Jörn Winter

Low temperature atmospheric pressure plasma sources for microbial decontamination

The aim of this paper is to provide a survey of plasma sources at atmospheric pressure used for microbicidal treatment. In order to consider the interdisciplinary character of this topic an introduction and definition of basic terms and procedures are given for plasma as well as for microbicidal issues. The list of plasma sources makes no claim to be complete, but to represent the main principles of plasma generation at atmospheric pressure and to give an example of their microbicidal efficiency. The interpretation of the microbicidal results remain difficult due to the non-standardized methods used by different authors and due to the fact that small variations in the setup can change the results dramatically.

Quantitative detection of plasma-generated radicals in liquids by electron paramagnetic resonance spectroscopy

In this paper the qualitative and quantitative detection of oxygen radicals in liquids after plasma treatment with an atmospheric pressure argon plasma jet by electron paramagnetic resonance spectroscopy is investigated. Absolute values for ?OH and radical concentration and their net production rate in plasma-treated liquids are determined without the use of additional scavenging chemicals such as superoxide dismutase (SOD) or mannitol (D-MAN). The main oxygen-centred radical generation in PBS was found to originate from the superoxide radical. It is shown that hidden parameters such as the manufacturer of chemical components could have a big influence on the comparability and reproducibility of the results. Finally, the effect of a shielding gas device for the investigated plasma jet with a shielding gas composition of varying oxygen-to-nitrogen ratio on radical generation after plasma treatment of phosphate-buffered saline solution was investigated.

Feed gas humidity: a vital parameter affecting a cold atmospheric-pressure plasma jet and plasma-treated human skin cells

In this study, the effect of feed gas humidity on the reactive component generation of an atmospheric-pressure argon plasma jet and its effect on human skin cells are investigated. Feed gas humidity is identified as one key parameter that strongly influences stability and reproducibility of plasma medical studies. The plasma jet is investigated by absorption spectroscopy in the ultraviolet and infrared spectral region for its ozone production depending on the humidity concentration in the feed gas. By optical emission spectroscopy the dependence of present excited plasma species such as hydroxyl radicals, molecular nitrogen, argon and atomic oxygen on the feed gas humidity is investigated. As an interface layer between the plasma jet effluent and the biological cell, a buffer solution is treated and the hydrogen peroxide (H2O2) production is studied with two independent colorimetric assays as a function of humidity admixture to the feed gas. Ultimately, the effect of varying feed gas humidity on the cell viability of indirect plasma treated adherent HaCAT cells is investigated. The highest viability is found for the driest feed gas condition. Furthermore, this work shows answers for the relevance of unwanted—or intended—feed gas humidity in plasma medical experiments and their comparatively large relevance with respect to ambient humidity. The findings will lead to more reproducible experiments in the field of plasma medicine.

Tracking plasma generated H2O2 from gas into liquid phase and revealing its dominant impact on human skin cells

The pathway of the biologically active molecule hydrogen peroxide (H2O2) from the plasma generation in the gas phase by an atmospheric pressure argon plasma jet, to its transition into the liquid phase and finally to its inhibiting effect on human skin cells is investigated for different feed gas humidity settings. Gas phase diagnostics like Fourier transformed infrared spectroscopy and laser induced fluorescence spectroscopy on hydroxyl radicals (OH) are combined with liquid analytics such as chemical assays and electron paramagnetic resonance spectroscopy. Furthermore, the viability of human skin cells is measured by Alamar Blue® assay. By comparing the gas phase results with chemical simulations in the far field, H2O2 generation and destruction processes are clearly identified. The net production rate of H2O2 in the gas phase is almost identical to the H2O2 net production rate in the liquid phase. Moreover, by mimicking the H2O2 generation of the plasma jet with the help of an H2O2 bubbler it is concluded that the solubility of gas phase H2O2 plays a major role in generating hydrogen peroxide in the liquid. Furthermore, it is shown that H2O2 concentration correlates remarkably well with the cell viability. Other species in the liquid like OH or superoxide anion radical do not vary significantly with feed gas humidity.

Atmospheric pressure plasma jets: an overview of devices and new directions

Atmospheric pressure plasma jets have a long history of more than 50 years. During this time their design and plasma generation mechanism has been developed and adapted to various fields of applications. This review aims at giving an overview of jet devices by starting with a brief history of their development. This is followed by an overview of commonly used terms and definitions as well as a survey of different classification schemes (e.g. geometry, excition frequency or specific energy input) described in literature. A selective update of new designs and novel research achievments on atmospheric pressure plasma jets published in 2012 or later shows the impressive variety and rapid development of the field. Finally, a brief outlook on the future trends and directions is given.

Detection of ozone in a MHz argon plasma bullet jet

This study for the first time confirms the presence of plasma bullets in a MHz argon atmospheric pressure plasma jet. Bullet characteristics are investigated by phase-resolved optical emission measurements. Regarding the jets reactive component output, its ozone production rates are investigated by two independent diagnostic techniques yielding complementary results. The first method—UV-absorption spectroscopy in the Hartley band—determines space-resolved distribution of the ozone concentration in the jet effluent. The second method—quantum cascade laser-absorption spectroscopy in the mid-infrared spectral region—yields high sensitivity results of the average ozone concentration in a multipass cell, in which the effluent is directed. The results of both diagnostic techniques show excellent agreement.

