Single layer of nickel hydroxide nanoparticles covered on a porous Ni foam and its application for highly sensitive non-enzymatic glucose sensor

Sensors and Actuators B 204 (2014) 159–166

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Single layer of nickel hydroxide nanoparticles covered on a porous Ni foam and its application for highly sensitive non-enzymatic glucose sensor Chung-Wei Kung a , Yu-Heng Cheng a , Kuo-Chuan Ho a,b,∗ a b

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

a r t i c l e

i n f o

Article history: Received 21 March 2014 Received in revised form 27 June 2014 Accepted 23 July 2014 Available online 1 August 2014 Keywords: Electrochemical treatment Glucose sensor Nanoparticles Ni foam Nickel hydroxide Non-enzymatic detection

a b s t r a c t A single layer of nickel hydroxide nanoparticles (Ni(OH)2 NPs) was covered on the full surface of a porous Ni foam by simply applying an electrochemical cyclic voltammetric (CV) treatment on a bare Ni foam substrate for 100 cycles in 1.0 M NaOH solution. The surface morphology of the obtained Ni(OH)2 NPs/Ni foam electrode was examined by scanning electron microscopy. The thickness of the Ni(OH)2 NPs layer on the electrode was estimated by X-ray photoelectron spectroscopy with Ar+ ion etching. In CV measurement, the Ni(OH)2 NPs/Ni foam electrode exhibited excellent electrocatalytic ability toward glucose in NaOH solution. The Ni(OH)2 NPs/Ni foam electrode was successfully used for the quantification of glucose by an amperometric method. The sensing parameters include a high sensitivity of 1950.3 ␮A/mM-cm2 , a linear range from 0.6 to 6.0 mM, a detection limit of 0.16 ␮M, and an applied potential of 0.45 V (vs. Ag/AgCl/KCl sat’d). The excellent performances obtained in the interference test, the long-term durability test in atmosphere, and the reproducibility test of the Ni(OH)2 NPs/Ni foam sensor indicate the applicability of the proposed electrode as a reliable non-enzymatic glucose sensor. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The detection of glucose is an important issue, because of the clinical significance of measuring blood glucose [1]. The normal concentration of blood glucose in human body ranges from 4 to 8 mM, but the concentration may become much higher or lower for a diabetic. Thus, diabetics need a tight monitoring of their blood glucose levels. Because the diabetes mellitus becomes a more serious health problem in recent years, glucose has been recognized as one of the most commonly tested analytes. Enzymatic glucose sensor, based on an enzyme electrode immobilized with glucose oxidase (GOx), has been proposed to measure the blood glucose level for several decades [2–5]. Although this kind of enzymatic glucose sensor shows the advantage of its high selectivity, the thermal and chemical instabilities of GOx are still the problem of the enzymatic glucose sensor [6]. The expensive price of GOx also leads to the high production cost of the enzymatic glucose sensor chip [7]. In addition, the immobilization of enzyme also further

∗ Corresponding author at: Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan. Tel.: +886 2 2366 0739; fax: +886 2 2362 3040. E-mail address: [email protected] (K.-C. Ho). http://dx.doi.org/10.1016/j.snb.2014.07.102 0925-4005/© 2014 Elsevier B.V. All rights reserved.

complicates the fabrication procedures of the glucose sensor chip and thus affects the sensing performance [8]. In order to solve the problems of the enzymatic glucose sensor, several types of non-enzymatic glucose sensors have been proposed and developed. Metal oxides or hydroxides are usually proposed as the catalytic materials in these kinds of sensors, e.g., MnO2 [9], CuO [10–12], Co3 O4 [13–15], and Ni(OH)2 [7,16,17] have been applied for the non-enzymatic glucose sensors. Among these materials, Ni(OH)2 based electrode has been found to show an excellent electrocatalytic ability toward glucose in alkaline medium; the enzymatic-free oxidation of glucose can be greatly enhanced by the redox couple of Ni(OH)2 /NiOOH formed on the electrode surface [7,16]. Thus, several kinds of synthesizing approaches were used to prepare the Ni(OH)2 based modified electrodes for the use of glucose detection, including solution-based synthesis [16,18], electrodeposition [19,20], sol–gel synthesis [17], and pulsed laser deposition [21]. In contract to the approaches mentioned above, Mu et al. proposed a direct electrochemical treatment to convert the NiO nanoparticles to Ni(OH)2 [7]. In that work, the commercial NiO nanoparticles were mixed with carbon paste and packed into a capillary with a copper wire to form the electrode first. Thereafter, the cyclic electrochemical treatment was used to convert the NiO

