Annealing temperature regulating the dispersity and composition of nickel-carbon nanoparticles for enhanced glucose sensing

Journal Pre-proof Annealing temperature regulating the dispersity and composition of nickel-carbon nanoparticles for enhanced glucose sensing

Sichao Zeng, Qiuping Wei, Hangyu Long, Lingcong Meng, Li Ma, Jun Cao, Haichao Li, Zhiming Yu, Cheng-Te Lin, Kechao Zhou, E. Sharel Pei PII:

S1572-6657(20)30010-2

DOI:

https://doi.org/10.1016/j.jelechem.2020.113827

Reference:

JEAC 113827

To appear in:

Journal of Electroanalytical Chemistry

Received date:

23 October 2019

Revised date:

25 December 2019

Accepted date:

5 January 2020

Please cite this article as: S. Zeng, Q. Wei, H. Long, et al., Annealing temperature regulating the dispersity and composition of nickel-carbon nanoparticles for enhanced glucose sensing, Journal of Electroanalytical Chemistry(2018), https://doi.org/10.1016/ j.jelechem.2020.113827

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© 2018 Published by Elsevier.

Journal Pre-proof Annealing Temperature Regulating the Dispersity and Composition of Nickel-Carbon Nanoparticles for Enhanced Glucose Sensing

Sichao Zeng a, Qiuping Wei a,*, Hangyu Long a, Lingcong Meng b, Li Ma a, Jun Cao a, Haichao Li a, Zhiming Yu a, Cheng-Te Lin c, Kechao Zhou a, Sharel Pei E d, *

State Key Laboratory of Powder Metallurgy, School of Materials Science and

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a

Engineering, Central South University, Changsha 410083, PR China

Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key

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c

School of Chemistry, University of Southampton, Southampton, SO171BJ

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b

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Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of

Ningbo 315201, PR China

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WMG, University of Warwick, Coventry, CV4 7AL

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d

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Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences,

* Corresponding author at: School of Materials Science and Engineering, Central South University, Changsha (Qiuping Wei); University of Warwick, Coventry (Sharel Pei E). E-mail addresses: [email protected] (Qiuping Wei); [email protected] (Sharel Pei E).

Journal Pre-proof Abstract: Design of electrodes with both excellent electrocatalytic activity and facile synthesis process is the pivotal challenge in the progress of non-enzymatic glucose sensors. In this work, a series of highly dispersed nickel-carbon nanoparticles modified boron doped diamond composite electrodes were fabricated after the boron doped diamond films with a sputtering nickel layer being annealed at different temperature in hydrogen atmosphere. The surface morphology and interface structure

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of nickel-carbon nanoparticles modified boron doped diamond samples were

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characterized detailedly with the increase of annealing temperature. The charge

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transfer of synthesized electrodes were improved with the increasing content of sp2 carbon and the improvement of crystallinity of nickel. Therefore, the volcano-type

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electrocatalytic performance trend was observed with the composite electrode

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prepared after annealing at 500 ℃ presenting the best glucose sensing performance

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with the sensitivities of 1730 and 1081 μA cm-2 mM-1 in glucose concentration ranges

(S/N = 3).

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of 0.01 - 2.12 and 2.12 - 9.06 mM respectively and a low detection limit of 0.2 μM

Keywords: Nickel-carbon nanoparticles; Boron doped diamond; Annealing temperature; Non-enzymatic glucose sensor

2

Journal Pre-proof 1. Introduction Accurate detection of glucose content is important in many fields, such as medical diagnosis, food industry, biotechnology, etc [1-4]. Glucose sensors with enzyme as sensitive medium have been widely used [5, 6]. However, due to the intrinsic characteristics of enzyme, it is vulnerable to be affected by pH, humidity and other environmental factors, which leads to poor reproducibility of enzyme immobilization

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and high cost [7-9]. In order to solve these problems, non-enzymatic glucose sensors

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have attracted extensive attentions for their high sensitivity and stability. The key

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point of fabricating high performance non-enzymatic sensors is to synthesize sensitive materials with excellent electrocatalytic performance and choose suitable substrate.

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In the past decades, various metal nanoparticles (NPs), such as Au [10, 11], Pt [11],

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Ni [10, 12], Cu [13-15] and Ag [16, 17], were used to fabricate non-enzymatic

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glucose sensors widely. Specially, Ni has attracted great attention because of its great

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catalytic activity towards glucose molecule and low cost. For example, Babu et al. fabricated the enzyme-free glucose sensor with a novel flowerlike architecture by depositing the nickel cobalt sulfide on the cellulose filter paper [18]. Archana et al. developed a flexible non-enzymatic glucose sensor by modifying the hierarchical CuO/NiO-Carbon nanocomposite on cello tape, and this sensor has the great real sample analysis performance [19]. In order to improve the performance of Ni-based non-enzymatic glucose sensors, researchers have regulated the interface structure, composition and surface morphology of the electrode materials by various synthesis methods. For example, Baghayeri et al. reported a strategy to fabricate the glucose 3

