Fast preparation of MoS₂ nanoflowers decorated with platinum nanoparticles for electrochemical detection of hydrogen peroxide

Abstract

Molybdenum disulfide (MoS₂) has shown increasing importance for the creation of functional nanomaterials. Here we demonstrate the quick preparation of MoS₂ nanoflowers decorated with platinum nanoparticles (PtNPs) by a simple one-step hydrothermal synthesis. Scanning and transmission electron microscopy techniques were utilized to investigate the morphology of the fabricated MoS₂–PtNP nanohybrids and the uniform decoration of PtNPs on MoS₂. Other techniques like X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction were used to measure the structure and properties of the synthesized MoS₂–PtNP nanohybrids. We expected the created MoS₂–PtNP nanohybrids to show excellent performance for biosensor applications. To prove this, the synthesized MoS₂–PtNP nanohybrids were utilized in the application of an electrochemical hydrogen peroxide (H₂O₂) sensor by using the nanohybrids to modify the glass carbon electrode. Our sensing result indicates that the fabricated MoS₂–PtNP based H₂O₂ sensor reveals a wide linear range (0.02 to 4.72 mM), low detection limit (0.345 μM, S/N = 3), high selectivity, and long-term stability (at least two weeks).

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1. Introduction

In recent years, various biosensors have been fabricated and applied in fields such as drug discovery, food analysis, and environmental monitoring.^1–4 Based on their fabrication strategy and function, biosensors can be classified as enzyme sensors,⁵ microbial sensors,⁶ DNA sensors,⁷ and so on. Among these different functional biosensors, the electrochemical biosensors have attracted continuous attention in the last few years owing to their excellent performance in various fields.^8–10

It is well known that hydrogen peroxide (H₂O₂) is an important substance in environmental and biological fields, and therefore it is very essential to detect H₂O₂ with high convenience and precise. Up to date, different electrochemical H₂O₂ sensors, including enzymatic and non-enzymatic sensors, have been created and applied in numerous fields owing to their unique merits like simplicity, high stability, and low cost.^11–13 Compared to the enzymatic electrochemical biosensors, the non-enzymatic electrochemical biosensors have a few obvious advantages such as higher sensitivity, stability, and briefness according to the previous studies.^14–16 Therefore, the applications of non-enzymatic electrochemical biosensors have attracted increasing attention in recent years.

To fabricate the electrochemical biosensors, a crucial step is to modify the electrode with a suitable electrode material. Recently, one of the two-dimensional (2D) nanomaterials, such as graphene, with their extremely novel physical and chemical properties, have been widely used in varieties of aspects such as nanotechnology,¹⁷ materials science,¹⁸ and biotechnology.^19,20 Following by the first fabrication of graphene in 2004,²¹ transition metal dichalcogenides (TMDs), the so-called graphene-like nanomaterials, such as WS₂, Sb₂Se₃, TiS₂, and so on, have been synthesized and used widely in the last few years due to their outstanding advantages.^22–24 Molybdenum disulfide (MoS₂), a member of the TMDs, owns unique thermal stability and high electrocatalytic activity, which promote the potential applications of MoS₂-based nanohybrids in material, analytical, energy, and environmental sciences.^25–28 As a typical 2D material, MoS₂ has many similar properties compared to graphene such as the excellent mechanical, optical, and electronic properties, which make MoS₂ apply in many fields like lithium batteries and hydrogen evolution reaction. However, as a typical layered material, MoS₂ has the semiconducting characters, which can open up the further applications of it in many fields such as the electrochemical sensors. For example, Wang et al. applied functionalized layered MoS₂ in electrochemical sensors for directly detecting of DNA and got a low limit of determination.²⁹ Su et al. made a glass carbon electrode modified with MoS₂ nanosheets decorated with gold nanoparticles to detect glucose directly.³⁰ In this work we would like to explore MoS₂ to detect H₂O₂ and hope to get a low detection limit.

