Cysteine detection using a high-fluorescence sensor based on a nitrogen-doped graphene quantum dot-mercury(II) system

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 L−1, with a detection limit of 1.3 nmol L−1. The sensor was successfully applied to detect cysteine in honey and beer samples, with a recovery range of 98-105%.