Journal Pre-proof sensor

based

on

NiO

decorated

Fe3O4

nanoparticles/poly(p-aminohippuric

acid)-sodium dodecyl sulfate nanocomposite with high glucose sensing performance due to the composite structure [20]. Karikalan et al. reported a novel and simple adsorption cum precipitation method to synthesize the glucose sensor based on Ni(OH)2 modified sulfur doped carbon composites [21]. Although the catalytic activity of Ni catalyst is important for the performance of

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Ni-based enzymatic glucose sensor, the supporting materials also play a great role

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because the substrates could disperse and stabilize the nanoparticles, and sometimes

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there may be synergistic effect between the carriers and the catalysts [16]. For example, Kumar et al. found the synergistic effect of shell-confined structure between

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Ni-Pd bimetal nanoparticles and carbon nanotubes could enhance the catalytic activity

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of nanoparticles remarkably [22]. Among various supporting materials, carbon

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materials, such as carbon nanotubes [23] and graphene [24], were used widely

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because of their high specific area, highly electrical conductivity and biocompatibility. For example, Yuan et al synthesized the three-dimensional nickel oxide nanoparticles on the graphene oxide modified glassy carbon electrode for glucose sensing by electrodeposition [25]. Jothi et al developed a kind of hybrid catalyst using Ni nanoparticles, graphene sheet and graphene nanoribbon, and the glassy carbon electrode was chosen to support the catalyst to fabricate a novel non-enzymatic glucose sensor [26]. However, the mechanical strength of these carriers is low and the secondary loading is essential generally, which causes a poor repeatability and stability of the composite electrodes [23]. 4

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Herein, boron doped diamond (BDD) is used as substrate because BDD has highly electrochemical and mechanical stability, low background current and good biocompatibility [27-30]. But due to the chemical inertness of BDD, it is difficult to modify catalysts on the surface of BDD electrode directly. Toghill et al constructed Ni nanoparticles modified BDD by electrodeposition and this composition electrode

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presented a sensitivity of 1040 μA mM−1 cm−2, but its performance decreased rapidly

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because of the weak interface bonding between Ni nanoparticles and BDD [12].

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Moreover, the physical adsorption interface between catalysts and BDD could cause

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the relatively high interface resistance and decrease the electron transfer rate. Dai et al

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prepared the nickel/nanodiamond modified BDD electrode with three-dimensional structures by electrodeposition to detect the glucose, but the maximum sensitivity was

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only 120 μA mM−1 cm−2 at the linear rage of 0.2 – 12 μM because of the relatively

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high interface resistance [31]. Therefore, it is essential to find a method to enhance the

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combination between Ni catalyst and BDD. It is proposed that Ni can etch the BDD during annealing to form porous BDD [32, 33], and there is a certain amount of sp2 carbon separating out during the reaction between Ni and BDD, which can enhance the combination between the NPs and supports [34]. Moreover, researchers found that the agglomeration of Ni NPs is greatly affected by the annealing temperature [35] and the Ni NPs would be changed to form nickel-carbon nanoparticles (Ni/C NPs) due to the formation of sp2 carbon during annealing [36]. Not only can the existence of sp2 carbon enhance the interface combination between Ni NPs and BDD and the electron transfer, but also decrease agglomeration of Ni NPs during electrochemical process. 5

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Therefore, changing the annealing temperature is a facile way to regulate the dispersity and composition of Ni/C NPs in order to improve the electrocatalytic activity. As far as we know, the relationship between the electrocatalytic activity and the dispersity and composition of the Ni/C NPs modified BDD composite electrodes (Ni/C-BDD) has not been systematically studied. Inspired by the above discussion, in this paper, a facile strategy was presented to

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regulate the dispersity and composition of Ni NPs which were modified on BDD. The

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certain thickness of nickel film was firstly loaded on the surface of BDD. Then, the

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dispersity and composition of Ni/C NPs were controlled by changing the annealing temperature from 200 to 800 ℃. Finally, the relativity of structure-performance for

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glucose sensors based on Ni/C NPs modified BDD has been revealed systematically.

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Journal Pre-proof 2.Experiment 2.1 Reagents Glucose, dopamine, galactose, fructose, ascorbic acid and uric acid were purchased from Sigma-Aldrich. Potassium ferricyanide, sodium hydroxide, potassium chloride (reagent purity) were purchased from Tianjin Recovery Technology Development Co., Ltd. All reagents were stored under standard conditions. Experiments were carried out

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in aqueous electrolyte using ultrapure water with the resistivity of 18.2 MΩ cm.

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2. 2 Preparation of composite electrode

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Firstly, the BDD films were deposited on the 4 × 4 × 0.5 mm3 silicon wafers (resistivity < 0.001 Ω cm) by hot filament chemical vapor deposition (HFCVD).

