Nanoscale characterization of nautilus shell structure: An example of natural self-assembly
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Reyes-Gasga Texas Materials Institute and Department of Chemical Engineering, University of Texas, Austin, Texas 78712; and Instituto de Física UNAM, México D.F. 01000, México
D.I. García-Gutierrez and M. Jose-Yacaman Texas Materials Institute and Department of Chemical Engineering, University of Texas, Austin, Texas 78712 (Received 29 November 2005; accepted 9 March 2006)
Structural characterization at the nanometric scale of the Nautilus sp shell was carried out by high-resolution transmission electron microscopy and high-angle annular dark field to understand how the organic and inorganic components are related. The inorganic phase that built the shell is made of calcium carbonate (CaCO3), with the orthorhombic unit cell of the aragonite, in a texturized arrangement in such a way that the c-axis is always perpendicular to the shell surface. The organic material forms films through the plates. We observed for a very first time some aragonite nanocrystals embedded in the organic matrix. This observation supports the hypothesis that the proteins and other organic compounds guide the crystal growth because the organic matrixes are the places where the nanocrystals grow. I. INTRODUCTION
The study of nature-made composite materials (biomineralization) offers an alternative way to develop controlled synthesis methods for new nanostructured materials with potential applications. In this direction, several authors have aimed their efforts at studying selfassembly in natural materials—the case of mollusk shells being one of the most interesting examples. However, most1–4of them have only 5–8 focused on the study of gastropod and bivalve nacre whose main components are aragonite, and some organic compounds such as poly9–13 anionic proteins and glycoproteins. For better understanding of how these compounds work in the natural crystal-growth process, some studies have also been made on the extraction, purification, and use of the proteins 1,11–13 from abalone shells to grow CaCO3 crystal in It has been known that organic components vitro. play an important role in the shell structure, not only in controlling the crystallization of CaCO3 into aragonite instead of calcite, which is more stable at low tempera14 tures and pressures, but also in guiding and ordering the aragonite crystals to form an alternating inorganic– organic layer structure of shell. However, the mechanism a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0190 1484
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 6, Jun 2006 Downloaded: 18 Mar 2015
FIG. 1. Shell inorganic component identification. Comparative x-ray diffraction patterns of the shell, (bottom) grid sample, and (top) the outer surface. More intense peaks are plane-indexed. Notice the remarkable preferential crystal orientation along the [002] direction. © 2006 Materials Research Society IP address: 169.230.243.252
R. Velázquez-Castillo et al.: Nanoscale characterization of nautilus shell structure: An example of natural sel
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