Previous studies suggested that the selection of a suitable electrocatalyst is very essential to improve the sensing performance of the non-enzymatic electrochemical biosensors.³¹ The conjugation of MoS₂ nanosheets with the noble metals, metal oxides, and carbon materials could be an ideal strategy to enhance the performances of the MoS₂-based materials for electronic devices.^28,32 It has been reported that platinum (Pt), gold (Au), copper (Cu), and silver (Ag) nanoparticles (NPs) possess very good electrochemical activity toward H₂O₂.^4,33–35 For instance, in our previous work, we demonstrated a facile one-step synthesis strategy for the preparation of a large-scale reduced graphene oxide multilayered film doped with AuNPs (RGO-AuNPs) and applied this film as a functional nanomaterial for H₂O₂ biosensor and achieved a good performance.⁸ Owing to their excellent electrochemical performances, PtNPs play an important role in the fabrication of biosensors.³⁶

We realized that the hybridization of MoS₂ with PtNPs and the creation of novel MoS₂ and PtNPs based nanohybrids will enhance their electrochemical sensor application. In this work, we synthesized the MoS₂ nanoflowers decorated with PtNPs (MoS₂–PtNP) through a facial one-step hydrothermal reaction, and further applied the created MoS₂–PtNP nanohybrids for electrochemical H₂O₂ sensor.

2. Experimental section

2.1. Regents and materials

Ammonium tetrathiomolybdate ((NH₄)₂MoS₄) was purchased from J&K Scientific Ltd. (Beijing, China). Chloroplatinic acid hydrate (≥99.9% purity), Nafion solution (∼5% in a mixture of lower aliphatic alcohols and water) and ethanol were purchased from Sigma-Aldrich (Beijing, China). Hydrazine (N₂H₄·H₂O), H₂O₂ (analytical grade, 30% aqueous solution), potassium hydroxide (KOH), disodium hydrogen phosphate (Na₂HPO₄), sodium dihydrogen phosphate (NaH₂PO₄), ascorbic acid (AA), uric acid (UA), and dopamine (DA) were supplied by Beijing Chemicals Co., Ltd. (Beijing, China). All chemicals used in this work were of analytical reagent grade and obtained from commercial sources and directly used without additional purification. The water used was purified through a Millipore system (∼18.2 MΩ cm).

2.2. Synthesis of MoS₂ nanoflowers and MoS₂–PtNP nanohybrids

The MoS₂ nanoflowers were prepared by a simple one-step hydrothermal synthesis. In brief, (NH₄)₂MoS₄ (22 mg) was added to 20 mL DI water and the mixture was sonicated for 10 min to obtain a clear and homogeneous solution. After that, hydrazine (50 μL) was added into the mixture solution and the reaction solution was further sonicated for 30 min before transferred to a 50 mL Teflon-lined autoclave. It was heated in an oven at 200 °C for 10 h with no intentional control of ramping or cooling rate.

To prepare the MoS₂–PtNP nanohybrids, H₂PtCl₆ (150 μL, 0.5 M) was added into 2 mL of DI water and sonicated for 10 min to achieve a homogeneous solution, and then 20 mg of (NH₄)₂MoS₄ and 18 mL of DI water were added into the above solution. After that, 100 μL of hydrazine was added into the reaction system and the reaction solution was further sonicated for 30 min before transferred to a 50 mL Teflon-lined autoclave. It was heated in an oven at 200 °C for 14 h without intentional control of ramping or cooling rate. All the products were centrifuged at 5000 rpm for 10 min with DI water and ethanol, and each washing step was repeated for at least 5 times. Finally, the obtained products were re-dispersed in 3 mL of DI water and frozen drying for next characterizations and use.

