Growth of Nacre Biocrystals by Self-Assembly of Aragonite Nanoparticles with Novel Subhedral Morphology

Abstract

Nacre has long served as a research model in the field of biomineralization and biomimetic materials. It is widely accepted that its basic components, aragonite biocrystals, namely, tablets, are formed by the nanoparticle-attachment pathway. However, the details of the nanoparticle morphology and arrangement in the tablets are still a matter of debate. Here, using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), we observed the nanostructure of the growing tablets at different growth stages and found that: (1) the first detectable tablet looked like a rod; (2) tablets consisted of subhedral nanoparticles (i.e., partly bounded by crystal facets and partly by irregular non-crystal facets) that were made of aragonite single crystals with a width of 160–180 nm; and (3) these nanoparticles were ordered in orientation but disordered in position, resulting in unique subhedral and jigsaw-like patterns from the top and side views, respectively. In short, we directly observed the growth of nacre biocrystals by the self-assembly of aragonite nanoparticles with a novel subhedral morphology.

Polyamines Promote Aragonite Nucleation and Generate Biomimetic Structures

“…Elongated crystals ≈ 250 nm in size and that exhibited granular structures were observed within the first 30 min of crystallization (Figure 3d,e) and then developed into spindle‐shaped crystals comprising faceted nanoparticles (Figure 3f) and ultimately crystals with wheatsheaf morphologies (Figure S8, Supporting Information). These subsequently evolved into beautiful structures comprising well‐defined hexagonal aragonite platelets [ 32,33 ] co‐oriented along their [001] axes (Figure 3g–i). The stacks of platelets appeared to be preceded by fibrillar structures.…”

Biomimetic Hierarchical Superstructures: Approaches Using Bicontinuous Microemulsions and Electrodeposition

“…The tablet-like shapes show diameters of around 200 nm, which corresponds to the mesoscale. Furthermore, these inorganic tablets assemble up to the microscale (or even at higher scales in some species), forming a composite with some biopolymers (Gao et al, 2019). The SEM image on the left in Figure 10 shows a composite containing 5 (wt%) of a polymer and 95 (wt%) of ceramic material (as in natural nacre) prepared by EPD, starting from an acrylamide-modified montmorillonite (MMT) aqueous suspension.…”

Nacre toughening due to cooperative plastic deformation of stacks of co-oriented aragonite platelets

“…In many shells, the outer layer is composed of calcite prisms and the inner layer-nacre-has a "brick-andmortar" structure, where~500 nm thick aragonite (CaCO 3 ) platelets (>95% of mass) are interspersed with an organic matrix, mainly composed of elastic biopolymers, such as chitin and proteins 9 . Each platelet behaves as a single crystal 10 , and only small misorientation was found owing to nanoparticles and twinning domains [11][12][13] .…”

“…Despite numerous studies on nacre toughening mechanisms at both macroscopic and submicron scales [6][7][8][9][10][11][12][13][14][15][16][17] , the potential mechanical contribution of aragonite stacks (i.e., crystallographically co-oriented stacks of platelets; 10-100 µm scale) 10,[46][47][48][49][50] , have yet to be explored. A hierarchical structure ("supertablet") with a characteristic length of about 10 µm was found to contribute to the material's toughness, but the structural characteristics that distinguish each supertablet were not identified 21 .…”