Electrochemical determination of methyl parathion using a molecularly imprinted polymer–ionic liquid–graphene composite film coated electrode

Sensors and Actuators B 176 (2013) 818–824

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Electrochemical determination of methyl parathion using a molecularly imprinted polymer–ionic liquid–graphene composite film coated electrode Lijuan Zhao, Faqiong Zhao, Baizhao Zeng ∗ Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei Province, PR China

a r t i c l e

i n f o

Article history: Received 11 July 2012 Received in revised form 23 September 2012 Accepted 1 October 2012 Available online 11 October 2012 Keywords: Methyl parathion Molecularly imprinted polymer Graphene Ionic liquid Electrochemical sensor

a b s t r a c t A molecularly imprinted polymer–ionic liquid–graphene composite film coated glassy carbon electrode (MIP–IL–EGN/GCE) was presented for the first time. It was fabricated by coating a GCE with IL-graphene oxide (GO) mixture, followed by MIP suspension. The resulting electrode was then conditioned at −1.3 V (vs SCE) in a Na2 SO4 solution to make the GO turn to graphene (EGN). The MIP was prepared by free radical polymerization using methyl parathion (MP) as template, methacrylic acid as functional monomer, ethyleneglycol dimethacrylate as cross-linking reagent and 2,2 -azobis(isobutyronitrile) as initiator. The response property of MIP–IL–EGN/GCE to MP was studied. Under the optimized conditions, the peak current of MP was linear to its concentration in the range of 0.010–7.0 ␮M with a sensitivity of 12.5 ␮A/␮M, and the detection limit was 6 nM (S/N = 3). When a 1.0 ␮M MP solution was determined for five times using a MIP–IL–EGN/GCE, the RSD of peak current was 2.3%; the electrode-to-electrode RSD was 6.4% (n = 5). The sensor also displayed high selectivity and stability. It was applied to the determination of MP in samples and the recovery was 97–110%. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Methyl parathion (MP) is an organophosphorus pesticide and it is widely used in agricultural production [1]. As organophosphorus pesticides have high toxicity, the maximum amount of their residues in food is limited by authorities and in China it is 0.1 mg/kg. Thus the sensitive detection of organophosphorus pesticide residues is generally necessary [2–4]. Now MP is currently determined by using gas chromatography–mass spectrometry, immunoassays, liquid chromatography–mass spectrometry and mass spectrometry [5–8]. But these methods usually suffer from complicated sample pretreatment and/or high cost. Electroanalytical method is rapid, simple and cheap [9,10], so it also has been exploited for the MP detection. For example, Du et al. [11] employed a bismuth film modified electrode to detect MP, the linear range was 1.1 × 10−8 –3.8 × 10−7 M and the detection limit was 4.6 × 10−9 M; Xu et al. [12] prepared a poly(malachite green)/graphene nanosheet–Nafion (PMG/GNs–NF) composite film modified glassy carbon electrode for MP detection, the linear range was 2 × 10−8 –1.5 × 10−6 M and the detection limit was 2 × 10−9 M; Li et al. [2] used a polyquercetin (Qu)–polyresorcinol (Re)–gold nanoparticle (AuNP) modified electrode to determine MP and a linear range of 7.0 × 10−8 –1.0 × 10−6 M was obtained. These

∗ Corresponding author. Tel.: +86 27 68752701; fax: +86 27 68754067. E-mail addresses: [email protected], [email protected] (B. Zeng). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.10.003

