A Facile Method To Obtain Highly Stable Silver Nanoplate Colloids

Mar 18, 2010 - addition of 10-5 M MHA. Panels a-d of Figure 8 show the. TEM images of the silver colloids corresponding to the curves b, d, f, and h, ...
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A Facile Method To Obtain Highly Stable Silver Nanoplate Colloids with Desired Surface Plasmon Resonance Wavelengths Bo-Hong Lee, Ming-Sheng Hsu, Yuan-Chin Hsu, Cheng-Wei Lo, and Cheng-Liang Huang* Department of Applied Chemistry, National Chiayi UniVersity, Chiayi City, Taiwan ReceiVed: October 22, 2009; ReVised Manuscript ReceiVed: March 4, 2010

The extinction spectra and TEM images show that the silver nanoprisms would undergo shape transformation in the presence of 5 × 10-6 M bromide ions. In this study, HS(CH2)15CO2H (MHA) were introduced to modify the surface of silver nanoprisms. The MHA-modified silver nanoprisms can prevent the shape transformation in the presence of bromide ions with a concentration as high as 0.1 M. Furthermore, it was found that the etching process on the unmodified silver nanoprisms caused by the bromide ions can be stopped immediately by adding MHA into the colloid solution. However, the rate of the etching process in the initial stage was too fast to be stopped by adding MHA to control the silver colloid with the desired SPR wavelength. PVP-modified silver nanoprisms cannot prevent the shape evolution in high concentration bromide ions, such as 10-3 M, but can slow down the etching rate. The silver nanostructures with the desired SPR wavelength can be obtained by the following simple method: KBr was added into the PVP-modified silver nanoprism colloid to initiate the shape evolution, and after a period of time, MHA was added to terminate the etching reaction. The PVP-KBr-MHA treated silver nanostructures with various colors produced by this simple method not only have long-term stability stored at room temperature but also can prevent shape transformation in highly concentrated chloride solutions. 1. Introduction The physical and chemical properties of metallic nanostructures are strongly related to their shapes.1,2 Various methods have been developed to successfully synthesize metal nanostructures with different shapes, such as spheres,3 rods,4 boxes,5 shells,6 tetrahedrons,7 cubes,8 disks,9 wires,10 hexagons,11 bipyramids,12 stars,13 and prisms.14,15 Among them, silver nanoprisms have attracted extensive interest in recent years because of their rich optical properties, originating from the extreme geometric anisotropy,16,17 which exhibit four characteristic peaks in the extinction spectra, corresponding to out-of-plane quadrupole, out-of-plane dipole, in-plane quadrupole, and in-plane dipole SPR modes.18 The wavelength of the in-plane dipole SPR mode of silver nanoprisms can be tuned by adjusting the aspect ratios and corner sharpness to cover almost the entire visible spectrum to the near-IR.18 The SPR wavelength of silver nanoprisms has been found to be linearly dependent on the surrounding refractive index both in discrete dipole approximation (DDA) calculation and experimental measurements.18,19 Van Duyne et al. reported that the in-plane dipole SPR mode red shifts almost linearly with respect to the thickness of the selfassembly monolayer (SAM) on the nanostructures.19 Because of these properties, silver nanoprisms can be used as good candidates for potential application in chemical and biological sensors.20 However, the silver nanoprisms would undergo a gradual shape change in some kinds of conditions. For example, Van Duyne et al. found that triangular silver nanostructures, which were fabricated by using the technique of nanosphere lithography (NSL), became more rounded at the tip and increased their height after exposure to water.19a Recently, it was found that the silver nanoprisms would also be etched by heat,21,22 * To whom correspondence should be addressed. Fax: +886-5-2717963. E-mail: [email protected].

acid,23 UV light irradiation,24 and in the presence of halide ions.25,26 Since the SPR shift is one of the most extensively utilized properties of nanoparticle-based biosensors and the peak positions of SPR are strongly dependent on the shapes of nanostructures, the fragile property of the corners of silver nanoprisms will limit the practical application. Therefore, it is important to keep the optical and morphological stability of nanoparticles in the biological buffers, which often contained highly concentrated chloride ions. Yu et al. have reported that shape evolution of silver nanoprisms can be prevented by modifying the particle surface with alkanethiol molecules.27 To stabilize the as-prepared silver nanoprisms, HS(CH2)15CO2H (MHA) instead of alkanethiols was employed in this study because the former is more soluble in water than the latter. The MHA-modified silver nanoprisms can endure a higher concentration of bromide ions than unmodified ones by a factor of about 105. Furthermore, we found that even the silver nanoprisms were in the sculpturing process in the presence of bromide ions; the addition of MHA can also stop the truncation process immediately. Bromide ions and MHA can be used as the reagents to start and stop the etching reaction to tailor the silver nanoprisms into the nanostructures with the desired color. However, the etching process in the initial stage is relatively too fast to be stopped at once even with the addition of low concentrations of bromide ions. Therefore, PVP was chosen to be put into silver colloid prior to adding the bromide ions, and then followed by the addition of MHA to stop the etching process. Because the sculpturing rate of PVP modified silver nanoprisms in the presence of bromide ions was slowed down, the silver nanoplates with the desired colors can be obtained easily by stopping the reaction with the addition of MHA at different stages of the etching process. The colorful PVP-KBrMHA treated silver nanoplate colloids remained stable at room temperature for more than one month and in highly concentrated chloride buffer solutions without any noticeable changes in their

