Preparation of spinel nickel-cobalt oxide

Electrochimica Acta 220 (2016) 545–553

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Preparation of spinel nickel-cobalt oxide nanowrinkles/reduced graphene oxide hybrid for nonenzymatic glucose detection at physiological level Guangran Ma, Min Yang, Chenyi Li, Haiyan Tan, Liang Deng, Shi Xie, Fugang Xu* , Li Wang* , Yonghai Song* Key Laboratory of Functional Small Organic Molecule, Ministry of Education, College of Chemistry and Chemical Engineering, Jiangxi Normal University, 99 Ziyang Road, Nanchang 330022, China

A R T I C L E I N F O

Article history: Received 10 August 2016 Received in revised form 23 October 2016 Accepted 24 October 2016 Available online 24 October 2016 Keywords: spinel nanocrystal nickel cobalt oxide glucose electrochemical sensor electrocatalysis

A B S T R A C T

Nickel-cobalt oxide nanowrinkles with spinel-type crystal structure supported on reduced graphene oxide (NiCo2O4 NWs-rGO) was prepared to develop a sensitive and stable nonenzymatic glucose sensor. The NiCo2O4 NWs-rGO hybrid were prepared by a facile one-pot hydrothermal reaction, and sequential calcination in air. The morphology, composition and crystal structure of the NiCo2O4 NWs-rGO hybrid were characterized by scanning electron microscope, transmission electron microscope, selected area electron diffraction, and energy-dispersive spectroscopy. The electrochemical behavior of the hybrid and its catalytic activity towards glucose oxidation were investigated by several electrochemical methods. Compared with single component NiO or Co3O4, spinel type NiCo2O4 NWs displayed higher catalysis towards glucose oxidation. Further integration of NiCo2O4 with graphene could reduce the overpotential and enhance the catalytic current due to the improved conductivity and dispersity of NiCo2O4. The NiCo2O4 NWs-rGO based glucose sensor showed a wide linear range of 0.005-8.6 mM, a low detection limit of 2 mM (S/N = 3), and an improved stability. A satisfactory recovery was also obtained for glucose detection in human serum at physiological level. Our results indicate rationally combine spinel type mixed metal oxide with graphene is a good alternative to fabricate advanced metal oxide based electrochemical sensors. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Electroactive nanocrystals of metal oxide have received considerable attentions in nonenzymatic electrochemical sensors and devices due to their easy preparation, high stability, low cost, high abundance and comparable catalysis with some noble metal materials [1,2]. However, most metal oxide has low conductivity, which seriously degrades the performance of electrochemical sensors [1,2]. Therefore, an effective approach to enhance the conductivity of metal oxide nanocrystals while maintain or even elevate their electrocatalytic activity is in great demand for developing advanced electrochemical sensors with low cost and high performance.

* Corresponding authors. Tel.: +86 791 88120861; fax: +86 791 88120861. E-mail addresses: [email protected] (F. Xu), [email protected] (L. Wang), [email protected] (Y. Song). http://dx.doi.org/10.1016/j.electacta.2016.10.163 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

Up to now, combining metal oxide with conducting materials is one of the most efficient and widely reported approach to enhance the performance of metal oxide based sensors [2–6]. Among various conducting materials, graphene has received considerable attention at present due to its large surface area, high conductivity and easy preparation [7]. Until now, many graphene-metal oxide hybrid or composites, such as Fe3O4-rGO [8,9], Co3O4-rGO [10], CuO-rGO [11], Cu2O-rGO [12,13], NiO-rGO [14,15] hybrids have been fabricated and applied in electrochemical sensors. These hybrids usually display much higher activity than that of corresponding single metal oxide component. However, two dimensional graphene or its hybrid with ordinary metal oxide nanoparticles tends to aggregate on electrode surface, and thus decreases the active surface area, and limits the further improvement of sensing performance. Besides integrating metal oxide with conducting materials, tailing the structure of metal oxide material itself is also an effective way to improve its conductivity, electrochemical activity and other properties. Recently, mixed transition metal oxides with

