Biomimetic Growth of Biomorphic CaCO3 with Hierarchically Ordered

Jul 24, 2007 - Institute of Surface Micro and Nano Materials, Xuchang UniVersity, Xuchang, Henan ProVince,. 461000, China, Department of Physics, The ...
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Biomimetic Growth of Biomorphic CaCO3 with Hierarchically Ordered Cellulosic Structures Zheng,*,†,‡

Huang,†

Ma,†

Zhi Baojun Huiqiang Xiaoping Zhuang Liu,‡ Ka Wai Wong,‡ and Woon Ming Lau*,‡,§

Zhang,†

Manying

Liu,†

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1912-1917

Institute of Surface Micro and Nano Materials, Xuchang UniVersity, Xuchang, Henan ProVince, 461000, China, Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, N. T., China, and Surface Science Western, The UniVersity of Western Ontario, London, Ontario, Canada ReceiVed December 3, 2006; ReVised Manuscript ReceiVed March 5, 2007

ABSTRACT: Biomorphic CaCO3 with different hierarchically ordered micro- and nanostructures was fabricated with natural cellulose substances as the host templates. The fabrication involves two mild sonication processes in which calcium and carbonate ions are sequentially added to the template, together with subsequent calcination in air. The specific pseudo-1D and pseudo-2D structures of the resultant crystalline calcite can be tailored by mimicking both natural and artificially woven cellulosic substances, with the former following the ribbon/tube form of cellulose fiber and the latter following the netlike architectures of the woven cotton cloth. The building block of these structures is a layer of CaCO3 grains grown on the surfaces of each cellulose fiber. By choosing Ca2+, CO32-, or HCO3- ions as the first adsorption species on the cellulose template, we show that the resultant CaCO3 grain size can be fine-tuned in a nanoscopic scale, most probably due to the differences in the nature of ion adsorption on the cellulose molecules and the resultant CaCO3 nucleation and growth. The impact of this new route is that we can precisely predict the morphologies of the final CaCO3 products that were not realized in other chemical approaches. Introduction The concepts of patterning inorganic or organic-inorganic hybrid materials with unusual hierarchical form, together with the emerging field of molecular tectonics, have fascinated synthetic chemists and materials scientists. Among the prevalent approaches to achieve these reaction goals, learning from how Nature does chemical synthesis, now known as biomimetic materials chemistry, is perhaps the most promising method toward future innovations in chemistry.1,2 Because biomineralization has played a critical role in forming and shaping our world, substantial efforts have been devoted to understanding it and harnessing it for future materials synthesis. An outstanding example of the recent advancement in biomineralization and associated biomimetic research is the study of formation of calcium carbonate in nature.3,4 We now know that metastable phases such as amorphous calcium carbonate (ACC), hydrated calcium carbonate, and vaterite are first formed, often with different strategies of nucleation as a function of molecular surface engineering of the host surface, and these metastable phases are then transformed into the stable phase of calcite. Many of these processes can now be carried out artificially with biomimetic designs to grow these different phases using precise molecular surface templates such as block copolymers with function-specific surfactants,5 self-assembled monolayer (SAM),6 Langmuir-Blodgett (LB) films,7 and other soluble matrices.8 Often, additives including appropriate proteins and biological growth facilitators such as fungi are adopted to control nucleation and growth. With these emerging strategies, biomimetic syntheses of various phases of calcium carbonate from macroscopic to nanoscopic scales have been demonstrated. A comprehensive review of this development was recently documented by Kato.9 * Address correspondence to these authors. E-mail addresses: [email protected]; [email protected]. † Xuchang University. ‡ The Chinese University of Hong Kong. § The University of Western Ontario.

