Recent Progress in Applications of Non-Thermal Plasma for Water Purification, Bio-Sterilization, and Decontamination

Various reactive oxygen and nitrogen species are accompanied by electrons, ultra-violet (UV) radiation, ions, photons, and electric fields in non-thermal atmospheric pressure plasma. Plasma technology is already used in diverse fields, such as biomedicine, dentistry, agriculture, ozone generation, chemical synthesis, surface treatment, and coating. Non-thermal atmospheric pressure plasma is also considered a promising technology in environmental pollution control. The degradation of organic and inorganic pollutants will be massively advanced by plasma-generated reactive species. Various investigations on the use of non-thermal atmospheric pressure plasma technology for organic wastewater purification have already been performed, and advancements are continuing to be made in this area. This work provides a critical review of the ongoing improvements related to the use of non-thermal plasma in wastewater control and outlines the operational principle, standards, parameters, and boundaries with a special focus on the degradation of organic compounds in wastewater treatment.

Effect of Dielectric Barrier Discharge Cold Plasma on Pea Seed Growth

Traditional seed pretreatment methods cause secondary pollution for the application of various chemicals. This study investigated the effect of dielectric barrier discharge (DBD) cold plasma on seedling growth. Effects of plasma-activated tap water (PATW) and plasma-activated seeds (PAS) were compared for germination rates, seedling height, dry weight, and chlorophyll content. Results show that compared with controls these growth parameters were all increased by more than 50%. The yields and contributions of hydrogen peroxide, nitrate, nitrite, and ammonium were quantified. Hydrogen peroxide and nitrate have an important role in seedling growth. By etching, the seed epidermis free radicals can reduce the apparent contact angle and increase the water absorption of the seeds. In addition to the low cost of PATW and PAS compared with commercial fertilizers, DBD does not involve any chemical addition. Thus, both PATW and PAS can be an alternative for improvement of agricultural production.

Chemistry and biochemistry of cold physical plasma derived reactive species in liquids

Reactive oxygen and nitrogen species deposited by cold physical plasma are proposed as predominant effectors in the interaction between discharge and biomedical application. Most reactive species found in plasma sources are known in biology for inter- and intracellular communication (redox signaling) and mammalian cells are equipped to interpret the plasma derived redox signal. As such, considerable effort has been put into the investigation of potential clinical applications and the underlying mechanism, with a special emphasis on conditions orchestrated significantly via redox signaling. Among these, immune system control in wound healing and cancer control stands out with promising in vitro and in vivo effects. From the fundamental point of view, further insight in the interaction of the plasma-derived species with biological systems is desired to (a) optimize treatment conditions, (b) identify new fields of application, (c) to improve plasma source design, and (d) to identify the trajectories of reactive species. Knowledge on the biochemical reactivity of non-thermal plasmas is compiled and discussed. While there is considerable knowledge on proteins, lipids and carbohydrates have not received the attention deserved. Nucleic acids have been profoundly investigated yet focusing on molecule functionality rather than chemistry. The data collected underline the efforts taken to understand the fundamentals of plasma medicine but also indicate 'no man's lands' waiting to be discovered.

Guava flavored whey-beverage processed by cold plasma: Physical characteristics, thermal behavior and microstructure

The present study aimed to compare the physicochemical (pH), physical (rheology parameters and particle size), microstructure (optical microscopy) and thermal properties (differential scanning calorimetry) of guava flavored whey-beverages submitted to cold plama technology in different processing time (5, 10, and 15 min) and gas flow (10, 20, and 30 mL min^-1) conditions with a conventional pasteurized product. Whey beverages treated by cold plasma presented higher pH values, lower consistency and lower viscosity, and a flow behavior index similar to Newtonian fluids. Milder cold plasma conditions resulted in whey beverages with higher pH, lower viscosity and consistency, and similar particle distribution and microstructure compared to the pasteurized product. In contrast, more severe processing conditions resulted in a higher particle surface area ([D 3,2]) and smaller particles (~10 μM), due to the decrease in the number of larger particles (1000 μM), cell rupture, the formation of cell fragments, and higher viscosity and consistency. The treatments did not affect the thermal properties (enthalpy and bound water) of any sample.

