Evaluation of cold atmospheric pressure plasma irradiation of water as a method of singlet oxygen generation.

We used cold atmospheric pressure plasma jet to examine in detail ¹O₂ generation in water. ESR with 2,2,5,5-tetramethyl-3-pyrroline-3-carboxamide, a secondary amine probe, was used for the detection of ¹O₂. Nitroxide radical formation was detected after cold atmospheric pressure plasma jet irradiation of a 2,2,5,5-tetramethyl-3-pyrroline-3-carboxamide solution. An ¹O₂ scavenger/quencher inhibited the ESR signal intensity induced by cold atmospheric pressure plasma jet irradiation, but this inhibition was not 100%. As 2,2,5,5-tetramethyl-3-pyrroline-3-carboxamide reacts with oxidizing species other than ¹O₂, it was assumed that the signal intensity inhibited by NaN₃ corresponds to only the nitroxide radical generated by ¹O₂. The concentration of ¹O₂ produced by cold atmospheric pressure plasma jet irradiation for 60 s was estimated at 8 μM. When this ¹O₂ generation was compared to methods of ¹O₂ generation like rose bengal photoirradiation and 4-methyl-1,4-etheno-2,3-benzodioxin-1(4H)-propanoic acid (endoperoxide) thermal decomposition, ¹O₂ generation was found to be, in decreasing order, rose bengal photoirradiation ≥ cold atmospheric pressure plasma jet > endoperoxide thermal decomposition. Cold atmospheric pressure plasma jet is presumed to not specifically generate ¹O₂, but can be used to mimic states of oxidative stress involving multiple ROS.