Fabrication, electrochemical characteristics and electrocatalytic activity of 4-((2-hydroxyphenylimino)methyl)benzene-1,2-diol electrodeposited on a carbon nanotube modified glassy carbon electrode as a hydrazine sensor

Abstract

This paper is a study of the fabrication, electrochemical characteristics, and electrocatalytic activity of 4-((2-hydroxyphenylimino)methyl)benzene-1,2-diol electrodeposited on a multi-walled carbon nanotube modified glassy carbon electrode (HPIMBD–MWCNT-GCE) as a hydrazine sensor. Cyclic voltammetric, chronoamperometric, and amperometric methods were used to probe the fabrication, and characterization of the modified electrode and its role as a sensor in the electrocatalytic determination of hydrazine. The cyclic voltammograms of HPIMBD–MWCNT-GCE show one pair of peaks with surface confined characteristics. The charge transfer coefficient, α, and the charge transfer rate constant, k_s, between electrodeposited HPIMBD and MWCNT-GCE were calculated at various pHs. The results show that the modified electrode exhibited a catalytic activity toward the electrooxidation of hydrazine. The catalytic rate constant of the hydrazine oxidation at the modified electrode surface was determined using cyclic voltammograms recorded at various potential scan rates. Furthermore, amperometry exhibits two linear dynamic ranges of 4.0–32.9 μM and 32.9–750.4 μM and a detection limit of 1.1 μM for hydrazine determination. Finally, the activity of the modified electrode was studied for hydrazine determination in tap water and in auxiliary cooling water of power generation and satisfactory results were obtained.

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1. Introduction

Hydrazine is an important chemical compound with a strong reducing ability and widely used in industrial applications as an emulsifier, catalyst, antioxidant, full cell and oxygen scavenger, reducing agent, and in insecticides, textile dyes, and photography chemicals.^1–4 Also, this material is applied as fuel cells because its high capacity and its electrooxidation do not produce CO₂.⁵ However, hydrazine has been known to be carcinogenic, mutagenic and toxic. This material can cause diseases such as headache, temporary blindness and DNA damage and can be absorbed through skin.^6,7 Therefore, the detection of trace hydrazine has significant importance in environmental and biological analysis.⁸ Unfortunately, the determination of hydrazine on the surface of carbon electrodes has a large overpotential for oxidation, low sensitivity, slow electron transfer, and low stability.⁹ So, in the recent decade, measurements have been directed towards the development of sensitive and selective methods for the detection and determination of hydrazine.¹⁰ In this regard, various complex compounds such as metal hexacyanoferrate,^11,12 cobalt phthalocyanine,¹³ an anthraquinone derivative¹⁴ and catechol derivatives including chlorogenic acid,¹ pyrocatechol violet,² a coumestan derivative,³ hematoxylin,⁴ caffeic acid,¹⁵ catechin hydrate¹⁶ and rutin¹⁷ have been used to construct modified electrodes to reduce the oxidation overpotential of hydrazine. Carbon nanotubes can be used to upgrade electron transfer reactions because these materials possess several unique properties such as good electrical conductivity and chemical inertness.^17–26 Furthermore, carbon nanotubes can be applied as protectors for the immobilization of various electron transfer mediators on electrode surfaces.²⁶

4-((2-Hydroxyphenylimino)methyl)benzene-1,2-diol (HPIMBD) has an o-hydroquinone moiety (see Scheme 1 for structure), and is expected to have the role of an excellent mediator in electrocatalytic reactions of some important analytes. Accordingly, in continuation of our studies to prepare modified electrodes for the determination of hydrazine,^{1–4,9,14–18} in this study, we employed 4-((2-hydroxyphenylimino)methyl)benzene-1,2-diol electrodeposited on a multi-walled carbon nanotube modified glassy carbon electrode (HPIMBD–MWCNT-GCE) for the electrocatalytic oxidation of hydrazine. The results indicate that a combination of HPIMBD and MWCNT remarkably improves the current response and decreases the overpotential of hydrazine oxidation. Our findings indicate that the modified electrode has advantages such as excellent stability, good reproducibility and repeatability, wide concentration linear ranges, and technical stability for the determination of hydrazine. Finally, to evaluate the utility of the modified electrode for analytical applications, it was used for hydrazine determination in two water samples, and satisfactory results were obtained.

Scheme 1 Structure of 4-((2-hydroxyphenylimino)methyl)benzene-1,2-diol (HPIMBD).

