Electrochemical behavior of metribuzin based on l -Norvaline modified electrode and its sensitive determination

    Electrochemical behavior of metribuzin based on L-Norvaline modified electrode and its sensitive determination Donglin Jia, Lu Wang, Yudong Gao, Lina Zou, Baoxian Ye PII: DOI: Reference:

S1572-6657(16)30019-4 doi: 10.1016/j.jelechem.2016.01.016 JEAC 2457

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

16 November 2015 12 January 2016 15 January 2016

Please cite this article as: Donglin Jia, Lu Wang, Yudong Gao, Lina Zou, Baoxian Ye, Electrochemical behavior of metribuzin based on L-Norvaline modified electrode and its sensitive determination, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.01.016

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Electrochemical behavior of metribuzin based on L-Norvaline modified electrode and its sensitive determination Lina Zoua,*

Baoxian Ye a,*

T

Lu Wanga,b, Yudong Gaoa,

IP

Donglin Jiaa,

SC R

a College of Chemistry and Molecular Engineering, School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, PR China

b Department of Environmental Engineering and Chemistry, Luoyang Institute of Science and

NU

Technology, Luoyang 471023, PR China.

*Corresponding author. Tel.: +86 0371 67781757; fax: +86 0371 67763654.

MA

E-mail address: [email protected] (B. Ye).

D

Abstract

TE

The electro-polymerization of L-Norvaline was investigated for the first time on glassy

CE P

carbon electrode. The polymeric conditions and mechanism were discussed in detail. The electrochemical behaviors of metribuzin were studied systematically at this sensor and the dynamic parameters of electrode process were calculated. This electrochemical sensor,

AC

fabricated simply and very easy surface update, showed good response for metribuzin with a wide linear range from 6.43×10-3 to 1.07 μg/mL and a low detection limit of 2.14×10-3 μg/mL (S/N=3). The proposed method was successfully applied to determine metribuzin in soil sample with satisfactory results. This work promoted the potential applications of amino acid materials in electrochemical sensors.

Key words: Metribuzin; L-Norvaline; cyclic voltammetric

ACCEPTED MANUSCRIPT

1. Introduction Pesticide contamination in environment is one of the major health concern. These

T

contaminants, for example, have been detected in the milk from lactating women [1].

IP

Therefore, it is important to monitor their residues in all environmental segments. Triazine

SC R

and organophosphorus pesticides are detected in the environment and their environmental behaviors are of great concern, although several members of these classes have been banned for years [2-6].

NU

Metribuzin, [4-amino-6-(1-1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one], is a s-triazine herbicide. Its molecular structure is shown in Scheme 1(a). This herbicide is

MA

considered to be moderate persistence in soils. Metribuzin have received a great deal of attention. Various studies about determination of metribuzin have been investigated by using chromatographic methods, such as liquid chromatography [7-10], gas chromatography [11],

D

micellar electro kinetic chromatography [12,13], capillary zone electrophoresis [14],

TE

molecularly imprinted polymer [15,16] and the spectrophotometric method based on its complexation with copper [17]. Nevertheless, chromatographic method is accurate but

CE P

time-consuming, expensive, cumbersome operation. For now, researchers have paid attention to electroanalytical technique due to its advantages, such as high sensitivity, good selectivity, rapid response and low cost. So far as we know, there have four literatures for determination

AC

of metribuzin by electroanalytical method [18-21]. However, the literature [19-21] was no enough sensitivity and the other literature [18] used mercury electrode as the electrochemical sensor for determination of metribuzin. Mercury is toxic and the use of mercury electrode has been banned in several situations due to environmental concerns and safety regulations. Therefore, to develope a more sensitive and environmentally friendly electroanalytical method for determination of metribuzin is still interesting and significant. Up to now, conducting polymer has attracted considerable interest because of their actual and potential applications in different fields. In addation, plenty of conducting polymers have been employed in the fields of ion recognition [22], electron transfer [23] and electrocnemical sensor [24,25]. Among them, there are quite a few reports about amino acid as material to build sensing interface for analytical application [26-28], which reveal that amino acids are

ACCEPTED MANUSCRIPT promising materials for electrochemical sensors. L-Norvaline (C5H11NO2, Scheme 1(b) ), also called L-(+)-2-Aminovaleric acid, is a white crystalline powder. It is soluble in hot water and dilute hydrochloric acid, but is insoluble in ethanol and ether. As far as we know, there has no

this

approach,

a

poly-L-Norvaline

modified

glassy

IP

In

T

report about L-Norvaline applied to electrode modified material so far.

carbon

electrode

SC R

(poly(L-Norvaline)/GCE) was prepared by cyclic voltammetric method, and used as a voltammetric sensor for sensitive determination of metribuzin. The electro-polymerization parameters of poly(L-Norvaline)/GCE were discussed in detail. Using this sensor, the

NU

electrochemical behavior of metribuzin were investigated systematically and dynamic parameters of electrode process were determinated using various electrochemical techniques.

