglia, V.; Sutherland, R.L.; Vezie, D.L.; Adams, W.W. Polymer, 1995, 36(14), 2699-2708. Belfield, K.D.; Schafer, K.J.; Liu, Y.; Liu, J.; Ren, X.; Van Stryland, E.W. J. Phys. Org. Chem., 2000, 13, 837-849. Brott, L.L.; Naik, R.R.; Kirkpatrick, S.M.; Pikas, D.J.; Stone, M.O. Polymer Preprints, 2001, 42(1), 675-676.


Defense Applications of Nanomaterials - American Chemical Society

Recommend Documents