JBCS

INTRODUCTION

The United Nations 2030 Agenda for Sustainable Development establishes 17 Sustainable Development Goals (SDGs), with SDG number 6 aiming to ensure the availability and sustainable management of water and sanitation for all. Target 6.1 of this goal encourages improving water quality by minimizing the release of hazardous chemicals.¹ One significant risk of contamination by hazardous chemicals is associated with pesticides. While pesticides are essential for ensuring agricultural production quantity and quality, they often have high toxicity and can contaminate the environment, particularly water resources. This vulnerability arises because agricultural practices frequently occur near rivers and lakes, which are necessary for irrigation.^2,3

The carbendazim (CBZ), methyl 1H-benzimidazol-2-yl carbamate, is a broad-spectrum benzimidazole fungicide (Figure 1). Despite being banned in Brazil according to RDC 739/2022⁴ of the National Health Surveillance Agency (ANVISA), it is widely used in agriculture to protect and eradicate various pathogens that affect fruits, vegetables, and cereals. It can be absorbed by plants and transferred to every part, interfering with bacterial cell mitosis and inhibiting growth.^5,6 The herbicide diuron (DI), 3-(3,4-dichorophenyl)-1, 1-dimethyl-urea, belongs to the phenyl-urea group and is widely used post-emergence in annual and perennial plants such as cotton, alfalfa, wheat, fruits and predominantly in sugarcane plantations, or as an antifouling biocide (Figure 1). It inhibits photosynthesis by blocking electron transfer in photosystem II and induces oxidative stress, resulting in chlorosis and necrosis.^7,8 The carbofuran (CF), 2,3-dihydro-2,2-dimethyl-7-benzofuranyl-N-methylcarbamate, is a carbamate pesticide widely used to inhibit the activity of insects, mites, and nematodes in crops and plants, with an acetylcholinesterase (AChE)-inhibiting mode of action (Figure 1).^9,10

 

 

Due to the toxicity and high environmental persistence of CBZ, DI, and CF pesticides, and the classification of CBZ as a fungicide and DI as herbicide by the US Environmental Protection Agency as potentially carcinogenic to humans, there is significant concern about the surface and groundwater contamination. This contamination can occur through direct agricultural runoff, application, and leaching, posing risks to human health. This justifies the increasing interest in monitoring these pesticides using fast, accurate, highly sensitive, and easy-to-operate methods.^11-13

Usually, the determination of pesticides has been carried out by different analytical methods. Ultraviolet spectroscopy exhibits simplicity, speed, sensitivity, reasonable accuracy and precision, and cost-effectiveness,^14-16 gas chromatography^17-19 or liquid chromatography exhibits high sensitivity in pesticide analysis, enabling wide linear ranges and limits of detection (LODs) down to the µg kg^-1 level.^17,20,21 Capillary electrophoresis is also used due to its relatively short analysis time, nL to pL sample volumes, and capability of determining pesticides at trace levels,^22,23 among others. These methods, among others, have a vast variety of applications and offer some advantages, such as high sensitivity and selectivity. However, the equipment is expensive and not frequently available in routine laboratories. These techniques are often time-consuming, have a high operating cost, involve excessive consumption of highly toxic organic solvents, and present difficulties in establishing on-site measurements.

By contrast, electrochemical sensors modified or fabricated through a cost-effective process can offer fast and continuous detection of CBZ, DI and CF in surface water samples with high sensitivities and robustness. Remarkable examples, include graphene-based sensor,²⁴ carbon paste electrode modified with hemin, nickel(II) 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine complex and graphene oxide (GO),²⁵ glassy carbon electrode (GCE) modified with cobalt(II) oxide (CoO)-decorated reduced graphene oxide (rGO),²⁶ GCE modified with zinc spinel ferrites/single multi-walled carbon nanotubes (ZnFe₂O₄/SWCNTs),²⁷ electrochemically reduced micellar graphene oxide (ERMGO),²⁸ zeolite-modified carbon paste electrode (ZMCPE),^29,30 GCE modified with nanoporous gold (NPG),³¹ GCE modified with reduced graphene oxide nanocomposite in organic iron terephthalate (35MIL(Fe)-101@GO),³² and GCE modified with platinum nanoparticles and chitosan (PtNPs/CS).³³ The properties of the functional materials used to modify the bare electrodes offer high analysis performance, with increased electrocatalytic activity, greater sensitivity, and electrochemical selectivity compared to unmodified electrodes.

