In Situ Electron Spin Resonance and Spin Trapping Study for Kinetics of Free Radical Reactions at Plasma/Liquid Interfaces*

Recently, the study about reactions at plasma/liquid interfaces becomes important in plasma technology, because of the various applications of plasmas in and in contact with liquid. Electron spin resonance (ESR) with spin-trapping agents is useful for detecting short-lived free radicals reacting in liquid phase. However, the accurate quantification of radicals requires kinetics of spin-trapping agents under plasma treatment rather than in thermal equilibrium. This study investigated such kinetics by developing in situ ESR coupled to a reactor of plasmas in liquid.

The plasma was generated in Ar gas bubbles surrounded by aqueous solution of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping agent at a concentration of 22 mM, by applying pulse-modulated AC voltage with frequency of 17 kHz and mean voltage of 0.6 kV on tungsten electrodes with a gap of 1.5 mm. The solution was circulated with flow rate of 30 ml/min and total amount of 50 ml through a quartz flat cell for ESR, which enabled high-sensitivity and time-resolved detection of DMPO adducts with hydrogen (H) atoms and hydroxyl (OH) radicals. As the result, concentrations of DMPO-H and DMPO-OH immediately after plasma generation were detected at 2.2 and 4.1 μM, and their increase during plasma generation and inactivation after plasma disappearing were also quantified. These results would elucidate the kinetics under plasma treatment and estimate the radical production and the reaction rates at plasma/liquid interfaces.