2. Experimental

2.1. Apparatus and materials

4-((2-Hydroxyphenylimino)methyl)benzene-1,2-diol (HPIMBD) (see Scheme 1 for structure) was synthesized as follows: to a stirred solution of 3,4-dihydroxybenzaldehyde (5.0 mmol, 0.69 g) in ethanol (5.0 mL) was added a solution of 2-aminophenol (5.1 mmol, 0.56 g) in ethanol (5.0 mL) drop by drop. The reaction mixture was stirred at room temperature for 2 hours. The pure 4-((2-hydroxyphenylimino)methyl)benzene-1,2-diol was crystallized after reducing the volume of ethanol and, in a consequent simple filtration, a 90% yield was obtained. The spectral data of 4-((2-hydroxyphenylimino)methyl)benzene-1,2-diol were as follows: ¹H NMR (500 MHz, CDCl₃): δ = 6.81 (td, J = 7.5, 1.3 Hz, 1H), 6.88 (d, J = 8.2 Hz, 1H), 6.90 (dd, J = 8.2, 1.3 Hz, 1H), 7.07 (td, J = 7.7, 1.4 Hz, 1H), 7.19 (d, J = 7.8 Hz, 1H), 7.21 (d, J = 8.4 Hz, 1H), 7.51 (d, J = 1.5 Hz, 1H), 7.52–8.45 (br, 3H, 3OH), 8.48 (s, 1H) ppm; ¹³C NMR (125 MHz, CDCl₃): δ = 114.8, 115.2, 115.8, 116.6, 120.4, 123.9, 128.3, 145.7, 149.8, 152.2, 158.0 ppm. Also the melting point of HPIMBD was obtained to be 183 °C.

Hydrazine, dimethyl formamide (DMF) and other chemicals with an analytical reagent were purchased from Merck Company. The multi-walled carbon nanotubes (10–20 nm diameter, 5–20 μm length, >95% purity) were acquired from NanoLab Inc. (Brighton, MA). The phosphate buffer solutions (0.1 M) were prepared from H₃PO₄ and the pH was adjusted with a NaOH solution. Double distilled water was used to make all the solutions.

All the electrochemical measurements were performed using an Autolab potentiostat/galvanostat model PGSTAT 30 (Eco chemic Utrecht, The Netherlands). A conventional three-electrode electrochemical system was used for all the electrochemical experiments. The working electrode was a 4-((2-hydroxyphenylimino)methyl)benzene-1,2-diol (HPIMBD) electrodeposited on a multi-walled carbon nanotube modified glassy carbon electrode (HPIMBD–MWCNT-GCE). The reference electrode was a saturated calomel electrode (SCE), and a platinum electrode was used as the counter electrode. All the potentials in the text were reported versus this reference electrode. A digital pH-meter Model 691 pH/mV meter from Metrohm was used for pH measurements. All the experiments were performed at room temperature.

2.2. Preparation of HPIMBD–MWCNT-GCE, MWCNT-GCE and HPIMBD-GCE

Prior to modification, the bare GCE was sequentially polished mechanically with a 0.05 μm Al₂O₃ slurry on a polishing cloth and then rinsed with doubly distilled water. After being cleaned, for the electrochemical activation of the electrode, it was activated by a continuous potential cycling from −1.45 to 1.70 V at a sweep rate of 100 mV s⁻¹, in a 0.1 M sodium bicarbonate solution. The activated GCE was rinsed with doubly distilled water. For the fabrication of the MWCNT modified GCE (MWCNT-GCE) 2 μl of a DMF–MWCNT (1.0 mg mL⁻¹) solution was placed directly onto the activated GCE surface and dried at room temperature to form a MWCNT film at the GCE surface. The HPIMBD modified GCE (HPIMBD-GCE) was prepared by immersing the activated GCE into a 1.0 mM solution of HPIMBD and using eight cycles of a potential sweep between −200 and 500 mV at 20 mV s⁻¹. Finally, for the preparation of HPIMBD–MWCNT-GCE, the MWCNT-GCE was modified by eight cycles of a potential sweep between −200 and 500 mV at 20 mV s⁻¹ in a 0.1 M phosphate buffer solution (pH 7.0) containing 1.0 mM of HPIMBD. Our previous experiences indicate that the pH has an important effect on the immobilization of HPIMBD on the MWCNT-GCE surface. In this direction, the effect of the modifier solution pH during the modification step was examined and the current response of the modified electrode was used as a measure of the surface deposited modifier. Based on the obtained results (not shown), the best current response was obtained when the modification was carried out in HPIMBD solution with pH 7.0. After the fabrication of various modified electrodes, they were rinsed thoroughly with water and maintained in a 0.1 M phosphate buffer solution. The effective surface areas of MWCNT-GCE and unmodified GCE estimated from the cyclic voltammograms of 0.1 M KNO₃ solution containing 3.0 mM K₃[Fe(CN)₆] at various potential scan rates³⁰ are 0.0314 cm² and 0.01 cm², respectively.

