Unraveling the interaction between singlet state atomic oxygen O(1D) and water: toward the formation of oxywater and hydrogen peroxide

We performed high-level quantum mechanical calculations to explore the interaction of atomic oxygen in the ground triplet state, O(3P), and the excited singlet state, O(1D), with water. We reported the potential energy curves for a few lowest electronic states when an atomic oxygen approaches the oxygen of a water molecule. Our results predict the formation of a singlet oxywater species as the product of O(1D) and H2O which lies about 149.33 kJ mol-1 below the total energy of a singlet oxygen atom and a water molecule. Our calculations predict that an O(3P) atom interacting with a water molecule forms a triplet oxywater complex with a shallow minimum on the triplet potential energy surfaces. We examined the transition of the singlet state oxywater species to hydrogen peroxide through the unimolecular reaction pathway, a (1,2)-hydrogen shift. We reported the structural properties, vibrational frequencies, and dipole moments of oxywater species, the transition state, and hydrogen peroxide. We also reported the energy barrier for the transition, and we provided an estimate for the respective reaction rate constant. In addition, we investigated the impact of solvents on the reaction pathway using an implicit solvation model of water. We predict that a singlet state oxywater species has a longer lifetime in a water environment than in the gas phase.

Production and transport of plasma-generated hydrogen peroxide from gas to liquid

In this work, the transport of hydroxyl radicals and hydrogen peroxide from a humid atmospheric pressure plasma jet into plasma-treated liquids is analysed. The concentration of H2O2 was measured by a spectrophotometric approach using the reagent ammonium metavanadate. OH was measured by the terephthalic acid dosimeter and the chemiluminescence of luminol. The plasma jet used is based on the design of the well-investigated COST reference jet and is extended by a capillary between the two electrodes. In addition to the experiments, the 0-dimensional plasma-chemical kinetics code GlobalKin was used to analyse the plasma chemistry in the gas phase in more detail. After 5 min plasma treatment, a maximum H2O2 concentration of 1 mM was found in the liquid, while the OH concentration was a factor 50 lower. The concentrations of both species in the liquid increased with plasma power, and the H2O2 concentration also increased with the humidity concentration of the feed gas, while the OH concentration first increased with humidity admixture and then decreased. The transport of both species could be controlled by the treatment distance, the gas flow rate and low frequency pulsing of the RF jet in such a way that the selectivity towards the long-lived species H2O2 was increased. Qualitative trends in the simulated number densities of gas phase H2O2 and OH at the location of the gas-liquid interface fit relatively well to the experimental measurements in the liquid.

In situ generation of cold atmospheric plasma-activated mist and its biocidal activity against surrogate viruses for COVID-19

AIMS

To provide an alternative to ultra violet light and vapourized hydrogen peroxide to enhance decontamination of surfaces as part of the response to the COVID-19 pandemic.

METHODS AND RESULTS

We developed an indirect method for in situ delivery of cold plasma and evaluated the anti-viral activity of plasma-activated mist (PAM) using bacteriophages phi6, MS2, and phiX174, surrogates for SARS-CoV-2. Exposure to ambient air atmospheric pressure derived PAM caused a 1.71 log10 PFU ml-1 reduction in phi6 titer within 5 min and a 7.4 log10 PFU ml-1 reduction after 10 min when the the PAM source was at 5 and 10 cm. With MS2 and phiX174, a 3.1 and 1.26 log10 PFU ml-1 reduction was achieved, respectively, after 30 min. The rate of killing was increased with longer exposure times but decreased when the PAM source was further away. Trace amounts of reactive species, hydrogen peroxide and nitrite were produced in the PAM, and the anti-viral activity was probably attributable to these and their secondary reactive species.

CONCLUSIONS

PAM exhibits virucidal activity against surrogate viruses for COVID-19, which is time and distance from the plasma source dependent.

Flexible Cold Atmospheric Plasma Jet Sources

The properties of non-thermal atmospheric pressure plasma jets (APPJs) make them suitable for industrial and biomedical applications. They show many advantages when it comes to local and precise surface treatments, and there is interest in upgrading their performance for irradiation on large areas and uneven surfaces. The generation of charged species (electrons and ions) and reactive species (radicals), together with emitted UV photons, enables a rich plasma chemistry that should be uniform on arbitrary sample profiles. Lateral gradients in plasma parameters from multi-jets should, therefore, be minimized and addressed by means of plasma monitoring techniques, such as electrical diagnostics and optical emission spectroscopy analysis (OES). This article briefly reviews the main strategies adopted to build morphing APPJ arrays and ultra-flexible and long tubes to project cold plasma jets. Basic aspects, such as inter-jet interactions and nozzle shape, have also been discussed, as well as potential applications in the fields of polymer processing and plasma medicine.

Argon Humidification Exacerbates Antimicrobial and Anti-MRSA kINPen Plasma Activity

Gas plasma is a medical technology with antimicrobial properties. Its main mode of action is oxidative damage via reactive species production. The clinical efficacy of gas plasma-reduced bacterial burden has been shown to be hampered in some cases. Since the reactive species profile produced by gas plasma jets, such as the kINPen used in this study, are thought to determine antimicrobial efficacy, we screened an array of feed gas settings in different types of bacteria. Antimicrobial analysis was performed by single-cell analysis using flow cytometry. We identified humidified feed gas to mediate significantly greater toxicity compared to dry argon and many other gas plasma conditions. The results were confirmed by inhibition zone analysis on gas-plasma-treated microbial lawns grown on agar plates. Our results may have vital implications for clinical wound management and potentially enhance antimicrobial efficacy of medical gas plasma therapy in patient treatment.

