Low-Temperature Chemical Synthesis of Three-Dimensional

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Low Temperature Chemical Synthesis of Three Dimensional Hierarchical Ni(OH)-Coated Ni Microflowers for High Performance Enzyme-Free Glucose Sensor 2

Arumugam Manikandan, Vediyappan Veeramani, Shen-Ming Chen, Rajesh Madhu, Ling Lee, Henry Medina, Chia-Wei Chen, Wei-Hsuan Hung, Zhiming Wang, Guozhen Shen, and Yu-Lun Chueh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07113 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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The Journal of Physical Chemistry

Low Temperature Chemical Synthesis of Three Dimensional Hierarchical Ni(OH)2-Coated Ni Microflowers for High Performance Enzyme-Free Glucose Sensor Arumugam Manikandan , Vediyappan Veeramani¥ Shen-Ming Chen¥*, Rajesh Madhu¥,#, †

Ling Lee†‡, Henry Medina†, Chia-Wei Chen†, Wei Hsuan Hung++, Zhiming M. Wang‡ and Yu-Lun Chueh†* †

Department of Materials Science & Engineering, National Tsing-Hua University,

Hsinchu 30013, Taiwan, R.O.C ¥

Department of Chemical Engineering and Biotechnology, National Taipei University

of Technology, Taipei 10608, Taiwan, R.O.C #

Department of Life, Environment, and Materials Science, Fukuoka Institute of

Technology, 3-30-1, Wajirohigashi, Higashiku, Fukuoka 811-0295, Japan. ‡

Institute of Fundamental and Frontier Sciences, University of Electronic Science and

Technology of China, Chengdu 611731, P. R. China. ++

Department of Materials Science and Engineering, Feng Chia University, Taichung

407, Taiwan, R.O.C *

E-mail: [email protected] and [email protected]

Phone: +886-35715131 (Yu-Lun Chueh); +886-2270 17147 (Shen-Ming Chen) Abstract- Since prevention methods of type-II diabetes and knowledge of prediabetes are lacking, the development of sensitive and accurate glucose sensors with an ultralow detection limit is imperative. In this work, the enzyme-free glucose sensor based on three dimensional (3D) hierarchical Ni microflowers with a Ni(OH)2 coating layer has been demonstrated in a simple one-step chemical reaction at a low temperature of 80 oC. The as-synthesized materials were characterized by several analytical and spectroscopic techniques. In addition, thin Ni(OH)2 layer formed at the surface of Ni microflower was evidenced by RAMAN, HRTEM and XPS, which is the key factor to achieve high sensitive enzyme-free glucose sensors based on low-cost materials such as copper, nickel and their oxide and hydroxide. Moreover, our modified electrode exhibits an outstanding detection limit as low as 2.4 nM with an ultra-high sensitivity of 2392 µA mM−1cm−2, which is attributed to not only the increased surface area due to the controlled formation of spikes but also the contribution of Ni(OH)2 coating layer.

1 ACS Paragon Plus Environment


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1. Introduction Diabetes mellitus is a group of metabolic disease, which is highly responsible for heart diseases, blindness and kidney failure. Therefore, it is imperative to develop prevention methods of type-II diabetes in particular to the prediabetes because fundamental knowledge is still not complete yet. Up to date, one of the effective techniques is the detection of glucose, since the abnormal levels of glucose in the human body can cause diabetes. For this purpose, electrochemical glucose biosensors are convenient, low cost, reliable, highly sensitive and easy to be operated, which are better than several other techniques such as surface plasmon resonance biosensors, calorimetric sensors and optical sensors.1 The detection of glucose by this technique can be divided into two major types. One is the enzymatic glucose sensor, which is based on the reaction between glucose and oxygen to produce glucolactone with help of enzyme and glucose oxidase (GOD). Another is the non-enzymatic sensors based on chemically modified electrodes, which can directly oxidize the glucose. The enzymatic glucose sensor possess a better selectivity, nevertheless, the poor chemical and thermal stability of enzymes as well as the expensive fabrication are major drawbacks.2,

