Ni doped Ag@C core–shell nanomaterials and their application in electrochemical H₂O₂ sensing

Abstract

In this work, attractive core–shell structured nanomaterials consisting of Ni doped Ag@C were synthesized. Further, a hydrogen peroxide (H₂O₂) sensor was fabricated by modifying the Ni doped Ag@C (Ni/Ag@C) nanocomposites onto the surface of a glassy carbon electrode (GCE). The composition and morphology of the Ni/Ag@C nanocomposites were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The results indicated that the nanocomposites were synthesized successfully and showed similar grain sizes and favorable dispersity. The electrochemical and electrocatalytic properties of the Ni/Ag@C nanocomposites were studied by cyclic voltammetry. An electrochemical investigation showed that the Ni/Ag@C nanocomposite modified GCE exhibited a good electrocatalytic ability for H₂O₂ reduction and it was used for the determination of H₂O₂. The linear range for H₂O₂ determination was from 0.03 mM to 17.0 mM with a detection limit of 0.01 mM (S/N = 3). The sensitivity was 22.94 μA mM⁻¹ cm⁻². It is expected that the application of the Ni/Ag@C nanocomposites could be extended to the construction of other sensors and applications in various sensing fields.

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1. Introduction

With such a wide application, many methods have been established for the detection of hydrogen peroxide (H₂O₂), such as chromatography,¹ chemiluminescence,² spectrophotometry,³ and electrochemistry.^4,5 The electrochemical method, including enzyme-based biosensors and non-enzymatic sensors, was used widely thanks to its high selectivity and fast response.^6–9 Because of the complicated immobilization procedure and instability of enzyme-based biosensors, non-enzymatic sensors have attracted more and more attention.

With the development of nanotechnology, many nanomaterials such as Au NPs,^10,11 Ag NPs,^12,13 and Pt NPs^14,15 were used in H₂O₂ electrochemical sensors. However, there have only been a few reports about Ni NPs in the application of H₂O₂ sensing. The aggregation of nanoparticles hinders their extensive application, so highly dispersed metal nanoparticles are important in the fabrication of sensors.¹⁶ Nowadays, core–shell nanomaterials are attracting much attention in the fabrication of sensors^17,18 because of their multifunctional properties.¹⁹ For example, core–shell Ag–Au bimetal nanoparticles exhibited good electrocatalytic ability towards the oxidation of L-methionine.²⁰ Core–shell CdSe@Ag₂Se was found to have excellent electrocatalytic ability towards methyldopa with a low detection limit of 0.04 μmol L⁻¹.²¹ By using the strategy of sacrificial templating, porous Pt nanowire cores with carbon shells (Pt PNW@C), using Te nanowires as a template, were achieved. It was found that the Pt PNW@C hybrids showed a highly sensitive response for H₂O₂ detection with a sensitivity of 0.35 μA μM⁻¹ cm⁻² and a detection limit of 0.1 μM.²² More recently, a novel core–double shell Au@C@Pt nanocomposite was synthesized and used to fabricate an enzyme-free electrochemical sensor for the rapid and sensitive detection of H₂O₂. Results showed that the sensor displayed two wide linear ranges towards H₂O₂ detection and a low detection limit of 0.13 μM was obtained.²³ Among those core–shell nanostructures, one of the most attractive shell layers was a carbon coating due to its chemical activity and low preparation cost.²⁴ Ag@C core–shell nanomaterials were one of the most used nanomaterials because the synthetic procedure was convenient.²⁵ The high surface to volume ratio of the Ag@C core–shell nanomaterial may alleviate the aggregation of nickel nanoparticles. Furthermore, the good conductivity of the Ag@C core–shell nanomaterial may enhance its application in the fabrication of sensors.

The aims of this work are to synthesize Ag@C core–shell nanomaterials through a one-pot hydrothermal method²⁶ and Ni/Ag@C nanocomposites through a reduction reaction,²⁷ then to fabricate an enzyme-free H₂O₂ sensor based on the nanocomposites produced with a simple casting method. Finally the sensing properties and the mechanism of the sensor were thoroughly investigated.

