Detection of Glucose Based on Bimetallic PtCu Nanochains Modified

Apr 21, 2013 - Li Wang , Xingping Lu , Cunjin Wen , Yingzhen Xie , Longfei Miao , Shouhui Chen , Hongbo Li ... Qiaofang Shi , Guowang Diao , Shaolin M...
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Detection of Glucose Based on Bimetallic PtCu Nanochains Modified Electrodes Xia Cao,†,‡ Ning Wang,*,§ Shu Jia,§ and Yuanhua Shao*,† †

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China School of Biochemical and Pharmaceutical Sciences, Capital Medical University, Beijing 100069, China § School of Chemistry and Environment, Beijing University of Aeronautics and Astronautics, Beijing 100191, China ‡

S Supporting Information *

ABSTRACT: A series of novel bimetallic PtCu nanochains have been synthesized through a water-based mild chemical route, and their compositions (PtxCu1−x) can be conveniently tuned at the mesoscopic scale by a facile dealloying process. These nanomaterials have been characterized by transmission electron microscope (TEM), high-resolution TEM (HRTEM), X-ray powder diffraction (XRD), and elemental analysis. They have different compositions (Pt88Cu12, Pt75Cu25, and Pt50Cu50) but have similar morphology. Electrochemical activity of these nanomaterials is compared to Pt and nanochains of Pt when they are chemically modified onto a glassy carbon electrode. Electrochemical measurements demonstrate that the sensors made by these PtCu nanomaterials are very sensitive and selective for glucose detection due to the wiring of dispersed crystals, porous nanostructure, clean surface, and synergetic electronic effects of the alloyed atoms. Among them, the modified electrode made of Pt75Cu25 shows the best performance. The superior catalytic activity and selectivity make nanomaterials, via the green synthesis, very promising for applications in direct biosensing of glucose.

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related to Pt.4 The mechanism of electro-oxidation of glucose has been investigated intensively on bare electrodes, and three main drawbacks have been unveiled: (1) The kinetics of electro-oxidation of glucose on a smooth Pt surface is too slow and therefore sensitivity is bad; (2) some endogenous species can be oxidized in the potential range similar to the oxidation of glucose and resulting in poor selectivity; and (3) the electroactivity of Pt electrode is strongly affected by adsorption of glucose oxidation intermediates and chloride. To alleviate these problems, the nanomaterials related to Pt have played a significant role. For example, mesoporous Pt and highly order Pt nanotube arrays have been employed to increase the active areas, which facilitates the kinetics of electro-oxidation of glucose in some extent and results in better sensitivity and selectivity.5 Another alternative is bimetallic nanoparticles (NPs), which are currently drawing much attention because of their intriguing catalytic behaviors with respect to that seen with monometallic systems.6 For example, as compared to single metal PtNPs, the PtxM1−x alloys present distinct synergetic characteristics, where Pt shows the specific electrocatalytic activity, and the other metal takes on either a biocompatibility or helps to increase the active sites and enhances the long-term stability.7 As a result, these bimetallic Pt alloys generally provide

iabetes is one of the leading diseases causing death and disability. It is a group of metabolic disorders in which the individual has a high concentration of glucose in their blood. The diagnosis is simply based on the following: if one’s plasma glucose concentration is ≥7 mM or ≥11.1 mM 2 h after a 75 g glucose drink, he (she) is a diabetes patient. There are about 347 million suffering from this disease worldwide according to the WHO in 2012.1 The number of patients diagnosed with diabetes in developing countries has accelerated in recent years. The treatment and management of diabetes is really challenging. Because the complications caused by this disease can be significantly reduced by tight control of blood glucose, the diagnosis and management of diabetes urgently require a fast, easy, sensitive, and selective approach to monitoring blood glucose level daily. The key technology involved in personal care of diabetes patients is biosensors. At present, most of the commercialized biosensors are related to electrochemical enzyme sensors (from the first to third generations). Although there have been already tremendous benefits from the use of those enzyme sensors, several drawbacks still remain, and they are mostly related to the enzymes employed, such as insufficient long-term stability and unsatisfactory reproducibility.2 There have been enormous efforts to try to overcome these problems in the past few decades.3 One of the appealing approaches is the enzymeless electrochemical sensor, which mainly depends upon the development of electrocatalysis, especially on the development and application of materials © 2013 American Chemical Society

