Ultrasensitive and selective hydrazine sensor development based on Sn/ZnO nanoparticles

Abstract

Tin-doped zinc oxide nanoparticles (Sn/ZnO NPs) were prepared by a facile wet-chemical method using reducing agents in alkaline medium. The Sn/ZnO NPs were characterized by UV/vis, FT-IR, energy-dispersive X-ray spectroscopy (XEDS), X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The Sn/ZnO NPs were deposited onto a flat glassy carbon electrode (GCE) with conducting binders (5% nafion) to result in a sensor that has a fast response towards selected hydrazine compounds by electrochemical approaches. Features including ultra-sensitivity, lower-detection limit, reliability, reproducibility, ease of integration, long-term stability, selective, and enhanced electrochemical performances were investigated in detail. The calibration plot is linear over different concentration ranges (2.0 nM to 20.0 mM). The sensitivity and detection limit were calculated as 5.0108 μA cm⁻² μM⁻¹ and 18.95 ± 0.02 pM (at a signal-to-noise-ratio, SNR of 3) respectively. Finally, the efficiency of the proposed chemical sensors can be applied and effectively utilized for the detection of toxic hazardous chemicals for the safety of the green environment on a broad scale.

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1. Introduction

Hydrazine hydrate (hydrazine monohydrate; N₂H₄·H₂O) is an inorganic compound, whose derivatives are used for a range of different applications in chemical industries, such as pharmaceutical intermediates, corrosion inhibitors, antioxidants, catalysts, emulsifiers, pesticides, insecticides, herbicides, rocket fuel, fuel cells, dyestuffs, and explosives.^1–5 However, hydrazine is a toxic and carcinogenic pollutant according to the Environmental Protection Agency (EPA) and the National Institute for Occupational Safety and Health (NIOSH), and it contaminates the environment. A high-level exposure of hydrazine affected the human central nervous system, nose, throat, eyes irritation, lungs, liver, and kidneys.^6–10 Several methods were already reported for the determination of hydrazine with various analytical techniques such as gas chromatography, coulometry, electrochemistry, fluorescent, and chemi-luminescence.^11–17 For the detection of ultra-level concentration of environmental pollutants efficiently, nanostructure materials have superior properties as compared to their bulk substances, such as mechanical strength, thermal stability, catalytic activity, electrical conductivity, magnetic properties, and optical properties. Development of chemical sensors based on semiconductor metal oxides, nanocomposite has involved a major study for the detection of various toxic chemicals by considering their catalytic and structural properties.^18–23 It is also extensively utilized as antioxidants, rocket fuels, catalysts, emulsifier, corrosion inhibitor, reducing agents, pesticides, and plant-growth regulators.²⁴ Hydrazine is one of the most common chemical used in industrial and environmental, which is not entirely safe due to its toxic effects in environment and human. It is a neurotoxin, which generate carcinogenic and mutagenic effects causing the damage of lungs, liver, and kidneys, respiratory tract infection as well as long-term diseases in the central nervous system.²⁵ In pharmaceuticals, hydrazine has much consequence as it has been already acknowledged as carcinogenic and hepatotoxic chemical which affects the liver and brain.^26,27 Electro-oxidation of hydrazine is used on the basis of development of fuel-cell due to the high capacity which exhibits no contamination. In addition to this, hydrazine is widely used as high-energy propellants in rockets and spacecrafts by military and aerospace industries.²⁸ Owing to abovementioned applications in industry, environment and pharmacology, it is very important to fabricate a reliable, cheap and effective technique for the efficient detection of hydrazine. Among several detection methodologies, the electrochemical I–V method offers an opportunity for portable, cheap and rapid detection of hydrazine. Therefore, numerous chemically modified electrodes based on different nanostructure materials, semiconductor doped or undoped nanomaterials, transition metal oxides, electrocatalytic moieties have been developed for the detection of hydrazine and reported elsewhere in literatures.²⁹ Recently, scientists have demonstrated the use of semiconductor nanostructures as electron mediators to modify the electrodes for the electrochemical detection of toxic hydrazine as well as other chemicals.³⁰ Zinc oxide (ZnO) is a multifunctional material because of their unique physical and chemical properties that developed the numerous chemical and physical applications. Industries like rubber, pharmaceutical, cosmetic, textile, electronic, and electro-technology were used zinc oxide to prepare many of their products, besides the uses as photocatalysis.^31–35 Doped zinc oxides with different metal and non-metal were enhanced the photocatalytic activity due to the lower band-gap energy of ZnO (∼3.37 eV), which can be lowered by maximizing the valence band, minimizing the conduction band, or introducing mid band-gap energy levels.^36,37 On the other hand, doping also result in high surface-to-volume ratio that causes of crystal defect and initiating charge carrier traps.^38–43 Undoped nanomaterials were also reported elsewhere for various applications such degradation of harmful and toxic organic dyes, decolorization of acids, humidity and ammonia sensors.^44–47

In this approach, Sn/ZnO NPs were synthesized by facile wet-chemical process which displayed structural and morphological transition-metal oxide nanostructures. The Sn/ZnO NPs were exhibited very sensitive recognition and transduction in the chemical interaction to change the electrochemical properties which investigated by reliable I–V method. Finally, Sn/ZnO NPs were fabricated to make a simple, reliable and efficient chemical sensor onto side-polished GCE surfaces and executed the chemical sensing performances with selective hydrazine at room conditions. To best of our knowledge, this is the first report for highly sensitive and selective detection of hydrazine with Sn/ZnO NPs using simple and reliable I–V method in short response time.

