d‐Glucuronic Acid‐Coated Ultrasmall Paramagnetic Ln₂O₃ (Ln = Tb, Dy, and Ho) Nanoparticles: Magnetic Properties, Water Proton Relaxivities, and Fluorescence Properties

Many of the lanthanide‐based nanoparticles are valuable in magnetic resonance imaging (MRI) because they have excellent magnetic properties that make them suitable for MRI contrast agents, while some of them have additional optical properties and thus can serve as fluorescence imaging (FI) agents as well. Herein, we synthesized d ‐glucuronic acid‐coated ultrasmall Ln 2 O 3 (Ln = Tb, Dy, and Ho) nanoparticles with average diameters of ∼ 2 nm. All the nanoparticles showed unsaturated but appreciable magnetization of ∼ 4 emu/g at an applied field of 1.8 T at room temperature, and consequently, high transverse water proton relaxivities ( r 2 ) of ∼ 7 s –1 m m –1 at 1.5 T, ∼ 35 s –1 m m –1 at 3.0 T, and ∼ 55 s –1 m m –1 at 9.4 T with negligible longitudinal water proton relaxivities ( r 1 ), thus being eligible for use as T 2 MRI contrast agents at high MR fields. In addition, d ‐glucuronic acid‐coated ultrasmall Tb 2 O 3 nanoparticles exhibited fluorescence in the green region, useful for dual‐modal imaging.