In Situ Observation of Dicalcium Phosphate Monohydrate Formation and Phase Transformation

Calcium orthophosphates (CaPs), as important minerals in biomineralization and biomedicine, have attracted wide attention. Dicalcium phosphate monohydrate (DCPM, CaHPO₄·H₂O), the recently discovered crystalline CaP phase, has a higher metastability than dihydrate (DCPD, CaHPO₄·2H₂O) and anhydrate (DCPA, CaHPO₄), which may lead to many potential applications in functional biomaterial development. However, the preparation of large-sized DCPM and the underlying mechanisms of its formation and phase evolution remain unclear. Herein, for the first time, we propose a method to prepare micrometer-sized DCPM under an acidic water-methanol mixture and using in situ time-resolved atomic force microscopy further explore its crystallization via dissolution of an acidic amorphous calcium phosphate. In support of the potential role of DCPM as the biomaterial, we demonstrate that DCPM can quickly evolve into more stable octacalcium phosphate in a near-physiological solution. This work provides a mechanistic understanding of the formation and phase transformation of DCPM, which may serve as a basis for subsequent synthesis and application of DCPM as functional biomaterials.