Fast Synthesis of Cu2O Hollow Microspheres and ... - ACS Publications

Nov 19, 2008 - Hugo M. Galindo , Yadira Carvajal , Eric Njagi , Roger A. Ristau and Steven L. Suib. Langmuir 2010 26 (16), 13677-13683. Abstract | Ful...
0 downloads 0 Views 999KB Size
Download PDF
Fast Synthesis of Cu2O Hollow Microspheres and Their Application in DNA Biosensor of Hepatitis B Virus Haitao Zhu,*,†,‡ Jixin Wang,†,‡ and Guiyun Xu§ College of Materials Science and Engineering, Qingdao UniVersity of Science and Technology, Qingdao 266042, P. R. China, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou UniVersity, Fuzhou 350002, P. R. China, and College of Chemistry and Molecular Engineering, Qingdao UniVersity of Science and Technology, Qingdao, 266042, P. R. China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 633–638

ReceiVed September 8, 2008

ABSTRACT: Well dispersed Cu2O hollow microspheres consisted of Cu2O nanoparticles were quickly synthesized in aqueous solution at room temperature (25 °C) with polyvinylpyrrolidone (PVP) as surfactant. The influences of the reaction time, PVP amount and pH value of NaOH solution were studied. The formation mechanism of Cu2O hollow spheres is that, with the modification and steric effect of PVP molecules, Cu2O nanoparticles aggregate to form loose aggregations and then quickly transform to hollow spheres through Ostwald ripening. Formation of loose aggregations is the key to the fast synthesis of hollow spheres at low temperature. The application of Cu2O hollow microspheres in DNA biosensor was also investigated. The hollow Cu2O microspheres greatly enhance the immobilization of the DNA probe on the electrode surface and improve the sensitivity of DNA biosensors.

1. Introduction In recent years, inorganic hollow microsphere and nanospheres have attracted considerable attention because of their potentially applications in drug delivery, catalysis, sensors, artificial cells, photonic crystals, light fillers and low-dielectricconstant materials.1-7 Various fabrication procedures for hollow structures have been developed that involve hard templates (e.g., polymeric, ceramic, and metallic spheres) or soft templates (e.g., emulsion droplets, surfactant and gas bubbles), as well as physical/chemical processes based on Kirkendall effect, Ostwald ripening, oriented attachment, chemically induced self-transformation, self-assembly, etc.8-20 Although much progress has been made in the synthesis of hollow spheres, it remains a major challenge to develop a fast and facile solution route for the preparation of inorganic hollow nanostructures. Cuprous oxide (Cu2O), a p-type semiconductor with unique chemical and physical properties, has potential applications in electronics, catalysis, optical devices, and gas sensors, etc.21-24 In the past decade, various morphological Cu2O products, such as hollow spheres, wires, cubes, cage and octahedron etc., have been synthesized by different methods.21-30 Among them, the Cu2O hollow sphere have attracted attention due to unique properties and potential application in many field.31-35 Several articles have reported the synthesis of Cu2O hollow spheres. For example, Wang’s group developed a novel method to synthesize multishelled hollow spheres with the assistance of CTAB multilamellar vesicles at 60 °C.32 Zeng’s group prepared hollow Cu2O nanospheres using solvothermal method under 150-180 °C for 20-40 h in N,N-dimethylformamide.34 Liu et al. synthesized submicrometer hollow Cu2O spheres using a multiple emulsion (O/W/O) as the template.36 Yang et al. prepared hollow Cu2O nanospheres by heating a solution of copper acetate and hydrazine as a reductant in 2-propanol under reflux conditions (81 °C, 30 min).37 Herein, we presented a rapid * To whom correspondence should be addressed. Telephone: +86 532 84022676. Fax: +86 532 84022787. E-mail: [email protected]. † College of Materials Science and Engineering, Qingdao University of Science and Technology. ‡ State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University. § College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology.

