Sensors and Actuators B 140 (2009) 51–57
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Sensors and Actuators B: Chemical
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Detection of nitrite using poly(3,4-ethylenedioxythiophene) modified SPCEs
Chia-Yu Lina, V.S. Vasanthaa, Kuo-Chuan Hoa,b,∗
aDepartment of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
bInstitute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
a r t i c l e i n f o
Received 15 September 2008
Received in revised form 12 April 2009
Accepted 24 April 2009
Available online 5 May 2009
Multi-wall carbon nanotubes (MWCNTs)
Screen-printed carbon electrodes (SPCEs)
a b s t r a c t
(PEDOT/MWCNTs) modified screen-printed carbon electrodes (SPCEs) were fabricated and their
catalytic properties towards nitrite were studied. Due to the electrostatic interaction between the
negatively-charged nitrite ions and the positively-charged PEDOT film, the operating potential for nitrite
oxidation was shifted about 160mV to negative side, compared to bare SPCE, as a PEDOT film was
deposited on the SPCE. The diffusion coefficient obtained from RDE experiment is 2.05×10−5cm2s−1.
The electron transfer coefficient (˛) was increased from 0.515 to 0.615 as the sensing electrode was
changed from PEDOT-modified to PEDOT/MWCNTs-modified electrode. Therefore, PEDOT/MWCNTs
composite shows the superior catalytic property towards nitrite and the operating potential was further
shifted about 100mV to the negative side. The sensitivity and limit of detection (LOD) for the PEDOT- and
PEDOT/MWCNTs-modified SPCEs are about 100mAcm−2M−1, 1.72?M and 140mAcm−2M−1, 0.96?M,
respectively. The possible interferences from several common ions were tested. The developed sensor
was also applied to the determination of nitrite concentration in tap water sample.
© 2009 Elsevier B.V. All rights reserved.
Nitrite is present ubiquitously in soils, waters, foods and physi-
ological systems and has been reported as a human health-hazard.
The excess uptake of nitrite would cause gastric cancer  and blue
sensor to detect nitrite in food, drinking water and environmental
Several techniques have been developed for nitrite determi-
nation, including spectrophotometry , chemiluminescence ,
chromatography  and capillary electrophoresis . However,
these determination methods usually have tedious detection pro-
cedures and therefore are time-consuming. Compared to these
methods, the electrochemical methods can provide cheaper, faster
and real-time analysis and thus have attracted more atten-
tion. The electrochemical oxidation of nitrite usually involves a
large overpotential at the surfaces of the bare electrodes, and
therefore, the determination of nitrite tends to suffer inter-
ferences from other more oxidizable compounds. To overcome
these problems, some electrochemically modified electrodes
based on porphyrin , Pt nanoparticles , metallophthalocya-
nine , and Nafion®/lead-ruthenate pyrochlore  have been
University, Taipei 10617, Taiwan. Tel.: +886 2 2366 0739; fax: +886 2 2362 3040.
E-mail address: email@example.com (K.-C. Ho).
explored to lower the operating potentials for nitrite oxida-
Since the discovery of carbon nanotubes (CNTs), they have
attracted more and more attention due to their excellent elec-
trical conductivity, chemical stability, high surface area and high
mechanical strength . Consequently, CNTs have been used as
an electrode material or modifier to promote electron transfer
reactions between biomolecules and the underlying electrodes
. Recently, the electrocatalytic oxidation of nitrite on carbon
nanotubes powder microelectrodes has been reported . On
the other hand, poly(3,4-ethylenedioxythiophene) (PEDOT) has
received significant amount of attention as an electrode material
in light emitting devices, electrochromic windows, polymer batter-
ies, etc. [14–15]. In addition, the PEDOT-modified electrodes have
also been used for detection of dopamine , ascorbic acid ,
pesticides , and cysteine  recently.
