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The transition from O3 to NOx discharge mode is common for air plasma with an increase of discharge power density, which greatly changes the chemistry of the plasma as well as its application effect. Although this discharge mode transition has been intensively studied in recent years, differences between plasmas subject to pulsed or sinusoidal excitations have rarely been reported on. In this study, a surface dielectric barrier discharge is excited using either a microsecond pulsed voltage or a sinusoidal voltage. The discharge mode transition of these two excitations is then comparatively studied. The results indicate that pulsed excitation produces more O3 in O3 mode discharge; the O3 concentration for pulsed excitation is 27.7% higher than that for sinusoidal excitation at the same power density of P = 0.08 W/cm2. On the contrary, it produces less NO and NO2 in NOx mode discharge; the NO concentration for pulsed excitation is 13.2% lower than that for sinusoidal excitation at P = 0.32 W/cm2. As a result, pulsed excitation delays the transition of the discharge mode when the discharge power density increases. This can be attributed to the higher electron and lower gas temperature of the pulse-excited plasma, which promotes O3 and inhibits NOx production.

With a change in ambient temperature and discharge power, air discharge plasma can switch between two typical modes, namely the ozone mode and the NO x mode. Here, we analyze the physicochemical properties and anticancer effects of air dielectric barrier discharge (DBD) plasma-activated saline (PAS) treated by ozone-riched gas produced in an ozone discharge mode, NO x -riched gas produced in a NO x discharge mode, and a mixture of both produced in the combined two modes simultaneously. The results show that gaseous and aqueous reactive oxygen species (ROS) and reactive nitrogen species (RNS) generated by plasma vary in different discharge modes, correlating to the amount of high-valance nitroxides. In the ozone mode, ozone and minor N2O5 are generated in the gaseous phase and are transformed to aqueous ROS and RNS. In the NO x mode, low-valance RNS are generated in the gaseous phase but are transformed into less aqueous RNS. For the PAS produced by the mixed gases, not only the highest concentrated ROS are generated, but also the most abundant gaseous high-valence RNS are produced and transformed to aqueous RNS in the saline. In vitro experiments reveal that, although A375 cells incubated with these three PASs (30%) could all inhibit their viability by enhancing intracellular oxidative stress and inducing cell apoptosis, the PAS produced by the mixed gases has the strongest anticancer effects. Further studies suggest that high-valence RNS play a key role in PAS-induced anticancer effects.

Plasma-activated hydrogels have promising application prospects in the field of transdermal penetration therapy, such as subcutaneous tumours, due to their long-lasting, slow-release properties and ability to be easily applied to skin. In this study, surface air plasma was used to prepare plasma-activated Aristoflex AVC hydrogel (PAH) and plasma-activated water (PAW) by irradiation. The characteristics of PAH and PAW were further studied in terms of reactive oxygen species (ROS) storage, transdermal permeation, and in vitro anticancer effects in an artificial skin model. The experimental results showed that PAH had a prolonged storage time of ROS and more transdermal permeation of ROS through artificial skin in Franz diffusion cell after 30 h compared to PAW, which means PAH could be superior to PAW for long-term applications on the skin surface. The in vitro anticancer effects of PAH in an artificial skin model were also better than those of PAW, reducing the viability of A375 cells without causing damage to HaCaT cells. Our study shows that PAH is superior to PAW in body-surface coating applications and can be used for subcutaneous anticancer applications.

N2O5, a reactive species produced by air discharge plasma, has recently attracted much attention. Due to its high reactivity and solubility, N2O5 is a key molecule in nitrogen fixation processes and exhibits promising prospects in plasma biomedicine. However, thus far, it is not well known how to produce N2O5 efficiently and then maintain its concentration under the action of fast removal reactions. In view of this, N2O5 production by dielectric barrier discharge (DBD) alone and by the combination of DBD and gliding arc discharge is compared in this paper. It is found that the combination method can yield over three times the concentration of N2O5 compared to the single DBD method with the optimum discharge power. Moreover, the concentration of N2O5 in the effluent gas can be maintained once O3 also exists because O3 can continually produce N2O5 to compensate for its reduction. Finally, the sterilization effects of both the plasma effluent gas and plasma-activated water have trends similar to the trend of the gaseous N2O5 concentration, implying that N2O5 plays an important role in sterilization. This paper enhances the understanding of N2O5 chemistry in air discharge plasma and provides an effective way to produce and maintain N2O5 for subsequent applications.

