FeCo nanoparticles-embedded carbon nanofibers as robust peroxidase mimics for sensitive colorimetric detection of L-cysteine

Zezhou Yang , Yun Zhu , Guangdi Nie , Meixuan Li , Ce Wang and Xiaofeng Lu *
Alan G. MacDiarmid Institute, College of Chemistry, Jilin University, Changchun, 130012, P. R. China. E-mail: xflu@jlu.edu.cn; Fax: +86-431-85168292; Tel: +86-431-85168292

Received 3rd May 2017 , Accepted 7th June 2017

First published on 8th June 2017


A simple and low cost detection of L-cysteine is essential in the fields of biosensors and medical diagnosis. In this study, we have developed a simple electrospinning, followed by calcination process to prepare FeCo nanoparticles embedded in carbon nanofibers (FeCo-CNFs) as an efficient peroxidase-like mimic for the detection of L-cysteine. FeCo nanoparticles are uniformly dispersed within CNFs, and their diameters are highly influenced by the calcination temperature. The calcination temperature also influences the peroxidase-like catalytic activity, and the maximum activity is achieved at a calcination temperature of 550 °C. Owing to the high catalytic activity of the as-prepared FeCo-CNFs, a colorimetric technique for the rapid and accurate determination of L-cysteine has been developed. The detection limit is about 0.15 μM with a wide linear range from 1 to 20 μM. In addition, a high selectivity for the detection of L-cysteine over other amino acids, glucose and common ions is achieved. This study provides a simple, rapid and sensitive sensing platform for the detection of L-cysteine, which is a promising candidate for potential applications in biosensing, medicine, environmental monitoring.


Introduction

L-Cysteine serves as a type of biothiol molecule, plays a crucial role in achieving homeostasis, metabolism and cellular growth in human body.1–3 Nevertheless, abnormal concentration of L-cysteine has been found to be linked to varied diseases such as retarded growth of children, depigmentation of hair, damage of organs and skin, and the well-known Alzheimer's disease.1–5 Accordingly, diverse detection approaches have been explored to assay the content of L-cysteine, involving high performance liquid chromatography (HPLC),6 fluorescence spectroscopy,7,8 electrochemical technique,9,10 and many other methods.11–17 However, there are certain limitations existing in the above mentioned methods, for instance, complex sample preparation process, time-consuming as well as requirement of expensive and sophisticated instruments, which dramatically restrict their practical applications. In order to overcome the aforementioned limitations, it is essential to develop a simple and effective route to determine L-cysteine with high sensitivity and selectivity.

Artificial enzymes, serving as candidates for natural enzymes, have gained widespread attention due to their tunable activity, high stability and low cost over the past few decades. A large variety of functional materials such as polymers, dendrimers, biomacromolecules, supramolecules have gradually been found to simulate the functions of natural enzymes and have shown promising applications in diverse fields including catalysis, sensors, environmental monitoring, and biomedicine.18–25 In recent years, as a type of burgeoning alternative to artificial enzymes, nanomaterial-based artificial enzymes have gradually become an important research topic owing to their terrific performance and size effect.26–28 The emergence of various synthetic methods has provided further opportunities for the progress of these materials, and a large types of nanomaterial-based artificial enzymes have been developed. Among these, carbon nanomaterials-based artificial enzymes are particularly prominent and have been extensively studied. For instance, graphene and graphene oxide,29–31 carbon nanotubes,32 carbon dots,33 nitrogen-doped carbon nanospheres@carbon nanofibers composite34 have been demonstrated to possess enzyme-like activities. Since carbon-based nanomaterials possess unique electron-transfer capacity along with superior stability, a number of them have also been utilized as catalyst carriers for enhancing the catalytic activity.35–40 It is generally known that noble metals as well as their alloys are excellent enzyme-like catalysts. However, high cost and low content have substantially limited their practical applications. Non-precious metals also possess some metallic characteristics, and various non-noble metals like Fe, Co, Cu, Ni accompanied with their alloys have been investigated and employed in diverse fields including enzyme-like catalysis.41,42

