Aqueous Reactive Oxygen Species Induced by He + O₂ Plasmas: Chemistry Pathways and Dosage Control Approaches

Plasma–liquid interaction has already been a hotspot in the research field of plasma medicine. Aqueous reactive oxygen species (ROS) generated in this process are widely accepted playing a crucial role in plasma biomedical effects. In this paper, chemistry pathways among various aqueous ROS induced by He + O₂ plasmas are investigated by a numerical model. Simulation results show that these aqueous ROS can be classified into two groups according to their production ways: the group of species including O, ¹O₂ and e directly produced in plasma, and the other group of species including O₂ ⁻, H₂O₂, O₃, etc. produced by liquid reactions. A key reaction chain of e → O₂ ⁻ → HO₂(→ HO₂ ⁻) → H₂O₂ is found to be important in the plasma-induced liquid chemistry. Furthermore, impacts of changes in plasma and solution conditions on aqueous ROS concentrations are studied as well. It is found that changes in plasma conditions (O₂ ratio in the discharge gas/power density) can globally influence the concentrations of almost every aqueous ROS, while conditions changes of the treated liquid (pH/dissolved oxygen) only partially influence the concentrations of some specific species including O₂ ⁻/HO₂, O₃ ⁻/HO₃ and H₂O₂. The revelations of the liquid chemistry pathways and the dependence of ROS dosage on the treatment conditions offer a better understanding on the plasma–liquid interactions, as well as provide optimized dosage control approaches for biomedical applications.