Introduction

Carbendazim (Cbz, methyl-1H-benzo-[d]-imidazol-2-yl-carbamate, Fig. 1), the worldwide used benzimidazole fungicide, is an inhibitor of the fungi growth by interfering with spindle formation at mitosis (cell division) [1, 2]. Carbendazim acts against broad spectrum of diseases on crops as cereals, cotton, tobacco, fruits, vegetables, and ornamentals. It is applied in post-harvest food storage and as the active compound in the seed pre-planting treatment as well [1].

Fig. 1
figure 1

Structure of carbendazim

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Due to its extensive application, the development of reliable analytical methods for monitoring the target fungicide in soil and water samples, marketed fruits, fruit juice concentrates, and vegetables is in the focus of many researches. Different analytical measurement techniques have already been applied to the analysis of Cbz, from which the favored one is liquid chromatography (LC) [3,4,5,6,7,8] in combination with different detector unites as diode array (DA) [3,4,5], fluorescent [6] or mass spectrometric (MS) [7, 8] detectors. Additionally, the capillary electrophoresis (CE) coupled with DAD was reported in the Cbz determination as well [9]. However, the above mentioned techniques are based on highly sophisticated instrumentation which often required complex and/or time-consuming sample preparation procedures. Therefore, some simpler and fast determination techniques, such as molecular spectroscopy (e.g., UV-vis spectrophotometry [10] and fluorimetry [11, 12]), immunoassays [13], or analytical voltammetry [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29], are also suitable and widely used for its determination.

The electrochemical oxidation of Cbz has been so far studied at different kinds of modified and unmodified working electrodes [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] mainly due to low costs and high sensitivity of voltammetric techniques. Based on the available literature data, the determination of Cbz was carried out using bare boron-doped diamond electrode (BDDE) [14]. To the best of our knowledge, there are no reports on the voltammetric determination of Cbz using a modified boron-doped diamond electrode. Additionally, unmodified carbon paste electrode containing mineral oil (MO-CPE) [15] was applied to determine Cbz. Moreover, the electrodes’ modification, as a common way to improve sensitivity, has been applied to determine Cbz. For this purpose, carbon paste electrode modified with thermally activated zeolite (ZM-CPE) [16], ordered mesoporous carbon paste electrode modified with pyrrolidinium ionic liquid (PIL-OMCPE) [17], carbon paste electrode containing tricresyl phosphate modified with 2-hydroxypropyl-β-cyclodextrin (2-HP-β-CD-TCP-CPE) [18], and glassy carbon paste electrode modified with graphene and amberlite XAD 2 resin (GNS-XAD2-GCPE) [19] have been applied. Commonly used to determine Cbz were glassy carbon electrodes modified with the following modifiers​: multiwalled carbon nanotubes (MWCNTs-GCE) [20], graphene oxide and multiwalled nanotubes (GO-MWNTs-GCE) [21], electrochemically reduced graphene oxide (ERGO-GCE) [22], β-cyclodextrin-functionalized reduced graphene oxide (β-CD-RGO-GCE) [23], β-cyclodextrin and graphene hybrid nanosheets (β-CD-GNs-GCE) [24], mesoporous silica containing multiwalled carbon nanotubes (SiO2-MWCNTs-GCE) [25], and carboxylic group-functionalized poly(3,4-ethylenedioxythiophene) film electrode (PC4-EDOT-COOH-GCE) [26]. Other electrodes used for the determination of Cbz were commercially available screen-printed carbon electrode modified with multiwalled carbon nanotubes (MWCNTs-SPCE) [27], screen-printed electrode modified with biosynthesized Au nanoparticles (Au-SPCE) [28], and multiwalled carbon nanotubes-polymeric methyl red film modified electrode (MWCNTs-PMRE) [29]. The elaborated working electrodes in combination with appropriate direct or adsorptive stripping voltammetric techniques achieved the linear dynamic range (LDR) of measurements mainly in micromole per liter and sub-micromole per liter concentration ranges, and some of them in nanomole per liter concentration ranges (see Table 1 for details). The lowest reported limit of detection is 1.00 × 10−9 mol L−1 which was obtained by ERGO-GCE in combination with a differential pulse voltammetry (DPV) method [22].

