CdS Bifunctional Ti@TiO2 Core–Shell Nanowire Electrode for High

Article pubs.acs.org/ac

Ni/CdS Bifunctional Ti@TiO2 Core−Shell Nanowire Electrode for HighPerformance Nonenzymatic Glucose Sensing Chunyan Guo, Huanhuan Huo, Xu Han, Cailing Xu,* and Hulin Li State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China S Supporting Information *

ABSTRACT: In this work, a Ni/CdS bifunctional Ti@TiO2 core−shell nanowire electrode with excellent electrochemical sensing property was successfully constructed through a hydrothermal and electrodeposition method. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were employed to confirm the synthesis and characterize the morphology of the as-prepared samples. The results revealed that the CdS layer between Ni and TiO2 plays an important role in the uniform nucleation and the following growth of highly dispersive Ni nanoparticle on the Ti@TiO2 core−shell nanowire surface. The bifunctional nanostructured electrode was applied to construct an electrochemical nonenzymatic sensor for the reliable detection of glucose. Under optimized conditions, this nonenzymatic glucose sensor displayed a high sensitivity up to 1136.67 μA mM−1 cm−2, a wider liner range of 0.005−12 mM, and a lower detection limit of 0.35 μM for glucose oxidation. The high dispersity of Ni nanoparticles, combined with the anti-poisoning faculty against the intermediate derived from the self-cleaning ability of CdS under the photoexcitation, was considered to be responsible for these enhanced electrochemical performances. Importantly, favorable reproducibility and long-term performance were also obtained thanks to the robust frameworks. All these results indicate this novel electrode is a promising candidate for nonenzymatic glucose sensing.

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from the electrode surface to the metal substrate.11,12 Mahshid et al.13 reported the preparation of Pt/Ni composite on TiO2 NTAs as an amperometric nonenzymatic glucose sensor and obtained a sensitivity of 1629 μA mM−1 cm−2 and a liner range of 0−0.12 mM. Chen et al.14 synthesized CuO nanofibers on TiO2 NTAs for sensitive nonenzymatic glucose detection, which exhibited a sensitivity of 79.79 μA mM−1 cm−2 and a liner range up to 2.0 mM at 0.5 V (vs SCE). Wang et al.15 prepared a Ti/TiO2/Ni composite by electrodepositing a Ni layer on the surface of TiO2 nanotube arrays (NTAs). The sensitivity of the composite electrode reached 200 μA mM−1 cm−2 at 0.55 V (vs SCE) and the liner range was 0.1−1.7 mM. Yu et al.16 developed a nonenzymatic glucose sensor based on Ni particles decorated TiO2 NTAs. The electrochemical sensor displayed a sensitivity of 700.2 μA mM−1 cm−2 and a liner range of 0.004−4.8 mM at an applied potential of 0.6 V (vs Ag/ AgCl). Unfortunately, there are some challenges encountered when the TiO2 NTAs are used as substrate, such as (1) the active materials generally existed on the top surface of TiO2 nanotubes and were seriously aggregated, which gave rise to a big discount of high surface area; (2) the long electron-transfer

lucose detection has been one of the most scientific and technological objects driven by not only the strikingly increasing and leading diabetes disease but also the urgent requirements in the fields of food, chemistry, biology, and environmental protection.1−4 This trend has been attracting tremendous academic and commercial efforts to develop a glucose sensor with high sensitivity, excellent selectivity, lowcost, and good reliability. As a category of glucose sensors, the nonenzymatic glucose sensor based on the direct electrocatalysis of glucose on the electrode has recently triggered considerable interests due to its efficient glucose sensing which is free from pH, temperature, limited stability, variable sensitivity, and high cost.5−7 For the construction of a nonenzymatic glucose sensor, the selection of substrate electrode is very important because the physical and chemical properties of the substrate material have an immense influence on the electrochemical performance of active material.8−10 Recently, TiO2 nanotube arrays (NTAs) fabricated by complicated anodization of Ti foils have recently captured considerable attentions as a substrate for the loading of active material in glucose detection owing to theirs large surface area, well-aligned nanostructures, good adhesion to the Ti substrate, and the natural properties from TiO2 such as high chemical stability, low cost, and excellent biocompatibility. Meanwhile, a Schottky-type contact can be formed between TiO2 and Ti which will facilitate the transport of the electron © 2013 American Chemical Society

