Study Of Biomimetic Minerialization Of Calcium Carbonate Regulated By Model Organic Molecules

Biogenetic minerals possess unique morphologies and complex architectures due to the precise regulation in the mineralization by the biomacromolecules in vivo and the growth of organisms. In this dissertation, nonionic pluronic triblock copolymer of F68, carboxylate-enriched sodium citrate and phosphatidylcholine（PC） were respectively served as model mineralized modifiers to study the effects of these modifiers on the polymorph selection and the formation of morphology of calcium carbonate via a biomimetic mineralization method. In addition, flower-shaped vaterite dendrites were successfully prepared by selecting suitable organic additives as the modifiers. Some important results were summarized as follows:1. A pluronic triblock copolymer of F68 was selected as model organic additive to influence the crystallization and growth of calcium carbonate by a biomimetic mineralization process. The results demonstrated that the model organic macromolecule of F68 not only preferentially interacted with the particular faces of calcite crystals to form elongated microcrystals, but also induced the oriented aggregation of these microcrystals along the crystallographic c direction into calcite prisms, i.e., forming mesocrystal architectures. In addition, a series of time-resolved experiments showed that the initial precipitate in the presence of higher concentration of F68 was unstable amorphous calcium carbonate（ACC）, and this transient precursor phase eventually transformed into prismatic mesocrystals of calcite via a mesoscale transformation process, which displays the same biomineralized sequence and features with the biomineralized process of biogenetic CaCO3. Although F68 only contains mass of ether-oxygen groups and a terminal hydroxyl group, its role played in the regulation of calcium carbonate crystallization process is considered analogous to an array of biomineralization-associated biomacromolecules such as some glycosylated proteins. Furthermore, the results of a series of control experiments via the replacement of F68 by ethylene glycol displayed that the minerallized products were calcitic rhombohedra even though the volume ratio of ethylene glycol was increased to 20% in the mineralized system. This indicated that the hydroxyl groups had no effect on the formation of the particular morphology of calcite. Hence, our experimental results suggest that apart from the polar carboxyl, nonionic -C-O-C- group （ether or glycosidic group） in the biomineralization-associated biomacromolecules of glycosidoproteins may not only influence the formation process but also contribute to the special morphogenesis of the biominerals. As such, these results provide another pathway towards a deeper insight into biomineralization mechanism.2. Multi-carboxylic sodium citrate was applied as the model acidic molecule to influence the mineralization of calcium carbonate. The results showed that with a low concentration of sodium citrate in the mineralized system, ACC nanoparticles were first formed and temporarily stabilized in the mineralized system, and then these ACC nanoparticles transformed into calcite with rod-like morphology. These calcite rods aggregated and finally evolved into hemispheres or spheres. With a high concentration, sodium citrate could stabilize the ACC nanoparticles for a long time and simultaneously made ACC nanoparticles assembly into spherical particles. In the following mineralization process, ACC nanoparticles gradually transformed into spheres with nuclear-shell structures, and the nuclei were composed of ACC nanoparticles while slim calcitic rods constituted the shells. The similar experiment by using of sodium tartrate to replace sodium citrate also show that sodium tartrate with a high concentration was capble of inducing the formation of ACC in the initial stage of mineralization as well. These results indicate that in biomineral formation process, some polar groups such as carboxyls in the biomineralization-associated biomacromolecules may not only contribute to the formation of amorphous precursor phases of the biominerals but also to the influence of the morphogenesis of the crystalline biominerals which transform from the amorphous precursor phases. These results provide us a deeper insight into the significances of the acidic proteins in vivo in the regulation of the formation process of biominerals.3. By use of phosphatidylcholine（PC） as the modifier of mineralization process, CaCO₃ was biomimetically synthesized in aqueous solution. The results showed that PC is capable of inducing the formation of the unusal anhydrous ACC and influencing the morphology of calcite which tansformed from ACC in the follow-up mineralization process. With a lower concentration of PC in solution, PC was insufficient to stabilize transient ACC which formed in the initial stage of mineralization, therefore, ACC was not detected in the as-obtained products. However, PC was capable of making calcitic crystals to form porous surface morphologies by the interaction between the polar phosphoric ester groups and the surfaces of calcite. With a higher concentration of PC in solution, PC can induce the formation of ACC and temporarily stablize this transient phase of calcium carbonate using the negatively charged phosphoric ester groups and affect the morphology of the mineral in the follow-up process of ACC transformed into calcite.These results indicate that PC is able to induce the formation of transient anhydrous ACC and influence the morphology of calcite which from the transformation of ACC. Since the great mass of biominerals undergo phase transitions from amorphous precursors into crystalline states, PC can stabilize ACC with certain concentration and affect the morphogenesis of CaCO₃4. Flower-shaped vaterite dendrites were successfully synthesized by a rapid microwave-assisted heating method. Vaterite is a thermodynamically unstable phase of calcium carbonate, so its formation is often controlled by kinetic factors. Recent studies of the mineralization of biogenetic calcium carbonate show that a large number of biological processes are capable of forming stable vaterite, which may indicate that the biological macromolecules can stabilize metastable vaterite. In addition, rapid microwave heating enables the temperature of the heated material to increase rapidly in a very short period of time without temperature gradient, which tends to favor the formation of the metastable phase, namely the kinetics control phase. Based on these thoughts, we successfully prepared pure flower-shaped vaterite dendrites through the coordinating process of by rapid microwave heating and suitable organic additives. The results showed that the rhombohedral calcite was the exclusive product when deionized water was applyed as the solvent; however, rhombohedral calcite and a spot of vaterite with planular discoid morphology were obtained by adding a small amount of ethylene glycol in the reaction system. With the introduction of 2-naphthyl oxygen acetic acid as an organic additive which contributes to the formation of metastable vaterite into the reaction system, the mixed phases which were composed of rhombohedral calcite, ball and flower-shaped vaterite were obtained in aqueous solution because of 2-naphthyl oxygen acetic acid poor solubility in water. However, with adding a small amount of ethylene glycol into the reaction system to resolve 2-naphthyl oxygen acetic acid, flower-shaped vaterite dendrites were obtained. In the formation process of metastable vaterite, 2-naphthyl oxygen acetic acid can apply the electronegativity of the polar carboxyl to effectively neutralize the charge of the high-energy （001） surface of vaterite and reduce the crystal surface energy. This results in the vaterite growth oriented along parallel to the , which is of great importance for the comprehension of the formation process of ACC and calcium carbonate with special morphologies in vivo. （001） plane and the formation of flower-shaped vaterite dendrites. Flower-like vaterite dendrites were successfully synthesized in this work and the formation mechanism of these dentrites were investigated as well.