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Microchim Acta 158, 151–157 (2007)
DOI 10.1007/s00604-006-0703-x
Printed in the Netherlands
Original Paper
Covalent modification of a glassy carbon electrode with penicillaminefor simultaneous determination of hydroquinone and catechol
Liang Wang1;�, Peng Fei Huang1;2, Jun Yue Bai1;2, Hong Jing Wang1,
Li Ying Zhang1, and Yu Qing Zhao1
1 College of Biological Science, Dalian Nationalities University, Dalian 116600, China2 School of Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, China
Received May 10, 2006; accepted September 13, 2006; published online December 14, 2006
# Springer-Verlag 2006
Abstract. A simple and highly selective electrochemi-
cal method has been developed for the simultaneous
determination of hydroquinone (HQ) and catechol
(CC) at a glassy carbon electrode covalently modified
with penicillamine (Pen). The electrode is used for the
simultaneous electrochemical determination of HQ and
CC and shows an excellent electrocatalytical effect on
the oxidation of HQ and CC upon cyclic voltammetry
in acetate buffer solution of pH 5.0. In differential pulse
voltammetric measurements, the modified electrode
was able to separate the oxidation peak potentials of
HQ and CC present in binary mixtures by about
103 mV although the bare electrode gave a single broad
response. The determination limit of HQ in the pres-
ence of 0.1 mmol L�1 CC was 1.0�10�6 mol L�1, and
the determination limit of CC in the presence of
0.1 mmol L�1 HQ was 6.0�10�7 mol L�1. The meth-
od was applied to the simultaneous determination of
HQ and CC in a water sample. It is simple and highly
selective.
Key words: Chemically modified electrode; penicillamine; hydro-
quinone; catechol; electrochemistry.
Hydroquinone (HQ) and catechol (CC) are two iso-
mers of dihydroxybenzenes. These polluting substances
that are hazardous to human health occur frequently
in the environment. They have been included in the
lists of priority pollutants to be monitored in the aquat-
ic environment by international bodies, such as the
US Environmental Protection Agency (EPA) and the
European Union (EU) [1]. Furthermore, because HQ
and CC have similar structures and properties, they
usually coexist. Therefore, it is very important to de-
velop simple and rapid analytical methods for dihy-
droxybenzene isomers [2]. The established methods
for the determination of HQ and CC are commonly
performed after pretreatment and separation [3]. Sam-
ple pretreatment and separation, as well as the signif-
icant operating complexity, the long times and large
volumes of reagents consumed by established techni-
ques make it important to develop a new method capa-
ble of simultaneous determination without the need
for prior separation of these compounds.
HQ and CC have a basic quinone structure that
might be electrochemically oxidized at a platinum or
carbon electrode [4]. The oxidation process to qui-
none has been widely studied from an electrochemical
point of view [5, 6]. However, there are a great num-
ber of difficulties when simultaneously determining
HQ and CC. The major difficulty is that the voltam-
metric peaks corresponding to the oxidation=reduction
of the two phenol isomers largely overlap in many� Author for correspondence. E-mail: qier102@hotmail.com
cases. Moreover, the competition of the phenolic iso-
mers at the electrode surface makes the relationship
between the voltammetric response and the isomer
concentrations in the mixtures non-linear [7].
Recently, an enormous amount of research has been
devoted to the development of new chemically mod-
ified electrodes (CMEs) for monitoring HQ or CC
[8–17]. The simultaneous determination of HQ and
CC at a glassy carbon electrode modified with multi-
wall carbon nanotubes has been proposed, the sepa-
rations between the oxidation peaks of HQ and CC
being about 102 mV [4]. Up to now, there have been
few reports about the simultaneous determination of
HQ and CC using a covalently modified glassy carbon
electrode (GCE). To the best of our knowledge, a glassy
carbon electrode covalently modified with penicilla-
mine for the simultaneous determination of HQ and
CC has not been reported before.
In the effort to develop a voltammetric method for
the simultaneous, selective and sensitive determina-
tion of HQ and CC, the present study employed a
glassy carbon electrode which was covalently modi-
fied with penicillamine. The modified electrode could
be used as a new sensor for selective and sensitive
determination of HQ and CC in binary mixtures. The
proposed method has been applied to the simulta-
neous determination of HQ and CC in water samples.
The method is simple and highly selective.
