In most applications helium-based plasma jets operate in an open-air environment. The presence of humid air in the plasma jet will influence the plasma chemistry and can lead to the production of a broader range of reactive species. We explore the influence of humid air on the reactive species in radio frequency (rf)-driven atmospheric-pressure helium-oxygen mixture plasmas (He-O 2 , helium with 5000 ppm admixture of oxygen) for wide air impurity levels of 0-500 ppm with relative humidities of from 0% to 100% using a zero-dimensional, time-dependent global model. Comparisons are made with experimental measurements in an rf-driven micro-scale atmospheric pressure plasma jet and with one-dimensional semi-kinetic simulations of the same plasma jet. These suggest that the plausible air impurity level is not more than hundreds of ppm in such systems. The evolution of species concentration is described for reactive oxygen species, metastable species, radical species and positively and negatively charged ions (and their clusters). Effects of the air impurity containing water humidity on electronegativity and overall plasma reactivity are clarified with particular emphasis on reactive oxygen species.

Plasma-induced non-equilibrium liquid chemistry is used to synthesize gold nanoparticles (AuNPs) without using any reducing or capping agents. The morphology and optical properties of the synthesized AuNPs are characterized by transmission electron microscopy (TEM) and ultraviolet-visible spectroscopy. Plasma processing parameters affect the particle shape and size and the rate of the AuNP synthesis process. Particles of different shapes (e.g. spherical, triangular, hexagonal, pentagonal, etc) are synthesized in aqueous solutions. In particular, the size of the AuNPs can be tuned from 5 nm to several hundred nanometres by varying the initial gold precursor (HAuCl4) concentration from 2.5 μM to 1 mM. In order to reveal details of the basic plasma-liquid interactions that lead to AuNP synthesis, we have measured the solution pH, conductivity and hydrogen peroxide (H2O2) concentration of the liquid after plasma processing, and conclude that H2O2 plays the role of the reducing agent which converts Au(+3) ions to Au(0) atoms, leading to nucleation growth of the AuNPs.

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Bacteria exist, in most environments, as complex, organised communities of sessile cells embedded within a matrix of self-produced, hydrated extracellular polymeric substances known as biofilms. Bacterial biofilms represent a ubiquitous and predominant cause of both chronic infections and infections associated with the use of indwelling medical devices such as catheters and prostheses. Such infections typically exhibit significantly enhanced tolerance to antimicrobial, biocidal and immunological challenge. This renders them difficult, sometimes impossible, to treat using conventional chemotherapeutic agents. Effective alternative approaches for prevention and eradication of biofilm associated chronic and device-associated infections are therefore urgently required. Atmospheric pressure non-thermal plasmas are gaining increasing attention as a potential approach for the eradication and control of bacterial infection and contamination. To date, however, the majority of studies have been conducted with reference to planktonic bacteria and rather less attention has been directed towards bacteria in the biofilm mode of growth. In this study, the activity of a kilohertz-driven atmospheric pressure non-thermal plasma jet, operated in a helium oxygen mixture, against Pseudomonas aeruginosa in vitro biofilms was evaluated. Pseudomonas aeruginosa biofilms exhibit marked susceptibility to exposure of the plasma jet effluent, following even relatively short (∼10′s s) exposure times. Manipulation of plasma operating conditions, for example, plasma operating frequency, had a significant effect on the bacterial inactivation rate. Survival curves exhibit a rapid decline in the number of surviving cells in the first 60 seconds followed by slower rate of cell number reduction. Excellent anti-biofilm activity of the plasma jet was also demonstrated by both confocal scanning laser microscopy and metabolism of the tetrazolium salt, XTT, a measure of bactericidal activity.

A dielectric barrier discharge (DBD) generated by flowing helium between the parallel-plate electrodes of an open air reactor has been characterized using time resolved optical and electrical measurements. A sinusoidal voltage of up to 5 kV (peak to peak) of frequencies from 3 to 50 kHz has been applied to the discharge electrodes. The helium flow rate is varied up to 10 litre min −1 . The adjustment of flow rate allows the creation of uniform DBDs with optimized input power equal to 120 ± 10 mW cm −3 . At flow rates from 4 to 6 litre min −1 a uniform DBD is obtained. The maxima in the line intensities of N + 2 and helium at 391.4 nm and 706.5 nm, respectively, under those conditions indicate the importance of helium metastables and He + 2 in sustaining such a discharge. The power efficiency and discharge current show maxima when the DBD in He/air is uniform. The gas temperature during the discharge has been measured as 360 ± 20 K.

The formation of reactive species in the afterglow of a radio-frequency-driven atmospheric-pressure plasma in a fixed helium-oxygen feed gas mixture (He+0.5%O 2 ) with humid air impurity (a few hundred ppm) is investigated by means of an extensive global plasma chemical kinetics model. As an original objective, we explore the effects of humid air impurity on the biologically relevant reactive species in an oxygen-dependent system. After a few milliseconds in the afterglow environment, the densities of atomic oxygen (O) decreases from 10 15 to 10 13 cm −3 and singlet delta molecular oxygen (O 2 ( 1 D)) of the order of 10 15 cm −3 decreases by a factor of two, while the ozone (O 3 ) density increases from 10 14 to 10 15 cm −3 . Electrons and oxygen ionic species, initially of the order of 10 11 cm −3 , recombine much faster on the time scale of some microseconds. The formation of atomic hydrogen (H), hydroxyl radical (OH), hydroperoxyl (HO 2 ), hydrogen peroxide (H 2 O 2 ), nitric oxide (NO) and nitric acid (HNO 3 ) resulting from the humid air impurity as well as the influence on the afterglow chemistry is clarified with particular emphasis on the formation of dominant reactive oxygen species (ROS). The model suggests that the reactive species predominantly formed in the afterglow are major ROS O 2 ( 1 D) and O 3 (of the order of 10 15 cm −3 ) and rather minor hydrogen-and nitrogen-based reactive species OH, H 2 O 2 , HNO 3 and NO 2 /NO 3 , of which densities are comparable to the O-atom density (of the order of 10 13 cm −3 ). Furthermore, the model quantitatively reproduces the experimental results of independent O and O 3 density measurements.

The effect of a cold (<40 °C) radio frequency-driven atmospheric pressure plasma jet on plasmid DNA has been investigated. Gel electrophoresis was used to analyze the DNA forms post-treatment. The experimental data are fitted to a rate equation model that allows for quantitative determination of the rates of single and double strand break formation. The formation of double strand breaks correlates well with the atomic oxygen density. Taken with other measurements, this indicates that neutral components in the jet are effective in inducing double strand breaks.

The development of a microcomputer-controlled electrostatic probe as a diagnostic technique is described. The technique uses cylindrical probes and the definitive theory of Laframboise to give rapid measurements of the plasma parameters in a medium density (1010 to 1012 cm−3) plasma including the fast electron density and temperature. The iterative technique described overcomes many of the problems associated with established methods of Langmuir probe analysis.