A sensitive hydrazine electrochemical sensor based on electrodeposition of gold nanoparticles on choline film modified glassy carbon electrode

Sensors and Actuators B 153 (2011) 239–245

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A sensitive hydrazine electrochemical sensor based on electrodeposition of gold nanoparticles on choline film modified glassy carbon electrode Jing Li, Huaqing Xie ∗ , Lifei Chen School of Urban Development and Environmental Engineering, Shanghai Second Polytechnic University, Shanghai 201209, PR China

a r t i c l e

i n f o

Article history: Received 1 September 2010 Received in revised form 24 October 2010 Accepted 25 October 2010 Available online 30 October 2010 Keywords: Electrocatalytic oxidation Electrodeposition Hydrazine Choline Gold nanoparticles

a b s t r a c t A highly sensitive hydrazine sensor was developed based on the electrodeposition of gold nanoparticles onto the choline film modified glassy carbon electrode (GNPs/Ch/GCE). The electrochemical experiments showed that the GNPs/Ch film exhibited a distinctly higher activity for the electro-oxidation of hydrazine than GNPs with 3.4-fold enhancement of peak current. The kinetic parameters such as the electron transfer coefficient (˛) and the rate of electron exchange (k) for the oxidation of hydrazine were determined. The diffusion coefficient (D) of hydrazine in solution was also calculated by chronoamperometry. The sensor exhibited two wide linear ranges of 5.0 × 10−7 –5.0 × 10−4 and 5.0 × 10−4 –9.3 × 10−3 M with the detection limit of 1.0 × 10−7 M (s/n = 3). The proposed electrode presented excellent operational and storage stability for the determination of hydrazine. Moreover, the sensor showed outstanding sensitivity, selectivity and reproducibility properties. All the results indicated a good potential application of this sensor in the detection of hydrazine. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Hydrazine and its derivatives are widely used in industrial applications and agriculture as fuel cells, explosives, antioxidants, rocket propellants, corrosion inhibitor, insecticides and plantgrowth regulators [1,2]. Hydrazine is also a toxic material, which has been recognized as a neurotoxin, carcinogenic mutagenic and hepatotoxic substance [3]. Because of the industrial and pharmacological significance, sensitive detection of hydrazine is practically important. Recent researches have been directed towards the development of rapid, sensitive and selective analytical methods for the detection of hydrazine [4–7]. Among numerous methods that have been reported for the determination of hydrazine, voltammetric method possesses many advantages such as high sensitivity, wide linear range, good selectivity, rapid response and simple operating procedure [8–10]. Unfortunately, hydrazine exhibits irreversible oxidation required large overpotential at conventional electrodes. Various inorganic and organic materials have been used to fabricate chemically modified electrodes (CMEs) for the detection of hydrazine, which can significantly lower the overpotential and enhance the electron transfer rate [11–15]. Metal nanoparticles have been received significant attention in recent years, due to their unique properties different from those

∗ Corresponding author. Tel.: +86 21 50217331; fax: +86 21 50217331. E-mail address: [email protected] (H. Xie). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.10.040

of the bulk materials, such as good conductivities, outstanding electrocatalytic activities, optical and magnetic properties [16–18]. Particularly, gold nanoparticles (GNPs) have been widely used in many electrochemical fields due to their advantages of catalysis, mass transport and high effective surface area [19]. The catalytic performance of GNPs is significantly dependent on the structure of the local microenvironment [20]. GNPs dispersed on a variety of substrate have been reported in literature, such as highly ordered pyrolytic graphite (HOPG) [21], carbon nanotubes [22–24], carbon paste electrode [25], self-assembled monolayer [26,27], conducting and non-conducting polymers [28–31]. Choline (Ch) is a precursor and a metabolic of acetylcholine (ACh), which plays an important role in maintaining the central nervous system and numerous metabolic functions [32]. Detection of Ch is critical to characterize cholinergic neurotransmission in normal and pathological physiology [33]. Recently, numerous techniques have been developed for the determination of Ch [33–37]. On the other hand, Ch has –OH group, which can be easily grafted on the carbon electrode surface through nucleophilic attack reaction between oxidized carbon and the hydroxyl [38]. Due to the –N+ (CH3 )3 polar head group of Ch, the Ch film provides a positively charged uniform surface, which plays an important role in the preparation of nanoparticles. This paper described the preparation, characterization and electrocatalytic applications of GNPs/Ch film. The positively charged Ch film could facilitate the formation of GNPs through the electrostatic interaction between –N+ (CH3 )3 and AuCl4 − . The nanostructured GNPs/Ch film exhibited high stability and remarkable catalytic

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activity for the electro-oxidation of hydrazine, which could be used as an effective sensor for the assessment of hydrazine.

