Aug 04, 2023

Enhanced electrocatalytic activity of fluorine doped tin oxide (FTO) by trimetallic spinel ZnMnFeO4/CoMnFeO4 nanoparticles as a hydrazine electrochemical sensor

Scientific Reports volume 13, Article number: 12188 (2023) Cite this article

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In the present study, ZnMnFeO4 and CoMnFeO4 tri-metallic spinel oxide nanoparticles (NPs) were provided using hydrothermal methods. The nanoparticles have been characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and electrochemical techniques. A reliable and reproducible electrochemical sensor based on ZnMnFeO4/CoMnFeO4/FTO was fabricated for rapid detection and highly sensitive determination of hydrazine by the DPV technique. It is observed that the modified electrode causes a sharp growth in the oxidation peak current and a decrease in the potential for oxidation, contrary to the bare electrode. The cyclic voltammetry technique showed that there is high electrocatalytic activity and excellent sensitivity in the suggested sensor for hydrazine oxidation. Under optimal experimental conditions, the DPV method was used for constructing the calibration curve, and a linear range of 1.23 × 10−6 M to 1.8 × 10−4 M with a limit of detection of 0.82 ± 0.09 μM was obtained. The obtained results indicate that ZnMnFeO4/CoMnFeO4/FTO nano sensors exhibit pleasant stability, reproducibility, and repeatability in hydrazine measurements. In addition, the suggested sensor was efficiently employed to ascertain the hydrazine in diverse samples of cigarette tobacco.

The application of metal oxide nanoparticles has recently grown significantly in photocatalytic and sensor applications1. Besides, given the high catalytic action, inexpensiveness, and chemical stability of these materials, many applications of them have been developed in energy2. Transition metal oxide nanoparticles also show great photocatalytic and electrical properties due to their shape, size, and area3,4. Spinel oxides are such materials that contain one or more transition metals in their structure, such as Fe3O45 and MgFe2O46, which are used as electrodes in rechargeable supercapacitors and batteries7,8.

A recent discovery has indicated that trimetallic spinel oxides exhibit enhanced properties in comparison to their monometallic and bimetallic counterparts when employed as electrode materials in lithium-ion batteries. Lavela and colleagues synthesized NiFeMnO4 utilizing a reverse micelle technique and achieved a substantial capacity of approximately 900 mAh/g, as reported in their study9. Stefan et al. synthesized the CoMnFeO4 nanoparticles and reported their superior electrochemical performance compared to several other binary oxides10. Based on a fundamental principle or set of principles, the following statement is made: the trimetallic oxides comprising Co, Fe, and Mn metals have been identified as a potentially effective catalyst for the development of a high-performing Advanced Oxidation Process (AOP) system, as stated previously. Conversely, the inclusion of the Fe oxide constituent will confer exceptional magnetic characteristics upon the catalyst, thereby facilitating its recyclability11. Among these structures, trimetallic spinel oxides, such as CoMnFe2O4, have been neglected, despite the fact that they are indeed likely to have a simple synthesis and morphology12, and given that the response of electrochemical sensors has a major dependence on the morphology and size of electrocatalyst particles and the effective area of the modified electrode, these materials can be considered intriguing and efficient catalysts13.

In this study, CoMnFeO4 and ZnMnFeO4 NPs, as modifiers for measuring hydrazine, were synthesized via the hydrothermal technique and deposited on the FTO glass. The ionic configuration of CoMnFeO4 is analogous to that of CoMnFe2O4, wherein (Fe3+Co2+) [Fe3+Mn3+Mn4+Co2+] O42− is present. The parentheses and brackets indicate the tetrahedral (A site) and octahedral (B site), respectively, while O represents oxygen. This information has been reported in Ref.12:

In addition, it has been observed that under different preparation conditions, such as changes in temperature, the stability of Fe2+, Mn2+ and Mn3+ ions are compromised, leading to their oxidation in the temperature range of 200–450 °C. This ultimately results in the formation of defective spinel ferrite. Furthermore, it has been observed that Mn4+ ions undergo reduction to Mn3+ ions at temperatures exceeding 450 °C14. The ionic configuration of ZnMnFeO4 exhibits a resemblance to that of CoMnFeO4, which was explained.