Table 1 Voltammetric determination of Cbz by different working electrodes
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Boron-doped diamond electrode is recognized as one of the best electrode materials for electrochemical research due to its advantages, such as the widest usable electrochemical potential window from all traditional electrode materials (up to ± 3 V), a characteristically low and stable background current resulting in higher sensitivity and improved limits of detection, long-term stability of response, extreme electrochemical stability in both alkaline and acidic media, and good mechanical robustness [30]. Up to date, BDDE has been extensively used in development of electroanalytical procedure for the quantification of the various biologically active compounds, such as drugs [31,32,33,34,35] and pesticides [36,37,38,39,40,41].

Over the past decade, an increased growth in the development and production of many commercial chemicals used in farming, including pesticides, was observed [42]. Although the application of pesticides solves the problem of unwanted pests and weeds, unfortunately, it also causes an environmental pollution [43]. At the same time, an enhanced growth in corrosion caused by pesticides was noticed due to its increasing use in agriculture. Corrosion is a common phenomenon and destroys all sectors of world economy including agricultural industry [42]. It was found that pesticides may be significantly corrosive and they can frequently damage farming machinery, and the most exposed elements to the corrosion effects of pesticides are the metal parts of agricultural equipment [44]. There are few reports in the literature concerning the corrosivity of pesticides when they are in contact with steel [36, 37, 44], aluminum [44], brass [44], and copper [45]. Although stainless steels are often chosen due to their resistance to corrosion, they are not immune to it [46]. Hence, it is very important to investigate the effect of pesticides on the corrosion properties of stainless steel used to produce agricultural machinery. The preliminary corrosion tests are usually conducted under laboratory conditions by means of electrochemical methods. Also, the microscopic imaging is very useful for evaluation of the corrosion effect concerning the size and morphology of surface damage.

This work shows the applicability of BDDE surface modified with MWCNTs, NAFION®, and β-CD as easy to use, simple to prepare, environmentally friendly, inexpensive, sensitive, and reliable promising working electrode with rapid response time for the voltammetric determination of Cbz. Basically, the potential window of BDDE is wide enough to allow the oxidative determination of the target analyte with known potential peak maxima nearly at + 1.3 V vs saturated calomel electrode (SCE). In addition, MWCNTs as a convenient modifier increases the electrode surface giving on this way a hybrid surface with BDDE substrate. The β-CD has ability to build inclusion complex with carbendazim in water media, and it could be expected that the immobilized β-cyclodextrin on the MWCNTs-BDDE surface at appropriate working potential(s) is able to give support for entrapment of the target analyte enhancing on such way the sensitivity of the method.

For the first time, the square-wave adsorptive stripping voltammetric (SWAdSV) procedure for Cbz determination based on its oxidation signal on β-CD-MWCNTs-BDDE was developed and then validated. Additionally, the developed procedure was successfully applied to determine the target fungicide in spiked river water sample. Moreover, for the first time, an investigation of the effect of Cbz on the corrosion properties of AISI type 316 L stainless steel used as a construction material in farming was studied under laboratory conditions using potentiodynamic methods and optical microscopic observations.

Experimental

Equipment

The voltammetric experiments for the development of the analytical methods were performed on a PalmSens electrochemical analyzer operated via a PSTrace software. The conventional three-electrode configuration with BDDE (Windsor Scientific Ltd., UK, diameter 3 mm), MWCNTs-BDDE, or β-CD-MWCNTs-BDDE as working electrodes was employed in combination with a saturated calomel electrode (SCE, Amel) as a reference electrode and a platinum (Amel) electrode as an auxiliary electrode. All electrochemical experiments were carried out in a one-compartment voltammetric vessel (ca. 50 mL) with reduced bottom (ca. 5 mL) at room temperature.

To conduct the corrosion measurements, an Autolab PGSTAT 30 potentiostat-galvanostat (EcoChemie Autolab B.V., Utrecht, the Netherlands) was used. Corrosion tests were performed in a three-electrode cell assembly consisting of SCE (Eurosensor, Gliwice, Poland) as a reference electrode and a platinum foil (Pt, 99.9%, The Mint of Poland, Warsaw, Poland) as a counter electrode. An AISI type 316 L stainless steel (Flanschenwerk Bebitz GmbH, Germany) was applied as a working electrode (an exposed area of 0.64 cm2). An MMT 800BT optical microscope (mikroLAB, Lublin, Poland) was used to characterize the corrosion damage.

