Research On The Diffusion Of OH Radicals In The Plasma On The Surface Of The Solution And Its Application In Plasma

High efficiency is the main reason for the wide application of atmospheric pressure cold plasma in many different fields,and the high efficiency is derived from the active particles generated by the plasma during the treatment.These particles have a short duration and strong activity,and have a great influence on the formation of many molecules,such as ONOOH,HO₂,H₂0₂ and many hydrated molecules in solution.However,these active particles can pass through the gas-liquid interface to reach the object to be treated,mainly including O,O:（a¹ △）,03,OH,hydroxide and NO_x oxide.The formation and interconversion of these active particles are related to OH radicals.The amount of OH radicals directly affects the production of these particles and determines the amount of them,so it has become the focus of research.In this paper,we respectively treat the 5 ml 2.0,5.0,10.0,20.0,50.0 mM DMSO solution for 5 minutes by dielectric barrier discharge in a discharge mode of Linear fields and cross fields.The treated solution was analyzed by an ultraviolet spectrophotometer,and the concentrations of formaldehyde and formic acid were measured,respectively,and the concentration of OH radicals was calculated.The experiment proves that the OH radical in the solution is not caused by the diffusion of OH radicals into the solution in the discharge region,but on the surface of the solution when the plasma acts on the solution.The content is proportional to the potential drop of the plasma on the surface of the solution,and the resulting OH radicals are converted into long-lived particles such as hydrogen peroxide,which can diffuse into the solution.In order to increase the OH radical content in the solution,we also treated the DMSO solution by 253.6nm UV-assisted plasma,and found that the OH radical content in the solution increased under the conditions of linear field and cross field,parallel field treatment.The increment is particularly significant under parallel field conditions,whereby it is known that the irradiation of ultraviolet light can promote the decomposition of hydrogen peroxide,thereby increasing the content of OH radicals in the solution.Interactions between effects generated by cold atmospheric pressure plasmas（CAPPs）and water have been widely investigated for water purification,chemical and nanomaterial synthesis,and more recently,medicine and biotechnology.Chemical species play a critical role in transferring the reactivity from gas plasmas to solutions to induce specific biochemical responses in living targets,e.g.pathogen inactivation and biofilm removal once a minimum level of reactive species is attained.While this approach works well in a single-organism system at lab scale,integration of plasma-enabled biofilm removal into complex real-life systems,e.g.a large aquaculture tank,is far from trivial.This is because it is difficult to deliver sufficient concentrations of the right kind of species to biofilm-covered surfaces in large aquatic tanks while carefully maintaining a fairly narrow range of pH that is healthy for its inhabitants,e.g.fish.In this work,we show that underwater microplasma bubbles（generated by a novel microplasma-bubble reactor that forms a dielectric barrier discharge（DBD）at the gas-liquid interface）act as transport vehicles to efficiently deliver reactive plasma species to target biofilm sites on artificial and living surfaces while keeping healthy water pH in a multi-species system.Thus-generated in situ plasma activated water（PAVW）effectively reduces existing pathogenic biofilm load and prevents any new biofilm from forming.Generation of underwater microplasma bubbles in a 25 cm-diameter fish tank for less than a minute per day can introduce sufficient quantities of reactive oxygen and nitrogen species（RONS）into PAW to induce biofilm reduction and improve the health status of fish.Species generated include hydrogen peroxide,ozone,nitrite,nitrate and nitric oxide.Using mimicked chemical solutions,we show that plasma-induced nitric oxide acts as a critical bioactive species that triggers the release of cells from biofilm and their inactivation.