Dendritic Nanostructures of Silver: Facile Synthesis, Structural

Apr 5, 2006 - Structural characterizations suggest the preferential growth along 〈100〉 and 〈111〉 directions by oriented attachment of silver n...
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Langmuir 2006, 22, 4836-4842

Dendritic Nanostructures of Silver: Facile Synthesis, Structural Characterizations, and Sensing Applications Xiaogang Wen, Yu-Tao Xie, Martin Wing Cheung Mak, Kwan Yee Cheung, Xiao-Yuan Li, Reinhard Renneberg, and Shihe Yang* Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ReceiVed January 27, 2006. In Final Form: March 3, 2006 Silver nanodendrites are synthesized by a simple surfactant-free method using a suspension of zinc microparticles as a heterogeneous reducing agent. Structural characterizations suggest the preferential growth along 〈100〉 and 〈111〉 directions by oriented attachment of silver nanocrystals in the diffusion limit, leading to the formation of silver nanodendrites 20-30 nm in stem and branch diameter and 5-50 µm in length. Surface-enhanced Raman scattering studies show that the silver nanodentrites give an intensive and enhanced Raman scattering when pyridine was used as a probing molecule. We have also demonstrated that the silver nanodendrites increase the sensitivity of an electrochemical glucose biosensor by as much as 1-2 orders of magnitude.

Introduction Due to their unique electrical and optical properties, onedimensional (1D) nanostructures of Ag have been extensively explored. Above all, various template techniques have been used, including porous alumina,1-4 mesoporous silica,5 calix[4]hydroquinone nanotubes,6 DNA,7 tubular protein assemblies,8 and polyol process.9-12 At the same time, a mechanism of nanoparticle-to-nanowire organization has been proposed in a study attempting to synthesize Ag nanowires in supercritical water.13 Using more or less similar methods, Ag dendritic nanostructures have also been synthesized, although such metallic dendrites are traditionally formed on electrode surfaces by electrochemical deposition.14 Such metallic nanodendrites are a type of higher level 1D nanomaterials, have a hierarchical structure, and could have potential applications in plasmonics and biosensors. Chen et al. produced Ag dendrites in the presence of poly(vinyl alcohol) (PVA) by a photoreduction technique.15 Using Reney nickel as a template and a reducing agent, Xie et al. synthesized Ag dendritic nanostructures.16 Although the mechanism of dendrite formation is still poorly understood, it * Corresponding author. E-mail: [email protected]. (1) Luo, J.; Huang, Z. P.; Zhao, Y. G.; Zhang, L.; Zhu, J. AdV. Mater. 2004, 16, 1512. (2) Choi, J.; Sauer, G.; Nielsch, K.; Wehrspohn, R. B.; Gosele, U. Chem. Mater. 2003, 15, 776. (3) Choi, J.; Sauer, G.; Goring, P.; Nielsch, K.; Wehrspohn, R. B.; Gosele, U. J. Mater. Chem. 2003, 13, 1100. (4) Cheng, Y. H.; Cheng, S. Y. Nanotechnology 2004, 15, 171. (5) Huang, M. H.; Choudrey, A.; Yang, P. D. Chem. Commun. 2000, 1063. (6) Hong, B. H.; Bae, S. B.; Lee, C. W.; Jeong, S.; Kim, K. S. Science 2001, 294, 348. (7) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (8) Behrens, S.; Wu, J.; Habicht, W.; Unger, E. Chem. Mater. 2004, 16, 3085. (9) Sun, Y. G.; Gates, B.; Mayers, B.; Xia, Y. N. Nano Lett. 2002, 2, 165. (10) Sun, Y. G.; Xia, Y. N. AdV. Mater. 2002, 14, 833. (11) Gao, Y.; Jiang, P.; Liu, D. F.; Yuan, H. J.; Yan, X. Q.; Zhou, Z. P.; Wang, J. X.; Song, L.; Liu, L. F.; Zhou, W. Y.; Wang, G.; Wang, C. Y.; Xie, S. S. Chem. Phys. Lett. 2003, 380, 146. (12) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R. R.; Sun, Y. G.; Xia, Y. N.; Yang, P. D. Nano Lett. 2003, 3, 1229. (13) Chang, J. Y.; Chang, J. J.; Lo, B.; Tzing, S. H.; Ling, Y. C. Chem. Phys. Lett. 2003, 379, 261. (14) Geddes, C. D.; Parfenov, A.; Gryczynski, I.; Lakowicz, J. R. J. Phys. Chem. B 2003, 107, 9989. (15) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z. Y. AdV. Mater. 1999, 11, 850. (16) Xiao, J. P.; Xie, Y.; Tang, R.; Chen, M.; Tian, X. B. AdV. Mater. 2001, 13, 1887.

