Preparation of flower-like Pt nanoparticles decorated chitosan-grafted graphene oxide and its electrocatalysis of hydrazine

Sensors and Actuators B 236 (2016) 192–200

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Preparation of flower-like Pt nanoparticles decorated chitosan-grafted graphene oxide and its electrocatalysis of hydrazine Dejiang Rao, Qinglin Sheng ∗ , Jianbin Zheng ∗ Institute of Analytical Science, Shaanxi Provincial Key Laboratory of Electroanalytical Chemistry, Northwest University, Xi’an, Shaanxi 710069, China

a r t i c l e

i n f o

Article history: Received 12 January 2016 Received in revised form 28 May 2016 Accepted 30 May 2016 Available online 31 May 2016 Keywords: Electrochemical sensor Flower-like Pt nanoparticles Chitosan Graphene oxide Hydrazine

a b s t r a c t In this paper, a novel graphene oxide/chitosan/platinum (GO/CTS/Pt) nanocomposite was successfully synthesized by using CTS as protective agent and dispersant. Further, a hydrazine electrochemical sensor was constructed by immobilizing GO/CTS/Pt nanocomposites on a glassy carbon electrode (GCE). The morphology and composition of the nanocomposites were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and Xray diffraction (XRD). The electrochemical investigations indicated that the GO/CTS/Pt modified GCE showed excellent electrocatalytic ability towards the oxidation of hydrazine. The current responses of the GO/CTS/Pt modified GCE towards the addition of hydrazine showed a wide linear range from 2.0 × 10−5 to 1.0 × 10−2 mol L−1 at a low applied potential of 0 V (versus SCE), and the low detection limit was 3.6 ␮mol L−1 at the signal-to-noise ratio of 3. Moreover, the sensor also exhibited good reproducibility, stability and selectivity for hydrazine sensing. This study suggests that the GO/CTS/Pt nanocomposites might be further applied to other fields with great potential. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Hydrazine (N2 H4 ) and its derivatives have attracted great attentions due to their auspicious applications in industrial catalysis, biomedicine intermediates and rocket fuels [1–3]. Besides, it is also a highly toxic chemical which is considered hepato-toxic, carcinogenic and neurotoxin substance [4,5]. Therefore, the accurate and sensitive method to detect N2 H4 is of great significance for environmental protection and human health. So far, many methods have been reported on the detection of N2 H4 , such as chemiluminescence, titrimetry, chromatography and electrochemical method [6–9]. Among those methods, the non-enzymatic electrochemical methods were absolutely required and effective for hydrazine detection of their superior merits of stability, fast, sensitive and low cost [10,11]. Recently, noble metal nanoparticles (NPs) have been widely used in the construction of N2 H4 electrochemical sensors [12,13]. The Pt-based nanocomposites have been extensively used to modify electrode because of their excellent electrocatalytic properties [14,15]. Graphene oxide (GO) as an important member of graphene family, usually produced by the oxidation of graphite [16]. Graphene,

∗ Corresponding author. E-mail addresses: [email protected] (Q. Sheng), [email protected] (J. Zheng). http://dx.doi.org/10.1016/j.snb.2016.05.160 0925-4005/© 2016 Elsevier B.V. All rights reserved.

which possess abundant phenol hydroxyl (-OH), oxygen groups and carboxylic groups (-COOH) [17]. Thus, GO is readily soluble in water and bring a highly negatively charged to its surface. It can be used as a good substrate material to synthesize functional nanocomposites because of its large theoretical specific surface area (2630 m2 g−1 ) [18]. Meanwhile, GO is widely used in electrochemical sensing because of its excellent electron transport capacity and variety of functionalization methods [19–21]. Chitosan (CTS), a natural polymer on the earth, has been proverbially investigated for electrochemical sensors because of its biocompatibility, multiple functional groups (amino groups and hydroxyl groups) and film forming properties [22,23]. In addition, CTS has a large number of amino groups (-NH2 ) and hydroxyl groups (-OH) that allow it to be easily applied in chemical modification [24]. The positively charged CTS is easily grafted onto the surface of GO by self-assembled method in aqueous solution under ultrasonic condition [25]. It can be attributed to the groups between GO and CTS form massive amide bond and hydrogen bond in aqueous solution, as shown in Scheme 1A [26]. Consequently, the GO/CTS composites possess both a remarkably graphitic property and favorable water solubility. Besides, the combination of GO and CTS results in the dispersion of GO at the individual sheet level in aqueous solution. Thus, a good dispersion of GO in CTS is expected [27].

