Water disinfection by indirect plasma treatment was investigated using a surface dielectric barrier discharge (DBD). Liquid was neither part of the discharge electrode configuration nor stirred during plasma treatment. High concentrations (106–108 cfu·mL−1) of Escherichia coli and Staphylococcus aureus have been completely inactivated within 5–15 min, depending on liquid sample volume. Inactivation occurred in non‐buffered liquids, only, where pH decrease was found. Measurements of pH, nitrate, and nitrite concentrations after DBD plasma or NO gas treatment lead to the conclusion that nitric acid formation from plasma‐generated reactive nitrogen species are the main source of liquid acidification. Incubation of bacteria in nitric acid alone did not result in comparable inactivation effects. Increase of H2O2 concentration was found as a result of plasma treatment of liquids but not after treatment by NO gas. Therefore, synergistic action of both reactive oxygen and nitrogen species are discussed to be responsible for antimicrobial plasma effects.

Plasma-based treatment of chronic wounds or skin diseases as well as tissue engineering or tumor treatment is an extremely promising field. First practical studies are promising, and plasma medicine as an independent medical field is emerging worldwide. While during the last years the basics of sterilizing effects of plasmas were well studied, concepts of tailor-made plasma sources which meet the technical requirements of medical instrumentation are still less developed. Indeed, studies on the verification of selective antiseptic effects of plasmas are required, but the development of advanced plasma sources for biomedical applications and a profound knowledge of their physics, chemistry, and parameters must be contributed by physical research. Considering atmospheric-pressure plasma sources, the determination of discharge development and plasma parameters is a great challenge, due to the high complexity and limited diagnostic approaches. This contribution gives an overview on plasma sources for therapeutic applications in plasma medicine. Selected specific plasma sources that are used for the investigation of various biological effects are presented and discussed. Furthermore, the needs, prospects, and approaches for its characterization from the fundamental plasma physical point of view will be discussed.

Treatment of aqueous liquids by surface‐DBD in atmospheric air resulted in bactericidal activity of the liquid itself. A 7 min treatment of sodium chloride (NaCl) solution and its immediate addition to Escherichia coli resulted in a complete bacteria inactivation (≥7 log) after 15 min exposure time. With a 30 min delay between plasma treatment of liquid and its addition to the bacteria, bactericidal effect was reduced but still detectable. Nitrate (${\rm NO}_{2}^{{-} } $), nitrite (${\rm NO}_{2}^{{-} } $), and hydrogen peroxide (H2O2), respectively, as well as strong acidification are detected in plasma treated liquids and can explain this bactericidal activity partially. Combination of 1.5 mg · L−1 ${\rm NO}_{2}^{{-} } $ and 2.5 mg · L−1 H2O2 at pH 3 results in maximum 3.5 log E. coli reduction within 60 min. Plasma diagnostics and liquid analytics are combined with theoretical considerations to focus possible reaction channels of plasma–water interactions. Using FT‐IR, stable molecules like nitrous oxide (N2O), ozone (O3), carbon dioxide (CO2), and traces of nitric acid (HNO3) and/or peroxynitrous acid (ONOOH) were measured. Reactions of these molecules from the plasma/gas phase with the aqueous liquid can result in acidification and generation of H2O2, ${\rm NO}_{2}^{{-} } $, and ${\rm NO}_{3}^{{-} } $ or peroxynitrite (ONOO−), respectively, via reactions which are associated with the occurrence of several more or less stable but biologically active chemical intermediates like ${\rm NO}^{ \bullet } $ or nitrogen dioxide (${\rm NO}_{2}^{ \bullet } $). On the other hand, H2O2, ${\rm NO}_{2}^{{-} } $, and ${\rm NO}_{3}^{{-} } $/ONOO− could serve as starting reaction partners to generate ${\rm NO}^{ \bullet } $, ${\rm HO}^{ \bullet } $, ${\rm NO}_{2}^{ \bullet } $, or hydroxyl radicals (${\rm HOO}^{ \bullet } $) in the liquid.

Cover: A lot of analytics and microbiological tests were done to get an insight into the complex chemistry of atmospheric pressure plasma in interaction with liquids and the resulting antimicrobial effects. Different species are detected and a variety of reaction channels is hypothesized which lead to bactericidal molecules. Further details can be found in the article by K. Oehmigen, J. Winter, Ch. Wilke, R. Brandenburg, M. Hähnel, K.‐D. Weltmann, Th. von Woedtke* .

From literature different plasma species are known for their possibility to inactivate microorganisms. Most studies report on the inactivation of germs by UV light emitted from rare gas discharges like plasma‐jets or low pressure discharges or disruption of cell membranes due to charging effects. Our aim was to show the influence of air humidity and active plasma power in a simple atmospheric pressure dielectric barrier surface discharge on the reduction of Bacillus atrophaeus spores within a few minutes of treatment time. The plasma treatment time was varied at humidity levels between 0 and 70% relative moisture with a distance of 0.6 mm between the dielectric barrier and test strip. The obtained results have shown an influence of the air humidity on the spore inactivation rate which has been reached by plasma treatment. The plasma does not emit a significant amount of UV light below 320 nm. From these results the role of reactive species, especially the effect of hydroxyl radicals, is discussed.