Ni Composite Electrodes for Nonenzymatic

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Ti/TiO2 Nanotube Array/Ni Composite Electrodes for Nonenzymatic Amperometric Glucose Sensing Chengxiang Wang, Longwei Yin,* Luyuan Zhang, and Rui Gao Key Laboratory for Liquid-Solid Structural EVolution & Processing of Materials, Ministry of Education, School of Materials Science & Engineering, Shandong UniVersity, Jinan 250061, People’s Republic of China ReceiVed: December 29, 2009; ReVised Manuscript ReceiVed: February 8, 2010

TiO2 nantube arrays (NTAs) with a tube diameter of about 100 nm were prepared by anodic oxidation, and a Ni layer about several tens of micrometers thick was electrodeposited on the surface of TiO2 NTAs. The synthesized Ti/TiO2 NTA/Ni composite electrodes were used for nonenzymatic glucose detection. The behavior of the anodic TiO2 NTA/substrate Ti metal is analogous to characteristics of an n-type semiconductor/metal Schottky barrier diode. The characteristic would enhance the rapid transport of surface reaction electrons to the metal substrate, thereby enhancing the performance of biosensors. Cyclic voltammogram investigations indicate that calcined Ti/TiO2 NTAs have a wider work potential window than that of Ti/TiO2 NTAs without calcinations and Ti foils. Moreover, the low saturated current in alkaline solutions under a constant potential makes Ti/TiO2 NTAs favorable as substrate materials for biosensors. For the Ti/TiO2 NTA/Ni composite electrodes, the relationship between the linear range and the potential was investigated. The positive line scans and the amperometric response curves to glucose indicated that a potential of 0.5-0.6 V would be favorable to get a wide linear range for detecting glucose. The sensitivity of the composite electrodes can reach a value as high as 200 µA mM-1 cm-2, and the detection limit was about 4 µM with a signal-to-noise ratio of 3 (S/N ) 3). The present Ti/TiO2 NTA/Ni composite nonenzymatic amperometric glucose sensor could find potential applications in a good biosensing platform for other redox proteins and enzymes. Introduction Glucose biosensors have been attracting great attention due to the practical medical needs and applications. Clark and Lyons first proposed the concept of a biosensor in which an enzyme is incorporated on an electrode surface in 1962.1 The enzymatic biosensors have then made a considerable progress due to their simplicity, high sensitivity, excellent selectivity, and potential ability for real-time and on-site analysis. Until now, they have passed through three generations.2 However, there are some disadvantages of the enzymatic electrodes. Enzymes are usually immobilized onto the solid electrodes by electropolymerization,3 cross-linking,4 casting,5 or self-assembly.6-8 Although these methods can provide an effective interface, denaturalization and leakage of enzymes lead to a poor stability and perturbed function.9 Enzymes can work effectively only under proper conditions, such as temperature, chemical environment (e.g., pH), and the chemical composition of the substrate.10 In some extreme environments, enzymatic electrodes would not work effectively. In contrast, nonenzymatic biosensors were advocated, especially the amperometric glucose sensors free from enzymes. The nonenzymatic electrooxidation of glucose on metal electrodes, such as Pt,11,12 Au,13,14 Ni,15,16 and Cu,17 has been previously investigated. Nonenzymatic electrooxidation of glucose is greatly enhanced at Ni and Cu compared with Pt and Au electrodes as a result of their electrocatalytic effect mediated by surface bond Ni2+/Ni3+ and Cu2+/Cu3+ redox couples.18 The effective Ni or Cu components were usually deposited on supporting materials. Of the numerous types of matrices used today, TiO2 has attracted considerable interest * To whom correspondence should be addressed. Phone: +86-53188396970. E-mail: [email protected].

