| In nature, biomineralization is a common phenomenon. Biominerals usually possess unique crystallographic morphologies and complex assembled architectures, which are significant different from their counterparts of inorganic origin. Calcium carbonate, as the most widely occurring biominerals among the more than70known different biogenic minerals known so far, has obtained considerable attentions of various fields including the material science, mineralogy, geology. A common consensus is that organic molecules induce the nucleation of special polymorph and control the unique morphogenesis of CaCO3crystals. In this dissertation, several organics are selected as model mineralized modifiers to influence the crystallization and growth of CaCO3under the conditions approximate biomineralization. Furthemore, it is noteworthy that the aragonite mineral itself contains almost no magnesium in its lattice. Nevertheless, Mg2+ions have been detected in various biogenic aragonites such as coral or bivalve shell. Therefore, the the aragonite precipitates were biomimetically formed from solutions with Mg2+ions and organics (sodium alginate and carboxylation chitosan) to explore the mode of Mg2+incorporation in aragonite. Several important results of this dissertation were summarized as follows:1. The biomimetic growth of CaCO3was carried out by a classic CO2gas diffusion technique, and sodium citrate (SC) and sodium dodecyl benzene sulfonate (SDBS) were selected as model organic additives to influence the crystallization and growth of CaCO3. Citrate with three-COO-groups has been identified as playing critical roles in interfering with crystal thickening and stabilizing apatite nanocrystal sizes in bone, and SDBS is an anion surfactant with a head group of sulfonic group. Therefore, they may mimic the mineralization of certain functional groups in macromolecules associated with biomineralization. FESEM and XRD analyses show that well-defined hexagonal prisms of vaterite mesocrystals stacked by hundreds of nanoflakes subunits have been successfully achieved in the presence of SC and SDBS. The results of TEM and SAED confirm that the vaterite hexagonal prisms are mesocrystals. Moreover, no hexagonal prism structures can be produced only with SC or SDBS, indicating that the cooperation of SC and SDBS is indispensable to the formation of hexagonal prismatic vaterite mesocrystals. Due to the remarkable resemblance of the vaterite hexagonal prism-like mesocrystals to the vaterite nacreous layers in freshwater cultured pearls or to the columnar/lamellar vaterite in the bivalve, current results may bring new insights into the biomineralization.2. A series of PASP-Mg-ACCs with different Mg2+contents were first biomimetically synthesized in the presence of polyaspartic acid (PASP) and Mg2+, and then their phase transformation processes were studied under different medium conditions. The goal of this study is to examine the role of biomolecule and Mg2+-regulated ACCs in carbonate mineralization and to reveal the possible biogenesis of MHC. The phase transformation experiments show that the PASP-Mg-ACCs with Mg2+contents lower than24mol%were transformed into magnesium calcite and aragonite, and MHC with a high Mg2+content was exclusively derived from the PASP-Mg-ACC with24.71mol%Mg2+. A series of time-resolved experiments unveiled that phase transformation experiences start with the dissolution of PASP-Mg-ACCs and the succeeding crystallization of the secondary mineral phases, and the PASP and Mg2+contents in the precursor PASP-Mg-ACCs are believed to be the crucial factors that control the secondary mineral phases. In addition, the PASP-Mg-ACC with24.71mol%Mg2+and MHC yielded very similar Ca L-edge NEXAFS features, indicating that the short-range structures of the PASP-Mg-ACC with24.71mol%Mg2+are the most similar to MHC. Therefore, PASP-Mg-ACC with24.71mol%Mg2+can transform to MHC. Furthemore, the PASP-Mg-ACCs with <24mol%Mg2+were most and least transformed into aragonite and magnesium calcite but not MHC, even in the high Mg2+transformation media, indicating that the Mg2+content in ACC is not the only reason for polymorph selection. The different structures of ACCs caused by PASP and Mg2+also have a significant impact on the ultimate products. Therefore, the formation of biogenic MHC may not only be regulated by the Mg2+but also the biomolecules in the mineralization system. This study may provide new insights into the biomineralization and natural origin of MHC.3. To explore the mode of Mg2+incorporation in aragonite, the aragonite precipitates were biomimetically formed from solutions with Mg2+ions and organics by a method of Ca(HCO3)2decomposition. Polysaccharides (sodium alginate and carboxylation chitosan) were selected as model organic additives to influence the crystallization and growth of CaCO3, respectively. These polysaccharides were chosen for their prevalence in biocalcification environments. As a consequence, the Mg2+ions have been detected in the aragonite obtained in the presence of organics, whereas almost no Mg2+is contained in the aragonite obtained without organics. Moreover, the Mg2+content in aragonite increases gradually with the increase of the content of organics. The TG analyses suggest that the Mg2+content in aragonite is direct proportion with organic content. The Mg K-edge NEXAFS spectra are applied to obtain information on magnesium environment in aragonite, and result shows that the magnesium environment in aragonite is the most similar to the organic-type environment. Therefore, it is reasonable to speculate that Mg2+ions are not in aragonite lattices but adheres to the organics. In addition, Mg2+ions are still existed in aragonite even at high temperature, suggesting that Mg2+ions in aragonite are not hosted in a highly disordered ACC or adsorbed on the surface of aragonite as impurities. Furthemore, the influences of Mg/Ca ratio and the Mg2+content in reaction solution on the content of Mg2+in the as-prepared aragonite samples are also investigated and discussed. The Mg/Ca ratio and the Mg2+content in reaction solution have a significant influence on the Mg2+content in aragonite, reiterating the controls of microenvironment on biomineralization of aragonite. These findings are relevant to present-day mineralogical distributions in carbonate systems and might be the key to elucidate the environmental conditions responsible for mineralogical changes through geologic time.4. The composition, structure and morphology of biomineral are not only regulated by the certain functional groups in macromolecules associated with biomineralization, but also regulated by the structure of biomacromolecular. Here, the the F68was chosed as model organic additive to study the conformation changes of F68on the influence of CaCO3crystallization and growth. The vacuum freeze-drying is used to influence the F68molecular conformation. The DLS analyses confirm the average size of F68treated by freeze-drying is nearly twice as large as that of the untreated one, indicating that the freeze drying treatment significantly enhances the hydrophobicity of the triblock copolymer F68so that an obvious aggregation occurs among the treated F68molecules. Therefore, the freeze-drying treatment really has an obvious influence on the microstructures of F68polymer. Moreover, a set of experiments with several-cycle freeze-dried F68were carried out to investigate the influence of freeze-drying treatment of F68on the crystallization and growth of calcite. The results show that different freeze-drying treatments for F68lead to the different influence of the F68on the crystallization and growth of calcite. Therefore, our experimental results suggest that the nonionic-C-O-C-group (ether or glycosidic group) in the biomineralization-associated biomacromolecules of glycosidoproteins may contribute to the special morphogenesis of the CaCO3, and the change of F68molecular conformation may obviously influence the crystallization and growth of calcite. |
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