10.1021/jp910100k  2010 American Chemical Society Published on Web 03/18/2010

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optical properties. This simple method can provide the highly reproducible and stable colorful silver nanostructures for the applications in multicolor diagnostic labeling as well as the SERS detection optimized by tuning the SPR wavelength to match the excitation laser. 2. Experimental Section 2.1. Materials. Silver nitrate, sodium citrate, sodium borohydride, KCl, KBr, 16-mercaptohexadecanoic (MHA), and PVP were all purchased from Sigma-Aldrich. The as-received chemicals were used without any further purification. Milli-Q grade water (>18 MΩ) was used for all solution preparation throughout all the experiments. 2.2. Instrumentation. Samples for transmission electron microscopy (TEM) images were examined with use of a Hitachi H-7100 TEM operated at 75 KV. Before analysis by TEM, the silver colloid was dripped onto the carbon-coated copper grid and air-dried at room temperature. All UV-vis-NIR extinction spectra were recorded at 25°C on a Hitachi U-2800 spectrophotometer, using a quartz cuvette with 10 mm optical path. 2.3. Colloids Preparation. Silver nanoprism colloids were prepared according to literature methods.14f A solution of sodium citrate (3.0 × 10-2 M, 1 mL) and a solution of silver nitrate (1.0 × 10-2 M, 1 mL) were added to 97 mL of pure water with rapid stirring. Then the solution of sodium borohydride (5.0 × 10-3 M, 1 mL) was added dropwise to the mixture, under vigorous magnetic stirring. The solution immediately turned yellow. After being stirred for 30 min, the prepared solution was irradiated with the sodium lamp (Philips 100-W, λ ) 589 nm). The typical power of the light on the solution was about 0.22 W/cm.2 After 12 h of irradiation, the solution turned blue. 2.4. Time-Dependent UV-Vis-NIR Extinction Spectra of Silver Nanoprism Colloids in the Presence of Bromide Ions. The as-prepared silver nanoprism colloids with and without the surface modification with PVP were mixed with KBr of different concentrations in a quartz cuvette. Then, the timedependent UV-vis-NIR extinction spectra were measured on a Hitachi U-2800 spectrophotometer. The typical resolution of the time course spectra that were recorded every 2 min was about 2 nm. 2.5. Synthesis of Silver Nanoplates with the Desired Color. A 200 µL sample of 10-3 M PVP was added into 1.8 mL of as-prepared silver nanoprism colloid solution in a vial. After 20 min, 200 µL of 10-3 M KBr was added into the PVP-treated silver colloid solution to initiate the etching process. The color of the colloid solution changed gradually from turquoise to blue, indigo, purple, purplish red, pink, orange, and yellow over about 30 min. A 100 µL sample of 10-4 M MHA was added into the colloid solution to terminate the etching reaction when the solution showed the desired color. Finally, the highly stable silver nanostructures with the desired SPR wavelength were obtained.

Figure 1. (a, top) Extinction spectrum of the as-prepared silver nanoprism colloid. (b, bottom) TEM image of the as-prepared silver nanoprisms.

3. Results and Discussion Figure 1a shows the extinction spectrum of as-prepared silver nanoprism colloid. According to the assignments in the literature, the peaks at 675, 470, and 330 nm correspond to in-plane dipole, in-plane quadrupole, and out-of-plane quadrupole SPR modes, respectively.18 Figure 1b shows the TEM image of the asprepared silver nanoprisms. More than 90% of the nanostructures are triangular in shape with the edge length ca. 40-100 nm. Figure 2a shows the extinction spectrum of as-prepared silver nanoprism colloid after the addition of 10-5 M KBr for 2 h.