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electrode to Ni(OH)2 in alkaline medium. The electrode obtained after the electrochemical treatment was applied for the non-enzymatic glucose detection; an excellent sensitivity of 5590 ␮A/mM-cm2 was achieved, with the linear range up to 0.11 mM and the limit of detection (LOD) of 0.16 ␮M. Although the direct electrochemical treatment provides a more facile way to prepare the Ni(OH)2 modified electrode compared to other synthesizing approaches, the capillary electrode-based glucose sensor shows the disadvantage of complex fabrication procedure, thus is not suitable for practical use. Another approach is using the electrochemical treatment to form the Ni(OH)2 layer from Ni metal substrate. In Toghill’s work, the surface of a bare Ni foil was converted to Ni(OH)2 via cyclic electrochemical treatment, and the obtained foil was served as the amperometric glucose sensor [22]. Compared to the capillary electrode-based glucose sensor, the direct use of Ni foil is more convenient and applicable to the glucose sensor chip, but the sensitivity of this sensor only achieved 670 ␮A/mM-cm2 [22]. Ni foam is one of the most commonly used porous substrates in the field of energy storage. Due to its high conductivity and threedimensional network structure, Ni foam shows several advantages, including a smaller diffusion resistance of electrolyte, an ideal electron pathway, and a much higher loading amount of active materials per unit electrode area [23]. In addition, Ni foam also shows the advantage of its low price. Thus, Ni foam has been applied for several energy-related applications, such as supercapacitor [24], lithium battery [25], hydrogen generation [26], and fuel cell [27]. However, the use of Ni foam for sensor applications is still insufficient. For the first time, Lu et al. proposed Ni foam as a new sensing platform for glucose detection [28]. In that work, the commercially available fresh Ni foam with a smooth surface was directly used both as the electrode substrate and active material for glucose determination. A linear range of 0.05–7.35 mM, a LOD of 2.2 ␮M, and a sensitivity of about 500 ␮A/mM-cm2 were achieved. In order to achieve a higher sensitivity toward glucose, Guo et al. utilized electrodeposition in a Ni(NO3 )2 solution followed by an annealing process to deposit NiO on the surface of Ni foam [29]. Although a high sensitivity of 6658 ␮A/mM-cm2 was achieved, high-temperature annealing process is required in this fabrication. In this study, a simple electrochemical treatment at room temperature was applied on the Ni foam in alkaline medium. After the current response became stable, a single layer of Ni(OH)2 nanoparticles (NPs), with a diameter of 10–15 nm, could cover uniformly on the full surface of the Ni foam by the self-limited precipitation reaction between Ni2+ and OH− . This single layer of Ni(OH)2 NPs was served as the active material for non-enzymatic glucose detection. With the help of the high surface area provided by the Ni(OH)2 NPs and the Ni foam backbone, a high sensitivity of 1950.3 ␮A/mMcm2 was achieved; this sensitivity is much higher than that shown in the literature using untreated Ni foam (about 500 ␮A/mM-cm2 ) [28], and much higher than that reported by using electrochemical treated Ni foil reported in the literature (670 ␮A/mM-cm2 ) [22]. In this study, a linear range from 0.6 to 6.0 mM and an excellent LOD of 0.16 ␮M were also obtained. The excellent performances obtained in the interference test, the long-term durability test, and the reproducibility test indicate the applicability of the Ni(OH)2 NPs/Ni foam sensor.

2. Material and methods 2.1. Chemicals Sodium hydroxide (puriss, Riedel-de Haën) and isopropyl alcohol (>99%, Sigma-Aldrich) were used as received. 50 mM of glucose

