Nonenzymatic Electrochemical Glucose Sensor Based on

Jan 16, 2008 - Here, we report on a novel nonenzymatic amperometric glucose sensor ... Their amperometric sensitivities increase in the order of Pt−...
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Anal. Chem. 2008, 80, 997-1004

Nonenzymatic Electrochemical Glucose Sensor Based on Nanoporous PtPb Networks Jingpeng Wang,†,‡ Dan F. Thomas,‡ and Aicheng Chen*,†

Department of Chemistry, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada, and Department of Chemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada

Here, we report on a novel nonenzymatic amperometric glucose sensor based on three-dimensional PtPb networks directly grown on Ti substrates using a reproducible one-step hydrothermal method. The surface morphology and bimetallic composition of the synthesized nanoporous PtPb materials were characterized using scanning electron microscopy and energy-dispersive X-ray spectrometry, respectively. Voltammetry and amperometric methods were used to evaluate the electrocatalytic activities of the synthesized electrodes toward nonenzymatic glucose oxidation in neutral media in the absence and in the presence of chloride ions. The synthesized nanoporous PtPb electrodes have strong and sensitive current responses to glucose. Their amperometric sensitivities increase in the order of Pt-Pb (0%) < Pt-Pb (30%) < Pt-Pb (70%) < Pt-Pb (50%). These nanoporous PtPb electrodes are also highly resistant toward poisoning by chloride ions and capable of sensing glucose amperometrically at a very low potential, -80 mV (Ag/AgCl), where the interference from the oxidation of common interfering species such as ascorbic acid, acetamidophenol, and uric acid is effectively avoided. Diabetes mellitus is a group of metabolic diseases afflicting about 200 million people worldwide. For these patients, frequent testing of physiological glucose levels is critical to confirm that treatment is working effectively and to avoid a diabetic emergency, such as hypoglycemia (low blood sugar, 7 mM).1 The rising demand for glucose sensors with high sensitivity, high reliability, fast response, and excellent selectivity has driven tremendous research efforts for decades. On one hand, a revolutionary noninvasive glucosesensing technique has been under development for more than 17 years, and yet, there is no resulting product on the market today.2 On the other hand, glucose sensors that employ enzymes and operate invasively and electrochemically have been developed over the last four decades. To date, although there have already been significant benefits apparent from the use of commercialized * Corresponding author. Fax: 1-807-3467775. E-mail: aicheng.chen@l akeheadu.ca. † Lakehead University. ‡ University of Guelph. (1) Diabetes Atlas, 2nd ed.; International Diabetes Federation: Brussels, Belgium, 2003. (2) Arnold, M. A.; Small, G. W. Anal. Chem. 2005, 77, 5429-5439. 10.1021/ac701790z CCC: $40.75 Published on Web 01/16/2008

© 2008 American Chemical Society

implanted enzyme biosensors,3 several drawbacks have been found in long-term clinical practice, such as impaired responses and unpredictable drift in the in vivo signal, which necessitate frequent calibration against finger-prick samples.4 The historical advances in the development of enzyme-based electrochemical glucose sensors commenced with Updike and Hicks reporting the first enzyme-based amperometric glucose sensor in 1967.5 Since then, a variety of experimental improvements on the immobilization technique of enzymes and designing of the redox systems have been reported. Several groups have reported on the covalent immobilization of glucose oxidase (GOx) on carbon nanotubes.6-8 Platinum was used to decorate carbon nanotubes for glucose sensing in attempts to improve sensor performance.9-12 Another approach to enzyme immobilization involves coating the electrode with a perm-selective polymer membrane to prevent the approach of the interference substances to the electrode substrate.13,14 A recent study has also found that a carbon fiber electrode with a composite layer of co-deposited Ru and GOx possesses high selectivity for glucose oxidation without any perm-selective membrane.15 However, carbon nanotubes have been reported to have drawbacks to their application as transducers in electrochemical biosensors. Chaniotakis et al. has recently demonstrated that carbon nanofibers are good matrixes for the immobilization of proteins and enzymes for the development of biosensors, far superior to carbon nanotubes or graphite powder.16 Another unique alternative approach to glucose (3) Wilson, G. S.; Gifford, R. Biosens. Bioelectron. 2005, 20, 2388-2403. (4) Sachedina, N.; Pickup, J. C. Diabetic Med. 2003, 20, 1012-1015. (5) Updike, S. J.; Hicks, G. P. Nature 1967, 214, 986-988. (6) Liu, J.; Chou, A. C.; Rahmat, W.; Paddon-Row, M. N.; Gooding, J. J. Electroanalysis 2005, 17, 38-46. (7) Wang, S. G.; Zhang, Q.; Wang, R.; Yoon, S. F.; Ahn, J.; Yang, D. J.; Tian, J. Z.; Li, J. Q.; Zhou, Q. Electrochem. Commun. 2003, 5, 800-803. (8) Lin, Y.; Lu, F.; Tu, Y.; Ren, Z. Nano Lett. 2004, 4, 191-195. (9) Hrapovic, S.; Luong, J. H. T. Anal. Chem. 2003, 75, 3308-3315. (10) Hrapovic, S.; Liu, Y.; Male, K. B.; Luong, J. H. T. Anal. Chem. 2004, 76, 1083-1088. (11) Yang, M.; Yang, Y.; Liu, Y.; Shen, G.; Yu, R. Biosens. Bioelectron. 2006, 21, 1125-1131. (12) Xie, J.; Wang, S.; Aryasomayajula, L.; Varadan, V. K. Nanotechnology 2007, 18, 065503-065511. (13) Yang, Q.; Atanasov, P.; Wilkins, E. Sens. Actuators, B 1998, 46, 249-256. (14) Schuvailo, O. M.; Soldatkin, O. O.; Lefebvre, A.; Cespuglio, R.; Soldatkin, A. P. Anal. Chim. Acta 2006, 573, 110-116. (15) Kohma, T.; Oyamatsu, D.; Kuwabata, S. Electrochem. Commun. 2007, 9, 1012-1016. (16) Vamvakaki, V.; Tsagaraki, K.; Chaniotakis, N. Anal. Chem. 2006, 78, 55385542.