In this study, a simple and effective electrospinning technique combined with calcination process has been used to prepare FeCo nanoparticles-embedded in carbon nanofibers (FeCo-CNFs) as an efficient peroxidase mimic. First, PVP/Fe(NO3)3/Co(Ac)2 nanofibers membrane was prepared via an electrospinning process. Then, the as-prepared membrane was calcined in Ar atmosphere to produce FeCo-CNFs. The calcination temperature has a high influence on the peroxidase-like catalytic activities and the size of the FeCo nanoparticles within CNFs. The apparent kinetic experiment suggested a ping-pong mechanism of the as-prepared FeCo-CNFs as peroxidase mimics, and a superior catalytic efficiency over HRP could be observed. In addition, we have developed a colorimetric method for assaying H2O2 and L-cysteine with a high sensitivity and excellent selectivity. The terrific catalytic activity of FeCo-CNFs as peroxidase mimics makes them promising candidates for applications in catalysis, biosensors, medical diagnostic and other fields.

Experimental

Chemicals and materials

Polyvinylpyrrolidone (PVP, Mw = 130[thin space (1/6-em)]000 g mol−1) and L-cysteine were purchased from Sigma-Aldrich. Fe(NO3)3·9H2O was commercially obtained from Tianjin East China Reagent Factory. Furthermore, 3,3′,5,5′-tetramethylbenzidine (TMB), Co(Ac)2·4H2O and PVP (K30) were bought from Sinopharm chemical reagent Beijing Co., Ltd. FeCl2·4H2O was obtained from XiLong Chemical Factory. CoCl2·6H2O was bought from Tianjin Kemiou Chemical Reagent Co., Ltd. Dimethyl sulfoxide (DMSO) was purchased from Aladdin. N,N-Dimethylformamide (DMF) was obtained from Tianjin Tiantai Fine Chemicals Co., Ltd. H2O2 (30%) and ethanol were acquired from Beijing Chemical Works. All the chemicals in our experiments were of analytical reagent grade and were used without further purification. Deionized water was used throughout the study.

Preparation of FeCo-CNFs

FeCo-CNFs were synthesized through an electrospinning technique, followed by a calcination process.43 First, 1.64 g of PVP was dissolved in a mixed solution consisting of 11.4 g of ethanol and 7.8 g of DMF; then, 0.50 g of Fe(NO3)3·9H2O and 0.154 g of Co(Ac)2·4H2O were added into the above mentioned solution. Vigorous stirring was operated for about 12 h to obtain a homogeneous solution. The mixture was electrospun into a precursor nanofibers membrane, which was defined as PVP/Fe(NO3)3/Co(Ac)2 under an applied DC voltage of 15 kV. An aluminium foil was used as the collector and the distance between the nozzle and the collector was fixed at about 20 cm. Subsequently, the nanofiber membrane mentioned above was calcined under Ar atmosphere at 450 °C, 550 °C, 650 °C and 750 °C for 3 h, which resulted in the production of FeCo-CNFs. For comparison, FeCo NPs were prepared through NaBH4 reduction of FeCl2 and CoCl2 with the assistance of PVP (K30) as stabilizer.44 CNFs were fabricated by a similar strategy involving electrospinning approach and annealing treatment, but in the absence of Fe(NO3)3·9H2O and Co(Ac)2·4H2O.

Investigation of the peroxidase-like catalytic activity of FeCo-CNFs

The peroxidase-like activity of the as-prepared FeCo-CNFs was characterized via a model catalytic reaction towards the oxidation of TMB. A typical experiment was realized as follows; 20 μL of TMB solution (15 mM in DMSO), which acted as peroxidase substrate, and 20 μL of H2O2 (30%) were added into 3 mL acetate buffer solution. Subsequently, 20 μL of catalyst suspension (3 mg mL−1) was injected into the above solution, followed by quick stirring to obtain a uniform solution. The catalytic property was examined via UV-vis spectrophotometry.

