Nickel-cobalt nanostructures coated reduced graphene oxide nanocomposite electrode for nonenzymatic glucose biosensing

Electrochimica Acta 114 (2013) 484–493

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

Nickel-cobalt nanostructures coated reduced graphene oxide nanocomposite electrode for nonenzymatic glucose biosensing Li Wang a , Xingping Lu a , Yinjian Ye a , Lanlan Sun b , Yonghai Song a,∗ a Key Laboratory of Functional Small Organic Molecule, Ministry of Education, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, People’s Republic of China b State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, 3888 East Nan-Hu Road, Changchun 130033, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 24 July 2013 Received in revised form 11 October 2013 Accepted 15 October 2013 Available online 29 October 2013 Keywords: Nickel-cobalt nanostructures Reduced graphene oxide Electrodeposition Electrocatalytic oxidation Glucose

a b s t r a c t Nickel-cobalt nanostructures (Ni-Co NSs) electrodeposited on reduced graphene oxide (RGO)-modified glassy carbon electrode (GCE) was prepared and used for highly sensitive glucose detection. RGO nanosheets were firstly assembled onto GCE surface by ␲–␲ interaction and then Ni-Co NSs were constructed on RGO/GCE by dynamic potential scan. The electrochemical and electrocatalytic behaviors of the Ni-Co NSs/RGO/GCE toward glucose oxidation were evaluated by cyclic voltammograms, chronoamperometry and amperometric method. The effects of some factors related to the fabrication of Ni-Co NSs/RGO/GCE, such as potential scan number and the molar ratio of Ni2+ /Co2+ in a solution, on the catalytic performance of the Ni-Co NSs/RGO/GCE were also explored. The results showed that the Ni-Co NSs/RGO/GCE exhibited the best catalytic activity at the potential scan number of 20 and the Ni2+ /Co2+ molar ratio of 1:1. The glucose concentration in the range of 10 ␮M to 2.65 mM linearly depended on the catalytic current (r = 0.9967, n = 17). The sensitivity was 1773.61 ␮A cm−2 mM−1 , and the detection limit was 3.79 ␮M (S/N = 3). This high catalytic activity, good sensitivity and stability of the Ni-Co NSs/RGO/GCE sensor opened up a new kind of hybrid materials in electrochemical detection of glucose. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Glucose is one of the indispensable substances for life activities, and it can be ingested directly in the metabolic process to provide energy so as to maintain the normal life activities. Glucose is extensively distributed in blood of being living [1], and the increase of glucose in blood could cause diabetes mellitus. The diabetes mellitus has become one of the major health afflictions worldwide [2]. Therefore, quantitative determination of glucose concentration both in blood and in other sources such as foods and pharmaceuticals is very important in biological and clinical analysis [3–5]. By now, glucose biosensors based on electrochemical enzymatic reaction or nonenzymatic sensors have attracted most of attentions. Although there have been already tremendous benefits from the use of those enzymatic sensors, several disadvantages still remain, such as high cost of enzymes, poor reproducibility and insufficient long-term stability. On the contrary, the nonenzymatic glucose sensors which can overcome these problems have become one of the most appealing approaches for the determination of glucose.

∗ Corresponding author. Tel.: +86 791 88120862; fax: +86 791 88120862. E-mail addresses: [email protected], [email protected] (Y. Song). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.10.125

Various low-cost metal or metal oxide nanostructures (NSs) materials, including CuO [6], Cu [7], Co3 O4 [8], NiO [9], etc., are widely used in nonenzymatic glucose sensor. Among these materials, nickel oxides or cobalt oxides NSs are particularly popular owing to their large specific surface areas, excellent conductivities and catalytic activities. Recently, Zhu et al. [10] have reported a stable and sensitive nonenzymatic glucose sensor based on a NiO/RGO modified glassy carbon electrode (GCE). Yuan et al. [11] reported a NiO/GO modified GCE for nonenzymatic glucose sensing. Lee et al. [12] presented the fabrication of a nonenzymatic glucose sensor based on CoOOH nanosheets directly grown on cobalt substrate. Wang et al. [13] reported the application of a novel graphene-Co3 O4 hybrid needle-like electrode for nonenzymatic glucose detection with high sensitivity. Another alternative is bimetallic crystal nanomaterials, which are also drawing much attention because of their better catalytic activities and anti-interference ability for glucose detection than their corresponding monometallic counterparts due to the coordination effect of two metallic materials. It was recently reported that Cu-Co alloy dendrite-based glucose and hydrogen peroxide sensors exhibited high sensitivity [14]. The carbon electrode-supported bimetallic Au-Ag alloy nanoparticles (NPs) were found effective for glucose oxidation [15]. Pt-Ni NP-graphene nanocomposites were used for highly sensitive glucose detection [16]. Pt-Ni nanowire arrays were also proposed

