Graphene Composites with Enhanced

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MOF-Derived Porous Ni2P/Graphene Composite with Enhanced Electrochemical Properties for Sensitive Nonenzymatic Glucose Sensing Yaxing Zhang, Jiaoyan Xu, Jianfei Xia, Feifei Zhang, and Zonghua Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11867 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Applied Materials & Interfaces

MOF-Derived Porous Ni2P/Graphene Composite with Enhanced Electrochemical Properties for Sensitive Nonenzymatic Glucose Sensing Yaxing Zhang, Jiaoyan Xu, Jianfei Xia*, Feifei Zhang, Zonghua Wang* College of Chemistry and Chemical Engineering, Shandong Sino-Japanese Center for Collaborative Research of Carbon Nanomaterials, Qingdao University, Qingdao 266071, P. R. China. KEYWORDS: Ni2P/Graphene Composite, glucose electrooxidation, nonenzymatic sensor, NiMOF-74 derived porous composite, electrocatalyst.

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ABSTRACT

Herein, we report a high-performance electrocatalyst by orientationally growing Ni2P nanoparticles in situ on graphene film (Ni2P/G) for nonenzymatic glucose sensor in alkaline media. The combination of highly active Ni2P and stable graphene film with rapid conductivity enables this composite display excellent electrochemical activity towards glucose with enhanced electron transfer rate and steadiness. With Ni-MOF-74 as precursor, Ni2P/G showed an even metal distribution, massive exposure of active sites and spatially ordered structure. Benefiting from the synergistic reaction of Ni2P particles and graphene, this MOF-derived composite exhibited highly electrocatalytic activity and specificity toward glucose electrooxidation. Under optimized conditions, a wide linear response was obtained from 5 μM to 1.4 mM with detection limit of 0.44 μM. Furthermore, an excellent linear response (R2 = 0.9897) was also obtained by Ni2P/G modified electrode in human serum with the concentration of glucose ranging from 1 mM to 8 mM, indicating that Ni2P/G platform could be utilized for glucose monitoring in practical life.


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INTRODUCTION Glucose has been increasingly recognized as an important small molecule analyte as a result of diabetes has become one of the most common and serious threats to human health 1. In order to rapidly diagnose diabetes and real-timely monitor blood sugar level, a large number of sensors have been researched and developed to test glucose level in blood

2-5.

Among these sensors, the

electrochemical sensor has been considered to be a more potential method for glucose detection due to its high sensitivity, low cost and variety in instrumentation

6-8.

At the initial stage, the

glucose electrochemical sensor had been based on glucose oxidase. Many kinds of enzymatic glucose biosensors have been widely developed by immobilizing glucose oxidase on nano materials

9, 10.

This class of electrochemical sensor inherits the specificity, sensitivity and fast

reaction speed of the enzyme. However, the costliness and vulnerability of biodevices hinder the practical application of this kind of glucose sensors. Considering the above obstacles, many researches are being paid to develop electrocatalytic materials with durability for fabrication of nonenzymatic glucose sensors 11-13. Noble metals such as platinum and gold were originally used as modified electrode materials 14. But the poor specificity and high cost still limit the practical application of these materials. Therefore, the potential of transition metals (such as copper and nickle) as electrocatalytic materials was then explored by researchers

15, 16.

Up to now, many

transition metals and their oxides and hydroxides have been used in the development of glucose sensors

17, 18.

However, these metals, metal oxides and hydroxides were hard to disperse

homogeneously on the surface of supporting materials, which limited their application. Nickle is an attractive transition metal for glucose electrooxidation with high catalytic activities. And its phosphide with excellent electrochemical performances has received deep research 19. Compared with the traditional platinum catalyst, Ni2P (001) possesses more excellent activity according to the density functional theory (DFT) 20. Therefore, exploring the synthesis of



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Ni2P with a single phase is significant for fabricating glucose sensors with excellent practical performances. Metal-organic frameworks (MOFs)

21are

new kinds of materials composed of

metal ions or small metallic clusters and organic ligands. Over the last few years, they have drawn tremendous attentions owing to their periodic topology structure, variable metal site and tunable open-framework structure 22, 23. Inspired by these merits, MOFs have been considered as an ideal precursor for synthesize metal compound with porous structure

24, 25.