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a spinel structure have attracted an upsurge of interest in many electrochemical applications [16]. The ternary nickel cobaltite NiCo2O4, as one typical example is widely studied due to its low cost, easy preparation [17], and much higher electrical conductivity and electrochemical activity than that of NiO or CoO [18] due to coexistence of active Ni and Co species [19]. These properties make spinel type NiCo2O4 an attractive electrochemical energy material with promising applications in lithium battery and supercapacitors [16–19]. However, the applications of NiCo2O4 in electrochemical sensors are rarely reported. Recently, NiCo2O4 was explored to achieve the non-enzymatic detection of glucose [20,21] and H2O2 [22], but these sensors suffer from poor catalysis and narrow linearly responsive range [20]. For example, the up limit of linear range of NiCo2O4 based glucose sensor is 65 mM [20] or 4.7 mM [21], which does not fully cover the physiological level of 4–7 mM glucose in human blood [23]. Besides, low active surface area, poor dispersity and complex preparation of these NiCo2O4 based nanomaterials also hinder the practical application of these sensors. In this paper, a new hybrid composed of spinel type NiCo2O4 nanowrinkles grown on reduced graphene oxide (NiCo2O4 NWsrGO) was prepared and its applications in catalytic oxidation and amperometric detection of glucose were investigated in detail. The advantages of the NiCo2O4 NWs-rGO based sensor include: (1) The hybrid material could be prepared by a facile and easy accessible one-pot hydrothermal reaction combined with subsequent calcination at low cost and high yield. (2) The NiCo2O4 NWs-rGO displays large surface area, good conductivity and thus much enhanced catalysis towards glucose oxidation than that of NiO, Co3O4 or NiCo2O4; (3) Most importantly, NiCo2O4 NWs-rGO with high stability and wide linear response range (up limit extended to 8.56 mM) could be used to directly detect glucose at physiological level without tedious dilution treatment. The NiCo2O4 NWs-rGO based sensor also shows a satisfactory recovery for glucose detection in human serum. 2. Experimental 2.1. Materials and reagents Nickel nitrate (Ni(NO3)26H2O), cobalt nitrate (Co(NO3)26H2O), sodium hydroxide (NaOH), hexamethylenetetramine (HMT), potassium permanganate (KMnO4) trisodium citrate (Na3Cit2H2O), and glucose were purchased from Shanghai Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Uric acid (UA), ascorbic acid (AA), H2O2 solution (30 wt %), phosphate acid (H3PO4) and graphite (325 mesh) were supplied by aladdin (Shanghai, China). All chemicals used were analytical grade. Ultrapure water (18.2 MVcm) was used for preparation of buffer and standard solutions. 2.2. Preparation of NiCo2O4 NWs-rGO hybrid Graphene oxide (GO) was first synthesized based on a modified Hummer’s method [24]. For the synthesis of NiCo2O4 NWs-rGO hybrid, 5 mg GO was dispersed in 20 mL H2O by ultrasonication for 30 min. Then 0.116 g Ni(NO3)26H2O, 0.232 g Co(NO3)26H2O, 0.28 g HMT and 0.0588 g Na3Cit2H2O were added into the above solution. The mixture was sonicated for another 5 min, and then refluxed with rigorous stirring in an oil bath at 90 C for 6 h. After that the black precipitate was harvested by centrifugation and washed with DI water and ethanol for several times. Finally, the obtained product was freeze-dried overnight and further annealed at 300 C for 3 h in air with a heating rate of 1 C min1 to obtain the final product NiCo2O4 NWs-rGO. In order to demonstrate the advantages of NiCo2O4 NWs-rGO hybrid, other nanomaterials including single component NiO or