sample solution was prepared before each experiment by direct dilution of D-(+)-glucose (99.5%, Sigma-Aldrich) in NaOH solution. The concentration of NaOH was optimized before glucose sensing. Uric acid (>99%, Sigma), acetaminophen (>99%, Sigma), ascorbic acid (>99%, Sigma), methanol (>99.8%, Sigma), ethanol (>99.8%, Sigma), D-(+)-galactose (>99%, Sigma-Aldrich), and D-(+)-mannose (>99%, Sigma) were used for the interference test. Deionized water (DIW) was used throughout the work. 2.2. Apparatus Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and amperometric experiments were performed with a CHI 440 electrochemical workstation (CH Instruments, Inc., USA), using a conventional three-electrode system. A commercial Ni foam (110PPI, thickness = 1.05 mm, Innovation Materials Co., Ltd, Taiwan), with an exposed area of 0.5 cm2 , was used as the working electrode. A Pt foil (4.0 cm × 1.0 cm) and a Ag/AgCl/saturated KCl (homemade) were used as the counter and reference electrodes, respectively. All electrochemical experiments were performed at room temperature and all the potentials were reported against the Ag/AgCl/sat’d KCl reference electrode. The surface morphologies of the Ni foams were observed by using scanning electron microscope (SEM, Nova NanoSEM 230). The surface compositions of the Ni foams at various depths were verified by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe system, ULVAC-PHI, Chigasaki, Japan), using a microfocused (100 ␮m, 25 W) Al X-ray beam, with a photoelectron take off angle of 45◦ . The Ar+ ion source for XPS (FIG-5CE) was used for the surface etching; it was controlled by using a floating voltage of 0.2 kV. The etching rate for Ni was calibrated to be 0.23 nm/s. The binding energies obtained in the XPS analyses were corrected for specimen charging, by referencing the C 1s peak to 285 eV. 2.3. Preparation of the Ni(OH)2 NPs/Ni foam electrode A commercial Ni foam substrate was cleaned first by the following procedure. The Ni foam was soaked and sonicated first in isopropyl alcohol, and then in DIW. The sonicated period for each step was 10 min. After finishing these two cleaning steps, the Ni foam was dried in an oven at 90 ◦ C for 30 min. Thereafter, the Ni foam was served as the working electrode in a three-electrode cell, with 1.0 M NaOH solution as the electrolyte. The CV treatment was then performed on the Ni foam for 100 cycles in the potential windows from −0.2 to 0.6 V, at a scan rate of 25 mV/s. After 100 cycles of CV scan, the CV curve became stable, and the Ni(OH)2 NPs/Ni foam electrode was obtained. 2.4. Amperometric detection of glucose For the detection of glucose by amperometry at a constant potential by using the Ni(OH)2 NPs/Ni foam electrode, a suitable sensing potential in the limiting current plateau region between 0 and 0.6 V was determined by applying LSV at a scan rate of 0.1 mV/s in 1.0 M bare NaOH solution and in the same solution containing 3.0 mM glucose. After determining the suitable sensing potential, amperometric detection of glucose was carried out under constant magnetic stirring. The current densities in the concentration range between 0.05 and 7.0 mM were collected and the calibration curve for glucose was constructed. For the durability test, the Ni(OH)2 NPs/Ni foam electrode after use was washed by DIW and stored in air. The amperometric current of the Ni(OH)2 NPs/Ni foam electrode was measured in 1.0 M NaOH solution containing 0.5 mM glucose after 7, 14, 21, and 28 days of storage.

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3. Results and discussion

increasing cycle of scan, the second anodic peak increases apparently and the first anodic peak starts to decrease; this observation can be explained by the increasing coverage of Ni(OH)2 on the electrode surface during the CV scan. After 100 cycles of CV scan, the first anodic peak disappears and the second anodic peak becomes stable, implying that the full surface of the Ni foam has been converted to Ni(OH)2 . Moreover, it should be noted that there are three cathodic peaks in all the CV curves; all the three cathodic peaks increase with the increasing cycle of CV scan and become stable after 100 cycles of scan. This observation can be explained by the various routes of phase transition from NiOOH to Ni(OH)2 , which have been reported in previous literature [30]. At a higher applied potential, ␤-NiOOH can be partially transformed to ␥NiOOH, which intercalates some water molecules and sodium ions in the layered structure. Thus, during the reversed scan, there are three possible routes for the reduction of Ni3+ . Namely, the transformation from ␤-NiOOH to ␤-Ni(OH)2 , the transformation from ␥-NiOOH to ␤-Ni(OH)2 , and the transformation from ␥-NiOOH to ␣-Ni(OH)2 . The three cathodic peaks in Fig. 1 may be attributed to these three kinds of phase transformation. After 100 cycles of the CV treatment, a stable CV curve with one anodic peak and three cathodic peaks can be obtained.

3.1. CV treatment for the preparation of the Ni(OH)2 NPs/Ni foam electrode

3.2. Material characterization of the Ni(OH)2 NPs/Ni foam electrode

The 100-cycle CV curves of a Ni foam recorded during the CV treatment for the preparation of the Ni(OH)2 NPs/Ni foam electrode are shown in Fig. 1. The treatment was performed in 1.0 M NaOH solution, at a scan rate of 25 mV/s. It can be seen that only one anodic peak emerges at about 0.38 V (vs. Ag/AgCl/KCl sat’d) and increases dramatically with the increasing cycle number during the first few cycles of scan. This anodic peak is attributed to the irreversible reaction of Ni to form Ni(OH)2 on the electrode surface. After several cycles of scan, the anodic peak at 0.38 V stops increasing, and the second anodic peak emerges at about 0.42 V (vs. Ag/AgCl/KCl sat’d). The anodic peak at 0.42 V corresponds to the reversible reaction from Ni(OH)2 to NiOOH. With further

