Nitrox surface discharge used for water activation: The reactive species and their correlation to the bactericidal effect

Plasma activated water (PAW) is a promising green antibacterial agent and the bactericidal effect is complicatedly affected by electron bombardment, ultraviolet radiation, interface reaction, and cascade chemical reaction. In this paper, a case of preparing PAW by treating aqueous solutions with afterglow gas is constructed based on surface micro-discharge (SMD), which focuses on the effect of afterglow gas-liquid mass transfer and liquid phase chemistry on PAW sterilization. The correlation of the bactericidal effect of PAW to the reactive species was studied based on the model of methicillin-resistant Staphylococcus aureus. The production of reactive oxygen and nitrogen species (RONS) in PAW with the regulation of N2/O2 ratios in the working gas for SMD. The RONS in both gas and liquid phases and the physicochemical properties of PAW were measured through optical and chemical methods. In addition, the effects of liquid types, liquid conductivity, and storage time on the bactericidal effects of PAW were explored. The key species for bacteria inactivation were identified by equivalent mixed solutions and specific scavengers. The results demonstrated that control of the N2/O2 ratios in the working gas can effectively improve the RONS in plasma and PAW. The bactericidal effect of PAW is correlated with peroxynitrite, superoxide anion, and their synergistic effects in an acidic liquid environment. This study provides a new strategy for insight into the bactericidal mechanism of PAW in biomedical applications.