Facile fabrication of nanoporous PdFe alloy for nonenzymatic electrochemical sensing of hydrogen peroxide and glucose.

G Model ACA 233243 No. of Pages 10

Analytica Chimica Acta xxx (2014) xxx–xxx

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Facile fabrication of nanoporous PdFe alloy for nonenzymatic electrochemical sensing of hydrogen peroxide and glucose Jinping Wang a , Zhihong Wang b , Dianyun Zhao a , Caixia Xu a, * a Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China b School of Basic Medical Sciences, Shandong University of Traditional Chinese Medicine, Jinan 250355, China

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

NP-PdFe alloy is fabricated by a simple dealloying method. NP-PdFe possesses open three-dimensional bicontinuous spongy morphology. NP-PdFe shows high electrochemical sensing activities towards H2O2 and glucose. NP-PdFe shows good long-term stability for H2O2 and glucose detection. NP-PdFe shows good reproducibility for H2O2 and glucose detection.

Nanoporous PdFe alloy, characterized by open three-dimensional bicontinuous nanospongy architecture, was easily fabricated by selectively dealloying PdFeAl source alloys, which exhibits greatly enhanced sensing performance and structure stability towards H2O2 and glucose compared with NP-Pd and Pd/C catalysts. ? ?

A R T I C L E I N F O

A B S T R A C T

Article history: Received 28 February 2014 Received in revised form 24 April 2014 Accepted 30 April 2014 Available online xxx

Nanoporous (NP) PdFe alloy is easily fabricated through one step mild dealloying of PdFeAl ternary source alloy in NaOH solution. Electron microscopy characterization demonstrates that selectively dissolving Al from PdFeAl alloy generates three-dimensional bicontinuous nanospongy architecture with the typical ligament size around 5 nm. Electrochemical measurements show that the NP-PdFe alloy exhibits the superior electrocatalytic activity and durability towards hydrogen peroxide (H2O2) detection compared with NP-Pd and commercial Pd/C catalysts. In addition, NP-PdFe performs high sensing performance towards H2O2 in a wide linear range from 0.5 to 6 mM with a low detection limit of 2.1 mM. This nanoporous structure also can sensitively detect glucose over a wide concentration range (1–32 mM) with a low detection limit of 1.6 mM and high resistance against chloride ions. Along with these attractive features, the as-made NP-PdFe alloy also has a good anti-interference towards ascorbic acid, uric acid, and dopamine. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Palladium Nanoporous Sensor Glucose Hydrogen peroxide Dealloying

1. Introduction In recent years, detection of H2O2 and glucose is widely needed in a variety of fields, such as in blood-glucose sensing, food industry, environmental monitoring, pharmaceutical, clinical, and industrial research, etc. [1–5]. Many efforts have been focused on

* Corresponding author. Tel.: +86 531 89736103. E-mail address: [email protected] (C. Xu).

developing suitable techniques to precisely monitor H2O2 and glucose, such as optical approaches, colorimetry, electrochemical approaches, etc. [6–9]. Among these methods, the electrocatalytic method based on nanomaterials has attracted enormous attention due to its simplicity, high sensitivity, and compatibility towards miniaturization [10–14]. In recent years, many nanostructured materials have been reported for developing novel nanomaterials with highly catalytic performances and sensitively electrochemical sensing activities, such as carbon nanomaterials, metal oxides, novel metallic

http://dx.doi.org/10.1016/j.aca.2014.04.062 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: J. Wang, et al., Facile fabrication of nanoporous PdFe alloy for nonenzymatic electrochemical sensing of hydrogen peroxide and glucose, Anal. Chim. Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.04.062


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J. Wang et al. / Analytica Chimica Acta xxx (2014) xxx–xxx

nanomaterials, and nanocomposites [15–22]. For instance, Yin et al. have fabricated Au cluster/graphene hybrids by adding HAuCl4 solution into reduced graphene oxide (rGO) solution with sonication, centrifuging, washing, and freeze-drying, as well as the prepreparation of graphene setting aside for at least two weeks to remove the residual N2H4. The as-synthesized Au cluster/rGO hybrids display an impressive eletrocatalytic performance toward oxygen reduction reaction [23]. Sun et al. have synthesized palladium nanoparticles– graphene–carbon nanotube (Pd–rGO–CNT) through peparing rGO– CNT nanocomposite, which is prepared by hydrothermal treatment of the GO and CNT aqueous dispersion via weak interactions to form rGO–CNT cylinder hydrogels, and then dipping the rGO–CNT nanocomposite in K2PdCl4 solution. Pd–rGO–CNT exhibited high catalytic activity toward the reduction of 4-nitrophenol [24]. Chen et al. have prepared palladium nanoparticles (PdNPs)–graphene nanosheets (PdNPGNs) by heating the as-synthesized GO aqueous solution in oil bath at 100 C for 24 h and mixing GO aqueous solution with K2PdCl6 under vigorous stirring for 60 min at 30 C. The PdNPs– PdNPGNs exhibited remarkably electrochemical sensing activity toward H2O2 [25]. Jiang et al. synthesized palladium/poly(3,4ethylenedioxythiophene) (Pd/PEDOT) nanocomposite through EDOT alcoholic solution added into H2PdCl4 solution under vigorous stirring for 3 h, followed by filtering, washing, and drying at 60 C for 12 h [26]. This Pd/PEDOT nanocomposite performed nonenzymatic electrochemical sensing performance towards H2O2. Among these various nanomaterials, nanostructured palladium materials have attracted great interests owing to their high catalytic activities and wide applications in the fields of hydrogen storage [27], the electrochemical oxidation of formic acid and methanol [28,29], the purification of automotive exhaust [30], etc. Nanostructured Pdbased materials have also been demonstrated to represent a class of