2.3. Fabrication of MoS₂-, PtNP-, and MoS₂–PtNP modified electrodes

Firstly, the glass carbon electrode (GCE, 3.0 mm in diameter) was polished with 1 and 0.3 μm alumina slurry. Secondly, the GCE was washed with ethanol and distilled water in an ultrasonic bath, and finally dried in air. For the preparation of MoS₂- (or PtNP-) modified GCE, 10 mg MoS₂ (or 100 mL PtNP solution), 1 mL ethanol, and 100 μL Nafion (0.1%) were mixed in an ultrasonic bath for 20 min. Then 10 μL of this mixture suspension was dropped onto the pre-treated GCE and dried in air. The MoS₂–PtNP modified GCE was made by directly transferring the nanohybrids onto the surface of GCE. These modified electrodes were used for the cyclic voltammograms (CVs) and amperometric response measurement.

2.4. Electrochemical experiments

All the electrochemical experiments were carried out using an electrochemical workstation (CHI760D, Chenhua, Shanghai) at room temperature. A conventional three-electrode system was employed with a modified GCE as a working electrode, a Pt wire as an auxiliary electrode, and a KCl saturated calomel electrode (SCE) or Ag/AgCl electrode as a reference electrode. The test solution in biosensor application was phosphate buffer solution (PBS, 0.1 M) with pH = 7.4, which was prepared with 0.1 M NaH₂PO₄ and 0.1 M Na₂HPO₄ and deoxygenated with highly pure nitrogen for 20 min before electrochemical experiments. CVs in this work were collected after six scan numbers under steady-state conditions. Amperometric measurements were carried out under stirred conditions at room temperature.

2.5. Experimental techniques

The scanning electron microscopy (SEM) morphologies of the MoS₂ nanoflowers and MoS₂–PtNP nanohybrids were taken on a JSM-6700F scanning electron microscope (JEOL) at 20 kV. X-ray diffraction (XRD, Rigaku D/max-2500 VB+/PC), X-ray photoelectron spectroscopy (XPS, ThermoVG ESCALAB 250), and Raman spectroscopy (LabRAM HORIBA JY, Edison, NJ) were used to measure the structure and properties of both MoS₂ nanoflowers and MoS₂–PtNP nanohybrids. In addition, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) characterizations of MoS₂ nanoflowers and MoS₂–PtNP were conducted on a JEM-2100F field emission transmission electron microscopy operated at 200 kV. The specific surface area was performed by 3H-2000PS1 static volume method with specific surface & pore analysis instrument (BeiShiDe, Beijing).

3. Results and discussion

3.1. Synthesis of MoS₂–PtNP nanohybrids

Fig. 1 presents the typical synthesis strategy of MoS₂–PtNP nanohybrids. In this process, the reductant, hydrazine was utilized to reduce (NH₄)₂MoS₄ and H₂PtCl₆ simultaneously to get MoS₂–PtNP nanohybrids. During this one-step hydrothermal reaction process, the H₂PtCl₆ precursor was reduced to PtNPs, which are attached onto the surface of MoS₂ nanoflowers that produced simultaneously by the formation of S–Pt bonds.

Fig. 1 Schematic presentation for the one-step hydrothermal synthesis of MoS₂–PtNP nanohybrids.

In the hydrothermal reaction process, hydrazine acted as a reducing agent to reduce (NH₄)₂MoS₄ and H₂PtCl₆ simultaneously and finally got the MoS₂–PtNP nanohybrids. Therefore, PtNPs were loaded onto the MoS₂ nanoflowers uniformly due to the 2D layered structure of MoS₂, where each layer composes of S–Mo–S stacks and the MoS₂ nanoflowers are actually accumulated by many single layers. In the process of forming MoS₂–PtNP nanohybrids, the S bonds of MoS₂ nanoflowers and the Pt atoms of PtNPs combine through the electrostatic attraction. Therefore, we suggest that the combination of MoS₂ nanoflowers and PtNPs is based on the S–Pt bond. Our method for fabricating MoS₂–PtNP nanohybrids is simple, efficient, and economic.

