22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Monitoring of OH generation at the gas-liquid interface by atmospheric
pressure helium plasma jet
H.M. Joh, E.J. Baek, S.J. Kim and T.H. Chung
Department of Physics, Dong-A University, KR-604-714 Busan, South Korea
Abstract: It has been known that plasma-induced reactive oxygen species (ROS) in the
gas phase could result in intracellular ROS generation and apoptotic cell death. It remains
unclear if there is plasma-induced ROS generation in the liquid phase environment. In this
work, an atmospheric pressure plasma jet was applied on the liquid containing cells. We
investigate the effect of the operating parameters such as applied voltage, repetition
frequency, duty ratio of a pulsed atmospheric pressure plasma jet (APPJ) on the generation
of OH radicals at the liquid surface.
Keywords: atmospheric pressure plasma jet (APPJ), reactive oxygen species (ROS),
hydroxyl (OH)
1. Introduction
Research on the applications of atmospheric pressure
plasmas (APPs) has rapidly expanded to biology and
medicine. The use of APPs in cancer therapies is drawing
an especially great amount of attention because plasmas
contain short lived free radicals, including reactive
oxygen species (ROS), charged species, and electric fields,
that can induce apoptosis in cancer cells [1]. Several
studies have reported that plasma-induced ROS in the gas
phase could result in intracellular ROS generation and
apoptotic cell death [2-4]. It remains unclear if there is
plasma-induced ROS generation in the liquid phase
environment [5]. In practice, diseased living tissues are
either moist or covered by a layer of liquid. When a
plasma jet is used to treat a living tissue, its plasma
species are delivered to the air-liquid interface and then
undergo transportation, and sometimes secondary
ROS/RNS generation within the liquid medium, before
reaching cells and tissues. In the previous work, we
observed that the richness of ROS may make plasma
operating condition produce better apoptotic rate [4].
Therefore, each plasma-generated agent that may have
biological implication should be identified and
quantitatively measured. These chemical species which
include O, O 2 -, OH, NO, and NO 2 exhibit strong
oxidative stress and/or trigger signalling pathways in
biological cells. It is widely known that OH plays an
important role in plasma chemistry among others [5]. In
this study, atmospheric pressure plasma jet was applied
on the surface of cell-containing liquid. The OH
generation in both the gas phase and liquid surface were
measured by optical emission spectroscopy. Since the
OH radicals strongly depend on the plasma control
parameter, we investigate the effect of the operating
parameters such as applied voltage, repetition frequency,
duty ratio of atmospheric pressure plasma jet (APPJ) on
the OH radical generation at the gas-liquid interface.
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2. Jet device and experimental setup
An atmospheric pressure plasma jet was fabricated and
characterized to provide a plasma environment well suited
for living tissue treatment. The inner cylindrical quartztube and outer quartz-tube with tapered-end were
employed. The helium plasma was generated by a pulsed
bipolar source of up to 13 kV with a repetition frequency
of several tens of kilohertz (APP020 EESYS). The
electrical characteristic of the plasma jets was measured
by a real time digital oscilloscope (WS44Xs-A LeCroy)
via high voltage probe (PPE 20 kV LeCroy) and current
probe (4100 Pearson). The optical characteristics of the
discharges were obtained by means of optical emission
spectroscopy (USB-2000+XR-ES OceanOptics) to
identify various excited plasma species produced from the
plasma jet.
To estimated OH radical density at the liquid surface,
the plasma jet was applied on the liquid and the spectra
were obtained using two different methods (emission
spectroscopy and ultraviolet absorption spectroscopy) as
shown in Figs. 1 and 2.
Fig. 1. Experimental setup for measurement of optical
transmission intensity into the liquid.
1
accelerates the decomposition of ozone and increases the
OH radical concentration in water [9].
-0.05
ln( Iv/ I0 )
-0.10
Fig. 2. Experiment setup of ultraviolet absorption
spectroscopy.