In addition to learning how Nature synthesizes materials, scientists have also recently spent increasing efforts on innovative ways of exploiting biomorphic structures in materials synthesis,10 because Nature is extremely advanced in making sophisticated morphologic structures, which may include composites and voids with a complex and yet cost-effective strategy of materials deployment. Particularly, the cellular anatomy of naturally grown biological systems as well as their derived products such as cellulose fibers, tissues, filter papers, cloth, and cardboard provides promising templates for the design of inorganic materials with hierarchically ordered structures on different length scales that cannot be exactly processed by conventional processing technologies engineered by humans. Through these so-called “morphosynthesis”,11 “morph-genetic”, or “artificial fossilization”1,12 processes, a variety of biological systems including wood,13,14 textiles,15 cotton,16 filter paper,17 pollen grains,11 bacteria,18 eggshell membranes,19,20 filter membranes,21 echinoids,22 viruses,23 and human and dogs hairs24 have been used as the morphologic and structural molds. Liquid/solgel or gaseous infiltrants of different compositions can be “injected” to these molds.1,14,16,19,20,25 Subsequent calcination can then be applied for consuming the organic host materials and leaving behind an inorganic replica of the original mold. In this paper, we describe the use of natural cellulose substances as the templates for fabricating hierarchically ordered calcite via mild coating processes with sonication followed by calcinations in air. Unlike the general sol-gel method, the present coating is accomplished by taking advantage of a simple inorganic reaction between adscititious Ca2+ and CO32- and assisted by a mild ultrasonic treatment. Cotton and cloth, which are composed of natural cellulose, were employed as the organic templates for fabricating their CaCO3 replicas. The important effects of reaction designs including the choices of using Ca2+ or CO32- as the reaction/adsorption initiator and of CO32- or HCO3- as the anion agent are explained. In addition, we also show the extension of the biomimetic synthesis to the use of cotton cloth for the production of mechanically rigid calcite

10.1021/cg0608763 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/24/2007

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Figure 1. SEM morphologies of the original cotton templates (a) and their CaCO3 replicas obtained from different reaction conditions: (b,c) 0.005 M Ca2+ as the reaction/adsorption initiator and 0.005 M CO32- as the anion agent; (d,e) 0.005 M Ca2+ as the reaction/adsorption initiator and 0.005 M HCO3- as the anion agent; (f) 0.01 M Ca2+ as the reaction/adsorption initiator and 0.01 M CO32- as the anion agent.

sheets having the artificial woven hierarchical structure of the cotton cloth. Experimental Section Synthesis. In the present experiments, CaCl2, Na2CO3, and NaHCO3 (all in A.R. grade) were used as received. Cotton threads and cloth were collected from the general market and cleaned with DI water several times to eliminate any contaminations. In a typical coating procedure with Ca2+ as the initiator reagent, purified cotton threads were immersed in a CaCl2 solution (0.005 M) in a beaker that was placed in an ultrasonic bath for 30 min. To speed up the adsorption of the initiator reagent, the temperature was then raised to 80 °C for 12 h. Subsequently the biomimetic reaction for calcium carbonate growth was completed by placing the pretreated cotton threads in a Na2CO3 solution (0.005 M) in the ultrasonic bath for 30 min. The samples were then rinsed with DI water and dried at room temperature. They were examined with scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) and Raman analysis. Finally the samples were calcined at 650 °C for 2 h with a temperature ram-rate of 15 °C/min. Variations of this process were conducted to track the effects of using Na2CO3 instead of CaCl2 as the reaction/adsorption initiator reagent, and using NaHCO3 to replace Na2CO3 as the agent for supplying the carbonate. Further, the effects of the reagent concentrations were also studied. After the verification of calcium carbonate growth on cotton threads prior to and after calcination, the biomorphic growth on cotton cloth was demonstrated with common cotton cloth as the biomorphic template. Characterization. In the course of the process development, X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance XRD diffractometer equipped with a Go¨bel mirror, using Cu KR radiation. Kratos AXIS-HS X-ray photoelectron spectroscopy (XPS)

equipped with a standard and monochromatic source (Al KR) operated at 150 W (15 kV, 10 mA) was employed for surface analysis. The binding energy (BE) scale was calibrated against the BE of Au 4f7/2 at 84.0 eV. The crystal morphologies were investigated by using a scanning electron microscope (SEM, LEO 1450VP with an energydispersive X-ray fluorescence analyzer). The microstructures and selected area electron diffraction (SAED) patterns of the products were measured by transmission electron microscopy (TEM, Philips CM120). Fourier transform infrared (FTIR) spectrometry was performed on KBr disks of the powdered samples using Thomas Nicolet 670 FTIR spectrometer. Raman experiments were carried out with a Renishaw model U1000B micro-Raman spectrograph at room temperature using the 514.5 nm line of an Ar ion laser as the excitation.