highly active electrocatalysts for H2O2 and glucose detection. Being indeed quite successful in achieving high activity, nevertheless, the preparation of these electrode materials is complicated involving organic agents, multi-step operations, and low production yield. In summary, the stated above is not desirable in terms of the everdemanding concerns for simplicity, green synthesis, and environmental protection. Recently, dealloying has been demonstrated to be a powerful method for scaling up the facile fabrication of nanoporous metal architecture [31,32]. Alloy foils with different components can be made by refining high-purity metals at high temperatures under the protection of inert gas, followed by melt-spinning. The more reactive atomic content in source alloys can be controlled to be at the suitable percentage and then be etched through chemical or electrochemical methods. For instance, Xu et al. have fabricated NP-PdCu and NP-PtCo by this simple dealloying method [4,33]. This nanoporous structure represents a class of interesting materials with rich surface chemistry and unique catalytic activity to allow functional integration [34]. Moreover, unlimited electron conductivity and macroscopic dimension in nanoporous metals make these materials ideal electrode materials to integrate multifunctionalities for sensing and catalysis [35]. Non-precious Fe as one of the 3d metals has exhibited synergistic catalytic effect for Pd to many electrocatalytic reactions such as oxygen reduction [36] and methanol electro-oxidation [37]. In present work, NP-PdFe alloy is fabricated through selectively dealloying ternary PdFeAl source alloy in mild alkaline solution. The electrochemical sensing activities of NP-PdFe alloy towards H2O2 and glucose were studied to explore the effect of Fe on the sensing performance of Pd with the aim to construct highly effective sensor.

Fig. 1. (a) SEM, (b) cross sectional SEM, (c) TEM, and (d) HRTEM images of the resulted sample after dealloying PdFeAl alloy in 0.5 M NaOH solution for 24 h at room temperature.




2. Experimental 2.1. Reagents PdFeAl alloy foils with the thickness at 50 mm were made by refining pure Pd, Fe, and Al (99.99%) in an arc-furnace, followed by melt-spinning under Ar-protected atmosphere. NP-PdFe alloy was prepared by etching PdFeAl alloy foils in 0.5 M NaOH solution for 24 h at room temperature. H2O2 solution (30%) and D-(+)-glucose were purchased from Sinopharm Chemical Reagent Co., Ltd. The glucose stock solution was kept overnight (at least 24 h) to allow mutarotation. Dopamine (DA), uric acid (UA), and ascorbic acid (AA) were purchased from Sigma–Aldrich. Other chemicals were of analytical grade and used without further purification. Ultra-pure water (18.2 MV) was used in all experiments. 2.2. Apparatus Powder X-ray diffraction (XRD) was carried out on a Bruker D8 advanced X-ray diffractometer using Cu KR radiation at a step rate of 0.04 s 1. The morphological and compositional characterizations of the sample were carried out on a JEM-2100 transmission electron microscope (TEM) and a JSM-6700 field-emission scanning electron microscope (SEM) equipped with an Oxford INCA x-sight energy dispersive X-ray spectrometer (EDS). The elemental mapping was obtained using a FEI QUANTA FEG250 scanning electron microscope equipped with an INCA Energy XMAX-50 X-ray spectroscopy analyzer. All electrochemical measurements were performed using CHI 760D electrochemical workstation (Shanghai CH Instruments Co., China). A conventional three-electrode cell was used with Pt foil as a counter electrode, mercury sulfate electrode as the reference electrode, and 4-mmdiameter glassy carbon electrode (GCE) as work electrode. All potentials were provided according to the reversible hydrogen electrode (RHE) scale.