2. Experimental sections

2.1. Materials and method

Tin(II) chloride dihydrate (SnCl₂·2H₂O), sodium hydroxide (NaOH), disodium phosphate, monosodium phosphate, and all other chemicals used were of analytical grade and purchased from Sigma-Aldrich Company. The dried Sn/ZnO NPs were investigated with UV/visible spectroscopy (Lamda-950, PerkinElmer, Germany). FT-IR spectra were measured for the sample Sn/ZnO NPs with a spectrophotometer (Spectrum-100 FT-IR) in the mid-IR range, which obtained from PerkinElmer, Germany. The XPS measurement of Sn/ZnO NPs was measured on a Thermo Scientific K-Alpha KA1066 spectrometer (http://www.thermoscientific.com). A monochromatic AlKα1 X-ray radiation source was used as excitation sources and the beam-spot size kept in 300.0 μm. The spectra were recorded in the fixed analyzer transmission mode, where pass-energy kept at 200.0 eV. The scanning of the spectra was performed at pressures less 10⁻⁸ Torr. The powder X-ray diffraction (XRD) prototypes was evaluated with an X-ray diffractometer (XRD, X'Pert Explorer, PANalytical diffractometer) prepared with CuKα1 radiation (λ = 1.5406 nm) using a generator voltage of 40.0 kV and current of 35.0 mA applied for the measurement. The morphology of Sn/ZnO NPs was examined on FE-SEM instrument (FESEM, JSM-7600F, Japan). Elemental analysis was investigated using EDS from JEOL, Japan. The size of the Sn/ZnO nanoparticles was determined using transmission electron microscopy (TEM; JEM-2100F, Japan). The TEM sample was prepared as follows: the synthesized Sn/ZnO NPs were dispersed into ethanol under ultrasonic vibration for 2 min, and then the TEM film was dipped in the solution and dried for investigation. I–V technique was employed with an Electrometer (Keithley, 6517A, Electrometer, USA). In I–V system, two electrodes were used working and counter electrodes connected directly in the electrometer. The current was measured against the applied potential of fabricated Sn/ZnO NPs sensor for selective hydrazine detection.

2.2. Preparation of Sn/ZnO nanoparticles

The reactions were carried out in 250.0 ml Erlenmeyer flasks. A volume of 50.0 ml of 0.1 M SnCl₂·2H₂O solution was added to a 50.0 ml of 0.1 M ZnCl₂ solution which is (1 : 1) ratio by volume. The pH slowly was adjusted using sodium hydroxide 2.0 M solution drop wise to approximately pH = 10.0. The solution in the flask was kept under stirring and heating at around 80.0 °C for 6 h. It was washed thoroughly with acetone and water then kept for drying at room condition. The as-grown products were dried in the furnace at 60.0 °C for 24 h. The final product was characterized in detail in terms of their morphological, structural, optical, and chemical properties. The following reactions (1)–(4) were summarized the formation of metal oxide materials,

NaOH_(s) → Na⁺_(aq) + OH⁻_(aq) (1)

SnCl₂·2H₂O_(s) → Sn²⁺_(aq) + 2Cl⁻_(aq) + 2H₂O_(l) (2)

ZnCl_2(s) → Zn²⁺_(aq) + 2Cl⁻_(aq) (3)

4Na⁺_(aq) + 4OH⁻_(aq) + 4Cl⁻_(aq) + Sn²⁺_(aq) + Zn²⁺_(aq) → 4NaCl_(aq) + Zn(OH)_2(aq) + Sn(OH)_2(aq) (4)

Zn(OH)_2(aq) + Sn(OH)_2(aq) → Sn/ZnO_(s)↓ + 2H₂O_(l) (5)

During preparation, pH plays a major role in the Sn/ZnO nanoparticles formation. At a particular pH, when SnCl₂·2H₂O and ZnCl₂ are hydrolyzed with reducing agent (NaOH), unstable tin- and zinc hydroxide formed instantly according to the eqn (5). During the whole synthesis route, NaOH operates a pH buffer to control the pH value of the solution and slow contribute of OH⁻ ions. When the concentrations of the Zn²⁺ and OH⁻ ions are reached above in critical value, the precipitation of Zn(OH)₂ nuclei begin to start. As there is high concentration of Sn²⁺ ion in the reaction system beside OH⁻ ions, the nucleation of Zn(OH)₂ crystals become easier due to the lower activation energy barrier of heterogeneous nucleation. However, as the concentration of Sn²⁺ existences, a number of larger materials with an aggregated precipitation form among the materials as Sn/ZnO.^46,47 In nanoparticles growth method, initially Sn/ZnO nucleus growth was taken place by self- and mutual-aggregation, which nano-crystal then re-aggregates and formed aggregated Sn/ZnO nanocrystal using Ostwald-ripening method. Nanocrystal crystallizes and re-aggregates with each other counter parts through vander-Waals forces and reforms Sn/ZnO spherical morphology, which is presented in Scheme 1. The calcined doped products were characterized in detail in terms of their morphological, structural, optical properties, elemental and applied for Hyd detection by reliable I–V method.

