Abstract widely used benzimidazolic IV class systemic
AbstractPoly(metacryloylhydrazide) was chemically synthesized using hydrazine hydrate andpoly methyl methacrylate through a simple reaction. The synthesized materialwas employed for fabrication of modified carbon paste electrode (CPE). The modified electrode was used as a novel impedimetricsensor for the determination of an important pesticide, Carbendazim. Theperformance of poly (metacryloyihydrazide) toward Carbendazim was examinedusing electrochemical impedance spectroscopy.
Binding of Carbendazim to thepolymer on the surface of the electrode changes the impedance of theelectrode/electrolyte interface which can be monitored as signal for theselective and sensitive determination of Carbendazim. Underoptimized experimental conditions, the proposed modified electrode shows alinear response range from 40 to 240 ppb Carbendazim with a detection limit of14 ppb (based on 3sb). Furthermore, the fabricated electrode wassuccessfully applied to determine Carbendazim in Carbendazim-added artificialsamples. Keywords: ElectrochemicalImpedance Spectroscopy; Ploy (Methyl methacrylate); Methacryloyl Hydrazide;Carbendazim; CPE. IntroductionFor many yearsnow, pesticides have been extensively employed to increase agriculture productsin order to supply human food.
Carbendazim (methyl 1H-benzodimidazol-2-ylcarbamate) (CBZ) is a widely used benzimidazolic IV class systemic fungicide onplants, seeds or soils that can be absorbed by plants through the roots, leavesand seeds. It is stable for long time and degrades slowly so can be accumulatedin the human body through consumption of infected drinking water, vegetablesand tree crops and fruits. Several reports have been published regarding thetoxic and carcinogenic effects of CBZ in human and animals.1 Hence, it is necessary to develop reliableanalytical methods to monitor and determine CBZ residues in water andfoodstuffs. Various quantitative analytical methods mainly based onchromatography technique have been established for determination of CBZ whichis expensive and time consumption2,.3.UV-vis spectrophotometry, spectrofluorometry and Raman spectroscopy have also beenused for this purpose.
4-6 On the otherhand electrochemical techniques such as differential pulse voltammetry (DPV) incombination with modified electrodes have been applied to determine theresidues of the CBZ in real samples.7-12 Great attention devoted to theelectroanalytical techniques is due to their high sensitivity and excellent selectivity,simple instrumentation, low cost, and miniaturization capability that areaccountable for these techniques. Several articles in literature report the useof modified electrodes recognizing pesticides selectively along with theelectrochemical techniques for quantified assay of toxic materials.
For examplean ionic liquid modified CPE was prepared and introduced for determination of CBZusing DPV by Ya et al.13 Theproposed modified electrode was applied for the determination of CBZ insugarcane samples. Petroni et al.
designed a commercial screen-printed carbon electrode modified withmulti-walled carbon nanotubes for square-wave voltammetric determination of thefungicide CBZ in the presence of an anionic surfactant.14They showed that the use of the anionic surfactant sodium dodecyl sulphateprovided a significant improvement on analytical sensitivity of themeasurement. The developed electrode was employed for determination of CBZ in aspiked orange juice sample. An electrochemically reduced nitrogen-dopedgraphene oxide-modified glassy carbon electrode (ERNGO/GCE) was developed forthe determination of CBZ in food samples by Ya et al.
15 The ERNGO/GCE was used to measure theconcentration of CBZ in Dendrobium candidum samples. For pre-preparation, thesamples was treated in ethanol and then centrifuged. The resulting ethanol solution was evaporatedto dryness using a mild nitrogen stream, and the residue was reconstituted in1.0 ml of ethanol. Recently a neat CPE (CPE) was used for the DPVquantification of CBZ in water and orange juice samples by Arruda et al.9 An oxidation peak was observed at high overpotential of 0.