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Scheme 1. Schematic illustration for the preparation of GO/CTS composites (A) and the GO/CTS/Pt/GCE (B).

The catalytic performance of Pt NPs is limited by their aggregation phenomenon. Many substances have been reported as dispersants for the preparation of Pt NPs, such as poly(diallyldimethylammonium chloride) (PDDA), polyvinyl pyrrolidone (PVP) and poly(methacrylic acid sodium salt) (PMAA) [28–30]. Nevertheless, the advantages of CTS will promote its application in the preparation of functional nanocomposites. According to our best knowledge, CTS was rarely used to disperse Pt NPs. The positively charged CTS can be combined with PtCl6 2− and further reduced to Pt NPs in situ more easily [31]. This proposed method is simpler than previous methods, for instance, electrodeposition and high temperature reduction [32,33]. Therefore, Pt NPs decorated chitosan-grafted graphene oxide nanocomposites were obtained by using CTS as protective agent and dispersant in our present study. The obtained nanocomposites can provide a large specific active site and effectively enhance electron transfer rate.

Herein, a novel graphene oxide/chitosan/platinum (GO/CTS/Pt) nanocomposite was designed and synthesized by a new synthetic strategy of CTS as protective agent and dispersant. Meanwhile, a novel N2 H4 electrochemical sensor was presented based on the GO/CTS/Pt nanocomposites. This synthetic strategy is promising application in the related fields of electrochemical sensing.

2. Experimental 2.1. Materials and reagents Ethanol, N2 H4 , NaOH, and KMnO4 were purchased from Tianjin Tianli Chemistry Reagent Co., Ltd. (Tianjin, China); NaBH4 was purchased from Guangdong Guanghua Chemical Factory Co., Ltd. (Guangdong, China); CTS (MW 5–6 × 105 , purity >90% deacetylation) and graphite powder (purity >99.9%, 325 mesh, Alfa Aesar)

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were purchased from Shanghai Yuanju Biotechnology Co., Ltd. (Shanghai, China); chloroplatinic acid hexahydrate (H2 PtCl6 ·6H2 O) was got from Shanghai Reagent Factory (Shanghai, China). The 0.1 mol L−1 phosphate buffered saline (PBS, pH 8.0) was used as the supporting electrolyte. The other reagents and chemicals used were of analytical grade. In addition, the aqueous solutions in the experiment were prepared with deionized water which made from a Millipore system (Milli-Q, China, resistivity >18 M cm). 2.2. Apparatus and electrochemical measurements The transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) were performed by a Tecnai G2 F20 S-TWIN (FEI, USA) measurement. The images of scanning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDX) were obtained by a JSM-6700 F (JEOL, Japan) measurement. The X-ray diffraction (XRD) patterns were observed by a D/MAX-3C (Rigaku, Japan) measurement. UV–vis spectrophotometric measurements were accomplished on an Agilent 8453 UV–vis spectrophotometer (Agilent, China). A conventional three-electrode electroanalysis system of CHI 660D electrochemical workstation (Shanghai CH Instrument Co., Ltd., China) was used to carry out the electrochemical study. The bare GCE (diameter: 3.0 mm), GO/CTS modified GCE (GO/CTS/GCE) and GO/CTS/Pt modified GCE (GO/CTS/Pt/GCE) were used as working electrodes, respectively. The Pt wire electrode and saturated calomel electrode (SCE) was used as counter electrode and reference electrode, respectively. The temperature in this study was performed at the room temperature (25 ± 2 ◦ C). 2.3. Fabrication of the sensor 2.3.1. Synthesis of GO and GO/CTS composites GO was synthesized by the Hummers’ method [34]. The GO/CTS composites were prepared by self-assembled method under ultrasonication. In a typical case, 40 mg of the as-prepared GO was added to 80 mL deionized water and exfoliated by ultrasonication to form a GO colloidal dispersion (0.5 mg/mL). After that, 8 mL of CTS (0.5%, mass ratio) was added to the above GO colloidal dispersion and kept ultrasonic agitation for 1 h. The products were collected by centrifugation at 7000 rpm for 10 min, washed with deionized water and ethanol for several times. Finally, the resulting products were placed in an oven and dried at 45 ◦ C for 6 h. 2.3.2. Synthesis of GO/CTS/Pt nanocomposites The GO/CTS/Pt nanocomposites were synthesized by chemical reduction method in alkaline aqueous solution. Briefly, 20 mg of GO/CTS composites were thoroughly dispersed in 40 mL deionized water by ultrasonication. After that, 50 ␮L of 1 mol L−1 NaOH solution and 1.2 mL of 38.68 mmol L−1 (mM) H2 PtCl6 aqueous solution were added to the GO/CTS colloidal dispersion under ultrasonic conditions. The mixture was then stirred for 30 min at room temperature. Further, excessive NaBH4 solution was subsequently added drop by drop with magnetic stirring. The reaction was carried out at room temperature for overnight. The expected products were obtained by centrifugation and abstersion, and then dried at 45 ◦ C for 6 h. 2.3.3. Modification of electrode Prior to use, the GCE was polished to a mirror-like surface using 1.0 and 0.3 ␮m alumina slurries. The GCE was washed with deionized water and completely cleaned in ethanol-water (1:1, v/v ratio) solution by ultrasonication. Further, 1 mg of GO/CTS/Pt nanocomposites was completely dispersed in 1 mL of deionized water. The obtained suspension (5 ␮L) was dropped to the mirror-like surface of GCE and dried in air at room temperature. The preparation of

