Tetraethoxysilane-Driven Surface Nanolayer Engineering of Apatite Nanoparticles and Their Uniform Film Formation

Implantable medical devices require long-term stability in vivo and high biocompatibility; however, precise control of the interfacial interactions between bioceramic-coated device surfaces and biological tissues remains a critical challenge. Herein, we investigated the influence of silicate ion introduction during the wet chemical synthesis of hydroxyapatite nanoparticles on their surface nanolayer states and electrophoretic deposition behavior. Using tetraethoxysilane, we synthesized two distinct types of silicate ion-containing hydroxyapatite nanoparticles: (1) silicate ion-substituted type, in which silicate ions were incorporated into the hydroxyapatite crystal structure, and (2) silicate-coated type, in which condensed silicate ions were partially formed on the nanoparticle surface. The spectroscopic analyses revealed that the silicate ion-substituted type would introduce hydroxyl vacancies within the hydroxyapatite crystal structure. This phenomenon induced the local lattice distortions, which directly influenced the surface states of the nanoparticles (i.e., the ion-containing states of the surface nanolayer) and their electrophoretic deposition properties. In contrast, the silicate-coated type did not occur in silicate ion substitution, and the silica oligomer was adsorbed on the nanoparticle surfaces, exhibiting a stabilized zeta potential and electrophoretic deposition properties. These findings demonstrated that the timing of tetraethoxysilane addition during the wet chemical synthesis critically dictates the surface nanolayer states on the nanoparticles, offering molecular-level guidelines for designing the initial interactions with biological tissues.