Received: October 24, 2013 Accepted: December 4, 2013 Published: December 4, 2013 876

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electrode system: a platinum plate as the auxiliary electrode, a standard Hg/HgO electrode as the reference electrode, and the prepared samples as the working electrode. Formation of Ti@TiO2 Core−Shell Nanowires. The details of the preparation process were similar to that reported in our previous publication.24 First, Ti foils with size of 0.5 cm ×1.5 cm were cleaned with acetone in an ultrasonic bath for 20 min and then rinsed with a large amount of water. In a typical procedure, Ti plates were loaded into a 50 mL Teflon-lined stainless steel autoclave filled with 10 mL of 2.5 wt % HCl aqueous solution and kept at 190 °C for 10 h to complete the hydrothermal reaction. After being cooled to room temperature, the as-prepared samples were completely washed with distilled water and dried in air. The as-prepared Ti nanowires were then annealed at 450 °C for 10 h to form the Ti@TiO2 core−shell nanowires. Fabrication of Ti@TiO2/Ni, Ti@TiO2/CdS, and Ti@TiO2/ CdS/Ni Electrodes. Ni nanoparticles were electrodeposited through a multipotential step method using a three-electrode system with Ti@TiO2 as the working electrode, a Pt plate as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The precursor solution was the mixture of 0.05 M NiSO4·6H2O, 0.01 M NiCl2·6H2O, and 0.025 M H3BO3 with the pH value of 3.0. The electrodeposition parameters of the multipotential step method include positive potential (2.0 V, 0.05 s), negative potential (−1.5 V, 0.5 s), and negative potential (−1.1 V). Before electrodeposition, the Ti@TiO2 electrode was first immersed into the solution for several minutes to ensure a complete diffusion of the precursor into the voids among Ti@TiO2 nanowires. After deposition, the electrode was first cleaned with distilled water and absolute alcohol for several times, respectively, in order to remove the precursor solution and then dried in air. The as-prepared sample was denoted as Ti@ TiO2/Ni. CdS nanoparticles were deposited on the surface of Ti@ TiO2 by a constant potential procedure via employing the same three-electrode system. The precursor solution was consisted of 0.15 M CdCl2 and 0.05 M Na2S2O3. The electrodeposition potential was kept at −0.65 V with the solution temperature at 50 °C and pH value at 2.0. Before electrodeposition, the Ti@ TiO2 electrode was also soaked into the precursor solution as described above. The loading of CdS was controlled by the electric quantity in the electrodeposition process. The asprepared sample was denoted as Ti@TiO2/CdS. With the Ti@TiO2/CdS as the working electrode in a threeelectrode setup, Ni nanoparticles were deposited onto the surface of Ti@TiO2/CdS and the as-prepared sample was denoted as Ti@TiO2/CdS/Ni. The amount of the Ni particles in both Ti@TiO2/CdS/Ni and Ti@TiO2/Ni was the same by controlling the electric quality in the electrodeposition procedure. At the same time, the influence of CdS content on the sensing performance of Ti@TiO2/CdS/Ni was investigated by cyclic voltammetry curves (CVs) when the electric quality of CdS was controlled at 0.02, 0.04, and 0.06 C, respectively. According to the results shown in Supporting Information Figure S1, the electrode was proved with the optimal electrocatalytic properties when the electric quality was 0.04 C. So it is employed in the following experiments.

