Innovative Antimicrobial Chitosan/ZnO/Ag NPs/Citronella Essential Oil Nanocomposite-Potential Coating for Grapes

Abstract

New packaging materials based on biopolymers are gaining increasing attention due to many advantages like biodegradability or existence of renewable sources. Grouping more antimicrobials agents in the same packaging can create a synergic effect, resulting in either a better antimicrobial activity against a wider spectrum of spoilage agents or a lower required quantity of antimicrobials. In the present work, we obtained a biodegradable antimicrobial film that can be used as packaging material for food. Films based on chitosan as biodegradable polymer, with ZnO and Ag nanoparticles as filler/antimicrobial agents were fabricated by a casting method. The nanoparticles were loaded with citronella essential oil (CEO) in order to enhance the antimicrobial activity of the nanocomposite films. The tests made on Gram-positive, Gram-negative, and fungal strains indicated a broad-spectrum antimicrobial activity, with inhibition diameters of over 30 mm for bacterial strains and over 20 mm for fungal strains. The synergic effect was evidenced by comparing the antimicrobial results with chitosan/ZnO/CEO or chitosan/Ag/CEO simple films. According to the literature and our preliminary studies, these formulations are suitable as coating for fruits. The obtained nanocomposite films presented lower water vapor permeability values when compared with the chitosan control film. The samples were characterized by SEM, fluorescence and UV-Vis spectroscopy, FTIR spectroscopy and microscopy, and thermal analysis.

Keywords: Ag nanoparticles; ZnO; antimicrobial packaging; chitosan; citronella essential oil.

Figure 1

The FTIR maps for CZ1–CZ4 films at wavenumbers 3275 cm⁻¹ (top row); 1743 cm⁻¹ (intermediate row); and 1643 cm⁻¹ (bottom row).

Figure 2

The FTIR maps for CA1–CA4 films at wavenumbers 3500 cm⁻¹ (top row) and 1740 cm⁻¹ (intermediate row); the last row represents the FTIR video image highlighting the morphology of the sample CA4 (in fact, all these membranes have the same aspect).

Figure 3

The FTIR maps for CZA1–CZA5 films at wavenumbers 3393 cm⁻¹ (top row) and 1643 cm⁻¹ (bottom row).

Figure 4

The UV-Vis spectra for chitosan and CZ1–CZ4 films (left) and CA1–CA4 films (right).

Figure 5

The UV-Vis spectra for CZA1–CZA5 films.

Figure 6

The PL spectra for chitosan and CZ1–CZ4 films (left) and CA1–CA4 films (right).

Figure 7

The photoluminescence spectrum (PL) spectra for chitosan and CZA1–CZA5 films.

Figure 8

The TG-DSC curves for chitosan and CZ1–CZ4 films.

Figure 9

The TG-DSC curves for CA1–CA4.

Figure 10

The TG-DSC curves for CZA1–CZA5.

Figure 11

The SEM images for chitosan, CZ1, CZ2, CZ3, and CZ4 films.

Figure 11

The SEM images for chitosan, CZ1, CZ2, CZ3, and CZ4 films.

Figure 12

The SEM images for chitosan, CA1, CA2, CA3, and CA4 films.

Figure 12

The SEM images for chitosan, CA1, CA2, CA3, and CA4 films.

Figure 13

The SEM images for chitosan, CZA1, CZA2, CZA3, CZA4, and CZA5 films.

Figure 13

The SEM images for chitosan, CZA1, CZA2, CZA3, CZA4, and CZA5 films.

Figure 14

The water vapor permeability (WVP) mechanism for the control film (left), chitosan–ZnO&AgNPs films (middle), and cracked films (right).

Figure 15

The antimicrobial activity of CZ1–CZ4 films and ZnO nanoparticles—different letters indicate statistically significant differences between films (p < 0.05).

Figure 16

The antimicrobial activity of CA1–CA4 films and CEO—different letters indicate statistically significant differences between films (p < 0.05).

Figure 17

The antimicrobial activity of CZA1–CZA5 films—different letters indicate statistically significant differences between films (p < 0.05).

Figure 18

Visual appearance of white grapes packaged in control (polyethylene film) and CZA1–CZA5 films, after 0, 7, and 14 days storage at 30 °C and 60% relative humidity.