1 Introduction

Antibiotic medicines are one of the great successes of medical science that have increased the life expectancy of humans. Recently, the largest amount of drug production is related to antibiotics, which are used against protozoa, microalgae, parasites, viruses, bacteria, and fungi [1]. However, the use of some antibiotics has been restricted in some countries, including China, the United States, and Canada, due to their harmful effects [2, 3]. Metronidazole (MNZ), a nitroimidazole derivative, is an antiprotozoal medication and an antibiotic that can be either taken by mouth or intravenously; it can also be used in the form of a cream. MNZ, 2-methyl-5-nitroimidazole-1-ethanol, belongs to the nitroimidazole group of pharmaceuticals. MNZ is applied for the treatment of certain bacterial diseases, such as vaginal bacterial, liver, stomach, and skin infections, as well as joints, brain, and respiratory tract [4]. Additionally, MNZ is a veterinary drug that is used to treat bacterial infections in swine and poultry [5]. It is also commonly used to prevent or treat echinococcosis caused by anaerobic bacteria and amoebic disease. Since this drug is not biodegradable, some undesirable effects like cardiogenic shock, pseudomembranous colitis, and peripheral neuritis can be diagnosed in clinical applications [6]. Overdose of the drug over a prolonged period can damage human health, wildlife, and the environment owing to its genotoxic, mutagenic, and carcinogenic side effects [7]. Headache, nausea, and loss of appetite are the other side effects of this medicine. In addition, a group of specialists believes that this drug should not be used in early pregnancy or lactation. Therefore, it is essential to extend a reliable, accurate, and sensitive procedure for the determination of MNZ. So far, it has been analyzed using various analytical methods, including chromatography [8, 9], spectrometric [10, 11], and electrochemical techniques [7, 12,13,14,15,16,17,18] Among the mentioned analytical techniques, electrochemical methods have been frequently used because of their fast response, low limit of detection, high sensitivity, lack of need for time-consuming sample preparation and complicated separation methods, and inexpensive, and ease of use [7, 19,20,21,22,23,24,25,26,27,28]. However, electrochemical methods using bare/unmodified electrodes show poor practicality and sensitivity with lower electrochemical activity. Therefore, it is necessary to improve the performance of electrodes by modifying their surface [29, 30]. In this research field, a glassy carbon electrode (GCE) modified with graphene and a magnetic molecular polymer (mag-MIP) has been reported as an electrochemical sensor for the determination of MNZ [12]. In addition, an activated screen-printed carbon electrode [13], a GCE/biocompatible chitosan-pectin polyelectrolyte complex [14], a carbon paste electrode coated with graphene and sodium dodecyl sulfate (SDS) [15], a GCE coated with an eggshell-like hollow alumina sphere [7], a GCE modified with a Fe3O4/N/C/C@MWCNTs nanocomposite derivative from H2N-Fe-MIL-88B metal–organic frameworks (MOFs) [16], a GCE modified with bismuth at a graphene via electrodeposition [17], a sensing platform based on green-synthesized magnetic Fe3O4 nanoparticles [18], a carbon nanofiber composite electrode decorated with composites of carbon nanofibers and orthorhombic iron-tungsten oxide (CNF/Fe2WO6) [27] and a GCE modified by binary mixed MOFs based on nickel and titanium [28] are only a part of the studies carried out for the electrochemical determination of MNZ.