L. Wang et al. / Electrochimica Acta 114 (2013) 484–493

for nonenzymatic glucose detection with good reproducibility and high stability [17]. The previous results indicate that the design and preparation of modified electrode with bimetallic crystal nanomaterials is important in achieving high activity and sensitivity for glucose detection. Ni-Co alloys have been used as important engineering materials not only due to their low-cost and high electrocatalytic activity, but also due to that the doping of Ni in Co crystal improves the mechanical [18], chemical [19], electrochemical properties [20], and structural stability [21]. The Ni-Co alloys modified electrode has been used for methanol oxidation [22] and urea oxidation [23,24]. Graphene, a single layer of carbon atoms in a closely packed honeycomb two-dimensional lattice, is emerging as a new type of functional supporting nanomaterials because of its excellent physical and chemical properties [25–27]. The unique properties of graphene, e.g. remarkable surface area, excellent conductivity and wide electrochemical window, have led it to be an ideal material in the field of electrochemical sensors [28,29]. In this work, Ni-Co NSs constructed on RGO nanosheets modified GCE as nonenzymatic glucose sensor was proposed for the first time. The parameters affecting the preparation process such as the potential scan number and the molar ratio of Ni2+ /Co2+ were optimized. The electrochemical and electrocatalytic behaviors of the Ni-Co NSs/RGO/GCE toward the oxidation of glucose were evaluated by cyclic voltammograms, chronoamperometry and amperometric method. The electron transfer rate constant, the analytical parameters (such as linear range, detection limit and sensitivity) and the kinetic parameters (such as the diffusion coefficient and the catalytic rate constant) of the Ni-Co NSs/RGO/GCE were explored.

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␲–␲ electronic interactions as previously reported [31]. Next, the RGO/GCE was immersed in a mixed solution of 0.1 M KCl + 0.005 M NiCl2 + 0.005 M CoCl2 [23] and a cyclic scan in the potential range from −0.05 V to −1.05 V [22] was performed to electrochemically deposit Ni-Co NSs onto the RGO/GCE surface. In a control experiment, Ni-Co NSs/GCE without RGO was prepared by the similar procedure as described above. 2.4. Instrumentations

2. Experimental

X-ray powder diffraction (XRD) data were collected on a D/Max 2500V/PC X-ray powder diffractometer using Cu K␣ radiation ( = 0.154056 nm, 40 kV, 200 mA). The scanning electron microscopy (SEM) image was taken using a XL30 ESEM-FEG SEM at an accelerating voltage of 20 kV equipped with a Phoenix energy dispersive X-ray analyzer. The samples for SEM observation were prepared by electrodepositing on GCE, followed by drying at room temperature. The chemical compositions of the synthesized Ni-Co NSs were determined by energy dispersive X-ray (EDX) using a HITACHI S-3400N attached to the SEM. All electrochemical measurements were performed on a CHI 430a electrochemical workstation (Shanghai, China) at the room temperature. A conventional three-electrode system was adopted including a bare or modified GCE as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum wire as auxiliary electrode. The cyclic voltammetric experiments were performed in a quiescent solution. The amperometric experiments were carried out under a continuous stirring. 0.1 M NaOH was chosen as the supporting electrolyte solution and purged with high purity nitrogen for 15 min prior to each measurement and then a nitrogen atmosphere was kept over the solution during measurements.

2.1. Chemicals and reagents

3. Results and discussion

Graphite powder, fructose, d-galactose, uric acid (UA) and lascorbic acid (AA), CoCl2 ·6H2 O and NiCl2 ·6H2 O were purchased from Sinopharm Group Chemical Reagent Co., Ltd (Shanghai, China). Glucose was obtained from the Tianjin Fuchen Chemical Reagent Factory (Tianjin, China). Other reagents were purchased from Shanghai Experimental Reagent Co., Ltd (Shanghai, China). All reagents were of analytical grade and used without further purification. All solutions were prepared with ultra-pure water, purified by a Millipore-Q system (18.2 M cm−1 ).

3.1. Electrodeposition of Ni-Co NSs on RGO/GCE

RGO was prepared by reduction of graphene oxide (GO) via hydrazine. The GO was synthesized from graphite powder based on a modified Hummers method [30]. The preparation of RGO from GO is carried out as follows: at first, a stable dispersion of GO was achieved by ultrasonicating 0.1 mg GO in 50 mL H2 O for 1 h (1000 W, 20% amplitude). Then, 0.1 mL of hydrazine solution (50 wt%) was added into the above supernatant. After being vigorously shaken or stirred for 5 min, the mixture was stirred for 1 h at 95 ◦ C. Finally, the stable black dispersion was centrifuged, filtered, washed, and dried under vacuum at 80 ◦ C to obtain RGO. 2.3. Preparation of Ni-Co NSs/RGO/GCE The GCE of 3.0 mm in diameter was polished carefully with 1.0, 0.3 and 0.05 ␮m alumina powder on felt pads, and then ultrasonically cleaned in water. To obtain the RGO modified GCE, 10 ␮L 0.01 mg mL−1 suspension of RGO was transferred onto a polished GCE surface, and then the solvent was evaporated in air. RGO nanosheets were assembled onto GCE surface through

