22G01
Plasma-Induced Chemical Processing in Solution by LF plasmajet
LFプラズマジェットを用いたプラズマ誘起液中化学プロセス
Katsuhisa Kitano 1, Satoshi Ikawa 2, Atsushi Tani 3,Tsuyoshi Kitamura 4, Ryuichi Arakawa 4, Yusuke Ono 1
北野勝久 1、井川聡 2、谷篤史 3、北村豪 4、荒川隆一 4、大野友輔 1
1

Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University,
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
2
Technology Research Institute of Osaka Prefecture
2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan3
Department of Earth and Space Science, Graduate School of Science, Osaka University
1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
4
Faculty of Chemistry, Materials and Bioengineering, Kansai University
3-3-35 Yamatecho, Suita, Osaka, 564-8680, Japan１
大阪大学工学研究科 原子分子イオン制御理工学センター 〒565-0871 大阪府吹田市山田丘2-1
２大阪府立産業技術総合研究所、３大阪大学理学研究科、４関西大学化学生命工学部
Atmospheric pressure plasmajet with low gas temperature has been produced by a simple system with adielectric tube through which He gas flows and a single electrode to which LF (Low Frequency)
high-voltage pulses (~10kV, ~10kHz) are applied. Supplied active species from air to liquid induce thechemical reactions in liquid. They were measured by ESR (electron spin resonance) and MS (MassSpectrometry). The formation of free radicals in liquid is discussed from various perspectives. This
attractive reaction fields have been utilized for biomaterial synthesis, surgery treatment and so on.

1. Introduction
Recent development of atmospheric plasmas with
low temperature has enabled non-conventional
plasma processing [1]. In this paper, formation
mechanisms of free radicals inside liquid by
non-thermal plasma jet are discussed.
Low gas temperature atmospheric pressureplasmas can be produced by a high-voltage (HV)
low-frequency (LF) power supply in the range ofkV and kHz in a tube where a discharge gas(typically He or Ar) flows. A schematic illustrationof the system is shown in Fig. 1. We call it a LF
microplasma jet as the plasma seemingly extends
from the tip of the tube into ambient air. Partial
discharges between the HV electrode and theelectrical ground in the surroundings occur in theHe/Ar gas flowing into the air [2]. Due to its lowgas temperature, as shown in Fig.2, this type of jets
is desirable for plasma processing in liquid.
We have successfully found that efficient
bactericidal activity can be achieved if the solution
is sufficiently acidic [3]. It is interesting to note that
there is a critical pH value of about 4.7 for thebactericidal effects, below which the bacteria areefficiently inactivated and above which the bacteria
are hardly affected by the plasma application. It has
been also found experimentally that the presence ofsuperoxide anion radicals (O2-•) in the solution isessential for bacterial inactivation by the plasmaapplication. The critical pH value may be associatedwith pKa of the dissociation equilibrium between
O2-• and hydroperoxy radicals (HOO•), which isknown to be approximately 4.8.

He gaselectrodeplasma jetquartz tube
HV power supply
Fig. 1 Schematic diagram of LF micro plasma jetwith a single electrode.

Fig. 2 Plasma jet exhausted to a finger withoutburning.

The fundamental question regarding such new
chemical reaction environments is what kinds ofspecies in plasmas cause these reactions. We havestudied free radical formation in water after plasmaexposure, using electron spin resonance (ESR) witha spin-trapping method [4]. Some atomic species in
the gas phase are converted to free radicals byplasma application. In this paper, we discuss about
the formation mechanisms of O2-•, which plays animportant role in the bacterial inactivation in thesolution by the plasma application.
2. Experimental Results
It is known that O2-• in the solution become
extinct by disproportionation reaction and its halflife time can be longer in alkaline solution. Thismeans that O2-• can be trapped in alkali solution.
We applied this alkali-trapping method for ESR
measurements, instead of spin-trapping method, forhigh sensitive measurement. ESR measurement wasdone at liquid-nitrogen temperature with X-band
ESR system (JEOL, JES-FA100).
The plasma exposure experiments were
performed against alkali solution (pH14) in the caseof contact and non-contact (distance is 30mm) with
LF plasmajets. ESR spectra after plasma exposure
are shown in Fig. 3. This shows the existence ofO2-• with clear signals. Unlike the usual chemical
spin-trapping method, trapping efficiency with thealkali-trapping method is theoretically almost unity.
In addition of the strong signal in the case ofcontact, weaker signal was seen also in that ofnon-contact. The fact suggests that the existence ofsome active species in the air.
To clarify the above experimental results, the air
100mm apart from the tip of the plasma jet wasanalyzed by MS (Mass Spectrometer) withdifferential pumping. As seen in Fig.4, the clear
peak signal of O2- • hydrated with one watermolecule is observed at m/z = 50. This shows theexistence of O2-• in the air. This is a sort of air ion.

Fig. 3 ESR spectra of the solution samples in thecases of contact and non-contact with the plasma.

Fig. 4 MS spectrum of the air 100mm apart from LF
plasmajet.

3. Summary
We developed the alkali trapping ESR
measurement method for O2- • formed in thesolution by the plasma exposure. Using thishigh-sensitive method, the O2-• was observed evenin the solution which is not in contact with theplasma. In the air, the existence of O2-• is also
confirmed with MS. These experimental results
strongly suggest that the air ions of O2-• are formed
from the air around the plasma and they aretransported into the solution.
In this paper, the generation of O2-•is focused
because of its interesting characteristics concerning
about plasma disinfection. Another active species
are also generated in gas and supplied to liquid. We
conclude that it is important to clarify the keyspecie(s) for each application and their fundamental
reaction, and to optimize their generation.
Acknowledgements
This study was supported by a Grant-in-Aid for
Scientific
ResearchonPriority
Areas“Microplasma” and a Grant-in-Aid for Young
Scientists (A) from MEXT, and A-STEP (JST).
References[1]K. Kitano, H. Furusho, Y. Nagasaki, S. Ikawa, S.
Hamaguchi,
ProceedingsofInternational
Conference on Phenomena in Ionized Gases
XXVIII,(Prague, July 2007), p. 1131-1134,(2007).
[2] K. Kitano and S. Hamaguchi, Proc. The 4thInternational Workshop on Microplasmas,(October 28-31, 2007, Tainan Taiwan), FP-017,55-56 (2007).
[3] S. Ikawa, K. Kitano, S. Hamaguchi, Plasma
Process. Polym., 7, 1, pp.33, (2010).
[4] A. Tani K. Kitano, K. Mizotani, S. Ikawa, S.
Hamaguchi, Proc. of Int. Interdisciplinary-Symposium on Gaseous and Liquid Plasmas,(ISGLP, Sep. 5-6, Sendai, Japan), p263, (2008).