Introduction

Electroanalytical methods have drawn researchers’ interest in the advancement of electrochemical sensors to examine electroactive molecules [1,2,3]. Voltammetric procedures have been extensively incorporated over the past few decades because of their ease of use, low price, excellent sensitivity, and rapid response [4, 5]. Carbon electrodes have been extensively employed for electrochemical studies of oxidizable substances [6]. One of its essential properties is the electrode’s catalytic function due to the electrode surface. Many surface modification approaches enhance the electrocatalytic behavior of an electrode. By adopting nanoparticles [7, 8], conducting polymer layers [9], nanotubes [10], and ionic liquids [11], the modification may be accomplished. Due to their enhanced physicochemical characteristics, including electrocatalysis throughout the redox mechanism, working electrodes covered with electro-polymerized conducting polymer layers have been essential [12, 13]. Electro-polymerization is a process that has many benefits, such as making the organic molecule immobilized evenly. By altering the electroanalytical parameters, film thickness could be easily managed, and more extended period stability of activity and charge transfer features could be achieved [14]. Several research experiments have shown that polymer film fabricated electrodes have improved reactions in identifying specific electroactive substances. Specifically, dyestuff polymerization may develop cross-linked oligomers, improving adsorption capacity [15].

In recent decades, intensive pesticide use has boosted agricultural output. However, at the same time, it has produced pesticides that rest in earthy water streams at points that go outside the judicial confines and have intensified risk complications in the atmosphere and nutrient security concerns [16]. Indeed, many chemicals are almost non-biodegradable, but these can be observable in soil, water bodies, air, and food supplies [17, 18]. There might be a significant risk of health complications such as respiratory issues, infertility, immunological issues, bone marrow defects, neurological abnormalities, and cytogenetic consequences owing to the toxic effect of the pesticide [19,20,21]. A benzimidazole fungicide, carbendazim (CBM), has been extensively utilized to prevent fungal infections and eliminate germs that damage the fruits and veggies in the food supply chain [10]. When two fungicides, benomyl (methyl 1-(butyl carbamoyl)benzimidazole-2-ylcaxbamate) and thiophanate-methyl [dimethyl 4,4′-(o-phenylene) bis(3-thioallophanate)], are sprinkled on field crops, they degrade to carbendazim and interact with water and damp soil [22]. When such two fungicides are converted into CBM for a short period, carbendazim (CBM) remains in the crops, water, and soil for a longer time [22,23,24]. Thus, CBM is investigated by the existence of its residues. For food products contaminated with carbendazim (fresh fruits and vegetables, oil seeds, cereals, spices, etc.), maximum residue limits (MRLs) have been established by regulatory authorities. For instance, the MRL for carbendazim in fruit juices is 200 ppb, while the European Union has established the MRL for benzimidazole insecticides at 500 ppb. Due to its widespread appeal and the fact that children are their main customers, the introduction of carbendazim in fruit juice products has generated a lot of concern [23]. It has become dangerous to living organisms and the environment because of its gradual degradation [25, 26]. Furthermore, developing low-cost, quick, sensitive, and accurate electrochemical methods is critical for monitoring CBM in general health and environmental pollution [27, 28]. Successively, different techniques for evaluating CBM have been used, including high-performance liquid chromatography [29], capillary electrophoresis [30], photometry method [31], fluorescence study [32], and solid-phase extraction-high-performance liquid chromatography [33]. Such procedures are lengthy and tedious and involve overpriced equipment. Electrochemical techniques attracted us to a larger extent; thus, we preferred the electroanalytical approach.

