Fabrication of RGO-NiCo2O4 nanorods composite from deep eutectic solvents for nonenzymatic amperometric sensing of glucose

Talanta 185 (2018) 335–343

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Fabrication of RGO-NiCo2O4 nanorods composite from deep eutectic solvents for nonenzymatic amperometric sensing of glucose Yue Nia,b, Jian Xua, Hong Liua, Shijun Shaoa,

T

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a CAS Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China b University of Chinese Academy of Sciences, Beijing 100049, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: NiCo2O4 nanorods Deep eutectic solvents Cyclic voltammetry Glucose electrooxidation Amperometric responses

A novel reduced graphene oxide supported nickel cobaltate nanorods composite (RGO-NiCo2O4) was prepared by a simple ionothermal method in deep eutectic solvents for the first time. Electrochemical results demonstrated that the obtained nanocomposite modified glassy carbon electrode exhibited excellent electrocatalytic performance towards the oxidation of glucose with a wide double-linear range from 1 μM to 25 mM and a low detection limit of 0.35 μM (S/N = 3). NiCo2O4 nanorods with many small interconnected nanoparticles provided many electrocatalytic active sites, while RGO with large surface area offered good electrical conductivity. The synergistic effect between NiCo2O4 nanorods and RGO contributed to the enhanced sensing ability of the hybrid nanostructure. This sensitive glucose sensor can be also used for the practical detection of glucose in human serum.

1. Introduction Glucose, which is one of the most important molecules, exists in blood to supply energy for human body. When the glucose concentration is at excess level in the blood, it might trigger diseases like diabetes mellitus. Therefore, it is necessary to develop effective sensors to accurately test blood glucose levels. To date, most of commercially available glucose biosensors are dependent on the electrochemical oxidation activity of glucose oxidase (GOD). Despite high selectivity of enzyme, it generally has a few disadvantages such as high cost, complex immobilization process and vulnerability to ambient conditions [1]. Alternatively, nonenzymatic glucose sensors based on inorganic nanocatalysts are promising to replace the enzyme-based sensors. Although noble metals and their alloys have been widely used for nonenzymatic glucose sensors, the high cost of such catalysts constrains their further application [2–5]. Meanwhile, transition metal oxides exhibit many advantages such as low cost, easy preparation and high stability. However, the low conductivity and easy aggregation characteristics of transition metal oxides on the electrode surface will not only decrease their specific surface area but also degrade their electrocatalytic performance [6]. Therefore, different supporting materials are expected to solve this problem via loading metal oxides and improving the performance of such sensors [7–9]. Recently, nickel cobaltite (NiCo2O4) with cubic spinel phase has

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been regarded as one of the most promising electrode materials for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) due to its excellent electrical conductivity and redox chemical valences [10–12]. As reported, NiCo2O4 materials possess higher electrochemical activity and conductivity than the pure NiO or Co3O4 materials [13]. However, due to the poor conductivity of NiCo2O4, redox reactions can be easily hindered, which further reduced the sensitivity. Graphene, which is a type of two dimensional carbon with large specific area, high conductivity and easy preparation properties, has been investigated as one of the most critical carbon based materials for wide applications [14–18]. Among all potential applications, graphene can be employed as support matrix for NiCo2O4 to enhance electrocatalytic activity of metal oxides [19,20]. The NiCo2O4-graphene composite showed superior electrochemical performance such as improved electrical conductivity, reduced aggregation and accelerated electrochemical reaction rate [21]. Herein, we synthesized RGO-NiCo2O4 nanorods composite for the first time via a facile ionothermal method in the deep eutectic solvents (DESs) which are simple eutectic-based ionic liquids prepared by eutectic mixing of choline chloride (ChCl) and hydrogen bond donors (acids, amides and alcohols) [22]. DESs are promising solvents to be used in the synthesis of nanomaterials due to their excellent physicochemical properties including thermal stability, surface tensions, negligible vapor pressure, nontoxicity, biodegradation, and easy

Corresponding author. E-mail address: [email protected] (S. Shao).

https://doi.org/10.1016/j.talanta.2018.03.097 Received 11 September 2017; Received in revised form 6 February 2018; Accepted 29 March 2018 Available online 30 March 2018 0039-9140/ © 2018 Elsevier B.V. All rights reserved.

