Green Synthesis of Zno Nanoparticles Using Japanese Knotweed Extract and Its Photocatalytic and Antimicrobial Properties

Zinc oxide (ZnO) is one of the best known photocatalysts in the field of environmental remediation. It is characterized by a facile synthesis, although it lacks mechanochemical and photocatalytic stability compared to some other photocatalysts. Herein, we prepared ZnO nanoparticles (NPs) by a facile and environmentally friendly synthesis using ethanolic extracts from the roots of Japanese knotweed (lat. Fallopia japonica). The extracts were used to support the reduction of zinc atoms during the synthesis and to control the growth of ZnO NPs. These NPs were characterized by slower and more controllable growth of crystallites during thermal annealing at 450 °C and showed better long-term photocatalytic activity for the degradation of ciprofloxacin. In addition, the modified synthetic route using plant extracts resulted in better antimicrobial activity against both Gram-positive bacteria, e.g., Staphylococcus aureus, and Gram-negative bacteria, e.g., Escherichia coli and Campylobacter jejuni. The materials were also tested for their antimicrobial activity against S. aureus under ultraviolet (UV) illumination. Also here, the photocatalyst prepared with plant extracts was found to be superior. The residues of the plant extract molecules on the surface of the catalyst were identified as the main cause of the observed differences, as proved using thermal gravimetry. From the results, it is implied the soluble elements present in the ethanolic extract of Fallopia japonica could have acted as capping agents preventing the aggregation of NPs in solution.