In some studies,²⁴ it is possible to observe important issues, such as a sharp decrease in the anodic current peak of CBZ if the amount of graphene in the electrochemical sensor is excessive, presumably due to aggregation of dispersed graphene, which results in a lower surface area and high resistance to electron transfer on the modified electrode. The direct electrochemical determination of carbamate residues such as CF is limited by the high oxidation potential that affects the detection limit and selectivity. For this reason, Wong and Sotomayor²⁵ and Wang et al.²⁶ hydrolyzed CF in an alkaline solution, obtaining much lower oxidation potentials and good electrochemical response, minimizing interference and increasing electrode sensitivity.

Although CBZ, DI, and CF pesticides can be determined individually by electrochemical techniques, recent studies^24-33 have shown the ability of electrochemical methods to identify and quantify these pesticides simultaneously. However, to the best of our knowledge, no methodology for the simultaneous electrochemical detection of CBZ, DI, and CF has been reported.

Composite electrodes offer substantial advantages in electroanalysis. For example, they can often be manufactured with great flexibility in the size and shape of the material, allowing easy adaptation to a variety of electrode configurations. They have a better signal-to-noise ratio and the versatility to incorporate selectivity and/or sensitivity enhancers into the bulk electrode material through chemical modification of the conductor and/or insulator phase.³⁴

Composite electrodes consist of at least one conductive phase, usually graphite powder, mixed in different proportions with an insulating phase, such as a polymer. The combined properties of these materials give the electrode greater mechanical stability, robustness, good signal/noise ratio, reproducibility, and the possibility of renewing the electrode surface.^35-38 Among conductive materials, graphite is one of the most used due to its attractive properties, such as a large surface area, excellent electrical and thermal conductivity.^39,40

The insulating materials agglomerate the dispersion of graphite powder and grant the material mechanical property and stability. Polymers are generally used as insulating materials, exhibiting high resistance to chemicals, flexibility for moulding, high electrical resistance, durability at high and low temperatures.⁴¹

Poly (ε-caprolactone) (PCL) is a biopolymer composed of hexanoate repeating units, included in the aliphatic polyester class. Strongly hydrophobic, semi-crystalline, tenacious, and flexible, PCL has good solubility in common organic solvents and can be degraded enzymatically. In addition to presenting excellent mechanical performance due to its rigidity and resistance.^42,43 Due to these characteristics, PCL is a great option as an insulating material in solid composites.⁴⁴

Electrodes obtained using a combination of a conductive graphite phase and a PCL insulating matrix represent an attractive approach to the fabrication electrochemical sensors,^44,45 due to their properties that improve the electrochemical behavior of the composite.

Notably, the use of composite electrodes based on graphite and PCL has not been reported in the literature for the determination of pesticides. Within this perspective, this work aims to develop an electroanalytical method for the simultaneous determination of the pesticides CBZ, DI, and CF in surface waters, given that the selected active principles present electroactive functional groups, using the CE.

 

EXPERIMENTAL

Reagents and solutions

Carbofuran and diuron were obtained from Sigma-Aldrich^® (97-99% purity) and prepared in acetonitrile (JT Baker^®, 99,98% purity) of analytical grade without prior purification. Carbendazim (Sigma-Aldrich^®, 97-99% purity) was prepared in acetonitrile/sulfuric acid 0.2 mol L^-1 (Dinâmica^®, 97% purity) 9:1 (v/v). The supporting electrolyte was a phosphate buffer solution prepared at various pH values using monobasic potassium phosphate (Sigma-Aldrich^®, 99.5% purity) and dibasic phosphate sodium (Sigma-Aldrich^®, 99% purity). The pH was achieved with sodium hydroxide solution (Sigma-Aldrich^®, 98% purity). The potassium ferrocyanide solution (Vetec^®, 98.5% purity) was prepared in potassium chloride (Dinâmica^®, 99.5% purity). Nitric acid (QEEL^®, 65% purity) and potassium chloride (Dinâmica^®, 99.5% purity) were used to prepare the Ag/AgCl reference electrode. Graphite (Sigma-Aldrich^®, less than 20 µm), PCL (Tech Tack^TM), chloroform (Dinâmica^®, 99% purity) all analytical grade, were used to prepare the composite electrode (working electrode).