pathway from the top of the TiO2 NTAs to the Ti foil in bottom and the semiconductor properties of TiO2 will lead to the fatal decline of electrochemical sensing performances. On the other hand, electrode active materials are also considered to be the determinant factor affecting the analytical properties of a nonenzymatic glucose sensor. Among various electrode materials for nonenzymatic glucose sensing, Ni nanoparticles are substantially undergoing a great deal of attention to be one of the most attractive candidates for glucose detection owing to their low-cost, environmental benignity, and outstanding catalytic behavior.12,17−19 Nonetheless, an assortment of theoretical as well as experimental studies indicate that the sensing performance of a nanostructured metal-based sensor is easily and seriously impaired by the absorption of reaction intermediates during glucose oxidation.20−23 Interference effects caused by the endogenous species also cannot be totally eliminated. Consequently, it will in effect diminish the sensitivity and selectivity of the sensor during longer duration operation and further drastically limit its commercial application. In this background, we attempt to construct a nonenzymatic sensor based on the Ni/CdS bifunctional Ti@TiO2 core−shell nanowire electrode to address the drawbacks suffered by TiO2 NTAs and Ni nanoparticles. Two approaches are proposed specifically as follows: (1) Ti@TiO2 core−shell nanowires grown on Ti foil are selected to replace TiO2 NTAs because the ultrathin TiO2 shell layer and conductive Ti core integrated onto a conductive substrate can effectively shorten the transfer pathway of the electron and improve the conductivity, as described in our previous report.24 (2) CdS nanolayers, as a dispersant and intermediate layer, are first deposited on the surface of Ti@TiO2 core−shell nanowires to decrease the aggregation of active material and enhance the antipoisoning ability of Ni nanoparticles against the intermediate produced in glucose sensing due to its high photosensitivity under natural light,25,26 good biocompatibility, and the feature of promoting the charge-transfer rate. As expected, the Ni nanoparticles are formed and densely scattered on the nanowire without aggregation. The resultant sensor based on the Ni/CdS bifunctional Ti@TiO2 core−shell nanowire electrode exhibits outstanding electrochemical sensing to glucose oxidation, including a high sensitivity up to 1136.67 μA mM−1 cm−2 and a wider liner range of 0.005−12 mM. Meanwhile, the electrode shows a perfect selectivity to glucose in the presence of common interfering species and good practical application. All of the above excellent performances make this novel electrode promising for nonenzymatic glucose detection.

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EXPERIMENTAL SECTION Apparatus and Reagents. Titanium (Ti) foils (purity, 99.0% Ti; thickness, 0.5 mm) were purchased from Sigma. Nickel chloride hexahydrate (NiCl2·6H2O), nickel sulfate hexahydrate (NiSO4·6H2O), β-D-glucose, ascorbic acid, uric acid, and other reagents were all of analytical grade and used without further purification. Field emission scanning electron microscopy (FESEM) analysis was performed using a Hitachi S-4800 microscope. Transmission electron microscopy (TEM) images were obtained with a FEI Tecnai G2F30 microscope. Xray diffraction patterns (XRD) were recorded with a Rigaku D/ M ax-2400 X-ray diffractometer at a 2θ range of 10−80°. All of the electrochemical performance experiments were performed using a CHI660D electrochemical workstation using a three-

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RESULTS AND DISCUSSION Figure 1A shows the top-view FESEM image of bare Ti@TiO2 nanowires. It can be seen that many uniform nanowires grow 877

