Hemin-functionalized MoS₂ nanosheets: enhanced peroxidase-like catalytic activity with a steady state in aqueous solution

Abstract

Hemin-functionalized MoS₂ nanosheets (hemin/MoS₂-NSs) are first obtained via van der Waals interactions between few-layered MoS₂ nanosheets (MoS₂-NSs) and hemin molecules. It is demonstrated that a portion of MoS₂-NSs undergoes a phase transition from semiconducting to metallic phase under the influence of hemin, which shows the coexistence of semiconducting and metallic phases in the crystal structure of hemin/MoS₂-NSs. MoS₂-NSs prepared from sonication-induced exfoliation of bulk MoS₂ crystals in aqueous surfactant solution exhibit intrinsic peroxidase-like activity for the oxidation of 3,3,5,5-tetramethylbenzidine in the presence of H₂O₂, which is further improved by the functionalization of hemin. Significantly, MoS₂-NSs are presented as a new support of hemin, and when compared to MoS₂-NSs, hemin/MoS₂-NSs exhibit better dispersity in aqueous solution, which is used in the development of H₂O₂ sensor based on the enhanced peroxidase-like activity.

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1. Introduction

Hydrogen peroxide is a product of enzymatic reactions and a chemical threat to the environment, which makes its determination important in various fields including environmental, food, clinic, and pharmaceutical analysis.¹ Up to now, a number of H₂O₂ sensors have been reported based on electrochemical, chemiluminescence, fluorometry, and colorimetry techniques.^1–4 Among these analytical methods, colorimetric measurement using the substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of artificial enzymes is considered as a low-cost, simple, fast, sensitive, and reliable technique to construct H₂O₂ sensors.⁴ Even though artificial enzymes have been applied in various fields, their deficiencies such as instability, high cost, and critical operating conditions may pose limitations to their applications. Hence, developing artificial enzymes, which are stable, can be easily prepared, and are cost-efficient still remains a central challenge.

With the discovery of striking properties and rapid development of graphene, two-dimensional (2D) layered materials such as hexagonal boron nitride (h-BN), metal chalcogenides, transition metal oxides, and transition metal dichalcogenides (TMDs) have attracted a significant amount of interest, which have been demonstrated to play a crucial role in developing advanced functional materials by exploiting their excellent properties.^5–8 Due to the specific 2D confinement of electron motion and the absence of interlayer perturbation, MoS₂ nanosheets (MoS₂-NSs) possess a direct band gap and show some remarkable properties, which offer new opportunities in various areas.^9–11 Recently, MoS₂-NSs have been fabricated into transistors,¹² integrated circuits,¹³ nanometer thick photovoltaics,¹⁴ sensors,¹⁵ and battery materials.^16,17 Furthermore, MoS₂-NSs have also been considered as an excellent catalytic material for hydrogen evolution reaction, which is induced by the active edge sites.^18,19 However, the catalytic ability of MoS₂-NSs in hydrogen evolution is significantly limited by their low conductivity and lack of active edge sites.^18–21 In order to expand the application and achieve the potential values of MoS₂-NSs, a number of MoS₂-based nanohybrids, which combine Au nanoparticles (NPs),²² iron oxide NPs,²³ graphene,²⁴ and carbon nanotubes,²⁵ have been synthesized. So far, the exfoliation of bulk MoS₂ crystals was considered as a direct route for obtaining MoS₂-NSs. It is known that bulk MoS₂ can be exfoliated by lithium intercalation but this method is sensitive to environmental conditions and results in structural deformations.^10,26 Alternatively, the sonication-assisted exfoliation of MoS₂ powder in aqueous surfactant solution has been considered as an efficient and environmentally friendly method, resulting in the preparation of small but high-quality exfoliated nanosheets, which can then be fabricated into composite materials.^26,27

