Streamer corona and transient spark in air for bio-decontamination of water and surfaces Z. Machala 1 , B. Tarabová 2 , M. Pelach 2 , K. Hensel 1 , M. Janda 1 , L. Šikurová 2 1 Division of Environmental Physics, 2 Division of Biomedical Physics, Faculty of Mathematics, Physics and Informatics, Comenius University, 84248 Bratislava, Slovakia Abstract: Bio-decontamination of water and agar surfaces contaminated by bacteria (Salmonella typhimurium, Bacillus cereus) was investigated in two types of positive DC-driven discharges in atmospheric pressure air, in needle-to-plane geometry: the streamer corona and the transient spark with short high current pulses of limited energy. Both discharges generate cold non-equilibrium plasma. Electro-spraying of the treated water through the needle electrode resulted in fast efficient bio-decontamination. Experiments comparing direct and indirect plasma effects, the emission spectra, oxidation stress measurements in the cell membranes, and chemical changes induced in the treated water helped better understanding of the plasma agents responsible for microbial inactivation. Radicals and reactive oxygen species seem to be dominant biocidal agents, although understanding of the plasma-induced water chemistry and the temporal evolution of the bio-decontamination mechanisms requires further research. Keywords: bio-decontamination, air plasma, streamer corona, transient spark, electro-spray, oxidative stress 1. Introduction Cold (nonequilibrium) plasmas at atmospheric pressure find recently numerous biological and bio- medical applications thanks to their reactive nature. In bio-decontamination by plasmas, it is crucial to understand the role of various mechanisms involved. In atmospheric pressure plasmas, the major role is typically attributed to radicals and reactive oxygen species (ROS, e.g. OH, O, O 3 ) and to charged particles, especially O 2 - , affecting the cell membranes. UV radiation plays a role only in UV C germicide region (220-280 nm) or in vacuum UV. In cold air discharges, UV C or VUV are usually not generated, so radicals and ROS are typically identified as dominant bio-inactivation agents. In this paper, the biocidal effects of two atmospheric air plasma sources treating water and agar surfaces are investigated – positive DC streamer corona (SC) and transient spark (TS). Despite DC applied voltage, these discharges have a pulsed character with nanosecond repetitive pulses. We focus on the identification of the dominant plasma agents in bio- inactivation (electric field, charged particles, neutral active species, UV radiation) by coupling the electrical discharge characteristics, their emission spectra, and biocidal effects and measurements of the oxidative stress induced in microbial cells. Comparing direct and indirect plasma effects enables further separation of various biocidal plasma agents. 2. Materials and methods The experimental set-up for DC discharges in point- to-plane geometry, with a high voltage hollow needle electrode enabling water flowing through the discharge zone was described previously in [1,2]. 2.2 Flowing water treatment The discharge set-up enabled the contaminated water to flow directly through the high voltage hollow needle electrode, and so through the corona active region in its proximity. The effect of electrostatic spraying occurred when the high voltage was applied on the needle electrode as shown in Figure 1.