is often believed that surfactants or templates are needed for the growth of Ag nanodendrites.15,16 Beside the diffusion-limited aggregation (DLA) effect, anisotropic crystal growth is thought to play an important role in the formation of Ag dendrites.17-19 However, it is not quite clear how the anisotropic growth affects the development of dendritic morphology. An interesting question is whether one can obtain dendritic metallic nanostructures by solution chemistry without the use of any surfactants or templates. It is well-known that hierarchical spatial-temporal patterns created far from equilibrium are ubiquitous in nature, especially in biological systems. In theory, a kinetic regime could be created by the balance of diffusion and reaction rates for the surfactant-free growth of dendrites. The investigation into this question is anticipated to advance the synthetic techniques for metallic nanodendrites. Here we report on our successful synthesis of Ag dendritic nanostructures using finely dispersed Zn microparticles as a reducing agent. By this method, phase-pure and well-defined Ag nanodendrites can be synthesized in high yield (>85%) without using any templates or surfactants. The products can be separated from the solution by a simple centrifugation step. This is the first time the surfactantfree Ag nanodendrites are synthesized by a method other than the electrochemical method. We have carefully analyzed the crystal structure of the Ag nanodendrites and discussed their formation mechanism on the basis of the structure and the reaction. We also moved on to explore the applications of the novel dendritic structures in chemical/biochemical sensors using spectroscopic and electrochemical methods. Experimental Section Preparation and Characterization of Ag Dendritic Nanostructures. A typical synthesis of the Ag dendrite nanostructures proceeds as follows. First, a 3 × 10-2 M AgNO3 (99.8%, RiedeldeHaen) aqueous solution was prepared. A 100 mg amount of Zn microparticles (98%, Aldrich) was dispersed in 10 mL of ethanol in an ultrasonic bath for 30 min. Into 20 mL of the AgNO3 solution, 1 mL of the Zn microparticle suspension was dropped under stirring at room temperature. After stirring for 6 h, the suspension was (17) Wang, S. Z.; Xin, H. W. J. Phys. Chem. 2000, 104, 5681. (18) Zhu, Y. C.; Zheng, H. G.; Li, Y. A. Mater. Res. Bull. 2003, 38, 1829. (19) Qiu, T.; Wu, X. L.; Mei, Y. F.; Chu, P. K. Appl. Phys. A 2005, 81, 669.

10.1021/la060267x CCC: $33.50 © 2006 American Chemical Society Published on Web 04/05/2006