Scheme 1 Schematic representation of growth mechanism of Sn/ZnO nanoparticles by facile wet-chemical process.

2.3. Fabrication of glassy carbon electrode with Sn/ZnO NPs

Phosphate buffer solution (PBS, 0.1 M, pH 7.0) was prepared by mixing 0.2 M Na₂HPO₄ and 0.2 M NaH₂PO₄ solution in 100.0 ml of deionized water. The glassy carbon electrode (GCE, surface area ∼ 0.0316 cm²) was fabricated by using Sn/ZnO NPs with conducting coating binders (5% nafion solution in ethanol). Then it was transferred into the oven at 50.0 °C for 2 h until the film uniformed and dried. An electrochemical cell was constructed with Sn/ZnO NPs coated GCE as a working electrode, and Pd wire used as a counter electrode. Hydrazine (20.0 mM; stock solution) was diluted at different concentrations in DI water and used as a target chemical. The amount of 0.1 M PBS was kept constant in the beaker at 10.0 ml throughout the chemical analysis. The analyte solution (25.0 μl) was dropped in 10.0 ml PBS solution systematically with lower to higher concentrations of hydrazine solution (2.0 nM to 20.0 mM). The sensitivity was calculated from the slope of voltage versus current from the calibration plot. An electrometer was used as a voltage source for the I–V method in the two-electrode system.

3. Results and discussions

3.1. Characterization of Sn/ZnO nanoparticles

FTIR study was performed in the range 400 to 4000 cm⁻¹ for the identification of the functional group attached on the surface of the Sn/ZnO NPs. Generally, FTIR spectroscopy is used extensively in the structural and functional determination of compounds or molecules. Here, Fig. 1 was shown the FTIR spectra of Sn/ZnO NPs. It was exhibited the spectrum of Sn/ZnO NPs at 760, 1120, 1624, 3064, and 3712 cm⁻¹ corresponding to Sn–O & Zn–O stretching indicates the formation of metal oxygen bond, C–O stretching, C=O stretching, and two O–H band stretching vibration, respectively.^5,48,49 So, the evidence for the formation of Sn–O & Zn–O group (peak at 760 cm⁻¹) in the Sn/ZnO NPs was investigated by FTIR spectroscopy.

Fig. 1 FT-IR spectrum for Sn/ZnO NPs.

The optical absorption spectra of Sn/ZnO NPs are accomplished by UV-visible spectrophotometer in the visible range (400.0–800.0 nm). Here, the absorption spectrum in the visible range in Fig. 2a was shown a broad absorption band around ∼350.0 nm. This absorption coefficient value is related with optical band-gap energy in accordance with the expression α = (hν − E_g)^1/2, where h is the Planck's constant and ν is the frequency of incident photon. Intercept on the energy axis is obtained by extrapolating the linear portion of the Tauc's plot (αhν)² vs. hν, as presented in Fig. 2b. It is measured the band-gap E_g of values of ∼2.8 eV, where band-gap energy is calculated in lower value in Sn/ZnO NPs compared to blanks of zinc oxide and tin oxide.^5,43

Fig. 2 Comparison of (a) UV/visible spectra and (b) band-gap energy (E_g) between Sn/ZnO and their individual single compositions.

X-ray powder diffraction is employed to measure the Sn/ZnO nanoparticles phases by comparing with the standard value of lattice parameters, crystal structures and crystallinity in Joint Committee on Powder Diffraction Standards (JCPDS) of ZnO and SnO₂. XRD spectra in Fig. 3 is revealed that all of the blank ZnO peaks (*) match with the Bragg reflections of the standard hexagonal phase structure of space group P6₃mc (186), a = b = 3.249 Å, c = 5.207 Å, (wurtzite, JCSPD 36-1451), for the diffraction planes at (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202). The blank tin oxide is matched SnO₂ peaks (#) for [JCPDS no. 41-1445] rutile phase of SnO₂ of tetragonal structure at reflections planes of (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), and (321). In comparison of the two blank pattern with the Sn/ZnO spectra it is exhibited the peaks (+) which indicate SnO doping in ZnO structure, and changed of intensities in some peaks.^4,5,50

Fig. 3 Typical XRD spectra of SnO₂, ZnO, and Sn/ZnO nanoparticles.

XPS measurements were performed for Sn/ZnO NPs to investigate the binding energy of tin, zinc, and oxygen, which confirmed the chemical state of the doped NPs and their depth. An XPS spectrum for binding energy of Sn/ZnO NPs in Fig. 4a was confirmed the presence of SnO. The peaks were appeared at 1025 eV for Zn 2p [Fig. 4b], at 534.4 eV for O 1s peak [Fig. 4c], at 490.2 eV for Sn 3d_5/2 peak, and 499.5 eV for Sn 3d_3/2 peak [Fig. 4d]. The shifting of binding energy could result of Sn-doping, where the values are obtained in good agreement with literatures.^48,50,51 Therefore, it is concluded that the prepared doped metal oxides have spherical particle decorated onto cubic-phases which contained two different elements. Also, this conclusion is reliable with the XRD data significantly in this investigation.

