Novel Dendritic Palladium Nanostructure and Its Application in

Aug 4, 2007 - Leyla Soleymani , Zhichao Fang , Xuping Sun , Hong Yang , Bradford J. Taft , Edward H. Sargent , Shana O. Kelley. Angewandte Chemie ...
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J. Phys. Chem. C 2007, 111, 12609-12616

12609

Novel Dendritic Palladium Nanostructure and Its Application in Biosensing Ping Zhou, Zhihui Dai,* Min Fang, Xiaohua Huang, and Jianchun Bao* Department of Chemistry, Laboratory of Materials Science, Nanjing Normal UniVersity, Nanjing 210097, People’s Republic of China

Jiangfeng Gong National Laboratory of Microstructures, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: April 13, 2007; In Final Form: June 28, 2007

A simple wet chemical route to the novel dendritic Pd nanostructure is reported. The nanostructure evolves from the initially generated spherical particles by the reaction of Pd(II) ions with hydrazine in the presence of polyglycol into urchinlike particles and then into the final dendrites. It is found that the polyglycol and hydrazine play the key roles on the formation of the novel structure. Only the urchinlike particles are obtained in the absence of polyglycol. The hydrazine acts as not only a reducing agent but also a matter to promote the dissolution of the initially generated Pd particles. A growth mechanism including the solid-liquid-solid (SLS) transformation and polyglycol-assisted oriented attachment process is proposed. This work reports the first example of the branched metallic Pd nanostructures, which might be of significance on the qualitative understanding of the morphology evolution of nanostructures and the achieving of desired nanostructures controllably. The electrocatalytic properties of the dendritic Pd nanostructure and its application for sensing H2O2 are also reported. The H2O2 sensor based on a dendritic Pd-modified electrode has good stability and reproducibility. It has a wider linear range and a lower detection limit than those from spherical Pd particles.

Introduction The shape-controlled synthesis of metal nanoparticles has attracted considerable interest because their properties and applications are influenced greatly by their morphologies (e.g., spherical vs linear, solid vs hollow).1-5 Various strategies have been developed for the fabrication of nanoparticles with different shapes such as rods, wires, plates, disks, cubes, prisms, and hollow structures. Current trends in nanomaterials are pointing toward the fabrication of the hierarchical assembly of nanoscale building blocks into ordered superstructures or complex architectures, for example, from one-dimensional nanorods to branched nanostructures and from plates to porous spherical nanostructures. They are critical for the success of bottom-up approaches toward integrated and functional nanosystems.6-10 Although quite a few papers have been published describing the preparation of metal nanoparticles with different morphologies, the synthesis of hierarchically ordered and/or branched metal nanostructures still remains very limited. One reason is probably due to the fact that most metals, such as Ni, Pd, Pt, Cu, Ag, Au, etc., have highly symmetric face-centered cubic (fcc) crystal structures, which will affect the growing progress of the crystals. To form a branched nanostructure of these metals, a surfactant such as cetyltrimethylammonium bromide or poly(N-vinyl-2-pyrrolidone), etc., is usually required as a shape-directing agent by preferential adsorbing on specific crystal planes.7 In addition, the branched structures are generally observed in far-from-equilibrium aggregation growth,11 meaning that such novel branched metal structures can be formed by adjusting reaction conditions such as solvent, reactant, concen* To whom correspondence should be addressed. Tel: 0086 25 83598260. Fax: 0086 25 83598280. E-mail: (Z.D.) [email protected] or (J.B.) [email protected].

