The Effects And Their Mechanisms Of Biomacromolecules During The Crystallization Of Calcium Carbonate

The biomineralization process provides unique inspiration to the design of man-made materials, as illustrated by the mechanical properties of many biominerals through their superior hierarchical organization to fulfill important biological functions. One of the most striking biomaterials, nacre has encouraged researchers to investigate the formation mechanisms involved which have found to be highly regular "brick-and-mortar" arrangement of aragonite tablets. As nacre is comprised of a small percentage of?-chitin, silk-fibroin-like proteins and acidic biopolymers, many approaches have been carried out to mimic the in vivo regulation action of these biopolymers in the growth process. These studies mainly include the exploration to the mutual interaction between organic/inorganic components within the formation process of natural shell and fabrication of hybrid materials with the similar morphology and structure to nacreous tablets through the assistance of other additives.Consensus favors that the initial assembly of organic components takes place before crystallization process, resulting in precise control over the morphology and polymorph of inorganic minerals, however, the detailed mediation mechanism still remains elusive. Therefore, the selected natural biomacromolecules, such as alginate and silk fiber proteins, have been used as the organic additives to synthesize nacre-like hybrid in this thesis, in order to investigate the involved organic/inorganic interaction, which would lay a foundation for the fabrication of compound materials with excellent mechanical properties.Accordingly, it is well known that polysaccharides have great effects upon the formation of biominerals. Polysaccharides with negatively charged functional groups are studied to mimic the behavior of acidic amino acids in the formation process of nacreous platelets. We made use of alginate to mediate the growth of CaCO3 for its negative -COOH groups and ability to form gel in the form of'egg-box'when combining with Ca2+. As the source of continuously and uniformly releasing alginate molecules and Ca2+, alginate/Ca nano-spherical gel was employed in the solution to induce the nucleation and growth of CaCO3. Time-resolved transmission electron microscope (TEM) was applied to study the crystallization at the very early stage of crystallization. It was found that the initially formed lens-like vaterite particles gradually dissolved from the middle of particle, and released alginate molecules and Ca2+ back into system. As reaction time increased, the released substances were involved in the next stage of crystallization of CaCO3, in the form of needle-like and shuttle-like aragonite particles sequentially depending on concentration of alginate molecules and Ca2+.'Egg-box' conformation of alginate and Ca2+ was considered as skeleton for the growth of such aragonite particles. Noticeably, shuttle-like aragonite particle was composed of'bricks'in size of several hundreds of nanometers, which were very similar to biogenetic nacreous layers in shells.Else, proteins in nacreous tablets, i.e., silk-fibroin-like protein and acid glycoproteins, are believed to play an important role in regulating the morphology and lattice structure of CaCO3 minerals. Therefore, silk fibroin (SF, one of the analogues of silk-fibroin-like protein) derived from silkworm silk of Bombyx mori, was applied to be involved into the crystallization process to investigate the mediation mechanism based on its sequence and conformation. Firstly, the crystallization process of CaCO3 mediated by the addition of SF at different crystalline stages was examined. During earlier stages of crystallization, time-resolved transmission electron microscopy (TEM) was applied to demonstrate that the crystallization of an amorphous precursor was based on randomly oriented domains. Different addition times of SF primarily led to two kinds of morphology of CaCO3, i.e. lens-like and multilayered vaterite. Additionally, the thickness or number of layers of such vaterite would increase with the delay of SF addition, ascribing to the control of SF over different basic units during the aggregation and reorientation process. It was found that those squeezed-out SF which probably resulted from the relatively weak interaction between SF chains and (001) plane of vaterite phase during the crystallization process could lead to the formation of oblate aggregates via vectorial assembly of units with consistent orientation (nanoparticle for lens-like vaterite or flake for layered vaterite) and inhibition to the growth of (001) faces of fused intermediates. For comparison, the crystallization process of CaCO3 regulated by poly (acrylic acid) (PAA) was observed by cryoSEM, presenting a "stepwise aggregation" pathway to form spherical polycrystals which may be attributed to strong electrostatic interaction between carboxyl groups in PAA chains and nanoparticles. Therefore, the extent of binding affinity between organic and inorganic substances was proposed to be relevant to the reconstructing process and the morphologies of final product.Moreover, we presented a detailed investigation into the role of SF in aqueous environment acting on the growth of CaCO3 at extremely slow crystallization speed over a wide component concentration. SF can be made to fold into regular?-sheets (mainly because of the motif of GAGAGS) and self-assemble to nano-fibrils spontaneously in an aqueous environment, showing a strong preference for the formation of the aragonite form of CaCO3 crystals and allowing fine control over the size and morphology of crystals. The aragonite phase could be generated via two different routes:direct growth or dissolution and recrystallization, depending on the concentration of Ca2+ and SF. Generally, lower concentration of Ca2+ and SF favored the formation of aragonite needles and their aggregates, of which the lattice structure of precursor was similar to that of organic matrix in natural shell. Higher concentrations of the components led to the formation of aragonite aggregates via a dissolution and recrystallization process through the intermediates of lens-like vaterite. Molecular modeling showed that the?-strand conformer of SF molecule has an excellent match with the ionic spacing in the aragonite (010) plane, which can promote growth along the (001) long axis of aragonite crystals. This synergy between SF and the aragonite phase may help our understanding of the function of organic matrixes involved in the biomineralization process and facilitate the fabrication of synthetic materials with the potential for high performance mechanical properties.Based on the preference of SF?-sheets to the aragonite phase, repeated spin-coating SF solution and depositing aragonite crystals could fabricate multilayered material with excellent mechanical property and superhydrophobic surface. The growth of single layer experienced gradually structural evolution from particles to needle units, along with the enhancement of hydrophobicity. The thickness and density of SF film could decide the growth direction of aragonite needle, and the mechanical property could be promoted through the increasing number of layers. Therefore, the orderly ranked needles and alternate arrangement of organic/inorganic phase endowed the mechanical properties of aragonite hybrid material approaching that of natural nacreous tablets.Furthermore, the spidroin (derived from the major ampullate silk of Nephila edulis spider) which has similar amino acid sequence as the silk-fibroin-like protein and Antheraea pernyi SF with primary sequence containing continual (A)n as well were used to mediate the growth of CaCO3. The spidroin could induce the deposition of aragonite while A. pernyi SF cannot, further demonstrating the preference selection of?-sheet conformation to the aragonite phase.