Fig. 4 XPS spectrum of (a) Sn/ZnO NPs, (b) Zn 2p, (c) O 1s, and (d) Sn 3d of acquired with MgKα1 radiations.

FESEM is one of the prominent methods to study the morphology of nanostructure or nanocomposite materials. High resolution FESEM images of Sn/ZnO nanoparticles onto cubes are displayed in Fig. 5. Fig. 5a–c shows typical shapes of as-prepared Sn/ZnO nanostructures at low-to-high magnified images. The average diameter of aggregated Sn/ZnO nanoparticles is calculated as 50.2 nm in the range of 25.0 to 65.0 nm. It is clearly displayed from FESEM that the Sn/ZnO nanoparticles is exhibited in regular white spotted particle onto cube-phase with high-density materials. It is concluded that the doped materials shows the formation of densely spherical particles onto cubic phases.

Fig. 5 FESEM images of Sn/ZnO nanomaterials. (a–c) Low-to-high magnified images.

Further morphological characterization of the Sn/ZnO nanomaterial was carried out and aggregated spherical particle-shaped morphology was found through TEM analysis. As shown in Fig. 6a and b, the TEM image of the Sn/ZnO nanoparticles displayed a particle shape with an average diameter of ∼49.5 nm in the range of 25.0 to 65.0 nm. The TEM observation shows the exact morphology of the Sn/ZnO nanoparticles assembled in spherical particle-shaped morphology, as found in FESEM, and showed full consistency in terms of shape and dimensions. FESEM and TEM were used to determine the morphology and diameter of the Sn/ZnO nanoparticles.

Fig. 6 Morphology and nanoparticle size analysis of Sn/ZnO nanostructure materials by TEM.

X-ray electron dispersive spectroscopy (EDS) gives quantitative presence of all elements in the prepared nanomaterials and nanocomposite samples. The EDS investigation of Sn/ZnO nanoparticles indicates the presence of Sn, Zn, and O compositions. Fig. 7a and b for Sn/ZnO indicates the presence of oxygen (39.89% wt), zinc (28.78% wt), and tin (31.33% wt). No other peak related with any impurity has been detected in the EDS, which confirms that the Sn/ZnO NPs products are composed only with Sn, Zn, and O elements.

Fig. 7 EDS of the Sn/ZnO nanoparticles. (a) FESEM images and (b) elemental analysis of selected area of Sn/ZnO NPs. (inset) Elemental compositions (wt% and at%).

3.2. Applications: detection of Hyd by Sn/ZnO nanoparticles

The potential application of Sn/ZnO NPs as chemical sensors for hydrazine been explored for measuring and detecting hazardous and risky chemical, which are not environmental friendly. The nanocomposite materials in various chemical sensors were reported elsewhere with different materials.^52–56 The Sn/ZnO NPs sensors have several advantages, such as consistency in air, non-toxicity, chemical stability, large-surface area, electrochemical activity, simplicity to assemble or construct, and bio-safe characteristics. As in the case of hydrazine sensors, the main reason is that the current response in the I–V method of Sn/ZnO NPs significantly is changed, when aqueous hydrazine adsorbed as the target analyte. The fabrication process and detection techniques are presented in the schematic diagram (Fig. 8). The fabricated surface of the Sn/ZnO NPs sensor was prepared with conducting coating binders on the CGE surface, which is presented in Fig. 8a. The fabricated electrode was kept in air for 2 h to make it smooth, dry, stable, and with a totally uniform surface. Theoretical I–V signal of the selective chemical sensor is expected with doped thin film as a function of current versus potential for hydrazine, which presented in Fig. 8b. The electrical responses of target hydrazine was investigated by a simple and reliable I–V technique using Sn/ZnO NPs fabricated GCE film, which presented in Fig. 8c. The time holding of the electrometer was set for 1.0 s. A considerable amplification in the current response with applied potential perceptibly is confirmed. The simple, reliable, possible reaction mechanisms are generalized (Fig. 8d) in the presence of hydrazine onto Sn/ZnO NPs sensor surfaces by the I–V method.

Fig. 8 Schematic representation of (a) fabrication of GCE with coating agents, (b) detection I–V method (theoretical), (c) outcomes of I–V experimental results, and (d) proposed reaction mechanisms of aqueous hydrazine detection in presence of Sn/ZnO/GCE nanomaterials.

Fig. 9a displays the current responses for bare GCE and with coating one with Sn/ZnO NPs on working electrode surfaces. The current signal is affected in comparison, which indicates that higher current response of the GCE than the coating one with Sn/ZnO NPs. The response of the resultant cell current (Fig. 9b) is shown before and after adding the target hydrazine analytes.

Fig. 9 I–V responses of (a) bare GCE and NPs/GCE (in 0.1 M PBS system); (b) Sn/ZnO/GCE with and without target hydrazine analytes. Potential range: 0 to +1.5 V.

