pubs.acs.org/Langmuir © 2009 American Chemical Society
Etching-Resistant Silver Nanoprisms by Epitaxial Deposition of a Protecting Layer of Gold at the Edges Damian Aherne,*,† Denise E. Charles,‡ Margaret E. Brennan-Fournet,§ John M. Kelly,† and Yurii K. Gun’ko*,† †
School of Chemistry and ‡School of Physics, Trinity College Dublin, Dublin 2, Ireland, and §School of Physics, National University of Ireland, Galway, Ireland Received March 18, 2009. Revised Manuscript Received June 10, 2009
The protection of silver nanoprisms against etching by the epitaxial deposition of a thin layer of gold in solution has been investigated. It has been found that at low Au/Ag ratios (∼0.08 to 0.17) a thin layer of gold is deposited on the edges of the nanoprisms as expected, but without the structural damage typically associated with galvanic replacement. Furthermore, this layer of gold provides robust protection against etching of the nanoprisms by chloride and is strong evidence that etching by chloride is face-selective and does not take place at the flat {111} faces of the nanoprisms. Additionally, the deposition of a protecting layer of gold results in only a small red shift in the position of the main plasmon resonance. We have investigated the sensitivity of the localized surface plasmon resonance (LSPR) to changes in the bulk refractive index of the solution and find that the gold-protected silver nanoprisms are promising candidates for the development of new refractive index-based biosensors.
1. Introduction Metal nanoparticles have been the focus of intense study because of many potential applications of their unique plasmonic properties.1 The plasmonic properties arise from the fact that when the dimensions of metal nanoparticles are much smaller than the wavelengths present in visible light the conduction electrons are capable of coherent oscillations known as a localized surface plasmon resonance (LSPR).2-6 Excitation of the conduction electrons occurs when there is resonance between the electrons and the oscillating electric field of the incident light, and in a macroscopic sample, this LSPR is observed as an extinction spectrum as light is passed through the sample. The spectral position of the LSPR and the scattering properties of metal nanoparticles can be tuned by adjusting the nanoparticle composition4 and morphology.2,3 The need to produce samples with finely tuned optical properties has therefore led to an enormous amount of research dedicated to developing versatile and reliable synthetic routes to the production of metal nanoparticles of a wide range of shapes and sizes from materials such as silver, gold, palladium, and platinum.7,8 The plasmonic properties of metal nanoparticles have made them very interesting candidates as key components of a range of novel chemical and biological sensors. For example, an exciting consequence of the LSPR phenomenon in highly shaped metal nanoparticles is that at the LSPR wavelengths the electric field *To whom correspondence should be addressed. E-mail: damian.aherne@ gmail.com,
[email protected]. (1) Larsson, E. M.; Alegret, J.; K€all, M.; Sutherland, D. S. Nano Lett. 2007, 7, 1256. (2) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (3) Wiley, B. J.; Im, S. H.; Li, Z.-Y.; McLellan, J.; Siekkinen, A.; Xia, Y. J. Phys. Chem. B 2006, 110, 15666. (4) Liz-Marzan, L. M. Langmuir 2006, 22, 32. (5) Hao, E.; Schatz, G. C. J. Chem. Phys. 2004, 120, 357. (6) Noguez, C. J. Phys. Chem. C 2007, 111, 3806. (7) Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310. (8) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60.
