Plasma-activated water: generation, origin of reactive species and biological applications

Abstract

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.

Antimicrobial effects of airborne acoustic ultrasound and plasma activated water from cold and thermal plasma systems on biofilms

“…Regarding the inactivation mechanism, one plausible explanation is that airborne acoustic ultrasound disrupts extracellular polymeric substances, provoking lesions at the surface of the biofilm, which eases the penetration of the reactive species generated in water into biofilm matrix, causing higher inactivation. Recent review on PAW proposed that the antibacterial efficacy of PAW is mainly attributed to the synergistic effects of plasma reactive species, pH and oxidation-reduction potential (ORP) 25,36 . The exact details and mechanisms for plasma on biofilms are still poorly understood.…”

“…*'C' were without any treatment; 'CW' were treated with sterile distilled water; 'cPAW' treated with cold plasma treated PAW; 'tPAW' treated with thermal plasma treated PAW; ' AA' treated with airborne acoustic ultrasound, ' AA + cPAW' treated with airborne acoustic ultrasound and cold plasma treated PAW; ' AA + tPAW' treated with airborne acoustic ultrasound and thermal plasma treated PAW. 36 . As demonstrated in Table 1, the concentration of nitrates and nitrites increased with plasma treatment, with higher levels of reactive nitrogen species (RNS) generated in cold PAW than thermal PAW.…”

“…The antimicrobial effect of PAW on bacterial biofilms are depended on the specifically involved plasma species, its composition, its interactions with biofilm matrix components and its diffusion into the biofilm cells 29 . Researcher have hypothesised that the interaction of plasma-induced reactive oxygen nitrogen species (RONS) with biofilm matrix components and solutes in the hydrated matrix could cause oxidation reaction as well as phosphodiesterase activity in cells which leads to physiological alterations and reduction in biofilm matrix area 36 , 37 . Eventually loss of matrix reduces the adhesiveness and dispersal of biofilm cells.…”

“…Studies by Lukes et al 39 and Xu et al 31 also demonstrated that RONS interact with intracellular components (DNAs, proteins, lipids) and influence its metabolic responses in microorganisms. These interactions cause oxidative stress on cell components thus initiating lipid and protein peroxidation on the cell membrane, followed by protein and/or DNA damage and cell death 36 .…”

“…The reason for acidification of PAW is predominantly due to the nitrate and nitrous acids generated by plasma treatment. Nitrates and nitrites in PAW are formed through dissolution of nitrogen oxides (NO X ) formed in the air into water 36 . As demonstrated in Table 1 , the concentration of nitrates and nitrites increased with plasma treatment, with higher levels of reactive nitrogen species (RNS) generated in cold PAW than thermal PAW.…”