Preparation of the composite and working electrodes

The composite electrodes were prepared by mixing graphite powder and PCL in a ratio of 70:30% (m/m) proportion, following procedures described by da Silva et al.⁴⁴ The PCL was solubilized in chloroform, and the final mixture was manually homogenized in an agate mortar for 20 min. The electrode assembly involved inserting the composite into a syringe (1.0 mm internal diameter). The composite inside the tube was pressed for 24 h until curing. The electric contact was established by connecting a copper wire to the composite. After curing for 7 days at room temperature, the electrodes were polished using a polisher/sander and with abrasive paper sheets with grit sizes of 600, 1200, and 2000 to expose and clean the electrode surface.

Instrumentation

The weighing was performed on a Shimadzu model AX200 analytical balance. The pH measurements of the buffer solutions were obtained using a PHB 500 ION pH meter. All aqueous solutions were prepared with high purity deionized water (18.2 MΩ cm^-1) obtained using a Milli-Q system (Millipore^®). The agitation of the solutions in the electrochemical cell was performed with Fisatom model 752A magnetic stirrer. A Marconi MA 033 oven was used for drying the material. An Arotec polisher (Aropol E) was used to polish the surface of the composite electrodes.

The electrochemical studies were carried out using an Autolab PGSTAT 30 potentiostat/galvanostat (Ecochemie-Metrohm) controlled with NOVA 2.1 software (Ecochemie). The measurements were performed using an electrochemical cell with 20 mL of supporting electrolyte, and the three electrodes system was composed of a platinum plate as the auxiliary electrode, Ag|AgCl (3.0 mol L^-1 KCl) as the reference electrode, and the composite electrode as the working electrode.

Ultra-high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) was used to compare the analytical results obtained from the proposed electroanalytical method. The chromatography analyses were performed using an UHPLC-MS/MS model Acquity H, coupled to the tandem model Xevo TQD mass spectrometer with electrospray ionization interface (ESI). Chromatographic separations were conducted on a Acquity UPLC^® BEH C18 column (100 mm × 2.1 mm × 1.7 m, Waters) at 40 ºC.

Electrochemical analysis

Electrochemical characterization and calculation of the reactive surface area of the electrodes were performed by cyclic voltammetry (CV) using 5.0 mmol L^-1 potassium ferricyanide in 0.5 mol L^-1 KCl, prepared just before use. For the electrochemical studies of the pesticides, CV and differential pulse voltammetry (DPV) methods were employed in a measurement cell containing 10.0 mL of phosphate buffer solution (pH 7.0). All measurements were carried out in triplicate for each concentration.

Analytical procedures

The applicability of the proposed method was tested in the analysis of spiked water samples. The surface water samples were collected approximately 7 km from the city of Campo Verde, MT, Brazil, at the source of the São Lourenço River. Samples were collected at the end of November 2019 and stored in amber flasks under refrigeration at approximately 4 ºC. The samples were filtered through a glass filter with a diameter of 47 mm and porosity of 44 μm, and fortified with CBZ e DI. The fortification was performed at three concentration levels: 25.0, 50.0, and 75.0 μg L^-1 for CBZ; and 250.0, 500.0, and 750.0 μg L^-1 for DI. The samples were analyzed in triplicate by the proposed and comparative chromatographic methods.⁴⁶ Accuracy was assessed by recovering samples of ultrapure water and surface water added with phosphate buffer, CBZ, and DI.