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intensity near the surface of the nanoparticle is enhanced strongly relative to the applied field.5,9-11 This property has spawned a great amount of research activity, both experimental and theoretical, in investigating metal-enhanced fluorescence (MEF)12-17 and surface-enhanced Raman spectroscopy (SERS).18-22 Another attractive property of the LSPR in metal nanoparticles is that its spectral position is very sensitive to changes in the local environment. Adjusting the distance of nearby metal nanoparticles23,24 or modifying the refractive index of the medium surrounding the nanoparticle25-28 will affect the spectral position of the LSPR. Indeed, the biomolecule-mediated assembly of metal nanoparticles has been extensively exploited to develop a (9) Messinger, B. J.; von Raben, K. U.; Chang, R. K.; Barber, P. W. Phys. Rev. B 1981, 24, 649. (10) Quinten, M. Appl. Phys. B: Laser Opt. 2001, 73, 245. (11) Evanoff, D. D.; White, R. L.; Chumanov, G. J. Phys. Chem. B 2004, 108, 1522. (12) Bharadwaj, P.; Anger, P.; Novotny, L. Nanotechnology 2007, 18, 044017. (13) Tam, F.; Goodrich, G. P.; Johnson, B. R.; Halas, N. J. Nano Lett. 2007, 7, 496. (14) Aslan, K.; Wu, M.; Lakowicz, J. R.; Geddes, C. D. J. Am. Chem. Soc. 2007, 129, 1524. (15) Chen, Y.; Munechika, K.; Ginger, D. S. Nano Lett. 2007, 7, 690. (16) Komarala, V. K.; Rakovich, Y. P.; Bradley, A. L.; Byrne, S. J.; Corr, S. A.; Gun’ko, Y. K. Nanotechnology 2006, 17, 4117. (17) Chen, Y.; Munechika, K.; Jen-La Plante, I.; Munro, A. M.; Skrabalak, S. E.; Xia, Y.; Ginger, D. S. Appl. Phys. Lett. 2008, 93, 053106. (18) Zou, X.; Dong, S. J. Phys. Chem. B 2006, 110, 21545. (19) Bell, S. E. J.; Sirimuthu, N. M. S. J. Am. Chem. Soc. 2006, 128, 15580. (20) Li, W.; Camargo, P. H. C.; Lu, X.; Xia, Y. Nano Lett. 2009, 9, 485. (21) Chen, X.; Li, S.; Xue, C.; Banholzer, M. J.; Schatz, G. C.; Mirkin, C. A. ACS Nano 2009, 3, 87. (22) Rodrı´ guez-Lorenzo, L.; Alvarez-Puebla, R. A.; Pastoriza-Santos, I.; Mazzucco, S.; Stephan, O.; Kociak, M.; Liz-Marzan, L. M.; Garcı´ a, F. J. J. Am. Chem. Soc. 2009, 131, 4616. (23) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (24) S€onnichsen, C.; Reinhard, B. M.; Liphart, J.; Alivisatos, A. P. Nat. Biotechnol. 2005, 23, 741. (25) Miller, M. M.; Lazarides, A. A. J. Phys. Chem. B 2005, 109, 21556. (26) Willets, K. A.; Van Duyne, R. P. Annu. Rev. Phys. Chem. 2007, 58, 267. (27) Medda, S. K.; De, S.; De, G. J. Mater. Chem. 2005, 15, 3278. (28) Sherry, L. J.; Jin, R.; Mirkin, C. A.; Schatz, G. C.; Van Duyne, R. P. Nano Lett. 2006, 6, 2060.
