Nitrogen-Doped Carbon Nanofibers as Highly Efficient

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Fe3C/nitrogen-doped carbon nanofibers as highly efficient biocatalyst with oxidase-mimicking activity for colorimetric sensing Na Song, Fuqiu Ma, Yun Zhu, Sihui Chen, Ce Wang, and Xiaofeng Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04036 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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ACS Sustainable Chemistry & Engineering

Fe3C/nitrogen-doped carbon nanofibers as highly efficient biocatalyst with oxidase-mimicking activity for colorimetric sensing Na Song,a Fuqiu Ma,b Yun Zhu,a Sihui Chen,a Ce Wang,a Xiaofeng Lua*

aAlan

G. MacDiarmid Institute, College of Chemistry, Jilin University, 2699 Qianjin Street, Gaoxin District, Changchun 130012, P. R. China.

bFundamental

Science of Nuclear Safety and Simulation Technology Laboratory, Harbin

Engineering University, 145 Nantong Street, Nangang District, Harbin 150001, P. R. China *Corresponding Author. Tel.: (+86)-431-8516-8292; fax: (+86)-431-8516-8292. E-mail address: [email protected] (Xiaofeng Lu).

ABSTRACT The synthesis of functional nanomaterials with unique structures and morphologies as efficient biocatalysts for sensing application has attracted tremendous interest. Herein, Fe3C nanoparticles encapsulated within nitrogen-doped carbon (Fe3C/N-C) nanofibers have been prepared through a facile electrospinning strategy and a carbonization process. The resulting Fe3C/N-C hybrid nanofibers display a superior oxidase-like performance towards the oxidation of 3,3′,5,5′1 ACS Paragon Plus Environment

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tetramethylbenzidine (TMB) and other substrates, which is dependent on the formation of Fe3C nanoparticles and their crystallinity. The obtained Fe3C/N-C hybrid nanofibers-based oxidaselike catalyst shows a good long-term stability and reusability. Thanks to the unique catalytic activity for oxidase mimicking, an efficient sensing platform to sensitively determine sulfite and L-cysteine with low detection limits of 24.9 and 23.0 nM (S/N=3), respectively, as well as excellent selectivity and anti-interference ability has been developed. This work demonstrates a versatile approach to fabricate Fe3C/N-C hybrid nanofibers as enzyme mimics with perfect catalytic efficiency, affording a facile and sensitive colorimetric approach for potential applications in biosensing and other biotechnologies.

KEYWORDS: Fe3C nanofibers, enzyme-like activity, colorimetric detection, sulfite, L-cysteine.

INTRODUCTION Over the past few decades, a great deal of research has been focused on the fabrication of novel artificial enzymes because of their low cost, desirable catalytic performance, easy preparation and separation, and high environmental stability in comparison with natural enzymes.1,2 Especially, since Fe3O4 magnetic nanoparticles (MNPs) were reported to possess an intrinsic catalytic activity for peroxidase mimicking, nanomaterials-based artificial enzymes (nanozymes) have captured tremendous interest because of the tunable enzyme-like catalytic efficiency from adjustable composition and chemical structure, and superior stability.3-6 To date, a large number of nanomaterial has been explored to exhibit enzyme-like activity, such as metal sulfides,7,8 metal oxides,9-11 noble metal nanoparticles,12,13 carbon nanomaterials,14,15 metalorganic frameworks (MOFs)16, and their nanocomposites.17-20 Owing to the unique catalytic

