Hydroxyapatite nanoparticles (HANP) have been actively used in the medical field due to their biocompatibility and are recently advocated as a slow-release fertilizer. The stochiometric HANPs do not meet the plant needs due to low solubility. The optimization of solubility of HANPs, however, is a major challenge. In the present study, we investigated the incorporation of protonated phosphate (HPO42-) into hexagonal apatite structure instead of PO43- with the end goal to enhance the solubility. A series of HANP were precipitated and crystalized in various pHs ranging from 6 to 12. The proton incorporation was confirmed by PXRD from partial phase transition of HANP to b-TCP at 1100°. The presence of HPO42- ions in the lattice was verified by absorption at 1213 cm-1 in FTIR spectra, which increases corresponding to the depletion of incorporated CO32- and increase of pH. A linear relationship was observed between b-TCP and HPO42- vibration. Though HPO42- is known to be present in the amorphous phase, which is undetectable by PXRD or FTIR, the absence of such phase in crystalline HANPs was validated by Raman and TEM methods. HANPs with a higher number of HPO42- (crystallized at pH 6) were found to be more soluble in the citric acid than unprotonated form (PO43-, crystallized at pH 12). These results provide clear evidence that enriching P63/M lattice of apatite with HPO42- ion enhances the solubility of HANPs and thus enhances the solubility of the potential nanofertilizer.

A novel route to enhance the solubility of apatite: Structural incorporation of protons

ACS Fall 2021

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