## 4 Discussion
### Effect of the Generated (Reactive) CAP Species on the Biofilms  
#### 4.2.1 Impact of the Generated UV Light  
Despite the lack of photon detection for most examined combinations of plasma characteristics using the UV-TEC strips (Figure 2), often no significant differences were observed between the log-reduction values obtained with and without the implementation of the quartz disc (Figure 6). Especially for the _Listeria monocytogenes_ model biofilms, this was observed for six out of eight examined treatment conditions. As a result, it can be concluded that this biofilm forming species was more vulnerable to inactivation by UV light and/or that the UV photons can more easily reach the biofilm-associated _Listeria monocytogenes_ cells. The latter can originate from the difference in thickness between the _Listeria monocytogenes_ (approximately 15 \(\upmu\)m) and _Salmonella_ Typhimurium (approximately 25 \(\upmu\)m) model biofilms [26]. However, one should be careful with the interpretation of the log-reduction results since (i) the standard deviations for _Listeria monocytogenes_ were quite high for some combinations of plasma characteristics and (ii) the average _Listeria monocytogenes_ log-reduction values without the implementation of the quartz disc were often 1.5 log\({}_{10}\) (CFU/cm\({}^{2}\)) higher than the corresponding values obtained with implementation of the quartz disc. Therefore, the contribution of UV light to the bactericidal effect of CAP was deemed to be only minor. The contradictory results observed in Figures 2 and 6 can possibly be explained based on the specifications of the applied UV-TEC strips. These strips can detect UV photons with a wavelength ranging between 250 and 420 nm, while the lethal wavelengths predominantly range between 200 and 280 nm (UV-C light) with a maximum absorption wavelength of DNA, the main target of UV photons (see Section 4.2.3), around 260 nm [19; 54; 55]. Consequently, UV photons emitted within the range of 200-250 nm were not detected by the applied UV-TEC strips. However, previous optical emission spectroscopy (OES) measurements performed within the team [56] indicated that the use of helium and oxygen as feeding gas barely resulted in the generation of photons with a wavelength below 300 nm. Similarly, the research of [PERSON] et al. [57], using an air-based surface micro-discharge plasma set-up for inactivation of _Deinococcus radiodurans_ planktonic cells, indicated that mainly UV-A (320-420 nm) photons were generated. Another possible explanation for these contradictory results can be that the sensitivity of the applied UV-TEC strips was not high enough to detect the UV photons with wavelengths higher than 300 nm. If these were generated, but not detected, biofilm-associated cells could also have been inactivated by these specific UV photons. For biofilms developed by _P. fluorescence_ and _S. epidermidis_, for example, treatment by LED light with a wavelength of 400 nm resulted in a significant reduction of the viable cell density, although only following more than 2 h of treatment [58]. It should be mentioned, however, that the intensity of this light treatment was much higher than the intensity of the UV photons possibly generated during CAP treatment. In combination with the very short treatment time, it is thus very unlikely that any of these UV photons resulted in a reduction of the viable _Listeria monocytogenes_ and _Salmonella_ Typhimurium cell density. Within the study of [PERSON] et al. [57], it was indeed observed that the UV-A component of the plasma was not able to inactivate the _Deinococcus radiodurans_ planktonic cells due to the low irradiation dose (\(<\)25 nW/cm\({}^{2}\)). Finally, the study of [PERSON] et al. [59], using an argon-based plasma jet, indicated that the generation of UV photons during CAP treatment resulted in the indirect generation of H\({}_{2}\)O\({}_{2}\) in deionized water.