3. Results and discussion

3.1. Electrochemical characteristics of HPIMBD–MWCNT-GCE

Fig. 1 shows the cyclic voltammograms of the HPIMBD–MWCNT-GCE in a 0.1 M phosphate buffer solution (pH 7.0) at potential scan rates ranging from 10 to 100 mV s⁻¹. When the potential was scanned between −10 and 350 mV, a surface-immobilized redox couple was observed with a formal potential (E⁰′) value of 117 mV. In addition, the formal potential, E⁰′, was almost independent of the potential scan rate for sweep rates ranging from 10 to 150 mV s⁻¹, suggesting facile charge transfer kinetics over this range of sweep rates.

Fig. 1 Cyclic voltammetric responses of HPIMBD–MWCNT-GCE in a 0.1 M phosphate buffer solution (pH 7.0) at different scan rates. Numbers 1–19 correspond to 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 and 100 mV s⁻¹ respectively. Insets: (A) plots of the anodic and cathodic peak currents vs. the potential scan rate. (B) Variation of the peak potentials vs. the logarithm of the potential scan rate. (C) Magnification of the plots of inset B for high potential scan rates.

The plots of the anodic and cathodic peak currents as a function of the potential sweep rate show a linear relation (Fig. 1, inset A) as predicted theoretically for a surface-immobilized redox couple.²⁷ The variation of the peak potentials versus the logarithm of the scan rate is shown in Fig. 1, inset B. The results show that the values of the anodic and cathodic peak potentials were proportional to the logarithm of the scan rate, and also nΔE_p is higher than 0.2 V for scan rates higher than 800 mV s⁻¹ (Fig. 1, inset C). Here, n is the number of electrons involved in the overall redox reaction of the modifier and ΔE_p is the peak potential separation (ΔE_p = E_pa − E_pc). Under these conditions, the surface electron transfer rate constant, k_s, and the charge transfer coefficient, α, for electron transfer between the electrodeposited HPIMBD and MWCNT-GCE can be estimated from the linear variation of the oxidation and the reduction peak potentials with the logarithm sweep rate according to the Laviron theory.²⁸ From the values of ΔE_p corresponding to different potential scan rates of 800 to 1000 mV s⁻¹, an average value of k_s was found to be 8.6 ± 0.1 s⁻¹ at pH 7.0. Also, value of α = 0.47 was obtained. It is well known that the electrochemical processes of o-hydroxybenzene derivatives are pH-dependent. Therefore, for the HPIMBD modifier the values of k_s and α were expected to be pH-dependent. Table 1 shows the values of k_s and α corresponding to different pHs. These data reveal that the k_s values increase at neutral pH (pH 7.0) and decrease at acidic or basic pHs. The average values obtained for k_s are comparable^29–33 or smaller^34–36 than those previously reported for other compounds with the o-hydroquinone ring.

Table 1 The surface charge transfer rate constant, k_s, and the charge transfer coefficient, α, for the electron transfer between the electrodeposited HPIMBD and MWCNT-GCE at different pHs

pH	α	k s /s^−1
3.0	0.49	3.6 ± 0.1
5.0	0.56	4.6 ± 0.1
7.0	0.47	8.6 ± 0.1
9.0	0.52	5.6 ± 0.1

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For quasireversible and irreversible systems, the shift values of the peak potential versus the variation of the potential scan rates are dependent on the electron-transfer kinetic as well as the Ohmic drop between the working (WE) and the reference (RE) electrode. In addition, the Ohmic drop is higher at higher current as a result of higher scan rates. In this work, all the electrochemical measurements were performed using a conventional three-electrode electrochemical system. In this system, the potential is measured between WE and RE and the current is measured between WE and the counter (CE) electrode. Since the current there is mainly between WE and CE and there is a negligible current between WE and RE, it is logical to conclude that the Ohmic drop values corresponding to the measured potentials are small. Also, in the design of the electrochemical system, the RE location was near to WE. However, there is an unavoidable error, due to the Ohmic drop together with measured potentials, in the reported k_s and α values. It is noted that the rate of heterogeneous electron transfer between an electrode and a modifier has a significant effect on the analytical parameters of an analyte determination in the solution. The variations of the anodic and cathodic peak potential values versus the logarithm of the scan rate as well as the obtained k_s values indicate that the electrodeposited modifier on the MWCNT-GCE has characteristics of a reversible system at the potential scan rates lower than about 200 mV s⁻¹,²⁷ which is a suitable potential scan rate range for study of the electrocatalytic activity of a modifier for redox reaction of an analyte. Thus, it is possible to use HPIMBD–MWCNT-GCE as an excellent sensor for quantitative determination of some of the species such that the modified electrode has an electrocatalytic activity for them.