3

An

alternative solution of the non-enzymatic glucose sensor utilizing a variety of electrocatalysts was proposed. For instance, transition metals (Au, Pt, Pd, etc.), metal oxides (CuO, NiO, Co3O4, Cu2O/Chitosan, etc.), metal hydroxides (Ni(OH)2), alloys (CuNi, PtPb, PtRu, etc.), carbon (carbon nanotube, graphene, boron doped diamond, etc.) and composites (Ni(OH)2-graphene, graphene/AuNP/Chitosan , Ni-MWNT, CuO-MWNT, etc.)4-11 have been reported. While transition metals are fouled by both high cost and poor stability after the chemisorption of intermediates and chloride ions,2 the nickel-based compound nanomaterials attract most interests because of high sensitivity, long stability, low toxicity, low cost and environment friendly features. The oxidation of glucose is mainly occurred by redox of Ni(OH)2/NiOOH coupled by the catalytic behavior of Ni(III) oxyhydroxide, resulting in the formation of radical intermediates and react with hydroxyl anions to form glucolactone. In


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brief, recent studies based on Ni(OH)2, including nanoflower α-Ni(OH)2 and Ni(OH)2 based nanostructures, NiO nanowalls,12 Ni(OH)2 nanosheets on Ni foam,13 and rose like Ni(OH)2,13 has improved glucose sensing performance compared to various Ni based nanostructures14 such as ring like self-assembled Ni nanoparticle15 carbon fibre decorated with Ni nanoparticles,2 Ni nanowire,5 Ni nanoflakes,16 NiO/MWCNT,17 Pt/Ni-graphene,18 Pt/Ni nanowire,19 porous Cu/NiO20 and flower like NiO hollow microsphere.3 These previous results encourage us that there is an opportunity to achieve both a high sensitivity and a good selectivity of non-enzymatic glucose sensor based on combination of Ni-based microstructure and Ni(OH)2. Although the aforementioned nickel based electrodes were avoided the oxidation of common interfering agents such as ascorbic acid and uric acids, its complex synthesis process, such as high temperature and the use of organic compounds, may damage the final product of those hierarchal nanostructures. In addition, template assisted growth of nanowires and nanowalls with addition of high cost material such as Pt may rise the production cost. In this regard, we demonstrated a novel non-enzymatic glucose sensor using low temperature surfactant assisted chemically synthesized Ni microflowers with the Ni(OH)2 coating layer by the well-known surfactant ethylenediamine (EDA)21, exhibiting the lowest detection limit of 2.4 nM among all the reported Ni-based non-enzymatic glucose sensor prepared by a single step surfactant assisted

reduction method. The contributions

from both the high surface-to-volume ratio and the Ni(OH)2 coating instead of the typical NiO are also discussed. These achievements provide a feasibility of cheap and effective glucose sensors for the early detection of glucose as a step forward in the prevention of type-II diabetes.

2. Experimental Section 2.1 Materials Nickel (II) nitrate hexahydrate (98%), ethylenediamine (99%), hydrazine monohydrates (64 wt %) were purchased from Alfa Aesar, sodium hydroxide was purchased from J.T. Baker chemicals and ethanol (98 %) was purchased from sigma aldrich. All the chemicals were