Hydrazine is an inorganic compound with the molecular formula N2H4 and is also a colorless liquid with an ammonia-like odor15. It is one of the environmental pollutants and a carcinogenic compound that can enter the body through the skin, lungs, and digestive system16,17. Furthermore, hydrazine is known as the stimulus of the nervous system, the gene mutation agent, and the cause of hematic abnormalities, and it has harmful side effects on the brain1,18. Consequently, the United States Environmental Protection Agency (EPA) has implemented a threshold level of 10 ppb for hydrazine19. However, hydrazine has a variety of applications in different industries, despite its toxicity. For example, it is used as fuel for spacecraft and missiles due to its rapid combustion reactions. Moreover, it is widely used to remove oxygen from steam boilers and to prevent the corrosion of boiler tubes in power plants1,20. More importantly, hydrazine is useful in the agricultural industry, in which tobacco products as well as insecticides are produced21. This compound is also a strong reducing agent17,22. Therefore, the measurement of hydrazine is important in terms of its being widely used and its toxicity, which has led to the appearance of a lot of methods to measure it, including potentiometric and titration23,24, spectrophotometry25, chromatography26, fluorimetry27, chemiluminescence28 and colorimetry29. According to the ability of hydrazine for electrochemical oxidation, electrochemical methods are also used as efficacious methods to measure it30.

Among the advantages of this method, the inexpensiveness, short response time, quick and easy sample preparation, high selectivity and sensitivity, and portability can be explicitly cited20,31,32. Despite the great benefits of this method, the measurement of hydrazine on unmodified electrodes has low sensitivity and great disturbance. By modifying these electrodes with proper materials that can catalyze the oxidation of hydrazine, the hydrazine oxidation over voltage can be diminished, followed by increasing the sensitivity of hydrazine and decreasing the disturbance in its measurement. A proper modifier can increase the efficient surface area of the electrode and the electron transfer velocity and can enhance and reinforce electrochemical reactions as a catalyst. In recent years, the use of nanomaterials with high electrocatalytic stability, such as metal and metal oxide nanoparticles33, conducting polymers34, carbon nano-tubes35 or graphene36, has been introduced for electrochemical measurement of hydrazine.

To measure hydrazine simply, sensitively, accurately, and cheaply, CoMnFeO4 and ZnMnFeO4 were used as modifiers to prepare an electrochemical sensor of hydrazine (Scheme 1). Afterward, the behavior of hydrazine on the modified FTO electrode was electrochemically examined by as-synthesized NPs and a bare FTO electrode. One notable benefit of utilizing FTO as a substrate in this study is the absence of a binder in the nanoparticle deposition process. The nanoparticles were deposited and stabilized on the electrode surface through heating. A temperature of 400° was deemed necessary for the completion of this task. Hence, it is not feasible to employ alternative substrates such as glassy carbon, nickel foam, copper foam, etc., which are unsuitable for employment in this context. One significant limitation of this substrate is its limited reusability, as it is not feasible for utilization beyond three repetitions.

Schematic representation of hydrazine detection using ZnMnFeO4/CoMnFeO4/FTO.

According to the literature, the trimetallic spinel oxide NPs have been used for the first time as a modifier to determine hydrazine, and this is also the first time that the electrocatalytic behavior of ZnMnFeO4 nanoparticles has been investigated. This modified electrode is used to determine the amount of hydrazine in diverse tobacco samples.

The apparatus, reagents, and solutions used in this paper are reported in the Electronic Supplementary Material File.

The process of hydrothermal synthesis of ZnMnFeO4 nanoparticles, which was carried out significantly in this study, was conducted similarly to the synthesis process of CoMnFeO412,37. In 50 mL of distilled water, a particular amount of Mn(NO3)2·4H2O (2 mM), Fe(NO3)3·9H2O (4 mM), and Zn(NO3)2·6H2O (6 mM) were dissolved and stirred by a magnetic stirrer for 30 min to obtain a homogenous solution. Following that, the pH was adjusted to 12.0 by adding the ammonia aqueous solution gradually and dropwise to the solution under stirring, and then the cations' hydroxides were coprecipitated. Next, the obtained mixture was transmitted into a 100-mL Teflon™-lined autoclave and heated at 180 °C for 24 h. After the final product reached ambient temperature, it was separated by a magnet, centrifuged, and washed with distilled water and ethanol several times until a neutral pH was achieved. Then, the product was placed at 70° inside the oven for 12 h and dried completely. Thereafter, the as-synthesized powder was calcined at 600 °C for 3 h.

