Summarizing results on the performance of a selective set of atmospheric plasma jets for separation of photons and reactive particles

Cold atmospheric plasma (CAP) jets are well known for various surface treatment applications. There are several relevant applications of CAP jets in the field of plasma medicine, such as inactivation of microorganisms like bacteria and bacterial and fungal spores, treatment of skin diseases or wounds, plasma surgery and cancer treatment [1–6]. Additionally, CAP jets could be applied as an alternative sterilization device for thermo-labile materials or applied to treat contaminated products that cannot be exposed to vacuum, such as fresh fruits or vegetable slices [7, 8].

Many plasma sources have been developed and characterized for the needs of these applications. These sources effectively produce reactive species, photons, charged particles and electric fields [9–13].

The effects of CAPs on prokaryotic or eukaryotic cells are induced by the above-mentioned plasma components, whereby reactive oxygen and nitrogen species (ROS and RNS, respectively) seem to be the dominant effectors [14–16]. Still, many experiments have demonstrated that plasma treatment is more effective than treatment with a combination of isolated components [1, 17, 18] or the effect of ROS alone [19]. The possible separation of different plasma components, such as reactive species, photons, ions or electric fields, and the possibility to test their isolated and combined effects present a great benefit for studies of the interaction mechanisms of plasma jets with biological substrates [15, 20]. Likewise, this separation is one of the main necessities for research on the inactivation mechanisms of microorganisms or effects on eukaryotic cells. Each plasma component can either act on microorganisms directly or participate in chemical reactions and therefore act on microorganisms indirectly by enhancing the flux or effect of other components [21]. Furthermore, it was demonstrated by different groups that synergies between treatment with different components can exist as well [14, 15, 22, 23]. Nevertheless, the effects cannot be traced back to the isolated plasma components because all plasma components act on the treated samples in parallel.

We developed plasma jets that allow the effects of two reactive components of plasma to be studied separately and in combination with identical fluxes. These components are plasma-generated reactive species and photons [15, 16, 20]. A previous version of a set of microscale atmospheric-pressure plasma jets (μAPPJs), which selectively allow this separated and combined treatment, was applied for studies on bacteria and bio-macromolecules. The μAPPJ is a remote plasma source, where the plasma is limited to a defined volume and only the effluent, not the active plasma or electric fields, is in contact with the substrate. In this case, the reactive species and photons are the main components involved in the treatment.

Here, we summarize our effort to develop a set of component-selective μAPPJs and report on the construction and quantitative characterization of an improved design. Photon and particle components emitted by the plasma are effectively separated by these sources and the treatment of both isolated plasma components is analyzed quantitatively and independently. Our characterization confirms that the fluxes of the selected plasma components, either photons or reactive particles, are each equal to a jet that combines the two components, while the other component is effectively blocked by the particular jet of this set. Moreover, we demonstrate how the selective jets can be applied to study the mechanisms of bacterial inactivation by plasma.

The microscale atmospheric-pressure plasma jet (μAPPJ) is a capacitively coupled plasma source, which consists of two stainless steel electrodes [24]. The electrodes (30 mm length, 1 mm thickness) are 1 mm apart, forming a 1  ×  1  ×  30 mm³ plasma volume. To confine the plasma zone, both electrodes are glued in between two quartz glass plates (thickness 1.5 mm). A helium (He) flow of 1.4 slm with small admixtures of oxygen (O₂) or nitrogen (N₂) (<1%, 99.999% purity) is applied. One electrode is powered by a 13.56 MHz sinusoidal voltage in the range 200–230 V root mean square voltage (absorbed power  <1 W) connected to an RF generator through an impedance matching network. The other electrode is grounded. This jet is very well characterized and modelled. Densities of atomic oxygen and ozone [25–28], atomic nitrogen [29, 30] and singlet delta oxygen [31] were determined using two-photon absorption laser-induced fluorescence spectroscopy (TALIF), optical emission spectroscopy (OES), phase-resolved optical emission spectroscopy (PROES) and molecular beam mass spectrometry (MBMS). The afterglow chemistry was simulated in a global kinetic model [32] and atomic oxygen densities were benchmarked by numerical simulations [33, 34].

