1 Introduction

Hydrazine (N2H4) has an extensive application in different areas like industrial factories, pharmaceuticals, rocket propellants, photography chemicals as well as corrosive inhibitors [1,2,3,4]. Despite these numerous applications, N2H4 has been reported to be poisonous, carcinogenic and cyanogenetic [5,6,7,8] by the United States Environmental Protection Agency (EPA) [9]. This material may lead to serious injuries to the liver, lungs, nervous system, spinal cord, pneumonia, kidney damaging and temporary blindness and dizziness [10, 11]. Moreover, severe exposures to hydrazine can lead to mortality [9, 12].

Because this material has been considered to be carcinogenic and mutagenic, experts in the field have considerably examined its determination [13, 14]. Therefore, various methods have been proposed for determination of this material, such as spectrophotometry [15], amperometry [16], titration [17], chemiluminescence [18], fluorimetry [19] as well as electroanalytical procedures [20,21,22]. Researchers have also presented some procedures other than electroanalytical ones that suffer from complexity, taking a lot of time to be performed, and incapability of determining the real-time concentration of N2H4 [23]. In fact, electrochemical approaches have provided opportunities for N2H4 determination that are cost-effective, sensitive, fast, and portable [24]. However, oxidation of hydrazine electrochemically because of the sluggish kinetics at the surface of the bare electrode was occurred in a high oxidation over-potential [25].

Hence, numerous studies addressed the elimination of the above challenge by chemical modification of surface of electrode using a novel group of materials, including metals and metal oxides [26, 27]. Today, the ability of modified electrodes to reduce over-potential and enhancing the oxidation currents has been proven [4, 28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47].

In this context, efforts have been made towards the development of different types of chemically modified electrodes based on nanostructure materials: Pt nanoparticles (NPs) supported on a nano porous gold electrode, reduced graphene oxide-cobalt oxide nano cube@gold nanocomposite, NiCo2O4 nano rod, carbon dots-Cu2O/CuO, Co3O4/N-doped carbon nanocomposites, ZrO2 NPs, NiCo2S4 porous sphere, Pt-Cu@silicon, Co-CeO2 nanoflake, multilayered reduced graphene oxide/gold–palladium nanohybrid, Ag@C core–shell and poly (alizarin yellow R), Material of Institute Lavoisier-53(Fe), cobalt NPs intercalated nitrogen-doped mesoporous carbon nanosheet networks [47], etc.

Sensors have wide applications in various fields: gas detecting [48], ammonia sensors [49], H2O2 or nitrite detection [50], filed of piezoresistive pressure sensors [51], nitrogen oxide detection [52], in the field of medical care [53, 54], glyphosate detection [55], etc.

More recently, Co3O4 and La2O3 NPs in the field of microwave absorption were also attracted wide attention [56, 57]. Therefore, with respect to these reports, the modified carbon paste electrode using La2O3–Co3O4 nanocomposite and ionic liquid, should be have potential applications in the microwave absorption.

As far as we know, there was no report on the hydrazine oxidation with the modified La2O3–Co3O4 nanoparticles-ionic liquid CPE (LaCoILCPE). The aim of this research is preparing of the LaCoILCPE as the novel electrode and assesses of its function for N2H4 determination. Then, applicability of the LaCoILCPE for voltammetric determination of hydrazine in the real samples was assessed.

2 Experimental

2.1 Devices and chemicals

Devices utilized in this research are similar to the devices reported in the previous article [58].

Hydrazine hydrate (N2H4·H2O, 80%), Na2HPO4 and NaH2PO4 bought from Sigma Aldrich Company. Moreover, we employed Na2HPO4 and NaH2PO4 for preparing phosphate buffer (PB) supporting electrolyte solution. In addition, all chemical compounds used in this work was analytical grade. Notably, deionized water was used for preparation of all solutions.

2.2 La2O3–Co3O4 nanocomposite synthesis

According to the research design, at first La2O3–Co3O4 nanocomposite was synthesized according to the previous report [58]. Briefly, we dissolved 0.46 mol of cobalt acetate in a beaker containing 80 mL of deionized water into a reaction bath, and 0.18 mol of thiourea was added to it with stirring and lastly 19.76 mL of ammonia was added to this solution, and five minutes shake. In the next step, the bath temperature enhanced up to 80 °C to form precipitate. This mixture put aside overnight, and then filtered. The filtrate was washed with ethanol and dried using an oven at 90 °C for 4 h. The obtained powder was Co3O4 NPs.

