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.