Hemin, a natural metalloporphyrin and the active center of the heme-protein family, is well-known for its properties of catalyzing a variety of oxidation reactions. Nevertheless, the direct application of hemin is challenging because of its molecular aggregation in aqueous solution to form catalytically inactive dimers and oxidative self-destruction in the oxidizing media.²⁴ Therefore, the development of novel materials as hemin supports to achieve biomimetic catalysts with enzyme-like activity is highly desired. Recently, it was reported that hemin could be adsorbed on graphene,^28,29 single-walled carbon nanotubes,³⁰ metal–organic framework,³¹ polypyrrole,³² and TiO₂ nanowires³³ to form composite nanomaterials, exhibiting good properties in various applications. Significantly, studies, which show that MoS₂-NSs can efficiently adsorb ssDNA molecules because of strong van der Waals interaction to achieve the construction of biosensors have been reported.^34,35 On the basis of the aforementioned works, we thought that a MoS₂ nanosheet possessing a similar 2D structure with graphene might serve as an available platform, which could be used to form composite nanomaterials with some plane-like molecules. Due to the important properties and potential applications of hemin, MoS₂-NSs were considered as a new support to form hemin/MoS₂-NSs. The properties and applications of MoS₂-NSs and hemin/MoS₂-NSs, which are used as artificial enzymes have never been explored before.

Herein, few-layered MoS₂-NSs were obtained from the exfoliation of MoS₂ powder in aqueous surfactant solution under sonication, which was followed by the interaction with hemin in methanol solution, leading to the formation of hemin/MoS₂-NSs. The catalytic activities of MoS₂-NSs and hemin/MoS₂-NSs in the reduction of H₂O₂ in the presence of 3,3,5,5-tetramethylbenzidine (TMB) were explored. It was found that when compared to MoS₂-NSs, the hemin/MoS₂-NSs composite exhibited higher peroxidase-like activity with greater dispersity in aqueous solution, which could have potential applications in various significant areas.

2. Experimental section

2.1. Reagents and chemicals

Hemin (>98%), molybdenum(IV) sulfide (MoS₂ crystalline powder, <2 μm, 99%), and horseradish peroxidase (HRP) were obtained from Sigma-Aldrich Co., USA. Sodium cholate and TMB (98%) were purchased from Aladdin Chemistry Co., Ltd., Shanghai, China. Other chemicals employed in this work were of analytical reagent grade, and were purchased from Kelong Chemical Reagent Co., Ltd., Chengdu, China. All the reagents were used as received. Doubly distilled water was used throughout the experiments.

2.2. Apparatus

All electrochemical processes were conducted on a CHI 660B electrochemical workstation (Chenhua Instruments Co., China). A three-electrode system consisted of a modified glassy carbon working electrode with a diameter of 3 mm, a saturated Ag/AgCl reference electrode, and an auxiliary electrode made of platinum. All potentials were given with respect to the Ag/AgCl electrode. The KQ-250B ultrasonic bath (250 W, Kun Shan Ultrasonic Instruments Co., Ltd, China) was used to exfoliate MoS₂ powder and prepare hemin/MoS₂-NSs. Transmission electron microscopy (TEM) measurements were performed on a Tecnai G2 F20 S-TWIN transmission electron microscope (FEI, USA) operated at 200 kV. Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4800 field emission scanning electron microscope (Japan). Atomic force microscopy (AFM) data were obtained in a Dimension Icon atomic force microscope (Bruker, Germany). X-ray diffraction (XRD) patterns were obtained with a D8 DISCOVER X-ray diffractometer (Bruker, Germany). X-ray photoelectron spectra (XPS) analyses were conducted using ESCALAB 250Xi X-ray photoelectron spectroscopy (Thermo Electron, USA). UV-visible spectra were recorded on a UV-2450 UV-vis spectrophotometer (Shimadzu, Japan). Centrifugation was carried out in a TGL-16M high-speed refrigerated centrifuge (Xiangyi, China).