In the present work, a glassy carbon electrode (GCE) was used to get a CBM response for detection and determination by electro-polymerization of GCE through a cyclic voltammetric method. We have selected Vat Violet 2R (VVR) dye for electro-polymerization on the GCE. Vat Violet 2R is a vat dye that can be dissolved in sodium hydroxide and sodium hydrogen sulfite. It can also be dissolved in water [34]. Vat dyes are rapid to wash and light. It is possible to find bright colors in many other shades [35]. These dyes originated in central Europe; due to the use of vats to reduce indigo plants through the fermentation process, they are specified as vat dyes. These dyes are essential for identifying and dyeing cellulosic fibers and cotton. They appear as direct dyes in their soluble regions so that dyeing can be done on cotton [34]. Using cyclic voltammetry (CV), square wave voltammetry (SWV), and differential pulse voltammetry (DPV), a poly(VVR) glassy carbon electrode, i.e., (poly(VVR)/GCE), demonstrated improved electrochemical performance at pH 3.0.

Experiment

Reagents and Apparatus

A phosphate buffer solution (PB) of an ionic concentration of 0.2 mol/L was prepared as a supporting electrolyte. The required pH from 3.0 to 11.2 was maintained by combining the necessary ratios of Na2HPO4 and NaH2PO4·H2O. The analyte CBM and the modifier Vat Violet 2R have been bought from Sigma-Aldrich, USA. The CBM stock solution (0.1 mM) was prepared in ethanol. The VVR solution was prepared with sodium hydroxide and sodium hydrogen sulfite solution mixture at a temperature of 50 °C by stirring the dye into the mix for 15 to 20 min. For each trial, double-distilled water was employed. Each purchased chemical was of analytical reagent grade and was employed without further refining. Electrochemical investigations were executed using the CHI-D630 model (CH Instrument-D630 electrochemical workstation). In a one-compartment electrode cell, a standard three-electrode system was used with an Ag|AgCl (3.0 M KCl) electrode as a reference, a platinum wire as a counter, and bare and poly(VVR)/GCE as working electrodes. The oxidation potential of the CBM versus the Ag|AgCl electrode has been reported.

Soil Sample

Many types of soil, namely black, brick, lake, farm, and red soil, have been collected from many places. Under the defined earth layers, these soil types were separated, considering sand particles and small and medium to large stones. Later, the collected samples were processed and homogenized. Effectively, a certain amount of soil samples was shifted to a beaker containing 25 mL of distilled water to mix evenly and reserved to remain for about 8–9 h to remove the contaminated part. Similarly, all sample solutions were processed and gathered in a volumetric flask. In addition, a CBM stock solution of 0.1 mM was added to the sample solution and then diluted using double-distilled water. The quantity of CBM is measured using the SWV method, the standard addition method.

Water Sample

Water samples from many places were collected, including a tap, a pond, reverse osmosis (RO), a dam, and a lake. The suspended and colloidal particles were removed from these samples through pre-treatment. SWVs were recorded in both blank and in the presence of a 0.1 mM CBM analyte in water samples. About 5.0 mL of 3.0 pH PB was added to 5.0 mL of a water sample taken in an electrochemical cell to record the voltammograms. Later, water samples of up to 50 mL, a known amount of CBM, i.e., a stock solution of 0.1 mM, were spiked, and SWVs of the water sample were recorded. Following the same procedure, an actual sample voltammogram was obtained. Therefore, the amount of CBM was quantified according to the standard addition method and calibration plot.

Results and Discussion

Surface Area of Electrodes

The cyclic voltammetry method was utilized to determine the surface area of the bare and modified GCE. The test was performed using 1.0 mM K4[Fe(CN)6] taken in 0.1 M KCl. The CVs were recorded at different scan rates [5, 36]. Corresponding values of current at different scan rates were noted. The correlation equation of Ip versus ʋ1/2 was substituted into the Randles–Sevcik equation for the reversible reaction, Ip = (\(2.69\times {10}^{5}\)) Do1/2 C* Ao n3/2 ʋ1/2 at 298 K, and the effective surface area (Ao) was calculated. Here, C* represents the concentration of K4[Fe(CN)6], Do is the diffusion coefficient of 7.6 × 10−6 cm2 s−1, Ip is anodic peak current, ʋ1/2 is the square root of scan rate, and n is the total number of electrons involved in the redox reaction of K4[Fe(CN)6], i.e., equal to one. The obtained Ao value for modified GCE was nearly twice higher (0.078 cm2) than bare GCE (0.043 cm2).

