Abstract
A series of PLA/ZnO bionanocomposite films were prepared by introducing positively surface charged zinc oxide nanoparticles (ZnO NPs) into biodegradable poly(lactic acid) (PLA) by the solvent casting method, and their physical properties and antibacterial activities were evaluated. The physical properties and antibacterial efficiencies of the bionanocomposite films were strongly dependent on the ZnO NPs content. The bionanocomposite films with over 3% ZnO NPs exhibited a rough surface, poor dispersion, hard agglomerates, and voids, leading to a reduction in the crystallinity and morphological defects. With the increasing ZnO NPs content, the thermal stability and barrier properties of the PLA/ZnO bionanocomposite films were decreased while their hydrophobicity increased. The bionanocomposite films showed appreciable antimicrobial activity against Staphylococcus aureus and Escherichia coli. Especially, the films with over 3% of ZnO NPs exhibited a complete growth inhibition of E. coli. The strong interactions between the positively charged surface ZnO NPs and negatively charged surface of the bacterial membrane led to the production of reactive oxygen species (ROS) and eventually bacterial cell death. Consequently, these PLA/ZnO bionanocomposite films can potentially be used as a food packaging material with excellent UV protective and antibacterial properties.
Highlights
Food packaging materials play an important role in protecting the food product from environmental factors such as moisture, light, oxygen, microbes, mechanical stress, and dust
Fourier-transform infrared (FT-IR) spectroscopy was employed to investigate the interfacial interactions between poly(lactic acid) (PLA) and zinc oxide nanoparticles (ZnO NPs) [21]
With the increasing ZnO NPs content, the intensities of the characteristic peaks, such as C=O stretching, –C–H– bending, and –C–O– stretching were increased because the nucleation density of PLA increased upon the introduction of ZnO NPs [3]
Summary
Food packaging materials play an important role in protecting the food product from environmental factors such as moisture, light, oxygen, microbes, mechanical stress, and dust. PLA has some drawbacks such as brittleness, a weak barrier property, poor UV-blocking ability, low heat distortion temperature, and low melt viscosity These properties need to be improved before application in food packaging. It has been reported that ZnO NPs with positively charged surface exhibit stronger antimicrobial activities against Gram-negative bacteria than Gram-positive bacteria. It is expected that negatively charged oxygen with unpaired electrons in PLA polymer would have higher interfacial electrostatic interactions with the positively charged surface ZnO NPs [11], resulting in a good dispersion in the PLA matrix and better physical properties in their bionanocomposite films [10]. To evaluate the feasibility of the PLA/ZnO bionanocomposite films as antimicrobial food packaging materials, their morphological, thermal, barrier, and antimicrobial properties were evaluated in detail
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