The transport of all species in the helium gas is dominated by convection, because the diffusion is limited by a very high collision rate. In contrast, the photons can propagate unhindered through the helium gas on line-of-sight to the plasma, being absorbed only weakly by the low-concentration species. These different transport mechanisms can be utilized to separate particle and photon components emanating from the plasma in the remote μAPPJ. The first version of the plasma jet for separation was a single selective jet produced by a crossed channel construction called X-jet (see figure 1) [14, 24]. It was shown that nearly all heavy particles can be steered into the crossing channel by an additional helium flow due to the construction of this crossing channel and that we can effectively separate photons and particles. Nevertheless, in the case of the combined photon and particle treatment, some vortexes appeared at the crossing resulting in the region with high gas residence time (a source of possible problems for some gas chemistries) and losses of particles into the side channel. This issue was demonstrated in a convection and diffusion simulation (COMSOL Multiphysics) of the X-jet geometry (see figure 2), where a test concentration (1 mol m⁻³) of reactive particles in the helium flow of 1.4 slm, only through the plasma channel, was simulated. The parameters for this simulation were atmospheric pressure and a dynamic helium viscosity of 2  ×  10⁵ Pa s. As mostly He/O₂ plasma was used for the biological treatment studies, the diffusion coefficient of atomic oxygen (1.3  ×  10⁻⁴ m² s⁻¹ [33]) was selected.

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It turned out that an improved selective source was needed, as the issues of vortexes at the crossing and losses of particles into the side channel appeared. To solve these issues, three independent selective jets were developed (see figure 3). Therefore, extending channel structures were added to the jets, which create the separation of the plasma-emitted components as described in [15]. One individual source was developed for each indicated treatment situation: a particle-jet, indicated for selective particle treatment, a photon-jet, indicated for selective photon treatment, and a complete-jet, indicated for combined particle and photon treatment. Because of the realization by individual sources, the complete-jet could be produced by a straight channel geometry and no flow interference with the side channel appears. Initial tests have shown that the separation of particles and photons was effective. Additionally, the particle fluxes of the particle-jet and the complete-jet were comparable (data not shown) and we used this version for our past studies of the separated and combined effects of particles and photons [15, 20, 40].

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Still, we decided to make additional small changes in the design to make the final performance better and to make sure that all possible sources of differences in the fluxes of photons from the photon-jet and combined jet on the one hand and reactive particles from the particle-jet and combined jet on the other hand are minimized. Since the particle fluxes are already comparable, this final development step improves mainly the comparability of the photon fluxes from the photon-jet and the combined jet. Identical fluxes are important because they will allow the study of possible synergistic effects between reactive particles and photons in the plasma treatment. The final version again consists of three independent selective jet sources. Similarly to the first set, extending channel structures are added to the jets, which create the separation of the plasma-emitted components, with the following improvements. All jet extensions are made of ceramics (Macor^® machinable glass ceramic) to achieve a comparable reflectivity and scattering of the photon-jet and complete-jet. Additionally, the use of the ceramic material avoids any reactions with reactive species and possible degradation through UV and vacuum UV photons of the polyoxymethylene (POM) material that was used in the previous version of the photon-jet. Furthermore, the channel of the particle-jet is now machined with smooth curves in place of edges. The design of the final set of the jets will be summarized in the following text for clarity and is shown in figures 4 and 5.

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The particle-jet is selectively optimized for the treatment with particle fluxes equal to those of the complete-jet without the influence of photons (see figures 4 and 5(a)). Its extended channel structure consists of a curved gas channel, which steers the effluent gas flow out of the line-of-sight to the plasma volume. Therefore, no photons can directly propagate to the surface of the treated sample, whereas all particles are transported by the convection of the flow to the exit nozzle. The curved structure of this jet extension was chosen to stabilize the flow of the effluent as turbulence and vortexes have to be avoided. A simulation of the gas flow (COMSOL Multiphysics, incompressible Navier–Stokes) is performed to confirm laminar flow inside the extended channel and in the effluent region assuming a helium flow in the range of 1–2 slm. Additionally, the length of the centreline of the curved channel is 8 mm, the same as the length of the channels in the other two jets. To reduce specular reflections in the additional channel structures, the glass plates are roughened in this region.