The effect of pH on the voltammetric responses of a HPIMBD–MWCNT-GCE was studied in buffer solutions with pH values varying from 2 to 11 (Fig. 2). As can be seen in the inset of Fig. 2, the formal potential of the surface redox couple (E⁰′), taken as the average of positive and negative peak potentials, was pH dependent. The slope was found to be −57.2 mV pH⁻¹ unit over a pH range from 2.0 to 8.3 which is very close to the Nernstian value of 59 mV pH⁻¹ corresponding to a two-electron, two-proton electrochemical reaction. There is also a change in the slope at pH values above 8.3, which can be ascribed to the deprotonation of the modifier, HPIMBD. The slope −30.6 mV pH⁻¹ was obtained at pH values greater than 8.3, which is close to the Nernstian value of −29.5 mV pH⁻¹ for a two-electron, one-proton process. The pH relative to the intersection of two linear segments (pH 8.3) should therefore correspond to pK_a1 of HPIMBD.

Fig. 2 Cyclic voltammograms of a HPIMBD–MWCNT-GCE (at 50 mV s⁻¹) in 0.1 M phosphate buffer solutions with different pHs. Numbers 1–10 correspond to pHs of 2 to 11. The inset shows a plot of the formal potential E⁰′ vs. pH.

The stability of HPIMBD–MWCNT-GCE was examined either by repetitive potential scans in a 0.1 M phosphate buffer solution (pH 7.0) or by keeping the modified electrode for a period of time in the pure supporting electrolyte (pH 7.0) and recording the cyclic voltammograms at different time intervals. The measurement of the current response of the modified electrode during continuous potential cycling indicates that the loss of the current response was about 15% after 40 potential cycles at 100 mV s⁻¹, while at the end of the second 40 cycles, this was approximately 4%. The experimental results show that although a significant decline in the current response of the modified electrode was observed during the initial potential scans, the rate of the current decline was then slowed significantly. Also, only a 12% decrease was observed in the current response of the modified electrode when it was kept in a 0.1 M phosphate buffer solution (pH 7.0), as a supporting electrolyte, for 2 days. The initial decay of the current response of the voltammograms observed in both cases might be due to the release of the modifiers that are weakly bonded to MWCNT deposited on the electrode surface and can be separated somewhat easily. However, the above results indicate that the chemical stability of the modified electrode is acceptable. It should be noted that there is no mechanical stability for the modified electrode. This fact is due to the structure of HPIMBD–MWCNT-GCE.

3.2. Electrocatalytic oxidation of hydrazine at HPIMBD–MWCNT-GCE

The ability of HPIMBD–MWCNT-GCE for the electrocatalytic oxidation of hydrazine was appraised by cyclic voltammetry. Fig. 3 shows the cyclic voltammograms of HPIMBD–MWCNT-GCE (curve a) and HPIMBD-GCE (curve c) in a 0.1 M phosphate buffer solution (pH 7.0) in the absence of hydrazine and also the voltammetric responses of a 0.1 M phosphate buffer solution (pH 7.0) containing 0.3 mM hydrazine at HPIMBD–MWCNT-GCE (curve b), HPIMBD-GCE (curve d), MWCNT-GCE (curve e) and activated GCE (curve f). As it can be seen, the anodic peak potentials for the oxidation of hydrazine at HPIMBD–MWCNT-GCE (curve b), HPIMBD-GCE (curve d), MWCNT-GCE (curve e) and activated-GCE (curve f) are about 240, 280, 310, and 315 mV respectively.

Fig. 3 Cyclic voltammograms of HPIMBD–MWCNT-GCE in a 0.1 M phosphate buffer solution (pH = 7) at a scan rate of 20 mV s⁻¹ in the absence (a) and the presence of 0.30 mM hydrazine (b). (c) as (a) and (d) as (b) for HPIMBD-GCE. (e) and (f) as (b) for MWCNT-GCE and activated GCE respectively.

Table 2 shows the electrochemical characteristics of hydrazine (0.3 mM) on the various modified electrode surfaces at pH 7.0. The results show that the largest current response and the most diminished peak potential correspond to hydrazine oxidation at the HPIMBD–MWCNT-GCE surface. For example, the results indicate that the peak potential of hydrazine oxidation at HPIMBD–MWCNT-GCE (curve b) shifted by about 40, 70, and 75 mV toward the negative values compared with those at HPIMBD-GCE (curve d), MWCNT-GCE (curve e) and activated-GCE (curve f) respectively. Also, there is an enhancement of the anodic peak current at the HPIMBD–MWCNT-GCE surface (curve b) relative to the values observed at the other various electrodes.