GO/CTS composites (A) and the GO/CTS/Pt/GCE (B) are schematically shown in Scheme 1. 3. Results and discussion 3.1. Characterization of GO/CTS/Pt nanocomposites Fig. 1 shows the SEM images of GO (A), GO/CTS (B), GO/CTS/Pt (C); TEM images of GO/CTS/Pt (D and E) and high-resolution transmission electron microscopy (HRTEM) images of Pt NPs (F). The sheet structure of GO with smooth surface can be observed in Fig. 1(A). However, for GO/CTS, it can be observed that the polymer films of CTS attached onto the surface of the GO and the surface of GO was not as smooth as before. As shown in Fig. 1(C), some white highlights were anchored on the sheet structure of GO/CTS composites, indicating that Pt NPs were successfully loaded on the surface of the GO/CTS composites. Besides, the morphology of GO/CTS/Pt nanocomposites was further confirmed by TEM. As shown in Fig. 1(D) and (E), the flower-like and well-dispersed Pt NPs were deposited onto the surface of GO/CTS composites without obvious aggregation. Further characterization on the special morphology of the Pt NPs was accomplished by HRTEM. The HRTEM image of Pt NPs (Fig. 1(F)) shows that the diameter of the flower-like Pt NPs is 20 ± 3 nm. The above results suggested that the Pt NPs can be effectively loaded on the surface of GO/CTS composites by this synthetic method. The component analysis can effectively reveal the element composition of nanocomposites. Furthermore, as shown in Fig. 1(G), the EDX spectrum indicates that the nanocomposites are composed of C, O, N and Pt elements. Additionally, the CTS mainly contain three elements of C, N and O. These results strongly indicated that the GO/CTS/Pt nanocomposites were successfully synthesized by this synthetic method. Fig. 2 shows the XRD patterns of (A) GO/CTS composites and (B) GO/CTS/Pt nanocomposites. Compared with GO/CTS, the XRD pattern of GO/CTS/Pt nanocomposites presents four highly obvious diffraction peaks at 40.3◦ , 45.9◦ , 67.4◦ and 81.1◦ , which consistent with the Pt face-centered cubic structure and corresponding to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystalline planes of Pt NPs [35,36]. This result further confirmed the successful functionalization of Pt nanoparticles on GO/CTS composites. 3.2. Electrochemical behavior of GO/CTS/Pt nanocomposites The electrochemical impedance spectroscopy (EIS) is an effective method to investigate the interfacial electron transfer characteristics of modified electrodes. The semicircle diameter of Nyquist plots is usually equal to electron transfer resistance (Ret ) [37]. The relationship between Ret and exchange current density (i0 ) is consistent with the equation Ret = (RT)/Fi0 . Here, R is the ideal gas constant (8.314 J mol−1 K−1 ) and T is the room temperature (298.15 K) [37]. Fig. 3 shows the Nyquist plots of the bare GCE (curve a), GO/CTS/GCE (curve b) and GO/CTS/Pt/GCE (curve c). As shown in Fig. 3, the bare GCE shows an extremely small Ret of 301.2 . Moreover, the Ret of GCE increases from 301.2  to 1049.6  after the GO/CTS composites fixed to the surface of GCE. However, the Ret of GO/CTS extremely decreases to 650.3  from 1049.6  after the Pt NPs were modified onto its surface. These results demonstrate that GO/CTS/Pt nanocomposites could effectively enhance the electron transfer efficiency. The electrochemical behavior of the GO/CTS/Pt/GCE was evaluated by the linear sweep voltammetry (LSV). For comparison, the electrochemical current responses at the bare GCE, GO/GCE and GO/CTS/GCE in the absence (a, c, e and g) and presence (b, d, f and h) of 4.0 mM N2 H4 were also recorded at the scan rate of 0.1 V s−1 . As shown in Fig. 4, the bare GCE shows no obvious electrocatalytic

