Nitrogen-Doped Carbon Dots with Free Radical Scavenging Abilities as Fluorescence Probes for Cr(VI) Sensing in Bio-Systems

Carbon dots (CDs) are a new class of fluorescent nanomaterials that can be used in optical devices, fluorescence sensing and bio-imaging. Recently, the preparation of CDs from biomass gained great interest because the source is plentiful and they are friendly to environment. However, there still exist some problems including low quantum yield (QY) and limited application. Although CDs have been successfully applied in detection of various ions, the sensing of Cr(VI) and the corresponding mechanism is rarely proposed. Here, a facile and effective preparation of N-doped carbon dots (N-CDs) from sweet potatoes was developed through hydrothermal synthesis, using urea as passivator. The characterization was performed to demonstrate the morphology and chemical composition of the N-CDs. In particular, it is confirmed that N element exists in NH2 group on the surface of the N-CDs. Because of doing with N element, the QY of these CDs significantly increases to 31%. In addition, the N-CDs exhibit favorable free radical scavenging activities toward DPPH, hydroxyl and superoxide anion free radicals. This antioxidant property, combined with inner filter effect, endows the N-CDs with florescence response to Cr(VI). The further study showed that linear range of Cr(VI) concentration is from 1 mu M to 80 mu M with a detection limit of 0.61 mu M. Subsequently, the N-CDs were competent to detect trace Cr(VI) both in human serum and pig liver. Based on the fact that the N-CDs are biocompatible by MTT experiment, the N-CDs were excellent in fluorescent sensing of Cr(VI) in living cells. In a word, the N-CDs are suitable to be applied for Cr(VI) detection in bio-systems, as well as in antioxidant fields.