Green and microwave-assisted synthesis of carbon dots and application for visual detection of cobalt(II) ions and pH sensing

A facile, economical, and green method for the preparation of carbon dots (CDs) was established via microwave irradiation using green natural material kelp as the main carbon source and ethylenediamine as nitrogen dopant. The as-prepared CDs were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), fluorescence spectrophotometer and UV–vis absorption spectra. Furthermore, the CDs showed high sensitivity toward pH and cobalt(II) ion(Co2+). To be specific, the fluorescence intensity of CDs exhibited a linear relationship over the pH range from 3 to 8, and the CDs were applied in the visual detection of Co2+. With the addition of Co2+, the color change (from colorless to brownish yellow under visible light and from bright blue to dark blue under ultraviolet light) of the solution can be clearly seen with the naked eye. Under optimal conditions, the CDs enabled selective detection of Co2+ with a linear range of 1–200 μmol/L and a limit of detection with 0.39 μmol/L. Finally, this method was applied in the analysis of real river water samples.