Ethylenediamine-assisted preparation of carbon nanofiber supported nickel oxide electrocatalysts for sensitive and durable detection of insulin

Abstract

A uniform nickel oxide (NiO) nanoparticle decorated on carbon nanofibers (CNFs) hybrid with the assistance of ethylenediamine (EDA) following a simple on-spot pyrolysis route has been fabricated for insulin electrocatalytic oxidation. The fabricated hybrid displayed superior catalytic performance due to the synergetic effects between NiO nanoparticles and CNFs. Excellent analytical features, including high sensitivity (1.55 μA μM⁻¹), short response time (<3 s), low detection limit (12.1 nM) and satisfactory linear dynamic range (20–1020 nM) were achieved. Moreover, this EDA-CNFs-NiO hybrid showed good stability and antifouling property in a 0.1 M NaOH electrolyte toward insulin after successive potential cycling, which is highly required for a promising insulin electrocatalyst.

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1. Introduction

Diabetes mellitus is a disorder of glucose regulation characterized by the body's inability to produce or properly use insulin, an important polypeptide hormone that regulates carbohydrates and metabolism in the body.^1–3 Diabetes mellitus is caused by too little insulin in the body (type 1 diabetes) or the body not responding to the effects of insulin (type 2 diabetes). The incidence of diabetes is rising and the total annual global costs associated with the treatment is rising too.² But when left untreated, consistently high blood glucose levels can lead to blindness, kidney and heart disease, nerve damage and increased susceptibility to infection.^2,4 Insulin therapy is essential in the treatment of diabetic patients.¹ Furthermore, diabetes patients are reliant on intake of adequate and accurate amount of insulin.^1,2 Therefore, insulin measurement is an important subject for clinical diagnostics and physiological studies of this disease. The present analytical procedures for the determination of insulin include bioassay,⁵ immunoassay,⁶ chromatography⁷ and capillary electrophoresis.⁸ These methods can detect insulin down to sub-nM or even pM concentrations, but on the other hand, they are time-consuming, cumbersome and expensive. The direct detection of insulin by electrochemical oxidation offers an excellent opportunity for the development of continuous real-time measurements compared to the above procedures. However, this approach also has its drawbacks because the direct oxidation of insulin is limited by slow kinetics, surface fouling, low sensitivity and reproducibility at a bare electrode surface. Therefore, several modified electrodes have been fabricated for the sensitive detection of insulin. They mainly rely on inorganic redox mediators and electrocatalysts, such as ruthenium or iridium-based materials.^9–12 However, these modified electrodes still suffer the problem of poor long-term stability of insulin oxidation response. Therefore, seeking novel electrocatalyst materials with good operational stability and antifouling performance remains a great challenge.

Supported metal oxide electrocatalysts have attracted increasing interest because of their novel properties and unique applicability, which cannot be achieved with single constitution.^13–15 Over the past few years, as an important kind of metal oxide, various nanostructured nickel oxide (NiO) electrocatalysts have been designed as electrochemical sensors due to the multiple oxidation states of the Ni components providing catalytic properties to the redox reactions.^16–20 For these nickel based nanomaterials, shape and size control is crucial to their sensing and catalytic functions. However, it is still suffering the problem to prevent nanostructure from forming agglomerates, which decreases surface area and hence attenuating the catalytic ability. To circumvent this problem and enhance electrocatalytical performance, it is favorable to utilize a suitable substrate to support NiO catalysts in a rational manner. In this strategy, the aggregation of NiO nanoparticles could be prevented efficiently by dispersing them finely on the support.

The advent of nanostructured support materials such as carbon nanotubes (CNTs),^21,22 carbon nanofibers (CNFs)^23,24 and graphene (GO)^14,15 provides opportunities for novel metal oxide electrocatalyst. As a member of carbon nanomaterials, CNFs have been widely applied as support materials because of their low production cost, good mechanical stability, excellent conductivity, Some pioneered work have been done in the field of carbon nanofiber-based nanocomposite by the groups of De Jong.^25,26 In this work, we will focus on the use of CNFs as support material by using an on-spot pyrolysis method for the preparation of supported NiO electrocatalysts. It is known that the properties of the metal oxides strongly depend on several factors such as their crystal sizes and stacking manners.^22,27 Herein, we report the synthesis of uniform NiO nanoparticles attached to CNFs with the assistance of ethylenediamine anhydrous (EDA). EDA was chosen because it can provide copious surface amino groups that allow loading of highly uniform dispersed NiO nanoparticles. Notably, the resulting EDA-CNFs-NiO nanocomposite exhibited good stability and excellent electrocatalytic activity for insulin oxidation with high sensitivity (1.55 μA μM⁻¹) and small overpotential (∼0.45 V).