electroanalytical methods were quite sensitive, but some of them showed poor selectivity. It is well known, molecularly imprinted polymers (MIPs) are biomimetic recognition materials. In comparison with natural antigen, antibody and enzyme, they possess many advantages such as superior stability, low cost, easy preparation and storage [13–15]. Hence, in order to improve the selectivity and stability of electrochemical sensors, it is a good way to fabricate MIPs based sensors. In fact, MIPs were widely used to prepare electrochemical sensors in recent years and they presented good performance [16–18]. Recently, Zhang et al. [19] constructed a MIP–vinyl group functionalized multiwalled carbon nanotube (MWCNTs) composite electrode for the voltammetric determination of MP. The electrode showed high selectivity; the linear range was 2.0 × 10−7 –1.0 × 10−5 M and the detection limit was 6.7 × 10−8 M. In general, MIPs are nonconductive. To fabricate MIPs based electrochemical sensors other conductive materials should be introduced, such as MWCNT, nano-gold, conductive organic polymers and inorganic polymers [19–22]. The conductive materials should possess high electrical conductivity, good stability and large surface area. Graphene (GR) is a novel nano-material with many advantages such as excellent electrical conductivity, high chemical stability and large surface [23]. Moreover, GR has the advantage of low manufacturing cost in comparison with other nanostructured carbon materials [24]. These make GR quite promising as novel electrode material [25]. In particular, the GR resulted from the

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electrochemical reduction of graphene oxide (GO) displays superior electronic property [26,27]. Hence it is suitable for fabricating MIPs based electrochemical sensors. In addition, ionic liquids (ILs) have wide electrochemical windows, high ionic conductivity and good solubility [28–30], and they are widely used in electroanalysis, material preparation and so on. Furthermore, ILs present shielding effect to the ␲–␲ stacking interaction among graphene sheets, thus they can promote the dispersion of graphene sheets [31–33] and IL-GR composite may show improved performance in fabricating electrochemical sensors. In this paper, MIPs, IL and graphene are combined to prepare MP sensor. Owing to their synergic action, the resulting electrochemical sensor exhibits high selectivity, high sensitivity and fast response towards MP. 2. Experimental 2.1. Reagents Methyl parathion and parathion were bought from Gamma Technology Development Co., Ltd. (Shenzhen, China) and their stock solutions (0.010 M) were prepared with ethanol and stored in a refrigerator at 4 ◦ C. 2,2 -Azobis-(isobutyronitrile) (AIBN) was obtained from Shanghai Shisihewei Chemical Industry Limited Company (China). Ethyleneglycol dimethacrylate (EDMA) was purchased from Energy-Chemical Company (Shanghai, China). Methacrylic acid (MAA) came from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The ionic liquid 1-(2 -hydroxylethyl)3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([HeMIM][NTf2 ]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6 ]) and N-methyl-piperidinium bis[(trifluoromethyl)sulfonyl]imide ([PP1][NTf2 ]) (purity: 99%, see Supplementary Materials Fig. S1) were provided by Lanzhou Institute of Chemical Physics (Lanzhou, China) and used as received. The graphene oxide came from Xianfeng Reagent Co., Ltd. (Nanjing, China). All other chemicals used were of analytical reagent grade. The water used was redistilled. 2.2. Apparatus Cyclic voltammetric and differential pulse voltammetric experiments were performed with a CHI 620D electrochemical workstation (CH Instrument Company, Shanghai, China). A conventional three-electrode system was adopted. The working electrode was a modified glassy carbon electrode (diameter: 2 mm) or a glass substrate (10 mm × 10 mm × 2.2 mm) coated with indium tin oxide (ITO), and the auxiliary and reference electrodes were a platinum wire and a saturated calomel electrode (SCE), respectively. The scanning electron microscope (SEM) image was obtained using a Hitachi X-650 SEM (Hitachi Co., Japan). Ultraviolet visible (UV–vis) absorption spectra were recorded by a U-3900 spectrometer (Hitachi Co., Japan). The Fourie transform infrared (FTIR) absorption spectra were recorded with a model Nexus-670 spectrometer (Nicolet, USA). The pH values of solutions were measured with a pHS-2 meter (Leici Instrumental Factory, Shanghai, China). All experiments were carried out at room temperature.

