Article pubs.acs.org/ac
Highly Sensitive and Selective Nonenzymatic Detection of Glucose Using Three-Dimensional Porous Nickel Nanostructures Xiangheng Niu,† Minbo Lan,*,†,‡ Hongli Zhao,†,§ and Chen Chen† †
Shanghai Key Laboratory of Functional Materials Chemistry, and Research Centre of Analysis and Test, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ‡ Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science and Technology, Shanghai 200237, People’s Republic of China § Institute of Applied Chemistry, East China University of Science and Technology, Shanghai 200237, People’s Republic of China S Supporting Information *
ABSTRACT: Highly sensitive and selective nonenzymatic detection of glucose has been achieved using a novel disposable electrochemical sensor based on three-dimensional (3D) porous nickel nanostructures. The enzyme-free sensor was fabricated through in situ growing porous nickel networks on a homemade screen-printed carbon electrode substrate via electrochemically reducing the Ni2+ precursor, along with continuously liberating hydrogen bubbles. The resulting nickelmodified electrode was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), powder X-ray diffractometry (XRD), and electrochemical techniques. Cyclic voltammetric, alternating-current impedance, and amperometric methods were used to investigate the catalytic properties of the assembled sensor for glucose electro-oxidation in alkaline media. Under optimized conditions, the enzymeless sensor exhibited excellent performance for glucose analysis selectively, offering a much wider linear range (from 0.5 μM to 4 mM), an extremely low detection limit (0.07 μM, signal-to-noise ratio (S/ N) of 3), and an ultrahigh sensitivity of 2.9 mA/(cm2 mM). Importantly, favorable reproducibility and long-term performance stability were obtained thanks to the robust frameworks. Application of the proposed sensor in monitoring blood glucose was also demonstrated.
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and the design of redox systems, sufficient sensitivity for clinical practice in diabetes management has been obtained.1a Favorable selectivity can be also guaranteed due to the highly catalytic specifity of bioenzymes. Nevertheless, the insufficient long-term stability, which originates from the nature of enzymes, turns out to be the most common and serious problem of enzymatic glucose sensors.7 Sometimes the conventional GOx-based glucose sensors may be exposed to the thermal and chemical deformations during fabrication, storage, and use. As reviewed by Wilson and Turner,7a the highly acid (below pH 2) or strongly basic (above pH 8) environment and the relatively high temperature (above 40 °C) can lead to a fatal loss of GOx activity. Therefore, the enzymebased glucose sensors might not be suitable for applications in some sophisticated and uncertain conditions, such as for ecological and food monitoring and for the control of bioprocesses. Besides, enzyme-based biosensors inevitably require enzyme immobilization procedures onto solid electro-
n the past decades, glucose has always been the most popular object in the analytical chemistry field.1 This hot trend, driven by not only the well-known rising demands for advanced blood sugar detection devices for clinical diagnosis and personal care but also the urgent requirements for ecological and food monitoring, pharmaceutical analysis, and control of bioprocess, has been attracting tremendous academic and commercial efforts to develop glucose sensors with high sensitivity, excellent selectivity, good reliability, fast response, and low cost. Although there are a few potential glucose sensing modalities including optical,2 acoustic,3 and transdemal technologies4 being explored, electrochemical sensors are still recognized universally as the most convenient and effective tool for glucose analysis to date, because they can exhibit many attractive features such as pre-eminent sensitivity, time efficiency, simple instrumentation, easy operation, and low production cost.5 As a category of electrochemical glucose sensors, enzyme electrodes based on the specific biocatalysis of enzymes toward zymolytes have drawn considerable attention since the pioneering work made by Clark and Lyons.6 Through improvements on the immobilization technique of enzymes © XXXX American Chemical Society
Received: October 23, 2012 Accepted: March 4, 2013
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for glucose oxidation were systematically assessed. It was found that the enzymeless sensor could provide ultrasensitive responses for glucose with favorable selectivity and excellent reliability.