Gliding arc is a promising plasma technology for water activation due to its high energy efficiency for producing reactive nitrogen species (RNS), which is believed as the key agent for the sustained bactericidal effect of plasma-activated water (PAW). Nitric oxide (NO) is the major product of gliding arc and also widely exists in PAW, but the production mechanism of aqueous NO and its role in sterilization have been little investigated before. In this paper, NO-rich plasma effluent gas is produced by gliding arc discharge and introduced into water to produce PAW. The concentrations of gaseous and aqueous reactive species are detected, which decrease with the increasing air flowrate of the gliding arc. To clarify the contribution of plasma-induced RNS on water activation, the NO + air mixed gas is used to simulate the plasma effluent gas, and the results show that the two gases have similar gaseous composition and aqueous NO yield. Compared with the NO + Ar mixed gas with the same NO proportion, the NO + air mixed gas produces much more aqueous NO, implying that the presence of O2 significantly enhances the production of aqueous NO. The sterilization experiments demonstrate the key role of aqueous NO in sterilization, but an acidic environment is necessary for aqueous NO to achieve a potent bactericidal effect.

The antibacterial ability of plasma-treated water (PTW) is affected by many factors, such as the type of equipment and operating conditions. These factors hinder the application of plasma technology, making it necessary to develop new methods that could prepare PTW with high efficiency for disinfection. In this study, a surface discharge plasma with a dielectric layer of polytetrafluorethylene (PTFE) was used to treat 110 mM H2O2 solution to prepare plasma-treated H2O2 solution (PTH). The bactericidal ability of PTW was evaluated by the inactivation of methicillin-resistant S. aureus (MRSA). The results show that the PTH treated by surface plasma for 3 min inactivated more than 6.3 orders of magnitude MRSA. Importantly, bubbles were produced when the MRSA suspension was incubated with the untreated H2O2 solution, while no bubbles were observed when the suspension was incubated with the PTH. Further experiments show the amounts of bubbles produced in this process were negatively correlated with the bactericidal effects. The concentrations of several reactive species in PTH were measured for antibacterial mechanism analysis and provided a clue that the synergism among hydrogen peroxide, peroxynitrite, as well as unstable reactive fluorides, derived from the gaseous fluorides from the etching of the PTFE dielectric, might play a key role in the bactericidal process. This work provides a new strategy to produce potent disinfectants with low irritation for the disinfection of the environment, object surfaces, and body surfaces.

Plasma-activated water (PAW) regarded as a green alternative is of great interest in biomedicine due to the presence of a rich diversity of aqueous reactive oxygen and nitrogen species (RONS). Optimizing the reactor parameter is an important approach to regulate RONS in PAW to obtain better biological effects. This work mainly focuses on the number of micro holes of the plasma device to realize underwater bubble discharge. By comparing single-hole and multi-hole plasma under the same pulse voltage, the multi-hole plasma would produce a better discharge characteristic, and the prepared PAW presents superior physicochemical properties to realize high level RONS production by means of microbubbles to increase the contact area with water molecules. Furthermore, the A549 lung carcinoma cells are used to verify the inactivation effect induced by PAW treated by underwater single-hole and multi-hole bubble plasma, respectively, and the results show that the multi-hole plasma would induce cell apoptosis reaching up to 57.4%, while only 31.2% apoptosis efficiency for single-hole plasma. This mainly contributes to the high concentration of key species (NO2 −, H2O2, ONOO−/O2 −, and ·OH) produced by multi-hole plasma. This study is helpful to insight into the influence of underwater bubble on chemistry activity of PAW, and promoting the wide applications in biomedical field.

