Synergistic effect of ternary electrospun TiO₂/Fe₂O₃/PPy composite nanofibers on peroxidase-like mimics with enhanced catalytic performance

Abstract

In this work, we demonstrate the fabrication of polypyrrole (PPy) decorated TiO₂/Fe₂O₃ (TiO₂/Fe₂O₃/PPy) composite nanofibers with a core–shell structure as an artificial enzyme system with a high peroxidase-like activity. The TiO₂/Fe₂O₃ nanofibers with a diameter in the range of 40–150 nm and a relatively narrow distribution of diameter size are prepared via an electrospinning technique and followed by a calcination process. Then we are able to in situ polymerize a layer of PPy on the surface of the TiO₂/Fe₂O₃ nanofibers by using Fe₂O₃ on the surface of nanofibers as an oxidant. The resulting ternary TiO₂/Fe₂O₃/PPy composite nanofibers exhibit an enhanced peroxidase-like activity toward the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂ over independent TiO₂, Fe₂O₃ and TiO₂/Fe₂O₃ composite nanofibers due to the synergistic effect. On the basis of the high peroxidase-like activity, a simple approach for the colorimetric detection of H₂O₂ with a detection limit of 2.4 μM and a linear detection range from 2 to 50 μM has been proposed. This work offers a new way for manipulating the enzyme-like performance of electrospun nanofibers for a wide range of potential applications in biosensing and environmental monitoring.

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Introduction

The past several decades have witnessed the remarkable progress in fabricating a large variety of artificial enzymes such as hemin, hematin, cyclodextrins, porphyrins, polymers, supramolecules, and biomolecules, etc.^1–8 Compared with natural enzymes, the artificial ones show the advantages of easy synthesis and purification, high stability, and low cost, which have shown a wide range of applications in biosensors, chemical industry, food processing, environmental science, and biotechnology. Recently, magnetic Fe₃O₄ nanoparticles with different sizes have been reported to display intrinsic peroxidase-like activity.⁹ Since then, a large variety of nanomaterials such as noble metal nanoparticles,^10,11 carbon nanomaterials,^12–15 metal oxide nanoparticles and nanowires,^16–20 chalcogenide nanoparticles,^21,22 ferromagnetic nanoparticles,²³ have been explored to possess unexpected enzyme-like activity. These nanomaterials-based artificial enzymes have extensively potential applications in biochemical analysis to detect hydrogen peroxide, glucose, metal ions, xanthine, acetylcholine, melamine, and ascorbic acid, etc.^24–27 In addition, they have also been widely used in immunoassays and pollutant removal.^24–27 However, a great challenge for the practical application of most of these nanomaterials is their relatively low enzyme-like activity. Additionally, bare small nanomaterials tend to agglomerate, which decreases their specific surface area and the peroxidase-like activity.

To enhance the enzyme-like catalytic activity and stability of the nanomaterials, it is a good choice to fabricate composite-based nanocatalysts with a synergistic effect. Actually, there is some work on the fabrication of multicomponent nanocomposites to achieve this purpose.^28–37 For instance, our group has developed a simple approach to fabricate conducting polyaniline/Cu₉S₅ composite nanowires which showed an enhanced peroxidase-like catalytic activity over independent PANI nanowires and Cu₉S₅ nanoparticles due to their synergistic effect.²⁸ Other nanocomposites, such as dumbbell-like Pt₄₈Pd₅₂–Fe₃O₄ nanoparticles,²⁹ Au/graphene composite nanosheets,³⁰ Co₃O₄/reduced graphene oxide,³¹ CNT/ZnFe₂O₄ nanocomposites,³² and CuS/graphene nanocomposites,³³ have also been prepared to exhibit an enhanced peroxidase-like activity. In recent years, one-dimensional nanostructures such as nanofibers, nanobelts, and nanotubes have been extensively explored for possible applications in high-performance catalysis due to their large specific surface area and active mass transport process, enabling faster reaction during the heterogeneous catalytic reaction.^38–41 Among those various kinds of strategies to fabricate 1D nanomaterials, electrospinning technique has become one of the hottest topics because it can easily produce 1D nanomaterials with controlled dimensions, tunable compositions, and designed architectures.^41–44

