Interactions of Non-Thermal Air Plasmas with Aqueous Solutions

Devices to produce non-thermal atmospheric-pressure plasma, or NTP, are gaining increasing attention for a wide range of applications. In addition to promising work in the field of plasma medicine, NTP has also been shown to effectively decontaminate food and surfaces, degrade organic pollutants, and open new routes for nanomaterials synthesis. In many of these applications, plasma-liquid interfaces play a key role in influencing reaction pathways and mediating the transport of reactive species to their intended targets in the liquid phase. 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 order to carry out this research, a device producing a pin-to-target plasma discharge in direct contact with a target solution was developed. NTP discharges in direct contact with an aqueous solution are known to produce reactive oxygen and nitrogen species in solution, including OH•, NO2• and O2•-. The type and geometry of the discharge employed, as well as the properties of the treated solution, affect the quantity and ratio of species produced. The resulting setup can be tuned for solution acidification, greater oxidative power, limited peak current flow or other attributes depending on the research priorities. The relationship between the voltage-current characteristics of the device, the properties of the solution (pH, conductivity), and the reactive species produced in solution was established. Methods to measure reactive species produced by NTP in liquids were reviewed and developed where necessary. This allowed the evolution of reactive species (e.g. H2O2, NO2-, and OH•) produced in solution by NTP to be determined as a function of time and solution pH. Subsequent reactions of these species were considered, and a model of peroxynitrous acid (O=NOOH) formation and degradation was developed and used to investigate the relevant reaction rates. Evidence is provided that near-surface reactions generating hydroxyl radicals (OH•) or other oxidizing species are the predominant source of active species in NTP-treated solution by tracking the degradation of indigo carmine, a common industrial dye. Selective use of buffers or other additives allows inactivation of certain reaction pathways, enabling contributions from either surface or bulk reactions to be identified. The observed rate of indigo oxidation in contact with the discharge far exceeded predictions from modeling reaction networks based on concentrations of species measured in the bulk solution, indicating a high concentration of highly oxidizing species within a very short distance (< 1 µM) of the plasma-liquid interface. The origin of the reactive species produced (gas phase, boundary layer, or bulk solution) and the relative importance of chemical kinetics versus transport were investigated during the oxidation of the target molecule in solution. Competition experiments and analysis of indigo carmine degradation products were used to identify active agents and properties of the NTP-treated solutions. Furthermore, the effects of NTP on simple biomolecules was investigated. The effect of NTP on solutions of free amino acids, nucleic acids, and the anti-oxidant glutathione was assessed to determine the extent to which irreversible oxidation of these species is induced. This work provides initial evidence of key mechanisms for both in vitro and in vivo studies reported in the literature. These results have important implications for NTP device design and operation for future applications of NTP