Eco-friendly synthesis of ZnO nanostructures from yeast strains isolated from kombucha and beetroot kwass for antimicrobial thin film applications

The increasing demand for sustainable antimicrobial materials has accelerated the development of environmentally friendly synthesis strategies that minimize the use of hazardous chemicals and energy-intensive processes. However, the application of unconventional yeast strains for integrated nanoparticle production and thin film fabrication remains largely unexplored. In this study, yeast strains isolated from kombucha and beetroot kvass were employed for the extracellular biosynthesis of zinc oxide nanoparticles (ZnO NPs), followed by thin film fabrication, enabling systematic investigation of structure-property-antibacterial relationships. This approach provides a sustainable route for developing biologically derived antimicrobial materials with potential relevance to food and biomedical applications. ZnO NPs were synthesized and deposited as thin films via spin coating. Ultraviolet-visible spectroscopy (UV-Vis) confirmed nanoparticle formation, while scanning electron microscopy (SEM) revealed agglomerated nanoparticles and homogeneous nanoscale films. Energy-dispersive X-ray spectroscopy (EDX) verified elemental composition, and X-ray diffraction (XRD) analysis identified monoclinic and cubic crystalline phases with crystallite sizes ranging from 8.78 to 19.40 nm, whereas thin films displayed predominantly amorphous characteristics. Fourier transform infrared spectroscopy (FTIR) identified characteristic absorption bands associated with Zn-O vibrations and surface functional groups. Water contact angle (WCA) analysis indicated hydrophilic surfaces. ZnO NPs exhibited pronounced antibacterial activity against Staphylococcus aureus ATCC 6538P and Escherichia coli O157:H7. Although thin films produced smaller inhibition zones, complete bacterial inactivation was achieved within 2 h under direct-contact conditions. These findings demonstrate the potential of biosynthesized ZnO nanostructures as sustainable antimicrobial platforms.