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Non-enzymatic electrochemical glucose sensor based on NiMoO4 nanorods
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 145501 (http://iopscience.iop.org/0957-4484/26/14/145501) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.122.253.212 This content was downloaded on 30/04/2015 at 05:25
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 145501 (7pp)
doi:10.1088/0957-4484/26/14/145501
Non-enzymatic electrochemical glucose sensor based on NiMoO4 nanorods Dandan Wang, Daoping Cai, Hui Huang, Bin Liu, Lingling Wang, Yuan Liu, Han Li, Yanrong Wang, Qiuhong Li and Taihong Wang Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, People’s Republic of China E-mail: [email protected] Received 8 November 2014, revised 2 February 2015 Accepted for publication 14 February 2015 Published 16 March 2015 Abstract
A non-enzymatic glucose sensor based on the NiMoO4 nanorods has been fabricated for the first time. The electrocatalytic performance of the NiMoO4 nanorods’ modified electrode toward glucose oxidation was evaluated by cyclic voltammetry and amperometry. The NiMoO4 nanorods’ modified electrode showed a greatly enhanced electrocatalytic property toward glucose oxidation, as well as an excellent anti-interference and a good stability. Impressively, good accuracy and high precision for detecting glucose concentration in human serum samples were obtained. These excellent sensing properties, combined with good reproducibility and low cost, indicate that NiMoO4 nanorods are a promising candidate for non-enzymatic glucose sensors. Keywords: NiMoO4, non-enzymatic, glucose sensor, electrochemical (Some figures may appear in colour only in the online journal) 1. Introduction
intermediates and chloride [9–12]. Transition metals, such as Cu [13] and Ni [14], have been noticed by many researchers since they can not only act as catalysts toward glucose oxidation like noble metals but also avoid surface poisoning and high cost due to the use of noble metals, but they are not stable and can be easily oxidized both in air and in a solution. So, their oxides have caught much attention because of the improved stability. Some metal oxides with exchanging valence states, such as CuO, NiO and CoOx [15–17], have been investigated to construct effective non-enzymatic glucose sensors; however, most of these electrodes have the drawback of low sensitivity because of their poor electrical conductivity. It has been reported that enhanced performance could be achieved benefiting from synergistic effects of multicomponent compositions; so, many efforts have been devoted to explore various compounds to obtain high performance such as NiO-Ag [18], NiO-Pt [19], CuO-Ag [20] and Ni-Cuoxide [21]. Among them, binary metal oxides seem to be the most favored candidates for non-enzymatic glucose detection since the ability to produce glucose sensors at low cost is also a consideration. Recently, metal molybdates have attracted great research interest due to their feasible oxidation state and high electrical
Reliable and fast glucose detection is vital due to its considerable importance in biotechnology, clinical diagnostics and the food industry [1–3]. Among a variety of detection mechanisms, electrochemical detection has been considered as one of the most promising candidates because of its high sensitivity, good selectivity and ease of operation [4, 5]. In general, glucose sensors can be divided into two kinds: enzyme-based sensors and non-enzymatic glucose sensors. Enzymatic detection usually exhibits good selectivity and high sensitivity, but due to the intrinsic nature of enzymes, enzyme-based sensors always suffer from stability problems caused by temperature, pH, humidity and toxic chemicals [6– 8]. Additionally, enzyme-based biosensors are not costeffective due to the high price of enzymes. So, many attempts have been made to construct a cheap, sensitive and selective non-enzymatic glucose sensor. Noble metals, such as Pt [9], Au [10] and Pd [11], have been widely investigated as the sensing materials for nonenzymatic glucose detection. However, most of these electrodes have the drawbacks of low sensitivity and poor selectivity, caused by surface poisoning from adsorbed 0957-4484/15/145501+07$33.00
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2.0 mL distilled water and sonicated for 1 h; then, 8 uL of asprepared suspension were dropped onto the surface of GCE. After drying in air overnight, 8 uL of Nafion solution (0.1 wt % in ethanol) were dropped on the layer of NiMoO4 nanorods so as to entrap the NiMoO4 nanorods. The as-prepared electrode was denoted as Nafion/NiMoO4 NRs/GCE. The control electrode (denoted as Nafion/GCE) was prepared by a similar procedure.