tration, temperature, etc. Finding novel approaches to architecturally control metal nanostructures is of a great challenge. More recently, a few branched metal nanostructures, especially Au and Ag, have been successfully obtained by using different wet chemical strategies.2b,2c,7,12 The development of the preparation of Pd nanoparticles with special morphologies becomes a very important issue because of their application in catalysis, sensing, and surface-enhanced Raman scattering (SERS).13-15 For instance, Pd hollow spheres of about 300 nm in diameter with high surface area were prepared by using silica spheres as a template, and they showed good catalytic activities in Suzuki coupling reactions. Pd triangular or hexagonal nanoplates could be selectively synthesized by manipulating the reduction kinetics of the polyol process and were found to be good active substrates for SERS because of their sharp corners and edges. These examples exhibit very interesting shape effects. Although various shapes of Pd nanoparticles have been synthesized,5a,13-16 there are few reports on the fabrication of hierarchically ordered structure, such as dendritic Pd nanostructures.17 Furthermore, the formation mechanisms remain to be clarified. Such novel nanostructures may have many potential applications in the fields of catalysis, sensing, microelectronic devices (nanometer-scale electrodes), and electrochemistry, stemming from their special characteristics, such as numerous branches, the open channels between the branches, large surface area, etc. In recent years, we have been exploring the possibility of the various nanostructures prepared by using inexpensive polyglycol as a structure-directing agent. Ni, Co, and CdS hollow spheres and PbS nanorods were successfully synthesized in polyglycol systems.18 As a part of our continuing effort to synthesize novel nanostructures, here, a polyglycol and hydrazine-assisted method has been further developed to prepare novel

10.1021/jp072898l CCC: $37.00 © 2007 American Chemical Society Published on Web 08/04/2007

12610 J. Phys. Chem. C, Vol. 111, No. 34, 2007 Pd dendrites with a high yield under mild conditions. It is found that such a dendritic Pd nanostructure evolves from the initially generated spherical particles by the reaction of Pd(II) ions with hydrazine in the presence of polyglycol, into urchinlike particles, and then into the final dendrites. A growth mechanism including the solid-liquid-solid (SLS) transformation and a polyglycolassisted oriented attachment process is proposed. In addition, the accurate determination of hydrogen peroxide (H2O2) is of great importance because it is an essential mediator in food, pharmaceutical, clinical, industrial, and environmental analyses. Herein, we also report a H2O2 sensor based on dendritic Pd immobilized on a glassy carbon electrode (GCE). The results demonstrate that such a H2O2 sensor is stable and has a wider linear range and a lower detection limit than those from spherical Pd particles, suggesting that this kind of Pd nanostructure is an efficient platform for applications in constructing sensors.

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Figure 1. XRD pattern of the Pd prepared by the typical reaction conditions.

Experimental Section All chemicals used in this work were used as received. In a typical synthesis, 20 mg of PdCl2 was dissolved in 1.0 mL of concentrated HCl to make solution A. Then, solution A, 33.3 mg of Na4P2O7, and 74 mg of NH4F were added to 4.0 mL of 0.33 g mL-1 polyglycol (Mw 20000) aqueous solution, followed by the addition of 25% ammonia. The pH value of the solution was then adjusted to 8.0 to obtain solution B. One milliliter of 80% hydrazine hydrate was added to 2 mL of 0.33 g mL-1 polyglycol aqueous solution to make solution C. Next, solution B was added to solution C under sonication at 60 °C. After the obtained mixture was further sonicated for about 10 min, it was aged for 3 h and then separated by centrifugation. The deposit was washed with deionized water and ethanol several times. After it was vacuum dried, the black dendritic Pd was obtained. The NH4F and Na4P2O7 used here act as a ligand and a stabilizer of the solution, respectively.19 The spherical Pd particles for comparison of electrochemical properties can be obtained by the typical condition mentioned above except for an aging time of 3 min. The phase characterization was performed by means of X-ray diffraction (XRD) using a D/Max-RA diffractometer with Cu KR radiation. The morphology and particle sizes of the samples were characterized by JEM-200CX transmission electron microscopy (TEM) and a JEOL 2010 high-resolution transmission electron microscopy (HRTEM) all working at 200 kV. A small amount of the black sample was dispersed in deionized water under sonification, and then, a drop of this solution was deposited on an amorphous carbon film on 300 mesh Cu grid for TEM observation. Field emission scanning electron microscopy (FESEM) characterization was carried out using a LEO1530VP microscopy at an acceleration of 15 KV. UVvis absorption spectra were obtained on a λ-17 spectrophotometer with a 1 cm optical length quartz cell. A Pd nanoparticle-modified GCE was prepared by dropping 2 µL of Pd nanoparticle suspension (0.2 mg mL-1) to the surface of a GCE and then letting it dry slowly at 4 °C. Cyclic voltammetric and amperometric measurements were performed on CHI660 electrochemical workstation (CH Instruments, United States). All electrochemical experiments were carried out in a cell containing 5.0 mL of 0.1 M, pH 7.0, phosphate buffer solution (PBS) at room temperature (25 ( 2 °C) and using a platinum wire as an auxiliary, a saturated calomel electrode as a reference, and the Pd-modified GCE as the working electrodes. All solutions were deoxygenated by bubbling highly pure nitrogen for at least 20 min and maintained