3.2. Morphology characterizations of MoS₂ nanoflowers and MoS₂–PtNP nanohybrids

Firstly, the morphologies of the as-prepared MoS₂ nanoflowers and MoS₂–PtNP nanohybrids were characterized with SEM, TEM, and HR-TEM. Fig. 2a shows the typical SEM image of the pure MoS₂ nanoflowers without adding the Pt salt into the reaction system. From this image, it can be clearly seen that the synthesized MoS₂ is uniform nanoflower, which has a size of about 150 nm. To see the detailed structure, the synthesized MoS₂ nanoflowers were further characterized with TEM and the image is shown in Fig. 2b. It can be found that all the MoS₂ nanoflowers reveal folded feature but not the layered nanosheets, which can be further confirmed by the HRTEM characterization (inside corresponding image). Fig. 2c gives the typical TEM image of the hydrothermal synthesized MoS₂–PtNP nanohybrids, and it is clear that the synthesized MoS₂ nanoflowers are decorated with a lot of uniform PtNPs. In addition, the energy dispersive X-ray spectroscopy (EDX) was used to characterize the elemental composition of the synthesized MoS₂–PtNP nanohybrids (Fig. 2d). It is found that the created nanohybrids contain of all three expected elements, Pt, Mo, and S. It should be noted that the peak around 2.3 keV can indicate all three elements that exist in the sample because the characteristic peaks of Pt, S, and Mo are very similar to each other at around 2.3 keV. Moreover, the peak around 1.9 keV with high intensity is a characteristic peak of Si, which is caused by the silicon wafer for supporting the MoS₂–PtNP nanohybrids. The corresponding HRTEM image of the created MoS₂–PtNP nanohybrids (Fig. 2e) further proves that the synthesized MoS₂ nanoflowers are decorated with PtNPs uniformly, and the lattice of MoS₂ (100) is also marked. The size of PtNPs for the decoration of MoS₂ nanoflowers is about 3–5 nm and the (111) lattice face of PtNPs can be clearly seen from the HRTEM image (Fig. 2f).³⁷

Fig. 2 (a and b) SEM and TEM images of MoS₂ nanoflowers; (c) TEM image of MoS₂–PtNP nanohybrids; (d) EDX of MoS₂–PtNP nanohybrids; (e and f) HRTEM images of MoS₂–PtNP nanohybrids.

3.3. Structural and property characterizations of MoS₂–PtNP nanohybrids

To further understand the structure and property of both MoS₂ nanoflowers and MoS₂–PtNP nanohybrids, the synthesized materials were further characterized by XRD, Raman spectroscopy, and XPS techniques.

Fig. 3a shows the typical XRD patterns of the prepared MoS₂ nanoflowers and MoS₂–PtNP nanohybrids. It is clear that the introduction of PtNPs into the nanohybrids enhances the XRD signals, and four strong diffraction peaks of the created MoS₂–PtNP nanohybrids can be found, which are associated to the (111), (200), (220), and (311) planes of the decorated PtNPs.⁴ It can also be found that both XRD patterns possess the broad diffraction peaks of MoS₂ nanosheets, and the enlarged XRD pattern of the MoS₂ nanoflowers shows the (002), (100), and (110) planes of pure MoS₂.²⁸

Fig. 3 Structural characterizations of MoS₂ and MoS₂–PtNP nanohybrids: (a) XRD patterns; (b) Raman spectrum; (c) XPS spectra; (d) XPS of PtNPs.

