We here describe a rapid method for synthesizing hollow core, porous crystalline calcium carbonate microspheres composed of vaterite using supercritical carbon dioxide in aqueous media, without surfactants. We show that the reaction in alkaline media rapidly conducts to the formation of microspheres with an average diameter of 5 µm. SEM, TEM and AFM observations reveal that the microspheres have a hollow core of around 0.7 µm width and are composed of nanograins with an average diameter of 40 nm. These nanograins are responsible for the high specific surface area of 16 m² g⁻¹ deduced from nitrogen absorption/desorption isotherms, which moreover confers an important porosity to the microspheres. We believe this work may pave the way for the elaboration of a biomaterial with a large potential for therapeutic as well as diagnostic applications.

The aim of the present work was to assess the merits of supercritical CO₂ (SC-CO₂) as a process for protein encapsulation into calcium carbonate microparticles. Lysozyme, chosen as a model protein, was entrapped during CaCO₃ precipitation in two different media: water (normal route) and SC-CO₂. The particles were characterized and compared in terms of size, zeta potential, morphology by SEM, crystal polymorph and lysozyme encapsulation. Fluorescent and confocal images suggested the encapsulation and core–shell distribution of lysozyme into CaCO₃ obtained by the SC-CO₂ process. A high encapsulation efficiency was reached by a supercritical CO₂ process (50%) as confirmed by the increased zeta potential value, lysozyme quantification by HPLC and a specific bioassay (M. lysodeikticus). Conversely, lysozyme was scarcely entrapped by the normal route (2%). Thus, supercritical CO₂ appears to be an effective process for protein encapsulation within nanostructured CaCO₃ particles. Moreover, this process may be used for encapsulation of a wide range of macromolecules and bioactive substances.