88 V vs. Ag/AgCl/Cl- reference electrode incitric acid-phosphate buffer solution adjusted at pH=5.0. The simultaneous determination of twopesticides, namely isoproturon and CBZ, by single drop analysis using a disposablegraphene based screen-printed electrochemical sensor with square wave strippingvoltammetry was developed by Noyrod et.
al.8The objective of the work was no requirement to use a conventional cell becausethe low-cost graphene-based sensor was used as an electrochemical cell withthree integrated electrodes. Also only a single 60 µL drop of the samplesolution is required to be dispensed onto the surface of the sensor for eachstripping voltammetric measurement. In all above mentioned works, measurementshave been conducted on the non-selective recognizing surfaces. Carboxylic group functionalized poly (3,4-ethylenedioxythiophene) (PC4-EDOT-COOH)electro synthesized in aqueous micro emulsion system exhibited goodelectrochemical recognition towards CBZ and was employed for theelectrochemical detection of CBZ by differential pulse adsorptive strippingvoltammetry.16. An oxidation peakwas observed at high over potential of 0.
87 V vs. Hg/Hg2Cl2/Cl-reference electrode in buffer solution adjusted at pH=7.0. Moreover theelectrode is stable for less than one week and the modifying polymer should beremoved and synthesized again to achieve new and fresh electrode. Recently Daiet al. reported an impedimetric determination of formaldehyde using a glassycarbon surface covered by poly metacryloylhydrazide/carbon nanotube (PMAH/CNT)electro spun nanofibers as recognizing agent.17The condensation reaction of -NH2 functional group on the PMAHattached to the electrode surface with carbonyl group of target molecules(formaldehyde) and formation of an imine (C=N) bond (due to the removing a H2Omolecule) leads to the change of impedance of the electrode/electrolyteinterface.
This change in impedance was quantitatively related to theconcentration of the target molecules.17In the present study we introduce a new modified CPE to recognize CBZsensitively and selectively at trace levels in real samples. PMAH issynthesized based on the method reported by Gong et al. with slightmodification.17 Then the polymeris well mixed with carbon powder and a mineral oil to achieve a conductivepaste. As it is well known, CPEs can be easily prepared and achievement to thenew surface and refreshing the electrode is very simple only by polishing thesurface of the electrode on a suitable polishing material. They are also usablefor long time applications.
The high efficient condensation reaction of -NH2functional group on the PMAH inside the electrode with carbonyl group of CBZ intest solution and elimination of a methanol molecule (instead of water) leadsto the change in impedance of electrode/electrolyte interface which quantitativelyis correlated to the concentration of CBZ in solution (Scheme1). Because, methoxy is an excellent leaving group in CBZ (compared tohydrogen in formaldehyde), the condensation reaction occurs efficiently. Here we utilize electrochemical impedancespectroscopy (EIS) technique that comprising all features associated withelectrochemical techniques mentioned above and also it can be well used to thedetection of interfacial binding events. Scheme1. Mechanismof reaction between poly methacryloyl hydrazide and CBZ.ExperimentalChemicalsPolymethylmethacrylate(average molecular weight Mw ~350,000) and CBZ standard werepurchased from Sigma-Aldrich.
Other chemical used wereof analytical grade and purchased from Merck. Doublydistilled water was used for preparation of all solutions. Instruments and methodsAllelectrochemical measurements were made using an Autolab general purposeelectrochemical potentiostat/galvanostat system PG302N and Autolab frequencyresponse analyser system (AUT20. FRA2- AUTOLAB, EcoChemie, B.V., Netherlands). Typical impedimetric measurements wereconducted in a conventional electrochemical cell housing three-electrode whichemployed PMAH modified CPE as the working electrode, an Ag/AgCl electrode asthe reference and a Pt rod as the counter electrode. The solution was containing10 mL 0.
1 mol l?1 KCl and 0.005 M Fe(CN)63-/4-. The working CPE electrode was immersed in 0.1 M phosphate buffer solution (pH7.0) containing certain amounts of CBZ for reaction time of 60 s before eachimpedimetric test. The frequency range used for the electrochemical impedancespectroscopy (EIS) measurement ranged from 0.
1 Hz to 100 KHz with analternating amplitude of 10 mV super imposed onto the formal potential ofFe(CN)63-/4- (0.25 V vs. Ag/AgCl reference electrode).Synthesis ofploy-methacryloylhydrazide (PMAH) PMAH wassynthesized based on the method was reported by Die et.
al. with somemodification (Scheme 2).17 1.0 g PMMA was dissolved in 10 mldichloromethane and added dropwise to the equimolar solution of hydrazine hydrate.During the addition, the mixture was stirred moderately at 35 °C. A white solidcrystalline material was extracted by addition of excess amounts of ethanol anddried at room temperature for subsequent uses.