Since the PEDOT film has been found to be in oxidized form
with high stability and conductivity at physiological pH , we
selected the PEDOT film with the expectation of forming elec-
trostatic interaction between PEDOT and nitrite ions. Besides, the
catalytic property and conductivity of PEDOT-modified electrode
towards nitrite can be enhanced by the application of MWC-
NTs undercoat layer before the PEDOT film formation. In this
study, PEDOT/MWCNTs-modified electrode has been fabricated by
electropolymerization of 3,4-ethylenedioxythiophene (EDOT) on
MWCNTs-modified SPCE electrode by cyclic voltammetry. Cyclic
voltammetry technique was used to study the catalytic oxidation
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C.-Y. Lin et al. / Sensors and Actuators B 140 (2009) 51–57
of nitrite on PEDOT/MWCNTs-modified electrode. The mechanism
for the electrochemical oxidation of nitrite anion was studied by
using a rotating disk electrode. The effects of PEDOT film thickness,
amount of MWCNTs and pH value of the electrolyte on the oxi-
dation current response of PEDOT/MWCNTs/SPCEs to nitrite have
been examined and discussed.
2.1. Chemicals and instruments
ethylenedioxythiophene (EDOT) were purchased from Aldrich
and used as received. 0.1M solution of nitrite was prepared
before every experiments by direct dissolution of sodium nitrite
(Riedel-de Haën) in deionized water. Multiwall carbon nano-tubes
(MWCNTs) were purchased from Nanotech Port Co. (Taiwan) and
these MWCNTs were produced via the chemical vapor deposition
(CVD) method. The diameter and the length of the MWCNTs are
40–50nm and 3–5?m, respectively. Other chemicals were of
analytical grade and used without further purification. Deionized
water was used throughout the work.
Electrochemical measurements were carried out at a CHI 440
electrochemical workstation (CH Instruments, Inc., USA) with a
conventional three-electrode system. A three-electrode type of
screen printed carbon electrode (SPCE, Zensor R&D, Taiwan), with
geometric area of 0.071cm2was used for the sensor preparation.
The working electrode, reference electrode and counter electrode
of the SPCE are carbon electrode, Ag/AgCl electrode and Pt wire,
respectively. All electrochemical experiments were performed at
room temperature and all the potentials are reported vs. the
Ag/AgCl. The rotating disk electrode (RDE) experiments were per-
formed by a Pine Instruments Co. electrode with a Pt disk of 0.5cm
in diameter in conjunction with a CH Instruments CHI-660 poten-
tiostat that was connected to a model AFMSRX analytical rotator.
The microstructure of the modified electrode was examined by
a field emission scanning electron microscopy (FESEM, JEOL JSM-
2.2. Preparation of PEDOT-modified electrode
The electropolymerization of EDOT, onto three-electrode type
SPCE, was carried out by cyclic voltammetric method in aqueous
solution containing 0.01M EDOT, 0.5mM (2-hydroxypropyl)-?-
cyclodextrin and 0.1M LiClO4between 0 and 0.95V at a scan rate
of 50mV/s for 1 to 4 cycles. After polymerization, the electrode was
treated with 0.1 PBS (pH 7.0) solution by repeated cycling in the
potential range of 0.3–0.9V at the scan rate of 25mV/s to obtain a
2.3. Preparation of PEDOT/MWCNTs-modified electrode
A MWCNTs suspension was prepared by dispersing MWCNTs in
hours. Experimentally, it was found that MWCNTs can be dispersed
well in DMF solvent as long as the concentration of MWCNTs is
equal to or below 2mg/ml. Thus, we chose 2mg/ml as the suitable
suspension was drop-casted on the surface of the electrode and
the electrode was then dried at 60◦C. To optimize the amount of
MWCNTs on the electrode, the above-mentioned procedure was
repeated for several times. After deposition of the MWCNTs, the
PEDOT film was then electrodeposited by the cyclic voltammetric
method mentioned in Section 2.2.
of the PEDOT film for 1, 2, 3, and 4 cycles in 0.1M PBS (pH 7) solution containing
1mM nitrite. Scan rare: 25mV/s.
2.4. Amperometric detection of nitrite
For detection of nitrite by using PEDOT-modified electrode, a
suitable sensing potential in the limit current plateau region was
determined between 0.47 and 0.8V by the linear sweep voltamme-
try at a scan rate of 0.3mV/s in the solution containing deaerated
0.1M PBS (pH 6.1) and 1mM nitrite. This suitable sensing potential
range between 50?M and 1.6mM were collected and calibration
curve for nitrite was constructed.