Accumulation Time

Using 0.3 μM of size α-alumina powder, GCE was polished with a wet polishing cloth until a shiny appearance was noticed. In addition to cleaning and activating the GCE surface, ultra-sonication was continuously performed in ethanol with 1:1 nitric acid and double-distilled water. Later, GCE was dehydrated in the nitrogen atmosphere. The 1.0 mM VVR was coated on GCE through electro-polymerization and left to dry to find out the accumulation time. Then, it was dipped in the analyte solution, and later, a CV process was chosen to test against peak current oxidation of 0.1 mM CBM in PB solution to study the accumulation time effect for 0 s and frequent until the 60 s (0, 10, 20, 30, 40, 50, 60) against peak current. In the 40 s, the highest current value was in the voltammogram (Fig. 1). Therefore, the 40 s is considered an appropriate accumulation time to research all parameters in the research work.

Fig. 1
figure 1

Cyclic voltammograms of 0.1 mM CBM at poly(VVR)/GCE at 0.05 mV/s in 0.2 M PB 3.0 pH. A Inset of accumulation time (i.e., 0 s, 10 s, 20 s, 30 s, 40 s, 50 s, and 60 s) for 0.1 mM CBM against anodic peak current

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Electro-polymerization of VVR on the GCE Surface

After cleaning and activating the GCE, a CV method was used to achieve electro-polymerization of the VVR layer over the GCE. GCE’s electrocatalytic activity is also influenced by the degree of thickness of the layer. The poly(VVR) modified glassy carbon electrode (poly(VVR)/GCE) was prepared by adding 1.0 mM VVR with 1.0 M NaOH solution in an electrochemical cell. As demonstrated in Fig. 2A, throughout several cycles (5, 10, 15, 20, and 25), the peak current of CBM increased gradually to 20 cycles, then decreased with cyclic periods. By varying the multiple cycles on the GCE, coverage was controlled. However, at 20 cycles, the peak current was higher. Electro-polymerization was achieved between − 0.6 V and + 1.6 V potential window. As the polymerization cycles are more than 20, the higher currents tend to decrease; increasing the layer thickness will inhibit the electron transport activity and slow the oxidation process. Thus, at 20 cycles, the CV technique was performed for the polymerization of 1.0 mM VVR with 1.0 M NaOH and 1.0 M NaHSO3 solution present in the electrochemical cell with a scanning value of 0.1 V/s with − 0.6 to + 1.6 V potential window (Fig. 2). This shows that the VVR film was developed on the GCE surface. The new poly(VVR)/GCE was used to study the carbendazim fungicide for further studies.

Fig. 2
figure 2

Cyclic voltammogram of poly(VVR) film modified GCE. Scan rate: 0.1 V/s; C (VVR): 1.0 mM; supporting electrolyte: a mixture of 1.0 M NaOH and 1.0 NaHSO3. A Anodic peak current plotted against the number of cycles of polymerization

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Electro-analytical Activity of CBM

The behavior of a 0.1 mM CBM fungicide at a developed GCE was studied using a CV technique at a bare glassy carbon electrode (BGCE) and at a poly(VVR)/GCE at a sweep rate of 50 mV/s in a pH of 3.0 PB at an accumulation time of the 40 s. In the presence of CBM at bare and at modified GCE, one intense oxidation peak and one reduction peak were observed. The anodic peak current at the poly(VVR)/GCE is 8.25 μA and at the bare GCE is 3.50 μA (Fig. 3). Also, there was a slight shift in the peak potential value. This kind of response is due to the electrocatalytic characteristics of the VVR dye. The peak potential (Ep) at bare GCE is 1.022 V, whereas the peak potential at poly(VVR)/GCE is 1.018 V. The electrode’s excellent surface area exposes the GCE’s active structure. The properties such as anti-fouling characteristics, large specific surface area, outstanding electrical conductivity, electro-catalytic effect, fast electron transmission capability, and remarkable chemical stability are enhanced by the excellent performance of CBM fungicide on poly(VVR)/GCE. Hence, further investigation of CBM was carried out using modified GCE.

