a simple and sensitive method for the determination of 4-n-octylphenol based on multi-walled carbon...

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.



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.



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)


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


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.




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



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)


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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