50 0

I/ A

2.2. Synthesis of RGO

Cyclic voltammograms (CVs) was utilized to deposit Ni-Co NSs on the RGO/GCE. Fig. 1 showed the CVs of the RGO/GCE scanned in potential range from −0.05 V to −1.05 V. There was a large cathodic peak at −0.85 V which resulted from the reduction of Ni2+ and Co2+ on the RGO/GCE surface to form Ni-Co NSs [22,32]. Two anodic peaks at −0.49 V and −0.17 V were related to the oxidation of the produced Ni-Co NSs [23]. With the increasing of the number of scan cycle, the current of cathodic peak gradually decreased, and the peak potential positively shifted as indicated by the black arrow in Fig. 1. The current decrease of cathodic peak with increasing

-50 -100 -150 -200 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

E vs SCE/V Fig. 1. CVs of RGO/GCE in 0.1 M KCl + 0.005 M NiCl2 + 0.005 M CoCl2 at 50 mV s−1 .

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cycle number indicated that Ni2+ and Co2+ have been reduced and deposited on the surface of RGO/GCE slowly. 3.2. Characteristics of the as-prepared Ni-Co NSs/RGO/GCE

at the rates of 10–700 mV s−1 , indicating that the electron transfer reaction involved a surface-controlled process (Fig. 4B). The electron-transfer coefficient (˛s ) and electron-transfer rate constant (ks ) could be determined based on Laviron’s theory [38]: 

Fig. 2A showed the SEM image of Ni-Co NSs/RGO/GCE obtained at the potential cycle number of 20, which clearly indicated that the Ni-Co NSs formed uniform flower-like NSs with average diameter of 300 nm on RGO/GCE. While only some bulks of Ni-Co NSs formed on the GCE surface without RGO (Fig. 2B). The result indicated that the RGO played an important role in the formation of flower-like Ni-Co NSs. The RGO provided a large surface area to direct the formation of uniform flower-like Ni-Co NSs. EDX was used to study the composition of the Ni-Co NSs/RGO/GCE (Fig. 2C). It revealed the peaks corresponding to Ni, Co, C, O, K and Cl, in which C-related peak in the EDX data comes from the RGO and GCE, the K-, Cl-, and O-related peaks come from the electrolyte. The ratio of the Ni/Co was calculated to be about 1:1, which is in accordance with the ratio of Ni2+ and Co2+ used in the electrodepositing solution. The crystal structure of the as-prepared flower-like Ni-Co NSs was characterized by XRD (Fig. 2D). As can be seen in the pattern, one characteristic peak at 40.9◦ corresponds to the (1 0 0) plane of the hexagonal closed packed (h.c.p.) Co crystal (JCPDS card No. 894308) [33]. Two characteristic diffraction peaks at 44.4◦ and 51.3◦ can be indexed to the (1 1 1) and (2 0 0) planes of face-centeredcubic (fcc) Ni-Co alloy, respectively (JCPDS card No. for fcc Co is 01-1259 and for fcc Ni is 04-0850) [33,34]. The broad peak might be ascribed to the small Ni-Co NSs, as it was well-known that the wider was the XRD peak, the smaller was the nanostructures. Above results indicated the formation of Ni-Co NSs crystal with high purity. It should be noted that the molar ratio of Ni2+ /Co2+ in above studies was 1:1, which might also play an important role in the formation of the flower-like Ni-Co NSs. To explore the effect of Ni2+ /Co2+ ratio on the morphology of Ni-Co NSs, different molar ratios (1:0, 4:1, 1:4 and 0:1) of Ni2+ /Co2+ were investigated. Fig. 3A–D showed the SEM images of Ni-Co NSs formed on RGO/GCE surface with different Ni2+ /Co2+ ratios. At the Ni2+ /Co2+ ratio of 1:0, the RGO sheets just became thick and rough and a few NSs with the average size of 100 nm were observed (Fig. 3A). At the Ni2+ /Co2+ ratio of 4:1, the amount and the size (200–500 nm) of the Ni-Co NSs obviously increased. At the Ni2+ /Co2+ ratio of 1:4, a small amount of flower-like NSs formed (Fig. 3C). At the Ni2+ /Co2+ ratio of 0:1, only thick Co films were formed (Fig. 3D). These results indicated that the Ni2+ /Co2+ ratio was one of the key factors affected formation of flower-like Ni-Co NSs, and the optimized Ni2+ /Co2+ ratio is 1:1. 3.3. Electrochemical behavior of Ni-Co NSs/RGO/GCE The electrochemical behaviors of Ni-Co NSs/RGO/GCE were investigated by CVs in 0.1 M NaOH solution (Fig. 4A). As can be seen from curve c (scan rate: 50 mV s−1 ), a pair of redox peaks with anodic peak at 0.27 V and the corresponding cathodic peak at 0.12 V was found. However, the anodic peak of Ni in alkaline medium appeared at about 0.5 V [22–24,35,36], while that of Co appeared at less than 0.3 V [8,12]. Therefore, the anodic peak at 0.27 V was probably attributed to the complex oxidation transformation of different species in alkaline medium (i.e. Ni(OH)2 , NiOOH, Co(OH)2 , CoOOH) [24,37]. The cathodic peak at 0.12 V was related to the reduction of Ni and Co oxides formed in the positive cycles. In order to understand the direct electrochemical process of NiCo NSs, the CVs of Ni-Co NSs/RGO/GCE were recorded in 0.1 M NaOH solution at different scan rates (curve a-p). Obviously, the peak current was enhanced with the increasing of the scan rate. The peak current was directly proportional to potential scan rate