Received: January 29, 2013 Accepted: April 20, 2013 Published: April 21, 2013 5040

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selectivity make this type of material very promising for applications in direct biosensing of glucose.

rapid response, good stability, and high catalytic efficiency toward the targeted species when employed as electrode materials, mainly due to the electron coupling and ligand effects between the different components.8 In the past few years, motivated by the belief that novel catalytic properties of bimetallic nanomaterials can be tuned by their morphology, size, and composition, the PtxM1−x alloy nanocrystals containing inexpensive and readily available late3d transition metals have been extensively investigated.6−9 Until now, numerous studies have been reported describing wet chemical methods such as coprecipitation, impregnation, reverse micelles, and polyol reduction for synthesizing PtCu nanomaterials.8a,10 Although they are effective in controlling the morphology and composition, these synthetic methods are generally carried out within high melting point organic solvents and with the help of vast quantity of surfactants. The surface adsorption of the long carbon chain hydrocarbons often results in contaminated crystal facets. The surface contamination frequently decreases their active sites and deteriorates the catalytic activity, which hinders their further catalytic or sensing applications.10c,11 The facile chemical synthesis of Pt containing alloys with novel nanostructures and a clean surface has so far been rather rare.10b,12 The current synthesis challenges are primarily associated with inducing anisotropic growth and ordered assembly while controlling multielement composition. In this sense, simultaneously tailoring the shape, composition, as well as the way that they are assembled at the macroscopic level in water-based solution still remains one of the most important issues. There have been several reports regarding enzymeless glucose sensors based on PtxM1−x alloy nanomaterials.13 The performances of those sensors were improved greatly. For instance, the catalytic glucose reaction on a Pt2Pb electrode showed not only at a much more negative potential than that on a pure Pt electrode, but also with a much higher current response.13e An enzyme-free electrochemical glucose sensor based on three-dimensional PtPb network directly grown on Ti substrates provided a sensitive and selective way for detection of glucose.13f Nanoporous PtCu alloys with hollow ligaments were also employed for glucose sensing.14 In such case, the foreigner element (M) may also enhance the catalytic activity of Pt via electronic effect or ligand or bifunctional effect. For example, the CuO could also be formed electrochemically at more negative potentials as in PtCu alloy as compared to Pt, which may help in the oxidation of intermediates formed via decomposition of glucose or oxidize glucose directly. These works at least point out an interesting direction for the development of enzymeless glucose electrochemical sensor. In this work, novel and composition tunable PtxCu1−x nanomaterials have been synthesized through a mild chemical route in water coupled with a gentle dealloying process. The synthetic processes are featured by a one-pot procedure, which combines sequential formation of precursor nucleation, selfassembly, and morphology shaping under mild solution conditions. The synthesized nanochains of PtxCu1−x have been modified onto a glassy carbon electrode (GCE). Their electrocatalytic behaviors have also been studied in detail. Electrochemical experimental results demonstrate that the PtxCu1−x nanochains modified electrode possesses sensitive and selective amperometric responses to glucose due to the synergetic effect between the Cu and Pt atoms, especially the electrode made by Pt75Cu25. The superior catalytic activity and