des. As a consequence, all kinds of enzyme electrodes are not totally free from the intrinsic uncertainty of biological components, regardless of the immobilization process and the sort of enzymes immobilized.7b Nowadays increasing interest is being paid on the development of nonenzymatic glucose detection devices. Nonenzymatic glucose sensors work based on the direct electrocatalytic oxidation of the analyte on an electrode surface, successfully avoiding the shortcomings that originate from the nature of bioenzymes. The electrode material is considered to be a crucial factor affecting the analytical properties of enzymefree sensors, and noble metals, including gold,8 palladium,9 silver,10 and especially platinum,11 are used the most to fabricate enzymeless glucose sensors. Although the sluggish kinetics of the glucose electro-oxidation reaction has been greatly facilitated by the introduction of some strategies, such as preparing various nanostructures,12 increasing active surfaces,11a and alloying,7c,13 the sensitivity collected on these precious metal electrodes, with only a magnitude of μA/(cm2 mM), is still far from commercialization. In addition, the catalytic performance of Pt-based materials is seriously impaired by adsorbed Cl− ions and chemisorbed intermediates that originate from the glucose oxidation process,7c resulting in poor operational stability. Interferential effects from endogenous species including ascorbic acid, dopamine, and uric acid cannot be totally eliminated. More importantly, concerns from the expensive cost, scarcity, and unbalanced distribution of precious metal resources drastically limit their commercial applications in a large scale. As a result, a major consideration in practical nonenzymatic glucose sensing is focused on fabricating high-performance devices using inexpensive and resourceful transition-metal catalysts.14 As the most attractive candidate, nickel materials have been intensively studied for the glucose electro-oxidation reaction, because of their impressed catalytic activity, which results from the redox couple of Ni3+/Ni2+ in alkaline media. Most Ni-based nonenzymatic glucose sensors are prepared by modifying substrates with nanoparticles,14b,15 scattered structures,14d,16 or nickel/carbon hybrids.17 An apparent pitfall of these fabricated sensors is the weak mechanical stability and durability of performance, because they easily suffer from the agglomeration, deformation, and collapse of Ni-based nanostructures for extended periods of time under applied potential. Furthermore, the desired maximization of sensitivity, as well as excellent selectivity, still remains a great challenge. Here, we report the ultrahighly sensitive and selective detection of glucose using a novel nonenzymatic electrochemical sensor based on three-dimensional (3D) porous nickel nanomaterials. The porous structures offer a large electrochemical surface area, not only fasting transport of electrolytes through the solid/liquid interface due to shortened diffusion length and beneficial diffusional regime,18 but also allowing them to come into contact with more active surfaces through abundant pores and channels. Crucially, the 3D selfsupported networks with interlaced bridging frameworks benefit the negligible change of Ni structures after numerous reduplicative measurements, effectively guaranteeing the favorable repeatability and long-term stability of performance. The enzyme-free sensor was fabricated by directly assembling porous Ni nanostructures on homemade screen-printed carbon electrode substrates via electrochemically reducing precursors along with continuously liberating hydrogen bubbles. The electrocatalytic properties of the fabricated porous Ni electrode
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EXPERIMENTAL SECTION Materials and Reagents. Screen-printed carbon electrodes (SPCEs) and screen-printed gold film electrodes (SPGFEs) as substrates for porous Ni electrodeposition were predominantly homemade, according to our previous reports,19 and the configurations of the prepared SPCEs and SPGFEs, with the working region being 3 mm in diameter, were presented in Figure S-1 in the Supporting Information. Indium tin oxide (ITO) conductive glasses, provided by Shenzhen Laibao HiTechnology Co., were also employed as substrates for Ni preparation. A bulk nickel foil purchased from Alfa Aesar was used for comparison. NiCl2, H2SO4, and NaOH (Sinopharm Chemical Reagent Co.) were directly utilized without further purification. Glucose, ascorbic acid (AA), dopamine (DA), uric acid (UA), acetamidophenol (AP), fructose, galactose, and lactose were purchased from Sigma−Aldrich. Glucose stock solutions were stored in 4 °C overnight for sufficient mutarotation before use. All other reagents used in this research were at least of analytical grade. Ultrapure water (18.2 MΩ cm, Laboratory Water Purification Systems) was utilized to prepare all solutions. Synthesis of Porous Ni Nanostructures. Three-dimensional (3D) porous Ni nanostructures were synthesized in situ on substrates, using a hydrogen-evolution-assisted electrodeposition strategy. All electrodeposition procedures were accomplished on a CHI440A electrochemical workstation (CH Instruments, Inc.) equipped with a three-electrode system consisting of a SPCE (or SPGFE, or ITO glass) working electrode, a Pt wire counter electrode, and a 3 M KCl saturated Ag/AgCl reference electrode. In a typical synthesis, a newly prepared SPCE, which served as the working electrode, was first anodized in 0.5 M H2SO4 for 20 cycles in the potential range from +1.5 V to +2.0 V using the cyclic voltammetric technique. After being rinsed with ultrapure water and dried at room temperature, the pretreated SPCE was immersed in a 0.2 M NiCl2 solution containing 1 M H2SO4 for 15 min, in order to allow the precursor solution to contact the substrate surface effectively. An electrodeposition process with a constant