Plasma activated water (PAW) is a promising green antibacterial agent and the bactericidal effect is complicatedly affected by electron bombardment, ultraviolet radiation, interface reaction, and cascade chemical reaction. In this paper, a case of preparing PAW by treating aqueous solutions with afterglow gas is constructed based on surface micro-discharge (SMD), which focuses on the effect of afterglow gas–liquid mass transfer and liquid phase chemistry on PAW sterilization. The correlation of the bactericidal effect of PAW to the reactive species was studied based on the model of methicillin-resistant Staphylococcus aureus. The production of reactive oxygen and nitrogen species (RONS) in PAW with the regulation of N2/O2 ratios in the working gas for SMD. The RONS in both gas and liquid phases and the physicochemical properties of PAW were measured through optical and chemical methods. In addition, the effects of liquid types, liquid conductivity, and storage time on the bactericidal effects of PAW were explored. The key species for bacteria inactivation were identified by equivalent mixed solutions and specific scavengers. The results demonstrated that control of the N2/O2 ratios in the working gas can effectively improve the RONS in plasma and PAW. The bactericidal effect of PAW is correlated with peroxynitrite, superoxide anion, and their synergistic effects in an acidic liquid environment. This study provides a new strategy for insight into the bactericidal mechanism of PAW in biomedical applications.

Atmospheric pressure plasma jets (APPJs) are widely used for the treatment of water-containing substances such as human tissue, leading to a necessity of understanding the interaction between APPJs and water solutions for the development of plasma biomedicine. The reported two- or three-dimensional fluid models are shown to be an effective method for this study. However, owing to the complex chemistry in APPJ-water interaction, little of them could provide a quantitative estimation of reactive species, which are difficult to be measured but of much interest in the applications. In this paper, a one-dimensional fluid model is developed to simulate the interaction between a helium APPJ and deionized water, which incorporates a relatively comprehensive chemistry both in gas and liquid phases but with a moderate computational load. The composition and distribution of reactive species are quantified during a plasma treatment time of 6 min, which is typical in practice. By considering the sidewise loss inside the quartz tube, the air mixing outside the quartz tube, the conductivity of deionized water, and the chlorine evolution reaction, the simulation results agree well with the experiments. It is found that the plasma could be divided into three regions with much different physicochemical properties, mainly due to the sidewise loss, the air mixing and the water evaporation. In plasma-activated water, H2O2aq and HNO2aq/NO2aq − are the dominant reactive species, and OHaq is the key intermediate species for the transformation among other reactive species. Finally, the chemical pathways for the production of aqueous reactive species are elucidated.

Plasma activated water (PAW) is a green prospective solution for a wide range of biotechnology applications. However, the drawback for the transient chemical activity of PAW would make it lose versatility in possible applications. Herein, we propose a promising approach for plasma re-activation aimed at enhancing the chemical activity of PAW again before it entirely decays. Our results indicate that the PAW generated by re-activation displays superior physicochemical properties and higher production of RONS. Especially, the concentrations of ONOO−/O2 − and H2O2 by re-activation can remarkably increase by 17.88% and 34.83% compared to the initial activation, respectively. Furthermore, the low temperature storage shows excellent synergetic effects with re-activation, indicating that the re-activation actually enhances the chemical activity of PAW to a higher level. The possible reasons for high chemical activity of PAW by plasma re-activation may be affected by the synergistic effect of multi-factors including the residual RONS in stored PAW and the lower pH value, temperatures of electrode and liquid during re-activation. This study provides good insight into the nature of liquid chemistry for PAW prepered by plasma re-activation, and demonstrates a new alternative for improving the chemical activity of PAW.

This study applied surface discharge plasma treated-air to deactivate the pseudovirus with the SARS-CoV-2 S protein in a cold-chain environment. The results of experiments show that the capability of infection of either the dried or the wet COVID-19 pseudovirus are inhibited after exposure to plasma treated-air regardless of whether it is attached to flakes of plastic or a copper sheet. Moreover, various plasma-generated reactive species provide it with an advantage over conventional ozone in deactivating the wet pseudovirus in a cold-chain environment. Our findings provide a potential disinfecting strategy to combat the spread of SARS-CoV-2 through cold-chain systems.