2.3. Apparatus
The phase purity and structure of the as-prepared NiMoO4 nanorods were characterized by x-ray diffraction (XRD) on a Philips X’pert pro x-ray diffractometer using Cu-Kα radiation at 40 kV and 30 mA. The morphology of the as-prepared NiMoO4 nanorods was characterized by field emission scanning electron microscopy (SEM) (CARL ZEISS Supra 55) and transmission electron microscopy (TEM) (JEOL JEM-2100) operating at 200 kV. Also, all the electrochemical measurements were carried out on a CHI660E electrochemical workstation. The modified electrode was used as the working electrode, a standard calomel electrode (SCE) was used as the reference electrode and a Pt wire was used as the counter electrode; all the experiments were done at ambient temperature. Before testing, the modified electrode was immersed in water for 1 h.
Figure 1. XRD pattern of the as-prepared NiMoO4 nanorods.
conductivity [22–24]. Among this family, NiMoO4 has drawn distinctive attention because of its chemical stability, enhanced electrochemical performance and low cost [23, 25]. To the best of our knowledge, there are few reports about sensors based on metal molybdates to detect glucose. Herein, a new non-enzymatic glucose sensor was fabricated by coating NiMoO4 nanorods onto the surface of a glass carbon electrode (GCE), which was prepared by a simple hydrothermal method [25]. It is worth mentioning that a NiMoO4 nanorod modified electrode exhibited a variety of attractive results, such as high sensitivity, low cost, low detection limit, wide liner range and anti-interference, as well as long-term stability. As far as we know, this is the first work employing NiMoO4 nanorods as sensing materials for non-enzymatic glucose detection.
3. Results and discussion 3.1. Characterization of the NiMoO4 nanorods
The phase purity and structure of the as-prepared product were characterized by XRD. Figure 1 displays the XRD pattern of the as-prepared product. Most of the diffraction peaks can be indexed to the monoclinic structured NiMoO4 (JCPDS Card No. 45-0142) with a space group of C2/m; the strongest peak at 2θ = 26.6° is the typical peak of β-NiMoO4 [28]; the rest of the diffraction peaks (marked with green lines) can be assigned to α-NiMoO4 (JCPDS Card No. 330948) [26–28], indicating that the as-prepared NiMoO4 nanorods possess the characteristics of both α-NiMoO4 and βNiMoO4. The morphology of the as-prepared product was investigated by SEM and TEM. Figures 2(a) and (b) show the SEM and the TEM image of the NiMoO4 nanorods; the diameter is about 80 nm, and the length is in the range of 300 nm to 1 μm. The high-resolution transmission electron microscopy HRTEM image (figure 2(c)) from one individual nanorod clearly shows interplanar spacings of 0.27 nm, corresponding to the (112) crystal planes. The selected area electron diffraction (SAED) pattern (inset in figure 2(c)) implies their polycrystalline characteristics. The composition of the asprepared product was also investigated by energy-dispersive x-ray spectroscopy (EDS) analysis. As shown in figure 2(d), Ni, Mo and O peaks are identified in the spectrum; the atom ratio of Ni/Mo is about 1:1.
2. Experimental details 2.1. Preparation of NiMoO4 nanorods
The NiMoO4 nanorods were prepared by a one-pot hydrothermal synthesis route, which was reported in our previous literature [25]. Briefly, 0.33 g of Na2MoO4.7H2O and 0.29 g of Ni(NO3)2.6H2O were added to 30 mL of mixed solvents (ethanol/distilled water volume ratio = 1:1) under ultrasonic treatment for several minutes; the homogeneous solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 140 °C for 12 h. Then, the product was centrifuged and washed with distilled water and ethanol three times and dried at 65 °C for 12 h. Finally, NiMoO4 nanorods were obtained by annealing the precursor in air at 500 °C for 4 h with a rate of 2 °C min−1. 2.2. Preparation of NiMoO4 nanorod modified electrodes
A simple drop-casting method was used to modify the electrodes. Prior to the surface casting, the GCE (dia. 3 mm) was polished with 1.0 and 0.3 um alumina powder in a sequence, rinsed with water and sonicated in ethanol and water, respectively. Then, the electrode was dried at room temperature in air. 2 mg of NiMoO4 nanorods were added in 2
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Figure 2. (a) SEM image and (b) TEM image of the as-prepared NiMoO4 nanorods. (c) HRTEM image with the inset showing the SAED pattern. (d) The EDS spectrum of the as-prepared NiMoO4 nanorods.