Fig. 3
figure 3

Cyclic voltammograms at the BGCE (a), at poly(VVR)/GCE (b) in the presence of 0.1 mM CBM at 0.05 V/s in 3.0 pH PB at 40-s accumulation time

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The pH Effect Study

Examining toxic chemical molecules’ electrochemical process is significant to understand better the reaction process, the speed, and the protons involved. The pH of a buffer solution is a vital research parameter that can affect the transfer of electrons from CBM and how it behaves electrochemically at poly(VVR)/GCE. The electrochemical reaction of 0.1 mM CBM at poly(VVR)/GCE at 40-s accumulation time with a scan rate of 50 mV/s within a pH 3.0–11.2 PB (0.2 M) was recorded using the CV technique. PB is treated as an inert electrolyte in the analysis with strong solvability in water and particular buffer resistance. It justifies the role of protons throughout the process. In the CBM molecule, the highest anodic peak current recorded was at 3.0 pH (Fig. 4) of its buffer solution.

Fig. 4
figure 4

CVs in the presence of 0.1 mM CBM at varying pH of PB at a poly(VVR)/GCE, (a–i; 3, 4.2, 5, 7, 8, 9.2, 10.4, 11.2) at an accumulation time of the 40 s at 50 mV/s scan rate. A Ipa and Epa values plotted against pH values

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Moreover, in the graph of Ep vs. pH, the linearity is expressed as Ep(V) = 1.170–0.054 (pH); r2 = 0.996; with pHs, increased linearity of peak potential was achieved (Fig. 4A). The slope of 0.054 is nearer to the Nernst equation value (0.059 Ep/pH), showing equal numbers of protons and electrons in the electro-oxidation reaction. As a result, throughout the research work, pH 3.0 was considered the ideal pH for the buffer solution.

Scan Rate Impact

The electrochemical process of the CBM fungicide was well understood at the poly(VVR)/GCE by analyzing the variation of scanning levels. The impact of the scan rate on 0.1 mM CBM oxidation in 0.2 M PB of pH 3.0 with an accumulation time of 40 s was assessed using a CV technique at poly(VVR)/GCE (Fig. 5). The Laviron equation is followed for the poly(VVR)/GCE and noted an improvement in the peak current and a reposition in the peak potential with a sweep rate. A linear increase in the maximum peak current was observed with sweep rate increase, for which the linear relationship is expressed as Ip(μA) = 91.30 (ʋ, V/s) + 2.60; r2 = 0.99 (Fig. 5A). The plot of the log of anodic peak current versus the log scan rate is shown as input to ensure the electrode process. The linear regression expression is presented with log Ip(μA) = 0.77 (log ʋ, V/s) − 4.13; r2 = 0.99 (Fig. 5B). The slope value is 0.77; this indicates that the electrode process is combined with adsorption and diffusion-controlled process. The Laviron Eq. (1) [36, 37] represents the relationship between scanning rate and anodic peak potential for an electrode reaction.

Fig. 5
figure 5

The influence of varying scan frequencies (a–l in V/s; 0.02, 0.04, 0.08, 0.1, 0.12, 0.14, 0.17, 0.20, 0.23, 0.27, 0.31, 0.35) at poly(VVR)/GCE at 40-s accumulation time in the involvement of 0.2 M 3.0 pH PB by CV technique for 0.1 mM CBM. A Ipa plotted against scan rates. B Log Ip plotted against log ʋ

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$${E}_{P}={E}^{0}+\left(\frac{2.303RT}{(1-\alpha )nF}\right)\mathrm{log}\left(\frac{\left(1-\alpha \right)nF}{RT{k}^{0}}\right)+\left(\frac{2\cdot 303RT}{(1-\alpha )nF}\right)\log\ v$$
(1)

Within the electrode’s process, the involvement of electrons is shown by n, ʋ is the sweep rate, and α is its charge transfer coefficient, whereas k0 shows the heterogeneous rate constant at standard conditions and E0 represents the formal redox potential. The heterogeneous constant, k0, was estimated as 2.73 s−1.