In the first phase, it’s extremely important to prepare and activate the substrate surface before modifying the electrode. As a consequence, FTO glass was degreased and cleaned by ultrasonication for 30 min, which was implemented in 3 phases, including cleaning in distilled water, acetone, and ethanol, respectively. The time spent on each phase is 10 min. At last, it was allowed to be dried under a flow of nitrogen gas immediately before use. Then, 1.5 mg of synthesized CoMnFeO4 nanoparticles were dispersed in 1 mL of N-methyl pyrrolidone via an ultrasonic bath for 10 min. This suspension (10 µL volume) was drop-cast on a 1 cm × 1 cm FTO glass substrate and dried in an oven at 80 °C for 1 h. Next, it was stabilized and annealed in air at 400 °C for 3 h. In the end, a light tan film was obtained on the FTO substrates. Following this, the same phases were carried out to deposit ZnMnFeO4 on the CoMnFeO4 electrode. After all, the modified electrode, which is indicated as ZnMnFeO4/CoMnFeO4/FTO was rinsed with deionized water.

The spinel ferrites are used in electrochemical sensors because of some features, including their superlative electrical and photoelectrochemical performance, high chemical stability, magnetic properties, low price, and good conductivity. Briefly, the modifier used in the present study causes HZ response enhancement and oxidation potential reduction considerably, which is due to its exceptional conductivity, high adsorption, and high electrochemical surface area of ZnMnFeO4/CoMnFeO4/FTO. By means of the FT-IR spectra presented in Fig. 1, the structural formation and functional groups that are provided in the as-synthesized samples, together with their metal-oxide vibrational modes, are acknowledged explicitly. In the IR spectra of all the ferrite samples, there are generally two metal-oxide bands observed, which both pertain to the nature of octahedral M–O stretching vibration and the nature of tetrahedral M–O stretching vibration38. In the spectrum of ZnMnFeO4 (Fig. 1a), the band with a greater wave number viewed at 584 cm−1 is ascribed to intrinsic stretching vibrations of the metal ions (Zn–O and Fe–O) at the tetrahedral site, while the other band viewed at 456 cm−1 correlates to the bending vibrations of the metal ions (Zn–O, Mn–O, and Fe–O) at the octahedral site38. In the FT-IR spectrum of CoMnFeO4 (Fig. 1b), the absorption band emerges at 589 cm−1 as a result of the intrinsic stretching vibrations of metal ions (Co–O and Fe–O) bonding the tetrahedral site12. The absorption peak that can be seen at 1630 and 3440 cm−1 corresponds to water molecules39. The CoMnFeO4 and ZnMnFeO4 ferrite samples are synthesized by the hydrothermal route, and in Fig. 1c,d, the patterns of X-ray diffraction for these samples can be clearly seen. For both samples, which have proper crystallinity and well-defined diffraction lines, a single-phase spinel structure was perceived without any unfavorable phase. There is no obvious peak detected for both samples. This bears witness to the high purity of the synthesized nanostructures. In Fig. 1c, the Apparent diffraction peaks for synthesized ZnMnFeO4 nanoparticles at planes of (111), (220), (311), (400), (422), (511), and (440) adapted well to the standard pattern reported in JCPDS card no. 01-074-2400. And in Fig. 1d, the relevant peaks of CoMnFeO4 at planes of (220), (311), (222), (400), (422), (511), and (440) in the XRD pattern could be indexed to the standard pattern reported in JCPDS card no. 00-001-1121. Based on the XRD diffractograms of both nanoparticles, it can be seen that every one of the peaks is either all even or all odd. This indicates that the samples are spinel in phase40.

FT-IR spectrum and XRD pattern of synthesized (a,c) ZnMnFeO4 NPs and (b,d) CoMnFeO4 NPs.

