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Electroanalytical determination of carbendazim by square wave adsorptivestripping voltammetry with a multiwalled carbon nanotubes modified electrode

Williame Farias Ribeiro,a Thiago Matheus Guimaraes Selva,b Ilanna Campelo Lopes,a

Elaine Cristina S. Coelho,b Sherlan Guimaraes Lemos,a Fabiane Caxico de Abreu,c Valberes Bernardo doNascimentob and M�ario C�esar Ugulino de Ara�ujo*a

Received 29th November 2010, Accepted 15th March 2011

DOI: 10.1039/c0ay00723d

A preconcentrating/voltammetric multiwalled carbon nanotube modified glassy carbon electrode

(MWCNT–GCE) has been developed for stripping analysis of carbendazim (Methyl Benzimidazol-2-yl

Carbamate—MBC), based on dispersing MWCNT in water. The effect of experimental variables, such

as the dispersion and loading of MWCNT, was assessed. A quasi-reversible behavior for MBC in acetic

acid/acetate buffer 0.1 mol L�1 (pH 4.7) was verified and its high effective pre-concentration was

attributed to the high adsorption capability and enormous surface area of the MWCNT. No evidence

of carry-over effect, combined with the easiness of electrode preparation, led to the development of

a highly sensitive and reliable method with an experimental work range from 0.256 to 3.11 mmol L�1

with a detection limit of 10.5 ppb for a short (60 s) accumulation period. Measurement of MBC in

a river water sample was demonstrated. The accuracy of the method for real sample analysis was

assessed by estimating the apparent recovery (93 � 2.9% and 86 � 4.1% for 4.3 � 10�7 mol L�1) for

a MBC spiked river water sample.

1 Introduction

Carbendazim (Methyl Benzimidazol-2-yl Carbamate—MBC) is

classified as a systemic fungicide due to its ability to control

a wide range of fungal diseases in crops of fruits and vegetables.

Studies have shown that MBC also appears as the active

substance of benomyl and thiophanate-methyl. Actually, it is the

main degradation product of these two compounds.1 Degrada-

tion of pesticides, fungicides and toxic effects of carbendazim in

humans and animals have been recently reviewed.2,3 As a widely

used fungicide, it has demanded research on developing sensitive

and rapid analytical methods for monitoring it in soil, water

samples, marketed fruits and vegetables. The ANVISA, a Bra-

zilian regulatory agency, sets a limit of 0.02 mg kg�1 as the human

acceptable daily intake of MBC.4 Analytical methods for benz-

imidazole fungicides and their residues have been reviewed.5 The

most common techniques for the determination of MBC in

a wide range of matrices such as soils, water, honey and some

kinds of fruits are UV spectrophotometry, spectrofluorimetry

and liquid chromatography.6–9

aUniversidade Federal da Para�ıba, CCEN, Departamento de Qu�ımica,Joao Pessoa, PB, Brazil. E-mail: laqa@quimica.ufpb.br; Fax: +55-83-3216-7437; Tel: +55-83-3216-7438bUniversidade Federal Rural de Pernambuco, Departamento de Qu�ımica,Recife, PE, BrazilcUniversidade Federal de Alagoas, CCEN, Departamento de Qu�ımica,Macei�o, AL, Brazil