Fig. 2(a)–(c) shows the SEM images of the bare Ni foam at three different magnitudes. The three-dimensional network structure of the Ni foam, with the pore size around 200 ␮m, is shown in Fig. 2(a). The medium-magnitude and high-magnitude SEM images of the frame of the Ni foam are shown in Fig. 2(b) and (c), respectively. It can be seen that the surface of the bare Ni foam is smooth even in nanoscale. The SEM images of the Ni foam after 100 cycles of the CV treatment in 1.0 M NaOH (Ni(OH)2 NPs/Ni foam electrode) are shown in Fig. 2(d)–(f). The surface morphologies of the Ni(OH)2 NPs/Ni foam electrode observed in the low-magnitude and medium-magnitude SEM images are nearly the same as those of the bare Ni foam, as shown in Fig. 2(d) and (e). However, from the

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Fig. 2. SEM images of the bare Ni foam at (a) low, (b) medium, and (c) high magnitude; SEM images of the Ni(OH)2 NPs/Ni foam electrode at (d) low, (e) medium, and (f) high magnitude.

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high-magnitude SEM image shown in Fig. 2(f), it can be observed that the surface of the Ni foam is covered with a layer of tiny Ni(OH)2 nanoparticles. The Ni(OH)2 nanoparticles, with particle size of 10–15 nm, arrange closely with each other and cover on the full surface of the Ni foam uniformly. XPS spectra measured at various depths were used to verify the composition of the nanoparticles and estimate the thickness of the nanoparticles’ layer on the surface of the Ni(OH)2 NPs/Ni foam electrode. The XPS spectra of the bare Ni foam and the Ni(OH)2 NPs/Ni foam electrode in the region of Ni 2p, measured after various periods of Ar+ etching, are shown in Fig. 3(a) and (b), respectively. The etching rate was calibrated to be 0.23 nm/s. Therefore, after 20 s of Ar+ etching, about 4.6 nm-thick materials could be removed from the surface of the electrode. Thus, the composition of the electrode at various depths can be determined from the XPS spectra measured after various periods of Ar+ etching. From the spectrum (i) in Fig. 3(a), it can be observed that there are five peaks in the XPS spectrum of the bare Ni foam before etching, located at 853.0 eV, 856.3 eV, 859.3 eV, 862.2 eV, and 870.5 eV. The XPS peaks at 853.0 eV, 859.3 eV, and 870.5 eV agree with the Ni 2p peaks of

Ni metal, as indicated by dashed lines in Fig. 3(a); the peaks at 856.3 eV, and 862.2 eV correspond to the Ni 2p peaks of Ni(OH)2 , as indicated by dotted lines in Fig. 3(a). All these peaks agree with the Ni 2p peaks of Ni metal and Ni(OH)2 observed in previous literature [31]. This observation indicates that the surface of the bare Ni foam substrate is composed of the mixture of Ni metal and Ni(OH)2 ; it can be explained by the partial oxidation of Ni occurred on the surface of the Ni foam in atmosphere. The XPS spectra of the bare Ni foam after 20, 40, and 60 s of etching are also shown in Fig. 3(a). Only the peaks of Ni metal can be observed in these three spectra. Fig. 3(b) shows the XPS spectra of the Ni(OH)2 NPs/Ni foam electrode, obtained after 100 cycles of the CV treatment. It can be seen that only the Ni 2p peaks of Ni(OH)2 can be observed in the spectrum measured before Ar+ etching (spectrum (i)). This observation confirms that the nanoparticles covered on the electrode surface, observed in Fig. 2(f), are composed of pure Ni(OH)2 . After Ar+ etching of 20 s and 40 s, all the Ni 2p peaks of Ni metal and Ni(OH)2 can be observed in the XPS spectra, as shown in spectra (ii) and (iii) in Fig. 3(b). The XPS peaks of Ni(OH)2 disappeared when the etching period increased to 60 s; only the peaks of Ni metal remain in spectrum (iv), indicating that the layer of Ni(OH)2 nanoparticles can be completely removed after 60 s of Ar+ etching. The result implies that the thickness of the Ni(OH)2 layer is between the etching depth for 40 s (about 9.2 nm) and that for 60 s (about 13.8 nm). This thickness range is comparable with the particle size of the Ni(OH)2 nanoparticles on the electrode surface observed in Fig. 2(f). This result confirms that there is only a single layer of Ni(OH)2 nanoparticles covered on the surface of the Ni foam after 100 cycles of the CV treatment. 3.3. Electrooxidation behavior of glucose at the Ni(OH)2 NPs/Ni foam electrode Fig. 4 shows CV curves of the Ni(OH)2 NPs/Ni foam electrode measured in 1.0 M NaOH solution containing various concentrations of glucose, i.e., 0, 1.0, 2.0, and 3.0 mM, at a scan rate of 10 mV/s. The geometric area of the Ni(OH)2 NPs/Ni foam electrode for all the following experiments is 0.5 cm2 . It can be seen that there are one anodic peak and three cathodic peaks in the background CV curve measured before glucose addition; this CV characterization is consistent with that of the last cycle CV curve shown in Fig. 1. The anodic peak corresponds to the oxidation from ␤-Ni(OH)2 to ␤NiOOH, and the three cathodic peaks correspond to the reduction