Recently, some

kinds of nickel phosphide nanoparticles have been prepared by the direct phosphorization of MOFs 26. However, the advantages of the MOF precursor were not well inherited by the obtained materials. Therefore, developing a universal method to synthetize MOF-derived Ni2P compound is important for expanding the application of Ni2P in electroanalysis of glucose. Herein, using graphene as the support and regulator, we have in situ prepared a threedimensional Ni2P/G composite with a single phase of Ni2P via the phosphorization of Ni-MOF74/G composite. In order to form the homogeneous Ni2P particles, Ni-MOF-74 was chose as the precursor. On the one hand, the substantially open nickle nodes facilitated the conversion of Ni nodes to Ni2P nanoparticles. On the other hand, the periodic framework of Ni-MOF-74 guaranteed the formation of uniform Ni2P nanoparticles. Meanwhile, in order to obtain a stable and efficient catalyst, graphene was chose as a support for immobilizing particles and enhancing conductivity. The synergistic reaction of Ni2P particles and graphene made as-prepared Ni2P/G composite exhibit highly electrocatalytic activity and enhanced the conductivity. Furthermore, benefiting from the addition of particle stabilizer (polyvinylpyrrolidone, PVP) in synthesis, NiMOF-74 could be formed on graphene with uniform size by the regulatory effect of PVP. To testify the catalytic ability of obtained Ni2P/G composite towards glucose, the Ni2P/G based sensing interface was fabricated to detect glucose in alkaline media. Satisfactorily, Ni2P/G


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showed excellent sensing performances in a wide detection range of 5 μM to 1.4 mM with detection limit of 0.44 μM. Furthermore, it also showed specificity for detecting glucose in human serum with an excellent linear response (R2 = 0.9897). Therefore, we could say that this strategy could not only ensure sensitive linear response in both laboratory sample and human serum by preventing the Ni2P from leaching, but also enable the glucose sensing possess excellent stability and reproducibility. 2. EXPERIMENTAL SECTION 2.1. Materials and Characterization. Nickel(II) acetate tetrahydrate (Ni(CH3COO)2·4H2O), sodium hypophosphite (NaH2PO2·H2O), 2,5-Dihydroxyterephthalic acid (DHTA) and tetrahydrofuran (THF) were ordered from Aladdin Reagent Co. Ltd. (Shanghai, China). The graphite, methanol (CH3OH), sodium hydroxide (NaOH) and hydrogen peroxide (H2O2) were obtained from Shanghai Reagent Factory (Shanghai, China). Potassium permanganate (KMnO4) and other reagents were analytical grade. Human serum samples were obtained from the Hospital of Qingdao University. The water was deionized water and all of the reagents were used without any pretreatment. Scanning electron microscopy (SEM) images were conducted on Zeiss merlin compact. High resolution transmission electron microscopy (HRTEM) images and transmission electron microscopy (TEM) images were taken by FEI Tecnai G2 F20. Powder X-ray diffraction (PXRD) data was obtained by Rigaku D-MAX 2500/PC. X-ray photoelectron spectroscope (K-Alpha, Thermo Fisher Scientific ESCALAB250Xi, USA) was applied to get the X-ray photoelectron spectra (XPS) with an Al Kα X-ray source. The automatic volumetric adsorption equipment (ASAP2020) was used to obtain the N2 adsorption/desorption isotherms at liquid nitrogen temperature (77 K). The synthesis processes of composites were carried out in a micro magnetic