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image (Fig. 1b), it is interesting to find that the resulted nanoporous architecture runs through the whole structure with high uniformity. Elemental mapping analyses indicate that Pd and Fe are homogeneously distributed throughout the surface of NP-PdFe alloy as shown in Fig. S1 (Supporting Information). The clear contrast in TEM image between the dark skeletons and the inner bright regions further demonstrates the formation of an open interconnected network architecture as shown in Fig. 1c. High resolution TEM (HRTEM) image of the dealloyed sample provides detailed structure information (Fig. 1d). The lattice spacing in the HRTEM image is calculated to be 2.16 Å, which corresponds to the (111) crystal plane of the PdFe alloy structure. It is interesting to find that the lattice fringes extend across several ligaments along (111) planes, indicating a single crystalline grain nature for the porous structure, which is similar to our previous observation [39]. TEM image in Fig. 1d indicates that this spongy nanostructure has a narrow ligament size around 5 nm. It is clear that the dissolution of Al from PdFeAl alloy in the source alloy resulted in the formation of nanoporous architecture. These electron spectroscopy characterizations demonstrate that during dealloying of PdFeAl ternary alloy, as Al is leached away (shown in Fig. 2), the remained Pd and Fe atoms can inter-diffuse at the solution/solid interface and selfassemble to form 3D bicontinuous nanoporous spongy structure with uniform channel size distribution.

2.3. Preparation of the modified electrodes Catalyst ink was prepared by mixing 2 mg carbon powder, 1 mg NP-PdFe alloy, 200 mL isopropanol, and 200 mL nafion solution (0.5 wt.%) under sonication for 20 min. The working electrode was made by dropping 4 mL the as-made catalyst inks and 4 mL nafion solution on a polished glassy carbon electrode. The NP-Pd and Pd/C catalysts were prepared in a similar way. The electrochemical active surface areas (ECSA) of the Pd-based catalysts were evaluated by integrating the reduction charges during the stripping of surface monolayer oxide in N2-purged 0.5 M H2SO4 solution in the range of 0 to 1.35 V at the scan rate of 50 mV s 1, in which the used charge density is 420 mC cm 2 Pd [38]. The ECSA of Pd/C, NP-Pd, and NP-PdFe was measured to be 74, 79, and 63 m2 g 1 by using the methods stated above. 3. Results and discussion 3.1. Characterization of NP-PdFe alloy Al-based PdFeAl alloy was chosen as the precursor alloy considering the more reactive property, rich supply, and low cost of Al. As an amphoteric element, Al atoms can be selectively removed from the PdFeAl ternary alloy in an alkaline solution with the other components such as Pd and Fe remained. The atomic percentage of Al is set at 80% to achieve easy dealloying and high porosity as shown in Fig. 2b. Fig. 1a illustrates a representative SEM image of the sample after dissolving Al from PdFeAl alloy. The resulted sample is characterized by an open spongy structure with uniform pore and ligament distributions. From the cross-sectional SEM

Fig. 2. (a) XRD patterns of the NP-PdFe and PdFeAl alloy. The standard patterns of pure Pd (JCPDS 65-2867), Fe (JCPDS 06-0696), Al (JCPDS 65-2869), and PdAl6 (JCPDS 42-1286) are attached for comparison. EDS spectra of (b) the PdFeAl alloy and (c) dealloyed sample.



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Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.04.062. The crystal structure of the dealloyed sample was examined by XRD. As shown in Fig. 2a, PdFeAl alloy exhibits a series of diffraction peaks which can be assigned to a PdAl6-type crystal structure. However, the relative intensity and position of these diffraction peaks are different from standard PdAl6 diffraction patterns, which may result from different alloy composition and rapid solidification during alloy refining. Compared with the complicated diffraction pattern of the PdFeAl source alloy, there are only a set of three diffraction peaks emerged around 40.7, 44.6, and 69.5 (2u) after selective leaching of Al from the ternary alloy, which can be ascribed to the (111), (2 0 0), (2 2 0) diffractions of face-centered cubic (fcc) alloy structure. The most pronounced diffraction peak can be assigned to the (111) reflection, while reflections corresponding to (2 0 0) and (2 2 0) planes become broader, and their relative intensity is barely discernibling in the XRD patterns. The very broad diffraction mainly features the large surface stress generated during dealloying process, since the grain size of the alloy sample is typically of order a few micrometers. The higher (111) reflection intensity reveals that the NP-PdFe alloy has preferentially oriented crystalline structure with (111) planes parallel to the supporting substrate, which is generally in good agreement with HRTEM observations [39]. The three peaks located between the projected 2u values of pure Pd and Fe without any peaks from pure metals, their oxides, or Al-based alloy species. Additionally, Al was not detected in the resulted sample after dealloying in NaOH aqueous solution, which can be clearly observed from the EDS result as shown in Fig. 2c. The above results indicate the formation of single phase PdFe alloy. As shown in Fig. 2b, EDS characterization of PdFeAl alloy demonstrates that the ratio of the three components is almost the same as the initial

