Hydrothermal synthesis of Mn²⁺- and Cu²⁺-doped calcium hydroxyapatite: morphological features and importance of EPR insights

This study aims to explore the effects of various hydroxyapatite (HA) doping techniques in depth. For this purpose, two ions with comparable sizes, namely Mn²⁺ and Cu²⁺, were selected based on their suitability as dopants for HA in biomedical applications. Moreover, their paramagnetic properties enable sample characterization through electron paramagnetic resonance (EPR) spectroscopy. Two different approaches were employed: (I) ion-substituted α-TCP was used as starting material; (II) foreign ions presented in the solution during the transformation of undoped α-TCP to HA. The foreign ions influenced the hydrolysis process of α-TCP, altering the phase purity and morphology of the products. Doped α-TCP showed a weaker effect, while adding ions to the synthesis solution had substantial impact. Mn-doped α-TCP transformed into plate-like HA particles while Mn²⁺ ions present in the solution led to the formation of both plate- and rod-shaped particles. In contrast, Cu²⁺ ions induced the formation of rod-like particles independently of the doping process. Plate-like particles demonstrated higher Brunauer–Emmet–Teller surface area (S_BET) than rod-like HA. Each sample exhibited a combination of mesopores and macropores, with mesopores in the range of 15–17 Å being dominant. EPR investigations revealed that Mn²⁺ and Cu²⁺ are excellent paramagnetic probes to monitor synthesis reactions of HA and determine the localisation of foreign ions in the material.