Investigation on the long‐term bactericidal effect and chemical composition of radical‐activated water

Abstract

We treat deionized water using an atmospheric‐pressure radical source to investigate the role of neutral radicals in the bactericidal efficacy of radical‐activated water (RAW). The bactericidal efficacy of RAW is investigated using a colony‐forming unit assay with Escherichia coli. Both the bactericidal efficacy and reactive oxygen and nitrogen species (RONS) concentration in stored RAW are investigated over a period of a month. E. coli was completely sterilized even in RAW stored for 7 days, indicating the strong bactericidal effect. However, deterioration of the bactericidal efficacy of RAW stored for 14 days is confirmed, which does not have a clear correlation with the time evolution of the RONS concentration. This result becomes a strong evidence that RAW has long‐term bactericidal efficacy.

Direct Treatment of Liquids Using Low-Current Arc in Ambient Air for Biomedical Applications

“…However, in the case of direct treatment with AALCA, not only long-living RONS could be involved in the bactericidal effect but also short-living RONS (e.g., ONOO − and HOO ), the strong electric field at the stage of corona and repeating spark discharges, and ultraviolet emissions, as it was reported elsewhere [18,41,58,59]. On the other hand, a long-term bactericidal effect produced by long-living RONS in PTW was reported elsewhere, which suggests a strong long-term bactericidal effect of PTW produced by AALCA owing to its high concentration of RONS [31,32,43]. To investigate the bactericidal effect of PTW, 5 mL of DI water was treated with AALCA for 1 min at an input power of 28 W. Then, 0.3 mL of the E. coli suspension was mixed with 2.7 mL of the produced PTW and incubated for various times.…”

“…The conductivity of the water sample increased from about 0.1 µS/cm for pristine DI water to 190 µS/cm after treatment for 20 s by spark discharges, which enabled the generation of a low-current arc that was transferred to the surface of the DI water, which initially had low conductivity. Moreover, the pH value gradually decreased from 6.76 for pristine DI water to 2.89 after treatment for 2 min, as shown in Figure 4, which is a considerably low value as compared with the PTW reported elsewhere [18,24,31,43]. The low pH value suggests a high concentration of RONS and possibly some ions (delivered from the electrodes by erosion) in the treated liquid, which could explain the increase of conductivity after the plasma treatment.…”

“…It could be clearly observed that the concentration of E. coli gradually decreased as the incubation time increased after suspension of E. coli in DI water treated with AALCA and reached the detection limit of 10 CFU/mL at 60 min, showing a 6-log reduction in the concentration of bacteria. Considering that the concentration of E. coli did not change in the control sample of E. coli suspended in pristine DI water even after 24 h of incubation, it could be concluded that the DI water treated with AALCA had a strong bactericidal effect that originated from the high concentration of RONS and the low pH level [31,32,43].…”

“…DI water (18.2 MΩ•cm at 25 • C) was produced by a Direct-Q 3UV (Millipore, Tokyo, Japan) system. To compare the performance of the developed AALCA to commonly used plasma sources, DI water was treated using AALCA, a commercially available NEAPPJ (PN-110/120TPG, NU Global Co., Ltd., Nagoya, Japan) [40], and a high-density nonequilibrium atmospheric pressure radical source (NEAPRS, Tough Plasma FPA10, Fuji MFG Co., Ltd., Tokyo, Japan) [4,31,41].…”

“…Typically, cold nonequilibrium plasmas are used for biomedical and agricultural applications to avoid thermal damage of the treated target [12,13,29,30]. However, in the case of using liquids treated by plasma, a plasma-treated medium (PTM) could be stored after treatment to reduce the temperature and for use after storage considering the presence of the long-living RONS [31,32]. For that reason, thermal and warm plasmas could be used for irradiation of medium following the cooling of PTM and use in biomedical applications.…”