Nano- and microscale ZnO demonstrate robust antibacterial action, although the driving mechanisms remain undetermined. In this study for commercial ZnO nano-powders and home-grown ZnO microparticles of varying morphologies we probe the response to bacterial growth media in isolation and with Staphylococcus aureus bacteria. ZnO microparticles are synthesized via a controllable hydrothermal method and subjected to biological assays with varying microbial environments. Changes in the optoelectronic, structural and chemical properties of these crystals before and after such exposure are characterized utilizing temperature-dependent photoluminescence spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. This is done to evaluate the impact of surface-surface interactions in antibacterial assays and the role ZnO surface and morphological properties play in these processes. In our experiments various bacterial environments are employed to elucidate the effects of media interactions on the cytotoxic efficacy of ZnO. In particular, minimum inhibitory concentration assays with Staphylococcus aureus reveal that microscale particles exhibit antibacterial efficacy comparable to that of the nano-powders, indicating that intra-bacterial internalization is not necessary for antimicrobial action. In our studies we determine that the nature of structural and optoelectronic changes in ZnO depends on both the media type and the presence (or absence) of bacteria in these media. Further evidence is provided to support significant cytotoxicity in the absence of particle internalization in bacteria, further highlighting the role of surface and media interactions in this process.

Nano- and microscale zinc oxide (ZnO) exhibits significant potential as a novel antibacterial agent in biomedical applications. However, the uncertainty regarding the underlying mechanisms of the observed antimicrobial action inhibits the realization of this potential. Particularly, the nature of interactions at the free crystalline surface and the influence of the local bacterial environment remains unclear. In this investigation, we utilize ZnO particles synthesized via tunable hydrothermal growth method as a platform to elucidate the effects of interactions with phosphate-rich environments and differentiate them from those with bacteria. This is achieved using the time- and energy-dependent surface photovoltage (SPV) to monitor modifications of the surface electronic structure and surface charge dynamics of the ZnO particles due to these interactions. It is found that there exists a dramatic change in the SPV transients after exposure to phosphate-rich environments. It also presents differences in the sub-bandgap surface electronic structure after these exposures. It can be suggested that these phenomena are a consequence of phosphate adsorption at surface traps corresponding to zinc deficiency defects. This effect is shown to be suppressed in the presence of Staphylococcus aureus bacteria. Our results support the previously proposed model of the competitive nature of interactions between S. aureus and aqueous phosphates with the free surface of ZnO and bring greater clarity to the effects of phosphate-rich environments on bacterial growth inhibition of ZnO.