and mild (room temperature) method for the synthesis of hollow Cu2O spheres. DNA biosensors play important roles in the detection of DNA. Electrochemical DNA biosensors maybe used in gene expression, disease diagnosis, and drug screening due to many advantages such as low cost, simple design, small dimension and high sensitivity.38,39 The key element of an electrochemical DNA biosensor is the detection electrode, in which immobilized single stranded DNA (ssDNA) acts as a probe to detect the target DNA. Therefore, the surface-immobilization of the ssDNA probe on the electrode is a key step to fabricate the electrochemical DNA biosensor. Recently, several metal oxides have been successfully employed in ssDNA probe immobilization. For example, Liu et al. reported the ssDNA probe immobilization based on ZrO2 gel.40 Yu’s group developed the CeO2/ Chitosan composite matrix for ssDNA probe immobilization.41 Inspired by the above success, using the DNA of Human hepatitis B virus (HBV, a most prevalent inflammation of liver in the developing country) as an example, we apply our synthesized Cu2O hollow spheres for the ssDNA probe immobilization. The results show that the electrode modified with Cu2O hollow spheres give a very high sensitivity for detecting the HBV DNA sequences.

2. Experimental Section All reagents are of analytical grade and used without further purification. In a typical procedure, 25 mL of 2 mM CuSO4 solution and 0.5 g of PVP-K30 were added into a conical flask under magnetic stirring at 25 °C. Then, 25 mL NaOH solution with pH value of 10 was added in the above mixture. After stirring for 2 min, 2.0 mL of 0.10 M N2H4 · H2O solution was added into it. The mixed solution soon turned to be yellow. After reacting for 5 min, the product was obtained by centrifuging, washing with distilled water and ethanol and then drying under vacuum at room temperature. To investigate the product formation mechanism, the synthesis parameter, such as the reaction time, PVP amount or pH value of NaOH solution was changed while keeping all other experimental parameters as in the typical run. The powder X-ray diffraction (XRD) analyses data were carried out on a Rigaku D/Max r-A diffractometer. The scanning electron microscopy (SEM) images were taken with FESEM-6700 field-emission microscope. The transmission electron microscopy (TEM) images and the selected area electron diffraction (SAED) were captured on a JEM2000EX transmission electron microscope. N2 adsorption-desorption

10.1021/cg801006g CCC: $40.75  2009 American Chemical Society Published on Web 11/19/2008

634 Crystal Growth & Design, Vol. 9, No. 1, 2009

Zhu et al.

Figure 1. Schematic representation of the electrochemical detection of DNA hybridization using electrode containing synthesized Cu2O hollow spheres.

Figure 2. XRD pattern (a), SEM (b), TEM (c), HRTEM image (d), N2 adsorption-desorption isotherm (e), and pore size distribution curve (f) of the typical sample. isotherms were measured on a NOVA 1000e apparatus at 77 K. The surface areas were calculated by the Brunauer-Emmett-Teller (BET) method, and the pore size distribution was calculated from the adsorption branch using the Barett-Joyner-Halenda (BJH) theory. Using the DNA sequences of HBV (see the Supporting Information) as an example, the application of Cu2O hollow microspheres in a DNA biosensor is briefly shown in Figure 1 (see the details in the Supporting Information). First, a carbon paste electrode (CPE) was fabricated according to the reference.39 Then, Cu2O hollow microspheres (typical samples) were attached to the surface of CPE. After immobilizing the probe DNA (ssDNA) of HBV on the surface of Cu2O/CPE, the ssDNA/ Cu2O/CPE was fabricated. For the detection of the target DNA, the ssDNA/Cu2O/CPE was immerged in the hybridization buffer solution containing the target DNA. After rinsing away the unhybridized DNA, a working electrode (WE) was prepared. The differential pulse voltammetry (DPV) were performed on a CHI832 Electrochemical Analyzer (CH Instruments, Shanghai, China). A three-electrode cell, consisting of the working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum wire auxiliary electrode (Pt), was used in the following electrochemical experiments. Methylene blue (MB) was used as the hybridization indicator. The response signal of MB was measured by using DPV in B-R buffer solution (see the Supporting Information). The scan rate was 50 mV s-1.

