A remote microscale atmospheric pressure plasma jet ( µ APPJ) with He, He/H ₂ O, He/O ₂ , and He/O ₂ /H ₂ O gas mixtures was used to study the transport of reactive species from the gas phase into the liquid and the following aqueous phase chemistry. The effects induced by the µ APPJ in water were quantitatively studied using phenol as a chemical probe and by measuring H ₂ O ₂ concentration and pH values. These results were combined with the analysis of the absolute densities of the reactive species and the modeling of convective/diffusion transport and recombination reactions in the effluent of the plasma jet. Additionally, modified plasma jets were used to show that the role of emitted photons in aqueous chemistry is negligible for these plasma sources. The fastest phenol degradation was measured for the He/O ₂ plasma, followed by He/H ₂ O, He/O ₂ /H ₂ O, and He plasmas. The modeled quantitative flux of O atoms into the liquid in the He/O ₂ plasma case was highly comparable with the phenol degradation rate and showed a very high transfer efficiency of reactive species from the plasma into the liquid, where more than half of the O atoms leaving the jet nozzle entered the liquid. The results indicate that the high oxidative effect of He/O ₂ plasma was primarily due to solvated O atoms, whereas OH radicals dominated the oxidative effects induced in water by plasmas with other gas mixtures. These findings help to understand, in a quantitative way, the complex interaction of cold atmospheric plasmas with aqueous solutions and will allow a better understanding of the interaction of these plasmas with water or buffered solutions containing biological macromolecules, microorganisms, or even eukaryotic cells. Additionally, the µ APPJ He/O ₂ plasma source seems to be an ideal tool for the generation of O atoms in aqueous solutions for any future studies of their reactivity.

Cold physical plasma has been suggested as a powerful new tool in oncology. However, some cancer cells such as THP-1 leukaemia cells have been shown to be resistant towards plasma-induced cell death, thereby serving as a good model for optimizing plasmas in order to foster pro-apoptotic anticancer effects. A helium/oxygen radio frequency driven atmospheric plasma profoundly induced apoptosis in THP-1 cells whereas helium, humidified helium, and humidified helium/oxygen plasmas were inefficient. Hydrogen peroxide – previously shown as central plasma-derived agent – did not participate in the killing reaction but our results suggest hypochlorous acid to be responsible for the effect observed. Proteomic analysis of THP-1 cells exposed to He/O₂ plasma emphasized a prominent growth retardation, cell stress, apoptosis, and a pro-immunogenic profile. Altogether, a plasma setting that inactivates previously unresponsive leukaemia cells is presented. Crucial reactive species in the plasma and liquid environment were identified and discussed, deciphering the complexity of plasma from the gas phase into the liquid down to the cellular response mechanism. These results may help tailoring plasmas for clinical applications such as oxidation-insensitive types of cancer.

It is demonstrated with help of¹⁸O₂labeling that O_(aq)is stable in water and can directly react with dissolved molecules.

Innovative technologies are needed to enhance access to clean water and avoid waterborne diseases. We investigated the performance of cold atmospheric plasma (CAP), a clean and sustainable approach for microbial inactivation and total organic carbon (TOC) degradation in environmental water. Water matrices played a crucial role in the performance of CAP efficacy; for example, complete removal of ɸX174 from dH₂O required 1 min of treatment, while ɸX174 reductions of ~ 2log₁₀ and 4log₁₀ were obtained after 10 min of CAP exposure in river water and wastewater samples, respectively. Similarly, after 10 min of CAP treatment, bacterial concentrations decreased by 3 log₁₀ and 4 log₁₀, in river and wastewater samples, respectively. In contrast, after 30 s of contact time, a 4 log₁₀ reduction of bacteria was accomplished in dH₂O. Complete removal of Acanthamoeba from dH₂O was found after 30 min of CAP treatment, whereas it was not removed from surface water or wastewater at the same exposure time. Additionally, the approach successfully reduced TOC, and the degradation kinetics of TOC were represented by pseudo-first-order. CAP showed higher rates of TOC degradation in the final effluent of the wastewater treatment plant compared to surface water. The difference in CAP performance between river water and wastewater could be attributed to the bulk structure of humic acids in river water compared to small organic byproducts in the final effluent of WWTP. Overall, the findings reported here support the idea that CAP holds promise as a sustainable solution for controlling pathogens, removing organic water pollution, and integrating with traditional purification processes. Low-cost systems may advance CAP technology and increase its widespread use.

A remote microscale atmospheric pressure plasma jet with a He/O ₂ gas mixture is used to etch a hydrogenated amorphous carbon layer. The etched profiles are measured by means of imaging spectroscopic reflectometry, a powerful technique providing a 2D map of the film thickness (etched profile) and also film properties. Additionally, the 2D axially symmetric fluid model of the gas flow and species transport combined with the basic kinetic model of the reaction of O atoms with O ₂ molecules has been solved to study the transport and surface reactivity of O atoms. The model provides a spatially resolved and surface-integrated O atom loss rate at the surface. The situation with convection-dominated species transport and fast recombination reactions of O atoms in the volume leads to a strong dependence of the etched profile on the O ₂ admixture and O atom surface loss probability β . By comparing etched profiles with the simulation results, the O atom surface reaction probability of β = 0.2%–0.6% could be estimated. The modeled O atom loss rate at the surface was always higher and with the same trend as the etching rate, corroborating that O atoms are the main etching species. The presented data and simulation results show that the fastest surface-integrated etching rate is achieved not under conditions with the highest O density on the jet axis, but at lower O ₂ admixtures due to reduced recombination losses in the gas phase.