Liquid-liquid extraction and purification of oil red O derived nitrogen-doped highly photoluminescent carbon dots and their application as multi-functional sensing platform for Cu2+ and tetracycline antibiotics

Oil red O was applied as precursor to synthesize nitrogen-doped carbon dots (NCDs) by a facile solvothermal method for the first time. Surprisingly, liquid–liquid extraction and purification resulted in the quantum yield (QY) of NCDs up to 68%. The purification mechanism is based on the difference in polarity and solubility between precursor, over-carbonized substances and NCDs. Further, optical properties and structure characterizations of NCDs were characterized by ultraviolet (UV) spectroscopy, fluorescence (FL) spectroscopy, transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Importantly, NCDs can be acted as an outstanding multi-functional sensing platform for Cu2+ and tetracycline antibiotics (TCs). Compared with other coexisting metal ions and antibiotics, this platform showed special responses to Cu2+ and TCs. NCDs showed high selectivity and sensitivity of fluorescence response to Cu2+ over a broad range of 10 nM −50 μM and a satisfactory detection limit (LOD)of 4 nM. Besides, NCDs also showed high sensitivity for the detection of TCs with a LOD range of 50 pM-100 nM. In addition, the fluorescent quenching mechanism of Cu2+ and TCs to NCDs were identified as IFE and static quenching. Finally, the platform was successful applied to detect Cu2+ and TCs in actual samples (e.g., water, milk, urine) with satisfactory results. All results indicated that NCDs appeared to have tremendous potential for Cu2+ and TCs detection in various fields.