Published on Web 07/07/2009
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new generation of biosensors.29,30 In addition, the development of biosensors that are based on a change in the local refractive index upon the selective binding of an analyte has also become very promising.1,31 Highly anisotropic silver nanoparticles32 such as nanoprisms have received considerable attention because of their ability to easily tune the in-plane dipole LSPR across the entire visible spectrum and into the NIR33,34 but also because of the superior optical properties of silver compared to other noble metals such as gold.35 Despite these advantages, unprotected silver nanoparticles are unstable under physiological conditions because of a catalytic oxidation (etching) of the surface in the presence of chloride and other anions.36,37 This etching process can have drastic consequences such as the etching of triangular silver nanoprisms to discs or, at higher concentrations of chloride, the etching to very small spherical nanoparticles of a few nanometres in diameter.38 Indeed, this etching phenomenon has been successfully employed to selectively etch silver nanoparticles that contain defects, leaving only single-crystal seeds that under the appropriate reaction conditions can be grown into single-crystal silver nanocubes.39 If the optical properties of highly shaped silver nanoparticles such as nanoprisms are to be exploited for the development of sensitive biological sensors, then effective ways must be devised to protect them so that they are stable under physiological conditions. In recent years, a number of approaches have been investigated for the protection of silver nanoparticles. Very stable, wellprotected silver nanoparticles have been reported by Doty et al. by using thiolalkylated oligo(ethylene glycol) moieties as capping ligands to produce spherical silver nanoparticles that are stable at NaCl concentrations of up to 1 M.40 Unfortunately, this is not extendable to larger, highly shaped nanoparticles because the capping ligands are uncharged and thus provide only steric stabilization, which is available only to spherical nanoparticles with diameters of less than about 20 nm. Xue et al. have employed 16-mercaptohexadecanoic acid as a charged thiol for the stabilization of silver nanoprisms against etching by amines during silica coating.41 The resulting silica coating can protect the silver nanoprisms from etching by chloride, but the minimum thickness of silica coating needed for complete coating is 15 nm. This limits the usefulness of the nanoprisms because their LSPR is much less sensitive to changes in the local environment outside the silica coating. Lee et al. have shown that small spherical silver nanoparticles can be protected against etching by chloride by utilizing oligonucleotides modified with three cyclic disulfide groups that can form a total of six thiol linkages to the silver surface.42 (29) Lee, J.-S.; Han, M. S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2007, 46, 4093. (30) De, M.; Ghosh, P. S.; Rotello, V. M. Adv. Mater. 2008, 20, 4225. (31) Haes, A. J.; Hall, W. P.; Chang, L.; Klein, W. L.; Van Duyne, R. P. Nano Lett. 2004, 4, 1029. (32) Ledwith, D. M.; Aherne, D.; Kelly, J. M. Approaches to the Synthesis and Characterization of Spherical and Anisotropic Silver Nanomaterials. In Metallic Nanomaterials; Kumar, C. S. S. R., Ed.; Wiley-VCH: Weinheim, Germany, 2009; Vol. 1, p 99. (33) Aherne, D.; Ledwith, D. M.; Gara, M.; Kelly, J. M. Adv. Funct. Mater. 2008, 18, 2005. (34) Ledwith, D. M.; Whelan, A. M.; Kelly, J. M. J. Mater. Chem. 2007, 17, 2459. (35) Wang, F.; Shen, Y. R. Phys. Rev. Lett. 2006, 97, 206806. (36) Mulvaney, P.; Linnert, T.; Henglein, A. J. Phys. Chem. 1991, 95, 7843. (37) Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481. (38) An, J.; Tang, B.; Zheng, X.; Zhou, J.; Dong, F.; Xu, S.; Wang, Y.; Zhao, B.; Xu, W. J. Phys. Chem. C 2008, 112, 15176. (39) Skrabalak, S. E.; Au, L.; Li, X.; Xia, Y. Nat. Protoc. 2007, 2, 2182. (40) Doty, R. C.; Tshikhudo, T. R.; Brust, M.; Fernig, D. G. Chem. Mater. 2005, 17, 4630. (41) Xue, C.; Chen, X.; Hurst, S. J.; Mirkin, C. A. Adv. Mater. 2007, 19, 4071. (42) Lee, J.-S.; Lytton-Jean, A. K. R.; Hurst, S. J.; Mirkin, C. A. Nano Lett. 2007, 7, 2112.
