Direct Observation of Spiral Growth, Particle Attachment, and Morphology Evolution of Hydroxyapatite

The two main pathways for the growth of calcium phosphates are either via the addition of monomeric chemical species to existing steps or via the attachment of precursor particles. Although recent experimental evidence suggests that the particle-attachment pathway is prevalent, real-time observations for the relative contributions of monomer-by-monomer addition or attachment of particles to seed crystals remain limited. Here we present an in situ study of hydroxyapatite (HAP) (100) surface growth with long imaging times by atomic force microscopy (AFM). We observed that HAP crystallization occurred by either classical spiral growth or nonclassical particle-attachment from various supersaturated solutions at near-physiological conditions, suggesting these mechanisms do not need to be mutually exclusive. We provided, to our knowledge, the first evidence of time-resolved morphology evolution during particle attachment processes, ranging from primary spheroidal particles of different sizes to triangular and hexagonal solids formed by kinetically accessible organized assembly and aggregation. These direct observations of HAP surface growth provide mechanistic and kinetic insights into the complex biomineralization of bone and open a way for the synthesis of higher-order and morphology-controlled biomimetic materials made of precursor nanoparticles