Development of PLA-based bilayer nanofibers containing ZIF (zeolitic imidazolate framework)-67 nanoparticles for active food packaging applications in citrus preservation

In this study, antimicrobial monolayer and bilayer nanofibers were developed from poly(lactic acid) (PLA) by incorporating zeolitic imidazolate framework-67 (ZIF-67) nanoparticles via electrospinning for active food packaging. ZIF-67 nanoparticles were synthesized under varying conditions, and their physicochemical properties were thoroughly characterized. Active monolayer PLA nanofibers were produced by directly electrospinning a PLA solution containing ZIF-67 nanoparticles, while bilayer fibers were formed by electrospinning ZIF-67 onto the surfaces of pure PLA nanofibers. The diameter and surface porosity of the fibers were affected by both the concentration of PLA and the content of ZIF-67 nanoparticles. Incorporation of ZIF-67 nanoparticles increased the tensile strength of monolayer and bilayer fibrous films by 3 and 4.5-fold, respectively, compared to pure PLA fibrous films, while slightly reducing their elongation at break. The composite bilayer films showed a controlled release of cobalt ions at pH 7.5 over 12 days. ZIF-67-loaded PLA nanofibers produced by two methods exhibited enhanced antimicrobial effects against Escherichia coli and Staphylococcus aureus compared to pure PLA. Citrus fruits coated with monolayer and bilayer PLA nanofibers had significantly lower weight losses of 1.18 % and 3.20 %, respectively, in comparison with uncoated citrus fruits, which reached 15.09 % after 20 days of storage at 25 degrees C. ZIF-67-loaded PLA fibrous films preserved color, reduced weight loss and delayed the decrease in firmness, and prevented mold growth on the surface over 20 days storage at 25 degrees C. These results demonstrate the potential of ZIF-67-loaded PLA nanofibers for active food packaging applications with good antimicrobial effects.