2. Experimental

2.1. Materials

Chitosan (CS, MW 5–6 × 10⁵, >90% deacetylation) was obtained from Shanghai Yuanju Biotechnology Co., Ltd (Shanghai, China). H₂O₂ (30%, v/v aqueous solution) was obtained from Tianjin Tianli Chemistry Reagent Co., Ltd (Tianjin, China). Glucose and AgNO₃ were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). Other chemicals and reagents were of analytical reagent grade. Doubly distilled water was used throughout the whole experimental process.

2.2. Apparatus and electrochemical measurements

Transmission electron microscope (TEM) images were obtained on a Tecnai G² F20 S-TWIN (FEI, USA). Fourier transform infrared spectra (FTIR) were observed using a TENSIR 27 (Bruker, Germany). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) images were acquired using a JSM-6700F (JEOL, Japan) and measured at an operating voltage of 10 kV. X-ray powder diffraction (XRD) data were obtained using Cu Kα radiation at a light tube power of 2.2 kW on a Bruker D8 Advance (Bruker AXS, Germany) measurement system.

The electrochemical measurements were carried out using a CHI 660 electrochemical workstation (Shanghai CH Instrument Co., Ltd., China) with a three-electrode electroanalysis system. The working electrode was a glassy carbon electrode (GC electrode) or a Ni/Ag@C nanocomposite modified glassy carbon electrode (Ni/Ag@C/GCE); the counter electrode and reference electrode were a platinum wire and a saturated calomel electrode (SCE), respectively. All electrochemical experiments were performed at room temperature (25 ± 2 °C).

2.3. Preparation of the sensor

2.3.1 Synthesis of the Ag@C core–shell nanomaterials. Glucose (3.5 g) was dissolved in doubly distilled water (20 mL). Subsequently, 0.03 M of a AgNO₃ aqueous solution (2.0 mL) was added into the above-mentioned solution drop-by-drop under vigorous stirring. After stirring for another 15 min, the mixture was transferred into a Teflon-lined autoclave and heated at 180 °C for 4 h. After the reaction, the autoclave was naturally cooled to room temperature. The precipitate was collected by centrifugation, washed with doubly distilled water, and dried at room temperature (25 ± 2 °C).

2.3.2 Synthesis of the Ni/Ag@C nanocomposites. Ag@C powders (10 mg) were dispersed in ethylene glycol (20 mL) by a hyperacoustic method. 1.0 M NiCl₂·6H₂O solution (0.1 mL), N₂H₄·H₂O (0.5 mL) and 1.0 M NaOH solution (1.0 mL) were added into the above-mentioned solution sequentially under magnetic stirring conditions. Then the mixture was heated in an oil bath at 60 °C for 1 h. The precipitate was separated by centrifugation, washed with ethanol several times, and dried at room temperature (25 ± 2 °C).

2.3.3 Electrode modification. Before modification, the GC electrode was polished with 1.0 and 0.3 μm alumina powder sequentially, and rinsed with doubly distilled water, followed by sonication in an ethanol solution and doubly distilled water successively. The GC electrode was dried in a stream of nitrogen. The Ni/Ag@C nanocomposites (1.0 mg) were dispersed into chitosan (1.0 mL, 0.5%) and sonicated for about 15 minutes to form a homogeneous suspension. Then the suspension was cast onto the GC electrode and dried at room temperature in air. The modified electrode can be expressed as a Ni/Ag@C/GC electrode (Scheme 1).

Scheme 1 Schematic illustration of the preparation of Ni/Ag@C and its H₂O₂ sensing.