Co3O4, and unsupported spinel NiCo2O4 were also prepared by a similar approach, except that graphene oxide or certain metal salt was not added in corresponding synthesis. 2.3. Preparation of modified electrode To fabricate modified electrode, 0.5 mg NiCo2O4 NWs-rGO was dispersed into 1 ml H2O to form a suspension, then 5 mL of the suspension was coated on the polished glassy carbon electrode (GCE) and dried in air. Finally, 5 mL nafion solution (0.05%) was coated on it to get the NiCo2O4 NWs-rGO modified GCE referred as NiCo2O4 NWs-rGO/GCE. Other material modified GCE was shorten as NiO/GCE, Co3O4/GCE, or NiCo2O4/GCE, respectively. 2.4. Characterization The morphologies of the products were observed by a scanning electron microscope S-3400N (Hitachi, Japan) or transmission microscope JEM 2100 (JEOL, Japan). Electrochemical experiments were performed on a CHI 760D electrochemical workstation (CH Instrument, Shanghai) using a three electrode system with platinum wire as counter electrode, Hg/Hg2Cl2 (KCl saturated) electrode as reference electrode and NiCo2O4-rGO/GCE or other material modified GCE as working electrode. Freshly prepared sodium hydroxide solution (0.1 M) was used as the supporting electrolyte. Electro-chemical impedance spectroscopy (EIS) was performed with the same three-electrode configuration in an electrolyte solution of 0.1 M KCl containing 1 mM [Fe(CN)6]4/3, in a frequency range from 0.1 Hz to 1 MHz with an amplitude of 5 mV. All electrochemical studies were performed under ambient conditions. 2.5. Real sample analysis The human serum was from local hospital. These samples were treated by trichloroacetic acid before used for analysis [25]. Briefly, the serum of human blood was separated by centrifugation. Then, 150.0 mL trichloroacetic acid (TCA) was added to the 5 mL serum sample in order to denature the proteins. Subsequently, the samples were centrifuged at 5000.0 rpm for 10.0 min. An amount of 1.9 mL of the supernatant was mixed with 0.1 mL NaOH (2.0 M) to produce the serum sample for electrochemical analysis. 3. Results and discussion 3.1. Preparation and characterization of NiCo2O4 NWs-rGO hybrid Spinel type mixed valence of transition metal oxide have been widely used in electrochemical capacitors and lithium battery [16– 19]. Here, the hybrid of spinel type NiCo2O4 with rGO was prepared to explore its electrochemical sensing performance. Fig. 1a,b shows the SEM images of the NiCo2O4 NWs-rGO hybrid. The low magnified image shows lots of sheet-like products piled on the substrate (Fig. 1a). Highly magnified image indicates that the surfaces of these sheets are flocky with many wrinkles and heave ridges decorated on the surface (Fig. 1b). The size of these sheets ranges from several hundred nanometers to more than one micrometer, which is similar to the size distribution of GO, implying that growth of NiCo2O4 may be guided by GO as template. In the absence of GO, NiCo2O4 micro-nanostructures with nanowrinkles decorated on surface were also observed. However, these NiCo2O4 wrinkles are connected with each other to form a large aggregate (Fig. 1c,d). The size of wrinkles is larger than that on the NiCo2O4 NWs-rGO hybrid (Fig. 1d). Compared with NiCo2O4, NiCo2O4 NWs-rGO displays improved dispersity as these NiCo2O4 wrinkles are grown on GO sheet as template. The improved


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Fig. 1. SEM images of NiCo2O4 NWs-rGO hybrid (a,b), and NiCo2O4 nanostructure (c,d).

dispersity and the small size of NiCo2O4 wrinkles are beneficial to increase the surface area and elevate the catalysis of NiCo2O4. The single component material NiO or Co3O4 without rGO support was also prepared. As can be seen in Fig. S1, both NiO and Co3O4 display wrinkled surfaces, but they have a spherical morphology, which is different from the sheet-like outline of NiCo2O4 or NiCo2O4 NWsrGO. Besides, both products display serious aggregation, which could largely decease the surface area and catalytic active sites. Compared these results, it is demonstrated that the formation of spinel NiCo2O4 could alter the morphology of the product, and further introducing GO could function as template to improve the dispersity and reduce aggregation. The interaction between NiCo2O4 and rGO may bring advanced properties. The morphology and crystal structure of NiCo2O4 NWs-rGO were further characterized by TEM. Fig. 2a shows the NiCo2O4 NWs-rGO are semi-transparent, indicating the NiCo2O4 NWs-rGO sheet are very thin. The surface are fully covered with wrinkle or ridge-like nanostructures. Highly magnified image in Fig. 2b shows the sheet are composed of many small NiCo2O4 particles, which connected with each other and form a mesoporous sheet-like morphology. The HRTEM reveals a crystal lattice of 0.24 nm and 0.20 nm (Fig. 2c), corresponding to the (311) or (400) face of NiCo2O4 nanocrystal [26]. Fig. 2b,c also reveal that the wrinkles or ridges seem to be the junction area of different crystal faces. SAED pattern of NiCo2O4 NWs-rGO shows several concentric circles (Fig. 2d), indicating a polycrystal structure of the product. The diffraction circles corresponds to the (220), (311), (400), and (422) crystal faces [27], confirming the formation of cubic spinel type NiCo2O4. The element component of the NiCo2O4 was analyzed by EDS. Fig. 3 shows the hybrid is mainly composed of Ni, Co, C, and O, and the atomic ratio for Ni to Co is near to 1:2, which is consistent to the formula of NiCo2O4. The results from SEM, TEM, EDS clearly confirmed the formation of spinel type NiCo2O4 NWs-rGO hybrid.

