Hollow Polypyrrole Nanocomposites

Oct 31, 2016 - Gopalan Anantha-Iyengar , Komathi Shanmugasundaram , Muthuchamy Nallal , Kwang-Pill Lee , Michael J. Whitcombe , Dhana Lakshmi ...
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Facile synthesis of prussian blue/hollow polypyrrole nanocomposites for enhanced hydrogen peroxide sensing Ziyin Yang, Xiaohui Zheng, and Jianbin Zheng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02953 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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Facile synthesis of prussian blue/hollow polypyrrole nanocomposites for enhanced hydrogen peroxide sensing Ziyin Yang, Xiaohui Zheng and Jianbin Zheng * Institute of Analytical Science, Shaanxi Provincial Key Laboratory of Electroanalytical Chemistry, Northwest University, Xi’an, Shaanxi 710069, China. E-mail address: [email protected]

Abstract A novel strategy is developed to synthesize prussian blue nanocubes/hollow polypyrrole (PB/H-PPy) nanocomposites for enhanced electrochemical determination of hydrogen peroxide (H2O2). PB/H-PPy were prepared through a facile approach, where Fe3O4 spheres acted as both the template and the source of Fe3+. It was found that the dissolving of Fe3O4 in acidic environment not only made the successful formation of hollow structure of PPy but also provided Fe3+ and assisted the formation of PB around H-PPy, therefore leading to the formation of PB/H-PPy. The morphology,

structure

and

electrochemical

properties

of

PB/H-PPy

were

characterized by transmission electron microscopy (TEM), field-emitting scanning electron microscope (FESEM), X-ray diffraction spectroscopy (XRD), fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and electrochemical techniques. The results indicated that a large number of PB nanocubes were densely distributed on the surface of H-PPy. Meanwhile, the combination of PB with H-PPy made the sensor based on PB/H-PPy exhibit an excellent performance toward H2O2 detection with a wide linear range of 5.0 µM to 2.775 mM, a high sensitivity of 484.4 µA mM-1 cm-2 and a low detection limit of 1.6 µM (S/N = 3). This work provided a new approach for rational design and fabrication of electrocatalytic material with improved catalytic activity. Keywords: Prussian blue; Hollow polypyrrole; Fe3O4; Hydrogen peroxide; Nonenzymatic sensor 1. Introduction The determination of hydrogen peroxide (H2O2) is important [1-3] and many H2O2 detection methods have been developed [4-6]. Among these methods,

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electrochemical sensors attracted much attention owning to their advantages of rapid response, high sensitivity and easy miniaturization [7]. Especially, enormous research effort was focused on non-enzymatic sensors [7]. Moreover, in order to improve the performance of sensors, nanomaterials including noble metals [8, 9], metal oxides [10, 11], alloys [12] were usually employed to fabricate non-enzymatic H2O2 sensors, which can be ascribed to their unique advantages of large surface area, excellent catalytic properties as well as enhanced mass transport. Among nanomaterials, prussian blue (PB) was extensively studied for its excellent catalytic activity toward the reduction of H2O2 [13, 14]. Moreover, in comparison with conventional nanomaterials, the sensor based on PB usually required a lower working potential [15], which was beneficial to prevent the interference of other species. However, PB possessed low stability and poor conductivity [15], which decreased its electrocatalytic activity and limited its efficient application in electrochemical sensors. As one of conductive polymers, polypyrrole (PPy) attracted much attention because of its good stability, excellent conductivity, low cost and ease of synthesis [16]. In particular, enormous research effort was paid on hollow PPy (H-PPy) for its particular chemical and physical properties [17]. For example,