due to the low cost, plenty of morphologies, and very important for biosensor substrates, the excellent biocompatibility, nontoxic, and good chemical and thermal stability.19-21 Anodic TiO2 NTAs have large specific surface areas, high uniformity, and the advantages of TiO2 materials mentioned above. Furthermore, the behavior of the anodic TiO2 NTA/substrate Ti metal is analogous to characteristics of an n-type semiconductor/metal Schottky barrier diode.22 These characteristics would enhance the rapid transport of surface reaction electrons to the metal substrate, thereby enhancing the performance of biosensors. The characteristics mentioned above indicate that Ti/TiO2 NTA electrodes would be promising substrate materials for biosensors. In this paper, Ti/TiO2 NTA/Ni composite electrodes were prepared by electrooxidation for glucose detection. Ti/TiO2 NTAs were first prepared by anodic oxidation of Ti foils in HF water solutions (0.5 wt %). A Ni layer was then electrodeposited on the Ti/TiO2 NTAs. The electrochemical tests were carried out, and the results indicated that a high potential would be favorable to obtain a wide linear range for these composite electrodes. The sensitivity can reach a value as high as ∼200 µA mM-1 cm-2 to glucose at 0.55 V with a detection limit of 4 µM (S/N ) 3). Experimental Section Material Synthesis. Ti foils (99.5% purity, 0.5 mm in thickness, A. Johnson Matthey Company) were cleaned by sonicating in acetone and isopropanol, followed by rinsing with deionized water and drying in a nitrogen stream. The anodic oxidation of Ti foils was carried out in the electrolytes consisting of 0.5 wt % HF water solutions, using a two-electrode system with a Ti foil as the anode and a Pt plate as the cathode. The potential between the two electrodes was kept at 20 V for 20

10.1021/jp912232p  2010 American Chemical Society Published on Web 02/24/2010

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Figure 1. (a) The SEM image of TiO2 NTAs by anodic oxidation. (b) The TEM image of TiO2 NTAs. (c) The electron diffraction (ED) pattern of TiO2 NTAs, indicating that the TiO2 NTAs are anatase structures.

min. After anodization, the sample was annealed at 500 °C for 3 h to crystallize the TiO2 NTAs. Ni was electrodeposited in the electrolytes consisting of 0.2 M NiSO4, 0.3 M KCl, and 0.3 M HBO3,23,24 using a three-electrode system with Ti/TiO2 NTAs as the working electrode, a Pt plate as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The electrodeposition potential was kept at -0.8 V (vs SCE) for 20 min, and the temperature was kept at 50 °C. The deposition method is a little from the references. Moreover, all the potentials in this paper were based on SCE as reference electrodes. Characterization. The morphologies and components of the products were analyzed using an SU70 high-resolution field emission scanning electron microscope (FE-SEM) and an attached X-ray energy-dispersive spectrometer (EDS). The structures of the products were analyzed using a high-resolution transmission electron microscope (HRTEM), a JEM-2100, at an acceleration voltage of 150 kV. The electrochemical tests were carried out on a PARSTAT 2273 electrochemical workstation.

Figure 2. (a, b) Top-view SEM images of Ti/TiO2 NTA/Ni composite electrodes; the deposition times were 200 s and 20 min, respectively. (c) EDS spectrum of the composite electrode.