Reduced graphene oxide (rGO), a carbon-based nanomaterial, has been recognized as an interesting material because it provides a larger surface area and accelerates the transfer of electrons between the analyte and the electrode, resulting in a faster and more sensitive current response [31]. Graphene oxide (GO) has a two-dimensional structure and exhibits excellent optical, thermal, electronic, and mechanical properties, with numerous applications in catalysis, sensors, electronics, energy conversion, storage, etc. With its unique structure and excellent properties, this carbon nanosheet provides an innovative way to prepare various multi-layer nanocomposites with better electrocatalytic activity [32], which have wide potential windows and suitable electrocatalytic activity for various redox reactions [33]. It is also worth mentioning the reduction of GO to rGO can create a single layer of graphene at a comparatively low cost. In recent years, sulfides of transition metals have been used as a fundamental category of functional materials. Among them, molybdenum disulfide is a layered material that has attracted considerable interest for application in the field of lithium-ion batteries owing to its improved cyclic stability and high theoretical capacity [34]. Moreover, MoS2, which has many active unsaturated sulfur atoms, has been recognized as an efficient catalyst based on experimental and computational studies [35,36,37]. Although its moderate conductivity limits the electron transfer efficiency, electrode kinetics, and volumetric expansion upon cycling [34, 37, 38], chemists are still working to improve the catalytic properties of MoS2 for various applications. One of the metallic transition metal sulfides with good thermal stability, comparatively high electrical conductivity, and theoretical capacity is cobalt sulfide (Co9S8), however, it shows poor electrochemical performance due to its slow mass transport kinetics [34,35,36,37,38]. It is worthwhile noting that cobalt sulfides exist in various phases, such as CoS, CoS2, Co3S4, Co9S8. The combination of MoS2 and Co9S8 provides many benefits, for instance, the synergistic effects can reduce the volume effect, improve the overall conductivity of the compounds, and improve the cycle performance and other electrochemical features of the sensor. The electrochemical efficiency of the nanocomposites can also be improved in a core–shell structure [34, 39,40,41]. Additionally, the combination of MoS2 and Co9S8 in various structures such as nanocomposite or core–shell structures can increase the energy storage capacity of lithium batteries [34, 42,43,44]. This nanostructure has also been used for electrocatalytic hydrogen evolution activity [45]. The hydrothermal method/thermal solution, sonochemical method, and microwave radiation are among the synthesis methods of transition metal sulfides [46, 47]. In this work, the core–shell structure of MoS2 and Co9S8 (Co9S8@MoS2) decorated on rGO was formed to catalyze the electrochemical determination of MNZ in blood plasma and tablet samples. To the best of our knowledge, this nanostructure has been used for the first time to modify the surface of the electrode to determine the concentration of electroactive compounds in the field of measurement.

2 Experimental

2.1 Materials and methods

All reagents required for the synthesis of nanostructures and other analyses in the current research work are of analytical grade. Phosphate buffer solutions (PBS) were prepared by dissolving K2HPO4, KH2PO4, and H3PO4 in distilled water. All utilized chemicals in the manufacturing of Co9S8@MoS2/rGO nanocomposite, H2SO4 (96%), H2O2 (30%), KMnO4, NaNO3, graphite powder, hydrazine hydrate, ammonia (25%), CoCl2·6H2O, urea ((NH2)2CO), ascorbic acid, thiourea (CH4N2S), sodium molybdate (Na2MoO4·2H2O), and ethanol (C2H5OH) were bought from Merck (Germany). Distilled water was procured from the Ghazi Company (Tabriz, Iran). Human blood serum samples were obtained from the Iranian Blood Transfusion Research Centre (Tabriz, Iran).

A Potentiostat Autolab instrument PGSTAT 30 ECO Chemie (The Netherlands) was used to collect electrochemical data and sensing analyses including, CVs, DPVs, and amperometric responses. Furthermore, the modified GCE was used alongside a conventional three-electrode system, including a platinum electrode as the auxiliary electrode, a saturated calomel electrode (SCE: Hg/Hg2Cl2/Saturated KCl solution) as the reference electrode, and an electrochemical cell. All electrodes used in this study were purchased from the Azar Electrode Company (Urmia, Iran). Electrochemical impedance spectroscopy was performed at an open circuit using Autolab potentiostat (model PGSTAT30) and impedance software ZView (II) and Nova 2.1. The morphological characterizations of the prepared Co9S8@MoS2/rGO core–shell nanocomposites were studied using field emission scanning electron microscopy (FESEM). The elemental composition was studied by energy dispersive X-ray spectroscopy (EDX) in a TESCAN MIRA3 and the core–shell structure of the synthesized nanostructure was investigated by an LEO 906 transmission electron microscope (TEM, Zeiss, Germany). Moreover, X-ray diffraction patterns were recorded using a Bruker AXS (D8 Advance) X-ray powder diffractometer (Cu Kα radiation source, λ = 0.154056 nm) in the 2θ range of 10–70° at 35 mA and 40 kV at room temperature (Germany). The FTIR spectrum was obtained using a Shimadzu 8400 series (Japan) Fourier transform infrared spectrometer. A pH meter Metrohm 827 was used to adjust the pH of the aqueous solutions (Switzerland Metrohm Company).