-0.15
-0.20
-0.25
0.8
Fig. 1 shows the schematic experimental setup for
measurement of optical transmission into the liquid. The
optical emission from the plasma is collected using a
400 µm diameter fiber placed in the bottom of the quartz
dish (diameter 60 mm). The distance from the plasma jet
nozzle to liquid surface was 10 mm.
The schematic experimental setup for the ultraviolet
absorption spectroscopy at the liquid surface is presented
in Fig. 2. This system consists of deuterium UV lamp,
plano-convex lens, collimator lens and a fiber optic
spectrometer. The OH density can be calculated using
Lambert-Beer’s Law [6]. The incident UV light on the
liquid has the intensity ( 𝐼0 ) and the transmitted light
intensity (𝐼𝜈 ). The light intensity has been absorbed by
OH radical species during the passing through the
distance 𝑥 in the liquid. The density of OH radicals from
plasma-liquid interaction is given by
𝑁=−
1
𝜎∙𝑥
ln(
𝐼ν
𝐼0
),
where N is the density or concentration for absorbing
species of OH, σ is the cross sectional area of about
1.2×10-16 cm2 for absorbing species of OH species [6, 7].
3. Results and Discussion
Helium jet is observed to efficiently produce the ROS
in the gas phase. The discharge produces a significant
UV radiation that belongs to transitions of the OH line at
309 nm, the atomic oxygen lines at 777 and 844 nm, the
N 2 emission bands at 310 - 440 nm, and the N 2 + emission
bands at 391 - 428 nm. OH is one of the most active
species generated in moist gas mixtures. Knowing the
production mechanisms and measuring the absolute
density of OH species will help the adjustment of
treatment doses, and allow for optimization of the plasma
process for a specific application. The dominant source
of OH radicals is related to the Penning and charge
transfer reactions of H 2 O molecules with excited and
charged helium species [8]. The measured ratio of the
transmitted to incident intensities 𝐼𝜈 /𝐼0 is observed to
decrease with the distance in the liquid medium, as shown
in Fig. 3. It is found that the densities of OH radical
species are 1.1×1015 cm-3 and 2.9×1015 cm-3 inside the
D.I water and H 2 O 2 , respectively. The presence of H 2 O 2
2
1.2
1.6
2.0
2.4
2.8
x (mm)
Fig. 3. Measured values of the intensity ratio 𝐼𝜈 /𝐼0 as a
function of the distance in the liquid.
It is found that the OH density measured by the second
method is about 0.3×1015 cm-3 at the gas-liquid interface.
These data indicate that the increase of OH radicals in the
liquid phase results from the plasma-liquid interaction.
Since the OH radicals in liquid contribute to cell death, a
strong correlation between the production of OH in liquid
phase and apoptosis rate of cancer cells is expected. In
this study, we investigate the effect of the operating
parameters such as applied voltage, repetition frequency,
duty ratio of a pulsed APPJ on the generation of OH
radicals at the liquid surface. Then we compare the
generation of OH radicals with the apoptosis rate of
cancer cells [4].
4. References
[1] M. Keidar, et al. Phys. Plasmas, 20, 057101 (2013)
[2] M. Vandamme, et al. Int. J. Cancer, 130, 2185
(2012)
[3] X. Yan, et al. Plasma Process. Polymers, 9, 59
(2012)
[4] H.M. Joh, et al. Appl. Phys. Lett., 101, 053703
(2012)
[5] K. Ninomiya, et al. J. Phys. D: Appl. Phys., 46,
425401 (2013)
[6] H.P. Dorn, et al. J. Geophys. Res., 100, 7397 (1995)
[7] Y.H. Kim, et al. Plasma Chem. Plasma Process.,
34, 457 (2014)
[8] X.Y. Liu, et al. Phys. Plasmas, 21, 093513 (2014)
[9] P. Jukes, et al. Plasma Sources Sci. Technol., 23,
015019 (2014)
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