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analytical grade and used as purchased without any further purification. The experiments were carried out by using doubly distilled water. 2.2 Synthesis of Ni(OH)2-coated Ni microflowers In a typical experiment, 0.1M nickel (II) nitrate hexahydrate aqueous solution, 0.15 and 0.3 ml ethylenediamine and 0.01, 0.05 and 0.1 ml hydrazine monohydrates were added to 15 M sodium hydroxide in 20 ml of distilled water. After vigorous stirring, the solution was kept in water bath and maintained 80 °C for 1 hour. The final black product floats on top of the solution which was separated by using magnet and washed with ethanol followed by centrifuge at 4000 rpm for 5 min and the process was continued for five cycles. Then, it was dried in room temperature over the night. 2.3 Synthesis of NiOx-coated Ni microflowers The dried powder from the above process was also used to prepare NiOx coated Ni microflower. The Ni(OH)2-coated Ni microflowers was oxidized by placing it into O2 plasma chamber for 3 minutes. The plasma was produced by a radio frequency (13.56 MHz) system with a power of 50 W at the pressure of 8.1×10-3 Torr. 2.4 Preparation modified electrode As-synthesized Ni(OH)2-coated Ni microflowers were first dispersed in ethanol under sonication treatment for 1 h. Meanwhile, the surface of the glassy carbon electrode (GCE) was carefully polished by using alumina powder and the ca. 8 µL of dispersed solution was drop casted on the active surface of the GCE followed by drying in an air oven at 30 °C. The modified GCE acts as a working electrode, while Ag/AgCl (sat.KCl) acts as a reference electrode and Pt wire acts as counter electrode. 2.5 Characterization The surface morphologies of Ni(OH)2- and NiOx-coated Ni microflowers were investigated using a field emission scanning electron microscope (FESEM, JSE-6500F, JEOL). Nanoscale structures and lattice spacings were examined by high resolution transmission electron microscope (FE-TEM, JEM-3000F, JEOL) operated at 300 kV with a point to point resolution


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of 0.17 nm. X-ray diffraction (XRD) was performed on TTRAX to investigate the crystallinity, the identification of the presence of Ni(OH)2 and its phase were confirmed by micro-Raman spectroscopy (HORIBA, LabRAM, HR800) with 633 nm laser and the X-ray photoemission spectroscopy (XPS) equipped with a monochromatic Al Kα X-ray source (XPS, Ulvac-PHI 1600). The cyclic voltammetry (CV) and amperometry (i-t) experiments were carried out by using an electrochemical work station (CHI 627).

3. Results and discussion Scheme 1 represents the simple procedure of Ni microflowers for enzyme free glucose sensor application. Ni microflowers were synthesized by the simple addition of EDA with aqueous solution of Ni(NO3) in the presence of high concentration NaOH and reductive agent N2H4. As-synthesized Ni(OH)2-coated Ni microflowers were first dispersed in ethanol under sonication treatment for the following fabrication of modified electrodes for sensing measurements. The detail synthetic processes and glucose measurements were described in experimental section. The morphologies of as synthesized Ni microflowers were characterized by field emission scanning electron microscopy (FESEM) as shown in Figure 1(a), in which the 3D hierarchical microflowers were observed with a uniform size in ranges between 400 and 840 nm. The spike of every microflower exhibits a sharp edge and the size is ranged between 20 to 50 nm as shown in the inset of Figure 1(a), which leads to the way with a high surface to volume ratio. In order to investigate the crystalline structure, the X-ray spectrometer was performed as shown in Figure 1(b). Diffraction peaks located at 44.38°, 51.71° and 76.3° correspond to (111), (200) and (220) planes of the face centered cubic phase of nickel with lattice constant a= 0.3523 nm (JCPDS No. 04-085, a=0.3524 nm). In addition, calculated average crystalline sizes of 18.68 nm, 11.9 nm and 13.41 nm using Scherrer equation (D= 0.94λ/β cos θ) can be achieved. The strong (111) reflection implies a preferred orientation of microflowers with an extrapolated internal spacing of 0.2039 nm. Furthermore, the high resolution transmission electron microscopy (HRTEM) was used to analyse the nanoscale structure as shown in Figure 1(c) to 1(e) with different magnifications. The



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HRTEM images show that the spikes have a rough surface with a highly crystalline nature. An internal spacing of 0.204 nm measured in Figure 1(f) taken from Figure 1(e), corresponding to (111) plane, is consistent with that obtained by X-ray results. The corresponding fast Fourier transformation (FFT) image as shown in Figure 1(g) confirms the single crystalline feature of the as-synthesized Ni microflowers. Accordingly, a successful syntheses method by a simple single chemical reaction was demonstrated to fabricate stable Ni-based microstructures at a low temperature without time consuming as previously reported by literatures.22-24

The growth of microflowers follows the redox reaction under the basic

condition, which is similar to synthesis of Ni nanoparticle25 given by: 2Ni2+ + N2H4 + 4OH-

2Ni + N2 + 4H2O.