2.3. Sonication-induced exfoliation of bulk MoS₂

MoS₂ nanosheets were prepared through sonicated-exfoliation of MoS₂ crystals in an aqueous surfactant solution.²⁶ The mixed water solution, containing 5 mg mL⁻¹ MoS₂ powder and 1.5 mg mL⁻¹ sodium cholate, was sonicated at room temperature (25 °C) for 10 h, which resulted in the dispersion of black MoS₂ nanosheets. To remove bulk MoS₂, the dispersed nanosheets were centrifuged at 3000 rpm for 30 min followed by the collection of a yellow-green supernatant. The separated supernatant was then centrifuged at 12 000 rpm for 30 min, and the MoS₂ nanosheets were collected. Exfoliated MoS₂ nanosheets were further dispersed in ultrapure water. Similarly, the regenerated dispersed nanosheets were centrifuged at 12 000 rpm for 30 min, accompanied by the collection of sediments to complete the washing process. The washing process was repeated two times to completely remove the surfactant sodium cholate. Ultimately, the sediments were dispersed in ultrapure water or methanol to prepare uniform exfoliated MoS₂ nanosheets aqueous or methanol solution, respectively.

2.4. Synthesis of hemin-functionalized MoS₂ nanosheets

The synthesis of hemin/MoS₂-NSs was carried out using methanol as the solvent. Initially, a mixed methanol solution containing 0.5 mg mL⁻¹ hemin and MoS₂-NSs was prepared and then sonicated (250 W) at room temperature (25 °C) for 2 h. The sonicated solution was then centrifuged at 12 000 rpm for 30 min and black sediments, hemin/MoS₂-NSs, were obtained. Furthermore, the as-prepared hemin/MoS₂-NSs were dispersed into methanol and the regenerated dispersed nanosheets were centrifuged at 12 000 rpm for 30 min followed by the collection of sediments for completing the washing process. Finally, the sediments were dispersed into water to prepare the hemin/MoS₂-NSs aqueous solution under sonication.

2.5. Preparation of MoS₂-NSs and hemin/MoS₂-NSs modified glassy carbon electrode

Prior to modification, the glassy carbon electrode (GCE, 3 mm) was polished with 0.3 and 0.05 μm alumina slurry to obtain a mirror finish, rinsed and sonicated in doubly distilled water for 5 min. The MoS₂-NSs modified GCE and the hemin/MoS₂-NSs modified GCE were obtained by dropping 5 μL of 50 μg mL⁻¹ MoS₂-NSs or hemin/MoS₂-NSs aqueous suspension on the surface of the well-polished GCE, which were then dried in air.

3. Results and discussion

In our experiments, MoS₂ nanosheets were prepared by the exfoliation of bulk MoS₂ crystals in an aqueous surfactant solution under sonication according to a previously reported method.²⁶ As we know, ultrasonic waves generate cavitation bubbles that collapse into high-energy jets, breaking up the MoS₂ crystallites and producing MoS₂ nanosheets. Meanwhile, the presence of surfactant molecules stabilized MoS₂ nanosheets in mixed solutions, avoiding the aggregation of nanosheets, which is induced by the large surface energy due to their van der Waals binding to the exfoliated sheets and the subsequent electrostatic stabilization.²⁶ The characterization of as-exfoliated MoS₂ nanosheets was carried out using SEM, TEM, and AFM techniques. As we can see from the SEM image (Fig. 1A), the exfoliated MoS₂ nanosheets exhibited a sheet structure, showing that the plane size of MoS₂-NSs was about 100 nm. The result of SEM measurement was further confirmed using TEM (Fig. 1B). As seen from the AFM image, the planar 2D structures of the as-prepared MoS₂-NSs with multiple monolayers are confirmed (Fig. 1C) and the sample with a thickness of about 3.4 nm corresponded to 4 and 5 monolayers of MoS₂ (inset of Fig. 1C).^14,36 A statistical analysis based on AFM measurements indicates that the MoS₂-NSs obtained from the sonication-assisted exfoliation have various thicknesses with the majority in the range of 3 to 5 monolayers (Fig. 1D). The average thickness of MoS₂-NSs was calculated to be 3.15 nm with a relative standard deviation (RSD) of 10.5%.

Fig. 1 (A) SEM, (B) TEM, and (C) AFM image of as-exfoliated MoS₂-NSs with the inset representing the height profile along the white line overlaid on the image (from point a to b). (D) Thickness distribution based on 100 randomly selected MoS₂-NSs.