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mixture was placed in a water bath at 60 ◦ C and let it polymerize for 24 h. When the polymerization was completed, the product was washed thoroughly with ethanol (with the aid of ultrasonication) and collected after centrifugation (10 min, 10,000 rpm). The washing and centrifugation procedures were repeated for several times until the product was free of template molecule, which was detected by ultraviolet visible spectrophotometry. The nonimprinted polymer (NIP) was synthesized through the same way except the template molecule was absent. 2.4. Preparation of MIP and NIP modified electrodes The bare GCE was polished to a mirror-finished with slurry alumina (0.05 mm), and then washed with water and ethanol each for 5 min, with the aid of ultrasonication. GO was dispersed in redistilled water to prepare 1 mg/mL GO suspension. Then 0.5 mL GO suspension was mixed with 0.5 mL [HeMIM][NTf2 ] solution (5 ␮L/mL, in N,N-dimethylformamide, DMF). A 2 mg MIP was dispersed in 1.0 mL DMF by ultrasonication for about 1 h. Then 4 ␮L of IL-GO solution was transferred on the cleaned GCE; after the solvent was evaporated under an infrared lamp, 6 ␮L of the MIP suspension was dropped onto the resulting IL–GO/GCE and the solvent was evaporated in air. Thus an MIP–IL–GO film coated electrode (MIPIL-GO/GCE) was obtained. The MIP–IL–GO/GCE was immersed in a 0.2 M Na2 SO4 aqueous solution and the potential was hold at −1.3 V (vs SCE) for 600 s. Thus GO was reduced to graphene and the obtained electrode was denoted as MIP–IL–EGN/GCE. Before measurement, the electrode was conditioned by repeating potential scan between −0.2 V and 0.6 V in phosphate buffer solution (PBS) until a stable CV curve was obtained. Similarly, the NIP-IL-GO/GCE and other electrodes were prepared. 2.5. Determination of adsorption amount The equilibrium adsorption amounts of MIP and NIP were determined as follows: 2 mg MIP (or NIP) was added to 10 mL MP solution (containing 3.80 × 10−5 –2.65 × 10−4 M). After stirred for 5 h, the mixture was centrifuged and the solution was collected and determined by ultraviolet visible spectrophotometry. The adsorption amount was calculated according to the formula: Q = V(C0 − Cs )/m. Where V represents the volume of solution, C0 and Cs are the MP concentration of solutions before and after adsorption, m is the mass of MIP (or NIP). Similarly, the adsorption amounts of some foreign compounds (i.e. parathion, 2,4-dinitrophenol, p-nitroluenem, p-nitrophenol and (2,4-dichlorophenoxy) acetic acid) on MIP were determined. 2.6. Electrochemical measurements Proper volumes of MP and PBS (pH 6.8) were transferred to a 10 mL cell, and the electrode-system was installed on it. After accumulation for 150 s under open-circuit, cyclic voltammograms (CV) or differential pulse voltammograms (DPV) were recorded. The potential scan ranges were 1.0 V to −0.4 V (for CV) and 0.8 V to −0.4 V (for DPV), respectively. After every measurement, the electrode was rinsed with ethanol to remove MP for reuse. 3. Results and discussion

2.3. Preparation of molecularly imprinted polymer 3.1. Morphological and structural characterization The MIP was prepared through a typical precipitation polymerization [34]. Briefly, 65.7 mg (0.25 mmol) of MP was dissolved in 35 mL dehydrated methanol, 87 ␮L of methacrylic acid (1 mmol), 0.95 mL EDMA (5 mmol) and 12 mg AIBN was then added. The solution was degassed for 10 min in an ultrasonic bath, purged with N2 for 10 min and then sealed in nitrogen atmosphere. Next, the

The SEMs of GO, EGN and IL–EGN are shown in Fig. 1. The GO film presents sheet-like shape with scrolled edges; after it is reduced to graphene, the scrolled edges reduce and it becomes more flat. The IL–EGN composite film is quite uniform due to the masking and adhesive effect of the IL. This composition is favorable for the

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L. Zhao et al. / Sensors and Actuators B 176 (2013) 818–824

Fig. 1. SEMs of GO(A), EGN(B), IL–EGN(C), MP imprinted polymer (D), nonimprinted polymer (E), and MIP– IL–EGN(F).