The Sn/ZnO NPs are implemented for the detection of a group analytes to study the selectivity in presence of various analytes. In Fig. 10a and b, it exhibits the suitability to use Sn/ZnO NPs as a good selective sensor towards hydrazine. The concentration of hydrazine is varied from 20.0 mM to 2.0 nM by adding de-ionized water at various proportions. A significant increased in the current value with applied potential are clearly demonstrated for the fabrication of thin NPs film on flat GCE, which shown in Fig. 10c. It is observed that the current of NPs as a function of hydrazine concentration at room condition is gradually increased from low-to-high value of current followed by higher concentration of target analytes. Fig. 10d shows the repeatability (R1 to R7) testing at 2.0 μM in a good consistent profile for the run set. A calibration curve is plotted (at +0.5 V) from the electrochemical responses in various hydrazine concentration and presented in Fig. 10e. The sensitivity and detection limit was calculated from the calibration curve based on the active surface area of fabricated GCE electrodes. The calibration plot is linear (r² = 0.8763) over the 2.0 nM to 20.0 mM hydrazine concentration ranges. The sensitivity and detection limit is ∼5.0108 μA cm⁻² μM⁻¹ and ∼18.95 ± 0.02 pM (at a signal-to-noise-ratio of 3) respectively. The fabricated hydrazine sensor also exhibits good sensitivity, lower detection limit of 2.0 nM to 0.20 mM, and long-term stability as well as enhanced electrochemical responses towards the target analytes. The response time was approximately 10.0 s for the Sn/ZnO NPs-coated GCE to achieve the saturated steady state current. The prominent sensitivity of hydrazine sensor could attribute to good absorption (porous surfaces fabricated with conducting binders) and adsorption ability (large surface area), high catalytic activity. The sensitivity of tin doped zinc oxide NPs affords high-electron communication features, which enhanced the direct electron communication between the active sites of NPs and sensor electrode surfaces. The modified thin NPs coated GCE had a better reliability as well as stability.^57–59

Fig. 10 Selectivity studied with various analytes using semiconductor Sn/ZnO nanocomposite materials. (a) I–V responses of various analytes; (b) current responses of analytes at +0.5 V (presented in percentage); analyte concentration was taken at 2.0 μM. Potential range: 0 to +1.5 V; delay time: 1.0 s; (c) concentration variations of hydrazine (2.0 nM to 20.0 mM); (d) repeatability runs for 2.0 μM of hydrazine, and (e) calibration plot (at +0.5 V) of Sn/ZnO/GCE.

Hydrazine is converted to nitrogen and hydrogen in complete decomposition reaction and in incomplete decomposition it produces nitrogen and ammonia. In other hand hydrazine in oxidation reduction reaction converted to nitrogen and water, oxygen (dissolved) chemisorbed onto the Sn/ZnO/GCE NPs surfaces, while NPs coated-film electrode is immersed into PBS system. During the chemisorption, the dissolved oxygen is converted to ionic species (such as O₂⁻ and O⁻) by gaining electrons from the conduction band of aggregated NPs this improve and enhance the current responses against potential during the I–V measurement at room conditions (eqn (6) & (7)).^25,60–62 The aqueous hydrazine sensing mechanism of Sn/ZnO/GCE NPs sensors based on the semiconductors oxides is presented, where the oxidation or reduction of the semiconductor oxide itself according to the dissolved O₂ in bulk-solution or surface-air of the surrounding atmosphere.

e⁻(Sn/ZnO/GCE) + O₂ → O₂⁻ (6)

e⁻(Sn/ZnO/GCE) + O₂⁻ → 2O⁻ (7)

These reactions are taken place in bulk-solution or air/liquid interface or surrounding air due to the low carrier concentration, which increased the resistance. Hydrazine sensitivity toward Sn/ZnO/GCE NPs (MO_x) could attribute to the higher oxygen deficiency and defect density leads to increase oxygen adsorption. Larger the amount of oxygen adsorbed on the surface, larger would be the oxidizing capability and faster would be the oxidation of hydrazine. The reactivity of hydrazine would have been very large as compared to other chemical with the surface under similar condition. When hydrazine reacts with the adsorbed O₂ on the doped-surface of the film, it was liberated the free electrons in the conduction band, which presented through the following reactions,

MO_x + N₂H_4ad + 4O⁻ → M(N₂H_4(OX))_ad + 4OH⁻ (8)

M(N₂H_4(OX))_ad + 4OH⁻ → N₂ + 4H₂O + 4e⁻ (9)

These reactions are corresponded to oxidation of the reducing carriers. These processes are increased the carrier concentration and consequently reduced the resistance on exposure to reducing liquids/analytes. At the room conditions, the exposure of Sn/ZnO NPs surface is to reduce the liquid/analytes results where surface mediated combustion process. The elimination of iono-sorbed oxygen is enhanced the electron density as well as the surface conductance of the surface film. Here, the reducing analyte hydrazine is donated electrons to Sn/ZnO NPs surface. Therefore, resistance is decreases as well as conductance is increases. This cause the analyte response (current response) increases with increasing applied potential, which presented in the Fig. 11. Thus it is produced electrons supply to rapid enhance in conductance of the large surface area of Sn/ZnO NPs. The unusual regions of Sn/ZnO NPs are dispersed on the surface would enhance the ability of material to absorb more oxygen species giving lower resistances.

Fig. 11 Mechanism of Sn/ZnO/GCE NPs chemical sensors at ambient conditions. (a) Before and (b and c) after injecting target hydrazine analytes in PBS system.