Then, 50 mL deionized water was given in a beaker and 0.2 mol of lanthanum nitrate, 0.1 g of Co3O4 NPs was added to it. In the following, NaOH solution (1.5 M) were poured dropwise to this solution until the pH of the solution reached to 10. Then, temperature of the solution was increased up to 80 °C and stirred for 1 h. The final product (La2O3–Co3O4 nanocomposite) was centrifuged at 3000 rpm for fifteen minutes, and washed with deionized water many times and ultimately vacuum-dried at a temperature of 60 °C overnight.

2.3 Preparation process of the electrode

In order to prepare the modified electrode, 0.9 g graphite powder with 0.1 g of La2O3–Co3O4 was mixed into a mortar. Then, 0.5 mL of paraffin and 0.2 mL ionic liquid was poured and again mixed for fifteen minutes. In the next step, the resultant paste entered into a glass tube (10 cm long and 3.4 mm i.d.) and packaged. In the following, a copper wire was inserted into the paste to make an electrical connection. After that, the excess paste pressed out of the glass tube and for obtaining the renewable surface a weighing paper was used to polish it. Furthermore, we applied a similar approach for preparing the La2O3–Co3O4 nanocomposite CPE (LaCoCPE) without ionic liquid, ionic liquid paste electrode (ILCPE) without the La2O3–Co3O4 nanocomposite, as well as the bare CPE without ionic liquid and the La2O3–Co3O4 nanocomposite for comparison.

2.4 Preparation process of the real samples

The water samples were collected from different places and used a 0.45 µm filter to filter them. In the next step, 10 mL of each water sample was transported into a 50 mL volumetric flask and diluted to the mark with PBS (pH of 7.0). Then, a certain volume of 100.0 μM hydrazine standard solution was poured into each sample before performing filtration for preparing the spiked samples.

3 Results and discussion

3.1 Structure and morphology of the La2O3–Co3O4 nanocomposite

The morphology of the La2O3–Co3O4 nanocomposite was examined via SEM image (Fig. 1) and revealed that the nano-Co3O4 has layered morphology consisting of nanosheets as reported by another researcher [56]. On the other hands, the nano-La2O3 displaying ball-like particles, with diameters of 20–1000 nm as reported by another researcher [59]. Therefore, as can be seen in Fig. 1, the La2O3 NPs was formed on the surface of the Co3O4 NPs.

Fig. 1
figure 1

The SEM image of La2O3–Co3O4 nanocomposite

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Figure 2 represents FT-IR spectra of the La2O3–Co3O4 nanocomposite (400–4000 cm−1). This spectrum showed two strong vibrational band at ~ 520 and 449 cm−1, which is attributed to metal–O vibration. Wide absorption peak appearing at nearly 3417 cm−1 could be assigned to symmetric vibration of –OH groups of the absorbed H2O molecules.

Fig. 2
figure 2

The FT-IR image of La2O3–Co3O4 nanocomposite

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Figure 3 demonstrates outputs of XRD for the synthesized La2O3–Co3O4 nanocomposite. As shown in the figure, each major diffraction peak in the spectra represents a pure phase for La2O3–Co3O4 nanocomposite and is completely consistent with the sample standard peaks. The diffraction peaks at 31.7°, 36.1°, 44.5°, 59.9° and 64.9° corresponding to (220), (311), (400), (511) and (440) planes of Co for Co3O4 NPs (Fig. 3) corresponded to values from JCPDS 74–2120 [60].

Fig. 3
figure 3

The XRD pattern of the La2O3–Co3O4 nanocomposite

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Moreover, the diffraction peaks at 15.1°, 28.5°, 29.1°, 39.8°, 49.1° and 55.2° corresponding to (100), (002), (101), (102), (211) and (201) plane of La2O3 corresponded to values from JCPDS 41–4019.

3.2 Electrochemical behavior of hydrazine at the LaCoILCPE surface

The obtained CVs for electrochemical oxidation of 50.0 μM hydrazine at the bare CPE, the LaCoCPE, the ILCPE, and the LaCoILCPE are presented in Figs. 4a, 4b, 4c and 4d, respectively.

