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Potential generation of nano-sized mist by passing a solution through dielectric barrier discharge

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Plasma generation and its optical characteristics

The applied voltage started to increase from − 2.0 kV to + 9.3 kV in 15 µs (Fig. 2a). The voltage then decreased to − 3.2 kV, oscillated, and gradually rose to − 2 kV, and the next pulse was applied. During the application of the voltage pulse, there were an oscillated displacement current of up to about 0.03 A and several current pulses corresponding to the plasma discharge. The discharge current pulses were seen in both positive and negative polarities, with a maximum of about 0.04 A (in absolute value).

Figure 2
figure 2

Characteristics of plasma generated by the DBD. (Characteristics of plasma generated by the DBD.a) Waveforms of the applied voltage (blue) and discharge current (red). Black arrows indicate discharge current pulses. ()b b) Light emitted by plasma generated in the electrode unit when no solution was infused. Scale bar, 3 mm. (c) Emission spectrum of the plasma discharge. Typical peaks are attributed to the N2-SPS (C–B) and N2+-FNS (B–X).

Focusing on the plasma emission during discharge, we can see that streamers were generated from the needle, which was the driven electrode, towards the inner surface of the glass tube with the ground electrode (Fig. 2b). The points at which these streamers were formed varied intermittently (Supplementary Movie S1). The spectra of the plasma emission exhibited peaks (Fig. 2c), and they were attributed to the second positive system of nitrogen [N2-SPS; N2(C–B)] and the first negative system of nitrogen [N2+-FNS; N2+(B–X)]19 19. These emission peaks are commonly found in atmospheric pressure plasma discharge20..

The discharge appeared to be of a DBD type, based on the observation of the discharge current, the light emission, and optical characteristics.

Dynamics and discharge characteristics in nano-sized mist

We then infused UPW into the dielectric barrier particles electrode unit, and succeeded in generating a nano-sized mist to be blown out from the tip of the applying electrode (Fig. 3 and Supplementary Movie S2). Based on the images taken with a 10 × or 100 × microscope lens, we measured the pixel size for relatively large mist particles and calculated the actual particle size by scale calibration. Particles clearly visible ranged in size from 5 to 50 μm (Fig. 3b, c). This study has the However, most of the mist particles was smaller than the minimum detectable limit. [at least 400 nm or less; the resolution of the optical system with 100 × microscope lens used in this study (Fig. 3c)]The space where the plasma is generated, ie, the field where the nano-sized mist is produced, has many molecules that are biased toward the same polar charge as the mist particles. . After the droplets flowing out of the needle tip were atomized by Coulomb repulsion similar to electrostatic spraytwenty onethey may have interacted with the surrounding plasma (molecules biased toward the same polarity charge) and then changed into even finer particles. The efficiency of mist generation depended on the infusion rate of the solution. Although the mist was generated even under conditions of high infusion rate (≥ 500 µL / min for UPW and PBS; ≥ 100 µL / min for castor oil, respectively), droplet formation at the tip of the unit occurred preferentially (Supplementary Fig. S1). The discharge characteristics at the time of mist generation exhibit that a number of discharge current pulses were induced during applying the voltage pulse (Supplementary Fig. S2). The discharge current pulse was larger (up to about 0.06 A) than in the case where no solution was infused.

Figure 3
figure 3

Nano-sized mist generated by passing a solution through DBD. (a) Time-course images of the UPW mist captured with a macro lens, visualized at 240 fps. The infusion rate of UPW was 50 µL / min. These images were obtained from Supplementary Movie 2. Scale bar, 5 mm. (b b) Magnified high-speed images of the UPW mist taken with the 10 × microscope lens. Scale bars, 100 µm. (c) Magnified high-speed images of the UPW mist taken with the 100 × microscope lens. Scale bars, 5 µm. These images show a single frame taken at a shutter speed of 1/8000 s and an ISO sensitivity of 8000.

The developed nano-size mist generator can be applied to atomization not only of UPW but also of PBS with high electrical conductivity and castor oil with high resistivity (Fig. 4 and Supplementary Movies S2, S3, and S4). There was no significant difference. We could see some differences in the generation efficiency of mist itself and its finer size depending on the solution type, but it was possible to produce a mist potentially in the nanometer size. range. The discharge characteristics of each solution in mist generation showed that discharge current pulses occurred as in the case of UPW (Fig. 5). The pulses in PBS were larger than those in the other solutions, exceeding 0.1 A. In PBS, the pulse width of the applied voltage was stretched by about 5 µs, and the oscillations in both voltage and current were smaller. The pulse widths of the discharge current for castor oil were, in contrast, smaller than t these differences in the discharge characteristics might be due to the electrical conductivitytwenty twoThis notion is supported by the fact described below that there is a marked difference in physical and chemical characteristics between UPW or castor oil, which has a low conductivity23,24We attribute that the physicochemical characteristics and the plasma-generated chemical species caused. We attribute that the physicochemical characteristics and the plasma-generated chemical species caused. differences in the discharge characteristics and ease of generating fine mist of each solution.

Figure 4
figure 4

Nano-sized mist generated from each solution. Time-sequential images of the mist captured with a macro lens, visualized at 240 fps. The infusion rate of (a) PBS or (d) castor oil was 50 µL / min. These images were obtained from Supplementary Movies 3 and 4. Scale bar, 5 mm. Gamma correction is applied to the image (d) Because the castor oil mist particles are too fine to be seen. Magnified images of the mist generated from (b b) PBS or (e) castor oil, taken with the 10 × microscope lens. Scale bars, 100 µm. (c) Magnified high-speed images of the PBS mist taken with the 100 × microscope lens. Scale bars, 5 µm. These images show a single frame taken at a shutter speed of 1/8000 s and an ISO sensitivity of 8000.

