st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Modification of inner walls of a commercial microfluidic chip by pulsed
microplasma treatment
H. Yoshiki1
1
Tsuruoka National College of Technology, 104 Sawada, Inooka, Tsuruoka, Yamagata 997-8511, Japan
Abstract: A pulsed microplasma was used to modify the surfaces of microchannel walls of
commercial COC microfluidic chips. A He microplasma was generated throughout a
60-mm-long microchannel at a crude vacuum of 30 kPa using a discharge pulse spacing of 4
ms and Vpp of 7–10 kV. X-ray photoelectron spectroscopy revealed that the oxygen-containing
polar groups such as –C–O– and C=O were considerably introduced into the plasma-treated
inner surfaces.
Keywords: pulsed He microplasma, surface modification, COC, microfluidic channel
1. Introduction
Microfluidic chips have recently been used for biomedical applications such as protein and DNA analysis [1,
2]. Miniaturized total chemical analysis systems (μ-TAS)
[3] are commonly fabricated from polymers such as
polydimethylsiloxane and polystyrene by replica molding
as well as from planar glasses. However, the hydrophobicity of polymer surfaces makes it difficult to fill microfluidic channels with liquid samples. It is thus necessary
to modify the polymer surfaces before they can be used in
practical applications. Treatment with plasmas such as
oxygen and air plasmas prior to sealing microchannels is
commonly used to make the microchannel surfaces hydrophilic [4-6]. On the other hand, treatment by
microdischarges or microplasmas (μplasma) generated in
a sealed microfluidic channel has the potential to control
the surface chemistry and the ζ-potential of microchannels.
Several atmospheric-pressure or low-pressure μplasm
sources have been reported for modifying sealed
microchannels [7-10]. However, these methods suffer
from technical problems such as causing damage to devices due to heating or high voltage breakdown, unstable
operation, and uniform modification of long channels.
In this study, a simple and damage-free technique for
modifying sealed commercial microfluidic chips is developed by using a pulsed He μplasma source. The
wettabilities, chemical binding states and surface morphologies on the plasma-treated micro-channels are examined.
2. Experimental
Figure 1 shows a schematic diagram of the experimental setup used for μplasma treatment of sealed microfluidic channels. The commercial microfluidic chips are
made from cyclic olefin copolymer (COC) (Sumitomo
Bakelite, BS-X2650). COC has recently been used for
high-performance analytical devices in life sciences because of its high chemical resistance and optical trans-
Fig.1 Schematic diagram of the experimental setup for
modifying the inner wall of a commercial microfluidic chip
using a pulsed μplasma.
parency. An imprinted COC piece and a flat COC plate
are thermally bonded tto form a sealed microfluidic chip
(30 mm × 70 mm × 1 mm). The chip has four straight
microchannels that are 340 μm wide, 86 μm deep, and 60
mm long. Two ports (i.e., reservoirs) used to introduce
liquid samples were connected to flexible plastic tubes.
He gas was introduced into one port through the plastic
tube at a flow rate of 56 ml/min and it was exhausted
from the other port using a rotary pump. The use of He
gas may be advantageous to plasma treatment in a sealed
microchannel, because He is light and thus mobile atomic
gas and has high thermal conductivity and high specific
heat. 0.25-mm-diameter Cu wires were inserted into the
ports through the plastic tubes and the wires were connected to a pulsed power generator using a compact induction coil (DC 6V, 12 W, 0.9 kg). The waveform of the
pulsed high voltage (P. H. V.) applied to the wire electrode
is shown in Fig. 2. The peak-to-peak discharge voltage
Vpp is 7–10 kV. The discharge pulse spacing is 4 ms, including a damped oscillation for 1.5 ms. Gas pressures
were measured by a crystal gauge (Anelva, M-320XG)
and were 30 kPa and 0.82 kPa at the inlet and outlet ports,
respectively. The microchannel was kept at a crude vac-
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
uum.
Fig.2 The waveform of the pulsed high voltage applied to
the wire electrode. The duration of a damped oscillation is
1.5 ms.
