Fabrication of a Highly Sensitive Hydrazine Electro- chemical Sensor Based on Bimetallic Au-Pt Hybrid Nanocomposite onto Modified Electrode

1Department of Chemistry, Faculty of Basic Science, Islamic Azad University, Khorramabad Branch, Khorramabad, Islamic Republic of Iran 2Faculty of Science and Engineering, Islamic Azad University, Doroud Branch, Doroud, Islamic Republic of Iran *Corresponding author. E-mail: Azadeh_Azadbakht@yahoo.com Fabrication of a Highly Sensitive Hydrazine Electrochemical Sensor Based on Bimetallic Au-Pt Hybrid Nanocomposite onto Modified Electrode Azadeh Azadbakht1,* and Amir Reza Abbasi2

Nanoparticles also facilitate the electron transfer and can be easily modified with a wide range of biomolecules and chemical ligands. Electrochemical behavior and applications of nanoparticles have witnessed a significant growth in the past few years. Therefore combination of nanowires and metal nanoparticle have received much interest because of their intriguing properties and potential applications in chemical sensing [24,25]. A 3,3,5,5-Tetramethylbenzidine (TMB), much less hazardous than benzidine and more sensitive as a chromogenic reagent, has been investigated for many years [26].
Doping of TMB-based organic nanofibers (NFs) with incorporating of metals ions is of particular interest. In this paper, we report the electrodeposition of Nickel-2, 6-Diaminopyridine

Synthesis of gold nanoparticle
In this study, colloidal gold nanoparticles were prepared in accordance with the literature published before [27]. 0.5 ml of 1% (w/v) of sodium citrate solution was added to 50 ml of 0.01% (w/v) of HAuCl 4 boiling solution. HAuCl 4 and sodium citrate aqueous solutions were filtered through a 0.22 μm microporous membrane filter before use. In this procedure, all glass wares used were cleaned in freshly prepared 1:3 of HNO 3 /HCl solution and then rinsed thoroughly with doubly distilled water. The mixture was boiled for 15 min and then stirred for 15 min after removing the heating source to produce colloidal gold nanoparticles. The solution was stored in a refrigerator in a dark-colored glass bottle before use. The synthesized colloidal gold nanoparticles show maximum absorbance intensity in UV-Vis spectra at 520 nm. In the current paper, colloidal gold nanoparticles were stable for 10 days and their colors were constant, approximately.

Electrode modification
To prepare an Au-PtNPs/NF modified electrode, glassy carbon electrode was polished with emery paper followed by alumina (1.0 and 0.05 µm) and then thoroughly washed with twice-distilled water. Then electrode was placed in ethanol container and used bath ultrasonic cleaner in order to remove adsorbed particles. TMB-based NFs were prepared according to the previous work [28]

Characterization of the modified electrode by SEM
To investigate the surface structure and the morphology of the modified electrode, we performed SEM. Figure 1 shows the SEM images of as-prepared NFs (A), Au-PtNP/NF/GCE (B) and Ni-DAP/Au-PtNPs/NFs/GC electrode (C). It can be seen that the as-prepared NFs have diameters of ~150 nm and lengths up to several micrometers. Results show that these nanofibers interlaced together. After the subsequent deposition process, one can see that uniform Au-PtNPs aligned along the surface of those NFs. The generated NPs were homogenously distributed in the matrix of interlaced NFs, constructing a 3D interlaced network ( Fig.1B). Figure 1C shows the SEM image of Ni-DAP/Au-PtNPs/NFs/GC electrode. As can be seen, the film has a globular structure with relatively homogeneous distribution. The presence of small nanoparticles leads to an increase in the surface coverage for more adsorption of hydrazine and OH ads . This for many electrooxidation processes [30,31]. The redox process of the modified electrode was expressed as follows,   Figure 5A illustrates cyclic voltammograms of 6.7 mM hydrazine using modified electrode recorded at potential sweep rates ranging from 5 to 200 mVs -1 . The oxidation current of hydrazine on the modified surface increases linearly with the square root of the potential sweep rate (Fig. 5B), which indicates the mass transfer controlled process. Also, it can be seen that, with the increasing scan rate, the anodic peak potential tend toward positive potentials, suggesting a kinetic limitation in the reaction between the redox sites of the Ni-DAP/Au-PtNPs/NFs/GC electrode and hydrazine. The α value of the electrodic reaction can be evaluated from the following equation [36], where b indicates the tafel slope. Using the dependency of anodic peak potential on the natural logarithm of the potential sweep rate (Fig. 5C), the value of electron transfer coefficient (α) is estimated as 0.47. Also, the obtained value of transfer coefficient from the recorded I-E curve of electrocatalytic oxidation of hydrazine (slope of log I vs. E plot) confirms the above reported value (0.508).

Rotating disk electrode (RDE) voltammetry
The indicates that the limiting current is mass-transport controlled (Fig. 6B). The Levich plots deviated from linearity at high rotation rates, suggesting a kinetic limitation. Under these conditions the Koutecky-Levich equation [37] can be used to determine the rate constant. The limiting current is given by Eq.
(5), l /I lim = 1 /I Lev + 1 /I K (5) where I Lev is the Levich current and I K is the kinetic current. I Lev and I K are defined by Eqs. (6) and (7),

Amperometric detection of hydrazine at modified electrode
Since amperometry under stirred conditions has a higher current sensitivity than cyclic voltammetry, it was used to estimate the low limit of detection. Figure 7A Table 1. As seen, the analytical parameters are comparable or better than results reported for hydrazine determination at the surface recently fabricated modified electrodes [38][39][40][41][42][43][44][45][46].

Interference effect for hydrazine determination
To apply this modified electrode to determine hydrazine in environmental water samples, the influence of common ions for the determination of 7 × 10 -6 M hydrazine was investigated. If these interfering ions cause a relative error of less than 5% for the determination of 7 × 10 -6 M of hydrazine, the interference of these species are negligible. The results are summarized in

Analytical applications
Since the present amperometric method is very sensitive and a small volume of the sample is adequate for the hydrazine determination, the standard addition method is suitable for simple and rapid evaluation of hydrazine. The reliability of the amperometric determination of hydrazine in photographic developer was verified using an iodimetric procedure described in the literature [47]. For this purpose, a 0.05 M iodine solution which was standardized in the usual way with a primary standard of As 2 O 3 or titrisol thiosulfate solution was used. The result of statistical calculation shown in Table 3 indicates good precision and good agreement between the repeatability of the proposed and official methods.

Stability and reproducibility
The stability and the reproducibility of the Ni-DAP/