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from ␤-NiOOH to ␤-Ni(OH)2 , the reduction from ␥-NiOOH to ␤Ni(OH)2 , and the reduction from ␥-NiOOH to ␣-Ni(OH)2 ; this redox mechanism has been discussed previously in section 3.1. After the addition of glucose, a remarkable increase in the anodic peak current density (Jpa ) can be observed in the corresponding CV curve, with reference to the anodic peak current density of the background CV curve obtained before glucose addition, as shown in Fig. 4. The catalytic mechanism of the Ni(OH)2 NPs toward glucose is shown as follow [7,16], Ni(OH)2 + OH− ↔ NiOOH + H2 O + e−

(1)

NiOOH + glucose → Ni(OH)2 + glucolactone

(2)

The dramatic increase in Jpa indicates that the Ni(OH)2 NPs/Ni foam electrode shows an excellent electrocatalytic ability toward glucose; this high catalytic activity may be attributed to both the excellent catalytic property of Ni(OH)2 NPs and the high surface area provided by the three-dimensional network structure of the Ni foam. The apparent and linear increase in Jpa at the applied potential of about 0.43 V (vs. Ag/AgCl/KCl sat’d) indicates that the Ni(OH)2 NPs/Ni foam electrode can be used for the glucose sensor with high sensitivity and excellent linearity. 3.4. Effect of NaOH concentration on glucose detection The concentration of NaOH in the electrolyte used for glucose detection significantly affects the sensing performance of the Ni(OH)2 NPs/Ni foam electrode because the OH− ion takes part in the oxidation of Ni(OH)2 . Thus, cyclic voltammetric experiments were used to discuss the effect of NaOH concentration on glucose detection and find the optimal NaOH concentration. Figure S1(a) in the supplementary data shows the background CV curves of the Ni(OH)2 NPs/Ni foam electrode measured in various concentrations of NaOH, ranging from 0.2 to 5.0 M, at a scan rate of 10 mV/s. Figure S1(b) in the supplementary data shows the CV curves of the Ni(OH)2 NPs/Ni foam electrode measured in the above NaOH solutions after 2.0 mM of glucose addition. The net change of the anodic peak current density before and after the glucose addition (Jpa ), as a function of the concentration of NaOH, is shown in Fig. S1(c). It can be observed that Jpa reached the maximum value when 1.0 M NaOH solution was used. If the concentration of OH− is too low, the reaction of Eq. (1) will shift to the left; it will limit the rate of the catalytic reaction (Eq. (2)) and thus lower the electrocatalytic ability of Ni(OH)2 toward glucose. On the other hand, if the concentration of OH− is too high, the catalytic reaction (Eq. (2)) will become the rate limiting step. Although the Jpa value of the background CV curve increases with the increasing concentration of OH− , the Jpa value of the CV curve measured after the glucose addition tends to approach a constant value with the increasing concentration of NaOH, as shown in Fig. S1(b); this reason causes the decrease in Jpa when the concentration of NaOH is higher than 1.0 M. The concentration of NaOH was optimized to be 1.0 M, and 1.0 M NaOH solution was used for all the following experiments. 3.5. Electrochemical process for glucose detection The CV curves of the Ni(OH)2 NPs/Ni foam electrode were measured in 1.0 M NaOH solution containing 2.0 mM glucose at various scan rates (), i.e., from 1 to 150 mV/s, and the result is shown in Fig. S2(a) in the supplementary data. Figure S2(b) shows the corresponding linear relationship between Jpa and 1/2 ; the square of correlation coefficient in Fig. S2(b) reaches 0.999, indicating that the rate of the electrochemical reaction is rather fast and the electrode process is controlled by the diffusion of the analyte from the solution to the electrode surface [32]. Moreover, the relationship