Results and Discussion Figure 1a depicts a typical SEM image of the synthesized TiO2 nanotube arrays on a Ti substrate prepared by anodic oxidation. The TiO2 nanotubes have a unique size distribution with a diameter of 100 nm. Figure 1b shows the TEM image of TiO2 NTAs peeled from the Ti substrate. The electron diffraction (ED) pattern of the (103), (200), and (101) rings (Figure 1c) from the TiO2 arrays indicates the anatase structure for the synthesized TiO2 products. At low temperatures less than 5 °C, it was hard to deposit nickel on the TiO2 NTAs. The nickel deposition temperature was selected at a high temperature of about 50 °C. After 200 s, the TiO2 NTAs were covered by many nickel particles and the inner surfaces of some TiO2 nanotubes were filled by nickel metals, as shown in Figure 2a. Figure 2b shows that the surface of TiO2 NTAs was completely covered by nickel layers after 20 min of deposition, forming the Ti/TiO2 NTA/Ni composite. The thickness of the Ni layer is about 40 µm. Figure 2c demonstrates a typical EDS pattern from the composite, indicating that the composites were composed of Ti, Ni, and O elements, with a chemical composition ratio of 46.43, 39.75, and 13.82%, respectively. Figure 3a shows the cyclic voltammogram (CV) curves of Ti/TiO2 NTAs with calcinations (curve 1), Ti/TiO2 NTAs without calcinations (curve 2), and Ti foils (curve 3). Changes of currents are observed under both cathodic and anodic polarization for three different electrodes. In the cathodic polarization range, three different electrodes show the similar shapes, as shown in Figure 3b. Onset potentials for reduction currents are all about -700 mV, corresponding to the reduction

Figure 3. (a) Cyclic voltammograms (CVs) of different electrodes in 0.1 M NaOH solution with a scan rate of 50 mV/s: (1) Ti/TiO2 NTAs with calcinations, (2) Ti/TiO2 NTAs without calcinations, and (3) Ti foils. (b, c) Parts of (a) at different ranges of voltages: -1.5 to -0.6 V and 0.5-1.0 V, respectively.

of Ti4+ to Ti3+. The second increase at a more negative potential (∼1200 mV) is assigned to H2 evolution (2H+ + 2e f H2).25 The currents of curves 1 and 2 are larger than that of curve 3, which is due to the high surface areas of TiO2 NTAs. Figure 3c shows the changes of current at 0.5-1.0 V. At about 550 mV, the current of the Ti electrode increases rapidly, which is due to the oxidation of Ti. However, the layers of anodic TiO2 NTAs on the Ti surface can protect the Ti substrate from oxidation at this potential. Until about 700 mV, the oxidation current of curve 2 increases obviously, while the current of curve 3 is still steady until about 1.0 V. The work potential window of curve 1 is about 1.7 V from -0.7 to 1.0 V, while curves 2 and 3 are -0.7 to 0.7 V and -0.7 to 0.55 V, respectively. The Ti/TiO2 NTA electrode with calcinations is more stable than the Ti foil and Ti/TiO2 NTA electrodes without calcinations in 0.1 M NaOH solution, and the wide positive work potential window of Ti/TiO2 NTA/Ni (curve 1) is favorable for the electrooxidation of glucose. The change of currents of different electrodes in 0.1 M NaOH solutions at 0.6 V is also investigated. As shown in Figure 4, the stable currents are about 2.3, 1.1, and 0.03 µA for Ti foils, Ti/TiO2 NTAs without calcinations,

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Figure 4. Change of currents on different electrodes: Ti foils (black line), Ti/TiO2 NTAs without calcinations (blue line), and Ti/TiO2 with calcinations (red line), at 0.6 V in 0.1 M NaOH solution without stirring.

Figure 5. Consecutive cyclic voltammograms of the Ti/TiO2 NTA/ Ni electrode composed of different scan numbers (1-10) at a scan rate of 50 mV/s in 0.1 M NaOH solutions.

and Ti/TiO2 NTAs with calcinations, respectively. The small background currents of Ti/TiO2 NTA electrodes make them appropriate as the substrate materials of biosensors, especially when the concentration of the detected object is low. Moreover, they can stabilize rapidly within about 10 s (not obvious in Figure 4), which is less than the other two electrodes. All the results indicate that Ti/TiO2 NTAs with calcinations are more stable than the other two electrodes. The nanotube arrays after calcinations are polycrystalline anatase TiO2 (Figure 1c), whereas the ones before calcinations are amorphous structured titanium oxides. Furthermore, Ti foils can be oxidized easily without the protection of titanium oxides. The anatase TiO2 nanotube arrays after calcinations have a wider work potential window than the other two electrodes. In the next electrochemical testing process, Ti/TiO2 NTAs with calcinations were selected as substrate materials. Figure 5 shows consecutive CVs of a Ti/TiO2 NTA/Ni electrode in 0.1 M NaOH solution at a scan rate of 50 mV/s. In the first scan, a pair of redox peaks appeared at 440 and 350 mV, assigned to the Ni2+/Ni3+ redox couple according to15

