Solvent dependent synthesis of edge-controlled graphene quantum dots with high photoluminescence quantum yield and their application in confocal imaging of cancer cells.

We report on the synthesis of edge-controlled and highly fluorescent few-layer graphene quantum dots (GQDs) using different solvents and explore their application in the confocal imaging of cancer cells. TEM and AFM imaging analysis reveal that GQDs of sizes in the range 5-8 nm and few-layer (1-4) thickness were grown using DMF, DMSO, and water as solvents. Micro-Raman analysis reveals that GQDs grown with DMF possess primarily the armchair edges, while that grown with water contains primarily the zigzag edges. The nature of oxygen functional groups on the edge/in-plane sites of carbon atoms was elucidated through thermogravimetric and FTIR analyses. The GQDs containing high density of armchair edges and oxygen functional group defects exhibited high photoluminescence (PL) quantum yield (∼32%). The time-resolved PL measurements suggest the charge transfer from the GQDs to the surrounding dielectric medium. Further, we explore the high PL quantum yield of GQDs in bio-imaging of A-375 and HeLa cancer cells. The cell viability of GQDs on A-375 cells was found to be considerably higher than that of HeLa cells at a GQD concentration of 44.4 µg/mL, which is very significant. Our results indicate the GQD edge site dependent cell viability, for the first time. These results will be useful for the development of highly fluorescent GQDs with specific edge structure and their exploration in the field of bio-imaging, bio-sensing, and drug delivery applications.