Electrochemical determination of hydrazine by using MoS2 nanostructure modified gold electrode

In this paper, MoS2 nanostructure was synthesized by using ammonium molybdate and thiourea as precursors through annealing in a tube furnace. The nanostructure was characterized for morphological, structural and elemental composition by using a field emission scanning electron microscope (FESEM), powder X-ray diffraction and energy-dispersive X-ray spectroscopy (EDS). The as-synthesized nanostructure was then immobilized on the gold electrode (working electrode) for the electrochemical detection of hydrazine. Cyclic voltammogram shows an intense peak at 22 μA, which proved the high electrocatalytic ability of the sensor. The strong electrocatalytic activity regarding the oxidation of hydrazine is ascribed to good electron transfer ability and high surface area of the nanoparticles. Further, the chronoamperometric study was conducted to estimate the sensitivity and the detection limit of the sensor. The sensor exhibited a detection limit and sensitivity of 196 nM and 5.71 μA/μM cm2 respectively. Promising results suchas highelectrical conductivity, lower detection limit and high sensitivity of the as-synthesized MoS2 nanostructure have proved its potential towards the electrochemical detection of hydrazine.


Introduction
Rapid industrial growth and the overpopulation of developing countries have posed a great threat to sustainable development. In the last decade, environmental pollution has increased many folds which substantially divert the focus of current research towards remediation of environmental pollution. Among these, water pollution has adversely affected the ecosystem and human health. Therefore, it is our prime focus to address water pollution to fulfill the clean water requirement worldwide [1]. After scrutinizing the major water polluting sources, it has been found that industrial waste, which is discarded directly into the aquatic system has created havoc [2]. These waste contaminants such as chemical and pesticides are generally stable and highly soluble in water as a result persist in the environment for a longer time and hence pollute the freshwater [3].
Among different water pollutants, hydrazine is one of the major contributors. It finds application in a wide range of sectors such as catalysis [4], emulsifier [5], high-power rocket propellant [6], reagent in industrial and agriculture sector [7], corrosion inhibitor in boilers [8], fuel cells [9] and a starting material in the production of different herbicides, insecticides [10], pesticides [11] and pharmaceutical compounds. Furthermore, it is also utilized as a precursor in the production of colorants and foaming agents [12]. It is classified under Group 2B carcinogen by the United States Environmental Protection Agency [13]. Adverse health impacts associated with exposure to a high quantity of hydrazine are nausea, headache, seizures, dizziness, irritation in the respiratory tract, abnormalities in blood and dysfunctioning of the central nervous system, brain and liver [14,15]. Its wide applicability in different fields in spite of its ill effects causes a threat to the living organism. Therefore, to avoid the health impacts of hydrazine it is the need of the hour to sense it below its permissible limits. Different techniques, which have already been reported in the literature regarding the sensing of hydrazine are flow injection with chemiluminescent detection [16], gas chromatography [17], high-performance liquid chromatography (HPLC) [18], absorption and emission spectrophotometry [19,20]. These methods require a long time for analysis, sophisti-cated instrumentation and complicated process regarding the preparation of the sample. Therefore, the electrochemical methods with advantageous properties such as accuracy, simplicity, cost-effectiveness and high sensitivity, are considered as the best alternative to these methods [21]. The mechanism associated with the electrochemical detection of hydrazine includes its oxidation on the electrode's surface. Generally, a modified working electrode is preferable over a bare electrode due to the slow kinetics associated with the latter. The key requirement regarding the modification of the working electrode is relatively low overpotential for the oxidation of hydrazine and high current response [22,23]. Although, noble metal-based materials such as gold [24], silver [25] and platinum [26] show high conductivity the high cost and scarcity limit their applications. Different 2D materials such as hexagonal boron nitride (h-BN) [27,28], black phosphorous (BP) [29], bismuthine [30], antimonene [31], borophene [32], mxene [33], graphene [34,35], transition metal carbides [36], transition metal oxide [37], pyridyl-based organochalcogen [38] have emerged as most promising after the discovery of graphene due to their wide applicability in the area of material science, condensed matter physics and engineering [39,40]. However, different limitations such as zero or low bandgap of graphene, BP and large bandgap of h-BN limit their use in the electronics applications. On the other side, transition metal dichalcogenides (TMDs), a new class of 2D materials with significant potential in different fields such as optoelectronics, electronics [41] and energy storage applications [42] has gained tremendous attention. These are atomically thin semiconductors of MX 2 type where M represents the transition metals such as molybdenum and tungsten and X refers to the sulfur atoms. These nanomaterials possess high structural stability owning to the covalent bond between the sulfur layer and the transition metals. Moreover, different layers of these materials are stacked through weak Van der Waals forces and the distance that exists between the molybdenum sheets is 0.65 nm [43]. Weak Van der Waals forces allow easy exfoliation of these materials through mechanical and chemical means. Generally, a bulk form of these material shows the properties of silicon-like semiconductors and possess an indirect bandgap (1.23 eV), while the direct bandgap of 1.89 eV is displayed by the monolayers of TMDs. The direct bandgap of TMD sheets is due to the confinement of electrons in the single plane [44]. Moreover, MoS 2 exhibits the tunable bandgap and different methods for adjusting its bandgap are chemical modification [45], applying compressive strain [46] and implication of vertical electrical field [47]. These materials also play an efficient role in chemical and biosensing. Due to these properties of MoS 2 , it has become the focus of researchers for electro-chemical applications. Moreover, literature has also proven the superiority of nanomaterials of TMDs over bulk MoS 2 regarding electrochemical performance.
Inspiring from the above facts, we have employed MoS 2 nanostructure for the electrochemical detection of hydrazine. The nanostructure was prepared through the thermal annealing method in a tube furnace. The flat 2D material is employed for the modification of gold electrodes, where it is proved to be an excellent electron mediator for the electrochemical oxidation of hydrazine. This work paves a way to design new 2D materials which provide more active sites on the edge and surface.

