Development of akaganéite (β-FeOOH, Cl) nanoparticles (Ak NPs), modified polyvinyl alcohol-Ak composites, and chitosan-Lemon Myrtle essential oil conjugates for green antioxidant and antimicrobial food packaging

Abstract

The development of effective food packaging materials not only promotes food transportation and marketing but also decreases food contamination and waste. There is a rising trend towards replacing petroleum-derived packaging plastics by bio-based materials with several functionalities such as antioxidant and antibacterial activities. According to green chemistry principles, this study investigated the possibility of manufacturing akaganéite (β-FeOOH) nanoparticles (Ak NPs) utilizing extracts from winery wastes. Selected Ak NPs were added into biodegradable poly(vinyl alcohol) (PVA) and modified to improve physical properties. The role of Ak NPs in the films was investigated regarding their performance as antimicrobial packaging materials and for their after-use disposal. Furthermore, Lemon Myrtle essential oil (LEO) was incorporated into chitosan (CS) films using a Pickering emulsion stabilized by chitosan-coated lignin (CS@AL) colloidal particles. The potential of these composite films as antioxidant and antimicrobial packaging materials was assessed. In addition, the impact of differently combined LEO and CS on the aggregation-induced emission (AIE) phenomenon of their composite films was revealed. The significant findings from the three studies are described below. For the green synthesis of Ak NPs, environmentally benign, readily scalable processes were developed based on gallic acid or grape seed tannins and urea. The addition of solutions of 0.1 % (w/v) grape seed tannins or gallic acid (at 640 mg L−1) yielded single-crystalline Ak NPs with reduced dimensions, increased porosities, and BET surface area, without formation of oxidized impurities such as hematite (Fe2O3). The added grape seed tannins (S) or gallic acid (G), together with relatively low urea (0.8 M), produced Ak NPs (S0.8 or G0.8) with higher activities as the peroxidase mimics compared to those prepared with only urea (C0.8). Moreover, S0.8 was more efficient in methylene blue (MB) discoloration than C0.8 at all three pH values of 4, 7, and 11. The S0.8-mediated MB degradation pathways at pH 4 and 7 differed from those at pH 11 due to the generation of different oxidants. The overall MB discoloration efficacies of S0.8 at pH 4, 7, and 11 were combinative effects of both physical adsorption and chemical reactions. In summary, the as-prepared Ak NPs can serve as peroxidase mimics to activate hydrogen peroxide (H2O2) for environmental remediation. The Ak NPs (above C0.8), showing moderate reactivities with H2O2, were selected and incorporated into the acidified aqueous ethanol solution of PVA. The initial PVA-Ak films were readily obtainable by solution casting and were heated at 140 ℃ for 10 min (optimal) to achieve in situ insolubilization and modification. The developed fabrication procedure used minimal harmful chemicals and was scalable and free from demanding processing steps. Due to the reactions between Ak NPs and H2O2, the modified PVA-Ak (MP-Ak) films were easily modifiable by immersing them in a dilute aqueous H2O2 solution. The most antimicrobial films enabled the killing of 80 – 100 % of Staphylococcus aureus and Escherichia coli during 10 min contact between film surfaces and the microbial inoculum. Irradiation under simulated sunlight enhanced the antimicrobial efficacies of the films. Moreover, the composite films were dissolved entirely in the aqueous H2O2 solution, owing to the Ak NPs-mediated degradation, achieving the desirable "zero solid waste". Composite films based on CS and LEO were prepared, exhibiting novel dose-dependent fluorescent patterns due to aggregation-induced emission (AIE) effects. Chitosan-coated alkali lignin (CS@AL) colloid particles were obtained by a facile one-pot ultrasonication and were used to prepare LEO-loaded Pickering emulsions (CS@ALPE). Different amounts of CS@ALPE were incorporated into CS-based film-forming solutions. The composite (CS@ALPE-CS) films contained LEO/CS (w/w) ratios around 0 %, 5.9 %, 14.8 %, 29.6 %, and 59.2 % after solution casting. The microstructures, optical properties, water affinity, mechanical attributes, fluorescent characteristics, and antimicrobial and antioxidant efficacies of CS@ALPE-CS films were assessed for multifunctional food packaging. 0.5 % (w/v) of CS@AL particles (pH ∼4.3 to 4.6) enabled stabilization of CS@ALPE (φ = 0.4) for at least 35 days. Adding CS@ALPE with appropriate LEO/CS (w/w) ratios modified the properties of composite films. The 0.5CS@ALPE-CS film (with LEO/CS (w/w) around 29.6 %) was the most promising candidate in this work, considering its optimal optical properties, resistance to moisture and mechanical stress, characteristic fluorescent attributes, and antimicrobial and antioxidant activities.