feeding ratio. After dealloying PdFeAl in NaOH solution, the EDS result shows that Al is etched to an undetectable level while the contents of Pd and Fe are basically consistent with the initial feeding ratio of around 4:1 between Pd and Fe in the ternary PdFeAl alloy (Fig. 2c), indicating the well control of the composition of the resulted sample by dealloying method. 3.2. Electrochemical sensing of H2O2 over NP-PdFe alloy Characterized by three-dimensional bicontinuous skeleton and hollow interconnected channels extending throughout the structure, NP-PdFe alloy is favorable for the unlimited transport of molecules and electron conductivity. The electrocatalytic activities of the NP-PdFe alloy towards H2O2 detection were examined to evaluate its potential application in H2O2 sensing. As shown in Fig. S2, the responses of H2O2 on NP-PdFe alloy increase with the pH in the range of 5.6–8.0, while it does not change remarkably from pH 7.0 to 8.0, which is similar to the previous reports [40,41]. Meanwhile, H2O2 is usually detected in neutral media, thus pH 7.0 was considered to be the optimized. Fig. 3 shows the cyclic voltammetric (CV) curves of commercial Pd/C, NP-Pd, and NP-PdFe modified electrodes in PBS solution with and without 1 mM H2O2. It is observed that in the presence of H2O2 NP-PdFe alloy exhibits not only high oxidation current from 0.44 V (Fig. 3c) but also evident reduction signals in the range of 0.2–0.7 V. Compared with Pd/C and NP-Pd catalysts, the onset oxidation potential on NP-PdFe toward H2O2 shifted negatively more than 100 mV, exhibiting superior oxidation activity towards H2O2 (Fig. 3a–c). It can be clearly observed that the NP-Pd and NP-PdFe exhibit more evident oxidation towards H2O2 in the forward scan compared to the Pd/C (Fig. S3), which can be ascribed to the specific nanoporous architecture with unlimited mass transport. The oxidation current

Fig. 3. CV curves of (a) Pd/C, (b) NP-Pd, and (c) NP-PdFe at 50 mV s 1; (d) CV curves of NP-PdFe modified GCE in PBS solution (0.1 M, pH 7.0) containing 1 mM H2O2 at different scan rates; Inset in (d) is the plots of current on NP-PdFe at 0.9 V vs. scan rate.




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Fig. 4. (a) Amperometric current responses of Pd/C, NP-Pd, and NP-PdFe on successive addition of 0.5 mM H2O2 into stirring PBS solution at 0.9 V; (b) plots of current vs. H2O2 concentrations; (c) sensing stability of Pd/C, NP-Pd, and NP-PdFe alloy in a stirring PBS solution containing 1 mM H2O2 for 2000 s at 0.9 V; (d) long-term stability of NP-PdFe alloy for continuous H2O2 detection for 14 days.

density on NP-PdFe is much higher than those on NP-Pd and Pd/C catalysts, indicating the higher catalytic activity of NP-PdFe alloy due to the synergistic catalytic effect of Pd and Fe. In other words, alloying Fe with Pd greatly enhanced the electrocatalytic performance toward H2O2 detection. Additionally, the unique architecture of the NP-PdFe alloy with uniform size distribution facilitates the mass transport and electron conductivity, leading to improved sensing performance. It can be concluded that NP-PdFe alloy is highly active toward H2O2 oxidation over a much wider potential range with a lower onset potential, providing a substantial basis for its electrochemical sensing application. Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.04.062. Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.04.062. The electrocatalytic behavior of NP-PdFe alloy towards H2O2 was further investigated by changing the scan rate. As shown in Fig. 3d, the oxidation currents of H2O2 gradually increase with the rise of the scan rate. As displayed in Fig. 3d inset, it is clear that the anodic currents at 0.9 V are linear as a function of the scan rate in a range from 20 to 100 mV s 1, and the corresponding linear equations for the currents is I (mA) = 60.47 + 1.98 y with a linear correlation coefficient of 0.998. It can be concluded that the electrochemical oxidation of H2O2 on the NP-PdFe modified electrode is a surface-controlled process [40]. Based on the high oxidation activity towards H2O2, the sensing performance of NP-PdFe alloy is evaluated by amperometric detection upon the successive addition of H2O2 into the electrolyte. As shown in Fig. 4a, the amperometric responses of the three catalysts increase sensitively and quickly after each addition of H2O2 into the stirring PBS solution. It is clear that the response currents generated on the NP-PdFe alloy are obviously higher than those on NP-Pd and Pd/C to the each addition of 0.5 mM H2O2, exhibiting much higher sensitivity, which is in good agreement with the CV results. As is observed, NP-PdFe responds rapidly to the