Plasma-surface interaction is a complex system by the codependency between the plasma and the substrate, especially for the substrate with different permittivities. Therefore, a better understanding of the dynamic evolution of interactions with different substrates is essential for optimizing the required changes of specific substrates. In this work, we report two modes of interfacial pattern formation by a pulsed helium plasma jet interacting with ITO glass and metal substrate, mainly focus on the dynamic evolution process for interfacial pattern modes during a transformation process from ITO glass to metal substrate. Pattern morphology, current-voltage characteristics, ICCD images, and reactive species distribution are used to examine the evolution mechanism of the plasma jet-substrate interaction. Results show that, for ITO glass substrate, a circular interfacial pattern with plenty of streamer channels is generated, while, for metal substrate, a constricted solid spot interfacial pattern is formed, these two pattern modes transformations are realized from glass dielectric to metal substrate. Furthermore, during the transformation process from dielectric to metal substrate, the pattern area is gradually decreased, while the discharge intensity, emission intensity, and local electric field are gradually stronger. Importantly, by observing the dynamic behavior of the interfacial pattern, the ionization wave on substrate surface would be stopped spreading toward the metal direction and a bright touch point appears at the interface between the dielectric and the metal, while surface ionization wave still spreads and has no influence toward other direction beside metal direction. Additionally, the evolution mechanism of pattern mode transformation is discussed, which may be attributed to the difference of the substrate conductivity leading to the different distribution of the local electric field. This study is beneficial to deep insights into the nature of the plasma-surface interaction.

Phosphate buffered saline (PBS) is a commonly used medium for in vitro experiments in plasma biomedicine; however, the mechanism for changes in PBS in response to plasma treatment is not well understood. Many kinds of reactive species are produced in plasma-activated PBS, but to date only a few of them can be quantified. In this paper, therefore, we aim to develop a fully coupled model for the interaction between surface air plasma and PBS, primarily to quantify its plasma-induced aqueous reactive species, as well as to elucidate their production mechanism. This model consists of a 0D sub-model for the surface plasma in air, and two 1D sub-models: for the PBS, and for the air gap between the plasma and the PBS. Similar models have been reported by our group fwith respect to the plasma treatment of deionized water. Here, by comparison, an additional 24 chlorine compounds, 17 phosphorous species and 123 chemical reactions are incorporated in our model. Our results indicate that the main aqueous reactive species are H2O2aq, O3aq, NO2aq −, NO3aq −, HClOaq, ClO2aq and ClO3aq −. During plasma treatment, the oxidation reduction potential of most reactive species increases within the first 50 s, then remains almost constant. The chemical profile of the plasma-activated PBS is also plotted, from which it can be observed that some reactive oxygen species, such as OHaq, H2O2aq, and O3aq play a crucial role in the production of chlorine compounds such as HClOaq and ClO3aq −.

In this paper, we employ UV absorption spectroscopy to monitor the generation and permeation of reactive oxygen and nitrogen species (RONS) in plasma-activated water (PAW) to revealthe dynamic variation mechanism of RONS chemistry. Parameters including gas impurity, pulse polarity and solution pH value are varied to explore their effects on the absorbance behavior and peak shift of absorption spectra as well as the permeation distribution of RONS. Regarding the absorbance behavior, experimental results show that introducing air and N2 into He working gas would effectively improve RONS absorbance, proportions of about 0.2% air and 0.5% N2 would result in the maximum absorbance, while the addition of O2 would result in a significant decrease in RONS absorbance. Under positive polarity, the RONS absorbance is about 20% higher than that under negative polarity. Changing the solution pH from acidic to alkaline is beneficial in increasing RONS absorbance, indicating that alkaline solution could effectively promote RONS formation. Regarding the characteristic peak shift, different parameter conditions seriously affect the shift of the absorption peak toward low wavelength or high wavelength due to the change in the ratio of the concentration of each component of RONS in PAW. Furthermore, with respect to the permeation distribution of H2O2 and NO2 −, the results show that the addition of O2 would result in the fastest production rate of H2O2 and introducing air and N2 would generate the fastest rate of NO2 − production. Interestingly, the NO2 − permeation distribution displays a ‘columnar mode’ and a ‘filamentous mode’ under positive and negative polarity, respectively. An alkaline solution promotes the formation of NO2 − while having an obvious inhibiting effect on the NO2 − permeation; conversely, an acidic solution has a promotional effect on NO2 −. This study provides a new in-depth understanding of the dynamic evolutionary behavior of RONS in PAW, helping to reveal the network relationship between RONS, and assisting in the development of applications of PAW.