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on the plane substrate with flat top surface and the diameter of the nanowires is about 40−90 nm. After the electrodeposition of Ni, it is observed (Figure 1B) that a few numbers of large nanoparticles appear on the side of Ti@TiO2 nanowires. Figure 1C depicts the structure image of Ti@TiO2 /CdS. It demonstrates the smooth surface of the Ti@TiO2 nanowire has transformed into a rough structure with amounts of tiny CdS particles. The increased surface roughness is believed to be beneficial for the nucleation and growth of active materials. In contrast with the greater particle aggregated on some spots of the nanowire in the Ti@TiO2/Ni sample, highly dispersed Ni nanoparticles are homogeneously distributed on the top and sides of the nanowire in the Ti@TiO2/CdS/Ni sample. Meanwhile, it can be seen that the size of the particle also obviously decreases. The elemental compositions of the asprepared samples were characterized by energy-dispersive X-ray spectroscopy (EDX) as indicated in Supporting Information Figure S2. It reveals the presence of Ti and O in all the samples. Besides, strong peaks of Ni appear in the Ti@TiO2/Ni sample and some peak signals that correspond to Cd and S are detected in the Ti@TiO2/CdS sample. Moreover, it is obvious that the peaks of Ni appeared in the Ti@TiO2/Ni sample; Cd and S that appeared in the Ti@TiO2/CdS sample are all observed in the Ti@TiO2/CdS/Ni sample, clearly suggesting the coexistence of Ni and CdS in this hybrid. In order to gain more details of structural information, further insights into the morphology and microstructure of the as-prepared samples were obtained by TEM. It can be seen from Figure 2A, the medial part of the Ti@TiO2 nanowire is wrapped by only one great Ni particle with a size up to 150 nm in the Ti@TiO2/Ni sample. For the Ti@TiO2/CdS sample (Figure 2B), a thin CdS layer that consisted of many tiny particles can be clearly identified along the whole Ti@TiO2 nanowire. Meanwhile, the rolling outline of the layer indicates the surface roughness as described in the SEM. In comparison with the Ti@TiO2/Ni sample, the deposition of Ni onto Ti@ TiO2/CdS nanowire displays a completely different morphology with high dispersity and decreased particle size as well as increased particle amount (Figure 2C). Figure 3 presents the XRD patterns of bare Ti@TiO2, Ti@ TiO2/Ni, and Ti@TiO2/CdS/Ni samples, respectively. Except for the diffraction peaks from the Ti substrate, a peak with 2θ of 25.3° which is assigned to an anatase crystal of TiO2 can be clearly detected. After Ni electrodeposition, two new diffraction peaks at 44.4° and 51.7° are emerging, which can be indexed as Ni(111) and (200) of cubic Ni phase (JCPDS 65-2865), respectively, while the pattern of Ti@TiO2/CdS/Ni not only shows the diffraction peak of Ni but also exhibits two diffraction

Figure 1. Top-view FESEM images of Ti@TiO2 (A), Ti@TiO2/Ni (B), Ti@TiO2/CdS (C), and Ti@TiO2/CdS/Ni (D) samples.

Figure 2. TEM images of Ti@TiO2/Ni (A), Ti@TiO2/CdS (B), and Ti@TiO2/CdS/Ni (C). 878

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of 1 mM glucose in 0.5 M NaOH solution, which are recorded at scan rate of 20 mV/s. For the Ti@TiO2/CdS electrode, no redox peak is present in 0.5 M NaOH solution and there is negligible current increase after the addition of glucose solution, which proves the electrode has no electrocatalytic ability for glucose oxidation. Compared with the Ti@TiO2/ CdS electrode, a pair of well-defined quasi-reversible redox peaks can be distinctly observed at both Ti@TiO2/Ni and Ti@ TiO2/CdS/Ni before the addition of glucose because metallic Ni can be oxidized into Ni(OH)2 in alkaline solution, then Ni(OH)2 was further oxidized to NiOOH.27 However, the peak current of the Ti@TiO2/CdS/Ni electrode is higher than that of the Ti@TiO2/Ni electrode, indicating the high utilization of active materials. It is in accordance with the description in SEM that the Ni particles in the Ti@TiO2/CdS/Ni electrode are higher in dispersity, more in quantity, and smaller in size than that in the Ti@TiO2/Ni electrode. Especially, compared to the Ti@TiO2/Ni electrode, the anodic peak of the Ti@TiO2/CdS/ Ni electrode has a shift to the negative direction, but the cathodic peaks are located at the same potential. That is to say, the potential difference (ΔEa,c) of the Ti@TiO2/CdS/Ni electrode is smaller than that of the Ti@TiO2/CdS/Ni electrode, which implies the excellent reversibility.28 The different redox potentials of the Ti@TiO2/CdS/Ni and Ti@

