Defect-Related Photoluminescence in Hydroxyapatite Nanoparticles Modulated by Carbonate Incorporation

The investigation of defect-related photoluminescence in hydroxyapatite (HA) nanoparticles (NPs) is essential for understanding their electronic structure and charge carrier recombination dynamics. These insights are important to advancing HA-based materials in photocatalysis, optical devices, hard tissue spectroscopy, and cellular bioimaging. In this study, we provide new evidence on the structural and compositional factors that govern the intrinsic photoluminescence of HA NPs synthesized by chemical precipitation at room temperature, and subjected to thermal treatment at 400 and 450 °C. Carbonate contents ranging from 0.6 to 10.9 wt % were introduced into HA nanorods during synthesis through AB-type substitution, replacing both OH^- (A-type) and PO₄ ^3- (B-type) groups. Increasing carbonate incorporation led to enhanced emissions under 405 nm excitation, with a primary band centered at 438 nm. Subsequent thermal treatment further amplified the emission intensity, with the strongest luminescence observed in samples containing higher carbonate content, which also exhibited a red-shift of the emission maximum to approximately 583 nm. These changes were mainly attributed to the progressive increase in carbonate concentration, particularly through B-type substitution, which promotes structural disorder, reduces crystallite size, and generates higher densities of vacancies defects, including V_Ca, V_OH, and V_O in PO₄ ^3- groups, as well as to the elimination of structural water during heating. These results confirm that carbonate, a frequent impurity in almost all HA NPs obtained by wet methods without strict experimental conditions (e.g., inert atmosphere), plays a central role in modulating the density of defects and, consequently, the photoluminescence properties in both as-synthesized and thermally treated forms. We also demonstrate the use of citrate-functionalized carbonated HA NPs for cellular bioimaging with HDFn cells as a model, underscoring their potential for biomedical applications.