Epc = E o + 

Epc = E o +

RT RT − ln v ˛s nF ˛s nF

(1)

RT RT + ln v (1 − ˛s )nF (1 − ˛s )nF

(2)

where n is the electron transfer number, v is the potential scan rate, R is the gas constant (R = 8.314 J mol−1 K−1 ), T is the temperature in Kelvin (T = 298 K) and F is the Faraday constant (F = 96493 C mol−1 ). For peak I and peak II, the ˛1 n was calculated to be 0.65 based on the plot of peak potential (Epc , Epa ) versus the natural logarithm of the scan rate (ln v) (Fig. 4C). According to the literature [39], if 0.3 < ˛ < 0.7, it could be concluded that n = 1 and ˛1 = 0.65. Thus, the possible mechanism involving the direct electrochemical process could be expressed as followed [35–37,40–43]: Ni(0) + 2OH− → Ni(OH)2 + 2e

(3)

Co(0) + 2OH− → Co(OH)2 + 2e

(4)

Ni(OH)2 + OH− → NiOOH + H2 O + e

(5)

Co(OH)2 + OH− → CoOOH + H2 O + e

(6)

First, the metal Co(0) and Ni(0) was transformed into Co(OH)2 and Ni(OH)2 in the alkaline conditions at the onset of the potential scan. Then, the oxides were further oxidized into CoOOH and NiOOH as potential shifted to the positive direction (peak I). When nEp < 200 mV, the ks could be estimated with the Laviron’s equation [38]: ks =

˛nF RT

(7)

At 50 mV s−1 , the ks was calculated to be 1.27 s−1 . The surface coverage of electro-active Ni-Co NSs ( * , mol cm−2 ) could be estimated by Faraday’s law [44]: Ip =

nFQ v n2 F 2 A ∗ v = 4RT 4RT

(8)

which can come to the expression as followed: ∗ =

Q nFA

(9)

where Q is the charge consumed in the CVs, A is the effective surface area of the electrode and the other symbols have their usual meaning. The A can be calculated based on Randles–Sevcik equation [45]. The calculated values of A were 0.047 cm2 for Ni-Co NSs/RGO/GCE and 0.041 cm2 for Ni-Co NSs/GCE, respectively. Then the value of  * was calculated to be about 8.99 × 10−8 mol cm−2 and 4.58 × 10−8 mol cm−2 for Ni-Co NSs/RGO/GCE and Ni-Co NSs/GCE, respectively. This result further confirmed that the RGO provided a large surface area to increase the quantity and reduce the dimension of the obtained Ni-Co NSs. 3.4. Electrocatalytic oxidation of glucose on Ni-Co NSs/RGO/GCE To explore the sensing activity of Ni-Co NSs/RGO/GCE, the CVs of different modified electrodes in 0.1 M NaOH in the presence (curve b, d, g and h) and absence (curve a, c, e and f) of 0.04 M glucose were shown in Fig. 5. There was no obvious oxidation peak at bare GCE (curve a, b) and RGO/GCE (curve c, d). In the presence of 0.04 M glucose, the oxidation peaks at about 0.27 V and 0.50 V obviously increased at the Ni-Co NSs/RGO/GCE (curve h) as compared with

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Fig. 2. SEM images of RGO/GCE (A) and RGO/GCE scanned in 0.1 M KCl + 0.005 M NiCl2 + 0.005 M CoCl2 at 50 mV s−1 for 20 CVs (B). (C) EDX analysis of the Ni-Co NSs/RGO/GCE. (D) XRD pattern of the Ni-Co NSs/RGO/GCE.

