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7/27/2019 A Simple and Sensitive Method for the Determination of 4-N-octylphenol Based on Multi-walled Carbon Nanotubes Modified Glassy Carbon Electrode
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JOURNAL OFENVIRONMENTALSCIENCES
ISSN 1001-0742
CN 11-2629/X
www.jesc.ac.cn
Available online at www.sciencedirect.com
Journal of Environmental Sciences 2012, 24(9) 1717–1722
A simple and sensitive method for the determination of 4- n-octylphenol based
on multi-walled carbon nanotubes modified glassy carbon electrode
Qiaoli Zheng1, Ping Yang1, He Xu1,∗, Jianshe Liu1,∗, Litong Jin2
1. School of Environmental Science and Engineering, Donghua University, Shanghai 201620, China. E-mail: [email protected]
2. Department of Chemistry, East China Normal University, Shanghai 200062, China
Received 21 October 2011; revised 02 March 2012; accepted 27 March 2012
AbstractA simple and sensitive electroanalytical method was presented for the determination of 4-n-octylphenol (OP) based on multi-walled
carbon nanotubes (MWCNTs) modified glassy carbon electrode (GCE). OP was directly oxidized on the MWCNTs / GCE, and the
electrochemical oxidation mechanism was demonstrated by a one-electron and one-proton process in the reaction. The oxidation peak
current of OP was significantly enhanced by the use of MWCNTs / GCE compared with those of bare glassy carbon electrode, suggesting
that the modified electrode can remarkably improve the performance for OP determination. Factors influencing the detection processes
were optimized. Under these optimal conditions, a linear relationship between concentration of OP and current response was obtained
in the range of 5 × 10−8 to 1 × 10−5 mol / L with a detection limit of 1.5 × 10−8 mol / L and correlation coefficient 0.9986. The modified
electrode showed good selectivity, sensitivity, reproducibility and high stability.
Key words: multi-walled carbon nanotubes; 4-n-octylphenol; electrochemical analysis; linear sweep voltammetry
DOI: 10.1016 / S1001-0742(11)60970-4
Introduction
Alkylphenols, especially nonylphenols (NP) and octylphe-
nols (OP) are widely distributed in the environment,
have estrogenic activity and can bio-accumulate in the
lipids of organisms. In order to control the level of
these compounds in environment, it is necessary to select
appropriate analytical methods. The proposed methods in-
cluding high performance liquid chromatography (HPLC),
gas chromatographic-mass spectrometric analysis (GC-
MS) and solid phase extraction (SPE) (Gadzala-Kopciuch
et al., 2008; Lopez-Espinosa et al., 2009; Cai et al., 2003)
have high sensitivity and low detection limits, however,they have the disadvantages of being complex, time-
consuming and expensive. Compared to other options,
electroanalysis has the advantages of quick response, sim-
plicity, time-saving, high sensitivity and selectivity. It has
been widely applied in various fields, in particular during
the determination of chemical substances with electroac-
tive groups, such as the hydroxide group in alkylphenols.
Carbon nanotubes (CNTs) have raised interest in
nanoscience and nanotechnology due to their large surface
area, unique structure, and remarkable mechanical and
electrical properties (Coleman et al., 2006; Valcarcel et al.,
2008). Moreover, since the discovery of their electrocat-alytic properties by Britto et al. (1996), CNTs have been
* Corresponding author. E-mail: [email protected] (He Xu); liujian-
[email protected] (Jianshe Liu)
widely used in both electrochemistry and electroanalytical
chemistry (Gooding et al., 2007; Tedim et al., 2008;
Alexeyeva et al., 2006). Multi-walled carbon nanotubes
(MWCNTs) modified glassy carbon electrodes have been
reported for the electrochemical determination of various
environmental pollutants. For example, Yi (2003) has
reported the determination of trace levels of mercury
based on a MWCNTs modified glassy carbon electrode.
