Session TR3: High Pressure Plasma Chemistry

Generation and remote delivery of plasma activated species

Plasma interactions with microdroplets offer new opportunities to deliver active chemical agents and nanoparticles to remote substrates downstream with many potential applications from cancer theranostics and wound healing in biomedicine, gentle food decontamination and seed germination in plasma agriculture to catalyst production and photonic structures fabrication, among others. We demonstrate plasma-liquid based pristine nanomaterials synthesis in flight and subsequent delivery up to 120mm from the atmospheric pressure plasma source. Monosized and non-aggregating metal nanoparticles are formed in the rf plasma in less than 100us, representing an increase in precursor reduction rate that is many (\textgreater 4) orders of magnitude faster than that observed with standard colloidal chemistry or via high energy radiolytic techniques. Also the collection and purification limitations of the latter are avoided. Plasma activated liquid including OH radicals and H2O2 are transported over 120mm and have demonstrated high efficacy bacterial decontamination. These results will be compared with charge species and radical transport from the rf plasma without microdroplets. Reaction models based on high solvated surface electron concentrations will be presented.   

Controlling the nitric and nitrous oxide production of an atmospheric pressure plasma jet

Atmospheric pressure plasma jets are non-thermal plasmas and have the ability to create reactive species. These features make it a very attractive tool for biomedical applications. In this work, we studied NO and N$_{\mathrm{2}}$O production, which are two species having biomedical properties. NO plays a role in the vascularization and in ulcer treatment, while N$_{\mathrm{2}}$O is used as anesthetic and analgesic gas. In this study, the plasma source is similar to the COST Reference Microplasma Jet (\textmu --APPJ). Helium is used as feed gas with small admixtures of molecular nitrogen and oxygen of below 1{\%}. The absolute densities of NO and N$_{\mathrm{2}}$O were measured in the effluent of an atmospheric pressure RF plasma jet by means of ex-situ quantum-cascade laser absorption spectroscopy via a multi-pass cell in Herriot configuration. We will show that the species' production is dependent on several parameters such as power, flow and oxygen and nitrogen admixture. The NO and N$_{\mathrm{2}}$O densities are strongly dependent on the N$_{\mathrm{2}}$-O$_{\mathrm{2}}$ ratio. Changing this ratio allows for choosing between a NO-rich or a N$_{\mathrm{2}}$O-rich regime [1]. [1] Douat \textit{et al}, \textit{PSST}, \textbf{25} (2016) 025027   

Plasma chemistry in electron-beam sustained discharges

There are many emerging applications that exploit the exotic chemical characteristics of plasmas. Some of these applications, if deployed on an industrial scale, involve processing much larger volumes of gas than seems reasonable using any atmospheric pressure plasma source in wide use today. We note that an electron-beam sustained discharge permits the creation of a atmospheric pressure plasma with reasonable uniformity, large volme, and widely controllable electron temperature. Robust and durable electron beam sources now exist that would facilitate such applications. In this paper we discuss the general advantages of this approach, and we present a modelling study concerned with the production of NO in mixtures of N$_2$ and O$_2$, looking towards plasma aided manufacturing of fertilizers.   

Chemistry of neutral species in the effluent of the micro atmospheric pressure plasma jet in water-helium admixture

A thorough understanding and good control of produced neutral and charged species by cold atmospheric plasmas is essential for potential environmental and/or bio-medical applications. In this study we use the COST reference micro plasma jet (\textmu -APPJ), which is a radio-frequency capacitive coupled plasma source with 1 mm electrode distance, which has been operated in helium-water vapour mixture and has been studied as a potential source of hydroxyl radicals and hydrogen peroxide molecules. The water vapour concentration was up to 1.2{\%}. Molecular Beam mass spectrometry is used as diagnostic tool. An absolute calibration of hydrogen peroxide was conducted using a double bubbler concept, because the ionization cross section for hydrogen peroxide is not available. Additionally the effluent chemistry was investigated by use of a 0D and 2D model. Absolute densities of hydrogen peroxide and hydroxyl radicals from atmospheric plasma will be presented. Their dependency on water vapour concentration in the carrier gas as well as distance to target have been investigated. The measured density is between 5E-13 cm-3 (2.4ppm) and 1.5E-14 cm-3 (7.2ppm) for both hydrogen peroxide molecules and hydroxyl radicals. The achieved results are in good agreement with other experiments.   

(Student Award Finalist) Reactive species in humidity containing atmospheric pressure plasma jets -- Numerical and experimental investigations

The formation and absolute densities of oxygen and hydrogen containing reactive species such as atomic oxygen (O), hydrogen (H), hydroxyl radicals (OH) and hydrogen peroxide (H$_{\mathrm{2}}$O$_{\mathrm{2}})$ in an atmospheric pressure plasma jet (APPJ) are investigated as a function of the humidity content in the helium feed gas. APPJs are effective sources for these species, which are known to be biologically active and form a central role in their potential for biomedical applications. To develop and tailor APPJs for therapeutics, quantification of the reactive species produced is necessary. In this work, different diagnostic techniques, such as UV and VUV absorption spectroscopy and picosecond two-photon absorption laser-induced fluorescence (ps-TALIF) and a 0-dimensional chemical kinetics model are applied. We find that the densities of hydrogen containing species increase non-linearly with increasing feed gas humidity. The trend of atomic oxygen depends strongly on impurities present in the APPJ. The model results show that the dominant formation and destruction mechanisms of the species of interest are strongly influenced by the humidity content with different processes dominating at high and low humidity.   

Control of ROS and RNS productions in liquid in atmospheric pressure plasma-jet system

Non-thermal plasma jets are of current interest in biomedical applications such as wound disinfection and even treatment of cancer tumors. Beneficial therapeutic effects in medical applications are attributed to excited species of oxygen and nitrogen from air. However, to control the production of these species in the plasma jet is difficult because their production is strongly dependent on concentration of nitrogen and oxygen from ambient air into the plasma jet. In this study, we analyze the discharge characteristics and the ROS and RNS productions in liquid in low- and high-frequency plasma-jet systems. Our experiments demonstrated the marked effects of surrounding gas near the plasma jet on ROS and RNS productions in liquid. By controlling the surround gas, the O$_{\mathrm{2}}$ and N$_{\mathrm{2}}$ main plasma jets are selectively produced even in open air. We also show that the concentration ratio of NO$_{\mathrm{2}}^{\mathrm{-}}$ to H$_{\mathrm{2}}$O$_{\mathrm{2}}$ in liquid is precisely tuned from 0 to 0.18 in deionized water by changing N$_{\mathrm{2}}$ gas ratio (N$_{\mathrm{2}}$/(N$_{\mathrm{2}} \quad +$ O$_{\mathrm{2)}})$ in the main discharge gas, where high NO$_{\mathrm{2}}^{\mathrm{-}}$ ratio is obtained at N$_{\mathrm{2}}$ gas ratio at N$_{\mathrm{2}}$/(N$_{\mathrm{2}} \quad +$ O$_{\mathrm{2}}) \quad =$ 0.8. The low-frequency plasma jet with controlled surrounding gas is an effective plasma source for ROS and RNS productions in liquid, and can be a useful tool for biomedical applications.