Unveiling the Formation Mechanism of Pseudo-Single-Crystal Aragonite Platelets in Nacre

We demonstrate direct evidence that a single-crystal-like aragonite platelet is essentially assembled with aragonite nanoparticles. The aragonite nanoparticles are readily oriented and assembled into pseudo-single-crystal aragonite platelets via screw dislocation and amorphous aggregation, which are two dominant mediating mechanisms between nanoparticles during biomineralization. These findings will advance our understanding of nacre’s biomineralization process and provide additional design guidelines for developing biomimetic materials.

Figure 1

Structural characterization of nacre. (a) TEM image of nacre’s cross section, showing a brick wall-like architecture with aragonite platelets sandwiched with organic biopolymer interlayers. (b) Electron diffraction pattern of aragonite platelets, exhibiting single-crystal diffraction characteristics.

Figure 2

Observation and analysis of screw dislocation. (a) TEM image of two aragonite platelets with an organic biopolymer interlayer and a mineral bridge. (b) HRTEM image of the boxed area in Fig. 2a, showing a screw dislocation lining two adjacent particles. (c) FFT pattern of the image in Fig. 2b. (d) Close-up view of the atomic arrangement of the interface between the two adjacent nanoparticles. (e) Schematic showing that crystallographic planes linking the two adjacent nanoparticles are determined to be aragonite (042) planes. (f) Screw dislocation is visualized by considering slipping across the two adjacent nanoparticles by one and a half of lattice vector $\frac{3}{2}[010]$.

Figure 3

Pseudosingle crystal effect. (a) HRTEM image of an aragonite platelet. (b)–(e) Close-up views of boxed areas b, c, d, and e in (a). $({\mathrm{a}}^{\prime })–({\mathrm{e}}^{\prime })$ Respective FFT patterns of (a)–(e). $({\mathrm{a}}^{\prime \prime })–({\mathrm{d}}^{\prime \prime })$ Schematics of FFT patterns from corresponding images showing $({\mathrm{b}}^{\prime \prime })+({\mathrm{c}}^{\prime \prime })+({\mathrm{d}}^{\prime \prime })=({\mathrm{a}}^{\prime \prime })$.

Figure 4

Nanoparticle assembly characteristics. (a) HRTEM image of an aragonite platelet. (b) Close-up view of two nanoparticles attached to the assembled aragonite platelet.