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Corresponding author: Fax 0086-571-87952625; Email address [email protected]. (HP He), [email protected] (JY Huang). Keywords: NaCl, porous 3D template...
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Functional Nanostructured Materials (including low-D carbon)

3D porous nickel frameworks anchored with crosslinked Ni(OH)2 nanosheets as a high sensitive non-enzymatic glucose sensor Weiwei Mao, Haiping He, Pengcheng Sun, Zhizhen Ye, and Jingyun Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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3D porous nickel frameworks anchored with crosslinked Ni(OH)2 nanosheets as a high sensitive non-enzymatic glucose sensor Weiwei Mao1, Haiping He1,*, Pengcheng Sun1, Zhizhen Ye1,2, Jingyun Huang1,2,*

1

School of Materials Science and Engineering, State Key Laboratory of Silicon

Materials, Zhejiang University, Hangzhou 310027, China 2

Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University

* Corresponding author: Fax 0086-571-87952625; Email address [email protected] (HP He), [email protected] (JY Huang)

Keywords: NaCl, porous 3D template, Ni(OH)2 nanosheets, hot corrosion, primary battery, non-enzymatic sensor

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Abstract A facile and scalable in situ micro-electrolysis nanofabrication technique is developed for preparing crosslinked Ni(OH)2 nanosheets on a novel 3D porous nickel template (Ni(OH)2@3DPN). For the constructed template, the porogen of NaCl particles not only induce a self-limiting surficial hot corrosion to claim the “start engine stop” mechanism, but also serve as the primary battery electrolyte to greatly accelerate the growth of Ni(OH)2. As far as we know, the micro-electrolysis nanofabrication is superior to the other reported Ni(OH)2 synthesis methods due to the mild condition (60 oC, 6 h, NaCl solution, ambient environment) and without any post treatment.

The

integrated

Ni(OH)2@3DPN

electrode

with

highly

suitable

microstructure and porous architecture implies a potential application in electrochemistry. As a proof of-concept demonstration, the electrode was employed for non-enzymatic glucose sensing, which exhibits an outstanding sensitivity of 2761.6 µA mM-1 cm-2 ranging from 0.46 µΜ to 2100 µM,fast response and low detection limit. The micro-electrolysis nanofabrication is a one-step, binder-free, entirely green, and therefore it has a distinct advantage to improve clean production and reduce energy consumption.

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1. Introduction Diabetes mellitus is a serious chronic disease worldwide, which can cause numerous complications and now occurs in younger patients even children

1-3

. The

requirement for constructing glucose sensors with high sensitivity and stability has been increasing to satisfy the urgent needs for application in medical diagnosis of diabetes particularly at early stage. Despite high selectivity of glucose oxidase, current enzyme-based sensors give rise to a limited sensitivity and suffer from the instability and high cost of enzymes procedures

4-5

as well as complicated immobilization

6-7

. Therefore, considerable attention has been paid to develop

transition-metal oxides or hydroxides as non-enzymatic sensors with superior sensitivity, high reliability, fast response and good selectivity

8-9

. Among them,

Ni-based materials especial for Ni(OH)2, which can generate NiOOH during the electrooxidation process, greatly improving catalytic activity towards glucose and thus leading to significant application in the design of enzyme mimic sensors 10-11. To meet these demands, substantial efforts have been made to enlarge the surface area and integrity of Ni(OH)2-based electrodes, such as designing porous nanostructures to improve the electron transport between active materials with the current collectors 12-14

. Consequently, design and synthesis of a binder-free Ni(OH)2/scaffold integrated

electrode opens a new strategy for obtaining non-enzymatic glucose sensors. Commercial nickel foam (NF), known as a kind of metal porous material scaffold, has drawn great attention in the past decades. Due to its high electronic conductivity and a desirable three-dimensional (3D) structure, NF not only serves as 3

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the current collector allowing for fast ion diffusion and electron transfer the ideal template to grow 3D interconnected graphene networks

15-16

but also

17-18

. However, NF

with large pore size (more than 100 microns) results in a limited loading of the active materials. Thus, a facile preparation technic with low cost and simple operation to fabricate porous 3D Ni template are highly desired. Under this situation, silica, which has highly ordered pore structure and tunable pore size, has been explored as 3D template for achieving mesoporous structure

19-20

. Nevertheless, the removal of silica

by HF acid is troublesome and causes critical problems in environment. To overcome these issues, employing ZnCl2 particles, a water-soluble salt, to develop pores is available thought for the design of carbonaceous materials

