Improving performance of MoS₂-based electrochemical sensors by decorating noble metallic nanoparticles on the surface of MoS₂ nanosheet

Abstract

Recently, the use of a molybdenum disulfide (MoS₂) nanosheet as a sensing platform has attracted more and more scientists' attention due to its graphene-like structure and excellent physical/chemical properties. Herein, the performances of MoS₂-based electrochemical sensors were improved by decorating noble metallic nanoparticles on the surface of the MoS₂ nanosheet. Taking catechol (CC) as an analyte, it could be found that the MoS₂ nanosheet and its derivatives have excellent electrocatalytic abilities toward CC. More interestingly, the gold nanoparticles-decorated MoS₂ nanosheet (Au–MoS₂), platinum nanoparticles-decorated MoS₂ nanosheet (Pt–MoS₂) and gold–platinum core–shell nanoparticles-decorated MoS₂ nanosheet (Au@Pt–MoS₂) modified electrodes exhibited better detection performances than the pure MoS₂ nanosheet and metallic nanoparticles modified electrodes due to their synergistic effect. As expected, the Au@Pt–MoS₂ nanocomposites modified electrode exhibited the linear range of 2–1000 μM and the detection limit of 0.44 μM for CC detection, which was better than all MoS₂-based nanomaterials. All the MoS₂-based electrochemical sensing platforms were employed to determine CC both in buffer and in real samples with satisfactory results, suggesting MoS₂-based nanomaterials maybe the potential candidates for constructing electrochemical sensors for chemical and biological molecules detection.

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1. Introduction

Molybdenum disulfide (MoS₂) is a member of the layered transition-metal dichalcogenide family, which easily exfoliated to a few layers or even to a single layer because of the weak van der Waals interactions.^1–4 As a graphene-like layered nanomaterial, MoS₂ has many properties similar to graphene, such as abundant storage, good mechanical strength, large surface area and easy functionalization.⁵ More importantly, the thickness of a MoS₂ nanosheet greatly influences its indirect-to-direct bandgap transition (from 1.2 to 1.9 eV),^6,7 which makes MoS₂ a charming material for applications in photoluminescence,⁸ transistors,⁹ catalysts for hydrogen evolution,¹⁰ sensors,^11–14 lithium batteries¹⁵ and so on. Among them, constructing MoS₂-based sensors combined with fluorescence, electrochemistry and surface-enhanced Raman scattering techniques for target molecules detection has gradually attracted more and more scientists' attention.¹ For example, Zhu et al. utilized the fluorescence quenching ability of MoS₂ to construct MoS₂-based biosensors for homogeneous detection of biomolecules.¹⁴ Li's group fabricated an electrochemical biosensor for DNA and RNA detection based on thionin–MoS₂ nanocomposite.¹⁶

As we know, nanohybrids always exhibit obvious synergistic effects, which possess better performance than single component. Therefore, MoS₂-based hybrid nanomaterials have been developed, such as MoS₂–graphene,¹⁷ MoS₂–organic compounds¹⁸ and MoS₂–noble metallic nanoparticles.¹⁹ Among these MoS₂-based nanohybrids, noble metallic nanoparticles-decorated MoS₂ nanohybrids received an increasing interest by researchers. Zhang's and Wang's groups systematically investigated four noble metallic nanoparticles decorated on the surface of MoS₂ nanosheets via different synthesized methods, such as Au, Ag, Pt and Pd nanoparticles. Moreover, they found such noble metallic nanoparticles–MoS₂ nanocomposites showed higher catalytic activity compared with commercial Pt catalysts.^20,21 Recently, Su et al. had successfully synthesized Au–Pt core–shell bimetallic nanoparticles-decorated MoS₂ nanocomposites (Au@Pt–MoS₂), which exhibited better catalytic activity toward methanol oxidation than that of Au–MoS₂ and Pt–MoS₂ nanocomposites.²² On the basis of successful synthesis, noble metallic nanoparticles-decorated MoS₂ nanocomposites have been extensively used to construct sensors for chemical and biological molecules detection. Kuru et al. used Pd–MoS₂ nanocomposites as a sensing platform for hydrogen detection.²³ Wang's and Sow's groups employed Au–MoS₂ nanocomposites as a surface-enhanced Raman scattering (SERS)-active substrate, which was used to high sensitive detection of Rhodamine 6G and methylene blue.^24,25 Our group also used Au–MoS₂ nanocomposites as electrochemical sensing platforms for neurotransmitters, H₂O₂, small biological molecules and protein detection.^12,26–29