Scheme2. Synthesisof poly metacryloyl hydrazide. Preparation of CPEConventional CPE(CPE) was prepared by thoroughly mixing 0.10 g graphite powder and three dropparaffin oil until a uniform wetted paste was obtained. The PMAH modified CPEswere prepared by hand mixing 0.10 g of graphite powder and 0.020, 0.
025, 0.033and 0.05 g of PMAH and three drop paraffin oil.
The resulting pastes werefirmly packed into a cylindrical Teflon tube (3.0 mm diameter) fitted with acopper piston, which serves as an inner electrical contact. The surface of theelectrodes was polished on a glossy paper prior to each electrochemical use. Results andDiscussionFig.
1 comparesthe IR spectra of the pristine PMMA and synthesized PMAH. Appearance of atwo-branch absorption band at 3350-3520 cm-1 characteristic of theNH2 functional group can be considered as a strong evidence ofsuccessful synthesis of PMAH through simple reaction of PMMA and hydrazinehydrate. Moreover, absorption bands corresponding to the bending vibration ofN-H and stretching of C-N are observable at 1618 and 1238 cm-1 respectivelyFig.
1Theelectrochemical impedance spectroscopic characterization of the neat CPE andits composites with various amounts of PMAH were conducted using Fe(CN)6?3/?4 as redoxprobe. Impedance spectra (Nyquist plots) for each electrode were obtained afterimmersing the electrode in phosphate buffer blank solution (pH=7.0) andphosphate buffer solution containing 80 ppb CBZ for 60 s. The resulted Nyquist plotsshow a semicircle with diameter corresponds to the charge-transfer resistance (Rct)and straight line corresponds to the Warburg impedance (Zw).
The changes incharge transfer resistance due to the reaction of CBZ with PMAH at the surfaceof the electrodes were taken into account according to the followingcalculation: ?R/R0=(R-R0)/R0where R0and R are charge transfer resistances at the surface of each electrode immersedin phosphate buffer blank solution (pH=7.0) and solution containing target molecules respectively. Fig 2 showsNyquist response of neat CPE (a and b) and composite electrodeincluding 33.3 wt% of PMAH (c and d). Comparison of plot a and b reveals that the Rct for neatCPE remain constant when the electrode is immersed in solution containingtarget molecules. However a great change of Rct occurs when the modifiedelectrode is immersed in solution containing 80 ppb CBZ target molecules (plotsc and d). This great change is attributed to the combination of CBZ moleculeswith active sites of the PMAH on the surface of the electrode. Comparison ofplot a and c indicates that the presence of PMAH in compositeelectrode increases the charge transfer resistance.
Fig. 2Fig3 (inset) showsNyquist response of the electrodes containing 0.0, 20.0, 25.0, 33.3, and 50.
0wt% of PMAH. Comparison of these plotsshows that with increasing the content of the PMAH, semicircle diameters(charge transfer resistance) increases because of the more combination oftarget molecules (CBZ) with active sites of the PMAH on the surface of theelectrode. Fig.
3 shows the changes inanalytical signal (?R/R0) vs. content of PMAH in compositeelectrodes. As it is observable, ?R/R0 increases rapidly withincreasing the amount of PMAH up to 33.
3 wt% and then the rate of increase isslowed. Then CPE electrode modified with 33.3% PMAH was selected and used forfurther optimization process. Fig. 3The effect of pH of buffer solutions on the reaction of CBZ withPMAH on the surface of the modified electrode was studied, and the relativecharge transfer resistance (?R/R0) as a function of pH ranging from 3.0to 10.
0 in the presence of 80 ppb CBZ are shown in Fig.4. Thecorresponding Nyquist plots are illustrated as inset. As shown in Fig.4, charge transfer resistance of the modified electrode increased graduallywith the increasing the pH value until it attained the maximum at pH 7.0, andthen decreased rapidly when pH increased further.