2. Experimental 2.1. Reagents Hydrazine, Ch, LiClO4 and HAuCl4 were obtained from Chemical Reagent Company of Shanghai (Shanghai, China). All other chemicals were of analytical-reagent grade and used without further purification. The 0.1 M phosphate buffer solutions of different pH values (PBSs) were prepared as the electrolytes. All experiments were carried out at ambient temperature. Doubly distilled water was used to prepare solutions.

2.2. Apparatus

2.3. Electrode preparation Prior to modification, the basal GCE was polished to a mirror finish using alumina slurries with different powder size down to 0.05 ␮m. After each polishing, the electrode was sonicated in ethanol and doubly distilled water for 5 min, successively, in order to remove any adsorbed substances on the electrode surface. After cleaning, the GCE was performed in 1 mM Ch solution containing 10 mM LiClO4 by cycling potential between −1.7 and 1.8 V at a scan rate of 20 mV/s for 6 cycles. Six CV scans were required to obtain a steady-state response. Then the electrode was rinsed with water and sonicated for 5 min to remove any physically adsorbed substance. The Ch film modified electrode was obtained and denoted as Ch/GCE. The formation of GNPs on the Ch/GCE was carried out by cyclic voltammetry (CV) scanning from 0.2 V to −1.0 V in 0.1 M KCl solution containing 0.5 mM HAuCl4 at a scan rate of 50 mV/s for 10 cycles. Through changing the cycle number in electrodeposition process, the amount and the size of the deposited GNPs can be controlled. Because the size of the nanoparticles significantly influenced the catalytic efficiency, 10 cycles was chosen as the optimum cycle number in gold electrodeposition process. The generated GNPs/Ch film modified electrode was obtained and denoted as GNPs/Ch/GCE. Similarly individual GNPs modified GCE was prepared for comparison, denoted as GNPs/GCE.

3. Results and discussion 3.1. Characterization of modified electrode The morphology of the GNPs/Ch/GCE surface was characterized by FE-SEM technique. As shown in Fig. 1, the GNPs were homogeneously distributed on the Ch film modified electrode surface with a diameter in the range of 30–80 nm. The larger GNPs might be the aggregations of much smaller nanoparticles due to the positively charged Ch film, which was favorable for the AuCl4 − reduction. XRD analysis was used to characterize the structure of GNPs. Fig. 2 displays the XRD pattern of GNPs/Ch film. Five diffraction peaks appeared at 2 of 38.3◦ , 44.5◦ , 64.7◦ , 77.7◦ and 81.9◦ , which can be ascribed to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of Au face-centered cubic crystallographic structure. All these patterns illustrate that GNPs has been successfully loaded onto the Ch film modified GCE. To further verify the Ch film and GNPs have been deposited on the GCE surface, XPS experiment was performed. As shown in Fig. 3A, the Au 4f spectrum consisting of two peaks located at 84.1 and 87.8 eV, corresponding to the Au 4f7/2 and Au 4f5/2 signals, respectively, which demonstrated the formation of metallic Au. Fig. 3B showed the XPS of Ch film. It can be observed N 1s signal appeared at 399.8 eV, which confirmed Ch has been successfully immobilized on the electrode surface. Moreover, the feature peak was found in the same position without any shift after the modified Ch/GCE was sonicated in 0.1 M PBS for 15 min, indicating that Ch has been strongly grafted on the GCE surface through cova-

400 {111} 300

intensity / a.u.

