Characterisation of volume and surface dielectric barrier discharges in N2_2–O2_2 mixtures using optical emission spectroscopy

A volume and a twin surface dielectric barrier discharge (VDBD and SDBD) are generated in different nitrogen–oxygen mixtures at atmospheric pressure by applying damped sinusoidal voltage waveforms with oscillation periods in the microsecond time scale. Both electrode configurations are located inside vacuum vessels and operated in a controlled atmosphere to exclude the influence of surrounding air. The discharges are characterised with different spatial and temporal resolution by applying absolutely calibrated optical emission spectroscopy in conjunction with numerical simulations and current–voltage measurements. Plasma parameters, namely the electron density and the reduced electric field, and the dissipated power are found to depend strongly on the oxygen content in the working gas mixture. Different spatial and temporal distributions of plasma parameters and dissipated power are explained by surface and residual volume charges for different O$_2$ admixtures due to their effects on the electron recombination rate. Thus, the oxygen admixture is found to strongly influence the breakdown process and plasma conditions of a VDBD and a SDBD

Fundamental properties and applications of dielectric barrier discharges in plasma-catalytic processes at atmospheric pressure

The combination of a nonthermal plasma and a heterogeneous catalyst provides unique opportunities for chemical transformations. High densities of reactive species, such as ions, radicals or vibrationally excited molecules, are generated by electron collisions and initiate a multitude of chemical reactions in the gas phase. By shifting the reaction site from the gas phase to the surface of the catalyst, the selectivity of these reactions can be significantly enhanced. Dielectric barrier discharges (DBDs) are a promising plasma source for these kinds of applications due to their non-equilibrium conditions and their simple construction. This review provides a brief introduction to the breakdown mechanism and the various geometries of DBDs and presents several plasma-catalytic DBD applications

Study on chemical modifications of glutathione by cold atmospheric pressure plasma (Cap) operated in air in the presence of Fe(II) and Fe(III) complexes

Cold atmospheric pressure plasma is an attractive new research area in clinical trials to treat skin diseases. However, the principles of plasma modification of biomolecules in aqueous solutions remain elusive. It is intriguing how reactive oxygen and nitrogen species (RONS) produced by plasma interact on a molecular level in a biological environment. Previously, we identified the chemical effects of dielectric barrier discharges (DBD) on the glutathione (GSH) and glutathione disulphide (GSSG) molecules as the most important redox pair in organisms responsible for detoxification of intracellular reactive species. However, in the human body there are also present redox-active metals such as iron, which is the most abundant transition metal in healthy humans. In the present study, the time-dependent chemical modifications on GSH and GSSG in the presence of iron(II) and iron(III) complexes caused by a dielectric barrier discharge (DBD) under ambient conditions were investigated by IR spectroscopy, mass spectrometry and High Performance Liquid Chromatography (HPLC). HPLC chromatograms revealed one clean peak after treatment of both GSH and GSSH with the dielectric barrier discharge (DBD) plasma, which corresponded to glutathione sulfonic acid GSO$_3$H. The ESI-MS measurements confirmed the presence of glutathione sulfonic acid. In our experiments, involving either iron(II) or iron(III) complexes, glutathione sulfonic acid GSO$_3$H appeared as the main oxidation product. This is in sharp contrast to GSH/GSSG treatment with DBD plasma in the absence of metal ions, which gave a wild mixture of products. Also interesting, no nitrosylation of GSH/GSSG was oberved in the presence of iron complexes, which seems to indicate a preferential oxygen activation chemistry by this transition metal ion

