Recently, nanomaterials based enzyme mimics have attracted much attention owing to their advantages over natural enzymes, such as easy preparation procedure, excellent stability, low cost, and high catalytic activity [1-6]. Up to now, a variety of nanomaterials [7-13] have been found to possess enzyme like activities. Among different nanocomposites, molybdenumdisulfide (MoS₂), were reported to own an intrinsic peroxidase-like activity [14]. However, the catalytic ability of MoS₂ is significantly hampered by their poor dispersibility and low conductivity [15, 16]. In order to achieve the potential values and expand the application of MoS₂, the MoS₂ hybrid composites have been developed [17-22], which showed synergistically enhanced peroxidase-like catalytic activity than individual component [23]. Despite these advances, it still remains a great challenge to fabricate MoS₂ hybrid composites with unique chemical structure and controllable composition.

On the other hand, nitrogen-doped carbon materials have gained much interest owing to their excellent electronic properties, exceptional conductivity and high catalytic performances [24]. It was reported that by doping heteroatom nitrogen (N) into reduced graphene oxide and mesoporous carbon, their peroxidase-mimicking activities were greatly enhanced [25]. Therefore, the enzyme-like catalytic activity of nitrogen-doped carbon materials and their hybrids had been extensively reported [26]. For instance, Lu et al. have reported the preparation of Fe₃O₄/nitrogen-doped carbon hybrid nanofibers, and constructed a sensitive platform to detect H₂O₂ and ascorbic acid (AA) [27]. Furthermore, they have fabricated FeCo nanoparticles embedded in nitrogen-doped carbon nanofibers as peroxidase-like mimic to detect L-cysteine [28]. Zhu's team has fabricated a honeycomb-like Pt/nitrogen-doped porous carbon, which showed a superior peroxidase-like catalytic activity [29]. Gao's team has developed nitrogen-doped porous carbon nanospheres which possess enzyme-like activities responsible for reactive oxygen species regulation, resulting in significant tumor regression [30].

Based on above mentioned, this greatly inspired us to take advantage of N-doped carbon materials as ideal supports for the formation of NCNTs@MoS₂ composites with serviceable properties to achieve synergistically enhanced performances. Furthermore, through combining electrically conducting substance of N-doped carbon materials, the charge transport capability was significantly improved to achieve the excellent performance of MoS₂. Therefore, the combination of MoS₂ with N-doped carbon materials is a good alternative for fabricating hybrid structure to obtain improved performance for electrocatalysis, hydrogen evolution reaction, and rechargeable batteries [31-33].

In this study, we demonstrated the controlled fabrication of N-doped carbon nanotubes (NCNTs) obtained from the polypyrrole (PPy) nanotubes precursor, which were exploited as the scaffold for anchoring MoS₂ nanosheets on its surface, producing NCNTs@MoS₂ composites. In serving as substrate, the NCNTs core acts as a host for MoS₂ deposition, while also offering an efficient electron pathway for the catalytic reaction. Meanwhile, the MoS₂ nanosheets help to promote the charge-transfer reaction through facilitating catalysis. In comparison with MoS₂ and NCNTs alone, the integration MoS₂ nanosheets with NCNTs cores induces a favorable synergistic effect, resulting in a higher catalytic activity than that of the single components. In view on this finding, a sensitive colorimetric detection system based on the NCNTs@MoS₂ composites for H₂O₂ and ascorbic acid was achieved.