3. Results and discussion

3.1. Characterization of the Ni/Ag@C nanocomposites

As shown in Fig. 1, the structures and morphologies of Ag@C and Ni/Ag@C were characterized by SEM and TEM. The Ag@C nanocomposites were spherical core–shell structures with an external diameter of about 800 nm (Fig. 1A and B), and have Ag cores with diameters of nearly 80 nm. The Ag core appeared lighter in the centre of the Ag@C nanocomposites in the SEM images (Fig. 1A) while appearing dark in the TEM images (Fig. 1B). From Fig. 1C–E, we can see that the granular nanonickel was attached onto the surface of the Ag@C nanocomposites. In addition, the EDS patterns showed that the nanocomposites were composed of C, Ag, and Ni elements (Fig. 1F), indicating that the Ag@C nanocomposites were synthesized successfully.

Fig. 1 SEM images of (A) Ag@C and (C) Ni/Ag@C. TEM images of (B) Ag@C and (D) Ni/Ag@C. A TEM image (E) and an EDS spectrum (F) of a single Ni/Ag@C nanocomposite.

The FTIR spectra of the Ag@C and Ni/Ag@C are shown in Fig. 2. As we can see from curve a, the characteristic peaks at about 1700 and 1620 cm⁻¹ are the result of C=O and C=C vibrations, respectively of the aromatization of glucose during hydrothermal treatment. The absorption band ranging from 1000 cm⁻¹ to 1300 cm⁻¹ is attributed to C–OH stretching and –OH bending vibrations, which indicates the existence of large numbers of residual hydroxyl groups.²⁶ After loading of the Ni NPs (curve b), the absorption peak intensity of the functional groups becomes lower, which may be due to the absorption peak of the Ni NPs covering the absorption peak of Ag@C.

Fig. 2 FTIR spectra of (a) Ag@C and (b) Ni/Ag@C nanocomposites.

3.2. Electrochemical properties of Ni/Ag@C

The electrochemical behavior of the Ni/Ag@C nanocomposites was investigated by cyclic voltammetry (CV). As can be seen from Fig. 3, in N₂-saturated 0.1 M PBS (pH 7.4), the bare GC, Ag@C/GC and Ni/Ag@C/GC electrodes (curve a, e, and c, respectively) showed no electrochemical response in the absence of H₂O₂. However, after the addition of 5.0 mM H₂O₂, the electrochemical response of the different electrodes increased by different degrees. Compared with the bare GC electrode (curve b) and the Ag@C/GC electrode (curve f), the Ni/Ag@C/GC electrode (curve d) showed a remarkable catalytic current peak about 37 μA in intensity at nearly −0.58 V. The results indicate that Ni NP doped Ag@C core–shell nanomaterials exhibit a favorable catalytic performance for the reduction of H₂O₂.

Fig. 3 CVs of (a and b) the bare GC electrode, (e and f) the Ag@C modified GC electrode and (c and d) the Ni/Ag@C modified GC electrode in N₂-saturated 0.1 M PBS (pH 7.4) in the absence (a, e and c) and presence (b, f and d) of 5.0 mM H₂O₂ at a scan rate of 100 mV s⁻¹.

As seen in Fig. 4, the catalytic activity of the Ni/Ag@C nanocomposites was studied by changing the concentration of H₂O₂. In N₂-saturated 0.1 M PBS (pH 7.4), the reduction current increased gradually with the increasing injection of H₂O₂ into the electrolyte, indicating that the Ni/Ag@C nanocomposites had favorable electrocatalytic activity towards H₂O₂. With an increase in concentration of H₂O₂, the reduction current increased gradually. As can be seen from Fig. 5, with the increase in scan rate from 20 to 140 mV s⁻¹, the cathodic peak current increased. The peak current increased in a linear relation with the square root of the scan rate between 20 and 200 mV s⁻¹ (inset of Fig. 5), indicating that the process was diffusion-controlled.

Fig. 4 CVs obtained by the Ni/Ag@C modified GC electrode in N₂-saturated 0.1 M PBS (pH 7.4) in the presence of different concentrations of H₂O₂ (from a to h: 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0 mM) at a scan rate of 100 mV s⁻¹.