2. Experimental section

2.1 Reagents

Nickel(II) sulfate hexahydrate (NiSO₄·6H₂O) was purchased from Tianjin Fuchen Chemical Reagent Company. Paracetamol (PCM) was purchased from Southwest Pharmaceutical Ltd. Sulfuric acid (H₂SO₄, 98%), nitric acid (HNO₃, 68%), ascorbic acid (AA), uric acid (UA) and sodium hydroxide were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. γ-globulins, myoglobin (Myb) and pepsin were purchased from Beijing BioDee Biotechnology Co. Ltd. Nafion per-fluorinated ion-exchange resin (Naf, 5 wt%) was from Aldrich-Sigma and was diluted to 1% in pure ethanol. Carbon nanofibers were purchased from Top Vendor Science & Technology Co., Ltd, Beijing. Ethylenediamine anhydrous (EDA) and NH₄HCO₃ was bought from Shanghai Lingfeng Chemical Reagent Co., Ltd. Bovine insulin (5800, >27 USP units per mg) was from Sigma. The stock solutions of insulin (1.0 mM) were prepared daily by dissolving powdered insulin in 0.01 M HCl. Insulin solutions were prepared by diluting aliquots of the insulin stock solutions with a background electrolyte solution. Unless otherwise stated, reagents were of analytical grade and used as received. Aqueous solutions were prepared with double-distilled water from a Millipore system.

2.2 Materials preparation

For a typical preparation of the CNFs-NiO nanocomposite, CNFs were firstly functionalized by using a mixture of concentrated H₂SO₄ and HNO₃ (3 : 1) under ultrasonic treatment at 50 °C for 8 h. After centrifugation from the mixture, the sediment was washed with doubly distilled water until pH reaches 7.0, and dried overnight at 60 °C in air. The acid-treated CNFs were denoted as A-CNFs. Then 20 mg A-CNFs and 5 mL of EDA were stirred vigorously in a beaker for 40 min, then added 20 mL of deionized water, stirred for 24 h as illustrated in Fig. 1. The solution was then centrifuged, washed with deionized water twice, dried at 60 °C and was collected as EDA-CNFs.

Fig. 1 Illustration of synthesis of EDA-CNFs-NiO nanocomposite.

The EDA-CNFs-NiO nanocomposites were synthesized as illustrated in Fig. 1. Firstly, 10 mg EDA-CNFs were dispersed into 10 mL of deionized water under ultrasonic treatment for 30 min. While the solution was stirred, 1.5 mL of NiSO₄·6H₂O (0.05 M) aqueous solution was added into the above suspention dropwise. After 5 min, the pH value of the solution was adjusted to 7.5 by adding NH₄HCO₃ (1 M) solution. The reaction was kept for 20 h. The resulting dark precipitate was purified by centrifugation, washed with deionized water and anhydrous ethanol for 3 times, respectively, and dried at 60 °C. Then the hybrid samples were obtained by pyrolysis of the precursor at 400 °C for 4 h in an argon atmosphere. For comparison, A-CNFs-NiO and pure NiO were also synthesized by the similar process except with A-CNFs as the supports and without CNFs as the support.

2.3 Characterization

Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) were obtained using Hitachi S-4800 field emission scanning electron microscope (operated at 5 kV and 15 kV, respectively). Transmission electron microscopy (TEM) images and the selected area electron diffraction (SAED) pattern were obtained on a JEOL JEM-200CX high resolution transmission electron microscope, employing an accelerating voltage of 200 kV. The X-ray powder diffraction pattern of the product was measured on a Shimadzu XRD-6000 X-ray diffractometer equipped with Cu-Kα radiation (λ = 0.154060 nm) with a scanning rate of 0.02° s⁻¹ and 2θ range from 10° to 80°. Electrochemical experiments were performed with CHI 660C electrochemical analyzer (ChenHua Instruments Co. Ltd., Shanghai, China) with a conventional three-electrode cell. A saturated calomel electrode (SCE) and a platinum electrode were used as the reference and the auxiliary electrode, respectively. Working electrodes were bare or modified glassy carbon (GC) electrodes (d = 3 mm). Experiments were carried out at room temperature.