2 ACS Paragon Plus Environment

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performance, these nanozymes have represented promising applications in varieties of fields including sensing, biomedicine, environmental science, and food industry. However, most of the reported enzyme-like nanomaterials show peroxidase-like activity, which is necessary involved with H2O2. The addition of H2O2 in the reaction system not only causes the damage to the environment, but also seriously limits their applications in the field of analytical science. Recently, several kinds of nanomaterials including nanostructured noble metals, metal oxides and their nanocomposites have been reported to be efficient oxidase mimics, which are able to realize the oxidation of the oxidase substrates without the introduction of H2O2 compared with peroxidase mimics.21-27 For example, citrate-capped Pt nanoparticles were reported to demonstrate oxidase-like activity to catalyze the oxidation of the oxidase substrates through a four-electron exchange process, which has been constructed as a valid sensing platform to detect therapeutic heparin level in real sample.28 However, it is still a difficult task to manufacture new type of functional nanomaterials with distinctive chemical structure and morphology as efficient oxidase mimics for a large variety applications. Fe3C is a member of the family of the iron carbides, processing extreme mechanical strength, sufficient thermal stability and excellent catalytic activity due to its special crystal structures.29,30 With these advantages, Fe3C-based nanomaterials have represented a large range of applications in lithium ion batteries,31 dye-sensitized solar cells,30 superior supercapacitors32 and oxygen reduction reaction33. Recently, Fe3C-based nanomaterials that are synthesized via a pyrolyzing process of the Prussian blue (PB) cubes under an Ar atmosphere have also been reported to display oxidase-like catalytic activity.34 Although the nature of the electrocatalytic or biocatalytic activity of Fe3C-based nanomaterials has been well studied, it is recognized that the huge specific surface area and unique microstructure contribute to the more active sites and rapid



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transfer of catalytic reaction-related specie. In addition, Fe3C nanoparticles encapsulated in carbon matrix will deliver a higher electronic conductivity and better stability. Especially, previous research has shown that carbon materials doped with heteroatom and their hybrid with metal oxide can be used as efficient electrochemical biosensors with a superior catalytic activity for the sensitive sensing applications.35-39 Therefore, the development of a simple and versatile strategy to fabricate Fe3C nanoparticles incorporated in heteroatom-doped carbon catalysts with unique composition and microstructure to achieve an optimized catalytic performance is highly appealing. Electrospinning technique is one of the most effective approach to prepare unique onedimensional structured nanomaterials.40,41 By using such an approach, the morphology and composition of the nanomaterials could be well controlled, which is beneficial for the regulation of the catalytic activity for enzyme-mimicking. In this study, we reported a facile synthesis of the Fe3C nanoparticles encapsulated within nitrogen-doped carbon (Fe3C/N-C) nanofibers through a facile electrospinning strategy and a carbonization treatment. As expected, the as-prepared Fe3C/N-C hybrid nanofibers possess distinctive oxidase-like activity towards 3,3′,5,5′tetramethylbenzidine (TMB) oxidation. The superior oxidase-like activity can be contributed to the smaller diameter, high ratio of length to diameter of the nanofibers and the synergistic interfacial effect between Fe3C nanoparticles and nitrogen-doped carbon matrix. On account of high catalytic property, a facile colorimetric assay of sulfite and L-cysteine has been developed. It is confirmed that the prepared Fe3C/N-C hybrid nanofibers are efficient oxidase mimics, which has displayed bright prospects in biosensors, biomedicine, environmental engineering and other fields.


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EXPERIMENTAL SECTION Chemicals Polyacrylonitrile (PAN, Mw=150 000), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonicacid) diammonium (ABTS), L-cysteine, 4-aminoantipyrine (4-AAP) and dihydroethidium (DHE) were commercially available from sigma-Aldrich. o-Phenylenediamine (OPD) was purchased from Tianjin kwangfu Fine Chemical Industry Research Institute. Fe(NO3)3·9H2O was provided by Tianjin East China Reagent Factory. Dimethyl sulfoxide (DMSO) was bought from Aladdin. TMB was obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Sodium acetate anhydrous (NaAc) was bought from Xilong Chemical Industry Co., Ltd. Sodium sulfite (Na2SO3), acetic acid, phenol, H2O2 and N,N-dimethylformamide (DMF) were purchased from Beijing Chemical Works. Preparation of the Fe3C/N-C hybrid nanofibers Fe3C/N-C hybrid nanofibers were prepared through a facile electrospinning strategy and a carbonization process. The typical procedure is as below: Firstly, a uniform PAN solution was prepared by dissolving 1.0 g of PAN in 8.8 g of DMF solvent at 90°C. Then, 0.4 g of Fe(NO3)3·9H2O was mixed with the above solution at room temperature. After magnetic stirring for further 12 h, the prepared precursor solution was loaded into a glass tube with a thin spinneret for electrospinning. The power supply was maintained at18 kV, and the distance between the spinneret and the aluminum foil collector was fixed about 20 cm. In the following, the prepared electrospun fibrous membrane was carbonized as the following process in tube furnace. First, the fibrous membrane was heated at 250 °C in air atmosphere for 3 h, and then carbonized in argon at 800 °C for 2 h with a heating rate of 2 °C/min. For comparison, the fibrous membrane was



