An “off-on” fluorescent sensor for copper ion using graphene quantum dots based on oxidation of l-cysteine

A simple and highly efficient “off-on” fluorescent sensor based on grapheme quantum dots (GQDs) for Cu 2+ was developed. In this sensing platform, the fluorescence of GQDs was quenched in the presence of 2,4-dinitrophenylcysteine (DNPC), which is the reaction product of 1-chloro-2,4-dinitrobenzene (CDNB) and l -cysteine, owing to the spectral overlap between the absorption of DNPC and the excitation of GQDs. In the presence of Cu 2+ , l -cysteine was catalytically oxidized to l -cystine by O 2 , resulting in the reduction of DNPC. Thus, the fluorescence of GQDs was recovery. Based on this, the fluorescent detection of Cu 2+ could be achieved. The proposed sensing strategy offered a selective identification of Cu 2+ with a detection limit of 4.5 nM. Additionally, the practical application of this assay for Cu 2+ determination in real water samples was also demonstrated.