Inspired by these exciting studies, it prompts us to explore the performances of MoS₂ nanosheet and its derivatives. Herein, we choose catechol (CC) as an analyte to study the performance of four MoS₂-based electrochemical sensing platforms, including MoS₂ nanosheet, gold nanoparticles-decorated MoS₂ nanocomposites (Au–MoS₂), platinum nanoparticles-decorated MoS₂ nanocomposites (Pt–MoS₂) and gold–platinum core–shell bimetallic nanoparticles-decorated MoS₂ nanocomposites (Au@Pt–MoS₂). CC is a typical heavy environmental pollutant due to its high toxicity and low degradability in the ecological environment. It can causes eczematous dermatitis, depression of the central nervous system (CNS) and a prolonged rise of blood pressure with the increasing doses of CC.^30,31 Therefore, accurate, sensitive, fast and low-cost detection of CC is very meaningful to environmental monitoring and human health. As shown in Fig. 1, all four MoS₂-based modified electrodes exhibit excellent electrocatalytic ability toward CC. It can be found that CC shows a pair of more well-defined and reversible redox peaks at Au@Pt–MoS₂ film modified electrode than that at other MoS₂-based modified electrodes. Moreover, Au@Pt–MoS₂-based electrochemical sensor exhibits the widest linear range (2–1000 μM) and the lowest detection limit (0.44 μM) for CC detection in four MoS₂-based sensors under the same condition. These results suggest that Au@Pt–MoS₂ nanocomposite has better electrocatalytic performance than other MoS₂-based nanocomposites. All MoS₂-based electrochemical sensors are successfully employed to determine CC in real water sample with satisfactory recovery, indicating those MoS₂-based nanomaterials maybe potential sensing platform candidates for chemical or biological molecules detection.

Fig. 1 Illustration of MoS₂-based electrochemical sensors for catechol detection.

2. Experimental section

2.1. Apparatus

Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential pulse voltammetry (DPV) were performed on an Autolab PGSTAT302 (Metrohm China Ltd, Switzerland). A conventional three-electrode system was used in this experiment. Different modified glassy carbon electrodes (GCE) (3.0 mm in diameter) was used as working electrode; a platinum wire and a saturated calomel electrode (SCE) were used as the auxiliary electrode and the reference electrode, respectively. The transmission electron microscopy (TEM) was employed to characterize MoS₂ nanosheet and MoS₂-based nanocomposites (Philips CM 200). The thickness of MoS₂ nanosheet was examined with atomic force microscope (AFM, Bruker).

2.2. Reagents and chemicals

Potassium hexacyanoferrate(III) (K₃Fe(CN)₆, ≥99.5%), potassium hexacyanoferrate(II) trihydrate (K₄Fe(CN)₆·3H₂O, ≥99.5%), disodium hydrogen phosphate (Na₂HPO₄, ≥99.0%), sodium dihydrogen phosphate dehydrate (NaH₂PO₄·2H₂O, ≥99.0%), catechol (CC, ≥98.0%) and ascorbic acid (AA, ≥99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Gold(III) tetrachloride trihydrate (HAuCl₄·3H₂O, ≥47.8%), sodium citrate tribasic dehydrate (Na₃C₆H₅O₇·2H₂O, ≥99.5%), potassium tetrachloroplatinate(II) (K₂PtCl₄, 99.99%) and hexadecyltrimethylammonium bromide (CTAB, 96%) were purchased from Sigma (USA). Polyvinyl pyrrolidone (PVP) was purchased from Alfa Aesar (Tianjin, China). Carboxymethyl cellulose sodium (CMC) and sodium borohydride (NaBH₄, 98%) were purchased from Aladdin Industrial Corporation (Shanghai, China). All chemicals were directly used without further purification. All solutions were prepared with Milli-Q water from a Milli-pore system.