Figure 2. FESEM image of the dendritic Pd nanoparticles.

under a nitrogen atmosphere during measurements. The amperometric experiments were carried out by applying a potential of -0.40 V for H2O2 on a stirred cell at 25 ( 2 °C. The sensor responses were measured as the difference between total and residual currents. Results and Discussion Structural Characteristics and Growth Mechanism of Pd Nanostructures. Figure 1 shows the XRD pattern of the Pd prepared by the typical reaction conditions. The diffraction peaks in the range of 20 < 2θ < 85° can be indexed as fcc structures Pd (111), (200), (220), and (311), and the lattice parameter is a ) 3.89 Å, all of which are in good accordance with the ASTM standard 05-0681. The sharpness of the peaks indicates that the product is well-crystallized. A FESEM image of the prepared Pd is shown in Figure 2, which shows that the Pd particles are dendritic structures. The aggregation of the dendrites is flowerlike, implying that these dendrites might grow radially from the same particle. Also, as can be seen from the figure, the side branches of the dendrite are in the same plane (marked with arrows in Figure 2). The morphology and structure of the Pd particles are further studied using TEM, HRTEM, and selected area electron diffraction (SAED) shown in Figure 3. Figure 3A also shows the dendritic structure. On the dendritic structure, numerous side branches grow neatly (parallel to each other) along the long shaft (main branch) (Figure 3B). The angle between the shaft and the side branch is about 54.7°. The maximum length of the shaft is about 2.5 µm, while the diameter varies gradually from about 70 to 30 nm along the shaft. The length of each side branch mainly ranges from 80 to 230 nm, with width ranging from 8 to 20 nm. On the basis of the high magnification image, it is also noted that the side branch growing

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Figure 3. TEM images of the dendritic Pd particles at lower (A) and higher (B) magnification, respectively. (C) The SAED pattern of a left whole dendrite in panel B. (D) HRTEM image of the marked area with a circle in panel B, which is a joint between the shaft and a side branch. (E) HRTEM image of the marked area with an arrow in panel B, which is a joint between a side branch and a secondary side branch.

on the shaft can also act as a secondary shaft to grow secondary side branches. This interesting pattern is rarely found in other metallic dendritic structures.20 The SAED pattern of a left whole dendrite in Figure 3B is shown in Figure 3C. The clear spots in the Figure 3C indicate that the whole dendrite is a single crystal and can be indexed to (111) and (200) reflections from fcc Pd, and the zone axis projection is along 〈011h〉. The HRTEM images of the part between the shaft and a side branch (circled in Figure 3B) and the part between a side branch and a secondary side branch (marked with arrow in Figure 3B) shown in Figure 3D,E, respectively, all clearly reveal the good crystalline and lattice fringes. The fringe spacings of the lattice are about 0.22 and 0.19 nm, which match well with the interplanar spacings of (111) and (200), respectively. The results mentioned above from Figure 3C-E indicate that the crystal orientation of the side branches is the same as that of the shaft, and the shaft and side branches grow along 〈200〉 and 〈111〉, respectively. It is necessary to point out that dendritic Pd nanostructures can be easily obtained in a high yield up to 90% by using the present method. To the best of our knowledge, this is the first report about the wet chemical route to the synthesis of Pd dendrites. The growth information of the dendritic Pd nanostructure based on the Figure 3 analysis is illustrated in Figure 4. The growth direction of the shaft (main branch, OA) is 〈200〉, and the growth directions of the side branches (OB, OC) are 〈111〉 and 〈1h11〉, respectively. The angle of 54.7° between shaft and side branch is consistent with that of theoretical value of growth direction between 〈111〉 or 〈1h11〉 and 〈200〉, meaning that the

