Antimicrobial and Conductive Nanocellulose-Based Films for Active and Intelligent Food Packaging

Abstract

Bacterial nanocellulose (BNC) is becoming an important substrate for engineering multifunctional nanomaterials with singular and tunable properties for application in several domains. Here, antimicrobial conductive nanocomposites composed of poly(sulfobetaine methacrylate) (PSBMA) and BNC were fabricated as freestanding films for application in food packaging. The nanocomposite films were prepared through the one-pot polymerization of sulfobetaine methacrylate (SBMA) inside the BNC nanofibrous network and in the presence of poly(ethylene glycol) diacrylate as cross-linking agent. The ensuing films are macroscopically homogeneous, more transparent than pristine BNC, and present thermal stability up to 265 °C in a nitrogen atmosphere. Furthermore, the films have good mechanical performance (Young's modulus ≥ 3.1 GPa), high water-uptake capacity (450-559%) and UV-blocking properties. The zwitterion film with 62 wt.% cross-linked PSBMA showed bactericidal activity against Staphylococcus aureus (4.3-log CFU mL^-1 reduction) and Escherichia coli (1.1-log CFU mL^-1 reduction), and proton conductivity ranging between 1.5 × 10^-4 mS cm^-1 (40 °C, 60% relative humidity (RH)) and 1.5 mS cm^-1 (94 °C, 98% RH). Considering the current set of properties, PSBMA/BNC nanocomposites disclose potential as films for active food packaging, due to their UV-barrier properties, moisture scavenging ability, and antimicrobial activity towards pathogenic microorganisms responsible for food spoilage and foodborne illness; and also for intelligent food packaging, due to the proton motion relevant for protonic-conduction humidity sensors that monitor food humidity levels.

Keywords: active food packaging; antimicrobial activity; bacterial nanocellulose; intelligent food packaging; moisture scavengers; nanocomposite films; poly(sulfobetaine methacrylate); protonic conductivity.

Figure 1

(A) Reactional scheme representing the radical polymerization of SBMA in the presence of poly(ethylene glycol) diacrylate (PEDGA) as cross-linker and potassium persulfate (KPS) as radical initiator, yielding cross-linked PSBMA, and (B) photographs of dry films of pristine BNC, and nanocomposites PSBMA/BNC_1 and PSBMA/BNC_2.

Figure 2

(A) ATR-FTIR (vibrational modes: ν = stretching, δ = bending) and (B) ¹³C CP/MAS NMR spectra of cross-linked PSBMA, pristine BNC, and nanocomposites PSBMA/BNC_1 and PSBMA/BNC_2.

Figure 3

SEM images with a magnification of ×10.0 k of the (A) surface and (B) cross-section of pristine BNC and nanocomposites PSBMA/BNC_1 and PSBMA/BNC_2.

Figure 4

Thermograms of (A) cross-linked PSBMA, pristine BNC and (B) nanocomposites PSBMA/BNC_1 and PSBMA/BNC_2. The inset curves represent the derivative.

Figure 5

UV-vis transmission spectra of pristine BNC and nanocomposites PSBMA/BNC_1 and PSBMA/BNC_2.

Figure 6

Effect of pristine BNC, PSBMA/BNC_1 and PSBMA/BNC_2 on the bacterial concentration of (A) S. aureus and (B) E. coli after 24 h of exposure; error bars represent the standard deviation; the asterisk (*) denotes statistically significant differences to the control treatment (p < 0.05).

Figure 7

(A) Arrhenius-type plot of the IP conductivity of the film PSBMA/BNC_2 at different RH; the straight lines are linear fits to the Arrhenius model, and (B) conductivity logarithm as a function of RH under variable temperature.