Because of the insolubility of hemin in neutral aqueous solution, the synthesis of hemin/MoS₂-NSs was carried out using methanol as the solvent (Fig. 2A). Initially, the mixed methanol solution containing 0.5 mg mL⁻¹ hemin and MoS₂-NSs was prepared followed by sonication at room temperature for 2 h. The sonicated solution was then centrifuged at 12 000 rpm for 30 min and the black sediments, hemin/MoS₂-NSs, were obtained (b₁, Fig. 2A). According to the Lambert–Beer theory, the concentration of hemin in supernatant was detected to be 0.38 mg mL⁻¹. Therefore, the mass fraction of hemin in hemin/MoS₂-NSs was calculated to be 19.4%. To conduct the characterizations and explore the properties of hemin/MoS₂-NSs, the sediments were dispersed into water to prepare the hemin/MoS₂-NSs aqueous solution.

Fig. 2 (A) Photographs of 0.5 mg mL⁻¹ MoS₂-NSs methanol solution (a₀), mixed methanol solution containing 0.5 mg mL⁻¹ MoS₂-NSs and 0.5 mg mL⁻¹ hemin (b₀), and 0.5 mg mL⁻¹ hemin methanol solution (c₀); photographs of 0.5 mg mL⁻¹ MoS₂-NSs (a₂) and 0.62 mg mL⁻¹ hemin/MoS₂-NSs (b₂) aqueous solution. (B) UV-vis absorption spectra of MoS₂-NSs (1) and hemin/MoS₂-NSs (2) dispersed in water, and hemin dispersed in methanol (3). (C) Cyclic voltammograms of bare GCE (a), MoS₂-NSs modified GCE (b), and hemin/MoS₂-NSs modified GCE (c) in 0.1 M PBS (pH 7.4).

The UV-vis absorption spectra of MoS₂-NSs and as-prepared hemin/MoS₂-NSs were obtained, which are shown in Fig. 2B. As we know, bulk MoS₂ is a semiconductor with an indirect band gap of about 1.29 eV where no characteristic absorption peaks could be observed in the UV-vis absorption spectrum. However, it was demonstrated that an indirect to direct band gap transition occurred in the d-electron system when the bulk MoS₂ is thinned to a single layer, while exfoliated MoS₂-NSs, consisting of one to several layers, possess two characteristic absorption peaks at approximately 610 and 670 nm,^6,37 which were observed in the as-prepared MoS₂-NSs aqueous solution (curve (1), Fig. 2B). The hemin/MoS₂-NSs aqueous dispersion exhibited two absorption peaks at around 610 and 670 nm, corresponding to the characteristic peaks of MoS₂-NSs, and a broad absorption peak was observed at around 400 nm (curve (2), Fig. 2B), close to the Soret band of hemin. Furthermore, because hemin is insoluble in water, the control experiment of free hemin was carried out in methanol solution, which exhibited an absorption peak at around 400 nm (curve (3), Fig. 2B). It is clearly demonstrated that MoS₂-NSs adsorbed hemin molecules at the surface of nanosheets through van der Waals interactions, which allowed them to be well dispersed in aqueous solution where a characteristic absorption of hemin was observed from complex nanosheets. The attachment of hemin onto the MoS₂-NSs surface was also characterized by an electrochemical method. Fig. 2C shows the cyclic voltammograms recorded at the bare GCE (a), the MoS₂-NSs modified GCE (b), and the hemin/MoS₂-NSs modified GCE in phosphate buffer solution (PBS). As expected, redox peaks were not observed on the bare GCE and MoS₂-NSs modified GCE, whereas a pair of well-defined redox peaks located at around −0.3 V were clearly seen on the hemin/MoS₂-NSs modified GCE. The redox peaks could be ascribed to the constitution of hemin, which has the characteristic of a single electron transfer process of iron at the core of hemin for the hemin_ox/hemin_red pair.²⁹ As is well-known, the strong van der Waals interactions between few-layered MoS₂-NSs lead to the aggregation of MoS₂-NSs in aqueous solution without stabilizers, which limits the application of MoS₂-NSs. However, compared to MoS₂-NSs, the hemin/MoS₂-NSs exhibited better dispersity in aqueous solution, which can be well dispersed without aggregation for at least 72 h (Fig. 3A). The electronegative hemin molecules adsorbed on the surface of MoS₂-NSs can prevent the aggregation of composite nanosheets due to the effect of electrostatic stabilization. The SEM and AFM images of the as-prepared hemin/MoS₂-NSs are shown in Fig. 3B and C, respectively, which were used to confirm the nanosheet structure of hemin/MoS₂-NSs, indicating that hemin molecules adsorbed on the surface of MoS₂-NSs would not break their original nanosheet morphology.

