Formation Mechanism Of Domed Platelets In Nacre

Nacre is a natural inorganic-organic composite material with superior structure and outstanding mechanical properties.However,it can be synthesized in the organism in the condition of room temperature and ordinary pressure,only by using calcium ion and carbanion from the environment.Therefore,nacre is an environmental material,which has long served as a classic model in biomineralization and the synthesis of biomimetic materials.To date,a lot of knowledge of nacre has been accumulated,but many questions still remain the subject of considerable debate,such as the morphology,arrangement,and inner structure of platelets(basic structural units of nacre)etc.In addition,nearly all the related knowledge comes from investigating mature flat platelets,neglecting the morphology and growth stage of platelets,which leads to deficiency of nacreous research.In our work,we focus on the platelets in the green mussel shells,particularly domed platelets with unique morphology.Using a field emission scanning electron microscope(SEM),high-resolution transmission electron microscope(HRTEM),X-ray diffractometer(XRD),differential scanning calorimetry(DSC),and Fourier transform infrared spectrometer(FTIR)etc.,we investigate the formation mechanism of domed platelets,including three dimensional morphology and arrangement,inner structure,nucleation and growth,crystallographic relationship and interface structure,and formation environment etc.The results are listed as follows:(1)Domed platelets and flat platelets coexist in green mussel shells,and the former is the product of the latter after deformation by absorbing force;(2)The platelets exhibit a gradient arrangement,including thickness and curvature.From the bottom to the top of nacre,the thickness of platelets varies from 0.84±0.01 ?m to 0.24±0.08 ?m,whereas the curvature varies from 0.77±0.12 ?m-1 to 0 ?m-1,indicating that the thickness and curvature change gradually,without any clear boundary.Based on observation,we hypothesize that the gradient in the thickness may probably result from the difference in growth rate between the edge and the center of the shell,whereas the gradient in the curvature result from the gradient in compressive stress imposed by the closing of the shells by the adductor muscles or the withdrawal of the periostracum by the mantle;(3)Flat and mature domed platelets are single crystals,whereas the immature domed platelets are aggregates of disordered aragonite nanoparticles.Among the immature domed platelets,the early immature ones consist of oval but partly crystalline colloidal nanoparticles with the diameter of 18-41 nm,whereas the late immature 'ones consist of fully crystalline colloidal nanoparticles with the diameter of 20-150 nm.We hypothesize that the single mature domed platelet forms via oriented attachment of ACC nanoparticles.(4)The new platelets always nucleate on the ring surface of the under platelet,and grow in a self-limiting manner.The horizontal growth rate of platelet is inconsistent and inversely proportional to the nucleation time.We hypothesize that the nucleation and horizontal growth rate of platelets are controlled by the lattice-mismatch-induced strain.(5)Domed platelets share the same crystallographic orientation,and their interlamellar membranes consist of ellipsoid crystalline protein with the diameter of 30-50 nm.Via a 10-20 nm thick superlattice region,the interlamellar membranes contact with platelets,then inherit and pass the crystallographic orientation of the under platelets,leading to the same crystallographic orientation of the whole platelets.(6)Domed and flat platelets are natural high-energy aragonite with 8 cm-1 and 12 cm-1 v2 frequency shift,which may due to the nanometer size effect,low degree of crystallinity,and lattice distortion.Compared with flat platelets,the energy of domed ones is relative low.which attribute to that domed platelets are the products of flat ones after deformation and realizing stress relief to a low-energy state.(7)There are differences in the structure between the ventral and posterior periostracum.The inner and middle layer possesses the thinnest and thickest thickness in the ventral periostracum respectively,and the latter consists of amorphous nanospheres with the diameter of 43-106 nm.While in the posterior periostracum,the thickness of structural layers increases from the outer to the inner,and the middle layer consists of fibers with the diameter of 178 nm.The nanospheres in the middle layer of the ventral periostracum consist of a small amount of protein,Ca,Mg,CO32-and amorphous phosphate,which may facilitate the mineralization of nacre for it can not only provide the raw materials but can stabilize the pH in the environment;(8)The nipples,inserting into the prismatic layer to form an unusual interlocking interface that resists sliding between the periostracum and prismatic layer,consist of proteins and usually display a paraboloid shape with an average base diameter of 2.5 ?m and height of 4.6 ?m,and act as the initial mineralized substrate instead of the periostracum,which may facilitate the mineralization for providing about 4 times larger surface area compared with the periostracum.Based on observation,we hypothesis that the nipples may probably result from the compressive stress imposed by the closing of the shells.In our work,we reconstruct the three dimensional morphology of domed platelets and establish the gradient arrangement model of platelets,and put forward the unclassical crystallization theory of ACC' oriented attachment.In addition,we directly observe the interface structure of domed platelets in bivalve shells for the first time,finding that crystalline interlamellar membranes contact with platelets via a superlattice region,then inherit and pass the crystallographic orientation of the under platelets.The findings will shed new light on the formation mechanisms of nacre and may help to design nacre-like materials.