3. Results and Discussions 3.1. Characterization of Cu2O Hollow Microspheres. Figure 2a is the XRD pattern of the typical sample. The four peaks in the pattern can be indexed to the 111, 200, 220, and 311 peaks of cubic Cu2O according to the JCPDS card (No. 65-3288). The average crystal size was calculated to be 9.7 nm using the Debye-Scherrer formula based on the broader peaks in XRD pattern. The SEM image (Figure 2b) shows that the obtained particles are spheres. The diameter of the hollow spheres is in the range of 150 to 220 nm. The high magnification image (inner Figure 2b) of the circle area in Figure 2b indicates the surface of the microspheres is rough and the microspheres are made up of Cu2O nanoparticles. The broken sphere implies that the spheres are hollow. A TEM image of the typical sample is shown in Figure 2c. The contrast between the dark edge and the pale center further provides convincing evidence of the hollow structure. Both the SAED pattern (inset in Figure 2c) and the HRTEM image (Figure 2d) of a single microsphere also imply that the microspheres are constructed by smaller nano-

Cu2O Hollow Microspheres

Crystal Growth & Design, Vol. 9, No. 1, 2009 635

Figure 3. TEM images and SAED patterns (inset) of interim samples collected at different reaction times after the addition of N2H4 · H2O: (a) 0 s; (b) 30 s; (c) 60 s; (d) 2 min. The rings in SAED patterns can be indexed to the 111, 200, 220, and 311 peaks of cubic Cu2O.

particles. Visible lattice fringes are observed in the HRTEM image with d spacing of about 2.48 Å, corresponding to the {111}plane of the cubic Cu2O. The N2 adsorption-desorption isotherm of the Cu2O hollow sphere shown in Figure 2e exhibits an acute uptake and a hysteresis loop in the relative pressure range of 0.8-1.0, indicating the presence of the inhomogeneous mesopores and macropores. The corresponding pore size distribution curve (Figure 2f) calculated by the BJH method based on the adsorption branch displays two peaks. The first peak in the range of 3 to 10nm is attributed to the aggregation of the nanoparticles within the shells of the hollow microspheres. The peak in the range 15-100 nm further confirms the hollow structure of the Cu2O microspheres. The specific surface area is 24.04 m2/g, calculated by the BET method. 3.2. Formation Mechanism of Cu2O Hollow Microspheres. To study the formation process of the Cu2O hollow spheres, samples were collected at different reaction time and were characterized by the TEM and SAED analysis. Figure 3 shows the TEM images and SAED patterns of the samples collected at different times after the addition of N2H4 · H2O solution. These images clearly reveal the evolution from nanoparticles to hollow spheres. Immediately after the addition of N2H4 · H2O, the reaction mixture quickly turns to be a light yellow suspension. Figure 3a shows that the nanoparticles are mainly produced in the reaction system at the initial stage. Figure 3b is a TEM image of the sample collected after 30s, showing that Cu2O nanoparticles aggregated to form loose spheres to minimize the surface energy. The modification effect of PVP (see the next paragraph) also plays a role in the formation of spheres. The SAED pattern (inset in Figure 3b) of an individual sphere confirms that the spherical particle consisted of very small Cu2O nanoparticles,

which is indicated by the weak and broaden diffraction rings. With increasing the reaction time to 60s, the compact density and the size of the spherical particles increase obviously (Figure 3c). However, the spheres are yet loose aggregates of very small Cu2O nanoparticles. When the reaction time is 2 min, the obtained spherical spheres present obvious hollow structure with lighter central parts and dark shell. The sharp and lighter diffraction rings (inset Figure 3d) suggest that the size of Cu2O nanoparticles which constructing the hollow spheres are larger at this stage. With a further increase in the reaction time to 5 min, as the typical sample, the hollow structure of spherical particles becomes more clearly and the cavity size increases (see Figure 2c). Based on the above results, the formation of Cu2O hollow microspheres can be explained by a self-transformation process of the metastable aggregated particles accompanied by the Ostwald ripening.19,42 Similar mechanisms have been involved in the preparation of TiO2, SnO2, CdMoO4, etc. hollow spheres.34,43-46 However, the formation of above hollow spheres usually took place under higher temperatures (140-220 °C) and long times (5-50 h) in a hydrothermal or solvothermal condition. The fast formation mechanism of hollow sphere in this work is proposed as illustrated in Figure 4. After adding the N2H4 · H2O solution, Cu2O nanoparticles is produced. The size of nanoparticle is very small due to the modification of PVP.47 Then the Cu2O nanoparticles aggregate quickly and form spherical particles to reduce the surface energy. However, due to the steric effect of PVP molecules,47 the aggregated particle is loosely compacted and there are many voids in the interior of spheric particles. In this stage, these aggregated Cu2O nanoparticles are not in thermodynamically equilibrium status