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EXPERIMENTAL SECTION Reagents. K2PtCl6, NiCl2·6H2O, tributyl phosphate, NaBH4, and CuCl2·2H2O were purchased from Sigma-Aldrich and used as received. All other reagents, unless otherwise stated, were of analytical grade and were purchase from Sinopharm Chemical Reagent Beijing Co. Ltd. and used without further purification. Deionized water from a Milli-Q system (18.2 MΩ cm at 25 °C) was used throughout the experiments. Synthesis of PtCu Nanochains. In the synthesis of PtCu nanomaterials, 2 mL of tributyl phosphate and 36 mg of NiCl2·6H2O were dissolved in 43 mL of deionized water. The reduction potential of Ni2+/Ni (−0.257 versus NHE) is much more negative than that of PtCl62−/Pt (+0.75 V versus NHE), Cu2+/Cu (+0.153 versus NHE), and even that of H+/H2; the reduction/deposition of Ni with Cu/Pt/CuPt is not preferred. The solution was sonicated for 15 min (40 kHz, 500 W, KQ500DE, Jiangsu Jintan HanKang Electronic Co.) while purged with N2, then 10 mg of NaBH4 dissolved in 20 mL of H2O was added drop by drop under intense stirring. Immediately after the addition of NaBH4, a mixture of 20 mL of K2PtCl6 (6 mM) and CuCl2·2H2O (6 mM) was added. After sonication for 30 min, the solution was kept at 60 °C for 2 h under constant stirring. All of the steps were carried out under nitrogen protection. The resulted solid powders were collected by centrifugation, and washed with water and ethanol six times, respectively. In fact, only amorphous porous NiB, instead of Ni, can be obtained before the addition of Au/Pt precursor. The amorphous porous NiB helps to form the nestle-like Pt, Cu, or CuPt nanochains, but it is easy to dissolve and has not been found deposited on the synthesized Pt, Cu, or alloyed AuPt chains. The obtained powders were then stored in ethanol for further characterization. Pt nanochains were also synthesized for comparison via a similar route without the addition of Cu precursor. PtCu alloys with different atomic ratio can be obtained by a mild dealloying process in 1:1 (v/v) HNO3 aqueous solution. Fabrication of PtCu Nanochains Modified Electrodes. To modify the electrode surface, an aliquot of 5 μL of 5 mg/mL PtCu nanochains suspension in ethanol was dropped onto a GCE electrode surface (d = 3 mm). After drying in air, 5 μL of Nafion solution (1 wt % in ethanol) was cast on the layer of PtCu nanochains to entrap PtCu nanochains. The as-prepared PtCu nanochains modified electrodes (denoted as PtxCu1−xNCE) were immersed in water for 1 h to wet the Nafion layer thoroughly before use. In addition, Pt wire (Pt) and Pt nanochains modified electrodes were made by the same way and used for comparison. Apparatus and Electrochemical Measurements. The morphologies and structures were characterized by a Hitachi S4800 cold field emission scanning electron microscope (CFESEM) and JEOL JEM-2100F transmission electron microscope (TEM) and high-resolution TEM (HRTEM) operated at 200 kV. The TEM samples were prepared by placing one drop of ethanol solution (containing nanoparticles) onto a carboncoated Ni grid and allowing them to dry in air. Elemental composition data were obtained by EDAX equipped within the JEOL JEM-2100F TEM. X-ray powder diffraction (XRD) pattern was collected by a Rigaku X-ray diffractometer (Rigaku Goniometer PMG-A2, CN2155D2, wavelength = 0.15147 nm) 5041

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and Pt, demonstrating the alloying nature of the nanochains.13−15 The energy dispersive spectroscopy (EDS) results show that the atomic percentage of Pt is about 50% (Pt50Cu50, see Figure 1C), which match well with the results of the SAED and HRTEM. Size distribution of the synthesized PtCu nanochains (Figure 1D) gives a diameter of approximately 4.8 nm. By dealloying the Pt50Cu50 nanochains in 1:1 (v/v) HNO3 aqueous solution, the composition of the bimetallic PtCu alloy can be conveniently tuned within a wide range (Figure 2A).

with Cu Kα radiation. The Brunauer−Emmett−Teller (BET) specific surface areas were determined on a constant volume adsorption apparatus (CHEMBET-3000) by the N2-BET method at liquid nitrogen temperature. All of the electrochemical experiments were carried out at room temperature (23 ± 2 °C) using a single compartment, three-electrode cell with the modified electrode as working electrode, and a saturated calomel electrode (SCE) and a Pt wire as respective reference and auxiliary electrodes. All potentials were measured and reported vs the SCE. The cyclic voltammetry measurements (CV) and chronoamperometry were performed on a model 660D electrochemical workstation (CH Instruments, Austin, TX).

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RESULTS AND DISCUSSION Characterization of Synthesized PtCu Nanochains. In Figure 1A it can be seen that the synthesized PtCu nanochains

Figure 2. Typical information of the dealloyed PtCu nanochains: atomic ratio of Cu in the PtCu alloy of the PtCu nanochains versus acidification time (A), and typical XRD patterns (B) taken from sample acidified for 1, 4, and 20 h. The inset in (A) is the morphology of the dealloyed CuPt nanochains, which remain almost unchanged as time varies.