Fig. 5 CVs obtained by the Ni/Ag@C modified GC electrode in N₂-saturated 0.1 M PBS (pH 7.4) in the presence of H₂O₂ (5.0 mM) at different scan rates (from a to g: 20, 40, 60, 80, 100, 120 and 140 mV s⁻¹). Inset: a plot of the electrocatalytic peak current of H₂O₂versus v^1/2.

The effect of applied potential on the steady state current of the Ni/Ag@C nanocomposite modified GC electrode was investigated. The results showed that the best electrocatalytic response for H₂O₂ sensing could be observed at −0.2 V. Although the modified electrode exhibited its highest catalytic activity at about −0.6 V, the background current was high. Meanwhile at −0.2 V, a lower background current, a good signal-to-noise ratio and less interference from other electroactive species could be achieved. Moreover, the influence of oxygen reduction can be limited. Thus, −0.2 V was selected as the working potential for amperometric H₂O₂ detection. Fig. 6A displays the current response of the Ni/Ag@C nanocomposite modified GC electrode in N₂-saturated 0.1 M PBS (pH 7.4) with different concentrations of H₂O₂ at a working potential of −0.2 V. The Ni/Ag@C nanocomposite modified GC electrode showed an amperometric current response towards H₂O₂ in less than 3 s, indicating that the response was fast.^28–30 The calibration curve of the H₂O₂ sensor is shown in Fig. 6B. The linear regression equation was found to be I_p (μA) = 0.1812 + 0.7202C (mM) with a correlation coefficient of 0.9979 in the concentration range of 0.03–17.0 mM. The sensitivity of this sensor was 22.94 μA mM⁻¹ cm⁻², which was higher than some H₂O₂ sensors.^31–33 The detection limit was 0.01 mM at a signal-to-noise ratio of 3. Table 1 summarizes a comparison of the analytical performances of different kinds of nanomaterials used for H₂O₂ sensing. It can be seen that our proposed sensor displayed a good performance with a higher applied potential, wider linear range and lower detection limit.

Fig. 6 (A) Current response of the Ni/Ag@C modified GC electrode in stirred N₂-saturated 0.1 M PBS (pH 7.4) with different concentrations of H₂O₂ at a working potential of −0.2 V. Inset: an amplification of the current versus time variation curve for low concentrations. (B) Calibration curve of H₂O₂versus its concentration.

Table 1 Comparison of the analytical performance of H₂O₂ sensors based on different nanomaterials

Nanomaterials	Applied potential (V)	Linear range (mM)	Detection limit (mM)	Ref.
a Poly[(2-ethyldimethylammonioethyl methacrylate ethyl sulfate)-co-(1-vinylpyrrolidone)] functionalized silver nanoparticles. b Ag nanoparticle decorated nanofibers. c Not available.
PQ11–Ag^a	−0.3	0.1–180.0	0.034	34
Pd hollow nanocube	−0.4	0.1–24.0	0.0074	35
Ag NPs-NFs^b	−0.3	0.1–80.0	0.062	36
Ag@C@Ag	NA^c	0.07–10.0	0.023	37
Au@C@Pt	0.0	0.009–1.86	0.0013	38
		1.86–7.11
Fe3O4@C–Cu	−0.3	0.08–372.0	0.033	39
Ni/Ag@C	−0.2	0.03–17.0	0.01	This work

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3.3. Interference study

In order to investigate the anti-interference and selectivity of the sensor, the interference effects were investigated. As shown in Fig. 7, a H₂O₂ solution (2.0 mM) was firstly added into the N₂-saturated 0.1 M PBS (pH 7.4) at a working potential of −0.2 V, then the interferents (1.0 mM, respectively) were added. There was an obvious amperometric response to the injection of H₂O₂, while no obvious response appeared when ascorbic acid (AA), glucose (Glu) and ethanol were injected. The results indicate that the sensor exhibits a good anti-interference ability and selectivity for the detection of H₂O₂.

Fig. 7 Amperometric response of the Ni/Ag@C modified GC electrode after adding 2.0 mM H₂O₂, 1.0 mM ascorbic acid (AA), glucose (Glu) and ethanol.

