Novel electrochemical sensing platform for detection of hydrazine based on modified screen-printed graphite electrode

The current work aimed to fabricate a screen-printed graphite electrode (SPGE) modified by MnO 2 nanorods (MnO 2 NRs) for sensing hydrazine. Thus, a facile protocol was adopted to construct the MnO 2 nanorods that were subsequently applied to modify the SPGE surface directly. As-synthesized MnO 2 NRs/SPGE sensor exhibited a strong sensing behavior towards the hydrazine, with a large peak current and small oxidation potential. This electrochemical sensor in the optimized conditions to detect the hydrazine possessed a low detection limit (0.02 μM), a broad linear dynamic range (0.05–275.0 μM) and an admirable sensitivity (0.0625 μA μM -1 ). The sensor applicability was practically estimated in real water samples, which revealed successful recovery values.


Introduction
Hydrazine, a vital chemical reagent, has attracted widespread research interest due to its industrial application and poisonousness. It is a robust reducing agent with various applications in the production of pesticides and medicine [1][2][3]. It is also a chemical deoxidizer with broad applications as an oxygen scavenger in water boilers [4]. Hydrazine is a key raw material in the construction of rockets and explosives [5,6]. Although hydrazine plays a great role in human production and life, it is easy to be absorbed by living organisms. The persistent contact with this agent may be associated with complications, such as some disturbances in the reproductive system, central nervous system, liver, kidneys and lungs [7][8][9]. According to the U.S. Environmental Protection Agency (US EPA), hydrazine is positioned in a class of possibly carcinogenic to humans, with a recommended threshold limit value (TLV) of less than 10 ppb [10].
The recent development of electrochemical approaches has been greatly simplified by screenprinted electrodes (SPEs) due to their high sensitivity, functionality and versatility [43][44][45]. The main performance features of a sensor are all gathered in SPEs, including cost-effectiveness, minimal sample preparation, ease of operation, high speed, small size, limited background, and comfortable surface modification. There is always a need for further research to develop electrode materials to improve the selectivity and sensitivity of electrochemical sensors [46][47][48][49][50][51]. Many advances in nanotechnology have been made in diverse fields in recent years , which have led to the introduction of highly efficient sensing platforms. Types of nano-scale materials have so far been identified with distinct physicochemical properties that can be employed in the electrochemical sensors to detect various analytes exhibiting admirable results [76][77][78][79][80][81][82].
A popular oxide material is manganese dioxide (MnO2), whose behavior can be enhanced by changing its morphology and surface area. MnO2 is a polymorph owing to an octahedral [MnO6] spatial arrangement. Nano-sized MnO2 exhibits commendable benefits due to a larger surface-tovolume ratio and further reactive surface for electrochemical reactions. The diverse application of this substance in electrochemistry and sensor fabrication can be attributed to the simple reduction of MnO2 to Mn2O3 and MnO and, at the proper potential, the re-oxidation to MnO2 as a catalytic circle for electrochemical detection [83][84][85][86][87].
The current work aimed to fabricate a new screen-printed graphite electrode (SPGE) supported by MnO2 nanorods (MnO2 NRs/SPGE) for sensitively sensing hydrazine. The sensor applicability was tested in real water samples, the results of which revealed successful recovery values.

Chemicals and instrumentations
All materials with analytical grades applied throughout this work were supplied from Aldrich and Merck. Electrochemical experiments were recorded using a PGSTAT-302N Autolab potentiostat/galvanostat (Eco Chemie, The Netherlands). The control of all experiments was carried out by a General Purpose Electrochemical System (GPES) software. The SPGEs were purchased from DropSens (Spain) and consisted of an Ag pseudo-reference electrode, graphite axillary electrode, and graphite working electrode. All pH values were measured by a digital Metrohm 710 pH meter.

Synthesis of MnO2 nanorods
The MnO2 NRs were obtained by dissolving KMnO4 (0.316 g) in deionized water (30 mL) while vigorously stirring, followed by the addition of 3 M HCl (1.4 mL) under vigorous stirring for another half hour. Then, the solution was placed in a 50-mL Teflon-lined autoclave at 160 °C for six hours. Next, the products were cooled down to room temperature and subsequently centrifuged and thoroughly rinsed with ethanol and deionized water to clean any impurities, followed by drying at 60 °C for 12 h.

Preparation of MnO2 NRs/SPGE
First, 1 mg of prepared MnO2 nanorods was added into an aqueous solution (1 ml), followed by sonication for 30 min to give a homogeneous solution. Then, 4 μL of MnO2 NRs was dispersed on the surface of SPGE dropwise. Following the solvent's evaporation, the sensor's surface was washed several times with deionized water to clean free modifier molecules and subsequently air-dried. The obtained electrode was noted as MnO2 NRs/SPGE.