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sensing is the fluorescence-based glucose sensors,17,18 in which several molecular receptors for glucose have been employed. These commonly used receptors include the lectin concanavalin A,19,20 enzymes (i.e., glucose oxidase,21 glucose dehydrogenase,22 and hexokinase/glucokinase23), bacterial glucose-binding protein,24 and boronic acid derivatives.25 However, there are few fluorescence-based glucose detection methods that have reached the stage of testing in vivo, and none has entered clinical practice in diabetes management yet.18 Although the aforementioned enzyme-based glucose sensors have been explored and improved to such extent that some of them offer high sensitivity and selectivity, the most common and serious problem with enzymatic glucose sensors is insufficient long-term stability, which originates from the nature of the enzymes.26 In addition, because the sensitivity of these glucose sensors essentially depends on the activity of the immobilized enzymes, reproducibility is still a critical issue in quality control.27 In contrast, many nonenzymatic glucose sensors have also been explored, especially Pt-based amperometric glucose sensors. The kinetics and mechanism of glucose electro-oxidation have been intensively investigated on bare Pt electrodes,28-30 showing three major pitfalls of the direct oxidation of glucose on a smooth Pt electrode: (i) the overall kinetics of glucose electro-oxidation is too sluggish to produce significant faradaic currents; (ii) in addition to the low sensitivity, the activity of Pt electrodes is strongly impaired by adsorbed chloride ions and chemisorbed intermediates that originate from glucose oxidation and quickly block the electroactive surface; and (iii) various endogenous interfering species, such as L-ascorbic acid (AA), uric acid (UA) and 4-acetamidophenol (AP) can also be oxidized in the potential range of glucose oxidation, thus resulting in a poor selectivity. However, taking advantage of nanostructured electrocatalysts, enzymeless Pt glucose sensors have been greatly improved by introducing mesoporous Pt surfaces,31 highly ordered Pt nanotube arrays,32 and ordered macroporous Pt templates.33 All of these nanostructured Pt electrodes possess a very high active surface area, thus favoring kinetically controlled sluggish reactions (i.e., the oxidation of glucose) to a greater extent than the diffusion(17) Moschou, E. A.; Sharma, B. V.; Deo, S. K.; Daunert, S. J. Fluoresc. 2004, 14, 513-520. (18) Pickup, J. C.; Hussain, F.; Evans, N. D.; Rolinski, O. J.; Birch, D. J. S. Biosens. Bioelectron. 2005, 20, 2555-2565. (19) McCartney, L. J.; Pickup, J. C.; Rolinski, O. J.; Birch, D. J. S. Anal. Biochem. 2001, 292, 216-221. (20) Barone, P. W.; Parker, R. S.; Strano, M. S. Anal. Chem. 2005, 77, 75567562. (21) Sierra, J. F.; Galban, J.; Castillo, J. R. Anal. Chem. 1997, 69, 1471-1476. (22) D’Auria, S.; Di Cesare, N.; Gryczynski, Z.; Rossi, M.; Lakowicz, J. R. Biochem. Biophys. Res. Commun. 2000, 274, 727-731. (23) Maity, H.; Maity, N. C.; Jarori, G. K. J. Photochem. Photobiol., B 2000, 55, 20-26. (24) Ye, K.; Schultz, J. S. Anal. Chem. 2003, 75, 3451-3459. (25) Karnati, V. V.; Gao, X.; Gao, S.; Yang, W.; Ni, W.; Sankar, S.; Wang, B. Bioorg. Med. Chem. Lett. 2002, 12, 3373-3377. (26) Wilson, R.; Turner, A. P. F. Biosens. Bioelectron. 1992, 7, 165-185. (27) Park, S.; Boo, H.; Chung, T. D. Anal. Chim. Acta 2006, 556, 46-57. (28) Vassilyev, Y. B.; Khazova, O. A.; Nikolaeva, N. N. J. Electroanal. Chem. 1985, 196, 105-125. (29) Beden, B.; Largeaud, F.; Kokoh, K. B.; Lamy, C. Electrochim. Acta 1996, 41, 701-709. (30) Bae, I. T.; Yeager, E.; Xing, X.; Liu, C. C. J. Electroanal. Chem. 1991, 309, 131-145. (31) Park, S. J.; Chung, T. D.; Kim, H. C. Anal. Chem. 2003, 75, 3046-3049. (32) Yuan, J.; Wang, K.; Xia, X. Adv. Funct. Mater. 2005, 15, 803-809. (33) Song, Y.; Zhang, D.; Gao, W.; Xia, X. Chem.sEur. J. 2005, 11, 2177-2182.

