Role of Maturation Temperature on Structural Substitution of Carbonate in Hydroxyapatite Nanoparticles

The phosphorus (P) release efficiency of hydroxyapatite nanoparticles (HANPs), a novel controlled-release fertilizer, could potentially be tuned by introducing carbonate ions into the crystal lattice. We used an atom-efficient synthesis method for carbonated HANPs from CO-oversaturated phosphoric acid before neutralization with Ca(OH). The effect of the maturation temperature on the physicochemical properties of the HANPs was analyzed by Fourier-transform infrared (FTIR) spectroscopy, nano-FTIR spectroscopy, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The overall content of carbonate incorporated into the apatite decreased significantly after maturation at 150°C to 2.2 wt.%, compare with 6.6 wt.% and 7.4 wt.% after maturation at 45°C and 95°C, respectively. The ratio of A- to B-type carbonate was 53:46 for HANPs maturated at 45°C, where all columnar hydroxyls were replaced by carbonate. The presence of an amorphous layer in the crystalline apatite was confirmed by modulating the laser strength in nano-FTIR spectroscopy. The ability to tune the carbonate content provides a basis for testing the resulting solubility changes in in vitro and field settings.