3.2. Electrochemical behavior of modified electrode The NiCo2O4 NWs-rGO hybrid and other metal oxide material were used to modify GCE. Fig. 4 shows the CVs of different materials modified electrodes in 0.1 M NaOH solution. Co3O4/GCE displays an anodic peak at 0.55 V and a cathodic peak at 0.45 V, corresponding to the redox of Co2+/Co3+ transformation [28] (Fig. 4a). NiO/GCE displays a pair of weak peaks at 0.42 V and 0.34 V due to the redox of Ni species [29] (Fig. 4b). NiCo2O4/GCE shows two pairs of peaks (Fig. 4c), with one pair peaks at 0.45 V and 0.34 V, the other at 0.25 V and 0.18 V, corresponding to the reversible Faradaic reaction of Co2+/Co3+ and Ni2+/Ni3+, respectively [30,31]. The redox peak potentials for Co or Ni species on NiCo2O4/ GCE decrease compared to those on single metal oxide NiO/GCE or Co3O4/GCE. Besides, the redox currents of these Ni/Co species increase. These results are ascribed to the unique spinel structure of NiCo2O4, which facilitates the electron transfer and enhance the conductivity [18,19]. After introducing graphene oxide, the redox peaks for Ni species and Co species are immerged together [30,31], thus only a strong anodic peak at 0.38 V and a cathodic peak at 0.29 V are observed on NiCo2O4 NWs-rGO/GCE (Fig. 4d). The redox peak potential are higher than Ni2+/Ni3+, but lower than Co2+/Co3+. Moreover, the peak current and capacitance current are higher than those of NiCo2O4/GCE due to the increased conductivity and enlarged surface area of the composite as a consequence of introducing reduced graphene oxide. Thus a good catalysis is expected on NiCo2O4 NWs-rGO/GCE. The electrochemical behavior of the NiCo2O4 NWs-rGO/GCE was further investigated by cyclic voltammetry at different scan rate (Fig. S2). The redox peak currents increase as scan rate increases (Fig. S2a). The peak currents are proportional to the scan rate (Fig. S2b), indicating a surface controlled process. The NiCo2O4 NWs-rGO/GCE is tolerant to a high scan rate, indicating good conductivity and good charge propagation within the electrodes [32].

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Fig. 2. (a,b) TEM images of NiCo2O4 NWs-rGO, (c) HRTEM of NiCo2O4 NWs crystal; and (d) the SAED pattern of the NiCo2O4 NWs.

3.3. Electrocatalytic oxidation of glucose on NiCo2O4 NWs-rGO/GCE Previously, many transitional metal oxide nanomaterials have been explored to fabricate electrochemical sensors due to their good catalysis towards some substances. Here, the electrocatalytic activity of materials prepared in this study towards the glucose oxidation were evaluated as an example. The results are presented in Fig. 5. After introducing glucose into the solution, the oxidation peak currents on these modified electrode increase, while the reduction peak currents decrease, indicating a typical catalytic behavior. From single component metal oxide Co3O4 (Fig. 5a) or NiO (Fig. 5b) to spinel metal oxide NiCo2O4 (Fig. 5c), the oxidation potential of glucose decreases while oxidation peak current