The better electrochemical sensor response is predominantly dependent on the electrode surface area, which was utilized in modified electrodes, and the size and morphology of the electrocatalysts. The purpose of the FE-SEM implementation was to investigate the morphological features and particle sizes of ZnMnFeO4 and CoMnFeO4 NPs. The synthesized ZnMnFeO4 sample’s SEM image is shown in Fig. 2a,b. In conformity with this delineation, the size of nanoparticles in these nanostructures tends to be smaller than CoMnFeO4, less than about 35 nm in diameter. The image of CoMnFeO4 nanoparticles less than 45 nm in size can be seen in Fig. 2c,d. According to the images, it can be stated that both synthesized nanoparticles resemble each other in some ways, including morphology, uniform size distribution, and compact arrangement. They can be illustrated as broccoli-bearing, multi-piece platelets that are connected to one object. The composition of the ZnMnFeO4 and CoMnFeO4 NPs electrode surface elements was designated with X-ray energy scattering spectroscopy. Both EDX spectra revealed a peak at 0.51 keV for O Kα. The revealed peak is because of the oxygen atoms in ZnMnFeO4 and CoMnFeO4 nanoparticles. There are three exclusive peaks for the Zn, Mn, and Fe elements in the ZnMnFeO4 graph (Fig. 2e) and also for the Co, Mn, and Fe elements in the CoMnFeO4 graph (Fig. 2f). The presence of desired elements was confirmed by the findings in the prepared compositions, which are uniformly distributed without the appearance of any impurities. Given that, it leads to the conclusion that the synthesis of ZnMnFeO4 and CoMnFeO4 has been efficiently accomplished. As shown in Fig. 2g,h, the morphologies of ZnMnFeO4 and CoMnFeO4 were examined by TEM. The stickled, truncated cubic particles were detectable in both images. The size of its particles for ZnMnFeO4 was less than 35 nm, and for CoMnFeO4, it was less than about 45 nm in diameter.

FE-SEM images and EDX spectra of synthesized (a,b,e) ZnMnFeO4 NPs and (c,d,f) CoMnFeO4 NPs with the scales of 200 nm and 500 nm; (g,h) TEM images of ZnMnFeO4 and CoMnFeO4 nanoparticles, respectively.

In order to modify the FTO substrate, the ZnMnFeO4/CoMnFeO4 NPs have been used as modifiers, and to control the tests, the FTO substrate was also modified with CoMnFeO4 and ZnMnFeO4 and then defined as CoMnFeO4/FTO and ZnMnFeO4/FTO. The electrochemical behavior of the three freshly modified electrodes, including ZnMnFeO4/CoMnFeO4/FTO, CoMnFeO4/FTO, and ZnMnFeO4/FTO, in 0.1 M KCl as a carrier electrolyte and in 5 mM [Fe(CN)6]−3/[Fe(CN)6]−4 as a redox probe at a scan rate of 50 mV s−1, were evaluated by using the cyclic voltammetry technique significantly and were compared to each other (Fig. 3a). According to the results, the modification of electrodes with ZnMnFeO4/CoMnFeO4 created a reduction in the peak-to-peak separation and had higher peak currents compared to the others. In fact, the features of the modified electrode, such as higher electrical conductivity and the large electroactive surface area, make these significant differences between the bare electrode and the ZnMnFeO4/CoMnFeO4/FTO electrode.

(a) Cyclic voltammograms for varied electrodes in 5mM [Fe(CN)6]−3/[Fe(CN)6]−4 with KCl 0.1 M as supporting electrolyte at the scan rate of 50 mV s−1. (a) Bare FTO; (b) CoMnFeO4/FTO; (c) ZnMnFeO4/FTO; (d) ZnMnFeO4/CoMnFeO4/FTO; (b) Electrochemical impedance spectroscopy (EIS) of Bare FTO and the other modified electrodes in 5 mM [Fe(CN)6]−3/[Fe(CN)6]−4 with KCl 0.1 M.

The electrical conductivity and electron transition resistance of the modified electrode surface have been checked by electrochemical impedance spectroscopy. The EIS spectrum record of the bare FTO electrode and the three modified electrodes cited above is shown in Fig. 3b. Studying the process of charge transfer on the surface of the modified electrode is a practical way to do so. The reason is that there is a double-layer capacitance and also a resistance to interfacial charge transfer after the modification on the electrode surface. The EIS Nyquist spectra involve a semicircular region and a linear region. The semicircular diameter in the high-frequency region indicates the resistance of the interface charge transfer (Rct) that is shown in Fig. 3b. The Warburg element represents the diffusion process and is associated with the low-frequency region linear section. The transition resistance of the electrodes is characterized by employing a semicircular diameter in the Nyquist designs.