Dendritic Nanostructures of SilVer centrifuged at 2000/min. The upper pellucid solution was decanted, and the precipitate was washed with ethanol 3 times and dried in air for further measurements. The morphology of the Ag nanodendrites was characterized using JEOL 6300 and JEOL 6300F scanning electron microscopes (SEMs) at an acceleration of 15 kV. The crystal phase was identified by powder X-ray diffraction (XRD, Philips, PW-1830 X-ray diffractometer with a 1.5405 Å Cu KR rotating anode point source operated at 40 kV and 40 mA). Further microstructure analysis was carried out using transmission electron microscopy (TEM, Philips CM20 and JEOL 2010F microscope operating at 200 kV). For TEM characterization, an ethanol suspension of the Ag nanodendrites was dropped on a carbon-coated Cu grid and dried in a vacuum. SERS Measurements. The silver electrode (i.d. ) 3 mm) was first polished mechanically with a Al2O3 slurry (0.03 µm) to a mirrorlike finish before being rinsed thoroughly with deionized water. Activation of the electrode was then performed on BAS-100 by electrochemical roughening with 10 oxidation-reduction cycles (ORCs) between the potentials of 400 and -600 mV at a scan rate of 50 mV s-1 in a 0.1 M KCl solution that had been thoroughly bubbled by N2. A conventional three-electrode electrochemical cell was used in all measurements. A platinum wire was used as the auxiliary electrode. A saturated calomel electrode (SCE) served as the reference electrode. All the potentials were reported with respect to the SCE. The silver-nanodendrite-film@glassy-carbon (GC) electrode was prepared by dropping a 40 µL aqueous suspension of the Ag nanodendrites directly onto a GC electrode (i.d. ) 3 mm), followed by solvent evaporation in air. The as-prepared working electrodes were rinsed with distilled water before being transferred to the spectroelectrochemical cell for surface-enhanced Raman spectroscopy (SERS) measurements. Potentiostatic control for SERS measurements was provided by a PAR model 273 A potentiostat. Surface-enhanced Raman spectroscopic measurements were performed with the micro-Raman spectrometer (Renishaw 1000, Gloucestershire, U.K.), equipped with CCD detectors and with an excitation line of 514.5 nm. The laser was focused onto a ca. 3 µm diameter spot on the sample surface via a 50× ultralong working distance objective (WD 8 mm and NA 0.55, Olympus). The laser power on the sample was ca. 1.1 mW, and three scans were taken with 10 s accumulation time for each scan. The electrolyte for SERS measurements contained 0.1 M KCl and 0.05 M pyridine. Biosensor Measurements. Enzyme electrodes were prepared by sol-gel technique according to Ren et al.20 In brief, an aqueous glucose oxidase (GOD) solution (1 U µL-1; one unit is defined as the amount of enzyme activity which catalyzes the transformation of 1 µmol of substrate/min under the standard condition) of 10 µL was mixed with a 200 µL ethanol suspension containing various amounts of silver-nanodendrites. Added into the enzyme-Agnanodendrite suspension mixture was 1 mL of polyvinyl butyral (PVB) dehydrated ethanal solution (2%) and 50 µL glutaraldehyde solution (1%). A platinum electrode was dipped into the suspension mixture prepared above and incubated for 8 min. The electrode was then pulled out from the suspension mixture and dried at ambient conditions. Electric current signals were measured and recorded with a potentiostat (Biometra EP-30, Go¨ttingen, Germany). Measurements were carried out inside a stirring cell filled with 5 mL of 0.05 M phosphate buffer saline (PBS), pH 6.85. A three-electrode configuration consisting of a Pt working electrode, a Ag/AgCl reference electrode, and a Pt counter electrode was employed for amperometric measurements at +600 mV. To characterize the response of the biosensor, we systematically added various amounts of a 100 mM stock glucose solution into the test electrolyte and monitored the current changes. Cyclic voltammetric (CV) experiments were carried out using a BAS model 100B (West Lafayette, IN) voltammetric analyzer driven by BAS 100W version 2.0 software. Again, the three-electrode configuration was used as described above. (20) Ren, X. L.; Meng, X. W.; Chen, D.; Tang, F. Q.; Jiao, J. Biosens. Bioelectron. 2005, 21, 433.

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Figure 1. XRD pattern of as-synthesized Ag nanodendrites.