2.2 Synthesis of the reduced graphene oxide (RGO)

Graphite powder was used to synthesize GO by Hamer’s method. Briefly, 1 g of graphite powder was added to 30 mL of H2SO4 in an ice bath and stirred. Then 1 g of the weighed NaNO3 powder was poured into the above suspension and stirred for an hour. Afterward, 3.6 g of KMnO4 was gradually added over 2 h. Then the temperature was increased to 35 °C and the mixture was stirred for 2 h. In the next step, 46 mL of double-distilled water was poured into the above mixture and stirred at 90 °C for 30 min. The reaction was completed by adding 12 mL of H2O2 (30%) and 120 mL of deionized water. Finally, the precipitated material was collected, washed, and dried in an oven at 60 °C.

To prepare rGO, the required amount of GO was dispersed in double distilled water for 2 h, and 20 mL of a 50:50 solution of hydrazine hydrate and 25% ammonia was poured into the above reaction vessel. Finally, the solution was refluxed at 85 °C for 1 h, collected, washed, and then dried in an oven at 60 °C [48].

2.3 Chemical synthesis of the Co9S8@MoS2/rGO core–shell nanocomposite

The Co9S8@MoS2/rGO core–shell nanoparticles were synthesized as described in the reference [34]. Accordingly, Co-rGO was synthesized in the first step as follows. First, 28 mg of rGO was dispersed in 70 mL of deionized water for 5 h to obtain a homogeneous solution. Then, 0.07 g of CoCl2.6H2O, 0.042 g of urea ((NH2)2CO), and 0.056 g of ascorbic acid were added to the above homogeneous solution with rapid stirring. The mixture was transferred to a Teflon-lined 100 mL autoclave and held at 160 °C for 16 h. The synthesize the Co9S8@MoS2/rGO core–shell nanocomposite, the resulting precipitate was collected by centrifugation and eluted three times with a 50:50 solution of ethanol and water and then dried at 60 °C. In the next step, 0.025 g of Na2MoO4·2H2O, and 0.08 g of thiourea were completely dissolved in 30 mL of solvent mixtures (H2O:C2H5OH = 1:1). Afterward, 0.05 g of the precursor was transferred to the solution under vigorous magnetic stirring. Subsequently, the mixture was transferred to a 50 mL Teflon autoclave and finally, it was placed in the oven at 200 °C for 24 h, which led to the formation of a dark deposit. Upon centrifugation, Co9S8@MoS2/rGO was obtained as the black product.

2.4 Synthesis of Co9S8/rGO

Initially, 0.05 g of thiourea was dissolved in 30 mL of a mixed solvent (H2O:C2H5OH = 1:1, in volume). Then, 0.05 g of the Co-GO precursor was added to the above mixture, stirred vigorously at 45 °C for 2 h, transferred into a 50 mL Teflon-lined autoclave, and kept at 200 °C for 24 h. The obtained black precipitate was washed with deionized water and ethanol 5 times and dried at 60 °C for 8 h.

2.5 Preparation of the Co9S8@MoS2/rGO/GCE sensor

To prepare the sensor, the surface of GCE was polished with alumina powder and ultrasonically rinsed in ethanol and water (50:50) for 15 min to eliminate impurities. Then a suspension of 0.005 g/mL of the synthesized nanostructure was prepared in distilled water. Next, an optimal value of 4 μL drops of the above suspension was dropped onto the GCE surface and dried at 25 °C.