Figure 6. (a) CVs of the Ti/TiO2 NTA/Ni electrode in the 0.1 M NaOH + 0.2 mM glucose (blue line) and 0.1 M NaOH (red line) solutions. (b) Positive line scans of the Ti/TiO2 NTA/Ni electrode in 0.1 M NaOH solution with different concentrations of glucose: 1, 2, 3, 4, and 5 mM. All the scan rates are 50 mV/s.

is slowed, and the anodic and cathodic peaks tend to stabilize at 410 and 340 mV. It would be reasonable to consider that there is a limit of the thickness of the nickel layers that could sense glucose effectively. When the thickness is more than the certain value, the performance of the nickel layers cannot be enhanced by increasing the thickness. Figure 6a shows the cyclic voltammogram of the Ti/TiO2 NTA/Ni electrode in 0.1 M NaOH solution in the absence (red line) and presence (blue line) of 0.2 mM glucose at a scan rate of 50 mV/s. The anodic peak current is obviously enhanced by addition of glucose, from 50 to 80 µA, while the cathodic current has no significant changes. The anodic currents in the positive scan are proportional to the bulk concentration of glucose, and any increase in the concentration of glucose causes an enhancement of the anodic currents, as shown in Figure 6b. In the range of 0.5-0.6 V, the currents show a good linear relationship with the concentration of glucose and the anodic peak currents shift positively as the concentration of glucose increases. The oxidation of glucose on the nickel surface could be expressed as a direct electrooxidation process as follows:15

Ni3+-glucose f Ni3+-intermediate + e-

Ni(OH)2 + OH- T NiOOH + H2O + e-

Ni3+-intermediate f Ni3+-products + e-

The first cycle needs higher potential for the nucleation of NiOOH. In the next cycles, the anodic and cathodic peaks shift to lower potentials due to the nucleation of NiOOH in the first cycle. However, the peak currents shift to high values as the cycle numbers increase. This indicates that the activation sites (Ni2+ or Ni3+) increase as the number of cycles increases, which could be used to activate the as-prepared nickel layer. Furthermore, it may be used to regenerate the nickel electrode after a long time of applications. It is a progressive oxidation of the nickel as the scan cycles increase.15 The oxidation rate of nickel

According to this model, a possible kinetic mechanism for this phenomenon was brought out. The coverage of Ni3+ on the nickel surface rose with the increase of potential. There are enough glucose molecules to adsorb on the Ni3+ sites initially, as illustrated in Figure 7a. It is an adsorption-controlled process that is not sensitive to the concentration. As shown in Figure 6b, the currents in the range before the 1 mM peak current have no significant difference. The current rises as the potential increases. When the potential is high enough, the glucose diffusing to the electrode surface cannot inhabit the active Ni3+

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Figure 7. Schematic illustration of the reaction process on the surface of electrodes at low and high potentials. At low potentials, there are enough glucose molecules to be adsorbed on the electrode surface. However, at high potentials, more Ni3+ sites formed and the glucose molecules that diffused to the electrode surface cannot inhabit the Ni3+ sites completely.