current of 0.1 A (corresponding to a current density of ∼1.4 A/cm2, if taking 0.071 cm2 as the electrodeposition area) then was carried out in the stationary electrolyte solution for 30 s; during this process, hydrogen evolution occurred continuously along with Ni2+ electroreduction; finally, the resulting electrode was carefully rinsed with adequate ultrapure water, and dried in air for further characterization and electrochemical measurements. Characterization. The surface morphology of nickel electrodeposited on different substrates was directly observed on a field-emission scanning electron microscopy (FESEM) system (Model S-4800, Hitachi High Technologies Co.). Transmission electron microscopy (TEM) images were captured on a TEM system (Model TEM-1400, JEOL), with nickel materials exfoliated from the SPCE substrate. The elemental composition was determined using an energydispersive X-ray spectrometer (EDS) (Model QUANTAX400-30, German Bruker Co.) that was attached to the FESEM system. Powder X-ray diffraction (XRD) patterns of B
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substrates, with no requirement for further assembling the prepared materials onto electrodes. To the best of our knowledge, this is the first fabrication of 3D porous Ni nanostructures using the extremely facile (but much more effective) approach. Figure 2A depicts FESEM images of the resulting nickel on SPCE substrates. Panoramic observations find that the substrate surface is completely covered by porous nanostructures. As shown in the FESEM proof, pores ranging from hundreds of nanometers to tens of nanometers in diameter are obtained. These pores connect each other with abundant interlaced bridging branches, and further form self-supported networks. Cross-section images demonstrate that the porous frameworks stretch from the bottom boundary to the top interface, and the thickness of the 3D Ni nanostructures is estimated to be 1.5 ± 0.1 μm. The synthesized nickel is supposed to be beneficial for the highly sensitive and favorably stable detection of analytes, because the porous nanostructures provide many large active surfaces, and the interlaced bridging skeletons maintain the robust mechanical stability. The homemade SPCE was employed as the substrate, because of its good electroconductibility, low cost, and negligible catalytic activity by itself. We also grew the porous Ni nanostructures on homemade SPGFE (A) and ITO glass (B) substrates successfully, as shown in Figure S-2 in the Supporting Information), indicating that the synthesis of porous Ni networks was independent of substrate materials. The TEM image (Figure 2B) also confirms the porous architecture of Ni synthesized by hydrogen-evolution-assisted electrodeposition. The EDS pattern presented in Figure 2C provides the typical signal of Ni. Quantitative analysis taking the average value of reading at three different spots on the sample surface reveals that the weight percent and atomic percent of nickel are 76.5% and 42.8%, respectively. The constitution of the porous Ni was further characterized by XRD. As shown in Figure 2D, three characteristic diffraction peaks positioned at 44.4°, 51.7°, and 76.6° should be attributed to the (111), (200), and (220) crystalline planes of the face-centered cubic (Fcc) Ni (JCPDS File Card No. 04-0805), respectively. Besides, a remarkable peak at 37.3° and a weak peak at 43.1° ascribed to the cubic phase (Fm3m) of NiO (JCPDS File Card No. 47-1049) are also observed, revealing that the synthesized Ni stored in air has been partially oxided. Electrochemical Behavior of the Porous Ni Electrode. Figure S-3 in the Supporting Information represents the first five cyclic voltammograms of the bulk Ni (Figure S-3(A)) and porous Ni (Figure S-3(B)) in 0.1 M H2SO4 at a scan rate of 20 mV/s. Similar to the bulk Ni foil, the fabricated porous Ni provides two anodic peaks in the sweep potential scope from −0.5 V to +0.8 V. The remarkable well-defined signal at +0.08 V results from the initial oxidation of Ni to Ni2+, and the peak at +0.18 V, which is slightly overlapped with the first peak, corresponds to the further oxidation of a small number of Ni2+ adsorbed on the electrode surface to Ni3+. However, in the cathodic round, no prominent response appears in the acidic environment. We further conducted cyclic voltammetric experiments in alkaline media using the fabricated porous Ni electrode, and found that the electrochemical behavior of the porous nickel was similar to that of the bulk nickel (Figure S-4 in the Supporting Information). As shown in Figure 3, a couple of well-defined redox peaks are observed for cyclic sweeping in 0.1 M NaOH. At a scan rate of 20 mV/s, an anodic peak at +0.45 V
the resulting samples were obtained utilizing a D/max2550 diffractometer (Rigaku International Co.) with a Cu Kα source. Electrochemical Measurements. All electrochemical measurements were carried out on the aforementioned workstation with the same three-electrode configuration at room temperature (26 ± 1 °C), unless otherwise stated. All solutions were deoxygenated with highly pure argon (99.9%) for 15 min before measurements. All potentials reported here were referred to the KCl saturated Ag/AgCl electrode (+0.194 V vs standard hydrogen electrode). The geometric surface area (0.071 cm2) of the fabricated Ni electrode was used to calculate all current densities. Cyclic voltammetric measurements were performed in stationary electrolyte solutions. Unless otherwise defined, chronoamperometric experiments were implemented in 0.1 M NaOH by injecting glucose or other species while the solution was stirred constantly (150 rpm). Electrochemical impedance spectroscopy (EIS) measurements were carried out on an IM6ex workstation (German Zahner Co.) at +0.5 V, with a disturbance potential of 5 mV and a frequency range from 1 MHz to 0.1 Hz. The equivalent circuit of Nyquist plots was simulated using the ZSimpWin software.