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Fig. 1. SEM images of GO (A), GO/CTS composites (B) and GO/CTS/Pt nanocomposites (C); TEM images of GO/CS/Pt nanocomposites (D, E); HRTEM images of Pt NPs (F) and EDX spectrum of GO/CTS/Pt nanocomposites (G).

A

B

Fig. 2. XRD patterns of (A) GO/CTS composites and (B) GO/CTS/Pt nanocomposites.

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2000 a b c

1000

500

0 0

500

1000

1500

2000

Z'/ohm Fig. 3. Nyquist plots of (a) the bare GCE, (b) GO/CTS/GCE, (c) GO/CTS/Pt/GCE in 0.1 mol·L−1 KCl solution containing 5.0 mM [Fe(CN)6 ]4−/3− at open-circuit potential conditions. (Frequency range: 10–10,000 Hz; AC amplitude: 5 mV).

response current whether or not containing N2 H4 in the potential range of −0.6 to 0.2 V, indicating no catalytic ability for N2 H4 . Similarly, it’s clear that both GO/GCE and GO/CTS/GCE show no obvious electrocatalytic response current in the absence (c and e) and presence (d and f) of 4.0 mM N2 H4 at the scan rate of 0.1 V s−1 . However, as shown in Fig. 4B (g and f), for GO/CTS/Pt/GCE, it can be derived that it exhibits a large electrocatalytic response current, indicating that the obtained Pt NPs show excellent catalytic ability towards the oxidation of N2 H4 . Moreover, a low over voltage of −0.05 V was obtained for the oxidation of N2 H4 at the GO/CTS/Pt/GCE. It may be attributed to GO with super-high specific surface area provides a large number of active sites to capture more N2 H4 and well-dispersed Pt NPs enhance the electron transfer efficiency. The response current is explained by supposing that the oxidation of N2 H4 was initiated by the dissociative adsorption of N2 H4 . The oxidation of N2 H4 at GO/CTS/Pt/GCE involved four electrons transferred reaction. Meanwhile, this process was controlled by thermodynamics rather than kinetics [38]. The catalytic decomposition of N2 H4 on the GO/CTS/Pt/GCE can be expressed as follows [5,38]: N2 H4 + 4OH−  N2 + 4H2 O + 4e−

(1)

In order to study the influences of pH on oxidation of N2 H4 at GO/CTS/Pt/GCE and improve the analytical characteristics of this N2 H4 sensor, the effect of the pH value of 0.1 mol L−1 PBS and the volume (VH2PtCl6 /mL) of H2 PtCl6 aqueous solution (38.68 mmol L−1 ) were carefully investigated. The influences of the pH and VH2PtCl6 (mL) were illustrated in Fig. 5. The effect of pH value (5.0, 6.0, 7.0,