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dispersion was cooled down to room temperature and dialyzed by the cellulose ester membrane bag (MD77, 8000–14000) for two weeks to purify RGO.

preparation [23–28]. The present paper reported an eco-friendly method to synthesize RGO-NiCo2O4 nanorods composite in the DESs of ChCl–urea mixture (CU-DESs). Further, RGO-NiCo2O4 nanorods composite was used to construct a nonenzymatic electrochemical sensor. Considering the working solution used and the process adopted for the composite production, this environmentally friendly sensor had more advantages than other sensors. Compared with RGO-NiO and RGOCo3O4 catalyst, the fabricated RGO-NiCo2O4 displayed a much better catalytic activity towards glucose electrooxidation in alkaline media.

2.4. Preparation of RGO-NiCo2O4 nanorods composite 0.5 mmol of NiCl2·6H2O and 1 mmol of CoCl2·6H2O were completely dissolved into CU-DESs and stirred at 80 °C for 30 min. Then, 30 mL RGO dispersion was added into the above solution under magnetic stirring. After reaction for 30 min and ultrasonication for 10 min, 20 mL of the homogeneous dispersion was transferred into 25 mL Teflon-lined stainless autoclave. The autoclave was kept at 110 °C for 16 h and cooled down to room temperature. Then, the precipitates were collected by centrifugation and washed 3 times with water. The final product was freeze-dried overnight and annealed at 300 °C for 3 h in air with a heating rate of 1 °C min−1. For comparison, RGO-NiO, RGOCo3O4 and unloaded RGO were also prepared by the same procedure without addition of certain metal salt.

2. Experimental section 2.1. Chemicals and apparatus Choline chloride (HOC2H4N(CH3)3Cl, 99%) and glucose were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd (Shanghai, China, http://www.sinoreagent.com/). Urea (NH2CONH2, > 99%), cobalt chloride (CoCl2·6H2O), nickel chloride (NiCl2·6H2O) and sodium hydroxide (NaOH) were supplied by Tianjin Kemiou Chemical Reagent Co., Ltd (Tianjin, China, http://www.chemreagent.com/). Nafion solution (5 wt%), uric acid (UA), L-ascorbic acid (AA), dopamine (DA) and graphite (99.99%, 325 mesh) were obtained from Sigma-Aldrich (http://www.sigmaaldrich.com). Other reagents were of analytical grade and used without purification. All water used in the experiments was filtered by Milli-Q (18.2 MΩ cm). The human serum obtained though centrifugation was provided by local hospital. The morphologies of the samples were analyzed by scanning electron microscope (SEM; JSM-5600LV) and transmission electron microscope (TEM; JEOL 2100 FEG). The element distribution and percentage were characterized by energy dispersive X-ray (EDX; JSM5600LV) spectrometry. X-ray photoelectron spectroscope (XPS, Physical Electronics, Perkin Elmer PHI-5702) and X-ray diffraction (XRD; Rigaku D/Max-2400, Cu-Kα radiation, λ = 0.15405 nm) were carried out to investigate the composition and phase of the samples. Electrochemical experiments were conducted on a CHI 660C Electrochemical Workstation (Shanghai Chenhua Instrument Corporation, Shanghai) with a conventional three-electrode system. A bare or modified glass carbon electrode (GCE) was used as the working electrode, a platinum wire as the auxiliary electrode and an Ag/AgCl (3 M KCl) electrode as the reference electrode. Freshly prepared sodium hydroxide (NaOH) solution with the concentration of 0.1 M was used as the supporting electrolyte. Amperometric measurements were carried out at an appropriate potential. After the background current decayed to a steady-state value, different concentrations of glucose solution was successive injected into 5 mL of 0.1 M NaOH solution under a magnetically stirred condition. Electrochemical impedance spectroscopy (EIS) was carried out in 0.10 M KCl solution containing 5.0 mM K3Fe(CN)6/ K4Fe(CN)6 (1:1) at its open circuit potential in the frequency range of 0.01–100 kHz.