The synthetic procedure of NCNTs@MoS₂ is illustrated in Scheme S1 (Supporting information). Firstly, MoO₃ nanorods were obtained via a hydrothermal method. Then, the MoO₃@PPy composites were prepared through a one-pot oxidative polymerization method. Subsequently, the PPy nanotubes were fabricated by etching the MoO₃ cores in ammonia solution through hydrothermal conditions. After hydrothermal reaction with (NH₄)₂MoS₄, a layer of MoS₂ nanosheets were grown on the surface of NCNTs by a solution-based method. Followed by annealing under nitrogen atmosphere to crystallize the sheath of MoS₂, yielding the final product of NCNTs@MoS₂ with high crystallinity. As shown in Fig. 1, the as-prepared hollow carbon nanotubes are uniform with a shell thickness of about 40 nm (Fig. 1a). The hollow interior of carbon nanotubes is further confirmed by TEM (Fig. 1b). The X-ray powder diffraction (XRD) pattern of the NCNTs is shown in Fig. S1A curve a (Supporting information), the broad peaks for graphitic carbon of N-doped carbon nanotube at 28° and 44° were obtained. While after the solvothermal process, the one dimensional morphology retains, while the surface of NCNTs becomes rough, indicating the successfully decoration with MoS₂ nanosheets (Figs. 1c and d). The SEM image shows that the MoS₂ layer is assembled by ultrathin nanosheets (Fig. 1c).These structural features are also verified by the TEM images (Fig. 1d). Moreover, the loaded MoS₂ could not be dislocated from the surface of the NCNTs after long time of sonication, which indicated an excellent adhesion between the MoS₂ and NCNTs. This can be attributed to the strong interactions between the doped nitrogen atom of NCNTs and the MoS₂. The uniformly nitrogen doped carbon nanotubes could not only supply growing sites for MoS₂ but also prevent the aggregation of the formed MoS₂ nanosheets. While without the support of NCNTs, severely aggregated nanosheets-assembled MoS₂ particles are obtained (Fig. S2 in Supporting information). Moreover, the coverage density of MoS₂ could be easily controlled by adjusting the (NH₄)₂MoS₄ concentration and the weight ratio to the NCNTs. Higher (NH₄)₂MoS₄ concentrations resulted in high coverage density of MoS₂ nanosheets on the surface of the NCNTs (Figs. 1e and f). Here, in our experiment, the weight ratio between (NH₄)₂MoS₄ and NCNTs was adjusted to 2:1. As shown in curve b of Fig. S1A, the formation of MoS₂ is verified by the XRD. The peaks at around 26°, 14.2°, 33.1°, 39.6°, 49.2°, and 59° can be attributed to [002] plane of graphitic carbon and [100], [103], [105], [108], and [110] planes of MoS₂, respectively [34]. This is further confirmed by the following XPS data. As shown in Fig. S1C (Supporting information), the XPS spectrum shows directly the presence of carbon, oxygen, nitrogen, molybdenum and sulfide elements. It was shown in Fig. S1D (Supporting information) that the peaks located at 229.8 and 232.9 eV are related to Mo 3d_5/2 and Mo 3d_3/2, the additional weak peak of 226.2 eV is assigned to S 2s [35]. The spectra of S 2p display two peaks at 163.6 and 162.5 eV relating to S 2p_1/2 and S 2p_3/2 (Fig. S3 in Supporting information), showing the valence of Mo and S are 4 and -2, respectively, and therefore confirming the stoichiometric ratio of Mo:S is 2:1 [21]. The BET surface area of the NCNTs@MoS₂ is acquired by the nitrogen adsorption/desorption curve (Fig. S1B in Supporting information). The as-synthesized NCNTs@MoS₂ possessed a BET surface area of 22.605 m²/g. The pore-size distribution obtained by density functional theory further demonstrated a hierarchical structure with mesopores, and macropores, which were attributed to the hollow tubes cores and the decorated MoS₂ nanosheets.

Beneficial from the highly active MoS₂ components integrated with NCNTs, the NCNTs@MoS₂ hybrids exhibited excellent peroxidase-like activity. Herein, we first employed the substrate of TMB-H₂O₂ to evaluate the peroxidase-like catalysis activities of the resultant NCNTs@MoS₂. The mechanism is interpreted that the NCNTs@MoS₂ hybrids can initiate the decomposition of the O—O bond between H₂O₂ and an ^·OH radical, then the ^·OH radical oxidizes TMB, which leads to a typical blue colour [36] (Fig. 2A). In order to demonstrate this conjecture, TA was applied in the experiment as a fluorescence probe to assess the effects of NCNT@MoS₂ on ^·OH signal intensity. As shown in Fig. S4 (Supporting information), a significant increased fluorescence intensity was observed in the presence of NCNTs@MoS₂, suggesting the generation of ^·OH radical during the catalysis.