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also carbonized at other temperatures including 600, 700 and 900°C. Finally, the Fe3C/N-C hybrid nanofibers were obtained. Oxidase-like catalytic property of Fe3C/N-C hybrid nanofibers Typically, 20 μL of TMB in DMSO solution (15 mM) and 20 μL of the as-prepared nanofibers suspension (3 mg mL-1) were successively injected into 3mL of acetate buffer solution (the pH value is fixed at 4.0, unless other stated) at room temperature. The formation of the oxidized TMB was recorded by monitoring the absorbance changes at 651 nm after ten– minute incubation. In addition, ABTS and OPD are also used as substrates for oxidase mimicking similar with that of TMB. In the experiment for steady state kinetic analysis, the concentrations of TMB were changed while the other conditions were fixed, and the absorbance values at 651 nm were immediately recorded in time course. Detection of sulfite and L-cysteine To detect sulfite, the Fe3C/N-C hybrid nanofibers suspension (20 μL, 3 mg mL-1) was mixed with TMB solution (20 μL, 15 mM in DMSO) and varied concentrations of sulfite (1-150 μM) in acetate buffer solution (3 mL, pH=4.0). The mixture was incubated for 10 min at ambient conditions, and then the catalytic reaction was monitored by measuring the changes of the absorbance at 651 nm of the system in a common spectrum mode. A similar procedure was performed to detect L-cysteine, while the concentrations of L-cysteine are fixed from 0 to 300 μM. Stability and reusability of the prepared Fe3C/N-C hybrid nanofibers In order to assess the usage stability of the prepared Fe3C/N-C hybrid nanofibers, they are stored in aqueous solution under ambient conditions, and then the catalytic activity was recorded every day. To further consider their reusability, a piece of the as-prepared Fe3C/N-C hybrid nanofibers


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membrane (1.5 × 1.5 cm) was dipped into an acetate buffer solution (3 mL, pH=4.0) containing 0.1 mM of TMB, and incubated for 15 min at ambient conditions with gentle shaking. Then the membrane was recovered from the reaction solution by tweezers or a magnet. After washing thoroughly with water, ethanol and the acetate buffer solution, the membrane was used for the next cycle. Detection of phenol Phenol with concentrations from 0 to 200 μM were injected into PBS buffer solution (0.1 M, 3.0 mL, pH=7.4) with H2O2 (260 mM), 4-AAP (10 mM), and Fe3C/N-C hybrid nanofibers (0.64 mg). Then the mixing solution was incubated for further 5 min at room temperature. After centrifuging at 12000 rpm for 10 min, the supernatant was recorded by monitoring the absorbance at around 500 nm. Characterization The morphology and structure of the prepared Fe3C/N-C hybrid nanofibers were characterized by a field-emission scanning microscopy (SEM, FEI Nova NanoSEM) and a transmission electron microscopy (TEM, JEOL JEM-1200 EX). High-resolution TEM (HRTEM) image and energy dispersive X-ray (EDX) analysis were obtained using a FEI Tecnai G2 F20 measurement. The crystallographic structures were characterized by X-ray diffraction (XRD, PAN-alytical B.V. Empyrean) with Cu Kα radiation and Raman spectra (Lab RAM HR Evolution). The surface elemental composition of the as-prepared samples was confirmed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB250). Furthermore, the catalytic properties were studied by ultraviolet-visible (UV-vis, Shimadzu UV-2501 PC spectrometer) measurement.

