Bactericidal and physicochemical properties of plasma activated water stored at different temperatures

Summary form only given. Water activated by non-thermal plasma creates an acidified solution containing reactive oxygen and nitrogen species, known as plasma-activated water (PAW). Physicochemical properties and bactericidal activities of PAW were evaluated at four different temperatures (25°C, 4°C, -20°C, -80°C). Interestingly, PAW at -80°C yielded the best antibacterial activity against Staphylococcus aureus, 3~4 log reduction over a 30-day period after PAW generation; meanwhile, PAW stored at 25°C, 4°C, and -20°C, respectively, yielded 0.2~2 log decrease in cell viability after the same exposure and storage time. These results were verified by scanning electron microscope (SEM). Then, physicochemical properties of PAW stored at different temperatures were evaluated, including pH, oxidation reduction potential (ORP), and hydrogen peroxide, nitrate, nitrite anion and NO radical levels. Furthermore, optical emission spectroscopy (OES) was employed to detect the most excited reactive species in PAW. We found similar pH and ORP for PAW for various storage times and temperatures. Nitrate composition varied significantly over time, diminishing within a few days. Importantly, hydrogen peroxide and nitrite concentrations in PAW at 25°C, 4°C, and -20°C showed a remarkably fast decrease while remaining stable at -80°C. These findings suggested that bacterial activity of PAW stored at 25°C, 4°C, -20°C decreased over time, and depended on three germicidal factors, specifically ORP, H
₂
O
₂
, and NO
₃
^-
. Moreover, PAW stored at -80 °C retained bactericidal activity, with NO
₂
^-
contributing to bactericidal ability in association with H
₂
O
₂
. Our findings provide a basis for PAW storage and practical applications in disinfection and food preservation.