636 Crystal Growth & Design, Vol. 9, No. 1, 2009

Figure 4. Schematic illustration of the formation process of Cu2O.

and become metastable because of their still quite large surface energy.19,42 Larger crystallites are thermodynamically favored. As a result, the loose spheres transform to hollow spheres through Ostwald ripening, it is similar to the formation of the TiO2, SnO2, CdMoO4 hollow spheres.43-46 However, the aggregated structure in our work is loosely, it makes the transformation quickly complete at low temperature. It can be confirmed by the following experimental results. Figure 5 shows the TEM images of samples obtained at different PVP additions while keeping all other experimental parameters as in the typical run. When the PVP amount was less than 0.10 g, solid cubic particles are the major products (Figure 5a). Increasing the amount to 0.24 g, solid Cu2O microspheres are obtained (Figure 5b). When the PVP amount is 0.30 g, a hollow microsphere emerges among the dominating solid microspheres (Figure 5c). However, the brightness of these solid microspheres is higher than that of the microspheres shown in Figure 5b, which indicates that these Cu2O microspheres are aggregated loosely. Further increase of PVP to 0.5 g (the typical sample), hollow Cu2O microsphere are obtained (Figure 2c). In this work, the PVP molecules modify the surface of Cu2O nanoparticles and restrain the aggregation of nanoparticles by steric effect.47 When the PVP amount is less than 0.1 g, the modification and restrain effect are deficient, so the Cu2O nanoparticles aggregate compactly and transform to cubic

Zhu et al.

particles according to the growth habit of Cu2O. With the increasing of PVP amount, the modification and steric effect increase, the density of aggregation decrease. When the PVP amount is less than 0.3 g, due to the density of aggregation is still high, it is could not obtain hollow interior structures quickly at room temperature. When the PVP amount is 0.5 g, loose aggregated structure is formed and hollow spheres are obtained through Ostwald ripening. The result shows that the loose aggregated structure is the key to fast synthesizes Cu2O hollow spheres at low temperature. The effect of the pH value of the NaOH solution on the product morphology was also investigated. Without the addition of the NaOH solution, the reaction is slow and little product is produced. When the pH value of NaOH solution is in the range 9-10, well dispersed Cu2O hollow spheres are synthesized quickly (see Figure 2). When the pH value is 11, octahedral-like solid particles with rough surface are the major product (Figure 6a). Regular octahedral Cu2O particles with smooth surface are obtained upon increasing the pH value to 12.0 (Figure 6b). The TEM image and SAED of an octahedron indicate that the octahedron has a singlecrystal structure (Figure 6c). The NaOH solution provides the necessary alkalinity for the use of N2H4 as reducing agent.48,49 Without adding NaOH solution, the reduction rate is low, a little product is collected. With a moderate pH value (9-10) of NaOH solution, the high reduction rate produces a large number of Cu2O nanoparticles. Large number of Cu2O nanoparticles aggregate quickly to form loose aggregations and then quickly transform to Cu2O hollow spheres through Ostwald ripening. When the pH value of NaOH solution is large than 11, NaOH will react with copper ions and produce Cu(OH)2 precipitation as intermediate product (see Supporting Information, Figure S2). It will decrease the reaction rate and the number of Cu2O nanoparticles. The slow reduction of the Cu(OH)2 precipitation and the slow growth of Cu2O

Figure 5. TEM images of Cu2O particles obtained with different PVP amount: (a) 0.10 g; (b) 0.24 g; (c) 0.30 g.

