Introduction

Zearalenone (ZEA), Fig. 1, is a secondary fungal metabolite produced by several species of Fusarium, mainly by Fusarium graminearum and Fusarium culmorum [1]. The acute toxicity of ZEA is relatively low (oral LD50 > 20,000 mg kg−1 body weight) upon oral administration in animals [2]. Nevertheless, this mycotoxin causes alterations in the reproductive tract of laboratory and domestic animals and, in humans; it has been associated with precocious puberty, neoplastic and hyperplasic endometrium, and human cervical cancer [3, 4]. Several countries established regulations for ZEA in food and feed in 1996. In 2004, there were a total of 16 countries [5], which indicates its significance. However, the maximum tolerable level is different greatly between countries and a common international maximum limit has not been set for ZEA in foodstuff. The maximum levels set by the European Commission are 20, 75, and 100 μg kg−1 for baby food, cereal flour, and unprocessed cereals, respectively [6–8]. Due to the very important implications of ZEA presence in human beings and animal health as well as from economic aspects, it is need to have economic and rapid techniques for its quantification [9]. The main analytical techniques for ZEA analysis include immunochemical approach [10–12], immunochromatographic assay [13], gas chromatography-triple quadruple mass spectrometry [14], high-performance liquid chromatography using fluorescence detection [15], Liquid chromatography-mass spectrometry [16], liquid-chromatography tandem mass spectrometry and ultra-high performance liquid chromatography high-resolution mass spectrometry [17], High-throughput screening enzyme-linked immunosorbent assay [18] and capillary electrophoresis [19]. Recently, voltammetry techniques have been adopted to study the redox mechanism and behavior of zearalenone [20–22]. These voltammetry techniques are faster and need shorter processing time compared to the conventional techniques [23, 24]. The advantages of electrochemical detection are evident in the development of chemical and biological sensors [25]. Pd nanoparticles (PdNPs) have the reputation of being one of the most attractive metals in many fields including sensor modification due to their high catalytic activities to different types of reaction, excellent economic benefits, good electrical conductivity and optical properties [26, 27]. Furthermore, polymeric ionic liquids (PILs) exhibit the advantages of both ionic liquids and polymers, such as enhanced ionic conductivity, thermal stability and excellent mechanical properties [28]. Owing to their unique properties, PILs have attracted great attention especially in sensor modification [29].

Fig. 1
figure 1

Chemical structure of zearalenone

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Herein, a glassy carbon electrode (GCE) is modified with Pd nanoparticles/polymeric ionic liquid nanocomposite which revealed high electrocatalytic activity towards the oxidation of ZEA. Under optimized experimental conditions, ZEA showed a well-defined oxidation peak with high current at the modified electrode. The modified electrode provided a low overpotential in comparison with traditional electrodes. High sensitivity toward ZEA, allowed successful using of the electrode for determination of this mycotoxin at low concentration level in food samples.

Experimental

Apparatus and chemicals

The electrochemical measurements were carried out with an Autolab potentiostat/galvanostat (PGSTAT 101, Eco Chemie, Netherlands, http://www.metrohm-autolab.com). A three-electrode cell was used at 22 ± 1 °C. An Ag/AgCl/KCl electrode, a platinum electrode and the PIL-Fe3O4/GCE were used as the reference, auxiliary and working electrodes, respectively. The conditions were controlled with Nov. 1.6 software. An ultrasonic water bath (Sonorex RK255, Germany, http://www.sonorex.com) was used for nanoparticles synthesis. The samples were characterized with a field emission scanning electron microscope (FESEM) (Hitachi S-4160) with gold coating.

Palladium (II) chloride (PdCl2), perchloric acid (HClO4), Zearalenone, Poly (ethylene glycol) methacrylate, 2-sulfobenzoic anhydride, 1-methylimidazole and azobis (2-methylpropionitrile) (AIBN) were purchased from Sigma-Aldrich (http://www.sigmaaldrich.com). Acetonitrile (ACN) was obtained from Merck (Darmstadt, Germany, http://www.merck.com). All solutions were freshly prepared with distilled water.

