Eco-friendly processes for the synthesis of amorphous calcium carbonate nanoparticles in ethanol and their stabilisation in aqueous media†
Amorphous calcium carbonate nanoparticles (ACC NPs) are promising multifunctional materials for healthcare applications. Due to their instability in aqueous media, pure ACC NPs for the biomedical field are increasingly synthesised in absolute ethanol, using the ammonia diffusion method (ADM). Although this method presents the advantage of providing stable ACC NPs without additives, it requires the use of pure ethanol as solvent. New insights into the formation mechanisms of ACC NPs in ethanol using gas diffusion are presented in this article. The optimisation of the process according to these findings can increase the mass concentration of ACC NPs by a factor of 3.5. As a result, the amount of ethanol required to produce a target mass of particles is significantly decreased, reducing the ecological impact of the process. The stabilisation of the resulting ACC NPs in aqueous media is achieved by a short-time process using phospholipids based on the ethanol injection method. By using the natural electrostatic affinity of negatively charged materials for the positive surface of ACC NPs in ethanol, we reduced the process time from 24 h to 2 minutes, compared with the closest state of the art, decreasing the operating time and corresponding energy consumption. The process does not require the use of synthetic PEGylated lipids for steric stabilisation. In addition, a natural egg-sourced phospholipid was identified as an efficient stabiliser for the first time. The upscaling of our process was successfully demonstrated using a 50 L reactor for bulk synthesis, as well as a continuous flow reactor for industrial continuous flow production.