Controllable preparation of Eu3+-doped octacalcium phosphate and fluorescence study of its hydrolysis into hydroxyapatite

Abstract

Octacalcium phosphate (OCP) is generally considered the intermediate of hydroxyapatite (HAP) during bone mineralization. Crystalline phase transformation of OCP to HAP is an important topic in the study of biomineralization processes. In this study, Eu³⁺-doped OCP (Eu-OCP) was synthesized controllably by an improved co-precipitation method. The results showed that Eu-OCP had different fluorescence properties (i.e. emission intensity and I₆₁₄/I₅₉₁ ratio) due to the varied crystal field environment compared with Eu³⁺-doped HAP. Furthermore, the relationship between the I₆₁₄/I₅₉₁ ratio and the mass ratio of HAP to OCP in a HAP/OCP mixture was revealed based on a Eu³⁺ fluorescent probe, which was verified by X-ray diffraction (XRD) analysis. The I₆₁₄/I₅₉₁ ratio was related to the content (x) of HAP in the HAP/OCP mixture according to I₆₁₄/I₅₉₁ = 2.0971 − 0.70246/(1 + exp(x − 0.68956)/0.11274) (R² = 0.970). By this fluorescent probe method, the crystal phase transition from OCP to HAP was quantitatively analyzed during the hydrolysis of OCP, which was in accordance with the XRD analysis. Therefore, this fluorescent probe method has potential to monitor crystal phase transition in biomineralization in situ.