Figure 2. (a, top) Extinction spectra of the as-prepared silver nanoprism colloid after the addition of 10-5 M KBr for 2 h. (b, bottom) TEM image of the as-prepared colloid in the presence of 10-5 M bromide ions for 2 h.

The in-plane dipole SPR band blue-shifted from 675 to 465 nm and the out-of-plane quadrupole SPR band red-shifted from 330 to 346 nm. The shifts of these two SPR bands originate from the geometry change of the silver nanostructures. Xu et

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Figure 3. (a, top) UV-vis-NIR spectrum of the as-prepared silver colloids in the absence and in the presence of 10-5 M MHA for 2 h. (b, bottom) TEM image of the MHA modified silver nanoprisms.

al. found that the chloride ions can sculpture the silver nanoprisms into the nanodisks.25 We recently also found that not only chloride ions but also bromide ions can sculpture the silver nanoprisms into silver nanodisks.26 The DDA calculations showed that snipping of the corners produces a blue shift in the in-plane dipole SPR band.18 Figure 2b shows the TEM images of the as-prepared colloid in the presence of 10-5 M bromide ions for 2 h. The TEM images show that most of the nanostructures are circular in shape, which is consistent with the above analysis of SPR shifts. Figure 3a shows the UV-vis-NIR spectrum of the asprepared silver colloids in the presence of 10-5 M MHA for 2 h. The peak position of the in-plane dipole SPR band shifted from 650 to 680 nm. Figure 3b shows the TEM image of the silver nanoprisms modified by MHA. The fact that most of the particles in Figure 3b are triangular in shape demonstrates that the geometry of nanoprisms did not change after the surface modification with MHA. It is well-known that the SPR wavelength is highly dependent on nanoparticle size and shape and also depends on the refractive index of the metal and the surrounding medium.18 Since the size and shape of silver nanoprisms did not change after MHA modification, the red shift of the in-plane dipole SPR band of silver nanoprism indicates that the nanostructures were covered by MHA, which has a larger refraction index than water molecules or citrate ions. This result is consistent with the work reported by Van Duyne et al.19 They found that the alkanethiols can form a selfassembled monolayer (SAM) on the silver nanostructure through the formation of the Ag-S chemisorption bond and cause a red shift of in-plane dipole SPR. It is very likely that MHA can also form a layer on the nanoprism surface since it also has a thiol group at one end of the molecule.

Figure 4. (a, top) UV-vis-NIR spectra of MHA modified silver nanoprism colloids in the absence and in the presence of 10-5, 10-3, and 0.1 M KBr for 2 h. (b, bottom) TEM image of silver nanoprisms which were immersed in 10-4 M MHA for 2 h prior to the addition of 0.1 M KBr for 6 h.

Figure 4a show the UV-vis-NIR spectra of the as-prepared colloids modified by the MHA before and after the addition of 10-5, 10-3, and 0.1 M KBr, respectively, for 2 h. It shows that the extinction spectra (curves b-d in Figure 4a) corresponding to MHA-modified silver nanoprism colloid in the presence of bromide ions are almost the same as that in the absence of bromide ions (curve a in Figure 4a). Figure 4b shows the TEM image of silver nanoprisms which were immersed in 10-4 M MHA for 2 h prior to the addition of 0.1 M KBr for 6 h. The fact that most nanostructures in Figure 4b are triangular in shape and that the extinction spectra of MHA-modified silver nanoprism colloids are almost the same in the presence and absence of bromide ions indicates that the MHA can really protect the corners of nanoprisms from the sculpturing effect of bromide ions. In our previous study, the threshold concentration of KBr to initiate the sculpturing process is about 10-6 M. It can be roughly estimated that the MHA-modified silver nanoprisms can endure a higher concentration of bromide ions than the unmodified ones by a factor of about 105. Figure 5a shows the time-dependent extinction spectra of asprepared nanoprism colloid in the presence of 5 × 10-6 M KBr. The fact that the in-plane dipole SPR band gradually blue-shifted indicates that the silver nanostructures undergo shape evolution in the presence of bromide ions. Figure 5b shows the timedependent extinction spectra of as-prepared nanoprism colloid in the addition of 5 × 10-6 M KBr for 2 min, followed by adding 10-4 M MHA. Comparing curves c, d, and e in Figure

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Figure 5. (a, top) Time-dependent extinction spectra of the as-prepared nanoprism colloid in the presence of 5 × 10-6 M KBr. (b, bottom) Time-dependent extinction spectra of the as-prepared nanoprism colloid in the addition of 5 × 10-6 M KBr and 10-4 M MHA. Curves a and b correspond to the silver nanoprism colloid in the absence and presence of KBr for 2 min, respectively. Curves c, d, and e correspond to silver nanoprism colloid in the presence of KBr for 2 min, followed by the addition of MHA for 0, 2, and 62 min, respectively.