MA

The electrode exhibited a good response on the electrochemical reduction of metribuzin, decreasing the peak potential and also increasing the peak current. A simple, sensitive and

D

stable electroanalytical method of metribuzin was proposed with wide linear range and low

TE

detect limit. Besides, the proposed method was used for metribuzin determination in natural

Scheme 1(a) Scheme 1(b)

AC

2. Experimental

CE P

soil with satisfactory results.

2.1 Instruments and reagents Electrochemical measurements were performed using a RST3000 electrochemical workstation (Zhengzhou Shiruisi Instrument Co., Ltd., Zhengzhou, China) with conventional three-electrode cell. A bare GCE (3 mm diameter) or modified GCE was used as the working electrode. An Ag/AgCl and a platinum (Pt) wire were used as reference and counter electrodes, respectively. All the potentials in this paper refer to Ag/AgCl. Electrochemical experiments were performed in 10 mL supporting electrolyte at room temperature. L-Norvaline and metribuzin were purchased from Aladdin (http://www.aladdine.com/). Standard stock solution of metribuzin (2.14×103 μg/mL) was prepared with methyl alcohol and kept under 4℃. It was diluted to necessary concentration before use. Working solutions were prepared daily by dilution with 0.1 mol L-1 Britton–Robinson buffer (B–R). All other

ACCEPTED MANUSCRIPT reagents were of analytical grade and were used directly without further purification. 0.1 mol L-1 B–R buffer was prepared using a mixed acid (0.04 mol L-1 H3PO4 + 0.04 mol L-1 HAc + 0.04 mol L-1 H3BO3) that was titrated to the desired pH with 0.2 mol L-1 NaOH. A pH 8.0 buffer

solution)

aqueous

L-Norvaline

was

used

for

SC R

IP

electro-polymerization.

solution

T

(phosphate

2.2 Fabrication of poly(L-Norvaline)/GCE

Prior to modification, the GCE was polished to a mirror finish using finer emery-paper

NU

and 0.5μm alumina slurry respectively. After rinsing thoroughly with water, the GCE was washed ultrasonically in absolute alcohol and double-distilled water again. Then L-Norvaline

MA

(2.5×10-3 mol L-1 in phosphate buffer solution, pH 8.0) was electrodeposited on the cleaned GCE surface by cyclic scanning between -1500 and +2500 mV with 100 mV s-1 for four

D

cycles. This was the optimal polymeric condition for fabricating the poly(L-Norvaline)/GCE

TE

from test. Prior to use, the poly(L- Norvaline)/GCE was pretreated in a 0.1 mol L-1 B-R buffer solution by cyclic scanning between potentials of -400 and -1000 mV (5 cycles).

CE P

2.3 Analytical Procedure

All electrochemical performences were carried out in B-R buffer solution (pH 1.82) at room temperature unless otherwise specified. Before measure, the poly(L-Norvaline)/GCE

AC

was put in pH 1.82 B–R solution for successive cyclic sweeps between -0.4 and -1.0 V at 100 mV s-1. When the voltammogram became steady, a known volume of metribuzin standard solution was added into the electrochemical cell. Following each measurement, the poly(L-Norvaline)/GCE was put in the original solution for two cyclic scanning to renew the electrode surface.

3. Results and discussion 3.1 Electro-polymerization of L-Norvaline on GCE For a polymer film modified electrode, its electrochemical response to the analyte was greatly affected by its polymerization conditions: polymeric potential window, pH of polymeric solution and thickness of polymer film. In current research, the polymerization

ACCEPTED MANUSCRIPT conditions of poly(L-Norvaline) were investigated in detail. The evaluation criteria was its electrochemical response for metribuzin (10.7 μg/mL in 0.1mol L-1 B-R, pH 1.82) by cyclic voltammetry.

T

3.1.1 Effects of electro-polymerization potential window

IP

By selection, the suitable potential window for electropolymerization of L-Norvaline

SC R

was between -1500 mV and +2500 mV. Fig. 1A showed the cyclic voltammograms for repetivive sweep 15 cycles. In the first cycle, two relatively weak anodic peaks (marked P1 and P2) at +180 mV and +1500 mV and a large cathodic peak (P3) at -694 mV were observed.

NU

From the second cycle on, the three peaks increased concurrently in subsequent cycles, suggesting that the amount of electroactive polymer increased on GCE surface. At the same

MA

time, it was also observed that the film growth was faster in the initial six cycles than that in subsequent cycles. From the tenth cycle on, the film grew more slow. After

TE

by bare eyes on the GCE surface.

D

electropolymerization, a uniform and greenish blue film of poly(L-Norvaline) could be seen

During the potential selection, we found that if the negative potential was put less than

CE P

-1000 mV, the electropolymerization of L-Norvaline could not take place on the electrode surface. It was also found that the smallest positive potential was +1500 mV for the electropolymerization of L-Norvaline on GCE. And the more positive potential was set, the

AC

faster electropolymerization of L-Norvaline took place and more sensitive electrochemical response for metribuzin was observed. Fig. 1B displayed the current response of metribuzin at poly(L-Norvaline)/GCE obtained under different positive potential ranged from +2000 to +2600 mV (negative potential of -1500 mV), each for 5 cycles. This might be attributed to the more positive potential was, the more amino acid cation radicals generated, contributing to polymer fixed on the electrode surface. Considering the tolerance of GCE to the applied positive potential, +2500 mV was chosen as the positive polymerization potential.