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controlled reactions (i.e., the oxidation of interfering species). In addition to this improved selectivity, the mesoporous Pt and Pt nanotube arrays also retain sufficient sensitivity, even in the presence of chloride ions.31,32 Great effort has also been made to modify Pt surfaces by using other metals, such as Tl, Bi, Pb, and W,34-37 in the hope of improving the electrocatalytic activity and selectivity toward the electro-oxidation of glucose. These attempts have brought much attention to the electrocatalytic oxidation of glucose. Recently, Yeo and Johnson reported on the anodic responses to glucose on the copper-based bimetallic alloys of Ni, Fe, and Mn in alkaline solutions.38,39 Mn5Cu95 showed a large enhancement in sensitivity for the anodic detection of glucose. Mallouk and co-workers40 reported on their combinatorial methods for screening active alloy electrocatalysts for glucose electro-oxidation, showing that the most active compositions contained both Pt and Pb. Their electrochemical measurements showed that a Pt2Pb electrode catalyzed glucose oxidation not only at a much more negative potential than a pure Pt electrode, but also with a much higher current response. However, a fatal flaw with the Pt2Pb electrode is its poor resistance to the poisoning of chloride ions. The high catalytic activity of the Pt-Pb alloy was further confirmed by Sheu el al.41 They electrodeposited Pt-Pb alloy nanoparticles onto multiwalled carbon nanotubes and used this nanocomposite as a testing electrode for glucose oxidation. The Pt-Pb nanocomposite produced a much higher current density than either the Au or Pt counterpart in both neutral and alkaline solutions. Unfortunately, neither the amperometric detection of glucose nor selectivity data was available.41 On the basis of this progress made on Pt-based electrocatalysts in nonenzymatic glucose sensing, one of the most important clues brought to our attention is that a high active surface area of the electrode materials plays a key role in the electro-oxidation of glucose, and apparently, that different synthesis method of Ptbased materials greatly affects the electrochemical performance of nonenzymatic glucose sensors. One conventional method to produce bimetallic alloys (such as Pt-Pb) is by vacuum arcmelting of two pure metal targets, followed by long-time annealing.42 The resulting PtPb button electrodes possess shiny and mirrorlike surfaces, which are not ideal for the electrochemical detection of glucose. The bulk Pt2Pb electrode from Mallouk and co-workers was prepared by hydrogen gas reduction of mixed Pt and Pb inorganic precursor solutions at high temperature, followed by transfer of the catalyst ink onto a smooth glassy carbon substrate. High surface area was not reported on the resulting alloy electrodes.40 Abrun ˜a et al. reported two new approaches to fabricating ordered intermetallic PtPb nanoparticles. However, since both of the methods involve co-reduction of Pt and Pb (34) Sakamoto, M.; Takamura, K. Bioelectrochem. Bioenerg. 1982, 9, 571-582. (35) Xonoglou, N.; Moumtzis, I.; Kokkinidis, G. J. Electroanal. Chem. 1987, 237, 93-104. (36) Wittstock, G.; Strubing, A.; Szargan, R.; Werner, G. J. Electroanal. Chem. 1998, 444, 61-73. (37) Zhang, X.; Chan, K.-Y.; You, J.-K.; Lin, Z.-G.; Tseung, A. C. C. J. Electroanal. Chem. 1997, 430, 147-153. (38) Yeo, I. H.; Johnson, D. C. J. Electroanal. Chem. 2000, 484, 157-163. (39) Yeo, I. H.; Johnson, D. C. J. Electroanal. Chem. 2001, 495, 110-119. (40) Sun, Y.; Buck, H.; Mallouk, T. E. Anal. Chem. 2001, 73, 1599-1604. (41) Cui, H. F.; Ye, J. S.; Liu, X.; Zhang, W. D.; Sheu, F. S. Nanotechnology 2006, 17, 2334-2339. (42) Zhang, L. J.; Wang, Z. Y.; Xia, D. G. J. Alloys Compd. 2006, 426, 268-271.