2.4 Electrode modification

The dispersed nanomaterials on the electrode were fabricated by the following ways. Firstly, the glass carbon electrode (GCE, d = 3 mm) was cleaned according to the literature.²⁸ Then, a total of 1 mg EDA-CNFs-NiO nanocomposites was dispersed into 1 mL of N,N-dimethylformamide (DMF) to give homogeneous dispersions under mild ultrasonication. Finally, 10 μL of the mixture was dropped onto the prepared electrode surface and allowed to dry in ambient air for 24 h. The same process was applied to prepare the A-CNFs-NiO nanocomposites and NiO nanoparticles modified electrodes. These resultant electrodes were noted as EDA-CNFs-NiO/GCE, A-CNFs-NiO/GCE, and NiO/GCE, respectively.

3. Results and discussion

3.1 Structure and morphology characterization

The morphologies and microstructures of as-prepared A-CNFs, A-CNFs-NiO, EDA-CNFs-NiO and pure NiO were characterized by SEM and TEM measurements. The TEM image of A-CNFs is shown in Fig. 2A. It can be seen that the resulting acid treated CNFs have the diameters between 150 and 250 nm. It is also clear that the nanofiber surface is smooth and no particles are loaded before deposition. The morphology is consistent with the SEM images in Fig. S1A and B.† In a further step, the A-CNFs-NiO conjugates were prepared by a facile on-spot pyrolysis of the precursor, which was developed by Jiang's group.¹⁴ As depicted in Fig. 2B, many nanoparticles grow on the surface of A-CNFs, but the size is not uniform. The morphology is also consistent with the SEM images in Fig. S1C and D.†

Fig. 2 TEM images of (A) A-CNFs and (B) deposit NiO nanoparticles on the A-CNFs.

The EDA modified CNFs were used as supports for the fabrication of EDA-CNFs-NiO, as illustrated in Fig. 1. The A-CNFs contain oxygen-containing groups, which facilitate the modification of CNFs by EDA. With the interaction of –NH₂ with Ni²⁺, anchoring of uniform NiO nanoparticles on the EDA-CNFs surface was easily achieved via on-spot pyrolysis process. Fig. 3A shows the TEM image of EDA-CNFs-NiO nanocomposites, which shows that the EDA modified CNFs are fairly well covered with the uniform NiO nanoparticles with an average size of ∼20 nm. SEM characterization also validated the uniform distribution of NiO nanoparticles on the EDA modified CNFs (Fig. S2A and B†). It can be seen that almost no free NiO nanoparticles were found in solution where homogeneous nucleation will take effect. The selected area electron diffraction (SAED) pattern of EDA-CNFs-NiO (the inset in Fig. 3A) shows two discernible rings indexed to (200), (220) planes of the tetragonal NiO phase (JCPDS 78-0429) and also a pair of strong arcs for CNFs matrix (002)^29,30 can be clearly detected. High-resolution TEM (HRTEM) reveals lattice fringes of NiO (200) planes and the other lattice fringes of the neighboring CNFs support (Fig. 3B).

Fig. 3 TEM (A) and HRTEM (B) image of deposit NiO nanoparticles on the EDA-CNFs. Inset shows the SAED pattern. (C) EDS spectrum and (D) XRD patterns of the hybrid. STEM and elemental mapping analysis of EDA-CNFs-NiO nanocomposites. (E) Typical STEM image. (F–H) Corresponding elemental mapping images of (F) C, (G) O, (H) Ni.