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During the deposition process, chamber pressure was maintained at 3 kPa by

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introducing reactive gas consisting of B 2 H6 (0.2 sccm), CH4 (1 sccm) and H2

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(49 sccm), and the substrate temperature was kept at 750 - 800 ℃. After 8-hour

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deposition, BDD films with the grain size of 6 - 10 μm were obtained, as shown in Figure S1. Then Ni layer (the thickness was about 20 nm as shown in Figure S1) was deposited on the BDD film by direct current (DC) magnetron sputtering. The sputtering process was carried out in Ar atmosphere (30 sccm) at the pressure of 0.5 Pa for 20 s, and the power was 150 W. This sample was labelled as Ni/BDD. Then the Ni/BDD samples were annealed in a tube furnace in H2 atmosphere at 10 kPa. The H2 flow rate was 98 sccm with annealing time of 30 min and annealing temperature of 200 ℃, 300 ℃, 400 ℃, 500 ℃, 600 ℃, 700 ℃ and 800 ℃, respectively. Finally, the composite electrodes were obtained after the encapsulation of different samples. 7

Journal Pre-proof These composite electrodes were labelled as 200-Ni/C/BDD, 300-Ni/C/BDD, 400-Ni/C/BDD, 500-Ni/C/BDD, 600-Ni/C/BDD, 700-Ni/C/BDD and 800-Ni/C/BDD with the numbers corresponding to the annealing temperature. The schematic for the

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preparation of the composite electrodes was shown in Figure 1.

Figure 1. The schematic for the preparation of the composite electrodes with BDD

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2.3. Characterization

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supporting highly dispersed nickel-carbon composite nanoparticles.

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Scanning electron microscopy (SEM, FEI, Nova Nano SEM 230) was used to characterize the surface morphology of different samples. Energy dispersive X-ray (EDX) analysis, Raman spectroscopy (HORIBA, LabRAM ARAMIS,532nm). Transmission electron microscope (TEM, Titan G2 60-300), X-ray diffraction (XRD, Brooke Company, Advance D8) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher-VG Scientific, ESCALAB250Xi) were used to characterize the structure and composition of the as-prepared samples. The sample for TEM was prepared by focused ion beam (FIB) system (FEI, Helios Nanolab G3 UC) and Pt was sputtered on the surface of the sample for protection. 8

Journal Pre-proof 2.4 Electrochemical Measurements Electrochemical measurements were carried out using CHI 660E electrochemical workstation (Shanghai Chenhua, China) in a three electrode system at room temperature (25 ℃). The Ag/AgCl, platinum foil (15 × 15 mm2) and the as-prepared composite electrode serve as the reference, counter and working electrode, respectively. Before electrochemical measurements, the electrode was pretreated by

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300 consecutive cyclic voltammetry (CV) (from 0.2 V to 0.6 V) in 0.5 M NaOH

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solution for obtainning a stable Ni(OH)2/NiOOH layer as shown in Figure S2. After

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the pretreatment, all the working electrodes were rinsed in the ultrapure water with the resistivity of 18.2 MΩ cm. Electrochemical impedance spectroscopy (EIS) was

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performed in 1 M KCl electrolyte containing 5 mM K3[Fe(CN)6]/K4[Fe(CN)6]

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solution at the frequency range from 0.1 Hz to 1×106 Hz with an amplitude of 7 mV.

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CV was performed in 0.5 M NaOH electrolyte solution, and chronoamperometric

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experiments (I - t) were performed in 0.5 M NaOH electrolyte solution by adding glucose or other species solution with continuous stirring (350 rpm).

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Journal Pre-proof 3. Results and discussion

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3.1. Characterization of the prepared samples

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Figure 2. SEM images of (a) Ni/BDD, (b) 200-Ni/C/BDD, (c) 300-Ni/C/BDD, (d) 400-Ni/C/BDD, (e) 500-Ni/C/BDD, (f) 600-Ni/C/BDD, (g) 700-Ni/C/BDD and (h)

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800-Ni/C/BDD samples. Inset images are the corresponding high resolution images. (I)

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EDX spectra 1 and 2 correspond to signal of Ni element on Ni/BDD and

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700-Ni/C/BDD surface, respectively. SEM characterization of the as-prepared samples is used to visualize the dispersion and particle size of the nanoparticles. Figure 2a illustrates that the surface roughness of Ni/BDD sample increases after sputtering nickel film on BDD film compared that of the pure BDD in Figure S1. As the annealing temperature increases from 200 ℃ to 600 ℃, highly dispersed and uniformly distributed Ni NPs (confirmed by the corresponding EDX spectra in Figure 2I are observed on the surfaces of all the samples, with a slight change of the surface roughness, as shown in Figure 2a-f. As the annealing temperature rises to 700 ℃ , larger Ni NPs (confirmed by 10

Journal Pre-proof corresponding EDX in Figure 2I) with diameter of about 70 - 100 nm is observed, scattered around by smaller Ni NPs, as shown in Figure 2g, which suggests that the melting of Ni due to the decrease of its melting point caused by nano-size effect [37], and the size of Ni NPs is further increased as the annealing temperature increases to

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800 ℃, as shown in Figure 2h.