The photon-jet is optimized for the treatment with photons of a flux equal to that of the complete-jet without the influence of the particle components (see figure 4(b) and 5(b)). This jet's extension consists of an additional helium channel crossing the effluent, making an angle of 90° between the additional channel and the jet axis. This crossing helium flow is able to steer the gas flow from the plasma, including reactive particles, into the crossing channel. Figure 5(d) shows a sectional view of the photon-jet model. To ensure that all particles are steered into the crossing channel, the cross section of this channel is larger (2  ×  1 mm²) and a greater helium flow (more than twice the gas flow through the jet) is applied to maintain higher gas velocity. As particles emanating from the jet could still diffuse along the boundary layer along the glass walls, transverse grooves (~1 mm depth) are cut into the quartz glass plate located at the position where the two channels cross each other (see the detailed view in figure 5(e)). Altogether, these modifications of the photon-jet ensure that only photons can propagate to a surface in line-of-sight to the plasma.

The complete-jet is the reference jet for treatment with particles and photons in combination to study possible interference and synergistic effects (see figure 4(c) and 5(c)). Its extended channel structure confines and guides the effluent additionally to achieve comparable treatment conditions to the particle-jet and photon-jet, with the same plasma–substrate distance and also with the same residence time (and therefore recombination time) in the effluent. All jet extensions are designed to prolong the confined volume by an additional 8 mm. Additionally, the quartz glass plates are roughened in the area of the extended jet geometries to avoid light reflections there.

The performance of the selective jets was tested by three different diagnostics, namely MBMS, OES and schlieren-imaging, and by a numerical fluid simulation of the gas flow.

First, MBMS was applied to compare the particle fluxes emanating from the different selective jets. MBMS enables the measurement of neutral species produced by the plasma at atmospheric pressure and was described in detail in previous publications [27, 29, 35, 36]. For the comparison of particle fluxes, the measurements of atomic oxygen (O), molecular oxygen (O₂) and ozone (O₃) produced by a helium (He) plasma with various admixtures of molecular oxygen (O₂) were chosen as reference values. These species were chosen because the μAPPJ is known for its effective production of oxygen species, and the production of oxygen species is weakly influenced by impurities in the gas supply. These are the only species detectable by mass spectrometry from the He/O₂ plasma.

Second, OES was performed to compare the photon fluxes of the different jet sources. An optical spectrometer (Ocean Optics USB 4000, 200 nm–850 nm, 16 bit) was connected through an optical fiber. The entrance of the fiber was positioned at a distance of 10 mm in front of each jet's nozzle in the direction of the jet axis and aligned to measure maximized emission intensity. In the case of the particle-jet, it was aligned parallel to the plasma channel's axis and positioned under the jet nozzle as well. As the plasma emission intensity is high, no lens was applied to collect the photons. The He/N₂ gas mixture is used for these measurements, because a lot of emission features are present in the UV–visible–IR spectral range of the spectrometer.

Moreover, the stability of the effluent flow was tested by schlieren-imaging because the flow emanating from the nozzle of the unmodified μAPPJ remains laminar. Quasi-parallelized white light emitted by a halogen lamp is reflected by a beam splitter onto the optical axis. The jet sources are positioned inside the beam. A concave mirror (f  =  500 mm) behind the jet focuses the light onto a horizontally oriented knife edge. Behind the knife edge, a camera is positioned on the beam axis. Changes of the optical density influence the beam due to refraction. Therefore, this setup visualizes the helium flow emanating from each jet of the set into air and can be applied to test whether any of the selective constructions causes turbulence in the effluent's flow.

The effluent flows of the different geometries were simulated in a 3D fluid dynamics simulation (COMSOL). To match the experimental conditions, a helium flow of 1.4 slm through the jet geometry was simulated, which emanates into helium atmosphere. A wall was set at 4 mm distance (typical treatment distance) to simulate the treated surface. The volume parameters used were atmospheric pressure and a dynamic helium viscosity of 2  ×  10⁵ Pa s. Both the schlieren-imaging and the simulation were performed to confirm and cross-check stable and laminar flow in the effluent of the selective jets.