From RONS to ROS: Tailoring Plasma Jet Treatment of Skin Cells

Finding a solution for air species contamination of atmospheric pressure plasmas in plasma medical treatment is a major task for the new field of plasma medicine. Several approaches use complex climate chambers to control the surrounding atmosphere. In this paper, ambient species are excluded in plasma-human-skin-cell treatment by ensheathing the plasma jet effluent with a shielding gas. Not only does this gas curtain protect the plasma jet effluent from inflow of air species but it also, more importantly, allows controlling the effluent reactive species composition by adjusting the mixture of the shielding gas. In the present investigations, the mixture of nitrogen to oxygen within the gas curtain around an argon atmospheric pressure plasma jet (kinpen) is varied. The resulting reactive plasma components produced in the jet effluent are thus either oxygen or nitrogen dominated. With this gas curtain, the effect of reactive oxygen species (ROS) and reactive nitrogen species (RNS) on the cell viability of indirectly plasma-treated HaCaT skin cells is studied. This human keratinocyte cell line is an established standard for a skin model system. The cell viability is determined by a fluorometric assay, where metabolically active cells transform nonfluorescent resazurin to the highly fluorescent resorufin. Plasma jet and gas curtain are characterized by numerical flow simulation as well as by optical emission spectroscopy. The generation of nitrite within the used standard cell culture medium serves as a measure for generated RNS. Measurements with the leukodye dichlorodihydrofluorescein diacetate show that, despite a variation of the shielding gas mixture, the total amount of generated reactive oxygen plus nitrogen species is constant. It is shown that a plasma dominated by RNS disrupts cellular growth less than a ROS-dominated plasma.

Controlling the Ambient Air Affected Reactive Species Composition in the Effluent of an Argon Plasma Jet

The influence of ambient air species is an ever-present problem for atmospheric pressure plasma jet applications. In particular, applications where the plasma-induced effects are extremely sensitive to specific types of ambient species (oxygen, nitrogen, humidity) - as, for example, in plasma medicine - require concepts to exclude or to control ambient species flux into the jet effluent that go beyond an environmental control via process chambers or even vacuum systems. In this paper, we demonstrate how to eliminate ambient species influence on effluent chemistry by ensheathing the effluent. With a designed shielding gas composition, we control the species flowing into the jet effluent and thus control the effluent chemistry. The proposed approach can be applied to the majority of possible jet plasma sources. Flow simulations as well as VUV-absorption spectroscopy measurements prove the gas curtain to be effective in shielding the jet gas from ambient species and show that a control of reactive species within the jet effluent is possible. On the example of plasma treatment of a NaCl solution, we demonstrate that, by adjusting the shielding gas composition, the generation of nitrite and nitrate in the solution can be finely controlled.

Reactive species output of a plasma jet with a shielding gas device?combination of FTIR absorption spectroscopy and gas phase modelling

In this work, a simple modelling approach combined with absorption spectroscopy of long living species generated by a cold atmospheric plasma jet yields insight into relevant gas phase chemistry. The reactive species output of the plasma jet is controlled using a shielding gas device. The shielding gas is varied using mixtures of oxygen and nitrogen at various humidity levels. Through the combination of Fourier transform infrared (FTIR) spectroscopy, computational fluid dynamics (CFD) simulations and zero dimensional kinetic modelling of the gas phase chemistry, insight into the underlying reaction mechanisms is gained. While the FTIR measurements yield absolute densities of ozone and nitrogen dioxide in the far field of the jet, the kinetic simulations give additional information on reaction pathways. The simulation is fitted to the experimentally obtained data, using the CFD simulations of the experimental setup to estimate the correct evaluation time for the kinetic simulation. It is shown that the ozone production of the plasma jet continuously rises with the oxygen content in the shielding gas, while it significantly drops as humidity is increased. The production of nitrogen dioxide reaches its maximum at about 30% oxygen content in the shielding gas. The underlying mechanisms are discussed based on the simulation results.

Ambient air particle transport into the effluent of a cold atmospheric-pressure argon plasma jet investigated by molecular beam mass spectrometry

Ambient air species, which are transported into the active effluent of an atmospheric-pressure plasma jet result in highly reactive oxygen and nitrogen species (RONS). Especially for the envisaged application field of plasma medicine, these RONS are responsible for strong biological responses. In this work, the effect of ambient air transport into the effluent of an atmospheric-pressure plasma argon jet on the on-axis densities of nitrogen, oxygen and argon was investigated by means of absolutely calibrated molecular beam mass spectrometry (MBMS). According to biomedical experiments a (bottomless) Petri dish was installed in front of the MBMS. In the following, the near flow field is referring to the region close to the nozzle exit and the far flow field is referring to the region beyond that. The absolute on-axis densities were obtained by three different methods, for the near flow field with VUV-absorption technique, for the far flow field with the MBMS and the total flow field was calculated with a computational fluid dynamics (CFD) simulation. The results of the ambient air particle densities of all independent methods were compared and showed an excellent agreement. Therefore the transport processes of ambient air species can be measured for the whole effluent of an atmospheric-pressure plasma jet. Additionally, with the validation of the simulation it is possible in future to calculate the ambient species transport for various gas fluxes in the same turbulent flow regime. Comparing the on-axis densities obtained with an ignited and with a non-ignited plasma jet shows that for the investigated parameters, the main influence on the ambient air species transport is due to the increased temperature in the case when the jet is switched on. Moreover, the presence of positive ions (e.g. ) formed due to the interaction of plasma-produced particles and ambient air species, which are transported into the effluent, is shown.