Functionalization of cellulose carbon dots with different elements (N, B and S) for mercury ion detection and anti-counterfeit applications

Mercury ion (Hg²⁺), as one of the most toxic heavy metal ions, accumulates easily in the environment, which can generate potential hazards to the ecosystem and human health. To effectively detect and remove Hg²⁺, we fabricated four types of carbon dots (CDs) using carboxymethyl nanocellulose as a carbon source doped with different elements using a hydrothermal method. All the CDs exhibited a strong fluorescence emission, excitation-dependent emission and possessed good water dispersibility. Moreover, the four fluorescent CDs were used for Hg²⁺ recognition in aqueous solution, where the CDs-N exhibited better sensitivity and selectivity for Hg²⁺ detection, with a low limit of detection of 8.29 × 10⁻⁶ mol/L. It was determined that the fluorescence quenching could be ascribed to a photoinduced charge-transfer processes between Hg²⁺ and the CDs. In addition, the CDs-N were used as a smart invisible ink for anti-counterfeiting, information encryption and decryption. Furthermore, the CDs-N were immersed into a cellulose (CMC)-based hydrogel network to prepare fluorescent hydrogels capable of simultaneously detecting and adsorbing Hg²⁺. We anticipate that this research will open possibilities for a green method to synthesize fluorescent CDs for metal ion detection and fluorescent ink production.