Green and sustainable preparation of functionalized carbon quantum dots and fluorescent hydrogel from exopolysaccharides for multiple fluorescence applications

Xanthan gum (XG) and oxidized xanthan gum (OXG) were used as raw materials to prepare two fluorescent carbon quantum dots (CQDs), XG-CQDs and OXG-CQDs, by hydrothermal method. Their microstructure, optical properties and the selectivity and accuracy in Cr6+ ion detection and fluorescent anti-counterfeiting applications were investigated. They indicated stable fluorescence in high concentration of salt (4 mol/L NaCl) and wide pH range, and had single selectivity to Cr6+ ion, good information encryption and anti-counterfeiting effect. OXG-PEI (polyetherimide) hydrogel was further built based on the cross-linking action of aldehyde groups in OXG-CQDs with amino groups of PEI. OXG-PEI hydrogel displayed the same photoluminescence properties as OXG-CQDs, and could be combined with a smartphone to build an RGB-based detection platform with lower limit of detection (3.67 mg/L) for Cr6+ ion detection. Based on its down-conversion luminescence properties, OXG-PEI hydrogel could absorb more than 99.0 % UV light (200-400 nm) for UV shielding application, and it as light conversion material layer could be coated in light emitting diode (LED) chip to construct a pure green LED device. This research provides a new insight into the preparation of multifunctional XG-derived CQDs materials with dazzling applications in environmental detection, UV shielding, LEDs construction and fluorescent anti -counterfeiting.