All electrochemical measurements were conducted on CHI 660C electrochemical workstation (ChenHua Instruments Co., Shanghai, China). A conventional three-electrode electrochemical system was used for all the electrochemical experiments, a saturated calomel electrode (SCE) as reference electrode, a platinum wire as counter electrode, glassy carbon electrode (GCE, Ø4.0 mm) and modified electrode as working electrodes. All potentials in the paper were reported versus SCE. Electrochemical impedance spectroscopy (EIS) were obtained in 0.1 mol/L KCl solution containing 10 mM K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1 mixture) in the frequency range from 0.1 Hz to 100 kHz with a signal amplitude of 10 mV in a cell with a netted Pt counter electrode of about 2 cm2 surface area. FE-SEM image was obtained on S-4800 field emission scanning electron microanalyser (Hitachi, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on ESCALAB MK2 spectrometer (VG Co., UK) equipped with Mg K␣ X-ray radiation as the source for excitation. Powder X-ray diffraction (XRD) spectra was recorded on a MXPAHF rotating anode X-ray diffractometer ˚ (Japan) with Cu-K␣ radiation source ( = 1.54056 A).

Fig. 1. FE-SEM image of GNPs/Ch composite film on bare GCE.

200 {200} 100 {220}

{311}

{222}

0 20

30

40

50

60

70

2θ (Degrees) Fig. 2. XRD pattern of GNPs/Ch/GCE.

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40

A

241

40

d

30

4f5/2

30

i / μA

Relative intensity / kcps

4f7/2

20

10

a

0 -10 80

82

84

86

88

90

92

-0.2

Binding Energy / eV 15

Relative intensity / kcps

b

10

0

B

0.0

0.2

0.4

0.6

0.8

E / V (vs. SCE) Fig. 5. CVs of 2.0 × 10−4 M hydrazine in 0.1 M PBS on the bare GCE (curve a), Ch/GCE (curve b), GNPs/GCE (curve c) and GNPs/Ch/GCE (curve d). Scan rate: 50 mV/s.

N1s

12

9

6

3 393

396

399

402

405

408

Binding Energy / eV Fig. 3. XPS spectrum of GNPs/Ch composite film in the Au 4f (A) and N 1s (B) region.

lent bond, not simply attached to the electrode surface by physical adsorption. The chemistry of Ch modification could be related to the covalent bonding of Ch with the anodized carbon surface [38]. The surface of the GCE is anodized at 1.8 V, generating cation radicals. Then, the surface cation radicals react with Ch by nucleophilic attack forming ether bound –O– linkages for Ch assembly. EIS is an effective method for investigating the features of surface modified electrode. Fig. 4 illustrated the EIS of the bare GCE (curve a), Ch/GCE (curve b) and GNPs/Ch/GCE (curve c), respectively. The semicircle diameter in the impedance spectrum is equal to the charge transfer resistance (Rct ). The value of Rct depends on the dielectric and insulting properties of the electrode/electrolyte interface. At the bare GCE, a semicircle of about 800  in diameter with an almost straight tail line was observed, which was

500

b

400

-Z" / Ω

c

20

a

c

300 200 100 0 0

200

400

600

800

1000

1200

Z' / Ω Fig. 4. EIS at bare GCE (a), Ch/GCE (b) and GNPs/Ch/GCE 10.0 mM K3 [Fe(CN)6 ]/K4 [Fe(CN)6 ] (1:1 mixture) + 0.1 M KCl.

(c)

in

characteristic of a diffusion limiting step of the electrochemical process [39]. The diameter of the high frequency semicircle was apparently reduced at the Ch/GCE and the Rct value was 410 . The decrease of Rct suggested that the immobilized Ch film was favorable for the approaching of ferricyanide anions to the electrode surface, because the positively charged Ch film would attract more Fe(CN)6 3− to the modified electrode surface due to the electrostatic interaction. While depositing GNPs on the Ch film, the diameter of the high frequency semicircle was further reduced. The Rct value was about 300 , which was lower than Ch/GCE. This result may be attributed to the good conductivity of GNPs, which can enhance the electron transfer rate. The impedance change of the modification process indicated that Ch and GNPs have been successfully deposited on the GCE surface.