autoclave (WCGF, Xi'an Taikang Biological Technology Co., Ltd.). Electrochemical experiments were performed on CHI-660C electrochemical workstation with a traditional threeelectrode cell (Shanghai Chenhua Instrument Co., Ltd., China). The cyclic voltammetry (CV) and amperometric technology were used to detect glucose. The Ni2P/G modified electrode was used as the working electrode. 2.2. Synthesis of MOF-74 Nanocrystals. According to the previous report, MOF-74 nanocrystals were prepared by one-pot reaction. Ni(CH3COO)2·4H2O (3 mmol, 0.747 g) was dissolved in 30 mL of deionized water. DHTA (3 mmol, 0.594 g) was dissolved in 30 mL of THF. Subsequently, the above mentioned solutions were mixed and transferred to an autoclave (Teflon lined). The mixtures were vigorously stirred for 10 minutes at room temperature. Then the autoclave was transferred to an oven and heated at 110 °C for 24 h. When the solution was cooled to room temperature, the bright yellow nanocrystals were collected by centrifugation and washed successively with methanol and deionized water for several times to remove residue. Then the synthesized sample was put in vacuum drying oven at 80 °C over night and the MOF-74 was obtained. 2.3. Synthesis of Porous Ni2P/Graphene Composite and C-MOF-74/Graphene Composite. Graphene oxide (GO) was prepared by the modified Hummer’s method as reported previously. The dispersion of GO (7 mg/mL) was obtained by ultrasonic dispersing 70 mg graphene oxide into 10 mL deionized water. GO (0.6 g) and PVP (61 mg) were dispersed in a mixed solution (THF: H2O = 1: 1, 50 mL), and the mixtures were stirred for 30 min to make the PVP be evenly dispersed on the surface of GO. After that, Ni(CH3COO)2·4H2O (3 mmol, 0.747g) was added into the mixture with a vigorous stirring for 30 min to coordinate Ni ions with GO by PVP, which thus facilitate the uniform nucleation of MOF-74 on GO films. Subsequently, DHTA (3


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mmol, 0.594 g) was dissolved in the mixture and the whole mixtures were transferred into reaction kettle at 110 °C for 24 h. Owing to the hydrothermal reaction, GO films were transformed into graphene films during the reaction period and a dark green suspension was obtained. After the reaction device cooled down, the dark green composites were collected by centrifugation and washed successively with methanol and deionized water to remove residue. Then the synthesized composite was put in vacuum drying oven at 80 °C over night and the MOF-74/G was obtained. In addition, MOF-74/G has also been prepared without PVP to confirm the important role of PVP in composite synthesis. To synthesize the Ni2P/G composite, MOF-74/G and sodium hypophosphite (the molar ratio of Ni to P is 1:20) were putted into a temperature-programmed furnace and heated at 275 °C for 5 hours under the constant flow of argon. After naturally cooling to ambient temperature, the dark gray powder was collected and washed with deionized water for several times. After during overnight in drying oven, the Ni2P/G composite was obtained. The complete synthesized process is shown in Scheme 1.

Scheme 1. A schematic diagram of the synthesized process of Ni2P/graphene



To investigate the great electrocatalytic activity of Ni2P/Graphene, the C-MOF-74/Graphene Composite (C-MOF-74/G) was prepared as comparison. It was synthesized by carbonizing 200 mg MOF-74/G with the previously described procedure, except for the remove of sodium hypophosphite from the reaction system. 2.4. Electrochemical Measurements. The Ni2P/G composite was dispersed in deionized water to form dispersion (1mg/mL) without further addition. In order to fabricate a Ni2P/G modified electrode (Ni2P/G/GCE), 5 μ L of dispersion was dropped on the surface of the polished GCE. After dried for 5 hours at room temperature, the Ni2P/G/GCE was fabricated and used to detect glucose in buffer solution and human serum sample. A platinum wire was used as the auxiliary electrode and Ag/AgCl electrode was used as the reference electrode. 2.5. Interference experiment and real sample determination In order to investigate the practicability of Ni2P/G composite, the anti-interference performance was researched by chronoamperometry with the successively addition of 5 mM AA, 5 mM UA, 5 mM DA, 5 mM Lac, 5 mM Fru and 1 mM glucose into the stirred NaOH solution. For real sample determination, human serum from healthy volunteer was used without pretreatment. Then the glucose with different concentrations (1, 2, 3, 4, 5, 6, 7, and 8 mM) in serum sample was detected using Ni2P/G/GCE.