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This approach, although successful, is not very practical because it is difficult and rather expensive to obtain significant quantities of such modified oligonucleotides. Furthermore, it has yet to be demonstrated that this approach can be extended to silver nanoprisms. One reason for this is that on flat surfaces there would be increased electrostatic repulsion between the negatively charged phosphate backbone of the oligonucleotides and this could limit surface coverage, leaving areas of the nanoparticle surface exposed to attack by chloride. Coating silver nanoparticles with less reactive noble metals such as gold has also been attempted. Gold is an attractive and obvious choice because it is inert, thiol ligands bind strongly,43 and it has the same crystal structure as silver with very similar lattice spacings, thus allowing the epitaxial growth of a layer of gold on silver.44 However, coating silver nanoparticles with gold is far from straightforward and is quite difficult because of galvanic replacement, a process whereby a metal is deposited as a result of the reduction of relevant ions by another metal with a lower reduction potential. For instance, hollow gold nanostructures such as nanoshells,45 nanocages,39,46 and nanorings47-49 can be produced by the consumption of sacrificial silver nanosphere, nanocube, and nanoprism templates by AuClh4 ions, respectively; pair is 0.99 V the standard reduction potential of the AuClh/Au 4 versus the standard hydrogen electrode (SHE) whereas that of the Agþ/Ag pair is 0.80 V versus SHE. There are a number of examples of the successful coating of silver nanoparticles with a layer of gold for spherical nanoparticles50,51 and for nanoprisms.52,53 The gold coatings on silver nanoprisms are usually relatively thick and can be quite rough. Also, there has been no reported study of the stability of goldcoated silver nanoprisms with respect to etching by chloride. A common factor in the gold coating procedures for nanoprisms is the presence of ascorbic acid to reduce the AuClh4 ion and thus avoid the oxidation of the silver. Here we carefully employ the ascorbic acid-mediated approach to epitaxially deposit a thin layer of gold at the edges of silver nanoprisms, and we clearly demonstrate their robust stabilization against oxidative etching by chloride. Furthermore, as will be shown below, this involves little change to the strong LSPR of the nanoprisms, and they display a very high sensitivity to changes in the refractive index of the dispersing medium.
2. Experimental Section Reagents were obtained from Sigma-Aldrich and used as received. Distilled water was used throughout. Unless otherwise stated, UV-vis measurements were carried out on a Varian Cary 50 UV-vis spectrophotometer. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken on a JEOL JEM-2100 LaB6 operating at 200 kV on samples (43) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (44) Fan, F.-R.; Liu, D.-Y.; Wu, Y.-F.; Duan, S.; Xie, Z.-X.; Jiang, Z.-Y.; Tian, Z.-Q. J. Am. Chem. Soc. 2008, 130, 6949. (45) Prevo, B. G.; Esakoff, S. A.; Mikhailovsky, A.; Zasadzinski, J. A. Small 2008, 4, 1183. (46) Chen, J.; McLellan, J. M.; Siekkinen, A.; Xiong, Y.; Li, Z.-Y.; Xia, Y. J. Am. Chem. Soc. 2006, 128, 14776. (47) Metraux, G. S.; Cao, Y. C.; Jin, R.; Mirkin, C. A. Nano Lett. 2003, 3, 519. (48) Sun, Y.; Xia, Y. Adv. Mater. 2003, 15, 695. (49) Jiang, L.-P.; Xu, S.; Zhu, J.-M.; Zhang, J.-R.; Zhu, J.-J.; Chen, H.-Y. Inorg. Chem. 2004, 43, 5877. (50) Cao, Y. W.; Jin, R.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7961. (51) Pande, S.; Ghosh, S. K.; Praharaj, S.; Panigrahi, S.; Basu, S.; Jana, S.; Pal, A.; Tsukuda, T.; Pal, T. J. Phys. Chem. C 2007, 111, 10806. (52) Sanedrin, R. G.; Georganopoulou, D. G.; Park, S.; Mirkin, C. A. Adv. Mater. 2005, 17, 1027. (53) Rodrı´ guez-Gonzalez, B.; Burrows, A.; Watanabe, M.; Kiely, C. J.; LizMarzan, L. M. J. Mater. Chem. 2005, 15, 1755.