Fig. 3b gives the Raman spectra of MoS₂ and MoS₂–PtNP nanohybrids. There are two characteristic peaks of MoS₂ (E_2g and A_1g) at 379.0 and 403.5 cm⁻¹, respectively, which are agreed well with the previously reported data.³⁸ In addition, the intensities of these two bands in the spectrum of MoS₂–PtNP nanohybrids are also largely enhanced due to the PtNP-caused signal enhancement. Fig. 3c presents the XPS spectra of MoS₂ nanoflowers and MoS₂–PtNP nanohybrids. For both samples, the bands located at 285.2, 393.0, and 531.8 eV are associated to the typical peaks of C1s, N1s, and O1s, respectively.^39,40 To make the peaks of Pt more distinguishable, we carried out another XPS analysis by scanning the range of binding energy from 65 to 83 eV, and the result is shown in Fig. 3d. Two significant characteristic peaks at 72.5 and 76 eV can be seen, which are assigned to the 4f_7/2 and 4f_5/2 planes of PtNPs, respectively.⁴¹ Based on the above characteristics, we suggest that the MoS₂–PtNP nanohybrids were synthesized successfully.

3.4. Non-enzymatic electrochemical H₂O₂ sensor

The fabricated MoS₂–PtNP nanohybrids were used as a potential electrode material to modify GCE, and the modified GCE was further utilized for the electrochemical detection of H₂O₂. To prove the importance of this kind of hybrid material on the sensing performance, the control experiments with bare GCE, MoS₂/GCE, and PtNP/GCE were also carried out. In this work, we hope the nanohybrids could reach a better electrochemical performance comparing to other similar studies.^8,10,13

Fig. 4 presents the electrochemical detecting H₂O₂ with the fabricated MoS₂–PtNP/GCE based sensor. Firstly, Fig. 4a displays the CVs of bare GCE, MoS₂/GCE, and MoS₂–PtNP/GCE in the presence of 2 mM H₂O₂. We found that both bare GCE and MoS₂/GCE show no redox processes, while the MoS₂–PtNP/GCE displays a reduction peak at about −0.35 V with an obvious positive shift for both the onset potential and the current peak. It is clear the introduction of PtNPs onto the 2D MoS₂ layers greatly improve the electrochemical properties of the created MoS₂–PtNP nanohybrids. We suggest that this reduction peak is originated from the electro-reduction of PtNPs but not MoS₂ sheets. In addition, the reduction peal originated from the reduction of H₂O₂ on the surface of GCE modified with MoS₂–PtNP nanohybrids.

Fig. 4 Electrochemical detection of H₂O₂: (a) CVs of bare GCE and GCEs modified with MoS₂, PtNP, and MoS₂–PtNP nanohybrids; (b) CVs of GCE modified with the MoS₂–PtNP nanohybrids under different H₂O₂ concentration; (c) I–T response of MoS₂–PtNP/GCE in 0.1 M PBS with successive addition of H₂O₂ at −0.35 V vs. SCE; (d) calibrated line.

Therefore, −0.35 V was selected as the applied potential for the I–T measurement. In addition, it should be noted the presented CV results indicate that the MoS₂–PtNP/GCE has better electrocatalytic activity than MoS₂-modified GCE toward the reduction of H₂O₂, which proves the importance of PtNPs in this hybrid material for enhancing the electrocatalytic activity on H₂O₂. Fig. 4b shows the CVs of the MoS₂–PtNP/GCE upon adding H₂O₂ solutions with different concentrations. It is clear that the peak potential located at −0.35 V keeps stable when increasing the concentrations of H₂O₂. The corresponding stable response over the long-term test (I–T response) is shown in Fig. 4c. The cathodic current was found to increase rapidly upon adding H₂O₂ due to the reduction of H₂O₂. The corresponding calibration curve (Fig. 4d) indicates a regular response to H₂O₂, and the linear range detection is calculated to be from 0.02 to 4.72 mM (R = 0.9945, S/N = 3), and the detection limit (LOD) of MoS₂–PtNP/GCE based sensor was further obtained by using the following equation:

LOD = 3σ/S.

In which σ is the standard deviation of the current response, and S is the slope of the calibration curve.⁴² Based on this equation, the LOD is calculated to be as low as 0.345 μM for the MoS₂–PtNP/GCE based H₂O₂ sensor.

Compared to the previous reports on the electrochemical H₂O₂ biosensors,^{1,4,8–10,13,43,44} Our fabricated MoS₂–PtNP based H₂O₂ sensor shows lower detection limit, as is shown in Table 1.