As shown in Fig. 2B, there is almost no absorption at 652 nm for the TMB+H₂O₂ system in the absence of NCNTs@MoS₂ (c). Also, the TMB + NCNTs@MoS₂ (b) system show almost no absorption at 652 nm at the same conditions. However, the TMB+H₂O₂+NCNTs@MoS₂ system indicates a significant increasing absorbance at 652 nm (a), showing that the NCNTs@MoS₂ composite can efficiently catalyze the oxidation of TMB in the presence of H₂O₂.

Furthermore, in order to probe more insight into the synergetic catalytic behaviors of the MoS₂-decorated N-doped carbon nanotube, the catalytic activities of NCNTs@MoS₂ were compared with that of single component of NCNTs, MoS₂ and NCNTs + MoS₂, respectively. Fig. S5 (Supporting information) shows the comparison of colorimetric TMB-H₂O₂ reaction results among NCNTs, MoS₂, NCNTs + MoS₂ and NCNTs@MoS₂ hybrids. In addition, the TMB-H₂O₂ reaction catalyzed by different nanomaterials can be visually observed (Fig. S5B inset). In view of the foregoing results, it was shown that the resulting NCNTs@MoS₂ hybrids showed much higher catalysis activity than MoS₂ sheet, NCNTs supports and NCNTs + MoS₂. In addition, the relative activities of bare MoS₂ NPs, NCNTs + MoS₂, NCNTs gave respective response of 67%, 49% and 40%, when that of NCNTs@MoS₂ was set as 100% (Fig. S5B). The results indicated that after the chemical interaction process of the fabrication of NCNTs@MoS₂, the catalytic activity of the composites was obviously improved. The greatly enhanced catalytic activity of NCNTs@MoS₂ was attributed to the integration MoS₂ with NCNTs as well as the one dimensional hierarchical structures of the hybrid composite. Firstly, the hollow N-doped carbon nanotubes endow it a nanoreactor of a homogeneous microenvironment for heterogeneous catalysis. The substrate can access the void space through the holes on the carbon nanotubes and form a local concentration effects, accelerating the catalytic reaction rate. In addition, the utilization of NCNTs can significantly enhance the stability and dispersion of the supports to allow for better distribution and more effective loading of MoS₂, which may facilitate to improve peroxidase-like catalysis activities of the nanocomposites. Thirdly, the freely movable MoS₂ nanosheets can provide more exposed active sites for catalysis.

Experimental results showed that the catalytic activity of these composites was dependent on pH, temperature, time and amount of nanocomposites respectively. As shown in Fig. S6 (Supporting information), the optimal condition was chosen for subsequent experiments. Also, in order to fully understand the key kinetic parameters and catalytic mechanism, steady-state kinetics were investigated (Fig. S7 in Supporting information) The Km and Vmax acquired by Lineweaver-Burk double reciprocal plots are summarized in Table S1 (Supporting information).

The sensitive detection of H₂O₂ plays an important role in the areas of industrial technology, biomedicine and food science [37, 38]. Various strategies have been proposed for the determination of H₂O₂ [39-43]. Owing to peroxidase mimics activity of the NCNTs@MoS₂, a facile and label-free colorimetric approach to detect H₂O₂ was developed. The dependence of the absorption values in the presence of various concentrations of H₂O₂ is presented in Fig. S8 (Supporting information). The linear detection range was 2.0–50 μmol/L with the limit of detection (LOD) of 0.14 μmol/L. This detection limit is better than previous reports on the some other nanomaterials based peroxidase-like nanocatalyst [44-46].

Ascorbic acid (AA), as a potent antioxidant, plays a vital role in many biochemical processes and is necessary for human health. AA is also extensively applied as an antioxidant in beverages, food, cosmetics, and pharmaceuticals [47]. Abnormal levels of AA are concerned with many diseases, like the common cold, mental illness, atherosclerosis, scurvy and cancers [48, 49]. Hence, the determination of AA is of vital importance. Up to now, a variety of methods have been developed for the determination of AA [50-52]. Owing to its simplicity, convenience, and low cost, the colorimetric assay has drawn considerable attention. Based on the antioxidant properties of AA, nanocomposites as peroxidase-like mimics based sensors are appealing for the determination of AA [53, 54].