2.6 Preparation of real samples

Five MNZ tablets (250 mg per tablet) purchased from a local pharmacy were carefully weighed and ground. The average weight of one tablet (0.4125 g) was transferred to a 100 mL flask containing distilled water and sonicated for 20 min. The centrifugation of a portion of the resulting solutions was followed by the supernatant solution analysis. MNZ was determined using the human blood plasma sample prepared by the Blood Transfusion Organization. One mL of the MNZ standard solution was added to a 15 mL falcon containing 1 mL of the blood plasma. After stirring the mixture, 1 mL of acetonitrile was added to precipitate the plasma proteins and then centrifuged at 6000 rpm for 15 min. Afterward, 0.5 mL of the clear supernatant solution was transferred to a 10 mL flask and made up with 0.1 M of PBS (pH = 7).

3 Results and discussion

3.1 Morphological characterizations

The crystal phase, structure, and morphology of the prepared sensor surface were characterized by SEM, EDX, and TEM. The SEM images of rGO (A) and Co9S8@MoS2/rGO (B) show the Co9S8@MoS2 core–shell nanoparticles on the surface and edges of the rGO plates (Fig. 1). Moreover, the quantitative EDX analysis of rGO and Co9S8@MoS2/rGO nanoparticles confirmed the presence of the mentioned elements in the modifier (Fig. 1a, b). Figure 1C shows the TEM image of the Co9S8@MoS2 core–shell nanostructure. A clear lattice fringe around the opaque core is a characteristic of the core–shell structure of the synthesized nanostructure. Moreover, the XRD patterns of Co9S8@MoS2/rGO and rGO (Fig. 2A and B) are consistent with the references used to synthesize this nanocomposite [34]. Co9S8 nanoparticles serve as a core, while MoS2 nanoparticles grow on the Co9S8 surface to form a core–shell structure. The XRD and SEM results support this conclusion. A distinct peak located at 14.1° corresponds to the (002) crystalline plane of MoS2. The peaks of 15.2, 25.5, 29.8, 31.1, 47.5, and 52.0° match well with the (111), (220), (311), (222), (511), and (440) crystalline planes of cubic-phase Co9S8. In addition, the broad peak centered at 26.0° can be assigned to the (002) crystalline plane of the rGO which is rich in graphene. Combined with the XRD and SEM results, it can be concluded that the Co9S8 nanoparticles serve as a core and the MoS2 nanosheets grow on the surface of the Co9S8 nanoparticles to form the core–shell structure [42].

Fig. 1
figure 1

SEM images of rGO (A), Co9S8@MoS2/rGO (B), EDX patterns rGO (a), Co9S8@MoS2/rGO (b)

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Fig. 2
figure 2

A XRD patterns of Co9S8@MoS2/rGO, B rGO, C FT-IR spectrum of Co9S8@MoS2/rGO

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X-ray diffraction is a suitable method for determining the average size of nanocrystallites in bulk materials. X-Ray diffraction is sensitive to the crystallite size inside the particles. From the Scherrer equation, the average size of nanoparticles, L, is as follows:

$$\mathrm{L}={\frac{K\uplambda }{\mathrm{\beta cos\theta }}}$$

where λ is the X-ray wavelength (0/154056 nm), β is the peak width of the diffraction peak profile at half maximum height resulting from the small crystallite size, in radians, and K is a constant related to the crystallite shape and usually assumed to be 0.9 [49]. By calculating L for all peaks and computing the average, an approximate nanoparticle size of 62 nm was obtained.