Additionally, a convection and diffusion simulation of the photon-jet geometry was performed. Similar to the simulation of the X-jet source, a test concentration (1 mol m⁻³) in the main channel was simulated. The parameters for this simulation were atmospheric pressure and a dynamic helium viscosity of 2  ×  10⁵ Pa s. As He/O₂ is the most important gas mixture for biological treatment studies, the diffusion coefficient of atomic oxygen (1.3  ×  10⁻⁴ m² s⁻¹ [33]) was selected.

Molecular beam mass spectrometry

The particle fluxes of atomic oxygen (O), molecular oxygen (O₂) and ozone (O₃) emanating from all jets were compared by means of MBMS. The data are presented separately for these three species, because no other neutral oxygen-containing species originating from the gas phase can be detected by the MS. Additionally, the atomic oxygen has to be measured with electrons in the ionizer with energy close to its ionization threshold to avoid dissociative ionization of O₂ or O₃ [27]. In all cases, the plasma parameters were 1.4 slm He flow, 3.0 slm He flow through the steering channel of the photon-jet, 230 V_RMS applied electrode voltage and 2 mm distance between jet nozzle and sampling orifice. Different admixtures of O₂ were added to the plasma. Selecting He/O₂ plasma is advantageous for the comparison of the particle production and separation, because the He/O₂ plasma is insensitive to variation of possible gas impurities (N₂/H₂O), because O₂ has low ionization energy among these species. The three selected species represent the different kinds that can emanate from the jet. Atomic oxygen (see figure 6) is a reactive species dominantly produced in the plasma volume and recombining with O₂ on the plasma effluent, with its density having an optimum around 0.6% of O₂ admixture [27]. The differences in measured signals for O atoms would indicate a longer residence time in one of the jets (vortexes) or differences is diffusion or surface reaction losses. Molecular oxygen (see figure 7) is a stable precursor gas with the highest density among the species presented here, and as such would most easily show any problems with the particle separation, especially in the photon-jet. Ozone (see figure 8) is a stable product of plasma chemistry dominantly generated in the effluent, the density of which increases with increasing O₂ admixture [27].

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As shown in figures 6–8, the particle fluxes emanating from the particle-jet and the complete-jet are comparable. In contrast, the photon-jet blocks the particles effectively, as the MBMS signal from the jet nozzle is zero within the calculated errors. Despite the differences among these species, the selective jets operate in a very similar way for all of them. The particle fluxes emanating from the particle-jet and the complete-jet are almost identical and no particles (except helium) emanate from the photon-jet. It should be noted here that for each comparison the three jets were measured one after the other and the data contain all possible errors due to variation in the jet alignment with respect to the MS sampling orifice, setting of the gas flows and applied voltage and reproducibility of the MS measurement, stressing the very good performance of these jets. A small systematic difference of the particle fluxes emanating from the particle-jet and complete-jet appears only in the case of ozone. This difference might be caused by the possible effect of photons as discussed later. Altogether, these measurements confirm that particles emanating from the plasma volume are very effectively steered into the crossing channel of the photon-jet, and the curved geometry of the particle-jet does not change the flux of particles compared to the complete-jet. Additionally, the absence of O atoms in the effluent of the photon-jet was confirmed by etching experiments for the previous version of the jets [15].

Optical emission spectroscopy

Figure 9 shows the emission spectra of the three selective jets. All measurements were performed at a distance of 10 mm between jet nozzle and optical fiber and the same plasma conditions, which were 1.4 slm He flow, 0.25% N₂ admixture and 210 V_RMS applied electrode voltage. For comparability, all emission spectra were normalized to 1 ms integration time. The obtained emission spectra of the photon-jet and the complete-jet are identical within the noise of the signal. Additionally, it is shown that the photons emitted from the plasma volume are effectively reduced by the extended channel structure of the particle-jet. Although the particle-jet is not completely free of photons due to scattered light, the flux intensity is 0.3% around 700 nm and around 0.1% in the biologically relevant wavelength range (<300 nm), which means that the treatment time of the particle-jet would have to be increased 1000-fold to achieve the same photon dose.