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prepared by the deposition and drying of a drop of an aqueous suspension onto a Formvar-coated 300 mesh copper grid. Silver nanoparticle seeds were prepared as described in a previously published report33 whereas the nanoprisms were produced via a slight modification of the nanoprism synthesis contained therein. Typically, nanoprisms are produced by combining 35 mL of distilled water, ascorbic acid ( 0.525 mL, 10 mM), and 3.5 mL of silver nanoparticle seed solution, followed by the addition of AgNO3 solution (21 mL, 0.5 mM) at a rate of 4 mL min-1 while stirring vigorously. After synthesis, trisodium citrate solution (17.5 mL, 25 mM) is added. This produces silver nanoprisms of approximately 21 nm perpendicular diameter across the {111} flat face. Larger nanoprisms can be produced by reducing the volume of seed solution used. The sample was then split into seven aliquots of equal volume, with each aliquot corresponding to 3 mL of added AgNO3 solution. One aliquot of silver nanoprism solution was taken, and to this was added 150 μL of 10 mM ascorbic acid solution. The appropriate volume of HAuCl4 solution (0.5 mM) corresponding to the desired Au/Ag ratio was then added at 0.2 mL min-1 while stirring vigorously. Samples of gold-coated nanoprisms were tested for stability against etching by chloride by making up to 10 mM NaCl. These samples were prepared by mixing equal volumes of gold-coated silver nanoprism solution and 20 mM NaCl solution. UV-vis spectra were taken at various time intervals to monitor the stability of the samples over time. The refractive index of the particles’ local environment was varied by dispersing the particles in sucrose solutions of different concentrations. The refractive indices of the sucrose solutions were measured on a temperature-controlled AR-2008 digital Abbe refractometer with a 589 nm LED light source and were compared to the Brix scale for accuracy. A small volume of the nanoparticle solution (50 μL) was mixed into 750 μL of each of the various sucrose concentrations, and the spectra were recorded (Varian Cary 6000i UV-vis-NIR).
3. Results and Discussion In the work presented here, the main goal was to utilize readily available reagents to coat silver nanoprisms with a thin coating of gold while avoiding etching of the silver by both the AuClh4 anion and the Clh ion that is liberated upon reduction of AuIII in the AuClh4 anion to Au. The approach adopted here is very straightforward and involves the use of ascorbic acid, a common mild reducing agent often used in the synthesis of silver nanoprisms, to reduce the gold salt and therefore avoid oxidizing the silver. It is important to use a reducing agent that is mild to avoid the spontaneous nucleation of gold nanoparticles and so that the surface of the silver nanoprisms can catalyze the reduction of AuClh4 by the reducing agent, a process more commonly known as electroless plating.54 In addition, we utilized citrate, a common capping agent for silver nanoparticles, to minimize the amount of damage done to the silver nanoprisms during the coating process. To investigate the effectiveness of gold coating for conferring stability against etching by chloride, a series of samples was prepared with a range of quantities of added gold. As outlined above, each sample was then tested for resistance to etching by making up aliquots to 10 mM NaCl. This concentration was chosen in order to use as high a concentration as possible to test the protection of the nanoprisms. Concentrations higher than 10 mM would typically result in aggregation of the particles as a result of the screening of the repulsive electrostatic interactions between the nanoprisms. (54) Satti, A.; Aherne, D.; Fitzmaurice, D. Chem. Mater. 2007, 19, 1543.
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Figure 1. (Top) UV-vis spectra of Ag nanoprisms (black) and five samples of Ag nanoprisms coated with increasing quantities of gold. The legend shows the ratio of gold to silver for each sample. (Bottom) UV-vis spectrum of each of the same samples after 5 days in a 10 mM solution of NaCl.
In the top panel of Figure 1, we can see the effect that adding increasing quantities of gold has on the optical properties. Increasing the amount of gold results in a small red-shift and a slight broadening of the UV-vis spectra. We largely attribute the red shifting of the LSPR to an increase in size as a result of the added gold. In the bottom panel of Figure 1, the effect of 10 mM NaCl on each of the samples after 5 days is clearly illustrated. The bare Ag nanoprisms (sample A) are etched back to small silver nanoprisms as indicated by the LSPR blue shifting to 393 nm, which is the position of the LSPR for small spherical silver nanoparticles.55 As the quantity of gold is increased, the degree of etching is decreased as can be seen in sample C. Samples D and E show a very high degree of stability with very little change in the LSPR over 5 days in 10 mM NaCl. This is a very significant result because we have managed to stabilize silver nanoprisms against etching by the addition of a relatively small amount of gold and without causing any significant deterioration of their optical properties. Surprisingly, sample F, the sample with the greatest amount of gold, shows a significant amount of broadening. A more complete set of data showing the shifts in position of the LSPR after soaking in 10 mM chloride over time is given in Table S1 in the Supporting Information section. Photographs of (55) Shirtcliffe, N.; Nickel, U.; Schneider, S. J. Colloid Interface Sci. 1999, 211, 122.