3.4. Repeatability and stability

The repeatability and stability of the sensor were evaluated. The amperometric current responses of five modified electrodes were investigated at −0.3 V towards 2.0 mM H₂O₂. The relative standard deviation (RSD) was less than 4%, indicating that the performance of the Ni/Ag@C nanocomposite modified GC electrode was reproducible. After three weeks, the modified electrode retained 90% of its initial current response towards 5.0 mM H₂O₂. Thus, this sensor exhibited acceptable repeatability and stability.

4. Conclusions

In conclusion, Ni/Ag@C nanocomposites were synthesized and an enzyme-free hydrogen peroxide sensor based on the nanocomposites was fabricated successfully. The synthesized nanocomposites showed similar grain sizes and favorable dispersity. The sensor exhibited favorable electrocatalytic properties towards H₂O₂, including high sensitivity and a fast response, as well as a good anti-interference ability. So this study may provide a possible method to develop new electrochemical sensors.

Acknowledgements

The authors gratefully acknowledge the financial support of this project by the National Science Fund of China (No. 21275116, 21575113), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20126101110013), the Natural Science Fund of Shaanxi Province in China (No. 2013KJXX-25), and the Scientific Research Foundation of Shaanxi Provincial Key Laboratory (14JS094, 15JS100 and 16JS099).

