The role of surface-surface interactions and growth media in antibacterial action of microscale ZnO

Robust antimicrobial action of ZnO is well known, thoroughly documented and actively studied, especially for the nano- and microscale geometries. Several driving mechanisms have been proposed behind this phenomenon; however, the most fundamental physical and chemical processes are still not well identified. Particularly, the nature of interactions between ZnO surfaces, cellular membranes and bacterial growth media remain unambiguous. To address this matter, we employ ZnO microparticles synthesized hydrothermally and subjected to Staphylococcus aureus biological assays with different microbial growth media. The ZnO microcrystals are produced using growth parameters for reliable control of morphology, and specifically surface polarity. The biological assays are used to examine the antibacterial action and also to run pre- and post-assay comparative studies of the ZnO crystals themselves. For the latter we employ a variety of characterization techniques, such as electron microscopy, energy-dispersive X-ray spectroscopy, time and wavelength dependent surface photovoltage, temperature-dependent photoluminescence spectroscopy, etc. Our experiments suggest that structural and optoelectronic changes of the ZnO surfaces strongly depend on the growth media type. There are indications of the influence of crystalline surface polarity, in particular in regard to the relevant concentration of surface defects and surface charge dynamics.