precursors in organic solvent, the as-synthesized alloy particles need extra effort to be separated and further purified.43 Ordered intermetallic PtPb nanorods have also been fabricated via organic phase co-reduction of organic Pt and Pb precursors.44 Again, some complicated separation and purification steps need to be taken before further making use of these PtPb nanorods. In the present work, we report on a facile and reproducible hydrothermal method to directly grow three-dimensional (3D) nanoporous PtPb networks on Ti substrates for nonenzymatic electrochemical glucose sensing. This one-step hydrothermal reduction process involves only inorganic metal precursors and diluted reducing agent in the water phase; thus, no further separation and purification are required. The electrocatalytic activity of the synthesized PtPb network electrodes toward glucose oxidation is studied; the sensitivity and selectivity of the PtPb network electrodes with various bimetallic Pt-Pb compositions for the nonenzymatic electrochemical detection of glucose are investigated and compared with a smooth Pt electrode and a nanoporous Pt electrode. The synthesized nanoporous PtPb electrodes have strong and sensitive current responses to glucose. EXPERIMENTAL SECTION Materials. Titanium plates (1.25 cm × 0.8 cm × 0.5 mm, 99.2%) were purchased from Alfa Aesar. H2PtCl6‚6H2O, Pb(NO3)2, D-glucose, L-ascorbic acid, uric acid, 4-acetamidophenol, potassium phosphate dibasic, potassium phosphate monobasic, sodium chloride, and formaldehyde solution (ACS reagent, 37 wt % in water) were used as received from Aldrich. Glucose stock solutions were allowed to mutarotate overnight before use. Nanopure water (18.2 MΩ cm) was used to prepare all solutions. All other chemicals were analytical grade and were used as received from commercial sources. Synthesis of Nanoporous Pt and PtPb Networks. The hydrothermal method used in fabricating the nanoporous Pt and PtPb networks is similar to our previous report.45,46 Briefly, a Ti plate was washed in acetone, followed by Nanopure water, and then etched in a 18 wt % HCl solution at 85 °C for 10 min to remove the oxide layer and roughen the Ti surface. The etched Ti substrate was transferred into a Teflon-lined autoclave containing 10 mL of 2.8 g‚L-1 H2PtCl6‚6H2O and 0.1 M formaldehyde solution. In the case of the fabrication of the PtPb networks, this 10-mL mixture solution also contained Pb(NO3)2 at concentrations that were stoichiometric to Pt to synthesize PtPb networks with various bimetallic compositions, then the autoclave was sealed and heated at 180 °C for 10 h. After cooling to room temperature, the coated Ti plate was annealed in a tube furnace at 250 °C under argon for 2 h. After final rinsing with pure water, the Ti plate coated with nanoporous PtPb networks was ready for further surface analysis and electrochemical measurements. Instruments and Electrochemical Experiments. The surface morphology and composition of the synthesized samples were characterized using scanning electron microscopy (SEM) (JEOL (43) Alden, L. R.; Roychowdhury, C.; Matsumoto, F.; Han, D. K.; Zeldovich, V. B.; Abrun ˜a, H. D.; DiSalvo, F. J. Langmuir 2006, 22, 10465-10471. (44) Maksimuk, S.; Yang, S.; Peng, Z.; Yang, H. J. Am. Chem. Soc. 2007, 129, 8684-8685. (45) Peng, X.; Koczkur, K.; Nigro, S.; Chen, A. Chem. Commun. 2004, 24, 28722873. (46) Koczkur, K.; Yi, Q.; Chen, A. Adv. Mater. 2007, 19, 2648-2652.

