Two-Dimensional Concentration Distribution of Hydrogen Peroxide Generated by Atmospheric-Pressure Plasma Jet Irradiation

The two-dimensional concentration distribution of hydrogen peroxide (H2O2) generated by atmospheric-pressure argon (Ar) plasma jet irradiation was semiquantitatively determined using H2O2 test strips. An Ar plasma jet was generated with different applied voltages (5–10 kV) and Ar gas flow rates (3–10 L/min) and was irradiated onto the H2O2 test strips at different irradiation distances (2–40 mm) and irradiation times (5–30 s). The shape of the distribution of H2O2 concentration depended on the combination of the applied voltage, gas flow rate, and the irradiation distance and was classified into four types: circular, double-circular, ring-shaped, and double-ringshaped. A circular distribution was observed for irradiation distances of ≥30 mm and a double-circular distribution was observed for irradiation distances of 10–20 mm. When the irradiation distance was ≤5 mm, a ring-shaped distribution was observed for applied voltages of ≤6 kV and a double-ring-shaped distribution was observed for applied voltages of ≥7 kV. [DOI: 10.1380/ejssnt.2015.474]


INTRODUCTION
Recently, research on the application of atmosphericpressure plasma to sterilization and medical useshas attracted attention [1][2][3][4].For sterilization, there are reports on the inactivation of bacteria, such as Escherichia coli [5,6], Micrococcus luteus [7], and Staphylococcus aureus [8], using atmospheric-pressure plasma and on the disruption of Bacillus subtilus spores [9,10] and Bacillus atrophaeus spores [11,12].For medical applications, it was reported that the irradiation of mammalian cells with atmospheric-pressure plasma caused their necrosis [13] or apoptosis [14] depending on the irradiation conditions.When melanoma skin cancer cells were irradiated with atmospheric-pressure plasma, the proliferation of the cells was inhibited [15].Irradiating ovarian cancer cells with atmospheric-pressure plasma initiated apoptosis [16].For endothelial cells, however, proliferation was promoted by atmospheric-pressure plasma irradiation [17].When a burned area of a rat was irradiated with atmosphericpressure plasma, the burn was rapidly and completely cured [18].Thus, plasma has been found both to inhibit the proliferation of cells and induce apoptosis as well asto activate the proliferation of cells.
When plasma reacts with oxygen molecules (O 2 ) in air and surrounding water molecules (H 2 O), active oxygen species, such as oxygen (O) radicals, superoxide anion (O − 2 ), ozone (O 3 ), hydroxyl (OH) radicals, and hydrogen peroxide (H 2 O 2 ) are generated.These species are considered to play a key role in the above effects.Understanding the reaction between plasma and water is particularly required because bacteria and cells are present in wet environments.
In our previous study, distilled water was irradiated with an argon (Ar) plasma jet to examine the reaction between plasma and water.We reported that H 2 O 2 was generated in the distilled water irradiated with the plasma jet and that the H 2 O 2 concentration increased with irradiation time according to results of flow injection analysis (FIA) [19].Some reports show that, similar to our results, the generation of H 2 O 2 in water was induced by on-water discharge [20,21] and on-water irradiation of atmospheric-pressure plasma [22,23].
The mechanism behind the generation of H 2 O 2 is considered to be as follows.First, high-energy electrons in streamers and plasma collide with H 2 O molecules to generate OH radicals and hydrogen (H) atoms [24,25].Next, the generated OH radicals are bound to each other to form H 2 O 2 [26,27].
Moreover, we carried out an experiment employing semiquantitative determination using H 2 O 2 test strips.We reported that when an agar medium was irradiated with an Ar plasma jet, H 2 O 2 was generated on the surface of the agar and a double-ring-shaped distribution of H 2 O 2 concentration was observed [28].