The functional groups of the synthesized compounds were identified using the FT-IR technique. The FT-IR spectrum of the Co9S8@MoS2/rGO nanostructure in the range of 400–4000 cm−1 is shown in Fig. 2C. The peaks at 1631 and 1408 cm−1 in the spectrum belong to the C–O stretching vibration of the epoxide [50], and the stretching vibration of tertiary C–OH groups, the peak at 1070 cm−1 belongs to carboxyl and carbonyl groups and the C=O stretching vibration [51]. A sharp vibrational peak in the region of 3403 cm−1 can be assigned to the stretching vibration of the O–H group bonding. In addition, two absorption bands at 3297 cm−1 and 1569 cm−1 can be assigned to the stretching and bending vibrations of water adsorbed on the surface of the samples, respectively [52]. A peak at 865 cm−1 is attributed to the Co-S bonding and its stretching vibration. The peak of the bending vibration observed at 1144 cm−1 belongs to a sulfonated group of Co9S8 and is merged with the CO2 absorption band [53]. The peaks associated with the bond (Mo–S), δ(C=S) and δ(C–S) are at 951 cm−1, 465 cm−1, and 1128 cm−1, respectively.

3.2 The electrical conductivity of the sensor

Electrochemical impedance spectroscopy (EIS) is a powerful method for testing the electrical conductivity properties of a modified electrode. The electrical conductivity of the bare GCE, and GCE modified with rGo, Co9S8/rGO, and Co9S8@MoS2/rGO was investigated using the EIS technique and compared with the bare GCE. ZView impedance software (II) and Nova 2.1 were used to fit the data to the Nyquist spectra of the impedance data. The Nyquist curve of the EIS (Nyquist plots corresponding to the real part Z′ versus the imaginary part -Z′′ of the complex impedance Z) contains a linear part and a semicircular part. A decrease in the radius of the semicircle in the Nyquist curve indicates high electrical conductivity and low resistance of electron transfer at the electrode surface. Figure 3A shows the Nyquist spectrum of the Co9S8@MoS2/rGO/GCE (d), Co9S8/rGO/GCE (c), rGO/GCE (b) sensors, and the bare GCE (a) in 1.0 mM [Fe(CN)6]3−/4− containing 0.1 M KCl. The Co9S8@MoS2/rGO/GCE sensor has a much smaller semicircle, the lowest resistance, and faster charge transfer than Co9S8/rGO/GCE, rGO/GCE sensors, and the bare GCE. Based on Fig. 3B, the reduction of the redox potential and the current improvement confirm the content above.

Fig. 3
figure 3

A EIS analysis of (a) the bare GCE, (b) rGO/GCE, (c) Co9S8/rGO/GCE, and (d) Co9S8@MoS2/rGO/GCE in 0.1 M KCl solution containing 1.0 mM [Fe(CN)6]3−/4−. Inset (c): Equivalent circuit applied to fit impedance measurements; B Corresponding CVs (a) Bare GCE, (b) rGO/GCE, (c) Co9S8/rGO/GCE, (d) Co9S8@MoS2/rGO/GCE

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3.3 Electrochemical behavior of MNZ on the surface of the bare GCE, rGO/GCE, Co9S8/rGO/GCE, and Co9S8@MoS2/rGO/GCE sensor

The electrochemical behavior of MNZ on the surface of the bare GCE, rGO/GCE, Co9S8/rGO/GCE, and Co9S8@MoS2/rGO/GCE sensor was compared by cyclic voltammetry (CV) in 0.1 M of the PBS solution (pH = 7) containing 1 × 10–4 M of MNZ. Figure 4 compares the CVs of the modified electrodes with those of GCE in 1.0 × 10–4 M of MNZ. Accordingly, improving the modifier and increasing its surface-to-volume ratio increases the cathodic peak current, and shifts the cathodic peak potential to a lower potential, indicating an enhancement in electron transfer due to the larger surface area of the Co9S8@MoS2/rGO/GCE sensor. It is worth noting that the cathodic peaks seen in Fig. 4 are related to the electrocatalytic reduction of MNZ.