Results and Discussion We first select our results on cotton threads as an example to demonstrate the feasibility of our biomimetic process for “pseudo-1D” replication. Figure 1 shows the SEM morphologies of the original cotton templates and their calcite replicas obtained from different reaction conditions including the choices of reaction/adsorption initiator, anion reactants, and reactant concentrations. As shown in Figure 1a, the cotton threads are fiber belts with a typical width of about 20 µm. After calcite formation, the resulting replicas faithfully duplicate the morphologic microstructures of the original cellulose bundles but with a significant shrinkage to 4-6 µm in diameter (Figure 1b,c). In addition, the “bundle replica” typically exhibits grainlike secondary structures with grain size of 500-800 nm, as shown in the magnified image of Figure 1c. When the

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Figure 2. SEM (a,b) and TEM (c,d) morphologies of the “cotton CaCO3” replicas obtained with 0.005 M CO32- as the reaction/adsorption initiator. The inset of panel d shows the SAED pattern of a single strip.

Figure 3. Characterization of the cotton and cloth replicas: (a) XRD patterns of typical cotton (below) and cloth replicas (above); (b) XPS survey spectrum and core level spectrum (inset) of the typical “cloth CaCO3”.

concentration of the two reactants was adjusted to 0.01 M, tube structures can be found in the resulting products as shown in Figure 1f. This can be attributed to the random convolution of the bundle replicas during the calcination. When Na2CO3 is replaced by NaHCO3, the morphology and microstructure of

the final calcite product change significantly in that, as shown in Figure 1d,e, interwoven and twisted structures consisting of small grains with diameter of 100-200 nm are formed. Finally, when the reactant/adsorption initiator is changed from CaCl2 to Na2CO3, the size and microstructure of the calcite product shrink toward the nanometer scale (100-500 nm in diameter) as shown in Figure 2a,b. The TEM image of a typical calcite bundle, as shown in Figure 2c, clearly illustrates the presence of strips of a layer of polycrystalline grains. A magnified image of a segment of one of the strips is included in Figure 2d, which indicates a strip width of ∼200 nm. When the electron beam is focused to less than 100 nm for the collection of selected area electron diffraction (SAED) data from the strip (as shown in the inset of Figure 2d), the diffraction pattern is the same as that of a single crystal of calcite. The SEM, TEM, and SAED results thus confirm that when Na2CO3 is used as the reactant/ adsorption initiator, our synthesis method gives a bundle replica comprising strips of nanoscopic single-crystalline calcite grains. The synthesis and analysis clearly demonstrate that our mild sonication approach with appropriate choices of reactants and processing conditions can yield calcium carbonate growth and calcite formation, together with the preservation of the morphology of a cotton fiber. The replicas of soft templates like cotton fibers have shown to be versatile. The size and microstructure of the final calcite products depend on the choices of reaction/ adsorption initiator, anion agent, and reactant concentrations. This suggests that these reaction parameters influence the adsorption of ions on the cellulose template surfaces and calcium carbonate nucleation and growth. The primary structure prior to calcination then influences the final structure of the calcite products. The micro-Raman spectra indicate that amorphous and hydrated CaCO3 were formed on the cellulose fibers at the first stage, and the calcination converts them to hexagonal calcite (S.G.: R3c (167), 81-2027). To demonstrate the feasibility of exploiting the above biomimetic process for molding the calcite product in a tailormade artificial “preform”, we choose man-made cotton cloth as a mold. As shown in Figure 4a,b, the original cloth used in

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Figure 4. SEM morphologies of the original cloth (a,b) and the “cloth CaCO3” obtained with 0.005 M CO32- as the reaction/adsorption initiator (c,f) and TEM images (g,h) obtained from a single strip of the “cloth CaCO3”. The inset of g shows the SAED pattern of a single strip.

this study is actually an artificial pseudo-2D network of cotton threads in compact bundles via simple weaving. Each bundle is composed of a large number of fiber belts (strips) with width of about 20-40 µm and thickness of about 5-10 µm. In a typical trial of using artificial cloth as templates, we immersed the cloth into a Na2CO3 solution first with mild ultrasonic treatment. After calcination, the resulting product exhibits perfect replication of the original cloth network as shown in Figure 4c,d, except that the dimension is shrunk. Although the shrinkage of the replica is significant, the basic 2D structural features are well maintained as shown in Figure 4d. In addition, Figure 4e clearly shows an individual bundle constructed with a large number of thin strips. Although the shrinkage is very significant, most of these thin strips still maintain the interwoven and the spiral features of the original cotton fibers. Further,