In Fig. 3b, it can be observed that the resistance to charge transfer for the modified CoMnFeO4/FTO electrode is less than that of the bare electrode. It undoubtedly shows that the CoMnFeO4 nanoparticles act as a stimulus and speed up the interfacial charge transfer. The diameter of the semicircle in the curve corresponding to the ZnMnFeO4/FTO electrode is decreased in proportion to the CoMnFeO4/FTO electrode, which represents the high electrical conductivity of the ZnMnFeO4 compared to the CoMnFeO4 NPs. In the curve related to ZnMnFeO4/CoMnFeO4/FTO, a decrease in charge transfer resistance can be seen after coating the ZnMnFeO4 on the CoMnFeO4/FTO, which is a result of a higher electrochemical active surface area and the enhanced charge transfer rate of ZnMnFeO4/CoMnFeO4/FTO compared to the other modified electrodes. The Warburg element is also seen in the impedance spectrum (Fig. 3b), indicating the electrolyte diffusion into the coating and showing the porosity of the modifier. This is an essential and critical parameter in the properties of catalysts. It indicates that these two NPs together are being used successfully and have a positive effect on each other.

The electrochemically active surface area of the modified electrode was studied, and results were reported in the supplementary file (Fig. S1).

The cyclic voltammetry technique was conducted to assess the electrochemical behavior of hydrazine on manifold-modified electrodes. The cyclic voltammetric response of 0.1 mM hydrazine in the 0.1 M ammonia buffer (pH = 9.0) was recorded in the potential range of − 0.1 to 0.85 V. As reflected in Fig. 4a, the oxidation of hydrazine on a bare FTO substrate demands a high positive potential (0.575 V), which shows a considerably lower peak current (curve a). In the b-curve, the hydrazine oxidation is shifted towards a less positive potential (0.423 V) by modifying the electrode surface with CoMnFeO4 nanoparticles, and the current is increased relative to the bare electrode state (22.0 μA). On the other side, in the c-curve that corresponds to the ZnMnFeO4/FTO electrode, the hydrazine oxidation potential is moderately shifted towards the positive potential (0.431 V) in comparison with the CoMnFeO4/FTO electrode, but the current is increased to about 29.0 μA. It does indicate that the electrodes modified with CoMnFeO4 NPs and ZnMnFeO4 NPs have catalytic properties on hydrazine. In the d-curve, it can be seen that when the electrode is modified with ZnMnFeO4/CoMnFeO4 NPs, there is a shift towards less positive potentials in the oxidation potential of hydrazine and also a sharp peak at potentials less than 0.4 V, about 0.34 V, and the current is increased to about 39.0 μA.

(a) CV responses of 100 µM hydrazine in 0.1 M ammonia buffer with pH = 9 for (a) Bare FTO (b) CoMnFeO4/FTO (c) ZnMnFeO4/FTO (d) ZnMnFeO4/CoMnFeO4/FTO at the scan rate of 50 mV s−1. (b) CV responses of ZnMnFeO4/CoMnFeO4/FTO electrode in phosphate buffer (0.1 M) containing 100 µM hydrazine at various pH from 5.0 to 9.0 (right to left) with the scan rate of 50 mV s−1, (c) Peak potential plot versus pH values, (d) Peak current plot versus pH values (e) checking out the buffer type; (a) Britton–Robinson, (b) Phosphate buffer, (c) Ammonia buffer. (f) CV responses of ZnMnFeO4/CoMnFeO4/FTO electrode in 0.1 M ammonia buffer (pH = 9.0) containing 100 µM hydrazine at various scan rates from 5 to 300 mV s−1 (from a to k). Inset: (g) and (h) diagrams of peak current vs. the square root of scan rates and scan rates, respectively. (i) Logarithm of peak current plot against peak potential, (j) Logarithm of peak current against logarithm of scan rates, (k) Logarithm of peak potential vs. logarithm of peak current.

Given that, the ZnMnFeO4/CoMnFeO4/FTO has a better electrocatalytic property for hydrazine when compared with the other modified electrodes. A stable current was achieved in less than 5 s by adopting a modified electrode. It indicates a rapid electron exchange on the modified electrode surface and satisfactory catalytic performance.