1202 | Anal. Methods, 2011, 3, 1202–1206

Electroanalytical techniques have not been largely used for

determination of MBC. However, electrochemical studies10 and

some applications such as electroanalytical sensors11–13 for

analysis of MBC following its adsorptive accumulation on elec-

trode surfaces can be found in the literature. These later ones

present a silicone modified graphite electrode,11 a glassy carbon

wall jet cell12 and a carbon fiber ultramicroelectrode13 as alter-

natives for simple and rapid determination of MBC with high

sensitivity. Polypyrrole14 and clay15 modified glassy carbon

electrodes have also been proposed for accumulation and strip-

ping analysis ofMBC. In addition, a pulse polarographic method

and the use of a catalytic wave of Co(II) for the voltammetric

determination of MBC have also been reported.16,17

Environmental pollution and food-safety have increased the

demand for reliable, rapid, sensitive and cost-effective pesticide

and fungicide assay methodologies. Selectivity, sensitivity, fast

response and broad linear dynamic range make electrochemical

sensors competitive to chromatographic methods toward

a variety of analytes in complex matrices.18 They also have the

advantage of being adaptable tomicrofabrication technology.19,20

Carbon nanotubes, discovered by Iijima,21 have attracted

enormous interest in several research areas. An increasing

number of reports on single and multiwalled carbon nanotube

modified electrodes can be found in the literature. Carbon

nanotubes are attractive materials for the development of elec-

trochemical sensors due to their capability to usually provide

strong electrocatalytic activity and minimize surface fouling of

the sensors,18 besides their high adsorption capacity,22,23 thus

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increasing selectivity and sensitivity of carbon nanotubes based

electrochemical detectors. Numerous applications of carbon

nanotubes on developing electrochemical sensors take advantage

of their electrocatalytic and adsorption properties. Carbon

nanotube modified electrodes have been applied for sensitive

detection of the pesticides isoproturon, voltage and dicofol,22

triazophos,23 thiocholine24 and the herbicides asulam25 and

amitrole,26 among others.

Recently, Chi and Li27 have studied the use of a polimeric

methyl-red film MWCNT modified glassy carbon electrode for

determination of MBC, and Manisankar et al.28 also presented

an application of functionalized carbon nanotubes for detection

of MBC in real samples.

This work takes advantage of the high adsorption capability

and low fouling tendency of the carbon nanotubes for the

development of a modified electrode for a sensitive voltammetric

stripping analysis of MBC at low concentration levels. The

electrode modification was carried out by simply evaporating

a suspension of MWCNT dispersed in water onto the GCE

surface. Such a procedure results a sensor in whichMBC presents

two quasi-reversible waves. In addition, the sensor also presents

no significant fouling effect, and the method possesses advan-

tages such as high sensitivity, rapid response, low cost and

simplicity.

2 Experimental

2.1 Apparatus and reagents

All voltammetric experiments were carried out using an Eco

Chemie, mAutolab� Type II, potentiostat coupled to a Met-

rohm, 663 VA Stand�, three-electrode module. A platinum wire,

a Ag/AgCl (3 mol L�1, KCl) and a 3 mm diameter Multi-Walled

Carbon Nanotubes modified Glassy Carbon Electrode

(MWCNT–GCE) were employed as counter, reference and

working electrodes, respectively.

MBC (97%) was purchased from Sigma–Aldrich. MWCNTs,

90% purity, 10–70 nm diameter, 20 mm length, were obtained

from CNT Co., Ltd (Songdo-Dong Yeonsu-Gu Incheon,

Korea). All other chemicals were of analytical-reagent grade and

used as received. The solutions and subsequent dilutions were

prepared daily with deionized water in a Milli-Q System.

Stock solutions of MBC (4.3 � 10�3 mol L�1) were prepared in

0.1 mol L�1 sulfuric acid. A 0.1 mol L�1 acetic acid/acetate buffer

(pH 4.7) was employed as supporting electrolyte.

Fig. 1 Cyclic voltammograms of MBC at GCE andMWCNT–GCE for

1.13 mmol L�1 MBC in 0.1 mol L�1 acetic acid/acetate buffer (pH 4.7).

Scan rate of 500 mV s�1.

2.2 Electrode preparation

Firstly, a bare GCE was polished to a mirror-like appearance in

a felt-pad using 0.3 and 0.05 mm alumina slurries, consecutively.