Wen et al. (2008) fabricated MWCNTs modified glassy
carbon electrode for the electrochemical analysis of the
endocrine-disrupting chemical trifluralin. Luo et al. (2008)
has reported the electrochemical reduction of nitrophenol
isomers at the carbon nanotubes modified glassy car-bon electrode. However, there are few reports about the
determination of 4-n-octylphenol (OP) with MWCNTs
modified glassy carbon electrode.
In this work, MWCNTs modified glassy carbon elec-
trode for the electrochemical determination of OP was
employed. The MWCNTs modified electrode significantly
enhanced the electrochemical response toward OP oxida-
tion, leading to better performance for OP detection. The
electrochemical reaction mechanism of OP on the modified
electrode was investigated. Factors influencing the detec-
tion processes, such as pH, scan rate and accumulation
time were optimized. The reproducibility and stability of the modified electrode were also tested.
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1718 Journal of Environmental Sciences 2012, 24(9) 1717–1722 / Qiaoli Zheng et al. Vol. 24
1 Materials and methods
1.1 Reagents and apparatus
4-n-Octylphenol was purchased from Alfa Aesar Co.
(USA). OP stock solution (10 mmol / L) was prepared with
anhydrous ethanol and kept in darkness at 4°C. Workingsolutions were freshly prepared by diluting the stock
solution. MWCNTs were purchased from Chengdu Or-
ganic Chemical Co. (China). The Britton-Robinson (BR)
buff er solution was prepared using H3PO4, CH3COOH and
H3BO3, and adjusting the pH with 0.2 mol / L NaOH. All
chemicals were analytical reagent grade and all solutions
were prepared with ultrapure water from EASY pure II
RF / UV (Thermo Science Co., USA).
Electrochemical experiments were performed on an
electrochemical workstation (CHI1230B, Shanghai Chen-
hua Co., China) with a conventional three-electrode cell.
A modified glassy carbon electrode was used as workingelectrode. A saturated calomel electrode (SCE) and a plat-
inum wire were used as reference electrode and auxiliary
electrode, respectively. The pH of the solution was mea-
sured using a pH-meter (Multi340i, WTW, Germany). All
the measurements were carried out at room temperature.
1.2 Preparation of MWCNTs / GCE
Before modification, a bare glassy carbon electrode (GCE)
was polished with 0.05 µm alumina slurry on polishing
cloth, successively washed with anhydrous ethanol and ul-
trapure water in an ultrasonic cleaner, and dried before use.
MWCNTs of 1.0 mg were added to 1 mL of N,N-dimethyl
formamide (DMF), and the mixture was sonicated for 30
min to obtain dispersed MWCNTs. The MWCNTs / GCE
was prepared by drop coating 5 µL of the above dispersed
MWCNTs on the surface of GCE before drying with an
infrared light.
2 Results and discussion
2.1 Characterization of MWCNTs / GCE
The scanning electron micrograph (SEM) of MWCNTs
on GCE surface is shown in Fig. 1. The MWCNTs with
general diameters of 20–50 nm can be observed.
The electrochemical properties of the MWCNTs / GCE
and GCE were studied in 5 mmol / L [Fe(CN)6]3−/4− solu-
tion with 0.2 mol / L KCl as supporting electrolyte using
cyclic voltammetry at 50 mV / sec. The cyclic voltam-
mogram of MWCNTs / GCE in the supporting electrolyte
shows a pair of well-defined redox couple curves (Fig. 2,
curve b) with large peak value of current (124.5 µA) and
a high capacitance current (1.868 × 10−4C), while GCE
shows a redox couple (Fig. 2, curve a) with low peak
current (58.6 µA) and low capacitance current (7.876 ×
10−5 C). The relationship between the peak current ( I p) and
electroactive area ( A, cm2) can be expressed according to
the Randles-Sevcik equation:
I p = 2.69 × 105n3 / 2 ACD1 / 2υ1 / 2 (1)
Fig. 1 SEM image of MWCNTs film.
