Defect density modulation in graphene quantum dots towards high magnetic resonance relaxivity

Abstract

According to the Solomon–Bloembergen–Morgan (SBM) theory, the number of coordinated water molecules (q) around the Gd³⁺ magnetic center plays an important role in the enhancement of magnetic resonance relaxivity. This coordination is significantly influenced by the surface structure of magnetic materials. Therefore, magnetic graphene quantum dots (GQDs), featuring diversified and controllable surface structures, have become a promising material for designing nanomaterials with high magnetic resonance relaxivity. In this work, we regulate the defect density of magnetic GQDs by selecting precursor topologies during the “bottom-up” preparation process, which modulates the number of coordinated water molecules in the magnetic Gd³⁺ center, thereby influencing its properties. The resulting longitudinal magnetic resonance relaxivity (r₁) of the magnetic GQDs is enhanced to 46.5 mM⁻¹ s⁻¹ at 114 µT, which is four times higher than that of commercial contrast agents at the same field strength (e.g., Gd-DTPA: r₁ = 10.4 ± 0.2 mM⁻¹ s⁻¹ at 114 µT). On the basis of the low Gd³⁺ release rate and fluorescence characteristics of magnetic GQDs, we further demonstrate their potential application in fluorescent–MRI bimodal imaging.