Figure 6. (a) SEM image of Cu2O particles when the pH value of NaOH solution is 11 and (b) SEM and (c) TEM images of Cu2O particles when the pH value of NaOH solution is 12. The inset image in part c is the SAED of a single octahedron.

Cu2O Hollow Microspheres

Figure 7. The DPV response of MB recorded using the CPE (a), Cu2O/ CPE (b), and ssDNA/Cu2O/CPE (c). Scan rate was 50 mV/s.

Figure 8. Resulting logarithmic curve obtained with different concentrations of target DNA sequences for hybridization.

nanoparticles will lead to the formation of single-crystal Cu2O octahedron.50 3.3. Application of Cu2O Hollow Microspheres in a DNA Biosensor. A DNA biosensor with MB as the hybridization indicator has been reported in earlier literature.51,52 It is based on the affinity of MB for the guanine bases of DNA. Thus, an increased loading of ssDNA probe can enhance the signal response of MB. Figure 7 displays the DPV response of MB recorded using the carbon paste electrode (CPE), the CPE modified with Cu2O hollow sphere (Cu2O/CPE) and the Cu2O hollow sphere modified CPE electrode then immobilized with ssDNA probe (ssDNA/Cu2O/CPE). As can be seen, the peak current increases in the order of CPE, Cu2O/CPE and ssDNA/Cu2O/CPE. The Cu2O hollow sphere modified electrode shows the notable electrochemical signal than the unmodified CPE. It indicates that the Cu2O hollow sphere could enhance the electrochemical response of the MB. Compared with the curve obtained with the Cu2O/CPE, the ssDNA/Cu2O/CPE offered an excellent enhanced signal, suggesting that the Cu2O hollow spheres film effectively enhance the loading of ssDNA probe. This was also confirmed by the UV-vis spectroscopy (Figure S4). The enhanced loading of ssDNA probe is ascribed to the affinity of the PVP-capped hollow Cu2O to ssDNA and the high surface area of Cu2O hollow spheres. The detection of the DNA biosensor was investigated by using the ssDNA/Cu2O/CPE to hybridize with different concentrations of target DNA sequences. The peak current values of MB were measured by DPV after ssDNA probe hybridized with the target DNA sequences. As can be seen from Figure 8, the peak current values were linear with the logarithmic value of the complementary target DNA sequences concentration ranging from 1

Crystal Growth & Design, Vol. 9, No. 1, 2009 637

Figure 9. The DPV response of MB recorded using the ssDNA/Cu2O/ CPE without hybridization (a) and after being hybridized with 1.0 × 10-10 mol · L-1 noncDNA sequences (b), 3-base-mismatched DNA sequences (c), or complementary target DNA sequences (d).

× 10-10 to 1 × 10-6 mol · L-1, with a detection limit of 1.0 × 10-10mol · L-1. The regression equation, with the regression coefficient (R) of 0.9951, is expressed as y ) -0.1079 log x + 1.4265 (x is the concentration value of target DNA 10-12 mol · L-1; y is the peak current value of MB by DPV, 10-5 A). The selectivity of the DNA biosensor was investigated by using the ssDNA/Cu2O/CPE electrode to hybridize with different kinds of DNA sequences. Three specific DNA sequences were the complementary sequences, 3-base mismatch sequences and noncomplementary sequences. Figure 9 shows the DPV curves of MB recorded using the ssDNA/Cu2O/CPE without hybridization (a) and after hybridized with 1.0 × 10-10 mol · L-1 noncDNA sequences (b), 3-base-mismatched DNA sequences (c) or complementary target DNA sequences (d). All the peaks obtained are well defined. One can see that ssDNA/Cu2O/CPE without hybridization present the highest signal due to the strong affinity of MB for the free guanine bases. After hybridization with complementary target DNA sequences, the signal decrease obviously because of the inaccessibility of MB to the guanine bases. While for the noncDNA sequences, the signal almost unchanged because the noncDNA could not be hybridized with the probe DNA. For the 3-base-mismatched sequences, the signal changes less than that for the complementary target DNA sequences, because only part of this DNA could be hybridized with the probe DNA. The results show that the biosensor has good selectivity for the hybridization detection.