References

1. U. Pinkernell, S. Effkemann and U. Karst, Simultaneous HPLC Determination of Peroxyacetic Acid and Hydrogen Peroxide, Anal. Chem., 1997, 69, 3623.
2. W. Lei and Z. H. Lin, Detection of Hydrogen Peroxide in River Water via a Microplate Luminescence Assay with Time-Resolved (“Gated”) Detection, Microchim. Acta, 2003, 143, 269.
3. Q. Chang, K. Deng, L. Zhu, G. Jiang, C. Yu and H. Tang, Determination of hydrogen peroxide with the aid of peroxidase-like Fe₃O₄ magnetic nanoparticles as the catalyst, Microchim. Acta, 2009, 165, 299.
4. S. J. He, B. Y. Zhang, M. M. Liu and W. Chen, Non-enzymatic hydrogen peroxide electrochemical sensor based on a three-dimensional MnO₂ nanosheets/carbon foam composite, RSC Adv., 2014, 4, 49315.
5. J. X. Zhang, L. P. Tu, S. Zhao, G. H. Liu, Y. Y. Wang, Y. Wang and Z. Yue, Fluorescent gold nanoclusters based photoelectrochemical sensors for detection of H₂O₂ and glucose, Biosens. Bioelectron., 2015, 67, 296.
6. K. M. Fang, Y. Y. Yang, L. Y. Fu, H. T. Zheng, J. H. Yuan and L. Niu, Highly selective H₂O₂ sensor based on 1-D nanoporous Pt@C hybrids with core–shell structure, Sens. Actuators, B, 2014, 191, 401.
7. H. Yue, J. He, D. Xiao and M. F. Choi, Biosensor for determination of hydrogen peroxide based on Yucca filamentosa membrane, Anal. Methods, 2013, 5, 5437.
8. E. Kurowska, A. Brzózka, M. Jarosz, G. D. Sulka and M. Jaskuła, Silver nanowire array sensor for sensitive and rapid detection of H₂O₂, Electrochim. Acta, 2013, 104, 439.
9. S. L. Yang, G. Li, G. F. Wang, J. H. Zhao, M. F. Hu and L. B. Qu, A novel nonenzymatic H₂O₂ sensor based on cobalt hexacyanoferrate nanoparticles and graphene composite modified electrode, Sens. Actuators, B, 2015, 208, 593.
10. P. J. Ni, Y. Zhang, Y. J. Sun, Y. Shi, H. C. Dai, J. T. Hu and Z. Li, Facile synthesis of Prussian blue@gold nanocomposite for nonenzymatic detection of hydrogen peroxide, RSC Adv., 2013, 3, 15987.
11. H. S. Li, Y. J. Li and S. Q. Wang, Water-soluble Au nanocages for enzyme-free H₂O₂ sensor and 4-nitrophenol reduction, CrystEngComm, 2015, 17, 2368.
12. J. S. Easow and T. Selvaraju, Unzipped catalytic activity of copper in realizing bimetallic Ag@Cu nanowires as a better amperometric H₂O₂ sensor, Electrochim. Acta, 2013, 112, 648.
13. P. M. Nia, F. Lorestani, W. P. Meng and Y. Alias, A novel non-enzymatic H₂O₂ sensor based on polypyrrole nanofibers–silver nanoparticles decorated reduced graphene oxide nano composites, Appl. Surf. Sci., 2015, 332, 648.
14. J. Zhang, J. Li, F. Yang, B. L. Zhang and X. R. Yang, Preparation of Prussian blue@Pt nanoparticles/carbon nanotubes composite material for efficient determination of H₂O₂, Sens. Actuators, B, 2009, 143, 373.
15. X. H. Niu, C. Chen, H. L. Zhao, Y. Chai and M. B. Lan, Novel snowflake-like Pt–Pd bimetallic clusters on screen-printed gold nanofilm electrode for H₂O₂ and glucose sensing, Biosens. Bioelectron., 2012, 36, 262.
16. C. Y. Liu and J. M. Hu, Hydrogen peroxide biosensor based on the direct electrochemistry of myoglobin immobilized on silver nanoparticles doped carbon nanotubes film, Biosens. Bioelectron., 2009, 24, 2149.
17. Z. Z. Cui, H. Y. Yin and Q. L. Nie, Controllable preparation of hierarchically core–shell structure NiO/C microspheres for non-enzymatic glucose sensor, J. Alloys Compd., 2015, 632, 402.
18. J. Q. Tian, Y. L. Luo, H. L. Li, W. B. Lu, G. H. Chang, X. Y. Qin and X. P. Sun, Ag@poly(m-phenylenediamine)–Ag core–shell nanoparticles: one-step preparation, characterization, and their application for H₂O₂ detection, Catal. Sci. Technol., 2011, 1, 1393.
19. X. Zhou, X. X. Dai, J. G. Li, Y. M. Long, W. F. Li and Y. F. Tu, A sensitive glucose biosensor based on Ag@C core–shell matrix, Mater. Sci. Eng., C, 2015, 49, 579.
20. M. Murugavelu and B. Karthikeyan, Synthesis, characterization of Ag–Au core–shell bimetal nanoparticles and its application for electrocatalytic oxidation/sensing of L-methionine, Mater. Sci. Eng., C, 2017, 70, 656.
