Transparent Aggregates of Nanocrystalline Hydroxyapatite

Abstract

Assemblies\nof nanoparticles into transparent aggregates have solicited strong\nresearch interest in the form of both crystalline or amorphous aggregates\nof nanoparticles. In the present work, we make short-range ordered\nseveral millimeter-sized transparent aggregates of citrate modified\ncalcium phosphate nanoparticles and discuss the mechanism of their\nformation. Microparticles of hydroxyapatite (HAP) nanocrystals and\namorphous calcium phosphate (ACP) were synthesized with citrate as\na growth and assembly modifier. Millimeter-sized transparent aggregates\nof these microparticles were made with 0 to 7.5% citrate/Ca²⁺. The degree of crystallinity, i.e., the ratio between nanocrystalline\nHAP and ACP in the microparticles, was determined by Rietveld refinement\nof powder X-ray diffraction data with an internal standard. It was\nfound to decrease with increasing citrate concentration. Citrate also\nreduced the nanocrystallite size at low citrate concentrations. Above\n∼3% added citrate, the crystallite size did not reduce further.\nTransparent aggregates were obtained by drying a suspension of particles.\nThe aggregates lacked long-range order and in many cases featured\nspiral fractures partially propagating through the aggregates. The\nassembly mechanisms were studied by in situ video imaging, polarized\noptical microscopy, transmission electron microscopy, and confocal\nmicroscopy. The transparent aggregates consisted of polydisperse microparticles.\nThe transparent aggregates form due to evaporation, but sedimentation\nleads to vertical size segregation with larger microparticles preferentially\nlocated at the bottom of the sample.