Table 2 Comparison of electrocatalytic oxidation of hydrazine (0.3 mM) on various electrode surfaces at pH 7.0

Name of electrode	Oxidation peak potential (mV)	Oxidation peak current density (μA cm^−2)
AGCE	315	18.15
MWCNT-GCE	310	21.02
HPIMBD-GCE	280	26.11
HPIMBD–MWCNT-GCE	240	34.39

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It should be noted that use of MWCNT in the structure of the modified electrodes causes an increase in the effective surface area of the modified electrode and, hence, an increase in the current response of the analyte. On the other hand, HPIMBD as a mediator of the electron transfer plays an effective role in decreasing the overpotential of hydrazine oxidation significantly. Also, a comparison of the data in Table 2 indicates that a combination of MWCNT and HPIMBD improves the electrochemical characteristics of hydrazine oxidation.

The effect of the potential scan rate on the electrocatalytic oxidation of hydrazine at the HPIMBD–MWCNT-GCE surface was used to obtain information about the oxidation mechanism of hydrazine. The linear sweep voltammograms of the modified electrode in a 0.1 M phosphate buffer solution (pH 7.0) containing 0.30 mM hydrazine at different scan rates are shown in Fig. 4. Inset A of Fig. 4 shows that the plot of the electrocatalytic peak current (I_p) versus the square root of potential scan rate (v^1/2) is linear, suggesting that at a sufficient positive overpotential, the electrochemical reaction is diffusion-limited. Under these conditions and using the following equation, the number of electrons in the overall oxidation reaction of hydrazine, n, can be estimated from the slope of the I_pversus v^1/2 plot (inset A of Fig. 4).^9,37

I_p = 3.01 × 10⁵n[(1 − α)n_α]^1/2AC_bD^1/2v^1/2 (1)

where C_b and D are the bulk concentration (0.3 mM) and the diffusion coefficient of hydrazine (5.23 × 10⁻⁶ cm² s⁻¹ obtained by chronoamperometry). Also, A = 0.0314 cm² and n_α(1 − α) = 0.33 (which is deduced from Tafel plots).

Fig. 4 Linear sweep voltammograms of HPIMBD–MWCNT-GCE in a 0.1 M phosphate buffer solution (pH 7.0) containing 0.30 mM hydrazine at different potential scan rates. The points are the data used in the Tafel plots. Numbers 1–9 correspond to 6, 8, 10, 12, 14, 16, 18, 20 and 22 mV s⁻¹ respectively. Insets: (A) variation of the electrocatalytic peak current vs. the square root of potential scan rate and (B) the Tafel plots derived from the linear sweep voltammograms.

Based on the above discussion, the total number of electrons, n, involved in the anodic oxidation of hydrazine was calculated to be n = 2.1 ≅ 2. This value was previously reported for the electrooxidation of hydrazine.^4,9,38,39 However, the value obtained for n is different from n = 4 which has been reported in some literature.^3,40–43 If one considers pK_a of N₂H₅⁺ (protonated form of hydrazine) and NH₃OH⁺ (protonated form of hydroxylamine) to be equal to 8.1 and 5.96, respectively,⁴⁴ it is logical to conclude that the overall electrocatalytic oxidation of hydrazine at pH 7.0 at the modified electrode surface will be as follows:^4,9

N₂H₅⁺ + 2H₂O → 2NH₂OH + 3H⁺ + 2e⁻ (2)

Based on the above results, the electrocatalytic oxidation of hydrazine at the HPIMBD–MWCNT-GCE surface can be written according to an E_rC_i catalytic mechanism as shown in the following equations:^4,9

As it can be seen, hydrazine is oxidized by the oxidized form of HPIMBD electrodeposited at the MWCNT surface. The symbols E_r and C_i imply reversible electrochemical and irreversible catalytic chemical reactions. For an mechanism, Andrieux and Saveant⁴⁵ developed a theoretical model and derived the following equation between the electrocatalytic peak current and the concentration of the analyte for a case of a slow scan rate, ν, and a large catalytic rate constant, k′:

I_cat = 0.496nFACD^1/2ν^1/2C*(nF/RT) (5)

where D and C are the diffusion coefficient (cm² s⁻¹) and the bulk concentration (mol cm⁻³) of the analyte (hydrazine in this study) respectively. Low values of k′ result in values lower than 0.496 for the constant. For low scan rates (6–22 mV s⁻¹), we find the value of this constant to be 0.21 for HPIMBD–MWCNT-GCE, in the presence of 0.3 mM of hydrazine. Using this value as well as Fig. 1 in the theoretical paper by Andrieux and Saveant,⁴⁵ an average value of k′ = (1.1 ± 0.03) × 10⁻³ cm s⁻¹ was obtained. The obtained k′ is in agreement with other values previously reported for hydrazine oxidation.^4,9,18 Also, this value of k′ explains a good catalytic feature for the oxidation of hydrazine at HPIMBD–MWCNT-GCE. Fig. 4, inset B, shows Tafel plots that were drawn from the data of the rising part of the current–voltage curves recorded at various scan rates. The data related to the Tafel region are affected by the electron transfer kinetics between the hydrazine and the surface-confined HPIMBD–MWCNT-GCE, assuming the deprotonation of the substrate to be a sufficiently fast step. Under this condition, the number of electrons involved in the rate-determining step can be estimated from the slope of the Tafel plot.²⁷ The results of polarization studies for the electrooxidation of hydrazine at HPIMBD–MWCNT-GCE show that the average value of the kinetic parameters of the electron transfer coefficient, α, is 0.67 ± 0.02 assuming one electron (n_α = 1) in the rate-determining step of the electron transfer process between hydrazine and the modified electrode.²⁷ Also, the exchange current density, j₀, for electrocatalytic oxidation of hydrazine at the HPIMBD–MWCNT-GCE surface is evaluated from the intercept of the Tafel plots.²⁷ The value of the exchange current density, j₀, is found to be 3.98 μA cm⁻¹.