Chemicals required
Ammonium molybdate tetrahydrate, (<99% purity) was taken from Sigma Aldrich. Thiourea (<97% purity), bought from CDH, was used as a source of sulfur. Hydrazine was supplied by Sigma Aldrich and used as received. A stock solution of hydrazine (1.0 mM) in 0.1 M phosphate buffer solution was prepared before being used in the experiment and the waste was discarded after the completion of the experiment. Deionized water used in the experiment was prepared in a double distillation unit.

Synthetic procedure of MoS 2 nanostructure
The MoS 2 nanostructure was prepared through the thermal annealing method using ammonium molybdate tetrahydrate and thiourea as starting materials. In brief, 200 mg of ammonium molybdate tetrahydrate and 1.0 g of thiourea were dissolved in 50 ml of distilled water separately. Then thiourea solution was mixed with ammonium molybdate under constant stirring. The solution mixture was then sonicated for thirty minutes and dried in the oven at 50 ∘ C after stirring on a magnetic stirrer. The resultant mixture was heated at 500 ∘ C for five hours under a nitrogen atmosphere leads to the formation of black colored fine powder.

Characterization
To investigate the morphology of the MoS 2 nanoparticles, field emission scanning electron microscopy (FESEM) was employed. FESEM images were recorded by using FESEM JEOL JSM-7610F (Tokyo, JAPAN). Further, the energy disper-sive spectroscopy (EDS) was employed to investigate the elemental and chemical compositions of the as-prepared nanomaterials by using OXFORD EDS, attached with FE-SEM. The crystallinity and crystal phases were studied by an X-ray diffractometer (XRD) (D8 Discover, Bruker AXS, Karlsruhe, Germany). The XRD spectra were measured with Cu-Kα radiation (k = 1.54056 Å) at 45 kV and 40 mA in the range of 2θ = 10-80 ∘ with a scan speed of 8 ∘ /min. The electrochemical characterization of the nanomaterial was performed at Autolab Type-III cyclic voltammeter.

Electrode preparation for the electrochemical sensing of hydrazine
In the first step, the gold electrode (working electrode) was polished with alumina slurry and activated through the ultrasonication method in a mixed solution of deionized water and ethanol. The electrode was then washed thoroughly to remove a trace amount of alumina. Then the electrode was left to dry in an oven at 60 ∘ C. In the second step, the MoS 2 nanostructure was immobilized onto the surface of the gold electrode (Au electrode). Before fabrication of the electrode, a slurry of the nanostructure of MoS 2 was prepared by adding butyl carbitol acetate (BCA). A 5.0 µL of the prepared slurry was then cast over the gold electrode, followed by dropping the solution of Nafion. After that, the modified Au electrode was subjected to a temperature of 60 ∘ C under a nitrogen atmosphere. The electrochemical study was preceded by Autolab Type-III cyclic voltammeter. Three electrodes-based systems was there in cyclic voltammeter where, in addition to the working electrode, counter (Pt wire) and reference electrodes (Ag/AgCl (sat. KCl) were also present. A 0.1 M Phosphate buffer solution (pH = 7) was also used in the experiment. Additionally, amperometric studies were performed to study the effect of the concentration of hydrazine on the current.

Characterizations of MoS 2 nanostructure
FESEM technique was employed for analysing the surface features of the as-synthesized MoS 2 nanostructure and the images are shown in Figure 1. Figure 1(a) and 1(b) represents the typical low and high magnification FESEM, which confirm the formation of flat sheet type nanostructure. Additionally, crumpled and flaky nanostructure over the nanosheet of MoS 2 was also observed, where different  Figure 1(c). Welldefined peaks in the EDS spectra were referred to as the molybdenum (Mo) and sulfur (S) atoms. Further, any other peak corresponding to other impurity elements was not found in the EDS spectra, which confirms the formation of pure MoS 2 .
The crystallinity and crystal phase associated with the as-synthesized MoS 2 nanostructure was analysed by using XRD studies. Figure 1(d) displays the XRD pattern of the MoS 2 nanostructure. Series of well-defined diffraction reflections at 2θ values 16.6, 33.2 and 58.8 ∘ are found corresponding to (002), (100) and (110) planes. The reflection plane (002) corresponds to a d-spacing of about 0.62 nm and the peak broadening suggested that the material is layered nanostructure. A Slight shift to the lower angle side has also been observed in comparison to JCPDS card 01-075-1539 for hexagonal molybdenum sulfide, which indicates a slight widening of the basal plane [48].