The sensor response time was around 10 s for the Sn/ZnO/GCE NPs to reach saturated steady state current. The higher sensitivity of coated-film is attributed to the good absorption (porous surfaces fabricated with coating), adsorption ability, high catalytic activity, and good biocompatibility of the Sn/ZnO nanomaterials. The estimated sensitivity of the fabricated sensor is relatively higher for hydrazine based on other composite or materials modified electrodes. Due to large surface area, the nanomaterials are provided a favorable nano-environment for the chemical detection with good quantity. The higher sensitivity of Sn/ZnO/GCE composite is provided high electron communication features which improved the direct electron transfer between the active sites of Sn/ZnO materials and GCE. The modified thin-film had a good stability due to high specific surface area, where the nanomaterials of Sn/ZnO imparted a favorable environment for the aqueous hydrazine detection (by adsorption) with large quantity. As for the nanomaterials, Sn/ZnO is provided a path to a new generation of chemical sensors, but a premeditate effort has to be expended for doped nanostructures to be taken critically for large-scale applications, and to achieving high applicable density with accessibility to individual sensors. Here, Table 1 shows some selected applications of nanomaterials or nanocomposites for sensing hydrazine compounds by various electrochemical approaches.

Table 1 Selected electroanalytical approaches used for sensing hydrazine performances with various nanomaterials

Sensing layer	Analyte	Transduction	Performances	Ref.
TiO2/CNT	Hyd	Amperometric	Sensitivity: 0.001497 μA cm^−2 μM^−1, DL: 0.22 μM, LDR: 0.35–162 μM, linearity, r^2 = 0.993	20
Ag/ZIF-8	Hyd	Amperometric	Sensitivity: 0.05446 μA cm^−2 μM^−1, DL: 1.57 μM (at SNR of 3), LDR: 6 to 5000 μM, linearity, r^2 = 0.998	23
CNT powder microelectrode	Hyd	CV	Sensitivity: 0.9944 μA μM^−1 cm^−2	63
Ag–ZnO nanoellipsoids	Hyd	CV	Sensitivity: 0.0946 mA μM^−1 cm^−2, DL: 0.07 nM, LDR: 0.07–1.0 μM	64
MWCNT and chlorogenic acid	Hyd	CV	Sensitivity: 0.0041 μA μM^−1 cm^−2, DL: 8.0 nM	65
Hierarchical micro/nanoarchitectures of ZnO	Hyd	CV	Sensitivity: 0.51 μA μM^−1 cm^−2, DL: 0.25 nM, LDR: 0.8–200 μM	66
Pristine ZnO NRs array	Hyd	CV	Sensitivity: 0.0448 mA μM^−1 cm^−2, DL: 0.2 nM	67
ZnO-II/Au	Hyd	CV	Sensitivity: 1.6 μA μM^−1 cm^−2, DL: 0.066 nM, LDR: 0.066–425 μM	68
ZnO/SWCNT	Hyd	CV	Sensitivity: 0.1 μA μM^−1 cm^−2, DL: 0.17 nM, DL: 0.5–50 μM	69
ZnO nanoflowers	Hyd	CV	Sensitivity: 0.0349 mA μM^−1 cm^−2, DL: 0.18 nM	70
Nano-Au ZnO-MWCNT	Hyd	CV	Sensitivity: 0.0428 μA μM^−1 cm^−2, DL: 0.15 nM, LDR: 0.5–1800 μM	71
Sn/ZnO NPs	Hyd	I–V method	Sensitivity: 5.0108 μA cm^−2 μM^−1, DL: 18.95 pM (at SNR of 3), LDR: 2.0 nM to 0.2 mM, linearity, r^2 = 0.8763, response time: 10 s	This work

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4. Conclusion

Finally, we have successfully fabricated hydrazine chemical sensor based on low-dimensional Sn/ZnO NPs immobilized flat GCE with conducting binders. Sn/ZnO NPs are prepared by wet-chemical method with reducing agents in aqueous alkaline system, which represents a simple, convenient, and economical approach. This hydrazine is sensor studied by simple I–V method at room conditions and investigated the analytical performances thoroughly in terms of sensitivity, detection limit, response time, selectivity, and storage stability as well as reproducibility. The present work provided extensive research activities that convened on the synthesis, characterization and hydrazine sensing application of Sn/ZnO NPs by GCE. The finally, the proposed I–V method provided reasonable and reliable results for a selective determination of hydrazine as a hazardous material. Thus, the method may play an important role for using it as an effective approach for a selective detection of hydrazine using Sn/ZnO/GCE NPs. This novel approach is introduced a well-organized route of efficient chemical sensor development for the detection of hydrazine chemicals in environmental fields in broad scales.

Acknowledgements

Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Saudi Arabia is highly acknowledged for the financial and instrumental facilities.