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Figure 2. TEM images of edge-on-oriented nanoprisms corresponding to samples A-F above.
samples A-F before and after 5 days at 10 mM NaCl are shown in Figure S1 of the Supporting Information. However, the question remains as to how this stabilization is achieved. Initially, it was thought that a complete thin coating of gold had been obtained with the amount of gold used to produce sample D, with a slightly thicker coating in the case of sample E. However, as will be shown below, this turns out not to be the case. An analysis of the gold-coated silver nanoprisms by TEM has allowed us to investigate the evolution of their structure as increasing quantities of gold are added. In Figure 2, we can see representative TEM images of an edgeon view of samples A-F. A statistical analysis of the thicknesses of these samples is shown in the set of histograms in Figure 3. From these histograms, we can plot the evolution of the average thickness as a function of the quantity of added gold; see Figure 4. The uncertainties in the determinations of the average thickness of a sample are obtained by dividing the standard deviation of the distribution by the square root of the number of particles measured. The plot in Figure 4 shows that from sample A to D the nanoprisms do not become progressively thinner as the amount of gold is increased, demonstrating the avoidance of galvanic replacement. This is remarkable because we have shown that sample D is resistant to etching by chloride. It is only with higher amounts of gold, as in samples E and F, that thinning of the nanoprisms is clearly significant. Therefore, as more gold is added, there is etching of silver from the nanoprisms at the flat 10168 DOI: 10.1021/la9009493
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Figure 3. (A-F) Histograms showing thickness measurements from edge-on-oriented nanoprisms as shown in Figure 2. In sample F, many nanoprisms have holes in the center, and these are recorded as having zero thickness.
Figure 4. Plot of nanoprism thickness against the ratio of gold to silver in samples A-F. The black error bars indicate the width of the distribution of measurements, and the red error bars indicate the uncertainties in the average values of thickness.
{111} faces, characteristic of a galvanic displacement reaction by direct analogy to the formation of hollow gold nanostructures as mentioned above. We can deduce from this that a gold layer is not being deposited on the flat {111} faces because if it were then the initial layer of gold would protect the flat {111} faces against galvanic replacement. It can also be seen that there is a slight increase in thickness for samples B and C. This is attributed to etching by ascorbic acid; see the following discussion. Figure 5 shows representative TEM images of flat-lying silver nanoprisms from samples A-F. In sample D, it is clear that some Langmuir 2009, 25(17), 10165–10173
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Figure 5. TEM images of flat-lying nanoprisms corresponding to samples A-F.
contrast has developed between the (gold-coated) edges and the centers of the nanoprisms. As seen above, sample D is the same thickness as the starting nanoprisms (sample A), thus the observed contrast in sample D is a result of the growth of a layer of gold at the edges; gold has a higher scattering efficiency for electrons than for silver. It is interesting that a distinct gold layer is visible in the flat-lying nanoprisms in sample D. At the Au/Ag ratio employed here (0.083), a gold layer of approximately 0.4 nm (i.e., about two atomic layers) is theoretically expected, yet the slightly darker edges of the nanoprisms are about 2 to 3 nm across. This can be explained by the morphology of the nanoprisms. The edges are not straight but rather are faceted as can be seen in the images in Figure 2. The gold layer will appear to be thicker laterally than it actually is as it curves around the faceted edges of the nanoprisms but will appear faint because the electron beam in the TEM is passing through less than 1 nm of gold. Also, there is perhaps some growth of gold from the edge toward the center of the nanoprism. Overall, this is further evidence that up to a certain point it is possible to establish a ring of gold around the silver nanoprisms while avoiding galvanic replacement and maintaining structural integrity. At the higher levels of gold, the gold layer at the edges become thicker, and the centers of the nanoprisms become thinner, resulting in a further increase in contrast. There is some hollowing out of the center of the nanoprisms with the formation of nanorings, as can be seen in sample F in Figure 2. This contributes to an increase in the polydispersity of structures and an increase in Langmuir 2009, 25(17), 10165–10173
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Figure 6. (A-F) Histograms of diameter measurements for flatlying nanoprisms as shown in Figure 5. The diameter is measured as the perpendicular diameter in the case of a (truncated) nanoprism or hexagon or simply the diameter in the case of a disk. The spherical gold nanoparticles in samples E and F are not included when calculating statistics.
