Gd³⁺-Doped Biocompatible Fluorinated Carbon Dots for Bimodal Bioimaging Applications

Fluorinated carbon dots (FCDs) represent a promising class of nanomaterials for biomedical applications owing to their biocompatibility, high fluorescence, and chemical stability. In this work, we report for the first time the synthesis and systematic investigation of Gd3+-doped FCDs (Gd-FCDs) obtained via a solvothermal route using urea, anhydrous citric acid, 3-(trifluoromethyl)aniline, and gadolinium(III) chloride hexahydrate. By incorporating paramagnetic Gd3+ ions into fluorinated fluorescent carbon dots, we aimed to create multifunctional nanoprobes capable of simultaneous fluorescence and magnetic resonance imaging (MRI). Comprehensive characterization demonstrated that Gd3+ doping significantly altered the structural and optical properties of the FCDs. While pristine FCDs were ultrasmall (2–8 nm), Gd-FCDs exhibited larger sizes (40–80 nm) due to ion-induced aggregation. During the synthesis process, Gd3+ ions are efficient positively charged centers stimulating formation of FCDs around them, resulting in bigger final complexes. Zeta potentials of FCDs and Gd-FCDs are −27.8 mV and −1.5 mV, respectively. UV–vis and fluorescence analyses revealed changes in electronic transitions and reduced fluorescence intensity, consistent with the introduction of nonradiative pathways by Gd3+ ions. Time-resolved fluorescence studies further confirmed the modified exciton dynamics. Importantly, proton relaxation measurements showed drastic reductions in both T1 and T2 relaxation times for Gd-FCDs in the concentration range 0.015–4 g/L, highlighting their strong MRI contrast capability across different magnetic field strengths. Cell toxicity measurements on 3T3-L1 biological cells show that all the FCDs revealed no toxicity against cells, indicating their complete biological compatibility at the concentration levels between 0.175 and 0.334 mg/mL. Its efficient penetration into cell nuclei enables robust fluorescent cell labeling across the visible spectrum.