For detection of nitrite by using PEDOT/MWCNTs-modified
electrode, a suitable sensing potential in the same way as PEDOT-
modified electrode, except that the scan rate and scan range were
set as 0.5mV/s and 0.3–0.8V, respectively. The suitable sensing
potential was determined as 0.6V.
3. Results and discussions
3.1. Electrooxidation behavior of nitrite on the PEDOT-modified
Fig. 1 shows the cyclic voltammetric response of bare SPCE and
PEDOT-modified SPCE in the presence of 1mM NaNO2at a scan
rate of 25mV/s. As it can be found that an increased oxidation cur-
rent density was observed and the oxidation potential of nitrite
shifted about 160mV to the negative side as the PEDOT film was
electrostatic attraction between the oxidized PEDOT film and the
negatively-charged nitrite anions that resulted in preconcentration
the cycle number during electropolymerization. As shown in Fig. 1,
as the cycle number is greater than 2. While the electrostatic inter-
action was enhanced by increasing the thickness of PEDOT film, the
was greatly reduced at greater film thickness. Therefore, the thick-
ness obtained at 2 cycles was found to be optimal thickness for
electrocatalytic oxidation of nitrite on PEDOT film.
in 0.1M PBS solution at pH 6.1, 7 and 8.1 was also examined. As
shown in Fig. 2, the peak potential shifted slightly to positive side
C.-Y. Lin et al. / Sensors and Actuators B 140 (2009) 51–57
Fig. 2. Cyclic voltammograms of the SPCE with electropolymerization of the PEDOT
film for 2 cycles in 0.1M PBS solution, with different pHs, containing 1mM nitrite.
Scan rare: 25mV/s.
as the solution pH increased. According to Guidelli , the whole
oxidation of nitrite into NO2followed by rapid disproportionation
of NO2into NO2−and NO3−as shown as Eqs. (1) and (2):
2NO2+H2O → NO3−+NO2−+2H+
The electro-oxidation of nitrite into NO2is the rate-determining
step which is proton independent, and therefore, the slight change
in peak potential can be attributed to the nature of the PEDOT film.
As the pH increased, the PEDOT film will become dedoped and thus
the electrostatic interaction will slightly decrease which resulting
in slight increase in peak potential for nitrite oxidation. Although
the maximum oxidation current density can be obtained at pH 7.0,
we chose solution pH of 6.1 for later experiments due to lower peak
potential for nitrite oxidation.
Fig. 3 depicts the cyclic voltammograms for the oxidation of
nitrite at the PEDOT-modified SPCE in the PBS solution (pH 6.1)
containing 1mM nitrite at various scan rates. Since the anodic peak
current density of nitrite increases linearly with the square root
of the scan rate, the oxidation of nitrite at the PEDOT/SPCE was
Fig. 3. Cyclic voltammograms of the PEDOT-modified SPCE in 0.1M PBS (pH 6.1)
solution containing 1mM nitrite at various potential scan rates from 5, 10, 15, 20,
25, 50, 75 and 100mV/s. Inset: Jpavs. scan rate.
Fig. 4. RDE voltammograms of the PEDOT-modified SPCE in 0.1M PBS solution
(pH 6.1) containing 2.8mM nitrite at different rotation rate ranging from 800 to
1800rpm. The inset shows a plot of the reciprocal of the limiting current density
(Jl,a) vs. the reciprocal of (rotation rate)1/2. Electrode=Pt disk. Scan rate=10mV/s.
diffusion-controlled. In addition, the electron transfer coefficient
(˛) can be obtained according to the Eq. (3) :
(1 − ˛)n˛=
where n? is the number of electron transfer involved in the
rate-determining step, all other parameters have their conven-
tional meanings. The estimated value of (1−˛)n?was found to be
0.97±0.02. Since the electro-oxidation of nitrite into NO2, involv-
ing 2 electrons transfer, is the rate-determining step , therefore
the values of n?and ˛ were estimated as 2 and 0.515, respectively.
3.2. Rotating disk voltammetry at the PEDOT-modified electrode
Fig. 4 portrays the electro-catalytic oxidation of nitrite at dif-
ferent rotation speeds at the PEDOT/GC-modified rotating disc
electrode in phosphate buffer solution (pH 6.1). The Kuoteck´ y-
Levich plot (the inset) that obtained for the voltammograms in the
Fig. 4 was found linear with square-root of different rotation rate.