Surface dielectric barrier discharge is capable of generating large-scale, flexible-shape and chemically active plasmas in open air, and hence has attracted much attention with relevance to clinical applications. An important application requirement is that the plasma treatment should have good homogeneity, which can be reflected by the surface distributions of reactive species on the treated sample. In this letter, a surface air discharge was used to treat a model tissue made of gelatin gel, and the surface distributions of H+, and O3 were detected. When the air gap width between the plasma and the model tissue is 3 mm, it is found that the distributions have mesh patterns similar to that of the ground electrode, but O3 concentrates on the edges of each mesh element, while and H+ concentrate in the center part. A wider air gap width benefits the mixing of reactive species but their surface distributions are more inhomogeneous, which should ascribe to a weak gas flow caused by the ionic wind and/or the gas heating effect. Gas flow can also be generated by rotating the plasma source, which is found to achieve a good homogeneity of treatment with a slow rotating speed of 1.2–2.4 rpm. This approach is easy to implement in clinical applications.

In this paper, a novel large-scale plasma source is put forward, which can generate two modes of discharge, i.e. the surface dielectric barrier discharge and the plasma jet array, by just varying the helium gas flow rate. It is found that the discharge power changes a little with the increasing gas flow rate, but the densities of reactive species in the gas phase and in the treated water change a lot. In particular, the gaseous O3 has its density decreasing while the aqueous O3 has its concentration increasing with the increasing gas flow rate. In the plasma-treated water, the reactive nitrogen species such as nitrite and nitrate have their concentrations first increasing and then decreasing, while the reactive oxygen species such as H2O2, O3 and OH have their concentrations increasing monotonically, implying that the plasma source is well-adaptive for different application requirements.

A comparative study on aqueous reactive species in deionized water treated by two types of plasma jets is presented. Classified by the direction of the electric field in the jet device, a linear-field jet and cross-field jet have been set up. Concentrations of several aqueous reactive species are measured quantitatively by chemical fluorescent assays and electron spin resonance spectrometer. Results show that these two-type plasma jets would generate approximately the same gaseous reactive species under the same discharge power, but the linear-field plasma jet is much more efficient at delivering those species to the remote deionized water. This leads to a much more aqueous short-lived species including OH and produced in water, which are mainly correlated to the solvation of gaseous short-lived species such as ions and electrons. Regarding the long-lived species of aqueous H2O2, the concentration grows faster when treated by the linear-field plasma jet in the initial stage, but after 10 min it is similar to that treated by the cross-field counterpart due to the vapor–liquid equilibrium. The aqueous peroxynitrite is also predicted to be produced as a result of the air inclusion in the feeding gas.

In this paper, atmospheric pressure N2/O2 plasma jets with homogeneous shielding gas excited by nanosecond pulse are obtained to generate simplex reactive nitrogen species (RNS) and reactive oxygen species (ROS), respectively, for the purpose of studying the simplex RNS and ROS to induce the myeloma cell apoptosis with the same discharge power. The results reveal that the cell death rate by the N2 plasma jet with N2 shielding gas is about two times that of the O2 plasma jet with O2 shielding gas for the equivalent treatment time. By diagnosing the reactive species of ONOO−, H2O2, OH and in medium, our findings suggest the cell death rate after plasma jets treatment has a positive correlation with the concentration of ONOO−. Therefore, the ONOO− in medium is thought to play an important role in the process of inducing myeloma cell apoptosis.

