Size Controlof Highly Monodisperse Citrate-StabilizedMagnetite Nanoparticles in Aqueous Media: The Role of Cerium Cations

Controlling the nucleation and growth of magnetite nanoparticles (NPs) via aqueous coprecipitation at room temperature is inherently challenging due to kinetic and thermodynamic constraints. Rapid pH increases trigger a burst nucleation event that depletes precursors before significant growth occurs, while NP surfaces are quickly passivated by water and hydroxyl ions, forming a dewetting barrier that impedes further growth. To address these challenges, we developed an innovative synthesis approach that incorporates lanthanide (Ln) cations, mainly Ce³⁺, together with sodium citrate into the coprecipitation process. These additives act synergistically to lower surface energy and stabilize both precursors and reaction intermediates, thereby facilitating controlled crystal growth. We systematically investigated the impact of Ln cations by fine-tuning the Ce³⁺ concentration, which enabled precise control over NP size, yielding diameters from 13 to 46 nm. Notably, at higher Ce³⁺ concentrations, multidomain nanowires are formed, with diameters reaching up to 90 nm and lengths on the micron scale. Moreover, the synthesized NPs exhibit enhanced performance in magnetic hyperthermia (MH) and peroxidase-like activity compared with those produced without Ce³⁺. The improved catalytic activity is attributed to accelerated Fe²⁺ regeneration in the reaction, mediated by the presence of the Ce⁴⁺/Ce³⁺ redox couple, which boosts ^•OH radical production. Importantly, this strategy is versatile and can be extended to other Ln cations (Eu³⁺, Gd³⁺, Er³⁺, Yb³⁺, and Lu³⁺), yielding single-crystal magnetite NPs of comparable sizes. Collectively, Ln-assisted synthesis addresses longstanding challenges in magnetite NP coprecipitation, providing a scalable route to high-performance magnetic NPs with tunable size and morphology. This approach not only improves control over NP growth but also broadens their potential applications in MH and catalysis.