Steady-state kinetics experiment

In order to measure the steady-state kinetics, the experiments were executed by adding 20 μL of constant concentration of one substrate into a spectrophotometer cuvette containing 3 mL of acetate buffer solution (pH = 4) as well as 20 μL of the other substrate with varied concentration, followed by injecting 20 μL of FeCo-CNFs catalyst suspension (3 mg mL−1). After rapid stirring for a while, the results were recorded by monitoring the absorbance values at 650 nm in time course. The apparent kinetic parameters were calculated by Michaelis–Menten equation, and the derived Lineweaver–Burk plot exhibited 1/ν = Km/Vmax·(1/[S] + 1/Km), where ν is the initial reaction velocity, Km is the Michaelis constant, Vmax is maximal initial reaction velocity, and [S] is the substrate concentration.

Detection of L-cysteine

A colorimetric method was developed to determine L-cysteine. In a model assay, unless otherwise stated, 20 μL of aqueous solution with varying concentrations of L-cysteine was mixed with 3 mL of acetate buffer solution (pH = 4) including the same amount of TMB and H2O2 reagents as described in the previous section. Afterwards, 20 μL of FeCo-CNFs catalyst suspension (3 mg mL−1) was added to the above mixture solution, followed by rapid stirring. The pertinent absorption variation at 650 nm was monitored via UV-vis spectrophotometry.

Characterization

The morphology of the as-prepared products was characterized via scanning electron microscopy (SEM, Nova NanoSEM 450) and transmission electron microscopy (TEM, JEOL JEM-1200 EX) operated at 15 and 100 kV, respectively. High-resolution TEM (HRTEM) images were examined using FEI Tecnai G2 F20 electron microscope with an acceleration voltage of 200 kV. The crystal phases of the samples were determined by X-ray diffractometer measurements (Empyrean, PANalytical B.V.) based on Cu-Kα radiation. The surface chemical compositions of the products were obtained using a Raman spectrometer (LabRAM HR Evolution). In order to confirm the chemical states of the substances on the surface of FeCo-CNFs, X-ray photoelectron spectra (XPS) were measured on a Thermo Scientific ESCALAB250 system. UV-vis spectra measurement was performed on a Shimadzu 2501 PC spectrometer in order to characterize the peroxidase-like catalytic property of the as-prepared FeCo-CNFs.

Results and discussion

A typical SEM image of the electrospun PVP/Fe(NO3)3/Co(Ac)2 nanofibers is presented in Fig. S1. It can be observed that relatively uniform nanofibers with a smooth surface are dominant in the product, and the size of the nanofibers ranges from 210 to 270 nm. After the calcination treatment, the obtained FeCo-CNFs maintain the nanofiber-like morphology, while their diameters decrease to a certain extent. Fig. 1 shows the SEM images of the as-prepared FeCo-CNFs calcined at 450, 550, 650 and 750 °C, respectively. The rough surfaces of these four types of fibrous nanomaterials are observed. In addition, it can be clearly seen in Fig. 1a that the sample possesses fine fibrous morphology as well as uniform diameters after being calcined at 450 °C, and the diameters of the as-prepared FeCo-CNFs are measured to be 130–210 nm. Moreover, the maintained fibrous morphology can be observed after annealing at higher temperatures (Fig. 1b–d). The diameters of the prepared FeCo-CNFs are in the range of 170–240 nm, 140–210 nm and 100–170 nm for the products calcined at 550, 650 and 750 °C, respectively.
image file: c7dt01611e-f1.tif
Fig. 1 SEM images of the as-prepared FeCo-CNFs calcined at 450, 550, 650 and 750 °C, respectively.