Zhang [18]

synthesized RGO-hollow PPy for supercapacitor application. The electrochemical results showed that the composite exhibited high capacitance and excellent stability, which benefited from the hollow structure of PPy spheres and sandwich structure of RGO-PPy composite. In addition, Wang [19] synthesized hollow carbonized PPy spheres which also exhibited excellent electrochemical properties. Therefore, in order to take full advantages of PB and H-PPy, it was desirable to synthesize PB/H-PPy. The combination of PB with H-PPy not only improved the conductivity and stability of PB and therefore increased its electrocatalytic activity but also minimized the diffusion resistance of analyte and provided a large surface for H2O2 molecules adsorption and reaction, making PB/H-PPy an advanced material for fabricating electrochemical sensors. Up to now, many methods have been employed to synthesize PB/PPy for enzymeless H2O2 detection [20, 21]. For example, Zou [20] synthesized

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PPy@PB composite by one-step chemical method, in which pyrrole was polymerized using acidic FeCl3-K3[Fe(CN)6] as oxidant. Meanwhile, pyrrole can be employed as the reductant for FeCl3-K3[Fe(CN)6] to generate PB. Jin [21] prepared MWCNT/PPy/PB by directly mixing Fe3+-[Fe(CN)6]3- with MWCNT/PPy. In this process, Fe3+ was reduced to Fe2+ by PPy and then Fe2+ reacted with [Fe(CN)6]3- to form PB nanoparticles. The results revealed that the sensor based on MWCNT/PPy/PB exhibited good response toward the detection of H2O2. In above methods, PB was formed and decorated on solid PPy through the reaction between Fe3+ and [Fe(CN)6]3- by adding reducing reagents. However, few studies have managed to synthesize PB through slowly releasing Fe3+ to [Fe(CN)6]4- solution without employing additional reducing reagents and it also remains a challenge to explore a facial approach to support PB on H-PPy for applications. In this paper, PB/H-PPy was synthesized through a facile method. In the method, PPy was coated on the surface of Fe3O4 spheres by chemical oxidation polymerization and then PB/H-PPy was obtained by adding acidic K4[Fe(CN)6] into PPy@Fe3O4 solution, where the dissolving of Fe3O4 in acidic environment not only made the successful formation of hollow structure of PPy but also provided Fe3+ and assisted the formation of PB around H-PPy. Then, the nonenzymatic H2O2 sensor based on PB/H-PPy was fabricated and its electrochemical performance toward H2O2 was investigated. 2. Experimental 2.1. Reagents and materials Pyrrole monomer (C4H5N), K4[Fe(CN)6]·3H2O and ferric chloride crystal (FeCl3·6H2O, 99.0%) were obtained from Shanghai Yuanju Biotechnology Co., Ltd (Shanghai, China). 2.2. Apparatus Transmission electron microscopic (TEM) images were carried out by Tecnai G2 F20 S-TWIN (FEI, USA). Field-emitting scanning electron microscope (FESEM) images were carried out by SU8020 (HITACHI, Japan). X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALAB MK II X-ray

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photoelectron spectrometer using Mg as the exciting source. X ray diffraction (XRD) patterns of the samples were taken by D/MAX 3C (Rigaku, Japan). Fourier transform infrared spectroscopy (FTIR) was recorded with TENSIR 27 (Bruker, German). Electrochemical measurements were carried out in a conventional three-electrode electroanalysis system controlled by CHI 660 electrochemical workstation (Shanghai CH Instrument Co. Ltd., China). A conventional three-electrode cell was used, including a glassy carbon electrode (GCE, geometric area = 0.07 cm2) as the working electrode, a saturated calomel electrode as the reference electrode and platinum foil as the counter electrode. All potentials given in this work were referred to the saturated calomel electrode. 2.3. Synthesis of PB/H-PPy 2.3.1 Preparation of Fe3O4@PPy Fe3O4 were prepared according to a typical method [22]. In order to prepare Fe3O4@PPy [23], 0.05 g SDS was added into 50 mL Fe3O4 solution (0.005 g/mL) and stirred for 2 h. Then, 0.075 g pyrrole monomer was added into above solution with stirring. After that, 5 mL FeCl3·6H2O solution (0.025 g) was added to begin the polymerization. After stirring for 2 h, Fe3O4@PPy was separated and washed with water for several times. 2.3.2 Preparation of PB/H-PPy 8.5 mL K4[Fe(CN)6]·3H2O solution (0.128 g) and 1.5 mL HCl solution (12 M) were added into 10 mL Fe3O4@PPy solution (0.03 g) and stirred for 24 h. After that, PB/H-PPy was separated and washed with water for several times. Then, the obtained powder was dried at 50 ℃ for 10 h. 2.4. Electrode modification GCE was prepared by the casting method. 2.0 mg of PB/H-PPy were added into 1 mL of water with sonicating. Then, 6 µL of PB/H-PPy solution and 3 µL of Nafion (0.05 wt%) was dropped onto the surface of GCE and dried in air at room temperature. 3. Results and Discussion 3.1 Characterizations of PB/H-PPy