Figure 4f shows their hierarchically ordered secondary structures, which indicate respective thickness and width of 100200 nm and 2-4 µm. Typically, these thin strips are made up of tiny calcite grains with diameter of less than 200 nm, with a secondary mesoporous feature. Although these features are seemingly fragile, they are somehow collectively supporting each other in the biomorphic hierarchical structure, and the product is a mechanically rigid white sheet that is strong enough to be treated with tweezers. The sheet size and thickness depend on the original cloth used and therefore can be well controlled by “weaving” and “tailoring” of the original cellulose templates. The macroscopic dimension of the cloth is reduced by ∼1/10 due to shrinkage after the removal of the templates during calcinations. It is also important to note that the resultant calcite

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Figure 6. Raman spectra of (a) pure cotton fibers from cloth, (b) “cloth CaCO3” prior to calcinations, and (c) “cloth CaCO3” after calcinations at 650 °C. Figure 5. FT-IR spectra of (a) pure cotton fibers from cloth, (b) “cloth CaCO3” prior to calcinations, (c) CaCO3 scratched from “cloth CaCO3” prior to calcinations, and (d) “cloth CaCO3” after calcinations at 650 °C.

sheet does not show creasing, which is generally observed in other biomorphic methods.26 It is known that organic templates such as proteins and larger macromolecular frameworks play a key role in directing the aggregation or nucleation of ions and their destination, particularly in relation to crystal growth.26 Chemical bonding, mechanical stress, and spatial confinement are also important for generating the specific reaction field at a defined length scale. In this way, macroscopic hierarchical structures are autonomously built up from assemblies of smaller units formed at nano- and mesoscale. The TEM image of a single strip indeed shows such hierarchical structures (Figure 4g,h). The typical SAED pattern from the area covering the whole strip of microand nanoparticles shows diffraction rings typical of calcite crystals of random orientations (Figure 4g, inset), which indicates the overall calcite polycrystalline characteristics of the replicas. The FTIR spectrum (Figure 5d) and XRD pattern (Figure 3a) obtained from the above 2D “cloth CaCO3” further confirmed the formation of polycrystalline calcite, which is thermodynamically the most stable form of calcium carbonate. The bands at 1440, 876, and 710 cm-1 (Figure 5d) are characteristic FTIR peaks of calcite. In the present study, we also used XPS to trace the composition change in the fabrication process. The XPS survey spectrum of the “cloth CaCO3” only indicates three elements C, O, and Ca, and no Na or Cl contaminations in the final products (Figure 3b). The inset of Figure 3b shows the C 1s core level spectrum of the CaCO3 film. The two peaks at the bonding energies of 284.9 (peak 2) and 289.6 eV (peak 1) can be assigned to the surface adventitious carbon and carbon as carbonates in calcites respectively.27 To track the CaCO3 growth and understand the growth mechanisms prior to and after the calcination step, we have characterized the corresponding specimens with a wide range of analysis tools including SEM, XRD, XPS, Raman, FTIR, and TEM. For all specimens prior to calcination, the detection

of CaCO3 is difficult because CaCO3 is only present as thin adsorbate layers on cotton fibers. SEM indeed shows images of adsorbed cotton fibers virtually the same as those of the virgin cotton (Supporting Information). Nevertheless, XPS analysis (Supporting Information) shows no detectable Cl- and Na+ and gives Ca2+ and CO32- with a concentration ratio close to unity. The intensity of both Ca2+ and CO32- are very weak, which confirms the thin adsorbate layer nature of CaCO3. For the specimens prepared with CaCO3 growth reagents at a low concentration of 0.005 M, the presence of CaCO3 is not obviously detected by FTIR; however, when CaCl2 and Na2CO3 solutions of 0.1 M are used, the FTIR spectrum of some CaCO3 scraped off from the “cloth CaCO3” can be verified and shown in Figure 5c. In comparison to the Raman results, the FTIR results are much less surface sensitive such that the raw FTIR signals from the “cloth CaCO3” (Figure 5b) mainly show the FTIR features of cellulose from the cloth substrate (Figure 5a). While the presence of CaCO3 on specimens prior to calcination can be confirmed with both XPS and FTIR, Raman and XRD do not definitely show the presence of either crystalline materials or any signature data of a pure form of the known CaCO3 phases. Compared with the Raman spectrum obtained from pure cotton fibers (Figure 6a), the CaCO3 deposited “cloth CaCO3” gives additional broad Raman signals (Figure 6b) near 1050-1100 cm-1 where the peak positions of amorphous CaCO3 and other CaCO3 phases are located.28 We speculate that a mixture of nanoclusters of CaCO3 with different structural configurations is present. After the calcination treatment, the cotton fibers are eliminated and no longer interfered with the detection of CaCO3, and all metastable CaCO3 phases are expected to be converted to calcite crystals (Figure 6c). The peaks at 1086, 712, and 284 cm-1 (Figure 6c) can be attributed to symmetric stretching (ν1), in-plane bending (ν4), and lattice modes of calcite.28 Indeed, XRD (Figure 3a), XPS (Figure 3b), FTIR (Figure 5d), Raman (Figure 6c), and TEM (Figure 4g) all confirm the presence of calcite. In addition, TEM and electron diffraction data clearly show that calcite is present in the form of a cohesive network of submicrometer crystalline grains. More importantly, SEM shows that the calcite grains are organized in a three-dimensional architecture resembling that of the cotton fibers (Figure 4c), which is clear evidence of