Chemicals and solutions

Cbz (99.9% purity) was obtained from Sigma-Aldrich. The Cbz stock solution (c Cbz  = 1.12 × 10−3 mol L−1) was made by dissolving this fungicide in methanol (Sigma-Aldrich) and kept in dark in refrigerator at 4 °C. The Britton-Robinson (B-R) buffer solutions of different pH levels (between 2.3 and 6.0) as supporting electrolytes were prepared by mixing solutions of 0.04 mol L−1 H3PO4 (Merck), 0.04 mol L−1 H3BO3 (Merck), and 0.04 mol L−1 CH3COOH (Merck) and adjusting pH by adding suitable amounts of 0.2 mol L−1 NaOH (Merck). The water sample was collected from Danube river (sampling point at Štrand recreation area, Novi Sad, Serbia). MWCNTs (Sigma-Aldrich, carbon > 95%, O. D. × L6 9 nm × 5 μm) were applied as surface modifiers of BDDE in combination with NAFION® (Fluka, NAFION® NR 50, H+ form, 7–9 mesh) via the drop coating of appropriately prepared ethanolic (ethanol absolute anhydrous, Carlo Erba) suspension of MWCNTs and NAFION® in accordance with our earlier published protocols [47, 48]. Namely, the suspension was prepared by dispersing 1.0 mg of MWCNTs in 0.9 mL of ethanol and 0.1 mL of ethanolic solution of NAFION® (5% of NAFION® was prepared from appropriate amount of solid NAFION®, ethanol, and doubly distilled water). Then, 3.0 μL of such prepared suspension was drop coated onto the electrochemically activated BDDE surface by micropipette. The agent used for further modification, β-cyclodextrin hydrate (Sigma-Aldrich), was of analytical grade. Solution of β-CD (c β-CD = 4 × 10−5 mol L−1) was prepared by dissolving of an appropriate amount of β-CD hydrate in doubly distilled water.

The 3.5% sodium chloride solution (NaCl, analytical reagent grade, POCh SA, Gliwice, Poland) without and with addition of Cbz (c Cbz  = 1.0 × 10−3 mol L−1) was prepared in triply distilled water-acetone mixture (1:1, v/v). Both solutions were used without further deoxygenation.

Preparation of modified electrodes

Before the modification of the electrode surface, the bare BDDE was immersed in the voltammetric vessel containing 0.1 mol L−1 H2SO4 solution, and its potential was scanned in the potential range from − 0.4 to + 1.2 V using cyclic voltammetric (CV) working mode with a scan rate of 100 mV s−1 until the stable and uniform signals were obtained. This phenomenon was observed usually after 10 consecutively repeated cyclic voltammograms in accordance with earlier reports [36, 37].

As for the preparation of MWCNTs-BDDE, the ethanolic suspension of MWCNTs and NAFION® was dropped on BDDE surface using a micropipette (3.0 μL of each suspension). MWCNTs-BDDE was left to dry in air at room temperature. For its further modification, β-CD molecules were accumulated onto MWCNTs-BDDE surface by optimized deposition potential (E acc) and time (t acc) using the solution which consists of 5.0 mL of β-CD solution (c β-CD = 4 × 10−5 mol L−1) and 5.0 mL of the supporting electrolyte, pH 4.0. The deposition conditions were considered in the potential range from − 0.8 to + 1.2 V, and the E acc = − 0.2 V and t acc = 120 s were found as optimal for the preparation of β-CD-MWCNTs-BDDE. Before their application, the β-CD-MWCNTs-BDDE and MWCNTs-BDDE were activated in a pure supporting electrolyte solution by cyclic voltammetric scans in potential range between − 0.4 and + 1.2 V with a scan rate of 100 mV s−1 until receiving stable and uniform voltammograms which were usually obtained after 10 repeated cyclic voltammograms. After each set of measurements, MWCNTs-BDDEs and β-CD-MWCNTs-BDDE were regenerated by successive repeated cyclic voltammograms (number of repetition, n = 10) in the supporting electrolyte solution to remove impurities from the electrode surface.

Procedures

Voltammetric procedure

In the case of the model system, an appropriate volume of Cbz stock solution was added by means of a micropipette into the voltammetric vessel with supporting electrolyte solution (5.0 mL of double-distilled water was mixed with 5.0 mL of corresponding Britton-Robinson buffer solution). If any reagents were subsequently added, the solution was mixed for 30 s under open circuit. Generally, square-wave voltammetric (SWV) scanning was performed in the potential range from + 0.4 to + 1.5 V with the following parameters: step potential (ΔE s) of 4 mV, amplitude (E SW) of 40 mV, and frequency (f) of 50 Hz. The above experiments were performed by CV on activated BDDE, MWCNTs-BDDE, and β-CD-MWCNTs-BDDE in the supporting electrolyte solutions from pH 2.3 to 6.0, which were selected in accordance with the target buffer media for Cbz characterization and determination.