To further shed light on surface of Ni microflowers, spectroscopic spectra, including Raman scattering and x-ray photoelectron spectroscopy (XPS), were performed. The Raman scattering spectrum of the as-synthesized microflower is shown in Figure 2(a), in which a broad peak located at 560 cm-1 can be deconvoluted by Gaussian Lorentzian fitting method into four major peaks at 454, 534, 568 and 596 cm-1, respectively. According to previous literatures26, 27 both the weak peak at 454 cm-1 and the wide peak at 534 cm-1 correspond to the symmetric of Ni-OH stretching mode of the α-Ni(OH)2 phase, while the peaks located at 568 and 596 cm-1 correspond to the Ni-O stretching mode of the β-Ni(OH)2 phase. Moreover, the Raman scattering peaks at 3264 and 3565 cm-1 as shown in the inset of Figure 2(a), correspond to the O-H stretch of α-Ni(OH)2 phase. These results confirm the existing of the mixture of α- and β-Ni(OH)2 phases in the as-synthesized Ni microflowers, while the slight difference of peak positions in comparison to those reported in the literatures could be attributed to the three-dimensional feature.28 Furthermore, a brief x-ray photoelectron spectroscopy of as-synthesized Ni microflowers is shown in Figure 2(b) with clear peaks corresponding to carbon (from sample holder), oxygen and nickel, respectively. For the Ni 1s spectrum shown in Figure 2(c), five peaks with the binding energies of 852.6, 855.7, 859.2, 869.9 and 872.8 eV were measured. The peaks at 852.6 and 869.9 eV correspond to 2p3/2 and


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2p1/2 transitions of metallic Ni with satellite peaks located at 859.2 eV and 872.8 eV. A weak peak at 855.7 eV is attributed to the 2p transition from Ni(OH)2, which is agreement with previous literatures.26, 29-33 For the O 1s spectrum shown in Figure 2(d), the main peak at 531.3 eV accompanied with a weaker shoulder located at 529.5 eV are attributed to the hydroxyl group (OH-). Since the XPS is a surface sensitive technique, it is a furthermore evidence that the Ni(OH)2 phases are formed at the surface of as-synthesized Ni microflowers. To further confirm the existence of Ni(OH)2, the O2 plasma treatment was performed to convert the Ni(OH)2 layer into the NiOx layer. After the plasma treatment for 3 minutes, the morphology and the size remain and no addition impurities are detected by XRD (Figure S1a and. S1b). Although the signal of the NiOx layer is too weak to be detected by XRD (Figure S1b), the HRTEM image as shown in Figure S1e indeed reveals the existence of the NiOx layer with a typical thickness of 2 nm. The NiOx layer can be distinguished by the high angle annular dark field (HAADF) image with a bright contrast on the surface of the Ni spike due to the different atomic number where a larger atomic number exhibits the brighter image contrast.34 In addition, energy dispersive (EDS) line scan profiles, which cross the entire spike region, reveal the uniform distribution of oxygen (O) and Nickel (Ni), confirming the solid evidence of the existed NiOx layer on the surface of the Ni spike (Figure S2). In addition, a clear evidence of the existed NiOx on the surface of Ni microflowers rather than Ni(OH)2 was confirmed by XPS measurements as shown in Figure S3 after the O2 plasma treatment, which is consistent with the results from XRD and HRTEM.8, 35-37 In addition, the dependence of the ethylenediamine (EDA) concentrations was investigated, where the growth of spikes in specific orientations is preferred as similar as synthesis of copper nanowire and copper nanodisc.38 Once the EDA concentration increases, twice flake-like nanostructures were generated as shown in Figure 3(a). On the contrary, the increase of the hydrazine concentration strongly leads to reduction of Ni ions and tends to form nanoparticles at elevated temperatures.25 For the high and the medium concentrations of hydrazine as mentioned in the method section, nanoparticles with very fine spike-like structures were