Fig. 1 A-B

3.1.2 Effects of supporting electrolyte pH Next, the influence of polymerization solution pH was investigated in 0.2 mol L-1

ACCEPTED MANUSCRIPT phosphate buffer solution (PBS) containing 2.5×10-3 mol L-1 L-Norvaline. The solution pH was changed from 5.5 to 8.0 and the electro-polymerization potential window was performed as selected above (-1500 mV ~ +2500 mV). A superimposed voltammograms was shown in

T

Fig. 2 (only the first and fifth cycle were given under each pH). As could be seen from Fig. 2,

IP

the electro-polymerization reaction of L-Norvaline could be carried out under each test pH

SC R

and the electro-polymerization diagram was similar to each other, which suggested that the mechanism of polymerization reaction might be the same. As is well known, amino acid monomer could be oxidized to amino free radical at higher potential and then subsequently

NU

form a carbon–nitrogen linkage at GCE surface. Scheme 2 was the proposed polymerization process of L-Norvaline at GCE surface. Moreover, with the increase of solution pH, the

chosen as the polymer solution pH.

MA

cathodic peak current of metribuzin was also increased (data not shown). Therefore, 8.0 was

Scheme 2

TE

D

Fig. 2

CE P

3.1.3 Effects of the film thickness on GCE For preparing a polymer film modified electrode, the film thickness is controlled by the polymerization cycles and it affects the electrochemical characteristic of prepared modified

AC

electrode. The current responses of metribuzin (10.71 μg/mL) were investigated at the poly(L-Norvaline)/GCE with different film thickness. As a result, the cathodic peak current of metribuzin changed gradually with increasing cycles. Moreover, bigger peak current of metribuzin was obtained for the four cycles. Therefore, four cycles of electro-polymerization was chosen as the optimized fabricating procedure of poly(L-Norvaline)/GCE and used in following experiments.

3.2 Electrochemical properties of poly(L-Norvaline)/GCE Fig. 3 was the chronocoulometric curve of the bare GCE (curve a) and poly(L-Norvaline)/GCE (curve b) in 1 mM K3Fe(CN)6 containing 0.1 M KCl. The corresponding Q–t1/2 plots were shown in the inset. According to the formula given by Anson[29] :

ACCEPTED MANUSCRIPT Qtotal 

2nFAc ( Dt )1 / 2

 

 Qdl  Qads

where A (cm2) is the area of the electrode, D (cm2 s-1) is the diffusion coefficient of the

T

species, c (mol cm-3) is the bulk concentration of the species, t (s) is the potential pulse width,

IP

Qdl (C) is the double-layer charge, Qads (C) is the Faradaic charge due to the redox of adsorbed

SC R

species. The slope of the Q–t1/2 plots could be expressed as 2nFAcD1/2/π1/2. For 1 mM K3Fe(CN)6, n = 1, D = 7.6×10-6 cm2 s-1 [30], from the slopes value of the straight lines, the electroactive surface areas of the the bare GCE and poly(L-Norvaline)/GCE were calculated

NU

to be 0.0543 cm2 and 0.0905 cm2, respectively.

To obtain more information about the poly(L-Norvaline)/GCE, the electrochemical probe,

MA

[Fe(CN)6]3-, was employed and cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed in a 5.0×10-3 mol L-1 [Fe(CN)6]3-/[Fe(CN)6]4- + 0.1 mol

D

L-1 KCl solution. Fig. 4A showed the cyclic voltammograms at bare GCE (curve a) and

TE

poly(L-Norvaline)/GCE (curve b), respectively. As was shown in Fig. 4A, a pair of well-defined redox peaks were observed at the bare GCE with ipa = -15.67 µA (-0.000289 A

CE P

cm-2), ipc = 15.65 µA (0.000288 A cm-2) and a peak-to-peak separation of Ep = 75 mV. It was a nearly reversible electrode process. When the poly(L-Norvaline)/GCE was used, the charging current increased greatly and the reversibility of [Fe(CN)6]3- decreased a little (Ep =

AC

80 mV). This may be due to the combination between amino (from L-Norvaline) and carboxyl (from electrode), which made the electrode surface negatively charged and thus some electrostatic repulsion existed with [Fe(CN)6]3-. Fig. 4B showed typical Nyquist plots of EIS obtained at bare GCE (curve a′) and poly(L-Norvaline)/GCE (curve b′) (all frequency regions: 0.1 MHz to 0.01 Hz), respectively. To show clearly the result, the magnifying Nyquist spectra of cure a' and cure b' was put in Fig. 4B. After fitting a Randle circuit and calculation, the Rct obtained were 24.6 Ω (1.34 Ω cm2) for bare GCE and 104.9 Ω (9.49 Ω cm2) for poly(L-Norvaline)/GCE. The above mentioned results were in accord with the cyclic voltammetry taken that the redox peak currents decreased weakly and background current increased. Fig. 3 Fig. 4

ACCEPTED MANUSCRIPT

3.3 Electrochemical behavior of metribuzin on poly(L-Norvaline)/GCE Fig. 5 displayed the cyclic voltammograms of metribuzin (10.71 μg/mL) at bare GCE

T

(curve b) and poly(L-Norvaline)/GCE (curve c) in 0.1mol L-1 B-R (pH 1.82), respectively.

