Amorphous solid

Mechanism (biology)

Gene

Quantum mechanics

Transformation (genetics)

Phase (matter)

Engineering

Transmission electron microscopy

Nanoparticle

Biology

Metallurgy

Materials science

Calcium

Solubility

Metastability

Recrystallization (geology)

Biochemistry

Electron microscope

Chemical engineering

Phosphate

Dissolution

Paleontology

Physics

Nanotechnology

Organic chemistry

Crystallization

Optics

Nanomaterials

Amorphous calcium phosphate

Crystallography

Chemistry

Biophysics

Crystal Growth & Design

Crystallization via phase transformation of a metastable precursor is a ubiquitous and effective strategy used by living systems to direct the growth of crystalline nanomaterials with remarkable functional properties. However, determining the exact process by which transformation occurs at the nanoscale is a difficult challenge. Here, the recrystallization process of amorphous calcium phosphate (ACP) to hydroxyapatite (HAP) is explored by liquid-cell transmission electron microscopy. The effect of confinement in the liquid-cell is found to increase the size of ACP nanoparticles. In the presence of Mg2+, these large ACP nanoparticles transform to HAP by first dissolving from the interior to create a hollow structure, after which HAP forms preferentially on the surface and then subsequently in the bulk solution. We propose that the preferential dissolution within ACP particles is due to a change in the structure and/or chemistry of the ACP surface, likely associated with dehydration before crystallization of HAP. These results imply an important role of the confined environment of the liquid-cell in regulating the size of ACP particles, which then affects the surface structure and the detailed dissolution–recrystallization pathway. Moreover, we stress the key role of Mg2+ in controlling HAP formation by stabilizing ACP via reduction in ACP solubility. This work provides a better understanding of the roles of additives and confinement during the phase transformation of ACP to HAP through dissolution and recrystallization.

Phase Transformation Mechanism of Amorphous Calcium Phosphate to Hydroxyapatite Investigated by Liquid-Cell Transmission Electron Microscopy