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A new type of atmospheric-pressure non-thermal plasma source was developed by a combining a low-frequency (LF) plasma jet with microwave plasma as a hybrid plasma. The LF-microwave hybrid plasma has a large amount of reactive species compared to conventional LF plasma jet owing to higher-energy electrons, which are accelerated by the oscillating electric fields of the microwave. Moreover, it can be sustained stably at a higher composition rate of the process gas in the discharge gas. The objective of this study is to investigate the effect of the admixture gas composition on surface sterilization by using our self-designed LF-microwave hybrid plasma source. The influence of the discharge gas composition ratio between oxygen and argon on the production of the reactive species and sterilization effect was investigated. The analysis of the optical emission spectra and the reaction of the chemical indicators reveal that reactive oxygen increases with the increasing ratio of oxygen in the oxygen-argon admixture discharge gas up to an oxygen ratio of 10% and then it saturates. In contrast, the results of the sterilization test using biological indicators indicate that a higher sterilization effect was obtained for a lower ratio of oxygen in the admixture discharge gas. It is considered that reactive species, except reactive oxygen, are related to surface sterilization using atmospheric-pressure non-thermal plasma.

The atmospheric pressure non-thermal plasma generated by low-frequency (LF)-microwave hybrid discharge has been employed to investigate the effects of plasma treatment on the germination and growth of radish (Raphanus sativus L.) sprout seeds. The plasma generated with admixture discharge gas of Ar-O2 was adopted for the irradiation of seeds. SEM and EDX analyses were performed to elucidate the physical and chemical state changes of seed surfaces after plasma treatment. The reactive species produced in the plasma were investigated using optical emission spectroscopy and chemical indicator. The results demonstrate that higher reactive oxygen species were generated from the LF-microwave hybrid plasma compared to conventional LF plasma jet. Moreover, the higher germination percentage and shorter germination time were obtained with the LF-microwave hybrid plasma compared to LF plasma jet and control group. It is considered that reactive species produced in the LF-microwave hybrid plasma are related to the germination time of the radish sprout seed.

Surface sterilization using LF-microwave hybrid plasma, which is a low-frequency (LF) plasma jet assisted by a microwave discharge, was investigated. Hybrid plasma is an atmospheric pressure non-thermal plasma and has a large number of reactive species. To improve the performance of surface sterilization using LF-microwave hybrid plasma, two types of discharge gas supply systems were employed. The first type (Type I) introduces argon and oxygen gas flows separately into the discharge region. The second type (Type II) introduces a mixture of two gases into the discharge region. The analysis of optical emission spectra and chemical indicators showed that more reactive oxygen species were produced by the Type II gas supply system compared to that by Type I. Moreover, a lower gas temperature was obtained with the Type II system compared to that with Type I. The results of the sterilization test show that a higher sterilization effect was obtained for the gas supply system Type II. The spore-forming bacteria (Geobacillus stearothermophilus) with a population of 2.4 × 10 6 CFU/carrier were sterilized for 50 s at the treatment temperature of 77.5 °C using the Type II gas supply system.

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