5b, it is found that the peak position of the in-plane SPR mode red-shifted slightly after the addition of MHA for a period of time. The red shift of the in-plane dipole resonance should originate the formation of a thin MHA layer on the nanoparticle surface. This result indicates that the addition of MHA can stop the sculpturing reaction at once by forming a protective layer on the surface of the nanostructures. Since the sculpturing reaction can be stopped immediately, one could think that the colloid with various colors could be easily achieved by terminating the reaction in different reaction stages. However, after several tries, the reaction rate still remained too fast in the initial reaction stage for us to obtain the colloid with SPR wavelength longer than 580 nm even when the low concentration of KBr was added. The sculpturing rate must be slowed down to stop the etching process in time at the very early stages of reaction. It has been found in previous studies that there exists a strong interaction between the surface of silver nanoparticles and PVP through coordinate bonding with the O and N atoms of the pyrollidone ring.28,29 It is believed that the sculpturing rate of PVP-modified silver nanoprisms in the presence of bromide ions can be slowed down or even totally blocked. Figure 6a shows the extinction spectra of as-prepared silver nanoprism colloids with and without the modification of 10-4 M PVP. These two spectra are almost the same. Figure 6b shows

Figure 6. (a, top) Extinction spectrum of the as-prepared silver nanoprism colloids with the addition of 10-5 M PVP. (b, middle) TEM image of the as-prepared silver nanoprisms modified with PVP. (c, bottom) Time-dependent extinction spectra of PVP-modified nanoprism colloid in the presence of 10-3 M KBr.

the TEM image of the as-prepared silver nanoprisms modified with PVP. The image also shows that most of the nanostructures are triangular in shape. These results demonstrate that the surface modification with PVP does not change the morphology of silver nanoprisms. Figure 6c shows the time-dependent extinction spectra of PVP-modified nanoprism colloid in the presence of 10-3 M KBr. These spectra show that the sculpturing reaction caused by the addition of bromide ions still existed; however, the sculpturing rate of the PVP-modified silver nanoprisms in the presence of bromide ions was slower than that of silver nanoprisms without PVP modification. The slower sculpturing rate possibly comes from the partial protection of PVP coated on the surface of silver nanoparticles. Since the sculpturing rate is slowed by the modification with PVP, we have enough time to finely control the color of silver nanoprism colloids. Figure 7 shows the picture and the corre-

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Figure 7. The picture and the corresponding extinction spectra of PVPmodified silver nanoprism colloid after the addition of 10-3 M KBr for various reaction periods, 0.5, 1, 2, 4, 6, 12, 24, and 48 min, followed by the addition of 10-5 M MHA.

Figure 8. TEM images of the PVP-modified silver colloids silver nanoprism colloid after the addition of 10-3 M KBr for various reaction periods, 1 (a), 4 (b), 12 (c), and 48 min (d), followed by the addition of 10-5 M MHA, corresponding to curves b, d, f, and h in Figure 7, respectively.

sponding extinction spectra of PVP-modified silver nanoprism colloid after the addition of 10-3 M KBr for various reaction periods, 0.5, 1, 2, 4, 8, 16, 32, and 64 min, followed by the addition of 10-5 M MHA. Panels a-d of Figure 8 show the TEM images of the silver colloids corresponding to the curves b, d, f, and h, respectively, in Figure 7. Comparing Figures 7 and 8, it can be seen clearly that the silver nanoplates with a higher degree of truncated corners have the shorter in-plane dipole SPR wavelengths, which is consistent with the previous studies.18 The silver colloids synthesized by this method show longterm stability stored at room temperature as well as in a highly concentrated chloride solution. Panels a and b of Figure 9 show the peak positions of in-plane dipole and out-of-plane quadrupole SPR bands, respectively, of the PVP-KBr-MHA treated silver nanoprism colloids versus the stored time. The peak positions remained almost the same for at least one month.

Figure 9. Peak positions of in-plane dipole SPR bands (a, top) and out-of-plane quadrupole SPR bands (b, middle) of the PVP-KBr-MHA treated silver nanoprism colloids versus the stored time. (c, bottom) Extinction spectra of PVP-KBr-MHA treated silver nanoprism colloids in the presence of 0.2 M KCl.