21. K. Asadpour-Zeynali and F. Mollarasouli, A novel and facile synthesis of TGA-capped CdSe@Ag₂Se core–shell quantum dots as a new substrate for high sensitive and selective methyldopa sensor, Sens. Actuators, B, 2016, 237, 387.
22. K. M. Fang, Y. Y. Yang, L. Y. Fu, H. T. Zheng, J. H. Yuan and L. Niu, Highly selective H₂O₂ sensor based on 1-D nanoporous Pt@C hybrids with core–shell structure, Sens. Actuators, B, 2014, 191, 401.
23. Y. Y. Zhang, Y. H. Li, Y. Y. Jiang, Y. C. Li and S. Q. Li, The synthesis of Au@C@Pt core–double shell nanocomposite and its application in enzyme-free hydrogen peroxide sensing, Appl. Surf. Sci., 2016, 378, 375.
24. J. X. Zhu, T. Sun, H. H. Hung, J. Ma, F. Y. C. Boey, X. W. Lou, H. Zhang, C. Xue, H. Y. Chen and Q. Y. Yan, Fabrication of Core–Shell Structure of M@C (M = Se, Au, Ag₂Se) and Transformation to Yolk–Shell Structure by Electron Beam Irradiation or Vacuum Annealing, Chem. Mater., 2009, 21, 3848.
25. X. M. Sun and Y. D. Li, Ag@C core/shell structured nanoparticles: controlled synthesis, characterization, and assembly, Langmuir, 2005, 21, 6019.
26. S. X. Mao, W. F. Li, Y. M. Long, Y. F. Tu and A. P. Deng, Sensitive electrochemical sensor of tryptophan based on Ag@C core–shell nanocomposite modified glassy carbon electrode, Anal. Chim. Acta, 2012, 738, 35.
27. J. W. Huang, Y. Q. He, J. Jin, Y. R. Li, Z. P. Dong and R. Li, A novel glucose sensor based on MoS₂ nanosheet functionalized with Ni nanoparticles, Electrochim. Acta, 2014, 136, 41.
28. W. Zhao, H. C. Wang and X. Qin, A novel nonenzymatic hydrogen peroxide sensor based on multi-wall carbon nanotube/silver nanoparticle nanohybrids modified gold electrode, Talanta, 2009, 80, 1029.
29. Y. D. Zhao, Y. H. Bi and W. D. Zhang, The interface behavior of hemoglobin at carbon nanotube and the detection for H₂O₂, Talanta, 2005, 65, 489.
30. W. Zheng, J. Li and Y. F. Zheng, An amperometric biosensor based on hemoglobin immobilized in poly(ε-caprolactone) film and its application, Biosens. Bioelectron., 2008, 23, 1562.
31. S. Abdollah, M. Monierosadat and N. Abdollah, A novel non-enzymatic hydrogen peroxide sensor based on single walled carbon nanotubes–manganese complex modified glassy carbon electrode, Electrochim. Acta, 2011, 56, 3387.
32. H. Heli, N. Sattarahmady, R. Dehdari Vais and A. R. Mehdizadeh, Enhanced electrocatalytic reduction and highly sensitive nonenzymatic detection of hydrogen peroxide using platinum hierarchical nanoflowers, Sens. Actuators, B, 2014, 192, 310.
33. S. Zhang, Q. L. Sheng and J. B. Zheng, Synthesis of Ag–HNTs–MnO₂ nanocomposites and their application for nonenzymatic hydrogen peroxide electrochemical sensing, RSC Adv., 2015, 5, 26878.
34. W. B. Lu, F. Liao, Y. L. Luo, G. H. Chang and X. P. Sun, Hydrothermal synthesis of well-stable silver nanoparticles and their application for enzymeless hydrogen peroxide detection, Electrochim. Acta, 2011, 56, 2295.
35. G. D. Nie, X. F. Lu, J. Y. Lei, L. Yang, X. J. Bian, Y. Tong and C. Wang, Sacrificial template-assisted fabrication of palladium hollow nanocubes and their application in electrochemical detection toward hydrogen peroxide, Electrochim. Acta, 2013, 99, 145.
36. J. Q. Tian, S. Liu and X. P. Sun, Supra molecular micro fibrils of o-phenylenediamine dimers: oxidation-induced morphology change and the spontaneous formation of Ag nanoparticle decorated nanofibers, Langmuir, 2010, 26, 15112.
37. Q. M. Wang, H. L. Niu, C. J. Mao, J. M. Song and S. Y. Zhang, Facile synthesis of trilaminar core–shell Ag@C@Ag nanospheres and their application for H₂O₂ detection, Electrochim. Acta, 2014, 127, 349.
38. Y. Y. Zhang, Y. H. Li, Y. Y. Jiang, Y. C. Li and S. X. Li, The synthesis of Au@C@Pt core–double shell nanocomposite and its application in enzyme-free hydrogen peroxide sensing, Appl. Surf. Sci., 2016, 378, 375.
39. M. Y. Zhang, Q. L. Sheng, F. Nie and J. B. Zheng, Synthesis of Cu nanoparticles-loaded Fe₃O₄@carbon core–shell nanocomposite and its application for electrochemical sensing of hydrogen peroxide, J. Electroanal. Chem., 2014, 730, 10.

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