The above results are in good agreement with the mixed phases checked by energy-dispersive X-ray spectroscopy (EDS) (Fig. 3C). In the EDS profile of EDA-CNFs-NiO (Fig. 3C), the clear peaks of C, Ni, O confirmed the existence of NiO and CNFs. The powder X-ray diffraction (XRD) pattern further confirmed the formation of NiO in the hybrids (Fig. 3D). The intensive diffraction peaks at 25.5° and 43.4° of CNFs (curve a) can be assigned to the diffractions of the (002) and (100) plane of typical graphite layers in CNFs.^30,31 After deposited with NiO (curve b), the intensity of diffraction peaks at 25.5° and 43.4° of CNFs decreased. At the same time, other new peaks that are similar to those of pure NiO appeared. Fig. 3D (curve c) shows the XRD pattern of pure NiO prepared without EDA-CNFs. All peaks are in good agreement with the standard profiles of NiO (JCPDS card no. 78-0429). Scanning TEM (STEM) and elemental mapping analysis of EDA-CNFs-NiO also suggested the presence of C, O and Ni components in the nanocomposites (Fig. 3E–H). It is notable that the elemental mapping patterns confirm that these NiO nanoparticles are dispersedly loaded on the whole surface of the EDA-CNFs. In contrast, the exact same synthesis strategy produced aggregated NiO nanoparticles with a diameter of 100–400 nm in the absence of EDA-CNFs as the supports (Fig. S2C and D†). These particles aggregated with each other. It also should be noted that the particle size of NiO grown on EDA-CNFs was much smaller than those of pure NiO and NiO grown on A-CNFs prepared by the same method. Such morphological difference highlights the important role of EDA-CNFs as a useful support for mediating the uniform growth of other functional nanomaterials.

Because the formation of the amine-terminated CNFs is a critical step in the preparation of nanocomposites, the surface bonding of the EDA to the CNFs was characterized using X-ray photoelectron spectroscopy (XPS). Fig. 4A displays the carbon XPS spectra of the A-CNFs and EDA-CNFs. A-CNFs (curve a) exhibited two peaks at 284.6 and 288.6 eV, which are the characteristics of C–C and COOH species,³² respectively. The peak at 288.6 eV confirmed the presence of carboxylic acid group on the surface of A-CNFs. For the EDA functionalized CNFs (curve b), besides the main C 1s peak at 284.6 eV, the XPS results show a shoulder binding energy peak at 286.5 eV, 1.9 eV higher than the main C 1s peak, which is assigned to an amide group.³² In addition, the apparent decrease of the peak around 288.6 eV confirms reduction of the carboxylic acid group as it is converted to an amide, due to electron donation from the adjacent N atom.^32,33 Fig. 4B shows the nitrogen XPS of A-CNFs and EDA-CNFs. The A-CNFs have a small nitrogen peak at 399.6 eV (curve a), which arises from nitrogen-containing species in the bulk nanofibers. Furthermore, a much higher N 1s binding energy at 399.5 eV was observed on the EDA-CNFs (curve b), which is essentially the value between 399–401 eV that has been reported for a wide range compounds containing primary and secondary amines.^34,35 These XPS data confirm the conversion of the carboxylic acid to the desired amide functionalization of CNFs.

Fig. 4 XPS measurements of C 1s (A) and N 1s (B) from the A-CNFs (a), and EDA-CNFs (b).

3.2 Electrocatalytic oxidation and amperometric detection of insulin at the EDA-CNFs-NiO modified electrode

To assess the catalytic properties of the EDA-CNFs-NiO nanocomposites for electrochemical oxidation of insulin, the film of EDA-CNFs-NiO nanocomposites was prepared onto glassy carbon electrodes for cyclic voltammetry (CV) in 0.1 M NaOH. A typical example of cyclic voltammograms obtained during continuous potential cycling between 0.15 and 0.65 V vs. SCE in 0.1 M NaOH is shown in Fig. S3.† In the early stages of potential cycling, a large anodic current was observed, which is ascribed to the formation of Ni(OH)₂.³⁶ As a characteristic of conducting film formation, the cathodic and anodic waves grew with the number of scans up to the 100th cycle and then a current plateau and stable voltammetric response reached. In these voltammograms, a pair of redox peaks is observed. Previous studies^37,38 have demonstrated that the couple of peaks are due to the conversion between Ni(II) and Ni(III) on the EDA-CNFs-NiO modified electrode surface:

Ni(OH)₂ + OH⁻ ↔ NiO(OH) + H₂O + e⁻ (1)

In order to understand the charge transport characteristics of the EDA-CNFs-NiO nanocomposite film, we recorded CV curves of the EDA-CNFs-NiO/GCE at various scan rates (Fig. S4 in the ESI†). The anodic response shifts positively with the increase of scan rate, while the cathodic peak moves negatively. Additionally, with the increase of scan rate, the anodic and cathodic currents increased linearly with the square root of scan rate, indicative of a diffusion-controlled process on the EDA-CNFs-NiO surface.

