Atmospheric pressure plasmas exhibit broad potential for application in nanomaterial synthesis, water purification, and biomedicine. Investigating the mechanisms of plasma-induced generation and transformation of liquid-phase reactive species, particularly hydroxyl radicals (OH) and hydrogen peroxide (H2O2), is crucial for optimizing plasma-based applications. In this study, two plasma discharge modes were established: (a) Nor mode: direct liquid contact discharge and (b) Cut mode: conductor cutoff discharge. The concentrations of liquid-phase OH and H2O2 induced by helium and helium/oxygen mixture plasma jets were measured and analyzed using a combination of optical emission spectroscopy (OES), chemical trapping-fluorescence spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. The results indicate significant differences in the generation of liquid-phase OH and H2O2 between the two discharge modes. Specifically, in (a) the Nor mode, the production of liquid-phase OH is considerably higher compared to (b) the Cut mode. Conversely, the concentration of liquid-phase H2O2 is higher in the cutoff discharge mode than in the direct contact mode. Additionally, the introduction of oxygen significantly suppresses the generation of both liquid-phase OH and H2O2. Comprehensive analysis of OES, fluorescence spectra, and EPR data reveals that the formation pathways of liquid-phase OH and H2O2 are closely related to the transport mechanisms of gas-phase reactive species in plasma, primarily resulting from the dissolution of gas-phase products into the liquid phase. Furthermore, dissolved atomic oxygen in the liquid phase interferes with HTA fluorescence detection, leading to inaccurate measurements of liquid-phase OH concentrations. This study elucidates the impact of oxygen introduction on the generation of OH and H2O2 induced by plasma and clarifies their generation and transformation mechanisms. It also discusses the limitations of conventional detection methods in complex atmospheres. These findings provide important theoretical insights and references for optimizing plasma applications in medicine and environmental fields.
Xi et al. (Fri,) studied this question.