2.3. Synthesis of different MoS₂-based nanomaterials

MoS₂ nanosheet was prepared according to our previous works.^25,29 The exfoliated MoS₂ nanosheet was stored at 4 °C for further application. Au–MoS₂, Pt–MoS₂ and Au@Pt–MoS₂ nanocomposites were synthesized by microwave-assisted hydrothermal method and oil-bath method. For Au–MoS₂ synthesis, 100 μL PVP (5%) was added into 4 mL MoS₂ (0.025 mg mL⁻¹) solution and mixed for several minutes. Then, 200 μL HAuCl₄ (10 mM) was injected into the solution. After this, the reaction mixture was heated to 60 °C for 5 minutes in the microwave reactor. Finally, the product of Au–MoS₂ was purified at least twice by centrifugation. It should be noted that the Au–MoS₂ nanoseed synthesized using the same method. Gold nanoparticle in Au–MoS₂ nanoseed (100 μL HAuCl₄) is smaller than that in Au–MoS₂ nanocomposite (200 μL HAuCl₄) due to the fewer volume of HAuCl₄ (Fig. S1†). For Pt–MoS₂ synthesis, 100 μL (50 mM) CMC, 50 μL (100 mM) NaBH₄ and 300 μL (5 mM) H₂PtCl₆ were added into 10 mL (10 μg mL⁻¹) MoS₂ nanosheets solution. Then the reaction mixture was heated to 100 °C for 5 minutes in the microwave reactor. Finally, the product of Pt–MoS₂ nanocomposite was centrifuged at least twice for the purification. Au@Pt–MoS₂ nanocomposite was synthesized according to our previous work.²² 0.5 mL 10 mM CTAB, 1 mL 100 mM AA and 100 μL 5 mM H₂PtCl₆ was added into 2 mL Au–MoS₂ seed solution. The mixture was then heated on a hotplate at approximately 100 °C for 6 min to form Au@Pt–MoS₂ nanocomposite. Finally, all the composites were stored at 4 °C for future use.

2.4. Preparation of different MoS₂-based modified electrodes

The GCE was firstly polished with 0.3 and 0.05 mm alumina slurry, respectively. After cleaning by ethanol and ultrapure water for several minutes, the GCE was dried under nitrogen stream. Then, 5 μL as-prepared MoS₂ nanosheet was dropped onto the cleaned electrode to form MoS₂ film modified electrode, which defined as MoS₂/GCE. Similarly, Au/GCE, Au–MoS₂/GCE, Pt/GCE, Pt–MoS₂/GCE, Au@Pt/GCE and Au@Pt–MoS₂/GCE were prepared by using the same procedure.

3. Results and discussion

3.1. Characterization of MoS₂-based nanomaterials

TEM was used to characterize the morphologies of MoS₂ nanosheet and noble metallic nanoparticles-decorated MoS₂ nanocomposites. As shown in Fig. 2A, MoS₂ nanosheet possessed a typical graphene-like layered and wrinkled nanostructure. AFM was utilized to determine the thickness of MoS₂ nanosheet. Fig. S2† showed the thickness of MoS₂ nanosheet was about 1.14 nm, indicating that the exfoliated MoS₂ nanosheet was almost single layer.^32,33 From Fig. 2B–D, it can be seen that 20 nm spherical Au nanoparticles, 21 nm spherical Pt nanoparticles and 19 nm core–shell Au–Pt nanoparticles were uniformly supported on the surface of MoS₂ nanosheet, respectively. These TEM images suggested that the expected Au–MoS₂, Pt–MoS₂ and Au@Pt–MoS₂ nanocomposites have been successfully synthesized.

Fig. 2 TEM images of the (A) MoS₂ nanosheet, (B) Au–MoS₂ nanocomposite, (C) Pt–MoS₂ nanocomposite and (D) Au@Pt–MoS₂ nanocomposite. Insert figures: histograms of Au, Pt and Au@Pt nanoparticles' diameters in the corresponding to MoS₂-based nanocomposites.