IP

The curve a in Fig. 5 was a blank voltammogram of poly(L-Norvaline)/GCE and there was no

SC R

redox reaction in potential window of -0.4 – -1.0 V. At bare GCE (Fig. 5b), the electrochemical response of metribuzin was very weak and only a very small bulge was observed at -0.69 V. However, in the case of poly(L-Norvaline)/GCE (Fig. 5c), the cathodic

NU

peak current dramatically increased with peak potential of -0.658V, which was positively moved of -0.032 V compared with that of bare GCE. The lower reduction potential and

MA

enhanced peak current could be reasonably ascribed to the electrocatalytic activity of poly(L-Norvaline) film for metribuzin.

TE

D

Fig. 5

3.4 Effect of pH and scan rate

CE P

To understand the reaction pathway and conjecture the reaction mechanism of metribuzin at poly(L-Norvaline)/GCE, it is necessary to investigate the effect of solution pH and scan rate. The voltammetric response of metribuzin in different supporting electrolytes

AC

was investigated, such as H2SO4, B-R, HAc-NaAc, and phosphate buffer solution. In terms of peak shape and the current response of metribuzin, B-R buffer solution was selected as the supporting electrolyte in following experiments. Experiments were performed by changing the solution pH from 1.82 to 5.31 (containing 10.71 μg/mL metribuzin). As was shown in Fig. 6A, with the increase of solution pH, peak current changed gradually and the cathodic peak potential of metribuzin was shifted negatively, meaning that the reduction of metribuzin was a process of getting proton. The better peak shape and bigger peak current of metribuzin were obtained at pH 1.82 (0.1 mol L-1 B-R). Besides, the relation between Ep and pH obeyed regression equation: Ep (mV) = -69.79 pH – 525.5 (R = 0.995) (Fig. 6B). The value of slope was 69 mV pH-1, indicated that the same amounts of electrons and protons took part in the electrode reaction. Fig. 6

ACCEPTED MANUSCRIPT

The following experiments were to investigate the effect of scan rate. As was shown in Fig. 7A, with the increase of scan rate from 10 to 250 mV s-1, the reduction peak potential

T

shifted to a more negative direction and the reduction peak current increased gradually.

IP

Moreover, a good linear relationship between peak potential and ln v (Fig. 7C) could be

SC R

described by the following equation: Ep (V) = -0.029ln v + 0.720 (R = 0.994). Meanwhile, a good linear relationship occurred between the logarithm of ip and the logarithm of v with the equation: log ip (μA) = 0.781 log v – 0.099 (R = 0.999, Fig. 7B). The slope of 0.781 in log ip ~

NU

log v relation indicating that the cathodic peak was controlled by both adsorption and

the following equation exists [31]: '

RT RTks RT ln  ln v αnF αnF αnF

TE

D

E p(V)  E 0 

MA

diffusion. According to Laviron's electrochemical theory for an irreversible electrode reaction,

where E0′ is formal standard potential and ks is the standard heterogeneous reaction rate

CE P

constant; n is the transfer of electron number;  refers to the charge transfer coefficient. From the slope of EP vs ln v relation, n = 2 could be achieved by assuming an electron transfer coefficient of α = 0.5. The results indicated that two electrons were involved in the

AC

electrochemical reduction process of metribuzin. In turn, the charge transfer coefficient could be computed to be 0.45. Considering the above results, the proton number taking part in the electrode reaction was 2 also. A possible reduction reaction mechanism of metribuzin was proposed and shown in Scheme 3. Besides, the heterogeneous electron transfer rate constant (ks = 1.021 s-1) could be calculated based on above mentioned equation. The formal standard potential (E0′ = 0.618 V) was obtained from another linear relation of Ep~ v by extrapolating v = 0. Fig. 7 Scheme 3

3.5 Chronocoulometry studies

ACCEPTED MANUSCRIPT For an adsorption and diffusion controlled electrode process, it is necessary to determine its diffusion coefficient and adsorption capacity. Chronocoulometry (CC) is the best technique to do this work. For this system, chronoamperometry was performed in a 10.71

T

μg/mL metribuzin solution and carried out with a step potential from – 400 mV to -1000 mV

IP

after the adsorption was saturated for 10 minutes. Fig. 8A shows the Q ~ t curves with