Although the longer is the dealloying time, the higher content of Pt is in the final PtCu alloy, there is always some fixed copper content even after strong acid dissolution (∼12%). This value should be the most stable percentage of copper in the PtCu alloys (i.e., Pt88Cu12). A consistent morphology for various compositions for the dealloyed PtCu nanochains emerges combining the XRD structural data (see the inset in Figure 2A). As shown in Figure 2B, all of the XRD patterns show a fcc structure with peak positions intermediate between those of Cu and Pt. According to Vegard’s law,15 the peak shifts indicate that the crystal structures are the substitution solid solution of Pt50Cu50, Pt75Cu25 (dealloyed for 4 h), and Pt88Cu12 (dealloyed for 20 h) with lattice constants of 3.76, 3.85, and 3.89 Å, respectively. The PtxCu1−x nanochains are chosen as the catalysts with different compositions for further study. When deposited, porous film made of ultrathin nanochains on the electrode surface can be clearly seen, as shown in Figure S1 in the Supporting Information. The porous film provides 3-D networks consisting of the chain-like nanowires and the interconnected nanopores. As a result, a larger contact area

Figure 1. (A) TEM image of the PtCu nanochains with average diameter of about 5 nm; (B) HRTEM image of one segment of PtCu nanochains and the SAED patterns (inset); (C) EDS spectrum; and (D) the size distribution of the PtCu nanochains.

are highly uniform on a large scale. Their average size is measured as about 5 nm in thickness and hundreds of nanometers in length. The lattice resolved HRTEM image (Figure 1B) shows that the PtCu nanochains are composed of well-crystallized nanoparticles of several nanometers. The visible lattice fringes of 0.21 nm can be ascribed to the expected d-spacing of the (111) plane of the PtCu alloy. Selected area electron diffraction (SAED) analyses show a typical polycrystalline fcc structure (inset of Figure 1B). The lattice parameter (0.38 ± 0.03 nm) lies between those of Cu 5042

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between sensing materials and sensed species is provided, which should promote the transportation and accessibility of target molecules onto the electrode surface and facilitate the electron transfer processes. Electrochemical Behaviors of the PtCu Nanochains Modified Electrodes. To evaluate the electrochemical behaviors of the PtCu nanochains with different compositions, their electroactive surface areas (ECSA) are estimated by counting the surface sites for hydrogen adsorption using cyclic voltammetry. Figure 3 presents the cyclic voltammograms

Figure 3. Cyclic voltammograms of the PtxCu1−xNCE electrodes and the Pt wire electrode (from outer to inner) in 0.5 M H2SO4 solution.

Figure 4. Cyclic voltammograms of the PtxCu1−xNCE electrodes and the Pt wire electrode (from outer to inner) in (A) 0.1 M PBS + 0.15 M NaCl and (B) 0.1 M PBS + 0.15 M NaCl + 10 mM glucose. Scan rate: 10 mV s−1.

obtained by the PtCu nanochains modified electrodes (PtxCu1−xNCE) in a 0.5 M H2SO4 solution at a scan rate of 10 mV/s. The integrated area under the desorption peak in the cyclic voltammogram represents the total charge relating to H+ adsorption/desorption (QH) and can be used to estimate their ECSA.16 It is found that the area of hydrogen adsorption/ desorption peak decreases in the order of Pt88Cu12NCE (8.39 mC/cm2) > Pt75Cu25NCE (8.21 mC/cm2) > Pt50Cu50NCE (7.02 mC/cm2) > PtNCE (6.91 mC/cm2) > polycrystalline Pt wire (Pt) (0.21 mC/cm2),17 implying that the ratio of Cu to Pt is a crucial factor affecting the electrochemical activity of bimetallic nanocatalysts. The ECSA of the PtxCu1−xNCE electrodes are all larger than the PtNCE electrode and more than 33 times larger than that of the polycrystalline Pt wire. The dissolution process of the PtCu alloy can be expected to increase both the available surface area and the active sites for the possible electrocatalytic reaction. In addition, the redox peak near 0.6 V in Figure 3 is probably caused by the redox reaction of PtO, which shows a classical Pt electrode behavior.11 The electrocatalytic properties of the PtxCu1−xNCE electrodes for glucose are investigated using cyclic voltammetry. To duplicate the physiological conditions required by most practical glucose sensor applications, the electrodes are all tested in 0.1 M phosphate-buffered saline (PBS) solution containing 0.15 M NaCl (0.1 M PBS/0.15 M NaCl, pH 7.4). Figure 4A shows the CVs of the PtxCu1−xNCE electrodes in the absence of glucose. The voltammograms obtained at all investigated electrodes are similar. The anodic peak corresponds to metal oxide formation at positive potentials, and the cathodic peak corresponds to metal oxide reduction at reversal scans. These results are in good agreement with those of the previous findings.9f,13c The PtxCu1−xNCE electrodes show greatly enhanced current densities, among which the Pt75Cu25NC electrode gives the largest current. Figure 4B

