Zinc oxide nanoparticles (ZnO NPs) have been investigated due to their distinct properties, variety of structures and sizes, and mainly for their antimicrobial activity. They have received a positive safety evaluation from the European Food Safety Authority (EFSA) for packaging applications as transparent ultraviolet (UV) light absorbers based on the absence of significant migration of zinc oxide in particulate form. ZnO NPs with different morphologies (spherical, flower, and sheet) have been synthesized via different sol–gel methods and extensively characterized by several solid-state techniques, namely vibrational spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), Fourier Transform Infrared Spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV-VIS), electron paramagnetic resonance (EPR), and nitrogen adsorption–desorption isotherms. The ZnO NPs were assessed for their antibacterial activity against Escherichia coli (gram-negative bacteria) and Staphylococcus aureus (gram-positive bacteria) to study the influence of morphology and size on efficacy. ZnO NPs with different morphologies and sizes demonstrated antimicrobial activity against both bacteria. The highest microbial cell reduction rate (7–8 log CFU mL⁻¹ for E. coli and 6–7 log CFU mL⁻¹ for S. aureus) was obtained for the sheet- and spherical-shaped NPs as a result of the high specific surface area. In fact, the higher surface areas of the sheet- and spherical-shaped nanoparticles (18.5 and 13.4 m² g⁻¹, respectively), compared to the flower-shaped NPs (5.3 m²g⁻¹), seem to promote more efficient bacterial cell reduction. The spherical-shaped particles were also smaller (31 nm) compared with the flower-shaped (233 × 249 nm) ones. The flower ZnO NP resulted in a 4–5 log CFU mL⁻¹ reduction for E. coli and 3–4 log CFU mL⁻¹ reduction for S. aureus. The lower apparent antibacterial activity of the flower-shaped could be associated with either the lack of defects on the particle core or the shape shielding effect. Compared to S. aureus, E. coli seems to be less resistant to ZnO NPs, which may be explained by the characteristics of its cell membrane. With simple synthesis techniques, which do not allow the size and shape of the nanoparticles to be controlled simultaneously, it is a challenge to elucidate the effect of each of these two parameters on antibacterial performance.

Zinc oxide nanoparticles (ZnO NPs) exhibit diverse morphologies and sizes, influencing their functional properties. However, the relationship between their morphology and behavior under varying conditions remains poorly understood. This study provides novel insights by linking ZnO NPs shape to generation of reactive oxygen species (ROS), and to antimicrobial efficacy under varying temperatures. ROS generation was confirmed via electron paramagnetic resonance, although no antioxidant activity was observed. Antibacterial tests against Escherichia coli and Staphylococcus aureus at different temperatures (4–22 °C) revealed that sheet-shaped NPs achieved complete bacterial reduction (7.5 log CFU mL⁻¹ for E. coli at 4 and 22 °C; 6.8 log CFU mL⁻¹ for S. aureus at 22 °C). Flower-shaped NPs were less effective due to larger size and reduced surface area. Zeta potential ranged from −44 to −58 mV, indicating high stability, with sheet-shaped particles being the most dispersed. Scanning electron microscopy confirmed closer interaction between sheet-shaped NPs and E. coli in agreement with the higher activity. Antibacterial efficacy decreased at 4 °C, highlighting implications for cold storage. The Weibull model successfully described E. coli reduction. These aspects were not previously addressed in the published work. The effect of temperature on the activity and its modeling are new insights into the morphology-dependent antimicrobial activity of ZnO NPs, supporting their integration into packaging materials for food applications.

Polybutylene adipate terephthalate (PBAT) and zinc oxide nanoparticles (ZnO NP) are among the most studied when it comes to address biodegradability and antimicrobial properties of materials for food packaging. This work presents a critical review of recent scientific literature with a focus on the impact in functional, physic‐mechanical properties, compostability and on safety. The properties of the nanoparticles, such as morphology and size reported are reviewed together with the range of concentrations and methods of incorporation in the PBAT matrix. The effect on antimicrobial and antioxidant activities, thermostability, tensile and optical properties, as well as on mass transfer properties (barrier to moisture and gases and migration into foods) is discussed. The properties of PBAT/ZnO depend on particle size, shape and concentration of the ZnO NP. Particles with higher surface area and smallest size are reported to have the best performance (usually the spherical shaped). However, information on the morphology/size of the nanoparticle is often not provided. The typical concentration of NP incorporation ranges from 0.5 to 5%, but the interpretation of results regarding the effect of concentration is highly limited due to poor information on the statistical significance of the results, particularly for low concentrations of incorporation. Another limitation found regards the film production method, because most of the studies are based in films prepared by solvent‐casting and not by extrusion. Antimicrobial activity is directly linked with the concentration of ZnO NP. Most studies applied in vitro experiments and not tests in real foods (in situ). Zinc migration can occur as the whole nanoparticle or in the ionic form. Studies indicate that migration of ZnO in the nano form is not expected if the nanoparticle is completely embedded in a polymeric matrix, which can be confirmed by some well‐established techniques. However, studies focusing on migration from other matrices are lacking. Further investigations are needed to achieve a balanced incorporation of ZnO NP considering the particle activity and zinc migration and to evaluate the impact on safety when used in food packaging.