Synthesis and characterization of luminescent cerium-doped hydroxyapatite

Hydroxyapatite (HAp) biomaterials are intended for use as bone substitutes, target functional agents in theranostics or bioimaging due to affinity for hard tissues, biocompatibility, and bioactivity. This study is devoted to HAp doped with cerium, having distinctive feature to change the oxidation state (3+/4+), which affects the physico-chemical and optical properties of the obtained powders. Cerium-doped HAp nanopowders with Ce content up to 0.5 wt% were obtained through precipitation technique and then were heat treated. The following conditions for heat treating were used: (i) heating in air atmosphere, (ii) hot pressing, (iii) heating in a carbon filling. For characterization, the complex methodology was implemented, including X-ray diffraction (XRD) analysis and high temperature XRD, X-ray absorption near edge structure (XANES) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, inductively coupled plasma atomic emission spectrometry (ICP-AES), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) spectroscopy, and, finally, photoluminescence (PL) spectroscopy. It was shown that the monophase cerium-HAp is stable up to a temperature of 1000 °C in the oxidizing atmosphere, above 1000 °C cerium(4+) oxide forms. Using reducing atmosphere during heating prevents the transition of Ce3+ to Ce4+. PL, EPR and XANES studding clarified obviously the Ce3+ content in the materials obtained under different processing conditions. It was shown, that Ce-HAp powders produced by precipitation and hot pressing contain Ce3+ and exhibit luminescence. The significant luminescence of hot pressed Ce-HAps was caused by a higher concentration of Ce3+ ions and incorporation of carbonate ions as a result of the processing.