Size dependent properties of Gd³⁺-free versus Gd³⁺-doped carbon dots for bioimaging application

Gd³⁺- free carbon dots (CDs) were synthesized by one-step solvothermal method using urea, citric acid and 3-(trifluoromethyl)aniline as precursors. Additionally, Gd³⁺-doped CDs were prepared by incorporating gadolinium chloride (Gd³⁺ ions) into the synthesis. Size selection of the purified CDs was achieved through filter membranes ranging from 3 kDa to 100 kDa. The chemical composition and optical properties of the obtained samples were characterized by Energy dispersive X-ray (EDX), Fourier-transform infrared spectroscopy (FTIR), Dynamic Light Scattering (DLS), proton relaxation time measurements, Ultraviolet-visible (UV-vis) and fluorescence spectroscopies. A comparative analysis revealed a strong size-dependent behavior in the optical properties of both Gd³⁺-doped and Gd³⁺-free CDs. Furthermore, in vitro tests confirmed the non-cytotoxicity of Gd³⁺-doped CDs, indicating their potential applicability in biomedicine for magnetic resonance imaging (MRI) and red fluorescence-based cell and tissue imaging.