Plasma–liquid interactions represent a growing interdisciplinary area of research involving plasma science, fluid dynamics, heat and mass transfer, photolysis, multiphase chemistry and aerosol science. This review provides an assessment of the state-of-the-art of this multidisciplinary area and identifies the key research challenges. The developments in diagnostics, modeling and further extensions of cross section and reaction rate databases that are necessary to address these challenges are discussed. The review focusses on non-equilibrium plasmas.

When cold atmospheric plasma comes into contact with water and biological media, antimicrobial or antitumor effects are induced, representing great potential for applications in biomedicine and agriculture. The need to control and tune the chemical composition and biomedical effects of plasma activated water/media (PAW/PAM) is emerging. By comparing two nonthermal air plasma sources, streamer corona and transient spark, interacting with water in open and closed reactors, and by enhancing the plasma–liquid interaction by water electrospray through these discharges, we demonstrate that the plasma gaseous products strongly depend on the discharge regime, its deposited power and gas flow conditions. The streamer corona strongly leads to the formation of ozone and hydrogen peroxide, while the more energetic transient spark leads to nitrogen oxides and hydrogen peroxide. The gaseous products then determine the chemical properties of the PAW and the dominant aqueous reactive oxygen and nitrogen species (RONS). The production of hydrogen peroxide depends on water evaporation and hydroxyl radical formation, which is determined by the discharge power. A transient spark produces higher concentrations of gaseous and aqueous RONS and induces stronger antibacterial effects than a streamer corona; however, the RONS production rates per joule of deposited energy are comparable for both studied discharge regimes. The net production rate per joule of gaseous nitrogen oxides strongly correlates with that of aqueous nitrites and nitrates. The antibacterial effects of the PAW tested on Escherichia coli bacteria are determined by the aqueous RONS: in the lower power streamer corona, this is ascertained mainly by the dissolved ozone and hydrogen peroxide, and in the higher power transient spark, by the combination of hydrogen peroxide, nitrite and acidic pH, while in the transient spark in the closed reactor it is determined by the acidified nitrites present.

The gliding arc discharge plasma reactors are known as a source of non-equilibrium plasma at atmospheric pressure. In the present study, generation of dominant reactive oxygen and nitrogen species in gaseous and liquid phase in water by the compact gliding arc device (mini-GAD) and corresponding bactericidal effects were investigated. Water and phosphate buffer solutions were used as model liquids. Mini-GAD is a strong source of nitrogen oxides (up to 800 ppm NO and 200 ppm NO₂) that result in high concentrations of nitrites and nitrates in water solutions. The highest bactericidal efficacy towards Escherichia coli was achieved for non-buffered water solution.

Atmospheric pressure DC-driven self-pulsing transient spark (TS) discharge operated in air and pulse-driven dielectric barrier discharge plasma jet (PJ) operated in helium in contact with water solutions were used for inducing chemical effects in water solutions, and the treatment of bacteria (Escherichia coli), mammalian cells (Vero line normal cells, HeLa line cancerous cells), deoxyribonucleic acid (dsDNA), and protein (bovine serum albumin). Two different methods of water solution supply were used in the TS: water electrode system and water spray system. The effects of both TS systems and the PJ were compared, as well as a direct exposure of the solution to the discharge with an indirect exposure to the discharge activated gas flow. The chemical analysis of water solutions was performed by using colorimetric methods of UV-VIS absorption spectrophotometry. The bactericidal effects of the discharges on bacteria were evaluated by standard microbiological plate count method. Viability, apoptosis and cell cycle were assessed in normal and cancerous cells. Viability of cells was evaluated by trypan blue exclusion test, apoptosis by Annexin V-FITC/propidium iodide assay, and cell cycle progression by propidium iodide/RNase test. The effect of the discharges on deoxyribonucleic acid and protein were evaluated by fluorescence and UV absorption spectroscopy. The results of bacterial and mammalian cell viability, apoptosis, and cell cycle clearly show that cold plasma can inactivate bacteria and selectively target cancerous cells, which is very important for possible future development of new plasma therapeutic strategies in biomedicine. The authors found that all investigated bio-effects were stronger with the air TS discharge than with the He PJ, even in indirect exposure.