Gadolinium-doped carbon nanoparticles: coordination, spectroscopic characterization and magnetic resonance relaxivity

Carbon nanoparticles (CNPs) are attracting great attention as potential multifunctional agents for biomedical applications because of their bright fluorescence, low toxicity and flexibility of their physico-chemical properties. In the present paper, aqueous solutions of CNPs doped with gadolinium (Gd) (Gd-CNPs) within a widely varying range of Gd concentrations were prepared by hydrothermal synthesis. The influence of Gd doping on the optical properties and magnetic resonance (MR) relaxivity of Gd-CNPs was revealed. The Gd content was determined using X-ray fluorescence and spectrophotometry analysis. The composition of surface functional groups and coordination of Gd ions in Gd-CNPs were established by means of IR absorption spectroscopy and X-ray photoemission spectroscopy (XPS). The optical properties of Gd-CNPs in aqueous solutions were characterized by means of UV-visible-near-IR absorption spectroscopy and photoluminescence measurements with different excitation wavelengths. The local surroundings of Gd ions and paramagnetic centers in Gd-CNPs were probed by using electron paramagnetic resonance (EPR) spectroscopy. MR proton relaxation measurements in aqueous solutions of Gd-CNPs were carried out to determine the effect of Gd concentration on their MR contrasting. The obtained results characterize the coordination of Gd ions in Gd-CNPs and demonstrate new insights for controlling the optical and MR contrast properties of these nanoparticles for biomedical applications.