Fig. 3 (A) Photographs of 0.2 mg mL⁻¹ hemin/MoS₂-NSs (a) and MoS₂-NSs (b) aqueous solutions in the stationary state at different times ranging from 0 to 72 h. (B) SEM and (C) AFM images of the as-prepared hemin/MoS₂-NSs with the inset representing the height distribution of individual MoS₂-NSs (from point (a) to (b)).

The crystal structures of exfoliated MoS₂-NSs and hemin/MoS₂-NSs were investigated using the X-ray diffraction system with bulk MoS₂ powder used as the reference. From the XRD patterns shown in Fig. 4A, it can be seen that both exfoliated MoS₂-NSs and hemin/MoS₂-NSs are mainly identified as 2H MoS₂, which have a dominant peak appearing at 14.4°, representing the (002) plane (ICDD card no. 77-1716).³⁸ In order to confirm the chemical composition, the XPS survey of MoS₂-NSs and as-prepared hemin/MoS₂-NSs were recorded (Fig. 4B). From Fig. 4B, it can be seen that the relative intensities of C 1s and O 1s binding energy at hemin/MoS₂-NSs increase significantly when compared to that of MoS₂-NSs. Meanwhile, a weak peak related to Fe 2p binding energy appears, indicating the formation of hemin/MoS₂-NSs. As is well-know, MoS₂-NSs have two different types of phases, i.e. stable hexagonal semiconducting phase (2H phase) and metastable metallic phase (1T), where it has been demonstrated that a transition from 2H to 1T phase occurs after the semiconducting MoS₂ was intercalated with Li⁺, K⁺, Na⁺, and H⁺.^36,38 To explore the phase changes of MoS₂-NSs under the influence of hemin molecules, high resolution Mo 3d and S 2p spectra of MoS₂-NSs and hemin/MoS₂-NSs were recorded. According to the XPS results of exfoliated MoS₂-NSs (Fig. 4C and D), Mo 3d_3/2, Mo 3d_5/2, S 2s, S 2p_1/2, and S 2p_3/2 peaks were observed at 232.6, 229.4, 226.5, 163.5, and 162.3 eV, respectively, showing the dominant 2H phase in MoS₂-NSs, which were obtained from the sonication-assisted exfoliation of MoS₂ powder. However, after MoS₂-NSs interacted with hemin, new peaks at 231.4 and 228.1 eV for Mo and 161.3 eV for S were observed at lower binding energies when compared to those of the 2H phase peaks (Fig. 4E and F). These new peaks were identified as 1T phase peaks, and it is suggested that a small portion of MoS₂-NSs undergoes a phase transition from semiconducting to metallic phase under the influence of hemin.³⁸ Meanwhile, the peaks representing the 2H phase can still be observed, indicating the coexistence of 1T and 2H phases in the crystal structure of hemin/MoS₂-NSs.

Fig. 4 (A) XRD patterns of exfoliated MoS₂-NSs and as-prepared hemin/MoS₂-NSs (peaks correspond to [*] 2H MoS₂). (B) XPS survey of MoS₂-NSs and hemin/MoS₂-NSs. High resolution Mo 3d (C and E) and S 2p (D and F) spectra of MoS₂-NSs (C and D) and hemin/MoS₂-NSs (E and F).

