Magnetic resonance imaging (MRI) contrast agents need to provide strong imaging signals while maintaining good safety. Oxygen vacancies can improve the magnetic properties of metal oxide nanoparticles, but creating and controlling these defects in ultrasmall particles is difficult.

In a study published in Nano Letters, a team led by Assoc. Prof. MA Kun from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences developed a magnetic-field-assisted method to produce ultrasmall gadolinium oxide (GdO_x) nanoparticles with abundant oxygen vacancies, enhancing their performance as T1-weighted MRI contrast agents.

Researchers introduced a 3 T steady magnetic field during the synthesis of GdO_x nanoparticles. The field altered the crystal growth process and promoted the formation of oxygen vacancies on particle surface. The GdO_x nanoparticles were about 2.29 nm in size, with a surface oxygen vacancy density about 3.5 times higher than that of conventionally synthesized samples.

Besides, the increased oxygen vacancies also changed the magnetic behavior of the nanoparticles, helping improve their interaction with surrounding water molecules. The GdO_x nanoprobes showed about 1.7 times higher T1 relaxivity than conventional Gd₂O₃ nanoparticles.

In tests on tumor-bearing mice, GdO_x produced stronger and longer-lasting T1-weighted tumor contrast than the clinical contrast agent Gd-DTPA and control samples. They also performed well in vascular imaging. Safety tests showed no evident liver or kidney toxicity, with little gadolinium released from the nanoprobes.

This study establishes magnetic-field-assisted synthesis as a generalizable platform for defect engineering in nanomaterials, offering precise control over spin structures and defect-property relationships.