Sustainable Synthesis of Fe³⁺-Responsive Fluorescent Probes from Crab Shell Waste-Derived Chitosan

It is crucial to develop advanced fluorescence sensing platforms for precise detection of metal pollutants in environmental monitoring. However, traditional fluorescent probes are often limited by aggregation-caused quenching (ACQ). In contrast, clusterization-triggered emission (CTE) probes, based on non-conjugated systems, exhibit excellent photostability by relying on spatial clusterization to restrict molecular motion. Crab shells, an abundant aquatic waste, are rich in chitin (20-30%). Following deacetylation, chitin is converted into chitosan, a biocompatible and biodegradable biopolymer with excellent potential for chemical modification. In this study, a novel chitosan-based CTE fluorescent probe (CS-FA) was synthesized via a facile cross-linking condensation reaction between chitosan and formaldehyde. This process successfully restricts intramolecular motion and promotes the tight clustering of electron-rich heteroatoms, thereby activating the CTE mechanism. The resultant CS-FA probe exhibits strong and stable blue fluorescence and demonstrates high selectivity and sensitivity toward Fe³⁺ in aqueous media. In the concentration range of 10-100 µM, the fluorescence intensity decreases linearly with the Fe³⁺ concentration, yielding a competitive limit of detection (LOD) of 0.52 µM. Mechanistically, the specific coordination between Fe³⁺ and the cross-linked polymer provides a dominant non-radiative decay pathway, leading to significant fluorescence quenching. Ultimately, this work not only proposes an innovative strategy for constructing sensitive and biomass-derived probes for Fe³⁺ monitoring but also broadens the high-value utilization pathways of marine waste, thereby providing a sustainable waste-to-resource strategy.