Kinetically-Controlled Morphologies of Magnetic Nanoparticles through Ligand and Precursor Chemistry

Kinetically-controlled morphologies of colloidal magnetic nanoparticles possess unique magnetic properties, making them highly promising for applications in magnetogenetics as magnetic torque probes. Yet, their size-controlled chemical synthesis is in its nascent state. Here, we present a capping-ligand-directed approach to tune the morphology and magnetic properties of CoxZnyFe3-(x+y) O4 nanoparticles by adding sodium oleate as a co-capping ligand to oleic acid during synthesis, resulting in the formation of monodisperse tetrahedral nanoparticles. Increasing the molar ratio of sodium oleate to oleic acid promotes facet-selective passivation along {111} facets, leading to progressive truncation of tetrahedra and yielding morphologies ranging from truncated tetrahedra to extremely truncated rod-like shapes. Our electron microscopy studies show that the synthesis of tetrahedron-shaped nanoparticles does not require a symmetry-breaking transformation from octahedra, as the initial crystallite formed is tetrahedra. When sodium oleate is removed from the synthesis, thermodynamically-driven monodisperse octahedral nanoparticles are formed. We find that ligand composition also influences the doping of ions into the crystal structure, with higher sodium oleate concentrations reducing Zn2+ incorporation due to modified metal-ligand coordination. Tetrahedral nanoparticles synthesized under optimal conditions exhibit higher room temperature saturation magnetization than bulk-like magnetite, highlighting their potential for magnetic nanoparticle-based biosensing applications. Our study underscores that not only morphology but also magnetic characteristics of nanoparticles can be tuned by a ligand-guided chemistry.