# Thin films coating with ZnO nanoparticles synthesized via acetic acid bacteria: Structural and antibacterial characterization for food applications

## TL;DR

This paper explores using bacteria to create zinc oxide nanoparticles and thin films that can kill bacteria, making them useful for food and medical applications.

## Contribution

The novel contribution is the green synthesis of ZnO nanoparticles using acetic acid bacteria and their application in antimicrobial thin films.

## Key findings

- ZnO nanoparticles synthesized using acetic acid bacteria showed strong antibacterial activity against S. aureus and E. coli.
- Thin films made from these nanoparticles exhibited hydrophilic properties and inhibited E. coli in direct contact tests.
- XRD and FTIR analyses confirmed the structural properties of ZnO nanoparticles and thin films.

## Abstract

In this study, zinc oxide nanoparticles (ZnO NPs) were synthesized using extracellular metabolites of acetic acid bacteria (AAB) derived from fermented beverages, and then a thin film coating of ZnO NPs was created via spin coating techniques. ZnO NPs and thin films obtained were characterized using physical, structural, and biological techniques. Firstly, two AAB isolates were obtained from olive leaf vinegar and kombucha with ginger, and genotypically identified. ZnO NPs synthesis by N. hansenii B1 and N. hansenii B2 was indicated by a white precipitate and confirmed for both NPs and thin films by UV-Vis absorption at 300–400 nm. SEM images revealed agglomerated ZnO NPs and homogeneous nanosized thin films, while EDX analysis confirmed their pure phase composition with zinc and oxygen peaks. XRD results showed the presence of monoclinic and cubic crystal ZnO NPs, with crystallite sizes ranging from 9.68 to 35.11 nm, while the thin films were amorphous. FTIR spectra revealed 519 and 3350 cm− 1 peaks corresponding to ZnO bonds. The measurements of water contact angles of ZnO thin films exhibited hydrophilic surface characteristics. Moreover, ZnO NPs exhibited strong antibacterial activity, being effective against S. aureus and E. coli O157:H7 (12.50–14.00 mm), with minimum inhibitory concentration values of 0.625–1.25 mg/mL. The inhibition zones of thin films were found between 0.50 and 2.38 mm, and both films completely inhibited E. coli O157:H7 in direct contact tests after 2 h. These findings suggest that green-synthesized ZnO NPs and their thin films have great potential as antimicrobial applications in various food, medicine, and related fields.