Graphene Quantum Dots as Fluorescent and Passivation Agents for Multimodal Bioimaging

Sammanfattning: Zero-dimensional graphene (carbon) quantum dots have been drawing attention in bio-related applications since their discovery, especially for their optical properties, chemical stability, and easily modifiable surface.  This thesis focuses on the green synthesis of nitrogen-doped graphene quantum dots (GQDs) for dual-mode bioimaging with X-ray fluorescence (XRF) and optical fluorescence. Both conventional and microwave- (MW-)assisted solvothermal methods were followed to investigate the precursors’ effect on the synthesized GQDs. The MW-assisted method permitted the synthesis of uniform GQDs with an excitation-independent behavior, due to highly controllable reaction conditions. It was demonstrated that the molecular structure of the precursors influenced the optical fluorescence properties of the GQDs. Thus, both blue- (BQDs) and red-emitting (RQDs) GQDs were obtained by selecting specific precursors, leading to emission maxima at 438 and 605 nm under the excitation wavelengths of 390 and 585 nm, respectively.  Amine-functionalized Rh nanoparticles (NPs) were chosen as the X-ray fluorescence (XRF) active core, synthesized via MW-assisted hydrothermal method with a custom designed sugar ligand as the reducing agent. These NPs were conjugated with BQDs using EDC-NHS treatment. The hybrid Rh-GQDs NPs exhibited green emission (520 nm) under 490 nm excitation and led to a reduced cytotoxicity with respect to bare Rh NPs, highlighting the passivation role of the GQDs via the real-time cell analysis (RTCA) assay. The hybrid complex constituted a multimodal bioimaging contrastagent, tested with confocal microscopy (in vitro) and XRF phantom experiments.