JSM 5900LV) equipped with an energy-dispersive X-ray spectrometer (EDS) (Oxford Links ISIS). Surface elemental compositions based on quantitative EDS analysis were reported in average values of readings taken at five different spots on each sample surface. All electrochemical experiments were performed using an electrochemical workstation (CHI660B, CH instrument Inc.), connected with an in-house-built, three-electrode glass cell (50 mL). A platinum coil was used as the counter electrode and was flame-annealed before each experiment. Ag/AgCl (saturated KCl) was used as the reference electrode. Titanium substrates coated with nanoporous Pt or PtPb networks were used as the working electrodes. For comparison, a polycrystalline platinum wire was also used as a working electrode. Unless otherwise specified, all potentials reported in this paper are referred to the Ag/AgCl (saturated KCl) reference electrode. Amperometric measurements of glucose were carried out in a 0.1 M phosphate buffer solution containing 0.15 M NaCl (pH 7.4) at selected potentials. Currents at each glucose concentration were recorded after the transient steady states reached while the solution was stirred constantly. The geometric surface area of each electrode was used to calculate the current density. All solutions were deaerated with ultrapure argon (99.999%) before measurements, and argon was passed over the top of the solution during the experiments. All measurements were conducted at room temperature (22 ( 2 °C). RESULTS AND DISCUSSION Synthesis and Characterization of Nanoporous Pt and PtPb Networks. For convenience in discussion, we report the element name of each sample followed by a value in parentheses, in which the percentage represents the normalized atomic ratio of the Pb content determined by quantitative EDS analysis. Figure 1 presents four SEM images of the synthesized nanoporous Pt and Pt-Pb materials with different Pt/Pb atomic ratios: (a) PtPb (0%), (b) Pt-Pb (30%), (c) Pt-Pb (50%), and (d) Pt-Pb (70%). It is apparent that the surface of titanium substrates is completely covered by the catalysts, and all the sample surfaces consist of irregular pores ranging from tens of nanometers to several micrometers in diameter. From the SEM micrographs, a unique trend in the size of particles is that as the Pb content is increased, particle size is increased, as well, from tens of nanometers (0% Pb) to hundreds of nanometers (70% Pb). Because the amount of the Pt in each Pt-Pb sample was the same, the proportional increase in the diameter of the networks, with the trend of PtPb (0%) < Pt-Pb (30%) < Pt-Pb (50%) < Pt-Pb (70%), is attributable to the addition of Pb to the Pt nanostructures. In all cases, the nanoporous networks were uniformly formed over the entire surface of the substrates, and the mass and thickness of coatings were controlled by the overall added reactants in the autoclave. In addition, no carbon and oxygen peaks were observed in the EDS spectra of the four nanoporous Pt and Pt-Pb samples, showing that the synthesized nanoporous networks were free of surface impurities. Titanium plate was used as the substrate because of its good electrical conductivity, no reported catalytic activity by itself, and its reasonable cost. We also grew the nanoporous networks on Teflon and graphite substrates, indicating that the synthesis of nanoporous PtPb networks is independent of substrate materials. The hydrogen adsorption and desorption on the Pt surface in an acidic media is an effective technique for Analytical Chemistry, Vol. 80, No. 4, February 15, 2008

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Figure 1. SEM images of the as-synthesized nanoporous materials: (a) Pt-Pb (0%), (b) Pt-Pb (30%), (c) Pt-Pb (50%), and (d) Pt-Pb (70%).

determining the active surface area of Pt-based electrodes.47,48 Figure 2 presents the CVs of two Pt electrodes (nanoporous Pt and polycrystalline Pt wire) and the Pt-Pb (50%) electrode in a 0.1 M H2SO4 solution at a potential scan rate of 20 mV/s. For comparison, the CV curve of the polycrystalline Pt electrode was magnified 10-fold. In the case of the two Pt electrodes, two pairs of reversible peaks are observed at approximately -0.11 and +0.025 V, corresponding to hydrogen adsorption/desorption on the electrode surface. It is well-known that the active surface areas of these Pt-containing electrodes can be determined by integrating the charge associated with the hydrogen adsorption or desorption peaks. It is assumed that the double layer capacitance is constant across the entire investigated potential range; the integrated hydrogen adsorption/desorption charge (QH) of the polycrystalline Pt wire was 0.21 mC/cm2. For the nanoporous Pt, the integrated QH is 6.42 mC/cm2. QH represents the number of the Pt sites available for hydrogen adsorption/desorption; thus, the active surface area of the nanoporous Pt networks is over 30 times larger than that of the polycrystalline Pt wire. As for the nanoporous Pt-Pb (50%) electrode, the hydrogen adsorption/desorption on the electrode surface is completely suppressed due to the modification of Pb. This is consistent with previous study.49Although

the active surface area of the Pt-Pb electrode cannot be directly determined using the hydrogen adsorption/desorption method, on the basis of the observation of the SEM images, the nanoporous

(47) Chen, A.; La Russa, D. J.; Miller, B. Langmuir 2004, 20, 9695-9702. (48) Paulus, U. A.; Wokaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Markovic, N. M.; Ross, P. N. Electrochim. Acta 2002, 47, 3787-3798.

(49) Casado-Rivera, E.; Volpe, D. J.; Alden, L.; Lind, C.; Downie, C.; Va´zquezAlvarez, T.; Angelo, A. C. D.; DiSalvo, F. J.; Abruna, H. D. J. Am. Chem. Soc. 2004, 126, 4043-4049.

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Figure 2. Cyclic voltammograms of selected electrodes in a 0.1 M H2SO4 solution at a potential scan rate of 20 mV/s: nanoporous Pt (red); Pt-Pb (50%) (blue); and polycrystalline Pt wire (black, magnified 10-fold).

Figure 3. Cyclic voltammograms (CV) of the different electrodes in (a) 0.1 M phosphate buffer (pH 7.4) and (b) 0.1 M phosphate buffer (pH 7.4) + 10 mM glucose at a potential scan rate of 10 mV s-1. Insets are the magnified CVs of the corresponding Pt wire electrode.

