Green synthesis of fluorescent carbon dots from Hongcaitai for selective detection of hypochlorite and mercuric ions and cell imaging

Natural source is an effective carbon precursor to produce carbon dots (CDs) with prominent physicochemical and photoluminescent properties. To enrich the CDs' type and widen their applications, more natural sources are desired. Herein, Hongcaitai (Brassica compestris L. var. purpurea Bailey), one of the popular and cheap vegetables in Hubei province of China, was attempted as the carbon precursor for synthesis of CDs by hydrothermal method. The obtained CDs were divided into two parts based on different solubilities in ethanol, CDs-A (ethanol soluble) and CDs-B (ethanol insoluble). Both CDs possessed multi functionalities; however, CDs-B contained sulfur element while CDs-A did not. Fluorescence quenching occured for both CDs in the presence of ClO− and Hg2+. However, CDs-A were more sensitive towards ClO−, while CDs-B were more sensitive towards Hg2+. Thus, CDs-A were successful to determine ClO− in tap water with limit of detection at 0.015 μM. CDs-B were successful to determine Hg2+ in river water with limit of detection at 0.06 μM. The results demonstrate that the sub-classfication of CDs by the subtle different properties, e.g. ethanol solubility, is necessary for better application performance of CDs. In addition, thanks to their low toxicity, both CDs were suitable for cellular labelling on HepG2 cells.