Polycarboxyl Fluorescent Hydrogel Prepared via Rapid Co-Crosslinking of Amino Carbon Dots and Composite Polysaccharide for Cr(VI) Detection and Efficient Adsorption of Neutral Red

Multifunctional hydrogels integrating fluorescence sensing and adsorption have application potential in water treatment, but often face the problem of overlap between crosslinking sites and functional sites. To address this, a “functional differentiation of groups” strategy is proposed in this study. Using carboxymethyl chitosan (CMCS) as the precursor, aminated carbon dots (CD-NH2) were synthesized by a hydrothermal method, and CMCS was used as the gel backbone. Using oxidized sodium alginate (OSA) as an aldehyde crosslinker, a fluorescent composite hydrogel (CD–gel) was prepared within 10 min via a room-temperature aqueous Schiff base reaction. In this network, –NH2 participates in Schiff base crosslinking, while –COOH is retained to provide adsorption sites; CD-NH2 are covalently anchored in the network, endowing the gel with fluorescence responsiveness. FTIR and XPS results support the formation of the Schiff base crosslinked network and the retention of carboxyl groups. SEM and N2 adsorption–desorption measurements show that the gel has a honeycomb-like meso–macroporous structure and is negatively charged over a broad pH range. The gel exhibits a fluorescence quenching response to Cr(VI) with a linear range of 0–80 μM (R2 = 0.990), a detection limit of 0.226 μM, rapid response, and anti-interference ability. Meanwhile, the gel exhibits rapid and efficient adsorption toward the cationic dye neutral red (NR), and the adsorption process follows the pseudo-second-order kinetic equation and the Langmuir isotherm equation, with a maximum theoretical adsorption capacity of 739.13 mg g−1. This study provides a group-engineering idea for the design of multifunctional hydrogels.