Direct synthesis of hydrogen peroxide from plasma-water interactions

Evidence for Superoxide-Initiated Oxidation of Aniline in Water by Pulsed, Atmospheric Pressure Plasma

Plasma-driven solution electrochemistry (PDSE) uses plasma-generated reactive species to drive redox reactions in solution. Nonthermal, atmospheric pressure plasmas, when irradiating water, produce many redox species. While PDSE is a promising chemical tool, there is limited insight into the mechanisms of the reactions due to the variety of short-lived reagents produced. In this study, we use aniline as a model system for studying redox mechanisms of PDSE. We show that the plasma irradiation of aqueous aniline solutions drives the formation of polyaniline oligomer, which is suppressed under acidic starting conditions. The addition of (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO), a radical scavenger, decreases the formation of oligomer by 80%, and the addition of superoxide dismutase fully hinders oligomerization. These results lead us to conclude that the oligomerization of aniline by plasma irradiation is initiated by superoxide. This discovery provides novel insights into PDSE mechanisms and illustrates a potential method of harnessing superoxide for chemical reactions.

Antimicrobial activity of cold atmospheric-pressure argon plasma combined with chicory (Cichorium intybus L.) extract against P. aeruginosa and E. coli biofilms

The present study reports a significant combined antibacterial activity of Cichorium intybus L. (known as Chicory) natural extract with cold atmospheric-pressure argon plasma treatment against multi-drug resistant (MDR) Gram-negative bacteria. To detect reactive species that are generated in the argon plasma, optical emission spectra were recorded. The molecular bands were allocated to the hydroxyl radicals (OH) and neutral nitrogen molecules (N2). Moreover, the atomic lines form the emitted spectra were determined to argon atoms (Ar) and the oxygen atoms (O), respectively. The results revealed that Chicory extract treatment at a concentration of 0.043 g/ml reduced the metabolic activity of P. aeruginosa cells by 42%, while, a reduced metabolic activity of 50.6% was found for E. coli biofilms. Moreover, the combination of Chicory extract with 3 min Ar-plasma introduced a synergistic effect, so that it exhibited a significantly reduced metabolic activity of P. aeruginosa to 84.1%, and E. coli ones to 86.7%, respectively. The relationship between cell viability and membrane integrity of P. aeruginosa and E. coli biofilms treated with Chicory extract and argon plasma jet were also analyzed by CLSM. It was found that after the combined treatment, a noticeable membrane disruption was formed. Besides, it was concluded that E. coli biofilms showed a higher sensitivity to Ar-plasma than P. aeruginosa biofilm at longer plasma exposure times. This study suggests that the anti-biofilm therapy based on a combined effect of Chicory extract and cold argon plasma treatment can serve as a considerable green method for treatment of antimicrobial MDR bacteria.

Argon Plasma Jet-Treated Poly (Vinyl Alcohol)/Chitosan and PEG 400 Plus Mangifera indica Leaf Extract for Electrospun Nanofiber Membranes: In Vitro Study

The wound-healing process can be disrupted at any stage due to various internal and external factors. The inflammatory stage of the process plays a vital role in determining the outcome of the wound. Prolonged inflammation due to bacterial infection can lead to tissue damage, slow healing, and complications. Wound dressings made using materials such as poly (vinyl alcohol) (PVA), chitosan (CS), and poly (ethylene glycol) (PEG) with Mangifera extract (ME) added can help reduce infection and inflammation, creating a conducive environment for faster healing. However, creating the electrospun membrane is challenging due to balancing various forces such as rheological behavior, conductivity, and surface tension. To improve the electrospinnability of the polymer solution, an atmospheric pressure plasma jet can induce chemistry in the solution and increase the polarity of the solvent. Thus, this research aims to investigate the effect of plasma treatment on PVA, CS, and PEG polymer solutions and fabricate ME wound dressing via electrospinning. The results indicated that increasing plasma treatment time increased the viscosity of the polymer solution, from 269 mPa∙to 331 mPa∙s after 60 min, and led to an increase in conductivity from 298 mS/cm to 330 mS/cm and an increase in nanofiber diameter from 90 ± 40 nm to 109 ± 49 nm. Incorporating 1% mangiferin extract into an electrospun nanofiber membrane has been found to increase the inhibition rates of Escherichia coli and Staphylococcus aureus by 29.2% and 61.2%, respectively. Additionally, the fiber diameter decreases when compared with the electrospun nanofiber membrane without ME. Our findings demonstrate that electrospun nanofiber membrane with ME has anti-infective properties and can promote faster wound healing.