4. Conclusions In summary, well dispersed Cu2O hollow microspheres could be fast synthesized in aqueous solution at room temperature using PVP as surfactant. The formation mechanism of Cu2O hollow spheres is that, with the modification and steric effect of PVP molecules, Cu2O nanoparticles aggregate to form loose aggregations and then quickly transform to hollow spheres through Ostwald ripening. The formation of the loose aggregations is the key to the fast synthesis of the hollow spheres at low temperature. It provides new insights into fast preparation of other hollow microspheres at low temperature. The application of Cu2O hollow microspheres in DNA biosensors of HBV shows that the hollow Cu2O microspheres greatly enhance the immobilization of the DNA probe on the electrode surface and improve the sensitivity of the DNA biosensors. Cu2O hollow microspheres may have great potentials in other DNA biosensors. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50872061), Young

638 Crystal Growth & Design, Vol. 9, No. 1, 2009

Scientist Foundation of Shandong Province (2006BS04025) and the Foundation of Qingdao Science and Technology (07-2-312-jch). The authors thank the reviewers for their valuable advice when the manuscript was first submitted to this journal. Supporting Information Available: Detailed electrochemical experimental informationi, including Figure S1, showing the schematic structure of CPE, Cu2O/CPE, and ssDNA/Cu2O/CPE electrode, Figure S2, showing the TEM image and SAED of the interim production when NaOH solution with pH ) 12 was added, and Figure S3, depicting the UV-vis spectra of ssDNA, Cu2O hollow sphere, and the Cu2O labeled ssDNA. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Caruso, F. AdV. Mater. 2001, 13, 11. (2) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Science 2002, 298, 1006. (3) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (4) Xu, L.; Chen, X.; Wu, Y.; Chen, C.; Li, W.; Pan, W.; Wang, Y. Nanotechnology 2006, 17, 1501. (5) Maji, T. K.; Matsuda, R.; Kitagawa, S. Nat. Mater. 2007, 6, 142. (6) Sander, M. S.; Cote, M. J.; Gu, W.; Kile, B. M.; Tripp, C. P. AdV. Mater. 2004, 16, 2052. (7) Liu, J.; Xue, D. AdV. Mater. 2008, 20, 2622. (8) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 3827. (9) Strohm, H.; Lobmann, P. J. Mater. Chem. 2004, 14, 2667. (10) Khopade, A. J.; Caruso, F. Chem. Mater. 2004, 16, 2107. (11) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schuth, F. Science 1996, 273, 768. (12) Fowler, C. E.; Khushalani, D.; Mann, S. J. Mater. Chem. 2001, 11, 1968. (13) Daiguji, H.; Makuta, T.; Kinoshita, H.; Oyabu, T.; Takemura, F. J. Phys. Chem. B 2007, 111, 8879. (14) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (15) Sun, Y.; Mayers, B.; Xia, Y. AdV. Mater. 2003, 15, 641. (16) Zeng, H. C. J. Mater. Chem. 2006, 16, 649. (17) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (18) Yu, J.; Yu, H.; Guo, H.; Li, M.; Mann, S. Small 2008, 4, 87. (19) Yu, H.; Yu, J.; Liu, S.; Mann, S. Chem. Mater. 2007, 19, 4327. (20) Zhang, L.; Wan, M. AdV.Funct.Mater. 2003, 13, 815. (21) Tan, Y.; Xue, X.; Peng, Q.; Zhao, H.; Wang, T.; Li, Y. Nano Lett. 2007, 7, 3723. (22) Hara, M.; Kondo, T.; Komoda, M.; Ikeda, S.; Shinohara, K.; Tanaka, A. Chem. Commun. 1998, 998, 357. (23) Zhang, J. T.; Liu, J. F.; Peng, Q.; Wang, X.; Li, Y. D. Chem. Mater. 2006, 18, 867.