μ\mus and ns twin surface dielectric barrier discharges operated in air

Electrode erosion through continual long-timescale operation (60 min) of identical twin surface dielectric barrier discharges (twin SDBDs) powered either by a microsecond ($\mu$s) or a nanosecond timescale (ns) voltage source is investigated. The twin SDBDs are characterized using current–voltage measurements, optical emission spectroscopy, and phase integrated ICCD imaging. The temporally and spatially averaged gas temperature, consumed electric power, and effective discharge parameters (reduced electric field, and electron density) are measured. The μ$\mu$s twin SDBD is shown to operate in a filamentary mode while the ns twin SDBD is shown to operate in a more homogeneous mode (i.e. non filamentary). Despite a similarity of the effective discharge parameters in both the μs and ns twin SDBD, erosion of the nickel coated electrodes caused by operation of the twin SDBD differs strongly. Only the formation of a moderate number of nickel oxide species is observed on the surface of the ns twin SDBD electrodes. In contrast, the nickel coated electrodes are locally melted and considerably higher densities of oxides are observed around the eroded areas of the μs twin SDBD, due to the filamentary nature of the discharge

Microscale atmospheric pressure plasma jet as a source for plasma-driven biocatalysis

The use of a microscale atmospheric pressure plasma jet ($\mu$APPJ) was investigated for its potential to supply hydrogen peroxide in biocatalysis. Compared to a previously employed dielectric barrier discharge (DBD), the $\mu$APPJ offered significantly higher H$_2$O$_2$ production rates and better handling of larger reaction volumes. The performance of the $\mu$APPJ was evaluated with recombinant unspecific peroxygenase from $\textit {Agrocybe aegerita}$ (r$\it Aae$UPO). Using plasma-treated buffer, no side reactions with other plasma-generated species were detected. For long-term treatment, r$\it Aae$UPO was immobilized, transferred to a rotating bed reactor, and reactions performed using the $\mu$APPJ. The enzyme had a turnover of 36,415 mol mol$^{−}$1 and retained almost full activity even after prolonged plasma treatment. Overall, the $\mu$APPJ presents a promising plasma source for plasma-driven biocatalysis

The role of humidity and UV-C emission in the inactivation of B. subtilis\textit {B. subtilis} spores during atmospheric-pressure dielectric barrier discharge treatment

Experiments are performed to assess the inactivation of $\textit {Bacillus subtilis}$ spores using a non-thermal atmospheric-pressure dielectric barrier discharge. The plasma source used in this study is mounted inside a vacuum vessel and operated in controlled gas mixtures. In this context, spore inactivation is measured under varying nitrogen/oxygen and humidity content and compared to spore inactivation using ambient air. Operating the dielectric barrier discharge in a sealed vessel offers the ability to distinguish between possible spore inactivation mechanisms since different process gas mixtures lead to the formation of distinct reactive species. The UV irradiance and the ozone density within the plasma volume are determined applying spectroscopic diagnostics with neither found to fully correlate with spore inactivation. It is found that spore inactivation is most strongly correlated with the humidity content in the feed gas, implying that reactive species formed, either directly or indirectly, from water molecules are strong mediators of spore inactivation

Sterilization of beehive material with a double inductively coupled low pressure plasma

American Foulbrood is a severe, notifiable disease of the honey bee. It is caused by infection of bee larvae with spores of the gram-positive bacterium $\textit {Paenibacillus larvae}$. Spores of this organism are found in high numbers in an infected hive and are highly resistant to physical and chemical inactivation methods. The procedures to rehabilitate affected apiaries often result in the destruction of beehive material. In this study we assess the suitability of a double inductively coupled low pressure plasma as a non-destructive, yet effective alternative inactivation method for bacterial spores of the model organism $\textit {Bacillus subtilis}$ on beehive material. Plasma treatment was able to effectively remove spores from wax, which, under protocols currently established in veterinary practice, normally is destroyed by ignition or autoclaved for sterilization. Spores were removed from wooden surfaces with efficacies significantly higher than methods currently used in veterinary practice, such as scorching by flame treatment. In addition, we were able to non-destructively remove spores from the highly delicate honeycomb wax structures, potentially making treatment of beehive material with double inductively coupled low pressure plasma part of a fast and reliable method to rehabilitate infected bee colonies with the potential to re-use honeycombs

Experimental investigations of plasma dynamics in the hysteresis regime of reactive RF sputter processes

