Graphene sheets with lateral dimensions less than 100 nm are known as GQDs, which have very low toxicity and exhibit high stability, excellent solubility, stable photoluminescence, and good biocompatibility. A fluorescence “off-to-on” mechanism of bright blue fluorescent glutathione (GSH)-functionalized GQDs (GQDs@GSH) for assaying adenosine triphosphate (ATP) was proposed