Synthesis of Calcium Sulfate Nanocrystals via Confined Reaction in Ion-Enriched Instantaneous Microdomains

Abstract Hemihydrate calcium sulfate nanocrystals hold promising potential in biomedical and industrial applications. Conventional synthesis methods typically necessitate high temperatures/pressures, acidic/alkaline media, or expensive organic reagents. This study proposes a facile approach: when dripping CuSO4 aqueous solution into CaCl2 ethanol solution, driven by SO42–, CuSO4 droplets are immediately transformed into numerous SO42–-enriched instantaneous microdomains, where Ca2+ and SO42– undergo a confined reaction and aggregate into calcium sulfate crystals, while ethanol suppresses crystal growth, thereby yielding hemihydrate calcium sulfate nanocrystals. Under optimized conditions, the prepared pure-phase calcium sulfate crystals belong to the α-hemihydrate phase and the trigonal space group P3221, having diameters of 30–80 nm and lengths of 100–350 nm. The process enables the cost-effective and environmentally friendly preparation of calcium sulfate nanocrystals under mild ambient conditions without acid or base, while allowing ethanol recycling and CuCl2 byproduct recovery.