RESULTS AND DISCUSSION



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With the advantage of the facile synthetic procedure on a large scale, electrospinning technique can conveniently generate 1D nanomaterials with a controllable composition and microstructure. In this study, PAN/Fe(NO3)3 hybrid nanofibers were firstly synthesized through an electrospinning process. Then a carbonization treatment under different calcination temperatures was performed to fabricate Fe3C/N-C hybrid nanofibers. The SEM images show that the prepared Fe3C/N-C hybrid samples maintain the nanofiber-like morphology with diameters of 180-460 nm (Fig. 1a, c, e and g). From the SEM images, it is also found that the Fe3C/N-C hybrid nanofibers carbonized at 600 and 700 °C are relatively smooth, while the Fe3C/N-C hybrid nanofibers carbonized at 800 and 900°C are much rougher. Especially, many defects are observed on the surface of the Fe3C/N-C hybrid nanofibers carbonized at 900°C. Furthermore, TEM images were used to characterize the formation of Fe3C nanoparticles encapsulated in carbon nanofibers (Fig. 1b, c, f and h). It is clearly found that the density of the Fe3C nanoparticles increases with the incremental carbonization temperature. When the calcination temperature is below 700 °C, few Fe3C nanoparticles are observed in the N-C hybrid nanofibers, while a large number of Fe3C nanoparticles appear as the carbonization temperature increases to 800 and 900 °C. The diameters of the Fe3C nanoparticles in the N-C nanofibers at the carbonization temperature of 800 and 900 °C are in the range of 10-70 nm.


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Fig.1 SEM and TEM images of Fe3C/N-C hybrid nanofibers carbonized at (a, b) 600 °C, (c, d) 700 °C, (e, f) 800 °C and (g, h) 900 °C.

The formation of the Fe3C nanoparticles within N-C nanofibers has further been characterized via a HRTEM measurement. As illustrated in Fig. 2a, the HRTEM image of a 9 ACS Paragon Plus Environment


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Fe3C/N-C hybrid nanofiber shows the uniform dispersion of Fe3C nanoparticles in the nanofibers. Fig. 2b shows a typical lattice fringe spacing at approximately 0.20 nm and 0.33 nm, which are ascribed to the (031) plane of the Fe3C (JCPDS 35-0772) and the (002) plane of the graphite42, respectively, demonstrating the successful synthesis of the Fe3C nanoparticles encapsulated within the N-C nanofibers. In Fig. 2c, a few lattice fringe spacing is also observed at about 0.255 nm, which is a result of the (311) crystal plane of the Fe3O4 particles (JCPDS 190629), indicating the existence of very few Fe3O4 nanoparticles in the Fe3C/N-C hybrid nanofibers. As shown in Fig. 2d, the EDX spectrum confirms the existence of the C, N, O and Fe elements in the product of the prepared Fe3C/N-C hybrid nanofibers. It is specially pointed out that the signals of Cu and Si are attributed to the carbon coated copper grid to support the TEM sample and the instrumental substrate. Furthermore, the HAADF-STEM image and elemental mapping exhibit that the Fe, C and N elements are uniformly distributed within the nanofibers (Fig. 2e-h).


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Fig.2 (a) TEM image, (b, c) HRTEM image, and (d) EDX spectrum of the prepared Fe3C/N-C hybrid nanofibers carbonized at 800 °C. (e) HAADF-STEM pattern and (f-h) EDX elemental mapping of Fe-L, C-K and N-K in Fe3C/N-C hybrid nanofibers carbonized at 800 °C.

The crystallographic features of the Fe3C/N-C hybrid nanofibers were further studied via a XRD measurement. In Fig. 3a, the diffraction peaks at around 37.8°, 39.9°, 40.7°, 42.9°, 43.8°, 44.5°, 44.9°, 45.9°, 48.7° and 49.1° can be assigned to the (210), (002), (201), (211), (102), (220), (031), (112), (131) and (221) planes of the Fe3C phase (JCPDS 35-0772), respectively. And the very weak band at 35.5° is attributed to the (311) plane of Fe3O4 phase (JCPDS 190629). It should be noted that there is an obvious peak at 44.7° appearing when the fibrous membrane was carbonized at 900°C, which can be assigned to the (110) plane of the metallic Fe phase (JCPDS 06-0696). The formation of the metallic Fe phase could be contributed to the decomposition of the Fe3C at a high temperature.43 It is found that the sharper peaks appear when 11 ACS Paragon Plus Environment


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a higher temperature is applied, suggesting a higher crystallinity is acquired at a higher carbonization temperature. Raman spectroscopy has also been used to verify the formation of the Fe3C/N-C hybrid nanofibers. As shown in Fig. 3b, the peaks at 222.3, 289.4 and 400.2 cm-1 are attributed to Fe3C nanoparticles.44 And there is a clearly pair of peaks at 1328.2 and 1596.2 cm-1, which are ascribed to a defect-evoked (D) band and a crystalline graphitic (G) band, respectively, confirming the presence of graphitic carbon in the Fe3C/N-C hybrid nanofibers.45

Fig. 3 (a) XRD pattern of the obtained Fe3C/N-C hybrid nanofibers carbonized at 600, 700, 800 and 900 °C; (b) Raman spectra of the obtained Fe3C/N-C hybrid nanofibers carbonized at 600, 700, 800 and 900 °C.

