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

Abstract

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 S. aureus and E. coli 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.