TEM images were further obtained to distinguish the prepared FeCo-CNFs that were calcined at 450, 550, 650 and 750 °C. As shown in Fig. 2a, it is clearly observed that FeCo nanoparticles are compactly distributed within CNFs with a diameter smaller than 5 nm at a calcination temperature of 450 °C. However, the diameters of FeCo nanoparticles increase to nearly 7 nm, 7–21 nm and 14–28 nm when the calcination temperature increases to 550, 650 and 750 °C, respectively (Fig. 2b, c and d). A typical HRTEM image of FeCo-CNFs calcined at 550 °C is displayed in Fig. 3a, and a distinct lattice fringe calculated as 0.202 nm is observed, which corresponds to the (110) crystalline facet of FeCo nanoparticles.45Fig. 3b presents the selective area electron diffraction (SAED) pattern of the above sample. The single diffraction ring is in accordance with the (110) crystal plane of FeCo nanoparticles, demonstrating the successful synthesis of FeCo nanoparticles with a polycrystalline characteristic within CNFs.


image file: c7dt01611e-f2.tif
Fig. 2 TEM images of the as-prepared FeCo-CNFs calcined at 450, 550, 650 and 750 °C, respectively.

image file: c7dt01611e-f3.tif
Fig. 3 (a) HRTEM image and (b) indexed SAED pattern of the as-prepared FeCo-CNFs calcined at 550 °C; (c) XRD patterns and (d) Raman spectra of the as-prepared FeCo-CNFs calcined at 450, 550, 650 and 750 °C.

To further clarify the crystal structure of the as-prepared FeCo-CNFs, XRD analysis is carried out, and the results are shown in Fig. 3c. No apparent peaks can be observed in the curve of FeCo-CNFs at 450 °C. However, it can be clearly seen that three conspicuous peaks appear at around 44.7, 65.1 and 82.5° in the other three characteristic signals, matching well with the (110), (200), (211) crystal facets of FeCo, respectively.43,46,47 It should be noted that the intensity of the three peaks becomes stronger with an increase in the calcination temperature, which is attributed to the superior crystallinity endowed by the higher temperature. In addition to this, no other distinct peaks appear in the curves, further proving that no other by-products have been generated. Raman spectroscopy was also used to confirm the chemical structrue of the as-prepared FeCo-CNFs. As shown in Fig. 3d, the strong characteristic peak emerging at about 1331 cm−1 is ascribed to the D band of carbon, while the peak appearing at about 1580 cm−1 corresponds to the G band of carbon. In addition, the broad band centered at about 2646 cm−1 is related to the G′ band of carbon. This peak can be observed more clearly when the temperature is increased to 750 °C, demonstrating the promotion of the degree of crystallinity.48,49 It is noteworthy that the appearance of several weak peaks lower than 1000 cm−1 in the curve of FeCo-CNFs calcined at 550 °C as well as 450 °C may be related to the oxidation of FeCo nanoparticles with small sizes, which are located on the surface of the carbon nanofibers.

Furthermore, XPS measurement is employed to study the chemical composition and atomic states on the surface of the as-prepared FeCo-CNFs. With regard to the FeCo-CNFs obtained at 550 °C, as shown in Fig. 4a, the typical signals at 780.9 and 796.3 eV originate from Co 2p3/2 and Co 2p1/2, respectively, accompanied with two satellite peaks centered at around 785.9 and 803.3 eV.50 In addition, the relatively weak peak that appears at about 777.8 eV can be observed, which is ascribed to Co0.51,52 In Fig. 4b, which illustrates the XPS spectrum of Fe 2p, it can be clearly seen that two characteristic peaks emerge at 711.4 and 724.4 eV, which are ascribed to Fe 2p3/2 and Fe 2p1/2, respectively.53 In addition, the appearance of a signal above 706.8 eV strongly indicates the existence of Fe0.52,54 As shown in Fig. 4c, a symmetrical peak centered at about 284.6 eV corresponding to C 1s is also observed. The XPS spectrum of O 1s can be deconvoluted into two peaks located at 530.4 and 532.2 eV, which can be indexed to lattice oxygen and hydroxide ions, respectively (Fig. 4d).55


image file: c7dt01611e-f4.tif
Fig. 4 XPS spectra of the as-prepared FeCo-CNFs calcined at 550 °C: (a) Co 2p, (b) Fe 2p, (c) C 1s, (d) O 1s.

