Rational hydrothermal synthesis of graphene quantum dots with optimized luminescent properties for sensing applications

Hydrothermal synthesis using graphene oxide (GO) as a precursor has been used to produce luminescent graphene quantum dots (GQDs). However, such a method usually requires many reagents and multistep pretreatments, while can give rise to GQDs with low quantum yield (QY). Here, we investigated the concentration, the temperature of synthesis, and the pH of the GO solution used in the hydrothermal method through factorial design experiments aiming to optimize the QY of GQDs to reach a better control of their luminescent properties. The best synthesis condition (2 mg/mL, 175 °C, and pH = 8.0) yielded GQDs with a relatively high QY (8.9%) without the need of using laborious steps or dopants. GQDs synthesized under different conditions were characterized to understand the role of each synthesis parameter in the materials' structure and luminescence properties. It was found that the control of the synthesis parameters enables the tailoring of the amount of specific oxygen functionalities onto the surface of the GQDs. By changing the synthesis' conditions, it was possible to prioritize the production of GQDs with more hydroxyl or carboxyl groups, which influence their luminescent properties. The as-developed GQDs with tailored composition were used as luminescent probes to detect Fe³⁺. The lowest limit of detection (0.136 μM) was achieved using GQDs with higher amounts of carboxylic groups, while wider linear range was obtained by GQDs with superior QY. Thus, our findings contribute to rationally produce GQDs with tailored properties for varied applications by simply adjusting the synthesis conditions and suggest a pathway to understand the mechanism of detection of GQDs-based optical sensors.