To compare the analytical results obtained from the proposed electroanalytical method, UHPLC-MS/MS method was employed, with an isocratic elution system using a mobile phase composed of acetonitrile/water (50:50, v/v) at a flow rate of 0.3 mL min^-1. Data were acquired in the multiple reaction monitoring (MRM) in the positive mode. Two specific transitions were optimized for each analyte to increase the selectivity and the reliability of the method (Table 1). The first MRM transition was used for quantification, whereas the second was used for qualitative identification. The source and operating parameters were optimized as follows: capillary voltage, 3.0 kV; desolvation gas flow rate, 1000 L h^-1 (N₂), at 500 ºC; and collision gas flow of 50 L h^-1 (Ar).

 

 

RESULTS AND DISCUSSION

Electrochemical performance

The composite electrode was electrochemically characterized using a CV in a solution of 5.0 mmol L^-1 K₄[Fe(CN)₆] in 0.5 mol L^-1 KCl. Successive measurements were conducted by varying the scan rate (v) from 10 to 200 mV s^-1 with a potential window of -0.2 to +0.8 V, as shown in Figure 2.

 

 

The electroactive surface area of the composite electrode was determined using the Randles-Sevcik formula (Equation 1) through CV, as shown in Figure 2.

where I_p (A) refers to the anodic peak current, n is the number of electron transfers in the redox reaction (n = 1), A (cm²) is the surface area of the electrode, C (mol cm^-3) is the concentration of K₄[Fe(CN)₆], D (cm² s^-1) is the diffusion coefficient (D = 7.6 × 10^-6 cm² s^-1), and v (V s^-1) is the scan rate. The electroactive area was calculated to be 0.178 ± 0.008 cm² for the bare CE and is related to the amount of electroactive sites on the electrode, that is, the area that effectively transfers the charge to the species in solution.⁴⁷

The anodic and cathodic peak currents were directly proportional to the square root of the scan rate (), indicating a diffusion-controlled redox process (Figure 2, inset).

For a reversible process, the peak potentials are independent of v and for a quasi-reversible process, cathodic peak potential (E_pc) decreases with the scan rate.⁴⁸ As can be seen in Figure 1S (presented in Supplementary Material), there is an increase in the anodic peak potential and a decrease in the E_pc, which is typical of a quasi-reversible process.

The theoretical difference between the anodic and cathodic peak potentials (ΔE_p) is 59 mV for a Nernstian behavior, an ideal one-electron reversible redox reaction, and an ideal surface electrode.⁴⁸ However, our results show an increase from ~ 75 mV at low scan rates to ~ 141 mV at the highest scan rates, indicating high response efficiency, possibly due to superior contact with the electrode surface.⁴⁹

Analytical potential of the composite electrode

Electrochemical behavior for CBZ, DI, and CF

Figure 2S (Supplementary Material) shows the CV profiles of the CE in 0.1 mol L^-1 phosphate buffer solution (pH 7.0) of the oxidation of 13.7 mg L^-1 carbendazim, 11.6 mg L^-1 diuron, and 144.8 mg L^-1 carbofuran, at a scan rate of 25 mV s^-1. From the voltammograms, a quasi-reversible oxidation peak of CBZ can be observed at +0.76 V, and an irreversible oxidation peak of DI and CF at +0.88 V and +1.32 V, respectively. Additionally, the voltammograms show a decrease in peak intensity from the second potential scan, indicating an adsorptive process on the EC surface, which decreases the electroactive area of the electrode surface with each scan.^50,51

The effect of scan rate (v) was studied based on the cyclic voltammetric responses of CBZ, DI, and CF on the CE in 0.1 mol L^-1 phosphate buffer solution (pH 7.0) (Figure 3). With the variation of v from 10 to 200 mV s^-1, it can be observed that the anodic peak currents (I_pa) of these pesticides all increase gradually with an increase in v. A linear relationship is observed between I_pa and the square root of the scan rates (v^1/2), indicating that the oxidation of these pesticides on the surface of the CE follows a diffusion-controlled process (Figure 3, inset).⁴⁸ The linear regression equations are presented as I_pa (A) = (-9.74 × 10^-6 ± 2 × 10^-6) + (1.42 × 10^-4 ± 6 × 10^-6) ν^1/2 (R² = 0.9902, CBZ), I_pa (A) = (-1.21 × 10^-6 ± 2 × 10^-7) + (3.44 × 10^-5 ± 8 × 10^-7) ν^1/2 (R² = 0.9972, DI) and I_pa (A) = (1.27 × 10^-6 ± 6 × 10^-6) + (2.77 × 10^-4 ± 2 × 10^-5) ν^1/2 (R² = 0.9775, CF).

