Enhancing photoluminescence performance and pH photostability of nitrogen-doped graphene quantum dots via surface-passivated by polyethylene glycol

The photoluminescence (PL) properties of nitrogen-doped graphene quantum dots (NGQDs) are highly dependent on reaction parameters. In this study, we developed new and fast protocols to surface passivate NGQDs with polyethylene glycol (PEG) using microwave irradiation. The resulting NGQDs-PEG exhibited highly stable PL with an emission peak at 424 nm when excited at 325 nm. The PL intensity of NGQDs with PEG increased with the volume of capping agents due to the enhancing luminescence effect of the surface functional group of polyethylene glycol. The optimal PEG amount of 2 wt% was found to improve the PL effect of NGQDs. Interestingly, the surface passivation of NGQDs with PEG enabled the NGQDs to exhibit strong PL performance across varying pH environments (from acidic to alkaline), addressing the issue of quenching of quantum dots in biological environments. Our research also focused on the ultrafast synthesis of NGQDs, where process parameters such as power value, catalyst volume, and reaction time were identified as important factors affecting NGQD luminescence. Our results showed that the optimal microwave power was 640 W, lower than that used in previous synthesis methods. Increasing the volume of catalyst promoted faster NGQD formation. The optimal reaction time was found to be 5 min, catalyst volume of 5 ml resulted in the highest PL intensity of NGQDs.