increases, demonstrating the spinel structure is good for elevating its catalysis. For NiCo2O4 NWs-rGO/GCE (Fig. 5d), the oxidation potential further decreases to about 0.4 V, and the oxidation current increases, confirming the vital role of rGO in enhancing its catalytic performance. As the scan rate increases, the oxidation peak current of glucose on this NiCo2O4 NWs-rGO/GCE also steadily grows (Fig. 6a). The derived plot reveals that the peak current is proportional to the square root of scanning rate (Fig. 6b), implying glucose oxidation on the modified electrode is a diffusion controlled process. From these cyclic voltammetry tests, it can be seen NiCo2O4 NW-rGO displays the highest catalysis towards glucose oxidation. The unique structure of NiCo2O4 NWs-rGO may be accounted for

Fig. 3. EDS result of NiCo2O4 NWs-rGO hybrid and its element percentage list.


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Fig. 4. CVs of different materials modified electrodes in 0.1 M NaOH, (a) Co3O4/GCE, (b) NiO/GCE, (c) NiCo2O4/GCE, and (d) NiCo2O4 NWs-rGO/GCE. Scan rate: 100 mV/s.

its enhanced catalysis. First, rGO as support changes the product morphology, decreases the crystal size and obviously reduces the aggregation, while decorated NiCo2O4 wrinkles prevent rGO sheet from aggregation. These factors lead to a larger surface area to

interact with glucose. Second, spinel structure of NiCo2O4 itself has more redox centers and higher electrochemical activity than single component metal oxide due to the combination of two transitional metal ions as stated in previous reports [18,19]. Thus an enhanced

Fig. 5. CVs obtained on different materials modified electrodes in 0.1 M NaOH containing glucose of 1–4 mM, (a) Co3O4/GCE, (b) NiO/GCE, (c) NiCo2O4/GCE, and (d) NiCo2O4 NWs-rGO/GCE. Scan rate: 100 mV/s.

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Fig. 6. Cyclic voltammetry curves obtained on NiCo2O4 NWs-rGO/GCE in 0.1 M NaOH containing glucose of 4 mM at different scan rate, and the derived plot of the current response versus the square root of scan rate.

voltammograms of NiCo2O4 are also frequently studied in the area of supercapacitor. These results show cyclic voltammetric scanning of NiCo2O4 in basic electrolyte could also produce these active NiOOH, CoOOH and CoO2 species [34]. Based on these information, the catalytic mechanism of NiCo2O4-rGO towards glucose oxidation can be described as following equations [21,35]:

catalysis is observed on NiCo2O4. Third, both the spinel type crystal and rGO support contribute to an increased conductivity of the hybrid. The EIS curve of Co3O4/GCE or NiO/GCE displays a large semi-circle, indicating a high electron transfer resistance (Fig. 7a, b). The EIS curve of NiCo2O4/GCE also displays a semi-circle pattern but with a much smaller diameter (Fig. 7c), confirming that spinel type NiCo2O4 has much higher conductivity than single component NiO or Co3O4. For NiCo2O4 NWs-rGO/GCE, no semi-circle pattern but a nearly straight line is observed (Fig. 7d), implying this material has a much smaller electron transfer resistance compared with other materials or even smaller than bare GCE (Fig. S3). All these factors, the increase active surface area, plenty of redox active sites, and enhanced conductivity, lead to the enhanced catalysis of NiCo2O4 NWs-rGO for glucose oxidation. Previously, the catalysis mechanism of NiO and Co3O4 towards glucose oxidation has been widely reported. It is revealed active intermediate such as NiOOH, CoOOH or CoO2 is responsible for the catalysis of these metal oxide [33]. Recently, the cyclic

50000

(1)

CoOOH + OH $ CoO2 + H2O + e

(2)

Ni(III) + Co (IV) + glucose $ Ni (II) + Co (III) + gluconolactone

(3)

The supporting matrix graphene may amplify the catalysis of NiCo2O4 by increase the active surface area and enhance the conductivity, although graphene is not an active catalyst for glucose oxidation.