Since the formation of NiO(OH) is important for providing catalytic properties to the redox reactions,^16–20 the EDA-CNFs-NiO nanocomposite modified electrodes were activated by 100 cycles of potential scan between 0.15 and 0.65 V vs. SCE at 50 mV s⁻¹ in 0.1 M NaOH, prior to electroanalytical use. After being pretreated, the electrochemical activity of the EDA-CNFs-NiO nanocomposite toward the insulin oxidation was studied by CVs. Fig. 5A shows the recorded CVs of EDA-CNFs-NiO modified electrode in the absence and presence of insulin. An anodic peak at +0.430 V and a cathodic peak at +0.342 V corresponding to the Ni(III)/Ni(II) couple appear in the absence of insulin, with a low peak potential separation (ΔE_p) of 88 mV. When insulin was added, enhancement of the oxidation peak current and anodic shift of the peak potential were observed. The cathodic peak current decreases slightly during the reverse scan, indicative of an irreversible electrochemical oxidation process.³⁸ This enhancement anodic current is dependent on the insulin concentration (Fig. 5A, curves b and c). The increase of oxidation current is attributed to the production of Ni(OH)₂ through the reaction between NiO(OH) and insulin, then produced Ni(OH)₂ is further oxidized to NiO(OH) at the electrode surface.^38,39 The decrease of the cathodic peak is probably related to the involvement of oxidized form of insulin. As indicated in literature,³⁹ the irreversible catalytic oxidation reaction can be expressed as follows:

NiO(OH) + insulin (reduced form) → Ni(OH)₂ + insulin (oxidized form) (2)

Fig. 5 (A) Cyclic voltammograms of EDA-CNFs-NiO modified GCE placed in 0.1 M NaOH solution with 0, 10 and 20 μM of insulin at the scan rate of 0.05 V s⁻¹: (curve a, b and c). (B) Electrochemical impedance plots of EDA-CNFs-NiO/GCE in the absence (a) and in the presence of 1 (b), 2 (c), 4 (d) μM of insulin in 0.1 M NaOH solution. Inset: equivalent circuit used for data fitting. (C) Amperometric responses of the EDA-CNFs-NiO/GCE upon the successive addition of insulin into gently stirred 0.1 M NaOH at 0.45 V. Inset: the linear relationships between the catalytic current and the concentration. (D) Cyclic voltammograms of EDA-CNFs-NiO/GCE in 0.1 M NaOH solution with 20 μM insulin at the scan rate of 0.05 V s⁻¹. The potential was continuously cycled between 0.15 and 0.60 V vs. SCE reference electrode.

For comparison, similar measurements for pure NiO nanoparticles and A-CNFs-NiO nanocomposites were also performed. These phenomena were also observed on the A-CNFs-NiO/GCE and NiO/GCE in the absence and presence of 10 μM insulin (Fig. S5 in the ESI†). At the surface of the NiO/GCE, the anodic wave for insulin oxidation starts at 0.44 V and anodic peak potential is ∼0.50 V, having ΔI_p = 11.6 μA. At the surface of the A-CNFs-NiO/GCE, the anodic wave for insulin oxidation has the similar catalytic ability with NiO/GCE, but ΔI_p = 9.5 μA, which is smaller than that of EDA-CNFs-NiO/GCE (ΔI_p = 14.8 μA). These results indicated that the EDA-CNFs-NiO electrode exhibited decreased overpotential and enhanced peak current for insulin oxidation when compared with the A-CNFs-NiO and NiO modified electrodes.