0.6 0.4 0.2 0.0 -0.2-120
-90
-60
-30
0
30
60
90
120 b
a
C u r r e n t ( µ A )
E (V)
Fig. 2 Cyclic voltammograms of GCE (curve a) and MWCNTs / GCE
(curve b) in 5 mmol / L [Fe(CN)6]3−/4− solution containing 0.2 mol / L KCl
at a scan rate of 50 mV / sec.
where, n is the number of electrons participating in the
redox reaction, C (mol / L) is the concentration of the redox
probe, D (cm2 / sec) is diff usion coefficient of the redox
probe, and υ (V / sec) is the scan rate. The [Fe(CN)6]3−/4−
redox system used in this study exhibited a one-electron
transfer, n is equal to 1, C is equal to 5 mmol / L and D is
6.7 × 10−6 cm2 / sec. Thus, A can be calculated to be 0.075
and 0.16 cm2 for GCE and MWCNTs / GCE, respectively,
indicating that MWCNTs could obviously increase the
active surface area of the electrode.
2.2 Cyclic voltammetric behavior of 4- n-octylphenol
The cyclic voltammograms of the GCE and MWC-
NTs / GCE in BR buff er solution (pH 5) in the presence
of 5 × 10−6 mol / L OP were obtained. Voltammograms as
shown in Fig. 3 (curves a and b) only have anodic peaks.
The absence of cathode peaks suggests that the oxidation
of OP at these electrodes is totally irreversible under
studied experimental conditions. Moreover, the oxidation
peak current of OP at the MWCNTs / GCE is much higher
thanat GCE ( I pMWCNTs / I pGCE is about 12.2), indicating that
MWCNTs could increase the active area and adsorptionsites of the electrode and improve the electrochemical
behavior of OP, leading to enhance current response for
OP detection.
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No. 9 A simple and sensitive method for the determination of 4-n-octylphenol based on multi-walled carbon nanotubes······ 1719
1.0 0.8 0.6 0.4 0.2 0.0
-30
-25
-20
-15
-10
-5
0
5
10
c
b
a
E (V)
C u r r e n t ( µ A )
Fig. 3 Cyclic voltammograms of GCE (curve a) and MWCNTs / GCE
(curve b) in BR buff er solution (pH 5) containing 5 × 10−6 mol / L OP at a
scan rate of 50 mV / sec. Curve c: blank solution.
2.3 Eff ect of pH
The eff ect of pH on the electrochemical oxidation of OP
at the MWCNTs / GCE was investigated in diff erent BR
buff er solutions (pH 3–10). As shown in Fig. 4a, the current
response of OP increases gradually from pH 3 to 5 and then
decreases at pH values higher than 5. It is likely that under
high pH conditions, hydroxyl ions (OH−) in the solution
increase and combine with the carboxyl group (–COOH)
of MWCNTs, which reduces the adsorption of OP on
MWCNTs, leading to decrease the current response (Chen
et al., 2000). Therefore, pH 5 was chosen as optimum pH
for the subsequent analytical experiments.
The relationship between the oxidation peak potential
( E p) and pH is shown in Fig. 4b. A linear shift of E ptowards the negative potential with increasing pH indicates
that protons were directly involved in the oxidation of OP.
It obeyed the following Eq. (2): E p = −0.0618 pH + 0.9553 ( R = 0.9976) (2)
The slope is close to the theoretical Nernstian value
of 0.059 V, indicating that the number of protons and
electrons involved during the oxidation reaction is the same
(Luczak, 2008).
2.4 Eff ect of scan rate
In order to investigate the eff ect of scan rate on the
oxidation of OP at MWCNTs / GCE, cyclic volammograms
of 5 × 10−6 mol / L OP at diff erent scan rates of 20–250
mV / sec were obtained (Fig. 5a). Figure 5b shows that the
plot of the oxidation peak current ( I p) versus the square
root of scan rate (υ1/2) is linear and can be expressed as
Eq. (3):
I p = −6.1942υ1 / 2+ 13.7030 ( R = 0.9992) (3)
This result suggests that the oxidation of OP at MWC-
NTs / GCE is a typical diff usion-controlled process.
