High-Frequency Immersed Plasma: Reactive Species Generation, Redox Transformations, and Competing Chemical Processes in Iron-Induced Oxidative Degradation in a Deoxyribose Model System

High-frequency immersed plasma discharge represents an efficient method for the generation of reactive oxygen and nitrogen species (RONS) in liquid media, leading to complex redox and oxidative processes in biologically relevant systems. Although plasma-generated reactive species in liquids have been widely investigated, it remains insufficiently understood how working-gas-dependent plasma chemistry translates into oxidative outcomes in iron-containing model systems, where plasma-derived species may interact with transition-metal redox cycling. The novelty of this study lies in the combined assessment of gas-dependent RONS accumulation, deoxyribose oxidative degradation, and plasma-induced changes in Fe(II) availability using a high-frequency immersed plasma discharge. Herein, we examined whether treatment with high-frequency immersed discharge influences the redox state of iron in a working gas-dependent manner, thereby affecting oxidative degradation in the deoxyribose model. Plasma treatment was performed under air and argon working gas conditions, and oxidative degradation was evaluated using the thiobarbituric acid reactive substances (TBA-RS) assay. In parallel, the concentrations of long-lived reactive species, including hydrogen peroxide, nitrites, and nitrates, were determined spectrophotometrically. The results demonstrated a treatment-time-dependent increase in oxidative degradation and reactive species accumulation, with more pronounced oxidative effects observed under argon plasma conditions. In the presence of ferrous ions, plasma treatment resulted in a gas-dependent effect, characterized by a synergistic enhancement of oxidative degradation under argon and a biphasic effect under air. Most notably, in Fe(II)-containing samples, 10 min of argon plasma treatment increased TBA-RS formation to approximately 2.7-fold of the Fe(II) control, whereas air plasma produced a biphasic response, with an initial decrease followed by an approximately 40% increase at the longest exposure time. Additional experiments suggest that plasma may influence the redox state and availability of ferrous ions, thereby affecting their participation in Fenton-type reactions and radical-mediated processes. The findings suggest that the overall oxidative outcome in plasma-treated systems is governed not only by the concentration of plasma-generated reactive species but also by plasma-induced modifications of transition metal redox chemistry. These preliminary results on the combined roles of plasma-generated reactive species and transition-metal chemistry contribute to understanding plasma–liquid interactions in such systems.