The formation of transient species (OH•, NO 2 •, NO radicals) and long-lived chemical products (O 3 , H 2 O 2 , NO − 3 , NO − 2 ) produced by a gas discharge plasma at the gas-liquid interface and directly in the liquid was measured in dependence on the gas atmosphere (20% oxygen mixtures with nitrogen or with argon) and pH of plasma-treated water (controlled by buffers at pH 3.3, 6.9 or 10.1). The aqueous-phase chemistry and specific contributions of these species to the chemical and biocidal effects of air discharge plasma in water were evaluated using phenol as a chemical probe and bacteria Escherichia coli. The nitrated and nitrosylated products of phenol (4-nitrophenol, 2-nitrophenol, 4-nitrocatechol, 4-nitrosophenol) in addition to the hydroxylated products (catechol, hydroquinone, 1,4-benzoquinone, hydroxy-1,4-benzoquinone) evidenced formation of NO 2 •, NO• and OH• radicals and NO + ions directly by the air plasma at the gas-liquid interface and through post-discharge processes in plasma-activated water (PAW) mediated by peroxynitrite (ONOOH). Kinetic study of post-discharge evolution of H 2 O 2 and NO − 2 in PAW has demonstrated excellent fit with the pseudo-second-order reaction between H 2 O 2 and NO − 2 . The third-order rate constant k = 1.1 × 10 3 M −2 s −1 for the reaction NO − 2 + H 2 O 2 + H + → ONOOH + H 2 O was determined in PAW at pH 3.3 with the rate of ONOOH formation in the range 10 −8 -10 −9 M s −1 . Peroxynitrite chemistry was shown to significantly participate in the antibacterial properties of PAW. Ozone presence in PAW was proved indirectly by pH-dependent degradation of phenol and detection of cis,cis-muconic acid, but contribution of ozone to the inactivation of bacteria by the air plasma was negligible.
The inactivation of Bacillus subtilis (ATCC 6633) spores deposited on a filter membrane was studied by using low-temperature plasma produced via surface dielectric barrier discharge. Spore samples were carefully prepared to avoid the formation of cell aggregates, and their inactivation was induced by multiple surface streamer discharge driven in a coplanar dielectric barrier discharge electrode geometry by an amplitude-modulated AC high voltage waveforms in humid air at atmospheric pressure. At a discharge duty cycle of 0.4, the surface dielectric barrier discharge is characterised by an average total power of 1.7 W (power density 1.5 W cm−2 and energy density ∼0.3 Wh l−1) and a low gas temperature of the plasma filaments of about 320 K. The spores were exposed by placing a sample holder at a fixed distance of 3 mm from the electrode surface covered by plasma filaments. Particular attention was paid to identifying sporicidal agents employed in the process of inactivation. Since treated samples did not come into direct contact with the streamer filaments and excessive heating was excluded thanks to the low energy density, our results indicate that the spores were inactivated mainly by reactive oxygen and nitrogen species such as O3, H2O2 and NO2–. Discharge-induced damage of the spore structure was evidenced via the detection of dipicolinic acid and leaking of intracellular components. We therefore conclude that B. subtilis spores were inactivated chemically, probably due to failure of the coat structure or membrane of the spore.
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