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between the scan rate-normalized current density (Jpa /0.5 ) and scan rate is shown in Fig. S2(c) in the supplementary data. The curve in Fig. S2(c) exhibits a typical shape of that for an ECcat process [33]; this observation further confirms the electrocatalytic mechanism shown in Eqs. (1) and (2). 3.6. Amperometric detection of glucose Linear sweep voltammetry (LSV) of the Ni(OH)2 NPs/Ni foam electrode was measured at a markedly slow scan rate, i.e., 0.1 mV/s, to find a suitable steady-state sensing potential for the amperometric detection of glucose. Figure S3 in the supplementary data shows the LSV curves of the Ni(OH)2 NPs/Ni foam electrode measured in 1.0 M bare NaOH solution and the same solution containing 3.0 mM glucose. From the net response of the two LSV curves, the suitable sensing potential was determined to be 0.45 V (vs. Ag/AgCl/KCl sat’d). Fig. 5(a) shows an amperometric response, i.e., a current density (J)–time plot, of the Ni(OH)2 NPs/Ni foam electrode for successive droppings of the glucose solution of various concentrations into 1.0 M NaOH solution, at an applied potential of 0.45 V vs. Ag/AgCl/KCl sat’d. The addition of glucose was incremented initially at the level of 50 ␮M till the total glucose concentration in the NaOH solution reached 0.2 mM. Thereafter, the glucose increment was made at the level of 0.2 mM till the total glucose concentration reached 1.0 mM. Finally, the level of 0.5 mM was used for the glucose increment till the last addition. The starting points of the additions of 50 ␮M, 0.2 mM, and 0.5 mM of glucose are indicated in Fig. 5(a). Fig. 5(b) shows the calibration curve for glucose, which was constructed by measuring the differences in current density with each addition of glucose at a specific concentration. Error bars were constructed based on the amperometric curves measured from three separated experiments. In Fig. 5(b), it can be observed that the anodic current density increases linearly with the increasing concentration of glucose from 0.6 to 6.0 mM (correlation coefficient, R2 = 0.9992). Besides, from the slope of the calibration curve in the linear region, the sensitivity of the Ni(OH)2 NPs/Ni foam electrode can be determined to be 1950.3 ␮A/mM-cm2 . The limit of detection (LOD) for the Ni(OH)2 NPs/Ni foam electrode was estimated as followed. A small region in the amperometric curve (Fig. 5(a)) before the first addition of glucose was enlarged. Thereafter, the average noise in current density was estimated from this enlarged curve. The signal for the LOD was calculated from this value of noise, based on signal-tonoise ratio of 3. Then, the LOD could be calculated to be 0.16 ␮M according to this signal value and the sensitivity obtained in Fig. 5(b). Compared with literature, the sensitivity toward glucose achieved by using the Ni(OH)2 NPs/Ni foam electrode is about three times higher than that achieved by using the Ni(OH)2 /Ni foil electrode obtained also by electrochemical treatment (670 ␮A/mMcm2 ) [22]. The remarkable increase in sensitivity may be attributed to the high surface area provided by the three-dimensional network of the Ni foam. In previous literatures, several kinds of high surface area materials, including graphene [34–36] and various kinds of carbon nanotubes [37–42], were used to fabricate the composite electrodes with Ni(OH)2 /NiOOH redox material for the use of non-enzymatic glucose detection. Compared to these composite electrodes, which used carbon materials to increase their surface area, the sensitivity toward glucose achieved with the help of the high surface area Ni foam in this study is higher or comparable compared to all the sensitivity values reported in the above literatures. A partial list of the reported non-enzymatic glucose sensors based on Ni(OH)2 is shown in Table 1; it can be seen that the analytical performance of the sensor proposed in this study is better than most previous studies in terms of sensitivity, LOD, and linear range.

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Time (s) Fig. 5. (a) Amperometric response of the Ni(OH)2 NPs/Ni foam electrode on successive droppings of the glucose solution of different concentrations into 1.0 M NaOH solution, (b) calibration curve for current density versus concentration of glucose, (c) amperometric response of the Ni(OH)2 NPs/Ni foam electrode on successive droppings of 0.5 mM glucose, 0.002 mM UA, 0.02 mM AP, and 0.01 mM AA into 1.0 M NaOH solution.

