Sulfur and nitrogen co-doped graphene quantum dot-assisted chemiluminescence for sensitive detection of tryptophan and mercury (II)

A simple one-step thermal treatment to prepare strong fluorescent sulfur and nitrogen co-doped graphene quantum dots (SN-GQD) using citric acid and l-cysteine as precursors was developed. The ultra-weak chemiluminescence (CL) from the reaction of hydrogen peroxide (H2O2) and periodate (IO4-) was significantly enhanced by SN-GQD in acidic medium. The enhanced CL was induced by excited-state SN-GQD (SN-GQD*), which was produced from the transfer energy of (O-2)(2)* and O-1(2) to SN-GQD and recombination of oxidant-injected holes and electrons in SN-GQD. In the presence of tryptophan (Trp), the CL intensity of the SN-GQD-H2O2-KIO4 system was greatly diminished. This finding was used to design a novel method for determination of Trp in the linear range 0.6-20.0 mu M, with a limit of detection (LOD) of 58.0 nM. Furthermore, Hg2+ was detectable in the range 0.1-9.0 mu M with a LOD of 64.0 nM, based on its marked enhancement of the SN-GQD-H2O2-KIO4 CL system. The proposed method was successfully applied to detect Trp in milk and human plasma samples and Hg2+ in drinking water samples, with recoveries in the range 95.7-107.0%.