Long-lived and short-lived reactive species produced by a cold atmospheric pressure plasma jet for the inactivation of Pseudomonas aeruginosa and Staphylococcus aureus

Different chemical pathways leading to the inactivation of Pseudomonas aeruginosa and Staphylococcus aureus by a cold atmospheric pressure plasma jet (APPJ) in buffered and non-buffered solutions are reported. As APPJs produce a complex mixture of reactive species in solution, a comprehensive set of diagnostics were used to assess the liquid phase chemistry. This includes absorption and electron paramagnetic resonance spectroscopy in addition to a scavenger study to assess the relative importance of the various plasma produced species involved in the inactivation of bacteria. Different modes of inactivation of bacteria were found for the same plasma source depending on the solution and the plasma feed gas. The inactivation of bacteria in saline is due to the production of short-lived species in the case of argon plasma when the plasma touches the liquid. Long-lived species (ClO^-) formed by the abundant amount of O^. radicals produced by the plasmas played a dominant role in the case of Ar + 1% O₂ and Ar + 1% air plasmas when the plasma is not in direct contact with the liquid. Inactivation of bacteria in distilled water was found to be due to the generation of short-lived species: O^. &O₂^.- for Ar + 1% O₂ plasma and O₂^.- (and ^.OH in absence of saline) for Ar plasma.

Roles of membrane protein damage and intracellular protein damage in death of bacteria induced by atmospheric-pressure air discharge plasmas

Although plasma sterilization has attracted much attention, the underlying mechanisms and biochemical pathways are still not fully understood. In this work, we investigate the molecular mechanism pertaining to the inactivation of Escherichia coli (E. coli) by air discharge plasmas. The membrane protein YgaP and intracellular protein swc7 are over-expressed in E. coli by genetic recombination and gene inducible expression techniques and plasma exposure is demonstrated to alter the structures of YgaP and swc7 in E. coli. The plasma-induced damage of YgaP and swc7 involves changes in the secondary and tertiary structures instead of the primary structure and the modification effectiveness depends on the storage time after the plasma treatment. Owing to the unique structure of E. coli, YgaP is more susceptible to the plasma treatment than intracellular swc7. Within 1 h after plasma exposure, YgaP is modified but not swc7, but after 1 h or longer, both YgaP and swc7 proteins are indeed modified. By analyzing the plasma-induced antimicrobial efficacy and modification of YgaP and swc7, plasma-induced modification of the membrane proteins is the major cause of bacterial death but there is no identifiable relationship with modification of the intracellular protein. The new results provide insights into the mechanism of multiple plasma-induced damage to bacteria and cells as well as the disinfection mechanism.

Plasma-activation of tap water using DBD for agronomy applications: Identification and quantification of long lifetime chemical species and production/consumption mechanisms

Cold atmospheric plasmas are weakly ionized gases that can be generated in ambient air. They produce energetic species (e.g. electrons, metastables) as well as reactive oxygen species, reactive nitrogen species, UV radiations and local electric field. Their interaction with a liquid such as tap water can hence change its chemical composition. The resulting "plasma-activated liquid" can meet many applications, including medicine and agriculture. Consequently, a complete experimental set of analytical techniques dedicated to the characterization of long lifetime chemical species has been implemented to characterize tap water treated using cold atmospheric plasma process and intended to agronomy applications. For that purpose, colorimetry and acid titrations are performed, considering acid-base equilibria, pH and temperature variations induced during plasma activation. 16 species are quantified and monitored: hydroxide and hydronium ions, ammonia and ammonium ions, orthophosphates, carbonate ions, nitrite and nitrate ions and hydrogen peroxide. The related consumption/production mechanisms are discussed. In parallel, a chemical model of electrical conductivity based on Kohlrausch's law has been developed to simulate the electrical conductivity of the plasma-activated tap water (PATW). Comparing its predictions with experimental measurements leads to a narrow fitting, hence supporting the self-sufficiency of the experimental set, I.e. the fact that all long lifetime radicals of interest present in PATW are characterized. Finally, to evaluate the potential of cold atmospheric plasmas for agriculture applications, tap water has been daily plasma-treated to irrigate lentils seeds. Then, seedlings lengths have been measured and compared with untreated tap water, showing an increase as high as 34.0% and 128.4% after 3 days and 6 days of activation respectively. The interaction mechanisms between plasma and tap water are discussed as well as their positive synergy on agronomic results.

