Multi-methodological characterization of Eu-doped polycrystalline Ca10(PO4)OH2 hydroxyapatite.

Eu-doped (mol 3%) and undoped polycrystalline Ca10(PO4)OH2 hydroxyapatite (HAp) [1-2] were synthesized by using the chemical-precipitation technique [3]. Some precautions were observed: the temperature was kept low at 25° C so that to inhibit the increasing of the average size of the particles; pH was kept constantly high (10 ± 0.05) in order to minimize the formation of secondary phases and to prevent the aggregation of the particles during their formation. The multi-methodological characterization achieved through powder X-Ray diffraction (PXRD) and photoluminescence (PL) techniques, showed that the Eu entered in Ca1 site in the dried (120 °C) doped samples and in those calcinated (450 °C) at low temperature: these samples show low crystallinity (3% and 7%, respectively), good luminescence and very low crystallite size (around 25 nm). On the contrary, Eu-doped sample calcinated at 900°C showed very high crystallinity (87%), with a crystallite size of 148 nm, while PL spectroscopy suggested that this sample presents the highest and narrowest emission bands. Specifically, the PL emissions peak at 573 nm, corresponding to the 5D0–7F0 of the Eu3+ transition in Ca2 site [4], was more than 10 times more intense than the emission peak at 592 of Eu3+ in Ca1 site, indicating the complete migration of Eu3+ ions in the Ca2 sites of HAp framework for the high temperature sample [3]. FTIR and Raman spectra showed slight band shifts with increasing annealing temperature of the samples [3]. Results show that low crystalline HAp obtained at 120° and 450°, could be employed as luminescent drug carriers, while high crystalline HAp, annealed at 900°, could be suitable materials for biological optical imaging. [1] Dorozhkin, S.V. Calcium Orthophosphates: Occurrence, Properties and Major Applications. Bioceram Dev Appl 2014, 4. [2] Hughes, J.M.; Rakovan, J. The Crystal Structure of Apatite, Ca5(PO4)3(F,OH,Cl). Reviews in Mineralogy and Geochemistry 2002, 48, 1–12. [3] Baldassarre, F.; Altomare, A.; Corriero, N.; Mesto, E.; Lacalamita, M.; Bruno, G.; Sacchetti, A.; Dida, B.; Karaj, D.; Ventura, G.D.; Capitelli, F.; Siliqi, D. Crystal Chemistry and Luminescence Properties of Eu-Doped Polycrystalline Hydroxyapatite Synthesized by Chemical Precipitation at Room Temperature. Crystals 2020, 10, 250. [4] Nikolaev, A.; Kolesnikov, I.; Frank-Kamenetskaya, O.; Kuz’mina, M. Europium concentration effect on characteristics and luminescent properties of hydroxyapatite nanocrystalline powders. Journal of Molecular Structure 2017, 1149, 323–331.