The oxidation peak potential ( E p) versus the natural
logarithm of scan rate (lnυ) is linear as shown in Fig. 5c
and the linear regression equation can be expressed as:
E p = 0.0512lnυ + 0.4177 ( R = 0.9930) (4)
1.0 0.8 0.6 0.4 0.2 0.0-90
-80
-70
-60
-50
-40
-30
-20
-10
0
a
C u r r e n t ( µ A )
E (V)
2 4 6 8 10 12
0.3
0.4
0.5
0.6
0.7
0.8
b
E p
( V )
pH
pH 3 pH 10
Fig. 4 (a) Linear sweep voltammograms (LSV) of MWCNTs / GCE in diff erent BR buff er solutions containing 5×10−6 mol / L OP at a scan rate of 100
mV / sec (pH: 3, 4, 5, 7, 8, 9, 10); (b) eff ect of pH on the oxidation potential ( E p).
1.0 0.8 0.6 0.4 0.2 0.0
-120
-100
-80
-60
-40
-20
0
20 a
20 mV/sec
250 mV/sec C u r r e n t ( µ A )
E (V)
4 6 8 10 12 14 16
-
90
-80
-70
-60
-50
-40
-30
-20
-10 b c
I p ( µ A )
υ1/2 (mV/sec)1/2
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
0.58
0.60
0.62
0.64
0.66
0.68
0.70
0.72
E p
( V )
lnυ
Fig. 5 (a) Cyclic voltammograms of 5 × 10−6 mol / L OP at MWVNTs / GCE with diff erent scan rates (20, 40, 60, 80, 120, 160, 200 and 250 mV / sec);
(b) the plot of the peak current ( I p) versus the square root of scan rate υ; (c) the relationship between E p and lnυ.
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1720 Journal of Environmental Sciences 2012, 24(9) 1717–1722 / Qiaoli Zheng et al. Vol. 24
For a totally irreversible electrode process, the rela-
tionship between the peak potential and scan rate can be
expressed as follows (Laviron, 1974):
E p = E 0 + RT
αnF
ln( RT k 0
αnF
) + RT
αnF
lnυ (5)
where, n is the number of electron transfer, α is the
electron transfer coefficient which is assumed to be 0.5
during a totally irreversible electrode process, E 0 is formal
potential, k 0 is standard rate constant of the reaction, and
R, T and F are gas-constant, temperature and Faraday
constant, respectively. In the present case, the calculated
value of n is 1.003, therefore the number of electron
transfer during the electrochemical oxidation of OP is 1.
It has been demonstrated that the number of electrons and
protons involved in the anodic oxidation reaction of OP
is the same (see Section 2.3), thus the electrochemical
oxidation of OP at MWCNTs / GCE is a one-electron and
one-proton process. The hydroxyl radicals might play an
important role in the electrochemical oxidation of OP
(Comninellis, 1994; Simod and Comninellis, 1997). It is
generally considered that OP oxidation begins with an
electron transfer that leads to phenoxy radicals. The radi-
cals reactions result in the formation of quinone structures,
which is believed to an important intermediate of OP
oxidation. Therefore, the oxidation reaction is promoted
and the oxidative current is obtained (Ngundi et al., 2003;
Li et al., 2005). The proposed reaction mechanism for the
electrochemical oxidation of OP is as the following.
OH
C8H
17C
8H
17C8H
17
O O O
·
C-C7H
16
-e-
-H+
·
(6)
2.5 Eff ect of accumulation time
The eff ect of accumulation time on the current response
of 5 × 10−6 mol / L OP obtained at the MWCNTs / GCE
was investigated. As shown in Fig. 6, the oxidation peak
current increases gradually with accumulation time (1 to
15 min), other conditions remaining unchanged indicating
that OP could be adsorbed on the electrode surface with
extending accumulation time. The peak current reaches a
maximum value with no indication of further increase with
accumulation time. This phenomenon can be attributed to
the saturated adsorption of OP at the electrode surface.
Accordingly, the optimal accumulation time was chosen as
15 min in the further experiments.
0 2 4 6 8 10 12 14 16 18 20 22
20
30
40
50
60
70
80
I p ( µ A
)
Tim (min)
Fig. 6 Eff ect of accumulation time on the oxidation current response of
5 × 10−6 mol / L OP.