presents the CVs of the PtxCu1−xNCE electrodes in 0.1 M PBS/0.15 M NaCl containing 10 mM glucose. Two anodic peaks attributed to the oxidation of glucose and resulting intermediates are observed for the positive scan. During the cathodic potential scan, the oxidation of glucose is suppressed in the high potential range due to the presence of surface oxide. With the reduction of Pt oxide, more surface-active sites are available for the oxidation of glucose, resulting in a large and broad oxidation peak. Another possibility is that the CuO is formed electrochemically at more negative potentials as compared to Pt and oxidizes the oxidation of intermediates formed via decomposition of glucose or glucose directly. However, copper oxides-based electrodes generally work at much higher pH solutions.8 The enhanced electrocatalytic activity should be explained by the well-accepted mechanism of glucose oxidation on a Pt electrode in neutral media.6f,9f For the Pt wire electrode, its voltammetric behavior is essentially similar to that of the PtxCu1−xNCE electrodes although it provides much less electrocatalytic activity. By normalizing the oxidation current peak currents to the electrochemical active surface, it can be seen that the current densities of glucose oxidation on the PtxCu1−xNCE electrodes are not only much higher than that on the Pt electrode, but also higher than that obtained on the PtNCE electrode. The ratio of the current density (the enhancement factor, referred as R) measured on PtxCu1−xNCE electrodes versus that obtained on the Pt electrode is bigger than 10. In a similar way, comparison of the electrocatalytic activity of PtxCu1−x obtained by the dealloying process can also be evaluated. These results show that the Pt75Cu25NCE electrode has the strongest electrocatalytic activity to glucose oxidation with R up to 20, although the Pt88Cu12 catalyst has the highest electrode active surface areas. It can be deduced that the catalytic performance of the electrodes is also related to the 5043

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competitive adsorption model.7b−e,9j,11,13b−f,17 In brief, because glucose cannot be completely oxidized at −10 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 electrode surface blocks some active surface sites and results in the short linear response to glucose concentration. However, the linear dependence of current response with glucose concentration at −10 mV gives rise to a sensitivity of 135 μA mM−1 cm−2, which is much higher than that at 545 mV. To the best of our knowledge, the amperometric responses of the Pt75Cu25NCE electrode at both −10 and +545 mV are rather high for Pt bimetallic electrodes under physiological conditions. The detection limit of the sensor is calculated to be 2.5 μM based on a signal-to-noise ratio of 3. The Pt75Cu25NCE electrode exhibits sensitive amperometric responses to glucose, which should be ascribed to the synergetic effect between the Cu and Pt atoms. Also, the performance is comparable, if not superior, to those reported in other literature4,13a,f,14,16c,17,18 (see Table 1), indicating that the Pt75Cu25NCE electrode is very promising for analytical application.