21-22

. Despite the advance,

it's hard to form a homogeneous mixture due to the low melting point of ZnCl2. Therefore, it is still challenging to seek an innocuous and harmless porogen that can be removed by water as well as keep the structural stability under high temperature during the sintering process of nickel. Herein, we report a facile and scalable in situ micro-electrolysis nanofabrication technique for one-step construction of 3D porous nickel template anchored with interconnected Ni(OH)2 nanosheets (designated as Ni(OH)2@3DPN) as an enzyme mimic glucose electrode. The template fabrication (Scheme 1) involves the pressure moulding of sodium chloride and nickel powders to form a compacted planchet, and the subsequent calcination in an argon/hydrogen environment. Nickel powders are sintered into interlinked backbones and sodium chloride penetrates the metal surface due to the chloride induced hot corrosion

23-27

. After hot-water immersion of the

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annealed planchet for a few hours, 2D crosslinked Ni(OH)2 nanosheets were directly grown on the interconnected 3D porous nickel template (small pores, about several microns). The micro-electrolysis nanofabrication is a one-step, binder-free, entirely green technique, avoiding the use of nickel salts, any acid or base, and no need of post treatments. Specifically, NaCl is not only the pore-forming material easy to remove in water but also served as electrolyte largely accelerating the growth of Ni(OH)2. The as-prepared Ni(OH)2@3DPN integrated electrode exhibits remarkably enhanced electrocatalysis, excellent selectivity as well as reliable stability during non-enzymatic sensing of glucose.

Scheme 1 The fabrication of 3D porous nickel template by a typical metal-powder metallurgy technique.

2. Experimental 2.1. Chemicals and characterizations Nickel power with particle size less than 5 µm, glucose, urea, citric acid, uric acid, dopamine and glycine were purchased from Aladdin Reagent (Shanghai, China). Sodium chloride (NaCl) and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Company Limited (Shanghai, China). Ni foams with a thickness of 1.5 mm and area density of 350 g cm-2 were purchased from Corun New 5

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Energy Co., Ltd. (Hunan, China). Ultra-pure water of 18.2 MΩ produced by a Milli-Q purification system (Bedford, USA) was used throughout the experiments. All chemicals are of analytical grade and used without further purification. The morphologies and energy-dispersive X-ray analysis spectroscope (EDS) mapping images were characterized by field-emission scanning electron microscopy (FESEM; Hitachi S-4800). X-ray diffraction (XRD) patterns over 2θ range of 15-85° were obtained by a D/Max-RBX-ray diffractomer (Rigaku, Japan). X-ray photoelectron spectra (XPS) were recorded on an Escalab 250Xi system (Thermo Scientific) using Mg Ka radiation. Electrochemical measurements were performed using a CHI760e electrochemical workstation (Chenhua, Shanghai). 2.2. Preparation of Ni(OH)2@3DPN integrated electrode The 3D porous nickel template was prepared by a metal-powder forming process. Typically, 0.25 g of nickel powders and 0.15 g of NaCl particles was grinded to a homogeneous mixture using an agate mortar. After pressing the chrome-steel tool filled with the above mixes under 20 MPa for 5 minutes, a compacted sheet with the diameter of 22 millimeters was obtained. Then, the preliminary sheet was calcinated to form connective metal frameworks at 700 oC for 4 h in a H2/Ar atmosphere with a heating rate of 10 oC per minute. After that, the macroscopic dark grey sheet together with the commercial NF (a control group for comparison), was immersed in deionized water at 60 ℃ for 6 h in the ambient air. Finally, the prepared circular sheet and NF were taken put and washed with deionized water several times, and then thoroughly dried in an oven at 60 oC for 12 h. 6

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2.3. Electrochemical measurements The electrochemical performance was performed by a CHI760e electrochemical workstation with a three-electrode system at room temperature. The as-prepared Ni(OH)2@3DPN (5 mm × 5 mm) or Ni(OH)2@NF (5 mm × 5 mm) electrode was served as the working electrode, while saturated calomel electrode (SCE, in saturated KCl) and a platinum wire electrode (Ф = 1 mm) were selected as the reference and counter electrodes respectively. Cyclic voltammograms (CVs) were obtained with a potential window of -0.2 V to 0.6 V and amperometric measurements were carried out at a constant potential of 0.46 V. All the electrochemical measurements were carried out in 0.1 M NaOH aqueous solution with magnetic stirring.