The surface elements and their electronic states of the Fe3C/N-C hybrid nanofibers are further identified by XPS measurement. The survey spectrum of the hybrid nanofibers is exhibited in Fig. 4a, indicating the existence of the C, N, O and Fe elements. As displayed in Fig. 4b, the Fe 2p region was deconvoluted into four components centered at 709.4, 713.0, 720.0, and 725.5 eV, which are corresponding to the Fe2+ 2p3/2, Fe3+ 2p3/2, Fe2+ 2p1/2 and Fe3+ 2p1/2, respectively, demonstrating the formation of Fe3C in the hybrid nanofibers.46 Fig. 4c shows the C 1s fine spectrum, in which the peaks are assigned to the C-Fe (284.2 eV), C-C/C=C (284.6 eV), C-N (285.0), C-O (285.4 eV) and C=O (287.0) bonding, confirming the presence of the N-C in 12 ACS Paragon Plus Environment

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the product.32,47,48 The N 1s spectrum is shown in Fig. 4d, which is fitted into three bands corresponded to the pyridinic N(398.6 eV), pyrrolic N (399.8 eV) and graphitic N (401.3 eV), suggesting the retention of the nitrogen atoms in the carbon structure.49 The spectrum of O 1s displays four characteristic peaks indexed to Fe-O (530.5 eV), C=O (531.8 eV), C-O (533.2 eV), and the chemisorbed O (534.2) (Fig. 4e).42 The XPS results further confirm the successful preparation of the Fe3C/N-C hybrid nanofibers.

Fig. 4 XPS spectra of the obtained Fe3C/N-C hybrid nanofibers carbonized at 800 °C: (a) full survey spectrum, (b) Fe 2p, (c) C 1s, (d) N1s, (e) O 1s.



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Previous reports have shown that the Fe-C-N compounds possess intrinsic enzyme-like catalytic properties.34,50 To evaluate the oxidase-like performance of the Fe3C/N-C hybrid nanofibers, we monitored the oxidation of TMB process via a colorimetric and UV-vis measurement. As shown in Fig. 5, the reaction system generates a blue color obviously after the addition of the prepared Fe3C/N-C hybrid nanofibers and TMB. However, there is no blue color appeared without the addition of either Fe3C/N-C hybrid nanofibers or TMB, demonstrating that the Fe3C/N-C hybrid nanofibers possess intrinsic oxidase-like activity. Similarly, the oxidaselike activity of the Fe3C/N-C hybrid nanofibers toward the oxidation of ABTS and OPD has also been evaluated. As shown in Fig. S1, the ABTS and OPD substrates display characteristic light blue and yellow colors and two maximum absorbance peaks appear at around 415 and 451 nm in the absorption spectra, respectively. Furthermore, the influence of the carbonization temperature on the catalytic activity of the Fe3C/N-C hybrid nanofibers has been studied. It is found that the Fe3C/N-C hybrid nanofibers carbonized at 800°C possess the best catalytic activity, which is owing to the formation of more Fe3C nanoparticles as well as their higher crystallinity at a higher carbonation temperature (Fig. 5b and 5c). However, when the carbonization temperature increases to 900 °C, Fe3C will decompose into metallic iron, which has also been confirmed by the XRD result. Therefore, the catalytic activity of the Fe3C/N-C hybrid nanofibers carbonized at 900°C is even lower than that carbonized at 800°C. In general, the oxidase-like catalytic property is strongly related to the pH value.27,51 Therefore, we have estimated the influence of the pH values on the oxidase-like activity of Fe3C/N-C hybrid nanofibers. As shown in Fig. 5d, the highest catalytic activity is acquired at pH = 4.0. Thus the pH value for the following oxidaselike catalytic studies is selected to be 4.0. We have tested the usage stability of the Fe3C/N-C hybrid nanofibers. The result shows that the oxidase-like activity of the obtained Fe3C/N-C


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hybrid nanofibers still held steady within 15 days storage period in aqueous solution (Fig. 6a), demostrating their good long-term stability. Furthermore, the as-synthesized Fe3C/N-C hybrid nanofibers can be fabricated as a freestanding membrane, showing an advantage of easy reusability (Fig. S2). From Fig. 6b，it is found that the catalytic activity of the Fe3C/N-C hybrid nanofibers membrane still remains more than 95% of the initial activity after the fifth cycle, indicating their excellent reusability.