As a new support of hemin, MoS₂-NSs were presented to make hemin/MoS₂-NSs, which were expected to be great biomimetic catalysts based on the effect of hemin functionalization. The catalytic activities of MoS₂-NSs and hemin/MoS₂-NSs were explored using TMB as the substrate, which could be oxidized in the presence of H₂O₂ to produce a blue oxidation product. When compared to the absorption spectra of blank TMB reaction solution (Fig. 5A), the absorbance of reaction solutions increased in the presence of MoS₂-NSs and as-prepared hemin/MoS₂-NSs for the same reaction time, indicating that both MoS₂-NSs and hemin/MoS₂-NSs have peroxidase-like catalytic activities. The catalytic activity of MoS₂-NSs in TMB oxidation reaction can be ascribed to the active sites located at the edges of sheets, which exhibit significant catalytic activity for hydrogen evolution reaction.^18–21 However, when compared to MoS₂-NSs (curve b, Fig. 5A), hemin/MoS₂-NSs (curve c, Fig. 5A) show a much stronger catalytic activity because of the contribution of hemin functionalization. Meanwhile, a higher catalytic activity of hemin/MoS₂-NSs was observed when compared to that of pure hemin, which is attributed to the support of MoS₂-NSs (Fig. S1, ESI†). In comparison to HRP, hemin/MoS₂-NSs exhibited significant catalytic activity over a broader pH range (from 2.0 to 6.0), and showed an optimal catalytic activity at the pH of 4.0 (Fig. 5B). The catalytic reaction can be detected by monitoring the change in TMB absorbance at 652 nm, and the time-dependent absorbance changes against the concentrations of MoS₂-NSs and hemin/MoS₂-NSs were recorded. It can be seen from Fig. 5C that with the addition of the same amount of H₂O₂, the TMB oxidation rate was increased with increasing MoS₂-NSs concentration, indicating that the catalytic activity to H₂O₂ reduction was related to the concentration of MoS₂-NSs. Fig. 5D shows the time-dependent absorption changes against the concentrations of hemin/MoS₂-NSs. In the presence of a constant concentration of H₂O₂, the TMB oxidation reaction rate also increased with increasing hemin/MoS₂-NSs concentration. The catalytic activity of hemin/MoS₂-NSs was inhibited at high H₂O₂ concentration. Moreover, the typical Michaelis–Menten curves can be obtained for hemin/MoS₂-NSs at a certain range of H₂O₂ concentrations (Fig. S2A and S2B, ESI†). For further analysis of the catalytic mechanism and the calculation of hemin/MoS₂-NSs catalytic activity, the apparent steady-state kinetic parameters for TMB oxidation reaction in the presence of H₂O₂ and hemin/MoS₂-NSs were determined. Meanwhile, the maximum initial velocity (V_max) and Michaelis–Menten constant (K_m) were recorded in Table 1 based on the Lineweaver–Burk plot and the double reciprocal plots of initial velocity against the concentration of one substrate were obtained over a range of concentrations of the second substrate (Fig. S2C and S2D, ESI†). These parallel lines are characteristic of a ping-pong mechanism, which is used to confirm the catalytic mechanism of hemin/MoS₂-NSs. The high catalytic activity of hemin/MoS₂-NSs is confirmed when compared to previous studies (Table 1).

Fig. 5 (A) UV-vis absorption spectra of TMB reaction solutions (a) in the presence of 120.0 μg mL⁻¹ MoS₂-NSs (b) or 12.0 μg mL⁻¹ hemin/MoS₂-NSs (c) after reaction for 10 min (pH 4.0). (B) Peroxidase-like activity of hemin/MoS₂-NSs is dependent on pH. Experiments were carried out using 50 μg mL⁻¹ hemin/MoS₂-NSs or 1 ng mL⁻¹ HRP in TMB reaction solutions with different pH. (C) Time-dependent absorption changes of TMB reaction solutions in the presence of different concentrations of MoS₂-NSs at room temperature and (D) Time-dependent absorption changes in the presence of different concentrations of hemin/MoS₂-NSs at room temperature (pH 4.0). TMB reaction solution: 2 mM H₂O₂ and 0.5 mM TMB in BR buffer solution.

