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.

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Covalent modification of a glassy carbon electrode with penicillamine 157