Experimental
Reagents
Penicillamine was purchased from Sigma (www.sigmaaldrich.com)
and was used as received. Hydroquinone and catechol were obtained
from Beijing Chemical Factory (Beijing, China). All other chemi-
cals were of analytical grade and were used without further purifi-
cation. An 0.1 mol L�1 acetate buffer solution (ABS) was used to
control the pH. All solutions were prepared with deionized water
treated in a Millipore water purification system (Millipore Corp.).
All experiments were carried out at room temperature.
Apparatus
Voltammetric measurements were performed with a CHI 440 elec-
trochemical analyzer (CH Instruments, Chenhua Co., Shanghai,
China) controlled by a personal computer. A conventional three-elec-
trode cell was used, including a saturated calomel electrode (SCE) as
reference electrode, a platinum wire counter-electrode and a bare or
modified glassy carbon disk working electrode (GCE). The pH
values were measured with a PB-10 pH meter (Sartorius). Unless
otherwise stated, the electrolyte solutions were thoroughly degassed
with N2 and kept under an N2 blanket.
Preparation of the modified electrode
The method used the electrooxidation of penicillamine to its analo-
gous cation radicals to form a chemically stable covalent linkage
between the nitrogen atom of the amine group and the edge plane
sites of the carbon electrode surface. The electrochemical modifica-
tion process is shown in Scheme 1. Typically, prior to electrochem-
ical modification, the bare GCE with a diameter of 3 mm was
polished with diamond pastes and alumina slurry down to 0.05mm
on a polishing cloth (Buehler, Lake Bluff, IL). It was rinsed with
water and sonicated in 1þ 1 HNO3, acetone and water for 10 min,
respectively. Then the electrode was finally cleaned by cycling be-
tween 0 and 1.6 V vs. SCE in 0.05 mol L�1 H2SO4 solution at a scan
rate of 100 mV s�1 until a reproducible cycle was achieved. The
treated GCE was immersed in phosphate buffer solution (pH 7.0)
containing 0.01 mol L�1 penicillamine, and subjected to cyclic vol-
tammetry between �1.5 and þ2.5 V at a scan rate of 100 mV s�1 for
5 min. Then the electrode was ready for use after final washing with
water and could be stored in phosphate buffer solution pH 7.0.
Hereafter the modified electrode will be referred to as Pen-modified
electrode. The effective area of the modified electrode was deter-
mined as 7.10�10�2 cm2 from a cyclic voltammogram of
1.0�10�3 M K3[Fe(CN)6] in 0.1 M KCl, which is more than that
of the bare GCE (6.85�10�2 cm2).
Results and discussion
Electrochemical oxidation of HQ
at the pen-modified electrode
Figure 1A shows the cyclic voltammograms (CVs) at
a bare GCE (Fig. 1A curve a) and a Pen-modified elec-
trode (Fig. 1A curve c) in the presence of 0.10 mmol L�1
HQ in ABS pH 5.0 at a scan rate of 100 mV s�1. At
Scheme 1
152 L. Wang et al.
the bare electrode, the oxidation and reduction of
HQ result in broad waves with corresponding peak
potentials of 222 and 92 mV, and �Ep, the difference
between the anodic peak potential (Epa) and the
cathodic peak potential (Epc), is 130 mV. However,
at the Pen-modified electrode, the reversibility of HQ
is significantly improved together with the current
signal increasing. The oxidation peak potential nega-
tively shifts to 195 mV, and the reduction peak posi-
tively shifts to 142 mV with �Ep ¼ 53 mV. The peak
current is 6.11-fold larger than the corresponding one
at the bare GCE. These findings suggest that Pen
can act as a promoter to enhance the electrochemical
reaction. Pen itself is weakly electroactive, which
shows in a broad, ill-defined redox peak in the poten-
tial range of �0.2–0.6 V (Fig. 1A curve b). Due to
the high porosity of Pen, the real surface area of the
modified electrode is far greater than that of bare
GCE. So the peak current increases evidently to-
gether with the background voltammetric response
at the Pen-coated GCE to a greater extent than that
at the bare surface.
Figure 1B shows the CVs of HQ at the Pen-modified
electrode at different scan rates. The oxidation peak
potential was observed to shift positively with the
increase in scan rate, and in addition, exhibited a linear
relation to the square root of the scan rate, v1=2, with the
linear regression equation ipa=mA¼�0.9545þ 1.5816
v1=2=(mV s�1)1=2 (correlation coefficient, r¼ 0.9969).