addition of H2O2 and reaches well-defined steady-state current within a period of 2 s, which is shorter than those of NP-Pd (2.4 s) and Pd/C (3 s). The fast response on NP-PdFe alloy can be attributed to the fact that H2O2 can diffuse unlimitedly into the three-dimensional bicontinuous nanoporous structure accompanied with synergistic catalytic effect of Pd and Fe. Fig. 4b shows the calibration plot of steady-state currents obtained from Pd/C, NP-Pd, and NP-PdFe relative to the concentration of H2O2. As shown in Fig. 4b, the electrode modified Pd/C has linear responses to H2O2 concentration in the range of 0.5–4 mM (linear equation: y = 6.11 + 20.67x, R = 0.991) with a detection limit of 3.3 mM (S/ N = 3). NP-Pd sample performs better linear relationship in the range of 0.5–4.5 mM (linear equation: y = 10.63 + 24.35x, R = 0.995) with a detection limit of 3.7 mM. From Fig. 4b, the calibration curves indicate that among these three catalysts NP-PdFe alloy exhibits the highest sensitivity and the widest linear range up to 6 mM (linear equation: y = 9.79 + 38.72x, R = 0.993) with a lowest detection limit of 2.1 mM and a highest sensitivity at 38.72 mA mM 1 cm 2. It is concluded that the NP-PdFe can be used as a sensor for H2O2 detection over a wide linear range between 0.5 and 6 mM. The remarkable performance of NP-PdFe modified electrode towards the detection of H2O2 indicates that NP-PdFe holds great potential to construct H2O2 sensor (Table 1). The long-term sensing stability of NP-PdFe electrocatalyst is essential for continuous and reliable H2O2 monitoring, which was evaluated by studying its long-term steady-state current response. Fig. 4c presents the chronoamperometry data under 0.9 V in a solution of 0.1 M PBS + 1 mM H2O2 for 2000 s. At the beginning, the rapid current decay for the catalysts is caused by the formation of double-layer capacitance. Upon a short time operation, the current gradually reached a quasi-equilibrium steady state. As shown in Fig. 4c, the steady-state current on the NP-PdFe alloy is much higher than those on NP-Pd and Pd/C catalysts, which is in good agreement with the results from the CV study and indicates the dramatically higher catalytic durability of the NP-PdFe alloy. The



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Table 1 Analytical parameters of different H2O2 biosensors. Modified electrode

Copper nanoclusters Doping ionic liquid into Prussian blue-multiwalled carbon nanotubes Nanotubular mesoporous PdCu NP-Fe2O3/CoO NP-PtCo NP-PtNi NP-PtAu NP-PdFe

Linear range (mM)

Detection limit (mM)

0.01–1 0.005–1.645

10 0.35

0.5–8.0 0.05–4.85 0.05–0.8 0.01–0.18 0.05–2.75 0.5–6

0.1 0.1 1.0 1.0 0.1 2.1

sensing stability of NP-PdFe alloy was further explored by continuous detecting H2O2 every day for a period of two weeks. Five NP-PdFe modified electrodes prepared in the same way were employed to detect 1 mM H2O2 at 0.9 V under the same condition. As shown in Fig. 4d, the recorded amperometric responses of one electrode have almost no change over a period of 13-day. Only 4.1% degradation in amperometric current was detected within two weeks, demonstrating that NP-PdF-based sensor is highly stable and superior to H2O2 detection. The reproducibility of NP-PdFe modified electrodes can also be obtained based on the measurement results in the period of 14 days, in which the relative standard deviation (RSD) for 1 mM H2O2 is 2.3%. For five NP-PdFe modified electrodes, the RSD is 3.1%. It is clear that the NP-PdFe shows good long-term stability and reproducibility for H2O2 detection. 3.3. Electrochemical detection of glucose over NP-PdFe alloy The electrocatalytic activities of the NP-PdFe alloy, NP-Pd, and commercial Pd/C towards glucose oxidation were then examined to evaluate their potential application in glucose detection. It has been reported that OH plays an important role in glucose oxidation. Although a higher concentration of OH in the solution caused higher electrocatalytic activity on the catalyst, too much

Sensitivity (mA mM 1 cm – 0.436 – 46.39 – 122.068 59.95 38.72

Refs. 2

) [1] [41] [34] [17] [4] [42] [43] This work

OH blocked the further electroadsorption of glucose anion, resulting in a decrease of current [44]. Chen et al. selected 0.1 M NaOH solution for the electrocatalytic oxidation of glucose, which is in agreement with the result shown in Fig. S4 in this work. It is obvious to observe that the NaOH concentration in the range of 0.05–1.0 M is preferable for the detection of glucose on NP-PdFe alloy at 0.35 V. Considering the stability of test system and mild detection condition, 0.1 M NaOH solution was chosen as the detection medium. Fig. 5a and b shows the CV curves of commercial Pd/C and NP-Pd catalysts in 0.1 M NaOH solution with and without 50 mM glucose. It is observed that in the presence of 50 mM glucose, the two catalysts exhibit wide oxidation area from the onset potential. In the positive scan, the first increase in current (starting from about 0.2 V) can be attributed to the electrochemical adsorption of glucose with the generation of adsorbed intermediates (per glucose molecule lost one proton in this electrochemical reaction), which generates the oxidation current. As the reaction continued, the accumulation of the intermediates on the electrode surface inhibits the electrochemical adsorption of glucose, resulting in a current decrease. When the applied potential is higher than 0.6 V, Pd-OH species are generated in the NaOH solution, which will oxidize the intermediates derived from the electrochemical adsorption of glucose.

Fig. 5. CV curves of (a) Pd/C, (b) NP-Pd, and (c) NP-PdFe in 0.1 M NaOH solution with and without 50 mM glucose at 50 mV s 1; (d) CV curves (100 cycles) of NP-PdFe alloy in 0.1 M NaOH + 50 mM glucose solution; Inset in (d) is the long-term stability on NP-PdFe alloy for glucose detection (peak current) vs. cycles.