3.2. Electrochemical characterization of MoS₂-based modified electrodes

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques were used to investigate the electron transfer properties of different MoS₂-based modified electrodes. As shown in Fig. 3A, MoS₂/GCE (curve b) showed a similar redox peaks with larger currents to the bare GCE (curve a), which ascribed to the large surface area of MoS₂ nanosheet. When Au, Pt and Au@Pt nanoparticles decorated on the surface of MoS₂ nanosheet, both anodic and cathodic peak currents obviously increased at Au–MoS₂/GCE (curve c), Pt–MoS₂/GCE (curve d) and Au@Pt–MoS₂/GCE (curve e). Moreover, the peak-to-peak potential separation (ΔE_p) of Au–MoS₂/GCE, Pt–MoS₂/GCE and Au@Pt–MoS₂/GCE were 93 mV, 96 mV and 101 mV, respectively, which was smaller than that of bare GCE (151 mV). These results indicated that Au–MoS₂, Pt–MoS₂ and Au@Pt–MoS₂ nanocomposites possessed excellent conductivity due to the synergistic effect of MoS₂ nanosheet and noble metallic nanoparticles, which greatly enhanced the electron transfer between redox probe and electrode surface. It was noted that Au@Pt–MoS₂/GCE had better electrochemical behavior than Au–MoS₂/GCE and Pt–MoS₂/GCE, ascribing to the dendritic-like structures of Au@Pt and the better electrocatalytic ability of Au@Pt–MoS₂ nanocomposites. As we all know, MoS₂ has high surface-to-volume ratio and noble metallic nanoparticles has wonderful electrical conductivity and electrocatalytic ability. Therefore, these MoS₂-based nanocomposites can efficiently facilitate the electron transfer between the redox probe and the electrode surface. The capacity of electron transfer at different electrodes was also investigated by EIS (Fig. 3B). The electron transfer resistance (R_ct) of the MoS₂/GCE (curve b) was much larger than that of the bare GCE (curve a). A large number of negative charges on the surface of MoS₂, which could hinder the electron transfer between the [Fe(CN)₆]^3−/4− and the electrode surface due to the electrostatic repulsion. Expectedly, once Au, Pt and Au–Pt nanoparticles decorated on the surface of MoS₂ nanosheet, the conductivity of MoS₂-based nanohybrids were significantly improved, resulting in the R_ct of Au–MoS₂/GCE (curve c), Pt–MoS₂/GCE (curve d) and Au@Pt–MoS₂/GCE (curve e) were obviously smaller than MoS₂/GCE. It should be pointed out that the R_ct of Au@Pt–MoS₂/GCE was smaller than that of Au–MoS₂/GCE and Pt–MoS₂/GCE, which was consistent with the CV curves. These experimental results suggested MoS₂-based nanocomposites possessed excellent conductivity, which could be used as sensing platform in electrochemistry field. To prove the repeatability of these MoS₂-based modified electrodes, at least three parallel electrodes were employed. The redox peak currents and R_ct of different MoS₂-based modified electrodes were listed in Fig. S3,† suggesting the as-prepared electrochemical platforms possessed excellent repeatability.

Fig. 3 (A) Cyclic voltammograms and (B) Nyquist plots of (a) bare GCE, (b) MoS₂/GCE, (c) Au–MoS₂/GCE, (d) Pt–MoS₂/GCE and (e) Au@Pt–MoS₂/GCE in 5 mM [Fe(CN)₆]^3−/4− solution containing 0.1 M KCl. Scan rate: 100 mV s⁻¹.