The peroxidase-like catalytic activity of the as-prepared FeCo-CNFs is evaluated via a typical catalytic reaction towards the oxidation of TMB. It can be clearly seen that the as-prepared FeCo-CNFs are able to catalyze the oxidation of TMB in the presence of H2O2 in acetate buffer solution (pH = 4), generating a blue color of the reaction system (inset in Fig. 5a). Accordingly, an evidently absorbance value at 650 nm is observed in the UV-vis absorption spectrum, which is attributed to the charge transfer complex and radical cation of TMB (TMB+),56,57 implying the peroxidase-like activity of FeCo-CNFs (Fig. 5a). For comparison, we recorded the absorbance change of the reaction systems of TMB + H2O2, H2O2 + FeCo-CNFs catalyst, and TMB + FeCo-CNFs catalyst for 10 min. No blue color and absorption peak at 650 nm are observed for the former two systems. However, a light blue band as well as a relatively weak absorption band at 650 nm is observed for the system of TMB + FeCo-CNFs catalyst. This result indicates a mild oxidase-like activity of the as-prepared FeCo-CNFs. Furthermore, we have studied the catalytic activities of FeCo-CNFs calcined at different temperatures as peroxidase mimics, which are displayed in Fig. 5b. It is observed that the characteristic curve, which represents FeCo-CNFs that was calcined at 550 °C, possesses maximum absorbance value at 650 nm, indicating that FeCo-CNFs calcined at 550 °C possess the best catalytic activity. The probable factors influencing the peroxidase-like activity of the FeCo/CNFs are the size of FeCo nanoparticles and their crystallinity. The small size and high crystallinity of FeCo nanoparticles are beneficial for the catalytic activities. In this study, the optimal calcination temperature at 550 °C produces FeCo/CNFs with both small size of FeCo nanoparticles and good crystallinity. Furthermore, control experiments have been performed to compare the peroxidase-like activity of the as-prepared FeCo-CNFs with individual FeCo nanoparticles and CNFs alone. As displayed in Fig. S2, it can be clearly seen that the FeCo-CNFs obtained at 550 °C possess significantly higher activities than FeCo NPs and CNFs alone as peroxidase mimics. Considering that the pH value has a great influence on the peroxidase catalytic activities, a control experiment for the oxidation of TMB in acetate buffer solution with diverse pH values was performed. Fig. 5c and d present the comparison of peroxidase-like catalytic activities at different pH values. It can be distinctly seen that the maximum catalytic activity is achieved at pH = 4.0. Therefore, the optimal pH value in the following experiments is chosen to be 4.0.


image file: c7dt01611e-f5.tif
Fig. 5 (a) UV-vis absorbance curves of absorption mode against different reaction systems at 10 min at pH = 4 acetate buffer solution, the inset presents the pertinent photographs; (b) UV-vis absorption plots of diverse systems containing FeCo-CNFs catalysts obtained at different calcination temperatures in the presence of fixed TMB (0.1 mM) and H2O2 (65 mM); (c) relationship toward the peroxidase-like catalytic activity of the FeCo-CNFs calcined at 550 °C against various pH values and (d) the relevant line chart of it.