Results and Discussion Figure 1 shows an XRD pattern of the Ag nanodendrites. All the diffraction peaks observed can be indexed to the cubic Ag with a unit cell parameter being a ) b ) c ) 4.0816 Å (4.0862 Å, from JCPDS 04-0783). No Zn peak is observed, indicating a complete reaction of the Zn precursor accompanied by the formation of Ag nanodendrites. A TEM image of a typical dendritic nanostructure is shown in Figure 2A at a low magnification. One is struck by a remarkable hierarchical structure sprawling to several generations with apparent self-similarity. The overall length of the dendrite is about 5 µm, and both the stem and the branches are 20-30 nm in diameter. It is clear that the nanodendrite is highly symmetric, and the angles between the stem and the branches are mostly about 50-60°. As can be seen from the inset of Figure 2A, the selected area electron diffraction (SAED) pattern displays discontinuous concentric rings characteristic of the cubic Ag, indicating that although the whole Ag nanodendrite is not a perfect single crystal, it has a certain extent of preferential crystal orientation. Figure 2B is a higher magnification TEM image from a subbranch (i.e., the third generation) of the Ag nanodendrite, which shows a robust connection between the branch and the subbranch. The HRTEM image in Figure 2C shows a connection region between a stem and one of the branches. It can be seen that continuous fringes run through both stem and branch. This suggests that the connection between branch and stem is not simply a physical contact but rather like an epitaxial growth; both belong to the same crystal system with nearly the same crystal orientation. The fringe spacing perpendicular to the stem measures 0.20 nm, which is very close to the interplanar spacing of (100). Another set of fringes perpendicular to the branch displays a fringe spacing of 0.23 nm, which matches well the interplanar spacing of (111). Figure 2D shows a HRTEM image of the Ag nanodendrite at another connection site between a branch and one of its subbranches (the part enclosed by the dashed white square in Figure 2B). The fringes perpendicular to the branch and to the subbranch can be seen clearly with spacings of 0.23 and 0.20 nm, which are assigned to the interplanar spacings of (111) and (100), respectively. According to the analysis above from Figure 2C,D, we can conclude that the successive generations from stem, to branch, to subbranch of the nanodendrite grow along 〈100〉, 〈111〉 and 〈100〉, respectively. Three types of dendritic structures can be found in the products in Figure 3, which differ mainly by the number of branching generations as well as the dendritic symmetry. Figure 3A displays a two-generation dendritic structure with sparse (1 branch/(100 nm)) branches (∼5%); both the stem and branches are uniform in diameter (20-30 nm) except for a thinning part formed at the connection site. In general, the branches are flanked by the stem symmetrically by an angle of 53-60°, and they become shorter

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Figure 2. (A) Low-magnification TEM image of a typical Ag nanodendrite together with an ED pattern (inset); (B) higher magnification TEM image of the Ag nanodendrite; (C) HRTEM image of the Ag nanodendrite in a joint between the stem and a branch; (D) HRTEM image of the Ag nanodendrite in a joint between a branch and a subbranch.

and shorter from the bottom to the tip of the stem. This structure is characterized by the fact that no subbranches are formed on the branches. The morphology of the main product (∼85%) is shown in Figure 3B. This structure is also symmetric and straight with a dendritic angle similar to that in Figure 3A. However, here the branching densities are much higher (1 branch/(10 nm)) than that in Figure 3A, and there are at least three branching generations. In addition, the trend becomes more pronounced that branches become shorter and shorter from the bottom to the tip of the stem. This typical structure has been described in detail in Figure 2. Still another different dendritic structure (∼5%) is shown in Figure 3C. Compared to the dendritic structures in Figure 3A,B, this structure is even more dendritic but less symmetric with less straight stems and branches/subbranches. More significantly, the angles between the stems and the branches vary in a wide range of 15-90° in marked contrast to the more or less fixed angle for the dendritic structures in Figure 3A,B. This hints at a somewhat different growth mechanism, as will be discussed below. It is reasonable to believe that the dendritic growth occurs at the tips of stem and branches. As the stem becomes longer and longer, new branches are formed near the tip region continuously; these new branches are shorter due to the shorter growth time. This is exactly what we observed in the dominant treelike product in Figure 3B. The nearly epitaxial joints of stem/branch and branch/subbranch in Figure 3A,B with a fixed angle of 52-62° indicates that the growth is globally diffusion-controlled but locally accomplished by oriented attachment. Such oriented attachment has recently been found to be responsible for the formation dislocations during early crystal growth.21 Notice that the angle between {100} and {111} is about 55°, which falls