to a higher value (from 0.478 V to 0.497 V) with the increment of glucose concentration, which can be attributed to the change in pH due to the glucolactone produced by glucose oxidation [17]. The electrocatalytic mechanism of the NiMoO4 nanorods toward glucose oxidation has been shown in figure 3(c), which can mainly be attributed to a Ni(II)/Ni (III) redox couple in an alkaline medium [29, 30]; in this process, the Mo is not involved in the redox reaction; the chief role of Mo is to improve the conductivity of metal molybdates and then enhance the electrocatalytic performance [24]. The CV responses of the Nafion/NiMoO4 NRs/GCE at different scanning rates were examined. As shown in figures 4(a) and (b), the peak current densities change obviously with the increment of the scanning rate, and both the anodic and cathodic peak current densities display a linear response to the square root of the scan rate in the range from 40 mV s−1 to 150 mV s−1, implying a diffusion-controlled process [31, 32]. The peak-to-peak potential separation (ΔEp) in the CV responses increases with the increase of the scanning rate, indicating charge-transferkinetic limitations [33].
3.2. Glucose-sensitive electrochemistry behavior of the Nafion/ NiMoO4 NRs/GCE
The Nafion/NiMoO4 NRs/GCE was characterized by cyclic voltammetry in the potential range of 0 V to 0.7 V in a 1.2 M NaOH solution; the Nafion/GCE was investigated as a control. As shown in figure 3(a), a pair of well-defined redox peaks can be observed at the Nafion/NiMoO4 NRs/GCE, which can be attributed to the transition between Ni(II) and Ni (III) in an alkaline medium [29, 30], but no CV peaks appear at the Nafion/GCE, indicating that the background of the Nafion/GCE can be ignored. Figure 3(b) shows the CVs of both electrodes in a 1.2 M NaOH solution containing x mM glucose (x = 0.0, 1.0, 2.0 or 3.0). On the Nafion/GCE (the inset in figure 3(b)), there is no significant difference with or without glucose, indicating that the Nafion/GCE displays negligible electrocatalytic activity toward the oxidation of glucose; however, the anodic peak current density increases obviously at the Nafion/NiMoO4 NRs/GCE with the increment of glucose concentration, confirming the remarkable catalytic activity of NiMoO4 nanorods toward glucose oxidation. Additionally, the anodic peak potential shifts slightly 3
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Figure 3. (a) CVs of the modified electrodes in 1.2 M NaOH. (b) CVs of the modified electrodes in 1.2 M NaOH with x mM glucose (the scan rate = 50 mV s−1). (c) The schematic of the Nafion/NiMoO4 NRs/GCE toward the oxidation of glucose.
In order to obtain the best sensing performance of the Nafion/NiMoO4 NRs/GCE, the influence of the NaOH solution concentration was investigated by cyclic voltammetry. The experiment results are shown in figure 5. The low concentration of the NaOH solution will lower the electrocatalytic ability of NiMoO4 nanorods toward the oxidation of glucose, but on the other side, too high of a concentration of NaOH solution will inhibit the transition from Ni(III) to Ni (II), thus limiting electrocatalytic sites on the NiMoO4 nanorods and then interfering the electrocatalytic reaction for glucose. A 1.2 M NaOH solution can be selected as the optimal concentration from the CV experiments (figure 5).
3.3. Amperometric detection of glucose at the Nafion/NiMoO4 NRs/GCE
The glucose sensing performance of the Nafion/NiMoO4 NRs/GCE was investigated by amperometry. The current– time (I–t) curve was carried out at +0.5 V (versus SCE) in a continuously stirred 1.2 M NaOH solution, as the change of the oxidation peak current displayed was the utmost obvious at about +0.5 V (versus SCE) with the addition of glucose. As shown in figure 6, a fast (within 4 s), stable and step-like increasing response can be observed at +0.5 V when various concentrations of the glucose are stepwise added. Furthermore, the calibration curve for Nafion/NiMoO4 NRs/GCE is shown in the bottom inset of figure 6. The Nafion/NiMoO4 NRs/GCE shows an excellent linear response to the glucose
Figure 4. (a) CVs of the Nafion/NiMoO4 NRs/GCE at different scan rates in 1.2 M NaOH. (b) Linear plots of peak current density versus the square root of the scan rate. 4
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physiological level of glucose is 3–8 mM, which is much higher than those of interfering species AA (∼0.1 mM) and UA (∼0.02 mM) [9, 10]. The amperometric responses to these interfering species at the Nafion/NiMoO4 NRs/GCE were carried out at + 0.5 V (versus SCE) in a continuously stirred 1.2 M NaOH solution with gradually injected 1 mM glucose, 0.1 mM AA, 0.02 mM UA and 0.1 mM NaCl; this was repeated once. Finally, 2 mM glucose were added. As shown in figure 7, the amperometric responses to these interfering species are almost insignificant in comparison with the well-defined oxidation currents of glucose, indicating that the Nafion/NiMoO4 NRs/GCE shows high selectivity for the glucose detection in the presence of interfering species.