The result indicates that the oxidation of HQ at the
Pen-modified electrode is a diffusion-controlled process.
The effect of the pH value of ABS on the response of
HQ was investigated by CV. The response of HQ is
regular in ABS. As the solution pH increases, the anodic
peak potential shifts to the negative, and the potential of
Epa vs. pH in ABS has a good linear relation in the
range of pH 3.01–6.88. The linear regression equation
Epa=V¼ 0.4759� 0.0527 pH (correlation coefficient,
r¼ 0.9969) was obtained, which showed that the up-
take of electrons is accompanied by an equal number
of protons.
Fig. 1. (A) Cyclic voltammograms at bare GCE (a) and p-Pen
modified electrode (b, c) in the presence of 0.1 mmol L�1 HQ
(a, c) and in the absence of HQ (b) in 0.1 mol L�1 ABS (pH 5.0);
scan rate, 100 mV s�1. (B) Cyclic voltammograms of 0.1 mmol L�1
HQ at p-Pen modified electrode in 0.1 mol L�1 ABS (pH 5.0) at
different scan rates: (a) 20, (b) 50, (c) 80, (d) 100, (e) 150, (f) 200,
(g) 300, (h) 350, (i) 400, (j) 450, (k) 500, (l) 550 mV s�1
Fig. 2. (A) Cyclic voltammograms at bare GCE (a) and p-Pen
modified electrode (b, c) in the presence of 0.1 mmol L�1 CC (a, c)
and in the absence of CC (b) in 0.1 mol L�1 ABS (pH 5.0); scan
rate, 100 mV s�1. (B) Cyclic voltammograms of 0.1 mmol L�1 CC
at p-Pen modified electrode in 0.1 mol L�1 ABS (pH 5.0) at dif-
ferent scan rates: (a) 20, (b) 50, (c) 80, (d) 100, (e) 150, (f) 200,
(g) 250, (h) 300, (i) 350, (j) 400, (k) 450 mV s�1
Covalent modification of a glassy carbon electrode with penicillamine 153
Electrochemical oxidation of CC
at the Pen-modified electrode
The CV of CC at the Pen-modified electrode is also
compared to that at a bare GCE. Figure 2A shows
the CVs at a bare GCE (Fig. 2A curve a) and a Pen-
modified electrode (Fig. 2A curve c) in the presence of
0.10 mmol L�1 CC in ABS pH 5.0 at a scan rate of
100 mV s�1. At the bare electrode, the oxidation and
reduction of CC result in broad waves with the cor-
responding peak potentials of 326 and 208 mV. So
it shows an irreversible behavior with �Ep, 118 mV.
However, at the Pen-modified electrode, the reversi-
bility of CC is significantly improved together with
the current signal increase. The oxidation peak poten-
tial shifts negatively to 314 mV, and the reduction
peak shifts positively to 255 mV with �Ep¼ 59 mV.
The peak current is 4.51-fold larger than the cor-
responding one at the bare GCE. The modified elec-
trode itself shows a broad, ill-defined redox peak in
the potential range of �0.2–0.8 V (Fig. 2A curve b).
These results indicate that Pen was able to accelerate
the rate of electron transfer of CC by a nonmediation
mechanism in pH 5.0 ABS, and thus may be called a
promoter.
Figure 2B shows the CVs of CC at the Pen-mod-
ified electrode at different scan rates. The oxida-
tion peak potential was observed to shift positively
with the increase in scan rate, and in addition, ex-
hibited a linear relation to the square root of the scan
rate, v1=2, with the linear regression equation ipa=mA¼�0.0444þ 1.2882 v1=2=(mV s�1)1=2 (correlation coef-
ficient, r¼ 0.9968). The result indicates that the oxi-
dation of CC at the Pen-modified electrode is a
diffusion-controlled process.
The effect of the pH value of ABS on the response
of CC was investigated by CV. The response of CC
is regular in ABS. As the solution pH increases, the
anodic peak potential shifts negatively, and the poten-
tial of Epa vs. pH in ABS has a good linear relation
in the range of pH 3.01–6.88. The linear regression
equation Epa=V¼ 0.5739� 0.0516 pH (correlation co-
efficient, r¼ 0.9974) was obtained, which showed that
the uptake of electrons is accompanied by an equal
number of protons.
