Size‐Dependent Oxygen Vacancy of Iron Oxide Nanoparticles

Abstract Prior research has highlighted the reduction of iron oxide nanoparticle (IONPs) sizes to the “ultra‐small” dimension as a pivotal approach in developing T 1 ‐MRI contrast agents, and the enhancement in T 1 contrast performance with the reducing size is usually attributed to the increased specific surface area and weakened magnetization. Nonetheless, as the size decreases, the variation in surface defects, particularly oxygen vacancy (V O ) defects, significantly impacts the T 1 imaging efficacy. In this study, the V O on IONPs is meticulously investigated through XPS, Raman, and EPR spectroscopy. As the nanoparticle size decreased, the V O concentration rose initially but subsequently declined, with the peak concentration observed in the size of 8.27 nm. Further insights gained from synchrotron XAS analysis and DFT calculations indicate that both surface tension and phase transition in IONPs contribute to alterations in the Fe─O bond length, thereby influencing the V O formation energy across varying nanoparticle sizes. The MRI tests reveal that the V O in IONPs serve as pivotal sites for the attachment of water molecules to iron ions, and IONPs with fewer V O exhibited a deterioration in T 1 ‐MRI contrast effects. This research may provide a deeper understanding of the relationship between T 1 contrast performance and the size of IONPs.