Besides, the plot did not pass through the origin suggesting that a
kinetic limitation is involved in the electron transfer reaction.
Fig. 5. SEM image of the surface of the PEDOT/MWCNTs-modified SPCE.
C.-Y. Lin et al. / Sensors and Actuators B 140 (2009) 51–57
Fig. 6. (A) Cyclic voltammograms of PEDOT- and PEDOT/MWCNTs-modified SPCE
in PBS solution (pH 6.1) containing 1mM nitrite ions. Cycle number for electropoly-
merization of PEDOT=2. (B) Cyclic voltammograms of PEDOT/MWCNTs-modified
SPCE with various layers of MWCNTs. Cycle number for electropolymerization of
PEDOT=2. (C) Cyclic voltammograms of the MWCNTs-modified SPCE with elec-
tropolymerization of the PEDOT film for 1, 2, 3, and 4 cycles in 0.1M PBS (pH 6.1)
solution containing 1mM nitrite. Scan rare: 25mV/s.
Fig. 7. Cyclic voltammograms of the PEDOT/MWCNTs-modified SPCE at different
pHs with PEDOT film electropolymerized for 2 cycles and 6 layers of MWCNTs in
0.1M PBS solution containing 1mM nitrite. Scan rare: 25mV/s.
The diffusion coefficient can then be calculated from the slope
of the Levich plot, shown as Eq. (4) :
where jK, jl,a, D, ?, ω and COare current density in the absence
of any mass-transfer effect, limiting current density, the diffusion
co-efficient, the kinematic viscosity, the rotation speed and the
bulk concentration of nitrite in the solution, respectively, and all
other parameters have their conventional meanings. The number
of the electron transfer and concentration of nitrite used are 2 and
2.05×10−5cm2s−1which is close to that reported in the literature
3.3. Electro-oxidation behavior of nitrite on the
Since the catalytic property of CNTs toward the electro-
oxidation of nitrite has been proven previously , the
PEDOT/MWCNTs electrode should have superior catalytic property
Fig. 8. Cyclic voltammograms of the PEDOT/MWCNTs-modified SPCE in 0.1M PBS
(pH 6.1) solution containing 1mM nitrite at various potential scan rates from 6.25,
12.5, 25, 50, and 100mV/s. Inset: Jpavs. scan rate.
C.-Y. Lin et al. / Sensors and Actuators B 140 (2009) 51–57
the PEDOT/MWCNTs-modified SPCE to various nitrite concentrations in 0.1M PBS
solution (pH 6.1). The applied potentials for PEDOT- and PEDOT/MWCNTs-modified
SPCEs are 0.7 and 0.6V vs. Ag/AgCl, respectively. The insets show the calibration
curves for the oxidation current density of nitrite vs. nitrite concentration.
for the electrochemical process of nitrite oxidation. To fabricated
PEDOT/MWCNTs film on the SPCE, a layer of MWCNTs was first
drop-coated on the surface of SPCE before the electro-deposition
of the PEDOT film (2 cycles). As shown in Fig. 5, the MWCNTs dis-
Effect of foreign ions on the amperometric detection of nitrite (1mM).
Ions addedRelative current responsea(%) at different
molar ratios of ([nitrite]:[added ions])
aRelative response (%)=Initrite+added ions/Initrite, which were obtained from at least
three repetitive experiments.
tribute uniformly and some of the MWCNTs were not covered by
the PEDOT film.
Fig. 6(A) shows the cyclic voltammetric response of PEDOT-
of 1mM NaNO2at a scan rate of 25mV/s. It can be found that
although the current density response decreased, the oxidation
potential shifted to more negative side (∼100mV) as a layer of
MWCNTs was deposited on the SPCE. Since the roughness of the
coverage would be lower on the MWCNTs/SPCE, if the same cycle
number was applied to both SPCEs. As a consequence, the over-
all pre-concentration effect caused by the electrostatic interaction
would be lower, thus resulting in a lower current response.
