Facile, gram-scale and eco-friendly synthesis of multi-color graphene quantum dots by thermal-driven advanced oxidation process

Graphene quantum dots (GQDs) have been demonstrated of great potential and benefits in the fields of bioimaging and white light-emitting-diodes (WLEDs). However, it is still highly demanding at the current level to solve the dilemma of achieving high-yield GQDs of good quality and superior fluorescent property using low-cost sustainable and industrializable production procedure. In this work, we for the first time report the gram-scale synthesis of well-crystalline GQDs with ultra-small size based on thermal-driven Advanced Oxidation Process (AOP) under facile green hydrothermal conditions. The average yield calculated from 20 trials reached up to 60%, and the average size of the dots was measured to be ~3.7 nm. Furtherly, GQDs with the photoluminescence (PL) emission of blue, green, yellow, orange, and red have been prepared by expanding the π-conjugation and introducing graphite nitrogen in the carbon skeleton based on chemical structure engineering. The PL-tunable GQDs have an average size distribution of 2–5 nm and a lamellar structure of 2–6 layers. Structure analysis results have indicated that the red shift of PL emission is attributed to bandgap narrowing. This approach successfully converts the easily available and cheap precursor into high-valued products with great application potentials. The PL-tunable GQDs have been successfully used as fluorescent probes of good biocompatibility for in vitro/ in vivo bio-imaging and to produce highly-photostable white-light-emitting composite film with a quantum yield (QY) of 24%.