Biomimetic Synthesis Of Nanometer Size Aragonite Templated By A New Class Of Protein

As an important inorganic filler, crystalline calcium carbonate appears three polymorphs:calcite, aragonite, and vaterite. Among these, aragonite is widely used in many fields for its high strength, high modulus, and excellent heat resistance and thermal insulation. However, it is very difficult to prepare the pure aragonite at ambient conditions. Therefore, we try to synthesize the pure aragonite with biomimetic method, which is defined as a process by which the inorganic meterials are prepared with the organic matter such as proteins as templates.Here, using a new class of protein-the fibrous protein from Solen grandis ligament as a template, we firstly synthesized pure aragonite crystals with spindle, sheaf, spherical, flower-like, radical, and sheet superstructures via gas diffusion method at ambient conditions. These as-synthesized crystals were characterized by Fourier transform infrared spectroscopy, Scanning electron microscopy, X-ray diffraction, and their formation and growth mechanisms were discussed.The main contents we studied and conclusions obtained are as follow: 1. Using the fibrous protein from S. grandis ligament as a template, the pure aragonite crystals can be synthesized in wide conditions for this reaction system at ambient conditions, i.e. dilute sulfuric acid of0.1mol·L-1, NH4HCO3power of19.0g, CaCl2concentration of0.9-9.0mmol·L-1, solution height of10-30mm, reaction temperature of20-30℃, and crystallization time of3-12hours. While, almost pure rhombic calcites were obtained on cover slips (blank control), accompany with small amount of aragonites and vaterites.2. Influence of the L-aspartic acid on the formation of aragonite crystals was also studied. The results show that, aragonite crystals can not be induced by sole aspartic acid. On the contrary, most of crystals obtained are rhombic calcites.3. We established a new model of aragonite crystal growth, and proposed that the aragonite crystals grow through unusual splitting manner. That is to say, it is a process that nanocrystals formed firstly grow into spherical superstructures (spherulites) as a splitting manner.4. The aragonite formation mechanism was discussed from four aspects, i.e. composition and structure of the protein, reaction temperature, concentration, and height of calcium chloride solution. The results show that, the protein substrate is the main factor, controlling aragonite formation. Further analysis reveals that the protein is homologous to cytoskeleton protein, with β-sheet structure and high acidic aspartic acid (14.15%). Therefore, we believed that the aragonite polymorph is controlled by the distance between negative charges of β-carboxyl in aspartic acid of fibrous protein from S. grandis ligament, which nearly matches the distance between Ca2+ions in aragonite crystals when the protein employs β-sheet conformation. In other words, the machnism of aragonite formation is supported by lattice-matched.The findings of our researches are not only of great significant for further understanding the biomineralization mechanism of CaCO3but also provide a new but easy and energy-saving method for synthesizing pure spherical aragonite materials.