Exploring the Emission Pathways in Nitrogen-Doped Graphene Quantum Dots for Bioimaging

Graphene quantum dots (GQDs) with tunable fluorescence emission promise excellent bioapplication potential, especially in bioimaging. We report the synthesis of nitrogen-doped GQDs (N-GQDs) from glucose and ethylenediamine, cheap and safe chemicals, using a one-step and fast microwave-assisted hydrothermal method. Our N-GQDs exhibit fluorescence in the entire visible spectral region, which extends to near-ultraviolet and slightly to near-infrared. Since the origin of fluorescence and its relation to the structure and synthesis conditions are not yet fully understood, we also concentrated on the fluorescence mechanism explanation. Structural characterization with steady-state and time-resolved photoluminescence measurements indicated that band-to-band transitions, size effect, and different nitrogen and oxygen functional groups play a role in this multicolor emission. Remarkably, we found for the first time the evidence that directly relates a change in the N-GQD work function to the change in oxygen groups under UV irradiation via ultraviolet photoelectron spectroscopy. Thus, we confirmed that for λex ≲ 380 nm, photooxidation processes occurred, which led to chemical modification, thereby lowering the work function in the N-GQDs. The N-GQDs were proved to be highly biocompatible by a cell viability assay using vascular smooth muscle cells. Together with the wide spectral range emission observed in confocal fluorescence imaging, it demonstrated the potential of the N-GQDs for in vitro bioimaging applications.