Phyto-Mediated Controllable Synthesis of ZnO Clusters with Bactericidal Activity

The rapid development of antibiotic resistance has been considered a major threat to public health. Nanomaterials have risen to be an effective weapon to tackle this problem through multiple antibacterial mechanisms. The improved and tailored physiochemical properties of fine-tuned secondary nanoarchitectures contribute to the superior bactericidal actions of metal oxide structures. However, it is still challenging to construct secondary structures through mild green manufacturing methods. Here, we report the preferred antibacterial ZnO nanocrystal clusters formed by a green structure-tuning synthesis process, in which the primary ZnO nanoparticles with sizes <10 nm were assembled into different forms of clusters depending on the zinc salt concentration and temperature. ZnO clusters with a stable loose-assembly structure and a rougher surface exhibited better bactericidal ability with minimal inhibitory concentrations of 0.5 and 0.1 mg/mL against Escherichia coli and Staphylococcus aureus, respectively. The underlying mechanism is related to enhancing contact with bacteria, releasing small ZnO nanoparticles, and generating additional reactive oxygen species, which could aggravate the damage to bacterial cell membrane and eventually lead to bacterial death. Furthermore, attachment of phenolic compounds from olive leaf extract would promote membrane penetration by ZnO nanoparticles, resulting in the improvement of antibacterial activities, which profit from the green route mediated by Olea europaea leaf extract that could structure-tune ZnO nanocrystal clusters in one simple step that retains the active ingredients on the nanoparticles. This work proposes a feasible and clean strategy to improve the structure-bioactivity relationship of ZnO by controlling its growth into a preferable structure, and the developed ZnO clusters have a good prospect in antibacterial applications because of their excellent performance and green fabrication method.