Devices to produce non-thermal (“cold”) atmospheric pressure plasma, or CAP, are gaining increasing attention for biomedical applications wherein the delivery of reactive nitrogen and oxygen species (RONS) is desired. Numerous studies have shown the importance of RONS in biological systems, from the inactivation of bacteria to the intricacies of cell signaling pathways.
While the effects of specific RONS on biological targets have been well studied, less is known about how reactive species are transported to or formed in an adjacent liquid phase by CAP. For example, the importance of convection and diffusion in establishing specie penetration depth, and the role of charged species at the interface are poorly understood. Understanding the mechanisms by which reactive species are formed in the liquid phase will aid in the rational design of plasma devices for treating aqueous systems or tissues, and may enable the delivery of tailored ‘cocktails’ of reactive species.
In this study, we focus on the interaction between a model CAP system, an air corona discharge, and an aqueous electrode (either distilled water or buffered solutions with a submerged grounded metal electrode). The geometry of the corona discharge allows direct contact between the air plasma and aqueous solution, and also produces a slight convective air flow (‘ionic wind’) towards the surface which aids in the transport of gaseous species into the solution. The bactericidal effects of a similar device have been previously demonstrated. The reactive species present in the treated aqueous solution (nitrites, nitrates, hydrogen peroxide, reactive oxygen species) are quantified after exposure to CAP or a non-plasma control mixture. Selective use of buffers or other additives allows inactivation of certain reaction pathways, enabling contributions from either surface or bulk reactions to be identified. Experimental results are compared to species transport simulations, which highlight the spatial inhomogeneity of these systems.