Figure 3. XRD patterns of Ti@TiO2, Ti@TiO2/Ni, and Ti@TiO2/ CdS/Ni.

peaks of CdS at 26.4° and 28.2°, which correspond to (002) and (101) reflections of hexagonal CdS (JCPDS 65-3414) structure, respectively. Figure 4A presents the CVs of Ti@TiO2/Ni, Ti@TiO2/CdS, and Ti@TiO2/CdS/Ni electrodes in the absence and presence

Figure 4. (A) Cyclic voltammetry curves (CVs) of the Ti@TiO2/CdS/Ni electrode (a1, a2), Ti@TiO2/Ni electrode (b1, b2), and Ti@TiO2/CdS electrode (c1, c2) in the absence (a1, b1, and c1) and presence (a2, b2, and c2) of 1 mM glucose in 0.5 M NaOH solution recorded at scan rate of 20 mV/s. (B) Linear sweep of the Ti@TiO2/CdS/Ni electrode in a range of glucose concentrations between 0 and 9 mM. (C) Amperometric response of the Ti@TiO2/CdS/Ni electrode at different potentials from 0.4 to 0.65 V with successive additions of 1 mM glucose. 879

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Figure 5. (A) Amperometric response of Ti@TiO2/CdS/Ni and Ti@TiO2/Ni electrodes to successive additions of glucose at an applied potential of 0.55 V. Inset: a partial magnification of the current response toward a low concentration of glucose solution. (B) The corresponding calibration curves.

potential and becomes stable over 0.55 V. But the background currents and noise signals are more pronounced at 0.6 and 0.65 V. So 0.55 V (0.47 V vs Ag/AgCl) is adopted as the optimum potential in the following experiments. This potential is more negative than those previously reported about nonenzymatic sensors,1,33−37 which will have an energetic effect on improving selectivity because most of the interfering species such as uric acid (UA) and ascorbic acid (AA) are not active when the potential is below 0.5 V (vs Ag/AgCl).17,38 The amperometric response of Ti@TiO2/CdS/Ni and Ti@ TiO2/Ni electrodes to glucose was carried out at 0.55 V for successive additions of various concentrations of glucose under vigorously stirred condition. Upon each addition of glucose (Figure 5A), Ti@TiO2/CdS/Ni and Ti@TiO2/Ni electrodes can both achieve 95% of steady-state current within 2 s. However, the current response of the Ti@TiO2/CdS/Ni electrode toward the same concentration of glucose solution is obviously greater than that of the Ti@TiO2/Ni electrode. Figure 5B shows the linear dependence of current response on glucose concentration. The sensitivity (calculated from calibration curves) of the Ti@TiO2 /CdS/Ni electrode (1136.67 μA mM−1 cm−2) is 10-fold greater than that of the Ti@TiO2/Ni electrode (114.03 μA mM−1 cm−2). Moreover, the detection limit (0.35 μM) is lower than that (1.0 μM) of the Ti@TiO2/Ni electrode and the liner range (0.005−12 mM) is much wider than that (0.005−9 mM) of the Ti@TiO2/Ni electrode. Importantly, the as-prepared Ti@TiO2/CdS/Ni electrode shows a good superiority in terms of sensitivity, applied potential, liner range, and detection limit when compared with some other nanostructure-based nonenzymatic glucose sensors.12,14−16,35,39−44 These fantastic properties of the Ti@TiO 2 /CdS/Ni electrode are not only derived from the well-dispersive Ni nanoparticles but also associated with the effect brought by CdS. As reported in a previous research study,16 the active materials with small size are proved to usually present outstanding catalytic activities due to more reactive sites. CdS has been regarded as a promising semiconductor because of its appropriate band gap, the proper position of the valence band and conduction band, excellent stability, and easy fabrication.45,46 As shown in Figure 6, under the natural light

