Sterilization by negative and positive DC plasma with a micro discharge gap at atmospheric pressure

A new needle-to-droplet electrode structure with a micro discharge gap (2 mm) was designed to achieve direct current (DC) discharge plasma in ambient air with the aim of using the plasma to sterilize liquids. Without using noble gases or an external air flow, we succeeded in generating both a negative and positive DC plasma at atmospheric pressure. The plasma was driven by a 0 to −20,000 V, 100 W DC power supply. A stainless steel needle with a tip diameter of [Formula: see text] and a 200-[Formula: see text] droplet of bacteria-containing liquid served as the electrodes. At atmospheric pressure and room temperature (23[Formula: see text]C), utilizing the negative DC plasma, the discharge time lasted 10 s; the results showed that the higher the discharge voltage, the more efficient the sterilization effect. Conversely, when we applied a voltage of −5.5 kV, we found that the sterilization effect was more efficient for longer discharge times. Our findings demonstrate that Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) can be killed in about 30 s. Our experiments show that our sterilization method required less time and was more efficient for positive than for negative DC plasma under the same conditions.