SC R

metribuzin present (curve b) and absent (curve a). The Fig. 8B shows the linearized plot for Q ~ t1/2. The linear equations of Q ~ t1/2 were Q (10-4 C) = 2.886 t1/2 (s1/2) + 0.345 (R = 0.997) and Q (10-4 C) = 6.885 t1/2 (s1/2) + 4.191 (R = 0.998) for curves (a) and (b) respectively. From

NU

the Fig. 8B, the difference of intercepts and slopes were obtained from curve a and curve b, further demonstrating that the reduction process of metribuzin was controlled by both

MA

adsorption and diffusion. Using Laviron's theory of Q = nFAΓ* and intercept difference of Q ~ t1/2 plots for curves (a) and (b), an Γ* value of 2.82×10-8 mol cm-2 was obtained. And

D

according to the formula given by Anson [29]:

TE

Qtotal 

2nFAc ( Dt )1/ 2

 

 Qdl  Qads

CE P

where D is the apparent diffusion coefficient, which is the measurement of mass transfer rate in electrode reaction; C is the metribuzin concentration; Qdl is double-layer charge; Qads is the Faradaic charge due to the reduction of adsorbed metribuzin. Other symbols have their

cm2 s-1.

AC

usual meanings. Based on the slope of the curve b (Fig. 8B), D was calculated to be 8.01×10-7

Fig. 8

3.6 Analytical applications and methods validation 3.6.1 Influence of accumulation time For an adsorpton driven electrode reaction, an accumulation step can be used to increase the detection sensitivity. This experiment was performed in a 0.643 μg/mL metribuzin solution with a CV technique. The results showed that the peak currents were heightening greatly by increasing the accumulation time (tacc) from 30 to 120 s and then diminishing. And it was found that accumulation potential (Eacc) was little influenced by the peak current.

ACCEPTED MANUSCRIPT Therefore, an accumulation condition for metribuzin was defined as accumulation of 120 s under open circuit.

IP

T

3.6.2 Calibration curve, detection limit, reproducibility and stability

SC R

Under selective conditions, the relationship between peak currents and metribuzin concentrations was investigated using linear sweep voltammetry technique. A superimposed voltammogram was shown in Fig. 9. The relationship between the reductive peak currents and the metribuzin concentrations was linear relationship in the range from 6.43×10-3 to 1.07

NU

μg/mL. The regression equation was ip (10-6 A) = 6.839 C (10-6 mol L-1) + 5.077 (R = 0.991)

MA

with a detection limit of 2.14×10-3 μg/mL. The comparison of poly(L-Norvaline)/GCE with other sensors for metribuzin determination was listed in Table 1. The use of the Hg electrode offers noticeably higher sensitivity than the poly(L-Norvaline)/GCE. However, Hg is toxic

D

and may occur secondary pollution. Therefore, the use of poly(L-Norvaline)/GCE for

CE P

TE

determination of metribuzin may be the suitable electrochemical sensor so far. Fig. 9 Table 1

A relative standard deviation (RSD) of 6.4% for 8 successive cycles detection of 0.643

AC

μg/mL metribuzin was obtained with one poly(L-Norvaline)/GCE, and 3.2% for five independent poly(L-Norvaline)/GCE, suggesting excellent repeatability and reproducibility of the proposed modified electrode. Furthermore, after a poly(L-Norvaline)/GCE was stored in air at 4°C for 15 days, the current response was retained of 95.6 % for its initial peak current value, which indicated this sensor had long-term stability.

3.7 Interference studies Under the optimized experimental conditions described above, the effects of some foreign species on the determination of metribuzin at 0.214 μg/mL level were evaluated in detail. The tolerance limit was defined as the maximum concentration of the interfering species which caused an error ≤±5%. 100-fold amounts of familiar ions, such as sodium

ACCEPTED MANUSCRIPT benzoate, glutamic acid, citric acid, Fe3+, SO42-, Mg2+, Cl-, Al3+, NO3-, Zn2+, were no influence on the current response of metribuzin. It was also found that no influence could be observed for ascorbic acid and uric acid (50-fold). These results indicate that the proposed method can

IP

SC R

3.8 Determination of metribuzin in soil samples and recovery

T

be used as a selective method for determination of metribuzin.

To evaluate the practical applicability of present method, natural soil was selected as sample. First, 1.0 g natural soil was weighed and put into a 50 ml volumetric flask. Then, a

NU

certain amount of metribuzin standard solution (2.14×103 μg/mL) was added to the aforementioned volumetric flask and metered volume by anhydrous methanol. Last, the soil

MA

sample was pretreated by ultrasonic treating for 30 minutes. After 10-min centrifugation with 10,000 rpm, the clear liquid phase was collected for further analysis. 1mL of

D

supernatant liquid was pipette into 9 mL 0.1 mol L-1 B–R solution (pH 1.82) to detect the

TE

content of metribuzin. The standard addition method was employed to detect three parallel samples and the average content detected was 0.50 µmol L-1 with an RSD of 2.0%. For

CE P

further checking of the accuracy, the recovery was detected in each sample solution by adding some standard metribuzin solution and then determining the total quantity of metribuzin. In the three determinations, the recovery obtained was 97.80%, 96.20% and 102.08%