To prepare the calibration curves, the following procedure was applied: the consecutive additions of the stock solution of Cbz were made by micropipette into the voltammetric vessel with 10.0 mL of the supporting electrolyte, and the SWV or SWAdSV signals were recorded. The calibration curves were recorded in two repetitions for MWCNTs-BDDE by SWV and in three repetitions for β-CD-MWCNTs-BDDE by SWAdSV. The results were analyzed by linear least square regression, where the oxidation peak maxima (I p) were correlated with the appropriate Cbz concentrations. The limit of detection and the limit of quantification (LOQ) of the target analyte in all investigated cases were calculated from the appropriate calibration curve on the basis of the following equation: LOD and LOQ = kSD a  / b, where k = 3 for LOD, k = 10 for LOQ, SD a is the standard deviation of the intercept (a), and b is the slope of the related calibration equation [49].

Determination of Cbz in spiked river water sample was carried out by the standard addition method on β-CD-MWCNTs-BDDE by optimized SWAdSV protocol. The river water sample was collected from the Danube river. The sample was spiked with the known amount of target analyte, homogenized by mixing with magnetic stirrer and filtrated before measurements by syringe filter 0.22 μm (Millex). The preparation procedure for the river water sample spiked with Cbz was as follows: 360.0 μL of the stock solution of Cbz was transferred to a 100.0-mL volumetric flask and filled up to the mark with the river water. Next, 5.0 mL of the prepared spiked solution was added into the voltammetric vessel which contained 5.0 mL of the supporting electrolyte solution, pH 4.0. Three subsequent standard additions of Cbz stock solution were poured to the vessel with the aid of a micropipette with the following final concentrations of Cbz standard in voltammetric vessel: 2.24 × 10−6, 4.48 × 10−6, and 6.72 × 10−6 mol L−1, and the SWAdS voltammograms were recorded after each addition. The measurements were performed in triplicate.

Corrosion procedure

In the case of corrosion measurements, the following procedure was applied to prepare the electrodes: AISI 316 L stainless steel round rod (diameter of 12 mm) was cut and the disks with a thickness of 5 mm were obtained. Before the corrosion tests, the surfaces of each steel sample were grounded with SiC abrasive paper from 80 to 1500 grits. Further, each sample was cleaned in water in an ultrasonic bath for 10 min, rinsed with ethanol, and dried with argon gas.

The corrosion tests were conducted in sodium chloride solutions in the presence and absence of Cbz. Three samples were used for each corrosion test, and the presented results are average values with the standard deviations. The steady-state potential (E OCP) measurements were investigated in an open circuit (OCP) over 2000 s, and the potentiodynamic characteristic was measured in the scanning potential range from + 0.2 V below E OCP up to + 0.5 V with a scan rate of 1.0 mV s−1. After the corrosion tests, the characterization of corrosion damage was examined using an optical microscope.

Results and discussion

Influence of pH on the voltammetric behavior of Cbz

Generally, the shape and value of the peak current of the signal of the investigated analyte can be strongly affected by pH of the supporting electrolyte. Based on this fact, the influence of pH of B-R buffer solutions over a pH range from 2.3 to 6.0 on I p and the peak potential (E p) of Cbz (c Cbz  = 4.77 × 10−5 mol L−1) were examined on bare BDDE and MWCNTs-BDDE (Fig. 2). It was found that Cbz oxidation signal(s) appeared between pH 2.3 and 6.0, with position(s) relatively close to the onset of the supporting electrolyte on both electrodes. This target analyte gave one peak at potential between ca. + 1.0 and + 1.3 V on bare BDDE (Fig. 2a). As can be seen from Fig. 2b (black diamond, left y-axis), an increase of the oxidation signal of Cbz on bare BDDE was observed when pH of the supporting electrolyte increases till pH 5.0. At pH 5.0 and 6.0, the oxidation peak with similar intensity was observed; nevertheless, Cbz peak with better symmetry was found at pH 5.0. In any case, the most reliable oxidation signal of Cbz was obtained at pH 5.0 on bare BDDE. On the other hand, one peak was observed on MWCNTs-BDDE in the pH range from 2.3 to 4.0 at potential range between + 1.0 and + 1.3 V. At pH levels higher than 4.0, the Cbz response measured by SWV protocol was not made by a simple single oxidation peak because of the recognizable splitted form of the received oxidation signal especially at pH > 4.0. Furthermore, at pH 6.0, the oxidation signal seems to be consisted of two distinguished peaks (Fig. 2c). Thus, in the pH range from 2.3 to 4.0, the peak currents slowly increased until attaining a maximum at pH 4.0, and at pH values higher than 4.0, the followed oxidation peak significantly decreased (Fig. 2d, left y-axis). Well-defined and the highest oxidation signal of Cbz with satisfactory repeatability (RSD = 1.8% for n = 5) on MWCNTs-BDDE was obtained in B-R buffer, pH 4.0. Based on the above arguments, pH 4.0 was selected as the supporting electrolyte in the further measurements on MWCNTs-BDDE. Additionally, in the case of surface modification of BDDE by ethanolic suspension of MWCNTs, the three times repeated drop coating procedure proved that the reproducibility of such electrode design is close to 9%.