In this paper, we report the observation of an interfacial pattern formation on the ITO surface by atmospheric pressure helium plasma jet-ITO interactions. By changing the voltage polarity of positive and negative pulses, the interfacial phenomenon displays two different pattern modes, i.e. a double ring pattern with a combination of homogeneous and filamentous modes as well as a single ring pattern with a homogeneous mode. The reasons may mainly be attributed to the spread of a radially outward traveling surface ionization wave that would cause electric field distributions and charge accumulations on the ITO surface. The spatial-temporal distribution of , He(3s3S), and O(3p5P) emissions are diagnosed to better understand the formation mechanism and the differences of plasma jet patterns under positive and negative polarities. Results show that the distribution of emission is the main contributor for generating the filament structure in a double ring pattern for positive polarity, the homogeneous mode pattern mainly depends on the distribution of O(3p5P) emission for positive and negative polarity. Additionally, in order to further systematically understand the behaviors of plasma jet patterns, some parametric results, such as behaviors versus pulse peak voltage, dielectric material, pulse repetition rate, and flow rate are investigated. Some interesting phenomena and additional insights for the plasma jet pattern are found with different parametric conditions. This study might help to better understand effects of plasma jets in interaction with surfaces, or its application in the medical sector.

In this letter, a 1D fluid model has been used to study the electron heating and particle transport in dual frequency atmospheric-pressure helium capacitive discharge with a high-frequency (HF) voltage of 10 MHz and a low-frequency (LF) voltage of 1 MHz. The electric field is decoupled to three components: the HF, the LF and the direct current (DC) ones, and they have much different effects on the plasmas. The eletrons in plasma bulk are mainly heated by the HF electric field, while in plasma sheath they are heated and cooled by the LF and DC electric fields, respectively. With a fixed total input power, the increase of LF power leads to great enhancement of the electrode fluxes of electrons and ions, especially for the energetic electrons of T e > 2 eV, because more power is dissipated in the vicinity of electrodes and the inelastic collision is more pronounced. Therefore, the particle transport on the treated sample can be greatly enhanced without additional gas heating in dual frequency plasmas, which meets the application requirements more compared to the single frequency plasmas.

Vacuum degree online detection of vacuum interrupter has been a great challenge for decades. In this letter, a novel approach based on laser induced breakdown spectroscopy (LIBS) was proposed to solve this tough problem, which is suitable for non-intrusive, electro-magnetic interference free and remote detection. The spectral lines of Cu, H, N and O elements from the interrupter shield were detected for a large gas pressure range from p = 1 × 10−3 Pa to 1 × 105 Pa. It was found that the spectral intensities of O and H increase monotonically with gas pressure, in contrast the spectral intensity of Cu first decreases slightly and then increases. Their intensity ratios, especially for that of Cu to O, change dramatically and monotonically with the gas pressure when p ≤ 0.1 Pa, indicating that they can be used for determining the vacuum degree values. Spectral ratio method fundamentally reduces the influences of the possible variation in measuring distance and the laser power fluctuation, making LIBS a promising method for vacuum degree online detection of vacuum interrupters.

We present in this study how DC bias voltage impacts on the fluxes of reactive species on the skin tissue by means of a plasma–tissue interaction model. The DC bias voltage inputs less than 2% of the total discharge power, and hence it has little influence on the whole plasma characteritics including the volume-averaged densities of reactive species and the heating effect. However, it pushes the plasma bulk towards the skin surface, which significantly changes the local plasma characteristics in the vicinity of the skin surface, and in consequence remarkably enhances the flux densities of reactive species on the skin tissue. With the consideration of plasma dosage and heat damage on the skin tissue, DC bias voltage is a better approach compared with the common approach of increasing the plasma power. Since the DC voltage is easy to apply on the human body, it is a promising approach for use in clincial applications.

Gelatin gels are used as surrogates of human tissues to study their barrier effects on incoming reactive oxygen and nitrogen species (RONS) generated by surface air discharge. The penetration depth of nitrite into gelatin gel is measured in real time during plasma treatment, and the permeabilities of nitrite, nitrate, O3 and H2O2 through gelatin gel films are quantified by measuring their concentrations in the water underneath such films after plasma treatment. It is found that the penetration speed of nitrite increases linearly with the mass fraction of water in the gelatin gels, and the permeabilities of nitrite and O3 are comparably smaller than that for H2O2 and nitrate due to differences in their chemistry in gelatin gels. These results provide a quantitative basis to estimate the penetration processes of RONS in human tissues, and they also confirm that the composition of RONS is strongly dependent on the tissue depth and the plasma treatment time. A small electric field of up to 20 V cm−1 can greatly reduce the barrier effects of the tissue model regardless of their directions, for which the underlying mechanism is unclear. However, the electric field force on the objective RONS should not be the dominant mechanism.