sites completely, as shown in Figure 7b. It is a diffusioncontrolled process that is sensitive to the concentration. There is a potential at which the glucose molecules diffusing to the electrode are equal to the amount of Ni3+ sites. A peak current is also obtained at this potential (Vp). It is obviously that a higher Vp is needed to form more Ni3+ sites for a higher concentration of glucose (Figure 7b). The anodic peak current then shifts positively as the glucose concentration increases. The diffusioncontrolled process would be favorable for biosensors because a good linear relationship between the current and concentration is shown in this section. A reasonable deduction is that it is hard to obtain a wide linear range under a low potential to these composite electrodes. However, a high potential can lead to more interference signals, such as ascorbic acid (AA) and uric acid (UA). A proper potential should be determined by compromising between the two factors. Figure 8 shows the amperometric responses to glucose at the Ti/TiO2 NTA/Ni electrode and the corresponding calibration curve at different potentials. Upon each addition of glucose of 1 mM, the electrochemical response was recorded as the solution was stirred constantly. The current increases as the potential increases as shown in Figure 8a,c,e. A high current is favorable to enhance the sensitivity of biosensors, and the suitable potential could be selected in the range of 0.5-0.6 V. The linear range at different potentials was investigated. The corresponding calibration curves of 0.4, 0.5, and 0.6 V are shown in Figure 8b,d,f with linear dependence Rb ) 0.969, Rd ) 0.979, and Rf ) 0.986. The linear ranges are about 1-4, 1-7, and 1-9 mM at 0.4, 0.5, and 0.6 V, respectively. As had been deduced in Figure 8b, it is hard to obtain a wide linear range at low potentials for Ti/TiO2 NTA/Ni electrodes. From Figure 8, we can see that high potential is useful to get a wide linear range to our composite electrodes. Further investigations are needed to determine whether this phenomenon could be used for other kinds of electrodes in different materials. The long-term stability of the Ti/TiO2 NTA/Ni electrode was investigated in 0.1 M NaOH solution with 1 mM glucose. Figure 9 shows the change of oxidation peak currents as a function of cycle numbers. The oxidation peak currents increase as the cycle numbers increase. The change of peak currents are about 16% after the first 90 cycles. In the next three 90 cycles, which are 100-180, 190-270, and 280-360, the changes of peak currents are about 5.1, 4.3, and 2.5%, respectively. The currents are going to be stabilized as the cycle numbers increase. Meanwhile, as the cycle numbers increase, the oxidation peak potentials shift positively, as shown in Figure 9. In 0.1 M NaOH solution with 1 mM glucose, the peak potential platforms are 418.2, 425.5, 432.8, and 440.0 mV. The differences between them were 7.3,

Figure 8. Amperometric responses in 0.1 M NaOH solution and the corresponding calibration curve of sequential additions of glucose at a Ti/TiO2 NTA/Ni electrode by 1 mM/step at (a, b) 0.4, (c, d) 0.5, and (e, f) 0.6 V, with Rb ) 0.969, Rd ) 0.979, and Rf ) 0.986. The current increases as the potential increases. The linear range of current vs concentration at the high potential is wider than that at the low ones.

Figure 9. Changes of oxidation peak currents of CVs as a function of cycle numbers (red void squares) and the corresponding potentials (blue solid circles) of the Ti/TiO2 NTA/Ni electrode in 0.1 M NaOH solution with 1 mM glucose. The scan rates are 50 mV/s.

7.3, and 7.2 mV. It may be due to the slow conversion of R-Ni(OH)2 to β-Ni(OH)2.15,26,27 From Figure 9, we can presume β-Ni(OH)2 is more active than R-Ni(OH)2 in the oxidation of glucose. Ascorbic acid and uric acid present are the most important interferences for the direct electrochemical oxidation of glucose on different electrodes, especially nonenzymatic sensors.11 The normal physiological level of glucose is about 3-8 mM, whereas that of interfering species like UA and AA are 0.1 and 0.1 mM.28 In this paper, we studied the influence of 0.1 mM AA and 0.1 mM UA to 1 mM glucose in the amperometric responses on Ti/TiO2 NTA/Ni electrodes. As shown in Figure 10, AA had a great influence on the detection of glucose, whereas UA did not. The selectivity of the composite electrode to AA needs further improvement. As has been mentioned above, the number of active Ni3+ sites is believed to be a

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Figure 10. Amperometric responses of Ti/TiO2 NTA/Ni electrodes to the successive addition of 1 mM glucose, 0.1 mM AA, and 0.1 mM UA at 0.5 V in 0.1 M NaOH solution.

