| Summary: | To optimize the iron oxide nanoparticles as <i>T</i><sub>1</sub>-weight contrast for in vivo magnetic resonance imaging (MRI), numbers of macromolecule ligands have been explored with considerable effort. However, reports refer to the comparison of the <i>T</i><sub>1</sub>-weight contrast performances of iron oxide nanoparticles modified with natural and artificial macromolecule ligands are still limited. In this work, we used a typical natural protein macromolecule (bovine serum albumin, BSA) and an artificial macromolecule (poly(acrylic acid)-poly(methacrylic acid), PMAA-PTTM) as surface ligands to fabricate Fe<sub>3</sub>O<sub>4</sub>-BSA and Fe<sub>3</sub>O<sub>4</sub>-PMAA-PTTM nanoparticles with similar size and magnetization by the coprecipitation method and compared their MRI performances. In vitro and in vivo experiments revealed that Fe<sub>3</sub>O<sub>4</sub>-BSA with lower cytotoxicity exhibited higher <i>r</i><sub>2</sub>/<i>r</i><sub>1</sub> ratio in solution and darkening contrast enhancement for liver and kidney sites of mice under <i>T</i><sub>1</sub>-weight imaging, while Fe<sub>3</sub>O<sub>4</sub>-PMAA-PTTM displayed much lower <i>r</i><sub>2</sub>/<i>r</i><sub>1</sub> ratio in solution and brighter contrast enhancement for liver and kidney sites. These remarkably different MRI behaviors demonstrated that the surface ligands play an important role for optimizing the MRI performance of Fe<sub>3</sub>O<sub>4</sub> nanoparticles. We expect these results may facilitate the design of macromolecule ligands for developing an iron oxide⁻based <i>T</i><sub>1</sub>-weight contrast agent.
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