2.5. Preparation of the modified electrodes Before the measurement, the bare GCE (3 mm) was successively polished with 1.0, 0.3 and 0.05 µm Al2O3 powder, and then ultrasonicated in water, ethanol and acetone for 5 min, respectively. The original 5 wt% Nafion solution was diluted 50 times with N,N-dimethylformamide (DMF) and used to entrap nanocomposites. 1 mg of RGO-NiCo2O4 nanorods composite was dispersed into 1 mL of the diluted Nafion solution under ultrasonic condition. Then, 5 μL of the RGO-NiCo2O4/Nafion suspension was dropped on the pretreated GCE surface and dried in the air. For comparison, RGO-NiO/Nafion, RGOCo3O4/Nafion and RGO/Nafion were prepared and coated in the same method. 3. Results and discussion 3.1. Characterization of RGO-NiCo2O4 nanorods composite When the mixture solution containing NiCl2, CoCl2 and CU-DESs was heated under stirring, the ammonia generated by the urea was not enough to make the nickel or cobalt ions precipitate [31]. After the addition of RGO dispersion into the hot solution, the nucleation gradually occurred in the mixture of aqueous and urea [32]. The strong hydrogen bond interactions of CU-DESs system made the water welldispersed. Therefore, the NiCo precursor uniformly distributed on the RGO surface and grown into highly oriented NiCo2O4 nanorods. It was worth mentioning that NiCo2O4 nanorod or other single metal oxide was impossible to be synthesized without the addition of RGO. Therefore, RGO played an important role in the formation of nanocomposites. The morphology and structure of RGO and RGO-NiCo2O4 nanorods composite were investigated by SEM, TEM, HR-TEM and SAED. In Fig. 1a, the SEM images of the RGO-NiCo2O4 nanorods composite showed that the surface of RGO was completely covered by NiCo2O4 nanorods. The corresponding elemental mapping images confirmed the homogeneous dispersion of Ni, Co, C and O elements throughout the whole nanocomposite. The EDX spectrometry further demonstrated that the nanocomposite consisted of Ni, Co, C and O elements (Fig. 1b). The TEM images of RGO displayed that its sheets were very thin with a few wrinkles, which indicated that they were ideal supports to load NiCo2O4 nanorods (Fig. S1). It can be seen in Fig. 2a that the nanorodlike NiCo2O4 sparsely and randomly grown on the surface of RGO. The NiCo2O4 nanorods, with an average diameter of 35 nm and average length of 272 nm, consisted of many small interconnected nanoparticles with a uniform size of 12 nm (Fig. 2b). The HRTEM image of RGONiCo2O4 nanorods composite exhibited typical crystallinity, with a interplanar distance of 0.47 nm and 0.29 nm, corresponding to the (111) and (220) planes of NiCo2O4, respectively (Fig. 2c) [33]. The well-

2.2. Preparation of CU-DESs CU-DESs were formed by stirring 2.793 g of choline chloride and 2.402 g of urea at 80 °C until a homogeneous colorless liquid formed [22]. 2.3. Synthesis of reduced graphene oxide sheets Graphene oxide (GO) was synthesized by a modified Hummers method [29]. Then reduced graphene oxide (RGO) was prepared according to the previous report [30]. In a typical process, 100 mg of GO was dispersed in 100 mL of ultrapure water by sonication. Then, the GO dispersion was transferred to a round bottom flask and heated to 95 °C under agitation. After the addition of 1 mL hydrazine (80 wt%) and 100 μL ammonium hydroxide (28 wt%), the resulting solution was refluxed at 95 °C for 1 h with rigorous stirring. Finally, the mixture 336

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Fig. 1. (a) SEM images and elemental mapping of RGO-NiCo2O4 nanorods composite and (b) EDX of nanocomposite.

