Low-Temperature Air Plasma Jet for Inactivation of Bacteria (S. Aureus and E. Coli) and Fungi (C. Albicans and T. Rubrum)

Abstract

Generally, most of the setups of plasma jet adopted rare gases (e.g., helium and argon) as the working gas. The drawback of using rare gases is that it cannot be continuously used as the gas tank volume is limited, and the cost of using rare gases to yield plasma jet is also higher. Thus, a plasma jet using solely air as the working gas could be a potentially promising solution. In this study, a low-temperature air plasma jet using only air as gas was developed. The device was optimized by adjusting the control circuit (deadband, current, voltage and duty ratio), frequencies (50-350 kHz), air pump flow-rate (0-8 L/min), and geometric size of each component. The spectrum of the low-temperature air plasma jet revealed that the main components are N₂, and the radicals N/NO occupy 92.5%, while O and O⁺ occupy less than 7.5%. Under the optimal conditions, where the discharging frequency was at 120 kHz and the output voltage was set to 5-10 kV, its inactivation ability toward microorganisms, such as Staphylococcus aureus, Escherichia coli, Trichophyton rubrum, and Candida albicans was assessed. In addition, the inactivation ability of low-temperature air plasma jet and ozone toward microorganisms was compared. Low-temperature air plasma jet and ozone were applied to microorganisms for a range of treatment times, and the results showed different degrees of microbiological inactivation. The inhibition zones formed on the agar plate were 3.8 cm (Staphylococcus aureus), 3.3 cm (Escherichia coli), 5.3 cm (Trichophyton rubrum), and 3.2 cm (Candida albicans) when a 15 min low-temperature air plasma jet was applied. Comparatively, the diameters of the inhibition zone after 15 min of ozone exposure were 3.8, 5.0, 0.9, and 0.0 cm, respectively. Collectively, the low-temperature air plasma jet showed comparable efficacy to ozone in terms of inactivation ability. This low-temperature air plasma jet demonstrated its potential to be used for medical applications involving the inactivation of microorganisms.