Thus, we have found that H 2 O 2 is generated in a wet region irradiated with an Ar plasma jet and that the distribution of H 2 O 2 concentration is likely to have certain patterns.However, details of the distribution pattern have been still unclear.Clarifying the details is important to help understand the reaction between plasma and water, which occurs during the irradiation of an Ar plasma jet in a wet region.Such understanding is expected to encourage the application of atmospheric-pressure plasma to sterilization and medical needs.
In this study, we clarified the two-dimensional concentration distribution of H 2 O 2 generated by Ar plasma jet irradiation.The dependences of the concentration distribution of H 2 O 2 on the irradiation time, gas flow rate, irradiation distance, and applied voltage were semiquantitatively determined using H 2 O 2 test strips.

II. EXPERIMENTAL PROCEDURE
Figure 1 shows a schematic of the experimental setup used in this study.In the unit to generate a plasma jet, a copper tube (inner diameter, 4 mm; outer diameter 6 mm) as the discharge electrode was inserted into a dielectric quartz tube (length, 50 mm; inner diameter, 6 mm; outer diameter, 8 mm) around which copper foil (thickness, 0.05 mm; width, 10 mm) was wrapped as the ground electrode [29].A high AC voltage was applied to induce dielectric barrier discharge inside the quartz tube between the electrodes and to convert the introduced Ar gas to plasma, which was ejected into air in the form of a jet.A high-voltage power LHV-10AC (Logy Electric Co., Ltd.) was used to generate an Ar plasma jet: frequency, 10 kHz; maximum applied voltage, 10 kV; and maximum gas flow rate, 10 L/min.Under these conditions, a plasma jet with a diameter of ∼6 mm and a length of ∼30 mm was ejected from the end of the quartz tube into air [30].The values of voltage and current were measured using a high-voltage probe P6015A and a current probe A621, respectively, connected to a digital oscilloscope TDS1001B (Tektronix, Inc.).The gas flow rate was adjusted using a gas flowmeter RK-1250 (KOFLOC, Kojima Instruments Inc.).Quantofix Peroxide 100 test strips (Macherry-Nagel GmbH & Co. KG) were used to semiquantitatively determine the distribution of H 2 O 2 concentration.Each six test strips (5×5 mm 2 ) were arrayed on a glass plate in the longitudinal and lateral directions and immersed in distilled water (Wako Pure Chemical Industries, Ltd.) for 1 s.The center of the test strip array was irradiated with a plasma jet.
The irradiation conditions were as follows.

A. Dependence on irradiation time
Figure 2 shows a photograph of a plasma jet irradiated onto the H 2 O 2 test strips.The emission of the Ar plasma jet was blue-white.Many streamers in the plasma within the quartz tube reached the test strips.Electrons were considered to collide with H 2 O molecules on the test strips because streamers are flows of electrons.
Figure 3 shows photographs of a plasma jet taken from the back of a quartz plate located in place of the H 2 O 2 test strips (irradiation distance, 2 mm).For small values of aperture, the inside of the quartz tube was white and a blue-white light-emitting ring with an inner diameter of ∼8 mm and an outer diameter of ∼13 mm was observed around the quartz tube.These results indicate that the plasma that reached the quartz plate extended in the lateral direction.Therefore, plasma that reaches the test strips is also considered to extend in the lateral direction.
The aperture was increased to observe the emission inside the quartz tube in more detail.As shown in Fig. 3(2), a blue-white light-emitting ring with an inner diameter of ∼5 mm and an outer diameter of ∼6 mm was observed; the inside of the ring was dark.The outer diameter of the blue-white ring was the same as the inner diameter of the quartz tube, indicating that the streamers were not uniformly distributed in the quartz tube; namely, they were localized near the inner wall of the quartz tube and rarely present in the central region of the tube.
