Development of green chitosan/gelatin coating reinforced with photothermal ZnO nanoparticles for light-responsive antibacterial packaging application

Abstract

Development of fruit packaging with antibacterial properties is of great interest. Zinc oxide nanoparticles (ZnO NPs) are widely used to enhance the antibacterial properties of chitosan-based packaging materials. ZnO NPs, with sizes ranging from 5 to 100 nm, exhibit strong antibacterial activity, though they also pose potential risks to human health. To address this concern, this study develops a novel approach that leverages the photothermal properties of larger ZnO NPs (>100 nm) to develop a light-responsive antibacterial coating. ZnO NPs (5 wt%) with a spindle morphology and photothermal activity were synthesized and incorporated into a chitosan/gelatin (1:1) blend, acting as light-to-heat converters. When exposed to light, these ZnO NPs induced a localized temperature increase of 10.3–14.2°C. This rise in temperature facilitated the movement of polymer chains, triggering a reversible thermal transition in the coating. Consequently, the encapsulated antibacterial agent polyhexamethylene guanidine (PHMG) was released. The release of PHMG was accelerated by up to 386% under light. The antibacterial rate of the coating, when exposed to light, achieved 97.3% against E. coli and 99.4% against S. aureus, respectively. Furthermore, blueberries packaged with the coating showed inhibited microbial growth by 29.1% and reduced weight loss under light. This approach presents a novel alternative to conventional strategies that rely on the reactive oxygen species (ROS)-mediated antibacterial activity of small ZnO NPs (<100 nm), thereby mitigating potential nanotoxicity concerns while enabling on-demand antimicrobial action, making it particularly suitable for food packaging applications.