Self-organized complex space charge structure configurations at plasma-liquid interfaces in pin-to-liquid DC discharges

Research in applying various atmospheric plasma discharges to cancer treatment has progressed significantly in recent years. Atmospheric plasma discharges (DC and AC discharges or RF jets) can initiate cell death via the production of reactive nitrogen and oxygen species (RONS) in the intercellular medium. However, there remains a need to find proper metrics with which a plasma dose can be defined for use in practical work. In this work a spectroscopic, electrical and calorimetric evaluation of DC atmospheric plasma discharges from a pin cathode and a liquid submerged cylindrical anode is presented in an attempt to help define a plasma dose. Self-organized patterns (SOPs), in the shape of complex space charge structure configurations, were formed at the liquid surface for various currents. Alterations in pattern type and shape were observed with variation in the cathode-anode separation gap and applied voltage. Discharge emission spectra identified the presence of N₂ (C³Π - B³Π) and the N₂⁺ (B₂∑_u⁺ - X₂∑_u⁺) transitions. Importantly both sodium and singlet oxygen were detected within discharges, at 589 and 777 nm, respectively, in typical cell culture medium as Dulbecco’s Modified Eagle Medium. The RONS concentrations, as reflected by the changes in their spectral signatures, were observed to increase with current supplied. Additionally, the SOPs presence in the DC discharges lead to an increase in the energy influx into the liquid and, consequently, to an increase in the temperature of the liquid anode.