The amount of MWCNTs was expected to affect the current
response of PEDOT/MWCNTs SPCE to nitrite, and therefore the
effect of the amount of MWCNTs, controlled by repeating drop-
coating procedure for sever times, was studied. As shown in
Fig. 6(B), the current response increased as the number of MWC-
NTs layer was increased. However, as the number of MWCNTs
layers exceeded 6 (with a thickness of ∼10?m for the MWCNTs
layer, as determined by the cross-sectional SEM image), the com-
posite film became unstable and fall off during the experiment.
Furthermore, the effect of the PEDOT layer thickness on the sensing
performance of the PEDOT/MWCNTs-modified SPCE was examined
after optimizing the layer thickness of MWCNTs. The results are
shown in Fig. 6(C), which indicates that the current response of
nitrite decreased as the deposition cycle number was greater than
2. Therefore, the optimal deposition cycle number for PEDOT and
the optimal layer number for MWCNTs were chosen to be 2 and 6,
A partial list of literatures on the electrochemical nitrite sensing using carbon nanotubes.
Type of the electrode Performance Ref.
Sens.g(mAM−1cm−2) LOD (?M) Linear range (mM)
aCarbon nanotube powder microelectrode.
dGrassy carbon electrode.
fRoom temperature ionic liquids.
C.-Y. Lin et al. / Sensors and Actuators B 140 (2009) 51–57
Fig. 10. (A) The calibration curves, monitoring over a period of 7 weeks, for the
PEDOT/MWCNTs-modified SPCEs obtained in 0.1M PBS solution (pH 6.1) containing
for the PEDOT/MWCNTs-modified SPCEs. Applied potential: 0.6V vs. Ag/AgCl.
The effect of solution pH on the current response of the
PEDOT/MWCNTs-modified SPCE was also examined in 1mM
nitrite+0.1M PBS solution at pH 6.1, 7.0 and 8.1. Unlike PEDOT-
modified SPCE, the PEDOT/MWCNTs-modified one did not show
obvious difference in current response for the solutions with dif-
ferent pHs, as seen in Fig. 7. Besides, the peak potential slightly
shifted to the positive side as the solution pH was increased, which
follows the same trend as that of the PEDOT-modified SPCE, as seen
in Fig. 2.
Fig. 8 shows the cyclic voltammograms for the oxidation of
nitrite at the PEDOT/MWCNTs-modified SPCE in the PBS solution
(pH 6.1) containing 1mM nitrite at various scan rates. It can be
found that the anodic peak current density of nitrite increases lin-
early with the square root of the scan rate, and thus the oxidation
of nitrite at the PEDOT/SPCE was diffusion-controlled. The value of
(1−˛)n?can also be extracted according the Eq. (3), and the esti-
values ˛ was estimated as 0.615 which is higher than that obtained
by using PEDOT-modified SPCE, indicating that the electron trans-
fer rate was enhanced by incorporating MWCNTs into the PEDOT
3.4. Amperometric detection of nitrite
The current densities of the PEDOT- and PEDOT/MWCNTs-
modified SPCE as a function of the nitrite concentration with a
sampling time of 200s at each concentration level were measured
and shown in Fig. 9(A) and (B), respectively. The current den-
sity increases linearly with the increased nitrite concentration in
both modified electrodes. The sensitivity of the PEDOT/MWCNTs-
modified SPCE is 140mAcm−2M−1, which is 1.4 times higher than
that of the PEDOT-modified SPCE. The limit of detection (LOD),
modified electrode are 1.72?M and 0.96?M, respectively. Table 1
is a partial list of literatures on the electrochemical nitrite sens-
ing using carbon nanotubes. It is noted that the limit of detection
is comparable with or lower than that obtained with other CNT-
based modified electrodes [13,23–29]. Besides, the LOD for our
proposed method is below the maximum admissible level (∼3?M)
of nitrite in drinking water established by the European Commu-
nity . This implies that our proposed method is applicable for
the detection of nitrite in drinking water.
Fig. 10 shows the stability data for the PEDOT/MWCNTs-
the same time and stored in a desiccator with a relative humidity of
posed sensor, evaluated in terms of the relative standard deviation
(n=3), was determined to be 4.4% (data not shown). The stability
data for a typical PEDOT/MWCNTs-modified electrode were col-
lected once every week or two weeks. A loss in the sensitivity of
17% was observed after 49 days.