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Figure 1. Experimental procedure of synthesis of PB/H-PPy.

Figure 1 revealed the schematic illustration of preparing PB/H-PPy. Core-shell Fe3O4@PPy was formed through the chemical oxidation polymerization using FeCl3 as oxidant under the assistance of anion surfactant. Then, Fe3O4@PPy were separated and washed with ultrapure water for several times. After that, K4[Fe(CN)6] and HCl were added into Fe3O4@PPy solution, in which both H+ ions and [Fe(CN)6]4− immersed into the interior of core-shell Fe3O4@PPy. The dissolving of Fe3O4 in acidic environment make the successful formation of hollow structure of PPy, meanwhile, Fe3+ released from Fe3O4 initiated the formation of PB around H-PPy. Therefore, Fe3O4 spheres acted as both template and source of Fe3+, making formation of hollow structure of PPy and growth of PB nanocubes around H-PPy occur simultaneously.

Figure 2. TEM images of nanocomposites: (A) Fe3O4, (B) Fe3O4@PPy, (C, D, E, F) PB/H-PPy.

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The morphologies of Fe3O4, Fe3O4@PPy and PB/H-PPy were characterized by TEM. Figure 2 showed TEM images of Fe3O4, Fe3O4@PPy and PB/H-PPy. As shown in Figure 2 (A and B), Fe3O4 displayed sphere-like morphology and Fe3O4@PPy revealed the core-shell-like morphology. Figure 2 (C, D, E, F) showed the typical TEM images of PB/H-PPy under different magnifications. Comparing with core-shell-like structure of Fe3O4@PPy, the higher magnification images of PB/H-PPy (Figure 2 (E and F)) showed that PPy revealed the hollow structure and a large number of nanocubes were encapsulated into PPy. The encapsulation of PB into H-PPy can improve the stability and conductivity of PB, therefore increasing its electrocatalytic activity toward H2O2 reduction. Moreover, Figure 3 showed SEM image of PB/H-PPy and also displayed the elemental mapping of C, N, Fe in PB/H-PPy. As shown in Figure 3, it can be seen that many nanocubes were decorated on spheres. In addition, the elemental mapping revealed that N and Fe were distributed across the whole structure, indicating the decoration of PB on PPy. Therefore, the results of TEM, SEM and elemental mapping characterizations confirmed that PB nanocubes were encapsulated into hollow PPy.

Figure 3. SEM image of PB/H-PPy and elemental mapping of C, N, Fe in PB/H-PPy.

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Figure 4. (A) XRD paterns of (a) Fe3O4, (b) Fe3O4@PPy and (c) PB/H-PPy; (B) FTIR spectra of (a) Fe3O4@PPy and (b) PB/H-PPy; (C) N 1s core-level spectra of Fe3O4@PPy; (D) N 1s core-level spectra of PB/H-PPy.