Reactive radio frequency (RF) sputter processes are highly relevant for thin film deposition, but there is no complete understanding of the fundamentals of their operation. While the Berg model describes the hysteresis regime considering the oxygen coverage of the boundary surfaces, a complete fundamental understanding of the plasma–surface interactions and their effects on the discharge is still missing. In this work, we provide such fundamental insights based on an extensive experimental analysis of the physics in the hysteresis regime of magnetized reactive sputter processes, where the same reactive gas admixture can lead to different discharge characteristics depending on the previous state of the plasma. A variety of plasma and surface diagnostics is used to reveal these insights. A low pressure capacitively coupled RF discharge (CCP, 13.56 MHz) with a magnetron-like magnetic field topology adjacent to the target is operated in argon gas with a variable admixture of O$_2$. The applied RF power, the gas flows/pumping speed, as well as the neutral gas pressure are changed systematically to understand the effects of these external control parameters on the hysteresis regime. The magnetic asymmetry effect is found to play an important role, since an axially non-uniform magnetic field is used to realize a local electron confinement at the target. Similar to process control in applications, the DC self-bias is measured to stabilize the surface composition using a feedback controller with the oxygen gas flow as the manipulated variable

Catalytic oxidation of small organic molecules by cold plasma in solution in the presence of molecular iron complexes†

The plasma-mediated decomposition of volatile organic compounds has previously been investigated in the gas phase with metal oxides as heterogeneous catalysts. While the reactive species in plasma itself are well investigated, very little is known about the influence of metal catalysts in solution. Here, we present initial investigations on the time-dependent plasma-supported oxidation of benzyl alcohol, benzaldehyde and phenol in the presence of molecular iron complexes $\textit {in solution}$. Products were identified by HPLC, ESI-MS, FT-IR, and $^{1}\textbf {H NMR}$ spectroscopy. Compared to metal-free oxidation of the substrates, which is caused by reactive oxygen species and leads to a mixture of products, the metal-mediated reactions lead to one product cleanly, and faster than in the metal-free reactions. Most noteworthy, even catalytic amounts of metal complexes induce these clean transformations. The findings described here bear important implications for plasma-supported industrial waste transformations, as well as for plasma-mediated applications in biomedicine, given the fact that iron is the most abundant redox-active metal in the human body

In-situ\textit {In-situ} control of microdischarge characteristics in unipolar pulsed plasma electrolytic oxidation of aluminum

Microdischarges occurring during plasma electrolytic oxidation are the main mechanism promoting oxide growth compared to classical anodization. When the dissipated energy by microdischarges during the coating process gets too large, high-intensity discharges might occur, which are detrimental to the oxide layer. In bipolar pulsed plasma electrolytic oxidation a so called "soft-sparking" mode limits microdischarge growth. This method is not available for unipolar pulsing and for all material combinations. In this work, the authors provide a method to control the size- and intensity distributions of microdischarges by utilizing a multivariable closed-loop control. $\textit {In-situ}$ detection of microdischarge properties by CCD-camera measurements and fast image processing algorithms are deployed. The visible size of microdischarges is controlled by adjusting the duty cycle in a closed-loop feedback scheme, utilizing a PI-controller. Uncontrolled measurements are compared to controlled cases. The microdischarge sizes are controlled to a mean value of $\it A$ = 5 ­­­­· $10^{-3}$ $mm^{2}$ and $\it A$ = 7 ­­­­· $10^{-3}$ $mm^{2}$, respectively. Results for controlled cases show, that size and intensity distributions remain constant over the processing time of 35 minutes. Larger, high-intensity discharges can be effectively prevented. Optical emission spectra reveal, that certain spectral lines can be influenced or controlled with this method. Calculated black body radiation fits with very good agreement to measured continuum emission spectra $\it T$ = 3200 K. Variance of microdischarge size, emission intensity and continuum radiation between consecutive measurements is reduced to a large extent, promoting uniform microdischarge and oxide layer properties. A reduced variance in surface defects can be seen in SEM measurements, after coating for 35 minutes, for controlled cases. Surface defect study shows increased number density of microdischarge impact regions, while at the same time reducing pancake diameters, implying reduced microdischarge energies compared to uncontrolled cases