Characterization And Biomimetic Synthesis Of Iogenic Fibrous Aragonite Aggregates

Calcium carbonate is one of nature’s most abundant biological materials, which widely exists in organism, such as skeletons, corals, and shells. It is also one of the most important inorganic materials in modern industrials and widely used in rubbers, medicines, new building materials and so on. In a variety of biological calcium carbonate, aragonite is the best choice for all kinds of catalysts and inorganic fillers because of its excellent property of machinery, chemistry and optics. So, the research and preparation of biogenic aragonite calcium carbonate materials are of great significance. Here, we analyze the chemical components of the ligament of C.nobilis and the periostracum of L.siebaldii by X-Ray Diffraction (XRD), Fourier Transform Infrared (FTIR) and Amino Acid Analysis. Then, we characterize the microstructure of them by Scanning Electron Microscopy (SEM) and Transmission Electron Microscope (TEM). After that, using the protein from the ligament of C.nobilis and bio-gel from the periostracum of L.siebaldii, we synthesize aragonite crystalline material via gas diffusion method at ambient condition, without any additives. What’s more, we characterize them by FITR, SEM and TEM, researching and imitating the forming process of them. Lastly, we discuss the mechanism of formation and the potential application value of starfriut-like aragonite aggregates which grown in the bio-gel.Main conclusions of this work are drawn as follows:(1) The aragonite fibers in the ligament are firstly found to be bead-like, not acicular or prismatic as previously reported. The mineral of ligament are mainly composed of aragonite fiber assembly which be covered with a small amount of protein films. The structure of the aragonite fibers (with diameter of 40-120 nm) is formed by a large number of attached nanoparticles along the long axis of the fiber and present a unique beaded-like. Studies have shown that the nanoparticles all are single aragonite crystals and their crystallographic c axis is oriented parallel to the growing orientation of the fibers ([001]).(2) We discover that there is a silvery white mineral layer on the inner surface of periostracum of L.siebaldii, which be confirmed to be aragonite (CaCO3). Analyzing by SEM and TEM, we demonstrate that there are two crystal aggregates on mineral layer; one is small spindle aggregates which grown on matrix, and the another one is big spherulitic aggregates (with diameter of 20μm-70μm) which grown on the spindle aggregates. Both of the aggregates are composed of aragonite crystals which be confirmed to be single crystals and covered with a thickness of less than 10 nm organic matter. The crystallographic c axis of aragonite crystals is also oriented parallel to the growing orientation of the fibers, and most of their cross section is long hexagon. According to all above analysis, we guess that the spherulitic aggregates are composed of the spindle aggregates by close packing theory.(3) With no additives and using the protein layer of ligament as matrix, we obtain pure aragonite crystals at ambient condition. The aragonite crystal, presenting bouquet-like, are formed by irregular hexagon fibers of aragonite. These fibers, which crystallographic c axis is oriented parallel to the growing orientation of fibers, are single crystals and very similar to the fibers in mineral layer of ligament. In addition, these fibers also are covered by organic matter. So, we infer that the organic matter play an important role in the formation of aragonite fibers.(4) Using the same method, we also obtain pure aragonite crystals by bio-gel from L.siebaldii. Comparing with above crystals structure, we first obtain starfriut-like aragonite aggregates, and these aggregates are formed by attached aragonite nanoparticles in the same crystallographic orientation.(5) We elucidate the formation mechanism of starfriut-like aragonite aggregates from three factors:1. Driving force. An increase in the bio-gel density leads to the increase of driving force which decides the growth mechanism and macroscopic morphology of crystals. Here, the growth mechanism is the adhesive-type, leading to perfectly symmetrical crystals.2. The structure of aragonite crystal. Almost all aragonite crystals are twinned growths of three individual crystals that form pseudo-hexagonal trilling. Thus, aragonite aggregates will be hexagonal.3. Directional adhesion mechanism. During the initial stage of crystallization, aragonite nanoparticles form needle-like crystals by oriented attachment. Crystals will parallel-axial grow to these needle-like crystals because their high surface energy and leading to form six edges of crystal aggregates. As a result, the starfruit-like crystals will be formed.