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observed as shown in Figure 3(b) and 3(c), respectively. Based on the obtained results, we claims that the growth mechanism involved surfactant assistance, which is similar as reported in these literatures.21, 39, 40 Accordingly, the optimized 3D hierarchal structure was achieved with the ethylenediamine and hydrazine concentrations of 0.15 and 0.01 ml, respectively. Since the formation of Ni(OH)2 can be blocked when synthesizing in a NaOH solution at a high concentration38, we intentionally washed our as-synthesized Ni microflowers by ethanol to ensure the existence of

the Ni(OH)2 layer on the surface. We also presented the Raman

scattering spectrum of as prepared Ni microflowers washed by water as shown in Figure 3(d). Note that the peaks can be deconvoluted by Gaussian Lorentzian fitting into three major peaks located at 472, 529 and 575 cm-1, corresponding to isostructural NiO2 units in Ni(OH)2 and Ni-O stretching modes.41-43 The results prove

the

formation of Ni(OH)2

when

washing with ethanol. To shed light on performance of mixed phases of Ni(OH)2-coated Ni microflowers as the glucose sensor, the results of cyclic voltammetry (CV) measurements operated in the absence and presence of 1.0 mM glucose in 0.1 M NaOH solution by utilizing sensors based on the bare glassy carbon electrode (GCE), and modified electrodes from Ni(OH)2- and NiOx-coated Ni microflowers are shown in Figure 4(a). In comparison to the CV curve of the bare GCE, the modified electrode from Ni(OH)2-coated Ni microflowers exhibits well defined the redox peak. In the absence of the glucose, the redox peak can be observed in the modified electrode from Ni(OH)2-coated Ni microflowers through Ni(OH)2+OH¯– e→NiOOH. Once a 1.0 mM of glucose was added into the solution, the anodic peak (Ipa) current increases intensely, which could be attributed to the production of gluconolactone when the glucose was oxidized through the reaction with NiOOH (NiOOH + glucose → Ni(OH)2 + gluconolactone).44 The modified electrode forms and NiOx-coated Ni microflowers also exhibits similar trend, while the peak intensity is much smaller than that from the Ni(OH)2-coated Ni microflowers as shown in inset of Figure 4(a). The results confirm that the spontaneously formed Ni(OH)2 phase on the surface of Ni microflowers during the


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synthesis process leads to exhibit stronger redox reaction. As for the device based on the modified electrode from Ni(OH)2-coated Ni microflowers, the increase of scan rate in ranges between 20 and 200 mV s−1 results in a widen peak-to-peak separation with a linear increment of anodic oxidation peak current and a correlation coefficients of 0.9939. The peak shifts toward the positive potential signifies the typical diffusion-controlled electrochemical process as shown in Figure 4(b). Once the concentration of glucose increases up to 6 mM, the corresponding oxidation peak current increases as shown in Figure 4(c). Furthermore, the active surface of the Ni(OH)2-coated Ni microflowers in Fe(CN)6 4- solution was determined to be 1.81 cm2 using the Randel Sevick equation shown in equation given by:45 Ip = 2.69 × 105 AD1/2 n 3/2γ1/2C Where, n is the number of electrons involved in the redox reaction (n = 1), A is the area of the electrode (0.072 cm2), D is the diffusion coefficient of the ferricyanide (cm2 s -1), C is the concentration of the ferricyanide in the bulk solution (mol cm-3) and γ is the scan rate (V s-1). Recently, Thapliyal et al., obtained the active surface area value is 0.651 cm2 for the NiO– ZrO2 nanocomposite, which has been lower than our modified electrode.46 Moreover, the surface concentration (SC) of the modified electrode is calculated using the Brown–Anson model based on the following equation:47, 48 Ip = n2F2I*AV/4RT where n is the number of electrons transferred in the electrode, F is the Faraday constant (96485 C/mol), I* is the surface concentration of the corresponding electrode (mol cm−2), A is the surface area of the electrode (0.072 cm2), V is the scan rate (V/s), R is the gas constant (8.314 J mol−1 K−1) and T is the room temperature (300 K). Hence, the calculated SC value is 8.1 x 10-11 mol cm−2. Moreover, we calculated the charge transfer rate constant (Ks) value \ using the Lavirons equations.49 log ks = α log(1 − α) + (1 − α) log α – log(RT/nFν) −α(1 − α) nF∆Ep/2.3 RT Hence, the Ni(OH)2-coated Ni microflowers has Ks= 3.65 s-1. An amperometric response was observed on the modified electrode from Ni(OH)2-coated Ni microflowers for the successive



