Bifunctional GQDs/Gd₂O₃:Co²⁺ Nanocomposites with Magneto-optical Properties as Potential MRI Contrast Agents

Gadolinium (Gd 3+ )-based composites are widely used as contrast agents in magnetic resonance (MR) imaging (MRI); however, they still suffer from modality homogeneity, insufficient imaging contrast, and potential biosafety issues. Here, cobalt ion (Co 2+ ) doping not only enhances the magnetization of graphene quantum dot (GQD)-modified gadolinium oxide (Gd 2 O 3 ) nanocomposites but also reduces their in vivo cytotoxicity. The synthesized GQDs/Gd 2 O 3:Co 2+ nanocomposites, prepared via polyol and hydrothermal methods, exhibit a cubic crystal structure with good dispersion and an average size of 18.21 nm. Their fluorescence intensity increases to ∼3.6 times that of pure Gd 2 O 3 . The magnetization of Co 2+ -doped GQDs/Gd 2 O 3 initially increases and then decreases, reaching a maximum value (2.713 emu/g) at 3% Co 2+ doping, while the longitudinal relaxation rate ( r 1 ) is 46.83 mM –1 s –1 . In vivo and in vitro MRI studies demonstrate that Co 2+ doping significantly enhances the brightness of T 1 -weighted images, thereby improving MRI performance. The biosafety of GQDs/Gd 2 O 3:Co 2+ is evaluated through hemolysis assays, cytotoxicity tests, and in vivo animal studies, confirming that the nanocomposites are nearly insoluble in blood, exhibit low cytotoxicity, and possess excellent in vivo safety. Thus, magneto-optical bifunctional GQDs/Gd 2 O 3:Co 2+ nanocomposites show great potential for MR imaging and fluorescence labeling.