Figure 9c shows the spectra of PVP-KBr-MHA treated silver nanoprism colloids in the presence of 0.2 M KCl. The peak positions of the spectra in Figure 9c are almost the same as those of the corresponding spectra in Figure 7. This result indicates that the PVP-KBr-MHA treated silver nanoprisms do not change their morphologies even in such a highly concentrated chloride solution, which is quite common in many kinds of cultivation liquids or biological buffer solutions. This property of high stability would make the PVP-KBr-MHA treated silver nanoprisms a promising candidate to be used in biosystems for the application of multicolor diagnostic labeling. 4. Conclusion The extinction spectra and TEM images show that the silver nanoprisms would be sculptured by bromide ions. The MHAmodified silver nanoprisms can endure a higher concentration of bromide ions than unmodified ones at least by a factor of

Highly Stable Silver Nanoplate Colloids about 105. Furthermore, it is found that the addition of MHA can stop the sculpturing reaction immediately. We tried to use bromide ions and MHA as the reagents to start and stop the etching reaction to tailor the silver nanoprisms into the nanostructures with the desired color. However, after several tries, the sculpturing rate was too fast in the initial reaction stage for us to obtain the colloid with longer SPR wavelength. We found that the sculpturing rate of PVP-modified silver nanoprisms in the presence of bromide ions was slower than that in unmodified ones. Therefore, the etching reaction can be stopped in time easily by adding MHA in such a slower shape evolution process, and the silver nanostructures with the desired SPR wavelength can be obtained. The PVP-KBr-MHA treated silver nanostructures with various colors produced by the present simple method not only have long-term stability stored at room temperature but also can prevent shape transformation in highly concentrated chloride solutions. Acknowledgment. This work was supported by the National Science Foundation of Taiwan (NSC 96-2113-M-415-007). The authors would like to thank Ms. Bi-Hua Su and Prof. Ming-Jen Lee for measuring the TEM images. C.L. is grateful to Profs. Wenlung Chen and Long-Liu Lin for their support and encouragement. References and Notes (1) El-sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (2) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (3) (a) Kim, M. H.; Lu, X.; Wiley, B.; Lee, E. P.; Xia, Y. J. Phys. Chem. C 2008, 112, 7872. (b) Wang, C.; Daimon, H.; Onodera, T.; Koda, T.; Sun, S. Angew. Chem., Int. Ed. 2008, 47, 3588. (c) Ghosh, D.; Chen, S. J. Mater. Chem. 2008, 18, 755. (d) Yin, Y.; Erdonmez, C. K.; Aloni, S.; Alivisatos, A. P. J. Am. Chem. Soc. 2006, 128, 12671. (e) Wang, Y.; Yang, H. Chem. Commun. 2006, 2545. (f) Tang, Y.; Ouyang, M. Nat. Mater. 2007, 6, 754. (4) (a) Orendorff, C. J.; Sau, T. K.; Murphy, C. J. Small 2006, 2, 636. (b) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L. F.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (c) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (d) Kim, F.; Song, J. H.; Yang, P. D. J. Am. Chem. Soc. 2002, 124, 14316. (e) Wiley, B.; Sun, Y.; Xia, Y. Langmuir 2005, 21, 8077. (5) Xiong, Y. J.; Wiley, B.; Chen, J. Y.; Li, Z. Y.; Yin, Y. D.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 7913. (6) (a) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (b) Radloff, C.; Halas, N. J. Nano Lett. 2004, 4, 1323. (c) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 206. (7) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; ElSayed, M. A Science 1996, 272, 1924. (b) Zhou, J.; An, J.; Tang, B.; Xu, S.; Cao, Y.; Zhao, B.; Xu, W.; Chang, J.; Lombardi, J. R. Langmuir 2008, 24, 10407. (8) (a) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Chem. 2000, 30, 545. (b) Nicewarner-Pena˜, S. R.; Freeman, R. G.; Reiss, B. D.; He, L.; Pena˜, D. J.; Walton, I. D.; Cromer, R.; Keating, C. D.; Natan, M. J. Science 2001, 294, 137. (c) Sun, Y. G.; Xia, Y. Science 2002, 298, 2176. (d) Yu, D.; Yam, V. W.-W. J. Am. Chem. Soc. 2004, 126, 13200. (e) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. D. Angew. Chem., Int. Ed. 2004, 43, 3673. (f) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. D. Angew. Chem., Int. Ed. 2004, 43, 3673. (g) Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 2154. (h) Song, H.; Kim, F.; Connor, S.; Somorjai, G. A.; Yang, P. J. Phys. Chem. B 2005, 109, 188. (i) Seo, D.; Park, J. C.; Song, H. J. Am. Chem. Soc. 2006, 128,

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