Wang et al. were synthesized by anodic oxidation of Ti foils. High corrosion resistance of TiO2 leads to a small background current when the Ti/TiO2 NTAs are used as the substrate materials in the dilute alkaline solution under a constant potential. This is very important in the detection of the object in low concentrations. Results indicate that Ti/TiO2 NTAs with calcinations had a wider work potential window than that of Ti foils and Ti/TiO2 NTAs without calcinations. For Ti/TiO2 NTA/Ni electrodes, the positive line scans in different concentrations and the calibration curves of I-t curves at different potentials indicated a potential of 0.5-0.6 V would be favorable to get a wide linear range in the direct oxidation of glucose. This result would have some positive significance in the investigations of nonenzymatic biosensors in different materials. A high sensitivity of about 200 µA mM-1 cm-2 can be obtained on the Ti/TiO2 NTA/Ni electrodes with a detection limit of 4 µM (S/N ) 3). Although the selectivity of the composite electrodes needs further improvements, the present Ti/TiO2 NTA/Ni composite nonenzymatic amperometric glucose sensor could find potential applications in a good biosensing platform for other redox proteins and enzymes. Acknowledgment. We acknowledge support from the National Nature Science Foundation of China (Nos. 50872071 and 50972079), the Shandong Natural Science Fund for Distinguished Young Scholars (JQ200915), the Nature Science Foundation of Shandong Province (Y2007F03 and Y2008F26), the Foundation of Outstanding Young Scientists in Shandong Province (No. 2006BS04030), the Tai Shan Scholar Foundation of Shandong Province, and the Gong Guan Foundation of Shandong Province (2008GG10003019).

Figure 11. (a) Amperometric responses in 0.1 M NaOH solution at 0.55 V of sequential additions of glucose at the Ti/TiO2 NTA/Ni electrode by 0.1 mM/step. (b) Calibration curve of the Ti/TiO2 NTA/ Ni electrode at a working potential of 0.55 V, with R ) 0.993.

constant at one certain potential. There is a competitive adsorption on the active Ni3+ sites among glucose, AA, and UA. Meanwhile, AA can be oxidized easily, whereas UA cannot. AA would then have much influence on the oxidation current in amperometric responses to glucose. The sensitivity and selectivity can be improved much in practical use by omitting the influence of AA. A high sensitivity could be obtained by this method. The potential was selected as 0.55 V (vs SCE). The amperometric response to glucose detection on another Ti/TiO2 NTA/Ni electrode performed in 0.1 M NaOH solutions at room temperature is shown in Figure 11a. Upon each addition of glucose of 0.1 mM, the electrochemical response was recorded as the solution was stirred constantly. The electrode had TiO2 nanotube arrays on both sides, about 25 mm2 for each side. The change of current is about 10 µA with 0.1 mM glucose addition for each time. The sensitivity of the Ti/TiO2 NTA/Ni electrode is about 200 µA mM-1 cm-2. The detection limit is about 4 µM with a signal-to-noise ratio of 3. Figure 11b shows the calibration curve of the Ti/TiO2 NTA/Ni electrode. The electrode responded to glucose with a line dependence (R ) 0.9929) in the range of 0.1-1.7 mM. Conclusions The Ti/TiO2 NTA/Ni composite electrodes were prepared by electrodepositing Ni on Ti/TiO2 NTA structure substrates that

