▎ 摘　　要

NOVELTY - Preparing nitrogen-doped graphene quantum dot comprises dissolving carbon source compound and nitrogen source compound in water to obtain mixed aqueous solution; carrying hydrothermal reaction under alkaline conditions to obtain nitrogen-doped graphene quantum dot; where the carbon source compound is 1,3,6-trinitropyrene, and the nitrogen source compound is tryptophan. USE - The quantum dot is useful in detecting mercury ions, and used in kit for detecting cysteine (all claimed). ADVANTAGE - The quantum dot: has blue fluorescence and unique mercury ion selectivity; can achieve sensitive fluorescence detection of mercury ions with low detection limits; and can achieves fluorescence detection of cysteine by combining with mercury ions. DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for: (1) nitrogen-doped graphene quantum dot is prepared by above mentioned method; (2) nitrogen-doped graphene quantum dot is useful in detection of mercury ions; and (3) use of mixture of nitrogen-doped graphene quantum dot and mercury ions in preparation of kit for detecting cysteine.