To the best of our knowledge, there are few reports on the fabrication of 1D electrospun semiconductor nanomaterials for peroxidase-like catalytic applications. In this study, we have successfully prepared TiO₂/Fe₂O₃ composite nanofibers by an electrospinning technique and following calcination process. It is found that the synthesized TiO₂/Fe₂O₃ composite nanofibers show an enhanced peroxidase-like catalytic activity over the independent TiO₂ and Fe₂O₃ nanofibers. More interesting, the peroxidase-like activity has been further enhanced by the decoration of a layer of PPy on the surface of TiO₂/Fe₂O₃ composite nanofibers through an in situ polymerization approach. This study investigates the influence of the composition of the electrospun nanofibers on the peroxidase-like catalytic performance, providing guidance for the fabrication of other novel 1D semiconductor nanocatalysts for applications in a wide range of fields involving enzyme-like catalysis.

Experimental

Chemicals

Poly(vinyl pyrrolidone) (PVP, M_w = 1 300 000 g mol⁻¹) was purchased from Sigma-Aldrich. Iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O) and tetra-n-butyl titanate were obtained from Tianjin East China reagent factory. 3,3′,5,5′-Tetramethylbenzidine (TMB) was purchased from Sinopharm Chemical Reagent Co., Ltd. Ethanol was provided by Tianjin FuYu Fine Chemical. N,N-Dimethylformamide (DMF), dimethylsulfoxide (DMSO) and H₂O₂ was purchased from Beijing Chemical Works. Acetic acid was obtained from Xilong Chemical Co., Ltd. Pyrrole was purchased from Aladdin. All of these chemicals are of analytical grade and used without further purification.

Fabrication of TiO₂/Fe₂O₃ composite nanofibers via an electrospinning technique and following calcination process

In a typical procedure, 2.4 g of Fe(NO₃)₃·9H₂O, 2.0 g of tetrabutyl titanate and 0.5 g of PVP were together dissolved in the mixed solvents composed of 3 ml of ethanol, 3 ml of DMF and 1 ml of acetic acid under vigorous stirring at room temperature to form a homogeneous and viscous solution as the electrospinning precursor. A single glass capillary with the tip fixed about 30 cm away from the aluminum foil collector was connected to a high-voltage power supply maintained at 18 kV. Then, referring to a defined heating program, the electrospun fibrous membrane that peeled off from the aluminum foil was calcined at 480 °C in air for 3 h. Finally, the yellow powder of the TiO₂/Fe₂O₃ composite nanofibers was obtained.

Fabrication of TiO₂/Fe₂O₃/PPy composite nanofibers through an in situ polymerization approach

10 mg of the as-prepared TiO₂/Fe₂O₃ nanofibers were dispersed in 12.5 ml of distilled water under ultrasonication for a few minutes. Then, 50 μg of pyrrole monomer was added to the above solution, followed by magnetic stirring at 0 °C for 2 h. The yellow precipitate was transferred into a 50 ml Teflon-lined stainless steel autoclave. The reaction proceeded in an electric oven at 150 °C for 24 h. After cooling down, the final product was centrifuged and washed thoroughly with water for several times, and then dried in a vacuum drying oven.

Catalytic activity measurement

In a typical colorimetric experiment, 20 μL of TMB solution (15 mM in dimethylsulfoxide, DMSO) as peroxidase substrate and 20 μL of H₂O₂ (30%) were added into 3 ml of acetate buffer solution (pH = 4.0, unless otherwise stated). Next, 20 μL of catalyst suspension (3 mg ml⁻¹) was injected to the solution mentioned above. The catalytic measurements proceeded to detect the absorbance changes at 651 nm in time course.

Characterization

The morphology and structure of the samples were observed using a scanning electron microscope (SEM, FEI Nove NanoSEM 450) and a transmission electron microscope (TEM, JEOL JEM-1200 EX) at 10 and 100 kV, respectively. High-resolution TEM (HRTEM) images, energy dispersive X-ray (EDX) analysis and elemental mapping analysis were acquired on a FEI Tecnai G2 F20 high-resolution transmission electron microscope. The crystallographic structure and chemical composition of the heterogeneous products were investigated by an X-ray diffraction (XRD, PAN-alytical B.V. Empyrean) with CuKα radiation and Raman spectra (LabRAM HR Evolution). X-ray photoelectron spectrum (XPS, Thermo Scientific ESCALAB250) was applied to characterize the chemical composition of TiO₂/Fe₂O₃ and TiO₂/Fe₂O₃/PPy composite nanofibers. The peroxidase-like catalytic activity was studied by ultraviolet-visible (UV-vis) spectra performed on Shimadzu UV-2501 PC spectrometer.