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RESULTS AND DISCUSSION Synthesis and Characterization of Porous Ni Nanostructures. In the present research, 3D porous Ni nanostructures were grown in situ on desired substrates (i.e., SPCE), using a reproducible hydrogen-evolution-assisted electrodeposition strategy. As illustrated in Figure 1, when a
Figure 1. Schematic illustration of the formation process of threedimensional (3D) porous Ni nanostructures.
very large current density (up to A/cm2) is applied to the electrode system, much negative potential is correspondingly set up at the working electrode. In this case, Ni2+ ions in the vicinity of the electrode surface are quickly exhausted, and, instead, numerous bubbles that originate from hydrogen evolution start to become liberated, disrupting the normal diffusion of reactive ions from the bulk solution to the iondepleted region. Consequently, no precursor reaches where hydrogen bubbles occupy, and only the interval spaces of gas bubbles allow a number of Ni2+ ions to diffuse into them. As hydrogen bubbles move off the electrode surface dynamically, the electrochemical reduction of the precursor takes its way between gas bubbles simultaneously, finally resulting in the formation of porous Ni architectures. Compared to common methods, including template-directed, hydrothermal, or solvothermal, as well as dealloying preparation for porous materials, the proposed electrodeposition strategy does not demand specific templates, high temperatures, reducing agents, organic solvents, and separation and purification procedures. In addition, this synthesis process involves only the direct electroreduction of an inorganic precursor on desired C
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Figure 2. (A) FESEM images of porous Ni nanostructures electrodeposited on SPCE substrates (precursor solution = 0.2 M NiCl2 + 1 M H2SO4; deposition current = 0.1 A, corresponding to a current density of 1.4 A/cm2; electrodeposition time = 30 s. (B) TEM image of porous Ni nanostructures. (C) EDS and (D) XRD patterns of the fabricated porous nickel.
surface area that nanomaterials can provide. In the present research, we evaluated the effective surface area (A) of the prepared electrodes using the chronocoulometric technique based on the Anson equation:20 Q = Q dl + Q ads + 2nFAC(Dt /π )1/2
where Qdl is the double layer charge, which can be eliminated by background subtraction; Qads is the Faradaic charge; n is the number of transferred electrons; F is the Faraday constant; C is the concentration of electrolytes; D is the diffusion coefficient, taking a value of 7.6 × 10−6 cm2/s (25 °C) in 0.1 mM K3[Fe(CN)6] containing 1 M KCl. Figure S-6(A) in the Supporting Information shows the plots of Q−t for the bare SPCE, bulk Ni, and porous Ni/SPCE. Based on the slopes of Q−t1/2 (Figure S-6(B) in the Supporting Information), A is determined to be 0.173 cm2 for the bare SPCE; while the porous nickel-modified SPCE provides an electrochemically effective surface area as high as 0.865 cm2. For comparison, the effective surface area of the bulk Ni is only 0.186 cm2. The roughness factors (a ratio of the real surface area to the geometric area) of the bare SPCE, bulk Ni, and porous Ni fabricated on SPCE are 2.44, 2.62, and 12.18, respectively. Electrocatalytic Oxidation of Glucose. The bare SPCE exhibits negligible catalytic effects on glucose electro-oxidation in alkaline media in the potential window from −0.1 V to +0.8 V, while the porous Ni electrodeposited on SPCE substrates can significantly promote the oxidation of glucose. We initially utilized the cyclic voltammetric technique to investigate the electrochemical behavior of the porous Ni electrode in 0.1 M NaOH with the absence and presence of glucose. As shown in Figure 4A, a pair of well-defined redox peaks corresponding to the Ni(III)/Ni(II) couple appear in the absence of glucose. When glucose is added into NaOH, notable enhancement of
Figure 3. Cyclic voltammograms of the fabricated porous Ni electrode in 0.1 M NaOH at different scan rates.
