Morphological and Crystallographic Control of Magnetic Nanoclusters as a Building Block of Magnetically Controllable Assembly for Bioassay

Magnetically responsive nanomaterials and their assemblies hold considerable promise for biomedical applications, including targeted therapy and image-guided intervention. Their performance is governed by the magnetic responsiveness of their constituent nanomaterials, which in turn depends on their particle size, shape, crystallinity, and assembly behavior. Iron-oxide nanoclusters (IOCs), which are composed of aggregated primary nanoparticles, are particularly suitable for magnetic actuation because of their high magnetic responsiveness and low coercivity. This study established a synthetic strategy for precisely controlling the morphology and internal structure of IOCs by tuning the reaction temperature and alkalinity. The synthesis conditions yielded spherical, cubic, and octahedral IOCs, each of which exhibited distinct crystalline and magnetic properties. Time-resolved X-ray diffraction and electron microscopy analyses revealed that the intermediate phases determined the crystal growth pathway, appreciably influencing the growth kinetics and final structure. The variously shaped IOCs exhibited superior alignment under external magnetic fields. Notably, the cubic particles readily formed directional arrays owing to their geometric regularity and strong magnetic responsiveness. We experimentally demonstrated that such assembled cubic nanostructures could be effectively used for solution mixing via field-induced rotation. These findings underscore the importance of shape-controlled synthesis in the design of highly responsive magnetic nanomaterials and provide valuable insights into the structural and functional design of magnetically controllable probes for bioassay.