Surface sterilization using LF-microwave hybrid plasma

Abstract

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.

Effect of LF-microwave hybrid plasma treatment on radish seed germination and sprout growth

“…The LF-microwave hybrid plasma source is composed of a rectangular waveguide (WRJ-2), which contains a cylindrical slotted antenna surrounding a discharge quartz tube, and a cylindrical copper electrode, which is installed in the discharge quartz tube. [27][28][29] Two slits are arranged on the cylindrical slotted antenna. The frequency of the microwave was 2.45 GHz and the power could be increased up to 2 kW.…”

“…A short plunger was installed for impedance matching between the plasma and the microwave circuit, and the forward, reflected powers were measured by using a directional coupler with power meters. [27][28][29] The flow rate of each gas was adjusted by each gas-flow controller. The LF plasma jet was generated at the beginning of the operation by applying an LF high voltage to a cylindrical copper electrode in the discharge quartz tube.…”

“…14,28) Consequently, the color of the CI changes from purple to green, depending on the dose of ROS interacting with the surface. [27][28][29] The CI was placed 25 cm away from the discharge region located at the seed treatment point.…”

“…[23][24][25][26] In our previous study, a new type of atmospheric pressure non-thermal plasma source, combining a low-frequency (LF) plasma jet with a microwave plasma source was developed as a hybrid plasma. [27][28][29] The LF-microwave hybrid plasma was initiated by the LF plasma and expanded by the dissipation of microwave power. The LFmicrowave hybrid plasma has a large number 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.…”

“…The results obtained by applying LF-microwave hybrid plasma on the surface sterilization of medical equipment demonstrated that the reactive species generated in the hybrid plasma are considered to play an important role in bacteria sterilization. [27][28][29] Therefore, a large number of reactive species generated in LF-microwave hybrid plasma, such as reactive oxygen species (ROS) and reactive nitrogen species (RNS), were considered to reduce the pathogenic contamination of seed surface and stimulate plant growth.…”