 

 

A plot of log I_pa versus log v shows linear behavior for pesticides with a slope of 0.59 for DI and, 0.45 for CF, suggesting a diffusion-controlled process (0.50).⁵² For CBZ, the slope of 0.71, indicates an intermediate behavior between a diffusion-controlled process (0.50) and adsorption (1.0), suggesting a combination of these processes on the surface of the CE. The linear regression equations are presented as log I_pa (A) = (-3.67 ± 0.01) + (0.71 ± 0.008) log ν (R² = 0.9994, CBZ), log I_pa (A) = (-4.43 ± 0.005) + (0.59 ± 0.004) log ν (R² = 0.9998, DI) and log I_pa (A) = (-3.55 ± 0.03) + (0.45 ± 0.02) log ν (R² = 0.987, CF).

The electrochemical behavior of simultaneously analyzed pesticides was also investigated (Figure 3S, Supplementary Material). The cyclic voltammograms show anodic peaks at +0.75, +0.88 and +1.32 V for CBZ, DI, and CF, respectively (Figure 3Sa). However, significant improvement in peak identification and separation was observed in the differential pulse voltammograms (Figure 3Sb).

The anodic peak of CBZ represents the oxidation of the double bond of carbamate in carbendazim to form an amide and heterocyclic radical (Scheme 1Sa, Supplementary Material).^53,54

In the voltammograms (Figure 3), it was observed that the oxidation peak of CF occurs at a very positive potential region. At lower concentrations, its presence compromises the baseline and makes it difficult to estimate the peak currents. Therefore, the responses obtained from the electrodes: CE and commercial glassy carbon electrode (GCE) were compared for the simultaneous determination of 1.0 mg L^-1 CBZ and 10.0 mg L^-1 DI in 0.1 mol L^-1 phosphate buffer solution (pH 7.0), under the same conditions. Since the electroactive areas of the electrodes differ, the voltammograms were recorded as a function of current density (J) (Figure 4).

 

 

It was found that the current density obtained using CE was higher compared to GCE (Table 1S, Supplementary Material), along with a better peak definition (Figure 4). Therefore, CE shows potential for application due to its greater sensitivity to the analytes of interest in this study.

Optimization of experimental parameters

Influence of pH

The pH of the electrolyte significantly affects the electrochemical determination of the target molecules. When redox processes on the electrode surface involve both electron transfer and proton transfer, both the potential and the peak current vary with changed in the pH of the medium. The pH of the solution can influence the distribution between neutral and/or ionic species of pesticides through protonation or deprotonation of the molecules.

Differential pulse voltammetry was employed to study the influence of pH on the oxidation peaks of CBZ, DI, and CF. Figure 5 shows the effect of pH on I_pa and the oxidation peak potentials (E_p) of CBZ, DI, and CF on CE across a pH range of 4.0 to 9.0. It can be observed that the E_p of the pesticides shifts to less positive values as the pH increases.

 

 

These results indicate a dependence on the electrodic process on the availability of protons in the medium, consistent with other studies,^52,55-58 and demonstrate that well pesticide molecules are protonated, oxidation becomes more difficult, resulting in more positive potentials. The slope values obtained from the E_p vs. pH graph of -57.33 mV pH^-1 for CBZ and -58.29 mV pH^-1 for DI are close to the theoretical value (Figure 6), which is, 59 mV pH^-1 at 25 ºC in a Nernstian system, indicating consistency in the number of protons and electrons participating in electrochemical oxidation processes (consistent with the theory).⁵² Thus, the oxidation of CBZ involves a process where 2 electrons and protons transferred, particularly at the protonated nitrogen of the imidazole ring (Scheme 1Sa).^53,54 The electrochemical reaction of DI follows a pathway involving one electron and one proton leading to the formation of radical intermediates and subsequent dimerization (Scheme 1Sb).⁵⁹