The selectivity of the Ni(OH)2 NPs/Ni foam glucose sensor was evaluated against uric acid (UA), acetaminophen (AP), and ascorbic acid (AA), which commonly appear in human blood as interferents for glucose detection. The normal physiological levels of glucose, UA, AP, and AA in human serum are about 5, 0.02, 0.2, and 0.1 mM, respectively [22,45,46]. Thus, in order to demonstrate the selectivity of the glucose sensor in human blood, the same concentration ratio for glucose, UA, AP, and AA (0.5, 0.002, 0.02, and 0.01 mM) was used for the interference test. Fig. 5(c) shows the amperometric response of the Ni(OH)2 NPs/Ni foam electrode for 0.5 mM glucose,

3.8. Long-term durability and reproducibility Fig. 6 shows the amperometric current density of the Ni(OH)2 NPs/Ni foam electrode measured in 1.0 M NaOH solution containing 0.5 mM glucose, after various periods of storage in air ranging from 0 to 28 days; these current densities were estimated from the amperometric curves shown in Fig. S5. From Fig. 6, it can be observed that the current density remains 92.5% after 28 days of storage. Besides, it can be seen that the current density becomes stable after 28 days of storage. These observations indicate that the Ni(OH)2 NPs/Ni foam electrode can provide an excellent long-term durability when it is stored in atmosphere,

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J/J0 (%)

3.7. Interference test and detection of glucose in serum samples

0.002 mM UA, 0.02 mM AP, and 0.01 mM AA in 1.0 M NaOH solution, at an applied potential of 0.45 V. It can be seen that there is nearly no current response for all the three interferents, compared to the response for 0.5 mM glucose. After the additions of the three interferents, 0.5 mM glucose was again added into the solution. As shown in Fig. 5(c), the current response still reached the similar value for the final addition of glucose, despite the previous additions of interferents; this observation indicates that the Ni(OH)2 NPs/Ni foam sensor has potential to become a reliable glucose sensor. The selectivity of the Ni(OH)2 NPs/Ni foam glucose sensor was also evaluated against methanol, ethanol, galactose, and mannose (Fig. S4). It can be seen that no signals can be observed after the additions of methanol and ethanol, but the catalytic currents for galactose and mannose appear in Fig. S4. Since the concentrations of galactose and mannose are neglectable compared to the concentration of glucose in human blood [47,48], the Ni(OH)2 NPs/Ni foam sensor should be promising for the detection of glucose in human serum. To verify the reliability of the Ni(OH)2 NPs/Ni foam sensor, glucose concentrations in three serum samples drawn from three different persons (CG ) were analyzed by a local hospital (National Taiwan University Hospital) and also analyzed by our sensor. 50 ␮L of 10 mM glucose was added into 5 mL of 1.0 M NaOH as a standard at the applied potential of 0.45 V vs. Ag/AgCl/KCl (sat’d), and then 50 ␮L of the serum sample was injected into the solution in order to measure CG in the serum sample. The detection of each serum sample was conducted three times, and the CG values obtained from these experiments are compared with those measured by the hospital (Table 2). The relative standard deviations (R.S.D.) are less than 8% for all serum samples, and the biases for all samples are less than 0.3 mM. This result indicates that the Ni(OH)2 NPs/Ni foam sensor can be applied for non-enzymatic detection of glucose in human blood serum.

2

14 (a)

J (µA/cm )

2

Current density (mA/cm )

164

20

200 0

0

7

14

21

28

0

Time (days) Fig. 6. Plot of the amperometric current density of the Ni(OH)2 NPs/Ni foam electrode measured in 1.0 M NaOH solution containing 0.5 mM of glucose, over a period of 28 days.

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165

Table 1 Partial list of the reported non-enzymatic glucose sensors based on Ni(OH)2 . Type of the electrode

Performance

Ni(OH)2 /CPEa Ni(OH)2 nanospheres/PI/CNTb Ni(OH)2 /ERGO-CNT/GCEc Ni(OH)2 NPs/Ni foam Ni(OH)2 NPs/CNT/GCE Ni nanowire arrays/GCE Ni(OH)2 /carbon nanofibers/GCE Ni(OH)2 nanowires/CNT Ni(OH)2 /Ni foil Ni(OH)2 nanoboxes Ni(OH)2 /graphene/GCE Ni(OH)2 /CILEd Ni/TiO2 nanotube arrays Ni/CNT/GCE Ni(OH)2 /graphene/GCE a b c d

Reference

Sensitivity (␮A/mM-cm2 )

LOD (␮M)

Linear range (up to, mM)

5590.0 2071.5 2042.0 1950.3 1438.0 1043.0 1038.6 857.0 670.0 487.3 328.0 202.0 200.0 67.2 11.4

0.16 0.36 2.7 0.16 0.5 0.1 0.76 5.0 1.8 0.07 0.6 6.0 4.0 0.89 0.6

0.11 0.8 1.5 6.0 1.0 7.0 1.2 0.5 10.0 5.0 1.0 23 1.7 17.5 3.1

[7] [42] [36] This study [37] [19] [43] [41] [22] [44] [34] [16] [20] [40] [18]

Carbon paste electrode. PI = polyimide; CNT = carbon nanotubes. ERGO = electroreduced graphene oxide; GCE = glass carbon electrode. Carbon ionic liquid electrode.