3.5. Reproducibility and stability
The reproducibility of the Nafion/NiMoO4 NRs/GCE was investigated. Five NiMoO4 nanorod modified electrodes were prepared following the same fabrication method, and their current responses to 1 mM glucose were investigated. The relative standard deviation (RSD) was 2.56%, confirming the high reproducibility of the Nafion/ NiMoO4 NRs/GCE in consideration of the fabrication method. The stability of the Nafion/NiMoO4 NRs/GCE was also investigated by amperometry. Figure 8(a) shows the amperometric response of the Nafion/NiMoO4 NRs/GCE to 14 mM glucose after the current–time (I–t) curve (in 3.3) for another long period of running time: 1500 s; there is little decay in the current signal during this period. Figure 8(b) shows the amperometric response to 1 mM glucose over a 20 day period. The electrode was stored in air at ambient temperature; its amperometric response was tested every 3 or 4 days. After 20 days, the final current signal accounted for about 92% of the original current signal,
Figure 5. CVs of the Nafion/NiMoO4 NRs/GCE in different concentrations of NaOH solutions: (a) without glucose and (b) with 1.0 mM glucose. (c) Relationship between ΔJpa and CNaOH.
concentration in the range from 0.05 mM to 14 mM with a correlation coefficient of 0.9996, a calibration sensitivity of 389.9 uA mM−1 cm−2 and a detection limit of 0.36 uM (S/ N = 3). As shown in table 1, the Nafion/NiMoO4 NRs/GCE shows a higher sensitivity and a lower detection limit in comparison to other previously reported glucose sensors based on nickel materials by the same fabrication method. 3.4. Interference test
Selectivity is an important evaluation factor for non-enzymatic glucose sensors due to the effect of some coexisting species such as UA, AA and NaCl. The normal 5
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Table 1. Comparison of analytical performance of our proposed glucose sensor with other published non-enzymatic glucose sensors.
Detection Limit (uM)
Materials NiMoO4 NRs NiO hollow nanospheres 5% NiO@Ag NWs NiO–Pt NFs NiOAt 0.2 V Au NBs At 0.6 V NiO/MWCNT
0.36 47 1.01 0.313 0.65 1.36 160
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Present work [16] [18] [19] [34] [35]
Table 2. Comparison between the values obtained in the hospital and those obtained using our biosensor for the determination of glucose in
the serum samples. Sample number 1 2
CG determined in the hospital (mM)
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9.40 6.22
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Figure 7. Amperometric response of the Nafion/NiMoO4 NRs/GCE at +0.5 V with successive additions of different analytes.
implying the good stability of the Nafion/NiMoO4 NRs/GCE. 3.6. Human serum sample detection
In order to confirm the practicality, the Nafion/NiMoO4 NRs/ GCE was further applied to detect glucose concentration in fresh human serum samples (from a local hospital). The test was carried out in a continuously stirred 1.2 M NaOH at +0.5 V (versus SCE) with stepwise injected 0.1 mM glucose, serum sample 1, serum sample 2 and 0.1 mM glucose. Based on the calibration curve in the bottom inset of figure 6, the corresponding glucose concentration could be figured out, as long as the current density was obtained. The test was done three times to confirm the reproducibility of the data. The experimental data was in comparison with that obtained from the local hospital. The results are shown in table 2; the good accuracy (recovery) and the high precision (low RSD) reveal the applicability of Nafion/NiMoO4 NRs/GCE for nonenzymatic detection of glucose.
Figure 8. (a) Amperometric response of the Nafion/NiMoO4 NRs/ GCE to 14 mM glucose after current–time (I–t) curve for another long period of running time: 1500 s. (b) Long-term stability of the Nafion/NiMoO4 NRs/GCE.