within the angle range of stem/branch and branch/subbranch. As such, the formation of the dendritic structure comes from the preferential growth along the 〈100〉 and 〈111〉 directions alternately in the successive branching generations. However, the angles between stem and branch are not exactly 55°, and therefore they are likely to be affected by the diffusion of Ag precursors other than the crystal growth direction. This actually culminates in the growth of the third type of dendritic structures shown in Figure 3C with wide-ranging angles between the neighboring generations (15-90°). In this case, the crystal growth direction plays an even less important role. In fact, a HRTEM study (Supporting Information, Figure S1) reveals that the fringes in the stem/ branch joints are not continuous. This indicates that the formation of these types of structures is dominated by diffusion control: the dendritic structure is obtained by physical adsorption of diffusing Ag nanocrystals once in contact with each other. This process may take place through the aggregation of Ag nanocrystals on the carbon-coated Cu grids accompanying the evaporation of ethanol, which was also proposed by Wang and Xin.17 This is supported by the presence of small isolated Ag nanocrystals (see Figure 3D) 4-10 nm in diameter with the fcc structure. Most probably, the Ag nanocrystals are a precursor, which arises from the reaction of Ag+ and Zn, to the nanorods and nanodendrites by combining a DLA mechanism to different extents. There is no doubt that diffusion of Ag nanocrystal precursors plays an important role in the formation of the Ag nanodendrites but to different extents in the cases shown in Figure 3A-C. For the nanodendrites in Figure 3A,B, diffusion and oriented attachment are both important, which most likely occur during (21) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969.

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Figure 3. TEM images of different Ag nanostructures obtained from the reaction: (A) sparse dendritic structure (∼5%); (B) dense, symmetric dendritic structure (∼85%); (C) dense asymmetric dendritic structure (∼5%); (D) Ag nanocrystals (∼3%)sprecursors of the dendritic structures.

the Ag+ reduction processes near the Zn microparticles. The nascent small Ag nanoparticles may be active enough to execute the oriented attachment. In addition, the concentration gradient of the Ag nanocrystal precursors sets up a uniform diffusion front, leading to the formation of the symmetrical dendrites with preferential crystal orientations, which dominate the reaction products. On the other hand, the nanodendrites in Figure 3C in small amount (∼5%) may be formed at a later stage such as during the TEM sample preparation. As a result, diffusion plays a dominant role here and the Ag nanocrystals are less active so that they are locked in position once in contact with each other. This makes the angles of stem/branch and branch/subbranch vary widely between 15 and 90° (Figure 3C) instead of 55° observed in the symmetrical dendrite in Figure 3A,B. In this case, there are perhaps large concentration differences on different growth sites, and as a result, asymmetrical dendrites will be formed as observed. We next examine the fractal phenomenon of the Ag nanodendrites. The fractal dimension (D) for a planar section of a Ag dendrite was obtained by using the box-counting algorithm.17,22 In this method, TEM images are covered with a grid of squares of size r and the total number N(r) of nonvacant boxes are described as follows: N(r) ∼ r-D. By plotting log N(r) vs log r, the value of D can be calculated from the slope. Figure 4 shows the double logarithmic plots of N(r) vs r for different dendrite morphologies shown in Figure 3. Curves b1 and b2 correspond to the TEM images in Figure 3B and the dashed white rectangular part of Figure 3B, respectively. Each curve is well-fitted by a straight line, and the fractal dimension is (22) Russel, D. A.; Hanson, J. D.; Ott, E. Phys. ReV. Lett. 1980, 45, 1175.

