Facile Syntheses of S,N-Codoped Carbon Quantum Dots and Their Applications to a Novel Off–On Nanoprobe for Detection of 6‑Thioguanine and Its Bioimaging

S,N-Codoped carbon quantum dots (SNCQDs) with a quantum yield (QY) of 23% have been synthesized using lotus root as a carbon source and glutathione (GSH) as a nitrogen and sulfur source. The obtained SNCQDs exhibited many excellent features such as a simple preparation process, high fluorescence QY, satisfactory photostability, superior water solubility, and biocompatibility. The SNCQDs combined with Cu2+ were used as a novel off–on nanoprobe for 6-thioguanine (TG). Initial fluorescence is significantly quenched in this sensing system via electron transfer from SNCQDs to Cu2+. Fluorescence is recovered after adding TG owing to Cu2+–TG complex formation between Cu2+ and TG. A novel off–on fluorescent nanoprobe was presented for highly sensitive determination of TG. Under optimal conditions, the nanoprobe exhibited a wide linearity range of 0.005–80 μmol/L for TG and a detection limit of 1.6 nmol/L. The designed off–on nanoprobe was successfully utilized for detecting TG in plasma and urine of a leukemia patient and imaging of TG in living T24 cells.