Long-lived species in plasma-activated water generated by an AC multi-needle-to-water discharge: effects of gas flow on chemical reactions

Plasma-activated water (PAW) represents a promising green antibacterial agent for biomedical and agricultural applications. In this study, a novel AC multi-needle-to-water discharge device was developed to investigate the effects of gas flow on the generation and chemical composition of PAW. It is shown that the concentrations of NO3- and N(III) (NO2- and HNO2) in the PAW both increased with an extension of the plasma-processing time and a reduction of the gas-flow rate. The absorption of gas-phase products carried by the gas flow from the discharge chamber was found to be beneficial for the generation of both NO3- and N(III) in the PAW at a gas flow rate of 20-60 L h-1, yet their concentrations were still lower than those without any feeding gas. As opposed to NO3- or N(III), the H2O2 concentration in the plasma-activated phosphate buffer solution (PAPBS) increased under stronger gas flows and was almost unaffected by absorption in PAPBS. The pH value of PAW increased at higher gas flow rates. A comparison of the N(III) in PAW and PAPBS reflects the effects of the reactions of NO2-H2O2 in the two different working liquids. To quantify the effects of gas flow on the discharge characteristics, gas temperatures were calculated from the optical emission spectra and were proven to be flow-independent near the discharge channel. Fourier transform infrared (FTIR) measurements of the gaseous products during the discharge, and further analysis of possible reaction pathways indicated that by controlling the gas flow in the multi-needle-to-water discharge system, the concentration of long-lived species in PAW could be tuned, which might favor the generation of ONOOH These findings contribute to a better understanding of effective electric discharge-related mechanisms for enhancing the biochemical and chemical activities of PAW.