43°, which were indexed as (002) and (100) planes, suggesting the amorphous behavior of RGO. The XRD spectrum of RGO-NiCo2O4 nanorods composite showed that the diffraction peaks at 18.9°, 31.1°, 36.6°, 44.5°, 59.1° and 64.9° were assigned to the lattice planes (111), (220), (311), (400), (511) and (440) of cubic spinel phase of NiCo2O4 (PDF#73-1702) [21]. The weak broad peak of RGO indicated that

defined rings, which belonged to the (111), (220) and (311) of spinel nickel cobaltite, in the selected area electron diffraction (SAED) pattern indicated the polycrystalline features of NiCo2O4 in the nanocomposite (Fig. 2d). Fig. 3a showed the XRD patterns of RGO and RGO-NiCo2O4 nanorods composite. RGO exhibited two broad peaks at around 23° and

Fig. 2. TEM images (a and b), HR-TEM image (c) and the SAED pattern (d) of the RGO-NiCo2O4 nanorods composite. 337

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Fig. 3. (a) Typical XRD patterns of RGO and RGO-NiCo2O4 nanorods composite. XPS spectra of RGO-NiCo2O4 nanorods composite: survey spectrum (b), Ni 2p (c) and Co 2p (d).

specific surface area. As reported by the literature [37], two pairs of redox peaks were observed at RGO-Co3O4/Nafion/GCE (Fig. 4b), among which one was attributed to the conversion between Co3O4 and CoOOH, and the other was between CoOOH and CoO2. The anodic peak current increased with the addition of 1.0 mM glucose, which confirmed the catalytic ability of Co3O4 for glucose oxidation. The RGONiO/Nafion/GCE displayed a well-defined redox peak without the addition of glucose due to the conversion between NiO and NiOOH [38] (Fig. 4c). When glucose was introduced, the increased anodic peak current was resulted from the oxidation of glucose by NiOOH. As exhibited in Fig. 4d, similar catalytic oxidation response was observed at RGO-NiCo2O4/Nafion/GCE. Compared with RGO-Co3O4/ Nafion/GCE and RGO-NiO/Nafion/GCE, the RGO-NiCo2O4/Nafion/ GCE displayed much wider anodic and cathodic peaks, due to the merged redox peaks of Ni and Co species, as well as higher peak current. In addition, the anodic peak current at 0.55 V remarkably increased after the addition of glucose. The results above indicated that the unique spinel structure of NiCo2O4 facilitated the electron transfer and possessed better catalytic performance [39]. In combination with the catalytic mechanism of NiO and Co3O4, the mechanism of electrochemical oxidation of glucose catalyzed by NiCo2O4 in basic electrolyte can be speculated as following equations [40,41]:

NiCo2O4 nanorods were well deposited on the surface of RGO. XPS measurement was conducted to investigate the surface element composition and bonding states of the nanocomposite. In the XPS survey spectra of RGO-NiCo2O4 nanorods composite (Fig. 3b), a series of sharp signals were assigned to the characteristic peaks of Ni 2p, Co 2p, O 1s and C 1s, respectively, which indicated the existence of Ni, Co, O and C elements. In the high resolution XPS spectra of Ni 2p (Fig. 3c) and Co 2p (Fig. 3d), there were two spin–orbit doublets and two shakeup satellites [34]. The peaks at 855.2 eV and 872.5 eV corresponded to Ni2+, while those at 856.4 eV and 874.2 eV were indexed to Ni3+. Analogously, the peaks at 780.8 eV and 796.5 eV were assigned to Co2+, while the peaks at 782.1 eV and 797 eV belonged to Co3+. The O1s (Fig. S2a) spectrum consisted of the lattice oxygen of pure NiCo2O4 (529.3 eV) and residual oxygen-containing functional groups of RGO (532.4 eV) [35]. In the XPS C1s spectrum (Fig. S2b), the peaks at 284.7, 286.6 and 288.7 eV were ascribed to the C˭C, C-O and C˭O bond, respectively [36]. The low intensity of C-O and C˭O suggested a lower content of the oxygen-containing groups in RGO-NiCo2O4 nanorods composite. 3.2. Electrocatalytic oxidation of glucose at different electrodes Cyclic voltammetry was used to characterize the electrocatalytic activity of different electrodes towards glucose oxidation in alkaline solution. No obvious peaks were observed at bare GCE (Fig. S3) and RGO/Nafion/GCE (Fig. 4a), which suggested that it was impossible to oxidize or reduce glucose at the surface of these electrodes. Moreover, after the introduction of RGO, the background current was larger than that of the bare GCE due to the enhanced conductivity and increased