Fig.5 (a) UV-vis absorbance and the corresponding optical photographs of the oxidation of TMB recorded at 10 min in three systems (TMB, Fe3C/N-C hybrid nanofibers, TMB + Fe3C/N-C hybrid nanofibers) in acetate buffer solution with a pH value of 4.0; (b) Comparison of the absorbance at 651 nm with time increasing in the presence of the Fe3C/N-C hybrid nanofibers catalyst carbonized at 600, 700, 800 and 900 °C. (c) The line chart to show the catalytic activity of the catalysts carbonized at different temperature. (d) Dependence of the oxidase-like activity of the obtained Fe3C/N-C hybrid nanofibers catalyst with varied pH values.



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Fig. 6 (a) Long-term stability of Fe3C/N-C hybrid nanofibers for oxidase mimicking; (b) The percentage values of relative catalytic activity of five cyclic experiments

The oxidase-like behavior of the Fe3C/N-C hybrid nanofibers has been profoundly investigated through the kinetic analysis using the concentration of TMB as the variable. As shown in Fig. 7, the data was calculated and in accordance with the Lineweaver–Burk double reciprocal plots. It can be clearly seen that the oxidation catalysis conforms to the typical Michaelis–Menten equation: 1 𝑣

𝐾𝑚

1

1

= 𝑉𝑚𝑎𝑥 ∙ [S] + 𝑉𝑚𝑎𝑥,

(1)

in which 𝐾𝑚 is the Michaelis constant, reflecting the affinity between the catalyst and the substrate, 𝑉𝑚𝑎𝑥 is the maximal reaction velocity. In this work, the Km and Vmax values of the Fe3C/N-C hybrid nanofibers catalyst for the substrate of TMB were calculated to be 0.225 mM and 3.25×10-7 Ms-1, respectively. The Km value is found to be lower than Fe3C/NGr (0.25 mM)50 and HRP (0.43 mM)5 in previous reports, suggesting the better affinity of the hybrid nanofibers towards TMB than Fe3C/NGr and HRP.


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Fig. 7 (a) Steady-state kinetic experiments of Fe3C/N-C hybrid nanofibers. (b) Lineweaver–Burk plots for TMB substrate. 20 μl of catalyst (3 mg·L-1) was used in this experiment, and the measurement is performed at room temperature.

It is generally known that the peroxidase mimics are able to catalyze H2O2 to produce hydroxyl radical which can oxidize TMB to generate a blue color.52 Nevertheless, the oxidaselike catalyst can catalyze molecular oxygen to decompose reactive oxygen species (ROS).27,53 It has been reported that the sulfite is a radical inhibitor which can react with ROS to prevent the oxidation of TMB.54 In order to prove the reaction mechanism between the Fe3C/N-C hybrid nanofibers and TMB, a control experiment was performed. As seen in Fig. 8(a), the reaction solution of Fe3C/N-C hybrid nanofibers + TMB appears a blue color and there is an obvious absorbance at 651 nm in the absorption spectrum, demonstrating the formation of the oxidized TMB. However, after the addition of a certain amount of sodium sulfite into the above reaction system, there is no distinct blue color as well as the absorbance at 651 nm. Therefore, it can be concluded that the Fe3C/N-C hybrid nanofibers can disintegrate the physically or chemically adsorbed O2 into ROS to oxidize the TMB molecules. The physically absorbed O2 comes from the dissolved oxygen in reaction solution and the chemically absorbed O2 refers to the absorbed oxygen on the surface of the Fe3C/N-C hybrid nanofibers which could be certified by the 17 ACS Paragon Plus Environment


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characteristic band at 534.2 eV in O 1s XPS spectrum (Fig. 4e). To further investigate the formation of the ROS species, dihydroethidine (DHE), a specific fluorescence probe for superoxide radical (O2˙ˉ), was used to react with the catalyst. Then an obvious fluorescent emission was observed at approximate 615 nm (Fig. 8(b)), demonstrating the generation of O2˙ˉ on the Fe3C/N-C hybrid nanofibers in the acetate buffer.55,56 This result shows an evidence that ROS plays an significant role for the oxidase-mimicking of the Fe3C/N-C hybrid nanofibers.

