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Synergistic effect of sterilization by microwave-excited atmosphericpressure plasma jet using photo-catalyst TiO2 with H2O2 and distilled water. H. Wk. Lee1, Y. S. Seo1, S. K. Kang1, H. J. Seo2, J. Choi1, Seung-Jin Park1, Jae-Yoon Sim1, S. M. Jeon2 and J. K. Lee1 1 Department of Electronic and Electrical Engineering, Pohang University of Science and Technology, Pohang, 790-784, S. Korea 2 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, S. Korea Abstract: A microwave-excited atmospheric-pressure plasma jet is applied to inactivate Gram-positive bacteria, S. mutans using photo-catalyst TiO2 with H2O2. Unlike UV-based sterilization methods, a significant synergy on the bactericidal effect is observed when the plasma is combined with TiO2 and H2O2 at a time. All the bacteria samples used in ~4´107 CFU/ml of concentration are destructed within 30 seconds. A kinetic simulation shows plenty of electrons having more than 5 eV in the high-frequency discharge, which can dissociate O-O and O-H bond in H2O2 and water molecules. This indicates that OH radical may contribute dominantly in this experiment. We conclude that the energetic electrons as well as UV photons from the plasma are likely to significantly contribute to the fast disinfection via effective interaction with TiO2 and H2O2 or H2O. Keywords: list 3-5 relevant keywords 1. Introduction Sterilization has been one of common research topics in biomedical applications [1]. Many researchers employed H2O2 based on conventional UV method with photo catalyst such as TiO2 to enhance bactericidal effect [2]. Low temperature plasma has been proposed as a noble sterilizer which is more effective than UV method, and the atmospheric non-thermal plasmas with TiO2 have also been suggested to enhance the inactivation of bacterial spores, B. subtilis [3]. In this study, a low-temperature microwaveexcited plasma technology using the coaxial transmission line resonator atmospheric pressure plasma jet (APPJ) [4] and either TiO2 or H2O2 (3 % H2O2 solution) or both is presented as a new powerful sterilization method. 2. Results and discussion Figure 1. (a) Survival curve of S. mutans according to the treatment method and time, and (b) survival curve of S. mutans for observing temperature and UV effect. Simulation result of (c) electron temperature, (d) percentage of high energy electron and (e)O radical emission intensity by OES measurement . Figure 1 (a) shows the result of Streptococcus mutans(S. mutans) sterilization experiment. While TiO2 with plasma case (TP treatment) did not show significant sterilization effect, H2O2 with plasma case (HP treatment) significantly promote the sterilization efficacy (filled square). In particular, the plasma treatment of using both H2O2 and TiO2 (HTP treatment) resulted in outstanding inactivation of S. mutans, all the bacteria were killed in 30 s. It should be noted that there was significantly synergistic bactericidal action of the HTP treatment in comparison with two cases, TiO2 with APPJ (TP treatment) and H2O2 with APPJ. Figure 1 (b) shows the effect of heat, UV and charged particles produced from the APPJ. A quartz was located between the APPJ and the sample to block the charged particles and other chemical species, and pass UV only. In the case of only UV, three log-reductions were achieved in 60 s, while the HTP treatment induced the destruction of all the bacteria in 30 s (25 s for grounded case). This indicates clearly that the charged particles of the plasma as well as UV photons significantly interact with H2O2 and TiO2 and consequently contribute to enhance the sterilization process by photo-catalyst with H2O2. It is well-known that TiO2 needs more than 3.2 eV for photolysis, and the bond energies of O-O and O-H on H2O2 are 146 (~1.51 eV) and 418.4 (~4.35 eV) kJ/mol, respectively. In the computational study based on particle-in-cell method, The percentage of energetic electrons more than 5 eV was 53.8 % in the 900-MHz discharge compared to 6 % in the 13.56-MHz discharge (Fig. 1(c)) [5]. A recent work also showed that microwave (2.45 GHz) plasma can generate a majority of energetic electrons (Fig. 1 (d)) [6]. The analysis of optical emission spectra also supports the result (Fig. 4c). The intensity of atomic oxygen lines at 777 nm of the HTP treatment was increased almost two-fold on average from the only plasma treatment (Fig. 1(e)). In summary, a synergistic effect on Grampositive bacteria sterilization by the microwaveexcited APPJ with TiO2 and H2O2 was observed. The energetic electrons more than 5 eV with UV photons in the APPJ may contribute to activate the photo-catalyst, TiO2 nanoparticles and dissociate the bonds of H2O2. As a result, the ROS generation is more enhanced and the outstanding rapid disinfection is achieved. Acknowledgement The authors thank Prof. T. H Chung and H. M. Joh of Dong-A university for the OES measurement. This work was supported by Midcareer Researcher Program through NRF grant funded by the MEST (2011-0015395) and the Korea Ministry of Education, Science, and Technology through its Brain Korea 21 program. References [1] M. Laroussi, Plas. Proc. and Polym., 2, 391, (2005) [2] M. Lanao et al., Solar energy, 84, 703, (2010) [3] H. Jung et al., A. Catal. B: Environ. 93, 212, (2010) [4] J. Choi et al., Plas. Sour. Sci. Tech., 18, 025029, (2010) [5] F. Iza et al., Phys. Rev. Lett., 99, 075004 (2007) [6] Y. S. Seo et al., IEEE Trans. Plasma Sci., 38, 5540307, (2010)