In order to further explore the peroxidase-like catalytic efficiency of the FeCo-CNFs, apparent kinetic experiments were performed on the FeCo-CNFs calcined at 550 °C. As displayed in Fig. 6a and b, the representative Michaelis–Menten plots related to fixed concentration of H2O2 (5 mM) and TMB (0.1 mM) can be observed, respectively. The decreasing growth of the initial velocity against the incremental concentration of substrate can be observed in both TMB and H2O2. In addition to this, Fig. 6c and d show the transformed double-reciprocal curves of initial velocity with respect to the concentration of one substrate accompanied with the other substrate fixed at three concentration levels. The kinetic parameters were calculated through Lineweaver–Burk plot, which was derived from Michaelis–Menten equation, and contrasted with those of HRP as well as several relevant nanomaterials-based artificial peroxidase mimics (Table 1).58–62 It is clearly seen that the apparent Km value of the as-prepared FeCo-CNFs for substrate with H2O2 is remarkably lower than that of HRP as well as the listed nanomaterials. Considering that Km value has been regarded as a standard of enzyme affinity toward substrate,63,64 the result indicates the preferable affinity of the obtained FeCo-CNFs against H2O2 to those materials. In addition, the Km value of the as-prepared FeCo-CNFs with TMB is less than that of HRP as well as most of the nanomaterials that are listed in Table 1, implying a superior affinity of the obtained FeCo-CNFs to TMB over the aforementioned substances. Furthermore, the approximate parallel linear lines in double-reciprocal plots reveal the existence of characteristic ping-pong mechanism in the as-prepared FeCo-CNFs; specifically, the FeCo-CNFs catalyst binds and reacts with one substrate, releases the first product and then reacts with the other substrate.65


image file: c7dt01611e-f6.tif
Fig. 6 Steady-state kinetic experiments of FeCo-CNFs calcined at 550 °C. The FeCo-CNFs catalyst suspension concentration was fixed at 20 μg mL−1 in 3 mL of acetate buffer solution (0.1 M, pH = 4). (a) H2O2 concentration was kept constant at 5 mM with varied concentrations of TMB; (b) the concentration of TMB was maintained at 0.1 mM and the H2O2 content was diverse; (c, d) double-reciprocal plots of catalyst in the condition of fixed one substrate (H2O2 or TMB) content with various concentrations of the other substrate.
Table 1 Comparison of the kinetic parameters of the FeCo-CNFs with different reported nanomaterials as well as HRP towards the substrates of H2O2 (A) and TMB (B)
A Catalyst K m [mM] V max [10−8 M s−1]
Substrate H2O2 FeCo-CNFs 0.124 5.37
HRP58 3.7 8.71
Co3O4 GNs59 245 28.5
GO-Fe3O4[thin space (1/6-em)]60 0.71 5.31
APTES-Fe3O4[thin space (1/6-em)]61 23.466 86.2
PB/γ-Fe2O3 MNPs62 323.6 117

B Catalyst K m [mM] V max [10−8 M s−1]
Substrate TMB FeCo-CNFs 0.228 21.28
HRP58 0.434 10.0
Co3O4 GNs59 0.12 33.2
GO-Fe3O4[thin space (1/6-em)]60 0.43 13.08
APTES-Fe3O4[thin space (1/6-em)]61 0.836 4.72
PB/γ-Fe2O3 MNPs62 0.307 106


As the absorbance value at 650 nm is dependent on the concentration of H2O2 in the system, a convenient approach for the detection of H2O2 has been established. Fig. 7a exhibits the absorption changes at 650 nm with optimal pH and varying H2O2 concentrations. It can be clearly seen that the reaction rate increases with the increasing concentration of H2O2, leading to the incremental absorbance value in 10 min. The typical dose–response curve of H2O2 concentration is presented in Fig. 7b, indicating a wide linear relationship of absorbance value at 10 min against H2O2 concentrations in the range of 2–50 μM (R2 = 0.99). Furthermore, the detection limit can be calculated as 1.87 μM (S/N = 3).


image file: c7dt01611e-f7.tif
Fig. 7 (a) Time-dependent absorbance changes at 650 nm of different systems involving fixed concentration of TMB (0.1 mM) as well as catalyst suspension (20 μg mL−1) in the absence or presence of H2O2 with varied concentrations (pH = 4); (b) the dependence of the corresponding absorbance changes at 650 nm on 10 min toward the concentration of H2O2 and the inset shows the linear calibration curve for the detection of H2O2.

