Article Electrochemistry, 84(4), 228–233 (2016)

Electrochemical Study and Determination of Dinitramine Using GlassyCarbon Electrodes Modified with Multi-walled Carbon NanotubesMohsen IRANDOUST* and Maryam HAGHIGHI

Department of Analytical Chemistry, Faculty of Chemistry, Razi University, Kermanshah, Iran

*Corresponding author: irandoust1341@yahoo.com

ABSTRACTA novel and sensitive method is described for voltammetric study and determination of dinitramine, commonly usedpesticide, based on its electrochemical reduction at a multi-walled carbon nanotube (MWCNT) modified glassycarbon electrode. Cyclic voltammetry (CV) was used to investigate the redox properties of this modified electrodeat various solutions pH values and various scan rates. CV studies indicated that the reduction process has anirreversible and diffusion-like behavior in a reduction mechanism with equal number of electrons and protons. Thesquare-wave voltammetry (SWV) was applied as a very sensitive voltammetric detection method for thedetermination of dinitramine. Under optimal conditions, the proposed method exhibited acceptable analyticalperformances in terms of linearity (over the concentration range from 4.0 × 10−8 to 1.4 × 10−6 and 1.4 × 10−6

to 2 × 10−5mol L−1, R2 = 0.999), detection limit (0.8 × 10−8mol L−1) and repeatability (RSD = 2.36%, n = 10,for 5.0 × 10−6mol L−1 dinitramine). To further validate its possible application, the method was used for thequantification of dinitramine in water samples.

© The Electrochemical Society of Japan, All rights reserved.

Keywords : Dinitramine, Pesticide, Voltammetric Determination, Multi-walled Carbon Nanotube

1. Introduction

Commonly used pesticides can be harmful to people, pets, andthe environment. Part of the problem is the toxicity of somepesticides, but even more important is the sheer volume ofpesticides used in many countries every year. Much of it findsits way to our water, air, and soil. Studies show that the mostcommonly used pesticides are the ones most likely to cause waterpollution. Dinitramine (Cobex) is a selective pre-plant herbicidewhich is incorporated into soil for the control of annual grasses andbroadleaf weeds. It has been field-tested in the USA for weedcontrol in cotton and soya beans. Cobex is being tested in thefollowing countries in addition to North America: Australia,Bolivia, Brazil, Columbia, Egypt, France, Germany, Greece,Holland, Hungary, India, Italy, Japan, Kenya, Morocco, Rumania,South Africa, Spain, Sudan, Syria, Turkey, Tanzania and UnitedKingdom. The active ingredient of Cobex is (N3,N3-diethyl-2,4-dinitro-6-trifluoromethyl-m-phenylenediamine) which has the pro-posed common name dinitramine.1 Dinitramine is irritating to skinand eyes (Scheme 1).

Dinitramine is detected mostly by chromatographic technology,such as gas chromatography,2,3 electron captured gas chromatog-

raphy,4 gas chromatography mass-spectrometry with solid-phasemicro extraction,5 and high-performance liquid chromatography,6

and computational analysis of the electrostatic potential,7 has alsobeen reported for the detection of dinitramine. However, seldomelectrochemical techniques, which are often simple and lessexpensive, have been used to determine dinitramine. The nitrogroups in dinitramine are easily reduced at the mercury electrodes.The major disadvantage was that the high toxicity of mercury mayendanger the health of the analyst and cause new contamination.Because sustainable disposal of mercury is very difficult andexpensive, the use of the material is strictly limited. Thereforeit seemed very desirable to find alternative electrode materialswhich are less toxic than mercury but display similar or moreexcellent characteristics. Consequently, an elaborate search forvarious non-mercury electrodes to be used in the electrochemicalanalysis has been going on during the past years and chemicallymodified electrodes play a great role in these inventions.8 To ourknowledge, no paper has appeared for the determination ofdinitramine using chemically modified electrode. Here, a novelsensitive voltammetric method based on a glassy carbon electrodewith multiwalled carbon nanotubes was developed for thedetermination of dinitramine.