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Fig. 6. (a) Amperometric current responses of Pd/C, NP-Pd, and NP-PdFe on successive addition of glucose into stirring NaOH solution at 0.35 V; (b) plots of current vs. glucose concentrations; (c) sensing stability of Pd/C, NP-Pd, and NP-PdFe alloy in stirring NaOH solution containing 5 mM glucose for 2000 s at 0.35 V; (d) CV curves of NP-PdFe alloy in 50 mM glucose + 0.1 M NaOH solution with and without 0.1 M NaCl at 50 mV s 1.

During this process, free active Pd sites are released for the direct oxidation of glucose, which results in the oxidation current between 0.6 and 1 V. In the negative scan, the oxidation of glucose is suppressed due to the block of the active Pd sites. The oxidized Pd surface begins to be reduced at about 0.1 V accompanied with a large anodic peak generated at about 0.07 V. With the negative scan, more surface-active sites are refreshed and available for the oxidation of glucose, which results in a large anodic peak in the range from 0.1 to 0.3 V. This reaction mechanism of glucose on Pd nanomaterails is similar to the previous work reported by Chen et al. [44]. As observed, the NP-PdFe alloy shows similar electrochemical behavior to NP-Pd and Pd/C for glucose oxidation in Fig. 5c. However, compared with NP-Pd and Pd/C catalysts (Fig. S5), the much higher current density on NP-PdFe indicates its greatly enhanced catalytic activities toward glucose electrooxidation due to the nanoporous structure and the synergetic effect of Pd and Fe. The alloying of Fe can modify the electronic properties of NP-PdFe alloy through the electron transfer from Fe towards Pd, since the electronegativity of Fe (1.83) is much smaller than that of Pd (2.20) [37]. The electron transfer can contribute to the d-band density of Pd [45], which is beneficial to generate OHads species on NP-PdFe surface [46]. The reason is that the electronic structure (the relative location of the d-bands in relation to the Fermi energy) governs the strength of bonding of OHads with metallic atoms [47]. For the glucose electro-oxidation, the desorption of OHads or reduction of Pd/Fe oxides regenerating the active metallic surface to favor the electro-oxidation of glucose [17]. Moreover, the unique three-dimensional bicontinuous nanoporous structure and ultrafine ligaments (5 nm) should also be profitable to the enhanced electrocatalytic activity of the NP-PdFe alloy. The interconnected channels of the NP-PdFe allow unblocked transport of small molecules, and the ligaments can provide more exposed surface atoms as active sites. The larger interconnected channels can significantly reduce the liquid sealing effect [48], making H2O2 and glucose easily diffuse into this nanoporous

material to reduce the concentration polarization [49]. Additionally, the Fe atoms can effectively stabilize the nanoporous structure of the NP-PdFe due to the formation of nearly pure Pd skin and PdFe alloy core after activating in N2-purged 0.5 M H2SO4 solution [50]. Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.04.062. Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.aca.2014.04.062. For practical applications, the long-term sensing stability of NPPdFe electrocatalysts is essential for continuous and reliable monitoring of glucose. As shown in Fig. 5d, during the CV cycles in 0.1 M NaOH + 50 mM glucose solution the activity loss is much less, and after 100 cycles the peak current remained 86% compared with the initial displayed in Fig. 5d inset. In addition, no obvious change is detected after the electrode was stored at room temperature for four weeks. These results indicate that the NP-PdFe alloy has good stability towards the electrochemical oxidation of glucose. Fig. 6a shows the typical amperometric response of the NP-PdFe alloy towards the successive addition of 1 mM glucose at 0.35 V including the responses of NP-Pd and Pd/C for comparison. It is clear that the response currents generated on the NP-PdFe alloy are obviously higher than those on NP-Pd and Pd/C to the each addition of 1 mM glucose, exhibiting much higher sensitivity, which is in good agreement with the CV results. The NP-PdFe electrode responds rapidly and sensitively to each addition of glucose and reaches the maximum steady-state current within 2 s, which is much faster than the responses on NP-Pd (ca. 2.5 s) and Pd/C (ca. 3 s). The rapid response can be attributed to the fact that glucose can diffuse freely into the three dimensional bicontinuous nanoporous structure as well as the synergistic effect between Pd and Fe. As shown in Fig. 6b, the Pd/C catalyst has linear responses to glucose concentration in the range of 1–13 mM (linear equation: y = 1.24 + 1.11x, R = 0.992) with a detection limit of 2.2 mM. And the NP-Pd sample performs better linear relationship in the range



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Table 2 Analytical parameters of different glucose biosensors. Modified electrode

Copper nanoclusters Gold@silica core–shell nanoparticle Ni(OH)2/electroreduced Grapheme oxide-multiwalled carbon nanotubefilm Nanotubular mesoporous PdCu coupled with glucose oxidase NP-PtCo NP-PtAu NP-PdFe

Linear range (mM)

Detection limit (mM)

Sensitivity (mA mM 1 cm

Refs. 2

)

