Sensitive and selective turn-off-on fluorescence detection of Hg^sup 2+^ and cysteine using nitrogen doped carbon nanodots derived from citron and urine

An environmentally benign and cost efficient cysteine fluorescence sensor is constructed on the basis of nitrogen doped carbon nanodots (NCNDs) prepared by using citron fruit extract and human urine waste as carbon and nitrogen sources, respectively. The turn-off-on fluorescence technique is exploited for the sensing of cysteine, in which the initial fluorescence of NCNDs is quenched with Hg2+via an electron transfer process. Owing to the competitive binding of Hg2+ with cysteine, the ‘turn-off’ fluorescence sensor is switched to the ‘turn-on’ state. Under optimized conditions, NCNDs-Hg2+ assembly demonstrates a lower detection limit and a wide linear range toward cysteine detection. Furthermore, NCNDs display the discriminative detection of cysteine from the similar bioactive molecules and the co-existence of interference species does not influence the detection of NCNDs toward the interest of an analyte. Furthermore, the practical application of NCNDs based fluorescence sensor is validated with the good recovery in human urine samples, which explores the new avenues in the fluorescence sensing of biomolecules.