Fig. 2
figure 2

Representative SW voltammograms of 4.77 × 10−5 mol L−1 Cbz recorded in B-R buffer solutions on bare BDDE (a) and MWCNTs-BDDE (c) (pH levels marked on the curves) with appropriate dependences of the average peak currents based on the triplicate of measurements (I p) (black diamond, left y-axis) on pH, and the plot of average peak potentials (E p) based on the triplicate of measurements (white diamond, right y-axis) vs pH on BDDE (b) and MWCNTs-BDDE (d). The error bars were constructed as standard deviations of three repeated measurements (in the case of b and d)

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In addition, the results showed that E p obtained at bare BDDE and MWCNTs-BDDE were shifted towards less positive values as it is evident from Fig. 2b, d (white diamond, right y-axes). The plots of E p vs pH were linear across the examined range with a slope of − 60.7 mV pH−1 for BDDE and − 65.6 mV pH−1 for MWCNTs-BDDE, which are close to the expected theoretical value of 59 mV pH−1 [50], and it is in accordance with our earlier research [18]. This fact indicates that the number of the exchanged protons and electrons is equal. Consistent to the earlier elaborated results concerning the oxidation process of Cbz on carbonaceous electrodes [18, 51] it can be proposed that the imidazol-2-yl-carbamate part of the Cbz molecule is oxidized via complex, 2e−, 2H+ exchanging, ErCi mechanism to imidazole carbonyl derivative.

It is known from our earlier work that appropriate amount of β-CD derivative added into the supporting electrolyte enhanced the oxidation signal of Cbz on carbon paste electrode made from graphite powder and tricresyl phosphate pasting liquid during the differential pulse adsorptive stripping voltammetric measurement [18]. Because of similar expectable signal enhancing effect, the comparison of the SWV signals of Cbz (c Cbz  = 4.77 × 10−5 mol L−1) in B-R buffer recorded at bare BDDE, MWCNTs-BDDE, and β-CD-MWCNTs-BDDE was performed and the obtained results are presented in Fig. 3. It was found that Cbz provided one oxidation peak at potential ca. + 1.2 V on all tested electrodes. However, both MWCNT-modified BDDEs showed more than two times higher current responses when compared to bare BDDE probably because of surface enlargement by MWCNTs. Additionally, it should also be noted that better-defined peak shape with the highest current response was obtained on β-CD-MWCNTs-BDDE, which can be explained by the additional oxidation signal enrichment due to the presence of the β-CD on the working electrode surface together with the immobilized MWCNTs by NAFION®. Namely, it can be proposed that besides the electrode surface enlargement by MWCNTs at optimized measurement parameters, the entrapped β-CD, which is able to build inclusion complex with Cbz in water media [52,53,54], additionally facilitates the accumulation of the target analyte on the electrode surface. Hence, based on the obtained results, the β-CD-MWCNTs-BDDE was considered to be the best choice for the subsequent study.