and a cathodic peak at +0.36 V are obtained, with a high peak current response (Ip) of 48.1 μA and a low peak potential separation (ΔEp) of 90 mV. Previous research15d,17c has demonstrated that the couple of peaks are due to the redox reaction of Ni(III)/Ni(II) couple on the electrode surface: Ni + 2OH− → Ni(OH)2 + 2e− −
Ni(OH)2 + OH ↔ NiO(OH) + H 2O + e
(1) −
(3)
(2)
In addition, the anodic response shifts positively with the increase of scan rate, while the cathodic peak moves negatively. It is calculated that the Ip is proportional to the square root of scan rate, as depicted in Figure S-5 in the Supporting Information), indicative of a diffusion-controlled process on the porous Ni surface. It is well-known that the electrocatalytic activity of catalysts can be significantly enhanced by introducing nanoscale structures compared to bulk materials. The improved performance partially results from the large electrochemically active D
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Figure 4. (A) Cyclic voltammograms of the porous Ni electrode in 0.1 M NaOH with the absence and presence of glucose. (B) Electrochemical impedance plots of the porous Ni electrode in 0.1 M NaOH containing different concentrations of glucose. (C) Chronoamperometric plots of the porous Ni electrode for different concentrations of glucose in 0.1 M NaOH recorded at +0.5 V.
charge-transfer resistance (Rct) of the Ni(III)/Ni(II) redox couple in the presence of glucose and the double layer capacitance (Qdl), and the low-frequency semicircle is attributed to the adsorption of reaction intermediates (Qads and Rads) on the electrode surface.22 In the equivalent circuit, the double layer capacitance Qdl changes little before and after adding glucose into 0.1 M NaOH, while the charge-transfer resistance Rct of the electrocatalytic reaction, which indicates how fast charge transfer occurs during glucose electro-oxidation on the electrode surface, decreases with glucose concentration increasing, as listed in Table S-1 in the Supporting Information). The similar behaviors are also observed on the bulk Ni electrode, as shown in Figure S-7(B) and Table S-2 in the Supporting Information). In order to better understand the electrocatalytic properties of the nonenzymatic sensor for glucose oxidation, cyclic voltammetric measurements at different scan rates were carried out. Figure S-9(A) in the Supporting Information exhibits the positive-going portion of cyclic voltammograms of the porous Ni electrode at different scan rates in 0.1 M NaOH containing 2.5 mM glucose. Obviously, the anodic response ascribed to glucose electro-oxidation increases correspondingly with sweep rate. It is further found that the anodic peak current is proportional to the square root of scan rate, as shown in Figure S-9(B) in the Supporting Information. It reveals that, at sufficient potential, the electrocatalytic process is controlled by glucose diffusion to the electrode/electrolyte interface and depends on the glucose concentration in bulk solutions, which is the very case for quantitative detection. The peak potential for the catalytic oxidation of glucose shifts positively with the increase of scan rate, suggesting that there is a kinetic limitation in the reaction between the redox sites of Ni and glucose. These results are similar to those obtained on the bulk nickel, as shown in Figure S-8 in the Supporting Information. We further conducted chronoamperometric measurements in
the oxidative peak current, as well as an anodic shift of the peak potential, is observed, and the cathodic peak current decreases slightly without potential shift, indicative of an irreversible electrochemical oxidation process. It is well-established that this enhancement anodic current is attributed to the electrooxidation of glucose with the participation of Ni(III).15−17 The irreversible catalytic oxidation reaction can be expressed as follows: NiO(OH) + glucose → Ni(OH)2 + H 2O + glucolactone (4)
Note that the electrocatalytic oxidation of glucose at the fabricated Ni electrode occurs not only in the anodic but also in the initial stage of the cathodic half cycle. This phenomenon implies that the catalytic active sites are limited at high positive potential, along with the accumulation of intermediates and reaction products on the electrode surface, thus resulting in the conclusion that glucose does not have enough time to undergo full oxidation in the forward anodic half cycle.21 With the increase of glucose concentration, the enhancement of the oxidative peak becomes more evident, and the cathodic peak ascribed to Ni(III) reduction gradually becomes unrecognizable. These phenomena are also observed on the bulk Ni electrode, as shown in Figure S-7(A) in the Supporting Information. Electrochemical impedance spectroscopy (EIS) analysis was further used to discover the catalytic procedures of glucose electro-oxidation on the porous Ni electrode. Figure 4B shows the Nyquist diagrams of the porous Ni/SPCE recorded at +0.5 V in the frequency range from 1 MHz to 0.1 Hz for some selected concentrations of glucose in 0.1 M NaOH. Similar to that of the bulk nickel, the Nyquist diagrams of the porous nickel consist of two depressed overlapping capacitive semicircles in the high- and low-frequency sides. The depressed semicircle in high frequency is related to the combination of E
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Figure 5. Effects of the (A) NiCl2 precursor concentration, (B) deposition current density, and (C) electrodeposition time on the amperometric response of 0.5 mM glucose in 0.1 M NaOH under a constant-rate stirring condition (150 rpm).