Figure 4. Illustration of the growth of the dendritic Pd nanostructure based in Figure 3.

shaft and side branch preferentially grow along two definite directions. In addition, the growth direction of some secondary side branches (growing along the side branches such as those marked with arrow in Figure 3B) is 〈200〉, parallel to that of the shaft. The formation of the anisotropic dendritic Pd nanostructure in solution is rather unexpected because Pd itself has a highly symmetric fcc crystal structure and the synthesis is carried out in an “isotropic” aqueous medium. How do the dendritic Pd nanocrystals form? To clarify the growth mechanism, it is essential to examine the different stages of the formation of Pd nanostructure. Figure 5A-D contains the TEM images of the Pd particles after reaction for 3, 15, 30, and 60 min, respectively. It is found that the spherical particles are initially formed at the

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Figure 6. UV-visible spectra of the reaction systems before and after reaction for 3 min.

Figure 5. TEM images of Pd nanostructures formed in polyglycol aqueous solution after reaction for (A) 3, (B) 15, (C) 30, and (D) 60 min, respectively.

earlier stages (Figure 5A). When aging for about 15 min, the studded spherical particles are formed and resemble urchins (Figure 5B). With increasing aging time to 30 min, the rudimentary shafts begin to form gradually (Figure 5C). At 60 min, most of the rudimentary shafts evolve into the structures with many pricks extruding toward both sides of the shafts (Figure 5D). Further prolonging the aging time to 180 min results in a perfect dendritic Pd nanostructure (Figure 3A). These results imply a two-stage growth mechanism of the dendritic Pd nanostructure, that is, the formation of the 〈200〉-oriented shaft and the subsequent growth of 〈111〉-oriented side branch on both sides of the shaft. In addition, it is worth noting from Figure 5C,D that a good many multiple arms grow from the same primary particle and eventually evolve to form welldefined dendritic nanostructures, which is consistent with the case in Figure 2. Such a novel phenomenon has not been observed previously. To learn about whether the unreacted Pd(II) ions are continuously reduced and deposited on initially formed Pd particles to directly form dendritic Pd nanostructures, we measured the UVvisible spectra of the solution before and after the reaction (Figure 6). Curves A and B are the spectra of typical reaction systems but without addition of Pd(II) ions or hydrazine, respectively. The absorption peak at about 300 nm in curve B is assigned to Pd(II) ions. No absorption peak at 300 nm was observed in the spectrum of a typical reaction after aging for 3 min (curve C in Figure 6), indicating that the Pd(II) ions are quickly reduced (no more than 3 min) to the metallic Pd. Thus, we can rule out the possibility of continuous reduction and deposition of unreacted Pd(II) ions on initially formed Pd particles. Kawai et al. achieved the dendritic gold nanostructure using CF3(CF2)7-SO2NH-(CH2)3N+(CH3)3I- (HFOTAI) not only as a reducing agent but also as a binder of the Au particles.12d In our case, it is also found that the polyglycol plays an important role for the formation of the dendritic Pd nanostructure. Without

Figure 7. TEM image of Pd nanoparticles obtained in the absence of polyglycol and keeping other typical conditions constant.