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Schlieren-imaging

Figure 10 shows schlieren-images of the particle-jet (a) and the complete-jet (b). Both images were taken at a helium flow of 1.4 slm emanating into ambient air. These images visualize the helium flow of the effluent of both jets. Effluent flows appear as gradient changes from black to white in these images. It is shown that the effluents of the jets remain laminar inside the ambient atmosphere. The effluent of the particle-jet is shifted by the channel construction. Nevertheless, the flow remains laminar. Red lines indicate the axes of both plasma volumes and the flow direction of the shifted effluent. As shown, the plasma volumes of the jets are each parallel to the effluent flow of the complete-jet or the shifted effluent flow of the particle-jet. At the end of the visible effluent region, the gas stream is curved due to the buoyancy force acting on helium in air. Quartz glass plates confine the plasma volume, thus it is visible as well. As the glass envelopes are roughened in the area of the extended structures to avoid reflections, the channels are not visible in this area. The effluent flow of the photon-jet does not need to be investigated by schlieren-imaging, as the MBMS measurement confirms that no particle components produced by the plasma emanate from this jet.

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Fluid dynamics simulations

3D fluid dynamics simulations of the geometry of the particle-jet (see figure 11) and the complete-jet (data not shown) were performed to confirm laminar flow in the effluent of both jets, which can be cross-checked with the schlieren-images. These simulations visualize the velocity field assuming an initial gas velocity of 1.4 slm through the plasma volume. In agreement with the schlieren-images, the simulation of the particle-jet shows that the axis of the shifted flow of the particle-jet geometry is parallel to the jet axis. In case of both jets, the flow remains laminar outside the jet nozzle. Both simulations show that a jet-like effluent structure is formed in agreement with previous simulations and measurements of the μAPPJ [26].

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Additionally, a convection and diffusion simulation of the photon-jet confirms that all particles are steered into the crossing channel construction (see figure 12). To match the required minimal conditions of the experiment, a flow of 2.8 slm through the crossing channel was selected. All further parameters of the simulation are equal to the convection and diffusion simulation of the X-jet source. Although stability of the fluid is not necessary for studies of the photon effects, a stable velocity field (indicated by white arrows) inside the photon-jet geometry was obtained.

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Overall, the performance tests are in good agreement and confirm that particle and photon components are effectively separated and the fluxes are the same. The photon emission intensity of the photon-jet and the complete-jet are equal, whereas the photons are blocked by the particle-jet as the OES measurements show. Reactive particle fluxes of the particle-jet and complete-jet are the same, whereas the photon-jet blocks the particles, which was confirmed by MBMS measurements. Additionally, the shifted particle beam emanating from the particle-jet remains laminar and parallel to the jet axis, which was shown by schlieren-imaging and confirmed by a 3D fluid dynamics simulation.

The photon-jet can be used to measure the vacuum UV (VUV) spectra down to ~50 nm if placed in front of a helium-filled spectrometer. A McPherson vacuum ultraviolet monochromator (0.2 m focal length, 2400 grooves/mm grating) was used for the detection of plasma emission in the range 50 nm to 300 nm. This spectrometer was modified to be operated at atmospheric pressure [36], where a flange with 1 mm diameter orifice has been placed in front of the entrance slit and a flange with a window covered by a thin sodium salicylate film (followed by a photomultiplier tube, where the photoluminescence intensity is recorded) is placed behind the exit slit. The monochromator is flushed with helium and therefore no window, which would otherwise absorb all photons under its cut-off wavelength (~115 nm for usual MgF₂ material), is needed.

As bacteria are mainly sensitive to photon treatment by UV and VUV photons, the emission spectra of the μAPPJ were measured in the range 50 nm to 300 nm and are compared in figure 13. The emission is dominated by the first and second continua of the $\text{He}_{2}^{*}$ excimer emission [37] when only helium gas is used. Addition of a molecular gas leads to effective quenching of this excimer emission. The emission intensity of the He/O₂ plasma is then dominated by atomic oxygen lines at 99 nm, 115 nm and 130 nm, which were previously detected by Bahre et al in a different monochromator setup [38] except for the O line at 99 nm as the entrance slit was covered by a MgF₂ window. The emission of the He/N₂ plasma is dominated by NO_γ lines in the spectral region from 200 nm to 280 nm. This surprising result can be explained by the water impurity, where N atoms quickly react with OH radicals to form NO.