Recently, multifarious nano-materials have been applied inelectro analytical chemistry.9 Since the discovery of carbonnanotubes (CNTs) by Iijima,10 carbon nanotubes have attractedenormous interest because of their unique structural, mechanical andelectronic properties. The first application of carbon nanotubes inelectrochemical analysis was by Britto11 where a paste of CNTsin bromoform was packed into a glass tube and used to study theoxidation of dopamine. The subtle electronic behaviors of carbonnanotubes reveal that they have the ability to promote electron-transfer reactions when used as an electrode material in elec-trochemical research. However, the hydrophobicity of CNTspresents a major challenge when it comes to disperse andmanipulate carbon nanotubes to give controlled modification ofelectrode surfaces. To exploit the potential applications in futurenano-devices, it is necessary to develop versatile approaches to

N

N N

NH2

O

O

O

O

FFF

Scheme 1. Chemical structure of dinitramine.

Electrochemistry Received: November 24, 2015Accepted: January 7, 2016Published: April 5, 2016

The Electrochemical Society of Japan http://dx.doi.org/10.5796/electrochemistry.84.228

228

assemble or integrate CNTs onto solid surfaces. In recent years,considerable efforts have been made to fabricate different CNTmorphologies and explore their application in various fieldsincluding composites, electrochemical devices and sensors, amongothers.12–14

In this article, dispersing multi-walled carbon nanotubes(MWNTs) is performed with the aid of dimethylformamide(DMF), and consequently, a MWNTs/DMF film was achieved onthe glassy carbon electrode (GCE) surface via solvent evaporation.It is found that the MWNTs/DMF film exhibits a sensitive responseto dinitramine.

2. Material and Methods

2.1 MaterialsDinitramine, purchased from Sigma–Aldrich, was used without

further purification. Stock solution of dinitramine (1.0 ©10¹2mol L¹1) was prepared by dissolving the compound inmethanol and kept in the dark at 4.0°C. Dimethylformamide(DMF) was purchased from Sigma–Aldrich. Carboxylated MWCNT(10–50 nm in diameter) was purchased from Notrino. All aqueoussolutions were prepared with doubly distilled deionized water.Voltammetric experiments were performed using a Metrohmpotentiostat/galvanostat model 797VA. A three electrode systemwas employed with a platinum wire as counter electrode, anAg/AgCl electrode as reference electrode and a MWCNT/GCE asworking electrode. All potentials in this paper refer to Ag/AgClelectrode.

2.2 Method2.2.1 Preparation of the MWCNT/DMF film modified GCE

A glassy carbon electrode of 3.0mm diameter was used. It waspolished successively with 0.3mm and 0.05mm alumina slurries(Metrohm) on silk, and sonicated subsequently in ethanol anddeionized water each for 10.0min. MWNTs (1mg) were added into2.0mL plastic centrifuge tube, and 1.0mL DMF was subsequentlyadded into it. A well-dispersed suspension of MWCNT/DMF wasobtained by ultrasonication for about 20.0min. The GCE was coatedby a drop of 3.0 µL MWCNT/DMF suspension and dried underinfrared lamp in the air. Then the uniform MWCNT/DMF filmcontaining a network of MWCNTwas formed. The freshly preparedMWCNT/GCE were activated in 0.02mol L¹1 Britton–Robinson(B–R) buffer solution by using successive cyclic scans from 1.0 to¹1.0V until a stable voltammogram was obtained. After eachmeasurement, the electrode surface was refreshed by the samemethods mentioned above.2.2.2 Procedure

Unless otherwise stated, a B–R buffer (0.02mol L¹1, pH = 2.0)was used as the supporting electrolyte for dinitramine determination.Certain volume of standard solution of dinitramine was added intothe 10.0mL cell containing B–R buffer. The solution was deaeratedwith nitrogen at least for 2.0min and finally the cyclic voltammo-grams or linear sweep voltammograms were recorded from 1.0 to¹1.0V after 5.0 s quiescence. The reduction peak currents around¹0.50V were measured for the quantification of dinitramine.

3. Results and Discussion

3.1 SEM characterizationThe suspension of MWNTs/DMF was cast on a pretreated glassy

carbon disk and the SEM image of thebare electrode and MWNTs/DMF film formed is shown in Fig. 1a, b. From Fig. 1b, it can befound that the glassy carbon disk surface is completely andhomogeneously coated by MWNTs. It also can be seen from thisimage that the MWNTs film contained very small portion ofamorphous carbon impurities.