3. Results and discussion 3.1. Morphology characterization and the underlying mechanisms The morphologies of Ni template before and after removing NaCl was presented by SEM images. Figure 1a shows an intact dark grey plane with some bright areas. As shown in the magnified image (Figure 1b), many bumps uniformly distribute on the seamless surface. NaCl particles dissolve in water to obtain throughout micropores, with an average diameter of 1-2 µm, and the corresponding results are displayed in Figure 2c and 2d. The obvious wrinkles on the Ni skeletons and the preserved cubic structure duplicated from NaCl (the red dotted bordered rectangle in Figure 1d), confirm the interdiffusion effect due to the chlorinated corrosion at high temperature.

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Figure 1 SEM images of 3D porous nickel template before (a, b) and after (c, d) removing NaCl by water washing.

The self-limiting surficial hot corrosion mechanism: during of the sintering process, NiO firstly dissolves at the low oxygen potential interface of oxidation film and salt to produce NiCl2: NiO + 2 Cl- → NiCl2 + O2-. Then, NiCl2 diffuses outward driven by concentration gradient and re-deposits at the high oxygen potential interface of salt and gas phase to produce NiO: NiCl2 + 1/2 O2 → NiO + Cl2. On the one hand, it is hard to protect against corrosive media getting in deeply due to the loose and porous structure of NiO film. On the other hand, NiO can stay stable in a low vacuum resulting from the high melting point of NiCl2. That is, it is a self-limiting surficial hot corrosion differing from the general hot corrosion in oxygen or air. In the end, H2 is introduced to reduce the NiO film while maintain 8

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the surface defects. The typical metal-powder forming technique is not only a quick and efficient way to sinter nickel powders into connective metal frameworks, but also determines a “start engine stop” mechanism for one-step and facile growth of Ni(OH)2 nanosheets.

Scheme 2 The growth mechanism for micro-electrolysis nanofabrication of the integrated Ni(OH)2@3DPN electrode.

During the Ni(OH)2 fabrication process, NaCl aqueous solution contacts with Ni skeletons to construct numerous primary cells, which could proceed spontaneously even at room temperature. The micro-electrolysis nanofabrication has distinctive advantages than the other reported Ni(OH)2 fabrication methods (Table S1). The as-prepared Ni(OH)2@3DPN electrode exhibits a considerably perfect duplicated macrostructure due to the coverage of the crosslinked nanosheets (Figure 2). Compared with NF, the self-supported 3D Ni framework provides much more surface 9

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area for loading active materials. Besides, the 3D porous Ni template is both the nickel source and the deposition scaffold, which avoid the use of binders, nickel salts, acids or bases. And, Ni and NaCl can be reused without any post treatment. The interconnected 3D structure effectively prevents the agglomeration or exfoliation of Ni(OH)2 nanosheets and offers large interspaces for the penetration of bioactive molecules, which reduce the contact resistance and enhance the mass/charge transfer rate at the electrode/electrolyte interface. “Start engine stop” mechanism under micro-electrolysis nanofabrication: the formation of Ni(OH)2 is a one-step process in salt solution under ambient atmosphere. It is the typical electrochemical reaction of primary batteries (Scheme 2): oxidation occurs at the cathode surface of Ni skeletons to generate Ni2+: Ni – 2 e- →

Ni2+; OH− ions are produced through the reduction reaction of anode: 2 H2O + O2 + 4 e- → 4 OH−. Ni(OH)2 nanosheets are subsequently deposited at the surface of 3D Ni backbones: Ni2+ + 2 OH− → Ni(OH)2. The morphological change of Ni(OH)2@3DPN was monitored at different reaction time, as shown in Figure S1. At initial stage (t = 2 h), scattered flakes are formed on the surface of Ni skeletons (Figure S1a and S1b). At the kinetically controlled growth stage, more flake crystals combine into large agglomerates (t = 4 h, Figure S1c and S1d). Then, anisotropic growth leads to complete formation of high-density crosslinked nanosheets (t = 6 h, Figure S1e and S1f). A significant morphology difference of Ni(OH)2@NF can be noticed in Figure S2. Under the same reaction condition, only sparse and small Ni(OH)2 hexagonal platelets were grown on the backbones of Ni foams. It experimentally 10

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demonstrates that the existence of “start engine stop” mechanism benefiting from the self-limiting hot corrosion.

Figure 2 (a) Low- and (b-d) high-resolution SEM images of the integrated Ni(OH)2@3DPN electrode with different magnifications.