Pt-Pb networks possess a large active surface area comparable to that of the nanoporous Pt networks. Electro-oxidation of Glucose in Neutral Media. Voltammetric methods were used to investigate and compare the catalytic activities of the as-synthesized electrodes. Figure 3a presents cyclic voltammograms (CVs) of a smooth Pt wire electrode, the assynthesized nanoporous Pt electrode and nanoporous Pt-Pb (50%) bimetallic electrode in a phosphate buffer in the absence of glucose. The voltammogram of the smooth Pt wire in the phosphate buffer is characterized by its well-known hydrogen adsorption/desorption peaks at negative potentials, a flat double layer region at intermediate potentials, and platinum oxide formation and reduction peaks at positive potentials. The nanoporous Pt electrode behaves similarly, except with greatly enhanced current densities. In contrast, the hydrogen adsorption/desorption peaks are completely suppressed at the nanoporous Pt-Pb (50%) electrode. The only anodic peak (corresponding to metal oxide formation) and the only cathodic peak (corresponding to metal oxide reduction) show up at +350 and -20 mV, respectively. These results are in good agreement with the previous findings of Mallouk and co-workers.40 Figure 3b presents three CVs recorded in 0.1 M phosphate buffer (pH 7.4) + 10 mM glucose at a potential scan rate of 10 mV/s. In the case of the Pt wire, multiple anodic peaks attributed to the oxidation of glucose and resulting intermediates are observed in the positive scan. In the negative scan, the oxidation

of glucose is suppressed in the high potential range because of the presence of surface oxide, which is reduced at a potential around +120 mV. With the reduction of surface Pt oxide, more and more surface-active sites are available for the oxidation of glucose again, resulting in large and broad anodic peaks in the potential range from 0.0 to -500 mV. These results can be further confirmed and explained by a well-accepted mechanism of glucose oxidation on a Pt electrode in neutral media.50,51 As for the nanoporous Pt electrode, its voltammetric behavior, essentially similar to the Pt wire, produces a substantially enhanced current response due to the significantly increased electroactive surface area of the nanoporous networks. In contrast, the voltammogram of the nanoporous Pt-Pb (50%) electrode in glucose solution is different from that of the Pt electrodes. The onset potential of glucose oxidation is more negative than that at Pt, and the anodic peak is very broad, implying a mechanistically complex oxidation process. In addition, the peak current densities of the nanoporous Pt-Pb (50%) are substantially higher than that of both Pt electrodes, indicating that the nanoporous Pt-Pb (50%) exhibits the best catalytic performance among these three electrodes. The nanoporous Pt-Pb (50%) electrode gives the highest catalytic activity toward glucose oxidation in a neutral phosphate buffer; thus, further challenge on its performance toward the tolerance of chloride ions is necessary to ensure its possible application in physiological environments. Figure 4 presents the CVs of the nanoporous Pt-Pb (50%) electrode recorded in (a) 0.1 M phosphate buffer (pH 7.4) + x mM glucose and (b) 0.1 M phosphate buffer (pH 7.4) + 0.15 M NaCl + x mM glucose, where x was varied in increments from 0 to 20. For the convenience of comparison, only the positive-going portion of each voltammogram is lined up. Two obvious, yet very important, differences between voltammograms a and b can be distinguished. First, in Figure 4a, the curve shape of all the anodic current peaks at the potentials around 0 mV looks less symmetric than that of the anodic current peaks at the potentials around -80 mV (Figure 4b). Indeed, in the case without chloride ions, there is a shoulderlike peak sitting at around -200 mV, just beside the large current peak at 0 mV. In the presence of chloride ions, the shoulderlike current peak disappears. Second, at each glucose concentration, the peak values of the current density at both 0 and +300 mV in Figure 4a are relatively higher than those at -80 and +400 mV in Figure 4b. These phenomena are both considered to be the major influence of chloride ions on the nanoporous Pt-Pb (50%) electrode. On the basis of the well-accepted mechanism of electrooxidation of glucose on Pt electrodes in neutral media,50,51 we propose the following hypothesis of a competitive adsorption model. In the case without chloride ions, glucose molecules would preferentially undergo electrosorption on the PtPb bimetallic surface, forming a layer of glucose intermediates (such as the enediol type) which can be easily oxidized. This process gives rise to the shoulderlike current peak (at ca. -200 mV). As the potential scans to a more positive value (i.e., ca. 0 mV), the adsorbed glucose intermediates are oxidized on the electrode surface, resulting in the large current peak. When the potential goes beyond this point, the accumulated intermediates block the (50) Ernst, X.; Heitbaum, J.; Hamann, C. H. J. Electroanal. Chem. 1979, 100, 173-183. (51) Ernst, X.; Heitbaum, J.; Hamann, C. H. Ber. Bunsenges. Phys. Chem. 1980, 84, 50-55.

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Figure 5. Chronoamperometry curves of Pt-Pb (50%) electrode in a solution of 0.1 M phosphate buffer (pH 7.4) + 0.15 M NaCl with successive addition of 1 mM glucose (1-16 mM). Potential was kept constant at (a) -80 and (b) +400 mV. Insets are plots of plateau currents versus concentrations of added glucose.