3.2 Electrochemical behavior of dinitramine at the MWCNTmodified GCE

Two successive cyclic voltammetry (CV) was performed in thepotential range of 1.0 to ¹1.0V to investigate the electrochemicalbehaviors of dinitramine (Fig. 2). It can be seen from peak a2, thatthe onset of the reduction of dinitramine occurs at ¹0.29V, and thecurrent reaches its maximum at about ¹0.47V at a bare GCE. Thesmall peak current indicates sluggish electrode kinetics at the bareGCE electrode for dinitramine reduction. On the other hand, at theMWNTs/DMF film modified GCE, the reduction of dinitraminestarts at about ¹0.27Vand a well-defined voltammetric current peakcan be seen at around ¹0.50V (peak b2). MWCNT greatly increasethe rate of electron transfer from dinitramine to the electrode,which is attributed to MWCNT can improve the reversibility ofthe electron-transfer process and the high aspect ratios of theMWCNT can present a strict effect for more efficient reduction ofdinitramine.15 Peak b2, can be related to the irreversible reductionofthe nitro groups (NO2) to hydroxylamine group (NHOH), andpeaks c1 and c2 are related to the irreversible oxidation of thehydroxylamine groups. Similar electrochemical behavior has beenpreviously reported for voltammetric determination of similar nitro-aromatic compounds such as, Azathioprine at thin carbon nano-particle composite film electrode16 parathion and chloramphenicolonthe surface of Nafion-coated glassy carbon17 and screenprintedelectrodes.18 Therefore, the electrochemical process of dinitraminecan be represented by the following equations (Scheme 2).

3.3 Electrode behavior of dinitramineThe electrochemical behavior of dinitramine at different scan

rates, from 0.01 to 0.2mV s¹1, in the potential range from 0.0 to¹0.6V was investigated by using CV in Fig. 3. Only a reduction

(a) (b)

200nm

Figure 1. SEM of the bare GCE (a) MWCNT/GCE (b).

-70

-50

-30

-10

10

30

50

-1 -0.5 0 0.5 1

I/μA

E/V vs. Ag/AgCl

b2

b1

a1

a2

c1c2

Figure 2. (Color online) Two successive CVs of a bare GCE inthe absence (a1) or in presence (a2) of 5.0 © 10¹6mol L¹1

dinitramine in B–R buffer (0.02mol L¹1, pH = 2.0) and MWCNT/GCE in the absence (b1) or presence (b2) of 5.0 © 10¹6mol L¹1

dinitramine in B–R buffer (0.02mol L¹1, pH = 2.0). Scan rate:100mV s¹1; potential range: from 1.0 to ¹1.0V.

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peak appears even at 10.0mV s¹1, and no corresponding oxidationpeak was observed on the reverse scan, which suggests thatthe electrode reaction of dinitramine is totally irreversible. Theexperimental results showed that the reduction peak currents wereproportional to square root of the scan rate at these electrodes,indicating that a diffusion-controlled process was involved in theelectrochemical behavior of dinitramine at the MWCNT/GCE.

Electrochemical impedance spectroscopy (EIS) was employed toinvestigate the impedance changes of the electrode surface due tothe modification procedure. Figure 4 shows the Nyquist plots ofK3Fe(CN)6/K4Fe(CN)6 at the bare GCE and the MWNTs/DMF filmmodified GCE. In these studies, high frequency zone, which appearsas a nearly semicircle plot, can be ascribed to the kinetic limitations(Rct) of the electrochemical reaction. On the other hand, the linearbehavior of ZIm versus ZRe in a low frequency region is character-istic of a diffusion-controlled electrode process. As can be seen in

F FF

H2N

NO

O N

NO

O

8e

F FF

H2N

N

NN

H

HO

H

OH

8H 2H2O

F FF

H2N

N

NN

H

OH

H

HO

F FF

H2N

N

NN

4e

O O4H

Scheme 2. Electrochemical process of dinitramine.

(a) (b)

y = -195.7x + 14.94R² = 0.998

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0

0 0.2 0.4 0.6

I/μA

V^1/2 /mV s-1-80

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-10

0

-0.5 -0.4 -0.3 -0.2 -0.1 0 I/μA

E/mV vs. Ag/AgCl

A↓T

Figure 3. (Color online) Cyclic voltammograms of dinitramine at the MWCNT/GCE in B–R buffer (0.02mol L¹1, pH = 2.0) containing of5 © 10¹6mol L¹1 dinitramine, at different scan rates from (A) to (T): 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13,0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20mV s¹1 (a), linear curve for reduction peak at about ¹0.30V (b).

