Magnetic Macroporous Microspheres from FeCO₃ via Water Vapor-Mediated Thermal Decomposition: Implications for Nanoparticle Capture and Biomedical Applications

The thermal decomposition of inorganic salts is a simple method to synthesize porous oxides. However, nanopore blocking during particle growth hinders the formation of macropores (>50 nm). In this study, macroporous magnetite (Fe 3 O 4 ) microspheres, composed of primary particles exceeding 500 nm in diameter, are synthesized via the thermal decomposition of FeCO 3 in the presence of water vapor. Under oxidizing (air) conditions, water vapor facilitates the formation of hematite (α-Fe 2 O 3 ) with bicontinuous macroporosity. In a reducing atmosphere containing water vapor, FeCO 3 directly yields magnetite microspheres with open macropores (∼210 nm). Water vapor promotes nanoparticle bonding and growth at low temperatures, thereby facilitating spontaneous macropore formation. These structures allow efficient nanoparticle capture in solution and facile magnetic manipulation, as demonstrated by the successful capture of Au nanoparticles approximately 15 nm in size. Their excellent biocompatibility was confirmed through cytotoxicity assays. The developed macroporous magnetic architecture holds significant potential in biomedical applications, particularly for encapsulating and delivering tumor markers and antibody drugs─typically several tens of nanometers in size─for targeted therapeutic delivery.