3.2. Structure and composition characterization The phase structure and purity were identified by XRD, and the results are shown in Figure 3a. The black curve represents the composite of NaCl and Ni after pressing and sintering. 3DPN obtained by water washing is in a good agreement with the characteristic peaks of nickel (marked by triangle, JCPDS 89-7128). The diffraction peaks of blue curve can be well attributed to NaCl (marked by circle, JCPDS 77-2064) and hexagonal β-Ni(OH)2 (marked by diamond, JCPDS 14-0117). 11

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No other diffraction peak is observed in the XRD pattern, confirming the purity of the integrated Ni(OH)2@3DPN electrode. X-ray photoelectron spectra (XPS) was further conducted (Figure 3b) and high-resolution XPS spectra of O 1s and Ni 2p were illustrated in Figure 3c and 3d. The tiny peak at 532.8 eV is associated with adsorbed oxygen species, and the strong peak at 531 eV can be attributed to the lattice oxygen of Ni(OH)2

28

. The peaks located at 873.6 eV and 856 eV, with an energy level

difference of 17.6 eV, belong to Ni 2p1/2 and Ni 2p3/2 respectively. Elementary composition was provided by energy-dispersive X-ray analysis in Figure 4. The uniform distribution of Ni and O reveals the successful construction of the integrated Ni(OH)2@3DPN electrode.

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Figure 3 (a) XRD patterns of different intermediates during the fabrication of Ni(OH)2@3DPN. (b-d) XPS spectra of Ni(OH)2@3DPN.

Figure 4 (a) A representative local area of Ni(OH)2@3DPN. (b-d) EDS elemental mapping images of Ni and O.

3.2 Cyclic voltammograms (CVs) and selectivity CVs were carried out to study the electrocatalysis at a scan rate of 20 mV s-1 in 0.1 M NaOH solution. According to Figure 5a, no remarkable change is observed for Ni(OH)2@NF (curve a and b) without and with glucose, which means an extremely limited catalytic effect. The appearance of the oxidation peak and a distinct improvement of redox current (curve c and d) demonstrate high electrochemical catalysis of the integrated Ni(OH)2@3DPN electrode. Compared with curve b, the marked current increase of curve d is strongly associated with the larger surface area provided by 2D crosslinked nanosheets and 3D network architecture. The electro-oxidation mechanism of the integrated Ni(OH)2@3DPN electrode can be 13

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ascribed to the highly active Ni(OH)2/NiO(OH) redox couple. NiO(OH) is obtained due to the electrooxidation of Ni(OH)2: Ni(OH)2 - e- + OH- → NiO(OH) + H2O; and then NiO(OH) catalyze the oxidation of glucose to produce glucolactone: NiO(OH) +

glucose → Ni(OH)2 + glucolactone. Selectivity was evaluated through the successive addition of other bioactive substances, as shown in Figure 5b. The Ni(OH)2@3DPN sensor presents good specificity to glucose, which can evade the interference resulting from glycine, urea, UA, CA, DA and AA.

Figure 5 Measurements of cyclic voltammograms and selectivity in 0.1 M NaOH solution under ambient atmosphere. (a) CVs of Ni(OH)2@NF (curve a and b), Ni(OH)2@3DPN (curve c and d) without and with 0.5 mM glucose at 20 mV s-1. (b) Selectivity of the integrated Ni(OH)2@3DPN electrode evaluated by comparing output current in response to glucose, urea, citric acid (CA), uric acid (UA), dopamine (DA), glucose, glycine and ascorbic acid (AA).

3.4 Amperometric measurements of the Ni(OH)2@3DPN electrode The electrochemical response of the Ni(OH)2@3DPN electrode was shown in Figure 6a. Because of overpotential increase and kinetic limitation in the 14

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electrooxidation of glucose

29-30

, not only peak currents increase gradually but also

peak potentials have a corresponding shift with the increasing scan rate. Figure 6b indicates the redox peak currents is proportional to the square root of the scan rate, demonstrating the diffusion-controlled reaction electro-oxidation

due

to the rapid

glucose

31-32

. The typical amperometric response of Ni(OH)2@3DPN and

Ni(OH)2@NF was achieved by the successive addition of glucose at a working potential of 0.46 V (Figure 6c). Compared with Ni(OH)2@NF, the integrated Ni(OH)2@3DPN electrode shows higher sensitivity and fast response of less than 2 s. The detection limit for glucose of the Ni(OH)2@3DPN electrode is about 0.46 µM (S/N = 3) with the sensitivity of 2761.6 µA mM−1 cm−2, which is nearly four times of Ni(OH)2@NF (660.5 µA mM−1 cm−2). The corresponding calibration curve of current versus the glucose concentration is plotted in Figure 6d:   = . +