50000

Co3O4

a

NiCo2O4 + OH + H2O !NiOOH + 2CoOOH + e

40000

NiO

b

-Z"/ohm

-Z"/ohm

40000 30000

30000

20000

20000

10000

10000 0

0 0

600

10000 20000 30000 40000 50000

0

Z'/ohm

c

10000 20000 30000 40000 50000

Z'/ohm

NiCo2O4

200

450

d

NiCo2O4 NWs-rGO

-Z'' / ohm

-Z'' / ohm

150

300

100

150

50 0

0 200

400

600

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Z' / ohm

1000

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120

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Z' /ohm

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Fig. 7. EIS spectrum of different materials modified electrodes in 5 mM K3Fe(CN)6-K4Fe(CN)6 containing 0.1 M KCl. (a) Co3O4/GCE, (b) NiO/GCE, (c) NiCo2O4/GCE, and (d) NiCo2O4 NWs-rGO/GCE.


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Fig. 8. (a) Amperometric response of NiCo2O4 NWs-rGO/GCE upon successive injection of glucose in 0.1 M NaOH solution, and (b) the corresponding calibration curve of current versus concentration of glucose. Working potential: 0.42 V.

3.4. Amperometric detection of glucose The high catalysis of NiCo2O4 NWs-rGO towards glucose oxidation implies that the hybrid can be used to develop a NiCo2O4 NWs-rGO based nonenzymatic glucose sensor. Here, these single component metal oxides NiO, Co3O4, or NiCo2O4 without graphene support prepared in our experiment were also used to construct glucose sensors for comparison. The results show Co3O4/GCE (Fig. S4a) or NiO (Fig. S4c) displays fast current response upon glucose injection. The linear response range is 5 mM to 1.76 mM for Co3O4/GCE (Fig. S4b) and 5 mM to 2.16 mM for NiO/ GCE (Fig. S4d), respectively. Fig. S4e,f show the NiCo2O4/GCE displays a wider linear range of 5 mM to 2.16 mM and higher sensitivity for glucose sensing than those of NiO or Co3O4 based sensors. For NiCo2O4 NWs-rGO/GCE (Fig. 8), a much higher and faster current response is observed (Fig. 8a). The current response exhibits a linear dependence on the concentration of glucose in the range of 5 mM to 8.56 mM (Fig. 8b). The sensitivity is 38.75 mA/mM, and the detection limit is 2 mM. Compared with NiO, Co3O4 or NiCo2O4, an obvious improvement on this sensor is the wide linear responsive range, which encompasses the normal glucose level (4– 7 mM) in human blood. In contrast, many previously reported glucose sensors based on NiO [36–40], Co3O4 [41–46], and NiCo2O4 [20,47] only display linear range with up limit of 1 mM–4 mM or even lower as shown in Table 1. The extended responsive range on NiCo2O4 NWs-rGO/GCE may be ascribed to the enhanced catalysis and improved stability of the hybrid compared with the single component NiO, Co3O4 or unsupported NiCo2O4. The synergistic interaction between NiCo2O4 and rGO brings enhanced catalysis, which could efficiently catalyze the oxidation of glucose and thus reduce the electrode fouling. The introduction of rGO as support could partially reduce the micro-cracking and disintegration

associated with the formation of the NiOOH/Ni(OH)2 or CoOOH/ CoO2 redox couple [39], and thus increases the stability of NiCo2O4 NWs-rGO/GCE. 3.5. Interference studies, reproducibility and stability The anti-interference ability is also important for a sensor. For glucose sensor, ascorbic acid, uric acid, or dopamine are often used as potential interference as they often co-exist with glucose in human blood. Fig. 9 shows 10 mM of AA, DA or UA, and 1 mM of NaCl does not show obvious interference for 0.1 mM glucose detection. Considering that the concentrations of these species is about 1/30 of that of glucose in human serum [40], the result in Fig. 9 indicates the NiCo2O4 NWs-rGO/GCE has a good substrate selectivity. The selectivity of the electrode may be ascribed to the following factors. First, the Nafion solution used to fix the nanomaterial onto electrode helps to improve the selectively of the sensor [48]. Nafion could partially prevent some interferents such as AA, DA, UA to access the electrode surface through electrostatic repulsion between Nafion (an anion polymer) and these interferents (AA, DA, UA in 0.1 M NaOH also bearing negative charge). Second, NiCo2O4 possess a very large real surface area (large ratio of real surface area to geometric surface area), thus favoring kinetically controlled reactions (i.e., the electrocatalytic oxidation of glucose) to a larger extent than the diffusion controlled reactions (i.e., the common electroactive species) [49]. Third, the concentration of interferent in serum sample is much lower than that of glucose [50], thus quite weak or negligible interfering signal was obtained. Combined these factors, a high selectivity of the NiCo2O4-rGO modified electrode for glucose detection can be expected.