immobilization of MIP and benefits electron transfer as IL is viscid and conductive. When the MIP is transferred to the composite film the MIP particles are well dispersed on it (Fig. 1F). Fig. 1 also displays the SEMs of the MIP and NIP. It can be seen that the MIP particles and NIP particles are similar, showing sphere-shape, with an average diameter of 3 ␮m; but NIP particles are not so regular. This means that the template molecule can influence the particle shape to some extent. Fig. 2 presents the FTIR spectra of different composition. GO exhibits five absorption bands at 3409 cm−1 (caused by O H), 1723 cm−1 (caused by C O), 1624 cm−1 (caused by aromatic C C), 1384 cm−1 (caused by carboxy C O) and 1078 cm−1 (caused by alkoxy/alkoxide C O stretches), respectively. After it is reduced, the absorption bands of C O and C O decrease, indicating that the exfoliated GO is turned to EGN. [C2 OHMIM][NTf2 ] produces absorption peaks at 2926, 1729 and 955 cm−1 . As can be seen, when the IL and EGN are mixed, the FTIR spectrum of the resulting composite is not the simple mixture of that of EGN and [C2 OHMIM][NTf2 ]. Some new peaks occur and some peaks of IL disappear. Part of this can be ascribed to the small amount of IL in the composite. On the other

hand, it indicates that EGN and [C2 OHMIM][NTf2 ] must interact with each other through forming hydrogen bond etc. 3.2. Adsorption curves The adsorption curves of MP on the MIP and NIP are determined (see Supplementary Materials, Fig. S2). For the MIP the adsorption amount of MP varies with MP concentration more rapidly. When it exceeds 2 × 10−4 M, the adsorption amount is close to the maximum value, which is considered the maximum adsorption capacity of MIP and it is about 3.5 × 10−3 mol/g. For the NIP the adsorption amount of MP changes more slowly with MP concentration changing. Similarly, when MP concentration exceeds 2 × 10−4 M, the adsorption curve exhibits a platform. This means that a saturated adsorption is achieved and the adsorption capacity of NIP is about 1.3 × 10−3 mol/g. It is much smaller than that of MIP. Obviously, through molecular imprinting the adsorption amount of MP is greatly enhanced. The adsorption curves of some foreign compounds with similar structure or group to MP are presented in Fig. 3. It is clear, except parathion other compounds show much weaker adsorption than

40

a -1

C-O C=C O-H

-4

b

Q/10 mol g

Transmittance(%)

35 C=O

c

30 25

methyl parathion parathion 2,4-dinitrophenol p-nitrotoluenem p-nitrophenol (2,4-dichlorophenoxy) acetic acid

20 15 10

d

5 0

4000 3500 3000 2500 2000 1500 1000

500

0.5

1.0

1.5 2.0 -4 c/10 M

2.5

3.0

-1

Wavenumber(cm ) Fig. 2. FTIR spectra of GO (a), EGN (b), IL (c) and IL–EGN (d).

Fig. 3. Adsorption curves of MP, parathion, 2,4-dinitrophenol, p-nitroluenem, pnitrophenol and (2,4-dichlorophenoxy) acetic acid (from upper to bottom) on the MIP.

L. Zhao et al. / Sensors and Actuators B 176 (2013) 818–824

-80

20

a

Peak current/µA

Current/ µA

-60 -40

b

-20

c

0

821

d

20 40

16 12 8 4

60 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

0.2

0.4

Potential/V

0

2

4

6 8 -1 c/µL mL

10

12

Fig. 4. Cyclic voltammograms (corresponding to the second scan) of MIP–IL–EGN/GCE (a), NIP–IL–EGN/GCE (b), MIP–IL–GO/GCE (c), MIP/GCE (d) in 0.1 M PBS (pH 6.8) containing 10 ␮M MP. Inset: the CVs corresponding to the first and second scans of MIP–IL–EGN/GCE. Scan rate: 50 mV/s.

Fig. 5. Effect of IL (i.e. [HeMIM][NTf2 ]) concentration on the peak current of 1 ␮M MP. Other conditions as in Fig. 4.