3.3. Electrochemical behavior of CC at MoS₂-based modified electrodes

Fig. 4 demonstrated the CVs of the bare GCE, MoS₂/GCE, Au–MoS₂/GCE, Pt–MoS₂/GCE and Au@Pt–MoS₂/GCE in 0.1 M PB (pH 7.0) containing 400 μM catechol, respectively. A pair of irreversible redox peaks of CC was obtained at the bare GCE (curve a). The anodic and cathodic peaks of CC were at 334 mV and 24 mV, respectively, leading to the ΔE_p was 310 mV. When MoS₂ modified on the bare GCE, the anodic and cathodic peak currents were slightly increased and the ΔE_p decreased to 191 mV (curve b), suggesting MoS₂ nanosheet could improve the electrochemical reversibility of CC. The ΔE_p at Au–MoS₂/GCE (curve c), Pt–MoS₂/GCE (curve d) and Au@Pt–MoS₂/GCE (curve e) were 188 mV, 167 mV and 136 mV, respectively, which were smaller than that at MoS₂/GCE and bare GCE. Furthermore, the anodic peak currents of Au–MoS₂/GCE, Pt–MoS₂/GCE and Au@Pt–MoS₂/GCE were 1.15, 1.25 and 1.41 folds than that of bare GCE, respectively. These results indicated that such noble metallic nanoparticles-decorated MoS₂ nanocomposites possessed strong electrocatalytic activity towards the redox reaction of CC. It also proved that bimetallic nanoparticle-decorated MoS₂ nanocomposites possessed better electrochemical performance than single metallic nanoparticles-decorated MoS₂ nanocomposites. Similarly, the reproducibility of MoS₂-based modified electrodes electrocatalytic ability towards CC was shown in Fig. S4.† To further prove the performance of noble metallic nanoparticles-decorated MoS₂ nanocomposites, Au, Pt and Au@Pt nanoparticles were synthesized to determine CC under the same condition. The TEM images of Au, Pt and Au@Pt nanoparticles and the electrochemical performances of Au/GCE, Pt/GCE and Au@Pt/GCE were shown in Fig. S5 and S6,† respectively. The experimental results also confirmed that the MoS₂-based nanocomposites had better electrochemical performances due to their synergistic effect. As we know, CC is an isomer of dihydroxybenzenes, which is easily adsorbed and aggregated on the surface of MoS₂ nanosheet vis π–π interaction. Therefore, the synergistic effect of the large surface of MoS₂ nanosheet and excellent electrocatalytic ability of noble metallic nanoparticles resulted in excellent electrochemical performance.

Fig. 4 Cyclic voltammograms of (a) bare GCE, (b) MoS₂/GCE, (c) Au–MoS₂/GCE, (d) Pt–MoS₂/GCE and (e) Au@Pt–MoS₂/GCE in 0.1 M PB (pH 7.0) solution containing 400 μM CC. Scan rate: 100 mV s⁻¹.

3.4. Effect of scan rate and pH value

The effect of scan rate on the redox peak currents of CC at different modified electrodes were investigated. As shown in Fig. S7,† both anodic and cathodic peak currents increased with the scan rate increasing. Moreover, the anodic peak currents of MoS₂/GCE, Au–MoS₂/GCE, Pt–MoS₂/GCE and Au@Pt–MoS₂/GCE were linear with the square root of the scan rate ranging from 2 mV s⁻¹ to 200 mV s⁻¹, suggesting that the reaction of CC at different MoS₂-based modified electrodes was a typical diffusion-controlled process (Fig. 5A).³⁴ The influence of pH value on the redox reaction of CC at different modified electrodes were also studied in the range of pH 6.0–8.0. Fig. S8† showed the redox peak potentials of CC at MoS₂-based modified electrodes shifted negatively with the pH value increased. The formal potential was linearly proportional to the pH value (Fig. S9†), suggesting two protons coupled with two electrons involved in the reaction process. More importantly, the anodic peak currents of MoS₂/GCE, Au–MoS₂/GCE, Pt–MoS₂/GCE and Au@Pt–MoS₂/GCE were largest when pH value was equal to 7.0 (Fig. 5B). Considering the determination sensitivity, pH 7.0 was chosen as the optimal condition in following experiments.

Fig. 5 Effect of (A) scan rate and (B) pH value on the peak currents of (a) MoS₂/GCE, (b) Au–MoS₂/GCE, (c) Pt–MoS₂/GCE and (d) Au@Pt–MoS₂/GCE in the presence of 400 μM CC.