4. Conclusion In summary, a novel non-enzymatic glucose sensor has been successfully fabricated using NiMoO4 nanorods as sensing 6
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materials. In comparison to previously reported Ni-contained glucose sensors following the same fabrication method, the Nafion/NiMoO4 NRs/GCE exhibited a faster response time (
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Non-enzymatic electrochemical glucose sensor based on NiMoO4 nanorods
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 145501 (http://iopscience.iop.org/0957-4484/26/14/145501) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 128.122.253.212 This content was downloaded on 30/04/2015 at 05:25
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 145501 (7pp)
doi:10.1088/0957-4484/26/14/145501
Non-enzymatic electrochemical glucose sensor based on NiMoO4 nanorods Dandan Wang, Daoping Cai, Hui Huang, Bin Liu, Lingling Wang, Yuan Liu, Han Li, Yanrong Wang, Qiuhong Li and Taihong Wang Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, People’s Republic of China E-mail: [email protected] Received 8 November 2014, revised 2 February 2015 Accepted for publication 14 February 2015 Published 16 March 2015 Abstract
A non-enzymatic glucose sensor based on the NiMoO4 nanorods has been fabricated for the first time. The electrocatalytic performance of the NiMoO4 nanorods’ modified electrode toward glucose oxidation was evaluated by cyclic voltammetry and amperometry. The NiMoO4 nanorods’ modified electrode showed a greatly enhanced electrocatalytic property toward glucose oxidation, as well as an excellent anti-interference and a good stability. Impressively, good accuracy and high precision for detecting glucose concentration in human serum samples were obtained. These excellent sensing properties, combined with good reproducibility and low cost, indicate that NiMoO4 nanorods are a promising candidate for non-enzymatic glucose sensors. Keywords: NiMoO4, non-enzymatic, glucose sensor, electrochemical (Some figures may appear in colour only in the online journal) 1. Introduction
intermediates and chloride [9–12]. Transition metals, such as Cu [13] and Ni [14], have been noticed by many researchers since they can not only act as catalysts toward glucose oxidation like noble metals but also avoid surface poisoning and high cost due to the use of noble metals, but they are not stable and can be easily oxidized both in air and in a solution. So, their oxides have caught much attention because of the improved stability. Some metal oxides with exchanging valence states, such as CuO, NiO and CoOx [15–17], have been investigated to construct effective non-enzymatic glucose sensors; however, most of these electrodes have the drawback of low sensitivity because of their poor electrical conductivity. It has been reported that enhanced performance could be achieved benefiting from synergistic effects of multicomponent compositions; so, many efforts have been devoted to explore various compounds to obtain high performance such as NiO-Ag [18], NiO-Pt [19], CuO-Ag [20] and Ni-Cuoxide [21]. Among them, binary metal oxides seem to be the most favored candidates for non-enzymatic glucose detection since the ability to produce glucose sensors at low cost is also a consideration. Recently, metal molybdates have attracted great research interest due to their feasible oxidation state and high electrical
Reliable and fast glucose detection is vital due to its considerable importance in biotechnology, clinical diagnostics and the food industry [1–3]. Among a variety of detection mechanisms, electrochemical detection has been considered as one of the most promising candidates because of its high sensitivity, good selectivity and ease of operation [4, 5]. In general, glucose sensors can be divided into two kinds: enzyme-based sensors and non-enzymatic glucose sensors. Enzymatic detection usually exhibits good selectivity and high sensitivity, but due to the intrinsic nature of enzymes, enzyme-based sensors always suffer from stability problems caused by temperature, pH, humidity and toxic chemicals [6– 8]. Additionally, enzyme-based biosensors are not costeffective due to the high price of enzymes. So, many attempts have been made to construct a cheap, sensitive and selective non-enzymatic glucose sensor. Noble metals, such as Pt [9], Au [10] and Pd [11], have been widely investigated as the sensing materials for nonenzymatic glucose detection. However, most of these electrodes have the drawbacks of low sensitivity and poor selectivity, caused by surface poisoning from adsorbed 0957-4484/15/145501+07$33.00
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2.0 mL distilled water and sonicated for 1 h; then, 8 uL of asprepared suspension were dropped onto the surface of GCE. After drying in air overnight, 8 uL of Nafion solution (0.1 wt % in ethanol) were dropped on the layer of NiMoO4 nanorods so as to entrap the NiMoO4 nanorods. The as-prepared electrode was denoted as Nafion/NiMoO4 NRs/GCE. The control electrode (denoted as Nafion/GCE) was prepared by a similar procedure.
2.3. Apparatus
The phase purity and structure of the as-prepared NiMoO4 nanorods were characterized by x-ray diffraction (XRD) on a Philips X’pert pro x-ray diffractometer using Cu-Kα radiation at 40 kV and 30 mA. The morphology of the as-prepared NiMoO4 nanorods was characterized by field emission scanning electron microscopy (SEM) (CARL ZEISS Supra 55) and transmission electron microscopy (TEM) (JEOL JEM-2100) operating at 200 kV. Also, all the electrochemical measurements were carried out on a CHI660E electrochemical workstation. The modified electrode was used as the working electrode, a standard calomel electrode (SCE) was used as the reference electrode and a Pt wire was used as the counter electrode; all the experiments were done at ambient temperature. Before testing, the modified electrode was immersed in water for 1 h.