Carbon-based metal-free electrocatalysts: from oxygen reduction to multifunctional electrocatalysis

Since the discovery of N-doped carbon nanotubes as the first carbon-based metal-free electrocatalyst (C-MFEC) for oxygen reduction reaction (ORR) in 2009, C-MFECs have shown multifunctional electrocatalytic activities for many reactions beyond ORR, such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and hydrogen peroxide production reaction (H₂O₂PR). Consequently, C-MFECs have attracted a great deal of interest for various applications, including metal-air batteries, water splitting devices, regenerative fuel cells, solar cells, fuel and chemical production, water purification, to mention a few. By altering the electronic configuration and/or modulating their spin angular momentum, both heteroatom(s) doping and structural defects (e.g., atomic vacancy, edge) have been demonstrated to create catalytic active sites in the skeleton of graphitic carbon materials. Although certain C-MFECs have been made to be comparable to or even better than their counterparts based on noble metals, transition metals and/or their hybrids, further research and development are necessary in order to translate C-MFECs for practical applications. In this article, we present a timely and comprehensive, but critical, review on recent advancements in the field of C-MFECs within the past five years or so by discussing various types of electrocatalytic reactions catalyzed by C-MFECs. An emphasis is given to potential applications of C-MFECs for energy conversion and storage. The structure-property relationship for and mechanistic understanding of C-MFECs will also be discussed, along with the current challenges and future perspectives.

Plasma-activated water production and its application in agriculture

The use of plasma-activated water (PAW) treatment is a promising technology that has many advantages, such as high efficiency, flexibility, environmental safety, and no residue. Thus, PAW has been applied in the agriculture industry to increase agricultural production. The application of PAW technology in agricultural production should emphasize its systematic nature, controllability, and operability, making it practical. This review systematically illustrates the production of PAW and the factors influencing it. The application of PAW in agriculture and its mechanism are discussed, including the effect on seed germination, the promotion of plant growth, and the control of plant diseases and pests. The implications of PAW for agriculture production and some of the related challenges are discussed. This review provides a deeper understanding of the viability of PAW technology in agriculture production. © 2021 Society of Chemical Industry.

Interactions of plasma-activated water with biofilms: inactivation, dispersal effects and mechanisms of action

Biofilms have several characteristics that ensure their survival in a range of adverse environmental conditions, including high cell numbers, close cell proximity to allow easy genetic exchange (e.g., for resistance genes), cell communication and protection through the production of an exopolysaccharide matrix. Together, these characteristics make it difficult to kill undesirable biofilms, despite the many studies aimed at improving the removal of biofilms. An elimination method that is safe, easy to deliver in physically complex environments and not prone to microbial resistance is highly desired. Cold atmospheric plasma, a lightning-like state generated from air or other gases with a high voltage can be used to make plasma-activated water (PAW) that contains many active species and radicals that have antimicrobial activity. Recent studies have shown the potential for PAW to be used for biofilm elimination without causing the bacteria to develop significant resistance. However, the precise mode of action is still the subject of debate. This review discusses the formation of PAW generated species and their impacts on biofilms. A focus is placed on the diffusion of reactive species into biofilms, the formation of gradients and the resulting interaction with the biofilm matrix and specific biofilm components. Such an understanding will provide significant benefits for tackling the ubiquitous problem of biofilm contamination in food, water and medical areas.

The emerging potential of cold atmospheric plasma in skin biology

The maintenance of skin integrity is crucial to ensure the physiological barrier against exogenous compounds, microorganisms and dehydration but also to fulfill social and aesthetic purposes. Besides the development of new actives intended to enter a formulation, innovative technologies based on physical principles have been proposed in the last years. Among them, Cold Atmospheric Plasma (CAP) technology, which already showed interesting results in dermatology, is currently being studied for its potential in skin treatments and cares. CAP bio-medical studies gather several different expertise ranging from physics to biology through chemistry and biochemistry, making this topic hard to pin. In this review we provide a broad survey of the interactions between CAP and skin. In the first section, we tried to give some fundamentals on skin structure and physiology, related to its essential functions, together with the main bases on cold plasma and its physicochemical properties. In the following parts we dissected and analyzed each CAP parameter to highlight the already known and the possible effects they can play on skin. This overview aims to get an idea of the potential of cold atmospheric plasma technology in skin biology for the future developments of dermo-cosmetic treatments, for example in aging prevention.