Fig. 3
figure 3

SW voltammograms of (a) bare BDDE, (b) MWCNT-BDDE, and (c) β-CD-MWCNTs-BDDE in B-R buffer at pH 5.0 (BDDE) and pH 4.0 (MWCNT-BDDE and β-CD-MWCNTs-BDDE) containing 4.77 × 10−5 mol L−1 Cbz

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Influence of accumulation parameters on the voltammetric behavior of Cbz on β-CD-MWCNTs-BDDE

To enhance the sensitivity of β-CD-MWCNTs-BDDE based on SWV method, the utility of enrichment procedure can be applied. It is well known that the stripping voltammetric techniques facilitate trace or ultra-trace level determination of biologically active compounds, and the accumulation parameters are the variables which significantly influence the analytical signal intensity. Therefore, the effect of E acc on I p of Cbz (c Cbz  = 1.70 × 10−5 mol L−1) on β-CD-MWCNTs-BDDE was examined over the range of − 0.8 to + 0.9 V at t acc of 120 s (Fig. 4a). It was observed that Cbz response was sensitive to the both mentioned deposition factors just in case of higher concentration of Cbz as 1.70 × 10−5 mol L−1. Namely, the highest I p was achieved at − 0.6 V, and a decrease in the oxidation peak intensity was observed at more anodic values (Fig. 4a). Hence, E acc of − 0.6 V was used throughout the present study. Further, the effect of t acc for Cbz was investigated in the accumulation time range from 10 to 220 s at E acc of − 0.6 V as illustrated in Fig. 4b. An increase in Cbz current response within the accumulation time from 10 to 100 s was observed, and the saturation coverage of β-CD-MWCNTs-BDDE occurred above 100 s. Therefore, t acc of 100 s was selected for SWAdSV determination of Cbz. It should be noted that in the case of MWCNTs-BDDE, the mentioned accumulation parameters are not as favorable for Cbz determination as it is the case for β-CD-MWCNTs-BDDE.

Fig. 4
figure 4

The effect of accumulation potential (E acc, a) with the initial accumulation time of 120 s and the impact of accumulation time (t acc, b) with the optimized accumulation potential of − 0.6 V, on the Cbz SWAdSV response (c Cbz  = 1.70 × 10−5 mol L−1) obtained on β-CD-MWCNTs-BDDE in B-R buffer at pH 4.0

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Development of voltammetric procedures at MWCNTs-BDD and β-CD-MWCNTs-BDDE for the determination of Cbz

To develop the voltammetric procedures to determine Cbz, MWCNTs-BDDEs and β-CD-MWCNTs-BDDE as working electrodes were considered in SWV and SWAdSV measurement mode, respectively. The exemplary voltammograms are shown in Fig. 5a, c. The SWV and SWAdSV determinations of Cbz in B-R buffer at pH 4.0 were performed based on the linear relationship between I p of Cbz and the appropriate increasing analyte concentrations of the target analyte (c Cbz ). The obtained results showed that there are linear correlations between c Cbz and I p in the concentration ranges of 5.04 × 10−6–3.63 × 10−5 mol L−1 for SWV and MWCNTs-BDDE (Fig. 5b), and 6.72 × 10−7–1.12 × 10−5 mol L−1 for SWAdSV and β-CD-MWCNTs-BDDE (Fig. 5d). At higher concentrations for both presented systems, the voltammograms from three derived calibration curves for each electrode showed a slight saturation. The linear regression equations can be described by the following equations: I p (μA) = 0.0458 c (μmol L−1) − 0.0485 (R 2 = 0.9976) for MWCNTs-BDDE, and I p (μA) = 0.1133 c (μmol L−1) + 0.0539 (R 2 = 0.9989) for β-CD-MWCNTs-BDDE. The LOD and LOQ of Cbz for MWCNTs-BDDE were 1.00 × 10−6 and 3.34 × 10−6 mol L−1, respectively, while for β-CD-MWCNTs-BDDE were 1.96 × 10−7 and 6.54 × 10−7 mol L−1, respectively. Based on the obtained results, it can be stated that due to the application of SWAdSV type of measurement and β-CD-MWCNTs-BDDE, LOD is lowered eight and five times in comparison with the appropriate values of SWV methods in combination with bare BDDE [14] and MWCNTs-BDDE, respectively. The improved LOD value on β-CD-MWCNTs-BDDE was probably obtained due to the applied technique, and additional signal enhancement thanks to the influence of β-CD on β-CD-MWCNTs-BDDE surface.