atomic ratio of the bimetal PtxCu1−x nanochains, and there exists an optimum composition for the detection of glucose. The enhanced catalytic activity of the alloyed PtxCu1−x for the oxidation of glucose might be attributed to the following mechanism. On one hand, the dealloying process increases the specific surface significantly and results in an abundance of active sites.12a On the other hand, the glucose oxidation can be simplified into two steps. The first is a fast decomposition of glucose to gluconate or other intermediates (RO), which can be adsorbed over active Pt sites and slowly oxidized.11 Kinetically, Cu atom in the PtxCu1−x alloy acts both as promoting centers for the generation of the Cu−OR species and as an electron donation to Pt in the PtxCu1−x alloy. The incorporation of Cu atom decreases the Pt 4f binding energies and consequently reduces the Pt−RO bond strength. Therefore, the intimate contact between Pt and Cu domains in the PtxCu1−x alloy greatly promotes the regeneration of Pt sites for high electrochemical activity toward glucose oxidation.13b−f On the basis of the above discussions and the BET data (see Figure S2 in the Supporting Information), which are in good agreement with the ECSA results, the Pt75Cu25 modified electrode having the best electrocatalysis toward glucose is mainly due to the synergetic effect. Detection of Glucose by the Pt75Cu25 Nanochains Modified Electrodes. On the basis of the above discussion, it is obvious that the electrode modified with Pt75Cu25 has the best performance toward the electro-oxidation of glucose. Here, we concentrate on this novel bimetal nanomaterial and its application for detection of glucose. Differential pulse voltammograms (DPVs) obtained at the Pt75Cu25NCE electrode in 0.1 M PBS/0.15 M NaCl solution containing different concentrations of glucose are shown in Figure 5A.

Table 1. Comparison of Different Electrode Materials for Glucose Determination electrode material Ni-BDD4

sensitivity (μA mM−1 cm−2) 104

PtPb nanowire array13a

11.25

nanoporous PtPb networks13f nanoporous PtCu14

10.8

nanoporous PtIr16c

93.7

PtAu nanowires17

101.2

NiO−Au hybrid anobelts18a CuO nanorod18b

48.35

Pt75Cu25NC (this work)

135

10.6

0.45

linear range 0.01−10 mM up to 11 mM up to 16 mM 0.6−15 mM up to 10 mM 20−140 μM NA 0.01−0.1 mM 0.01−17 mM

detection limit (μM) 2.7 8 NA 50 NA NA 1.32 1.2 2.5

An important parameter for a biosensor is its ability to discriminate between the interfering species commonly present in similar physiological environment and the target analyte. As mentioned previously, one of the major challenges in nonenzymatic glucose detection is the interfering electrochemical signals caused by the oxidation of endogenous substances such as uric acid (UA), ascorbyl palimitate (AP), and ascorbic acid (AA). We further investigate the amperometric detection of glucose at −10 mV on Pt75Cu25NCE electrode to determine the selectivity for glucose sensing because the Pt75Cu25NCE electrode possesses better sensitivity at −10 mV than that at +545 mV. Figure 6 presents the testing results on selectivity of the Pt75Cu25NCE electrode with successive additions of AA, AP, UA, frucose, and glucose in 0.1 M PBS containing 0.15 M NaCl. Interestingly, the Pt75Cu25NCE electrode produces negligible current signals for all four common interfering agents, yet still provides significant responses to incremental glucose concentrations. However, taking the amperometric responses of the Pt75Cu25NCE electrode to successive addition of 1 mM glucose without

Figure 5. The DPVs of the P75Cu25NCE electrode in the potential range −0.2 to 1.0 V at scan rate of 10 mV s−1 in 0.1 M PBS + 0.15 M NaCl (pH 7.4) solution containing glucose at concentrations of 0.4, 0.8, 1.2, 1.6, 2, 5, and 10 mM (A), and the relationship between the oxidation peak current and the glucose concentrations recorded at −10 mV (B) and at 545 mV (C).

With addition of glucose, the oxidation peak currents increase gradually, suggesting that these peaks are indeed the oxidation of glucose. Figure 5B and C represent the calibration curves of the Pt75Cu25NCE electrode for the determination of glucose at −10 and 545 mV, respectively. A wider linear range dependency between the current response and concentration of glucose is observed at 545 mV (up to 2.0 mM with a correlation coefficient of 0.99) than that at −10 mV (in the range of 1.0 × 10−2 to 14 mM with a correlation coefficient of 0.99). This phenomenon can be reasonably interpreted by the 5044

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Information). The reproducibility of the Pt75Cu25NCE electrodes is examined by analysis of the same concentration of glucose (10 mM) using five-paralleled prepared electrodes. The five electrodes, made independently, showed the current responses of 1.77, 1.73, 1.79, 1.74, and 1.78 mA cm−2 (see Figure S4 in the Supporting Information). It was observed that the Pt75Cu25NCE electrodes had acceptable reproducibility with a relative standard deviation of 3.2%. The storage stability is examined at the same modified electrode for a consecutive 30 days. While not in use, the modified electrode is stored in air. In the repeated measurements in consecutive days, the response to the oxidation of the same concentration of glucose at the optimum potential is maintained 98% of the initial values. The good long-term stability could be attributed to both the structural stability and the good adsorption of the Pt75Cu25 nanochains on the surface of GCE electrode with the help of Nafion.