Results and discussion
Characterization of MnO2 nanorods   Figure 2 shows the application of the cyclic voltammetry (CV) method to evaluate the electrochemical behavior of 200.0 μM hydrazine at different electrodes (unmodified SPGE, and MnO2 NRs/SPGE) in PBS (0.1 M, pH 7.0) at the scan rate of 50 mV/s. Based on the results, there was an oxidation peak on the surfaces of the electrodes, but no reduction peak, highlighting an irreversible electrochemical response of hydrazine on the electrodes. A relatively wide and weak peak current (Ipa) of hydrazine oxidation was found on the unmodified SPGE (at 1000 mV with 3.0 μA), which reveals that the electrochemical oxidation does not happen spontaneously due to high activation overpotential. The hydrazine Ipa on MnO2 NRs/SPGE, when compared with unmodified SPGE, displayed further elevation to 13.0 μA, meaning an increase up to 4.3 times that on the unmodified SPGE. In addition, hydrazine oxidation occurred at a lower potential than unmodified SPGE.

Effect of the scan rate (ʋ) on the results
The influence of various scan rates between 10 and 400 mV/s on the anodic peak currents for hydrazine (100.0 μM) was studied using the MnO2 NRs/SPGE (Figure 3). The regression equation was Ipa (hydrazine) = 1.7036  ½ -4.2319 (R 2 =0.9994) (Figure 3, inset). This result indicates that the oxidation process is controlled by diffusion. Further, there was a shift in the oxidation peak potential of hydrazine toward a more positive potential by increasing the scan rates. To study the rate-determining step as shown in Figure 4, the data of the rising part of the currentvoltage curve obtained at 10 mV/s scan rate were applied to draw a Tafel plot for 100.0 μM of hydrazine. The linearity of the E versus log I plot, implies the intervention of the kinetics of the electrode process. The slope of this plot was utilized to estimate the number of electrons transferred in the rate-determining step. Figure 4 shows the Tafel slope of 0.2153 V for the linear section of the plot, which means the rate-limiting step of one-electron transfer with a transfer coefficient of α = 0.72.  Figure 5. For an electroactive material (hydrazine in this case) with a diffusion coefficient of D, the current observed for the electrochemical reaction at the mass transport limited condition is described by the Cottrell equation [88]. As shown in Figure 5A, I versus t −1/2 plots were used with the optimal fit for various hydrazine concentrations. We drew the slopes from straight lines against different concentrations of hydrazine, see Figure 5B. According to the Cottrell equation and obtained slope, the mean D value was 2.7×10 −5 cm 2 /s. MnO2 NRs/SPGE sensor was used to electrochemically detect different hydrazine concentrations ( Figure 6). A gradual elevation was observed for the peak currents of hydrazine oxidation by raising its concentrations, which means an advanced performance of our sensor in the electrocatalytic oxidation of hydrazine. Oxidation peak currents of hydrazine versus Chydrazine (Figure 6, inset) showed a wide linear range from 0.05 to 275.0 μM. The detection limit (LOD=3σ/S; where σ is the standard deviation of blank response, and S is the slope of the calibration curve with a linear range of concentrations of the analyte) was calculated to be 0.02 μM.

Interference study
The effect of some interference species on the determination of hydrazine was studied. The results show that the interfering effects of glucose, sucrose, urea, uric acid, Na + , Cl -, No3 -, pb 2+ , and Ag + on the anodic peak current of hydrazine is less than 5%. Hence, the MnO2 NRs/SPGE has a superior selectivity for hydrazine.

Analytical application
The detection of hydrazine in the water samples (drinking water and tap water) was performed using MnO2 NRs/SPGE sensor. The concentration values of hydrazine were calculated via the method of standard addition. Attained findings are summarized in Table 1, the recovery is between 96.7 and 102.5 %, and the relative standard deviations are all less than or equal to 3.0%. The experimental results confirmed that the MnO2 NRs/SPGE sensor has great potential for analytical application.

Conclusion
The present work utilized an ultra-facile protocol to construct MnO2 nanorods-modified SPGE (MnO2 NRs/SPGE) for the electrochemical determination of hydrazine. According to CV findings, the as-fabricated sensor exhibited an electrocatalytic performance compared with the unmodified SPGE for the oxidation of hydrazine. The linear current response to the hydrazine level was between 0.05 and 275.0 μM, and the limit of detection was 0.02 μM with a sensitivity of 0.0625 μA μM -1 . The diffusion coefficient for hydrazine using MnO2 NRs/SPGE, 2.7×10 −5 cm 2 s -1 , was obtained. The developed sensor applicability was practically tested to detect the concentrations of hydrazine in real water samples, which revealed successful recovery values.