3.3. Chronoamperometric and amperometric studies of electrocatalytic oxidation of hydrazine at the HPIMBD–MWCNT-GCE surface

Chronoamperometry was used for the evaluation of the diffusion coefficient, D, of hydrazine. As Fig. 5 shows, the chronoamperograms were obtained by setting the working electrode potential at 300 mV at various concentrations of hydrazine. Fig. 5, inset A, shows the experimental plots of I versus t^−1/2 with the best fits at different concentrations of hydrazine. The slopes of the resulting straight lines were then plotted versus the hydrazine concentration (Fig. 5, inset B). Based on the Cottrell equation²⁷ and using the plot slope of Fig. 5, inset B, the diffusion coefficient of hydrazine was calculated to be 5.23 × 10⁻⁶ cm² s⁻¹. This value of diffusion coefficient is in good agreement with 8.2 × 10⁻⁶ cm² s⁻¹,¹ 1.15 × 10⁻⁵ cm² s⁻¹,² 3.2 × 10⁻⁶ cm² s⁻¹,³ 4.58 × 10⁻⁶ cm² s⁻¹,⁴ 1.6 × 10⁻⁶ cm² s⁻¹,¹⁶ 4 × 10⁻⁵ cm² s⁻¹,^40,41 and 8.03 × 10⁻⁶ cm² s⁻¹, which were previously reported for hydrazine.

Fig. 5 Chronoamperometric responses of HPIMBD–MWCNT-GCE in a 0.1 M phosphate buffer solution (pH 7.0) containing different concentrations of hydrazine at a potential step of 300 mV. Numbers 1–5 correspond to 0.20, 0.40, 0.60, 0.80 and 1.0 mM hydrazine. Insets: (A) plots of I vs. t^−1/2 obtained from the chronoamperograms and (B) plot of the straight lines against the hydrazine concentration.

It is noted that the diffusion coefficient, D, value of hydrazine is necessary for calculation of the number of electrons in the overall oxidation reaction of hydrazine, n, based on eqn (1) as well as the heterogeneous electron transfer rate constant, k′, between the modified electrode surface and hydrazine based on eqn (5).

Since amperometry under stirring conditions has a much higher current sensitivity than cyclic voltammetry, it was used to determine the linear ranges and the detection limits of hydrazine at the HPIMBD–MWCNT-GCE surface. Fig. 6 shows the amperograms which were obtained for a rotating HPIMBD–MWCNT-GCE (rotation speed 2000 rpm), under conditions where the potential was held at 300 mV in a 0.1 M phosphate buffer solution (pH 7.0) containing different concentrations (4.0–32.9 μM (Fig. 6A) and 32.9–750.4 μM (Fig. 6B)) of hydrazine. Also the stability of HPIMBD–MWCNT-GCE under working conditions was investigated in the presence of 60.0 μM of hydrazine. Fig. 6C indicates the response stability of HPIMBD–MWCNT-GCE to 60.0 μM of hydrazine solution during 600 s. As shown, the amperometric current of hydrazine oxidation remained almost constant during the experiment.

Fig. 6 (A) Amperometric response at the rotating HPIMBD–MWCNT-GCE (rotation speed 2000 rpm) surface held at 300 mV in 10 ml of a 0.1 M phosphate buffer solution (pH = 7.0) for successive 9 additions of 10 μl, successive 10 additions of 20 μl and successive 10 additions of 50 μl hydrazine of 1.0 mM and successive 10 additions of 10 μl hydrazine of 10.0 mM. (B) Successive 10 additions of 20 μl, successive 10 additions of 50 μl hydrazine of 10.0 mM to A. (C) Stability of the response to 60.0 μM hydrazine using HPIMBD–MWCNT-GCE.