$$\Delta {E}_{p}={E}_{p}-{E}_{P/2}=\frac{47\cdot 7}{\alpha n}$$
(2)

The Bard and Faulkner Formula (2) determines the value of αn as 1.41, and α was found to be equal to 0.58. In this, Ep/2 represents half of the peak potential, where current is proportional to half the maximum peak value of the current. The electrons and protons participated in the electro-oxidation process of about 2.0. This was estimated using the Bard and Faulkner expression alpha value for the CBM molecule at poly(VVR)/GCE. A possible electro-oxidation mechanism of CBM is shown in Scheme 1.

Scheme 1
scheme 1

Possible electro-oxidation mechanism of CBM

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Analytical Applications

CBM Concentration

The DPV technique was used to obtain the peaks of good resolution at lower analyte concentrations. CBM concentrations varied from 0.05 to 0.7 μM to determine the sensitivity of poly(VVR)/GCE in 3.0 pH of PB at 40-s accumulation time (Fig. 6). Linear relationship achieved according to Ip(μA) = 10.50 (C0, μM/L) + 1.28; r2 = 0.99, with its current value compared to the CBM concentration range. From the formulas, LOD = 3S/M and LOQ = 10S/M, the detection limit and the quantity limit have been calculated. Here, S indicates blank standard deviation, and M indicates the slope of the graph. The detection limit for this activity is 0.31 × 10−8 M. All previously reported LOD values can be seen in Table 1. The findings of LOD and LOQ from fabricated electrode showed low value indicating that it has excellent sensitivity, effective for CBM detection and determination [39,40,41,42,43,44].

Fig. 6
figure 6

DPVs recorded for different concentrations of CBM at accumulation time 40 s in 3.0 pH PB (a–n in μM; 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7). A Inset graph of Ipa/μA versus [CBM]/μM

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Table 1 LOD comparison with other modified electrodes
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Analysis of Soil Samples

Testing of soil samples was performed to influence the environmental and agricultural use of the generated electrode. The established sensor is sensitive to CBM analysis results based on Table 2. The SWV method was used at ambient temperature for CBM molecule studies. The recovery percentage is 90.00% and 99.00%.

Table 2 Determination of CBM in spiked soil samples
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Analysis of Water Samples

Water sample analysis was carried out to determine the presence of CBM in the water sample using the SWV method. Pre-treated water samples were then taken with a known CBM concentration to test the detection of CBM analysis. Table 3 shows the test results for other water samples. The recovery percentage is 90.00% and 100%.

Table 3 Finding of CBM in spiked water samples
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Impact of Temperature Variation

Using a CV method, the effect of temperature on the electro-oxidation of CBM analyte was studied at poly(VVR)/GCE in 3.0 pH PB for the 40 s at a scan rate of 50 mV/s. The current values of CBM are elevated linearly due to the temperature being enhanced between 288 and 313 K with a decrease in peak current, which can be seen in Fig. 7. For thermodynamic expressions 3 and 4, the log (Ipa) of CBM vs. 1/temperature is linear. Due to the diffusibility differences of the CBM molecule, there is an increase in current oxidation (Eq. 3). As the temperature rises, the GCE-generated behavior plays an essential role in the activation energy, i.e., Ea, for the distribution of the CBM molecule. Using the anodic peak current of the CBM molecule, the Arrhenius equation was used to determine that the activation energy was 11.19 kJ/mol.

