Structure elucidation of multicolor emissive graphene quantum dots towards cell guidance†
Graphene quantum dots (GQDs) can become excellent bioimaging tools when tuned to emit at larger wavelengths due to the minimal tissue absorbance and emission in this range. Tuning the GQD structure can help but understanding the chemical structure responsible for their properties remains challenging. Herein, we elucidated the structure of GQDs synthesized from glucose and ammonium hydroxide using a fast microwave-assisted hydrothermal protocol. Remarkably, these GQDs exhibited emission from the NUV-Vis up to the NIR range. The structure and chemical composition were elucidated using advanced NMR techniques, such as two-dimensional nuclear magnetic resonance, combined with traditional spectroscopy and electron microscopy. The graphitic core composed of pyrazines presented localized defects and lower rotation mobility compared with their edges that were mainly formed by hydroxyl, acid, and amine functional groups, which paved the way for the observed multicolor red-shifted fluorescence emission. Confocal laser scanning microscopy revealed functional cell imaging in a wide spectral range of fluorescence from bright purple to red, confirming the uptake of GQDs by the cells without any observable toxicity. The non-cytotoxicity was further proved by the chemiluminescence cell viability adenosine triphosphate (ATP) assay. Combined with the tunable GQD emission, it gives them the potential to act as bioimaging carriers starting a new phase for their use in vivo.