At pHs lower than 7.0, NH2functional groups on the PMAH react with hydronium ions in acidic buffersolution and are converted to NH3+ with littlenucleophilic properties that are not able to react with CBZ target molecules.More over in this range of pH CBZ is also converted into soluble salt. Withincreasing the pH of buffer solution to the values higher than 7.0 CBZmolecules degrade slowly.
16,18resulted in the rapid decrease in response function. Therefore, pH of 7.0 waschosen for the impedimetric determination of CBZ.Fig.
4To achieve a highly sensitive detection of CBZ, immersing time ofthe PMAH modified electrode in sample solution (buffered at pH 7.0) has beenoptimized. Immersion time can influence on the transport of the CBZ to theelectrode surface and its more reaction with PMAH immobilized on the surface ofthe electrode. Fig. 5 shows the Nyquistplots (inset) and corresponding analytical signal against reaction time forPMAH modified electrode dipped in CBZ molecules for various immersion times.
The enhancement of Rct value indicates that the amount of CBZ reacting in therecognition sites of the PMAH modified electrode increased with the increase ofthe immersion time. It shows that with increasing the immersion time up to 120s the analytical response function increases rapidly and then with moreincreasing the immersion time the rate of increase of ?R/R0decreases and finally reaches the platform after 120 s. Therefore, the 120 s immersion time wasemployed in subsequent experiments.Fig. 5Theeffects of some potential interfering species on the determination of 80 ppbCBZ were examined. The tolerance limit was taken as the maximum concentrationof the foreign substances that caused an approximately ±5% relative error inthe determination. The results revealed that more than 100-fold all examinedspecies indicated in Table 1 did not affectthe determination of CBZ.
Table 1Inthe determination step of CBZ, after the PMAH modified electrode was immersedin buffer solution containing various amounts of target molecules for 60 sNyquist plots were recorded in 5 mM Fe(CN)63?/Fe(CN)64?containing 0.1 M KCl. Calibration studies include the successively dipping theelectrode into CBZ solutions with a surface regeneration step, in which theelectrode surface was polished on a glassy paper to achieve a smooth surfaceafter each impedance run. The values of relative change of electron transferresistance were plotted as a function of CBZ concentration (Fig. 6). A linear relationship between the CBZconcentration and charge transfer value was obtained over the concentrationrange from 0.0 to 240 ppb CBZ. The linearity of this method was described bythe equations ?R/R0% = 1.
048 C (ppb) – 3.0714, R2= 0.997.The limit of detection (LOD) was found to be 14 ppb CBZ using equation LOD = 3sb/m,where sb is the standard deviation of the blank response and m isthe slope of the calibration plot.Fig. 6Todemonstrate the feasibility of the electrode, the PMAH modified CPE was usedfor the detection of CBZ in well water, commercial formulation and wheatsamples under optimized conditions by the standard addition method. Results aremeasure of the precision and accuracy of the method for analysis of the CBZ inreal samples are listed in Table 2.
Therecoveries range were 92.5-112.5%, 95.0-104.0 and 95.
0-104.0% in well water,commercial formulation and Wheat samples respectively, indicating that theprepared sensor could be employed for practical application without sample purificationstep. Moreover, satisfactory results revealed that the proposed method wasaccurate, suitable and efficient for the trace level detection of CBZ inpractical samples.
Table 2Table 3 compares the responsecharacteristics for the determination of CBZ at various modified electrodesreported in recent studies. As can be seen from Table 3, the PMAH/CPE exhibitedhigh sensitivity, low detection limit, and wide linearity toward CBZ.Table 3ConclusionInsummary, we successfully fabricated a modified CPE for impedimetricdetermination of carbendazim by introducing methacryloyl hydrazide in carbonpaste. The high reactivity of the carbonyl hydrazide functional group towardesters such as carbendazim was employed to change the electrode/electrolyteinterface charge transfer resistance.19The electrochemical impedance spectroscopy was employed for the first time inmonitoring the electrode/electrolyte charge transfer resistance resulted fromthe reaction taking place between carbendazim molecules and binding sites onthe surface of the poly methacryloyl hydrazide modified CPE.
Theelectrode was successfully applied for the determination of carbendazim inspiked well water, wheat and commercial formulation samples with high precisionand recovery. The proposed method offered linearity in a concentration rangefrom 40 to 240 ppb with detection limit of 14 ppb.