0.1–2 0.025–25 0.01–1.5

100 16 2.7

– – 2042

[1] [6] [52]

0.5–20 0.05–3.0 0.2–5.4 1–32

1 0.1 0.5 1.6

– 0.499 22.77 2.7

[34] [4] [43] This work

of 1–19 mM (linear equation: y = 2.03 + 1.57x, R = 0.994) with a detection limit of 1.9 mM. However, the NP-PdFe alloy exhibits the highest sensitivity and the widest linear range up to 32 mM (linear equation: y = 0.39 + 2.7x, R = 0.998) with the lowest detection limit of 1.6 mM and the sensitivity of 2.7 mA mM 1 cm 2. The remarkable performance of NP-PdFe modified electrode towards the detection of glucose indicates that NP-PdFe holds great potential to fabricate glucose sensor (Table 2). The long-term sensing stability of NP-PdFe alloy is significant for continuous and reliable monitoring of glucose, which was estimated by detecting the steady-state current by using potentiostatic method. As illustrated in Fig. 6c, the addition of 5 mM glucose in stirring alkaline solution shows a stable amperometric response after running for 2000 s. At the beginning, the rapid current decay for the catalyst is caused by the formation of double-layer capacitance. However, the current gradually reaches a quasi-equilibrium steady state. In comparison, the steady-state current on the NP-PdFe alloy is much higher than those on Pd/C and NP-Pd catalysts, which is in good agreement with the results from the CV study, indicating that the dramatically enhanced catalytic durability of NP-PdFe alloy. It has been reported that chloride ions have a serious poisoning effect on some single metallic and alloy electrocatalysts, such as Au, Pt, and their alloys, resulting in the activity loss towards the electrochemical oxidation of glucose [40,51]. Thus, it is significant to study the effect of Cl on the electrocatalytic activity of the NP-PdFe alloy. Fig. 6d shows the CV curves of the NP-PdFe alloy in 0.1 M NaOH + 50 mM glucose solution with and without 0.1 M NaCl. It is observed that, in the positive scan, the electrochemical adsorption of glucose is slightly inhibited owing to the presence of a large amount of Cl at low potential (lower than 0.2 V). However, as the potential increased, the nanoporous alloy shows similar current responses towards

glucose oxidation in the presence and absence of Cl . This result demonstrates that Cl (0.1 M) only has a neglectable poisoning effect on the NP-PdFe alloy at low potentials and that the catalyst can be used for glucose sensing, even in the presence of a high concentration of Cl (0.1 M). AA, UA, and DA usually coexist with glucose and H2O2 and can be easily oxidized at a relatively positive potential during the detection process of glucose and H2O2. Thus, it is essential to detect the anti-interference of NP-PdFe toward the glucose and H2O2 sensing. And the interferences of these electroactive molecules were tested by adding 0.1 mM AA, 0.02 mM UA, and 0.01 mM DA during glucose and H2O2 sensing processes, respectively. For a better comparison, the current densities to glucose and H2O2 are set at 100%, respectively, and the other electrochemical responses are normalized by these values. During the glucose detection, interferences from AA, UA, and DA are 8.7%, 2.1%, and 0.07%, respectively, without coating additional nafion (Fig. 7a). And in the H2O2 sensing process, interferences from AA, UA, and DA are 6.2%, 0.2%, and 0.89%, respectively (Fig. 7b). It is clear that there is little response current towards DA and UA generated on NP-PdFe alloy, which can be ignored. The interference from AA slightly affects the response currents of glucose and H2O2 on NP-PdFe alloy. Consequently, the extra nafion (0.5%, 2 mL) solution was coated on the electrodes as a thin layer to exclude the interference of these electroactive molecules [53]. After coating 2 mL of nafion solution on the NP-PdFe modified electrode, the interference current from AA was almost eliminated during the detections of glucose and H2O2. It can be concluded that the as-prepared NP-PdFe is preferable to construct the nonenzymatic biosensor towards glucose and H2O2 with good anti-interference performance towards DA, UA, and AA by coating the additional nafion solution with no current decay of electrooxidation.

Fig. 7. Interference (0.1 mM AA, 0.02 mM UA, and 0.01 mM DA) on the response of (a) 5 mM glucose in 0.1 M NaOH solution at 0.35 V and (b) 1 mM H2O2 in 0.1 M PBS solution at 0.9 V.