Figure 1. XRD pattern of the as-prepared NiMoO4 nanorods.
conductivity [22–24]. Among this family, NiMoO4 has drawn distinctive attention because of its chemical stability, enhanced electrochemical performance and low cost [23, 25]. To the best of our knowledge, there are few reports about sensors based on metal molybdates to detect glucose. Herein, a new non-enzymatic glucose sensor was fabricated by coating NiMoO4 nanorods onto the surface of a glass carbon electrode (GCE), which was prepared by a simple hydrothermal method [25]. It is worth mentioning that a NiMoO4 nanorod modified electrode exhibited a variety of attractive results, such as high sensitivity, low cost, low detection limit, wide liner range and anti-interference, as well as long-term stability. As far as we know, this is the first work employing NiMoO4 nanorods as sensing materials for non-enzymatic glucose detection.
3. Results and discussion 3.1. Characterization of the NiMoO4 nanorods
The phase purity and structure of the as-prepared product were characterized by XRD. Figure 1 displays the XRD pattern of the as-prepared product. Most of the diffraction peaks can be indexed to the monoclinic structured NiMoO4 (JCPDS Card No. 45-0142) with a space group of C2/m; the strongest peak at 2θ = 26.6° is the typical peak of β-NiMoO4 [28]; the rest of the diffraction peaks (marked with green lines) can be assigned to α-NiMoO4 (JCPDS Card No. 330948) [26–28], indicating that the as-prepared NiMoO4 nanorods possess the characteristics of both α-NiMoO4 and βNiMoO4. The morphology of the as-prepared product was investigated by SEM and TEM. Figures 2(a) and (b) show the SEM and the TEM image of the NiMoO4 nanorods; the diameter is about 80 nm, and the length is in the range of 300 nm to 1 μm. The high-resolution transmission electron microscopy HRTEM image (figure 2(c)) from one individual nanorod clearly shows interplanar spacings of 0.27 nm, corresponding to the (112) crystal planes. The selected area electron diffraction (SAED) pattern (inset in figure 2(c)) implies their polycrystalline characteristics. The composition of the asprepared product was also investigated by energy-dispersive x-ray spectroscopy (EDS) analysis. As shown in figure 2(d), Ni, Mo and O peaks are identified in the spectrum; the atom ratio of Ni/Mo is about 1:1.
2. Experimental details 2.1. Preparation of NiMoO4 nanorods
The NiMoO4 nanorods were prepared by a one-pot hydrothermal synthesis route, which was reported in our previous literature [25]. Briefly, 0.33 g of Na2MoO4.7H2O and 0.29 g of Ni(NO3)2.6H2O were added to 30 mL of mixed solvents (ethanol/distilled water volume ratio = 1:1) under ultrasonic treatment for several minutes; the homogeneous solution was transferred into a 50 mL Teflon-lined stainless-steel autoclave and maintained at 140 °C for 12 h. Then, the product was centrifuged and washed with distilled water and ethanol three times and dried at 65 °C for 12 h. Finally, NiMoO4 nanorods were obtained by annealing the precursor in air at 500 °C for 4 h with a rate of 2 °C min−1. 2.2. Preparation of NiMoO4 nanorod modified electrodes
A simple drop-casting method was used to modify the electrodes. Prior to the surface casting, the GCE (dia. 3 mm) was polished with 1.0 and 0.3 um alumina powder in a sequence, rinsed with water and sonicated in ethanol and water, respectively. Then, the electrode was dried at room temperature in air. 2 mg of NiMoO4 nanorods were added in 2
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Figure 2. (a) SEM image and (b) TEM image of the as-prepared NiMoO4 nanorods. (c) HRTEM image with the inset showing the SAED pattern. (d) The EDS spectrum of the as-prepared NiMoO4 nanorods.
