Real-Time Formation of Nitrate and Nitrite Species in Plasma-Activated Liquids: From Distilled Water to Cell Culture Solutions

Abstract Plasma-activated liquids (PALs) bridge plasma chemistry and aqueous reactivity, offering a controllable platform for biomedical, agricultural, and environmental applications. Their reactivity stems from gas-phase reactive oxygen and nitrogen species dissolving into the liquid; however, the role of the choice of solution in governing this chemistry remains insufficiently investigated. Here, we systematically compare deionized water (DW), mineral water (MW), tap water (TW), phosphate‑buffered saline (PBS), and Dulbecco’s modified Eagle medium (DMEM) treated under identical conditions using a surface dielectric barrier discharge (sDBD) in a gas‑tight reactor. By coupling in situ optical absorption spectroscopy with liquid-phase analysis, we reveal a direct link between the gas-phase O 3 –NO x transition and the formation of NO 2 ⁻, HNO 2 , and NO 3 ⁻ in each solution. Among the tested media, PBS exhibited the fastest transition, occurring about 82 s earlier than in DW, driven by its high ionic strength and rapid O 3 consumption. While nitrate concentrations converged across solutions, nitrite and nitrous acid were strongly modulated by initial pH, buffering capacity, and ionic composition. In particular, PBS produced significantly higher NO 2 ⁻ levels, reaching 0.453 mM, which is about 6.6 times higher than the lowest concentration observed in DMEM. Dilution of PBS (0.5X, 0.1X) delayed the transition and reduced buffering, favoring HNO 2 accumulation. These findings demonstrate that PAL chemistry emerges from the interplay between plasma conditions and solution properties.