3. RESULTS 3.1. Material Preparation and Characterization. The morphological evolution of Ni-MOF-74/G and Ni2P/G are presented by using SEM images. As displayed in Figure 1a, uniform Ni-MOF-74 crystals with column shaped polyhedral


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morphology are interpolated and wrapped by graphene film, indicating that Ni-MOF-74/G has successfully formed via one-pot reaction. Compared with original Ni-MOF-74 (Figure S1a), NiMOF-74/G presents a uniform morphology, which can promote the formation of homogeneous reactive sites. Compared with the prepared MOF-74/G without the help of PVP (Figure S1c and S1d), MOF-74/G with the help of PVP shows more regular and uniform morphology. The reason is that the PVP can connect with the oxygen-containing functional groups of GO due to the electrostatic force. And GO and PVP can provide more binding sites for nickel ion. Therefore, the PVP not only acts as a particle stabilizer, but also an effective structure-directing agent to regulate the size of composite during the solvothermal process

27.

After phosphorization, the

original morphology of the Ni-MOF-74/G precursor is mostly maintained by Ni2P/G, but slight contraction and somewhat rough surface can also be observed in Figure 1b. The changes result from the formation of Ni2P nanoparticles and the partial pyrolysis of organic ligands. To further confirm the formation of Ni2P, HRTEM images have been applied to explore the detailed structural information. As shown in Figure 1c, uniform Ni2P nanocrystals can be observed disperse evenly in 3D framework wrapped with graphene, which confirm the in situ formation of Ni2P/G. The average particle length of Ni2P nanoparticles is 30 ± 10 nm (Figure S2). From the high magnification HRTEM image, we can see the crystal lattice fringes clearly (Figure 1d). The distance of the fringes is about 0.203 nm. The distance is respectively corresponding to the (201) plane of Ni2P, indicating that the Ni2P/G has been successfully synthesized.



Figure 1. SEM images of (a) Ni-MOF-74/G and (b) Ni2P/G, (c) low magnification HRTEM images of Ni2P/G, and (d) high magnification HRTEM images of the Ni2P. The PXRD patterns have been shown in Figure 2a. Compared with the simulation curve of NiMOF-74, the profiles of Ni-MOF-74 and Ni-MOF-74/G present a good accordance. The results indicate that Ni-MOF-74 with a good crystal structure has been formed in both materials. Compared with pure GO, there is no peak at 11.11° in the PXRD patterns of Ni-MOF-74/G, indicating that the GO has been transformed to the reduced form by the solvothermal reaction and Ni-MOF-74/G has been successfully obtained. After phosphorization, Ni2P/G shows different diffraction peaks at 30.42°, 31.72°, 35.34°, 40.6°, 44.54°, 47.33°, 54.16° and 54.9°, which corresponding to (110), (101), (200), (111), (201), (210), (300), and (211) planes of hexagonal Ni2P phase (JPCDS No. 03-0953). But the two strong diffraction peaks of Ni-MOF-74 at 6.76° and 11.78°can be also detected, indicating the existence of MOF-74 framework. N2 ACS Paragon Plus Environment

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adsorption-desorption isotherms and pore-size distribution curves of Ni2P/G (Figure 2b and 2c), Ni-MOF-74 and Ni-MOF-74/graphene (Figure S4) have been studied through N2 adsorption/desorption experiments. As displayed in Figure 2b, a similarly Type II isotherm of Ni2P/G with a petty obscure hysteresis loop has been shown by N2 adsorption-desorption isotherms. The BET area is 334 m2/g. It can provide more reactive sites to improve the analytical performances. This pattern indicates that Ni2P/G mainly has a mesoporous structure larger than 20 nm which has been confirmed by Figure 2c. Moreover, Ni2P/G have an aperture at around 4 nm, which enables Ni2P/G play the role of molecular sieve and effectively eliminate the interference of macromolecules in actual samples.

