Ultrasmall Ferrite Nanoparticles Synthesized via Dynamic Simultaneous Thermal Decomposition for High-Performance and Multifunctional T₁ Magnetic Resonance Imaging Contrast Agent

Large-scale synthesis of monodisperse ultrasmall metal ferrite nanoparticles as well as understanding the correlations between chemical composition and MR signal enhancement is critical for developing next-generation, ultrasensitive T₁ magnetic resonance imaging (MRI) nanoprobes. Herein, taking ultrasmall MnFe₂O₄ nanoparticles (UMFNPs) as a model system, we report a general dynamic simultaneous thermal decomposition (DSTD) strategy for controllable synthesis of monodisperse ultrasmall metal ferrite nanoparticles with sizes smaller than 4 nm. The comparison study revealed that the DSTD using the iron-eruciate paired with a metal-oleate precursor enabled a nucleation-doping process, which is crucial for particle size and distribution control of ultrasmall metal ferrite nanoparticles. The principle of DSTD synthesis has been further confirmed by synthesizing NiFe₂O₄ and CoFe₂O₄ nanoparticles with well-controlled sizes of ∼3 nm. More significantly, the success in DSTD synthesis allows us to tune both MR and biochemical properties of magnetic iron oxide nanoprobes by adjusting their chemical composition. Beneficial from the Mn²⁺ dopant, the synthesized UMFNPs exhibited the highest r₁ relaxivity (up to 8.43 mM^-1 s^-1) among the ferrite nanoparticles with similar sizes reported so far and demonstrated a multifunctional T₁ MR nanoprobe for in vivo high-resolution blood pool and liver-specific MRI simultaneously. Our study provides a general strategy to synthesize ultrasmall multicomponent magnetic nanoparticles, which offers possibilities for the chemical design of a highly sensitive ultrasmall magnetic nanoparticle based T₁ MRI probe for various clinical diagnosis applications.