Synthesis of Pd nanoparticles

The chemical pretreatment of Vulcan XC-72R carbon (VC) powder was carried out using Senthil Kumar method [30]. The treated carbon support and ethylene glycol were added into a beaker and irradiated with strong ultrasonic wave (300 W, 30 % amplitude) for 30 min at room temperature while PdCl2 solution was dropped continuous to support solution. At end, the obtained blackish slurry was filtered, washed and dried in vacuum for 12 h at room temperature. The XRD patterns of obtained PdNPs/VC nanocomposites are shown in Fig. 2. The peak at 2θ value of 25° is related to carbon support. The three peaks, corresponding with the planes (111), (220) and (322) at ca. 40°, 47° and 68° respectively were characteristic of cubic crystalline PdNPs (JCPDS, Card No. 87–0639). The average sizes of the PdNPs were calculated 6.0 nm by Debye- Scherrer formula.

Fig. 2
figure 2

XRD pattern of the PdNPs/VC nanocomposite

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FESEM image of synthesized PdNPs is shown in the Supplementary content (Fig. S1). The white visible spots on the images are believed to be PdNPs which were deposited on the Vulcan carbon supports with compact and granulated structure.

Synthesis of PdVC-PIL nanocomposite

Monomer of ionic liquid was prepared and purified using the Ohno method [31]. Poly (ethylene glycol) methacrylate and 2-sulfobenzoic anhydride were mixed and stirred in chloroform at room temperature for 24 h. Then, 1-methylimidazole, equimolar to the vinyl group, was added slowly to this solution under stirring at 0 °C. After 12 h, this solution was dropped in excess diethyl ether for purification. The oily precipitate was washed several times with diethyl ether and dried in vacuum at room temperature. This monomer was polymerized in presence of AIBN and 1 % wt. PdNPs in ethanol. Polymerization of these ionic liquid monomers was initiated at 70 °C for 2 h under N2 atmosphere. Obtained polymer was washed with dehydrated ethanol two times and then dried in vacuum. FESEM image of nanocomposite is shown in the supplementary content (Fig. S2).

Preparation of the modified electrode

Prior to coating, the GC electrode was carefully polished with 3 μm alumina powder on polishing cloth and sonicated in 1:1 ethanol and distilled water ratio in order to remove adsorbed particles. After that, GCE surface was activated in 0.1 M NaHCO3 solution. 1 μL solution containing 0.5 mg mL−1 nanocomposite and 1 % nafion was cast on GCE surface and allowed to dry at room temperature. A PIL/GCE and a PdVC/GCE were fabricated under above conditions with 0.5 mg mL−1 polymer without PdNPs and 0.5 mg mL−1 PdVC without polymer, respectively.

Procedure of cereal derived products preparation

A total of 6 cereal-derived samples, including 2 baby food samples, 2 breakfast cereal samples and 2 beer samples were purchased randomly from different supermarkets and small shops in Kerman, Iran. The cereal contents in the baby food samples ranged from 30 to 80 % wt. Breakfast cereal samples included the following major ingredients, alone or mixed: maize, bran, chocolate, cereals (wheat and/or rice), fruits and oats. All of the beer samples were non-alcohol. 25 g of each Breakfast cereals and baby foods was extracted with 100 mL of methanol: water extraction solvent (80:20, v/v). The extract was filtered through a filter paper and then 10 mL of filtrate was diluted. 100 mL of beer samples were boiled for 15 min until foaming was stopped. Then the residue liquid was diluted. Then each sample was stirred for about 10 min and the pH was measured directly and adjusted to 7.2.

Results and discussion

Choice of materials

VC powder was chemical treatment before nanoparticles synthesis. Acid treatment creates surface oxygen groups which form anchoring sites for metallic precursors as well as for metals on the carbon surface. The pretreatment VC provides excellent conductivity in a range of applications at relatively low loading levels. Therefore, measured amounts of PdCl2 and VC powder were used in reaction to make 30 wt.% of the metal content in the final product. Pd nanoparticles were synthesized by sonochemical reactions in ethylene glycol on carbon support. It must be noted that the reactions have been run without any additives. The solvent ethylene glycol functions as a surface stabilizer as well as the expected reducing agent. The use of conductive polymeric ionic liquid for synthesis of nanocomposite can increase the modified electrode conductivity.