3.5. The interference studies
The interference effect of the foreign ions on the determination
of nitrite was examined by addition of various ions into the PBS
solution (pH 6.1) containing 1mM nitrite and the results are sum-
marized in Table 2. It can be found that most ions did not show
any interference effect during the determination of nitrite by using
PEDOT/MWCNTs-modified electrode. However, 10-fold amount of
SO32−ion shows serious interference.
Finally, the PEDOT/MWCNTs-modified SPCE was applied to
determine nitrite concentration in tap water sample. The Griess
method was used for comparison. Equal volumes of 1X Griess
reagent (Sigma–Aldrich) and tap water sample were mixed and
incubated for 15min. Thereafter, the concentration of nitrite was
determined by reading the absorbance at 540nm. The results
obtained by both our method and the Griess method were sum-
marized in Table 3. It can be found that the results obtained by our
method are in good agreement with those obtained by the Griess
method. It should be noted that the original nitrite concentration
The determination of nitrite concentration in tap water sample by comparing the proposed method and the Griess method.
Sample/methodActual value (?M)Added (?M) Value found after addingRSDb(%, n=3) Recoveryc(%, n=3)
Tap water/Griess method
Tap water/electrochemicaldetection (This work)
bRelative standard deviation.
cConcentration found after adding 20?M of nitrite/20?M of nitrite.
C.-Y. Lin et al. / Sensors and Actuators B 140 (2009) 51–57 Download full-text
is undetectable by both methods, so we added 20?M nitrite in
tape water sample and recorded the results as “value found after
The PEDOT- and PEDOT/MWCNTs-modified SPCEs were fab-
ricated and their catalytic properties towards nitrite were
investigated. With modification of PEDOT and PEDOT/MWCNTs,
the operating potential can be reduced about 160 and 260mV,
respectively, compared to that of bare SPCE electrode. The sensitiv-
ity and limit of detection for PEDOT/MWCNTs-modified SPCE are
140mAcm−2M−1and 0.96?M, respectively. The lower operating
potential and practical detection limit allows the PEDOT/MWCNTs-
modified electrode for practical use.
This work was sponsored by the National Research Council of
Taiwan, the Republic of China, under grant number NSC 96-2220-
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ing from National Cheng Kung University, Tainan, Taiwan,
in 2003. He received his M.S. degree in Chemical Engi-
neering from National Taiwan University, Taipei, Taiwan
in 2005. Now, he is a Ph. D. student in second grade in
Chemical Engineering at National Taiwan University. His
research interest mainly surrounds nanomaterials for gas
V. S. Vasantha received B.S. in Chemistry in the year
1985 at Madurai Kamaraj University and M.S. in Indus-
trial Chemistry with specialization of electrochemistry in
the year 1988, at Alagappa University. She was awarded
Ph. D. in electrochemistry by Alagappa University while
working as Senior Research Fellow at CECRI, in the year
1994. She joined as a Research Associate and worked
till 2000 in conducting polymers modified electrodes at
CECRI. She was engaged in teaching of Chemistry for
three years in the Junior college. She joined in National
Taipei University of Technology as Post Doctoral Fellow
in the year 2003 and the National Taiwan University in
the year 2005. Currently, Associate Professor at Alagappa University, Karaikudi in
Kuo-Chuan Ho received B.S. and M.S. degrees in Chem-
ical Engineering from National Cheng Kung University,
Tainan, Taiwan, in 1978 and 1980, respectively. In 1986,
he received the Ph. D. degree in Chemical Engineering at
the University of Rochester. The same year he joined PPG
Industries, Inc., first as a Senior Research Engineer and
then, from 1990 until 1993, as a Research Project Engi-
neer. He has worked on the electrochemical properties of
various electrode materials, with emphasis on improving
the performances of sensor devices. Following a six-year
industrial career at PPG Industries, Inc., he joined his alma
mater at National Cheng Kung University in 1993 as an
Associate Professor in the Chemical Engineering Department. In 1994, he moved to
the Department of Chemical Engineering at National Taiwan University. Currently,
he is a Professor jointly appointed by the Department of Chemical Engineering and
Institute of Polymer Science and Engineering at National Taiwan University.