Furthermore, the chemical structural characterizations of Fe3O4, Fe3O4@PPy and PB/H-PPy nanocomposites were studied by XRD. As shown in Figure 4 (A), both Fe3O4 (curve a) Fe3O4@PPy (curve b) revealed some peaks at 29.6°, 36.5°, 42.4°, 57.1° and 61.5°, which can be ascribed to the diffraction from (220), (311), (400), (511) and (440) planes of Fe3O4, respectively, indicating the presence of Fe3O4. In the case of PB/H-PPy (curve c), the peaks which were assigned to diffraction of Fe3O4 disappeared, implying the successful dissolving of Fe3O4. Meanwhile, some new peaks at 17.4°, 24.8°, 35.1°, 39.2°, 43.6°, 50.7°, 54.0°, 57.2° were observed, which were indexed to (2 0 0), (2 2 0), (4 0 0), (4 2 0), (4 2 2), (4 4 0), (600) and (620) planes of PB, confirming the presence of PB. In addition, the compositions of Fe3O4@PPy and PB/H-PPy were further investigated by FTIR (Figure 4 (B)). For the spectrum of Fe3O4@PPy (curve a), the band observed at 592 cm-1 was due to Fe-O stretching of Fe3O4 [24], while the peak at 1561 cm-1 was associated with the

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fundamental vibration of pyrrole ring. The peak of 1210 cm-1 was due to C-N stretching vibration. These peaks were consistent with the characterizations of Fe3O4@PPy [25]. For the spectrum of PB/H-PPy (curve b), the peak at 592 cm-1 disappeared due to the dissolving of Fe3O4. The new absorption at 2081 cm-1 was related to the stretching vibration of the CN groups and the peak at 498 cm-1 can be attributed to the formation of FeII-CN-FeIII [26], implying the formation of PB. The XPS experiments were employed to study the chemical structure of PPy. Figure 3 (C) showed N 1s spectra of Fe3O4@PPy, which can be deconvoluted into three peaks at 397.3, 399.4 and 401.25 eV. These peaks were related to imine-like (=N–), amine-like (–NH–) and positively charged nitrogen (N+) structure, respectively. Figure 3 (D) showed N 1s spectra of PB/H-PPy. The high-resolution N1s spectra could be deconvolved into four peaks with binding energies of 397.05, 397.6, 399.25 and 402.05 eV, which corresponded to C≡N of PB nanocubes, =N–, –NH– and N+ structure, respectively, indicating the existence of PB and PPy. The binding energy of –N+– in Fe3O4@PPy appeared at 401.25 eV, while it locates at 402.05 eV in PB/H-PPy. This indicated the red shift of N 1s binding energy, which may be due to the electron donating effect of amino groups in PPy, suggesting Fe3+ ions coordinated with amino groups through sharing the electron pairs of amino groups. Such interaction was beneficial for increasing the loading and preserving the dispersion of PB. Therefore, the results of TEM, XRD, FTIR, XPS characterizations confirmed the successful synthesis Fe3O4@PPy and PB/H-PPy nanocomposites. 3.2 Electrochemical properties of PB/H-PPy nanocomposites Cyclic voltammograms (CVs) were employed to study the electrochemical behaviors of PB/H-PPy/GCE. As shown in Figure 5 (A), PB/H-PPy/GCE revealed a pair of

redox peaks, which can be ascribed to the conversion of prussian white and

prussian blue and also confirmed the formation of PB. Meanwhile, the electrochemical stability of PB/H-PPy/GCE was studied. As shown in Figure 5 (B), the current decreased little after thirty cycles. This implied the good stability of PB, which benefited from the protection of H-PPy. Furthermore, the possible kinetic mechanism of PB/H-PPy/GCE was studied. Figure 5 (C) showed the effect of scan

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rates on the current response. From Figure 5 (D), it can be seen that the current increased in a linear relationship with the square root of scan rates (from 20 to 200 mV/s), indicating the diffusion controlled process.