Electrocatalytic oxidation of ZEA at modified GCE

The voltammetry activity of modified GCE was studied in 20 % ACN + 80 % 1 M HClO4 aqueous solution and scan rate of 20 mV s−1. Figure 3 illustrates the cyclic voltammograms (CV) of a) bare GCE, b) PdVC/GCE, d) PIL/GCE and e) PdVC-PIL/GCE in 35.0 ng mL−1 ZEA. No peaks were observed at the reverse scan which suggests that the oxidation process is chemically irreversible for ZEA under the experimental conditions. As shown, the anodic peak potential of ZEA on bare, PdVC, PIL and PdVC/PIL GCE surface is ~1.1, ~1.0, ~0.8, ~0.69, respectively. Also, it can be seen that the peak current is enhanced in case of the modified GCE. Therefore the PdVC/PIL can catalyze the electrode reaction by decreasing overpotential. This modified electrode increases the measurements sensitivity in comparison to other electrodes.

Fig. 3
figure 3

Cyclic voltammograms of a bare GCE, c PdVC/GCE, d PIL/GCE and e PdVC-PIL/GCE in 35.0 ng mL−1 zearalenone. Scan rate: 20 mV s-1

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Calibration curve and detection limit

Square wave voltammetry (SWV) method was applied to determine the concentration of ZEA (Fig. 4). The plot of peak current vs. concentration was linear in the concentration ranges of 0.03 to 35.0 ng mL−1 with slopes of 0.092 ng mL−1 (inset). The detection limit of ZEA was calculated by making seven replicate current measurements for a blank solution, the detection limit based on the mean of these measurements and 3σ gave a value of 0.01 ng mL−1. The relative standard deviations from seven determinations of zearalenone at each of two concentrations of 1.0 and 20.0 ng mL−1 were 3.5 and 2.0 % respectively.

Fig. 4
figure 4

SWV of PdVC-PIL/GCE in 20 % ACN + 80 % 1 M HClO4 aqueous solution containing different concentrations of ZEA. Numbers 1–8 correspond to 0.03, 0.5, 5.0, 10.0, 15.0, 20.0, 25.0 and 35.0 ng mL−1 zearalenone. Inset: Plot of the electrocatalytic peak current at the working potential of +0.69 V versus Ag/AgCl as a function of zearalenone concentration

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Interference study

In order to check the selectivity of the modified electrode, the influence of various interferences on the determination of ZEA was studied under optimum conditions in the concentration of 35.0 ng mL−1. The tolerance limit was defined as the maximum concentration of the interfering substance that caused an error of less than ±5 % in the determination of ZEA. The results showed that less than 1000 fold excess of Na+, K+, Ca2+, Mg2+, CO3 2−, PO4 3−, SO4 2−, Cl−, Br−, NO3 −, ClO4 −, ACN, Ethanol; 500 fold excess of ascorbic acid, citric acid, sucrose, lactic acid; 5 fold excess of Aflatoxin, Ochratoxin and Deoxynivalenol did not have any significant effect on the ZEA signal.

Real sample analysis

This method was applied to the determination of ZEA in food samples. The analytical results are shown in Table 1, 2, and 3. The method was checked by spiking ZEA in samples. Also, the accuracy of method was evaluated by analyzing a Maize Reference Material (ERM-BC717) with zearalenone content of 83 ± 9 μg kg−1.

Table 1 Determination of ZEA in baby food
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Table 2 Determination of ZEA in breakfast cereals
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Table 3 Determination of ZEA in malt beverage samples
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Comparison of the method with literature methods

A comparison of the main characteristics of this method with others reported in the literatures is given in Table 4. Generally, the results obtained by the present method are comparable to or better than the other methods.

Table 4 Comparison of the published determination methods for ZEA with the proposed method
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Conclusion

This article described the development of an electrochemical sensor for zearalenone detection by the facility construction of a conductive polymeric ionic liquid containing the synthesized palladium nanoparticles on Vulcan XC-72R carbon. In presence of this nanocomposite, the surface area of electrode is increased and more active sites are provided for adsorption of zearalenone. The contribution of each component in the nanocomposite was specified to increase the response of the electrode. This nanocomposite decrease oxidation potential and accelerate the oxidation of ZEA. These properties make this modified electrode very useful for accurate determination of ZEA in food samples. The approach is compatible with high throughput analysis, saving time, labor, and money. While the using toxic solvent, acetonitrile is a method limitation, but 1 M HClO4 containing 20 % acetonitrile, as electrolyte, increased the electrode response and method sensitivity.

The only limitation of this method is using of the toxic solvent, acetonitrile, as the electrolyte, that increased the electrode response and method sensitivity.