Results and discussion

Morphology and characterization of electrospun TiO₂/Fe₂O₃ composite nanofibers prepared by electrospinning technique

One of the most important advantages of the electrospinning technique is that it can easily produce 1D nanostructured composites with a tunable composition by using the mixed precursors in the initial electrospun solution. In the present study, the TiO₂/Fe₂O₃ composite nanofibers are fabricated by electrospinning a PVP solution in ethanol, DMF and acetic acid mixed solvent containing iron nitrate and tetrabutyl titanate precursors, and followed by a calcination process at 480 °C. Fig. 1a shows a SEM image of the as-prepared electrospun TiO₂/Fe₂O₃ composite nanofibers. It is found that uniform and smooth nanofiber morphology is dominated in the product and most of the synthesized composite nanofibers possess a diameter in the range of 40–150 nm. The morphology of the TiO₂/Fe₂O₃ composite nanofibers has also been characterized by TEM image. As shown in Fig. 1b, it seems that most of the nanofibers exhibit a fiber-in-tube hierarchical structure, which might be due to the different phase separation behaviors of the two types of precursors during the calcination process.^45,46 Fig. 1c presents a typical HRTEM image of the TiO₂/Fe₂O₃ composite nanofiber, which shows crystalline nature of the two metal oxides. The HRTEM image displays a lattice fringe spacing of 0.35 nm which is consistent with the (101) crystallographic plane of anatase TiO₂,⁴⁷ while the lattice fringe spacing that measured to be around 0.27 nm is related to the (104) crystal plane of α-Fe₂O₃.⁴⁸ In Fig. 1d, the energy dispersive X-ray (EDX) spectrum confirms the existence of Ti, Fe, O and C elements in the TiO₂/Fe₂O₃ composite nanofiber product, in which the signal of Cu should be ascribed to the carbon-coated copper grid. These results indicate the successful fabrication of TiO₂/Fe₂O₃ composite nanofiber. Furthermore, elemental mapping analysis provides the evidence that TiO₂ phase is homogeneously distributed throughout the whole composite nanofibers, while Fe₂O₃ component is rich outside of the composite nanofibers (Fig. 1e–h).

Fig. 1 (a) SEM image, (b) TEM image, (c) HRTEM image, (d) EDX spectrum, (e–h) EDX element mapping images of the synthesized TiO₂/Fe₂O₃ nanofibers. The images in (e–h) stand for the HAADF pattern, Ti–K, Fe–K and O–K successively.

The crystal structure and chemical components of the prepared TiO₂/Fe₂O₃ composite nanofibers were characterized by XRD and Raman measurements. In the XRD pattern of TiO₂/Fe₂O₃ composite nanofibers showed in Fig. 2a, the peak centered at around 25.4° is attributed to the (101) plane of the crystallographic nature of anatase TiO₂,⁴⁹ while the peaks centered at around 32.5, 35.6, 40.8, 54.0, 62.5 and 63.4° are ascribed to the (104), (110), (113), (116), (214), and (300) planes of the crystallographic α-Fe₂O₃.⁵⁰ This result demonstrates the successful fabrication of TiO₂/Fe₂O₃ composite nanofibers. However, it is noted that a small peak at 27.1° corresponding to the rutile phase is observed, indicating a transformation of anatase to rutile takes place, which should be attributed to the effect of the iron element.⁴⁶ Fig. 2b shows the Raman spectrum of the synthesized TiO₂/Fe₂O₃ composite nanofibers, evidently corresponding to phonons from α-Fe₂O₃ and anatase TiO₂, confirming the formation of the composite nanofibers. In detail, the typical band at 224.0 cm⁻¹ is attributed to A_1g mode of α-Fe₂O₃, the peaks at 292.0, 408.9 and 612.0 cm⁻¹ are ascribed to E_g modes of α-Fe₂O₃, and the peak at 1322.1 cm⁻¹ is related to the second harmonic vibration of α-Fe₂O₃.⁵⁰ On the other hand, the bands at 145.6 and 664.0 cm⁻¹ are attributed to B_1g and E_g modes of anatase TiO₂, respectively.⁵¹ This result indicates that both α-Fe₂O₃ and anatase TiO₂ phase exist in the TiO₂/Fe₂O₃ composite nanofibers.