Figure 4 shows photographs of the H 2 O 2 test strips irradiated with a plasma jet for different irradiation times.The test strips become blue as a result of the reaction between the coloring reagent and H 2 O 2 , and the blue color becomes more intensewith increasing H 2 O 2 concentration.The H 2 O 2 concentration was determined by referring to the color chart printed on the test strip case.After 5 s of irradiation, a blue ring with an inner diameter of ∼5 mm and an outer diameter of ∼6 mm (denoted as the inner ring) and a light blue ring with an inner diameter of ∼13 mm and an outer diameter of ∼21 mm (denoted as the outer ring) were formed, confirming a double-ringshaped distribution of H 2 O 2 concentration.The ring between the inner and outer rings was referred to as the white ring.The region outside the outer ring was slightly colored.Therefore, when a wet region is irradiated with an Ar plasma jet, H 2 O 2 is generated not only in the irradiated region but also around the region.The H 2 O 2 concentrations were ∼30 mg/L in the inner ring, ∼3 mg/L in the outer ring, and ∼1 mg/L outside the outer ring.The inside of the inner ring was white, where little H 2 O 2 was generated.After 10 s of irradiation, a double-ringshaped distribution was similarly observed; an inner ring with an inner diameter of ∼5 mm and an outer diameter of ∼7 mm and an outer ring with an inner diameter of ∼13 mm and an outer diameter of ∼23 mm were formed.The H 2 O 2 concentrations were 30-100 mg/L in the inner ring, ∼3 mg/L in the outer ring, and ∼1 mg/L outside the outer ring.After 20 s of irradiation, an inner ring with an inner diameter of ∼5 mm and an outer diameter of ∼8 mm and an outer ring with an inner diameter of ∼13 mm and an outer diameter of ∼24 mm were formed.The H 2 O 2 concentrations were 30-100 mg/L in the inner ring, ∼3 mg/L in the outer ring, and ∼1 mg/L outside the outer ring.After 30 s of irradiation, an inner ring with an inner diameter of ∼5 mm and an outer diameter of ∼8 mm and an outer ring with an inner diameter of ∼13 mm and an outer diameter of ∼26 mm were formed.The H 2 O 2 concentrations were 30-100 mg/L in the inner ring, ∼10 mg/L in the outer ring, and 1-3 mg/L outside the outer ring.
The width of the white ring remained almost unchanged regardless of the irradiation time: it was ∼3.5 mm after 5 s of irradiation, and ∼3 mm after 10, 20, and 30 s of irradiation.However, the width of the outer ring increased with increasing irradiation time: it was ∼4 mm after 5 s of irradiation, ∼5 mm after 10 s of irradiation, ∼5.5 mm after 20 s of irradiation, and ∼6.5 mm after 30 s of irradiation.The colors of the inner ring, the outer ring, and outside the outer ring intensified with increasing irradiation time, indicating that the H 2 O 2 concentration increased with irradiation time.Therefore, the concentration distribution of H 2 O 2 generated by plasma jet irradiation depends on the irradiation time.
The outer diameter of the inner ring was the same as the inner diameter of the quartz tube and the outer diameter of the blue-white light-emitting ring formed by the streamers as shown in Fig. 3. Therefore, the inner ring was considered to be formed because the streamers flowing along the inner wall of the quartz tube reached the test strips.High-energy electrons in the plasma collided with H 2 O molecules on the test strips to generate OH radicals, which were reacted together to form H 2 O 2 .Hence, we consider that the concentration of the OH radicals generated by plasma jet irradiation was highest in the inner ring and lower inside the plasma jet and in the outer region.Similar results had already been reported by Yonemori et al. [31].They measured the distribution of OH radicals generated by helium (He) plasma jet irradiation under atmospheric pressure using laser-induced fluorescence and found that the OH radical concentration was highest theimmediately adjacent to the plasma jet and lower inside the plasma jet and in the outer region.
The formation of the outer ring indicates that H 2 O 2 was generated even when the streamers did not reach the test strips.Therefore, we consider that the mechanisms behind the formation of the inner and outer rings are different.