3.2. Electrocatalytic oxidation of hydrazine In order to investigate the electrocatalytic behavior of GNPs/Ch/GCE towards hydrazine oxidation, CVs were obtained in comparison with GNPs/GCE, Ch/GCE and bare GCE in the presence of 2.0 × 10−4 M hydrazine. As shown in Fig. 5, there was no defined anodic peaks for hydrazine oxidation on the bare GCE (curve a). Under the same experimental conditions, the CV obtained at Ch/GCE presented a large overpotential with a very drown out peak at about 0.57 V (curve b). The peak was rather broad due to the slow electron transfer kinetics of hydrazine oxidation process. However, in comparison with those on the Ch/GCE and bare GCE, a remarkable increase in oxidation current and negative shift of the peak potential can be observed at the GNPs/GCE (curve c). The 370 mV reduction of overpotential indicated an efficiently catalytic ability of the GNPs/GCE towards the oxidation of hydrazine. This large decrease in oxidation overpotential corresponds to the presence of high density arrays of GNPs and their electrocatalytic behavior. A well-defined oxidation peak appeared at 0.15 V on the GNPs/Ch/GCE (curve d). The peak current was about 3.4-fold enhancement of that at the GNPs/GCE. The increase of current response could be attributed to the high specific surface area and the increase of reversibility of the electron transfer process. Furthermore, it may be also generated from a surface accumulation of the electro-active species. The comparisons reveal that the GNPs have much stronger catalytic activity toward the oxidation of hydrazine than Ch film, and Ch supported GNPs have further enhanced catalytic activity. On the other hand, the electrocatalytic oxidation of hydrazine at low potential (0.15 V) is very useful for practical applications, since there are less risk for interference.

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50 40

1/2

50

slope / μA s

60

i / μA

In most cases, the electrochemical reaction is dependent on the pH value of aqueous solutions. The electrochemical behavior of hydrazine in PBS with different pH values was explored. The catalytic peak current (ip ) increased with increasing pH from 4.0 to 7.0 and remained almost constant between 7.0 and 9.0 in our experiments. Since the pKa of hydrazine is 7.9, the decrease in peak current at lower pH was attributed to the protonation of hydrazine. The protonated form is less active. Considering the practically applications, pH 7.4 PBS was chosen for the experiment. The scan rate effect on the oxidation of 2.0 × 10−4 M hydrazine at the GNPs/Ch/GCE was investigated by CV. A plot of anodic peak current versus the square root of scan rate (v1/2 ) yielded a straight line in the range of 10–400 mV/s, ip (␮A) = 1.54 + 3.42v1/2 (mV1/2 /s1/2 ) (R2 = 0.998). Moreover, the peak current did not increase with increasing the immersed time, indicating the electrocatalytic oxidation of hydrazine at the GNPs/Ch/GCE was a typical diffusion-controlled electron transfer mechanism. Additionally, the oxidation peak potential (Epa ) shifted toward positive direction with increasing the scan rate (v). A linear relationship between Epa and log(v) was obtained, indicating the oxidation of hydrazine was an irreversible electrode process. The Tafel slope (b) of 116 mV/decade was achieved based on the slope of Epa versus log(v). Assuming one electron transfer in the rate-determining step (n␣ = 1), the charge transfer coefficient (˛) was calculated as 0.5 according to the equation of b = 2.3RT/(1 − ˛)n␣ F. From the plot of Ip versus v1/2 , the number of electrons (n) involved in the overall reaction can be obtained according to the following equation for a totally irreversible diffusion-controlled process [40]:

30 20

a

10 0

30

0

100

200

N2 H4 + H2 O → N2 H3 + H3 O+ + e− +

N2 H3 + 3H2 O → N2 + 3H3 O + 3e

−

10 0 0.0

0.2

0.4

0.6

-1/2

t

(slow)

(I)

(fast)

(II)

N2 H4 + 4H2 O → N2 + 4H3 O+ + 4e− In order to get more information about the electrocatalytic process, chronoamperometry was used to evaluate the diffusion coefficient of hydrazine. Chronoamperometric measurements were carried out at different concentrations of hydrazine on the GNPs/Ch/GCE by setting the working electrode potential at 0.15 V. The diffusion coefficient was obtained according to the Cottrell equation [40]: nFAD1/2 c 1/2 t 1/2

where D and c are the diffusion coefficient (cm2 /s) and bulk concentrations (mol/cm3 ) of hydrazine, respectively. i is the current controlled by the diffusion of hydrazine from the bulk solution to the electrode/solution interface.