MP on the MIP. This means that the MIP has good selectivity. As to parathion, its structure is very similar to that of MP, thus its adsorption amount is also larger.

hence the resultant MIP–IL–EGN/GCE displays weaker accumulation effect and response to MP. Meanwhile, we also studied the effect of IL concentration on the peak current of MP (Fig. 5). With the addition of IL, the peak current increases rapidly at first, then slowly and keeps almost unchanged at last. Here, 5 ␮L/mL IL is adopted.

3.3. Voltammetric behavior of MP The cyclic voltammograms of MP at different electrodes are recorded (Fig. 4). MP can exhibit a sharp irreversible cathodic peak at −0.59 V(I) during the first scan (Fig. 4, Inset), which results from the irreversible reduction of the nitrophenyl group to hydroxylamine [35]; during the repetitive scan a pair of quasi-reversible peaks occur at −0.053 V(II) and −0.104 V(III), corresponding to the redox of nitroso compound and hydroxylamine (see Supplementary Materials, Fig. S3). At the same time, the peak I decreases significantly. It is clear in Fig. 4, MP produces small peaks at the MIP/GCE and MIP–IL–GO/GCE. This can be ascribed to their poor conductivity and small effective surface area. The small background signal can support this point to some extent. When GO is reduced it becomes conductive and its structure also changes; thus the resulting electrodes present enhanced response to MP. As can be seen, MP exhibits much bigger peaks at the MIP–IL–EGN/GCE than at the NIP–IL–EGN/GCE, indicating that the MIP plays an important role in sensing MP. This is because the MIP can provide many specific sites for MP. Without IL the obtained electrodes present weaker response to MP; furthermore, the electrodes are not so stable. Undoubtedly, this is related to the accumulation and immobilization effect of IL. We think that the IL must interact with MP as it is liquid and it can be incorporated into the MIP. Therefore, the MIP-IL-EGN/GCE virtually combines the effect of MIP, EGN and IL, and thus gives excellent electrochemical response to MP. 3.4. Optimization of conditions 3.4.1. Ionic liquid Considering the influence of IL, several ILs are compared (see Supplementary Materials, Fig. S4). When [HeMIM][NTf2 ] is used, the resulting MIP-IL-EGN/GCE produces big peaks for MP; while it is replaced by [BMIM][PF6 ] and [PP1][NTf2 ] the peaks become small. Part of this can be ascribed to the interaction among IL, MP and EGN. [HeMIM][NTf2 ] contains imidazole ring and hydroxyl, giving rise to the ␲–␲ bond and hydrogen bond among them, thus MIP–[HeMIM][NTf2 ]–EGN/GCE has good accumulation effect and response to MP. The interaction between MP and [BMIM][PF6 ] (or [PP1][NTf2 ]) is relatively weak due to the short of hydrogen bond,

3.4.2. The amount of GO and MIP To explore the influence of GO amount the concentration of IL (5 ␮L/mL) and the volume of IL–GO suspension used for modifying the GC are kept unchanged. As a result, the peak current of MP increases with increasing the concentration of GO suspension up to 1.0 mg/mL, then it decreases slowly (see Supplementary Materials, Fig. S5). This is related to the change of electrode area and the electron transfer resistance. When the GO amount is too large the electrode area keeps almost unchanged, but the resistance increases. In this case, 4.0 ␮L of 1.0 mg/mL GO suspension was selected. The amount of MIP is also varied to examine its influence on the peak current of MP. The experiment result shows when the MIP amount is smaller (e.g. 1 mg/mL, 6.0 ␮L), the peak current is bigger, but the selectivity is poor; when the amount of MIP is too large (e.g. 3 mg/mL, 6.0 ␮L), the MIP beads easily peel off the electrode surface. Here 6.0 ␮L of 2.0 mg/mL MIP suspension is optimal. 3.4.3. Solution pH As the electrochemical reaction of MP involves proton transfer, the solution pH does influence the peak current. Herein we study the influence of pH in the range of 4.5–8.5 (Fig. 6). The peak current increases with pH rising up to 6.8, and then it decreases with further increasing pH. Therefore, pH 6.8 phosphate buffer solution is adopted. At the same time, the peak potential (Ep ) changes with pH. Furthermore, they show a linear relationship as Ep (V) = −0.0601pH + 0.237, with a correlation coefficient of 0.9884 and a slope of 60.1 mV/pH. According to the Nernstian equation: E ∝ 59.16 m/npH (m represents proton-transfer number, n represents electron-transfer number), we can see that the electrochemical reaction involves equal numbers of proton-transfer and electron-transfer. 3.4.4. Accumulation time As shown in Fig. 7, the peak current of 1 ␮M MP increases with time going. When it is above 150 s, the peak current keeps almost unchanged, meaning that the saturated rebinding of MP onto the MIP-IL-EGN/GCE is achieved. Therefore, 150 s is selected as the optimal accumulation time for MP determination.