Figure 5. (A) Cyclic voltammograms obtained by PB/H-PPy/GCE in 0.5 M KCl solution (HCl, pH 3.0) at a scan rate of 100 mV/s. (B) Cyclic voltammograms obtained by PB/H-PPy/GCE in 0.5 M KCl solution (pH 3.0) after thirty cycles at a scan rate of 100 mV/s. (C) Cyclic voltammograms obtained by PB/H-PPy/GCE in 0.5 M KCl solution (pH 3.0) at different scan rates (from a to g: 20, 40, 60, 80, 100, 120 and 140 mV/s). (D) Linear fitting program of current versus the square root of scan rate.

Figure 6. Cyclic voltammograms obtained by bare GCE (a, a’), Fe3O4/GCE (b, b’),

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Fe3O4@PPy/GCE (c, c’) and PB/H-PPy/GCE (d, d’) in the absence (a, b, c, d) and presence of (a’, b’, c’, d’) 2.0 mM H2O2 at a scan rate of 40 mV/s.

Cyclic voltammograms (CVs) were employed to study the electrocatalytic activity of modified electrode toward reduction of H2O2. Figure 6 displayed CVs obtained by bare GCE (a, a’), Fe3O4/GCE (b, b’), Fe3O4@PPy/GCE (c, c’) and PB/H-PPy/GCE (d, d’) in the absence (a, b, c, d) and presence of (a’, b’, c’, d’) 2.0 mM H2O2. In the absence of H2O2, bare GCE (curve a), Fe3O4/GCE (b) and Fe3O4@PPy/GCE (c) exhibited almost no electrochemical response. PB/H-PPy/GCE (curve d) showed a stable redox peaks. After adding H2O2, comparing with that of bare GCE (curve a’), Fe3O4/GCE (b’) and Fe3O4@PPy/GCE (c’), PB/H-PPy/GCE (curve d’) revealed a decrease of anodic peak and an increase of cathodic peak, confirming the excellent electrocatalytic activity of PB/H-PPy toward H2O2 reduction. The mechanism of electroreduction of H2O2 can be expressed as follows [14]: 2K2FeII[FeII(CN)6] + H2O2 → 2KFeIII[FeII(CN)6] + 2OH- +2K+ KFeIII[FeII(CN)6] + e- + K+ → K2FeII[FeII(CN)6] The effect of working potential on the amperometric response was studied. Figure 7 showed the amperometric curve obtained at PB/H-PPy/GCE on successive addition of 0.05 mM H2O2 at various working potentials. From Figure 7, it can be seen that the reduction current of H2O2 obtained at working potential of 0.1 V was higher than that of 0.2 V, 0 V and - 0.1 V. Moreover, the background noise was low at such low working potential. Therefore, 0.1 V was chosen as the working potential for determination of H2O2.

Figure 7. Amperometric response obtained at PB/H-PPy/GCE on successive addition of 0.05 mM

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at various applied potentials.

Figure 8. (A) Amperometric curve obtained by PB/H-PPy/GCE for successive additions of H2O2 in 0.5 M KCl solution (HCl, pH 3.0) at the work potential of 0.1 V under constant stirring. (B) Calibration curve of H2O2 versus its concentration.

Moreover, the amperometric current-time curve was employed to study the sensing property of PB/H-PPy. Figure 8 (A) displayed the amperometric current-time curve obtained by PB/H-PPy/GCE at 0.1 V. Such a low working potential was beneficial to prevent the interference of other species. From Figure 8 (A), it can be seen that the reduction current exhibited a rapid increase with the successive addition of H2O2. Figure 8 (B) revealed the calibration curve of PB/H-PPy. The linear regression equation was expressed as Ip (µA) = -2.665 + (-33.91)·C (µM) with a wide linear range of 5.0 µM to 2.775 mM, a sensitivity of 484.4 µA mM-1cm-2 and a low detection limit of 1.6 µM at a signal-to-noise ratio of 3. In addition, as shown in Table 1, comparing with other H2O2 sensors, PB/H-PPy/GCE exhibited a high sensitivity and a wide linear range, which benefited from the combination of PB and H-PPy. PB combined with H-PPy improved the stability and conductivity of PB, therefore increasing the catalytic activity for H2O2 reduction. Meanwhile, the encapsulation of PB into H-PPy minimized the diffusion resistance of analyte and provided a large surface for H2O2 molecules adsorption and reaction.

