The chemical characteristics of atmospheric pressure dielectric barrier discharge plasma generated in air are very sensitive to some factors, such as environment temperature and discharge power. There are two typical modes, namely the ozone mode and nitrogen oxides mode. In this paper, the air surface micro-discharge plasma in the ozone mode and nitrogen oxides mode are respectively generated at environment temperatures of 5 °C and 50 °C when discharge power is almost kept at 6 W, and the plasma-activated water (PAW) is prepared by means of passing the plasma exhaust gases into deionized water. By comparing the reactive species in PAW under these two modes and their sterilization effects, it is found that the concentrations of H ⁺ , H ₂ O ₂ , NO ₂ ⁻ , NO ₃ ⁻ and ONOO ⁻ in PAW for the nitrogen oxides mode are about 2–3 fold over those for the ozone mode, while the concentration of O ₃ in PAW for the ozone mode is more than 28 times that in the nitrogen oxides mode. The sterilization effect under the nitrogen oxides mode is two orders of magnitude higher than that in the ozone mode. For the biological effects, it is found that treatment of a methicillin-resistant Staphylococcus aureus suspension by PAW leads to approximately a four-log reduction in the nitrogen oxides mode, which is higher than that in the ozone mode, leading to speculations that the reactive nitrogen species represented by ONOO ⁻ in PAW may be the critical species in sterilization. Furthermore, the transition between ozone mode and nitrogen oxides mode can be achieved by adjusting the discharge power. Interestingly, both of the compositions and concentrations of the reactive species in gas and liquid phases under 5 °C & 9.5 W and 50 °C & 6 W conditions are roughly identical, indicating that the effect of environment temperature on the mode transition is consistent with that of discharge power.

Air plasma is commonly used to treat water for activation or purification, and plasma-activated water (PAW) is a promising green disinfectant that has attracted much attention in recent years. However, either the O ₃ discharge mode or the NO _x discharge mode of air plasma lacks efficiency for water activation, mainly due to the low solubility of O ₃ , NO and NO ₂ . The transition process between those two modes could produce high-valence NO _x such as N ₂ O ₅ which should be more efficient for water activation, but this process is not easily controlled and water activation by N ₂ O ₅ has rarely been reported before. In this letter, N ₂ O ₅ is found to be produced effectively and stably by mixing the effluent gases of a NO _x mode air plasma, produced by gliding arc discharge, and an O ₃ mode air plasma, produced by dielectric barrier discharge. The mixed gas rich in N ₂ O ₅ is found to be very efficient for water activation. A colony-forming unit reduction of nearly 6 logs was achieved for a methicillin-resistant Staphylococcus aureus suspension by using PAW treated by the mixed gas, and the high bactericidal effect of the PAW could persist for more than 1 h after the plasma activation. The main reactive species for the bactericidal effect in the PAW are discussed, and the chemical pathways for N ₂ O ₅ production and its usage for water activation are illustrated.

A continuous risk from microbial infections poses a major environmental and public health challenge. As an emerging strategy for inhibiting bacterial infections, plasma‐activated water (PAW) has proved to be highly effective, environmental‐friendly, and non‐drug resistant to a broad range of microorganisms. However, the relatively short lifetime of reactive oxygen and nitrogen species (RONS) and the high spreadability of liquid PAW inevitably limit its real‐life applications. In this study, plasma‐activated hydrogel (PAH) is developed to act as reactive species carrier that allow good storage and controlled slow‐release of RONS to achieve long‐term antibacterial effects. Three hydrogel materials, including hydroxyethyl cellulose (HEC), carbomer 940 (Carbomer), and acryloyldimethylammonium taurate/VP copolymer (AVC) are selected, and their antibacterial performances under different plasma activation conditions are investigated. It is shown that the composition of the gels plays the key role in determining their biochemical functions after the plasma activation. The antimicrobial performance of AVC is much better than that of PAW and the other two hydrogels, along with the excellent stability to maintain the antimicrobial activity for more than 14 days. The revealed mechanism of the antibacterial ability of the PAH identifies the unique combination of short‐lived species ( ¹ O ₂ , ∙OH, ONOO ⁻ and O ₂ ⁻ ) stored in hydrogels. Overall, this study demonstrates the efficacy and reveals the mechanisms of the PAH as an effective and long‐term disinfectant capable of delivering and preserving antibacterial chemistries for biomedical applications.

N₂O₅, a reactive species produced by air discharge plasma, has recently attracted much attention. Due to its high reactivity and solubility, N₂O₅ is a key molecule in nitrogen fixation processes and exhibits promising prospects in plasma biomedicine. However, thus far, it is not well known how to produce N₂O₅ efficiently and then maintain its concentration under the action of fast removal reactions. In view of this, N₂O₅ production by dielectric barrier discharge (DBD) alone and by the combination of DBD and gliding arc discharge is compared in this paper. It is found that the combination method can yield over three times the concentration of N₂O₅ compared to the single DBD method with the optimum discharge power. Moreover, the concentration of N₂O₅ in the effluent gas can be maintained once O₃ also exists because O₃ can continually produce N₂O₅ to compensate for its reduction. Finally, the sterilization effects of both the plasma effluent gas and plasma-activated water have trends similar to the trend of the gaseous N₂O₅ concentration, implying that N₂O₅ plays an important role in sterilization. This paper enhances the understanding of N₂O₅ chemistry in air discharge plasma and provides an effective way to produce and maintain N₂O₅ for subsequent applications.