Fig. 4
figure 4

The cyclic voltammograms of the unmodified CPE (a), the LaCoCPE (b), the ILCPE (c) and the LaCoILCPE (d) in the presence of 50.0 μM hydrazine into 0.1 M PBS (pH of 7.0). Scan rate equaled 50 mV/s in all cases

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As illustrated in the figure, the anodic oxidation potential of hydrazine at the surface of the LaCoILCPE and the bare CPE is 810 mV and 1060 mV, respectively, which reflects the capability of the La2O3–Co3O4 nanocomposite as well as IL as one of the proper mediators. Compared to the bare CPE, the anodic oxidation potential of hydrazine at the LaCoILCPE switched around 250 mV to negative values. Nonetheless, as compared to the bare CPE, the anodic peak current (curve d) considerably increased, which can be attributed to the probable electrocatalytic impacts of the LaCoILCPE on hydrazine.

In the next step, optimizing the solution pH would be crucial to achieve electrocatalytic oxidation of hydrazine. Therefore, the dependence of electrochemical activity of hydrazine on the pH value of the aqueous solution was also assessed. For this purpose, we used CV to investigate electrochemical activity of hydrazine into 0.1 M PBS with different pH-values (3.0 < pH < 9.0) at the LaCoILCPE surface. The results showed that the electrochemical oxidation of hydrazine at the LaCoILCPE surface in neutral conditions is better than acidic or alkaline conditions (Fig. 5). Therefore, pH of 7.0 was chosen as optimum pH for electrochemical oxidation of hydrazine at the LaCoILCPE surface.

Fig. 5
figure 5

The plot of Ip vs. pH

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3.3 Effects of the scan rate

The impact of the potential scan rates on the oxidation current of hydrazine was second parameter that was assessed (Fig. 6) and observed that with increase of the scan rate, the peak current was also enhanced. Due to the linear dependence of the anodic peak current (Ip) against the square root of the scanning rate (υ1/2), it can be concluded that oxidation process is diffusion-controlled.

Fig. 6
figure 6

CVs of the LaCoILCPE into 0.1 M PBS (pH of 7.0) containing 50.0 µM hydrazine at different scan rates. Notably, 1 to 9 are corresponding to 10, 25, 50, 75, 100, 200, 300, 400 and 500 mV s−1. Inset: Variations in the anodic peak current versus the square root of the scan rates

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In the following, using the data of the CV recorded at a scan rate of 10 mV s−1, the TOEFL curve was plotted (Fig. 7). This section of voltammogram that is called Tafel region would be influenced by the electron transfer kinetic between substrate (hydrazine) and the LaCoILCPE. The Tafel slope is 0.1084 that completely match with the involvement of 1 electron in the rate-determining step of the electrode [61], considering the charge transfer coefficient, α = 0.46 for hydrazine.

Fig. 7
figure 7

A Tafel plot extracted from CV of the LaCoILCPE into 0.1 M PBS (pH of 7.0) containing 50.0 µM hydrazine at a scan rate equal to 10 mV s−1

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3.4 Chronoamperometric measurement

In the next step, to perform the chronoamperometric measurements of hydrazine at the surface of the LaCoILCPE, the working electrode potential was adjusted at 900 mV versus Ag/AgCl/KCl (3.0 M) (Fig. 8). The current produced from electrochemical reaction at the mass transport limited condition for electroactive material (in this case: hydrazine), that having a diffusion coefficient of D can be ascribed by Cottrell equation [61]:

$$I = nFAD^{1/2} C_{b} \pi^{ - 1/2} t^{ - 1/2}$$
(1)
Fig. 8
figure 8

Chronoamperograms observed at the LaCoILCPE into 0.1 M PBS (pH of 7.0) containing different concentrations of hydrazine. It is notable that 1 to 4 are corresponding to 0.1, 0.4, 0.9 and 1.5 mM of hydrazine. Inset: The plot of I versus t−1/2 obtained from the chronoamperograms 1–4 (a), the slope of straight lines against concentration of hydrazine (b)

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here Cb and D imply the bulk concentration (mM) and diffusion coefficient (cm2 s−1). The chronoamperometric measurements of hydrazine are performed in diverse concentrations of hydrazine (0.1 M of PBS at pH of 7.0). Then, the plots of I versus t−1/2 were plotted (Fig. 8a). In the following, the slopes of the resultant straight lines versus (Fig. 8b) concentration of hydrazine were plotted. Based on the final slope and the Cottrell equation, the mean value of D for hydrazine was calculated as 5.0 × 10–5 cm2 s−1 which is consistent with the reported results by another researchers [62, 63].