stationary solutions to evaluate the catalytic rate constant (kcat). As shown in Figure 4C, when the oxidation current is dominated by the rate of the electrocatalytic reaction, the catalytic current (Icat) can be written as eq 5:
Icat = π 1/2(kcatC0t )1/2 IL
(5)
where Icat and IL are the currents in the presence and absence of glucose, respectively; kcat is the catalytic rate constant (M−1 s−1), C0 the bulk concentration (M) of glucose, and t the elapsed time (s). Based on the slope of Icat/IL vs t1/2, the kcat value is calculated to be 1.20 × 103 M−1 s−1 taking the glucose concentration C0 = 1.0 mM into account, while the kcat obtained on the bulk Ni is calculated to be 0.83 × 103 M−1 s−1, according to Figure S-7(C) in the Supporting Information. Amperometric Detection of Glucose. Prior to nonenzymatic glucose detection, experimental parameters possibly influencing the analytical performance of the fabricated nonenzymatic sensor were optimized. We studied the effects of Ni2+ precursor concentration, deposition current density, electrodeposition time, and detection potential on the oxidation current response for 0.5 mM glucose. The single-factor experiment results, as shown in Figure 5, reveal that the Ni2+ concentration, deposition current, and electrodeposition time have significant effects on the obtained current response upon glucose. The maximum current response toward glucose oxidation is obtained with a 0.8 M Ni2+ precursor solution and a 3.5 A/cm2 deposition current density (corresponding to a deposition current of 0.25 A), as shown in Figures 5A and 5B. According to the results shown in Figure 6, a detection potential of +0.5 V is regarded as the favorable choice considering the compromise between sensitivity and selectivity. Considering these electrodeposition conditions affect the porous Ni structure with each other, we investigated the effects of those factors on the Ni structure (the pore size, the
Figure 6. Effects of the electrodeposition time on the amperometric response of 0.5 mM glucose in 0.1 M NaOH under a constant-rate stirring condition (150 rpm).
pore density, and the thickness and mass of porous Ni) first using orthogonal experiments, as shown in Table S-3 in the Supporting Information. FESEM observations show that all of these electrodeposition conditions result in similar porous Ni nanostructures, as shown in Figure S-10 in the Supporting Information. Because of the random agglomeration of hydrogen bubbles, the pore size (in the range from hundreds to tens of nanometers) and density of porous Ni fabricated under various electrodeposition conditions show no apparent difference. However, the thickness and mass of porous Ni on SPCE substrates increase with the precursor concentration, deposition current, and electrodeposition time, as shown in Table S-4 in the Supporting Information. The results of orthogonal experiments indicate that the nonenzymatic sensor based on porous Ni networks prepared with 0.8 M Ni2+ precursor, a deposition current of 0.25 A, and an electrodeposition time of 30 s should be used for the following amperometric detection of glucose in alkaline media. F
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Figure 7. (A) Chronoamperometric responses of the fabricated porous Ni electrode upon successive addition of glucose into 0.1 M NaOH at +0.5 V under a constant-rate stirring condition (150 rpm). (B) Relationship between the amperometric responses and the glucose concentrations.
Under the above optimized conditions, amperometric responses upon successive addition of glucose into constantly stirred NaOH solutions were recorded. As shown in Figure 7A, well-defined amperometric currents increasing stepwise with the level of glucose are obtained, with a response time of