Biomimetic Growth of Biomorphic CaCO3

the biomorphic nature of the specimen preparation procedures in this work. With all these results, we propose that the growth mechanism includes the following steps: (a) adsorption of the growth initiating reagent (Ca2+ or CO32-, depending on the reaction design) on the cotton thread surfaces with a mild sonication treatment, which effectively removes the air trapped in the fibers and drives the Ca2+ or CO32- ions to homogeneous adsorption of the ionic species in the cellulose matrix; (b) diffusion of the second growth reagent to some of the adsorption sites; (c) formation of CaCO3 nuclei; (d) diffusion of the first reagent and the second reagent to these nuclei to facilitate the subsequent CaCO3 growth. At this stage, most of the CaCO3 nuclei and clusters are amorphous in nature, with some of them participating in phase transformation into the metastable CaCO3 polytypes. Hence, a mixture of amorphous and nanocrystalline phases give Raman signals of broad peaks with no clear fingerprint Raman signatures of the known CaCO3 polytypes. The SEM images at this stage also show no presence of microcrystalline or polycrystalline phases. When the specimens are subject to the calcination treatment, the CaCO3 nuclei grow and transform to calcite while the cotton fibers are oxidized and being removed. Yet extensive diffusion of CaCO3 spanning a substantial distance is not possible under the calcination condition such that the three-dimensional architectures of the cotton fibers are preserved, with a shrinkage of the overall dimensions due to the loss of cotton fibers. The conversion of the original adsorbate layers of amorphous or nanocrystalline CaCO3 nuclei prior to calcination to submicrometer grains of calcite during calcination by chance yields a three-dimensional cohesive network with a mechanical strength strong enough to survive the specimen handling and analysis in this work. The chemical, crystalline, and biomorphic nature of the CaCO3 are therefore revealed and recorded. Conclusion We have successfully fabricated the “cellulose CaCO3” with hierarchically ordered micro- and nanostructures. The specific 1D or 2D structures of the biomorphic crystalline CaCO3 can be tailored by mimicking both natural and artificial cellulose substances. The growth mechanisms prior to and after the calcination step are also speculated. Unlike sol-gel coating processes, which heavily rely on the physical (electrostatic) attraction between the precursor molecules and the biological templates, this approach takes advantage of an inorganic ion reaction occurring in the cellulose fibers with assistance of a mild sonication treatment, which facilitates the reaction and dispersion of the CaCO3 particles avoiding aggregation and overdeposition on the surface of the cellulose fibers. Other key factors for this exact duplication include the choices of the reaction/adsorption initiator, anion or cation agent, and reactant concentrations. The impact of this new route is that we can precisely predict the morphologies of the final CaCO3 products, which was not realized in other chemical approaches. Biological materials with hierarchically ordered cellulose structures are quantitatively abundant and morphologically complex. With the simple route described in this report, one can weave or compact these materials in any tailor-made forms and shapes and then