Simultaneous determination HQ and CC
In order to evaluate the sensitivity and selectivity of
the Pen-modified electrode for the quantification of
HQ and CC, the electrochemical behavior of binary
mixtures of 0.1 mmol L�1 HQ and 0.1 mmol L�1 CC
at the Pen-modified electrode was investigated using
CV and differential pulse voltammetry (DPV). For the
mixtures containing HQ and CC, 0.1 mmol L�1 ABS
was used to control the pH of mixtures, and pH 5.0
was chosen because at this pH the oxidations of the
two compounds have a high electrochemical response.
Figure 3 shows the CV and DPV voltammograms
obtained for HQ and CC coexisting at a bare GCE and
a Pen-modified electrode. As shown in Fig. 3, the bare
electrode cannot separate the voltammetric signals of
HQ and CC. Only one broad voltammetric signal was
observed for both analytes. Therefore, it is impossible
to use the bare electrode for the voltammetric deter-
mination of CC in the presence of HQ.
Moreover, the Pen-modified electrode resolved the
mixed voltammetric signals into two well-defined vol-
tammetric peaks. The Pen-modified electrode exhib-
its good selectivity and excellent sensitivity in the
simultaneous determination of HQ and CC. The peaks
observed at 264 and 159 mV in DPV recording corre-
spond to the oxidation of CC and HQ, respectively
(Fig. 3B). In theory, the density of the electron cloud
Fig. 3. CVs (A) and DPVs (B) for the homogeneous solution of
0.1 mmol L�1 HQ and 0.1 mmol L�1 CC at bare (a) and p-Pen
modified electrode (b) in 0.1 mol L�1 ABS (pH 5.0). (A) Scan rate:
100 mV s�1; (B) scan rate: 4 mV s�1; pulse amplitude: 50 mV;
pulse width: 50 ms; pulse time: 200 ms
154 L. Wang et al.
is lower from HQ to CC, therefore their electroactivity
decreases and the oxidation of the HQ is easier than
that of CC, which shows that the potentials of their
oxidation peaks increase. The experimental results
accord with this theory [2]. As the oxidation potential
of HQ is shifted to the less positive side, the anodic
current of CC has no contribution from HQ, because
HQ is readily oxidized well before the oxidation po-
tential of CC is reached. Thus, the precise determi-
nation of CC in the presence of HQ or the precise
determination of HQ in the presence of CC is possi-
ble at the Pen-modified electrode. Furthermore, the
separation between the DPV oxidative peaks of HQ
and CC is large (�105 mV), and thus the simulta-
neous determination of HQ and CC or the selective
determination of CC in the presence of HQ is feasible
at the Pen-modified electrode.
The next attempt was taken to determine HQ and
CC simultaneously by using the Pen-modified electrode
with DPV. Figure 4 represents the DPV recordings at
different concentrations of HQ, where the concentra-
tion of CC was kept constant. The oxidative peak cur-
rent for HQ increased linearly with the increase in HQ
concentration. Furthermore, while the HQ peak current
increased with the increase in HQ concentration, the
peak current of CC kept almost constant. Thus, it is
confirmed that the responses of HQ and CC at the
Pen-modified electrode are independent. The determi-
nation limit of HQ in the presence of 0.1 mmol L�1 CC
was found to be 1.0�10�6 mol L�1.
The overall facility of the Pen-modified electrode for
simultaneous determination of HQ and CC was demon-
strated by simultaneously changing the concentration
of HQ and CC. Figure 5 illustrates the DPV responses
of the Pen-modified electrode while simultaneously
varying the concentrations of both HQ and CC. The
calibration curves for HQ and CC were linear over a
wide range of concentrations (15–115mmol L�1 for
HQ and 25–175 mmol L�1 for CC), with correlation
coefficients 0.9953 and 0.9971, respectively. The de-
termination limits for HQ and CC were found to
be 1.0�10�6 and 6.0�10�7 mol L�1, respectively.
The slopes (�I=�C) of the linear calibration curves
were estimated to be about 1.57 and 1.06mA=mmol L�1
for HQ and CC, respectively. Thus, a simultaneous,
selective and sensitive determination of HQ and CC
was achieved at the Pen-modified electrode.