Fig. 3. SEM images of RGO/GCE scanned in 0.1 M KCl + NiCl2 + CoCl2 (cNiCl2 + cCoCl2 = 00.1 M) at 50 mV s−1 for 20 cycles with different Ni2+ /Co2+ ratio: (A) 1:0, (B) 1:4, (C) 4:1 and (D) 0:1.

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A

I/ A

p

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R=-0.9912

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R=0.9936

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B

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a

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/mv s

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Epc

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ln /v s

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Fig. 4. (A) CVs of Ni-Co NSs/RGO/GCE in 0.1 M NaOH at different scan rate: (a) 10, (b) 30, (c) 50, (d) 70, (e) 100, (f) 120, (g) 150, (h) 200, (i) 250, (j) 300, (k) 350, (l) 400, (m) 450, (n) 500, (o) 600 and (p) 700 mV s−1 . (B) Plot of peak current versus the potential scan rate. (C) Plot of peak potential versus the natural logarithm of scan rate (ln v).

that in absence of glucose (curve f). While the oxidation peaks at 0.27 V and 0.50 V were much smaller at Ni-Co NSs/GCE (curve e, g) than that at the Ni-Co NSs/RGO/GCE (curve f, h). These results implied that the catalytic current mainly resulted from active NiCo NSs oxide toward the catalytic oxidation of glucose, and the RGO played a crucial role in the sensor’s performance. RGO provided a large surface area to increase the quantity and reduce the dimension of the active Ni-Co NSs oxide. The large surface-to-volume ratio of the formed Ni-Co NSs oxide led to a large total surface area to promote the oxidation of glucose. The glucose was oxidized by active Ni-Co NSs oxide with cyclic mediation redox process [35,36,43]. The NiOOH can be used as heterogeneous catalysts and showed good chemical stability and electrocatalytic activity [40–43]. At higher potential, CoOOH was further oxidized into CoO2 [40–43]. CoOOH + OH− → CoO2 + H2 O + e

(10)

Then the NiOOH and CoO2 can catalyze glucose oxidation to form glucolactone. The possible redox mechanism can be assumed as followed [8,13,35,36,43]: NiOOH + glucose → Ni(OH)2 + glucolactone

(11)

2CoO2 + glucose → 2CoOOH + glucolactone

(12)

The performance of Ni-Co NSs/RGO/GCE obtained at different potential cycle number (2, 10, 20 and 35) was investigated by CVs in 0.1 M NaOH in the presence of 0.04 M glucose (Fig. 6), since the amount and morphology of Ni-Co NSs on the modified electrode depended on the potential cycle number. When the potential cycle number was 2 (curve a), the glucose oxidation current was low, which was due to the relatively small number of Ni-Co NSs formed on the surface of RGO/GCE. After 10 cycles (curve b), the glucose oxidation peak current became larger because the amount of NiCo NSs electrodeposited on the surface of RGO/GCE increased and the catalytic performance was better. The glucose oxidation peak current reached a largest value at the potential cycle number of 20 (curve c). The result confirmed that the regular flower-like Ni-Co NSs provided the largest surface area to promote the oxidation of glucose. When the potential cycle number further increased to 35 (curve d), the oxidation peak current decreased, even the shape of peak changed. It was because that the flower-like Ni-Co NSs disappeared and only thick films formed, which were not good for the electron transfer during the glucose oxidation process. These results showed that the flower-like Ni-Co NSs/RGO/GCE obtained at the potential cycle number of 20 exhibited the best catalytic performance of glucose oxidation. The effects of Ni2+ /Co2+ ratio on the catalytic performance of NiCo NSs/RGO/GCE were also investigated. Fig. 7A showed the CVs of 300

300

h

200

2 cycles 10 20 35

c d b a

g

I/ A

I/ A

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a b c d

100 f c,d a,b

e 0

100

0

-100

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0.1

0.2

0.3

0.4

0.5

0.6

0.7

E vs SCE/V Fig. 5. CVs of different electrodes in 0.1 M NaOH in the absence (a, c, e, f) and presence (b, d, g, h) of 0.04 M glucose at 50 mV s−1 : (a, b) bare GCE, (c, d) RGO/GCE, (e, g) Ni-Co NSs/GCE and (f, h) Ni-Co NSs/RGO/GCE.

-0.1

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0.2

0.3

0.4

0.5

0.6

0.7

E vs SCE/V Fig. 6. CVs of Ni-Co NSs/RGO/GCE obtained at the potential cycle number of 2 (curve a), 10 (curve b), 20 (curve c) and 35 (curve d) in 0.1 M NaOH in the presence of 0.04 M glucose at 50 mV s−1 .

