Valorization of prawn shell waste-derived chitosan into hydrazone functionalized Schiff base for colorimetric ion sensing, solid-state fluorescence, antimicrobial, antifungal, and antioxidant applications

In this study, we converted prawn shell waste-derived chitosan into a hydrazone-functionalized Schiff base containing 2-hydroxy-1-naphthaldehyde moieties through sequential chemical modifications. We characterized the synthesized chitosan-based Schiff base using FTIR, UV-visible spectroscopy, ¹H NMR, ¹³C NMR, HR-MS, and HPLC. The hydrazone Schiff-base formation was confirmed by characteristic CN stretching in the FTIR spectrum and azomethine (CHN) proton signals in the ¹H NMR spectrum, consistent with hydrazide condensation with 2-hydroxy-1-naphthaldehyde. The material exhibited colorimetric sensing, solid-state fluorescence, antimicrobial, antifungal, and antioxidant properties. Naked-eye color changes were observed in the presence of Fe²⁺, Fe³⁺, Pb²⁺, OH^-, CN^-, and CO₃^2- ions, while fluorescence changes occurred with Al³⁺ and Ag ⁺ ions. The detection limits for Al³⁺, Ag⁺, Fe³⁺, Fe²⁺, and Pb²⁺ were 3, 5, 9, 10, and 12 ppm, respectively, while those for OH^-, CN^-, and CO₃^2- were 8, 4, and 3 ppm, respectively. The compound also exhibited strong yellow-green solid-state fluorescence under UV irradiation. Furthermore, the compound showed antibacterial activity against Escherichia coli and Staphylococcus aureus, antifungal activity against Candida albicans, and 76.11% DPPH radical-scavenging activity. Molecular docking studies using a representative oligomeric model of the material indicated predicted binding affinities of -6.8, -6.5, and -8.7 kcal/mol for C. albicans CYP51, E. coli DNA gyrase B, and S. aureus Sortase A, respectively. Overall, this work demonstrates an eco-friendly, sustainable approach to valorizing prawn shell waste into advanced, multifunctional bio-based materials with promising environmental, analytical, and biomedical applications.