Figure 4. Double logarithmic plots of N(r) vs r for the different dendritic morphologies shown in Figure 3: curve a, Figure 3C; curve b, Figure 3B; curve c, enclosure of the dashed white rectangle in Figure 3B.

determined to be 1.85 and 1.84, respectively. The close D values obtained at different length scales indicate that the nanodrendrite structures exhibit good symmetry and self-similarity characteristic of fractals. Curve a is obtained from the morphology shown in Figure 3C. It is also fitted to an excellent straight line, and the fractal dimension is found to be 1.96. This is somewhat larger than those from curves b1 and b2. This is consistent with the fact that the nanodendrites in Figure 3C are less well-defined compared with those in Figure 3B due to the larger and more asymmetric concentration gradients as well as the more arbitrary positions for the docking of the Ag nanoparticles. Because the Ag nanoparticles serve as a precursor, their concentration is expected to directly affect the growth of the

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Figure 5. SEM images of Ag dendritic nanostructures grown at different distances away from the Zn source: (A) Zn source region; (B) region 3 mm away from the Zn source; (C) region 6 mm away from the Zn source.

Ag nanodentrites. To demonstrate this point, we fixed the Zn microparticles in a confined area (2 × 2 mm) at an edge of a Si slide (10 × 10 mm) and carefully immersed the substrate into the reaction solution for the growth of Ag nanodendrites. The reaction proceeded in a static condition without stirring. Figure 5 shows the SEM image of the resulting Ag products formed at different distances away from the Zn microparticle source. Right at the area where the Zn source was placed, plenty of dendritic structures (∼80%) are produced (Figure 5A) and the length of the backbone can reach up to 40-50 µm. Note that some particles are also formed. For the products 3 mm away from the Zn source, nearly all the Ag products have dendritic structure although the total amount is less than that in the Zn source area. In general, the dendrites in this area are smaller (about 10-15 µm) in length due obviously to the limited supply of Ag nanocrystals. Finally, in the region 6 mm away from the Zn, only a tiny amount of small dendrites are formed (Figure 5C), and instead, many particles can be found, which suggest that the short supply of the Ag nanocrystals cannot sustain the dendritic growth. These results suggest that, in the static solution, an appropriate concentration gradient of the Ag nanocrystals is necessary for the successful growth of well-defined nanodendrites. As mentioned above, the overall process of dendrite growth can be described as: Ag+ + Zn f Ag f Ag nanocrystals f Ag nanodendrite. Near the Zn source, Ag nanocrystals are produced near the Zn source with the largest concentration gradient in this

Figure 6. SERS spectra of pyridine adsorbed on a Ag-nanodendrite-film@GC electrode (A) and a randomly roughened silver electrode (B) obtained at different applied electrical potentials.

region. The aggregated Ag particles including dendrites are mostly formed in this region due to the copious supply of Ag nanocrystals. At some distance (3 mm) away from the Zn source, the yield of dendrites is higher, although the total products are less. This is attributed to the appropriate concentration gradient of the Ag nanocrystals for the dendritic growth. However, when the growth site is too far away from the Zn source (6 mm), the supply of the Ag nanocrystals is too small for the dendritic growth. The small size, large surface area, and hierarchical structure of the Ag nanodendrites provide a unique substrate for surfaceenhanced Raman scattering (SERS). For this purpose, the Ag nanodendrites are deposited on a glassy carbon (GC) electrode and pyridine is employed as a probe molecule. Figure 6 shows SERS spectra of pyridine adsorbed on the silver-nanodendritefilm@GC electrode (A) and, for comparison, a randomly roughened silver electrode (B) obtained at different applied electrical potentials. On both electrodes, when the applied potential is increased in the negative direction, the Raman peak intensities increase markedly. SERS spectra are dominated by the totally symmetric A1 mode of pyridine, suggesting that the enhancement is along the C2 axis of the pyridine molecule. The peaks corresponding to ring breathing and stretch modes at ∼1005 (a1), 1034 (a1), and 1591 cm-1 (a1) reach the maximum intensities

