Atmospheric-pressure plasma jet interacting with saline and non-saline medium: plasma characteristics & liquid-phase chemistry

The application of atmospheric-pressure plasma jets (APPJs) in medicine often involves interaction with saline environments. This study examines how the salinity of the target liquid influences APPJ behaviour and plasma-induced liquid chemistry. The APPJ is used to treat two types of targets: (a) deionized water (DIW) and (b) an agarose gel made in DIW, with both targets made with and without the inclusion of physiologically-relevant 150 mM NaCl. APPJ is operated with argon gas at an applied voltage of 1.60 kV (rms), current of 1.95 mA (rms), frequency of ≈35kHz, and output power of ≈0.55W. The coupling of the APPJ with the DIW or NaCl solution establishes a stronger conductive path for the flow of current/charge towards the target. The magnitude of charge/current flow is higher for the 150 mM NaCl solution due to the presence of more conductive ions inside the solution. The higher flow of charges towards NaCl solution leads to a stronger emission from excited species at the plasma–liquid interface and this is confirmed through optical emission spectroscopy. It was found that the APPJ produced more hydrogen peroxide (H2O2) and nitrites (NO− 2 ), as markers of reactive oxygen and nitrogen species, respectively, in DIW compared to 150 mM NaCl.However, the oxidation potential of agarose gel was qualitatively enhanced with the addition of NaCl. These contrasting results suggest that measurement of H2O2 and NO− 2 is not reliable to predict the oxidative potential of all media, particularly with saline media. We attribute the discrepancy in our result to the formation of potent oxidant hypochlorous acid (HOCl) in saline media, which inhibits the formation of H2O2 as described in the manuscript through the differing chemical reaction pathways that take place when the APPJ interacts with DIW and NaCl solution, and their respective rate coefficients.