Fig. 5
figure 5

Characteristic of the developed methods: a Representative SW voltammograms recorded on MWCNTs-BDDE in the concentration range of 5.04 × 10−6–3.63 × 10−5 mol L−1 (1–13) in B-R buffer pH 4.0 (0) with the average calibration curve for SWV from two repeated sets of measurements (b), and c representative SWAdS voltammograms registered on β-CD-MWCNTs-BDDE in the range of 6.72 × 10−7–1.12 × 10−5 mol L−1 (1–12) with the average calibration curve for SWAdSV from three repeated sets of measurements (d). The error bars were constructed as standard deviations of three repeated measurements (in the case of b and d)

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Application of proposed method to analysis of model sample—the river water

Subsequently, the assessment of the applicability of the previously elaborated SWAdSV method employing β-CD-MWCNTs-BDDE was performed by quantifying Cbz in spiked river water sample. To this purpose, the sample was analyzed using the standard addition procedure. No detectable interfering peaks were found in the investigated potential range derived from the river water components. The experiments were performed in triplicate, and the example of series of SWAdS voltammograms obtained by standard additions along with a corresponding standard addition plot is presented in Fig. 6. The contents of Cbz in river water samples obtained in three repeated measurements (2.04 × 10−6, 2.05 × 10−6, and 2.03 × 10−6 mol L−1, respectively) are in good agreement with spiked amounts of Cbz to the river water samples, which are equal to 2.02 × 10−6 mol L−1. The RSD value provided high repeatability of the determination of Cbz (RSD = 0.5%) at β-CD-MWCNTs-BDDE. The recovery values for all three samples were equal to 101.0, 101.5, and 100.5%, which proves that there is no significant matrix effect of river water samples. The average values of the results are given in Table 2, and the results indicated that the developed procedure using β-CD-MWCNTs-BDDE can be successfully utilized to determine Cbz in river water samples.

Fig. 6
figure 6

a An example of SWAdSV sets of Cbz determination in the spiked river water sample on β-CD-MWCNTs-BDDE using the standard addition method. Bottom to top: (0) B-R buffer (pH 4.0) with river water sample (1:1, v:v), (1) B-R buffer (pH 4.0) with spiked river water sample (1:1, v:v), and three successive standard additions of Cbz with the final concentrations in vessels of 2.24 × 10−6, 4.48 × 10−6, and 6.72 × 10−6 mol L−1. b The analytical curve presents the average values of current intensities obtained for three times repeated measurements, while the error bars were constructed as standard deviations of these three repeated measurements

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Table 2 Determination of Cbz in the spiked river water by the proposed SWAdSV method on β-CD-MWCNTs-BDDE (n = 3)
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Based on the above, it can be concluded that the environmentally friendly BDDE is an excellent substrate electrode material with wide potential window in the anodic potential range, which reflects on Cbz oxidation responses either in the case of its bare or modified forms. It is easily applicable, in combination with optimized amount of MWCNTs and NAFION@ as surface modifiers, for the further modification which encompasses the electrodeposition of β-CD on MWCNTs-BDDE surface forming on such way β-CD-MWCNTs-BDDE. In the case of such β-CD-MWCNTs-BDDE electrode configuration, only a very simple drop coating step is required together with the electrochemical cycling of the working electrodes before the drop coating, a constant potential treatment after for β-CD deposition, and the electrochemical activation of the working electrodes. Furthermore, the building unites of β-CD-MWCNTs-BDDE are commercially available and easily accessible. The surface modification contained only very simple and fast preparation steps, and the whole procedure takes no more times than ca. 20 min. β-CD-MWCNTs-BDDE is an easy to prepare and reliable analytical tool, which in combination with the optimized SWAdSV protocol results in comparable outlines concerning the broad spectra of earlier introduced methods either based on a very classical type of carbon paste electrodes or more sophisticated complex materials for Cbz determination (see in Table 1).

Corrosion tests

The corrosion measurements were focused on an examination of general and pitting corrosion in presence of Cbz as the possible corrosion-inducting agent in water:acetone media (1:1, v:v). General corrosion parameters were determined from the registered anodic polarization characteristics analyzed for AISI 316 L in 3.5% sodium chloride solution in the presence and absence of 1.0 × 10−3 mol L−1 Cbz (Fig. 7). By the extrapolation of the Tafel lines, the parameters, such as corrosion potential (E cor), and corrosion current density (j cor), were calculated. Some parameters characterizing the corrosive properties for the tested solutions (without and with Cbz), such as steady-state potential (E OCP), E cor, j cor, pitting potential (E pit), and corrosion rate (CR), are collected in Table 3.