Then, the electrode was washed with plenty of water and soni-

cated for 3 min in nitric acid (1 : 1), acetone and water, respec-

tively. Next, the electrode was activated in 0.1 mol L�1

supporting electrolyte by 10 successive cyclic scans from �0.8 to

1.5 V at a scan rate of 250 mV s�1. Concurrently, MWCNTs were

dispersed in water or dimethylformamide (DMF) (1–4 mg mL�1)

by sonication for 2 hours. Cast films were prepared by placing

a droplet of 20 mL of that suspension onto the GCE surface and

evaporating the solvent in an oven at 50 �C for 30 min.

This journal is ª The Royal Society of Chemistry 2011

The MWCNT–GCE was then activated by successive scans in

the same way as described above for the bare GCE.

2.3 Procedure

All experiments were performed at room temperature. For the

accumulation step, the electrode was kept immersed in 5 mL of

a deaerated sample solution under stirring, while holding its

potential at open circuit. An accumulation time of 60 s, at open

circuit, was used for the quantitative measurements. The accu-

mulated MBC was measured by square wave voltammetry

(SWV). The scan parameters included a pulse potential

frequency (f) of 25 Hz, a pulse height (Ep) of 50 mV and a scan

increment of (DE) 5 mV, using acetate buffer 0.1 mol L�1

(pH 4.7) as supporting electrolyte. Following each measurement,

the electrode was cleaned by rinsing it with plenty of water.

2.4 Recovery

The accuracy was evaluated by the apparent recovery29 in pure

electrolyte and river water samples spiked with MBC.

3 Results and discussion

3.1 Voltammetry of MBC

Cyclic voltammograms of MBC on bare and MWCNT–GCE,

Fig. 1, in acetate buffer (0.1 mol L�1, pH 4.7) at a scan rate of

500 mV s�1, shows a much higher sensitivity for the modified

electrode. In addition, two quasi-reversible waves are observed

for the modified electrode (Ep,1a ¼ 0.76 V and Ep,2a ¼ 1.3 V;

Ep,1c ¼ 0.45 V and Ep,2c ¼ 0.86 V). Hern�andez et al.11 have

also observed a quasi-reversible oxidation wave for MBC

around 1.0 V.

The effect of scan rate in the range of 20 to 500 mV s�1 for

MBC on MWCNT–GCE in acetate buffer (0.1 mol L�1, pH 4.7)

is shown in Fig. 2. A displacement of the anodic and cathodic

peak potentials (2a and 2c) to more positive and more negative

potentials, respectively, is clearly observed, and both waves show

a linear dependence of the peak current to the scan rate (inset in

Fig. 2), characterizing an adsorption mass transfer electro-

chemical process.

Anal. Methods, 2011, 3, 1202–1206 | 1203

Fig. 2 Cyclic voltammograms of MBC at MWCNT–GCE. Effect of

scan rate: 20, 50, 100, 200 and 500 mV s�1. Inset: plot (:) Ip,2a and (C)

�Ip,2c vs. scan rate. MBC 1.13 mmol L�1 in 0.1 mol L�1 acetic acid/acetate

buffer (pH 4.7).

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A comparison study between the sensitivity of the voltam-

metric response for the bare and the modified GCE is presented

in Fig. 3. The remarkable higher sensitivity observed for the

modified electrode is attributed to the high adsorption capability

and enormous surface area of the carbon nanotubes,22 since no

electrocatalytic effect was observed, which is in accordance to the

literature.27,28

3.2 Optimization of experimental variables

3.2.1 Dispersion of MWCNT. Unbundling and dispersion of

pristineMWCNT can be assisted by the use of dispersants and/or

surfactants.18,30,31 On preparing a MWCNT modified electrode

by cast, the concentration of the applied dispersion and thickness

of the deposited layer have a direct influence on the morphology

and properties of the surface. It is important to note that

MWCNTs are insoluble at all solvents and have a high tendency

to aggregate due to strong van der Waals binding forces.32 Thus,

the degree of dispersion deeply depends on the nature of the

solvent, dispersants or surfactants and on the physical and

chemical treatments. A number of researches have focused on

Fig. 3 Square wave voltammograms of MBC at (----) GCE and (——)

MWCNT–GCE. MBC 5.21 mmol L�1 in 0.1 mol L�1 acetic acid/acetate

buffer (pH 4.7). Scan rate of 125 mV s�1.