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addition of glucose at various concentrations in 0.1 M NaOH solution at an applied potential of +0.55 V under an optimum condition as shown in Figure 4(d). Clearly, a clear linear relationship was observed between 0.5 and 29959 µM, following a relationship of I(µA), y=189.98x+9.4365, with a correlation coefficient of 0.9942 as shown in the inset. This result indicates glucose sensor on the modified electrode from Ni(OH)2-coated Ni microflowers, providing a much better response. Furthermore, the detection limit and sensitivity of 2.4 nM and 2392 µA mM−1 cm−2 can be further extrapolated, respectively. To shed light the outstanding performance as the sensor using Ni(OH)2-coated Ni microflowers, different Ni-based modified electrodes were listed in Table 1 for comparison. Obviously, the sensor based on Ni(OH)2-coated Ni microflowers exhibits an ultralow detection limit with the outstanding sensitivity among all Ni-based modified electrodes because of the existed mixed phases of Ni(OH)2 on the high surface area of Ni microflowers. In addition, the growth of Ni(OH)2-coated Ni microflowers can be formed at the low temperature of 80 oC. Furthermore, the specificity is an important parameter for the glucose sensor in real-time applications. As shown in Figure 5(a), the sensor based on modified electrode from Ni(OH)2-coated Ni microflowers is insensitive to human serum, which contains a large variety of proteins and other molecules, while it is more sensitive to the serum mixed glucose. Moreover, we have tested the stability of the fabricated electrode after the real sample (serum sample) analysis as shown in Figure S5. The result reveals that electrode would be reusable for the reported sensor. In addition, the selectivity of the glucose sensor was examined by ascorbic acid, uric acid, dopamine and many common interfering agents such as NaCl, KCl, Na2SO3, NaNo3, ZnCl2, CuCl2 and CaCl2 as shown in Figure 5(b). A small response occurs for ascorbic acid because of the oxidation of ascorbic acid at a high over-potential and the intensity is much less than that for glucose. Hence, the modified electrode from Ni(OH)2-coated Ni microflowers is vastly useful for the detection of glucose in the real samples without any interference of the other biological species. Clearly, the corresponding relative standard deviation (RSD) of the measurements is 2.5 % for the modified electrode


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from Ni(OH)2-coated Ni microflowers, indicating a good reproducibility. Furthermore, the modified electrode from Ni(OH)2-coated Ni microflowers exhibits only ~10 % degradation of its original current after 25 days as shown in Figure 5(c), which suggests practical catalytic applications of the modified electrode from Ni(OH)2-coated Ni microflowers.

4. Conclusion In summary, we successfully synthesized the Ni(OH)2-coated Ni microflowers by using a simply single step reduction method at a low temperature of 80 oC. The Ni(OH)2 containing the mixture of α- and β-Ni(OH)2 was confirmed by Raman spectra. As for the glucose sensor based on the modified electrode from Ni(OH)2-coated Ni microflowers, To our knowledge, the detection limit of 2.4 nM with the sensitivity of 2392 µA mM−1cm−2 was measured, which is the lowest detection limit of the enzyme free electrochemical glucose sensor based on among all Ni-based modified electrodes. These achievements open the chance for spontaneously formed Ni(OH)2-coated Ni microflowers as the novel modified electrode with low cost as the highly sensitive glucose sensor.