Structure and Physicochemical Properties of Water Treated under Nitrogen with Low-Temperature Glow Plasma

Water treated with low-temperature, low-pressure glow plasma (GP) in contact with air stimulates various microorganisms, the growth of various plants and provides healthy breeding of various animals. In this paper, we present water treated with GP under oxygen-free nitrogen. It is potentially suitable for breeding anaerobic microorganisms, and increasing the crops of plants utilizing atmospheric nitrogen. Deionized water saturated with oxygen-free nitrogen was treated for 5 to 90 min with low-temperature glow plasma (GP). That operation produced nitrogen in various exited states depending on the treatment time. These excited nitrogen molecules built aqueous clathrates around them. The number and structure of those clathrates depended on the time of the treatment with GP. In terms of mass, density, pH, conductivity, surface tension, Ultraviolet-Visible (UV-VIS), Fourier Transformation Infrared (FTIR), Raman and Electron Spin Resonance (ESR) spectra as well as Differential Scanning Calorimetry (DSC), the macrostructure of water saturated with nitrogen treated with GP strongly depended on the treatment time. Based on the entropy criterion, the macrostructure formed on 30 and 5 min treatment was the most and least organized, respectively.

Spontaneous generation of hydrogen peroxide from aqueous microdroplets

Water is considered to be a stable and relatively inert molecule in bulk solution. We report an exceptional behavior of water: Water molecules are spontaneously oxidized to form hydrogen peroxide near the water−air interface of micron-sized water droplets. This process does not require any chemical reagent, catalyst, applied electric potential, or radiation. Only pure water in the form of microdroplets in air is necessary for the appearance of hydrogen peroxide. We suggest that this discovery opens various innovative opportunities including green and inexpensive production of hydrogen peroxide, green chemical synthesis, safe cleaning, and food processing.

We show H₂O₂ is spontaneously produced from pure water by atomizing bulk water into microdroplets (1 μm to 20 µm in diameter). Production of H₂O₂, as assayed by H₂O₂-sensitve fluorescence dye peroxyfluor-1, increased with decreasing microdroplet size. Cleavage of 4-carboxyphenylboronic acid and conversion of phenylboronic acid to phenols in microdroplets further confirmed the generation of H₂O₂. The generated H₂O₂ concentration was ∼30 µM (∼1 part per million) as determined by titration with potassium titanium oxalate. Changing the spray gas to O₂ or bubbling O₂ decreased the yield of H₂O₂ in microdroplets, indicating that pure water microdroplets directly generate H₂O₂ without help from O₂ either in air surrounding the droplet or dissolved in water. We consider various possible mechanisms for H₂O₂ formation and report a number of different experiments exploring this issue. We suggest that hydroxyl radical (OH) recombination is the most likely source, in which OH is generated by loss of an electron from OH⁻ at or near the surface of the water microdroplet. This catalyst-free and voltage-free H₂O₂ production method provides innovative opportunities for green production of hydrogen peroxide.

Plasma-Assisted Synthesis and Surface Modification of Electrode Materials for Renewable Energy

Renewable energy technology has been considered as a "MUST" option to lower the use of fossil fuels for industry and daily life. Designing critical and sophisticated materials is of great importance in order to realize high-performance energy technology. Typically, efficient synthesis and soft surface modification of nanomaterials are important for energy technology. Therefore, there are increasing demands on the rational design of efficient electrocatalysts or electrode materials, which are the key for scalable and practical electrochemical energy devices. Nevertheless, the development of versatile and cheap strategies is one of the main challenges to achieve the aforementioned goals. Accordingly, plasma technology has recently appeared as an extremely promising alternative for the synthesis and surface modification of nanomaterials for electrochemical devices. Here, the recent progress on the development of nonthermal plasma technology is highlighted for the synthesis and surface modification of advanced electrode materials for renewable energy technology including electrocatalysts for fuel cells, water splitting, metal-air batteries, and electrode materials for batteries and supercapacitors, etc.

Plasma-Functionalized Solution: A Potent Antimicrobial Agent for Biomedical Applications from Antibacterial Therapeutics to Biomaterial Surface Engineering

Deadly diseases caused by pathogenic bacteria and viruses have increasingly victimized humans; thus, the importance of disinfection has increased in medical settings as well as in food and agricultural industries. Plasma contains multiple bactericidal agents, including reactive species, charged particles, and photons, which can have synergistic effects. In particular, the chemicals formed in aqueous solution during plasma exposure have the potential for high antibacterial activity against various bacterial infections. Here, we report the antibiotic potency of plasma-treated water (PTW). To illustrate the applicability of PTW for disinfecting biological substances, an Escherichia coli biofilm was used. We sought to identify the chemical species in PTW and investigate their separate effects on biofilm removal. Dielectric barrier discharge in ambient air was used to prepare the PTW and treat the biofilm directly. Hydrogen peroxide, ozone, and nitrites were identified as the long-lived reactive species in the PTW, whereas hydroxyl radicals and superoxide anions were identified as the short-lived reactive species in the PTW; all these species showed an ability to disinfect in biofilm removal.