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 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, l-cysteine was catalytically oxidized to l-cystine by O, resulting in the reduction of DNPC. Thus, the fluorescence of GQDs was recovery. Based on this, the fluorescent detection of Cu could be achieved. The proposed sensing strategy offered a selective identification of Cu with a detection limit of 4.5 nM. Additionally, the practical application of this assay for Cu determination in real water samples was also demonstrated.