L-Cysteine is an indispensable amino acid in the human body and makes vital contributions to numerous physiological functions. Considering that the aberrant content of L-cysteine results in various diseases, sensitive detection of L-cysteine has attracted widespread attention. In the past decades, diverse determination methods have been established as well. Owing to the ability of L-cysteine for reducing oxidation state of TMB to TMB, a rapid and effective colorimetric route to assay concentration of L-cysteine has been developed via monitoring the absorbance change at 650 nm. It can be observed in Fig. 8a that the absorption peak at 650 nm descends along with the increased concentration of L-cysteine. In addition, the distinct decaying color of the corresponding photographs can be seen in Fig. 8c, demonstrating the inhibitory effect of L-cysteine toward catalytic oxidation of TMB. Fig. 8b displays the dose–response plot of absorbance value at 10 min against L-cysteine concentrations; a linear relationship within a wide range of 1–20 μM (R2 = 0.99) is observed. Furthermore, the detection limit can be calculated to be 0.15 μM (S/N = 3).


image file: c7dt01611e-f8.tif
Fig. 8 (a) The absorbance changes at 10 min of different TMB systems in the condition of constant TMB (0.1 mM), H2O2 (65 mM), catalyst suspension (20 μg mL−1) with absence or presence of various L-cysteine content; (b) the dose–response curve for determination of L-cysteine and the inset displays the linear calibration plot for detection of L-cysteine; (c) the corresponding photographs of the above systems.

Taking into consideration the interference of other amino acids as well as ions in the human body during the determination experiment of L-cysteine, selectivity measurement was carried out, and the results are depicted in Fig. 9. On comparing the inhibitory effect of L-cysteine with the most common amino acids and ions in human body, it can be clearly observed that the colorless solution is obtained only when L-cysteine exists in the system, while no significant color change can be observed among other interferential substances (Fig. 9b). Accordingly, the unique strong absorption change can only be observed in the presence of L-cysteine (Fig. 9a). This result suggests that the presence of these biological interferential substances has virtually no effect on the assaying of L-cysteine. This approach provides a convincing evidence for the terrific selectivity with regard to the detection of L-cysteine in the biological system.


image file: c7dt01611e-f9.tif
Fig. 9 (a) The difference values of absorbance at 650 nm between 0 min and 10 min in diverse systems containing fixed concentrations of TMB (0.1 mM), H2O2 (65 mM), catalyst solution (20 μg mL−1) with L-cysteine (1 mM) or other different interferential substances (1 mM); (b) the corresponding photographs of the above reaction solutions on 10 min.

Conclusions

In summary, we have developed a facile and effective route to synthesize FeCo-CNFs via electrospinning, followed by a calcination process in Ar atmosphere. The uniform morphology and the size of FeCo nanoparticles within CNFs can be controlled by the calcination temperature. Through comparing the peroxidase-like catalytic activity of the as-prepared FeCo-CNFs calcined at different temperatures, the maximum activity is achieved at a calcination temperature of 550 °C. The steady-state kinetic experiment demonstrates a good catalytic efficiency of the FeCo-CNFs. Owing to the high catalytic activity of the as-prepared FeCo-CNFs, a convenient and rapid approach for the sensitive detection of H2O2 as well as L-cysteine with a low detection limit and wide linear range has been established. High selectivity has been achieved by comparing the inhibitory effect of L-cysteine with other biological interferential substances. It is anticipated that this study offers a meaningful route for the fabrication of biosensors with promising potential applications in biosensing, medical diagnostic and environment monitoring.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (51473065, 21474043) and the Graduate Innovation Fund of Jilin University (2015011).

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7dt01611e

This journal is © The Royal Society of Chemistry 2017