Figure 3. a) and b) TEM images of 500-Ni/C/BDD sample. c) and d) HRTEM images corresponding to region 1 and 2 in Figure 3a respectively. e-f) EDX elemental 11

Journal Pre-proof mappings of 500-Ni/C/BDD sample. TEM and EDX elemental mappings were used to characterize the composition and structure of the synthesized Ni NPs and the interface characteristic of the 500-Ni/C/BDD sample. As shown in Figure 3a and Figure 3b, it is obvious that the Ni NPs are well dispersed and supported by diamond. The HRTEM images of the region 1 and 2 in Figure 3a are shown in Figure 3c and Figure 3d respectively. The

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crystal lattice fringe with interplanar spacing of 0.146 nm is attributed to the (112)

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planes of cubic diamond. The lattice fringe with interplanar spacing of 0.21 nm is

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assigned to the (111) planes of Ni crystal. It is obvious that the presence of amorphous carbon at the surface of Ni NPs to form Ni/C NPs. This amorphous carbon was

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formed by the reaction between the Ni NPs and diamond during annealing [34], and it

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enhances interface bonding, which is in favor of dispersing and stabilizing Ni/C NPs.

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As shown in Figure 3e-h, the EDX elemental mappings also show the presence of the

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carbon atoms in Ni layer.

Figure 4. a) Glancing angle X-ray diffraction (GAXRD) spectra of the BDD sample 12

Journal Pre-proof as-prepared samples with different annealing temperature. The incident angle was 1.2 degree. b) the partial GAXRD spectra of 400-Ni/C/BDD sample. GAXRD was used to obtain accurate characterization of thin nickel layer (around 20 nm, as shown in Figure S1b. Figure 4 illustrates the GAXRD spectra of all the samples, and the characteristic peak at 2θ = 44° angle reflects the cubic diamond (111) diffraction peak (PDF 06-0675). Without annealing or at annealing temperature below

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300 ℃, there are no obvious Ni (111) diffraction peak in the GAXRD spectra of

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Ni/BDD, 200-Ni/C/BDD and 300-Ni/C/BDD. However, as the annealing temperature

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rises to 400 ℃, the fcc Ni (111) diffraction peak is observed at 2θ = 44.5° angle (PDF 01-1258). These results indicate that the as-grown Ni layer prepared by magnetron

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sputtering is amorphous and crystallized at annealing temperature of 400 ℃. While

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the annealing temperature is above 400 ℃, the intensity of the Ni (111) diffraction

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peaks increases, indicating the improvement of crystallinity of Ni NPs.

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Figure 5. C 1s peak of X-ray photoelectron spectroscopy (XPS) spectra of the a)

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Ni/BDD, b) 200-Ni/C/BDD, c) 300-Ni/C/BDD, d) 400-Ni/C/BDD, e) 500-Ni/C/BDD,

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f) 600-Ni/C/BDD, g) 700-Ni/C/BDD and h) 800-Ni/C/BDD samples. The dots

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represent the original data and color lines are fitted lines. i) Percentage content of sp 2 carbon for different samples.

Figure 5 illustrates the XPS spectra of C 1s peak of as-prepared samples. As shown in Figure 5a, three main peak components, C1 peak at 283.5 eV, C2 peak at 285.0 eV and C3 peak at 286.6 eV, are observed, which are attributed to the carbon in carbide [38, 39], sp3 C-C bond from BDD [40, 41], carbon functional group on the electrode surface [42], respectively. It is obvious that there is a certain amount of nickel carbide in the Ni/BDD sample. The reason for the generation of carbide is that sputtered nickel atoms possessed high instantaneous temperature and energy, and the nickel 14

Journal Pre-proof layer was deposited on the surface of the BDD film at a high cooling speed. As a result, these carbon atoms existed in the nickel nanoparticles with the form of supersaturated solid solution. However, as shown in Figure 5b-h, there is a new peak component (C4 peak) being observed at 284.4 eV (shifted -0.6 eV from the C 1s peak of sp3 C-C bond) in the C 1s spectrum of the samples after annealing, which is attributed to sp2 C=C bond [43]. As shown in Figure 5b, there is the signal of little

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amount of sp2 carbon shown in the XPS spectrum in 200-Ni/C/BDD, which is

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ascribed to the precipitation of supersaturated carbon atoms in the sputtered nickel

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layer during annealing. However, as shown in Figure 5i, the sp2 carbon content of 300-Ni/C/BDD is much higher than that of 200-Ni/C/BDD because Ni and BDD

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began to react during annealing at 300 ℃[44] and as the annealing temperature

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increased gradually from 300 – 800 ℃, the reaction between Ni and BDD is

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accelerated and the percentage content (calculated by using the area of C4 peak

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divided by the area of C1s peak) of sp2 carbon increases in the samples as shown in Figure 5c-i. Moreover, the wide scan spectra of all the samples are shown in Figure S3. The atomic content of C 1s increased gradually, indicating the increasing amount of sp2 carbon with the increase of annealing temperature, which is in accordance with Figure 5i.

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Figure 6. Raman spectra of the BDD and the as-prepared samples at different annealing temperature.