Developing arrays of parallel microplasma jets is an attractive route to scaling the area available for the treatment of surfaces with low temperature plasma. Increasing the packing density of the arrays may lead to electrical and gas kinetic jet–jet interactions, but previous work has focused almost exclusively on electrostatic coupling between the jets. Chemical interactions ('crosstalk') have received considerably less attention. We report here the results of an investigation of chemical crosstalk in 4 × 4 arrays of microplasma jets, produced in flowing helium at atmospheric pressure. Oxidation damage to an Escherichia coli lawn serves as a diagnostic of the spatial distribution of molecular radicals and other reactive plasma species, produced at the plasma jet/ambient background interface or between the jets, and incident on the surface. Spatial maps of bacterial inactivation by the microplasma jet array for 20 s show the destruction of E. coli at distances as large as 2.7 jet diameter from the nearest plasma perimeter, compared to typically less than 0.5 jet diameter in the single jet case. Extending to 30 s of plasma exposure leads to destruction of the entire bacterial sample. This 'action at a distance' effect, the production of long-lived species such as O, O2(a1Δg) and O3 that are responsible for bacterial deactivation, peaks along a line bisecting columns and rows of plasma jets. The data illustrate the synergistic effect of adjacent jets on off-axis formation of reactive species, and show that the chemical and biological impact of an array cannot be inferred from the plasma chemistry of a single jet.

The dielectric breakdown properties of hot SF6 gas during the dielectric recovery phase play an important role in understanding gas breakdown occurring in high-voltage circuit breakers. This paper is devoted to the theoretical investigation of dielectric breakdown properties of hot SF6 gas contaminated by copper at temperatures of 300–3500 K and pressures of 0.01–1.6 MPa. The equilibrium compositions of SF6–Cu mixtures are obtained with the consideration of condensed species. The unknown ionization cross sections for CuS, CuF and CuF2 are calculated using a Deutsch–Mark (DM) formalism based on quantum chemistry. The two-term Boltzmann equation is adopted to numerically calculate the electron energy distribution function, collision ionization coefficient and electron attachment coefficient. Then the reduced critical electric field strength is determined when the effective ionization coefficient equals to zero. The influences of the Cu proportion and gas pressure on the dielectric breakdown properties are investigated. It is shown that the existence of copper compounds increases the concentration of high-energy electrons significantly, even for the case with a very low percentage (e.g. 1% Cu). With the increase of copper content, the value of (E/N)cr is reduced remarkably at temperatures below 3000 K, but enhanced slightly above 3000 K. It is also found that the increase of pressure can improve the dielectric breakdown performance of hot SF6–Cu mixtures.

Evolution of electron-heating processes with driving frequency and their effects on production reactive oxygen species (ROS) are studied for low-temperature radio-frequency atmospheric helium–oxygen plasma. With increasing frequency from 13.54 to 108.48 MHz at a constant power density of 40 W cm−3, the space-averaged electron density is found to increase by 35% but this is accompanied by marked decrease in the sheath thickness, the maximum electron temperature, and the electric field in the sheath by 76%, 46% and four-folds, respectively. As a result, plasma species generated via reactions with low threshold electron energy (e.g. ) experience more abundant production and those generated by energetic electrons (e.g. O and O*) undergo a reduction in their number densities as the driving frequency increases. These frequency dependences directly regulate power dissipation in species produced directly by electrons, ROS concentrations, and ultimately plasma chemistry. Delivery of ROS is shown to be effective only from a boundary layer immediately above a sample surface, due to severely compromised diffusion and drift at atmospheric pressure, introducing a further frequency-dependent factor. In general, the increase of the driving frequency affects energetic and low-energy electrons differently and the contrast in frequency effects of different ROS is desirable for manipulating plasma chemistry as experienced by the sample.

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, 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 3–300 µm 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.

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.