Biobased chitosan films incorporating rosemary-mediated silver nanoparticles for enhanced antimicrobial activity and catalytic applications.

Chitosan (CS), a naturally occurring polysaccharide derived from chitin, is widely recognized for its biodegradability, biocompatibility, and non-toxicity, making it an attractive candidate for sustainable functional materials. Silver nanoparticles (AgNPs) possess remarkable antimicrobial and catalytic properties but are often produced through energy-intensive and non-ecofriendly chemical routes. In this work, we report the green synthesis of AgNPs using Rosmarinus officinalis (rosemary) extract as both a reducing and stabilizing agent, and their subsequent incorporation into CS matrices to prepare CS@R-AgNPs films via a simple ex-situ solution casting approach. The resulting nanocomposite films were comprehensively characterized by UV-Vis spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). TEM micrographs revealed a uniform dispersion of R-AgNPs within the CS matrix, while FTIR analysis confirmed electrostatic interactions between the polymeric chains and the nanoparticles. The CS@R-AgNPs films exhibited strong antibacterial activity against both Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) strains even at a low AgNPs loading (0.5%). Moreover, the films efficiently catalyzed the reduction of 4-nitroaniline (4-NA) to 4-phenylenediamine in the presence of NaBH₄, following pseudo-first-order kinetics with rate constants increasing from 0.0871 to 0.196 min-1 as AgNPs content increased from 1% to 3%. The catalytic system demonstrated excellent stability, retaining 96% conversion after ten successive cycles. These findings underscore the potential of CS@R-AgNPs films as dual-function, environmentally benign materials for antimicrobial and catalytic applications.