Figure 5
figure 5

Comparison of waveforms of the applied voltage (blue) and discharge current (red) when each solution is infused at a flow rate capable of generating nano-sized mist. Black arrows indicate discharge current pulses. UPW, ultrapure water; PBS, phosphate buffered saline ..

Physical and chemical characteristics of solutions from which the nano-sized mist was produced

After passing UPW through the DBD, its pH value decreased from 6.5 to below 3.0, with some differences depending on the infusion rate (Fig. 6a). In contrast, the pH of PBS slightly decreased from 7.4 to about 6.8 due to the interaction with the plasma (Fig. 6b). The conductivity of each solution tended to increase with changes in its pH value, and reached the maximum value of 7.9 mS / cm (UPW, Fig. 6c) or 18.6 mS / cm (PBS, Fig. 6d), respectively, at the infusion rate of 50 µL / min. The changes in these characteristics can be attributed to the influence of chemical species produced in each solution by interaction with the plasma.

Figure 6
figure 6

Physical characteristics of the nano-sized mist generated by the DBD. (a, b) The pH values ​​of UPW and PBS before and after passing through the plasma discharge at each infusion rate (mean ± SD, n = 5). ((c, d) Electrical conductivity of UPW and PBS before and after passing through the plasma discharge at each infusion rate (mean ± SD, n= 5). UPW, ultrapure water; PBS, phosphate buffered saline. NT, samples without plasma treatment (no treatment).

Various chemical reactions occurred at the gas–liquid interface where plasma discharge occurs, leading to the formation of oxidative products in the solution18 18Our plasma source also produced oxidative compounds such as hydrogen peroxide, nitrite ion, and nitrate ion in each solution (Fig. 7). The amount of hydrogen peroxide produced in UPW increases with decreasing its infusion rate, reaching a maximum value (446.4 ± 19.0 mg / L) in the rate of 100 µL / min, and the produced amount (314.4 ± 17.6 mg / L) decreases under the condition with the lowest rate (50 µL / min) (Fig. 7a). Hydrogen peroxide is produced by the coupling of hydroxyl radicals, which are generated from water molecules in solution by the plasma, as shown in the following chemical reactiontwenty five..

$$ { text {OH}} cdot , + { text {OH}} cdot , to { text {H}} _ {{2}} { text {O}} _ {{ 2}} $$

Hydrogen peroxide also acts as an oxidant under acidic conditions as followed26,27..

$$ { text {H}} _ {{2}} { text {O}} _ {{2}} + { text {2H}} ^ {+} + { text {2e}} ^ { -} to { text {2H}} _ {{2}} { text {O}} $$

This half-reaction of hydrogen peroxide as an oxidant might induce a decrease in its produced amount at the 50 µL / min condition. This notion is supported by the result that the concentration of hydrogen peroxide in PBS, which had a small change in pH value. On the other hand, nitrite and nitrate ions are produced due to the reaction of nitric oxide, hydrogen peroxide, and hydroxyl radicals generated by the plasma, as shown in the following reactions.28 28..

$$ begin {aligned} & { text {NO}} + { text {OH}} cdot , + M to { text {HNO}} _ {{2}} + M \ & { text {NO}} _ {{2}} + { text {OH}} cdot , + M to { text {HNO}} _ {{3}} + M \ & { text { 2NO}} _ {{2}} + { text {H}} _ {{2}} { text {O}} to { text {HNO}} _ {{3}} + { text { HNO}} _ {{2}} \ end {aligned} $$

Here, Mindicates the third body, which is typically H2O. The generated nitric acid and nitrous acid dissociate in the solution to yield nitrite and nitrate ions29..

$$ begin {gathered} { text {HNO}} _ {{2}} , leftrightarrows , ​​{ text {H}} ^ {+} + { text {NO}} _ {{2} } ^ {-} hfill \ { text {HNO}} _ {{3}} , leftrightarrows , ​​{ text {H}} ^ {+} + { text {NO}} _ {{ 3}} ^ {-} hfill \ end {gathered} $$

The smaller the infusion rate of the solution, the more nitrogen compounds were produced, with some exceptions. At the rate of 1000 µL / min, the concentration of the produced nitrite ions was 6.80 ± 0.75 mg / L in UPW (Fig. 7c) or 41.0 ± 3.3 mg / L in PBS (Fig. 7d), while it was less than 0.15 mg / L under other rate conditions. Nitrate ions were most abundantly produced under the condition of 50 µL / min solution infusion rate, and their concentrations. were 2759.2 ± 489.5 mg / L in UPW (Fig. 7e). In PBS, the amount of nitrite ions produced was large, and they must be reduced and converted to nitrate ions to measure the produced amount of nitrate ions independently. The reduction agent We therefore measured the total amount of nitrite and nitrate ions generated in PBS. As a result, their concentration was 774.0 ± 122.1 mg / L (Fig. 7f). The reason why there are fewer oxidative compounds prod uced by the plasma in PBS than those in UPW is probably because phosphoric acid compounds such as the disodium hydrogen phosphate and potassium dihydrogen phosphate in PBS acted as scavengers for them. depend on the presence or absence of buffering related to changes in pH value of each solution.

Figure 7
figure 7

Chemical species in the nano-sized mist generated by the DBD. Concentrations of dissolved (a, d) H2O2(b, e) NO2(c) NO3and (f) NO2 + NO3in UPW or PBS after passing through the plasma discharge at each infusion rate (mean ± SD, n= 5). UPW, ultrapure water; PBS, phosphate buffered saline.

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