 

 

In contrast, the oxidation process of the CF shows little dependence on pH variation, as evidenced by a slope value of -10.25 mV pH^-1, which is below the theoretical value. This suggests that proton participation in the oxidation within the studied pH range is minimal.⁵² The proposed mechanism for CF begins with the hydrolysis of CF to carbofuran phenol, an electrochemically active compound. Subsequently, electrochemical oxidation of carbofuran phenol occurs via a single electron and proton transfer process (Scheme 1Sc).⁶⁰

As the pH of the electrolyte increases, it was observed that the highest I_p intensity occurred at pH 6.0 for CBZ of 6.55 µA and DI of 10.87 µA, and pH 9.0 for CF of 21.54 µA (Figure 4S, Supplementary Material). This is attributed to CBZ being in a deprotonated form at higher pH values (pH > 4.5), while DI and CF are partially protonated considering their second pK_a 10.3 for DI and 11.96 for CF.⁵⁸ Additionally, peak symmetry, peak intensity and resolution between the peaks were optimal at pH 7.0, with E_p values shifting to less positive values and improvement in baseline determination for CF. Therefore, pH 7.0 was chosen for the simultaneous analysis of CBZ, DI and CF using CE.

Optimization of DPV parameters

The influence of pulse amplitude (∆E_p), scan increment (E_s) and pulse time (t_p) on the different pulse voltammograms of CBZ, DI, and CF was evaluated to optimize the proposed method (results not shown). The optimal values selected were ∆E_p = 50 mV, E_s = 5 mV and t_p = 0.2 s, considering the best peak current amplitude and minimizing capacitive current influence.

Analytical curves from simultaneous determination of pesticides

Under optimized experimental conditions, simultaneous quantitative determination of CBZ, DI and CF at various concentrations was studied on CE in pH 7.0 phosphate buffer solution. The typical differential pulse voltammetric curves of different concentrations of these pesticides are shown in Figure 7. The anodic peak current of CBZ increases with increasing concentration (Figure 7a). A linear region of 10-105 μg L^-1 (R² = 0.9967) is observed in the analytical curve (Figure 8a). The limit of detection (LOD) is 4 μg L^-1 based on 3.3S_d/s, where S_d is the calculated standard deviation of the mean current of measurements at the lowest concentration (ten runs) and s is the slope value of the linear calibration curve. The limit of quantification (LOQ) is 13 μg L^-1 based on 10S_d/s.⁶¹ Similar trends were observed for DI (Figure 8b) and CF (Figure 8c). As shown in the insets of these three figures, calibration graphs exhibit linear relationships from 100 to 1059 μg L^-1 (R² = 0.9979) for DI, and from 4 to 43 mg L^-1 (R² = 0.9993) for CF. The LODs are 35 μg L^-1 and 1 mg L^-1 and the LOQs are 107 μg L^-1 and 3 mg L^-1 for DI, and CF, respectively. According to the maximum allowed values established by Ministry of Health of Brazil in Consolidation Ordinance No. 5 of 2017,⁶² the LOQs are higher for DI (90 µg L^-1) and CF (7 µg L^-1), and lower for CBZ (120 µg L^-1).

 

 

 

 

The simultaneous determination of only CBZ and DI was also evaluated (Figure 8b) because in the simultaneous analysis with all three pesticides (including CF), the oxidation peak of CF appears in a much more positive potential compared to CBZ and DI, compromising the baseline of the potential scan and leading to lower accuracy in peak current measurements. Additionally, comparisons were made between the results of analytical curves generated with CBZ and DI, with and without CF. Linear response ranges were observed over the entire concentration range of 5 to 105 µg L^-1 (R² = 0.9987) for CBZ and 50 to 1059 µg L^-1 (R² = 0.9934) for DI, with LODs of 4 and 18 µg L^-1, and LOQs of 13 and 55 µg L^-1 for CBZ and DI, respectively (Figure 8).

