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 ^∘C. The kinetics of long-lived reactive oxygen and nitrogen species were also studied, with H 2 O 2 measured by visible spectrophotometry and NO 2 − and NO 3 − analyzed via ion chromatography. While H 2 O 2 and NO 2 − concentrations decreased during the storage, NO 3 − 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.

Plasma-activated water (PAW) is an emerging antimicrobial agent with promising applications in bacterial inactivation. The PAW samples are generated by non-contact exposure of deionized water to a surface dielectric barrier discharge plasma and are characterized by a reactive nitrogen species-rich chemistry. In this work, fluorescence flow cytometry is applied to assess the viability of Escherichia coli treated with PAW. The results indicate that PAW exhibits a strong bactericidal effect, significantly increasing propidium iodide positive populations and leading to cell shrinkage. Comparative colony-forming unit counting confirmed these findings, showing agreement between both techniques and ruling out the possibility of a viable but non-culturable (VBNC) bacteria state. These results underscore the potential of the PAW samples produced with the present setup for safe decontamination applications, while also offering insights into the mechanisms of bacterial inactivation.

While atmospheric pressure plasma sources are emerging as potentially innovative instruments in medicine, some aspects of the interaction between plasma and biological substrates remain unclear. The high diversity in both sources and applications in the literature, and the lack of a systematic testing protocol, has resulted in a wide variety of devices that cannot be efficiently compared with one another. In this work, an integrated benchmark involving physical, chemical, and biological diagnostics is proposed. The setup is designed to be stable and fixed, while remaining adaptable to different sources. Three different sources, for a total of five configurations, are compared, demonstrating the possibility of obtaining multimodal data. Comparing the biological effects in terms of E. coli abatement between direct and indirect treatments allowed for the exclusion of short-timescale species and phenomena to have a key role in the abatement. The chemical characterisation describes the equilibrium of reactive oxygen and nitrogen species in treated samples, whose presence in the water has been found to be coherent with the plasma operating gas and the nitrogen vibrational temperatures. Nitrate, nitrite and peroxide are excluded from having an autonomous role in the inactivation biochemistry, suggesting the presence of a synergistic effect.