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at ca. -0.6 V, similar to that observed from a randomly roughened Ag electrode.23 The overall SERS intensities start to decrease when the applied potential is switched to more negative. However, the in-plane C-H deformation mode at 1214 cm-1 still keeps increasing until ∼-0.8 V. The relative intensities of the totally symmetric 622 cm-1 (a1) mode and the non-totally symmetric 650 cm-1 (b2) mode vary significantly with the applied potential. The intensity of the 622 cm-1 (a1) mode is much stronger than that of the 650 cm-1 (b2) mode at -0.6 V, indicating that the pyridine is adsorbed on the substrate in a stand-on manner with its C2 axis perpendicular to the substrate surface. The Ag-Cl stretch mode at ca. 237 cm-1 is shifted to 221 cm-1 and becomes much weaker (Figure 6A) when the electrical potential is switched to -0.6 V from open-circuit potential (OCP), indicating that the chloride ion adsorbed on the silver nanodendrite surface has begun to desorb and the pyridine has already started to adsorb on the Ag surface. When the electrical potential is set to -0.8 V, the adsorption of pyridine on silver is clearly evidenced by the appearance of peaks at 249 cm-1 in Figure 6A and 241 cm-1 in Figure 6B (Ag-Npyridine stretch mode),24 suggesting that the adsorption of pyridine on the silver nanodendrites is slightly stronger than that on the OCR roughened silver electrode. In addition to the characteristic strong bands observed at 1591 (a1), 1034 (a1), and 1005 cm-1 (a1), weak peaks are also observed at ca. 1628, 1569, 1479, 1442, 1382, 1238, 1150, 1094, 1064, 938, 750, 705, 650, 622, and 410 cm-1, more clearly for pyridine adsorbed on the silver-nanodendrite-film@GC electrode than on the OCR roughened silver electrode. These peaks are well in agreement with those reported in the literature.25 Comparing the SERS results in Figure 6A,B, it is found that the overall intensity from the silver-nanodendrite-film@GC electrode is generally higher than that from the randomly roughened silver electrode, presumably due to the property of special potential distribution of the silver nanodendrite.26 At the OCP, the intensity ratio (IA/ IB) is calculated to be ∼4. As the negative applied potential is increased, both the varied orientation of the adsorbed pyridine and the negative potential itself contribute to the SERS enhancement, so some of the peaks reached the maxima. When the negative applied potential was increased to -0.4 V, the ratio of (IA/IB) is increased to about 4.7. Further increase in the negative applied electrical potential is followed by a decrease in the intensity ratio of (IA/IB). Our results demonstrate that the overall scattering efficiency of the silver-nanodendrite-film@GC electrode is much higher than that of the OCR roughened silver electrode. Moreover, the silver-nanodendrite-film@GC electrode has the advantage of better and stable reproducibility than the randomly roughened silver electrode, a property highly desirable for any application of SERS in analytical detection. We reasoned that the Ag nanodendrites are an ideal conducting matrix for immobilization of enzymes for the development of redox biosensors. The performance of a biosensor is determined by the Km value (Michaelis-Menten constant) of the enzyme and the rate of electron transfer between the enzyme and electrode surface. One can imagine that the Ag nanodendrites, being biocompatible and mechanically stable, provide not only finely divided supports for the enzymes but also viable conducting pathways for shuttling the electrons between the enzymes, substrates, and the electrode. It was estimated that when the distance between an electron donor and acceptor of a protein (23) Golab, J. T.; Sprague, J. R.; Carron, K. T.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. 1988, 88, 7942. (24) Infrared and Raman Spectra of Inorganic and Coordination Compounds; Nakamoto, K., Ed.; Wiley: New York, 1997. (25) Characteristic Raman Frequencies of Organic Compounds; Dollish, F. R., Fateley, W. G., Bentley, F. F., Eds.; Wiley: New York, 1983; p 265. (26) Mandelbrot, B. B.; Evertsz, C. J. G. Nature 1990, 348, 143.