Electrochemical impedance spectroscopy (EIS) analysis was further used to discover the catalytic procedures of insulin electro-oxidation on the EDA-CNFs-NiO modified GC electrode. Fig. 5B shows the Nyquist plots of the EDA-CNFs-NiO/GCE recorded at 0.45 V in the frequency range from 0.1 Hz to 1 MHz for some selected concentrations of insulin in 0.1 M NaOH. In the absence of insulin, a depressed semicircle was observed. With the insulin concentrations increasing from 1–4 μM, a steady decrease in the diameter of the depressed semicircle was observed. An equivalent circuit compatible with the results is designed (the inset of Fig. 5B). In this electrical equivalent circuit, R_s is solution resistance. The depressed semicircle is related to the combination of a constant phase element corresponding to the double layer capacitance (Q_dl) and charge transfer resistance (R_ct), which indicates how fast charge transfer occurs during insulin electrocatalytical oxidation on the electrode surface. In this equivalent circuit, the double layer capacitance Q_dl is a constant phase element, while the charge transfer resistant R_ct decreases with insulin concentration increasing. This phenomenon is similar with the EIS of the porous Ni electrode in 0.1 M NaOH containing different concentrations of glucose.³⁸ This may be attributed to the similar electrocatalytical mechanism associated with the Ni(III)/Ni(II) redox couple in the presence of target analytes (glucose or insulin). They react with NiO(OH) and produce Ni(OH)₂ on the electrode surface, leading to the target analytes concentration dependency on charge transfer R_ct.

Amperometry under stirred conditions with amperometric detection was employed for sensitive detection of insulin. According to the potential dependence of the insulin electrocatalytic oxidation current, the optimum electrode potential was chosen at 0.45 V in order to obtain constant and high sensitivity. Fig. 5C shows a typical amperometric response upon successive addition of insulin into constantly stirred NaOH solutions. Well-defined amperometric currents increasing stepwise with the level of insulin were obtained with a response time <3 s, suggesting efficient catalytic ability of the EDA-CNFs-NiO/GCE for insulin electro-oxidation. It was found that the current response is linear with the insulin concentrations from 20 nM to 1020 nM, with a correlation coefficient of 0.9989. The limit of detection was calculated to be as low as 12.1 nM. The sensitivity of the insulin sensor is 1.55 μA μM⁻¹. It is worth mentioning that the limit of detection, linear calibration range and sensitivity for insulin determination at EDA-CNFs-NiO/GCE are comparable or even better than those obtained by other kinds of modified electrodes.^9–12,40,41 Since the protein fouling of electrode surfaces represents a significant problem in diagnostic system,⁴² the antifouling property and stability of electrocatalytic activity of EDA-CNFs-NiO/GCE for oxidation of insulin were examined by repetitive scanning at the scan rate of 50 mV s⁻¹. As shown in Fig. 5D, the electrocatalytic currents are constant under different scan numbers and the currents can remain about 99.2% of the initial current value after 100 cycle numbers, indicating the long-term stability and antifouling property of the built electrode for insulin electrooxidation.

Several kinds of interfering compounds that exist in biological liquids were investigated for the determination of insulin. These interferences include ascorbic acid (AA), uric acid (UA), paracetamol (PCM), γ-globulins, myoglobin (Myb) and pepsin, which were compared to that of 0.24 μM insulin at the potential of 0.45 V (Fig. S6†). It can be seen that the addition of γ-globulins (75 μM), Myb (6.0 μM) and pepsin (2.9 μM) brought out hardly discernible current response, whereas obvious catalytic currents were observed for 0.24 μM insulin (Fig. S6,† red line). It can also be observed that the EDA-CNFs-NiO modified GC electrode (EDA-CNFs-NiO/GCE) has very strong electrocatalytic activity to 0.1 mM AA, UA and PCM. This result indicates that the nanocomposite film cannot completely block the diffusion of ascorbate, urate and paracetamol into the composite films.

A common method used for diminution or elimination of interferences is to cover the electrode surface with an additional membrane for blocking the diffusion of interferences into the sensor catalytic layer. Nafion (Naf) contains negatively charged sulfonate groups, which can effectively restrict the anionic interferences from permeating into the electrode surface. It has been confirmed that the Nafion coating prevents interference of anionic substances such as ascorbic acid and uric acid, and decreases acetaminophen interference.^43–45

Herein, we cover the surface of EDA-CNFs-NiO/GCE with a layer of 5 μL Nafion (1 wt%) and then the electrode (Naf/EDA-CNFs-NiO/GCE) was treated in alkaline solution as explained before and tested in similar conditions as in previous experiments. As show in Fig. S6† (black line), the response to 0.24 μM insulin is reduced, which has ∼8% diminution of the sensitivity for insulin. In contrast, the response to ascorbic acid at the Nafion-covered electrode is significantly diminished (∼54%). A similar phenomenon can also be observed for uric acid (∼75%) and paracetamol (∼38%). These results indicate that the Nafion membrane-covered electrodes can efficiently reduce the interference of ascorbic acid, uric acid and paracetamol.