AC

respectively (Table 2). These data demonstrated that the proposed method has good accuracy for the determination of metribuzin. Table 2

4.Conclusion In summary, we have successfully fabricated a new voltammetric sensor, poly(L-Norvaline)/GCE, which can be applied to the detection of metribuzin with excellent sensitivity and selectivity. In addition to the mercury electrode, poly(L-Norvaline)/GCE exhibited significant advantages of wide linear range and low detection limit for metribuzin compared with previous results. Moreover, the detailed electrochemical characteristics of metribuzin were studied systematically and the dynamic parameters of electrode process were calculated. The present method also showed good recoveries and could be applied to

ACCEPTED MANUSCRIPT determine metribuzin in soil sample with satisfactory results. Acknowledgements The authors are very grateful for the financial support from the National Natural Science

IP

T

Foundation of China (Grant no. 21275132; 21575130). References

SC R

[1] F.M. Jochen, H. Fiona, T. Leisa-Maree, S. Robert and F. Peter, Persistent organochlorine pesticides in human milk samples from Australia, Chemosphere 70 (2008) 712-720. [2] A.S. Stephen, S.L. Linda, B. Marianne and F.T. Ronald, Assessing N,N′-Dibutylurea

NU

(DBU) Formation in Soils after Application of n-Butylisocyanate and Benlate Fungicides, J. Agric. Food Chem. 52 (2004) 747-754.

MA

[3] B. Elisabet, T. Lennart, New methods for determination of glyphosate and (aminomethyl) phosphonic acid in water and soil, J. Chromatogr. A 886 (2000) 207-216.

D

[4] D. Ruey-An, P.L. Liao, Determination of organochlorine pesticides and their metabolites

TE

in soil samples using headspace solid-phase microextraction, J. Chromatogr. A 918 (2001) 177-188.

from

an

CE P

[5] C. Gonccalves, M.F. Alpendurada, Assessment of pesticide contamination in soil samples intensive

horticulture

area,

using

ultrasonic

extraction

and

gas

chromatography–mass spectrometry, Talanta 65 (2005) 1179-1189. P. Asterios, P.M. Euphemia, Occurrence and Spatial and Temporal Distribution of

AC

[6]

Pesticide Residues in Groundwater of Major Corn-Growing Areas of Greece (1996-1997), Environ. Sci. Technol. 35 (2001) 63-69. [7] N. Ruiz de Erenchun, M.A. Goicolea, Z. Gomez de Balugera, M.J. Portela and R.J. Barrio, Determination

of

herbicides

by

reductive

amperometric

detection

in

liquid

chromatography, J. Chromatogr. A 763 (1997) 227-235. [8] F.L. James, M. Cathie, H. Marie-Claire, P. Valene, L. Francois and D. Nathalie, Evaluation of immunoaffinity chromatography as a replacement for organic solvent clean-up of plant extracts for the determination of trazine herbicides by liquid chromatography, J. Chromatogr. A 752 (1996) 147-154. [9] N.P. Emmanuil, P.M. Euphemia, Determination of metribuzin and major conversion products in soils by microwave-assisted water extraction followed by liquid

ACCEPTED MANUSCRIPT chromatographic analysis of extracts, J. Chromatogr. A 962 (2002) 9-20. [10] S. Jasmin, M.R. Jan, A. Behisht, S. Farhat-un-Nisa, Quantification of triazine herbicides in soil by microwave-assisted extraction and high-performance liquid chromatography,

T

Environ. Monit. Assess. 178 (2011) 111-119.

IP

[11] J. Beltran, F.J. Lopez, M. Forcada, F. Hernandez, Microextraction procedures combined

SC R

with large volume injection in capillary gas chromatography for the determination of pesticide residues in environmental aqueous samples, Anal. Chim. Acta 356 (1997) 125-133.

NU

[12] F. H. Jose, O.I. Monsalud del, M.G. C. Ana, G.C. Antonio and S.N. Antonio, Determination of the herbicide metribuzin and its major conversion products in soil by

MA

micellar electrokinetic chromatography, J. Chromatogr. A 1102 (2006) 280-286. [13] R. Carabias-Martınez, E. Rodrıguez-Gonzalo, P. Revilla-Ruiz and J. Domınguez-Alvarez, extraction

and

sample

stacking–micellar

electrokinetic

capillary

D

Solid-phase

TE

chromatography for the determination of multiresidues of herbicides and metabolites, J. Chromatogr. A 990 (2003) 291-302.

CE P

[14] Q.M. Carolina, M.G.C. Ana, O.I. Laura del, O. Monsalud del, Large volume sample stacking in capillary zone electrophoresis for the monitoring of the degradation products of metribuzin in environmental samples, J. Chromatogr. A 1164 (2007) 320-328.