Fig. 7
figure 7

Impact of varying temperatures (288 K, 293 K, 298 K, 303 K, 308 K, and 313 K) on 0.1 mM CBM anodic peak current at poly(VVR)/GCE in 3.0 pH PB. A Inset graph of log (Ipa) plotted against 1/T

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$$\sigma ={\sigma }^{0}{\mathrm{e}}^{-\frac{{E}_{a}}{RT }}$$
(3)
$$D={D}^{0}{\mathrm{e}}^{-\frac{{E}_{a}}{RT}}$$
(4)

Here, conductivity/diffusibility is σ/D, the activation energy is Ea, σ0/D is standard conductivity/initial diffusibility, the temperature is T, and the universal gas constant is R (8.314 J mol−1 k−1). In a practical means of investigating the applications, parameters must be conducted at ambient temperature, even though the peak current of the analyte increases as the temperature goes high [45, 46]. With the Erying equation as well as the thermodynamic equations, the change in enthalpy (H*) and entropy (S*) of CBM at poly(VVR)/GCE was calculated:

$${\Delta G}^{*} = -RT\ln\left(\frac{k h}{T {k}_{\mathrm{B}}}\right)$$
(5)
$${\Delta H}^{*}= {\Delta G}^{*}+T\Delta {S}^{*}$$
(6)

Variable H* is utilized to determine whether the electrode reaction is spontaneous. For S* positive variable, G* negative, and H* negative based upon the Gibbs free energy [47,48,49]. The Boltzmann constant, kB (1.381 × 10−23 m2 kg s−2 k−1), and the Planck constant, h (6.66 × 10−34 J s), are also included in the above 6 and 7 equations. As polymerization of VVR was carried out, the working electrode expedited the processes like an electrocatalyst which increased the current in the system. The Ea* value, as well as the thermodynamic variables, are listed in Table 4.

Table 4 Thermodynamic parameters of CBM at poly(VVR)/GCE
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Selectivity of the Poly(VVR)/GCE

Selectivity of the electrode is identifying specific molecules or ions in the presence of other molecules or ions. Since the sensor is frequently used to detect a molecule or ion in a mixture of all other ions or molecules, it is a crucial parameter to consider when applied to study the actual samples. It has been discovered that the modified electrode is very sensitive to the CBM chemical. By analyzing the interference effect with various interfering metal ions (1.0 mM), such as calcium chloride, copper sulphate, ferrous sulphate, potassium nitrate, manganese sulphate, and zinc chloride, the electrochemical performance of CBM at poly(VVR)/GCE was studied using the SWV technique. Considering the 100-fold metal ion concentrations, there was no interference in the CBM peak potential, indicating that the suggested approach possesses a higher selectivity for CBM detection.

Repeatability and Reproducibility of the Poly(VVR)/GCE

The essential parameters for assessing the sensitivity and reliability of the fabricated working electrode are reproducibility and repeatability. Reproducibility of poly(VVR)/GCE was tested by preserving the electrode in a closed area for approximately 20 days. At ambient temperature, the reproducibility was processed on that same day, and its RSD was found to be 1.97%, indicating that the electrode shows excellent reproducibility. Repeatability has been achieved in ideal circumstances so far. The repeatability interaction was set up by five consecutive 0.1 mM CBM standard solution estimations. The poly(VVR)/GCE maintained 98% of the initial peak current, confirming the stability of the fabricated poly(VVR)/GCE. So, the fabricated poly(VVR)/GCE is suitable for long-term consistency and reproducibility for analytical applications.

Conclusions

The proposed work focuses on developing a simple and effective poly(VVR)/GCE working electrode for detecting and determining CBM. With the poly(VVR)/GCE, nano-molar CBM detection was performed. Due to the electro-polymerization of the VVR and its layer growth on the GCE, an increased peak current of CBM was observed, double the peak current of CBM at a bare GCE. The electrode process was observed to be quasi-reversible. Vat dyes are the vital dyes commonly employed for printing, which include dyeing cellulosic fibers and cotton. The VVR dye has been used as a modifier of GCE electro-polymerization. The fabricated electrode showed CBM sensing and quantification in actual samples. The sensor has successfully demonstrated the detection limit, high sensitivity, quick reaction, reproducibility, electro-catalytic performance, and repeatability. The fabricated poly(VVR)/GCE revealed a minimum detecting limit of 0.312 × 10−8 M for the CBM analyte. Based on the temperature variation effect, activation energy and other thermodynamic variables like ΔS*, ΔH*, and ΔG* have been investigated.