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the Ni NSs/RGO/GCE in 0.1 M NaOH solution in the absence (curve a) and presence (curve b) of 0.04 M glucose. In the presence of 0.04 M glucose, an obvious oxidation peak appeared at about 0.54 V (curve a) as compared with that in absence of glucose (curve b). Fig. 7B–D showed the CVs of Ni-Co NSs/RGO/GCE obtained at different Ni2+ /Co2+ ratio for the oxidation of glucose. The glucose oxidation started at 0.43 V (Fig. 7B) at the Ni-Co NSs/RGO/GCE obtained at Ni2+ /Co2+ ratio of 1:4, showing a negative shift of 0.11 V as compared with the Ni NSs/RGO/GCE. An additional negative-shift of 0.17 V was observed at the Ni-Co NSs/RGO/GCE obtained at Ni2+ /Co2+ ratio of 1:1 (Fig. 7C). The onset potentials of glucose oxidation were decreased gradually with increasing of Co2+ content in the electrodepositing solution. It can be explained by the following reasons. The doping of Co atoms could prevent from the formation of less electrochemical active ␤-phase Ni(OH)2 in the subsequent transformation from Ni-Co NSs to hydroxide and the formation of Co3+ could also increase the hydroxide conductivity and reduce the

1200

redox peaks potentials of Ni(OH)2 [24,46,47]. Moreover, Co facilitates the Ni to reach a higher oxidation state during the oxidation process and promotes the electron transfer of glucose oxidation [48,49]. In addition, the incorporation of Co resulted in the formation of flower-like Ni-Co NSs. The flower-like Ni-Co NSs resulted in the large electrode surface area to facilitate the glucose oxidation. However, the onset potential of glucose oxidation showed a little change as the content of Co further increased (Fig. 7D and E). It may be because that the Co3+ species are inactive for glucose oxidation (Fig. 7E) and the increase of Co might decrease the exposed Ni active sites and inhibit the glucose oxidation [23]. Fig. 7F showed the changes of the onset potential (Eoxidation ) and the catalytic current (Icat , obtained at the same potential of 0.3 V) for glucose oxidation at Ni-Co NSs/RGO/GCE versus the Co2+ contents. As the Co2+ content increased to 50%, the Eoxidation of glucose oxidation decreased gradually. After that, the Eoxidation showed a little change. The Icat was increased to 117 ␮A at the Ni2+ /Co2+ molar ratio of 1:1.

1000

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E vs SCE/V 250

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0.45

80

0.40 60 0.35 40

Eoxidation

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Icat / A

I/ A

100

I/ A

150

0 0

20

40

60

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Cobalt ion(molar ratio%)

Fig. 7. CVs of different electrodes in 0.1 M NaOH in the absence (a) and presence (b) of 0.04 M glucose at 50 mV s−1 : (A) Ni NSs/RGO/GCE, (B) Ni-Co NSs/RGO/GCE (cNiCl2 /cCoCl2 = 1 : 4), (C) Ni-Co NSs/RGO/GCE (cNiCl2 /cCoCl2 = 1 : 1), (D) Ni-Co NSs/RGO/GCE (cNiCl2 /cCoCl2 = 4 : 1) and (E) Co NSs/RGO/GCE. (F) Onset potential and catalytic current for glucose oxidation on Ni-Co NSs/RGO/GCE versus the Co2+ contents in the deposition solution.

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350

A210 B 180 I/ A

250

150 120

200

I/ A

28

y=47.54+105.71x R=0.9985

Icat/Id

300

30

26

where  > 1.5, erf(1/2 ) is almost equal to unity, the above equation can be reduced to:

C

Icat = 1/2 1/2 = 1/2 (Kcat Ct)1/2 Id

y=3.38+10.60x R=0.9983

24 22

90

150

0.4

e d c b a

100 50 0

0.8 1.2 t-1/2/s-1/2

20

1.6

1.8

2.0 2.2 t1/2/s1/2

2.4

a-e

-50 0

20

40

60

80

100

t/s Fig. 8. (A) Double steps chronoamperograms of Ni-Co NSs/RGO/GCE in 0.1 M NaOH solution with different concentrations of glucose: (a) 0 mM, (b) 1 mM, (c) 2 mM, (d) 3 mM and (e) 4 mM. Potential steps were 500 mV and 250 mV, respectively. (B) Dependency of transient current on t−1/2 . (C) Dependency of Icat /Id on t1/2 derived from the data of chronoamperograms of curve a and c in panel (A).