AC

[15] F. Breton, P. Euzet, S.A. Piletsky, M.T. Giardi, R. Rouillon, Integration of photosynthetic biosensor with molecularly imprinted polymer-based solid phase extraction cartridge, Anal. Chim. Acta 569 (2006) 50-57. [16] A. Behisht, Z.Y. Chen, S.Jasmin, M. R. Jan, L. Ye, Preparation and Characterization of Uniform Molecularly Imprinted Polymer Beads for Separation of Triazine Herbicides, J. Appl. Polym. Sci. 126 (2012) 315-321. [17] S. Jasmin, M. R. Jan, A. Behisht, M. Mian, Extractive spectrophotometric method for determination of metribuzin herbicide and application of factorial design in optimization of various factors, J. Hazard. Mater. 164 (2009) 918-922. [18] J. Skopalová, K. Lemr, M. Kotouček, L. Čáp, P. Barták, Electrochemical behavior and voltammetric determination of the herbicide metribuzin at mercury electrodes, Fresenius J. Anal. Chem. 370 (2001) 963-969.

ACCEPTED MANUSCRIPT [19] A.L. Augusto César de, G.S. Emanuella, O.F.G. Marília, T.Josealdo, T.S Tiago, C.A. Fabiane, Electrochemical Behavior of Metribuzin on a Glassy Carbon Electrode in an Aqueous Medium including Quantitative Studies by Anodic Stripping Voltammetry, J.

T

Braz. Chem. Soc. 9 (2009) 1698-1704.

IP

[20] T. Bogdan, S. Adriana, M. Anca, I. Cătălina, T. Cristian, Electrochemical Study of

SC R

Metribuzin Pesticide Degradation on Bismuth Electrode in Aqueous Solution, Int. J. Electrochem. Sci. 10 (2015) 223-234.

[21] M. Monica, B. Esperanza, C. Manuel, Z. Antonio and S. A. Alberto, Cathodic

NU

Electrochemical Determination of Herbicides in Acid Media Using a Bismuth Film Electrode, Electroanalysis 21 (2009) 415-421.

MA

[22] G.Y. Jin, Y.Z. Zhang, W.X. Cheng, Poly(p-aminobenzene sulfonic acid)-modified glassy carbon electrode for simultaneous detection of dopamine and ascorbic acid, Sens.

D

Actuators. B 107 (2005) 528-534.

TE

[23] J.H. Chen, J. Zhang, Q. Zhuang, S.B. Zhang and X.H. Lin, Electrochemical study of bergenin on a poly(4-(2-pyridylazo)-resorcinol) modified glassy carbon electrode and its

CE P

determination in tablets and urine, Talanta 72 (2007) 1805-1810. [24] K. Palraj, S. A. John, Electropolymerized film of functionalized thiadiazole on glassy carbon electrode for the simultaneous determination of ascorbic acid, dopamine and uric

AC

acid, Bioelectrochemistry 77 (2009) 13-18. [25] A.T. Stanislav, H. Dietmar, L. Roland, G. Lo, A.K. Arkady, Improvement of direct bioelectrocatalysis by cellobiose dehydrogenase on screen printed graphite electrodes using polyaniline modification, Bioelectrochemistry 76 (2009) 87-92. [26] Q. Cao, H. Zhao, Y.M. Yang, Y.J. He, N. Ding, J. Wang, Z.J. Wu, K.X. Xiang and G.W. Wang, Electrochemical immunosensor for casein based on gold nanoparticles and poly(l-Arginine)/multi-walled carbon nanotubes composite film functionalized interface, Biosens. Bioelectron. 26 (2011) 3469-3474. [27] N.B. Minh-Phuong, C.A. Li, N.H. Kwi, X.H. Pham and H.S. Gi, Determination of acetaminophen by electrochemical co-deposition of glutamic acid and gold nanoparticles, Sensors and Actuators B: Chemical. 174 (2012) 318-324. [28] Y.F. Li, J. Liu, G. Song, K.J. Li, K. Zhang, B.X. Ye, Sensitive voltammetric sensor for

ACCEPTED MANUSCRIPT bergenin based on poly(L-lysine)/graphene modified glassy carbon electrode, Anal. Methods 5 (2013) 3895-3902. [29] F. C. Anson, Application of Potentiostatic Current Integration to the Study of the

T

Adsorption of CobaIt(lll)-( Ethylenedinitri1o)tetraacetate on Mercury Electrodes, Anal.

IP

Chem. 36 (1964) 932-934.

SC R

[30] A.J. Bard, L.R. Fanlkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York (1980).

[31] E. Laviron, General expression of the linear potential sweep voltammogram in the case

AC

CE P

TE

D

MA

NU

of diffusionless electrochemical system, J. Electroanal. Chem. 101 (1979) 19-28.

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

Scheme 1(a). Chemical structure of metribuzin.

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

Scheme 1(b). Chemical structure of L-Norvaline

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

Fig. 1. (A): Voltammograms of L-Norvaline (2.5×10-3 mol L-1) with repetitive cyclic scan for 15 circles in potential window of -1500 mV and +2500 mV. (B): the relationship between the response to metribuzin and the positive potential set-up; scan rate: 100 mV s-1.