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convert them to a product of calcite, apatite, or other biomineral substances by a combination of biomimetics and conventional chemistry. Acknowledgment. This work was supported by National Natural Science Foundation of China (Grant No. 20574058) and Natural Science Foundation of Henan Province, China (Grant No. 611021900). Supporting Information Available: Detection of CaCO3 growth prior to calcinations including SEM and XPS results. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (a) Mann, S. Biomimetic Materials Chemistry; VCH Publishers, Inc.: New York, 1996. (b) See the Focus article, Chem. Commun. 2004, 1. (2) (a) Walsh, D.; Lebeau, B.; Mann, S. AdV. Mater. 1999, 11, 324. (b) Faatz, M.; Grohn, F.; Wegner, G. AdV. Mater. 2004, 16, 996. (c) Addadi, L.; Raz, S.; Weiner, S. AdV. Mater. 2003, 15, 959. (d) Kuther, J.; Seshadri, R.; Tremel, W. Angew. Chem., Int. Ed. 1998, 37, 3044. (e) Mukkamala, S. B.; Powell, A. K. Chem. Commun. 2004, 918. (f) Zhan, J. H.; Lin, H. P.; Mou, C. Y. AdV. Mater. 2003, 15, 621. (3) Mann, S. Angew. Chem., Int. Ed. 2000, 39, 3392. (4) Meldrum, F. C. Int. Mater. ReV. 2003, 48, 187. (5) (a) Qi, L. M.; Li, J.; Ma, J. M. AdV. Mater. 2002, 14, 300. (b) Yu, J. G.; Yu, J. C.; Zhang, Z. L.; Wang, X. C.; Wu, L. Chem. Commun. 2004, 2414. (6) Lee, I.; Han, S. W.; Lee, S. J.; Choi, H. J.; Kim, K. AdV. Mater. 2002, 14, 1640. (7) Zhang, Y.; Jin, R. J.; Zhang, L.; Liu, M. H. New J. Chem. 2004, 28, 614. (8) Raz, S.; Weiner, S.; Addadi, L. AdV. Mater. 2000, 12, 38. (9) Kato, T.; Sugawara, A.; Hosoda, N. AdV. Mater. 2002, 14, 869. (10) Huang, J. G.; Kunitake, T. J. Am. Chem. Soc. 2003, 125, 11834. (11) Hall, S. R.; Bolger, H.; Mann, S. Chem. Commun. 2003, 2784. (12) Huang, J. G.; Kunitake, T.; Onoue, S. Y. Chem. Commun. 2004, 1008. (13) Dong, A. G.; Wang, Y. J.; Tang, Y.; Ren, N.; Zhang, Y. H.; Yue, Y. H.; Gao, Z. AdV. Mater. 2002, 14, 926. (14) Shin, Y. S.; Liu, J.; Chang, J. H.; Nie, Z. M.; Exarhos, G. J. AdV. Mater. 2001, 13, 728. (15) Imai, H.; Matsuta, M.; Shibizu, K.; Hirashima, H.; Negishi, N. J. Mater. Chem. 2000, 10, 2005. (16) Caruso, R. A. Angew. Chem., Int. Ed. 2004, 43, 2746. (17) Shin, Y. S.; Li, X. S.; Wang, C. M.; Coleman, J. R.; Exarhos, G. J. AdV. Mater. 2004, 16, 1212. (18) Davis, S. A.; Burkett, S. L.; Mendelson, N. H.; Mann, S. Nature 1997, 385, 420. (19) Yang, D.; Qi, L. M.; Ma, J. M. AdV. Mater. 2002, 14, 1543. (20) Yang, D.; Qi, L. M.; Ma, J. M. J. Mater. Chem. 2003, 137, 1119. (21) Caruso, R. A.; Schattka, J. H. AdV. Mater. 2000, 12, 1921. (22) Meldrum, F. C.; Seshadri, R. Chem. Commun. 2000, 29. (23) Shenton, W.; Douglas, T.; Young, M.; Stubbs, G.; Mann, S. AdV. Mater. 1999, 11, 253. (24) Kim, Y. Biomacromolecules 2003, 4, 908. (25) Caruso, R. A.; Schattka, J. H.; Greiner, A. AdV. Mater. 2001, 13, 1577. (26) Cook, G.; Timms, P. L.; Spickermann, C. G. Angew. Chem., Int. Ed. 2003, 42, 557. (27) Gopinath, C. S.; Hegde, S. G.; Ramaswamy, A. V.; Mahapatra, S. Mater. Res. Bull. 2002, 37, 1323. (28) (a) Tlili, M. M.; Ben Amor, M.; Gabrielli, C.; Joiret, S.; Maurin, G.; Rousseau, P. J. Raman Spectrosc. 2001, 33, 10. (b) Gabrielli, C.; Jaouhari, R.; Joiret, S.; Maurin, G. J. Raman Spectrosc. 2000, 31, 497.

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