Unveiling the Fluorescence Origin in Pyrene Derived Graphene Quantum Dots: A Single Particle Level Analysis

The bottom-up approach has been the preferred route for large-scale synthesis of graphene quantum dots (GQDs). However, the structure and origin of photoluminescence in these dots synthesized by the bottom-up approach are still a subject of debate. Here, using a series of separation techniques like solvent extraction, column chromatography, gel electrophoresis and dialysis, we present three distinct fluorescent materials in GQDs synthesized from pyrene, a well-known precursor molecule. The subtle balance between the extent of graphitization and molecular fluorophores determines the nature of fluorescence emission in GQDs verified using a suite of optical and vibrational spectroscopy techniques in combination with electron microscopy. The single-particle level emission properties strongly support our observation. Using cell imaging studies, we also verify that all three materials are suitable for fluorescent staining of biological samples. Our results resolve a long-standing debate on the true structural character of GQDs and their source of emission.