the addition of polyglycol and keeping other typical conditions constant, the obtained products are not dendritic nanocrystals but have a studded urchinlike morphology (Figure 7). Because polyol can be used as a weak reducing agent of noble metal salts, another test is carried out to further learn about the role of the polyglycol in the reaction system. Figure 8 shows the TEM images of the Pd particles prepared by using polyglycol instead of hydrazine as a reducing agent after reaction for 3 days. It reveals the formation of chainlike (Figure 8A) or circled chainlike (Figure 8B) structures composed of small particles. It is reasonable because the reduction rate of polyglycol with Pd(II) ions is very slow, which gives the generated Pd particles enough time to aggregate and form chainlike structures directed by polyglycol. Similar examples of one-dimensional nanostructures prepared using polyglycol as a structural directing agent were reported previously.18d,21 Thus, it is considered that polyglycol plays the important inducing role in the shaft formation of the dendritic Pd structure. It was found that the hydrazine also had an important effect on the formation of the dendritic Pd nanostructure. When the amount of the hydrazine was decreased from 1.0 to 0.1 or 0.5 mL and other typical conditions were kept constant, in the former case, only the particles with the morphology similar to Figure 5A were obtained after reaction for 3 h while in the latter only a few dendritic Pd particles were generated, implying that the formation of the dendritic Pd nanostructure was related to the presence of a great excess of hydrazine. Combined with the experimental observation that the initially formed spherical particles (Figure 5A) were through urchinlike particles (Figure 5B) to the final perfect dendritic nanostructure (Figure 3A), we speculated that the role of the greatly excess hydrazine was as a matter to dissolve the Pd particles, namely, some initially generated Pd particles redissolve under the action of hydrazine and then redeposit on the others to form the dendritic Pd nanoparticles. A similar case of Pd dissolving was reported by Arai et al. when palladium was used as a catalyst for the Heck

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Figure 8. TEM images of chainlike (A) and circled chainlike (B) Pd nanostructures obtained only using polyglycol as a reductant after reaction for 3 days and keeping other typical conditions constant.

Figure 9. TEM images of the Pd nanostructure synthesized under the typical conditions except the reaction temperature at 25 °C and aging time for 3 (A and B) and 8 h (D), respectively. (C) HRTEM image of the marked area with the arrowhead in panel B.

reaction; the reduced Pd could be more easily leached out into the solvent and then redeposited onto the supports in the presence of Na2CO3 and triethylamine.22 The amount of Pd that leached into the solvent increased with an increase of triethylamine use. Thus, in the present case, hydrazine is chosen not only as a reducing agent but also as a matter to promote the dissolution of the Pd particles and even the formation of dendritic nanostructures. On the basis of the observation and analysis mentioned above, the formation mechanism of the dendritic Pd structure can be interpreted as a SLS transformation and a polyglycol-assisted oriented attachment process. In the mechanism, Pd(II) ions are rapidly reduced with hydrazine to yield spherical particles, which is followed by the dissolving of some Pd particles under the action of excess hydrazine and then redepositing on the undissolved Pd particles in radial directions to form urchinlike particles. Each branch in the urchinlike particles further grows along the 〈200〉 direction to form shaft; meanwhile, 〈111〉oriented pricks grow on both sides of the shaft by the oriented attachment process with the assistance of polyglycol and hydrazine. Such a structure facilitates the formation of dendrite. Thus, the dendritic structure of Pd can still be formed through

the oriented attachment of nanoparticles despite the highly symmetric fcc crystal structure of the metal itself. To further learning about the oriented attachment growth mechanism, another temperature-dependent experiment was designed. Figure 9 displays TEM images of the samples synthesized under the typical conditions except the reaction temperature (at 25 °C) with an aging time of 3 (Figure 9A,B) and 8 h (Figure 9D). An obvious barb structure marked with arrows in Figure 9A and also a relatively delayed growth of Pd nanostructure, which has a rough surface with many short pricks growing along the shaft (marked with arrow in Figure 9B), are observed. The HRTEM image (Figure 9C) shows the microstructure information of a small particle on the tip of the shaft in Figure 9B (marked with an arrowhead). It displays continuous fringes, and the spacings of 0.19 and 0.22 nm are assigned to the interplanar spacings of (200) and (111), respectively. This suggests that the small particle is not a physical contact with the shaft but an epitaxial growth. In other words, the small particle and the shaft belong to the same crystal system with the same crystal orientation. The growth case of the Pd nanostructure further confirms the formation of the shaft and the dendrite by an oriented attachment mechanism. A similar