Fig. 7A and B clearly show that the plot of the peak currents versus hydrazine concentration is constituted from two linear segments corresponding to concentration ranges of 4.0–32.9 μM and 32.9–750.4 μM of hydrazine. The results refer to a well-defined response during the successive addition of even 0.09 μM of hydrazine into the stirred buffer solution. This high sensitivity also demonstrates the stability and efficient catalytic ability of the modifier immobilized on the MWCNT-GCE surface.

Fig. 7 Plots of the current responses of amperograms in Fig. 6vs. hydrazine concentrations in the ranges of (A) 4.0–32.9 μM and (B) 32.9–750.4 μM.

According to the method mentioned in ref. 46, the lower detection limit, C_m, was obtained using the equation C_m = 3s_bl/m, where s_bl is the standard deviation of the blank response (μA) and m is the slope of the calibration plot (0.016 μA μM⁻¹). In this experiment, fourteen replicate measurements were made in the blank solution and the resulting data are then treated statistically to obtain s_bl = 0.006 μA. From the analysis of these data, the detection limit of 1.1 μM was obtained for hydrazine determination at the modified electrode surface. The average amperometric current measured (μA) and the precision estimated in terms of the relative standard deviation (%RSD) for nine repeated measurements (n = 9) of 12.8 μM hydrazine were 0.23 μA and 2.0%, respectively. In Table 3, some of the electroanalytical parameters obtained in this research are compared with those previously reported by others.^{1–4,7,9,12,14–18,47–55} A comparison of the analytical parameters of hydrazine determination at various modified electrode surfaces shows that the proposed modified electrode has advantages such as wide linear dynamic range (4.0–750.4 μM) and good detection limit (1.1 μM) for hydrazine determination.

Table 3 Comparison of the analytical parameters of several modified electrodes for hydrazine electrocatalytic determination

Modifier	Method	Linear range (μM)	Detection limit (μM)	Sensitivity (μA μM^−1)	Ref.
Chlorogenic acid	Chronoamperometry	50–3000	—	0.0054	1
Pyrocatechol violet	Amperometry	5.0–500.0	4.2	—	2
A coumestan derivative	Differential pulse voltammetry	1–40	0.66	6.1	3
		40–400		3.0
Hematoxylin	Amperometry	2.0–122.8	0.68	0.0208	4
Ruthenium oxide nanoparticles	Amperometry	2.0–268.3	0.15	0.0974	7
		268.3–417.3
An indenedione derivative	Differential pulse voltammetry	0.6–8.0	0.29	0.167	9
		8.0–100.0		0.014
Manganese hexacyanoferrate	Amperometry	33.3–8 180 000	6.65	0.0475	12
Quinizarine	Differential pulse voltammetry	0.2–1.0	0.14	—	14
		2.0–10
Caffeic acid	Amperometry	2.5–1000	0.4	3.16	15
Catechin	Amperometry	2.0–58.4	0.16	0.0084	16
		58.4–237.2		0.0052
Rutin	Amperometry	2.0–190.9	0.61	0.0656	17
4-Hydroxy-2-(triphenylphosphonio)phenolate	Differential pulse voltammetry	1.0–20.0	0.13	0.0122	18
		20.0–1000.0		0.0042
		1000.0–6000.0		0.002
Nickel hexacyanoferrate	Amperometry	2.0–5000	0.28	0.26	47
Iron–phthalocyanine complex	OSWV	13.0–92.0	5.0	0.062	48
Gold nanoparticle–polypyrrole nanowire	Differential pulse voltammetry	1–500	0.2	0.126	49
		500–7500		0.035
Tetracyanoquinodimethanide–titanium oxide	Amperometry	2–100	0.6	0.36	50
A porphyrin derivative	Amperometry	0.25–250	0.03	—	51
A ruthenium complex	Amperometry	10–90	8.5	—	52
Rhodium acetamidate	Amperometry	10–10 000	5.0	0.0003	53
Nickel tetrasulfonated phthalocyanine	Amperometry	100–600	10	—	54
Curcumin	Amperometry	2.0–44.0	1.4	22.9	55
4-((2-Hydroxyphenylimino)methyl)benzene-1,2-diol	Amperometry	4.0–32.9	1.1	0.016	This work
		32.9–750.4

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3.4. Application of HPIMBD–MWCNT-GCE for determination of hydrazine in real samples