.   , which shows a good linearity (correction coefficient, R = 0.9996) ranging from 0.46 to 2100 µM. Based on the Randles-Sevcik equation

33

, the

enhanced sensing performance obtained by the integrated Ni(OH)2@3DPN electrode is due to the larger electrochemical active surface area. The comparison of analytical performance of Ni(OH)2@3DPN electrode with other published Ni(OH)2-based glucose sensors is shown in Table 1. It should be noted that the performance of the sensor proposed in this study is better than most previous studies in terms of sensitivity, detection limit and linear range, indicating the competitive edges of the integrated Ni(OH)2@3DPN electrode to potentially use in practice. The improvement of sensing performance can be essentially attributed to the 15

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in-situ preparation technique, porous macrostructure of 3D Ni networks and highly stable

microstructure

of

crosslinked

nanosheets.

The

micro-electrolysis

nanofabrication is spontaneous due to the formation of countless primary batteries. The as-prepared integrated electrode with 3D porous architecture provides much more surface area for loading active materials, resulting in a better current response. The crosslinked microstructure prevents the agglomeration or exfoliation of Ni(OH)2 nanosheets and offers abundant sites for electro-catalysis of glucose, which improve the sensor stability and sensitivity significantly.

Figure 6 Amperometric measurements in 0.1 M NaOH solution under ambient atmosphere. (a) CVs of the Ni(OH)2@3DPN electrode from -0.2 V to 0.6 V at different scan rates with 0.4 mM glucose. (b) The corresponding plot of peak currents 16

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and the square root of the scan rate. (c) A typical steady-state response of Ni(OH)2@3DPN and Ni(OH)2@NF by successive injections of glucose at a potential of 0.46 V under stirring. (d) The calibration curve of the response current versus the glucose concentration.

Table 1 Comparison the present electrode with other reported Ni(OH)2 based non-enzymatic glucose sensors. Electrode

Sensitivity

Detection Linear range References

(µA mM-1 cm-2) limit (µM) (up to, µM) Ni(OH)2@3DPN

2761.6

0.46

2100

This work

Ni/NF

2370

5

700

34

Ni(OH)2/NiO

2300

0.2

1000

35

2071.5

0.36

800

36

1950.3

0.16

6000

11

Ni(OH)2 /CNT/GCE

1438

0.5

1000

37

Co3O4Ni(OH)2/GCE

1089

1.08

40

38

Ni(OH)2/PEDOT-rGO

346

0.6

7100

39

192

8

14

40

Ni(OH)2 NS/PI/CNT Ni(OH)2 NPs/NF

Ni(OH)2/TiO2

4. Conclusion To summarize, a in situ micro-electrolysis nanofabrication technique is proposed to grow crosslinked Ni(OH)2 nanosheets on a novel 3D porous nickel template. What is particularly unusual is that it claims a self-limiting surficial hot corrosion effect due 17

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to the existence of NaCl, which determines the “start engine stop” mechanism for the one-step growth of Ni(OH)2. The micro-electrolysis nanofabrication for Ni(OH)2 is an entirely green method featured in zero-pollution, zero-emission and no need for complex equipment. As a proof of-concept demonstration, the integrated Ni(OH)2@3DPN electrode was served as a non-enzymatic glucose sensor, which presents a significantly enhanced sensing performance with high sensitivity, short response time, low detection limit and good selectivity. The performance improvement can be attributed to the synergistic effect of the macroscopic 3D porous structure and large specific area provided by microscopic flake-like morphology. This facile strategy for the fabrication of metal hydroxides based integrated electrodes potentially gets popularization and application in future.

Supporting Information SEM images of Ni(OH)2@3DPN at different reaction time to investigate the growth process. Typical SEM images of Ni(OH)2@NF under the same preparation condition of Ni(OH)2@3DPN.

Acknowledgments This work was financially supported by Zhejiang Provincial Public Technology Research (LGG18E020001) and the Fundamental Research Funds for the Central Universities.

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Notes The authors declare no competing financial interest.

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Table of contents Title: 3D porous nickel frameworks anchored with crosslinked Ni(OH)2 nanosheets as

a high sensitive non-enzymatic glucose sensor

The growth mechanism for micro-electrolysis nanofabrication of the integrated Ni(OH)2@3DPN electrode.

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