Capillary force and concentration gradient promote the bioprocessing-inspired formation of ultralong fluorapatite nanorods under confinement†

Abstract

Crystallization within small volumes of solutions rather than bulk solutions is a common phenomenon found during material synthesis and biomineralization processes. However, the driving forces for mass transport and crystallization in confined environments remain elusive. Herein, inspired by the intrafibrillar collagen mineralization process, we investigate the infiltration and crystallization mechanisms of fluorapatite (FAP) within confined channels by comparing anodic aluminum oxide and track-etched templates with different surface properties. The results demonstrate that similar to intrafibrillar collagen mineralization, capillary force, along with a specific interaction between the confined channel surface and mineral precursors, is the main driving force for the initial infiltration of liquid precursors into confined channels, leading to the nucleation of FAP nanocrystals on the surface of the channels. We elucidate the critical role of negatively charged polyacrylic acid in promoting the formation of liquid precursors for successful infiltration into confined channels and controlling crystallization kinetics within the channels. The formation of FAP nanorods, followed by further promoting ion diffusion via a concentration gradient, resulted from the lower local concentration surrounding the FAP crystals. Furthermore, FAP nanocrystals exhibit progressive alignment along the channel direction during the subsequent crystal growth stage, and ultralong FAP nanorods with a length of more than 25 μm could be obtained. The collective findings underscore the pivotal role of the structure and surface properties of nanoscale confined environments in controlling the infiltration and crystallization pathways of inorganic crystals and establishing a foundation for the controlled synthesis of biomimetic materials under confinement.