Stabilization of Vaterite in Nanostructured CaCO₃ via Precipitation: Implications for Drug Delivery Systems

Polycrystalline vaterite aggregates of approximately 2000 nm composed of 50–100 nm nanoparticles were grown by precipitation routes. Different aggregate morphologies─including raspberry-shaped, star-like, spindle, lenticular, and spherical forms─were stabilized by adjusting synthesis parameters such as pH, temperature, order of the limiting and excess reactants, and the use of simple organic additives such as polyethylene glycol (PEG) and gelatin. Vaterite with a raspberry-shaped morphology and > 99% purity was obtained by adding CaCl2 dropwise to a PEG-Na2CO3 solution using an infusion pump at 50 mL/h, under continuous stirring at pH 10 and 35 °C. The resulting batch was homogeneous and remained stable at normal laboratory conditions for at least 3 years, becoming an interesting material for drug delivery in human cancer therapies. Finally, a systematic correlation between aggregate morphology, crystallite size, and the stability of the vaterite phase was established, highlighting the role of nanostructuring in preventing its transformation into more stable CaCO3 polymorphs.