An amperometric method was used for the determination of hydrazine concentration in two samples of auxiliary cooling water from Yazd Power Generation Management Company (Yazd, Iran) and tap water at the HPIMBD–MWCNT-GCE surface. To determine hydrazine in the auxiliary cooling water of power generation, 2.5 mL of the water sample was diluted to 10.0 mL with a 0.1 M phosphate buffer solution (pH 7.0) before the measurements. Then, the diluted solution was placed in an electrochemical cell to determine hydrazine using an amperometric method. Based on the average current of the amperometric response (n = 4) and using the calibration plot shown in Fig. 7A, the hydrazine concentration in the diluted real sample solution was obtained to be 5.1 ± 0.3 μM (Table 4). The credibility of the results obtained for hydrazine determination at the modified electrode surface was also evaluated by comparing the results with those obtained from the standard ASTM method⁵⁶ for the determination of hydrazine in the same real sample. The total concentration of hydrazine in the cooling water sample was found to be 0.66 ± 0.04 ppm (n = 4) using the proposed modified electrode, which is in good agreement with the value obtained using the ASTM method (0.64 ± 0.03 ppm (n = 4)). A statistical t-test was performed to evaluate the accuracy of the proposed method. Based on the t-test, it can be concluded that t_crit (t_crit = 2.45) is larger than t_exp (t_exp = 1.76) at a 95% confidence level at 6 degrees of freedom. This result suggests that there is no evidence of systematic difference between the results obtained by either of the methods. Also, to confirm the results, the auxiliary cooling water was spiked with different amounts of hydrazine as shown in Table 4. The results in Table 4 show that RSD% and the recovery rates of the spiked hydrazine are acceptable.

Table 4 Determination and recovery rates of hydrazine for two water samples that were spiked with specific concentrations of hydrazine at the HPIMBD–MWCNT-GCE surface^a

Samples	Added (μM)	Found (μM)	RSD (%)	Recovery (%)
a Three replicate measurements were made on the same samples. b Dilution factor is 4. c Dilution factor is 2.
Auxiliary cooling water from power generation^b	—	5.1	5.9	—
	5.0	10.2	3.1	101.0
	15.0	20.0	3.0	99.5
	30.0	34.6	2.0	98.6
Tap water^c	—	—	—	—
	5.0	5.1	2.6	102.0
	15.0	15.4	2.4	102.7
	30.0	29.8	2.1	99.3

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Also, the results clearly suggest that the matrix of the cooling water sample does not make any interference in the determination of hydrazine by the proposed method. Similar experiments were done to determine hydrazine in tap water. At first, the tap water was diluted 2 times with a 0.1 M phosphate buffer solution and the pH was adjusted to 7.0. Then, the water sample was spiked with 5, 15 and 30 μL of 10 mM hydrazine, and their RSD% and recovery rates were determined by amperometric measurements at HPIMBD–MWCNT-GCE and utilizing a calibration plot for hydrazine within a range of 4.0–32.9 μM (Fig. 7A). As it can be seen in Table 4, the RSD% and the recovery rates of the spiked hydrazine were acceptable. Thus, it is concluded that the matrix of water samples does not make any interference in the determination of hydrazine at the proposed modified electrode.

3.5. Interference study

An important problem in determining hydrazine is the effect of potential interfering ions. A survey on the influence of various substances as potential interfering compounds on the hydrazine determination under optimum conditions at the HPIMBD–MWCNT-GCE surface was done. It was done by analyzing 100.0 μM hydrazine in a 0.1 M phosphate buffer solution (pH 7.0) containing concomitant ions at different concentrations. The tolerance limit was defined as the maximum concentration of the interfering substance that made an error less than 3% for determination of hydrazine. Table 5 shows the results of these measurements. The results indicate that hydrazine recovery was almost quantitative in the presence of an excess amount of possible interfering species at the HPIMBD–MWCNT-GCE surface.

Table 5 Effect of interfering species on the accuracy of hydrazine concentration determination at the HPIMBD–MWCNT-GCE surface

Foreign species	Molar ratio (species/hydrazine)
Na^+, K^+, Ca^2+, NH4^+, Mg^2+, CO3^2−, PO4^3−, SO4^2−, Cl^−, Br^−, NO^3−, NO^2−, IO^4−, SCN^−, S2O3^2−, C2O4^2−	1000
NH2OH	1

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4. Conclusions

The results of this study indicate that HPIMBD–MWCNT-GCE presents a stable and excellent electrocatalytic activity for hydrazine determination. The charge transfer coefficient, α, and the charge transfer rate constant, k_s, of HPIMBD electrodeposited on MWCNT-GCE were calculated at various pHs. The diffusion coefficient of hydrazine was calculated to be 5.23 × 10⁻⁶ cm² s⁻¹ under experimental conditions, using chronoamperometric results. The electron transfer catalytic rate constant, k′, the charge transfer coefficient, α, the number of electrons involved in the rate-determining step, n_α, and the overall number of electrons involved in the catalytic oxidation of hydrazine at the modified electrode surface were also determined using cyclic voltammetry. In amperometric measurements, there appeared two linear calibration ranges for hydrazine. The detection limit of hydrazine was obtained to be 1.1 μM. Also, the activity of HPIMBD–MWCNT-GCE was investigated for hydrazine determination in two water samples.

Acknowledgements

The authors gratefully acknowledge Yazd University for providing the research facilities required for this work.

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