Fig. 4
figure 4

(A) CVs of 1 × 10–4 M of MNZ at (a) the bare GCE, (b) the rGO/GCE, (c) Co9S8/rGO/GCE, (d) Co9S8@MoS2/rGO/GCE, and Curve of (e) is the CV of the Co9S8@MoS2/rGO/GCE nanosensor without MNZ in 0.1 M of PBS (pH = 7), scan rate: 0.05 Vs.−1

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3.4 Effective surface area of the sensor

The active surface area of the electrode was increased by modifying the GCE surface with semiconductor nanocomposites Co9S8@MoS2/rGO. Increasing the active surface of the electrode makes it easy for electrons to reach the electrode and exchange electrons in the reactions. The active surface area of the modified electrode was calculated using the cyclic voltammetry technique in the solution of 0.1 M supporting the electrolyte of KCl containing 1.0 mM of [Fe(CN)6]3−/4−. The area of the active surface can be calculated according to the CVs shown in Fig. 5A and the following equation (Randles–Sevcik):

$${\text{I}}_{{\text{p}}} = \, \left( {{2}.{69} \times {1}0^{{5}} } \right){\text{n}}^{{{3}/{2}}} {\text{AD}}^{{{1}/{2}}} {\text{v}}^{{{1}/{2}}} {\text{C}}_{0}$$

where Ip is the peak current, C0 refers to the concentration of [Fe(CN)6]3−/4−, n is the number of electrons exchanged, A is the area of the active surface of the modified electrode, v is scan rate, and D is the diffusion coefficient of [Fe(CN)6]3−/4−. Ip should have a linear relationship with the v1/2. By recording CVs at different scan rates and drawing the Ipa-v1/2 diagram (D = 7.6 × 10−6cm2 s−1 and n = 1), the surface area of the Co9S8@MoS2/rGO/GCE sensor was calculated from the slope of the diagram in Fig. 5B (slope = 0.0000707 A/Vs−1). Eventually, A was found to be 0.0945 cm2, which is a threefold increase over the bare GCE (0.0314 cm2).

Fig. 5
figure 5

A CVs of the modified GCE with Co9S8@MoS2/rGO in 0.1 M KCl solution containing 1.0 mM [Fe(CN)6]3−/4− at scan rates (inner to outer) of 10, 25, 45, and 70 mVs−1, B peak current diagram against the square root of the scan rate

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3.5 Scan rate and kinetic investigation

To study the effect of scan rate and reaction kinetics as well as the nature of the electrode process, the cyclic voltammograms of the sensor were recorded at the scan rates of 5–210 mVs−1 in 0.1 M of PBS (pH = 7) solution containing 1 × 10–4 M of MNZ. The peak potential showed a slight shift toward the cathodic direction (Fig. 6A), revealing that the reduction of MNZ at the Co9S8@MoS2/rGO/GCE sensor is an irreversible process. Figure 6B shows a linear relationship between the cathodic peak current (Ipc) and the square root of the scan rate (V1/2), hence the reduction of MNZ is a diffusion-controlled process.

Fig. 6
figure 6

A CVs of 1 × 10–4 M MNZ in 0.1 M of PBS (pH = 7) on the Co9S8@MoS2/rGO/GCE sensor at different scan rates from 5 to 210 mVs−1. B Linear relationship of cathodic peak current (Ipc) of MNZ Vs scan rate. C Relationship of cathodic peak current (Ipc) of MNZ Vs square root of scan rate

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The electrochemical behavior of MNZ on the surface of Co9S8@MoS2/rGO/GCE was studied by the chronoamperometric method. To this aim, the chronoamperograms were recorded for the modified electrode with a known surface area in the static solution of MNZ with a concentration range of 0.1–0.7 mM in 0.1 M of PBS (pH = 7), and in a potential of − 0.7 V. The recorded chronoamperograms are shown in Fig. 7A. In Fig. 7B, was plotted I versus t−1/2. Finally, the slope of the graphs in Fig. 7B was plotted versus the MNZ concentration (Fig. 7C). Diffusion coefficient of MNZ can be obtained according to the recorded charts and the Cottrell equation:

$${\text{I }} = {\text{ nFAD}}^{{1/2}} {\text{C}}\pi ^{{ - 1/2}} {\text{t}}^{{ - 1/2}}$$

where I is the current intensity in amperes, n is the total number of MNZ electrons exchanged (n = 4), F is the Faraday number (96,489 C), C is the concentration of the electroactive compound in mol/cm3, t is the time in seconds, D is the diffusion coefficient, and A is the surface area of the modified electrode (0.0945 cm2). The diffusion coefficient of MNZ was 4.98 × 10−6cm2 s−1, which is comparable to the values obtained in previous articles [7, 23].