To verify the effect of the electrolyte solution pH value, the 0.1 M phosphate buffer was used in the pH range of 5.0–9.0, including 0.1 mM hydrazine, reaching the greatest current response and the best oxidation potential of the sensor for hydrazine. In Fig. 4b, it is apparent that the peak potential and peak current of the ZnMnFeO4/CoMnFeO4/FTO electrode are highly dependent on the pH of the solution. This is due to the shift of hydrazine oxidation peak potentials to negative potentials with increasing pH of the solution, based on the following Eq. (1):

The slope of − 61.3 mV was obtained from the potential of the Ep–pH diagram, which is close to the theoretical Nernst value (Fig. 4c). Accordingly, it indicates that equal numbers of electrons and protons were engaged in the oxidation reaction of hydrazine (Eq. 2).

The pH value of the supporting electrolyte is a significant parameter for the effective electrocatalytic behavior of hydrazine, and as shown in Fig. 4d, the response of hydrazine increased along with an increasing pH value. It has been suggested that the enhancement of the current response in alkaline solutions is caused by the adsorption of hydrazine to the electrode surface. In this study, pH = 9.0 was chosen as the eligible pH, and indeed, the effect of buffer type on the electrooxidation of hydrazine was studied at the same pH value. Consequently, diverse buffers, namely ammonia, phosphate, and Britton–Robinson with a concentration of 0.1 M, were used to accomplish this. Figure 4e shows that the voltammetric response of the ammonia buffer is superior to that of buffers. The reason is that the hydrazine oxidation peak emerged at the lower potential in contradiction to other buffers and increased the current.

To investigate the kinetic reaction of the hydrazine oxidation and its electron transfer mechanism, the cyclic voltammetry technique was used at 5–300 mV s−1 scan rates by ZnMnFeO4/CoMnFeO4/FTO electrodes in 0.1 M ammonia buffer (pH 9.0) containing 0.1 mM hydrazine (Fig. 4f). According to the results, the peak current increases progressively as the scan rate increases. The diagrams of peak currents (Ip) versus scan rate (ν) and second root of scan rate (ν 1/2) were plotted to perceive and interpret the diffusion or absorption nature of the electrode process. Considering that the peak currents are linearly proportional to the square root of the scan rate, the electrocatalytic oxidation of hydrazine on the ZnMnFeO4/CoMnFeO4/FTO was controlled by a diffusion-controlled process (Fig. 4g,h). As the scan rate increases, the peak potential of hydrazine electrooxidation shifts towards a positive potential. In other words, there are kinetic constraints at high scan rates41.

A Tafel plot was depicted at the scan rate of 5 mV s−1 (Fig. 4i) for further studying the kinetic parameters (α). According to Tafel Eq. (3):

where α is the transfer coefficient, T is the temperature (K), F is the Faraday constant (96,485 C mol−1), R is the universal gas constant (8.314 J K−1 mol−1) and na refers to the number of transferred electrons in determining step, The Tafel plot slope is 6.2156 mV decade−1. On this account, by substituting these values in Eq. (3), the α parameter is obtained at 0.77 assuming na equals 1. The relationship between peak potential (Ep) and the natural logarithm of scan rate (log ν) can be defined by the Laviron equation.

According to this equation and also by using the slope value of the Epa vs. log ν diagram (Fig. 4j) and the α value that was attained to be 0.77 from Eq. (3), the number of electrons engaged in the rate-limiting step of hydrazine (n) was estimated to be 1.

Additionally, a linear relationship between the Epa and Ipa was proposed by depicting the diagram of log Epa versus log Ipa (Fig. 4k) in the scan rates work. Accordingly, the regression equation was: log Epa (V) = 0.6347 log Ipa (μA) − 1.5122 (R2 = 0.9693). The hydrazine oxidation, however, becomes more challenging in high-rate scanning because, by increasing the scan rate, a considerable change occurs with expanding peak currents in the hydrazine oxidation potential displacement towards more anodic potentials.

Based on all previous findings and results, the hydrazine electrochemical oxidation mechanism has been reported to be conducted via a 4-electron process and results in the production of nitrogen gasses by the following equations (Eq. 6)18,42.