0100200300400500600700800

0 500 1000 1500

Z"(Ω

)

Z' (Ω)

a

b

Figure 4. Nyquist diagram (ZBB versus ZB) for the EIS measure-ments in 1mM K3Fe(CN)6/K4Fe(CN)6 + 0.1M KCl at the formalpotential 0.2V for the bare GCE (a) and at the MWCNT/GCE (b).

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Fig. 4, a semicircle with a very large diameter is observed at thebare GCE. However, the diameter of the semicircle is significantlyreduced with MWNTs/DMF film modified GCE, which suggeststhat the surface of the modified electrode exhibits lower electrontransfer resistance and greatly increases the electron transfer rate.

3.4 Effect of pHThe pH of the supporting electrolyte has a significant influence

on the electroreduction of nitro groups at the modified electrode.Cyclic voltammograms of the modified electrode at different pHvalues in the range of 2.0–10.0 were recorded at the potential scanrate of 100mV s¹1 (Fig. 5a). It was found that the peak potentialshifted negatively with increasing pH, which suggests that H+

participates in the reduction process. A good linear relationshipwas observed between the Ep and pH values in the range of 2.0–10.0 (Fig. 5b). This relationship can be described by the followingequation:

Ep ¼ -0:057 pH- 0:310 ðR2 ¼ 0:995ÞThe slope of the linear variation of Ep versus pH under acidicconditions showed a value of about ¹57mV per pH unit. Thisclearly indicates that equal numbers of electrons and protons areinvolved in the electroreduction of nitro groups on the surface ofthe modified electrode. In solutions with a pH value greater than 6.0the slope is changed because the mechanism itself changes to wherethere are no protons involved before the rate determining step.Therefore, phosphate buffer at pH 2.0 was used as supportingelectrolyte in all voltammetric determinations. On the other hand,the cathodic peak current increased by decreasing the pH from 10.0to 2.0 (Fig. 5c).

3.5 Effect of the amount of MWNTs/DMF suspensionThe thickness of the MWCNT–DMF film on the GCE surface is

determined by the amount of MWCNT–DMF suspension droppedon the GCE surface. The peak current significantly increased withincreasing the amount from 1.0 to 3.0 µL. As the amount of

suspension further increased, the peak current changed very slightly,and when the amount of suspension exceeded 6.0 µL, the peakcurrent conversely decreased. This is probably attributed to thecompromising effects of DMF on the electrochemical performanceof the composite film due to the hydrophobic and insulating actionsof DMF. As a result, an appropriate amount for the fabrication ofMWCNT modified GCEs was determined as 3.0 µL of 1.0mgmL¹1

MWCNT–DMF suspension.

3.6 Analytical application and the response repeatability ofMWCNT modified GCE

The analytical applicability of the above mentioned mechanismfor the determination of dinitramine was examined through usingcyclic voltammetry and square-wave voltammetry (SWV) upon thecatodic peak of dinitramine. The voltammetric detection of dinitr-amine via the application of SWV has been used to achieve moreaccurate and sensitive voltammetric responses. In this regard, theCV and SWV response of dinitramine in buffered solutions withpH 2.0 under the voltammetric experimental conditions (voltagestep: 0.005V, pulse amplitude: 0.05V, and frequency: 50.0Hz) isrecorded in the presence of various concentrations of dinitraminespecies. The results showed that during the addition of dinitramineto the buffered solution, an increase in catodic peak current isresulted. A linear dynamic range is obtained from 4.0 © 10¹7 to5.5 © 10¹5mol L¹1 of dinitramine for CV and two intervals in therange of 4.0 © 10¹8 to 1.4 © 10¹6 and 1.4 © 10¹6 to 2 ©10¹5mol L¹1 of dinitramine for SWV, with a detection limit of0.1 © 10¹7 for CV and 0.8 © 10¹8mol L¹1 for SWV (S/N 3)(Fig. 6).

