Off-site production of plasma-activated water for efficient disinfection: The crucial role of high valence NO_x and new chemical pathways

Efficient disinfection of pathogens is a critical concern for environmental disinfection and clinical anti-infective treatment. Plasma-activated water (PAW) is a promising alternative to chemical disinfectants and antibiotics for its strong disinfection ability and not inducing any acute toxicity. Previous plasma sources are commonly placed near or fully in contact with water as possible for more efficient activation, but the risk of electrode corrosion and metal particle contamination of water threatens the safety and stability of PAW. In this work, plasma-activated gas (PAG) rich in high-valence NO_x is generated by a hybrid plasma configuration and introduced into water for off-site PAW production. It is found that plasma-generated O₃ dominates the gas-phase reactions for the formation of high-valence NO_x. With the time-evolution of O₃ concentration, the gaseous NO₃ radicals are produced behind N₂O₅ formation, but will be decomposed before N₂O₅ quenching. By decoupling the roles of gaseous NO₃, N₂O₅, and O₃ in the water activation, results show that short-lived aqueous species induced by gaseous NO₃ radicals play the most crucial role in PAW disinfection, and the acidic environment induced by N₂O₅ is also beneficial for microbial inactivation. Moreover, SEM photographs and biomacromolecule leakage assays demonstrate that PAW disrupts the cell membranes of bacteria and thus achieves inactivation. In real-life applications, an integrated device for off-site PAW production with a yield of 2 L/h and a bactericidal efficiency of >99.9 % is developed. The PAW of 50 mL produced in 3 min using this device is more effective in disinfection than 0.5 % NaClO and 3 % H₂O₂ with the same bacterial contact time. Overall, this work provides new avenues for efficient PAW production and deepens insights into the fundamental chemical processes that govern the reactive chemistry in PAW for environmental and biomedical applications.