Plasma-activated water retains antimicrobial properties against <i>Escherichia coli</i> after 72 h of storage

Plasma-activated water (PAW) is increasingly recognized for its bactericidal properties. To advance this technology toward practical applications and deepen the understanding of its mechanisms, it is crucial to study the storage stability of PAW, focusing on both its chemical composition and antimicrobial properties over time. In this study, PAW was produced using a surface dielectric barrier discharge applied to ultra-pure water. Six PAW samples were analyzed, with plasma exposure times of 10, 20, or 30 min, and with or without water recirculation. Chemical properties such as pH, electrical conductivity, and oxidation-reduction potential were monitored over 72 h of storage at 25 degrees C. The kinetics of long-lived reactive oxygen and nitrogen species were also studied, with H2O2 measured by visible spectrophotometry and NO2- and NO3- analyzed via ion chromatography. While H2O2 and NO2- concentrations decreased during the storage, NO3- levels increased in all samples, reaching similar final concentrations independently on the water recirculation during plasma exposure. After 72 h, the bactericidal effect of PAW on Escherichia coli was evaluated for 10 and 30 min treatment times and compared to fresh samples. Although water recirculation initially provided stronger antimicrobial effects, after storage, both recirculated and non-recirculated PAW samples exhibited similar antimicrobial activity. This study demonstrates that PAW samples, regardless of the initial chemical composition, associated to water recirculation, achieve comparable antimicrobial properties after 72 h of storage, providing valuable insights for practical PAW applications.