12614 J. Phys. Chem. C, Vol. 111, No. 34, 2007 pricky structure was reported by Shi et al. in the case of penniform BaWO4 nanocrystals.23 Other novel branched PbSe nanowires and Ag dendrites formed through oriented attachment of nanocrystals were also demonstrated.12a,24 The oriented attachment mechanism originally put forward by Penn and Banfield has been applied to explain the anisotropic growth of a variety of nanorods/nanowires via self-assembly of quasispherical nanoparticles.25 The present formation of the Pd shafts via self-assembly of Pd nanoparticles in the presence of polyglycol and hydrazine may represent another example of the oriented attachment. In addition, the growth rate of the Pd dendrite is markedly slow as compared with the case at 60 °C. Even with the aging time up to 8 h, only a few dendritic structures are formed (Figure 9D). Obviously, at the relatively low temperature, the solubility (under the action of hydrazine) of the Pd decreases, which results in the lower growing rate of the dendritic Pd nanostructure. While at a temperature of 60 °C, there is enough thermal energy and solubility to relatively quickly arrange the Pd atoms to form dendritic structures with the aid of polyglycol. In addition, it is worthy of pointing out from the timedependent experiments that various morphologies of the presently evolved Pd nanocrystals at different aging periods such as urchins or multipods can be kept after centrifugation and dried, respectively, suggesting that we provide a simple and facile approach to various novel Pd nanostructures. Electrocatalysis of Pd Modified GCE to Reduction of H2O2. From a technological point of view, these obtained dendritic Pd nanostructures may have important applications in microelectronic devices, such as nanometer-scale electrodes and electrochemistry. Recently, trends in the research and development of noble nanoparticles have increasingly emphasized the application in biosensing. Generated electroactive species can be measured amperometrically by these sensors. The detection of H2O2 has become very important because of its wide and varied applications in the textile, paper, cleaning product, and food industries. H2O2 and its derivatives are powerful oxidizing agents. Therefore, it can be employed in the synthesis of many organic compounds26 and used to treat environmental pollutants such as chlorine, aldehydes, phenols, and other aromatic compounds. Also, many enzymatic reactions create H2O2 as a product so its concentration may be used as an indicator of the progress of a reaction.27 Investigations have taken place into determining H2O2 concentrationviatitrimetry,28 spectrometry,29 andchemiluminescence.18c An alternative is the electrochemical detection of H2O2. Attempts have been made to modify electrodes with suitable electrocatalytic metals, such as platinum, rhodium, and palladium.30 Among these available transition metals, palladium is often chosen to modify the electrode because it is electrocatalytically active, relatively inexpensive, and difficult to oxidize.31 Furthermore, electrochemical determinations of H2O2 are generally performed by oxidation of H2O2 at a positive potential.32 However, such amperometric biosensors often suffer from electrochemical interference by oxidizable species, such as L-ascorbic acid, uric acid, and acetaminophen. In our present work, we measure the reduction of H2O2 at a negative potential at the dendritic Pd-modified electrode, which can avoid the interference from other electroactive species. Figure 10 shows the typical cyclic voltammetric curves for the H2O2 sensor. In 0.1 M, pH 7.0, PBS, the cyclic voltammogram of the dendritic Pd-modified GCE shows no peaks. Upon addition of H2O2 to the solution, the shape of the cyclic voltammogram changes dramatically with an increase of the

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Figure 10. Cyclic voltammograms of the dendritic Pd-modified GCE in 0.1 M, pH 7.0, PBS containing 0, 0.1, 0.3, and 0.5 mM H2O2 (from the bottom to the top) at 0.020 V s-1. Inset: Cyclic voltammograms of the spherical Pd particles-modified GCE in 0.1 M, pH 7.0, PBS containing 0, 0.1, 0.3, and 0.5 mM H2O2 (from the bottom to the top) at 0.020 V s-1.