TiO2/Ni electrodes may be caused by the following two reasons. (1) According to a previous report, the CdS layer between Ni and TiO2 can effectively increase the chargetransfer rate due to the proper band energy difference between CdS and TiO2.29,30 (2) The multichannel provided by the highly dispersive nanoparticles on the nanowires can facilitate the transport of the ions on the interface of active materials and electrolyte. This can be further verified by the electrochemical impedance spectroscopy (EIS) as shown in Supporting Information Figure S3. It depicts that the Ti@TiO2/CdS/Ni electrode has the lower charge-transfer resistance and diffusive resistance than that of the Ti@TiO2/Ni electrode, indicating the fast reaction kinetic process. Upon the addition of glucose, the anodic peak current of both electrodes has a dramatic increase, which can be attributed to the catalytic effect of NiOOH toward glucose (2NiOOH + glucose → 2NiO + gluconolactone + H2O). But the current increase of the Ti@ TiO2/CdS/Ni electrode is about 3 times higher than that of the Ti@TiO2/Ni electrode. It is demonstrated from Supporting Information Figure S3 that the Ti@TiO2/CdS/Ni electrode has the lower semicircle diameter than that of the Ti@TiO2/Ni electrode, indicating quick electron transfer from the reactive sites to the current collector. Figure 4B presents the liner sweep voltammograms collected from the Ti@TiO2/CdS/Ni electrode in 0.5 M NaOH solution in the range of glucose concentration from 0 to 9 mM. As expected, the current increases with the increase of glucose concentration. But the peak potential displays insignificant shift even when the glucose concentration is up to 9 mM, which has a great difference from the notably positive shift in a previous report.31,32 The results effectively confirm the excellent antipoisoning of the Ti@TiO2/ CdS/Ni electrode against the intermediate. It is known that the applied potential has a strong effect on the sensing performance of the electrochemical sensor, so the applied potential is optimized accordingly. The potential optimization was conducted at the Ti@TiO2/CdS/Ni electrode around the peak potential through a typical I−t technique (Figure 4C). Though the current response is relatively weak, the steady step current can be observed even at a potential as low as 0.4 V. When the applied potential is higher than 0.4 V, the current response increases with the increase of applied 880

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consumed by the holes in the CdS valence band as described above. The ability to discriminate the interfering species which have similar electroactivity to the target analyte is one of the most significant analytical factors for a biosensor. Ascorbic acid and uric acid presented in physiological fluids are the most important interferences for the direct electrochemical oxidation of glucose on different electrodes, especially for a nonenzymatic sensor.48 The blood glucose level of normal human body is between 4 and 7 mM, which is much higher than the concentration of the interfering species such as UA (0.1 mM) and AA (0.1 mM). Figure 7A shows the amperometric response of the Ti@TiO2/CdS/Ni electrode toward the addition of 1 mM glucose, 0.05 mM AA, and 0.05 mM UA in a stirred 0.5 M NaOH solution at the applied potential of 0.55 V. Unlike the result reported by Wang et al.,15 the current responses induced by UA and AA are negligible compared to the sharp current response of glucose, indicating the proposed sensor is responsible for good selectivity toward the detection of glucose. The long-term stability is also an important parameter for evaluating the performance of the sensor. The proposed sensor based on the Ti@TiO2/CdS/Ni electrode was stored in air at ambient conditions, and its current response to 1 mM glucose was checked every 3 days within a 30-day period through amperometric response at 0.55 V. The data collected from the repeated experiments (Figure 7B) depicts the proposed sensor maintains at least 92.5% of the initial sensitivity in the continuous tests, suggesting the good long-term stability. The reproducibility of the sensor was determined by conducting seven successive amperometric measurements of 1.0 mM glucose with one single electrode. The relative standard deviation (RSD) is 1.65%, demonstrating that the electrode was not poisoned by the oxidation product and can be used repeatedly for the detection of glucose. For five electrodes prepared through the same way, an RSD of 2.85% was obtained, indicating the proposed method was reliable. The applicability was performed by determining the glucose concentration in the human serum samples. The results tested using the Ti@TiO2/CdS/Ni electrode are in agreement with that read from the commercial GOD-based sensor (Table 1), indicating the fabricated nonenzymatic glucose sensor can be utilized for practical sample testing with favorable accuracy and precision.