Fig. 7
figure 7

A Chronoamperograms in the presence 0.1–0.7 mM of HZ at potential − 0.5 V vs. SCE, B Plots of I − t−1/2 for various MNZ concentrations and C slope values of I − t−1/2 plots versus HZ concentrations

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3.6 The effect of pH

The pH of biological samples is very important, and this parameter of the electrolyte plays an essential role in redox reactions. Therefore, the effect of the electrolyte pH on the reduction peak of MNZ was studied in the range of 3–10 with the prepared sensor for 1 × 10–4 MNZ at a scan rate of 50 mVs−1. Figure 8 shows the resulting CVs of MNZ reduction in the solutions with different pH values. As shown in Fig. 8A, the cathodic peak potential shifted to a negative potential with increasing pH, which is due to the participation of proton(s) in the reduction reaction of MNZ. According to the slope of the graph in Fig. 8B(0.0554 V/pH), which shows the Nernst behavior of the sensor, the number of exchanged electrons is equal to that of protons participating in the reduction reaction of MNZ and 1‒ (2hydroxyethyl) ‒2 methyl 5 nitroimidazole to 1‒ (2hydroxyethyl) ‒2 methyl hydroxyamide imidazole. Since MNZ has two pka, it is likely to be involved in acidic and basic reactions in acidic and basic solutions, and the reduction reaction is not easily done (Scheme 1).

Fig. 8
figure 8

A CVs of 1 × 10–4 M of MNZ on the Co9S8@MoS2/rGO/GCE sensor in 0.1 M of PBS at different pH values (3–10): B Linear relationship of Epa Vs pH. C Effect of pH on the reduction peak currents

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Scheme 1
scheme 1

Electrochemical Reduction Mechanism of MTZ [13, 54]

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Due to the importance of pH measurement in the biological environment and referring to the diagram of MNZ cathodic peak currents (Ipc) versus different pH values, an optimum pH value of 7 was chosen in this work (Fig. 8C).

3.7 The amperometric detection of MNZ on the Co9S8@MoS2/rGO/GCE sensor

In this study, the hydrodynamic amperometry technique was used at a constant electrode potential to study the performance of the Co9S8@MoS2/rGO/GCE sensor as an amperometric sensor in the determination of MNZ. Figure 9A shows current–time responses obtained by the Co9S8@MoS2/rGO/GCE sensor at the applied potential of -0.8 V for the successive additions of MNZ to a 10 mL supporting electrolyte which was stirred with different increments. The linear responses of the Co9S8@MoS2/rGO/GCE sensor to MNZ were in the concentration ranges of 5.00 × 10–7–3.50 × 10–5 M and 3.50 × 10–5–1.80 × 10–4 M (Fig. 9B). Moreover, the calculated limit of detection (LOD) was 0.38 × 10–6 M based on the signal-to-noise ratio (S/N) of 3 in the initial part of the calibration curve. At the higher concentrations for the analyte, the active sites of Co9S8@MoS2/rGO on the surface of the modified electrode maybe not be enough to reduce MNZ, and two linear ranges were observed.

Fig. 9
figure 9

A Amperometric response of Co9S8@MoS2/rGO/GCE sensor for the increasing concentrations of MNZ in 0.1 M of PBS (pH = 7). Scan rate: 0.05 Vs−1 and pulse amplitude: 50 mV. B Calibration curve for the different concentrations of MNZ

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3.8 The application of the Co9S8@MoS2/rGO/GCE sensor in real samples

The capability of the fabricated sensor was examined by determining the concentration of MNZ in real samples of MNZ tablets and human serum samples with MNZ spiked by hydrodynamic amperometry. When MNZ was determined in the tablet sample, the MNZ concentration in the electrochemical cell was estimated to be 1.5 µM and the MNZ-spiked in the human serum sample was estimated to be 2.3 µM, as indicated by the information on the drug package and the calculations. Therefore, the same signal with the proposed sensor was observed with the least error. These measurements were repeated thrice, and the obtained relative standard deviation (RSD) values (2.6% and 3.3%, respectively) indicated the satisfactory repeatability of the electrode. (Table 1). Table 2 compares the results of the efficiency of the sensor prepared in this research study with the methods reported in previous articles.

