Synthesis And Assembly Of Carbonate Minerals With Hierarchical Micro/Nanostructures And Implication For Biomineralization

Biominerlization in nature is a common phenomenon.Numerous living organisms form biominerals.The nucleation and growth of mineral crystals formed by organisms in the course of biomineralization are precisely controlled,and therefore these crystals are usually exquisitely shaped and are significantly different in their appearance from their counterparts of inorganic origin.These features have been regarded as one of the criteria for the mineral biogenic origin.Therefore,biogenetic minerals can be used as the biological hallmarks for tracing the evolution of microbial ecosystem in geological environment,and searching for the information of early terrestrial life origin and extraterrestrial life.In this dissertation,in the absence of organic/biomoleculs,a series of carbonate minerals including aragonite,witherite,strontianite and vaterite with hierarchical micro/nanostructured architectures were synthesized or assembled in control.Microstructure analysis and characterization for these synthesized minerals indicate that the formation of the hierarchically assembled superstructures can be controlled by the intrinsic anisotropy dipole-dipole interactions between the assembled subuints,and the intrinsic dipole-dipole interactions from the crystals can play an important role in biomineralization and natural mineralization.Therefore,the intrinsic dipole-dipole interactions should contribute the formation of hierarchical structures of biominerals.Some important results can be summarized as follows:1.The microstructure investigations for nacre layers in molluscan shell have revealed that the nacre layers are alternatively stacked by aragonite tablets with a pseudo-hexagon and organic metrix,such that the cross section resembles a brick-mortar wall.Moreover,the aragonite nanocrystals in a pseudo-hexagonal aragonite tablet and the aragonite tablets in a nacre layer share the same crystallographic a,b,c orientations of aragonite crystal.Nevertheless,the theories or models proposed so far cannot explain the biofabrication of this architecture.Here,highly organized aragonite rods can be prepared over a broad pH ranging from 1.5 to 6.9 and in the absence of any bio-or organic macromolecules.The organized rods were characterized by XRD,FTIR,FESEM,TEM,SAED,and EDX techniques.FESEM results reveal that the mesoscale aragonite rods are not only assembled with aragonite micro rods by end-to-end,and side-to-side,but are also partially fused to one another,forming flat,faceted surfaces,i.e.,mesocrystal structure.TEM and SAED analyses confirm that the organized rods have the same crystallographic symmetry as single-crystal aragonite,and thus the self-assembly process is energetically favorable.Similar assembly process also occurs for the mineral strontianite and witherite of the aragonite group,revealing the occurrence of a general self-assembly process.The driving force controlling the self-assembly process may originate from the inherent anisotropic dipole-dipole interactions between the assembled units.Such dipole interaction may generally occur in biomineralization of nacre layers in molluscan shell,and orchestrate aragonite nanocrystals in an aragonite tablet and aragonite tablets in a nacre layer to coherently orient and array.Furthermore,the dipole-dipole interactions may also contribute to the co-orientation of the aragonite tablets in the same nacreous column,and are potentially responsible for the amorphous calcium carbonate-assisted rotation of the nanoaragonite in an aragonite platelet.Therefore,the experimental results may provide insight into biomineralization mechanisms.It appears that biological genetic and crystallochemical factors may synergistically operate in biomineralization.2.In order to donate further proof to support the dipole driven self-assembly model,witherite,which is isostructural with aragonite,was selected as a model mineral to examine the conception.Herein,nanoscale witherite rods(nanorods)were first synthesized by use of dimethyl sulfoxide(DMSO)as a stabilizer,and then the completely washed up nanorods were redispersed in deionized water to obtain organized mesoscale witherite rods(mesocrystals or mesorods)at ambient temperature.The nano-or meso-rods obtained under different conditions were characterized by a range of techniques involving XRD,FTIR,FESEM,TEM,SAED,and HRTEM.The SEM,TEM and SAED results demonstrate that the witherite nanorods can be spontaneously organized into mesorods in deionized water at ambient temperature via an three dimensional oriented attachment growth process,indicating that the intrinsic anisotropic dipole-dipole interactions between the assembled nanorods should be responsible for the self-assembly of the nanorods into the mesorods of witherite.Therefore,the results can provide a direct examination for the dipole driven self-assembly,and that dipole-dipole interactions can reasonably account for some aspects of formation of hierarchical biominerals.3.Vaterite is a thermodynamically unstable polymorph of calcium carbonate among the three anhydrous polymorphs(calcite,aragonite and vertire).Herein,vaterite with different morphologies was successfully synthesized by a simple injection-precipitation method.The experiments were performed at 37 and 25 ?using pH values of 1.5,3.0,and 6.9.At 37 ?,precipitated vaterite has spindle-like morphology at the low pH 1.5 of the initial CaCl2 solution,and shows coexistence of the spindle-,spheroid-,and cauliflower-like morphologies at the intermediate pH 3.0,whereas it is spheroidal at pH 6.9.SAED analyses revealed that the spindle-like vaterite superstructures were self-assembled by the oriented aggregation of vaterite microcrystals along the crystallographic c direction.The formation of the ordered spindle-like aggregates of vaterite is still controlled by the inherent dipole-dipole interactions along the crystallographic c direction of vaterite crystal,conforming that the dipole interactions should generally occur in natural mineralization including biomieralization.At 25 ?,however,the low pH(1.5)led to coexistence of cauliflower-and spheroidshaped vaterite,whereas spherulite-like vaterite was always obtained at either pH 3.0 or 6.9.The results show that simple inorganic precipitation processes lead to complex and unusual morphologies with hierarchical structure.Therefore,some caution must be taken when morphological criteria are claimed as proof for biogenic origin of minerals.Moreover,time studies of the polymorphic transition revealed that,although solution-mediated dissolution of precursor vaterite and reprecipitation of secondary calcite always occur,Ostwald ripening finally contributes to the formation of rhombohedral calcite.Therefore,the polymorphic transition sequentially proceeds from vaterite through irregular calcite aggregates to stable calcite rhombohedra.