Supporting Information Available: Side view of Ti/TiO2 NTA/Ni composite electrodes with a deposition time of 20 min and the corresponding Ni-element mapping and a single TiO2 nanotube and its ED pattern. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Clark, L. C.; Lyons, C. Ann. N. Y. Acad. Sci. 1962, 102, 29–45. (2) Wang, J. Chem. ReV. 2008, 108, 814–825. (3) Xue, M.-H.; Xu, Q.; Zhou, M.; Zhu, J.-J. Electrochem. Commun. 2006, 8, 1468–1474. (4) You, T. Y.; Niwa, O.; Tomita, M.; Hirono, S. Anal. Chem. 2003, 75, 2080–2085. (5) Benvenuto, P.; Kafi, A. K. M.; Chen, A. J. Electroanal. Chem. 2009, 627, 76–81. (6) Pang, X.; He, D.; Luo, S.; Cai, Q. Sens. Actuators, B 2009, 137, 134–138. (7) Cao, H.; Zhu, Y.; Tang, L.; Yang, X.; Li, C. Electroanalysis 2008, 20, 2223–2228. (8) Zhang, S. X.; Wang, N.; Niu, Y. M.; Sun, C. Q. Sens. Actuators, B 2005, 109, 367–374. (9) Wang, B.; Li, B.; Wang, Z.; Xu, G.; Wang, Q.; Dong, S. Anal. Chem. 1999, 71, 1935–1939. (10) This information was obtained on the following Web site: http:// en.wikipedia.org/wiki/Enzyme. (11) Yuan, J. H.; Wang, K.; Xia, X. H. AdV. Funct. Mater. 2005, 15, 803–809. (12) Attard, G. A.; Ahmadi, A.; Feliu, J.; Rodes, A.; Herrero, E.; Blais, S.; Jerkiewicz, G. J. Phys. Chem. B 1999, 103, 1381–1385. (13) Yi, Q.; Yu, W. Microchim. Acta 2009, 165, 381–386. (14) Martins, A.; Ferreira, V.; Queiro´s, A.; Aroso, I.; Silva, F.; Feliu, J. Electrochem. Commun. 2003, 5, 741–746. (15) Danaee, I.; Jafarian, M.; Forouzandeh, F.; Gobal, F.; Mahjani, M. G. Electrochim. Acta 2008, 53, 6602–6609. (16) Lu, L.-M.; Zhang, L.; Qu, F.-L.; Lu, H.-X.; Zhang, X.-B.; Wu, Z.-S.; Huan, S.-Y.; Wang, Q.-A.; Shen, G.-L.; Yu, R.-Q. Biosens. Bioelectron. 2009, 25, 218–223. (17) Zhao, J.; Wang, F.; Yu, J.; Hua, S. Talanta 2006, 70, 449–454. (18) Luo, P. F.; Kuwana, T. Anal. Chem. 1994, 66, 2775–2782. (19) Yu, J.; Ju, H. Anal. Chem. 2002, 74, 3579–3585.

Ti/TiO2 Nanotube Array/Ni Composite Electrodes (20) Zhang, T.; Tian, B. Z.; Kong, J. L.; Yang, P. Y.; Liu, B. H. Anal. Chim. Acta 2003, 489, 199–206. (21) Zheng, W.; Zheng, Y. F.; Jin, K. W.; Wang, N. Talanta 2008, 74, 1414–1419. (22) Zheng, Q.; Zhou, B.; Bai, J.; Li, L.; Jin, Z.; Zhang, J.; Li, J.; Liu, Y.; Cai, W.; Zhu, X. AdV. Mater. 2008, 20, 1044–1049. (23) Wang, Q.; Wang, G.; Han, X.; Wang, X.; Hou, J. G. J. Phys. Chem. B 2005, 109, 23326–23329. (24) Pan, H.; Liu, B.; Yi, J.; Poh, C.; Lim, S.; Ding, J.; Feng, Y.; Huan, C. H. A.; Lin, J. J. Phys. Chem. B 2005, 109, 3094–3098.

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