400-Ni/BDD,

500-Ni/BDD,

600-Ni/BDD,

700-Ni/BDD

and

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300-Ni/BDD,

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Figure 6 illustrates the Raman spectra of BDD, Ni/BDD, 200-Ni/BDD,

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800-Ni/BDD, respectively. As shown in the Raman spectrum of Ni/BDD sample, the

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presence of the peaks at 500 cm-1 and 1200 cm-1 is due to the boron doping in diamond [45], and the peak appears at 1332 cm-1 is ascribed to the sp3 bond of diamond [46, 47]. It is noted that the Raman spectrum of Ni/BDD is similar to that of BDD and no obvious D peak at 1350 cm-1 and G peak at 1580 cm-1, which represents the defect and disorder state of the sp2 carbon and the content of sp2 carbon, respectively [48, 49], are observed for both of them, indicating that the sputtered Ni layer would not induce the formation of sp2 carbon, which is consistent with the XPS shown in Figure 5a. The G peak is less intense in the Raman spectrum of 200-Ni/BDD sample, implying that the little sp2 carbon, which is also in accordance 16

Journal Pre-proof with the XPS in Figure 5b. With the increase of annealing temperature, the peak intensity at 500 cm-1 and 1200 cm-1 decreases, while the intensity of G peak increases, which means that the sp2 carbon increases. Moreover, D peaks are observed in the Raman spectrum for the samples annealed at the temperature above 300 ℃ , indicating an increase in amorphous and disorder sp2 carbon during the reaction between Ni and BDD. As shown in Figure 3c and Figure 3d, the HRTEM of the

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500-Ni/C/BDD demonstrates the existence of amorphous carbon.

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Figure 7. a) Electrochemical impedance spectroscopy (EIS) of the different electrodes

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in 1 M KCl electrolyte containing 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] solution at the frequency range of 0.1 Hz to 106 Hz with an amplitude of 7 mV. Inset illustrates the equivalent circuit used to fit the EIS for all the electrodes: R L, Rfilm and Rct represent the resistance of electrolyte solution, film electrode resistance and charge transfer resistance, respectively. C represents the double layer capacitance, Q represents the phase angle element, which is attributed to the absorption of the intermediates, and Zw represents the Warburg element in the low frequency region. b) The R film and Rct value of the as-synthesized electrodes obtained by the equivalent circuit fitting. Figure 7a depicts the Nyquist diagrams of the different electrodes and the 17

Journal Pre-proof equivalent circuit used for fitting the experimental data (the fitting information is listed in Table S1). The Ni/BDD electrode has larger Rfilm (1892 Ω cm2) and Rct (2789 Ω cm2) than of 200-Ni/C/BDD electrode (Rfilm (1256 Ω cm2) and Rct (1037 Ω cm2)), indicating that the sp2 carbon in the 200-Ni/BDD electrode improves the charge transfer. As shown in Figure 7b, Rfilm and Rct values of the as-synthesized electrodes decrease gradually with the increase of annealing temperature, which is attributed to

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the improved charge transfer due to the augment of the sp 2 carbon and the

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crystallization of the Ni, as shown in Figure 5i and Figure 4a. Particularly,

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500-Ni/C/BDD electrode has very low Rfilm (0.1795 Ω cm2) and Rct (9.89 Ω cm2) value, and the Rfilm and Rct of 500-Ni/C/BDD, 600-Ni/C/BDD, 700-Ni/C/BDD and

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800-Ni/C/BDD electrodes reached a relative stable value, suggesting that these

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electrodes have an excellent charge transfer performance

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3.2 Electrocatalytic performance of different electrodes for glucose detection

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Figure 8. CV curves of the a1) Ni/BDD, a2) 200-Ni/C/BDD, a3) 300-Ni/C/BDD, a4)

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400-Ni/C/BDD, a5) 500-Ni/C/BDD, a6) 600-Ni/C/BDD, a7)700-Ni/C/BDD and a8) 800-Ni/C/BDD in 0.5 M NaOH with 1 mM glucose at different scan rates (from inner to outer): 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180 and 200 mV s -1. b) Linear dependence of the redox peak currents versus the square root of scan rate. Figure 8a1-a8 showed the CV curves of different electrodes at different scanning rates in 0.5 M NaOH electrolyte containing 1 mM glucose. With the increase of the scanning rate, the oxidation peak potentials shift positively due to the electrode requiring a greater overpotential to achieve the same electron transfer rate in the higher scanning rate. The linear fitting curves of oxidation and reduction peak 19

Journal Pre-proof currents vs. the square root of scan rate of different electrodes are shown in Figure 8b and it is obvious that the redox peak currents are proportional to the square root of the scan rate (the corresponding linear equations are shown in Table S2), indicating a

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characteristic diffusion-controlled reaction in this solution [15, 21, 50].

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Journal Pre-proof Figure 9. a) CVs of the 500-Ni/C/BDD electrode recorded in 0.5 M NaOH solution (green line) and in 0.5 M NaOH solution with 1 mM glucose (orange line) between 0.2 and 0.6 V at the scan rate of 50 mV s -1. b) The corresponding catalytic current of different electrodes. c) CVs of the 500-Ni/C/BDD electrode in 0.5 M NaOH electrolyte with different concentration of glucose from 1 to 5 mM. Inset image is the corresponding linear fitting curve of the oxidation peaks current vs. glucose

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concentration. d) The slope of the linear fitting curve of different electrodes. e) the

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schematic of the glucose oxidation on the modified electrode.