to a higher value (from 0.478 V to 0.497 V) with the increment of glucose concentration, which can be attributed to the change in pH due to the glucolactone produced by glucose oxidation [17]. The electrocatalytic mechanism of the NiMoO4 nanorods toward glucose oxidation has been shown in figure 3(c), which can mainly be attributed to a Ni(II)/Ni (III) redox couple in an alkaline medium [29, 30]; in this process, the Mo is not involved in the redox reaction; the chief role of Mo is to improve the conductivity of metal molybdates and then enhance the electrocatalytic performance [24]. The CV responses of the Nafion/NiMoO4 NRs/GCE at different scanning rates were examined. As shown in figures 4(a) and (b), the peak current densities change obviously with the increment of the scanning rate, and both the anodic and cathodic peak current densities display a linear response to the square root of the scan rate in the range from 40 mV s−1 to 150 mV s−1, implying a diffusion-controlled process [31, 32]. The peak-to-peak potential separation (ΔEp) in the CV responses increases with the increase of the scanning rate, indicating charge-transferkinetic limitations [33].
3.2. Glucose-sensitive electrochemistry behavior of the Nafion/ NiMoO4 NRs/GCE
The Nafion/NiMoO4 NRs/GCE was characterized by cyclic voltammetry in the potential range of 0 V to 0.7 V in a 1.2 M NaOH solution; the Nafion/GCE was investigated as a control. As shown in figure 3(a), a pair of well-defined redox peaks can be observed at the Nafion/NiMoO4 NRs/GCE, which can be attributed to the transition between Ni(II) and Ni (III) in an alkaline medium [29, 30], but no CV peaks appear at the Nafion/GCE, indicating that the background of the Nafion/GCE can be ignored. Figure 3(b) shows the CVs of both electrodes in a 1.2 M NaOH solution containing x mM glucose (x = 0.0, 1.0, 2.0 or 3.0). On the Nafion/GCE (the inset in figure 3(b)), there is no significant difference with or without glucose, indicating that the Nafion/GCE displays negligible electrocatalytic activity toward the oxidation of glucose; however, the anodic peak current density increases obviously at the Nafion/NiMoO4 NRs/GCE with the increment of glucose concentration, confirming the remarkable catalytic activity of NiMoO4 nanorods toward glucose oxidation. Additionally, the anodic peak potential shifts slightly 3
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Figure 3. (a) CVs of the modified electrodes in 1.2 M NaOH. (b) CVs of the modified electrodes in 1.2 M NaOH with x mM glucose (the scan rate = 50 mV s−1). (c) The schematic of the Nafion/NiMoO4 NRs/GCE toward the oxidation of glucose.
In order to obtain the best sensing performance of the Nafion/NiMoO4 NRs/GCE, the influence of the NaOH solution concentration was investigated by cyclic voltammetry. The experiment results are shown in figure 5. The low concentration of the NaOH solution will lower the electrocatalytic ability of NiMoO4 nanorods toward the oxidation of glucose, but on the other side, too high of a concentration of NaOH solution will inhibit the transition from Ni(III) to Ni (II), thus limiting electrocatalytic sites on the NiMoO4 nanorods and then interfering the electrocatalytic reaction for glucose. A 1.2 M NaOH solution can be selected as the optimal concentration from the CV experiments (figure 5).
3.3. Amperometric detection of glucose at the Nafion/NiMoO4 NRs/GCE
The glucose sensing performance of the Nafion/NiMoO4 NRs/GCE was investigated by amperometry. The current– time (I–t) curve was carried out at +0.5 V (versus SCE) in a continuously stirred 1.2 M NaOH solution, as the change of the oxidation peak current displayed was the utmost obvious at about +0.5 V (versus SCE) with the addition of glucose. As shown in figure 6, a fast (within 4 s), stable and step-like increasing response can be observed at +0.5 V when various concentrations of the glucose are stepwise added. Furthermore, the calibration curve for Nafion/NiMoO4 NRs/GCE is shown in the bottom inset of figure 6. The Nafion/NiMoO4 NRs/GCE shows an excellent linear response to the glucose
Figure 4. (a) CVs of the Nafion/NiMoO4 NRs/GCE at different scan rates in 1.2 M NaOH. (b) Linear plots of peak current density versus the square root of the scan rate. 4
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physiological level of glucose is 3–8 mM, which is much higher than those of interfering species AA (∼0.1 mM) and UA (∼0.02 mM) [9, 10]. The amperometric responses to these interfering species at the Nafion/NiMoO4 NRs/GCE were carried out at + 0.5 V (versus SCE) in a continuously stirred 1.2 M NaOH solution with gradually injected 1 mM glucose, 0.1 mM AA, 0.02 mM UA and 0.1 mM NaCl; this was repeated once. Finally, 2 mM glucose were added. As shown in figure 7, the amperometric responses to these interfering species are almost insignificant in comparison with the well-defined oxidation currents of glucose, indicating that the Nafion/NiMoO4 NRs/GCE shows high selectivity for the glucose detection in the presence of interfering species.