L. Zhao et al. / Sensors and Actuators B 176 (2013) 818–824

18 16 14 12 10 8 6 4 2 0

-40 -20 0 20

4

5

6

7

8

-0.8

9

Fig. 8 shows the differential pulse voltammograms of MP at MIP-IL-EGN/GCE under the optimized experimental conditions. It can be seen that the peak current increases with MP concentration increasing. Furthermore, they present fine linear relationship in the range from 1.0 × 10−8 M to 7 × 10−6 M, the regression equation is Ip (␮A) = 12.52c (␮M) + 0.0852 (R2 = 0.9957), with a sensitivity of 12.5 ␮A/␮M. But when MP concentration is above 7 × 10−6 M, the calibration curve gradually deviates from the straight line, indicating that the saturated adsorption is gradually reached. The limit of detection is 6.0 × 10−9 M (S/N = 3). These are quite good in comparison with other electrodes, such as ordered mesoporous carbon modified electrode, ZrO2 -nanoparticles modified carbon paste electrode and poly(malachite green)/graphene nanosheet–Nafion electrode etc. [11,36–39] (see Supplementary Materials, Table S1). Compared with the MIP-IL-EGN/GCE, the NIP-IL-EGN/GCE shows a smaller linear range and lower sensitivity, which can be explained by the lack of specific binding site on the NIP film. 3.6. Selectivity, repeatability and stability of MIP-IL-EGN/GCE

16 14 12 10

80 MIP-IL-EGN/GCE

60 40 20

20

90

120 150 180 210 t/s

Fig. 7. Effect of accumulation time on the peak current of 1 ␮M MP. Other conditions as in Fig. 4.

4 6 c/µM

8

10

a

b

c

d

e

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m e th

60

2

(2,4-dichlorophenoxy) acetic acid. As shown in Fig. 9, when 10-fold foreign substance is present the change of the peak current of MP is less than 5% (i.e. 96–104%). This indicates that the MIP–IL–EGN/GCE has good selectivity. However, as mentioned previously, parathion

8 4 0

30

NIP-IL-EGN/GCE

Fig. 8. (A) Different pulse voltammograms of MP at MIP–IL–EGN/GCE. MP concentration: 0.01, 0.02, 0.04, 0.06, 0.08, 0.14, 0.2, 0.6, 1.0, 2.0, 3.0, 4.0, 5.0, 7.0 ␮M (from a to n). Other conditions as in Fig. 4. (B) The calibration curves for MP at MIP–IL–EGN/GCE and NIP–IL–EGN/GCE. Inset: the calibration curve for 0.01–0.20 ␮M MP at MIP–IL–EGN/GCE.

8 6

0.4

B

0

Peak current/µA

18

0.2

0

To evaluate the selectivity of MIP–IL–EGN/GCE some foreign compounds with similar structure or group to MP are tested, such as 2,4-dinitrophenol, p-nitroluenem p-nitrophenol and

20

-0.4 -0.2 0.0 Potential/V

100 Peak current/ µA

3.5. Calibration curve

-0.6

120

Fig. 6. Effect of pH on the peak current of MP at MIP–IL–EGN/GCE. Inset: the plot of peak potential versus pH. Other conditions as in Fig. 4.