1204 | Anal. Methods, 2011, 3, 1202–1206

new dispersants and dispersion protocols to pursue high

concentrated and stable dispersions, although some degree of

dispersion can be directly obtained in water,30,31 other

solvents33,34 or aqueous solutions of surfactants18 by a simple

ultrasonication of suspended MWCNT.

It is known that stirring MWCNT in a mixture of nitric and

sulfuric acids introduces carboxylic and sulfonic groups at the

ends or at sidewall defects,31 improving its solubility in water and

also its interaction with surfactants and dispersants.35 In addi-

tion, the degree of dispersion can be reinforced by preparation of

MWCNT/polymer composites, such as in the cellulose deriva-

tives36 and amylose37 examples reported as alternatives to obtain

high concentrated and stable dispersions.

In this work, the performance of a very simple dispersion

treatment was evaluated for the preparation of a MWCNT–

GCE. The MWCNTs were ultrasonicated for two hours in water

or DMF. The resulting dispersion was applied onto a GCE

surface and left to dry at 50 �C in an oven. Water produced

a better dispersion, which gave a higher homogeneity and a much

more sensitive voltammetric response. Different loadings

(1–4 mg mL�1) did not reveal significant difference in their

analytical responses, in disagreement with studies accomplished

by Ulloa et al.30 Thus, a concentration of 1.0 mg mL�1 of

MWCNT dispersed in water was employed at all subsequent

studies.

3.2.2 Film casting. A high relative standard deviation (rsd) of

8.23% (n ¼ 5) was verified for the measurement of MBC at low

concentration (1.25 � 10�6 mol L�1). It was attributed to the

modifying electrode procedure, based on placing a droplet of the

MWCNT suspension onto the GCE surface and evaporating the

solvent in an oven. Obviously that procedure influences the

electrode to electrode reproducibility and, actually, such repro-

ducibility requires a uniform dispersion of theMWCNT onto the

GCE, which also depends directly on the efficiency of their

previous dispersion treatment and the nature of the employed

dispersant. Thus, the reproducibility could be further improved.

3.2.3 Influence of pH. As it can be seen in the Fig. 4, there is

a strong influence of the pH on the electrochemistry of MBC. A

linear dependence (�64.7 mV per pH) between the peak potential

and the solution pH was verified. A protonation step in the

oxidation mechanism of MBC molecules is then expected.10 The

protonation step can be attributed to the presence of nitrogen

atoms in the MBC molecule structure.10

Although a much higher sensitive response was verified for

strong acid solutions, a pH 4.7 (0.1 mol L�1 acetic acid/sodium

acetate buffer) was chosen for analytical purposes, since the well

shaped peak at this range indicates a voltammetric behavior less

susceptible to kinetic effects.

3.2.4 Pre-concentration and deposition time studies. The

adsorption capacity of MWCNT was evaluated to pre-concen-

trate MBC on the electrode surface at fixed potentials (�0.2 V to

0.6 V) and at open circuit. The study revealed no significant

difference for the two conditions. Open circuit was then chosen

for the pre-concentration step. Saturation of the electrode

surface was not observed in a range of 0 to 150 s for MBC

concentrations in the micromolar range. A pre-concentration

This journal is ª The Royal Society of Chemistry 2011

Fig. 6 Square wave voltammograms for additions of MBC in

0.1 mol L�1 acetic acid/acetate buffer (pH 4.7). (a) 0, (b) 0.26, (c) 0.76,

(d) 1.01, (e) 1.25, (f) 1.73, (g) 2.20, (h) 2.66, and (i) 3.11 mmol L�1.

The corresponding calibration plot forMBC is also shown (inset). Stirred

solution; 60 s of accumulation.

Fig. 4 Influence of pH on the IPa and EPa. Inset: SWV for MBC at

different pHs. MBC 2.56 mmol L�1 in 0.1 mol L�1 acetic acid/acetate

buffer (pH 4.7).

