A novel and highly sensitive fluorescence sensor, which was based on the recovered fluorescence of a
nitrogen-doped graphene quantum dot–Hg(II) system, was developed for cysteine detection. An easy,
green, one-pot synthesis of nitrogen-doped graphene quantum dots was established by using citric acid
and urea as carbon and nitrogen sources, respectively. The fluorescence of nitrogen-doped graphene
quantum dots was significantly quenched by Hg(II) because of the efficient electron transfer between
nitrogen-doped graphene quantum dots and Hg(II). Subsequently, fluorescence was recovered gradually
upon cysteine addition to form a stable complex with Hg(II). The fluorescence sensor showed a response
to cysteine within a wide concentration range of 0.05–30 μmol L1
, with a detection limit of
1.3 nmol L1
. The sensor was successfully applied to detect cysteine in honey and beer samples, with a
recovery range of 98–105%.