0.8

1.0

-1/2

/s

B

18

16

14

12

10 0.3

0.6

0.9

1.2

t

The rate-determining step is one electron transfer followed by a 3-electron process to give N2 as a final product. The overall reaction of hydrazine oxidation can be expressed as following reaction:

i=

300

c / μM

Ip = 3.01 × 105 n[(1 − ˛)n␣ ]1/2 Acb D1/2 v1/2 where A is electrode surface area, cb is hydrazine concentration and D is diffusion coefficient. Considering D = 2.46 × 10−5 cm2 /s (D was calculated from chronoamperometry), from the slope of Ip versus v1/2 plot, the total number of electrons (n) involved in the oxidation is evaluated to be 4. The mechanism of hydrazine oxidation depends significantly on the electrolyte solution and the nature of the electrodes. Under solution conditions whereas hydrazine was mainly present in its unprotonated form, and the protonated form which presented only a small extent, therefore, following mechanism could be proposed for the oxidation of unprotonated hydrazine at the GNPs/Ch/GCE.

A

i

40

20

Ic / Id

242

1/2

1.5

1.8

2.1

1/2

/s

Fig. 6. (A) Plot of i versus t−1/2 obtained from chronoamperometric measurements at the GNPs/Ch/GCE with different concentrations of hydrazine (a–i): 5.0 × 10−6 –3.0 × 10−4 M. Inset showed the relationship between the slopes of the linear segments and corresponding concentrations. (B) Plots of Ic /Id versus t1/2 , obtained from the data of chronoamperometric response at the GNPs/Ch/GCE in the presence and absence of 1.0 × 10−4 M hydrazine.

A calibration curve was recorded in 0.1 M PBS containing various concentrations of hydrazine in the range of 5–300 ␮M. The experimental plot of i versus t−1/2 at different concentrations of hydrazine was linear, as shown in Fig. 6A. The slopes of the resulting straight line were then plotted versus the concentrations of hydrazine (inset in Fig. 6A). From the slope of the plot, the D of hydrazine was calculated as 2.46 × 10−5 cm2 /s. The value is larger than that reported previously for hydrazine [13]. The overall electrochemical oxidation of hydrazine on the GNPs/Ch/GCE was controlled by diffusion of the species in bulk solution and the cross-exchange process between the species and the redox sites of GNPs/Ch composite film. Thus, the catalytic current was dominated by the rate of electron exchange (k) between the redox sites of the GNPs/Ch film and substrate. The value k can be evaluated using chronoamperometry based on the equation of Ic /Id = (kct)1/2 , where Id is the diffusion limited current in the absence of hydrazine, Ic is the catalytic current in the presence of hydrazine, and c is the bulk concentration. On the slope of the Ic /Id versus t1/2 plot, as shown in Fig. 6B, the value of k was estimated as 4.7 × 107 cm3 /mol s for hydrazine. 3.3. Determination of hydrazine Under the optimum conditions, the oxidation peak currents of various hydrazine concentrations at the GNPs/Ch/GCE were recorded by linear sweep voltammetry (LSV) in static solutions. Fig. 7A illustrated the effect of various hydrazine concentrations on the LSVs at the GNPs/Ch/GCE. Very well-defined voltammo-

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800

600

A

r

k

45

l

30

i / μA

15

400

a

0 -0.2 0.0 0.2 0.4 0.6

200

0 -0.4

-0.2

0.0

0.2

0.4

0.6

E / V (vs. SCE) 1000

45

B

30

ip / μA

800 600

15

243

negligible and have no effect on the diffusion of hydrazine toward the electrode surface. While hydrazine concentration increases, N2 evolution at the catalytic sites affects the normal diffusion of new analyte molecules to the electrode surface, in result of the decay of the current sensitivity. However, N2 bubble at the electrode surface has not been observed in our experiments. The current response was reproducible with a relative standard deviation (R.S.D.) of 2.8% for 10 repetitive determinations. The detection limit was found as 1.0 × 10−7 M (s/n = 3). According with these results, the linear response obtained at low concentrations of hydrazine can be used to construct hydrazine sensor, in that gas effect is negligible. The comparison between previously reported chemically modified electrodes for the determination of hydrazine was listed in Table 1. As can be seen, the designed GNPs/Ch/GCE exhibited relatively low detection limit, high current sensitivity and wide linear range with two linear segments. The advantages of the GNPs/Ch/GCE can be attributed to the composite structure of GNPs/Ch film, which has synergic effect toward hydrazine oxidation.