Table 1 Comparison of different materials modified biosensors

Materials	LR (mM)	LOD (μM)	Ref.
MWCNT–AgNP	0.5–30	18.6	1
MWCNT–PtNP	0.1–75	0.61	4
Graphene–AuNP	0.25–22.5	6.2	8
PVA–NG/AgNP	0.005–47	0.56	9
Graphene–Cu2O	0.005–2.78	10.8	10
CNF–PtNP	0.01–9.38	1.9	13
Mesoporous Pt	0.02–40	4.5	41
Graphene/NF/AgNP	0.05–5	10.4	42
MoS2–PtNP	0.02–4.72	0.345	This work

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From the above result, we can find that the detection sensitivity of the H₂O₂ sensor has been largely improved. We suggest that the improvement are likely relative to two factors. The first factor is the utilization of the MoS₂ nanoflowers, which can greatly increase the surface-to-volume ratio of the synthesized electrode material.

In order to reveal the specific surface area of the samples, we conducted the N₂ adsorption–desorption measurement. As shown in Fig. 5, we obtain two distinct hysteresis loops in the N₂ adsorption–desorption isotherm. It is obvious that the Brunauer–Emmett–Teller (BET) surface area of the MoS₂–PtNP nanohybrids is larger than that of MoS₂ nanoflowers. The other factor is the decoration of MoS₂ by PtNPs with uniform size, which can obviously enhance the electrocatalytic activity of pure MoS₂.

Fig. 5 Nitrogen-adsorption–desorption isotherms of MoS₂ nanoflowers and MoS₂–PtNP nanohybrids.

The selectivity, reuse ability, and long-term stability of the fabricated MoS₂–PtNP/GCE based sensor were further evaluated by testing the steady-state response of the MoS₂–PtNP/GCE in N₂ saturated PBS (0.1 M, pH 7.4) at an applied potential of −0.35 V vs. SCE, and the corresponding results are shown in Fig. 6. The selectivity of the MoS₂–PtNP/GCE was evaluated by adding the normal used interfering substances like AA, UA, and DA into the tested system.⁴⁵ Fig. 6a presents the amperometric responses of the MoS₂–PtNP/GCE for the successive addition of 2.0 mM of H₂O₂, 0.1 mM of AA, 0.1 mM of UA, 0.1 mM of DA, and 2.0 mM of H₂O₂. It can be found that the interferences with these species on the detection of H₂O₂ are negligible, which approves the high selectivity of our sensor toward H₂O₂. The reuse stability of the MoS₂–PtNP/GCE was further assessed, and the data is shown in Fig. 6b, which indicates that the fabricated GCE can be used at least 6 times. Finally, we further measured the long-term stability of the MoS₂–PtNP/GCE over a 15 day period. To achieve this aim, the fabricated GCE was stored in the refrigerator at 4 °C and measured every 1–4 days. We found that the intensity of the catalytic current maintains almost steady compared to its initial value in response to 2 mM H₂O₂ after 15 days, indicating an acceptable stability of our H₂O₂ sensor.

Fig. 6 Typical steady-state response of the MoS₂–PtNP modified GCE in N₂-saturated PBS (0.1 M, pH 7.4) at an applied potential of −0.35 V vs. SCE: (a) amperometric responses upon successive addition of 2 mM of H₂O₂, 0.10 mM of AA, 0.10 mM of UA, 0.10 mM of DA, and 2 mM of H₂O₂; (b) reusability in the response to 2 mM H₂O₂; (c) long-term storage stability in response to 2 mM H₂O₂.