Fig. 8 (a) UV-vis absorption spectra and the optical photographs of the oxidase-like catalytic system by Fe3C/N-C hybrid nanofibers in the absence and the presence of 300 µM sodium sulfite. (b) Specific determination of O2˙ˉwith DHE, λex = 500 nm.

As we all know that the sulfite is usually acted as an antioxidant and antiseptic in food industry. However, it has been proven that the sulfite could cause asthma, dystrophy and skin allergy.57,58 Hence, it is indispensable to find a sensitive approach to detect sulfite. In this study, we have performed a simple colorimetric route for the determination of sulfite. From Fig. 9a, the absorbance at 651 nm of the reaction solution containing Fe3C/N-C hybrid nanofibers and TMB decreases with sulfite concentration increasing. Fig. 9b exhibits a linear relationship curve between ΔA (A

(651nm, absence)

-A

(651nm, sulfite))

and the concentration of sulfite with a wide range

from 0.1 to 10 μM (R2=0.999). And the detection limit of sulfite is estimated to be around 24.9


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nM (S/N=3). This value is much better than many other reports of nanomaterials-based enzymelike colorimetric determination of sulfite27,53,54,59 (Table. 1).

Fig.9 (a) The absorbance changes of the mixing solution consisting of TMB (0.1mM), Fe3C/N-C hybrid nanofibers suspension (20 μgmL-1) in the absence or presence of varied concentrations of sulfite. (b) The dose–response curve for the detection of sulfite, and the inset displays a linear calibration plot to detect sulfite. ΔA: A (651nm, absence) - A (651nm, sulfite).

Materials

Linear

Limit of

range (μM)

detection (nM)

substrate

Ref.

MnCo2O4 NFs

TMB

1-6

27.9

27

Co3O4 NPs

TMB

2-16

53

53

Ag2O NPs

TMB

100-500

1000

54

PS-MnO2

TMB

0-250

1000

59

Fe3C/N-C NFs

TMB

0.1-10

24.9

This work

Table 1. Comparison of the sensing performance for sulfite based on this work and other similar enzyme-like detection approaches.



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L-cysteine is one of the vital amino acids in many organisms, exhibiting a broad application in biomedicine, food industry and cosmetic, etc. For example, it can be employed in natural juice to protect vitamin being oxidize and turning brown. Therefore, it is indispensable to sensitively detect the concentration of L-cysteine for real applications. It is well known that L-cysteine processes a reductive sulfur group (-SH), which can consume the ROS and reduce TMB oxidation, resulting in a blue color fading of the solution.60-62. Thus we developed a simple and efficient approach to sensitively detect L-cysteine. As shown in Fig. 10a, it is observed that the absorbance at 651 nm reduces as the concentration of the L-cysteine increases. As depicted in Fig. 10b, the relationship between the ΔA values and the concentration of the L-cysteine from 0.1 to 10 μM shows a linear relationship (R2=0.988). And the limit of detection is approximately 23.0 nM (S/N=3). From table 2, it can be found that the limit of detection value is better than many previously reported nanomaterials-based enzyme-like determination methods.27, 60, 63,64

Fig.10 (a) The absorbance changes of the mixing solution consisting of TMB (0.1mM), Fe3C/NC catalyst suspension (20 μgmL-1) in the absence or presence of varied concentrations of Lcysteine. (b) The dose–response curve to detect L-cysteine, and the inset displays a linear calibration plot to detect L-cysteine. ΔA: A (651nm, absence) - A (651nm, L-cysteine).


Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Linear Materials

Limit of

substrate

Ref. range (μM)

detection (nM)

MnCo2O4 NFs

TMB

0.5-10

34.3

27

FeCo NPs/CNFs

TMB

1-20

150

60

Ce MOF

TMB

0-40

135

63

Fe-MIL-88NH2

TMB

1-80

390

64

Fe3C/N-C nanofibers

TMB

0.1-10

23.0

This work

Table 2. Comparison of the sensing performance for L-cysteine based on this work and other similar enzyme-like detection approaches.

