| Mollusk shell and pearl are well-known examples of biomaterials that exhibit distinctive optical behavior and an orders-of-magnitude increase in toughness and strength over their predominant constituent material:CaCCO. These remarkable properties have attracted scientists to study the architecture and nucleation mechanism of shell. However, the growth mechanism and optical effects of pearl are still not clear. The structure and optical effects of pearl have been studied by micro-techniques in this work and the formation mechanism of pearl has been discussed. What’s more, synthesis of CaCO3through CaCl2and (NH2)2CO solution was also tried.First, the initial formation stage and succedent biomineralization of pearl were studied using scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and atomic force microscopy. Another initial formation phase with needle-like structure which is found to be nanocrystallites of aragonite was discovered. As a result, two possible formation modes are proposed to describe the initial formation stage of pearl. As for the succedent mineralization of "brick and mortar" structure, nanostripes were first discovered inside the "brick"(aragonite platelet), compared with the foregoing finding of nanograins. The various nanostructure of aragonite platelet allow us to reconsider the role of the inter-and intracrystalline organic material surrounding CaCO3, and a possible biomineralization mechanism was proposed.Second, the relationship between optical behavior (optical effects, body color and structural color) and microstructure of pearl was studied by laser scanning confocal microscopy, high-resolution scanning electron microscopy and atomic force microscopy.There are three optical effects in pearls, i.e., a) the reflection-diffraction effect; b) a chromatic diffusion halo; c) the whispering gallery effect. From the observation for the structure of pearls, we proposed that the first optical effect exhibited by pearls is dominated by the surface diffuse-reflection. In addition, the multilayer interference and diffraction have little contribution. With the delicate observation for the structure of pearl by atomic force microscope, it is revealed that the polygon platelets of pearl are not single crystals but nano-grains of aragonite in the network of organic matrix, which is responsible for the chromatic diffusion halo. As for the third optical effect, it originates from the stratifications structure which is composed of aragonite platelets. When the light falls on the surface of pearl, it will travel along the laminations of its structure rather than through the whole pearl, so the rim of pearl is bright even if no light falls on the surface directly.In order to find out the relationship between body color and structure for pearl, four fresh-water cultured pearls with different colors were measured and observed by Raman spectroscopy and high resolution scanning electron microscopy. It was found that the morphologies of the exterior surface and fracture profile for the four pearls showed different characteristics. The surface of white and yellow pearls presents trim growth steps, but the growth steps of violet and purple pearls are disordered. Especially for the purple pearl, not only the growth steps are irregular, but also the surface owns more bunching steps and defects than others. As for the fracture surface, we found that the aragonite platelets thicknesses of white and yellow pearls were in the range of200nm-400nm whereas the aragonite lamina of violet and purple pearls ranked more densely and the thicknesses were in the range of100nm-300nm. After etching by EDTA, it was discovered that there is organic matrix left between the lamina. The violet pearl was the easiest one to be etched by EDTA, and the purple pearl was the most difficult one, so it can be concluded that the purple pearl was very dense. That’s why the purple pearl owns the best luster in the situation of bad exterior surface.We also investigated the structural color of a purple pearl that exhibits angle-dependent yellow-green iridescence. Surprisingly, a hierarchical hexagonal photonic crystal structure was discovered on the pearl surface through HRSEM observation. A finite-difference time-domain calculation was performed on this structure, and also a multilayer structure which was located beneath the surface layer. The calculations show that the nested hexagonal photonic crystal structure is actually the origin of the yellowish-green iridescence of the pearl. We investigated the effect of environmental conditions on the synthesis of CaCO3through CaCl2and (NH2)2CO solution. It was found that more aragonite can be synthesized at around80℃and PH=8. However, it is hard to obtained pure aragonite.By adding ethyl silicate to CaCl2and (NH2)2CO solution, the structure of CaCO3can be tailored by controlling the hydrolytic polycondensation and synthesis procedure. At room temperature(22℃, clusters of calcite platelets in peony shape were formed with stirring; but at80℃, chrysanthemum-like aragonite clusters were obtained. Stable amorphous calcium carbonate supported by mesoporous silica gel was successfully synthesized. The silica gel support is prepared through the hydrolytic polycondensation of ethyl silicate under suitable conditions. LSCM observations reveal that the morphology of the product is branched with cruciform-like and flower-like structure. Raman spectroscopic analysis and SEM observation for the products confirm the combination of stable ACC nanoparticles and mesoporous silica gel. A possible growth mechanism for the branched structure has been proposed. Results indicate potential application of this work to ACC storage, crystal engineering, biomimetic synthesis, etc. |
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