To further ascertain the reproducibility of the re-
sults, three different GCE were modified with Pen,
and their responses to the oxidation of HQ and CC
Fig. 4. DPVs of HQ and CC at p-Pen modified electrode in
0.1 mol L�1 ABS (pH 5.0), [CC] was kept constant and [HQ]
was changed (i.e., [CC]¼ 0.1 mol L�1, [HQ]: (a) 5, (b) 10, (c) 20,
(d) 40, (e) 60, (f) 80, (g) 100 mmol L�1). The inset shows the re-
lationship between the anodic peak current and the concentration
of HQ. Scan rate: 4 mV s�1; pulse amplitude: 50 mV; pulse width:
50 ms; pulse time: 200 ms
Fig. 5. (A) DPVs for HQ and CC at p-Pen modified electrode in
0.1 mol L�1 ABS (pH 5.0) while simultaneously changing their
concentration (i.e., [CC]: (a) 25, (b) 40, (c) 55, (d) 70, (e) 85,
(f) 100, (g) 115, (h) 130, (i) 145, (j) 160, (k) 175 mmol L�1; [HQ]:
(a) 15, (b) 25, (c) 35, (d) 45, (e) 55, (f) 65, (g) 75, (h) 85, (i) 95,
(j) 105, (k) 115 mmol L�1). Scan rate: 4 mV s�1; pulse amplitude:
50 mV; pulse width: 50 ms; pulse time: 200 ms. (B) Corresponding
calibration plots for CC (a) and HQ (b)
Covalent modification of a glassy carbon electrode with penicillamine 155
were tested. The separation between the voltammetric
signals of HQ and CC and the sensitivities remained
the same at all three modified electrodes, confirming
that the results are reproducible.
Analytical applications
The analytical utility of the Pen-modified electrode
for the simultaneous determination of HQ and CC
has been examined using synthetic samples consisting
of HQ and CC in local tap water. The determination
of HQ and CC in the samples was carried out using
DPV at the Pen-modified electrode in 0.10 mmol L�1
ABS (pH 5.0). The results are listed in Tables 1 and 2.
When known amounts of CC were added to the water
control samples containing HQ, quantitative recoveries
of 97.5–103.1% were obtained. When known amounts
of HQ were added to the water control samples con-
taining CC, quantitative recoveries of 98.3–102.6%
were obtained. The feasibility of the Pen-modified elec-
trode for the simultaneous determination of HQ and
CC is evident.
Conclusions
The present study demonstrates an excellent approach
to the development of a novel voltammetric sensor of
hydroquinone and catechol based on a glassy carbon
electrode covalently modified with penicillamine. Fast
electron transfer, high selectivity and excellent sensi-
tivity for the oxidation of hydroquinone and catechol
are achieved at the Pen-modified electrode. The pre-
sent modified electrode showed excellent sensitivity
and selectivity properties and can separate oxidation
peaks of hydroquinone and catechol, which are in-
distinguishable at the bare electrode. Since the vol-
tammetric signals of hydroquinone and catechol are
well-separated at the Pen-modified electrode, sensitive
determination of catechol in the presence of hydro-
quinone or the simultaneous determination of hydro-
quinone and catechol can be achieved. The covalently
modified glassy carbon electrode with aspartic acid is
a promising approach to determination of isomers.
Acknowledgements. This project was supported by the Doctor
Foundation of Dalian Nationalities University (20056101).
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Table 1. Simultaneous determination results for CC in local tap
water containing HQ
Sample
no.
Tap water
containing
HQ (mmol L�1)
CC added
(mmol L�1)
CC
founda
(mmol L�1)
Recovery
(%)
1 20.0 60.0 58.5 97.5
2 20.0 70.0 69.0 98.6
3 20.0 80.0 82.5 103.1
a Average of five determinations.
Table 2. Simultaneous determination results for HQ in local tap
water containing CC
Sample
no.
Tap water
containing
CC (mmol L�1)
HQ added
(mmol L�1)
HQ founda
(mmol L�1)
Recovery
(%)
1 20.0 40.0 39.3 98.3
2 20.0 50.0 51.3 102.6
3 20.0 60.0 60.7 101.2
a Average of five determinations.
156 L. Wang et al.
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Covalent modification of a glassy carbon electrode with penicillamine 157
Recommended