Double-layer structures in low-temperature atmospheric-pressure electronegative RF microplasmas: separation of electrons and anions

Stratification of negatively charged species in electronegative discharges is a well-known phenomenon that can lead to various double-layer structures. Here, we report on the separation of electrons and anions in atmospheric-pressure electronegative microdischarges. In these discharges, electrons oscillate between the electrodes, moving across and beyond an electronegative core. As a result of this motion, positively charged regions form between the oscillating electron ensemble and the central electronegative discharge

Atmospheric pressure plasmas: generation and delivery of reactive oxygen species for biomedical applications

Reactive oxygen species (ROS) that can trigger biological responses are readily attainable in atmospheric pressure plasma sources. Admixtures of oxygen and water can act as precursors for the generation of these ROS and lead to the production of O, OH, O3, 1 O2, OOH and H2O2. The dynamics and chemistry in these discharges is complex and result in intricate spatiotemporal profiles of the species that cannot be accurately captured by zero dimensional analysis. Besides fluxes of neutral ROS, ionic fluxes including anions are also observed. The high reactivity of most of the ROS, however, limits their penetration into the treated sample and therefore encapsulation of the ROS and/or triggering of a secondary chemistry is required for the plasma treatment to reach beyond the first layers of biomolecules

Hydrogen peroxide production in an atmospheric pressure RF glow discharge: comparison of models and experiments

The production of (Formula presented.) in an atmospheric pressure RF glow discharge in helium-water vapor mixtures has been investigated as a function of plasma dissipated power, water concentration, gas flow (residence time) and power modulation of the plasma. (Formula presented.) concentrations up to 8 ppm in the gas phase and a maximum energy efficiency of 0.12 g/kWh are found. The experimental results are compared with a previously reported global chemical kinetics model and a one dimensional (1D) fluid model to investigate the chemical processes involved in (Formula presented.) production. An analytical balance of the main production and destruction mechanisms of (Formula presented.) is made which is refined by a comparison of the experimental data with a previously published global kinetic model and a 1D fluid model. In addition, the experiments are used to validate and refine the computational models. Accuracies of both model and experiment are discussed

Generation and loss of reactive oxygen species in low-temperature atmospheric-pressure RF He+O2+H2O plasma

This study focuses on the generation and loss of reactive oxygen species (ROS) in lowtemperature atmospheric‐pressure rf (13.56MHz) He+O2+H2O plasmas, which are of interest for many biomedical applications. Pure He+O2 plasmas are a good source of ozone, singlet oxygen and atomic oxygen, with densities of these species increasing as oxygen content increases1. He+H2O plasmas offer an interesting alternative to He+O2 plasmas as a source of reactive oxygen species (ROS), and they produce significant amounts of hydrogen peroxide, hydroxyl radicals and hydroperoxyl radicals, which increase with increasing water content2. Admixtures of O2 and H2O lead to richer cocktails of ROS that combine all these species

Generation and loss of reactive oxygen species in low-temperature atmospheric-pressure RF He + O2 + H2O plasmas

This study focuses on the generation and loss of reactive oxygen species (ROS) in low-temperature atmospheric-pressure RF (13.56 MHz) He + O2 + H2O plasmas, which are of interest for many biomedical applications. These plasmas create cocktails of ROS containing ozone, singlet oxygen, atomic oxygen, hydroxyl radicals, hydrogen peroxide and hydroperoxyl radicals, i.e. ROS of great significance as recognized by the free-radical biology community. By means of one-dimensional fluid simulations (61 species, 878 reactions), the key ROS and their generation and loss mechanisms are identified as a function of the oxygen and water content in the feed gas. Identification of the main chemical pathways can guide the optimization of He + O2 + H2O plasmas for the production of particular ROS. It is found that for a given oxygen concentration, the presence of water in the feed gas decreases the net production of oxygen-derived ROS, while for a given water concentration, the presence of oxygen enhances the net production of water-derived ROS. Although most ROS can be generated in a wide range of oxygen and water admixtures, the chemical pathways leading to their generation change significantly as a function of the feed gas composition. Therefore, care must be taken when selecting reduced chemical sets to study these plasmas

Dynamics of atmospheric pressure He/H2O microplasmas: a new double layer structure

Dynamics of atmospheric pressure He/H2O microplasmas: a new double layer structur

Electron-anion separation in electronegative rf microdischarges

Several types of double-layer structures have been reported previously in the literature and the stratification of negatively charged species in electronegative discharges is a well known phenomena. Here we report on the evolution of a different type of double layer structure found in electronegative microplasmas. In these microdischarges, the electron ensemble oscillates between the electrodes forming sheaths that are larger than half the discharge gap. As the electrons oscillate, they move across and beyond a central electronegative core formed by anions. As a result of their different motion, electrons and anions are completely separated and regions of positive space charge form between the oscillating electron ensemble and the central electronegative discharge

Chemical pathways governing the production of Reactive Oxygen Species (ROS) in atmospheric pressure He+O2+H2O plasmas

It is well-known that atmospheric-pressure plasmas can be engineered to produce reactive oxygen species (ROS) and reactive nitrogen species (RNS) known to play important roles in biological systems. Here we concentrate on the generation of ROS, and in particular on the chemical pathways that govern the generation and loss of ROS in atmospheric pressure rf (13.56MHZ) plasmas sustained in helium with admixtures of O2 and H2O

1-D fluid model of atmospheric-pressure rf He+O2 cold plasmas: parametric study and critical evaluation

In this paper atmospheric-pressure rf He+O2 cold plasmas are studied by means of a 1-D fluid model. 17 species and 60 key reactions selected from a study of 250+ reactions are incorporated in the model.O + 2 , O − 3 , and O are the dominant positive ion, negative ion, and reactive oxygen species, respectively. Ground state O is mainly generated by electron induced reactions and quenching of atomic and molecular oxygen metastables, while three-body reactions leading to the formation of O2 and O3 are the main mechanisms responsible for O destruction. The fraction of input power dissipated by ions is ∼20%. For the conditions considered in the study ∼6% of the input power is coupled to ions in the bulk and this amount will increase with increasing electronegativity. Radial and electrode losses of neutral species are in most cases negligible when compared to gas phase processes as these losses are diffusion limited due to the large collisionality of the plasma. The electrode loss rate of neutral species is found to be nearly independent of the surface adsorption probability p for p > 0.001 and therefore plasma dosage can be quantified even if p is not known precisely

Wall fluxes of reactive oxygen species of an rf atmospheric-pressure plasma and their dependence on sheath dynamics

A radio-frequency (rf) atmospheric-pressure discharge in He–O2 mixture is studied using a fluid model for its wall fluxes and their dependence on electron and chemical kinetics in the sheath region. It is shown that ground-state O, O+2 and O− are the dominant wall fluxes of neutral species, cations and anions, respectively. Detailed analysis of particle transport shows that wall fluxes are supplied from a boundary layer of 3–300μm immediately next to an electrode, a fraction of the thickness of the sheath region. The width of the boundary layer mirrors the effective excursion distance during lifetime of plasma species, and is a result of much reduced length scale of particle transport at elevated gas pressures. As a result, plasma species supplying their wall fluxes are produced locally within the boundary layer and the chemical composition of the overall wall flux depends critically on spatio-temporal characteristics of electron temperature and density within the sheath. Wall fluxes of cations and ions are found to consist of a train of nanosecond pulses, whereas wall fluxes of neutral species are largely time-invariant