Peak current/ µA

a

-60

pH

4

n

A

-80 Current/ µA

Peak c urrent/ µA

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yl p a

l on id r a th i tr o p h e n o to l u e n e m o p h e n o l ti c a c ) ace in i tr o n i tr i y d n x p 4 o , p 2 h en lo ro p d ich (2 ,4 -

Fig. 9. Influence of coexistent substance on the electrochemical response of MIP–IL–EGN/GCE to MP. Solution composition: (a) 1.0 ␮M MP + 0.1 M PBS (pH = 6.8), (b) a + 10 ␮M 2,4-dinitrophenol, (c) a + 10 ␮M p-nitroluenem, (d) a + 10 ␮M pnitrophenol. (e) a + 10 ␮M (2,4-dichlorophenoxy) acetic acid. Other conditions as in Fig. 7.

L. Zhao et al. / Sensors and Actuators B 176 (2013) 818–824 Table 1 Determination results of MP in samples using an MIP–IL–EGN/GCE (n = 3). Samples

MP added (␮M)

Cabbage

0 0.10 0.50 1.00 5.00

– 0.10 ± 0.49 ± 0.97 ± 5.1 ±

0.07 0.03 0.06 0.3

– 100 98 97 102

0 0.10 0.50 1.00 5.00

– 0.11 ± 0.52 ± 0.98 ± 5.1 ±

0.09 0.03 0.05 0.3

– 110 104 98 102

Apple peel

MP found (␮M)

Recovery (%)

can also be strongly adsorbed by the MIP, thus it shows serious influence on the peak current of MP. When equal concentration of parathion is present the peak current of MP increases by 28%. To check the repeatability of the MIP-IL-EGN/GCE, a 1 ␮M MP solution is determined for five times using an electrode and the relative standard deviation (RSD) of the peak current is calculated to be 2.3%. A 1 ␮M MP solution is also determined every day using an electrode and an inter-day RSD of 4.5% (n = 5) is obtained. When a 1 ␮M MP solution is detected with five different electrodes prepared by the same way a RSD of 6.4% is obtained. When the MIP–IL–EGN/GCE is stored in a refrigerator at 4 ◦ C for 5 days, the peak current retains 93% of its initial value; After one month-store it becomes 86%. These indicate that the MIP-IL-EGN/GCE has good reproducibility and stability.

3.7. Application In order to evaluate the practical feasibility of the MIP-ILEGN/GCE for the determination of MP, cabbage and apple peel were determined. Prior to determination they were grinded to slurry and then centrifuged, 5.0 mL of the supernatant was weighed and diluted to 50 mL with 0.1 M PBS (pH = 6.8) for determination. But no MP was detected in these samples. Standard MP solutions were added to the sample solutions and the recovery was estimated. The results are summarized in Table 1 and the recovery is acceptable.

4. Conclusions A novel molecularly imprinted polymer–ionic liquid–graphene composite film coated glassy carbon electrode (MIP–IL–EGN/GCE) is fabricated for MP sensing. The electrode combines the effect of MIP, EGN and IL, and exhibited high stability, reproducibility and sensitivity. The electrode can be applied to the determination of MP in practical samples. This work provides a new way for constructing sensitive MIP sensors.

Acknowledgements The authors appreciate the financial support of the National Natural Science Foundation of China (Grant No. 21075092, 21277105) and the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, Grant No. 2010-KF-12).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2012.10.003.

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Biographies Lijuan Zhao is currently a postgraduate student of Department of Chemistry, Wuhan University. She majors in electrochemical sensor. Faqiong Zhao is an associate professor of chemistry, Department of Chemistry, Wuhan University. She graduated from Sichuan Normal University in 1998 and received her PhD in analytical chemistry from Wuhan University in 2002. Her research interests cover bioelectrochemistry and biosensor. Baizhao Zeng is a professor of chemistry, Department of Chemistry, Wuhan University, Wuhan, China. He graduated from the Department of Chemistry, Wuhan University in 1986. He received his PhD in the analytical chemistry from Wuhan University in 1992. He worked as a postdoctoral fellow from 1996 to 1998 at the Chemistry Department of McGill University, Montreal, Canada. His research interests cover electrochemical sensor and solid phase microextraction.