Figure 11. Amperometric responses of the dendritic Pd (a) and the spherical Pd (b) modified GCEs at -0.40 V upon successive additions of 5 µL of 2 mM H2O2 to 5.0 mL of 0.1 M, pH 7.0, PBS. Inset: Plot of catalytic currents of dendritic Pd (a) and the spherical Pd (b) modified GCEs vs H2O2 concentration.

Figure 12. Stability of the biosensors for H2O2 stored in 0.1 M, pH 7.0, PBS at 4 °C (a) and in air at room temperature (b).

reduction current, displaying an obvious electrocatalytic behavior of the dendritic Pd to the reduction of H2O2. No electrocatalytic current is observable at a bare GCE when H2O2 is added to pH 7.0 PBS (Figures not shown), indicating that the reduction of H2O2 results from dendritic Pd. Furthermore, the reduction current increases with an increase of H2O2 concentration, suggesting that the dendritic Pd has a potential application in biosensing. The spherical Pd particles modified GCE also displayed electrocatalytic responses to H2O2 shown as an inset in Figure 10. However, after the H2O2 concentration was increased to 0.3 mM, the increase of the electrocatalytical response became very

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SCHEME 1: Schematic Illustration of the Formation of Dendritic Pd Nanostructure

small, indicating that the linear range of H2O2 was narrower than that of the dendritic Pd particles modified GCE. The amperometric response of dendritic Pd-modified GCE with successive additions of H2O2 to 0.1 M, pH 7.0, PBS at an applied potential of -0.40 V is shown as curve a in Figure 11. Upon addition of an aliquot of H2O2 to PBS, the reduction current increases steeply to reach a stable value. The electrode achieves 95% of the steady-state current in less than 10 s. The results clearly demonstrate that the electrocatalytic response is very fast. Under the optimal conditions, the linear response range of the sensor to H2O2 concentration is from 1.0 µM to 1.02 mM with a correlation coefficient of 0.9999 (curve a in the inset in Figure 11), which is wider than 1.0 µM to 0.1 mM at the Pt-modified Au electrode.33 The sensitivity is 78.3 nA cm-2 µM-1, which is higher than 29.3 nA cm-2 µM-1 on Pd/PPD/ GluOx at 700 mV.34 The detection limit is estimated to be 2.4 × 10-7 M at a signal-to-noise ratio of 3, which is lower than 1.0 µM on a silver macroelectrode35 and similar to 2.4 × 10-7 M on a Pt-modified Au electrode.33 As a comparison, the amperomatric response of the spherical Pd-modified GCE was evaluated as curve b in Figure 11. Under optimal conditions, the linear range of the spherical Pd particles modified GCE was from 1.0 to 300 µM with a correlation coefficient of 0.9990 (curve b in inset in Figure 11). The sensitivity was 70.8 nA cm-2 µM-1. The detection limit was estimated to be 3.2 × 10-7 M at a signal-to-noise of 3. From Figure 11, we can see that the signal of the spherical Pd particles modified GCE was noisy, which resulted in the detection limit of H2O2 being not very low. These results might be due to the dendritic structures of Pd with large surface-to-volume ratios and a hierarchical textures acting as a nanoscale bridge to promote the electron transfer between substrates and electrode surfaces and increase the sensitivity of the biosensors.12a Therefore, the dendritic Pd particles modified GCE showed a good electrocatalytic response to the reduction of H2O2. The storage stabilities of H2O2 biosensors stored in 0.1 M, pH 7.0, PBS at 4 °C or in air at room temperature were examined by periodically checking their relative response currents (the ratios of the catalytic currents detected at different times to the initial current value) in 0.1 M, pH 7.0, PBS containing 10 µM H2O2 (Figure 12). After 1 month, the sensors could retain 95% of activity to H2O2 in 0.1 M, pH 7.0, PBS at 4 °C, while only 84.5% of activity to H2O2 was retained when stored in air at room temperature, respectively. Thus, the biosensors were stored in 0.1 M, pH 7.0, PBS at 4 °C when not in use. The fabrication reproducibility of six electrodes, made independently, showed an acceptable reproducibility with a relative standard deviation of 3.8% for the current determined at 10 µM H2O2. Conclusions The novel dendritic Pd nanostructure has been synthesized with a high yield by a simple wet chemical method. Polyglycol and hydrazine play the key role on the formation of the Pd