2

A

120

b a

h

90

I/µA

I/ µA

0

B

-2

1

c

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ef

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E vs SCE/V

-4 -0.6

d

g

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-Z"/ohm

1500

7.5, 8.0, 8.5, 9.0 and 10.0) on the oxidation peak current of N2 H4 (3.0 mM) was investigated by LSV with the GO/CTS/Pt/GCE. These results were shown in Fig. 5(A). As shown in Fig. 5(A), the oxidation peak current increases from pH 5.0–8.0 and decreases from pH 8.0–10.0 in the 0.1 mol L−1 PBS. The maximum response current was obtained at pH 8.0. Therefore, the pH 8.0 of 0.1 mol L−1 PBS was used as the support electrolyte in the electrochemical experiments. The amount of Pt NPs in the nanocomposites is a critical factor to get a higher analytical sensitivity for the detection of N2 H4 because Pt NPs can dramatically enhance the electron transfer efficiency. Similarly, the effect of the volume (V/mL) of H2 PtCl6 aqueous solution was also investigated. The GO/CTS/Pt nanocomposites with different amount of Pt NPs were prepared, and then constructed different GO/CTS/Pt/GCE with these nanocomposites to detect N2 H4 (3.0 mM). The results were shown in Fig. 5(B). As shown in Fig. 5(B), the current response to the electrocatalytic oxidation of N2 H4 was the highest at the VH2PtCl6 of 1.2 mL. This may be attributed to the excessive Pt NPs have been aggregated, and the electrocatalytic properties have been constrained significantly. Therefore, 1.2 mL of H2 PtCl6 aqueous solution is more appropriate. The oxidation peak current increases with raising the concentration of N2 H4 from 1.5 to 4.5 mM on the GO/CTS/Pt/GCE were shown in Fig. 6(A). As shown in Fig. 6(A), the oxidation peak currents of N2 H4 increase gradually after gradient addition of N2 H4 to 0.1 mol L−1 PBS (pH 8.0). These results demonstrate that the GO/CTS/Pt nanocomposites exhibited excellent catalytic ability for N2 H4 oxidation. The oxidation peak currents of the GO/CTS/Pt/GCE in the presence of 4.0 mM N2 H4 in 0.1 mol L−1 PBS (pH 8.0) at different scan rates (v) were investigated. As shown in Fig. 6(B), the catalytic current increases with increasing the scan rate from 0.02 to 0.14 V s−1 . The oxidation peak current and the square root of the scan rate (v1/2 ) show a good linear relationship (R = 0.9992, n = 7) which adheres to the Randles-Sevcik equation: Ip = 0.4463n3/2 F3/2 ADapp 1/2 Cv1/2 /(RT)1/2 [39,40]. Here, n is the number of electron transfer, R is the ideal gas constant (8.314 J mol−1 K−1 ), F is the Faraday’s constant (96,485C mol−1 ), A is the effective surface area of the electrode, T is the room temperature (298.15 K), Dapp is the apparent electron diffusion coefficient and C is used as the concentration of effective electroactive site [39]. As shown in Fig. 6(B), the relationship of Ip with v1/2 is observed by the linear relation equations Ip (␮A) = 15.8 + 311.6 v1/2 ((V s−1 )1/2 ). According to the slope of the Ip − v1/2 curve and the Randles-Sevcik equation, the apparent electron diffusion coefficient (Dapp ) can be accurately evaluated in this experiment. Thus, the apparent electron diffusion of GO/CTS/Pt/GCE could be controlled. Some significant sensing characteristics of this sensor were evaluated by the current-time (I-t) curves. Fig. 7(A) shows the typ-

0 -0.4

-0.2

E vs SCE/V

0.0

0.2

-0.6

-0.4

-0.2

0.0

0.2

E vs SCE/V

Fig. 4. LSV curves obtained by (a, b) bare GCE, (c, d) GO/GCE, (e, f) GO/CTS/GCE and (g, h) GO/CTS/Pt/GCE in 0.1 mol L−1 PBS (pH 8.0) in the absence (a, c, e and g) and presence (b, d, f and h) of 4.0 mM N2 H4 . (Scan rate: 0.1 V s−1 ).

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A

197

B

Fig. 5. (A) Effect of pH value of 0.1 mol L−1 PBS on the oxidation peak current of N2 H4 (2.5 mM); (B) The oxidation peak current of GO/CTS/Pt/GCE with different amounts of Pt NPs: 0.5, 1.0, 1.2, 1.5 and 1.8 mL H2 PtCl6 aqueous solution (38.68 mM).

A

B

Fig. 6. (A) LSV curves obtained by GO/CTS/Pt/GCE in the presence of different N2 H4 concentrations (From a to h: 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 and 4.5 mM) in 0.1 mol L−1 PBS (pH 8.0) at a scan rate of 0.1 V s−1 ; (B) LSV curves obtained by GO/CTS/Pt/GCE in 0.1 mol L−1 PBS (pH 8.0) containing 4.0 mM N2 H4 with different scan rates (From a to g: 0.02, 0.04, 0.06, 0.08, 0.10, 0.12 and 0.14 V s−1 ). Inset: Linear fitting chart of oxidation peak currents of N2 H4 versus v1/2 .