Compared to the simultaneous determination with all three pesticides, the results of simultaneous analyses involving only CBZ and DI pesticides show increased sensitivity, a relatively wide detection range, and a lower LOQ value for DI that meets the legislation requirements for pesticide limits in water.⁵⁷ The detection limit for CBZ and DI at CE was compared with data previously reported for other electrodes and the comparative results are shown in Table 2. It can be observed that the detection limit for CBZ and DI with CE is lower than those reported for other electrodes. The advantages of the electrode reported in this work, which makes it highly promising, include the use of simple and low-cost materials in its manufacture. Moreover, none of the previous publications enable the simultaneous determination of the tested pesticides. Based on these observations, other analytical performance variables for the method were evaluated for the binary analysis of CBZ and DI.

 

 

Repeatability and intermediate precision

The precision of the proposed method was determined by comparing the oxidation peak currents of 50 µg L^-1 CBZ and 500 µg L^-1 DI. The repeatability (intra-day precision) was evaluated by analysing ten successive measurements using the same electrode on a single day. Repeatability was determined with the same analyst, equipment and laboratory conditions over a short time scale, and was calculation using the relative standard deviation (RSD) at the corresponding concentration level.⁶¹ The RSD was found to be 3.39% for CBZ and 2.48% for DI, indicating good repeatability. Similarly, intermediate precision (inter-day precision) was evaluated by conducting experiments with three electrodes on different days.⁶¹ The RSD values were 4.89% for CBZ and 3.70% for DI for inter-day operation (n = 3 days), demonstrating good intermediate precision for electrode manufacturing. These values are quite reasonable, considering the renewal of the electrode surface between analyses.⁶¹

Analysis of samples

To demonstrate the applicability of the proposed method, a recovery test of CBZ and DI was performed on spiked water samples from the São Lourenço River. The recovery of CBZ and DI was determined using the standard addition method, with three levels of fortification: 25.0, 50.0, and 75.0 µg L^-1 for CBZ and 250.0, 500.0, and 750.0 µg L^-1 for DI. In this study, pesticides were also quantified by liquid chromatographic analysis, and the findings were compared with obtained by the electrochemical method using the composite electrode. The averages recovery percentages for the electrochemical method ranged from 78.00 to 105.98%, while for the UHPLC-MS/MS method, the average recovery values were closer to 100%. However, the RSDs were higher than those achieved with the proposed method, compromises the accuracy of the comparative method (Table 3).

 

 

According to the Student's t-test at the 95% confidence level, there was no significant difference between the results obtained from the two methods. These average recoveries indicate that the composite electrode is a reliable, effective, and applicable sensor for CBZ and DI determination in natural water.

 

CONCLUSIONS

In this study, a voltammetric method was development using the composite electrode based on graphite and polycaprolactone (CE), which was successfully applied to the simultaneous determination of the pesticides carbendazim (CBZ), diuron (DI) and carbofuran (CF).

The results demonstrated that the composite electrode exhibited good analytical performance in terms of precision for the simultaneous determination of CBZ and DI (RSD below 5%). It showed adequate linearity over ranges of 5-105 μg L^-1 for CBZ and 50-1059 μg L^-1 for DI, with low limits of quantification (LOQs) (13 μg L^-1 for CBZ and 55 μg L^-1 for DI) and limits of detection (LODs) (4 μg L^-1 for CBZ and 18 μg L^-1 for DI), comparison to the simultaneous analysis of CBZ, DI and CF.

The simultaneous determination of CBZ and DI in surface water samples fortified using the developed voltammetric method was satisfactory, showing statistically similar results (95% confidence) to those obtained by the reference chromatographic method, within the limit established by Association of Official Analytical Chemists (AOAC) (70 to 120% recovery). The main advantage of the developed method for the analyzing surface water samples is its capability to work with samples without requiring special pretreatment.

The results indicate that CE is a highly effective material for the simultaneous electroanalysis of these three pesticides, and potentially others pesticides, in complex environments such as natural water. The CE offers advantages such as low cost, easy fabrication, and simple operation.

 

SUPPLEMENTARY MATERIAL

The supplementary material is available at http://quimicanova.sbq.org.br in pdf format, with free access.

 

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Guest Editor handled this article: Marco T. Grassi