Figure 2. (a) XRD patterns of simulated Ni-MOF-74, Ni-MOF-74, Ni-MOF-74/G, and Ni2P/G, (b) N2 adsorption-desorption isotherms and pore-size distribution curves of Ni2P/G (c).



The XPS was performed to further explore the change in elemental and structural information of Ni-MOF-74/G and Ni2P/G (Figure 3 and Figure S5). Compared with its precursor (Figure 3a), Ni2P/G presents observable change in the contents of C and Ni due to the pyrolysis of organic ligands. The appearance of phosphorus in Ni2P/G indicates that a phosphorization has been done at 275 °C. As shown in Figure 3c, the appearance of P (8.99%) in Ni2P/G indicates that the Ni nodes of MOFs have been partially conversed into Ni2P. The transformation of nickel compound is further revealed by XPS spectra in the Ni 2p and P 2p regions. From the Figure 3d, we can see the Ni 2p3/2 peaks at around 852.7 eV, which is corresponding to the Niδ+ in Ni2P. And we can see the Ni 2p3/2 peaks at around 856.4 eV, which is corresponding to the Ni2+ in the surface of NiO 28. And the peak at around 861.5 eV can also be seen, which is the satellite of Ni 2p3/2 29. Moreover, from the Ni 2p1/2 spectra, two peaks at 874.1 and 874.3 eV can be seen, which are along with a satellite peak at around 880.4 eV 28. Compared with Ni2+ in Ni-MOF-74 (Figure 3b), the binding energy of Ni 2p3/2 in Ni2P/G shifts from 856.8 eV to 856.4 eV, demonstrating that the Ni2P have been formed from the MOF precursor. As shown in Figure 3e, two peaks at 128.7 eV and 129.5 eV appear in the P 2p region, which can be attributed to the binding energy of P 2p3/2 and P 2p1/2 in Ni2P 28, 30. And at 132.7 eV, another peak can also be observed. This peak indicates the existence of oxidized phosphate species, which is attributed to the contact between air and the surface of Ni2P 31. Besides, the binding energy of Ni 2p3/2 (852.7 eV) and P 2p3/2 (129.5 eV) in Ni2P have shown the shifts of Ni and elemental P. These shifts are the results of charge transfer from Ni to P, which can greatly promote the catalytic performance of Ni2P/G in the electrocatalytic process.


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Figure 3. XPS spectra of (a) Ni-MOF-74/G and (c) Ni2P/G, Ni 2p spectrum of (b) Ni-MOF-74/G and (d) Ni2P/G and (e) P 2p spectrum of Ni2P/G. 3.2 Characterization and Electrochemical performance of C-Ni-MOF-74/G. To study the effects of phosphorus on the electrocatalytic performances of Ni2P/G, C-NiMOF-74/G was also prepared and its properties were studied. As shown in Figure 4 (a, b), C-NiMOF-74/G shows an almost similar morphology with the Ni-MOF-74/G in SEM and TEM images except the black spot. Furthermore, the consistency of material color before and after carbonization also proved the conclusion (Figure S3). The results confirm the stability of NiMOF-74/G and further testify the strong combination of Ni-MOF-74 and graphene with the aid



of PVP. To explore the transformation, XPS of C-Ni-MOF-74/G had been done for explore the change of element content (Figure 4c). After carbonization at 275 °C, the automatic ratio of C and Ni showed a great change compared with its precursor, which maybe a result of pyrolysis of organic ligands and evaporation of bound water confined in the holes of MOFs. In addition, we also studied the electrochemical properties of C-Ni-MOF-74 (Figure S6) and C-Ni-MOF-74/G (Figure 4d). As shown in Figure, C-Ni-MOF-74/GCE and C-Ni-MOF-74/G/GCE shows a poor electrocatalytic activity in NaOH solution compared with Ni2P/G/GCE. The reason is Ni2P (001) possesses more excellent activity according to the density functional theory (DFT). And the synergistic reaction between phosphorus and nickle improve the electrochemical properties of Ni-based material.

