Sterilization in liquids by air plasma under intermittent discharge

In recent years, cold atmospheric plasma has received attention for medical applications like sterilization, injure treatment, and cancer cell treatment. Plasma sterilization is effective against a wide range of microorganisms. Plasma sterilization devices for contact lens have developed over the years. The purpose of this study is to clarify sterilization characteristics under high ozone, low nitric acid, and nitrous acid concentration in liquids. These concentrations are caused by a new cycle of intermittent discharges. Sterilization efficacy is evaluated by inactivating bacillus spores. Concentrations of HNO3, HNO2 and O-3 in water were measured using a pack test. Dielectric surface temperature is measured using a thermocouple and thermography to study relationships between the concentration and the temperature. Sterilization of spores in water was achieved for 80 min. Increasing ozone concentration promotes the generation of hydrogen peroxide, which is a well-known factor for sterilization, and (OH)-O-center dot, which has strong oxidizing power, however, the concentration of hydrogen peroxide is too low for sterilization, and the life time of (OH)-O-center dot is as short as microseconds. Meanwhile, a decrease in the concentration of nitric acid and nitrous acid suppresses the formation of peroxynitrite, which has very strong oxidizing powers and a lifetime of 1 sec. These results suggest a critical role of peroxynitrite in sterilization.