3.5 Calibration plot and detection limit

It is possible to use electrooxidation peak current of hydrazine at the LaCoILCPE surface for measurement of hydrazine in solution. Since DPV has a higher sensitivity compared to other quantitative methods, therefore, this method was used to investigate the linear range of the method. To perform this test, the LaCoILCPE was placed in a series of hydrazine solutions with different concentrations and peak current was measured. According to the findings (Fig. 9), the oxidation current of hydrazine at the LaCoILCPE surface has a linear dependence on the hydrazine concentration (at range 1.0 × 10–7–1.2 × 10–4 M), with a correlation coefficient of 0.9983. Finally based on 3 s, a detection limit of 0.1 × 10–7 M for hydrazine was calculated.

Fig. 9
figure 9

The DPV of the LaCoILCPE into 0.1 M PBS (pH of 7.0) containing different concentrations of hydrazine (0.1, 5.0, 10.0, 20.0, 30.0, 40.0, 50.0, 60.0, 70.0, 80.0, 90.0, 100.0 and 120.0 μM). Inset: The peak current plot versus concentration of hydrazine in ranges from 0.1–120.0 μM

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Table 1 compares analytical function of the LaCoILCPE with the other modified electrodes [9, 30, 33, 36, 45, 47, 62, 64,65,66,67,68]. As seen in the table, the sensitivity, the detection limit and the linear range obtained using the LaCoILCPE is in the range of many reports or even better. As can be seen in Table 1, the LaCoILCPE sensor shows lower detection limit and higher sensitivity than others. The high sensitivity may be ascribed to the high porous structure of the La2O3–Co3O4 nanocomposite, which facilitates the transport of electroactive molecules. The low detection limit may be the result of the synergistic effect of the La2O3 NPs, the Co3O4 NPs and ionic liquid.

Table 1 The comparison of the effectiveness of some approaches employed to detect hydrazine
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3.6 Reproducibility and stability

To evaluate the reproducibility of the LaCoILCPE sensor, DPV was used. The calculated RSD for ten measurements of 50 μM of hydrazine was 2.9%, which demonstrates reasonable reproducibility of the LaCoILCPE sensor. For assessment of stability of the LaCoILCPE, a modified electrode was storage for 21 days, and then used for hydrazine determination. The obtained data showed that current response of the LaCoILCPE to the hydrazine after 21 days was 96.1% of its initial value, which illustrates a very good storage stability.

3.7 Interference effect

The interference effect is a very important factor in the analytical chemistry. For this research, we determined the impact of some possible interfering species on the detection of 50.0 μM hydrazine by DPV. Results (Fig. 10) were shown that tenfold KCl, NaCl, MgCl2, CaCl2, Na2CO3, Uric acid, Glucose, Saccarose and Ascorbic acid had no interference in detecting hydrazine, and the variation in the current caused by the interference species was less than ± 5%. Therefore, it can be concluded that, the LaCoILCPE enjoys an acceptable selectivity to detect hydrazine.

Fig. 10
figure 10

The bar chart shows the current response of the LaCoILCPE to 50 µM of hydrazine coexisting in the solution with 0.5 mM of each interfering species (KCl, NaCl, MgCl2, CaCl2, Na2CO3, Uric acid, Glucose, Saccarose and Ascorbic acid)

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3.8 Real-sample analysis

For this step, the LaCoILCPE was employed to measure hydrazine in different water samples. After the sample pretreatment (Sect. 2.4), hydrazine was determined by DPV. Since, the real samples were assessed and are without hydrazine, therefore a known concentration of hydrazine was added into the samples and was determined by DPV. Table 2 gives the outputs and showed that the recoveries of this new method are in the range of 96.0–103.0%. Moreover, RSD values (n = 5) are <  ± 3.4%, demonstrating hydrazine detection in the water samples using the LaCoILCPE is highly reliable.

Table 2 Utilization of the LaCoILCPE to detect hydrazine in the water samples (n = 5)
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4 Conclusion

As mentioned earlier, we applied the La2O3–Co3O4 nanocomposite as well as an ionic liquid as the modifier for modification of carbon paste electrode, and designed a new sensor providing a sensitive approach to detect hydrazine. This new approach is simplified, portable, and economical to measure the concentration of hydrazine in different water samples with an acceptable analytical function. As a result of the good combination of La2O3–Co3O4 nanocomposite and ionic liquid, this sensor shows dramatic electrochemical activities for hydrazine oxidation.