4. Conclusions

The assaying of insulin has important implications in the doping control of human athletics, diabetes diagnosis, adequate and accurate amount of insulin delivery. Herein, we have designed and synthesized uniform NiO nanoparticles decorated CNFs with the assistance of EDA via a simple on-spot pyrolysis strategy for sensitive and durable detection of insulin. The obtained EDA-CNFs-NiO nanocomposite possesses a well-defined structure with the uniform nanoparticles attached to CNFs closely. Compared to the pure NiO nanoparticles and the A-CNFs-NiO nanocomposite, the EDA-CNFs-NiO modified GC electrode exhibited better electrocatalytic ability toward insulin oxidation without using any specific reagent or electron-transfer mediator. It can also minimize surface fouling effect of insulin, demonstrating long-term stability and high sensibility for insulin determination. This work not only provides a facile strategy to prepare a highly efficient electrocatalyst for detection of insulin that potentially improves the management of diabetes, but also is easily expanded to prepare other carbon materials combining inexpensive, earth-abundant metal oxide for practical applications in catalysis, sensors and energy conversion technologies.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21001004, 20975001) for the financial supports.

References

1. R. Mo, T. Jiang, J. Di, W. Tai and Z. Gu, Chem. Soc. Rev., 2014, 43, 3595.
2. O. Veiseh, B. C. Tang, K. A. Whitehead, D. G. Anderson and R. Langer, Nat. Rev., 2015, 14, 45.
3. M. Stumvoll, B. J. Goldstein and T. W. Van Haeften, Lancet, 2005, 365, 1333.
4. N. Sarwar, P. Gao, S. R. K. Seshasai, R. Gobin, S. Kaptoge and E. D. Angelantonio, Lancet, 2010, 375, 2215.
5. L. Vu, W. F. Pralong, F. Cerini, A. Gjinovci, R. Stocklin, K. Rose, R. E. Offord and A. D. Kippen, Anal. Biochem., 1998, 262, 17.
6. M. G. Roper, J. G. Shackman, G. M. Dahlgren and R. T. Kennedy, Anal. Chem., 2003, 75, 4711.
7. L. Mercolini, A. Musenga, B. Saladini, F. Bigucci, B. Luppi, V. Zecchi and M. A. Raggi, J. Pharm. Biomed., 2008, 48, 1303.
8. K. Ortner, W. Buchberger and M. Himmelsbach, J. Chromatogr. A, 2009, 1216, 2953.
9. R. T. Kennedy, L. Hung, M. A. Atkinson and P. Dush, Anal. Chem., 1993, 65, 1882.
10. J. Wang and X. Zhang, Anal. Chem., 2001, 73, 844.
11. A. Salimi, S. Pourbeyram and H. Haddadzadeh, J. Electroanal. Chem., 2003, 542, 39.
12. M. Pikulski and W. Gorski, Anal. Chem., 2000, 72, 2696.
13. M. R. Gao, Y. F. Xu, J. Jiang, Y. R. Zheng and S. H. Yu, J. Am. Chem. Soc., 2012, 134, 2930.
14. B. Zhao, J. Song, P. Liu, W. Xu, T. Fang, Z. Jiao, H. Zhang and Y. Jiang, J. Mater. Chem., 2011, 21, 18792.
15. Z. S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng and K. Müllen, J. Am. Chem. Soc., 2012, 134, 9082.
16. R. A. Soomro, Z. H. Ibupoto, Sirajuddin, M. I. Abro and M. Willander, Sens. Actuators, B, 2015, 209, 966.
