Non-touching plasma-liquid interaction - where is aqueous nitric oxide generated?

Nitric oxide is a relatively stable free radical and an important signal molecule in plants, animals, and humans with high relevance for biological processes involving inflammatory processes, e.g. wound healing or cancer. The molecule can be detected in the gas phase of non-thermal plasma jets making it a valuable tool for clinical intervention, but transport efficiency from the gas phase into the liquid phase or tissue remains to be clarified. To elucidate this fact, the nitric oxide concentration in buffered solutions is determined using electron paramagnetic resonance spectroscopy. The origin of the nitric oxide in the liquid could be excluded, therefore, potential precursors such as hydroxyl radicals, superoxide anions, atomic hydrogen and stable species (nitrite, nitrate and hydrogen peroxide) were detected and the potential formation pathway as well as ways of enhancing the production of nitric oxide by alteration of the feed gas and the surrounding gas composition during plasma treatment of the liquid have been pointed out.