0 0

200

400

600

3.4. Interference studies

400 200 0 0

2000

4000

6000

8000

10000

c /μM Fig. 7. (A) LSVs of various concentrations of hydrazine at the GNPs/Ch/GCE (a–k): 5.0 × 10−4 , 1.0 × 10−3 , 1.5 × 10−3 , 2.0 × 10−3 , 3.0 × 10−3 , 4.0 × 10−3 , 5.0 × 10−3 , 6.0 × 10−3 , 7.0 × 10−3 , 8.0 × 10−3 , 9.3 × 10−3 M. Inset showed LSVs of low concentrations (l–r): 5.0 × 10−7 , 5.0 × 10−6 , 2.5 × 10−5 , 5.0 × 10−5 , 1.0 × 10−4 , 3.0 × 10−4 , 5.0 × 10−4 M. Scan rate: 50 mV/s. (B) The corresponding calibration curves for hydrazine.

grams were obtained. The height of the anodic peak increased with increasing concentrations. Fig. 7B showed the plot of the peak current (ip ) versus concentrations of hydrazine, which consisted of two linear segments with different slopes. A current sensitivity of 89.1 ␮A/mM in the range of 5.0 × 10−7 –5.0 × 10−4 M and sensitivity of 84.3 ␮A/mM in the range of 5.0 × 10−4 –9.3 × 10−3 M were observed. The facts agree with the work previously reported for the electrocatalytic oxidation of hydrazine [29,41,42], which is attributed to the change in the surface catalytic reaction conditions arising from the generation of N2 bubbles on the catalytic active sites. At low hydrazine concentrations, the formed N2 bubbles are

Nitrogen-containing compounds such as nitrate, nitrite and ammonia, often accompany with hydrazine in the nitrogen cycle process, as well as in a variety of industrial processes. Possible interferences for the determination of hydrazine at the GNPs/Ch/GCE were investigated by adding various foreign species into the solution. The tolerance limit is defined as the maximum concentration of the interfering substance that causes relative error less than 5% for the determination of 1.0 × 10−5 M hydrazine. The results showed that common ions had no interference on hydrazine determination, such as 400-fold quantities of NO3 − , NO2 − , Cl− , Br− , F− , I− , HCO3 − , PO4 3− , CO3 2− , C2 O4 2− , SO4 2− , Na+ , K+ , Mg2+ , Ca2+ , Ba2+ , Cu2+ , Ni2+ , Zn2+ , Fe3+ , Fe2+ and Al3+ , 150-fold quantities of glucose, sucrose, fructose, oxalic acid, tartaric acid, citric acid and malic acid, 25-fold quantities of l-tyrosine, l-arginine and l-glutamic acid, 10fold quantities of NH3 + , and 1-fold quantities of NH2 OH.

3.5. Real samples In order to evaluate the performance and feasibility of the proposed method for analysis of hydrazine, six distilled water samples were spiked with different standard concentrations of hydrazine. 3.0 ml of this solution was placed in the electrochemical cell for the determination using LSV method. The data given in Table 2 showed the satisfactory results.

Table 1 Comparisons of the responses of some hydrazine sensors constructed based on different modified electrodes. Electrode

Detection limit (␮M)

Sensitivity (␮A/mM)

Linear range (␮M)

Ref.

Ni(II)-BA-MWCNT-PEa BiHCF/CCEb Au/PPy/GCEc

0.8 3 0.2

69.9 4.2 126 35.6 17.0 – 38 12.2 89.1 84.3

2.5–200 7–1100 1–500 500–7500 10–400 500–2000 1–1050 5–800 0.5–500 500–9300

[9] [13] [29]

3,4-DHsalophen/GCEd

1.6 e

Mn(II)-complex/MWNTs/GCE Pd-CILEf GNPs/Ch/GCE a b c d e f

0.5 0.82 0.1

Ni(II)-BA-MWCNT-PEP: Ni(II)-baicalein complex modified multi-wall carbon nanotube paste electrode. BiHCF/CCE: bismuth hexacyanoferrate modified carbon ceramic electrode. Au/PPy/GCE: gold nanoparticle-polypyrrole nanowire modified glassy carbon electrode. 3,4-DHsalophen/GCE: N,N -bis(dihydroxybenzylidene)-1,2-diaminobenzene modified glassy carbon electrode. Mn(II)-complex/MWNTs/GCE: carbon nanotube and terpyridine manganese(II) complex modified glassy carbon electrode. Pd-CILE: palladium nanoparticles modified carbon ionic liquid electrode.