The mechanism behind the formation of the outer ring is discussed in the following.Previously, we obtained an emission spectrum of an Ar plasma jet and found an emission peak corresponding to the OH radicals in the spectrum [28].We suggest that OH radicals form because high-energy electrons in the plasma collide with H 2 O molecules present in air and in the Ar gas cylinder.Therefore, when a plasma jet is ejected into air, OH radicals are generated at the boundary between the plasma jet and the immediately adjacent air, and react together to form H 2 O 2 in air.
In this experiment, the plasma that reached the test strips extended in the lateral direction.In accordance with this extension, H 2 O 2 was transported in the lateral direction and fell onto the test strips to form a blue outer ring.With increasing irradiation time, the amount of H 2 O 2 transported in the lateral direction increased and the blue color intensified.
The formation of the white ring is considered to be caused by the laterally extended part of the plasma because the width of the white ring is almost the same as that of the lateral extension part of the plasma shown in Fig. 3(1).This result suggests that the laterally extended part of the plasma does not generate H 2 O 2 .High-energy electrons in the plasma that reaches the test strips lose energy when colliding with the H 2 O molecules on the surface of the test strips.The laterally extended part of the plasma has few high-energy electrons and is incapable of generating H 2 O 2 .

B. Dependence on gas flow rate
Figure 5 shows a plasma jet irradiated onto the H 2 O 2 test strips for different gas flow rates.For a gas flow rate of 3 L/min, streamers were not uniformly distributed, and those localized at the right side reached the test strips.For gas flow rates of ≥5 L/min, however, the streamers were uniformly distributed and reached the test strips.
Figure 6 shows photographs of the H 2 O 2 test strips irradiated with plasma jets for different gas flow rates.For a gas flow rate of 3 L/min, an inner ring with a diameter of ∼6 mm and an outer ring with an inner diameter of ∼8 mm and an outer diameter of ∼13 mm were formed in the plasma-irradiated region, confirming a double-ringshaped distribution.For a gas flow rate of 5 L/min, an inner ring with a diameter of ∼7 mm and an outer ring with an inner diameter of ∼9 mm and an outer diameter of ∼14 mm were formed, also confirming a double-ringshaped distribution.For a gas flow rate of 7 L/min, an inner ring with a diameter of ∼7 mm and an outer ring with an inner diameter of ∼11 mm and an outer diameter of ∼18 mm were formed.For a gas flow rate of 10 L/min, an inner ring with a diameter of ∼7 mm and an outer ring with an inner diameter of ∼13 mm and an outer diameter of ∼27 mm were formed.These results indicate that theconcentration distribution of H 2 O 2 generated by plasma jet irradiation depends on the gas flow rate.The width of the white ring increased with gas flow rate: it was ∼0.5 mm for 3 L/min, ∼1 mm for 5 L/min, ∼2 mm for 7 L/min, and ∼3 mm for 10 L/min.The width of the outer ring also increased with gas flow rate: it was ∼2 mm for 3 L/min, ∼3 mm for 5 L/min, ∼4 mm for 7 L/min, and ∼7 mm for 10 L/min.
The H 2 O 2 concentrations in the inner and outer rings were 10-30 and 1-3 mg/L for 3 L/min, ∼30 and 1-3 mg/L for 5 L/min, ∼30 and 1-3 mg/L for 7 L/min, and ∼30 As shown in Fig. 6(1), the lower right region of the inner ring expands, which corresponds to the localization of thestreamers at the right side shown in Fig. 5(1).Therefore, this observation supports our assumption that the inner ring is formed because the streamers flowing along the inner wall of the quartz tube reach the test strips.