The selectivity to determine L-cysteine via monitoring the absorbance of the reaction systems has also been studied by replacing L-cysteine with some other common ions, such as Na+, Cl–, Ca2+, K+, HCO3–, NO3–, and some common amino acids including valine, tryptophan, threonine, histidine, lysine, methionine, and phenylalanine, as well as glucose. Fig. 11a shows the ΔA values of those systems. It can be clearly observed that there is a significant difference of the ΔA values between those interferences and L-cysteine with the same concentration. And as shown in Fig. 11b, the color of the solution containing L-cysteine is basically colorless, but the other solutions containing the typical interferences are still retaining blue, demonstrating a high selectivity towards L-cysteine detection based on the Fe3C/N-C hybrid nanofibers–TMB system. In addition, Fig. S3 shows the ΔA values of the Fe3C/N-C hybrid nanofibers-TMB system at 651 nm with L-cysteine and other interferential substances, demonstrating an excellent antiinterfering ability to detect L-cysteine using the colorimetric sensing approach.



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Fig. 11 (a) The ΔA values of the Fe3C/N-C-TMB system at 651 nm with L-cysteine (0.3 mM) or other interferential substances (0.3 mM), respectively. The reaction timeis fixed at 10 min; (b) the optical photographs corresponding to the above reaction systems.

It is noted that the prepared Fe3C/N-C hybrid nanofibers also possess a peroxidase-like activity, which are able to catalyze the TMB oxidation with the assistance of H2O2 (Fig. S4). It has been reported that phenol and 4-AAP could be oxidized by H2O2 to form quinone-imine complex.65 Therefore we have developed a colorimetric assay to detect phenol based on the peroxidase-like property of Fe3C/N-C hybrid nanofibers. From Fig. S5a, it is found that the absorbance at 500 nm increases with the increment of phenol concentrations, corresponding to the changes of the reaction solution from colorless to pink. A linear relationship between A500 and the phenol concentration from 1 to 200 μM (R2=0.998) is exhibited in Fig. S5b, and the limit 22 ACS Paragon Plus Environment

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of detection is estimated to be 0.705 μM (S/N=3). This result implies that the Fe3C/N-C hybrid nanofibers are outstanding artificial enzyme catalysts for promising biosensing applications.

CONCLUSIONS In summary, we have prepared Fe3C/N-C hybrid nanofibers via a simple electrospinning and calcination process. The synthesized Fe3C/N-C hybrid nanofibers exhibited an excellent intrinsic oxidase-like catalytic property with favorable stability and reusability, which could be used to sensitively determine sulfite and L-cysteine with a low limit of detection. The detection system also displayed an excellent selectivity and anti-interference ability towards L-cysteine. In addition, the prepared Fe3C/N-C hybrid nanofibers showed a good peroxidase-like catalytic property, which has been used for the determination of phenol with a detection limit of 0.705 μM (S/N=3). This work provides a simple and effective way for the synthesis of the Fe3C/N-C hybrid nanofibers, which can be generalized to prepare many types of other functional materials for enzyme mimicking, showing promising potentials in medicine, biosensing and environmental science.

ASSOCIATED CONTENT Supporting Information UV-vis absorption spectra and photographs of the oxidized ABTS (0.1 mM) and OPD (0.1 mM); schematic illustration for the reusability experiment process; anti-interference test for the detection of L-cysteine; the peroxidase-like study of Fe3C/N-C hybrid nanofibers; UV-vis absorption curves for the detection of phenol.



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AUTHOR INFORMATION Corresponding authors: *(X.F.L.) Tel/Fax +86-431-85168292; email: [email protected]; ORCID Xiaofeng Lu: 0000-0001-8900-9594 Notes Authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (51773075, 51473065, 21474043), the Foundation of Heilongjiang Postdoctoral Science (LBHZ17050), the Natural Science Foundation of Heilongjiang Province (B201317) and Decommissioning of nuclear facilities and special funds for radioactive waste management ([2017]955)

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Graphic for manuscript Synopsis

Fe3C/nitrogen doped carbon nanofibers as efficient enzyme-like mimicsexhibit an excellent colorimetric biosensing performance.


Nitrogen-Doped Carbon Nanofibers as Highly Efficient

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