Metal-Ion-Cross-Linked Nitrogen-Doped Carbon Dot Hydrogels for Dual-Spectral Detection and Extractable Removal of Divalent Heavy Metal Ions

Nitrogen-doped carbon dots (NCDs) synthesized by the low-temperature sintering method show electronegativity rich in -COO-, -CO-NH-, -NH-, and -OH. The structure, morphology, and fluorescence performance of NCDs were successively characterized by Fourier-transform, infrared, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, photoluminescence, ultraviolet (UV), and X-ray fluorescence analyses. On the one hand, NCDs can sensitively detect Pb2+ in water with a detection limit of 3 ppb through the dual response of fluorescence emission spectra and UV absorbance spectra. On the other hand, the negatively charged NCDs combine with various positively charged heavy metal ions (Pb2+, Cu2+, Ni2+, Co2+, and Cd2+) through charge attraction and coordination to form a hydrogel, thereby removing heavy metal ions from the water. Among them, the removal rate of Pb2+ by NCDs is as high as 94.8%. It is the first time that pure NCDs have been used to remove heavy metal ions quickly and easily in the form of a reversible hydrogel. A simple way is to achieve ultrasensitive detection and high-efficiency removal of heavy metal ions in water with unitary NCDs.