The on-off-on Fluorescence Sensor of Hollow Carbon Dots for Detecting Hg²⁺ and Ascorbic Acid

Carbon dots (CDs) have excellent fluorescence properties and can be used in many research fields. In this paper, carbon dots were prepared by microwave-assisted pyrolysis of citric acid and urea, characterized by transmission electron microscope (TEM), X-ray diffractometer (XRD), ¹³C-NMR spectrum, zeta potential, Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible (UV-vis) absorption and fluorescence spectra, and detected the Hg²⁺ and ascorbic acid (AA) sequentially. It showed that carbon dots were hollow, spherical particles and less than 10 nm, photoluminescence quantum yield of carbon dots was about 15%. The CDs were selective and sensitive to Hg²⁺ and AA based on the "on-off-on" fluorescence behavior. The detection limits of CDs for Hg²⁺ and AA were 0.138 μM and 0.212 μM, respectively. Fluorescence response mechanism of CDs was also discussed.