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    Engineered zinc oxide-based nanotherapeutics boost systemic antibacterial efficacy against phloem-restricted diseases
    (Royal Soc Chemistry, 2022) Soliman, Mikhael; Lee, Briana; Özcan, Ali; Rawal, Takat B.; Young, Mikaeel; Mendis, Hajeewaka C.
    Delivering active ingredients with high antibacterial efficacy at infected sites in plants is essential to reach global goals for food security and sustainable agriculture productivity. Engineering nanomaterials is a suggested approach to attain systemic delivery of antibacterial active ingredients, thereby improving the treatment efficacy and minimizing harmful effects related to leaching in the environment. Herein, surface defect engineering of nanotherapeutics is used as a new form of active ingredient for systemic antimicrobial action in the phloem. The nanoparticle-based formulation, called Zinkicide (R), features a spherical particle composed of a zinc oxide (ZnO) core and zinc peroxide (ZnO2) shell with a total diameter below 5 nm. This formulation exhibits significant efficacy to manage citrus huanglongbing (HLB) disease as seen by the decrease in severity of symptoms and the increase from similar to 7% to 19% of medium and large fruits in HLB infected citrus groves, during field trials. Further analysis of the bacterial responses to Zinkicide (R) in situ indicates high potency at a concentration as low as 9-18 mu g mL(-1) and biofilm growth inhibition at a concentration of 50 mu g mL(-1). Nanoscale infrared spectroscopy reveals morphology and secondary structure changes of the bacterial membrane upon treatment. The origin of the changes is considered, based on the optical signatures of the nanoparticles, indicative of surface defects. These inform a theoretical description of the participation of a ZnO2/ZnO surface containing a pair of missing O atoms in the production of reactive oxygen species (ROS). The key participation of defects in the antibacterial action is confirmed experimentally by the slow decrease in antibacterial efficacy as nanoparticles age in media with passivation effects on the surface. This study reveals the importance of size of the nanoparticle and nature of surface defects in the design of nanotherapeutics for targeted delivery and antibacterial activity.
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    Öğe
    Role of capping agents in the synthesis of salicylate-capped zinc oxide nanoparticles
    (American Physical Society, 2020) Rawal, Takat B.; Smith, Micholas D.; Özcan, Ali; Smith, Jeremy C.; Tetard, Laurene; Santra, Swadeshmukul
    Capping agents are often used for controlling the size, aggregation, and properties of nanoparticles. To guide the design of improved nanomaterials for targeted performance, one can use mechanistic insights into the interactions between capping agents and nanoparticles. Here, we employ density functional theory (DFT), reactive force-field molecular dynamics (ReaxFF MD) simulations, and optical spectroscopy to study the interactions between salicylate, as a model capping agent, and zinc oxide (ZnO) nanoparticles. We find that salicylate strongly interacts with the nanoparticle via the formation of O-Zn bonds in a distorted six-membered coordination ring structure. We describe the mechanisms of capping of ZnO nanoparticles by salicylate via three different binding modes. Simulations indicate that salicylate undergoes dissociative adsorption at the highly active surface Zn sites via a hydrogen-transfer process, thereby forming a tridentate configuration. The water-mediated interaction also facilitates the dissociative adsorption, leading to two salicylate O atoms coordinating with a surface Zn atom, while the other salicylate O atom bonds with another surface Zn atom. For molecular adsorption, binding free energies indicate that salicylate binds more strongly to ZnO, often in a bidentate configuration, than water does. The formation of the salicylate-ZnO complex is substantiated by UV-visible and Fourier transform infrared spectra. We find that the C.O stretching mode of salicylate becomes softened when it interacts with the nanoparticle, suggesting chemisorption of salicylate on ZnO. Although DFT predicts strong interaction between salicylate and ZnO, ReaxFF MD simulation indicates the moderate interaction between these two components in aqueous solution. Water molecules in close contact with the nanoparticle surface undergo dissociation, thus resulting in a surface hydroxyl, a reactive oxygen species that may influence the nanoparticle's catalytic properties. The atomic-level information provided here can guide the selection process of salicylate as an appropriate agent for ZnO nanoparticle synthesis when strong interactions of particles with capping agents are required.