Aqueous ball milling of nacre constituents facilitates directional self-assembly of aragonite nanoparticles of the gastropod <i>Haliotis glabra</i>.

Abstract

A ball-milling approach was developed to investigate the constituents of isolated nacre tablets of the gastropod <i>Haliotis glabra</i> in aqueous suspension without additional chemical additives. The obtained particle mixtures were characterized using X-ray crystallography as well as scanning and transmission electron microscopy. Aragonite nanoparticles retained their crystal structure even after 14 h of ball milling. The long-term stability of the particle mixtures varied as a function of the ball-milling duration. An increased milling time led to rod-like stable assemblies of aragonite nanoparticles. Selected area electron diffraction investigations revealed that the longitudinal axes in about one-third of these nanoparticle rods were oriented along the crystallographic <i>c</i>-axis of aragonite, indicating oriented attachment of the aragonite nanoparticles. These <i>in vitro</i> observations support the idea that a two-stage process, separated into crystallization of nanoparticles and oriented assembly of nanocrystals, could also occur <i>in vivo</i>.