NiCo2O4+OH-+H2O↔NiOOH+2CoOOH+e-

(1)

CoOOH+OH ↔CoO2+H2O+e

(2)

CoO2+NiOOH+C6H12O6↔CoOOH+NiO+H2O+C6H10O6

(3)

-

-

Therefore, the increased anodic peak current accompanied by the 338

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Fig. 4. Cyclic voltammograms (CVs) of RGO/Nafion/GCE (a), RGO-Co3O4/Nafion/GCE (b), RGO-NiO/Nafion/GCE (c) and RGO-NiCo2O4/Nafion/GCE (d) with the absence (black curves) and presence (red curves) of 1.0 mM glucose in 0.1 M NaOH solution at a scan rate of 100 mV/s. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Simultaneously, the cathodic peak current increased slowly, indicating that anodic peak current was more sensitive in the determination of glucose. Fig. 5a showed the CVs of the RGO-NiCo2O4/Nafion/GCE in 0.1 M NaOH solution containing 1.0 mM glucose at different scan rates. The anodic peak current increased with the scan rate in the range from 40 to 500 mV/s, while the peak potential gradually shifted to more positive values owing to a kinetic limitation in the reaction between the redox sites of NiCo2O4 and glucose [42]. Moreover, the peak current was proportional to the square root of the scan rate (Fig. 5b), indicating that the electrochemical reaction on RGO-NiCo2O4/Nafion/GCE was a typical diffusion controlled process.

electrooxidation of glucose was mainly because of the consumption of CoO2 and NiOOH. It deserved to be mentioned that the introduction of RGO matrix not only increased the modified electrode surface and stabilized NiCo2O4 nanorods, but also facilitated the electron transfer during the catalysis process. The synergistic effect of NiCo2O4 nanorods and RGO made the nanocomposite effectively enhance the electrochemical oxidation of glucose. The cyclic voltammograms (CVs) of the RGO-NiCo2O4/Nafion/GCE were recorded in different glucose concentrations in the range of 0–17 mM (Fig. S4). Compared to the CV in 0.1 M NaOH solution at the absence of glucose, an obvious increase of oxidation current can be observed upon the stepwise addition of glucose into the electrolyte.

Fig. 5. (a) CVs of RGO-NiCo2O4/Nafion/GCE in 0.1 M NaOH solution containing 1 mM glucose at different scan rates (40, 60, 80, 100, 120, 140, 160, 180, 200, 300, 400 and 500 mV/s). (b) Plot of electrocatalytic oxidation current of glucose vs the square root of scan rate. 339

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Fig. 6. (a) Amperometric responses of RGO-NiCo2O4/Nafion/GCE at different potentials with successive injection of 0.2 mM glucose into 0.1 M NaOH solution. (b) Amperometric response of RGO-NiCo2O4, RGO-Co3O4, RGO-NiO and RGO modified GCE at 0.55 V (vs Ag/AgCl) with successive injection of 0.2 mM glucose into 0.1 M NaOH solution.

Fig. 7. (a) Amperometric responses of the RGO-NiCo2O4/Nafion/GCE with successive addition of the glucose of different concentrations into 0.1 M NaOH solution at an applied potential of 0.55 V. The inset shows the magnified image of the region from 0 to 900 s of the sensing curve. (b) The corresponding calibration curve of the amperometric response of RGO-NiCo2O4/Nafion/GC electrode to the concentration of glucose from 1 μM to 6.3 mM. (c) The calibration curve for the concentration of glucose from 6.3 mM to 25 mM. Error bars represent one standard deviation of the means.

