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 Co _<i>x</i> Zn _<i>y</i> Fe_{3‑(<i>x</i>+<i>y</i>)}O₄ nanoparticles by adding sodium oleate as a cocapping 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 the highest room temperature saturation magnetization among other morphologies, 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.