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

Fig. 2. Polymerized cyclic voltammograms of L-Norvaline (2.5×10-3 mol L-1) in PBS with different pH: 5.5, 6.0, 6.5, 7.0, 8.0; cycle number of polymerization in each pH: 5 cycles (only 1st and 5th were shown). Scan rate: 100 mV s-1.

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

Scheme 2. A possible electro-polymerization process of L-Norvaline on the GCE.

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

Fig. 3. Chronocoulometric curves of the 1.0 mM K3[Fe(CN)6] containing 0.1 M KCl at the bare GCE (a) and poly(L-Norvaline)/GCE (b). Inset: the corresponding Q–t1/2 plots.

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

Fig. 4. (A) Cyclic voltammograms of [Fe(CN)6]3− (1.0×10−3 mol L−1 containing 0.1 mol L−1 KCl) at bare GCE (a) and poly(L-Norvaline)/GCE (b). Scan rate: 100 mV s-1. (B) Nyquist plots of [Fe(CN)6]3−/4– (5.0×10-3 mol L-1 containing 0.1 mol L−1 KCl) at bare GCE (a') and poly(L-Norvaline)/GCE (b'). Inset: the magnifying Nyquist spectra of cure a' and cure b'.

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

Fig. 5. Cyclic voltammograms of (a) GCE in suporting electrolyte: 0.1 mol L-1 B-R (pH 1.82)-blank voltammogram; Scan rate: 100 mV s-1; (b) GCE in metribuzin (10.71 μg/mL) solution; (c) poly(L-Norvaline)/GCE in metribuzin (10.71 μg/mL) solution.

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Fig. 6. (A) Cyclic voltammograms of metribuzin (10.71 μg/mL) in different pH B-R (pH from curve a to h): 1.82, 2.30, 2.81, 3.32, 3.81, 4.31, 4.81, 5.31; (B) the relationship between the peak potential and pH; Scan rate: 100 mV s-1.

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Fig. 7. (A) Cyclic voltammograms of metribuzin (10.71 μg/mL) under different scan rates (from inner to outer): 10, 20, 40, 60, 80, 100, 150, 200 and 250 mV s-1. (B) The relationship between log ip and log v; (C) the relationship between Ep and lnv.

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU

Scheme 3. Possible reduction reaction mechanism of metribuzin.

AC

CE P

TE

D

MA

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

Fig. 8. (A) Chronocoulometric curves with absence (curve a) and presence (curve b) of 10.71 μg/mL metribuzin; (B) the corresponding Q~ t1/2 plots. Other conditions were same as in Fig. 5.

NU

SC R

IP

T

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

Fig. 9. The superposed LSV curves of metribuzin with different concentrations (from a to i): 6.43×10-3, 1.5×10-2, 2.14×10-2, 1.07×10-1, 1.50×10-1, 2.14×10-1, 4.29×10-1, 6.43×10-1, 1.07 μg/mL.

ACCEPTED MANUSCRIPT

24.86

carbon paste/castor oil Bismuth Electrode —

0.267

Bismuth Film 2.14 ~ 42.86 Electrode poly(L-Norvaline)/ 0.00643 ~ 1.07 GCE

SWV

[19]

—

CV

[20]

1.29

SWV

[21]

LSV

This work

AC

CE P

TE

D

MA

NU

0.804

SC R

Glassy carbon electrode (GCE) carbon paste/Nujol 0.214 ~ 85.72

IP

T

Table 1 Comparison of different electrochemical sensors for the determination of metribuzin. Different metribuzin Analytical range Detection limit Electrochemical Reference sensors (μg/mL) (μg/mL) technique mercury electrode 0.00101 ~ 0.030 0.000270 DPAdSV [18]

0.00214

ACCEPTED MANUSCRIPT

0.107

IP

Original Found(μg/mL)

T

Table 2 Results of the soil sample analysis indicating percentage recovery.

2.0

0.214

0.429

0.643

Total found(μg/mL)

0.317

0.519

0.763

97.8

96.2

102.08

1.81

3.66

2.25

MA

NU

Standard added (μg/mL)

Recovery/%

CE P

TE

D

RSD/%

AC

SC R

RSD/%

ACCEPTED MANUSCRIPT

CE P

TE

D

MA

NU

SC R

IP

T

Graphical abstract

AC

Cyclic voltammograms of (a) GCE in suporting electrolyte: 0.1 mol L-1 B-R (pH 1.82)-blank voltammogram; Scan rate: 100 mV s-1; (b) GCE in metribuzin (10.71 μg/mL) solution; (c) poly(L-Norvaline)/GCE in metribuzin (10.71 μg/mL) solution.

ACCEPTED MANUSCRIPT Highlights

IP

T

► A new electroanalytical method for determination of metribuzin was established with

SC R

low detection limit and wide linear range.

►The proposed method was also available to detect metribuzin in soil with satisfactory results.

NU

►It is worth mentioning that the reduction reaction mechanism of metribuzin by using

MA

electrochemical method is for first time discussed in detail. ► The electro-polymerization of L-Norvaline was investigated for the first time on

AC

CE P

TE

D

glassy carbon electrode.