Table 1 Results of recoveries of MNZ in drug samples and Human serum
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Table 2 Comparison of the performance of the proposed method with previously reported methods
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3.9 Interference studies

Electrochemical sensors for the detection of MNZ are interfered with by compounds with similar chemical structures. Therefore, possible interference must be carefully investigated to verify the sensitivity of the proposed sensor. Under optimal conditions, biological samples such as uric acid (UA), oxalic acid (OA), malic acid (MA), glucose, ascorbic acid (AA), and acetaminophen (AC), as well as metal ion samples such as Cl−, NO3−, Cu2+, Fe2+, Fe3+, Na+, and K+ were selected for interference studies (Fig. 10). The results of this analysis are shown in Table 3. The presence of the aforementioned biological samples and metal ions results in a slight change in the current value. In the presence of biological samples 10 times higher than the MNZ concentration (50 µM), the electrochemical signal related to MNZ shows a slight change of about 2.3%. In the presence of six-fold metal ions, this signal change was 2.8%. Therefore, the proposed sensor is suitable for the determination of MNZ.

Fig. 10
figure 10

Amperometric responses of different concentrations of interfering compounds on Co9S8@MoS2/rGO/GCE sensor in 0.1 M of PBS (pH = 7) with 50 µM of MNZ

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Table 3 Effects of interfering species on the electrochemical determination of MNZ at Co9S8@MoS2/rGO/GCE
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3.10 The stability and reproducibility of the Co9S8@MoS2/rGO/GCE sensor

Cyclic voltammetry tests were performed to examine and determine the stability, repeatability, and reproducibility in 1 × 10–4 M of MNZ (0.1 M of PBS pH = 7), in the potential range of − 0.3 to − 0.9 V, and a scan rate of 50 mVs−1. Besides, 100 cyclic voltammetry scans were applied in the stability test. The peak current and peak potential of the first scan and the 100th scan were compared with each other and the differences in both cases were less than 2.1% (Fig. 11A). To investigate the effect of aging on the electrode, an electrode was prepared in optimal conditions and examined on different days from 1 to 7 days. It was observed that the peak current decreased by 4.8% after 7 days (Fig. 11B). To study reproducibility, the cyclic voltammograms were recorded individually for five electrodes prepared in optimal conditions; the standard deviation of the peak currents was 3.5% (Fig. 11C).

Fig. 11
figure 11

CVs of the 1 × 10–4 M MNZ in 0.1 M of PBS (pH = 7) on the Co9S8@MoS2/rGO/GCE sensor with a scanning speed of 50 mV/s, A repeated 100 times. B with five electrodes. C The first, third, fifth, and seventh days

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4 Conclusion

In this study, the synthesis of an efficient electrochemical sensor to measure MNZ was demonstrated by synthesizing Co9S8@MoS2 core–shell nanoparticles and modifying the surface of glassy carbon electrodes with these nanoparticles. The synthesized Co9S8@MoS2 core–shell nanoparticles were systematically characterized by SEM, TEM, EDX, XRD, and FT-IR. The electrochemical tests indicated good electrocatalytic activity of the as-prepared sensor toward MNZ. Amperometry was used for the quantification of MNZ, which led to good results in wide linear ranges and at a low LOD. In addition, a Co9S8@MoS2/rGO/GCE sensor was developed for the first time, and the results show that this sensor can detect MNZ in real samples at low concentrations. In addition to being simple and sensitive, this newly developed sensor is low-cost, stable, and offers a large surface area.