Figure 4. Positive-going portions of cyclic voltammograms (CV) of nanoporous Pt-Pb (50%) electrode at a potential scan rate of 10 mV s-1 in 0.1 M phosphate buffer (pH 7.4) solution containing glucose (at concentrations of 0, 1, 2, 5, 10, 15, 20 mM) and (a) without the presence of NaCl and (b) with the presence of 0.15 M NaCl.

surface active sites of the PtPb electrode, causing the sudden drop of current. At potentials more positive than +300 mV, the adsorbed intermediates are oxidized, forming products such as gluconolactone or gluconic acid.50,51 Further increasing the electrode potential, metal oxide formation occurs, and the surface becomes less active, causing the current response to decrease again. In contrast, when large amounts of chloride ions are present, this strong surface poisoning species competes with the adsorbing glucose intermediates, adsorbed intermediate products, or both. Cl- preferentially blocks part of the surface active sites, causing less chance of reactive species approaching the electrode surface. Hence, at each glucose concentration, the presence of chloride ions not only eliminates the shoulderlike current peak originating from the electroadsorbed glucose intermediates but also diminishes peak current densities by oxidation of fewer amounts of the surface reactive species, as compared to the chloride-free media. Nevertheless, the nanoporous Pt-Pb (50%) electrode still possesses a rather strong current response in the presence of physiological levels of chloride ions, and no downgrade performance of Pt-Pb electrode caused by long time exposure to chloride solutions is observed. Such uncompromising current response to glucose in the presence of chloride ions was also found with mesoporous Pt31 and ordered Pt nanotubes arrays electrodes.32 The mechanistical explanation of why these Pt-based 1002 Analytical Chemistry, Vol. 80, No. 4, February 15, 2008

electrodes with extremely high surface area can offer much better sensitivities than those with a smooth surface is still unclear. Upon the basis of our aforementioned hypothesis, the large active surface area may be the key factor in the nanoporous PtPb networks’ surviving chloride poisoning. This may also give a possible explanation of the poor resistance to chloride poisoning reported by Mallouk et al. on their Pt2Pb electrode.40 In addition, the most obvious common feature in Figure 4a and b is that there are always certain potential ranges in which the incrementally increasing concentrations of glucose lead to a proportional increase in current densities. Specifically, when chloride ions are absent, anodic currents at each glucose concentration peak at the potentials around 0 and +300 mV, respectively; whereas in the case with chloride ions, these two peak-current potentials slightly migrate to around -80 and +400 mV, respectively. In both cases, the two potential ranges are apparently associated with the events of glucose electrosorption or oxidation, thus allowing us to choose the potentials to be used in our amperometric sensing. Amperometric Detection of Glucose under Physiological Conditions. For amperometric sensing applications, electrodes are generally evaluated by measuring current response at a fixed potential and within a certain time after adding the analyte and possible interfering species.31,32,40 To duplicate the physiological conditions required by most practical glucose sensor applications, our nanoporous Pt-Pb electrodes were further tested exclusively in a 0.1 M phosphate-buffered saline (PBS) solution containing 0.15 M NaCl. Figure 5 compares the amperometric responses of the nanoporous Pt-Pb (50%) electrode at the aforementioned -80 and +400 mV with successive additions to the glucose concentration in increments from 0 to 16 mM. In both cases, current signals increase rapidly and sensitively after each addition of glucose to the stirred solution. However, each increased amount of current

Figure 6. Chronoamperometry curves of the different electrodes in 0.1 M phosphate buffer (pH 7.4) containing 0.15 M NaCl with the successive addition of 1 mM glucose (1-16 mM) at -80 mV.

density at -80 mV tends to drop a little after each addition of 1 mM glucose, whereas the current response at +400 mV increases linearly with the incremental glucose concentration. This feature is more obviously demonstrated in the inset plots of plateau current densities versus glucose concentration (Figure 5a and b); it can be reasonably interpreted by our aforementioned competitive adsorption model. Because glucose cannot be completely oxidized at -80 mV, some intermediates resulting from the glucose oxidation accumulated on the electrode surface during the first several additions. The accumulation of the intermediates on the Pt-Pb surface blocks some active surface sites, thus resulting in the nonlinear chronoamperometric response to glucose concentration. While the electrode potential was held at the high electrode potential +400 mV, the reactive species of glucose (such as the glucose intermediates and/or intermediate products) could be continuously oxidized on the Pt-Pb surface; a linear chronoamperometric response to glucose concentration at +400 mV was thus observed. In the inset of Figure 5b, the linear dependence of current response with glucose concentration at +400 mV gives rise to a sensitivity of 10.8 µA cm-2 mM-1. Although the current densities at -80 mV do not respond linearly with the added glucose, they are much higher than the current densities at +400 mV for each glucose concentration, implying a better sensitivity in the glucose range of 1-16 mM. To the best of our knowledge, the amperometric responses of our nanoporous Pt-Pb (50%) electrode at both -80 and +400 mV are the highest observed so far in the literature for Pt-Pb bimetallic electrodes in neutral media under physiological conditions.40,41 Since the nanoporous Pt-Pb (50%) electrode possesses a much better sensitivity at -80 than at +400 mV, we further investigated the amperometric detection of glucose at -80 mV on nanoporous Pt-Pb (0%), Pt-Pb (30%), and Pt-Pb (70%) electrodes to determine the optimal electrode materials for glucose sensing. As shown in Figure 6, the smooth Pt wire electrode shows the worst sensitivity among all the tested electrodes toward the successive addition of glucose in 0.1 M PBS containing 0.15 M NaCl due to the poisoning by surface-adsorbed intermediates (dehydrogenation type) and chloride ions. The nanoporous Pt electrode gives much better current responses than Pt wire by virtue of its significantly increased surface area. However, at such