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Table 1. Comparison of non-enzymatic H2O2 sensors.

Sensors

Applied potential (V)

Nanoporous gold

-0.4

0.01 – 8



3.26

27

GO/PB

0.1

0.005 – 1.2

408.7

0.22

28

PB/CA

-0.2

0.01 – 0.25

190

2.2

29

PBNPs/Nafion

-0.05

0.0021 – 0.14

138.6

1

30

MWCNT/PB

0

0.01 – 0.4

153.7

0.567

31

Ag/AlOOH/GCE

-0.2

0.005 – 9

64.4

1.1

32

Ag NPs/ATP/GCE

-0.3

0.01 – 21.53



2.4

33

PB/H-PPy

0.1

0.005 – 2.775

484.4

1.6

This work

Linear range Sensitivity Detection References (mM) (µA mM-1 cm-2) limit (µM)

Figure 9. Amperometric response of 0.05 mM H2O2, 0.05 mM Glu, UA, AA, DA and L-cys on PB/H-PPy/GCE in 0.5 M KCl solution (HCl, pH 3.0) at the work potential of 0.1 V under constant stirring.

The selectivity of PB/H-PPy/GCE was studied. As shown in Figure 9, 0.05 mM glucose (Glu), uric acid (UA), ascorbic acid (AA), dopamine (DA) and L-cysteine (L-cys) showed little interference for 0.05 mM H2O2 detection, which may benefited from the low working potential. The reproducibility and stability of PB/H-PPy/GCE was evaluated. The relative standard deviation was about 6.4% for six PB/H-PPy/GCE. Additionally, PB/H-PPy/GCE preserved about 81.7% of current response after three weeks. In order to verify the reliability of the sensor, The real sample of disinfectant was detected through the standard addition method. As shown in Table 2. It can be seen obviously that the recovery obtained at PB/H-PPy/GCE was good, indicating that the

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sensor can prevent the interference of other substances and can be used for sample analysis. Table 2. Determination of H2O2 in disinfector sample. Sample

Added (µM)

Found (µM)

Recovery (%)

1

15.0

14.2

94.6

2

90.0

94.3

104.8

3

250.0

241.2

96.5

4. Conclusion Prussian blue/hollow polypyrrole (PB/H-PPy) nanocomposites were successfully synthesized through a facile approach without employing additional reducing agents, template, surface modifier or complicated apparatus. Meanwhile, the hollow structure of PPy minimized the diffusion resistance of analyte and the encapsulation of PB into H-PPy improved the stability and conductivity of PB, which made PB/H-PPy/GCE exhibit an excellent performance for hydrogen peroxide analysis. Considering the facile preparation route and excellent experimental results, the present study may paved the way for the facial synthesis of other nanocomposites with unique morphology and properties for applications. Acknowledgments The authors gratefully acknowledge the financial support of this project by the National Science Foundation of China (21575113, 21275116 and 21105080), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20126101120023), the Natural Science Foundation of Shaanxi Province in China (2013KJXX-25), and the Scientific Research Foundation of Shaanxi Provincial Key Laboratory (13JS097, 13JS098, 14JS094, 15JS100).

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Table of Contents Graphic:

A novel strategy is developed to synthesize prussian blue nanocubes/hollow polypyrrole (PB/H-PPy) nanocomposites for enhanced electrochemical determination of hydrogen peroxide.

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