3.5. Chronoamperometric response and calibration curve The catalytic rate constants (Kcat ) of the as-prepared Ni-Co NSs/RGO/GCE were measured with double steps chronoamperograms under suitable working electrode potentials. Fig. 8A showed the double steps chronoamperograms for the Ni-Co NSs/RGO/GCE in the absence (curve a) and presence (curve b–e: 1–4 mM) of glucose. The applied potential steps were 500 mV and 250 mV, respectively. Fig. 8B showed the plot of net current versus the minus square roots of time, and it presented a linear dependency. It demonstrated that the electrocatalytic oxidation of glucose was a diffusion-controlled process. The diffusion coefficient (D) of glucose could be estimated according to Cottrell equation [50]: I = nFAD1/2 C−1/2 t −1/2

(13)

where C is the bulk concentration. The mean value of the diffusion coefficient of glucose was found to be 4.33 × 10−4 cm2 s−1 by using the slope of the line in Fig. 8B. Chronoamperometry could also be used for the evaluation of the catalytic rate constant with the help of the following equation [51]:



Icat exp (−) = 1/2 1/2 erf (1/2 ) + Id 1/2

 (14)

where Icat and Id are the currents in the presence and absence of glucose, respectively,  = Kcat Ct is the argument of the error function, Kcat is the catalytic rate constant, and t is elapsed time. In the case

(15)

From the slope of the plot: Icat /Id versus t1/2 , as shown in Fig. 8C, the mean value of Kcat for glucose oxidation was calculated to be 1.79 × 107 cm3 mol−1 s−1 . Yi et al. [52] have used the same method to evaluate the catalytic rate constant of glucose on nanoNi/Ti electrode. In addition, the chronoamperometric response of Ni-Co NSs/GCE was also investigated for comparison. All of the kinetic parameters obtained in this work were listed in Table 1. To study the effect of applied potentials on electrocatalytic oxidation of glucose at the Ni-Co NSs/RGO/GCE, the amperometric responses of the electrode to six successive injection of 10 ␮M glucose (Fig. 9A) were recorded at various applied potentials. The oxidation of glucose at the Ni-Co NSs/RGO/GCE were started at about 0.3 V, and then increased sharply toward the positive potential. The amperometric response matched the CVs results very well. Thus 0.5 V was chosen as the working potential in the following experiments. Amperometric measurements were carried out at 0.5 V at NiCo NSs/RGO/GCE by successive injection of glucose (curve a in Fig. 9B) into 0.1 M NaOH under stirring. The oxidation current reached a maximum steady-state value and achieved 95% of the steady-state current within 2 s. The linear range of the glucose detection was from 0.01 mM to 2.65 mM (r = 0.9967) with a slope of 1773.61 ␮A cm−2 mM−1 , and the detection limit was estimated to be 3.79 ␮M based on the criterion of a signal-to-noise ratio of 3 (S/N = 3) (curve a in Fig. 9C). To study the effect of RGO on the catalytic activity and sensitivity for the oxidation of glucose, the amperometric response of the Ni-Co NSs/GCE toward the oxidation of glucose (curve b in Fig. 9C) was investigated. The comparison between Ni-Co NSs/RGO/GCE and Ni-Co NSs/GCE in Table 1 indicated that Ni-Co NSs/RGO/GCE exhibited a better catalytic activity and sensitivity for the oxidation of glucose. The better catalytic activity and sensitivity might result from the large surface area of Ni-Co NSs/RGO/GCE and the synergetic effect between RGO and Ni-Co NSs. Up to now, many nonenzymatic sensors have been developed for the detection of glucose, and all of them have some advantages and limitations [8–13,17,36,43,52]. A comparison of the glucose assay performance of our newly designed sensor with those already reported in literature was also shown in Table 1. Taking Co3 O4 nanofibers/GCE [8] as an example, the sensitivity was lower (36.25 ␮A cm−2 mmol L−1 ) than that of as-prepared Ni-Co NSs/RGO/GCE, although the detection limit was very low (0.97 ␮M). By comparing, it could be clearly seen that the Ni-Co NSs/RGO/GCE offered a reasonable linear range and detection limit, but the sensitivity was highest among these sensors except for nanoNi/Ti electrode which exhibited a narrow linear range.

Table 1 Comparison of the performance of various non-enzymatic glucose sensors. Modified electrode

D cm2 s−1

Kcat cm3 mol−1 s−1

Detection limit ␮mol L−1

Linear range mmol L−1

Sensitivity ␮A cm−2 mmol−1

References

Ni-Co NSs/RGO/GCE Ni-Co NSs/GCE Co3 O4 nanofibers-Nafion/GCE NiO nanofibers/GCE NiO/graphene/GCE NiO nanoparticles/GO/GCE CoOOH nanosheet electrode graphene-Co3 O4 nanoneedle electrode Pt/Ni nanowires/GCE Ni nanowire array electrode Pt/Ni-Co nanowires electrode nanoNi/Ti electrode

4.33 × 10−4 2.94 × 10−4 – – – – – – – – – –

1.79 × 107 7.86 × 106 – – – – – – – – – 1.67 × 106

3.79 6.83 0.97 1.28 5 1 10.9 <10 1.5 0.1 1 1.2

0.01–2.65 0.01–2.65 Up to 2.04 Up to 1.94 0.02–2 0.00313–3.05 Up to 0.5 0.05–0.3 0.002–2.0 0.0005–7 0–0.2 0.05–0.6