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Table 1 Comparison of the sensing performance of different electrode materials for glucose detection. Electrode materials

Detection limit (μM)

Linear range (mM)

Sensitivity (μA mM−1 cm−2)

Reference

NiO/MWCNTs NiO–Au hybrid nanobelts

2 0.65 1.32 0.2 0.7 1.8 0.08 0.1 1 0.38 0.3833 0.16 2 0.35

0.01–7 Up to 2.79 Up to 4.55 0.05–5.66 0.1–5.0 0.0299–6.44 0.005–12 0.0002–0.2; 0.5–20 0.002–0.2; 1–20 0.005–0.065 0.015–4.735 0.0003–1.0 0.005–8.56 0.001–6.3 6.3–25

1768.8 23.88 (+ 0.22 V) 48.35 (+ 0.6 V) 668.2 1915 1997 1440 6000 1330 6690 4550 1685 548.9 960.37 216.7

[44] [45]

NiO/Pt/ERGO NiO nanoskein Ni/NiO-rGO CTAB−Co3O4 cobalt oxide/Au cobalt oxide NiCo2O4 nanosheet arrays NiCo2O4 @Polyaniline Hollow NiCo2O4 nanorod NiCo2O4 nanowrinkle-rGO RGO-NiCo2O4 nanorods

[46] [47] [48] [49] [6] [50] [40] [51] [20] This work

phase of NiCo2O4, as demonstrated by other reports [39,43], possessed more redox centers and better electrocatalytic activity than NiO or Co3O4. Moreover, graphene, which can provide large surface area, induce the generation of nanorod-like NiCo2O4 and avoid its aggregation, also played an important role in the glucose catalysis process. Amperometric responses of the RGO-NiCo2O4/Nafion/GCE were carried out at 0.55 V by successive injection of glucose into 0.1 M NaOH solution under stirring. As shown in Fig. 7a, the recorded current reached 95% of steady state within 5 s and exhibited a step-style increase with the injection of glucose, indicating the rapid oxidation of glucose molecule on the electrode surface. There were two calibration curves for RGO-NiCo2O4/Nafion/GCE: IGlu(μA) = (21.81 ± 1.45) + (67.85 ± 0.66)CGlu(mM)

(4)

IGlu(μA) = (331.86 ± 35.55) + (15.31 ± 2.00)CGlu(mM)

(5)

One concentration range was from 1 μM to 6.3 mM (Fig. 7b), and the other was from 6.3 mM to 25 mM (Fig. 7c), with the sensitivity of 960.37 and 216.7 μA mM−1 cm−2, respectively. The detection limit was calculated to be 0.35 μM (S/N = 3). Compared with other nonenzymatic glucose sensors reported in literature based on NiO, Co3O4 or NiCo2O4 (see Table 1), the RGO-NiCo2O4/Nafion/GCE sensor also displayed a good electrocatalytic activity and sensitivity towards glucose oxidation. EIS was utilized to evaluate the charge transfer resistance of electrode surfaces and the Nyquist plots of Nafion/GCE, RGO/Nafion/GCE, RGO-NiO/Nafion/GCE, RGO-Co3O4/Nafion/GCE and RGO-NiCo2O4/ Nafion/GCE were obtained (Fig. S5). The semicircle at the high frequency region represents the electron transfer limited process. The variation of semicircular diameter in Nyquist plot corresponds to the change in charge-transfer resistance (Rct) which dictates the electron transfer kinetics of the redox probe at the electrode/electrolyte interface. It can be seen that the Nafion/GCE had the largest impedance among all the modified electrodes. The incorporation of RGO accelerated the diffusion of redox probe [Fe(CN)6]3-/4- toward the electrode surface and improved the charge transfer ability. The semicircular diameter of RGO-NiO/Nafion/GCE, RGO-Co3O4/Nafion/GCE and RGONiCo2O4/Nafion/GCE were bigger than that of RGO/Nafion/GCE,

Fig. 8. Amperometric responses of the RGO-NiCo2O4/Nafion/GCE with successive addition of 1 mM glucose, 0.1 mM AA, 0.1 mM UA, 0.1 mM DA and 1 mM glucose into 0.1 M NaOH solution with a constant potential of 0.55 V.

