22G04
Radical Species in Aqueous Solution for Plasma-Induced Chemical Processing
プラズマ誘起液中化学プロセスにおける液中ラジカル種の診断	 
Atsushi Tani1, Yusuke Ono2, Satoshi Fukui1, Satoshi Ikawa3 and Katsuhisa Kitano2
谷	  篤史1, 大野友輔2, 福井	  智1, 井川	  聡3, 北野勝久2
1

Department of Earth and Space Science, Graduate School of Science, Osaka University
1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
大阪大学大学院理学研究科宇宙地球科学専攻	  〒560-0043	  大阪府豊中市待兼山1-1
2
Center for atomic and Molecular Technologies, Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
大阪大学大学院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
3Technology Research Institute of Osaka Prefecture
2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan
大阪府立産業技術総合研究所	  〒594-1157	  大阪府和泉市あゆみ野 2-7-1
Free radicals formed in water during atmospheric-pressure plasma processing with a low-frequency (LF)
HV power supply have been investigated by electron spin resonance (ESR) using a spin-trapping
compound, CYPMPO. Even if the end of plasma jet does not reach the surface of water sample, hydroxyl
radical (OH·) and superoxide anion radical (O2-·) are formed in the solution if surrounding gas containsoxygen. It suggests that plasma does not need to contact with liquid in plasma-induced chemical
processing.

1. Introduction
A low-frequency (LF) plasma jet [1,2] isatmospheric-pressure plasma with low gas
temperature and can be used for plasma
processing in liquids or on liquid surfaces. With
the LF plasma injected into liquids, advanced
reaction fields may be created in the liquids,
where reduction, polymerization, or sterilization
can occur [3]. Although active ions, molecules,
and radicals are formed by plasma exposure in thefields, they are usually unstable and not easily
observed.
For the medical application to control theinfection diseases, inactivation of bacteria in liquid
is distinctly important. Successful bactericidal
activity was found to be achieved if the solution is
sufficiently acidic [4,5]. It is interesting to note that
there is a critical pH value of about 4.7 for thebactericidal effects, below which the bacteria are
efficiently inactivated and above which they are
hardly affected by the plasma application. It has
been also found that the presence of superoxideanion radicals (O2–·) in water and the air is essential.
The critical pH value may be associated with pKa ofthe dissociation equilibrium between O2–· andhydroperoxy radicals (HOO·), which is known tobe approximately 4.8. This means that O2–· can bechanged into HOO·, which have much stronger
bactericidal activity, in lower pH. To clarify theelementary processes, plasma-produced activespecies were measured and their reaction tobiomacromolecules was evaluated [6]. In this paper,

we discuss about the production of radical species
in aqueous solution.
The spin trapping technique is one of methods
to observe unstable radicals in solution. Unstable(short lifetime) radicals in liquid are trapped byspin trapping compounds to form spin adductswith a longer lifetime. In previous reports, O 2-·
and hydroxyl radical (OH·) were formed in water
after plasma exposure if surrounding gas includes
oxygen gas, based on electron spin resonance(ESR) measurements [7] with a spin trapping
compound, CYPMPO [8]. Since the end ofplasma jet reached the surface of water, intense
signal intensities for both radicals were observed.
Recently, helium and oxygen mixed gases
through discharge keep high activity and can beused for sterilization [9]. This suggests that suchgases may also induce active species in water
solution, even if the plasma does not touch thesurface of water. In this work, free radicals
formed in water with non-contact plasma havebeen investigated using a spin-trapping
compound, CYPMPO with ESR.
2. Experimental
One end of the glass tube was inserted to thechamber where a glass cup was sealed with anO-ring. Oxygen gas was supplied from one of theside ports to the chamber. High voltage (~10 kHzand ~10 kV) was applied to the electrode on theglass tube 19 cm apart from the lower end of thetube. Most discharge was terminated near the otherelectrodes for electric ground and did not reached tothe chamber where oxygen flowed into the system.
This means that meta-stable helium produced byplasma may react with oxygen gas in the chamber.
After purging gas in the camber with helium andoxygen, the plasma discharge was kept for 3 min.
The flow rates were controlled by flow controller(KOFLOC, 8500MC).
The sample was aqueous solution of a spintrapping compound, CYPMPO (5-(2,2-dimethyl1,3-propoxycyclophosphoryl)-5-methyl-1-pyrrolineN-oxide). Reagent of CYPMPO was supplied byRADICAL RESEARCH and used without any
further purification. ESR measurements werepreformed at room temperature with a flat glass cellusing the X-band ESR system (JEOL, JES-FA200).
HeElectrode~10 kV

PlasmaGlass tube

ExhaustO2
Glass
SampleGroundFig. 1. A schematic illustration of the experimental
setup with a straight glass tube. Oxygen wassupplied to the chamber through a side port.

335

336

337

338

339340Magnetic Field / mT

Fig. 2. Representative ESR spectrum of the solution.
All signals are due to spin adducts of hydroxyl
radical (OH·) and superoxide anion radical (O2-·).
The signals can be distinguished using four peaks inthe middle (◆ for OH· adduct and ▼ for O2-·adduct).

3. Results and discussion
Figure 2 shows a representative ESR spectrum ofthe solution. ESR signals for OH· and O 2-· adducts
are observed. It indicates that active species areinduced in water even by non-contact plasma.
The peak height for O 2-· adduct is larger thanthat for OH· adduct in non-contact plasma. This
result is strongly different from the previous one
by plasma exposure directly to the water (i.e.
contact plasma with the surface of the water)where an intense signal for OH· and a weaksignal for O 2-· [8]. This means that amounts ofplasma-induced active species in liquid maychange with the distance from the plasma
discharge to the water surface.
Both radicals are reactive oxygen species. As
their reactivity is different, suitable radicals
induced in liquid should depend on an application.
For example, sterilization in liquid with the LF jetneeds O 2-· [4]. In the case of non-contact plasma,O 2-· may produce effectively into liquid with low
yield of OH· in comparison with the case ofcontact plasma.
4. Summary
Free radicals formed in water with non-contact
plasma have been investigated by ESR using a
spin-trapping compound, CYPMPO. Hydroxyl
radical (OH·) and superoxide anion radical (O2-·)
are formed in the solution if surrounding gas
contains oxygen. Plasma does not need to contact
with liquid in plasma-induced chemicalprocessing.
References[1] 北野勝久、応用物理学会誌、77(4), pp. 383 (2008).
[2] 北野勝久、日本国特許第 4677530 号.
[3] 北野勝久、現代化学，7 月号，p25，(2009).
[4] S. Ikawa, Plasma Process. Polym., 7, pp.33, (2010).
[5] 井川聡、日本国特許第 4408957 号.
[6] E. Takai, Plasma Processes and Polymers. in press.
[7] M. Kamibayashi, Free Rad. Res. 40, 1166, (2006).
[8] 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).
[9] K. Kitano, Proc. of ISPC-20 (The 20th International
Symposium on Plasma Chemistry), Philadelphia,
July, (2011).