Table 2 Comparison between the glucose concentrations in serum samples measured in the hospital and those measured by using our Ni(OH)2 NPs/Ni foam sensor. Sample number

CG a determined in the hospital (mM)

CG a determined by our sensor (mM)

%R.S.D.b

Bias (mM)

1 2 3

4.50 4.33 5.17

4.56 4.29 5.44

7.1 5.8 7.5

0.06 −0.04 0.27

a b

Glucose concentration in serum samples. % R.S.D. calculated from three separated experiments using our sensor.

which is one of the important requirements for the use in glucose sensor chip. The reproducibility of the sensor was tested by measuring the sensitivities of four Ni(OH)2 NPs/Ni foam electrodes toward glucose from their amperometric responses. The relative standard deviation of sensitivity is only 4.06% for the four electrodes, demonstrating excellent electrode reproducibility. Both the excellent performances obtained in the long-term durability and reproducibility tests imply the applicability of the Ni(OH)2 NPs/Ni foam electrode as a reliable non-enzymatic glucose sensor. 4. Conclusions A Ni(OH)2 NPs/Ni foam electrode was prepared by applying 100 cycles of electrochemical CV treatment to a bare Ni foam in 1.0 M NaOH solution. The CV curve of the electrode became stable after the CV treatment, with one anodic peak and three cathodic peaks. Verified by SEM images and XPS spectra, a single layer of Ni(OH)2 NPs, with particle size of 10–15 nm, could cover on the full surface of the Ni foam after the CV treatment. The Ni(OH)2 NPs/Ni foam electrode offered an excellent electrocatalytic ability toward glucose. The concentration of NaOH was optimized to be 1.0 M for obtaining the best sensitivity toward glucose. The electrocatalytic process toward glucose on the Ni(OH)2 NPs/Ni foam electrode was verified to be ECcat process and controlled by the diffusion of glucose. The Ni(OH)2 NPs/Ni foam electrode was used for the amperometric detection of glucose; the suitable sensing potential was determined to be 0.45 V. In the amperometric detection of glucose, the linear range, sensitivity, and limit of detection were 0.6–6.0 mM (correlation coefficient, R2 = 0.9992), 1950.3 ␮A/mM-cm2 , and 0.16 ␮M, respectively. Interference from uric acid (UA), acetaminophen (AP), and ascorbic acid (AA) was negligible compared to the current signal from glucose. The electrocatalytic current of the Ni(OH)2 NPs/Ni foam electrode toward glucose still remained 92.5% and became

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Biographies Mr. Chung-Wei Kung received the BS degree in Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, in 2011. He is now pursuing his PhD degree in Department of Chemical Engineering, National Taiwan University. He worked as a visiting scholar in Department of Chemistry, Northwestern University from March 2013 to March 2014. His research focuses on the synthesis of nanostructural thin films of various materials including metal oxides, metal sulfides, conducting polymers and metal-organic frameworks, and their electrochemical applications, such as electrochemical sensors, dye-sensitized solar cells, supercapacitors, and electrochromic thin films. Mr. Yu-Heng Cheng currently is a senior BS student in Department of Chemical Engineering from National Taiwan University, studying under the guidance of Prof. Kuo-Chuan Ho. His research interests focus on synthesizing the novel structures of transition metallic compounds, novel structures of conducting polymers and their further application as supercapacitors and electrochemical sensors. Dr. Kuo-Chuan Ho received BS and MS degrees from National Cheng Kung University, Taiwan, in 1978 and 1980, respectively. In 1986, he received the PhD degree in Chemical Engineering at the University of Rochester. Thereafter, he joined PPG Industries, Inc., from 1986 to 1993. He joined National Cheng Kung University in 1993 as an Associate Professor, and then moved to National Taiwan University in 1994. Currently, he is a Distinguished Professor in Department of Chemical Engineering and Institute of Polymer Science and Engineering at National Taiwan University. His researches mainly focus on electrochemical sensors, electrochromic devices, and dye-sensitized solar cells.