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The capabilities of the application of these jets were demonstrated by treating E. coli MG1655 bacteria on agar medium similar to our previous works [14, 15, 24, 39]. It should be noted that the treatment shown and discussed in the following has been performed with the first set of selective jets (see [15]), where it cannot be guaranteed that the photon fluxes from the photon-jet and combined jet are identical.

Sample preparation

Results and discussion

Inhibition zones resulting from treatment with the different plasma jets were measured. The plasma parameters were 1.4 slm He flow, 230 V_RMS applied electrode voltage and a distance of 4 mm between the sample and jet nozzle. To analyze the influence of the gas mixture, a He-only plasma, a He/N₂ plasma and He/O₂ were applied. The admixture of N₂ and O₂ was set to 0.6%.

Figure 14 compares the effects of the photon-jet treatments. The fastest inactivation appears with the photons from the He/N₂ plasma. This fact has to be a result of the NO_γ lines in the range 200 nm to 280 nm, which directly influence the inactivation of biological samples because nucleic acids as well as proteins absorb in this range. Regarding the treatment with helium and He/O₂ plasmas, the former leads to a marginally faster inactivation. This result could be explained by the highly energetic emission of $\text{He}_{2}^{*}$ excimer continuum. A long treatment time of 360 s led to an inhibition zone that was larger than the irradiated area of the samples. This result indicates that the photons also induced production of reactants from impurities in the gas phase, as already reported before [14]. It can be concluded that these reactants lead to bacterial inactivation after long treatment times. The emission intensity of the He/O₂ plasma was limited to the atomic oxygen lines at 99 nm, 115 nm and 130 nm, and is weaker then the integrated emission of the $\text{He}_{2}^{*}$ excimer and results in the slowest inactivation.

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In comparison to the photon-jet treatment, the particle-jet treatment (see figure 15) inactivated the bacteria faster independent of the chosen gas mixture. The particle-jet treatment was most effective when treating with the He/O₂ plasma. This result underlines the effective inactivation of bacteria by reactive oxygen species. Photons can be excluded in the case of particle-jet treatment. Therefore, ROS have to be the major players for bacterial inactivation in case of a He/O₂ plasma. The reactive nitrogen species emitted from the particle-jet when driven with He/N₂ were less effective in inactivating E. coli. Treatment with He/O₂ plasma leads to the production of ozone, which inactivates E. coli likewise.

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Ozone remaining inside the chamber leads to an increasing concentration in the helium atmosphere. Additionally, ozone dominantly remains at the sample's surface due to the lower mass of helium. As a result, bacteria at large distances from the jet axis can be inactivated. After long treatment times (⩾360 s), the produced ozone affected the entire sample due to its stability, but survivors remained in the area beyond the formed inhibition zones. The corresponding samples are indicated by '*'. As bacteria on the entire agar plate become affected, the ozone-induced inactivation can be called a long-range effect.

Compared to the He/O₂ and He/N₂ plasma, the He-only plasma led to a less effective inactivation of E. coli. This result could be expected as the He-only plasma produces a low density of chemical reactants originating mainly from impurities (mainly H₂O resulting in OH radicals in He or NO in He/N₂).

The complete-jet treatment (see figure 16) was the most effective treatment for all gas mixtures used. Especially after short treatment times, the differences were visible as the combined treatment led to larger inhibition zones than particle treatment whereas no inhibition zones were formed in case of photon treatment. Samples treated for 10 s using a He/N₂ plasma showed a synergistic behavior as photon treatment did not lead to an inhibition zone, but 10 s combined treatment led to a spot in the samples' center surrounded by a shaped region. This result indicates that combined treatment is more effective than particle treatment whereas no effect of photon treatment is detected for the same treatment time. Additionally, the inactivation rate outside the photon-irradiated area is also faster, strongly indicating that some gas-phase species are excited or generated in the photochemical reactions and accelerate the inactivation. However, it is not yet known which species and which inactivation mechanism are involved. Similar effects are observed also for the He/O₂ plasma, where even the long-range effect ascribed to ozone is accelerated. This result is not in agreement with the observed, slightly lower O₃ counts in figure 8. One could speculate that the VUV photons could result in O₃ dissociation or excitation, which would lead to more ozone generation in the region outside the main gas channel, but further analysis is needed to better understand this issue.