17. D. Xie, W. Yuan, Z. Dong, Q. Su, J. Zhang and G. Du, Electrochim. Acta, 2013, 92, 87.
18. S. Liu, B. Yu and T. Zhang, Electrochim. Acta, 2013, 102, 104.
19. P. Liao and E. A. Carter, Chem. Soc. Rev., 2013, 42, 2401.
20. V. O. Williams, E. J. DeMarco, M. J. Katz, J. A. Libera, S. C. Riha, D. W. Kim, J. R. Avila, A. B. F. Martinson, J. W. Elam, M. J. Pellin, O. K. Farha and J. T. Hupp, ACS Appl. Mater. Interfaces, 2014, 6, 12290.
21. M. Mazloumi, S. Shadmehr, Y. Rangom, L. F. Nazar and X. Tang, ACS Nano, 2013, 7, 4281.
22. Y. X. Zhang, X. Guo, X. Zhai, Y. M. Yan and K. N. Sun, J. Mater. Chem. A, 2015, 3, 1761.
23. C. Xu, M. Lu, Y. Zhan and J. Y. Lee, RSC Adv., 2014, 4, 25089.
24. L. Ji, A. J. Medford and X. Zhang, J. Mater. Chem., 2009, 19, 5593.
25. M. K. Van Der Lee, A. J. Van Dillen, J. H. Bitter and K. P. De Jong, J. Am. Chem. Soc., 2005, 127, 13573.
26. M. L. Toebes, Y. Zhang, J. Hajek, T. A. Nijhuis, J. H. Bitter, A. J. Van Dillen, D. Y. Murzin, D. C. Koningsberger and K. P. De Jong, J. Catal., 2004, 226, 215.
27. C. Y. Ma, Z. Mu, J. J. Li, Y. G. Jin, J. Cheng, G. Q. Lu, Z. P. Hao and S. Z. Qiao, J. Am. Chem. Soc., 2010, 132, 2608.
28. L. Zhang, Y. H. Ni and H. Li, Microchim. Acta, 2010, 171, 103.
29. T. Kyotani, L. F. Tsai and A. Tomita, Chem. Mater., 1996, 8, 2109.
30. T. Luo, J. Liu, L. Chen, S. Zeng and Y. Qian, Carbon, 2005, 43, 755.
31. H. Yang, Y. Yan, Y. Liu, F. Zhang, R. Zhang, Y. Meng, M. Li, S. Xie, B. Tu and D. Zhao, J. Phys. Chem. B, 2004, 108, 17320.
32. S. E. Baker, W. Cai, T. L. Lasseter, K. P. Weidkamp and R. J. Hamers, Nano Lett., 2002, 2, 1413.
33. T. Ramanathan, F. T. Fisher, R. S. Ruoff and L. C. Brinson, Chem. Mater., 2005, 17, 1290.
34. H. Jiang, F. Chen, M. G. Lagally and F. S. Denes, Langmuir, 2010, 26, 1991.
35. Z. Lin, T. Strother, W. Cai, X. Cao, L. M. Smith and R. J. Hamers, Langmuir, 2002, 18, 788.
36. A. Salimi, M. Roushani, S. Soltanian and R. Hallaj, Anal. Chem., 2007, 79, 7431.
37. S. J. Yu, X. Peng, G. Z. Cao, M. Zhou, L. Qiao, J. Y. Yao and H. C. He, Electrochim. Acta, 2012, 76, 512.
38. X. Niu, M. Lan, H. Zhao and C. Chen, Anal. Chem., 2013, 85, 3561.
39. M. Asgari, M. G. Maragheh, R. Davarkhah and E. Lohrasbi, J. Electrochem. Soc., 2011, 158, K225.
40. M. Zhang, C. Mullens and W. Gorski, Anal. Chem., 2005, 77, 6396.
41. B. Rafiee and A. R. Fakhari, Biosens. Bioelectron., 2013, 46, 130.
42. P. Asuri, S. S. Karajanagi, R. S. Kane and J. S. Dordick, Small, 2007, 3, 50.
43. L. Zhang, S. Yuan, L. Yang, Z. Fang and G. C. Zhao, Microchim. Acta, 2013, 180, 627.
44. A. Arvinte, A. C. Westermann, A. M. Sesay and V. Virtanen, Sens. Actuators, B, 2010, 150, 756.
45. R. Vaidya, P. Atanasov and E. Wilkins, Med. Eng. Phys., 1995, 17, 416.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03306c

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