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time of 60 s represented a good compromise condition between

sensitivity and the overall time of analysis.

3.3 Carry over study

Consecutive square wave scans of a solution containing

2.56 mmol L�1 of MBC were obtained to evaluate the existence of

memory effect. As can be seen in Fig. 5, no significant carry over

is presented by the sensor, which is in agreement with the low

fouling tendency expected for the carbon nanotubes.1 It

demonstrates the prompt regeneration of the electrode surface

between experiments. Thus, the sensor could be used several

times without loosing activity, and a simple water rinsing was

sufficient to clean the electrode.

Fig. 7 Recovery study in different water samples spiked with MBC: (-)

pure water, (C) river water (point A) and (:) river water (point B). A

corresponding set of square wave voltammograms of the river water

(point B) is shown in the inset.

3.4 Analytical curve

Fig. 6 displays the square-wave stripping voltammetric responses

and the respective analytical curve for MBC in the optimized

experimental conditions. A good linear relationship between peak

current and concentration was verified for the concentration

range from 2.56 � 10�7 to 3.11 � 10�6 mol L�1 (r ¼ 0.9976 for

n ¼ 8, and IPa/mA ¼ �0.62 (�0.23) + 4.28 � 106 (�0.12 � 106)

Fig. 5 Carry-over experiment using (a) 0 and (b) 60 s deposition times

(rsd of 4.0% and 2.6%, respectively). SWV at MWCNT–GCE of MBC

2.56 mmol L�1 in 0.1 mol L�1 acetic acid/acetate buffer (pH 4.7).

This journal is ª The Royal Society of Chemistry 2011

[MBC]/mol L�1). A detection limit of 10.5 ppb (5.49 �10�8 mol L�1) and a quantification limit of 35 ppb (1.83 �10�7 mol L�1) were, then, estimated.38 The World Health Orga-

nizationmentions onemaximum allowable concentration limit of

100 ppb for water samples.39

Table 1 Results for the recovery study of MBC in different watersamples

Sample

MBC/10�7 molL�1

Apparentrecovery (%) rsda (%)Added Founda

Pure water 4.3 4.6 106 1.3River water (point A) 4.3 4.0 93 2.9River water (point B) 4.3 3.7 86 4.1

a n ¼ 3.

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Attempts to reuse the modified electrode have been explored

by repeating the calibration experiment with intermittent water

rinsing (figure not shown). The low fouling tendency of the

MWCNT permitted the reuse of the modified electrode for

several weeks. A standard deviation of 7.5% for 10 successive

measurements of a 1.25 � 10�6 mol L�1 MBC solution demon-

strated a good repeatability of the proposed sensor.

3.5 Practical application

The performance of the developed sensor was evaluated by

measuring MBC concentration in pure electrolyte and two river

water samples collected from distinct points (A and B) of the

Paraiba River, from Cruz do Esp�ırito Santo City—Paraiba

State—Brazil (Fig. 7). Although no official documentation was

found about contamination of that river by MBC, there are huge

pineapple farms, where MBC is largely used to control fungal

diseases,40 in its near region. The modified electrode responded

efficiently to increments in the MBC concentration. Recoveries

of 93 � 2.9% and 86 � 4.1% were verified for 4.3 � 10�7 mol L�1

MBC in the real samples studied (Table 1).

4 Conclusions

MWCNT is a highly suitable matrix for adsorptive accumulation

and stripping analysis of MBC. Two well defined quasi-reversible

oxidation–reduction waves were observed for a MWCNT–GCE.

The developed surface is well suited for monitoring micromolar

concentrations of MBC in real samples by square wave stripping

analysis with short (60 s) pre-concentration times. The sensor

presents no significant fouling effect, and the method possesses

advantages such as high sensitivity, rapid response and

simplicity.

Acknowledgements

The financial support for this work by CNPq is gratefully

acknowledged.

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