Surface Defect Mediated Interactions of Bacterial Growth Media on Microcrystalline ZnO

Abstract

Nano- and microscale ZnO exhibits significant potential in biomedical applications as an antibacterial agent. This is attributed to the selective toxicity and growth inhibition for a wide range of both Gram-positive and Gram-negative bacteria as well as for microbial strains resistant to traditional antibiotics. Pursuit of novel bactericidal applications, however, is inhibited by uncertainty surrounding the underlying mechanisms of the observed antimicrobial action. The most common suggested mechanisms include generation of various reactive oxygen species, release of Zn ions and surface-surface interactions between the bacterial cell wall and free crystalline surface of ZnO. Recent work has placed an emphasis on the role of the free crystalline surface in bacterial growth inhibition. Additionally, the cytotoxicity of ZnO particles has long been known to be heavily dependent on environmental conditions, yet detailed descriptions of these phenomena are lacking. Herein we investigate the effects of interactions with both bacteria and the bacterial environments on the physicochemical and optoelectronic properties of the free crystalline surface of ZnO microparticles (MPs). This is done to gain insight into the nature of interactions with the bacterial growth media and the influence such interactions have on bacterial growth inhibition. Hereby, we expose hydrothermally grown ZnO MPs to phosphate-buffered saline (PBS) media both with and without the presence of Newman strain S. aureus bacteria. The surface electronic structure and charge dynamics are probed via both time and energy dependent surface photovoltage (SPV) conducted prior to and following minimum inhibitory concentration assays. We demonstrate significant changes in the characteristic timescales of long-lived processes in the SPV transients after exposure to phosphate-rich environments. We also note that the presence of bacteria has an impact on shorter lived processes and suppresses the effect of media at longer timescales. The SPV spectra point to a significant adsorption of phosphate compounds on the crystalline surface. We also demonstrate changes in the electronic structure as a result of exposure to PBS.