Dielectric barrier discharge (DBD) devices are known ozone generators. Authors have previously demonstrated a DBD surface plasma source, operating in air at atmospheric pressure, to achieve killing of vegetative cells in 2-3 min and sterilization in 20 min (bacterial spores). The aim of this paper is to examine the role of the ozone in surface DBD plasma sterilization. The role of ozone in plasma killing is examined by a) characterizing the rate of production/decay of ozone during DBD plasma generation, b) studying the effect of exposing bacteria (Escherichia coli) solely to the ozone thus produced. Our results indicate that while ozone plays a major role, the energy flux delivered to the electrodes is also crucial in the process of plasma sterilization.

Ozone is a well-known disinfecting agent that is used as an alternative for chlorine in many applications, including water decontamination. However, the utility of ozone in water decontamination is limited by high electrical power consumption and expensive, bulky equipment associated with ozone generation. This study investigates the effectiveness of a lightweight, compact surface dielectric barrier discharge (SDBD) reactor as an ozone generator to inactivate Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus (MRSA) in an open water system. Experimental details are provided for ozone generation technique, mixing method, ozone concentrations in air and water, and input energy required to produce adequate ozone concentrations for bacterial inactivation in a contaminated, open water system. Specifically, an active plasma module (APM) reactor system of size 48 cubic centimeters, weighing 55 grams, with a maximum ozone yield of 68.6 g/KWh was used in atmospheric conditions as the source of ozone along with an air pump and a diffusion stone for mixing the ozone in water. Over 4-log reduction in P. aeruginosa concentration was achieved in 4 minutes with 0.1 mg/L ozone concentration in an open water system using 8.8 ± 1.48 J input energy. Also, over 5-log reduction in MRSA concentration was achieved in 2 minutes with 0.04 mg/L ozone concentration in an open water system using 4.4 ± 0.74 J input energy.

Plasma sterilization offers a faster, less toxic and versatile alternative to conventional sterilization methods. Using a relatively small, low temperature, atmospheric, dielectric barrier discharge surface plasma generator, we achieved ≥6 log reduction in concentration of vegetative bacterial and yeast cells within 4 minutes and ≥6 log reduction of Geobacillus stearothermophilus spores within 20 minutes. Plasma sterilization is influenced by a wide variety of factors. Two factors studied in this particular paper are the effect of using different dielectric substrates and the significance of the amount of liquid on the dielectric surface. Of the two dielectric substrates tested (FR4 and semi-ceramic (SC)), it is noted that the FR4 is more efficient in terms of time taken for complete inactivation. FR4 is more efficient at generating plasma as shown by the intensity of spectral peaks, amount of ozone generated, the power used and the speed of killing vegetative cells. The surface temperature during plasma generation is also higher in the case of FR4. An inoculated FR4 or SC device produces less ozone than the respective clean devices. Temperature studies show that the surface temperatures reached during plasma generation are in the range of 30°C–66°C (for FR4) and 20°C–49°C (for SC). Surface temperatures during plasma generation of inoculated devices are lower than the corresponding temperatures of clean devices. pH studies indicate a slight reduction in pH value due to plasma generation, which implies that while temperature and acidification may play a minor role in DBD plasma sterilization, the presence of the liquid on the dielectric surface hampers sterilization and as the liquid evaporates, sterilization improves.

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Cover: Dielectric Barrier Discharge (DBD) plasma devices are known ozone generators. The role of produced ozone in DBD sterilization was studied by examining its lethality to Escherichia coli. Measuring the levels of ozone produced during and after plasma generation identifies three phases: production, diffusion and decay. Each of these phases is characterized by an empirical formula. A correlation between the ozone production and the input energy flux to the plasma devices is also developed that is key to surface plasma sterilization. Further details can be found in the article by Subrata Roy et. al. http://doi.wiley.com/10.1002/ppap.201300108.