Correlations between gaseous and liquid phase chemistries induced by cold atmospheric plasmas in a physiological buffer

The understanding of plasma-liquid interactions is of major importance, not only in physical chemistry, chemical engineering and polymer science, but in biomedicine as well as to better control the biological processes induced on/in biological samples by Cold Atmospheric Plasmas (CAPs). Moreover, plasma-air interactions have to be particularly considered since these CAPs propagate in the ambient air. Herein, we developed a helium-based CAP setup equipped with a shielding-gas device, which allows the control of plasma-air interactions. Thanks to this device, we obtained specific diffuse CAPs, with the ability to propagate along several centimetres in the ambient air at atmospheric pressure. Optical Emission Spectroscopy (OES) measurements were performed on these CAPs during their interaction with a liquid medium (phosphate-buffered saline PBS 10 mM, pH 7.4) giving valuable information about the induced chemistry as a function of the shielding gas composition (variable O2/(O2 + N2) ratio). Several excited species were detected including N2+(First Negative System, FNS), N2(Second Positive System, SPS) and HO˙ radical. The ratios between nitrogen/oxygen excited species strongly depend on the O2/(O2 + N2) ratio. The liquid chemistry developed after CAP treatment was investigated by combining electrochemical and UV-visible absorption spectroscopy methods. We detected and quantified stable oxygen and nitrogen species (H2O2, NO2-, NO3-) along with Reactive Nitrogen Species (RNS) such as the peroxynitrite anion ONOO-. It appears that the RNS/ROS (Reactive Oxygen Species) ratio in the treated liquid depends also on the shielding gas composition. Eventually, the composition of the surrounding environment of CAPs seems to be crucial for the induced plasma chemistry and consequently, for the liquid chemistry. All these results demonstrate clearly that for physical, chemical and biomedical applications, which are usually achieved in ambient air environments, it is necessary to realize an effective control of plasma-air interactions.

Oxidative modification and electrochemical inactivation of Escherichia coli upon cold atmospheric pressure plasma exposure

Cold atmospheric pressure plasmas (CAPPs) are known to have bactericidal effects but the mechanism of their interaction with microorganisms remains poorly understood. In this study the bacteria Escherichia coli were used as a model and were exposed to CAPPs. Different gas compositions, helium with or without adjunctions of nitrogen or oxygen, were used. Our results indicated that CAPP induced bacterial death at decontamination levels depend on the duration, post-treatment storage and the gas mixture composition used for the treatment. The plasma containing O2 in the feeding gas was the most aggressive and showed faster bactericidal effects. Structural modifications of treated bacteria were observed, especially significant was membrane leakage and morphological changes. Oxidative stress caused by plasma treatment led to significant damage of E. coli. Biochemical analyses of bacterial macromolecules indicated massive intracellular protein oxidation. However, reactive oxygen and nitrogen species (RONS) are not the only actors involved in E. coli's death, electrical field and charged particles could play a significant role especially for He-O2 CAPP.

Mechanistic insights into the impact of Cold Atmospheric Pressure Plasma on human epithelial cell lines

Compelling evidence suggests that Cold Atmospheric Pressure Plasma (CAPP) has potential as a new cancer therapy. However, knowledge about cellular signaling events and toxicity subsequent to plasma treatment is still poorly documented. The aim of this study was to focus on the interaction between 3 different types of plasma (He, He-O₂, He-N₂) and human epithelial cell lines to gain better insight into plasma-cell interaction. We provide evidence that reactive oxygen and nitrogen species (RONS) are inducing cell death by apoptosis and that the proteasome, a major intracellular proteolytic system which is important for tumor cell growth and survival, is a target of (He or He-N₂) CAPP. However, RONS are not the only actors involved in cell death; electric field and charged particles could play a significant role especially for He-O₂ CAPP. By differential label-free quantitative proteomic analysis we found that CAPP triggers antioxidant and cellular defense but is also affecting extracellular matrix in keratinocytes. Moreover, we found that malignant cells are more resistant to CAPP treatment than normal cells. Taken together, our findings provide insight into potential mechanisms of CAPP-induced proteasome inactivation and the cellular consequences of these events.

Formation of reactive nitrogen species including peroxynitrite in physiological buffer exposed to cold atmospheric plasma

Cold Atmospheric Plasmas (CAPs) are increasingly used for biomedical applications, their various reactive components must be then better determined. We demonstrate that peroxynitrite (ONOO⁻) is effectively a major reactive species generated by CAPs.

Disinfection of water by pulsed power technique: a mechanistic perspective

A detailed sub-cellular level bacterial disinfection mechanism and perturbation of bacterial surface potential due to ROS/RNS in pulsed plasma treatment.