Basic Study of Bacteria Inactivation at Low Discharge Voltage by Using Microplasmas

Inactivation of microorganisms, such as Escherichia coli, by exposure to a microplasma is experimentally investigated. A microplasma is an atmospheric-pressure nonthermal plasma. Microplasmas, which generate high-intensity electric fields, can be formed using relatively low discharge voltages (0.7-1.1 kV) across small discharge gaps (0-100 ?m). The key benefits of the practical application of exposure to a microplasma are as follows: 1) the low discharge voltage and 2) the simple apparatus because a vacuum enclosure is not required. Hence, the apparatus for generating a microplasma could be relatively small and inexpensive and could be integrated into a portable device. The ozone generated by a microplasma at a low power level was measured, although the specific power density of the microplasma was larger than that of large-scale conventional plasmas. The emission spectra of the microplasma discharge in N₂ was measured: 1) to confirm the UV light emission and 2) to identify the active chemical species generated by the microplasma discharge. The emission spectra was also measured with the presence of water droplets. The UV light from the microplasma discharge showed excited nitrogen molecules and OH radicals. In this paper, two cultures of bacteria, i.e., gram-negative Escherichia coli HB101 and gram-positive Bacillus subtilis JCB 20036 were the target microorganisms to be inactivated. In the experiments reported here, the number of bacteria decreased after microplasma treatment. The inactivation rate increases as the discharge voltage increases. Escherichia coli is completely inactivated when air is used as carrier gas at a plasma discharge voltage of 1.05 kV. Using nitrogen as carrier gas, the highest inactivation rate is 77% at a discharge voltage of 1.15 kV. In addition, Bacillus subtilis is inactivated with a rate of 97% at 1.07 kV with air as carrier gas. Using nitrogen as carrier gas and a discharge voltage of 1 kV- - results in an inactivation rate of 70% of bacteria. The inactivation of microorganisms by microplasma may be due to several factors either individually or in combination of the following: 1) the excited molecules and ions; 2) ozone; 3) high electrical fields; and 4) UV light. The effect of active species such as OH radicals may also be important since all the bacteria were carried within a small water droplet in between the electrodes.