Microstructure evolution in Stöber-type silica nanoparticles and their in vitro apatite deposition

Prepared via Stöber-type sol–gel routes were three types of silica particles of <1 μm in size: pure silica, Ca-involving silica, and chitosan/alginate-coated silica with a polymershell-silica core structure. Calcium ions were impregnated in the organic layers of the polymer-coated silica particle. The sol–gel procedure was applied to tetraethoxysilane dissolved in an ethanol/water mixture, while Ca–silica was derived from CaCl₂-containing ethanol/water solutions. Scanning and transmission electron micrograph analyses indicated that those silica particles consisted of ~10 nm primary particles, the Ca–silica particles (~500 nm) were larger than the Ca-free ones (~200 nm) and that their size increased with the Ca concentration in the precursor solutions. From ¹H- and ²⁹Si- magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra and ²⁹Si cross-polarization NMR spectra, the mechanism of primary particle agglomeration and degradation of the secondary particles in saline were discussed in terms of the content of H₂O molecules and >Si–OH as well as hydrogen bonding interactions among them. In addition, the Ca–silica and core-shell silica deposited apatite in Kokubo’s simulated body fluid. Thus, the present Ca–silica and polymer-coated silica particles were suggested to be applicable to injectable bone fillers for bone generation.