Fig. 2 (a) XRD patterns of TiO₂/Fe₂O₃ nanofibers and TiO₂/Fe₂O₃/PPy composite nanofibers, (b) Raman spectra of TiO₂/Fe₂O₃ nanofibers and TiO₂/Fe₂O₃/PPy composite nanofibers.

To acquire more detailed information on the chemical composition and surface electronic state of the prepared TiO₂/Fe₂O₃ composite nanofibers, XPS spectrum was performed. As shown in Fig. 3a, the wide-scan XPS spectrum demonstrates the presence of Ti, Fe, O and C elements, where the signal of C should be mainly attributed to the absorbed carbon dioxide. To deeply evaluate the binding behavior of the elements in TiO₂/Fe₂O₃ composite nanofibers, high-resolution XPS spectra for Ti 2p, Fe 2p and O 1s regions are used. As shown in Fig. 3b, two predominant peaks are observed at around 458.4 and 464.2 eV, which are related to the core levels of Ti⁴⁺ 2p_3/2 and Ti⁴⁺ 2p_1/2, signifying the formation of the TiO₂ structure.⁵² Fe³⁺ as the exclusive valence state existing in the as-prepared TiO₂/Fe₂O₃ composite nanofibers was expounded, since two predominant peaks at 711.3 and 725.1 eV attributed to Fe 2p_3/2 and Fe 2p_1/2 are perspicuously detected, while representative shake-up satellite lines are situated at approximately 720.2 and 733.1 eV (Fig. 3c).⁵³ The O 1s fine spectrum shows the characteristic peaks at around 530.1 and 532.0 eV, which are related to the lattice oxygen and hydroxyl groups, respectively (Fig. 3d).

Fig. 3 XPS spectra of the prepared TiO₂/Fe₂O₃ nanofibers: (a) full survey spectrum, (b) Ti 2p, (c) Fe 2p and (d) O 1s regions.

Decoration of PPy on the surface of TiO₂/Fe₂O₃ composite nanofibers through an in situ polymerization approach

By using Fe₂O₃ on the surface of TiO₂/Fe₂O₃ composite nanofibers as an oxidant, a layer of PPy will be formed through the in situ polymerization approach to generate TiO₂/Fe₂O₃/PPy composite nanofibers. Fig. 4a shows a typical SEM image of the prepared TiO₂/Fe₂O₃/PPy composite nanofibers, which present a nanofiber-like morphology with the similar diameter but a relatively coarse surface comparing with the TiO₂/Fe₂O₃ nanofibers. The TEM image also clearly shows the rough surface of the TiO₂/Fe₂O₃/PPy composite nanofibers (Fig. 4b). The HRTEM image illustrates the lattice fringe spacings of 0.35 nm and 0.27 nm, which are consistence with the (101) crystallographic plane of anatase TiO₂ and (104) crystal plane of α-Fe₂O₃, demonstrating that the crystal structure of TiO₂ and Fe₂O₃ do not change after the formation of PPy. Compared with TiO₂/Fe₂O₃ nanofibers, both EDX spectrum and elemental mapping analysis of TiO₂/Fe₂O₃/PPy composite nanofibers give an obvious N element, fully proving the formation of PPy on the surface of TiO₂/Fe₂O₃ nanofibers.

Fig. 4 (a) SEM image, (b) TEM image, (c) HRTEM image, (d) EDX spectrum and (e–j) EDX element mapping images of the synthesized TiO₂/Fe₂O₃/PPy composite nanofibers. The images in (e–j) stand for the HAADF pattern, Ti–K, Fe–K, O–K, N–K and C–K elements successively.

