Abstract
The interaction between a cold gas plasma and water creates a plasma activated liquid, a solution rich in highly reactive chemical species. Such liquids have garnered considerable attention due to their powerful antimicrobial properties and ease of production. In this contribution, air plasma was used to activate potable water samples from five different countries, including the UK, France, Norway, Slovenia and Palestine. All water samples had an initial pH in the range of 7.9 to 8.2, following plasma activation samples from the UK and Norway reached a pH below 3, whereas water from France and Palestine remained stable at 8. The concentration of NO3− increased in all samples, reaching a maximum concentration of 3 mM after 25 min plasma exposure; whereas the concentration of NO2− showed a non-linear dependence with exposure time, reaching between 10 and 25 µM after 25 min of exposure. To demonstrate the impact of water origin on the antimicrobial potential of each solution, the inactivation of Staphylococcus aureus and Escherichia coli was considered. It was found that activated water from the UK was capable of achieving > 6 log reduction, whereas water from Palestine was only able to achieve a 0.4 log reduction, despite both liquids receiving an identical plasma exposure. The study demonstrates the importance of initial water composition on the level of plasma activation, indicating that additional purification steps prior to activation may be necessary to ensure efficacy and repeatability.
Highlights
Microbial decontamination remains a key challenge in the healthcare and food production sectors
As the surface barrier discharge (SBD) electrode used in this study was confined within a sealed volume, quantification of species densities was limited to the measurement of Ozone using optical absorption, a process that required no gas to be drawn from the enclosure
Ozone quenching in an air fed atmospheric pressure SBD has been widely studied [36], and it has been posited that the rapid formation of NO is attributed to the reaction between vibrationally excited N2 and O within the discharge region, which subsequently reacts with O 3 to form NO2, R6–8 [37]
Summary
Microbial decontamination remains a key challenge in the healthcare and food production sectors. A host of novel, non-thermal disinfection technologies are currently under active investigation and have shown great promise for efficient and effective microbial inactivation, without many of the drawbacks associated with conventional methods [4] One such approach is the use of plasma activated water (PAW), where a non-equilibrium gas discharge is used to generate reactive chemical species directly within a liquid volume [5]. When humid air is used, typical reactive species created in the plasma include O, OH, NO, O3, O−2 and H2O2, these are either dissolved on contact with a liquid or react at the interface to form secondary compounds such as H NO2, HNO3 and ONOOH [8] The generation of these reactive compounds, using only air and electricity, has sparked enormous interest in PAW technology over the last decade. An interest that has driven PAW researchers to explore a wide variety of different decontamination applications, ranging from the inactivation of fungal spores on food products [9], right through to the inactivation of biofilms on medical devices [10]
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