Table 3 Values of corrosion parameters of AISI 316 L stainless steel in 3.5% NaCl solution and Cbz-containing NaCl solution (c Cbz  = 1.0 × 10−3 mol L−1)
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The obtained results showed that Cbz caused shift of the steady-state potential to more positive value. Based on this fact, it can be supposed that presence of this fungicide improves the corrosion properties of tested stainless steel. The elaborated results (see in Table 3) showed that the presence of Cbz in NaCl solution has beneficial effect on the corrosion resistance of AISI 316 L stainless steel. It can be observed that the addition of pesticide to NaCl solution shifted the value of the corrosion potential to more anodic direction and caused twofold decrease of corrosion current density. These two facts confirmed that the investigated steel has a relatively better general corrosion resistance in Cbz-containing solution. Additionally, CR value (calculated using CorrView 2.9 software, where equivalent weight (EW) was 25, and density (ρ) was equal to 8 g cm−3) was equal to 1.17 and 0.69 μm year−1 for 3.5% sodium chloride solution in the absence and in the presence of Cbz, respectively. Thus, it can be stated that this compound improved the corrosion properties, i.e., caused twofold decrease in this parameter.

Moreover, the value of pitting potential (E pit, also called as breakdown potential) was obtained from potentiodynamic characteristics. The pitting potential was taken as the potential value where an abrupt increase in the anodic current density was observed. It can be stated that the presence of Cbz does not change the value of pitting potential, what is clearly visible in Fig. 7.

Fig. 7
figure 7

The effect of Cbz on the potentiodynamic polarization curves for AISI 316 L in 3.5% NaCl solution (dashed line) and in 3.5% NaCl solution with 1.0 × 10−3 mol L−1 Cbz (full line). Scan rate of 1.0 mV s−1

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After the anodic polarization in sodium chloride solution, plenty of small pits were observed on the stainless steel surface, while in the case of Cbz-containing NaCl solution, the corrosion damages were a bit smaller in size as depicted on Fig. 8a, b. Moreover, the pits formed on the periphery of the active surface close to the silicone gasket were deeper and more clearly visible when compared to the entire surface (Fig. 8c, d). Generally, the formed pits on the stainless steel surface after polarization in both solutions showed a very similar morphology, and it can be concluded that AISI 316 L exhibited a similar pitting corrosion behavior in the presence and absence of Cbz.

Fig. 8
figure 8

The optical microscope images of pits formed on AISI 316 L stainless steel surface due to the anodic polarization in the presence of 3.5% NaCl solution (a, c) and in 3.5% NaCl solution with 1.0 × 10−3 mol L−1 Cbz (b, d). The magnitude ×100 (a, b) and ×200 (c, d)

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In summary, it can be concluded that the tested stainless steel exhibited better resistance to general corrosion in Cbz-containing sodium chloride solution; therefore, Cbz acts as an inhibitor of corrosion, but it seems that Cbz is not affected significantly on pitting corrosion.

Conclusions

In this work, the SWV characterization of carbendazim fungicide was performed using boron-doped diamond electrode (BDDE) in its unmodified and surface modified forms with multiwalled carbon nanotubes (MWCNTs, MWCNTs-BDDE) in Britton-Robinson buffer as supporting electrolyte in the pH range between pH 2.3 and 6.0, where the optimal shape and intensity of its oxidation peak at pH 4.0 were obtained in the case of MWCNTs-BDDE. The analytical signal intensity was significantly increased due to the presence of MWCNTs on BDDE surface and much more thanks to the additional influence of β-CD when β-CD-MWCNTs-BDDE was used as the working electrode. In all investigated cases, the oxidation peak(s) appeared in the potential range between + 1.0 and + 1.3 V. After the development of the SWV method, the accumulation potential and time were optimized by β-CD-MWCNTs-BDDE offering the opportunity for the development of highly sensitive SWAdSV method for trace level analysis of Cbz. The developed method was tested for the determination of Cbz in spiked Danube river water without special pretreatment of samples except filtration and suitable dilution with satisfactory results. The proposed SWAdSV method with β-CD-MWCNTs-BDDE could be serving as a good alternative to other complexes and costly analytical techniques used for pesticide analysis and could also be adopted for the use in quality control laboratories.

Additionally, the effect of Cbz on the corrosion properties of an AISI type 316 L stainless steel used to fabricate farming equipment and tools was tested by means of general and pitting corrosion. It can be seen that the tested stainless steel exhibited better resistance to general corrosion in Cbz-containing sodium chloride solution. In this case, Cbz acts as a corrosion inhibitor, but it seems that Cbz is not affected significantly on pitting corrosion. The information about the corrosivity of pesticides towards stainless steel used to produce farming tools and equipment should be performed due to frequent use of pesticides in agriculture.