Inactivation of Escherichia coli Cells in Aqueous Solution by Atmospheric-Pressure N ₂ , He, Air, and O ₂ Microplasmas

Abstract

bAtmospheric-pressure N 2 , He, air, and O 2 microplasma arrays have been used to inactivate Escherichia coli cells suspended in aqueous solution. Measurements show that the efficiency of inactivation of E. coli cells is strongly dependent on the feed gases used, the plasma treatment time, and the discharge power. Compared to atmospheric-pressure N 2 and He microplasma arrays, air and O 2 microplasma arrays may be utilized to more efficiently kill E. coli cells in aqueous solution. The efficiencies of inactivation of E. coli cells in water can be well described by using the chemical reaction rate model, where reactive oxygen species play a crucial role in the inactivation process. Analysis indicates that plasma-generated reactive species can react with E. coli cells in water by direct or indirect interactions.Plasma, called the fourth fundamental state of matter, in addition to solids, liquids, and gases, consists of equal numbers of positive ions and negative electrons (negative ions in some cases) and other reactive species, generally resulting from the ionization of neutral gases. Due to the reactive species in plasma, gas-based reactive plasmas are thought to be effective in killing various microorganisms (1-3). Therefore, recently, the inactivation of microorganisms in water by atmospheric-pressure cold plasmas (APCP) has attracted great attention for biomedical and environmental applications due to their lethal effects on bacteria and fungi (4-8), since APCP include many reactive species similar to those in the conventional methods of microorganism inactivation, such as ozone generation (9), UV irradiation (10), chemical agents (11), electrical fields (12, 13), and microwave irradiation (14).The chemical reaction rates of these plasma-activated species may be improved greatly when atmospheric-pressure nonequilibrium plasmas are generated in water (4-7, 15). These short-lived species can be formed in the vicinity of microorganisms and efficiently kill microorganisms in water. Usually, it is relatively hard to generate stable atmospheric-pressure plasmas in water. Among various plasma sources (4, 5, 7), atmospheric-pressure arc discharge has frequently been used for killing microorganisms in water. Compared to other sources, the strong arc discharges are less influenced by the aqueous environment while obviously leading to an increase in the water temperature with high energy consumption. This arc discharge can also cause serious damage to heat-sensitive materials, and the volume of treated aqueous solution is limited, since the arc plasma is usually controllable only in a small processing space.Previously (16), we designed an atmospheric-pressure air microplasma array to inactivate Pseudomonas fluorescens cells in aqueous media. The microplasma produced by hollow-fiberbased microplasma jets is stable and extremely efficient in killing P. fluorescens cells in aqueous media. This design demonstrates potential application for large-volume plasma inactivation of bacterial cells in water. In this study, we repor...

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