4. Conclusions

In summary, we demonstrated a simple one-step hydrothermal synthesis of MoS₂–PtNP nanohybrids and tested their potential applications for fabricating H₂O₂ sensor. The fabricated H₂O₂ sensor shows high sensitivity, excellent reproducibility, and long-term stability. Compared to the previous reports on the synthesis of MoS₂-based hybrid materials, our strategy for creating MoS₂–PtNP nanohybrids is much more simple, efficient, and economic. We expect this one-step hydrothermal synthesis can be further utilized to prepare other functional hybrid nanomaterials based on the single-layered TMD materials and metal nanoparticles, and further applied in the energy and environmental fields.

Acknowledgements

We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (Grant no. 51573013) and the China Scholarship Council (CSC) PhD scholarship.

Notes and references

1. Z. Ouyang, J. Li, J. Wang, Q. Li, T. Ni, X. Zhang, H. Wang, Q. Li, Z. Su and G. Wei, J. Mater. Chem. B, 2013, 1, 2415.
2. Y. Tang, B. L. Allen, D. R. Kauffman and A. Star, J. Am. Chem. Soc., 2009, 131, 13200.
3. Q. Sun, Q. Chen, D. Blackstock and W. Chen, ACS Nano, 2015, 9, 8554.
4. P. Zhang, X. Zhao, X. Zhang, Y. Lai, X. Wang, J. Li, G. Wei and Z. Su, ACS Appl. Mater. Interfaces, 2014, 6, 7563.
5. T. Itoh, T. Shimomura, A. Hayashi, A. Yamaguchi, N. Teramae, M. Ono, T. Tsunoda, F. Mizukami, G. D. Stucky and T. Hanaoka, Analyst, 2014, 139, 4654.
6. H. W. Tseng, Y. J. Tsai, J. H. Yen, P. H. Chen and Y. C. Yeh, Chem. Commun., 2014, 50, 1735.
7. N. Dai and E. T. Kool, Chem. Soc. Rev., 2011, 40, 5756.
8. P. Zhang, X. Zhang, S. Zhang, X. Lu, Q. Li, Z. Su and G. Wei, J. Mater. Chem. B, 2013, 1, 6525.
9. Y. Li, P. Zhang, Z. Ouyang, M. Zhang, Z. Lin, J. Li, Z. Su and G. Wei, Adv. Funct. Mater., 2016, 26, 2122.
10. J. Ding, S. Zhu, T. Zhu, W. Sun, Q. Li, G. Wei and Z. Su, RSC Adv., 2015, 5, 22935.
11. F. Wang, X. Liu, C. H. Lu and I. Willner, ACS Nano, 2013, 7, 7278.
12. Z. Wang, X. Luo, Q. Wan, K. Wu and N. Yang, ACS Appl. Mater. Interfaces, 2014, 6, 17296.
13. Y. Li, M. Zhang, X. Zhang, G. Xie, Z. Su and G. Wei, Nanomaterials, 2015, 5, 1891.
14. I. G. Thakkar, K. L. Lear, J. Vickers, B. C. Heinzec and K. F. Reardonbc, Lab Chip, 2013, 13, 4775.
15. S. Radhakrishnan and S. J. Kim, RSC Adv., 2015, 5, 12937.
16. P. Zhang, X. Zhao, Y. Ji, Z. Ouyang, X. Wen, J. Li, Z. Su and G. Wei, J. Mater. Chem. B, 2015, 3, 2487.
17. X. Zhao, P. Zhang, Y. Chen, Z. Su and G. Wei, Nanoscale, 2015, 7, 5080.
18. Z. S. Wu, W. Ren, D. W. Wang, F. Li, B. Liu and H. M. Cheng, ACS Nano, 2010, 4, 5835.
