Temperature Stability and Effectiveness of Plasma-Activated Liquids over an 18 Months Period

Abstract

Non-buffered plasma-activated liquids such as water and saline have shown bactericidal effects. In the present study, we investigated the anti-bacterial efficacy and stability of plasma-activated water (PAW) and plasma-activated saline (PAS), generated using a high voltage dielectric barrier discharge system. This study compares the potential of non-buffered plasma-activated liquids (PAL) for the inactivation of Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) after storage of the solutions at five different temperatures for a storage time up to 18 months after their generation. The temperatures used were room temperature, 4 °C, −16 °C, −80 °C, −150 °C. Both PAW and PAS achieved 6 log reduction for both bacteria on the first day of their generation after 60 min contact time and they retained these effects after 18 months when stored at the lowest temperatures. Chemical analysis of PAL showed that plasma caused a drop in pH, generation of reactive oxygen species and nitrates, whereas no nitrites are detected in the system used. The concentrations of chemical species were affected by the storage at different temperatures and a thermocouple probe was used to investigate the freezing behaviour of the PAL.

Distinct Chemistries Define the Diverse Biological Effects of Plasma Activated Water Generated with Spark and Glow Plasma Discharges

“…Decrease of pH can occur due to presence of nitrogen oxides, which can be produced from atmospheric air by the plasma and dissolve in liquids [31]. In our study, pH decreased with increasing plasma exposure time of deionised water and this result is in accordance with other reports [5,6,31] and remained stable for at least a week. Concentrations of some of the chemical species were altered in the same storage period, indicating that the chemical reactive species in PAW may have reacted to form other secondary products, with pH and conductivity values remaining the same.…”

“…A total of 50 µL of Griess reagent was added to 50 µL of PAW/standard curve sample and absorbance was read at 548 nm, after 30 min of incubation. Nitrate concentrations were determined photometrically by using the Spectroquant ® nitrate assay kit (Merck Chemicals, Darmstadt, Germany) adapted to a 96-well plate format, as described in [6].…”

“…In addition to these species, other potentially less stable reactive species with concentrations that vary depending on the target liquid solution type, are also important for their biological effects. Different PAL such as nonbuffered and buffered solutions may carry different concentrations of reactive chemical species and maintain diverse cytotoxic and antimicrobial properties [5] which can be retained for up to 18 months of storage at low temperatures [6]. Differences in reactive species chemistry and biological effect between various PAL can derive from different composition of the liquids, or antioxidants within the solutions as well as the plasma device and discharge conditions.…”

Influence of Feeding Gases on the Composition of Plasma Activated Water

“…This review article aims to present the impact of these feed gases on the composition of plasma activated water. Moreover, Evanthia Tsoukou et al in their study on the impact of non-buffered Plasma Activated Liquids (PAL) such as Plasma Activated Saline (PAS) and Plasma Activated Water (PAW) showed their antibactericidal efficacy and stability of PAW for up to 18 months when stored at the lowest temperatures (Figure 2) [8]. Influence of Feeding Gases on the Composition of Plasma Activated Water by using different feeding gases like N2, He, O2 and air on sterile distilled water and reported that air PAW had lower pH compared to other PAW [10].…”

“…Graphs represent averages of two independent plasma treatments (Sets 1 and 2); standard deviations can be found in Supplementary Table S1. (A) PAW E. coli (60 min), (B) PAW S. aureus (60 min) (Courtesy[8]).…”