Figure 6. Chronoamperometric curves of the Pt75Cu25NCE electrode in a 0.1 M phosphate buffer (pH 7.4) containing 0.15 M NaCl with successive addition of 1 mM AA, 1 mM AP, 1 mM UA, 1 mM fructose, and 1 mM glucose with a constant potential at −10 mV. The inset is the corresponding chronoamperometric curves of the Pt75Cu25NCE electrode to successive addition of 1 mM glucose without any interfering species in the same experimental conditions.

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CONCLUSIONS In summary, the orderly nanostructured PtCu chains with a clean surface have been conveniently synthesized and assembled through a simple wet chemical process. The synthesized PtCu nanochains with tunable bimetallic compositions are then obtained by a facile dealloying process by treating the PtCu precursor with acidifications. Because of the high specific surface areas and synergetic effects between the alloyed PtCu atoms, catalytic activity for glucose oxidation has been significantly enhanced. A sensitivity of 135 μA mM−1 cm−2 has been achieved by using the Pt75Cu25 nanomaterials for electrode modification. The mechanism of the electro-oxidation of glucose on these PtCu electrodes has also been discussed in detail. This work demonstrates that wiring the dispersed nanocrystals is indeed a promising route to improve NP activity and durability, and the reported self-assembly method can be generalized to produce various wired Pt-based bimetal nanocrystals with improved electrocatalytic activity for biosensing applications.

interferences for comparison, there is a 5% decrease in sensitivity of each current response of the added glucose with the presence of the three interfering species. This enhanced selectivity may be attributed to the nonpoisoned PtCu surfaces, which not only prevent the adsorption of poisoning intermediates at Pt surfaces, but also provide a repelling effect toward the negatively charged AA, UA, and AP with the help of Nafion. Another possible reason is that the oxidation of AA, AP, and UA happens at more positive potentials (see Figure S3 in the Supporting Information). The determination of glucose in human serum samples is also performed on the Pt75Cu25NCE electrode by utilizing the calibration curve method. In brief, the blood samples obtained from the hospitalized patients are first diluted with 100 mL of the PBS buffer (pH 7.4), at which the Pt75Cu25NCE electrode is used to monitor the glucose content. The concentration of glucose in human blood serum sample is calibrated by standard glucose solution. The standard colorimetric enzymatic procedure is used as a reference for checking the biosensor accuracy. The results obtained from the glucose sensor agree well with those obtained by the standard colorimetric enzymatic method. The relative standard deviations (RSD) listed in Table 2 indicate most of the results are accurate and credible. Thus, it can be concluded that the developed sensor performs very well in the detection of glucose in serum samples. The operational stability of the Pt75Cu25NCE electrode is tested by monitoring the current response after 100 successive cycles. It can be seen that the peak current intensity remains 96.2% of its initial value (see Figure S4 in the Supporting

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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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added (mmol L−1)

found (mmol L−1)

RSD (%)

recovery (%)

5.19 5.59 5.80 5.98 6.65 9.44 5.76

0.50 0.50 0.50 1.00 1.00 1.00 1.00

5.92 6.23 6.15 7.19 7.49 9.63 6.64

4.0 2.3 2.9 3.2 3.0 3.3 2.8

104.5 102.1 97.6 103.0 97.9 102.0 98.2

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-62759394. Fax: +86-10-62751708. E-mail: [email protected] (Y.S.); [email protected] (N.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21275102, 211075004, 21173017, and 51272011), the China Postdoctoral Science Foundation (2011M500182 and 2012T50014), and the Program for New Century Excellent Talents in Chinese Universities (NCET-12-0610) for financial support.

Table 2. Determination of Glucose in Human Blood Serum serum samples (mmol L−1)

ASSOCIATED CONTENT

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REFERENCES

(1) http://www.who.int/mediacentre/factsheets/fs312/en/index. html. 5045

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dx.doi.org/10.1021/ac400292n | Anal. Chem. 2013, 85, 5040−5046