Figure 6. Scheme diagram of the Ti@TiO2/CdS/Ni electrode toward glucose oxidation and the consumption of intermediate by photoexcited holes.

illumination, CdS with narrow band gap energy (Eg = 2.17 eV) can be easily excited by light. Additionally, the conduction band energy of anatase TiO2 lies more positive than that of CdS;47 the electron from the CdS conduction band can be readily injected into the conduction band of TiO2, and then it is transported to the external circuit through the Ti core nanowires, whereas holes with high oxidability accumulate in the valence band of CdS and form the hole centers, which can effectively oxidize the intermediate produced in the glucose oxidation. As for the glucose oxidation on the Ni-based electrode, the accepted mechanism is that metallic Ni was first oxidized into Ni(OH)2 in alkaline solution, then Ni(OH)2 was further oxidized to NiOOH. The oxidative NiOOH could then catalyze the glucose oxidation and generate gluconolactone, which is the result of deprotonation of the glucose and isomerization to its enediol form. This isomer absorbs on the surface of Ni particle and is difficult to be further oxidized by NiOOH. Nevertheless, the intermediate can be effectively

Figure 7. (A) Current response of the Ti@TiO2/CdS/Ni electrode upon successive addition of 1.0 mM glucose, 0.05 mM AA, 0.05 mM UA, and 1.0 mM glucose. (B) Normalized sensitivity of the Ti@TiO2/CdS/Ni electrode based sensor to glucose tested every 3 days over 1 month. 881

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Table 1. Determination of Glucose in Human Serum Samples by the Ti@TiO2/CdS/Ni Electrode samples

determined values

reference values

relative deviations (%) (n = 3)

1 2 3

15.2 6.0 4.1

15.14 5.88 4.23

0.8 1.2 0.9

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CONCLUSION In summary, the Ni/CdS bifunctional Ti@TiO2 core−shell nanowire electrode was fabricated via the typical hydrothermal method and electrodeposition technique. The results proved that the CdS layer between TiO2 and Ni could not only manipulate the morphology and structure of Ni particles but also have an active effect on glucose sensing. The Ni nanoparticles with good dispersibility and small size were obtained by the predeposition of a CdS nanolayer on a Ti@ TiO2 nanowire. It was favorable for the enhanced electrochemical properties. On the other hand, the photoexcited holes of CdS generated under the irradiation of visible light could timely consume the reaction intermediate, which endowed the Ti@TiO2/CdS/Ni electrode with the higher sensitivity, lower detection limit, and a wider linear range than that of the Ti@ TiO2/Ni electrode. In addition, the Ti@TiO2/CdS/Ni electrode was confirmed to have excellent selectivity, good reproducibility, and long-term stability as well as superior practical application ability for glucose detection.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-931-891-2589. Fax: +86931-891-2582. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the National Natural Science Foundation of China (NNSFC No. 20903050), the Fundamental Research Funds for the Central University (Lzujbky-2012-22 and Lzujbky-2012-79), and the National Science Foundation for Fostering Talents in Basic Research of the National Natural Science Foundation of China (Grant No. J1103307), the Basic Scientific Research Business Expenses of the Central University and Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University (LZUMMM2013004), and the National College Students’ Innovative Entrepreneurial Training Program of Lanzhou University (No. 20130730096).

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