Table 1 Comparison the analytical performance of sensors for glucose detection. Electrode material

Linear range (mM)

Sensitivity (mAmM1cm2)

Detection limit (mM)

Ref.

NiO nanofilm Hedgehog like NiO NiO-carbon sphere NiO/Ni foam NiO nanrod/Ni foam Co3O4 nanofiber Co3O4 nanododecahedra Co3O4 nanofibers Co3O4-TiO2 Co3O4 NPs Co3O4-OMC NiCo2O4 nanosheet Hollow NiCo2O4 nanorod NiCo2O4 nanowrinkle-rGO

0–1 0.5–4.5 0.002–1.279 0.018–1.2 0.05–0.5 0–0.4 0.002–2.1 Up to 2.04 0.01–3 0.006–0.8 0.01–0.9 0.005–0.065 0.0003–1.0 0.005–8.56

1680 1053 30190 – – 1050 708.4 36.25 2008.82 520.7 2597 6690 1685 548.9

0.34 1.2 2 6.15 20 – 0.58 0.97 0.34 0.13 1 0.38 0.16 2

36 37 38 39 40 41 42 43 44 45 46 20 47 This work

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Acknowledgements This work is financially supported by the Science Foundation of Jiangxi Province (20161BAB203088), Scientific Research Funds of Jiangxi Normal University (No. 4506 and No. 4304), and Open Project Program of Key Laboratory of Functional Small Organic Molecule, Ministry of Education, Jiangxi Normal University (No. KLFS-KF-201428). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.10.163. References Fig. 9. Amperometric response of NiCo2O4 NWs-rGO/GCE upon injection of glucose and potential interference species in 0.1 M NaOH solution.

Table 2 Determination of glucose in human serum by proposed sensor (n = 3). Human Serum Samples

Determined (mM)

Added (mM)

Found (mM)

Recovery (%)

1 2 3

4.56 5.30 7.18

2.0 1.0 1.0

6.61 6.35 8.15

102.5 105.0 97.0

RSD (%) 4.6 3.7 4.9

For five successive injection of 2 mM glucose, the RSD of current responses of the sensor is 3.9%, indicating a good reproducibility. The RSD of current response on five independent sensors for 2 mM glucose is 4.7%, implying an acceptable repeatability. After storing the NiCo2O4 NWs-rGO/GCE in ambient conditions for one month, the current response for the same concentration glucose retains 91.5% of its original value, demonstrating that the sensor has a good stability. 3.6. Real sample analysis The application of the NiCo2O4 NWs-rGO/GCE in real sample analysis was also evaluated by detection of glucose in human serum. The measured value and the recovery were listed in Table 2. For the standard addition test, the recovery for glucose ranges from 97% to 105%, indicating a satisfactory recovery. The results demonstrate that the NiCo2O4 NWs-rGO based sensor have potential in monitoring glucose level in human blood. 4. Conclusions In this paper, a novel hybrid NiCo2O4 NWs-rGO was prepared, and its application in glucose detection was evaluated. The formation of spinel type NiCo2O4 brings improved conductivity and enhanced catalysis. Graphene oxide could act as template to guide the growth of NiCo2O4 NWs, and thus improves the dispersity of NiCo2O4, increases its conductivity, and further enhances its catalysis towards glucose oxidation. Glucose sensor based on the NiCo2O4 NWs-rGO hybrid displays a higher sensitivity and wider linear range than sensors based on single component metal oxide or NiCo2O4. The NiCo2O4 NWs-rGO based sensor could also be used for glucose sensing in human serum at physiological level. Our results indicate combining spinel type mixed metal oxide with graphene is a good alternative to fabricate advanced metal oxide based hybrids for promising applications in catalysis and electrochemical sensors.

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