## 3 Results  
Intra- and Extra-Microbial OH- and H\({}_{2}\)O\({}_{2}\) after CAP Treatment with H\({}_{2}\)O\({}_{2}\)  
The synergistic effects of H\({}_{2}\)O\({}_{2}\) with CAP are thought to be attributed to the newly generated molecules from H\({}_{2}\)O\({}_{2}\) in contact with CAP. For example, an OH- radical can be a strong candidate for enhanced bactericidal efficiency, since it can be formed easily from H\({}_{2}\)O\({}_{2}\). At first, we tried to find the highly generated OES peaks right above the surface of the 1% H\({}_{2}\)O\({}_{2}\) solution when CAP was applied (Figure 1b). The height of most peaks increases on liquids in comparison with those on the dry plate. The peaks at 309 nm (OH-radical) and 763 nm (excited Ar) were enhanced above both surfaces of deionized (DI) water and H\({}_{2}\)O\({}_{2}\), but the peaks at 777 nm and 884 nm (O atom) were not affected by the solutions. We could not find big differences in OES from the gas above the CAP irradiated solutions.  
At second, we tried to evaluate the generation of OH- radicals in a liquid environment. We assessed the amounts of H\({}_{2}\)O\({}_{2}\) and OH- radicals in solutions after CAP treatment (Figure 1c). When CAP was applied on DI water, the amount of generated OH- and H\({}_{2}\)O\({}_{2}\) increased with treatment time and reached 5.5 mM and 4.6 mM after 5 min of treatment, respectively (Figure 1c). In the case of nitrate and nitrite, the concentrations were very low, as expected from the OES, which was measured to be around 80 uM after 5 min of CAP treatment. On the contrary, H\({}_{2}\)O\({}_{2}\) was slightly decreased in the 0.05% H\({}_{2}\)O\({}_{2}\) solution with the increase in CAP treatment time. The decrease in H\({}_{2}\)O\({}_{2}\) in H\({}_{2}\)O\({}_{2}\) solution means that the more amount of H\({}_{2}\)O\({}_{2}\) might be dissociated than the newly generated H\({}_{2}\)O\({}_{2}\) by CAP in the 0.05% H\({}_{2}\)O\({}_{2}\) solution. Contrary to our expectations, the measured amounts of OH- radicals were not significantly different in the two solutions.  
Additionally, we measured the amount of extracellular OH- and H\({}_{2}\)O\({}_{2}\) when bacteria were present in solutions during CAP treatment. Figure 2a shows that the measured amounts of OH- were not affected by the presence of bacterial cells. However, a significant difference in the amount of H\({}_{2}\)O\({}_{2}\) was observed when bacteria were present in the 0.05%H\({}_{2}\)O\({}_{2}\) solution as shown in Figure 2a. About 3 mM (0.009%) H\({}_{2}\)O\({}_{2}\) disappeared when there were bacterial cells. These results imply that the CAP-induced dissociation of H\({}_{2}\)O\({}_{2}\) may not be related to the OH- radical generation, and the induced cytotoxicity by combined treatment may be attributed to the action of H\({}_{2}\)O\({}_{2}\) itself on cells.  
To confirm the role of the OH- radical in the synergistic bactericidal effects, 40 uM OH- specific scavenger mannitol was used in a CFU test. Figure 2b shows the CFU of _E. faecalis_ in DI and 1% H\({}_{2}\)O\({}_{2}\) solution with and without CAP treatment for 45 s. Graphs show that the addition of OH- scavenger mannitol did not affect CFU values in all the cases. This demonstrates that OH- radical did not play a key role in the synergistic bacterial killing effects.