Figure 4. (a) SEM images of C-Ni-MOF-74/G, (b) TEM images of C-Ni-MOF-74/G, (c) XPS spectra of C-Ni-MOF-74/G and (d) CVs of Ni2P/G/GCE (black) and C-Ni-MOF-74/G/GCE (red) in 0.1 M NaOH with pH of 13 without glucose (The scan rate is 50 mV s-1).


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Figure 5. (a) CV curves of blank GCE (solid blue, dot green) and Ni2P/G/GCE (dash black, dash dot red) in 0.1 M NaOH with the pH of 13 in the presence (green, red) and absence (blue, black) of glucose (1 mM). The scan rate is 50 mV s-1. (b) CV curves of Ni2P/G/GCE in glucose (1 mM) at different scan rates from 10 to 200 mV s-1. (c) The linear relationship between the square root of scan rate and current density. (d) Plot of anodic peak current density vs pH for Ni2P/G/GCE electrode in the presence of glucose (1 mM) at 0.5 V. The scan rate is 50 mV s-1. 3.3 Electrocatalytic Performances of Ni2P/G toward the glucose. CV was utilized to examine the electrochemical performance of Ni2P/G in 0.1 M NaOH with the pH of 13 in the presence and absence of glucose (1 mM). The scan rate was 50 mVs-1 and the potential range was 0.2 to 0.7 V. As shown in Figure 5a, bare GCE is electrochemically inactive toward NaOH over this potential range. On the contrary, a pair of redox peaks can be observed at about 0.513 V and 0.406 V for Ni2P/G/GCE, which could be attributed to the conversion between Ni2+ and Ni3+ 32. The reaction can be formulated as: NiOx/Ni(OH)x + OH- - e- → NiOOH



During the anodic scanning, NiOx/Ni(OH)x was formed on the surface of Ni2P in NaOH solution. Then the above peaks arose subsequently due to the surface Faradaic reaction between NiOx/Ni(OH)x and NiOOH

33, 34.

After the addition of 1 mM glucose, a notable increase of the

anodic response could be immediately observed on the Ni2P/G/GCE surface. But the change on bare GCE was negligible. It is reasonable to conclude that Ni2P/G/GCE has an excellent electrocatalytic performance for glucose electrooxidation. This enhanced anodic response results from the oxidation of glucose, which is due to the electrocatalytic activity of NiOOH. The oxidation of glucose can be demonstrated by following formulation: NiOOH+glucose→NiOx/Ni(OH)x+H2O+glucolacton+eDuring the oxidation process of NiO to NiOOH in alkaline environment, glucose was oxidized to glucolacton by obtained NiOOH. Meanwhile, the NiOOH was further reduced to NiOx/Ni(OH)x. The influences of scan rate were also studied in the range of 10 to 200 mV s-1. As shown in Figure 5b, with the increase of scan rate, the intensities of currents are increased. And the current is proportional to the square root of scan rates (Figure 5c). This linear relationship shows that the glucose oxidation on Ni2P/G/GCE is controlled by diffusion process. We further investigated the effect of pH on electrochemical responses of Ni2P/G/GCE in presence of 1mM glucose. As shown in Figure 5d, the anodic peak current linearly increases with raising pH from 10 to 13. With further increasing pH value, a sharp raise can be observed at pH 14, which is a result of the oxygen evolution reaction. Considering the practical application, pH 13 was chose for our research. Real-time amperometric method was performed for exploring the electrocatalytic activity of Ni2P/G/GCE to glucose. Under optimized conditions, glucose solution was successively injected


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into the stirred NaOH solution and the amperometric responses of Ni2P/G/GCE were recorded (Figure 6a). Under the potential of 0.5 V, the steady current can be quickly achieved less than 5 s after the addition of glucose. The fast electrode response is attributed to the cooperation of highly active Ni2P nanoparticles and graphene with excellent conductivity. The corresponding standard curve displayed in Figure 6b shows that the Ni2P/G/GCE establishes a linear relationship range from 5 μM to 1.4 mM with regression coefficient of 0.9995. The linear regression equation is I = 7.234 Cglu + 2.902. Furthermore, the limited detection and sensitivity of Ni2P/G/GCE are 0.44 μM and 7234 mA mM-1 cm-2, respectively. The performances of our glucose sensor have been compared with other electrochemical glucose sensors reported previously. The data have been summarized in Table 1. The comparison results demonstrate that Ni2P/G/GCE shows more satisfactory performances than other glucose sensor with a low detection limit as well as high sensitivity and stability.

