22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Measurement of hydroxyl radicals generated by pulsed atmospheric pressure
plasma jet in liquids
S.J. Kim, H.M. Joh, E.J. Baek and T.H. Chung
Department of Physics, Dong-A University, KR-604-714 Busan, South Korea
Abstract: An atmospheric-pressure plasma jet source driven by a pulsed bipolar wave of
several tens of kilohertz was designed and characterized. This source exhibits plasma
stability maintaining efficient reaction chemistry and low gas temperature. The hydroxyl
radicals dissolved in the liquid were monitored indirectly in various operating conditions.
Keywords: non-thermal plasma, plasma treatment, plasma source
1. Introduction
Non-thermal plasmas have received great attention in
plasma research due to their immense potential in
biomedical applications [1]. A plasma jet, in which
atmospheric-pressure plasma in the form of a bullet is
released as a train of bullets into the atmosphere, is
notable as it enables the generation of a stable discharge
that transports reactive species beyond the plasmagenerating electrode [2]. Due to the possibility of a
targeted application, development of new jet devices has
been one of the most important elements that have
contributed to the rapid advancement of technologies in
plasma medicine. Each plasma-generated agent should be
identified and quantitatively measured. The radical
species emitted by a gas-plasma operation are reactive
nitrogen species like nitric oxide (NO) and nitrogen
dioxide (NO 2 ) and reactive oxygen species like ozone
(O 3 ), hydroxyl radicals (OH-), superoxide (O 2 -), and
singlet oxygen (1O 2 ) mainly [3]. 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 generation of
reactive radicals within the liquid medium, before
reaching cells and tissues [4]. The interactions between
the plasma and the liquid are of great interest and
importance, and the generation of plasmas at the interface
of or inside liquids has been investigated for a range of
applications [5]. In the presence of moisture, the
hydroxyl-radical (OH) is one of the most active species
generated in gas mixtures and so reactive that it is thought
to react with essentially any target [1]. 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 [6]. Measuring the absolute
density of OH species will improve the adjustment of
treatment doses, and allow for optimization of the plasma
process for a specific application. In this work, the
properties of a specially designed helium plasma jet
driven by a pulsed bipolar wave of several tens of
kilohertz are reported. The OH radicals dissolved in the
liquid were monitored indirectly using the fluorescent
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properties of hydroxyterephthalic acid (HTA) formed by
the reaction of terephthalic acid (TA) with OH radicals.
2. Materials and methods
Fig. 1 shows a photograph of the plasma plume and the
schematic of the experimental setup of the jet source
driven by a pulsed bipolar high voltage with a repetition
rate of 50 kHz (FT-Lab PDS 4000). The plasma jet
consists of copper wire electrodes (2 mm diameter),
PEEK plastic housing, and a pencil-shaped ring-grounded
electrode (copper, 6 mm inner diameter and 16 mm outer
diameter at the exit) covered with dielectric PEEK. The
wire is inserted coaxially in the housing and covered with
a Teflon tube, leaving a length of 6 mm of the wire
exposed to gas (being surrounded by the grounded
electrode). A pulsed bipolar source is applied to the
copper wire. The helium gas is controlled by a flow meter
(Kofloc RK1600R). The optical emission spectra were
recorded from the jet in the wavelength range of 200 to
1000 nm using a fiber optic spectrometer
The plume
(USB-2000+XR1-ES OceanOptics).
temperature was measured using a fiber optic temperature
sensor (Luxtron, M601-DM&STF).
Fig. 1. Schematic of experimental setup and photograph
of plasma plume.
1
As a method of OH radical detection, we utilized the
hydroxylation of terephthalic acid (TA), which is a typical
photocatalytic reaction that specifically oxidizes TA.
That is, the OH radical reacts with TA to form
hydroxyterephthalic acid which fluoresces. When the
solution containing TA and HTA molecules is irradiated
by UV light, the HTA molecules emit light at λ = 425 nm,
while the TA molecules do not [7].
From the
fluorescence intensity, the amount of OH radicals could
be estimated in plasma-treated liquids.
Fig. 3(a) shows the fluorescence spectra of HTA as a
function of the treatment time. As the time increased, the
fluorescence intensity increased, indicating an increase in
the total amount of OH radicals trapped by TA. It was
observed that a mixture containing 95% helium and 5%
oxygen led to an increase of the fluorescence intensity
(Fig. 3b). Since the various discharge parameters and
ambient gas can influence the amount of OH radicals that
can cause different plasma-induced chemistry effects,
further experimental investigations are needed to consider
the effects of variable plasma properties.
3. Results and Discussion
Fig. 2a shows the measured gas temperature as a
function of applied voltage.
The applied voltage,
excitation frequency, and gas flow rate were
1.1 - 1.8 kV pp , 50 kHz, and 2 L/min, respectively. With
an increase in the applied voltage, the plume temperature
was increased. The temperature was less than 35 ℃
despite the high applied voltage. Fig. 2b illustrates the
optical emission spectra recorded. The optical spectra of
plasma exhibited an enhanced intensity level of highly
reactive radicals, such as OH, NO, O, and H α . The
richness of these species, such as ROS and RNS, in our
jet source could provide efficient applications in various
plasma treatments.
Fig. 3. Fluorescence spectra of aqueous TA solutions
exposed to pulsed discharge as function of (a) treatment
time, and (b) additive oxygen flow rate.
Fig. 2. (a) Measured gas temperature as function of
applied voltage, and (b) optical emission spectra.
2
4. References
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(2012)
[2] X. Lu, M. Laroussi and V. Puech. Plasma Sources
Sci. Technol., 21, 034005 (2012)
[3] J. Liebmann, et al. Nitric Oxide, 24, 8 (2011)
[4] M.G. Kong, M. Keidar and K. Ostrikov. J. Phys. D:
Appl. Phys., 44, 174018 (2011)
[5] D. Mariotti, et al. Plasma Process. Polymers, 9,
1074 (2012)
[6] X.Y. Liu, et al. Phys. Plasmas, 21, 093513 (2014)
[7] S. Kanazawa, et al. Plasma Sources Sci. Technol.,
20, 034010 (2011)
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