The XRD pattern of the prepared TiO₂/Fe₂O₃/PPy composite nanofibers is similar with that of TiO₂/Fe₂O₃ nanofibers, but the peak corresponding to the (101) plane of anatase becomes weaker, which might be due to the rich existence of the TiO₂ in the core and the decoration of PPy layer on the surface of the TiO₂/Fe₂O₃ composite nanofibers. The Raman spectroscopy was used to characterize the chemical structure of the prepared PPy on the surface of TiO₂/Fe₂O₃ nanofibers. It is found that two strong characteristic peaks at around 1322.8 and 1582.0 cm⁻¹ appear in the spectrum, which are attributed to the ring vibrations and C=C vibration of PPy, respectively.⁵⁴ This result indicates that PPy is decorated on the surface of TiO₂/Fe₂O₃ nanofibers through the in situ polymerization process.

XPS was used to characterize the chemical state of the prepared PPy on the surface of TiO₂/Fe₂O₃/PPy composite nanofibers. The wide-scan XPS spectrum clearly illustrates the presence of Fe, O, Ti, N and C elements in the product (Fig. 5a). The binding behaviors of the elements for Ti 2p, Fe 2p and O 1s in the high-resolution XPS spectra of TiO₂/Fe₂O₃/PPy composite nanofibers are similar with that of TiO₂/Fe₂O₃ nanofibers. However, the obvious appearance of N 1s in the XPS spectrum clearly proved the formation of PPy on the surface of TiO₂/Fe₂O₃ nanofibers. As shown in Fig. 5e, the N 1s fine spectrum can be deconvoluted into two peaks centered at 399.7 and 400.4 eV, which are ascribed to the binding energies of –N–H– and –N= states in PPy.⁵⁵ In addition, the C 1s spectrum can be deconvoluted into three peaks positioned at 284.2, 284.6 and 285.2 eV, which are attributed to C=C, C–N and C–O groups.

Fig. 5 XPS spectra of the prepared TiO₂/Fe₂O₃/PPy composite nanofibers: (a) full survey spectrum, (b) Ti 2p, (c) Fe 2p, (d) O 1s, (e) C 1s and (f) N 1s regions.

Peroxidase-like catalytic activity

To investigate the peroxidase-like activity of the as-prepared TiO₂/Fe₂O₃/PPy composite nanofibers, we performed a model reaction of the oxidation of colorimetric substrate TMB catalyzed by the composite nanofibers in the presence of H₂O₂. As shown in Fig. 6a, an obvious blue color originating from the oxidation product of TMB with a maximum absorbance at 651 nm is observed after the addition of the TiO₂/Fe₂O₃/PPy composite nanofibers, demonstrating an excellent peroxidase-like catalytic behavior of TiO₂/Fe₂O₃/PPy composite nanofibers. By contrast, in the absence of the as-prepared catalyst, TMB could not be oxidized by H₂O₂. In addition, in the absence of TMB or H₂O₂, no distinct color changes as well as absorbance peaks at 651 nm were observed. This result supports that TiO₂/Fe₂O₃/PPy composite nanofibers are able to behave like peroxidase toward the oxidation of peroxidase substrate such as TMB in the presence of H₂O₂.

Fig. 6 (a) UV-vis absorption curves of TMB solutions oxidized by different systems in acetate buffer solution (pH = 4.0) recorded on 750 seconds of the catalytic reaction, the inset shows a typical photograph of TMB solution oxidized by TiO₂/Fe₂O₃/PPy composite nanofibers, (b) the time-dependent absorbance changes at 651 nm with the addition of different kinds of catalysts (20 μg ml⁻¹) in the presence of 100 μM TMB and 65 mM H₂O₂, (c) the peroxidase-like catalytic activity of the prepared TiO₂/Fe₂O₃/PPy composite nanofibers in acetate buffer solution with varied pH values and (d) the corresponding line chart.

