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.

Helium metastable densities in the COST Reference Microplasma Jet are estimated for a variety of He/N2 admixtures and dissipated powers by applying a collisional-radiative model to absolutely calibrated optical emission spectroscopy measurements. This is accomplished by delineating the excitation mechanisms that result in the N2(C–B) and N2+(B–X) emission bands, the latter of which is strongly coupled to the presence of helium metastables. A number of other plasma parameters are established and discussed for each operating condition including the electron energy distribution function, reduced electric field, rate constants, and electron density. With these parameters, the reaction rates for the primary ionization pathways are also calculated, emphasizing the importance of helium metastables for discharge sustainment. Good agreement with the existing literature is found for most plasma parameters and for helium metastable densities, in particular. A clear [N2]−1 relationship between the nitrogen concentration and density of helium metastables is demonstrated, as has been identified in previous studies in analogous atmospheric pressure plasma jets. This validates the efficacy of this optical technique for determining helium metastable densities and establishes it as a viable, and in many cases, more accessible alternative to other means of quantifying helium metastables in low-temperature plasmas.

Single-cell impedance flow cytometry (IFC) is a rapid, label-free diagnostic capable of resolving the dielectric properties of individual cells. Here, single-cell IFC is employed to investigate the effects of plasma-activated water (PAW) on E. coli. This diagnostic makes use of the frequency-dependent dielectric response of cells. By recording the dielectric spectra of E. coli, the subcellular dielectric properties can be estimated. Following exposure to PAW, significant shifts in the phase angle of the impedance of E. coli were observed compared to untreated populations. The phase shift increased with frequency and when cells were exposed to PAW with higher concentrations of reactive species. A reduction in the amplitude of the impedance signal was also noted, corresponding to a decrease in cell volume. After signal calibration with stationary-phase E. coli and a three-shell dielectric model, a sensitivity analysis and genetic algorithm were used to find the relevant subcellular dielectric parameters. The results show that the conductivity of the outer membrane, periplasm, and cytoplasm change considerably in response to PAW treatment. This indicates that PAW modifies multiple layers of the cell simultaneously and that the inactivation pathway of PAW-treated E. coli is likely not confined to a single subcellular layer. For the first time, this study demonstrates that single-cell IFC can extract meaningful and detailed information about the cellular response to plasma treatment in an efficient and label-free manner. This establishes it as a promising diagnostic for uncovering the mechanisms of a variety of biological applications of low-temperature plasmas.

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.

As a powerful oxidant, atomic oxygen (O) holds considerable promise for a variety of biomedical and industrial applications. However, the inability to quantify solvated oxygen atoms has prevented the determination of the fundamental parameters governing its behaviour in relevant aqueous environments. Here, we directly image ground-state oxygen atoms in water using femtosecond two-photon absorption laser-induced fluorescence. Measurements show that oxygen atoms persist for tens of microseconds in water, penetrating hundreds of micrometres into the liquid. This observed longevity has significant implications, suggesting a need to re-evaluate existing models of solvated atomic oxygen reactivity and transport. Beyond atomic oxygen, this technique is broadly applicable to other solvated atomic species of interest, including nitrogen (N) and hydrogen (H). This work establishes that radical atomic species can be quantified in liquid with ultrafast laser spectroscopy, providing the basis for the determination of key properties including reaction rates, chemical lifetimes, and Henry’s law constants.

This article was originally published online on 23 July 2024 with an error in all instances of “N2+.” These have all been corrected as “ N 2 +” throughout the t This article was originally published online on 23 July 2024 with an error in all instances of “N2+.” These have all been corrected as “ N 2 +” throughout the t