Table 1. Comparison of electrochemical performances of MOF-derived porous Ni2P/G with other reports on the nonenzymatic detection of glucose. Electrode

Linear (mol dm−3)

LOD (μM)

Sensitivity (μA mM−1 cm−2)

Ref.

3D porous Co3O4

1×10-6 to 3×10-4; 4×10-3 to 1.25×10-2

0.1

471.5

[35]

CuO nanorods

4×10-6 to 8×10-3

4

371.43

[36]

NiO/rGO

5×10-6 to 2.8×10-3

1

1571

[37]

Au network

1×10-6 to 5×10-4; 4×10-3 to 1.2×10-2

0.2

Ni2P/NA

1×10-6 to 3×10-3

0.18

7792

[39]

Ni2P/G

5×10-6 to 1.4×10-3

0.44

7234

This work

[38]



Figure 6. (a) Amperometric responses of Ni2P/G/GCE to successive additions of glucose (in 0.1 M NaOH solution with the potential of 0.5 V vs. Ag/AgCl electrode) and (b) the corresponding calibration curve of current vs. glucose concentration (R2 = 0.995). In order to investigate the selectivity of Ni2P/G/GCE, the anti-interference performance of Ni2P/G was studied by chronoamperometry. 1 mM glucose, 5 mM AA, 5 mM UA, 5 mM DA, 5 mM Lac, 5 mM Fru and 1 mM glucose were successively added into the stirred NaOH solution after the injection of glucose (Figure 7a). It is found that there is no response toward coexisting substances, indicating that Ni2P/G has great selectivity for glucose. In order to further evaluate the potential of the Ni2P/G modified GCE in practical application, glucose was detected in human serum sample with different concentrate of glucose (1, 2, 3, 4, 5, 6, 7, and 8 mM). As observed, with the increase of glucose concentrations, the anodic peak currents increase linearly (Figure 7b). And the corresponding calibration curve is obtained with the correlation coefficient of 0.9897 (Figure 7c). The results indicate that the Ni2P/G composites have a promising future in the construction of nonenzymatic electrochemical glucose sensors.


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Figure 7. (a) Amperometric responses of Ni2P/G/GCE to the successive addition of 1 mM glucose, 5 mM AA, 5 mM UA, 5 mM DA, 5 mM Lac, 5 mM Fru and 1 mM glucose at 0.5 V in 0.1 M NaOH, (b) CV curves of Ni2P/GCE to the glucose with different concentrations (1, 2, 3, 4, 5, 6, 7, and 8 mM) in human serum sample (with the potential of 0.5 V). (c) The corresponding linear plot of current vs. glucose concentration in human serum sample. CONCLUSIONS In conclusion, an efficient electrocatalyst for nonenzymatic glucose sensor based on MOFderived composite have been reported. As an electrocatalytic reaction platform, Ni2P/G not only exhibits excellent electrochemical performance towards glucose, but also presents high selectivity and specificity to glucose in both buffer solution and human serum. According to above result, Ni2P/G composite has been confirmed as an efficient catalyst for glucose monitoring under alkaline conditions. In addition, the low cost and environmental friendly preparation further increased the feasibility in the application of this composite in real samples.



Thus, it is believed that the Ni2P/G composite has great electrochemical properties and good potential as electrochemical reaction platform. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experiments details of the preparation of compounds, the fabrication of modified electrode and characterization information.(PDF) AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We really appreciate the financial support from the National Natural Science Foundation of China (21645007 and 21475071), the Taishan Scholar Program of Shandong Province (No.ts201511027) and the Natural Science Foundation of Shandong (ZR2016BM21). REFERENCES


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