In the following, we compared the catalytic activity of the prepared TiO₂/Fe₂O₃/PPy composite nanofibers with independent TiO₂, Fe₂O₃ and TiO₂/Fe₂O₃ nanofibers. As shown in Fig. 6b, the individual TiO₂ nanofibers almost show no peroxidase-like catalytic activity, and Fe₂O₃ nanofibers alone possess a low catalytic activity. However, the electrospun TiO₂/Fe₂O₃ nanofibers display an obvious enhanced catalytic activity over the independent TiO₂ and Fe₂O₃ nanofibers. The high catalytic activity should be due to the synergistic effect between TiO₂ and Fe₂O₃ components in the TiO₂/Fe₂O₃ composite nanofibers. A possible mechanism for the enhancement of the absorbance of TiO₂/Fe₂O₃ and TiO₂/Fe₂O₃/PPy for the oxidation of TMB has been proposed. Based on the previous reports, Fe(III) on the surface of Fe₂O₃ can initiate a reaction to generate ˙OH radical, which involve in the oxidation of TMB to generate TMB˙⁺.⁵⁶ Compared with the individual Fe₂O₃ nanofibers, the TiO₂/Fe₂O₃ composite nanofibers with Fe₂O₃ phase rich on the shell might show increased electron density and superior electron transfer, resulting in an enhanced peroxidase-like activity.⁵⁷ Interestingly, the peroxidase-like catalytic activity of TiO₂/Fe₂O₃/PPy composite nanofibers is further enhanced compared with that of TiO₂/Fe₂O₃ composite nanofibers. This enhancement might be due to that the photogenerated electrons excited by the PPy under visible irradiation tend to transfer to the semiconducting metal oxides because of their low Fermi levels, which prevents the recombination of the electrons and holes. Therefore, the electrons are easily captured by H₂O₂ to form ˙OH, which extensively increases the oxidation ability of TMB.⁵⁸ As a result, the peroxidase-like catalytic activity of TiO₂/Fe₂O₃/PPy composite nanofibers is much higher than independent TiO₂, Fe₂O₃ and TiO₂/Fe₂O₃ nanofibers. Based on the previous reports, the peroxidase-like catalytic activity is strongly related to the pH values in the solution. Therefore, we have studied the influence of pH values on the catalytic activity of the TiO₂/Fe₂O₃/PPy composite nanofibers. It is found that the catalytic activity that obtained at pH = 4.0 is higher than that at other pH values (Fig. 6c and d), and thus the optimal pH for the oxidation of TMB is chosen at pH = 4.0.

On the basis of the excellent peroxidase-like catalytic property of TiO₂/Fe₂O₃/PPy composite nanofibers, a simple and accurate colorimetric approach for the H₂O₂ detection has been developed. As the absorbance of the oxidized TMB is H₂O₂ concentration dependent, H₂O₂ can be detected by monitoring the absorption changes at 651 nm by UV-vis spectrum. Fig. 7a displays a time-dependent absorbance changes at 651 nm facing various concentrations of H₂O₂ in acetate buffer solution (pH = 4.0). It is obviously observed that the increase of H₂O₂ concentration leads to an increased reaction rate. From a typical dose–response curve in time course mode, it is calculated that H₂O₂ can be detected as low as 2.4 μM (S/M = 3) with a wide linear range from 2.0 to 50.0 μM (R² = 0.980), implying that the as-prepared TiO₂/Fe₂O₃/PPy composite nanofibers are good catalysts as an artificial enzyme for their potential applications in biosensing and environmental monitoring.

Fig. 7 (a) The time-dependent absorbance changes at 651 nm in the absence and presence of H₂O₂ with varied concentrations in acetate buffer solution (pH = 4.0) and (b) a dose–response curve for the detection of H₂O₂ using TiO₂/Fe₂O₃/PPy composite nanofibers as an artificial peroxidase. The inset in (b) is the linear calibration plot for H₂O₂.

Conclusions

In summary, TiO₂/Fe₂O₃/PPy composite nanofibers with well-defined morphology have been successfully fabricated via an electrospinning technique and in situ polymerization process. The as-prepared TiO₂/Fe₂O₃/PPy composite nanofibers show a higher peroxidase-like catalytic activity than independent TiO₂, Fe₂O₃ and TiO₂/Fe₂O₃ composite nanofibers in acetate buffer solution (pH = 4.0), which may result from the synergistic effect among the components in the composite nanofibers. This peroxidase-like catalytic reaction provides a simple and accurate determination of H₂O₂ with a low detection limit and a wide linear range. It is believed that the TiO₂/Fe₂O₃ composite nanofibers may become a useful artificial peroxidase for biosensing, medical diagnostics, and environmental monitoring.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51473065, 51273075, 21474043), the National Key Technology Research and Development Program (2013BAC01B02) and Graduate Innovation Fund of Jilin University (2015011).

Notes and references

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