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In contrast to He/O₂ plasma treatment, no long-range effects were visible after treatment with He/N₂ or He-only, indicating that no long-lived reactive products were produced that could inactivate E. coli. He/O₂ was the most effective gas mixture for bacterial inactivation. However, for studies that focus on the mechanisms at the cellular level and not just on inactivation of the bacteria, He/N₂ could be a better choice.

Inhibition zone tests present only one possibility for the large range of potential applications of the set of selective μAPPJs to study the induced effects of plasma components on biological materials separately and in combination. Further studies of the effects of plasma components can be found in [14, 15, 20, 39, 41].

This article summarizes our effort to develop a set of selective atmospheric plasma jets that separate photons and particles emanating from the plasma. This selective set enables plasma treatment experiments on microorganisms or biological macromolecules, which can reveal the isolated or synergistic interaction mechanisms among reactive species and photons. The first of the three jets is the particle-jet, which separates all heavy particles from photons; the second jet is the photon-jet, which separates photons from particles; and the third jet is the complete-jet, which enables treatment by the combination of almost identical fluxes of particles and photons to study interaction mechanisms of particle and photon components in combination.

The performance of the separation was tested by MBMS, OES, schlieren-imaging and a fluid dynamics simulation. All methods show with good agreement that the selective μAPPJs effectively separate particles and photons emanating from the plasma and that the fluxes are laminar and comparable to the complete-jet. The effective exclusion of photon emission by the particle-jet was demonstrated by optical emission spectroscopy. It was shown that only 0.1% of photons are scattered and leave the particle-jet in the wavelength range below 300 nm, while the photon flux intensities of the photon-jet and the complete-jet are the same. Conversely, the effective exclusion of reactive particles by the photon-jet was checked by molecular beam mass spectrometry. It was shown that no reactive particles emanate from the photon-jet, while the particle fluxes of the particle-jet and the complete-jet are the same. The measured fluxes of the two jets differ only within the calculated error. Moreover, schlieren-images were taken to confirm laminar flow in the effluent region of the particle-jet and complete-jet. These images verify that the effluent of the shifted particle-jet remains parallel to the jet axis and that the gas flow is laminar. This is also confirmed by 3D fluid dynamics simulations of the effluent of the particle-jet and the complete-jet.

The photon-jet allowed measurement of the plasma-emitted photons in the vacuum UV range down to 50 nm, where the $\text{He}_{2}^{*}$ excimer emission was observed. Treatment of E. coli bacteria with the different selective jets was directly studied by inhibition zone tests for He-only, He/N₂ and He/O₂. He/N₂ treatment resulted in the fastest photon-induced damage to bacteria due to the emission of the NO_γ lines in the range 200 nm to 280 nm. He-only plasma was less effective, as there is no NO_γ emission. However, highly energetic $\text{He}_{2}^{*}$ excimer emission is emitted by He-only plasma. He/O₂ did not effectively induce photon damage, as the emission in the relevant range (50–300 nm) is limited to three atomic oxygen lines. However, He/O₂ plasma was the most effective plasma mixture for particle-induced damage on bacteria, as reactive oxygen species are important effectors on bacteria. After long treatment times, the whole sample was affected, as the stable ozone is bactericidal. He/N₂ was less effective. In particular, no long-range effects appeared, indicating that no stable components are produced. He-only plasma induced particle damage only after long treatment times, which was presumed to be caused by water impurities. The combination of particle- and photon-induced damage was always most effective, very probably due to some additional photochemical reactions in the gas phase.

This work has been funded by the German Research Foundation (DFG, grant PlasmaDecon PAK 728 (BE 4349/2-1) to Jan Benedikt, Julia Bandow (BA 4193/3-1) and PLACID (PAK 816) to Volker Schulz-von der Gathen), as well as FOR 1123 and the Research Department 'Plasmas with Complex Interactions' of the Ruhr-University Bochum (RUB).