Detection of hydrazine by using MoS 2 modified Au electrode
Electrocatalytic activity of the as-synthesized MoS 2 nanocomposite towards hydrazine was assessed by cyclic voltammetry (CV). The experiment was run in 0.1 M phosphate buffer solution (PBS) at pH = 7. Figure 2 displayed The absence of cathodic peak shows the irreversible nature, which is due to the excellent electrocatalytic effect regarding the detection of hydrazine. Hydrazine donates electrons to MoS 2 nanoparticles and increases the conductivity of the latter, hence improving the electrocatalytic activity by facilitating the charge transfer kinetics. Hydrazine, as a reducing agent, is increasing the intra-particle conductivity of the MoS 2 nanoparticles. Based on the above results, it can be concluded that MoS 2 nanomaterial is suitable for electrocatalytic and sensing applications. Electrochemical properties of the MoS 2 modified Au electrode have been further investigated by observing its current response at different potential sweeps ranging from 100 to 1000 mV/sec with a successive increment of 100 mV/s (Figure 3). It was inferred from the peak that the anodic current peak increased in a linear pattern (Figure 4) with the increased scan rate. Additionally, a slight shift in peak potential toward the positive side has also been observed. The positive intercept of Figure 4 distinctly reveals that the process of oxidation is diffusion-controlled and ruled out any possibility of adsorption of hydrazine molecules on the electrode surface [49].
Further, the number of electrons that participated in the hydrazine's oxidation was estimated through the Randles-Sevcik equation [50] Ip = (︁ 2.99 × 10 5 )︁ n 3/2 A D 1/2 ν 1/2 C where Ip = peak current, A = surface area of the electrode (cm 2 ), n = sum of electrons that participated in redox cycle, ν = potential of scan rate (V/s), D = diffusion coefficient (cm 2 /s) and C = the amount of N 2 H 4 in bulk solution (mol cm −3 ). On the basis of the above equation, the number of electrons estimated to participate in the reaction was two and the suggested chemical reaction was [50] The main purpose of this study is to explore the reliability or the efficiency of the MoS 2 as a sensor for the detection of hydrazine. Chronoamperometry study was performed to record the current response of MoS 2 modified gold electrode with the change in hydrazine concentration at a constant potential of 0.27 V. The transient in current with the alteration in the concentration (0.1 to 1.0 µM) of hydrazine  was recorded ( Figure 5). The current response of the hydrazine displayed a linear response with the increasing amount of hydrazine. Further, different analytical properties such as limit of detection and sensitivity were calculated from the calibration curve ( Figure 5). The detection limit found for MoS 2 regarding the detection of hydrazine was 196 nM. The value of correlation coefficient (R) calculated was 0.991 for the MoS 2 modified Au electrode. A sensitivity of 5.71 µA/µM cm 2 for MoS 2 was obtained after applying the equation i.e., the slope of calibration curve/area of the electrode [51].
Further, the detection limit of as-synthesized MoS 2 nanostructure was compared with the already reported sensors. Table 1 depicts the comparison of different nanomaterials used for the sensing of hydrazine in terms of the limit of detection.

Selectivity study of MoS 2 nanostructure
Further, as synthesized MoS 2 nanostructure was tested for the interference of various other potential ions (Na + , Ca 2+ , K + , NO 3− , SO 2− 4 ) and electro-active species (glucose and urea), through chronoamperometric studies. In this study, the current response of these species (concentration 10-fold) was recorded at the same condition. No interference of these competing ions and electro-active species was recorded. Therefore, it can be concluded that MoS 2 based sensors displayed high selectivity toward the detection of hydrazine.

Conclusion
This study represents the sensing of hydrazine through MoS 2 nanostructure. The MoS 2 nanostructure is synthesized through the thermal annealing method. The presence of sulfur atoms in the MoS 2 nanoparticles acts as a catalyst for the oxidation of hydrazine on its surface due to its electron-donating ability. MoS 2 modified gold electrode exhibited an excellent electrocatalytic response toward the sensing of hydrazine with the low limit of detection (196 nM) and high sensitivity (5.71 µA/µM cm 2 ). Based on the above result, it can be concluded that MoS 2 nanoparticles have a high electrocatalytic ability and can be utilized as a sensor for sensing environmental pollutants.

Conflict of interest:
There are no conflicts to declare.
Author contributions: DS is involved at every stage of compilation starting from identifying the problem, compilation of manuscript and internal revision. DSR has collected and compiled all the literature, drawn the figures and written the manuscript under the supervision of DS and NG. ST has assisted the internal revision of the manuscript.
Ethical approval: The conducted research is not related to either human or animal use.