Green Preparation of S and N Co-Doped Carbon Dots from <i>Water Chestnut</i> and <i>Onion</i> as Well as Their Use as an Off–On Fluorescent Probe for the Quantification and Imaging of Coenzyme A

Fluorescent carbon dots (CDs) originated from natural biomass have been of great interest in recent years because of their superior optical and chemical properties. However, previously reported CDs used only one natural biomass as precursor, and the fluorescence quantum yield (QY) and long-wavelength emissions are usually weak, which restrict their further applications in biology-relevant fields. Here a green method was demonstrated for the preparation of S and N codoped fluorescent CDs (S,N/CDs) by adopting two natural biomasses (water chestnut and onion) as precursors. The fabrication process is simple and environmentally friendly. By hydrothermal heating of water chestnut and onion, monodispersed, highly fluorescent S,N/CDs (diameter 3.5 nm) were obtained. The carboxyl on the surface of S,N/CDs can bind to Cu(II) ion, resulting in the luminescence quenching of S,N/CDs. And coenzyme A (CoA) can restore the luminescence of S,N/CDs. Based on the above features of S,N/CDs, an innovative off–on fluorescence probe was presented for high sensitivity determination of CoA. Under optimum conditions, the linear range for CoA detection is 0.03–40 μM with a detection limit of 0.01 μM. The developed off–on nanoprobe was applied for the quantification of CoA in pig liver, and imaging of CoA in living T24 cells.