References

1. J. Ding, S. Zhu, T. Zhu, W. Sun, Q. Li, G. Wei and Z. Su, RSC Adv., 2015, 5, 22935.
2. S. Ameen, M. Shaheer Akhtar and H. S. Shin, Talanta, 2012, 100, 377.
3. S. K. Mehta, K. Singh, A. Umar, G. R. Chaudhary and S. Singh, Electrochim. Acta, 2012, 69, 128.
4. S. Kumar, G. Bhanjana, N. Dilbaghi and A. Umar, Ceram. Int., 2015, 41, 3101.
5. P. Malik, M. Srivastava, R. Verma, M. Kumar, D. Kumar and J. Singh, Mater. Sci. Eng. C, 2016, 58, 432.
6. M. Abdul Aziz and A. N. Kawde, Talanta, 2013, 115, 214.
7. X. Gu, X. Li, S. Wu, J. Shi, G. Jiang, G. Jiang and S. Tian, RSC Adv., 2016, 6, 8070.
8. I. Tiwari, M. Gupta, P. Sinha and C. E. Banks, Mater. Res. Bull., 2014, 60, 166.
9. A. D. Arulraj, M. Vijayan and V. S. Vasantha, Spectrochim. Acta, Part A, 2015, 148, 355.
10. S. Amlathe and V. K. Gupta, Analyst, 1988, 113, 1481.
11. Y. Liu, I. Schmeltz and D. Hoffmann, Anal. Chem., 1974, 46, 885.
12. A. Fogg, A. Chamsi, A. Barros and J. Cabral, Analyst, 1984, 109, 901.
13. Y. Tan, J. Yu, J. Gao, Y. Cui, Y. Yang and G. Qian, Dye. Pigm., 2013, 99, 966.
14. A. Safavi and M. A. Karimi, Talanta, 2002, 58, 785.
15. S. Reja, N. Gupta, V. Bhalla, D. Kaur, S. Arora and M. Kumar, Sens. Actuators, B, 2016, 222, 923.
16. Y. Ma, H. Li, R. Wang, H. Wang, W. Lv and S. Ji, J. Power Sources, 2015, 289, 22.
17. A. Benvidi, S. Jahanbani, A. Akbari and H. Zare, J. Electroanal. Chem., 2015, 758, 68.
18. S. P. Kim and H. C. Choi, Sens. Actuators, B, 2014, 207, 424.
19. S. Mutyala and J. Mathiyarasu, Sens. Actuators, B, 2015, 210, 692.
20. X. Yue, W. Yang, M. Xu, X. Liu and J. Jia, Talanta, 2015, 144, 1296.
21. A. Samadi-Maybodi, S. Ghasemi and H. Ghaffari-Rad, Sens. Actuators, B, 2015, 220, 627.
22. A. Umar, M. M. Rahman and Y.-B. Hahn, J. Nanosci. Nanotechnol., 2009, 9, 4686.
23. A. Umar, M. M. Rahman, S. H. Kim and Y. B. Hahn, Chem. Commun., 2008, 7345, 166.
24. H. R. Zare and N. Nasirizadeh, Electrochim. Acta, 2007, 52, 4153.
25. E. H. Vernot, J. D. MacEwen, R. H. Bruner, C. C. Haun, E. R. Kinkead, D. E. Prentice, A. Hall, R. E. Schmidt, R. L. Eason, G. B. Hubbard and J. T. Young, Fundam. Appl. Toxicol., 1985, 5, 1050.
26. K. Yamada, K. Yasuda, N. Fujiwara, Z. Siroma, H. Tanaka, Y. Miyazaki and T. Kobayashi, Electrochem. Commun., 2003, 5, 892.
27. S. V Guerra, L. T. Kubota, C. R. Xavier and S. Nakagaki, Anal. Sci., 1999, 15, 1231.
28. J. Zhang, W. Gao, M. Dou, F. Wang, J. Liu, Z. Li and J. Ji, Analyst, 2015, 140, 1686.
29. A. Kołodziejczak-Radzimska and T. Jesionowski, Materials, 2014, 7, 2833.
30. S. Sabir, M. Arshad and S. K. Chaudhari, Sci. World J., 2014, 2014, 1.
31. A. Sirelkhatim, S. Mahmud, A. Seeni, N. H. M. Kaus, L. C. Ann, S. K. M. Bakhori, H. Hasan and D. Mohamad, Nano-Micro Lett., 2015, 7, 219.
32. K. M. Lee, C. W. Lai, K. S. Ngai and J. C. Juan, Water Res., 2016, 88, 428.
33. A. Hatamie, A. Khan, M. Golabi, A. P. F. Turner, V. Beni, W. C. Mak, A. Sadollahkhani, H. Alnoor, B. Zargar, S. Bano, O. Nur and M. Willander, Langmuir, 2015, 31, 10913.
34. Z. Wang, Y. Liu, B. Huang, Y. Dai, Z. Lou, G. Wang, X. Zhang and X. Qin, Phys. Chem. Chem. Phys., 2014, 16, 2758.
35. J. Coronado, Design of advanced photocatalytic materials for energy and environmental applications, Springer, London, 2013.
36. K. C. Barick, S. Singh, M. Aslam and D. Bahadur, Microporous Mesoporous Mater., 2010, 134, 195.
37. G. Thennarasu and A. Sivasamy, Powder Technol., 2013, 250, 1.