To study the plasma gas phase, the light emitted from
the μplasma was analyzed by optical emission spectroscopy (Hamamatsu Photonics, PMA-11). X-ray photoelectron spectroscopy (XPS; Ulvac-Phi, ESCA 5600Ci) was
used to characterize microchannels that had been treated
by the He μplasma and had then been mechanically delaminated. A monochromated Al Kα X-ray source was
used as the excitation source. The spectrometer was operated at 350 W and 15 kV. The aperture used for the measurement had a diameter of 120 μm and the angle between
the X-ray beam and the analyzer was fixed at 45°. An
electron flood gun was used to suppress surface charging
during the XPS measurements. The surface morphologies
of inner microchannel walls after μplasma treatment were
examined by digital microscopy (KEYENCE; VHX-100)
and scanning electron microscopy (SEM; TOPCON,
ABT-32).
3. Results and discussion
Figure 3 shows a photograph of a pulsed He μplasma
generated in a commercial COC microfluidic channel.
The discharge mechanism is as follows; a corona discharge generates at a tip of a wire electrode at the first
impulse of 4 kV or more and then electrons creep across
the microchannel toward the opposite electrode. At the
next stage, a surface discharge occurs between two ports
for the succeeding damped oscillation of the applied
voltage. An intermittent discharge was generated between
two ports and a bright light was emitted. Figure 4 shows
an optical emission spectrum of the pulsed corona He
μplasma. O I (777 and 845 nm), Hα, and OH lines are
clearly visible as well as many He I lines. The He atomic
excitation temperature Texc was estimated from Boltzmann
plots obtained using He I lines (318.7, 361.3, 381.9, 388.8,
396.4, 438.7, 447.1, 471.3, 501.5, 587.5, 667.8, 706.5,
and 728.1 nm). The average Texc was approximately 9570
K. This indicates that the pulsed corona μplasma is highly
Fig.3 He pulsed μplasma generated in a COC microchannel
between two Cu wire electrodes.
energetic and that the electron temperature is over 1 eV.
Therefore, any residual moisture in a microchannel or a
gas line will be dissociated by the energetic corona discharge, producing O, H, and OH radicals. On the other
hand, an increase in the surface temperature of a
microchannel during discharge was measured by an infrared thermometer and it was 1℃ or less. Therefore, the
increase the gas was not thermalized and remained at
room temperature since the pulsed discharge had the millisecond pulse spacing, i.e., long plasma off-time. Therefore, the microchannels did not melt or fracture.
After μplasma exposure, the COC microchannels exhibited highly hydrophilic properties. Figure 5(a) and 5(b)
show the wettabilities of microchannels being in right
after and two weeks after the He μplasma treatments, respectively. In the plasma-treated microchannel, water
dyed red flowed very rapidly between the two ports.
Ageing effect on the hydrophilic properties could not be
observed. This qualitative observation of the wettability is
consistent with the introduction of oxygen-containing
functional groups onto the COC microchannel walls by O
Fig.4 Optical emission spectrum of a He pulsed μplasma
generated in a COC microchannel.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig.5 Wettabilities of COC microchannels that had and had
not been treated with He pulsed μplasma. Plasma treatment
time was 30 min. (a) right after and (b) two weeks after the
plasma treatment.
and OH radicals in the plasma gas phase. To confirm this
conjecture, the atomic content ratio of the microchannels
that had and had not been treated with the He μplasma
was evaluated by high-resolution XPS peak analysis. Table 1 shows the atomic concentrations before and after
μplasma treatment. It reveals that oxygen was introduced
to the COC surface from the oxygen-containing μplasma
(see Fig. 4) because COC contains only carbon and hydrogen. In addition, Cu was not detected in any of the
μplasma-treated microchannels. It means that the contamination due to sputtering and redeposition of the Cu
electrode material was apparently negligible for the present plasma treatment conditions. To investigate the kinds
of functional groups that were introduced to the COC
surface, deconvolution analysis of C1s peaks was performed. The inner surface after He μplasma treatment has
four potential carbon-containing components with binding
energies of 285 eV (C–C), 286.4 eV (–C–O–), 288 eV
(C=O), and 289 eV (–COO–) [11]. Table 2 presents the
deconvolution analysis results for the C 1s peak; it reveals
that the C–C content is lower after μplasma treatment,
whereas most oxygen-containing polar groups (such as
–C–O–, the carbonyl group C=O, and the carboxyl group
–COOH) increase. This result indicates that the μplasma
treatment broke some of the C–C bonds in the polymer
surface and that these broken C–C bonds recombined with
oxygen atoms producing oxygen-containing polar groups.