Technologies to decontaminate bacterial biofilm on hospital surfaces: a potential new role for cold plasma?

Healthcare-associated infections (HCAIs) are a major challenge and the near patient surface is important in harbouring causes such as methicillin-resistant Staphylococcus aureus (MRSA) and Clostridioides difficile. Current approaches to decontamination are sub-optimal and many studies have demonstrated that microbial causes of HCAIs may persist with onward transmission. This may be due to the capacity of these microbes to survive in biofilms on surfaces. New technologies to enhance hospital decontamination may have a role in addressing this challenge. We have reviewed current technologies such as UV light and hydrogen peroxide and also assessed the potential use of cold atmospheric pressure plasma (CAPP) in surface decontamination. The antimicrobial mechanisms of CAPP are not fully understood but the production of reactive oxygen and other species is believed to be important. CAPP systems have been shown to partially or completely remove a variety of biofilms including those caused by Candida albicans, and multi-drug-resistant bacteria such as MRSA. There are some studies that suggest promise for CAPP in the challenge of surface decontamination in the healthcare setting. However, further work is required to define better the mechanism of action. We need to know what surfaces are most amenable to treatment, how microbial components and the maturity of biofilms may affect successful treatment, and how would CAPP be used in the clinical setting.

Nanosecond-scale impulse generator for biomedical applications of atmospheric-pressure plasma technology

This study proposes an improved high-voltage fast impulse generator based on an inductive energy storage system with a 4 kV static induction thyristor. Nanosecond-scale impulses with pulse widths below 100 ns and a peak voltage of up to 15 kV can be generated by modifying the high-voltage transformer in the circuit and tuning the circuit capacitor. The resulting device is highly stable and can perform continuously if the discharge parameters are chosen within the recommended range. A plasma jet was operated using the generator at low temperature (below 37 °C). Together with its high stability and potential for continuous operation, the proposed generator offers promise for use in biomedical and agricultural applications. Furthermore, the nanosecond-scale high-voltage impulses produced by the generator enable it to achieve an electron density in the plasma one order of magnitude higher than the commercially available radio frequency plasma jet analog. We also show how to reduce the total cost of the generator.

A low power flexible dielectric barrier discharge disinfects surfaces and improves the action of hydrogen peroxide

There is an urgent need for disinfection and sterilization devices accessible to the public that can be fulfilled by innovative strategies for using cold atmospheric pressure plasmas. Here, we demonstrate a successful novel combination of a flexible printed circuit design of a dielectric barrier discharge (flex-DBD) with an environmentally safe chemical reagent for surface decontamination from bacterial contaminants. Flex-DBD operates in ambient air, atmospheric pressure, and room temperature without any additional gas flow at a power density not exceeding 0.5 W/cm2. The flex-DBD activation of a 3% hydrogen peroxide solution results in the reduction in the bacterial load of a surface contaminant of > 6log10 in 90 s, about 3log10 and 2log10 better than hydrogen peroxide alone or the flex-DBD alone, respectively, for the same treatment time. We propose that the synergy between plasma and hydrogen peroxide is based on the combined action of plasma-generated OH· radicals in the hydrogen peroxide solution and the reactive nitrogen species supplied by the plasma effluent. A scavenger method verified a significant increase in OH· concentration due to plasma treatment. Novel in-situ FTIR absorption spectra show the presence of O3, NO2, N2O, and other nitrogen species. Ozone dissolving in the H2O2 solution can effectively generate OH· through a peroxone process. The addition of the reactive nitrogen species increases the disinfection efficiency of the hydroxyl radicals and other oxygen species. Hence, plasma activation of a low concentration hydrogen peroxide solution, using a hand-held flexible DBD device results in a dramatic improvement in disinfection.

Reproducibility of 'COST reference microplasma jets'

Atmospheric pressure plasmas have been ground-breaking for plasma science and technologies, due to their significant application potential in many fields, including medicinal, biological, and environmental applications. This is predominantly due to their efficient production and delivery of chemically reactive species under ambient conditions. One of the challenges in progressing the field is comparing plasma sources and results across the community and the literature. To address this a reference plasma source was established during the 'biomedical applications of atmospheric pressure plasmas' EU COST Action MP1101. It is crucial that reference sources are reproducible. Here, we present the reproducibility and variance across multiple sources through examining various characteristics, including: absolute atomic oxygen densities, absolute ozone densities, electrical characteristics, optical emission spectroscopy, temperature measurements, and bactericidal activity. The measurements demonstrate that the tested COST jets are mainly reproducible within the intrinsic uncertainty of each measurement technique.

Low Temperature Plasma Jets: Characterization and Biomedical Applications

For many decades non-equilibrium plasmas (NEPs) that can be generated at atmospheric pressure have played important roles in various material and surface processing applications [...]