Zhu et al. (24) Zhang, W.; Luan, C.; Yang, Z.; Liu, X.; Zhang, D.; Yang, S. Appl. Surf. Sci. 2007, 253, 6063. (25) Xiong, Y. J.; Li, Z. Q.; Zhang, R.; Xie, Y.; Yang, J.; Wu, C. Z. J. Phys. Chem. B 2003, 107, 3697. (26) Gou, L. F.; Murphy, C. J. Nano Lett. 2003, 3, 231. (27) Li, X. D.; Gao, H. S.; Murphy, C. J.; Gou, L. F. Nano Lett. 2004, 4, 1903. (28) Wang, Z.; Chen, X.; Liu, J.; Mo, M.; Yang, L.; Qian, Y. Solid State Commun. 2004, 130, 585. (29) Zhang, Z. M.; Sui, J.; Zhang, L. J.; Wan, M. X.; Wei, Y.; Yu, L. M. AdV. Mater. 2005, 17, 2854. (30) Lu, C.; Qi, L.; Yang, J.; Wang, X.; Zhang, D.; Xie, J.; Ma, J. AdV. Mater. 2005, 17, 2562. (31) Xu, Y.; Chen, D.; Jiao, X.; Xue, K. J. Phys. Chem. C 2007, 111, 16284. (32) Xu, H. L.; Wang, W. Z. Angew. Chem., Int. Ed. 2007, 46, 1489. (33) Zhang, H.; Zhu, Q.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. AdV. Funct. Mater. 2007, 17, 2766. (34) Chang, Y.; Teo, J. J.; Zeng, H. C. Langmuir 2005, 21, 1074. (35) Zhang, H.; Shen, C.; Chen, S.; Xu, Z.; Liu, F.; Li, J.; Gao, H. Nanotechnology 2005, 16, 267. (36) Liu, H.; Ni, Y.; Wang, F.; Yin, G.; Hong, J.; Ma, Q.; Xu, Z. Colloid Surf. A 2004, 235, 79. (37) Yang, M.; Zhang, Y.; Pang, G.; Feng, S. Eur. J. Inorg. Chem. 2007, 3841. (38) Ren, Y.; Jiao, K.; Xu, G. Y.; Sun, W.; Gao, H. W. Electroanal. 2005, 17, 2182. (39) Xu, G. Y.; Jiao, K.; Fan, J. S.; Zhang, B. Asian J. Chem. 2007, 19, 4161. (40) Liu, S. Q.; Xu, J. J.; Chen, H. Y. Bioelectrochemistry 2002, 57, 149. (41) Feng, K. J.; Yang, Y. H.; Wang, Z. J.; Jiang, J. H.; Shen, G. L.; Yu, R. Q. Talanta 2006, 70, 561. (42) Yu, J.; Guo, H.; Davis, S. A.; Mann, S. AdV. Funct. Mater. 2006, 16, 2305. (43) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492. (44) Lou, X. W.; Wang, Y.; Yuan, C.; Lee, J. Y.; Archer, A. AdV. Mater. 2006, 18, 2325. (45) Yu, D.; Sun, X.; Zou, J.; Wang, Z.; Wang, F.; Tang, K. J. Phys. Chem. B 2006, 110, 21667. (46) Wang, W. S.; Zhen, L.; Xu, C. Y.; Zhang, B. Y.; Shao, W. Z. J. Phys. Chem. B 2006, 110, 23154. (47) Zhu, H. T.; Zhang, C. Y.; Yin, Y. S. J. Cryst. Growth 2004, 270, 722. (48) Zhu, H. T.; Zhang, C. Y.; Yin, Y. S. Nanotechnology 2005, 16, 3079. (49) Wu, S. H.; Chen, D. H. J. Colloid Interface Sci. 2003, 259, 282. (50) Xu, H. L.; Wang, W. Z.; Zhu, W. J. Phys. Chem. B 2006, 110, 13829. (51) Kerman, K.; Ozkan, D.; Kara, P.; Meric, B.; Gooding, J.; Ozsoz, M. Anal. Chim. Acta 2002, 462, 39. (52) Kara, P.; Kerman, K.; Ozkan, D.; Meric, B.; Erdem, A.; Ozkan, Z.; Ozsoz, M. Electrochem. Commun. 2002, 4, 705.

CG801006G