38. Y. Peng, S. Qin, W.-S. Wang and A.-W. Xu, CrystEngComm, 2013, 15, 6518.
39. S. Chakma and V. S. Moholkar, Ultrason. Sonochem., 2015, 22, 287.
40. Y. Yang, Y. Li, L. Zhu, H. He, L. Hu, J. Huang, F. Hu, B. He and Z. Ye, Nanoscale, 2013, 5, 10461.
41. M. Samadi, M. Zirak, A. Naseri, E. Khorashadizade and A. Moshfegh, Thin Solid Films, 2016 DOI:10.1016/j.tsf.2015.12.064 .
42. R. Lamba, A. Umar, S. K. Mehta and S. K. Kansal, J. Alloys Compd., 2015, 653, 327.
43. N. Md Sin, M. Mamat, M. Malek and M. Rusop, Appl. Nanosci., 2014, 4, 829.
44. S. Sinha, Sens. Actuators, B, 2015, 219, 192.
45. N. Modirshahla, A. Hassani, M. A. Behnajady and R. Rahbarfam, Desalination, 2011, 271, 187.
46. X. Li, P. Sun, T. Yang, J. Zhao, Z. Wang, W. Wang, Y. Liu, G. Lu and Y. Du, CrystEngComm, 2013, 15, 2949.
47. V. Galstyan, E. Comini, C. Baratto, A. Ponzoni, E. Bontempi, M. Brisotto, G. Faglia and G. Sberveglieri, CrystEngComm, 2013, 15, 2881.
48. V. Vinoth, J. Wu, A. Asiri, T. Lana-Villarreal, P. Bonete and S. Anandan, Ultrason. Sonochem., 2016, 29, 205.
49. N. Tripathi and S. Rath, Mater. Charact., 2013, 86, 263.
50. Y. G. Zhu, Y. Wang, J. Xie, G. S. Cao, T. J. Zhu, X. Zhao and H. Y. Yang, Electrochim. Acta, 2015, 154, 338.
51. X. Deng, Q. Zhang, E. Zhou, C. Ji, J. Huang, M. Shao, M. Ding and X. Xu, J. Alloys Compd., 2015, 649, 1124.
52. C. Karuppiah, S. Palanisamy, S. M. Chen, S. K. Ramaraj and P. Periakaruppan, Electrochim. Acta, 2014, 139, 157.
53. P. K. Rastogi, V. Ganesan and S. Krishnamoorthi, Electrochim. Acta, 2014, 125, 593.
54. J. Zhao, J. Liu, S. Tricard, L. Wang, Y. Liang, L. Cao, J. Fang and W. Shen, Electrochim. Acta, 2015, 171, 121.
55. S. Shukla, S. Chaudhary, A. Umar, G. R. Chaudhary and S. K. Mehta, Sens. Actuators, B, 2014, 196, 231.
56. M. Mazloum-Ardakani, A. Khoshroo and L. Hosseinzadeh, Sens. Actuators, B, 2015, 214, 132.
57. M. M. Rahman, G. Gruner, M. S. Al-Ghamdi, M. A. Daous, S. B. Khan and A. M. Asiri, Chem. Cent. J., 2013, 7, 60.
58. M. M. Rahman, S. B. Khan, G. Gruner, M. S. Al-Ghamdi, M. A. Daous and A. M. Asiri, Electrochim. Acta, 2013, 103, 143.
59. M. M. Rahman, A. Jamal, S. B. Khan and M. Faisal, ACS Appl. Mater. Interfaces, 2011, 3, 1346.
60. T. Sakamoto, D. Matsumura, K. Asazawa, U. Martinez, A. Serov, K. Artyushkova, P. Atanassov, K. Tamura, Y. Nishihata and H. Tanaka, Electrochim. Acta, 2015, 163, 116.
61. K. V. Manukyan, A. Cross, S. Rouvimov, J. Miller, A. S. Mukasyan and E. E. Wolf, Appl. Catal., A, 2014, 476, 47.
62. L. Tamasauskaite-Tamasiunaite, J. Rakauskas, A. Balčiunaite, A. Zabielaite, J. Vaičiuniene, A. Selskis, R. Juškenas, V. Pakštas and E. Norkus, J. Power Sources, 2014, 272, 362.
63. Y. Di Zhao, W. De Zhang, H. Chen and Q. M. Luo, Talanta, 2002, 58, 529.
64. R. Kumar, D. Rana, A. Umar, P. Sharma, S. Chauhan and M. S. Chauhan, Talanta, 2015, 137, 204.
65. A. Salimi, L. Miranzadeh and R. Hallaj, Talanta, 2008, 75, 147.
66. Y. Ni, J. Zhu, L. Zhang and J. Hong, CrystEngComm, 2010, 12, 2213.
67. J. Liu, Y. Li, J. Jiang and X. Huang, Dalton Trans., 2010, 39, 8693.
68. W. Sultana, S. Ghosh and B. Eraiah, Electroanalysis, 2012, 24, 1869.
69. K. N. Han, C. A. Li, M.-P. N. Bui, X.-H. Pham and G. H. Seong, Chem. Commun., 2011, 47, 938.
70. B. Fang, C. Zhang, W. Zhang and G. Wang, Electrochim. Acta, 2009, 55, 178.
71. C. Zhang, G. Wang, Y. Ji, M. Liu, Y. Feng, Z. Zhang and B. Fang, Sens. Actuators, B, 2010, 150, 247.

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