Figure 7. (a) Cyclic voltammograms of a Pt-Pb (50%) electrode in a 0.1 M phosphate buffer (pH 7.4) containing 0.15 M NaCl, and 5 mM ascorbic acid (dotted line) and 10 mM glucose (solid line), at a potential scan rate of 10 mV s-1. (b) Chronoamperometry curve of Pt-Pb (50%) electrode in a 0.1 M phosphate buffer (pH 7.4) containing 0.15 M NaCl with successive addition of 0.02 mM UA, 0.1 mM AP, 0.1 mM AA, and 1 mM glucose at 60 s intervals with a potential set constant at -80 mV.

negative potential, its amperometric response toward the successive addition of glucose still suffers from the accumulated surfaceadsorbed intermediates. The other three Pb-containing electrodes all possess much improved sensitivity toward the added glucose as compared to the nanoporous Pt electrode. In particular, their amperometric sensitivities increase in the following order: nanoporous Pt < Pt-Pb (30%) < Pt-Pb (70%) < Pt-Pb (50%). The bimetallic atomic ratios of the PtPb networks are optimal at 50% for amperometric glucose detection. We speculate that more than 50% Pt on the surface would increase the amount of accumulated surface-poisoning intermediates, whereas more than 50% Pb in surface composition would decrease the overall electroactive surface area due to the enlarged particle sizes, thus rendering compromised amperometric sensitivity toward the successive addition of glucose in PBS containing NaCl. As discussed in the introduction, the avoidance of endogenous interfering species is a big challenge in nonenzymatic glucose detection because a few of the structurally similar organic substances (for instance, UA, AP and AA) are also simultaneously oxidized along with glucose at the electrode surface and, hence, give interfering electrochemical signals. Figure 7a presents the CVs of the nanoporous Pt-Pb (50%) electrode recorded in a glucose solution and in a solution containing 5 mM ascorbic acid, which is one of the most serious interfering agents. The onset of the ascorbic acid oxidation is at around -50 mV; the peak potential of glucose oxidation is -80 mV, which is 30 mV lower than the onset potential of ascorbic acid oxidation. This indicates that sensing glucose at -80 mV on the nanoporous Pt-Pb (50%) electrode would successfully avoid the interfering current signal contributed from ascorbic acid. Analytical Chemistry, Vol. 80, No. 4, February 15, 2008

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Figure 7b presents the selectivity testing results of the nanoporous Pt-Pb (50%) electrode with successive additions of UA, AP, AA, and glucose in 0.1 M PBS containing 0.15 M NaCl. Interestingly, the nanoporous PtPb electrode produces negligible current signals for all three common interfering agents, yet still gives out significant responses to incremental glucose concentrations. However, taking the current responses in Figure 5a (glucose solution without interferences) for comparison, there has been an ∼5% decrease in sensitivity of each current response of the added glucose with the presence of the three interfering species. Another control experiment was also conducted on the nanoporous Pt-Pb (50%) electrode with successive additions of UA, AP, AA, and glucose at +400 mV, in which the current responds linearly with the glucose concentration, as seen in Figure 5b. As predicted in the voltammogram of Figure 7a at +400 mV, this time the electrode did not survive the impact of interfering species due to a strong current signal produced from ascorbic acid. CONCLUSIONS We have successfully grown three-dimensional nanoporous PtPb bimetallic networks with different Pt/Pb ratios directly on Ti substrates using a one-step hydrothermal method. The facile approach described in this study not only allows controllable variation of the bimetallic Pt-Pb compositions to achieve optimum performance, but also eliminates complicated separation and

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purification steps. The as-synthesized nanoporous PtPb electrodes have strong and sensitive current responses to glucose. They are also highly resistant toward poisoning by chloride ions and are capable of sensing glucose amperometrically at a very negative potential, -80 mV (Ag/AgCl), where the interference from the oxidation of common interfering species such as AA, AP, and UA is effectively avoided. Ultrahigh electroactive surface area combined with the optimal PtPb composition are considered to be the key factors responsible for the excellent performance of the nanoporous Pt-Pb (50%) electrodes. Further investigation of the mechanism of glucose oxidation on nanoporous PtPb electrodes at the molecular level as well as their practical applications as glucose sensors is in progress. ACKNOWLEDGMENT This work was supported by a discovery grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). A. Chen acknowledges the Canada Foundation for Innovation (CFI) and NSERC for a Canada Research Chair Award.

Received for review August 23, 2007. Accepted November 24, 2007. AC701790Z