2.6 Calibration curve
The determination of OP at the MWCNTs / GCE was
performed using linear sweep voltammetry (LSV) at a scan
rate of 100 mV / sec. Figure 7 shows that the oxidation
peak current ( I p) is proportional to OP concentration in
the range of 5 × 10−8 to 1 × 10−5 mol / L (about 10 to
2060 µg / L) with a detection limit of 1.5 × 10−8 mol / L
(about 3 µg / L), and the linear regression equation can be
expressed as I p = –6.5406 OP concentration – 4.204 ( R
= 0.9986). The result of the analytical determination of
OP by this method shows a low detection limit, which is
much better than some of the previous reports based on
chromatography (Table 1), suggesting that this proposed
method could potentially be used for monitoring of trace
OPs in environment.
2.7 Reproducibility, stability and interference
In the reproducibility tests, it was found that the relative
standard deviations of linear sweep voltammetric respons-
es of 5 × 10−6 mol / L OP obtained at the MWCNTs / GCE
for 10 replicates was 2.4%, exhibiting an excellent repro-
ducibility. The stability of the modified electrode was also
investigated by measuring the current response of 5 × 10−6
mol / L OP every 10 days by LSV. Between measurements
the electrode was stored at 4°C in a refrigerator. The
current response decreased to 98% after 10 days, while90% of the initial response retained after 20 days. The
response still retained 85% after over 30 days. In addition,
the selectivity of MWCNTs / GCE for the detection of OP
was tested in the presence of various interferents in BR
buff er solution containing 1 × 10−6 mol / L OP. The results
suggested that phenol, hydroquinone, dichlorophenol and
some ions such as Ca2+, Mg2+, Al3+, Cu2+, Cl− and SO42−
did not show interference to the determination of OP.
Table 1 Comparison of the analytical parameters obtained using other methods for the determination of OP
Method Concentration range Limit of detection Reference
Gas chromatography mass spectro metry 0.02– 1.00 mg / L 0.01 mg / L Tsuda et al., 1999
Micellar electr okinetic chromatograph y 5–20 0 mg / L 5 mg / L Cai et al., 2004
Liquid chromatography mass spectrometry 0.1–10 mg / kg 0.03 mg / kg Andreu et al., 2007
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No. 9 A simple and sensitive method for the determination of 4-n-octylphenol based on multi-walled carbon nanotubes······ 1721
1.0 0.8 0.6 0.4 0.2 0.0
-80
-70
-60
-50
-40
-30
-20
-10
0 a
0
10 μmol/L
E (V)
0 2 4 6 8 10
-70
-60
-50
-40
-30
-20
-10
0 b
OP concentration (µmol/L)
C u r r e n t ( µ A )
I p ( µ A
)
Fig. 7 (a) Linear sweep voltammograms of MWCNTs / GCE in BR buff er solution (pH 5) containing diff erent concentration of OPs (0, 0.05, 0.1, 0.2,
0.4, 0.8, 1.0, 2.0, 4.0, 6.0, 8.0, 10 µmol / L); (b) calibration curve of the peak current against the concentration of OP. Accumulation time: 15 min, scan
rate: 100 mV / sec, potential: 0–1.0 V.
3 Conclusions
In this work, a simple and sensitive electrochemical
method was proposed for the determination of 4-n-
octylphenol based on MWCNTs / GCE. The oxidation peak
current of OP was significantly enhanced after electrode
modification. The electrochemical oxidation mechanism of
OP was demonstrated by a one-electron and one-proton
process in the electrode reaction. The method exhibited
some obvious advantages, such as simple preparation
process, high sensitivity, low cost and good stability,
suggesting that it has a potential application for trace OPs
detection in environment.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (No. 21005014, 41073060), the Fun-
damental Research Funds for the Central Universities (No.
2011D11307) and the ‘Chen Guang’ project of Shanghai
Municipal Education Commission and Shanghai Educa-
tion Development Foundation (No. 11CG34).
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