1773.61 878.05 36.25 32.91 15.94 ␮A mmol−1 1087 967 – 920 1043 1125 7320

This work This work [8] [9] [10] [11] [12] [13] [17] [36] [43] [52]

L. Wang et al. / Electrochimica Acta 114 (2013) 484–493

491

10

A 8

B a

0.6V

200

0.4V

I/ A

6

I/ A

250

0.5V

4

500 M

150

b

100 0.3V

2

50 0.25V 0.1V

0 100

200

300

400

100 M 20 M

10 M

0

500

200

400

600

800

t/s C y=6.70+83.36x R=0.9967

200

I/ A

D

150

a b

100

D-galactose AA

UA 3+ fructose Fe _ 2BrO3 SO4 _ 2+ NO2 Co

y=3.24+36.00x R=0.9983

50

1200

glucose

I/ A

250

1000

t/s

Ni _

NO3 glucose

0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

concentration/mmol L

_

IO3

+

2+

Fe

2+

2+

Zn

2-

SO3

K

glucose

100 200 300 400 500 600 700 800 900

-1

E vs SCE/V

Fig. 9. (A) Electrocatalytic oxidation of 10 ␮M glucose on Ni-Co NSs/RGO/GCE at different applied potentials. (B) Typical amperometric response of Ni-Co NSs/RGO/GCE (a) and Ni-Co NSs/GCE (b) to successive injection of glucose into the stirred NaOH. (C) The plots of steady-state current versus glucose concentration. (D) The effects of some electroactive substance on glucose detection. Applied potential: 0.5 V.

Table 2 Determination of glucose in blood serum samples. Blood serum samples (mM)

Diluted samples (mM)

Added (mM)

Determined by colorimetric enzymatic method (mM)

Determined by our nonenzymatic sensor (mM)

Recovery (%)

RSD (%, n = 5)

9.6

0.57 0.57 0.57 1.05 1.05 1.05

0.50 0.70 1.20 0.50 1.00 1.50

1.06 1.29 1.79 1.58 2.09 2.52

1.05 1.31 1.72 1.62 2.13 2.49

98.13 103.15 97.18 104.52 103.90 97.65

3.75 3.64 2.93 3.27 3.87 4.21

3.6. Selectivity, stability and repeatability of Ni-Co NSs/RGO/GCE The interference was also investigated. Fig. 9D showed the current responses of the sensor to different chemicals. For Fe3+ , Fe2+ , SO4 2− , BrO3 − , IO3 − , NO2 − , NO3 − and Cl− in a 10-fold concentration, no obvious interference to glucose detection was observed. While only for SO3 2− in 10-fold concentration, there was a significant interference for the glucose detection. The interference of organic compounds, including fructose, d-galactose, UA and AA have also been investigated. Amperometric response of the sensor to consecutive injection 0.1 mM of glucose, fructose, d-galactose, UA and AA showed that the addition of UA hardly provided notable interference for glucose sensing, and 0.1 mM fructose, d-galactose and AA only marked a poor increase of currents (<10%). These results implied the good selectivity of Ni-Co NSs/RGO/GCE. The determination of glucose in blood serum samples was also performed on the Ni-Co NSs/RGO/GCE. In brief, the blood samples obtained from the hospitalized patients were first diluted with 1 M NaOH (the final solution was adjusted to pH 13), at which the NiCo NSs/RGO/GCE was used to monitor the glucose content. The standard colorimetric enzymatic procedure was used as a reference for checking the sensor accuracy. The results obtained from

the glucose sensor agree well with those obtained by the standard colorimetric enzymatic method. The relative standard deviations (RSD) listed in Table 2 indicate most of the results are accurate and credible. Thus, it could be concluded that the developed sensor performs very well in the detection of glucose in serum samples. The stability and repeatability of the resulted sensor were also investigated. After the sensor was stored in the inverted beaker at room temperature for 45 days, the current response to 0.04 M glucose decreased by 3.8%. To evaluate the repeatability of the same sample, the same sensor was used to detect 0.04 M glucose for 10 times and a RSD of 4.3% was obtained. To test the electrode-toelectrode repeatability, six sensors were prepared under the same condition. The responses of the six sensors toward 0.04 M glucose were measured with a RSD of 5.6%. The good repeatability of the results indicated the reliability of the sensor results. 4. Conclusions Flower-like Ni-Co NSs have been electrodeposited on the RGO/GCE by CVs. It was found that RGO could promote the formation of a large number of flower-like Ni-Co NSs on the surface of GCE. The Ni-Co NSs/RGO/GCE showed good catalytic activity

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