A simple fluorescent “Turn off-on” sensor based on P, N-doped graphene quantum dots for Hg2+ and Cysteine determination

A new dual-doped, Phosphorus(P) and Nitrogen(N), fluorescent graphene quantum dots (P, N-GQDs) have been synthesized and used to detect “Mercury(II)-Cysteine (Hg2+-Cys)” as a “Turn off-on” fluorescent sensor. First, blue-green emitting P, N-GQDs with high quantum efficiency were successfully prepared for the first time from lemon salt by thermal pyrolysis method, which is simple and fast. The structure of P, N-GQDs was analyzed by the spectroscopic techniques. Next, P, N-GQDs were examined as a “Turn-off” fluorescent sensor to detect various metal ions. It was determined that the fluorescence intensity of P, N-GQDs was selectivity quenched only with Hg2+ ions. Afterwards, Hg2+ loaded P, N-GQDs (P, N-GQDs@Hg) were studied as a “Turn-on” fluorescent sensor to detect various amino acids/biomolecules. The fluorescence intensity of P, N-GQDs@Hg was determined to be almost completely (about 95%) and selectively recovered with cysteine. Thus, it was determined that P, N-GQDs could be used selectively and sensitively as “Turn off-on” fluorescence sensors for “Hg2+-Cys”. Furthermore, the limit of detection (LOD) was determined as 0.13 µM for Hg2+ and 0.65 µM for Cys and the interference experiments were carried out for each ion and molecule. In addition, the designed method was successfully applied to the real samples. Consequently, the P, N-GQDs could be applied as a very precise and selective “Turn-off-on” fluorescence sensor for Hg2+ and Cys.