Sustainable antioxidant and antibacterial activities of zinc ion and caffeic acid-coupled carboxymethyl chitosan

Carboxymethyl chitosan (CMCS), a water-soluble derivative of chitosan, exhibits good biocompatibility. However, the limited bioactivities impede its potential applications. This work presents a novel CMCS derivative, termed CMCS-Zn-CA, which was prepared by complexing CMCS with Zn²⁺ and caffeic acid (CA) under microwave irradiation. Structural characterization was conducted using FT-IR, UV, XRD, TGA, and XPS analyses. Results showed that the scavenging rates of CMCS-Zn-CA on DPPH and hydroxyl radical were 95.45% and 92.45%, respectively, while those of CMCS were only 12.62% and 9.21%, respectively. The antibacterial assay on <i>S. aureus</i> and <i>E. coli</i> revealed that CMCS-Zn-CA demonstrated much lower minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) than CMCS, indicating the excellent inhibition ability of CMCS-Zn-CA on both strains. The time-kill curves and plate count confirmed the antibacterial activity of CMCS-Zn-CA, and SEM results validated its efficacious ability in disrupting the cell barrier. Furthermore, CMCS-Zn-CA demonstrated sustainable antioxidant activity by retaining 93.9% of the CA content and 95.4% of the DPPH radical scavenging rate and prolonged the milk storage time by maintaining a higher pH, lower acidity and lower bacterial count. In conclusion, the CMCS complex displayed sustainable antioxidant and antibacterial activities, suggesting its great potential in practical applications.