Fast precipitation of uniform CaCO₃ nanospheres and their transformation to hollow hydroxyapatite nanospheres

Shape-controlled synthesis of calcium carbonate with specific polymorphs can be achieved by the assistance of organic additives. In this study, highly uniform nanosized calcium carbonate spheres were synthesized by a fast precipitation method in the presence of a simple polymer, poly(styrene sulfonate) (PSS). The polymorph of the synthesized calcium carbonate products changes from pure calcite in PSS-free reactions to vaterite in PSS-containing (1-50g/L) reactions. The effect of PSS on the formation of vaterite can be attributed to the two aspects: decrease of driving force by reducing the interfacial energy, and phase stabilization effect caused by the adsorbed PSS. A higher PSS concentration (50g/L) results in highly uniform vaterite nanospheres of 400-500nm in diameter. Furthermore, PSS is found more effective to induce the formation of vaterite in the Ca²⁺-rich reaction condition (Ca²⁺:CO₃²-=5:1) than in the CO₃²--rich conditions (Ca²⁺:CO₃²-=1:5). It has also been found that different mixing mode of the calcium and carbonate precursor solutions has a significant influence on the size distribution of the products. Finally, with a controlled anion-exchange method, the as-prepared vaterite nanospheres can be easily transformed to hollow hydroxyapatite spheres, which exhibit great potential to be used as the drug carriers due to their considerably high surface area and biocompatibility.