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 Cu2+ 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 Cu2+, l-cysteine was catalytically oxidized to l-cystine by O2, resulting in the reduction of DNPC. Thus, the fluorescence of GQDs was recovery. Based on this, the fluorescent detection of Cu2+ could be achieved. The proposed sensing strategy offered a selective identification of Cu2+ with a detection limit of 4.5 nM. Additionally, the practical application of this assay for Cu2+ determination in real water samples was also demonstrated.