3.3. Amperometric detection of glucose at the RGO-NiCo2O4/Nafion/GCE The effect of applied potentials (from 0.4 to 0.65 V vs Ag/AgCl) on amperometric currents was investigated by successive injection of 0.2 mM glucose into 5 mL of 0.1 M NaOH electrolyte solution (Fig. 6a). The amperometric current increased with increased potential in the range from 0.4 to 0.55 V, while it began to decrease when the potential was over 0.55 V. The results demonstrated that the amperometric response at 0.55 V was the largest among all of the above potentials. Therefore, considering the high detection sensitivity, the potential of 0.55 V was chosen as the optimum applied potential in the following experiments. In Fig. 6b, the amperometric response of RGO, RGO-NiO, RGOCo3O4 and RGO-NiCo2O4 modified GCE at 0.55 V were compared by successive injection of 0.2 mM glucose into 0.1 M NaOH solution. It can be seen that the RGO-NiCo2O4/Nafion/GCE exhibited higher current response to glucose than other modified electrodes, while RGO/Nafion/ GCE almost displayed no response. These results confirmed that spinel

Table 2 Recovery data for the determination of glucose in human blood serum using the calibration curve method. Sample

Detected by spectrophotometric method (mM)

Detected by our method (mM)

Added (mM)

Found (mM)

Recovery (%)

RSDa (%)

human blood serum

5.25

5.13

0.50 1.00 2.00

5.44 6.17 7.45

94.6 98.7 102.8

2.59 3.85 4.27

a

Calculated from four separate measurements. 341

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indicating the conductivity of these modified electrodes were reduced after introducing the transition metal oxides. Moreover, compared with RGO-NiO or RGO-Co3O4, RGO-NiCo2O4 modified GCE showed a much smaller diameter, which revealed that the conductivity of spinel phase NiCo2O4 was better than its single component metal oxide. The excellent sensing performance likely resulted from the structural characteristics of RGO-NiCo2O4. First, there were numerous contact sites between NiCo2O4 nanorods and RGO, which could avoid formation of stacked structure, improve stability of the nanocomposite and prompt more NiCo2O4 nanorods to load. Second, 2D graphene with large specific surface area provided much adsorption sites for glucose, which could enhance the electrocatalytic capability of NiCo2O4 nanorods. Third, the synergistic effect between NiCo2O4 nanorods and RGO facilitated electron transfer on the electrode surface.

wide linear range, low detection limit and high sensitivity, towards the oxidation of glucose in alkaline solution. The good performance was attributed to the excellent catalytic ability of NiCo2O4 nanorods, good conductivity of RGO, as well as their synergistic effect. Therefore, it was promising for practical glucose detection in human blood serum. Our work provided an eco-friendly method to fabricate nanocomposite with specific composition and morphology for sensitive nonenzymatic biosensors.

3.4. Selectivity of the RGO-NiCo2O4/Nafion/GCE

Appendix A. Supporting information

One of the major challenges in applying amperometric sensors to actual measurement is to minimize the interference effect. As is wellknown, ascorbic acid (AA), dopamine (DA) and uric acid (AA), with concentrations no more than 0.1 mM, coexist with glucose in human blood serum [6]. Amperometric measurement was carried out on the RGO-NiCo2O4/Nafion/GCE at 0.55 V with successive injecting 1 mM glucose, 0.1 mM AA, 0.1 mM UA, 0.1 mM DA and 1 mM glucose into 0.1 M NaOH solution (Fig. 8). It can be seen that RGO-NiCo2O4/Nafion/ GCE exhibited remarkable current response for glucose, while small current response for the above interferences. These results clearly demonstrated that the RGO-NiCo2O4 modified electrode had good selectivity towards glucose.

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.talanta.2018.03.097.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21572239 and 21505145) and the Funds for Distinguished Young Scientists of Gansu (1210RJDA013).

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