Crystallization from amorphous phases is an emerging pathway for making advanced materials. Biology has made use of amorphous precursor phases for eons and used them to produce structures with remarkable properties. Herein, we show how the design of the amorphous phase greatly influences the nanocrystals formed therefrom. We investigate the transformation of mixed amorphous calcium phosphate/amorphous calcium carbonate phases into bone-like nanocrystalline apatite using in situ synchrotron X-ray diffraction and IR spectroscopy. The speciation of phosphate was controlled by pH to favor HPO4 (2-) . In a carbonate free system, the reaction produces anisotropic apatite crystallites with large aspect ratios. The first formed crystallites are highly calcium deficient and hydrogen phosphate rich, consistent with thin octacalcium phosphate (OCP)-like needles. During growth, the crystallites become increasingly stoichiometric, which indicates that the crystallites grow through addition of near-stoichiometric apatite to the OCP-like initial crystals through a process that involves either crystallite fusion/aggregation or Ostwald ripening. The mixed amorphous phases were found to be more stable against phase transformations, hence, the crystallization was inhibited. The resulting crystallites were smaller and less anisotropic. This is rationalized by the idea that a local phosphate-depletion zone formed around the growing crystal until it was surrounded by amorphous calcium carbonate, which stopped the crystallization.

Osteopontin, which is a phosphoprotein with strong ties to in vivo bone mineralization, is shown to change the precipitation pathway of calcium phosphate. We show that the presence of the phosphoprotein, even in minute concentrations, can stabilize an otherwise oversaturated mixture against precipitation. At moderate concentrations, we find that the protein introduces a new intermediate state into the reaction pathway leading to apatite formation. This new intermediate was found to share many characteristics of a coacervate or polymer-induced liquid-like precursor (PILP) phase. Our results show that these types of complex phases should be considered when discussing the mechanisms of bone mineralization on a subcellular level.

The formation of nanocrystals in biomineralization such as in bone occurs under the influence of organic molecules. Prompted by this fact, the effect of alizarin red S, a dye used in in vivo bone labeling methods, on bone-like carbonated apatite nanocrystal formation was investigated as a function of alizarin red S additive concentration. The obtained nanoparticles were investigated by powder X-ray diffraction (XRD), FTIR as well thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) while the kinetics of nanoparticle formation was investigated by in situ pH and synchrotron XRD measurements. Increasing alizarin red S concentration lead to amorphous particles over a threshold concentration and to smaller crystallites in a dose-dependent fashion. Alizarin red S induced a macroscopic lattice strain that scaled linearly with the alizarin red S concentration; this effect is reminiscent of that seen in biogenic calcium carbonates. TGA showed that the amorphous particles contained significantly more water than the crystalline samples and the DSC data showed that crystallization occurs after loss of most of the included organic material. The in situ studies showed that the formation of apatite goes via the very rapid formation of an amorphous precursor that after a certain nucleation time crystallizes into apatite. This nucleation time increased exponentially with alizarin red S concentration showing that this additive strongly stabilizes the amorphous precursor phase.

Organic additives influence crystallization processes in a multitude of ways. In biomineralization, e.g. bone or shell, such additives play a crucial role in morphology, and in polymorph and size control. However, the specific interactions between the additives and the growing mineral are in general unknown. Here, a model of bone mineralization, namely the formation of apatite nanocrystals under the influence of poly(acrylic acid), is studied using in situ X-ray diffraction. Since the kinetics of these reactions are very temperature dependent, a new X-ray scattering reaction cell has been developed that allows very high temperature precision, with an r.m.s. variation during operation of $0.05 K. The performance of the cell and its use in studying the apatite/ poly(acrylic acid) system are discussed. The apatite formation process proceeds via the formation of an amorphous precursor which then crystallizes. It is found that poly(acrylic acid) retards crystallization and reduces the growth rate of the forming crystallites.

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