Electron paramagnetic resonance and luminescence spectroscopy of transition metal ion impurities and X-ray-induced radicals in brushite

Calcium phosphates hold paramount importance as the constituents of hard tissue of living organisms. Among them, brushite (CaHPO4·2H2O), also known as dicalcium phosphate dihydrate (DCPD), is a common precursor in phosphate synthesis and is becoming increasingly appealing as a biomaterial for bone regeneration applications. This study presents a comparative analysis of structural, morphological, thermal, and spectroscopic properties of synthetic brushite prepared by the wet precipitation method with several brushite products available from commercial vendors. Electron paramagnetic resonance (EPR) spectroscopy revealed that Mn2+ and Fe3+ ions are common trace impurities in commercial products. Following exposure to X-rays, EPR signals of several phosphorous-related radicals emerge. The annealing of the irradiated samples gives rise to broad thermally stimulated luminescence (TSL) signals in the 40-120 °C range, which correlates with the annealing kinetics of the radicals. The stability of radiation-induced EPR and TSL signals is limited by the phase transformation temperature from brushite to monetite (CaHPO4). The results of this study provide novel insights into the formation and stability of phosphorous-related radicals and demonstrate how minuscule amounts of impurity ions can strongly influence the spectroscopic properties of brushite.