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The CVs obtained in the absence and presence of 1 mM glucose with 0.5 M NaOH electrolyte is shown in Figure 9. Figure 9a shows CVs of the 500-Ni/C/BDD

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electrode in the absence and presence of 1 mM glucose in 0.5 M NaOH electrolyte

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(CVs of the other composite electrodes are shown in Figure S4). A well-behaved

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redox peaks of Ni(OH)2/NiO(OH) can be observed in the glucose-free solution. With

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the addition of glucose, the oxidation peak current increases and the oxidation peak potential shifts positively, and the reduction peak current decreases. Other electrodes also show the same trend at Figure S4 for glucose being oxidized specifically to gluconolactone by NiO(OH). The reaction could be explained as follow [48, 51]: 𝐍𝐢(𝐎𝐇)𝟐 + (𝐎𝐇)− → 𝐍𝐢𝐎(𝐎𝐇) + 𝐇𝟐 𝐎 + 𝐞−

(1)

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𝐍𝐢𝐎(𝐎𝐇) + 𝒈𝒍𝒖𝒄𝒐𝒔𝒆 → 𝐍𝐢(𝐎𝐇)𝟐 + 𝒈𝒍𝒖𝒄𝒐𝒏𝒐𝒍𝒂𝒄𝒕𝒐𝒏𝒆

(2)

In the catalytic oxidation of glucose, NiO(OH) on the nickel nanoparticles is rapidly reduced by glucose, increasing the content of Ni(OH) 2 and decreasing the content of

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NiO(OH). As a result, oxidation peak current increases while the reduction peak

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current decreases. The schematic of the glucose oxidation on the modified electrode is

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shown in Figure 9e.

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Figure 9b shows the catalytic current values for glucose oxidation at different

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electrodes by calculating the difference of oxidation peak current (∆Ip) in the absence and presence of 1 mM glucose with 0.5 M NaOH electrolyte. The ∆Ip value firstly

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increases and then decreases with increase of the annealing temperature. The catalytic

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current for glucose oxidation of 200-Ni/C/BDD (0.0588 mA) is 4.39 times higher than

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that of the Ni/BDD electrode (0.0134 mA), attributed to the improvement of interfacial charge transfer by the sp2 carbon in the electrode. 500-Ni/C/BDD has the optimal catalytic current (0.168 mA) because of the highly dispersed Ni/C NPs and excellent charge transfer rate, shown in Figure 2e and Figure 7. In order to measure the electrocatalytic activity for different concentrations of glucose solution of the composite electrodes, the CVs of the 500-Ni/C/BDD electrode in 0.5 M NaOH electrolyte with glucose concentrations from 1 to 5 mM are illustrated in Figure 9c (CVs of the other composite electrodes are in Figure S5). The oxidation peak currents for glucose of 500-Ni/C/BDD electrode gradually increase with the 22

Journal Pre-proof increase of glucose concentrations, obeying a linear calibration equation of Ip (μA) = 161.94 Cglucose (mM) + 705.54 (R2 = 0.999). The slope of linear calibration equation is used to estimate the glucose catalytic performance of composite electrodes. The oxidation peak potential shifts to higher potential attributing to the diffusion limitation of glucose at the electrode surface [52]. Furthermore, Figure 9d shows the slope values of the linear fitting equation for different electrodes, which also represents a

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volcano-type trend as the annealing temperature increases. This is attributed to the

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influence of both interface electron transfer and specific surface area of composite

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electrodes. The electron transfer rate of composite electrodes increases with the increase of annealing temperature because of the formation of sp2 carbon and the

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crystallization of nickel, but for specific surface area, it decreases with the increase of

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annealing temperature because of the aggregation of nickel layer.

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3.3 Amperometric response of different electrodes for glucose sensing

Figure 10. a) Amperometric response to the successive additions of different concentrations of glucose solution of the electrodes: bare BDD, a) Ni/BDD, b) 200-Ni/C/BDD, c) 300-Ni/C/BDD, d) 400-Ni/C/BDD, e) 500-Ni/C/BDD, f) 600-Ni/C/BDD, g) 700-Ni/C/BDD and h) 800-Ni/C/BDD. (inset: a partial 23

Journal Pre-proof magnification of the current response toward the low concentration glucose solution). b) The linear fitting curve of corresponding currents vs. glucose concentration for 500-Ni/C/BDD electrode. A series of amperometric response of the electrodes under the potential of 0.5 V in 0.5 M NaOH electrolyte are in Figure 10a, which presents clear step-like changes with the addition of glucose. As can be seen in Figure 10a, the bare BDD electrode

oo

f

hardly has the current response towards glucose. The 500-Ni/C/BDD has the highest

pr

current response, which is consistent with the trend of CV curves in Figure 9. It is

e-

noted that the current response improved immediately and achieved 95% of the steady-state current within 5 s after the glucose adding to the solution, indicating the

Pr

efficient catalytic performance of the 500-Ni/C/BDD electrode for glucose. The slight

al

fluctuation of the current response of 500-Ni/C/BDD and 600-Ni/C/BDD electrodes is