nanostructure. The formation process of the dendritic Pd can be shown in Scheme 1; that is, Pd(II) ions are rapidly reduced to yield spherical particles with hydrazine (A). Under the action of hydrazine, some Pd particles form urchinlike particles by SLS transformation (B); then, these particles further grow to form shafts with many short pricks extruding toward both sides (C) by oriental attachment with the aid of polyglycol, which is followed by growing to the dendritic Pd (D). Hydrazine is acts as not only a reducing agent but also a matter to promote the dissolution of the initially generated Pd particles. The current understanding of the growth mechanism of the nanostructure may be potentially applied for designing new strategies to control synthesized novel hierarchical and/or complex nanostructures with desired architectures. A H2O2 sensor based on the dendriric Pd was constructed, showing good stability and a wider linear range and a lower detection limit than those from spherical Pd particles. This application study suggests that this novel Pd nanostructure is an efficient platform for constructing sensors and such patterned nanostructures will further find applications in other related fields. The preparation and properties of Pd and other hierarchically ordered metal nanostructures are underway in our laboratory. Acknowledgment. This work was supported by the National Natural Science Foundation of China for the project (Nos. 20473038, 20505010, and 20471030). References and Notes (1) (a) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (b) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (c) Sun, Y.; Mayers, B.; Xia, Y. AdV. Mater. 2003, 15, 641. (2) (a) Payne, E. K.; Shuford, K. L.; Park, S.; Schatz, G. C.; Mirkin, C. A. J. Phys. Chem. B 2006, 110, 2150. (b) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327. (c) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (3) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (b) Mirkin, C. A.; Me´traux, G. S. AdV. Mater. 2005, 17, 412. (c) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (4) (a) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (b) Dumestre, F.; Chaudret, B.; Amiens, C.; Fromen, M. C.; Casanove Renaud, M. J. P.; Zurcher, P. Angew. Chem., Int. Ed. 2002, 41, 4286. (5) (a) Bradley, J. S.; Tesche, B.; Busser, W.; Maase, M.; Reetz, M. T. J. Am. Chem. Soc. 2000, 122, 4631. (b) Bao, J.; Tie, C.; Xu, Z.; Zhou, Q.; Shen, D.; Ma, Q. AdV. Mater. 2001, 13, 1631. (6) (a) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L.; Alivisatos, A. P. Nature 2004, 430, 190. (b) Kanaras, A. G.; So¨nnichsen, C.; Liu, H.; Alivisatos, A. P. Nano Lett. 2005, 5, 2164. (7) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (8) (a) Kovtyukhova, N. I.; Mallouk, T. E. Chem. Eur. J. 2002, 8, 4355. (b) Yan, H.; He, R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. J. Am. Chem. Soc. 2003, 125, 4728. (9) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303, 348. (10) Hou, Y.; Kondoh, H.; Ohta, T. Chem. Mater. 2005, 17, 3994. (11) (a) Wang, M.; Liu, X.-Y.; Strom, C. S.; Bennema, P.; van Enckevort, W.; Ming, N.-B. Phys. ReV. Lett. 1998, 80, 3089. (b) Wang, S.; Xin, H. J. Phys. Chem. B 2000, 104, 5681.

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