4. Conclusions NP-PdFe alloy with 3D bicontinuous nanoporous structure has been successfully fabricated by one-step dealloying PdFeAl alloy. This bimetallic nanomaterial exhibits superior electrocatalytic activity towards H2O2 and glucose detection compared with the NP-Pd and commercial Pd/C catalysts, which can be used for sensitively detecting these two small molecules in wide ranges of 0.5–6 mM for H2O2 and 1–32 mM for glucose. In addition to the high sensing performance and reproducibility, the nanoporous NPPdFe alloy also has other advantages, such as high resistance towards Cl , high anti-interference, easy preparation, and clean surface etc. The novel nanoporous spongy alloy is not only scientifically interesting but also has great application potential in constructing nonenzymatic electrochemical sensors. Acknowledgment This work was supported by the National Science Foundation of China (51001053 and 21271085). References [1] L.Z. Hu, Y.L. Yuan, L. Zhang, J.M. Zhao, S. Majeed, G.B. Xu, Copper nanoclusters as peroxidase mimetics and their applications to H2O2 and glucose detection, Anal. Chim. Acta 762 (2013) 83–86. [2] J. Zhao, Y.L. Yan, L. Zhu, X.X. Li, G.X. Li, An amperometric biosensor for the detection of hydrogen peroxide released from human breast cancer cells, Biosens. Bioelectron. 41 (2013) 815–819. [3] Y.Z. He, X.X. Wang, J. Sun, S.F. Jiao, H.Q. Chen, F. Gao, L. Wang, Fluorescent blood glucose monitor by hemin-functionalized grapheme quantum dots based sensing system, Anal. Chim. Acta 810 (2014) 71–78. [4] C.X. Xu, F.L. Sun, H. Gao, J.P. Wang, Nanoporous platinum –cobalt alloy for electrochemical sensing for ethanol, hydrogen peroxide, and glucose, Anal. Chim. Acta 780 (2013) 20 –27. [5] C.H. Nieh, S. Tsujimura, O. Shirai, K. Kano, Amperometric biosensor based on reductive H2O2 detection using pentacyanoferrate-bound polymer for creatinine determination, Anal. Chim. Acta 767 (2013) 128–133. [6] I. Al-Ogaidi, H.L. Gou, A.K.A. Al-kazaz, Z.P. Aguilar, A.K. Melconian, P. Zheng, N. Q. Wu, A gold@silica core–shell nanoparticle-based surface-enhanced Raman scattering biosensor for label-free glucose detection, Anal. Chim. Acta 811 (2014) 76–80. [7] Y.S. Xia, J.J. Ye, K.H. Tan, J.J. Wang, G. Yang, Colorimetric visualization of glucose at the submicromole level in serum by a homogenous silver nanoprism– glucose oxidase system, Anal. Chem. 85 (2013) 6241–6247. [8] J. Luo, S.S. Jiang, H.Y. Zhang, J.Q. Jiang, X.Y. Liu, A novel non-enzymatic glucose sensor based on Cu nanoparticle modified graphene sheets electrode, Anal. Chim. Acta 709 (2012) 47–53. [9] Y.B. Zhou, G. Yu, F.F. Chang, B.N. Hu, C.J. Zhong, Gold-platinum alloy nanowires as highly sensitive materials for electrochemical detection of hydrogen peroxide, Anal. Chim. Acta 757 (2012) 56–62. [10] L.J. Zhong, S.Y. Gan, X.G. Fu, F.H. Li, D.X. Han, L.P. Guo, L. Niu, Electrochemically controlled growth of silver nanocrystals on graphene thin film and applications for efficient nonenzymatic H2O2 biosensor, Electrochim. Acta 89 (2013) 222–228. [11] L. Meng, J. Jin, G.X. Yang, T.H. Lu, H. Zhang, C.X. Cai, Nonenzymatic electrochemical detection of glucose based on palladium-single-walled carbon nanotube hybrid nanostructures, Anal. Chem. 81 (2009) 7271–7280. [12] S.D. Sun, X.Z. Zhang, Y.X. Sun, S.C. Yang, X.P. Song, Z.M. Yang, Facile waterassisted synthesis of cupric oxide nanourchins and their application as nonenzymatic glucose biosensor, ACS Appl. Mater. Interfaces 5 (2013) 4429– 4437. [13] Q. Xu, L.N. Yin, C.T. Hou, X.X. Liu, X.Y. Hu, Facile fabrication of nanoporous platinum by alloying-dealloying process and its application in glucose sensing, Sens. Actuat. B: Chem. 173 (2012) 716–723. [14] M. Yuan, A.P. Liu, M. Zhao, W.J. Dong, T.Y. Zhao, J.J. Wang, W.H. Tang, Bimetallic PdCu nanoparticle decorated three-dimensional grapheme hydrogel for nonenzymatic amperometric glucose sensor, Sens. Actuat. B: Chem. 190 (2014) 707–714. [15] M.M. Liu, R. Liu, W. Chen, Graphene wrapped Cu2O nanocubes: non-enzymatic electrochemical sensors for the detection of glucose and hydrogen peroxide with enhanced stability, Biosens. Bioelectron. 45 (2013) 206–212. [16] L.M. Lu, X.B. Zhang, G.L. Shen, R.Q. Yu, Seed-mediated synthesis of copper nanoparticles on carbon nanotubes and their application in nonenzymatic glucose biosensors, Anal. Chim. Acta 715 (2012) 99–104. [17] J.P. Wang, H. Gao, F.L. Sun, Q. Hao, C.X. Xu, Highly sensitive detection of hydrogen peroxide based on nanoporous Fe2O3/CoO composites, Biosens. Bioelectron. 42 (2013) 550–555.

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