It is concluded that the oxygen-containing polar groups
play an important role in increasing the hydrophilicity of
the COC microchannel surface.
Finally, the surface morphologies of He μplasma
treated inner microchannels were investigated. Figure 6(a)
and 6(b) show a digital microscope and SEM images of
the inner surface of a 30-min-He-μplasma treated
microchannel, respectively. No damage was observed on
the microchannels. Furthermore, the inner surface showed
smooth and thus had optical transparency. In contrast, the
microchannel treated by a He μplasma for more than 60
min had a rough surface and thus showed milk-white as is
shown in Fig. 6(c) and 6(d). This is caused by etching by
Fig.6 (a) Digital microscope and (b) SEM images of surface
morphologies of the microchannel inner walls after He
μplasma treatment for 30 min. (c) Digital microscope and
(d) SEM images of surface morphologies of the
microchannel inner walls after He μplasma treatment over
60 min.
O radicals. It implies that there is the optimum plasma
treatment condition for COC microchannels.
4. Conclusions
A pulsed He μplasma was used to modify the surfaces
of microchannel walls in COC microfluidic chips. An
intermittent surface discharge occurred throughout a
60-mm-long microchannel at a crude vacuum of 30 kPa
using a discharge pulse spacing of 4 ms, including a
damped oscillation for 1.5 ms, and the Vpp value of 7–10
kV. The He atomic excitation temperature was approximately 9570 K, whereas the gas remained at a room temperature. Plasma-treated microchannels exhibited highly
hydrophilic properties. XPS analysis revealed that the
oxygen-containing polar groups such as –C–O– and C=O
remarkably increased on the inner surfaces after plasma
treatment. In addition, the microchannels irradiated by the
energetic corona discharge exhibited no damage and had
smooth surfaces.
Table 1. XPS elemental analysis of untreated and He
μplasma-treated COC microchannel inner walls. Plasma
treatment time was 30 min.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture,
Sport, Science and Technology of Japan. The author
thanks Dr. Y. Tanno for the use of the XPS system at
Yamagata Research Institute of Technology.
References
[1] C. T. Culbertson, S. C. Jacobson, J. M. Ramsey, Anal.
Chem., 72, 5814 (2000).
[2] L. C. Waters, S. C. Jacobson, N. Kroutchinina, J.
Khandurina, R. S. Foote, J. M. Ramsey, Anal. Chem.,
70, 158 (1998).
[3] D. J. Harrison, A. Manz, Z. Fan, H. Ludi, H. M. Wid
mer, Anal. Chem., 64, 1926 (1992).
[4] G. B. Lee, C. H. Lin, K. H. Lee, Y. F. Lin, Electropho
Resis, 26, 4616 (2005).
[5] J. Hyun, A. Chilkoti, J. Am. Chem. Soc., 123, 6943
(2001).
[6] D. M. Cannon, T. C. Kuo, P. W. Bohn, J. V. Sweedler,
Anal. Chem., 75, 2224 (2003).
[7] H. Yoshiki, A. Oki, H. Ogawa, Y. Horiike, Thin Solid
Films, 407, 156 (2002).
[8] J.K. Evju, P.B. Howell, L.E. Locascio, M.J. Tarlov, J.J.
Hickman, Appl. Phys. Lett., 84, 1668 (2004).
[9] M. Kadowaki, H. Yoshizawa, S. Mori, M. Suzuki,
Thin Solid Films, 506-507, 123 (2006).
[10] C.-P. Klages, A. Hinze, K. Lachmann, C. Berger, J.
Borris, M. Eichler, M. von Hausen, A. Zanker, M. Thomas, Plasma Process. Polym., 4, 208 (20079.
[11] G. Beamson, D. Briggs, “High Resolution XPS of
Organic Polymers”, John Wiley&Sons, Chichester, New
York, Brisbane, Toronto, Singapore 1992.
Table 2. Deconvolution analysis results for C1s peaks of
COC microchannel inner walls.