As explained previously, the widths of the white and outer rings increased with gas flow rate.This may be because the amount of plasma that extends in the lateral direction is proportional to the gas flow rate.As the gas flow rate increases, H 2 O 2 generated in the boundary between the plasma jet and the immediately adjacent air is transported farther.http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) Volume 13 (2015) Kuwahata and Yamaguchi

C. on irradiation distance
Figure 7 shows a plasma jet irradiated onto H 2 O 2 test strips at different irradiation distances.For an irradiation distance of 2 mm, streamers flowing along the inner wall of the quartz tube reached the test strips in a straightforward manner.For an irradiation distance of 5 mm, streamers also reached the test strips in the same way.For an irradiation distance of 10 mm, streamers were localized around the centerline outside the quartz tube and reached the test strips.For an irradiation distance of 20 mm, streamers were localized around the centerline outside the quartz tube and reached the test strips while extending in the lateral direction.The color of the plasma near the test strips was purple.For an irradiation distance of 30 mm, streamers were localized around the centerline outside the quartz tube and the ends of the streamers just reached the test strips.For an irradiation distance of 40 mm, no streamers reached the test strips.These results indicate that how the plasma jet (streamers) reaches the test strips differs depending on the irradiation distance.The convergence of the streamers observed for irradiation distances of ≥10 mm may be because the speed of the plasma flow in the quartz tube was highest along the centerline and decreased for the regions closer to the inner wall of the quartz tube owing to the increased friction between the plasma and the quartz tube.
Figure 8 shows photographs of the H 2 O 2 test strips irradiated with a plasma jet at different irradiation distances.At an irradiation distance of 2 mm, an inner ring with a diameter of ∼7 mm and an outer ring with an inner diameter of ∼13 mm and an outer diameter of ∼27 mm were formed in the irradiated region, confirming a doublering-shaped distribution.The region outside the outer ring was light blue.At an irradiation distance of 5 mm, a double-ring-shaped distribution was similarly observed: an inner ring with a diameter of ∼7 mm and an outer ring with an inner diameter of ∼13 mm and an outer diameter of ∼21 mm were formed.At an irradiation distance of 10 mm, no double-ring-shaped distribution was observed but a blue circle with a diameter of ∼6 mm was formed in the irradiated region (referred to as the inner circle).Around the inner circle, another blue circle with an outer diameter of ∼10 mm was formed (referred toas the outer circle).At an irradiation distance of 20 mm, an inner circle with a diameter of ∼5 mm was formed, around which an outer circle with an outer diameter of ∼10 mm was formed.At an irradiation distance of 30 mm, a light blue inner circle with a diameter of ∼8 mm was formedin the irradiated region.At an irradiation distance of 40 mm, the vicinity of the plasma-irradiated region became slightly light blue.These results also indicate that the concentration distribution of H 2 O 2 generated by plasma jet irradiation depends on the irradiation distance.The H 2 O 2 concentration distributions were double-ring-shaped for irradiation distances of ≤5 mm and circular for irradiation distances of 10-30 mm.
At an irradiation distance of 2 mm, the H 2 O 2 concentrations were ∼30 mg/L in the inner ring, ∼10 and ∼3 mg/L at the inner and outer sides of the outer ring, respectively, and ∼1 mg/L in the region outside the outer ring.At an irradiation distance of 5 mm, the H 2 O 2 con-  jet did not reach test strips but they turned slightly blue, probably because H 2 O 2 generated in air fell onto the test strips.
Kawasaki et al. [32] reported experimental results similar to our results shown in Figs.8(3) and 8(4).They determined the distribution of the oxidizing substances generated around a He plasma jet under atmospheric pressure usinga gel visualization reagent and found that their concentration was low inside the plasma jet, highest in the immediatevicinity of the plasma jet, and decreased with increasing distance from the plasma jet.However, they did not identify the oxidizing substances.The reason why no white ring was formed in the cases in Figs.8(3) or 8(4) may be that the plasma reaching the test strips did not extend in the lateral direction because of the large irradiation distance.

D. Dependence on applied voltage
Figure 9 shows a plasma jet irradiated onto H 2 O 2 test strips atdifferent applied voltages.For applied voltages of 5 and 6 kV, streamers did not reach the test strips but weak blue-purple light did.At an applied voltage of 7 kV, however, streamers converged near the end of the quartz tube and reached the test strips.