4. Interference Study

A systematic study was carried out to evaluate the interferencesof foreign ions and pesticides on the determination of dinitramine atthe level of 1.0 © 10¹6mol L¹1. We found that 500-fold concen-tration of trifluralin, 2-methyl,4,6-dinitropheol and 1000-fold

(a)

(b) (c)

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AE/mV vs. Ag/AgCl

y = -0.057x - 0.310R² = 0.995

-1-0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2

0 5 10 15

E/ m

V v

s. A

g/A

gCl

pH

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1 6 11I/μ

ApH

Figure 5. (Color online) Cyclic voltammograms of dinitramine at the MWCNT/GCE in B–R buffer (0.02mol L¹1) containing of5 © 10¹6mol L¹1 dinitramine, at different pH from 2.0 to 10.0 (a), linear curve for reduction peak on pH solution (b), Ip with pH solution (c);the potential sweep rate was 100mV s¹1 for reduction peak.

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concentration of Na+, K+, mg2+, Ca2+, Pb2+, Al3+, Fe3+, Cl¹, Br¹,I¹, NO3

¹,CO32¹ or SO4

2¹ in the solution had almost no influenceson the determination of dinitramine (signal change below 5%).

5. Application of the Method

In order to examine the suitability of the recommended methodfor determination of dinitramine in real samples, it was applied totap water and waste water samples. The results of the analyses arecollected in Table 1. The data in Table 1 show that the method isaccurate and percent recoveries between 99.0 and 102%. To provethe precision of the method, electrochemical experiments wererepeatedly performed 10 times with the same MWCNT/GCE inthe solution containing 5 © 10¹6mol L¹1 of dinitramine and RSDwas calculated to be lower than 2.50% which revealed that therepeatability of the MWCNT/GCE was excellent.

6. Conclusions

It has been demonstrated that dinitramine, a nitroaromaticpesticide compound, can be determined using a glassy carbonelectrodes modified with multi-walled carbon nanotubes. The resultsshowed that the presence of carbon nanotube film on the surface ofthe electrode dramatically affect the kinetics and sensitivity of theelectrochemical responses toward dinitramine. In these investiga-tions, the electrochemical response corresponding to the reductionof nitro functional group was used as a sensitive procedure forthe determination of dinitramine within two concentration ranges;4.0 © 10¹8 to 1.4 © 10¹6 and 1.4 © 10¹6 to 2 © 10¹5mol L¹1 with adetection limit of 0.8 © 10¹8mol L¹1. High sensitivity and improveddetection limit of the MWCNT/GCE are promising for thedetermination of trace amounts of dinitramine. Good precisionand good recovery were obtained when the proposed method wasapplied to real water samples.

Acknowledgments

The authors gratefully acknowledge the support of this work bythe Department of Analytical Chemistry, Faculty of Chemistry,Razi University Kermanshah, Iran. The authors acknowledge RaziUniversity Center for Scientific Instrument.

References

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(a) (b)

(c) (d)

-180-160-140-120-100-80-60-40-200

-0.8 -0.6 -0.4 -0.2 0

I/μA

E/V vs. Ag/AgCly = -2.573x - 24.30

R² = 0.998

-180-160-140-120-100-80-60-40-200

0 20 40 60

I/μA

C/μM

-150-145-140-135-130-125-120-115-110-105

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0 I/μA

E/V vs. Ag/AgCl

y = 0.476x - 118.8R² = 0.999

y = 9.451x - 128.2R² = 0.999

-130-128-126-124-122-120-118-116-114-112-110

0 10

I/μA

C/μM

Figure 6. (Color online) CVs for various concentrations of dinitramine in the range of 4.0 © 10¹7 to 5.5 © 10¹5 at at the MWCNT/GCEin B–R buffer (0.02mol L¹1) (a) corresponding linear calibration curve of Ip versus dinitramineconcentration (b), SWVs for variousconcentrations of dinitramine in the range of 4.0 © 10¹8 to 1.4 © 10¹6 and 1.4 © 10¹6 to 2 © 10¹5mol L¹1 at the MWCNT/GCE in B–Rbuffer (0.02mol L¹1) (c), corresponding linear calibration curve of Ip versus dinitramine concentration (d); scan rate 100mV s¹1.

Table 1. Application of the recommended method to determina-tion of dinitramine in water samples.

SampleDinitramine

added(mol L¹1)

Dinitraminefound

(mol L¹1)

Relativestandarddeviation(RSD%)

Recovery%

Tap water 0.005 © 10¹6

Not detected4.96 © 10¹6

2.36 99

Waste water 0.005 © 10¹6

Not detected5.16 © 10¹6

2.49 102

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