Novel plasma-based technologies that offer maximum efficiency at minimal environmental costs are expected to further promote the sustainable societal and economic development. Unique transfer of chemical reactivity and energy from gaseous plasmas to water takes place in the absence of any other chemicals, but results in a product with a notable transient broad-spectrum biological activity, referred to as plasma-activated water (PAW). These features make PAW a green prospective solution for a wide range of biotechnology applications, from water purification to biomedicine. Here, we present a succinct review of how novel, efficient methods based on non-equilibrium reactive plasma chemistries can be applied to low-cost natural water sources to produce a prospective product with a wide range of applications while at the same time minimising the process steps and dramatically reducing the use of expensive and/or hazardous reagents. Despite the recent exciting developments in this field, there presently is no topical review which specifically focuses on the underlying physics and chemistry related to plasma-activated water. We focus specifically on the PAW generation, origin of reactive species present in PAW, its related analytical chemistry and potentially different mechanisms that regulate the bio-activities of PAW in different biotech-applications and their roles in determining PAW efficacy and selectivity. We then review recent advances in our understanding of plasma-water interactions, briefly outlining current and proposed applications of PAW in agriculture, food and biomedicine. Finally, we outline future research directions and challenges that may hinder translation of these technologies into real-life applications. Overall, this review will provide much needed insights into the fundamental aspects of PAW chemistry required for optimization of the biochemical activity of PAW and translation of this environment- and human-health-friendly, and energy-efficient strategy into real life applications.

The socio-economic, environmental, and health implications of diseases caused by pathogenic microorganisms and their treatment using conventional antimicrobials are significant. The increasing resistance to antibiotics and detrimental biological side effects of many common antibiotics on human health and on the ecosystem have driven the search for new cost-effective and highly-efficient sterilization treatments and agents that are more environmentally benign. Plasma activated water (PAW), a product of cold atmospheric plasma reacting with water, is a promising broad-spectrum biocidal agent whose biochemical activity is attributed to the presence of a rich diversity of highly reactive oxygen and nitrogen species (RONS). The transient activity of PAW, where PAW reverts to water within days of storage and application, suggests that it can become a green alternative to conventional chemical treatment methods, yet the issues of scale up and the not fully understood mechanism of activity remain. In this study, we sought to explore the antibiotic potential of PAW generated from a plasma jet in a continuous flow reactor and determine the individual and combined contribution of thus-generated reactive chemistries in PAW for organism inactivation. Treatment of Escherichia coli with PAW led to more than a 4-log reduction, while exposure to an equivalent single dose of hydrogen peroxide (H₂O₂), nitrate (NO₃ ⁻) or nitrite (NO₂ ⁻) to that found in PAW failed to attain the same level of reduction. Peroxynitrite was identified as a critical bioactive species, particularly under acidic conditions, originating from the synergistic plasma effects (like the reactions of H₂O₂, NO₃ ⁻, NO₂ ⁻ and other existing short-lived species like OH radicals in PAW). This research successfully demonstrated the possibility of PAW being an effective environmentally benign disinfectant, the activity of which is closely linked to the generation of peroxynitrite, providing much needed insights into the fundamental aspects of PAW chemistry required for optimisation of the biochemical activity of PAW and translation of this decontamination strategy into real life applications.

Sustainable ammonia production using water and air through the coupling of plasma-driven intermediary NO_x generation and their electrocatalytic conversion.

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.

Atmospheric-pressure N₂, He, air, and O₂ microplasma arrays have been used to investigate the effects of plasma treatment on seed germination and seedling growth of mung bean in aqueous solution. Seed germination and growth of mung bean were found to strongly depend on the feed gases used to generate plasma and plasma treatment time. Compared to the treatment with atmospheric-pressure O₂, N₂ and He microplasma arrays, treatment with air microplasma arrays was shown to be more efficient in improving both the seed germination rate and seedling growth, the effect attributed to solution acidification and interactions with plasma-generated reactive oxygen and nitrogen species. Acidic environment caused by air discharge in water may promote leathering of seed chaps, thus enhancing the germination rate of mung bean, and stimulating the growth of hypocotyl and radicle. The interactions between plasma-generated reactive species, such as hydrogen peroxide (H₂O₂) and nitrogen compounds, and seeds led to a significant acceleration of seed germination and an increase in seedling length of mung bean. Electrolyte leakage rate of mung bean seeds soaked in solution activated using air microplasma was the lowest, while the catalase activity of thus-treated mung bean seeds was the highest compared to other types of microplasma.

Plasma medicine is a relatively new field that investigates potential applications of cold atmospheric-pressure plasmas in bioengineering, such as for bacterial inactivation and degradation of organic molecules in water. In order to enunciate mechanisms of bacterial inactivation at molecular or atomic levels, we investigated the interaction of atmospheric-pressure air microplasmas with amino acids in aqueous solution by using high-resolution mass spectrometry (HRMS). Results show that the oxidation effect of plasma-induced species on the side chains of the amino acids can be categorized into four types, namely hydroxylation, nitration, dehydrogenation and dimerization. In addition, relative activities of amino acids resulting from plasma treatment come in descending order as follows: sulfur-containing carbon-chain amino acids > aromatic amino acids > five-membered ring amino acids > basic carbon-chain amino acids. Since amino acids are building blocks of proteins vital to the growth and reproduction of bacteria, these results provide an insight into the mechanism of bacterial inactivation by plasma.