3.5. Reproducibility and stability
The reproducibility of the Nafion/NiMoO4 NRs/GCE was investigated. Five NiMoO4 nanorod modified electrodes were prepared following the same fabrication method, and their current responses to 1 mM glucose were investigated. The relative standard deviation (RSD) was 2.56%, confirming the high reproducibility of the Nafion/ NiMoO4 NRs/GCE in consideration of the fabrication method. The stability of the Nafion/NiMoO4 NRs/GCE was also investigated by amperometry. Figure 8(a) shows the amperometric response of the Nafion/NiMoO4 NRs/GCE to 14 mM glucose after the current–time (I–t) curve (in 3.3) for another long period of running time: 1500 s; there is little decay in the current signal during this period. Figure 8(b) shows the amperometric response to 1 mM glucose over a 20 day period. The electrode was stored in air at ambient temperature; its amperometric response was tested every 3 or 4 days. After 20 days, the final current signal accounted for about 92% of the original current signal,
Figure 5. CVs of the Nafion/NiMoO4 NRs/GCE in different concentrations of NaOH solutions: (a) without glucose and (b) with 1.0 mM glucose. (c) Relationship between ΔJpa and CNaOH.
concentration in the range from 0.05 mM to 14 mM with a correlation coefficient of 0.9996, a calibration sensitivity of 389.9 uA mM−1 cm−2 and a detection limit of 0.36 uM (S/ N = 3). As shown in table 1, the Nafion/NiMoO4 NRs/GCE shows a higher sensitivity and a lower detection limit in comparison to other previously reported glucose sensors based on nickel materials by the same fabrication method. 3.4. Interference test
Selectivity is an important evaluation factor for non-enzymatic glucose sensors due to the effect of some coexisting species such as UA, AA and NaCl. The normal 5
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Table 1. Comparison of analytical performance of our proposed glucose sensor with other published non-enzymatic glucose sensors.
Detection Limit (uM)
Materials NiMoO4 NRs NiO hollow nanospheres 5% NiO@Ag NWs NiO–Pt NFs NiOAt 0.2 V Au NBs At 0.6 V NiO/MWCNT
0.36 47 1.01 0.313 0.65 1.36 160
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Sensitivity
References −1
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389.9 uA mM cm (27.6 uA mM ) 3.43 uA mM−1 67.51 uA mM−1 cm−2 180.80 uA mM−1 cm−2 23.88 uA Mm−1 cm−2 48.35 uA mM−1 cm−2 13.7 uA mM−1
Present work [16] [18] [19] [34] [35]
Table 2. Comparison between the values obtained in the hospital and those obtained using our biosensor for the determination of glucose in
the serum samples. Sample number 1 2
CG determined in the hospital (mM)
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9.40 6.22
9.68 6.38
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Figure 7. Amperometric response of the Nafion/NiMoO4 NRs/GCE at +0.5 V with successive additions of different analytes.
implying the good stability of the Nafion/NiMoO4 NRs/GCE. 3.6. Human serum sample detection
In order to confirm the practicality, the Nafion/NiMoO4 NRs/ GCE was further applied to detect glucose concentration in fresh human serum samples (from a local hospital). The test was carried out in a continuously stirred 1.2 M NaOH at +0.5 V (versus SCE) with stepwise injected 0.1 mM glucose, serum sample 1, serum sample 2 and 0.1 mM glucose. Based on the calibration curve in the bottom inset of figure 6, the corresponding glucose concentration could be figured out, as long as the current density was obtained. The test was done three times to confirm the reproducibility of the data. The experimental data was in comparison with that obtained from the local hospital. The results are shown in table 2; the good accuracy (recovery) and the high precision (low RSD) reveal the applicability of Nafion/NiMoO4 NRs/GCE for nonenzymatic detection of glucose.
Figure 8. (a) Amperometric response of the Nafion/NiMoO4 NRs/ GCE to 14 mM glucose after current–time (I–t) curve for another long period of running time: 1500 s. (b) Long-term stability of the Nafion/NiMoO4 NRs/GCE.
4. Conclusion In summary, a novel non-enzymatic glucose sensor has been successfully fabricated using NiMoO4 nanorods as sensing 6
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materials. In comparison to previously reported Ni-contained glucose sensors following the same fabrication method, the Nafion/NiMoO4 NRs/GCE exhibited a faster response time (