Table 1 Comparison of the apparent Michaelis–Menten constant (K_m) and maximum reaction rate (V_m)^a

	Km [mM]		Vm [10^−8 M s^−1]
Catalyst	TMB	H2O2	TMB	H2O2
a SWCNT: single-walled carbon nanotube; MOF: metal–organic framework (MTL-101(Al)–NH2); GNs: graphene nanosheets.
Hemin/MoS2-NSs	0.119	0.15	6.52	3.21
MoS2-NSs	3.74	7.36	1.28	0.64
Hemin-SWCNT^30	—	0.08	—	4.79
Hemin@MOF^31	0.068	10.9	6.07	8.98
Hemin-GNs^29	5.1	2.256	4.55	5.06

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On the basis of the intrinsic peroxidase property of hemin/MoS₂-NSs, a novel H₂O₂ sensor was fabricated using spectrophotometry (Fig. 6A). Fig. 6B shows the time-dependent absorption changes of TMB reaction solutions at 652 nm with different concentrations of H₂O₂ using hemin/MoS₂-NSs as the catalyst. It can be seen from Fig. 6B that the absorbance of TMB reaction solutions increases with increasing H₂O₂ concentration after reacting for 10 min. Because the catalytic activity of hemin/MoS₂-NSs is dependent on H₂O₂ concentration, a H₂O₂ sensor is constructed. Fig. 6C exhibits a typical H₂O₂ concentration–response curve under optimal conditions after reacting for 10 min. As we can see from the inset of Fig. 6C, the absorbance of reaction solution has a good linear relationship with the concentration of objective H₂O₂ in the range of 2.0 × 10⁻⁷ to 4.0 × 10⁻⁶ M. The detection limit, which is defined as the concentration of the analyte giving signals equivalent to three times the standard deviation of the blank signals (S/N = 3) was calculated to be 4.3 × 10⁻⁸ M, which was lower than that reported in most of the previous reports (Table S1, ESI†).

Fig. 6 (A) Schematic illustration of the spectrophotometric detection of H₂O₂ based on the catalytic activity of hemin/MoS₂-NSs in TMB oxidation reaction. (B)Time-dependent absorption changes at 652 nm in the absence or presence of H₂O₂ in BR buffer (pH 4.0). (C) A dose–response curve for H₂O₂ detection using hemin/MoS₂-NSs as an artificial enzyme. Inset: linear calibration plot for H₂O₂.

In order to explore the selectivity of the novel H₂O₂ sensor based on hemin/MoS₂-NSs, influences from other impurities such as NaCl, KCl, CaCl₂, glucose, urine, ascorbic acid, dopamine, and cysteine were studied (Table S2, ESI†). We found that when the concentrations of inorganic salts were higher than 0.5 M, hemin/MoS₂-NSs were observed to aggregate in salt solution because of the charge screening effects, which have been used to account for the decrease in the stability of hemin-graphene in salt solutions.²⁹ Furthermore, the aggregation of hemin/MoS₂-NSs reduced numerous active sites, which would disturb the H₂O₂ detection. It was verified that small biological molecules did not influence the determination of H₂O₂ when their concentrations were lower than 5 mM. Furthermore, the influences of other common oxidants, such as ClO⁻ and Cr₂O₇²⁻, were also explored. It was demonstrated that the developed sensor exhibited a great selectivity, which could be used to detect H₂O₂ in the presence of other oxidants.

4. Conclusion

In summary, few-layered MoS₂-NSs prepared from the sonication-induced exfoliation of bulk MoS₂ have been demonstrated to possess peroxidase-like activity. Meanwhile, hemin/MoS₂-NSs were synthesized through the van der Waals interaction between MoS₂-NSs and hemin molecules in mixed methanol solution with the assistance of sonication. The hemin/MoS₂-NSs had high dispersity in aqueous solution, and exhibited a high catalytic activity in the oxidation of TMB in the presence of H₂O₂. As a new support of hemin, MoS₂-NSs were presented, showing their strong van der Waals force when interacting with plane-like molecules. On the basis of high catalytic activity of hemin/MoS₂-NSs, a novel H₂O₂ sensor was fabricated using spectrophotometric method. In further research, hemin/MoS₂-NSs can be considered a reliable biomimetic catalyst for other oxidation reactions and it can also be taken into consideration as a novel sensing platform for biomolecular detection, raising new potentials concerning MoS₂-NSs applications in various significant areas.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (nos 20975083 and 21273174) and the Municipal Science Foundation of Chongqing City (no. CSTC-2013jjB00002).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 and S2, Tables S1 and S2. See DOI: 10.1039/c4ra01746c

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