Supplementary Information Electronic Supplementary Information (ESI) available: Detail material characterizations, including SEM, XRD, TEM and XPS for NiOx-coated Ni microflower. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The research is supported by the Ministry of Science and Technology through Grant through

grants

no

104-2628-M-007-004-MY3,

104-2221-E-007-048-MY3,

104-2633-M-007-001, 104-2622-M-007-002-CC2, and the National Tsing Hua University through Grant no. 104N2022E1. Y.L. Chueh greatly appreciates the use of facility at CNMM, National Tsing Hua University through Grant No. 104N2744E1

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Immobilization

of

myoglobin

on

Au

nanoparticle-decorated

carbon

nanotube/polytyramine composite as a mediator-free H2O2 and nitrite biosensor. Sci. Rep.



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Table caption. Table 1. Comparisons of analytical parameters for enzyme free glucose sensor from different modified electrodes.

Materials

Linear range (µM)

Limit of Detection (µM)

HAC/NiO

5–4793

0.055

Sensitivity (µA mM−1cm−2) 1721.5

Ni(OH)2/rGO

2–3100

0.6

11.43

44

NiO

2–10, 50–3300

0.3

-

50

Ni NPs/SMWNTs

1–1000

0.5

1438

51

Graphene/NiO

5–2800

1

1571

52

NiO-Ag

1–590

1.37

19.3

8

NiO

10-800

1.2

8500

53

RGO/Ni(OH)2

0.01-30

15

11400

54

Ni/NiO

0.01-8300

10

4490

55

0.5–2959

0.002

2392.4

This work

Ni(OH)2-coated Ni microflowers /GCE


Ref 37


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Figure Caption: Scheme 1 Schematic illustration of synthesis process and glucose detection mechanism of Ni(OH)2-coated Ni microflowers. Figure 1 (a) A SEM image of the modified electrode from Ni(OH)2-coated Ni microflowers. Inset of (a) is a high magnification image for a spike. (b) XRD spectrum. (c-e) HRTEM images with different magnifications. (f) A HRTEM image of the Ni spike and (g) the corresponding FFT image taken from (f). Figure 2 (a) Raman spectra of as-prepared Ni(OH)2-coated Ni microflowers. The higher order range is shown in the inset. (b) XPS spectra of Ni(OH)2-coated Ni microflowers. (c) and (d) core level spectra of Ni 2p and O1s for Ni(OH)2-coated Ni microflowers. Figure 3 SEM images of sample prepared with (a) 0.3 ml EDA, (b) 0.1 ml hydrazine and (c) 0.05 ml hydrazine. (d) Raman spectra of Ni(OH)2-coated Ni microflowers after the DI water wash. Figure 4 (a) CV curves observed for the bare and the as the modified electrode from Ni(OH)2-coated Ni microflowers in the presence of 100 µM glucose solutions. Inset shows the CV curve of the modified electrode from N-coated Ni microflowers. (b) CV profiles at different scan rates from 20 to 200 mVs-1. The inset shows the variations of peak current as the function of the square root of scan rate. (c) CV profiles for the modified electrode from Ni(OH)2-coated Ni microflowers measured at varied glucose concentrations from 0.002 to 6 mM. Inset shows its corresponding calibration plots at a scan rates 50 mVs-1. (d) Amperometic response for the modified electrode from Ni(OH)2-coated Ni microflowers at varied glucose concentrations from 0.005-2.9 mM. Inset shows its corresponding calibration plots. Supporting electrolytes: 0.1 M NaOH aqueous solutions; rpm: 1200; Applied potential: 0.55 V.


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Figure 5 (a) Real sample analysis in human serum (10 µL) for the modified electrode from Ni(OH)2-coated Ni microflowers with and without glucose on the response of 10 µM glucose. (b) Effect of interference study for the modified electrode from Ni(OH)2-coated Ni microflowers at 10 µM mM glucose concentration with the addition of various species. (c) Stability test after 25 days for the modified electrodes based on Ni(OH)2 coated Ni microflowers. Supporting electrolytes: 0.1 M NaOH saturated with nitrogen gas; rpm: 1200; app. potential: +0.55 V.



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Scheme 1


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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TOC


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Low-Temperature Chemical Synthesis of Three-Dimensional

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