▎ 摘　　要

NOVELTY - Synthesizing nitrogen-doped graphene quantum dot comprises (i) adding 20 mg citric acid into a reaction vessel, and adding 20 ml secondary water to the reaction vessel to obtain 1 mg/ml citric acid solution, and (ii) adding aqueous ammonia into the reaction kettle, and where the solid-to-liquid ratio of citric acid to ammonia water is 20:0-20:1, and mixing the aqueous ammonia with the citric acid solution in the reaction kettle, placing the reaction kettle into an oven, and making the solution in the reaction kettle react under the condition that the reaction temperature is 170-210 degrees C, after the reaction time is 2-4 hours to obtain synthetic liquid, and dialyzing the synthetic liquid after overnight. USE - The quantum dot is useful for detecting mercuric cation (Hg2+) in water (claimed). ADVANTAGE - The method is simple and convenient, and is economical.