Real‐Time Visualization of Calcium Phosphate Formation on Titanium Dioxide Nanoparticles Using Liquid Transmission Electron Microscopy

The integration of titanium dioxide (TiO₂) with calcium phosphate (CaP)-based hydroxyapatite (HAP) is a promising strategy for enhancing the bioactivity of bone implants. However, a fundamental understanding of the interfacial reactions governing CaP mineralization on TiO₂ remains limited due to lack of characterization techniques with sufficient spatial and temporal resolution in hydrated state. In this study, in situ liquid transmission electron microscopy (TEM) imaging and correlative ex situ TEM analyses are combined to investigate the nucleation, aggregation, and crystallization of the CaP layer on TiO₂ nanoparticle surfaces. The findings reveal a three-step mineralization process: 1) aggregation of TiO₂ nanoparticles in solution, 2) formation of an amorphous CaP-like (ACP-like) layer, leading to an ACP-like coated TiO₂ nanoparticle structure, and 3) progressive crystallization of ACP into HAP, forming a HAP-like coated TiO₂ nanoparticle structure. Liquid-TEM imaging captured dynamic transformations, including nanoparticle aggregation, structural evolution, and phase transitions, providing unprecedented insights into the physicochemical interactions underlying mineralization. Additionally, the effects of electron beam exposure on TiO₂ nanoparticles are evaluated, demonstrating that high electron flux densities can induce morphological instability. This study advances the understanding of CaP-TiO₂ interfacial mineralization and offers valuable guidance for optimizing bioactive coatings to improve osseointegration and the stability of bone implants.