The present survey found that the hydrazine oxidation process is an irreversible oxidation process in which hydrazine oxidizes to produce N2 and hydronium ions (H3O+) along with four electrons. In this part, the rate-limiting step is the first step containing one-electron transfer (Eq. 4), followed by the fast second step containing the three-electron transfer process (Eq. 5)34,42:

The chronoamperometry technique was performed in a pH 9.0 ammonia buffer (0.1 M) containing different concentrations of hydrazine (from 15 to 75 µmol L−1) to ascertain the diffusion coefficient of the hydrazine. The potential of the working electrode was set at 0.4 V, and the Cottrell equation was applied (Eq. 7) to define the current response for an electroactive compound that is controlled by a diffusion mechanism:

C and D are the bulk concentration (mol cm−3) and diffusion coefficient (cm2 s−1), respectively; A is the active area of the modified electrode that was obtained to be 1.72 cm2, and I refers to the diffusion current of hydrazine from the bulk solution to the interface of the solution/electrode, and the other signs have their specific conventional meanings. The experimental diagrams that have been plotted for the several hydrazine concentrations are viewed in Fig. 5a. The diagram of I versus t−1/2 can be observed in Fig. 5b. For hydrazine, the average diffusion coefficient was computed by plotting the slope values versus different hydrazine concentrations (Fig. 5c); D = 4.29 × 10−6 (cm2 s−1).

(a) Chronoamperograms achieved from ZnMnFeO4/CoMnFeO4/FTO electrode in the presence of 16, 48, 64 and 80 µM hydrazine (a–d) at the potential of 0.4 V vs. SCE, (b) Peak current of a, b, c and d curves vs. time−1/2 for varied concentrations of hydrazine (16–80 µM); and (c) is slope values of I–t−1/2 plot vs. various concentrations of hydrazine.

To assign the detection limit and the linear range as the critical parameters of an electrochemical nano-sensor, the differential pulse voltammetry procedure was implemented with optimized parameters such as a scan rate of 50 mV s−1, pulse amplitude as 0.05 V, pulse time as 0.05 s, step potential as 0.005 V, quiet time as 3 s, and interval time as 0.5 s, and the DPV curves of ZnMnFeO4/CoMnFeO4/FTO were recorded with different concentrations of hydrazine in 0.1 M ammonia solution (pH 9.0).

Figure 6a shows that as the concentration of hydrazine increases, the electrocatalytic response gets sharper. A linear calibration plot was also attained between the hydrazine concentrations and associated peak current (Ip) as I (μA) = 0.347 [hydrazine] (μA/mM) + 0.021; R2 = 0.997. The linear range was achieved at the concentration range of 1.2–184.7 µM. Sensitivity, LOD, and LOQ are estimated as 0.347 μA mM−1, 0.82 μM and 2.75 μM for hydrazine, respectively, through the slope of the calibration plot (Fig. 6b).

(a) Differential pulse voltammograms (DPV) for varied concentrations of hydrazine (a–p) in 0.1 M ammonia buffer with pH = 9.0 and (b) is the calibration plot of hydrazine with a linear range of 1.23–184.7 µM.

ZnMnFeO4/CoMnFeO4 as a modifier provides an acceptable ambiance for hydrazine indication since it has a high electroactive surface area. It can be noted that the high electron communication features are caused by the high sensitivity of this sensor, which enhances the direct charge transfer between the active area of the modifier and the FTO substrate. The analytical performance of the suggested sensor that has been used to detect hydrazine compared to the electrodes reported earlier is shown in Table 1.

The detection limit of the suggested sensor is better or at least similar to other modifiers of electrodes that have been previously stated in the table. The findings demonstrate that the sensor was adequate and qualified for hydrazine detection.

The concentrations of some anions and cations and several chemicals, including Cl−, Br−, NO3−, SO42−, (CH3CO2)−, Na+, K+, Cu2+, Ni2+, ethanol, citric acid, uric acid, glucose (GL), Lactose, Fructose, and ascorbic acid (AA), were added separately to the ammonia buffer solution (0.1 M, pH 9.0) containing 100 µM hydrazine. This was done to evaluate the selectivity of the ZnMnFeO4/CoMnFeO4/FTO and examine the absence of interference, which is an influential and essential parameter in practical applications. For the maximum concentration of the foreign substances, the limit of tolerance was taken, which produced an approximate error in the analytical response of the analyte of less than ± 5% (Fig. S2). As can be observed in Table 2, the small change in the DPV response of hydrazine is caused by the eightfold ascorbic acid and uric acid, tenfold citric acid, 300-fold glucose, Lactose and Fructose, and 500-fold ethanol, K+, Na+, Cu2+, Ni2+, Br−, Cl−, SO42−, (CH3CO2)−, and NO3−. According to the findings, the suggested sensor has been chosen properly. Therefore, it can be applied to detect hydrazine in the presence of biological molecules and environmental pollutants.

