Surface Photovoltage Response of ZnO to Phosphate-Buffered Saline Solution with and without Presence of Staphylococcus aureus

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.