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due to the diffusional limitation under stirring throughout measurements [52]. As

Jo u

shown in Figure 10b, the corresponding calibration curve for current response vs. glucose concentration of 500-Ni/C/BDD is obtained, which corresponds to the low and high concentration regions, respectively. In the range from 0.01 to 2.12 mM with a sensitivity of 1730 μA cm-2 mM-1 (I (mA) = 0.277 × C (mM) + 0.006 (R= 0.999)) and from 2.12 to 9.06 mM with a sensitivity of 1081 μA cm-2 mM-1 (I (mA) = 0.173 × C (mM) + 0.251 (R=0.998)) (Corresponding calibration curves of other electrodes are shown in Figure S6). The decrease of sensitivity is attributed to the adsorption of intermediates on the surface of active materials at a high concentration of glucose

24

Journal Pre-proof [53, 54]. Moreover, the limit of detection (LOD) of 500-Ni/BDD electrode is calculated to be as low as 0.2 μM (S/N = 3, the specific values for calculating the LOD are summarized in Table S3). The electrochemical performance of all electrodes is summarized in Table S4. The excellent catalytic activity of the 500-Ni/C/BDD electrode for glucose sensing are attributable to several factors: (1) highly dispersed Ni/C NPs with tens of nanometers could significantly increase active area. (2) the

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f

intrinsic catalytic activity of the Ni/C NPs was enhanced by the synergistic interaction

pr

between Ni and sp2 carbon. (3) The charge transfer of the composite electrodes was

e-

improved by the sp2 carbon and the improvement of crystallinity of Ni. Additionally, the Comparison of the glucose sensing performance of the 500-Ni/C/BDD electrode

Pr

with other Ni-based non-enzymatic glucose sensors was shown in Table 1.

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Table 1. Comparison of the glucose sensing performance of the 500-Ni/C/BDD

Linear range (mM)

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Electrode materials

rn

electrode with other Ni-based non-enzymatic glucose sensors. Sensitivity (μA mM-1 cm-2)

LOD (μM)

Reference

Ni-NPs/TiO2NTs

0.004-4.8

700

2.0

[55]

Ni(OH)2/TiO2NTs

-

961

0.1

[56]

Ni(OH)2/Au/GCE

0.005-2.2

371.2

0.92

[57]

Pt/Ni(OH)2/MWCNTs/GC

0.4-5.67

145

0.13

[58]

NiO nanostructures

0.1-10.0

206.9

1.16

[59]

120; 35.6 1040

0.05

[31]

Ni-microparticle/BDD

0.0002-0.012; 0.0313-1.06 0.1-10

2.7

[12]

Cu@Ni NPs

0.001-4.1

780

0.5

[60]

CuO/NiO-CT

0.0001-4.5

586.7

0.037

[19]

NiO-APTS@SBA/CNT

0.00006-0.4

-

0.023

[23]

3800; 1297 1087

0.028

[21]

NiONPs/GO/GC

0.0001-5.22; 5.22-10.20 0.00313-3.05

-

[25]

GS/GNR/Ni/GCE

0.000005-5

2320

0.0025

[26]

Ni-ND/BDD

NiSDCN/GCE

25

Journal Pre-proof 0.0005-1.0

6161

0.46

[24]

Ni-ITO

0.02-3.0

610

3.74

[61]

Ni NP/graphene

0.005-0.55

865

1.85

[62]

NiO/TiO2

0.005-12.1

252.0

1

[63]

NiCo-MOF NS

0.001-8

684.4

0.29

[64]

Ni/C/BDD

0.01-2.12; 2.12-9.06

1730; 1081

0.2

This work

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pr

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f

Cu/Ni-EG/pNi

26

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3.4 Stability, selectivity and reproducibility

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Figure 11. Amperometric response of the a) Ni/BDD and b) 500-Ni/C/BDD in 0.5 M

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NaOH and 1 mM glucose solution at 0.5 V before the ultrasonic processing (orange line) and after the ultrasonic processing (green line). The paramaters of ultrasonic processing is 100 W for 5 mins. c) The ratio value of amperometric response (I and I0 are amperometric response values after the ultrasonic treatment and before the ultrasonic treatment, respectively) for different electrodes. d) Amperometric response of the composite electrode after successively adding 2 mM glucose, 0.1 mM DA, 0.1 mM UA, 0.1 mM AA, 0.1 mM fructose, 1 mM glucose in 0.5 M NaOH solution at 0.5 V.

27

Journal Pre-proof The stability of non-enzymatic glucose sensor directly affects its practical application. In previous studies, researchers have found that the shedding of sensitive materials in the use of non-enzymatic glucose sensor is an important constraint factor for long-term utilization. In order to test the stability of different electrodes, all the electrodes were placed in ultrasonic pool for ultrasonic processing for 5 min at 100 W. Amperometric response of Ni/BDD and 500-Ni/C/BDD electrodes before and after

oo

f

ultrasonic treatment are shown in Figure 11a and Figure 11b, respectively. The

pr

stability of different electrodes is evaluated by the value (η=I/I0) after and before

e-

ultrasonic treatment. As shown in Figure 11c, current response of the Ni/BDD electrode is attenuated by 70%, and the response current attenuation of all the

Pr

electrodes is greatly reduced after annealing, which is ascribed to the release of stress

al

in nickel layer and intensive interface binding between synthesized nickel-carbon

rn

composite nanoparticles and BDD.