Due to the superior catalytic activity of the prepared NCNTs@MoS₂ hybrid composites, a colorimetric assay for AA has been developed. The principle of colorimetric assay of AA was showed in Fig. 3A. In brief, the NCNTs@MoS₂ served as an electron transfer mediator. It can catalyze the oxidation of TMB in the presence of H₂O₂ to produce a blue colored production (oxTMB), while AA is present, which can quickly reduce oxTMB to TMB, and result in the blue color fading. As shown in Fig. 3B, the addition of AA led to an obvious decrease in the absorbance and fading of the blue color. Hence, based on this finding, a colorimetric assay for the detection of AA with its inhibitory effect has been developed. Fig. 3C shows the absorbance changes at 652 nm of TMB solution catalyzed by NCNTs@MoS₂ in the presence of H₂O₂ and various concentrations of AA. The results show that the absorbance intensity decreased gradually with the increment of AA concentration, which could be observed directly by naked eyes (Fig. 3C inset). There is a good linear relationship (R = 0.993) between the absorbance and the AA concentration with a linear range from 0.2 μmol/L to 80 μmol/L (Fig. 3D). The resulting color change allowed for visual detection of AA down to as low as 0.12 μmol/L, which is comparable to other nanocomposites based colorimetric detection of AA (Table S2 in Supporting information). The peroxidase-like property NCNTs@MoS₂ composites were demonstrated to be an excellent platform for the detection of AA. In general, owing to the hollow structure, superior conductivity and excellent adsorption ability of the NCNTs, it can gather signals from a vast range of reaction sites to minimize the diffusion distance, thus, resulting in facilitating electron transfer between substrates and composite. In addition, the two dimensional MoS₂ nanosheets can boost the response of AA owing to their large surface area and superior catalytic activity. Hence, the synergistic integration of NCNTs and MoS₂ nanosheets induces the excellent sensing performance. Based on the controllable interface of these peroxidase-like catalytic nanocomposites, it was feasible to develop a series of visible colorimetric assays for specific targets in real samples.

In the experiment, the selectivity of assay was also conducted, Fig. S9 (Supporting information) confirms that the present assay has an excellent selectivity for the detection of AA, demonstrating further applicable to the biological samples. To examine the practicality of this designed sensing platform, this AA colorimetric sensor was preliminarily employed for the detection of AA in commercial ascorbic acid tablets and orange juice. AA level in commercial ascorbic acid tablets and orange juice obtained from the experiments were shown in Table S3 (Supporting information). The results indicate that concentration of AA in commercial tablets and orange juice are consistent with that claimed, showing that this NCNTs@MoS₂ based sensor is practicable and reliable for examining AA in real samples. Furthermore, this NCNTs@MoS₂ based sensor was also applied to detect AA in human plasma samples (Table S3). Taking the 10-fold dilution into calculation, AA concentration in human plasma obtained was 65 μmol/L, within the scope of 45–80 μmol/L, which were consistent with the reported [55]. In order to further test the accuracy of the result, different amounts of AA were spiked into the human plasma sample for recovery experiments. The acceptable satisfactory recoveries suggested that this NCNTs@MoS₂ based sensor is feasible for detecting AA in practical samples.

In conclusion, we have developed an approach to fabricate NCNTs@MoS₂ based system through MoS₂ nanosheets anchoring on the surface of NCNTs, and constructed a sensing system based on intrinsic peroxidase-like activity to colorimetrically detect H₂O₂ and AA. The outstanding catalytic activity is presumably attributed to the synergistic effects between MoS₂ and NCNTs. Owing to the high catalytic activity of the as-prepared NCNTs@MoS₂ hybrids, a simple, convenient and rapid way for the sensitive detection of H₂O₂ and AA has been developed. This sensing system also exhibits a high selectivity toward AA with other biological interferences. Therefore, this approach will provide promising candidates for designing nanoenzyme materials made of transition metal sulfide@N-doped carbon materials with core/shell architectures for colorimetric detection of H₂O₂ and AA as well as other applications.

The authors are grateful to the financial support by the Natural Science Foundation of Shanghai City (No. 18ZR1416400), the National Natural Science Foundation of China (No. 21305086) and Science and Technology Development Fund of Shanghai Municipal Public Security Bureau (No. 2018003).

Supplementary material related to this article can befound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.09.037.