3.5. Electrochemical determination of CC at MoS₂-based modified electrodes

DPV was performed to accurately determine CC at different MoS₂-based modified electrodes. As shown in Fig. 6A, the oxidation peak current of CC was increased with the increasing CC concentration at MoS₂/GCE. The oxidation peak current of was proportional to the concentration of CC in the range from 20 to 600 μM (inset in Fig. 6A). The regression equation was obtained as I_pc (μA) = 0.0057c (μM) + 0.3767 (R² = 0.9991) and the detection limit was estimated to be 0.6 μM (S/N = 3). Similarly, the oxidation peak currents of Au–MoS₂/GCE, Pt–MoS₂/GCE and Au@Pt–MoS₂/GCE were linear with the CC concentration ranging from 10 μM to 1000 μM, 10 μM to 800 μM and 2 μM to 1000 μM with estimated detection limits of 0.57 μM, 0.47 μM and 0.44 μM, respectively (Fig. 6B–D). These linear ranges and detection limits of different MoS₂-based electrodes were comparable to or better than some nanomaterials-based electrodes, which is shown in Table S1.† From the results, it can be found that noble metallic nanoparticles-decorated MoS₂ nanocomposites exhibited better electrochemical performance than MoS₂ nanosheet, including linear range and detection limit. It was also noted that the bimetallic nanoparticles-decorated MoS₂ nanocomposite (Au@Pt–MoS₂) showed better performance (wider linear range) than single nanoparticles-decorated MoS₂ nanocomposites (Au–MoS₂ and Pt–MoS₂), which was consistent with the result in Fig. 3.

Fig. 6 The differential pulse voltammograms of various concentrations of CC at (A) MoS₂/GCE, (B) Au–MoS₂/GCE, (C) Pt–MoS₂/GCE and (D) Au@Pt–MoS₂/GCE in 0.1 M PB (pH 7.0). Inset figures: plot of oxidation peak currents derived from DPVs vs. CC concentrations.

For further application, the stability of these MoS₂-based electrochemical sensors should be investigated. As shown in Fig. S10 and S11,† the cyclic stability and long-term stability of these MoS₂-based sensors were studied. The peak currents of MoS₂-based sensors could retain over 70% after 100 successively scanning and 20 days storage, suggesting that such MoS₂-based sensors had excellent stability.

3.6. Analytical applications

To explore the feasibility of MoS₂-based sensing platforms to detect CC in practice, CC was added into river water and local tap water to simulate actual samples. The founded amount and recovery of CC detection were summarized in Table 1. The recovery were varied from 96.6% to 105.4% and relative standard deviation (RSD) was less 5% for MoS₂-based electrodes, suggesting these MoS₂-based electrochemical sensing platforms could be used to determine CC in real samples.

Table 1 Determination results for CC in river water and local tap water (average of three measurements)

Materials	CC added (μM)	River water			Local tap water
		CC founded (μM)	Recovery (%)	RSD (%)	CC founded (μM)	Recovery (%)	RSD (%)
MoS2	1	1.03	103.2	2.12	1.04	104.1	3.24
	2	1.99	99.4	2.51	1.95	97.7	1.93
	5	5.01	100.2	1.31	4.85	96.9	2.23
Au–MoS2	1	1.05	105.4	3.01	1.03	103.4	1.39
	2	1.98	98.9	1.73	1.04	103.6	2.74
	5	4.83	96.6	2.22	4.85	96.9	0.98
Pt–MoS2	1	1.03	103.7	2.63	0.99	99.1	3.42
	2	2.03	102.1	4.78	2.02	101.3	2.94
	5	5.17	103.4	3.84	5.06	101.1	1.49
Au@Pt–MoS2	1	1.02	102.4	1.94	1.03	102.9	3.97
	2	2.06	103.1	2.91	2.03	101.6	1.89
	5	4.83	96.6	0.96	4.92	98.5	2.06

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4. Conclusion

In this work, CC was chosen as analyte to investigate the electrochemical performances of MoS₂ nanosheet and three MoS₂-based nanocomposites. If combining the advantageous characteristics of MoS₂ and noble metallic nanoparticles, the nanohybrids exhibited better electrochemical performance than pure MoS₂ nanosheet and noble metallic nanoparticles. For example, Au–MoS₂, Pt–MoS₂ and Au@Pt–MoS₂ modified electrodes exhibited a wider detection range and a lower detection limit than MoS₂/GCE under the same experimental condition. Moreover, these MoS₂-based electrochemical sensors could be employed to determine CC in water samples with satisfactory results. All the results suggested that properties of MoS₂-based nanocomposites can be tuned according to the natural properties of MoS₂ nanosheet and other components. A MoS₂-based nanocomposite may be a promising candidate for the future study of electrochemical sensors for chemical and biological molecules detection.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (2012CB933301), the National Natural Science Foundation of China (21305070, 21475064), the Natural Science Foundation of Jiangsu Province (BK20130861), the Sci-tech Support Plan of Jiangsu Province (BE2014719), the Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R37), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, YX03001).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12401a

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