A

B

Fig. 7. (A) I-t curves obtained by the GO/CTS/Pt/GCE upon the successive injection of N2 H4 in 0.1 mol L−1 PBS (pH 8.0) with stable stirring (Applied potential: 0 V). Inset: Amplification of current versus time variation curves for low concentration. (B) Linear fitting chart of oxidation peak currents of N2 H4 versus its concentration.

ical amperometric responses of the GO/CTS/Pt/GCE in 0.1 mol·L−1 PBS (pH 8.0) with the successive injection of N2 H4 . Although the GO/CTS/Pt/GCE exhibited the maximum oxidation peak current at −0.05 V, detection of the N2 H4 was carried out at 0 V. Such a low applied potential can reduce the measurement background and avoid the interference of other coexisting electroactive substances [41,42]. The calibration curve of the N2 H4 sensor was shown in Fig. 7(B). Here, the GO/CTS/Pt/GCE shows a fast response time (about 3s) towards the oxidation of N2 H4 . The linear regression equation for this response is Ip (␮A) = 7.39 C (mM) + 3.68 with a correlation coefficient of 0.9997 (n = 19). The GO/CTS/Pt/GCE catalyzed N2 H4 within a wide linear range from 2.0 × 10−5 to 1.0 × 10−2 mol L−1 at a low applied potential of 0 V. The sensitivity was calculated to be 104.6 ␮A mM−1 cm−2 . The limit of detection (LOD) was estimated to be 3.6 ␮M at a signal-to-noise ratio of

3. On the basis of the above excellent electrochemical sensing characteristics, it can be known that the GO/CTS/Pt nanocomposites exhibited good catalytic properties towards the oxidation of N2 H4 . The comparisons of analytical characteristics of the proposed sensor with other reported N2 H4 electrochemical sensors were listed in Table 1. It is noticeable that our sensor has an acceptable detection limit and linear range. Moreover, the sensitivity of our sensor is more comparable with previous sensors towards the oxidation of N2 H4 [43,44,47]. This good analytical characteristics of GO/CTS/Pt/GCE for the oxidation of N2 H4 are attributed to the super-high specific surface area of GO and excellent electrical conductivity of highly dispersed Pt NPs.

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Table 1 Comparison of analytical performances of the proposed N2 H4 sensor with other N2 H4 electrochemical sensors. Sensors

Linear range (mM)

LOD (␮M)

Sensitivity

References

Ni(II)-BAa -MWCNT-PE MWCNT/Chlorogenic/GCE PdNPs-EDACb /GCE nPtc -MWCNT/GCE Nano-Au/Tid GO/CTS/Pt/GCE

0.02–1.0 0.0025–5 0.005–0.15 – 0.5–4 0.02–10

9.2 8 1.5 0.0005 42 3.6

3.92 ␮A mM−1 4.1 ␮A mM−1 cm−2 – 19.5 ± 0.6 ␮A mM−1 1.117 ␮A ␮M−1 104.6 ␮A mM−1 cm−2

[43] [44] [45] [46] [47] This work

– Not provided; a baicalein; b ethylenediamine cellulosec nanosized Pt; d nanoporous gold particles modified titanium electrode.

Table 2 The results of determinations of N2 H4 in industrial waste water sample. Sample number (No.)

This sensor (␮M)

RSD (%)

Ultraviolet spectrophotometry (␮M)

RSD (%)

No. 1 No. 2 No. 3

16.21 41.42 130.46

3.6 4.7 1.8

15.92 41.33 130.57

2.7 1.5 3.8

(The result of average of five determinations by the GO/CTS/Pt/GCE).