Figure 10 shows photographs of the H 2 O 2 test strips irradiated with plasma jet at different applied voltages.At an applied voltage of 5 kV, only a blue outer ring with an inner diameter of ∼12 mm and an outer diameter of ∼20 mm was formed and no double-ring-shaped distribution was observed.At an applied voltage of 6 kV, no double-ring-shaped distribution was observed and only a blue outer ring with an inner diameter of ∼12 mm and an outer diameter of ∼22 mm was formed.At an applied voltage of 7 kV, however, an inner ring with a diameter of ∼7 mm and a blue outer ring with an inner diameter of ∼13 mm and an outer diameter of ∼26 mm were formed, showing a double-ring-shaped distribution.The inside of the inner ring was also blue.These results indicate that the concentration distribution of H 2 O 2 generated by plasma jet irradiation depends on the applied voltage.The distribution was ring-shaped at applied voltages of ≤6 kV and double-ring-shaped at ≥7 kV.The width of the outer ring increased with applied voltage: it was ∼4 mm at 5 kV, ∼5 mm at 6 kV, and ∼6 mm at 7 kV.In contrast, the diameter of the white circle where H 2 O 2 was not generated remained almost unchanged: it was ∼12 mm at 5 and 6 kV and ∼13 mm at 7 kV.
The H 2 O 2 concentration in the outer ring increased with applied voltage: it was ∼1-3 mg/L at 5 kV, ∼3 mg/L at 6 kV, and ∼10 mg/L at 7 kV.The H 2 O 2 concentration in the inner ring was ∼30 mg/L at 7 kV.The H 2 O 2 concentration inside the inner ring was ∼10-30 mg/L.This may be due to H 2 O 2 generated because the streamers converged near the end of the quartz tube, as shown in Fig. 9(3), and reached the test strips.
No inner ring was formed at 5 and 6 kV, probably because the streamers did not reach the test strips.This result supports the fact that the inner ring is formed because streamers flowing along the inner wall of the quartz tube reach the test strips.In contrast, it was confirmed that the outer ring was formed even if the streamers do

IV. CONCLUSIONS
To clarify the two-dimensional concentration distribution of H 2 O 2 generated by atmospheric-pressure Ar plasma jet irradiation, the dependence of the distribution on the applied voltage, gas flow rate, irradiation distance, and irradiation time was determined semiquantitatively using H 2 O 2 test strips.The shape of the distribution of H 2 O 2 concentration depended on the combination of the applied voltage, gas flow rate, and irradiation distance and was classified four types: circular (an inner circle alone), double-circular (inner and outer circles), ringshaped (an outer ring alone), and double-ring-shaped (inner and outer rings).A circular distribution was observed when the irradiation distance was large.When the irradiation distance was small, the ring-shaped distribution was observed for lower applied voltages, and the doublering-shaped distribution was observed for higher applied voltages.
An inner circle and ring were formed owing to H 2 O 2 generated because the streamers in the plasma jet reached the H 2 O molecules on the test strips.Anouter circle and ring were formed because H 2 O 2 was generated at the boundary between the plasma and the immediately adjacent air, transported towards the outside by the plasma (that was ejected from the quartz tube, reached the test strips, and extended laterally), and fell onto the test strips.The white ring was formed because of the laterally extended plasma where no H 2 O 2 was generated.
From these results, we confirmed that when a wet region is irradiated with an Ar plasma jet under atmospheric pressure, H 2 O 2 is generated not only in the irradiated region but also around the region.

4 .
To determine the dependence of the concentration distribution of H 2 O 2 on the applied voltage, the applied voltage was varied in the range of 5-7 kV (gas flow rate, 10 L/min; irradiation distance, 2 mm; irradiation time, 30 s).