For real samples of hydrazine, selective detection was provided by the suggested sensor in the present study, and it has been shown whether this method is applicable. The hydrazine and derivatives that concurrently exist in tobacco products. To assess the amount of hydrazine, however, the diluted tobacco solution was injected directly into the electrolyte solution. Next, the amount of hydrazine standard solution that had been assessed earlier was added, and then, to detect the concentration of hydrazine, the standard addition method by differential pulse voltammetry (DPV) technique was accurately implemented. In Fig. S3, it can be observed the test of common hydrazine in real samples for three brands of cigarettes (C1–C3) using ZnMnFeO4/CoMnFeO4/FTO. Considering that the findings of the calculated recovery were in the range of 97.77–100.71% with a R.S.D. (for three repetitions) of below 3%, one can deduce that the suggested sensor has a desirable selectivity (Table 3).

The electrode was kept in the air for one month to analyze the modified electrode’s stability. Afterward, required assessments were conducted on the 7th and 30th days. Based on the findings of this study, it can be declared that the modified electrode has 97.25% of its initial current response after 7 days and 95.88% for a month, which has desirable stability in view of its high specific surface area (Fig. S4a). Employing five consecutive tests with the same modified electrode and applying cyclic voltammetry, the modified electrode response repeatability in hydrazine measurement was precisely measured. The R.S.D. of peak currents was estimated at 1.78%, which demonstrates satisfactory and adequate repeatability for the modified electrode (Fig. S4b). In addition, four different electrodes were modified with ZnMnFeO4/CoMnFeO4 for studying the reproducibility of the sensor, the CVs were registered, and the R.S.D. was calculated at 2.05% (Fig. S4c). The findings conclusively reveal that ZnMnFeO4/CoMnFeO4/FTO has good stability as a sensor and also possesses adequate quality of repeatability and reproducibility in hydrazine measurement.

The sensor's low reusability is one of the work's limitations. After four washes, the modified electrode's CV response recovered as much as 98.4%, 94.6%, 91%, and 82.9% of the original response signal (Fig. S4d).

In this study, a new assay for the oxidation of hydrazine was prepared based on the modified FTO electrode. The modified FTO electrode was prepared as ZnMnFeO4/CoMnFeO4/FTO through deposition of the ZnMnFeO4 NPs on the surface of the CoMnFeO4 NPs. The results showed that this modified electrode provides significantly enhanced electrolytic activity with a remarkable decrease in overvoltage and provides better peak current intensity when compared with bare FTO electrodes. To detect hydrazine using the DPV technique, the modified electrode showed high sensitivity and selectivity and a low detection limit. Furthermore, there are several remarkable advantages to the suggested sensor, including long-term stability and repeatability, simple preparation, and low cost.

The interference from current co-existing chemical species, which are present in excess concentration, was tolerated by the examined electrode. In the final analysis, the modified electrode was employed to detect the amount of hydrazine in cigarette samples, and the findings were satisfactory.

The datasets generated during the current study are available from the corresponding author on reasonable request.

Differential pulse voltammetry


Fluorine doped tin oxide


Limit of detection

Limit of quantification

Relative standard deviation


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The authors wish to thank the financial support from the University of Tabriz, Tabriz, Iran.

The authors received financial support from the University of Tabriz, Tabriz, Iran.

Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, 51666-16471, Iran

Jalal Niazi Saei & Karim Asadpour-Zeynali

Pharmaceutical Analysis Research Center, Faculty of Pharmacy, Tabriz University of Medical Sciences, Tabriz, 51664, Iran

Karim Asadpour-Zeynali

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J.N.S. carried out the experiment and wrote the manuscript, K.A.Z. supervised the project.

Correspondence to Karim Asadpour-Zeynali.

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Saei, J.N., Asadpour-Zeynali, K. Enhanced electrocatalytic activity of fluorine doped tin oxide (FTO) by trimetallic spinel ZnMnFeO4/CoMnFeO4 nanoparticles as a hydrazine electrochemical sensor. Sci Rep 13, 12188 (2023).

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Received: 24 March 2023

Accepted: 23 July 2023

Published: 27 July 2023


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