3.3. Reproducibility and stability study

45

3.4. Selectivity study The selectivity of this N2 H4 sensor was studied by the chronoamperometry. The amperometric response of the GO/CTS/Pt/GCE upon the injection of N2 H4 and other electroactive species in 0.1 mol L−1 PBS (pH 8.0) with the applied potential of 0 V were shown in Fig. 8. Here, glucose (Glu), glutamic acid (GA), NaNO3 and ascorbic acid (AA) were used as correlative electroactive species. This four electroactive species (1.0 mM, respectively) and N2 H4 (1.0 mM) were added to the 0.1 mol L−1 PBS (pH 8.0), respectively. The results are shown in Fig. 8. As illustrated in Fig. 8, the electroactive species of Glu, GA, NaNO3 and AA exhibited no interference even if the concentrations of the interference substances are the same as the N2 H4 . These results show that GO/CTS/Pt/GCE has a good selectivity against other electroactive species. 3.5. Real sample analysis in industrial waste water samples The ability of the sensor to detect N2 H4 in a real sample was studied by using the GO/CTS/Pt/GCE, and the results were compared with ultraviolet spectrophotometry. The industrial waste water (In Xi’an, China) was used as real samples. Briefly, the industrial waste water sample was added to 0.1 mol L−1 PBS (pH 8.0) and recorded the amperometric responses to the oxidation of N2 H4 on the GO/CTS/Pt/GCE at the applied potential of 0 V. The reaction between N2 H4 and p-dimethylaminobenzaldehyde can produce

30 I/ A

The reproducibility and stability of the GO/CTS/Pt/GCE play an important role in the electrochemical experiment. The reproducibility of this N2 H4 sensor was investigated by the well-known chronoamperometry. Five GO/CTS/Pt/GCEs were fabricated and their current responses to 2.0 mM N2 H4 were measured. Results showed that the relative standard deviation (RSD) of this reproducibility verification experiments were less than 4.0%, suggesting that the GO/CTS/Pt/GCE exhibited a good repeatability. The stability of GO/CTS/Pt/GCE was also evaluated by adding 2.0 mM N2 H4 into the 0.1 mol L−1 PBS (pH 8.0) with three GO/CTS/Pt/GCEs as the working electrodes. Results showed that the current responses of the GO/CTS/Pt/GCEs remained more than 90% of its initial amperometric current responses after four weeks. Thus, the above results indicate that the GO/CTS/Pt/GCE represents good reproducibility and stability.

N2H4

GA

AA

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0 100

Glu NaNO3 N2H4

200

300

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t/s Fig. 8. Amperometric responses obtained by the GO/CTS/Pt modified GCE on successive addition of N2 H4 , Glu, GA, NaNO3 and AA (1.0 mM, respectively) in 0.1 mol L−1 PBS (pH 8.0) under stirring conditions. (Applied potential: 0 V).

brown compounds in aqueous solution. Thus, the ultraviolet spectrophotometry is suitable for the accurate determination of N2 H4 concentration. Further, evaluation of N2 H4 in this industrial waste water sample was studied by the ultraviolet spectrophotometry. The experimental results are listed in Table 2. As shown in Table 2, the noteworthy RSD from 1.8% to 4.7% were calculated. Moreover, the detection results of our proposed N2 H4 sensor are consistent with that by ultraviolet spectrophotometry. Thus, it can be concluded that the present sensor could be used to detect N2 H4 in real samples. 4. Conclusions In summary, we report a simple synthetic strategy to prepare GO/CTS/Pt nanocomposites. Meanwhile, a non-enzymatic N2 H4 electrochemical sensor was fabricated based on GO/CTS/Pt nanocomposites. The flower-like Pt nanoparticles decorated chitosan-grafted graphene oxide have been vindicated to possess excellent electrocatalytic ability. The electrochemical study suggested that the obtained GO/CTS/Pt nanocomposites have high electrocatalytic activity for N2 H4 oxidation and they can be adopted as a catalyst for the detection of N2 H4 with high sensitivity and selectivity. Moreover, the sensor also exhibits a low LOD of 3.6 ␮M

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Biographies Dejiang Rao is a master degree candidate of Institute of Analytical Science, Northwest University, China. One of his research interests is non-enzymatic sensors based on carbon-based functional materials. Qinglin Sheng is an associate professor of Institute of Analytical Science, Northwest University, China. He received his doctor degree from Northwest University, China, in 2009. The current research interests are synthesis of nanomaterials and its application in biosensors.

Jianbin Zheng is a professor of Institute of Analytical Science, Northwest University, China. He obtained his Ph.D. in 1997 at the department of chemistry of Northwest University, China. He finished his postdoctoral work in 2000 in Xi’an, Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, China. His research interests include electroanalytical chemistry, bioelectrochemical sensor, ionic liquid electrochemistry, HPLC electrochemistry and chemometrics.