19. H. Wang, D. Sun, N. Zhao, X. Yang, Y. Shi, J. Li, Z. Su and G. Wei, J. Mater. Chem. B, 2014, 2, 1362.
20. Z. Su, H. Shen, H. Wang, J. Wang, J. Li, G. U. Nienhaus, L. Shang and G. Wei, Adv. Funct. Mater., 2015, 25, 5472.
21. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666.
22. N. Peimyoo, J. Shang, C. Cong, X. Shen, X. Wu, E. K. L. Yeow and T. Yu, ACS Nano, 2013, 7, 10985.
23. Q. Zhang, Z. Zhang, Z. Zhu, U. Schwingenschlogl and Y. Cui, ACS Nano, 2012, 6, 2345.
24. C. Wan, Y. Kodama, M. Kondo, R. Sasai, X. Qian, X. Gu, K. Koga, K. Yabuki, R. Yang and K. Koumoto, Nano Lett., 2015, 15, 6302.
25. W. Zhang, P. Zhang, Z. Su and G. Wei, Nanoscale, 2015, 7, 18364.
26. N. Choudhary, M. Patel, Y. H. Ho, N. B. Dahotre, W. Lee, J. Y. Hwangb and W. Choi, J. Mater. Chem. A, 2015, 3, 24049.
27. M. Nguyen, P. D. Tran, S. S. Pramana, R. L. Lee, S. K. Batabyal, N. Mathews, L. H. Wong and M. Graetzelde, Nanoscale, 2013, 5, 1479.
28. P. Zhang, X. Lu, Y. Huang, J. Deng, L. Zhang, F. Ding, Z. Su, G. Wei and O. G. Schmidt, J. Mater. Chem. A, 2015, 3, 14562.
29. T. Wang, R. Zhu, J. Zhuo, Z. Zhu, Y. Shao and M. Li, Anal. Chem., 2014, 86, 12064.
30. S. Su, H. Sun, F. Xu, L. Yuwen, C. Fan and L. Wang, Microchim. Acta, 2014, 181, 1497.
31. J. Ding, W. Sun, G. Wei and Z. Su, RSC Adv., 2015, 5, 35338.
32. L. Hu, Y. Ren, H. Yang and Q. Xu, ACS Appl. Mater. Interfaces, 2014, 6, 14644.
33. M. Frasconi, C. Tortolini, F. Botrè and F. Mazzei, Anal. Chem., 2010, 82, 7335.
34. F. Li, C. Lei, Q. Shen, L. Li, M. Wang, M. Guo, Y. Huang, Z. Nie and S. Yao, Nanoscale, 2013, 5, 653.
35. X. Qin, W. Lu, Y. Luo, G. Chang, A. M. Asiri, A. O. A. Youbi and X. Sun, Analyst, 2012, 137, 939.
36. G. Wei, F. Xu, Z. Li and K. D. Jandt, J. Phys. Chem. C, 2011, 115, 11453.
37. Z. Cheng, B. He and L. Zhou, J. Mater. Chem. A, 2015, 3, 1042.
38. J. Deng, H. Li, J. Xiao, Y. Tu, D. Deng, H. Yang, H. Tian, J. Li, P. Ren and X. Bao, Energy Environ. Sci., 2015, 8, 1594.
39. L. Zhou, B. He, Y. Yang and Y. He, RSC Adv., 2014, 4, 32570.
40. S. V. P. Vattikuti, C. Byon, C. V. Reddya and R. V. S. S. N. Ravikumarb, RSC Adv., 2015, 5, 86675.
41. S. H. Patil, B. Anothumakkool, S. D. Sathayec and K. R. Patil, Phys. Chem. Chem. Phys., 2015, 17, 26101.
42. X. Bian, K. Guo, L. Liao, J. Xiao, J. Kong, C. Ji and B. Liu, Talanta, 2012, 99, 256.
43. S. A. G. Evans, J. M. Elliott, L. M. Andrews, P. N. Bartlett, P. J. Doyle and G. Denuault, Anal. Chem., 2002, 74, 1322.
44. J. Wang, X. Zhao, J. Li, X. Kuang, Y. Fan, G. Wei and Z. Su, ACS Macro Lett., 2014, 3, 529.
45. P. M. Nia, F. Lorestani, W. P. Meng and Y. Alias, Appl. Surf. Sci., 2015, 332, 648.

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