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Selectivity is important for non-enzymatic glucose sensors because there are many interfering species in practical application. Therefore, 2 mM glucose, 0.1 mM DA, 0.1 mM UA, 0.1 mM AA, 0.1 mM fructose, 1 mM glucose were successively added in 0.5 M NaOH solution, and the amperometric response of the composite electrode was recorded at 0.5 V. As shown in Figure 11d, the composite electrode shows good selectivity. In order to investigate the reproducibility of the 500-Ni/C/BDD electrode, four extra electrodes were fabricated using the same process. The relative standard deviation (RSD) of the current responses were 1.46% and 2.34% in low and high 28

Journal Pre-proof concentration ranges respectively, as can be seen in Table S5, demonstrating an excellent reproducibility of the electrode fabrication method.

3.5 Analytical application In order to compare the determination performance between the as-prepared non-enzymatic glucose sensor and the commercial glucose sensor (purchased

from

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f

Sinocare Co., Ltd.), the analysis of human blood serum samples (as for the

pr

500-Ni/C/BDD electrode, the human blood serum was added in 0.5 M NaOH

e-

electrolyte solution and tested by amperometric method) was carried out by these two kinds of sensors, as shown in Figure S7. The glucose concentrations of sample 1 and

Pr

sample 2 were calculated according to amperometric response and corresponding

al

calibration equation of the 500-Ni/C/BDD electrode in Table S4. The results were

rn

compared with those obtained by the commercial blood glucose meter as shown in

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Table 2, confirming the prepared non-enzymatic sensor has the potentially practical application value.

Table 2. Results of the test concentration of glucose and the relative standard deviation (RSD) for human blood serum samples analysis. sample

Tested by 500-Ni/C/BDD

Tested by blood glucose meter

RSD

1

5.3 mM

5.1 mM

2.7%

2

1.8 mM

1.7 mM

4.0%

29

Journal Pre-proof 4. Conclusions In conclusion, we have synthesized highly dispersed Ni/C NPs on BDD by annealing

process

for

enhanced

glucose

sensing.

The

relativity of

the

structure-performance of the Ni/C NPs modified BDD composite electrodes towards glucose has been investigated detailedly. The electrocatalytic activity of composite electrodes is dependent on both interface electron transfer and specific surface area of

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f

them. As annealing temperature is below 600℃, the highly dispersed Ni/C NPs could

pr

be obtained. The content of sp2 carbon increases and the crystallinity of Ni improves

e-

with the increase of annealing temperature, which could improve the interface charge transfer of the composite electrodes. As a result, a volcano-type glucose sensing

Pr

performance trend of the as-prepared composite electrodes was observed with

al

augment of annealing temperature, and the 500-Ni/C/BDD electrode possessed two

rn

linear dependence of current response with glucose concentration ranges of 0.01 -

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2.12 and 2.12 - 9.06 mM, and the sensitivities of 1730 and 1081 μA cm-2 mM-1, respectively, and the LOD was as low as 0.2 μM (S/N = 3). In addition, the composite electrodes have good stability and selectivity. The best electrocatalytic performance for glucose of the 500-Ni/C/BDD electrode is attributed to the large specific area due to well-dispersed Ni/C NPs and the enhanced charge transfer due to the presence of sp2 carbon and the crystallization of Ni.

30

Journal Pre-proof

Acknowledgement We gratefully acknowledge the National Key Research and Development Program of China (No. 2016YFB0301402, No. 2016YFB0402705), the National Natural Science Foundation of China (No. 51601226, No. 51874370, No. 51302173), the State Key Laboratory of Powder Metallurgy, the Fundamental Research Funds for the Central Universities of Central South University (2018zzts014 and 2017gczd024),

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f

Hunan Provincial Natural Science Foundation of China (No. 2019JJ50796), Hunan

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Provincial Innovation Foundation For Postgraduate (CX2018B085) and the Open-End

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Fund for Valuable and Precision Instruments of Central South University for financial support. The authors also wish to thank the reviewers and editor for kindly giving

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revising suggestions.

31

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Credit Author Statement Sichao Zeng: Writing - original draft, Writing - review and editing, Methodology Qiuping Wei: Resources, Supervision, Funding acquisition Hangyu Long: Conceptualization Lingcong Meng: Software Li Ma: Formal analysis

f

Jun Cao: Writing - Review and Editing

oo

Haichao Li: Writing - Review and Editing Zhiming Yu: Funding acquisition

pr

Cheng-Te Lin: Writing - Review and Editing, Formal analysis

e-

Kechao Zhou: Funding acquisition

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Sharel Pei E: Writing – Review and Editing, Software

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Journal Pre-proof Declaration of Interest Statement

The authors declare that they have no known competing financial interests or personal relationships that could have

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appeared to influence the work reported in this paper.

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