[41] [43] [44] This work

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Table 2 Results of the recovery test. Sample distilled water

Added (␮M)

Found (␮M)

Recovery (%)

R.S.D. (%, N = 15)

1 2 3 4 5 6

5.0 10.0 15.0 20.0 25.0 30.0

5.1 10.3 14.8 19.7 25.3 30.6

102 103 98.7 98.5 101 102

3.8 3.5 3.0 3.3 3.1 3.4

3.6. Stability and reproducibility The stability and the reproducibility of the GNPs/Ch/GCE were also investigated. A set of 15 replicate determinations for 10 ␮M hydrazine yield a relative standard deviation (R.S.D.) of 2.8%. Six pieces of the modified electrode were prepared and the R.S.D. for the individual determination of 100 ␮M hydrazine was 3.5%. These results indicated that the designed sensor had excellent reproducibility and anti-fouling ability. The stability of the GNPs/Ch/GCE was examined by monitoring the remained amount of current response after successive potential cycling the modified electrode in the potential range of −0.2 to 0.5 V in 0.1 M PBS for 200 cycles. The peak heights of the cycle voltammogram retained 96% of its initial value and no obvious potential shift was observed. On the other hand, the storage ability of the modified electrode was also studied. The current response to hydrazine was no apparent decrease in the first continuous 10 days by every day use and stored in 0.1 M PBS at 4 ◦ C. Only 15% leakage was found after two months. Under the selected conditions, the method showed the GNPs/Ch/GCE had good reproducibility and long-term stability. 4. Conclusions In this paper, we have developed a facile route to fabricate a new hydrazine sensor based on the GNPs/Ch composite film comprised of Ch layer and GNPs. The constructed GNPs/Ch film provided a nanostructure with large effective surface area, which can act as electron transfer medium and enhance charge transfer rate. The research work demonstrated that the electrochemical sensor exhibited stable and excellent electrocatalytic behavior toward hydrazine oxidation at a very low overpotential compared to that obtained on the Ch/GCE and GNPs/GCE. The sensor showed promising determination of hydrazine with many desirable properties, such as high sensitivity, wide linear range, low detection limit, long-term storage ability and good anti-interference ability. The strategy of the combination of Ch film and GNPs exhibited significant advantages of synergic effect of organic and metallic nanoparticles composite for construction of chemical and biochemical sensor. Acknowledgements This work was supported by the Specialized Research Fund for Shanghai Second Polytechnic University (XQD208014), Excellent Young Scholars Research Fund of Shanghai (egd08014), Program for New Century Excellent Talents in University (NCET-10-883), and Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. References [1] K. Yamada, K. Yasuda, N. Fujiwara, Z. Siroma, H. Tanaka, Y. Miyazaki, T. Kobayashi, Potential application of anion-exchange membrane for hydrazine fuel cell electrolyte, Electrochem. Commun. 5 (2003) 892–896. [2] S. Amlathe, V.K. Gupta, Spectrophotometric determination of trace amounts of hydrazine in polluted water, Analyst 113 (1988) 1481–1483.

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Biographies Jing Li received her Ph.D. in chemistry from University of Science and Technology of China, in 2008. His current research interest is in the area of electrochemistry and electrochemical sensors. Huaqing Xie received his Ph.D. in Materials of Science and Engineering from Shanghai Institute of Ceramics, Chinese Academy of Sciences, in 2002. He is a professor in Shanghai Second Polytechnic University. His current research interests are mainly in the area of nanofluids and electrochemistry. His is also interested in the fabrication and characterization of graphene and its application to nano-electronic devices. Lifei Chen received her Ph.D. degree in 2007 from East China University of Science and Technology. Now, she is an associate professor in Shanghai Second Polytechnic University. Her current research is on the thermal transport properties of carbon nanotube nanofluids.