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Figure 7. Calibration curves of Ag nanodendrite biosensors prepared with sol-gel solutions containing different Ag concentrations: curve a, 60 µg; curve b, 30 µg; curve c, blank sample without Ag nanodendrites. The data are normalized by subtracting the corresponding blank signal with PBS.

molecule (e.g. oxidoreductase) increases from 8 to 17 Å, the electron-transfer rate decreases by 4 orders of magnitude.21 Glucose oxidase is a structurally rigid enzyme molecule in which the redox-active center is deeply embedded in the apoenzyme. Our Ag nanodendrites could act as a bridge between the enzyme redox-active center and the electrode surface and enhance their electronic communication. We have fabricated a glucose biosensor with the Ag nanodendrites and studied the sensing characteristics of this biosensor. Figure 7 shows the calibration curves of the enzyme-immobilized electrodes as a function of the glucose concentration (see also Figures S2 and S3 in the Supporting Information for raw data and CVs). Curves a-c are obtained from the electrodes prepared from Ag-enzyme sol-gel solution containing 60 and 30 µg and without Ag nanodendrite materials, respectively. The data are normalized by subtracting the corresponding blank signal with PBS. The curves between the current response and the concentration of glucose are approximately linear from 1 to 5 mM. It is estimated that the GOD electrodes prepared with the sol-gel solutions containing 30 and 60 µg Ag nanodendrites are, respectively, 19 and 32 times more sensitive for glucose detection. Clearly, the dendritic structures of silver with large surface-to-volume ratio and a hierarchical texture can act as a nanoscale bridge between enzymes and electrode surfaces. This accelerates electron transfer and thus increases the sensitivity of the biosensors. The stability of the silver nanodendrite modified electrode was investigated. The cyclic voltammogram shows a peak anodic current at +350 mV for the Ag nanodendrites. This indicates that the Ag nanodendrite will undergo oxidation at +600 mV. Indeed, the average steady background currents obtained by electrodes prepared from sol-gel solution containing 60 and 30 µg of the Ag dendrites were 3.3 and 3.1 nA higher than that without it. However, the oxidation of Ag dendrite only increases the steady background current of the biosensor, and the increase in the sensitivity of the biosensors (Figure 7) strongly suggests the enhancement of the electron-transfer rate between the enzyme molecules and the electrode surface by the Ag nanodendrites. Further improvement can be made by using a mediated enzymatic system (e.g., ferrocene) that can lower the working potential of the biosensor and overcoating the silver nanodendrites with other inert metals (e.g., platinum).

Conclusions Using a new reaction scheme employing a reducing Zn microparticle suspension, we have successfully synthesized Ag

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nanodendrites. The advantages of this method include the simplicity (no need for surfactants), the heterogeneous nature, the high yield, and the amenability to control. The main product (∼90%) is Ag nanodendrite with 20-30 nm in both stem and branch diameter and 5-50 µm in length. Through detailed studies, we have revealed that the stem, branch, and subbranch grow along 〈100〉, 〈111〉, and 〈100〉 directions, respectively. Such a preferential growth pattern along 〈100〉 and 〈111〉 in alternation leads to the formation of the Ag nanodendrites. It is argued that the growth of the Ag nanostructures is controlled by two important factors: diffusion and oriented attachment of Ag nanocrystal precursors. The competition of the two factors dictates the resulting morphologies of the Ag nanostructures. This exemplifies a unique strategy for assembling sophisticated hierarchical structures from nanoparticle precursors. SERS studies show that the Ag nanodentrites provide intense and enhanced Raman scattering, when pyridine is used as a probing molecule. Finally,

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the Ag nanodendrite structure has been tested as a conducting matrix for a glucose biosensor. Preliminary results have shown a marked sensitivity increase by a factor of as much as 32 compared to the biosensor without using the Ag nanodendritic matrix. Acknowledgment. We are grateful to the Hong Kong University of Science and Technology for financial support under Grant HIA05/06.SC02. Support from the Research Grant Council of Hong Kong are also acknowledged. S.Y. thanks the Hong Kong Young Scholar Cooperation Research Foundation of NSFC. Supporting Information Available: TEM image (Figure S1), biosensor response vs glucose concentration (Figure S2), and CVs of the biosensor (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. LA060267X