Figure 7. Plot of nanoprism diameters as a function of the ratio of gold to silver in the samples. The linear fit is a fit to the diameters for samples B-F. The black error bars indicate the width of the distribution of measurements, while the red error bars indicate the uncertainties in the average values of thickness.
the ratio of gold to silver in the nanoparticles, which both lead to a broadening of the LSPR. A statistical analysis of the diameters of the samples is shown in the set of histograms in Figure 6. It can be seen that there is no obvious increase in the size distribution between samples A and F. In samples E and F, the presence of some spherical gold nanoparDOI: 10.1021/la9009493
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Aherne et al. Scheme 1. Gold Coating of Silver Nanoprismsa
a Up to sample D, an increasingly thick layer of gold is deposited without galvanic replacement of the silver, rendering these gold-coated nanoprisms stable against etching by chloride. Further addition of gold results in galvanic replacement with the resultant thinning of the silver in the center.
ticles has been detected. These arise either from the self-nucleation of gold in solution or the growth of gold upon residual silver seed nanoparticles. The evolution of the average diameter as a function of the quantity of added gold is plotted in Figure 7. As before, the uncertainties in the determinations of the average diameter of a sample are obtained by dividing the width of the distribution by the square root of the number of particles measured. The first thing to note is the apparent decrease in diameter for the smallest amount of added gold (sample B). This is attributed to etching of the nanoprisms by ascorbic acid in the minutes before the addition of the solution of HAuCl4; etching of gold nanorods by ascorbic acid has been reported previously.56 An example of this etching can be seen in the Supporting Information section (Figure S2), where clearly many nanoprisms have been etched to discs. Statistical information from this sample is unreliable because once the ascorbic acid is added the etching continues throughout the preparation of the sample for TEM, leading to an exaggerated picture of the extent of etching. We attribute the presence of many disklike particles in the samples, such as in Figure 5E, to this etching. This etching by ascorbic acid might also explain the apparent increase in thickness for samples B and C. Silver that is etched away from the edges could be deposited on the flat {111} face, resulting in thicker nanoprisms. Nanoprism thickening such as this has been observed previously for silver nanoprisms that have been etched to discs by chloride.38 Next, it is clear that there is an increase in diameter from samples B-F as a result of the deposition of a gold layer at the edges. A linear fit to the diameter data for samples B-F has been carried out where ascorbic acid-etched silver nanoprisms are the starting point for the gold deposition, and the increase in diameter with the Au/Ag ratio is approximately linear in this range. Thus, we have determined that the increase in diameter is Δd=5.54 Au/Ag, which translates into an increase in diameter of approximately 1.8 nm for a Au/Ag ratio of 0.333. This increase of almost 2 nm is statistically significant and establishes that there is an increase in diameter due to the deposition of a gold layer at the edges. If we consider a disk with straight sides (i.e., a very short cylinder) that is 21 nm in diameter and 5 nm in height, then a Au/Ag ratio of 0.333 (sample F) should result in an increase in diameter of approximately 3.2 nm. A similar prediction is obtained for a 5-nm-thick nanoprism with a perpendicular diameter of 21 nm. Because the variation in expected thickness is approximately linear in the range of the amount of gold used in this work, the thickness would vary proportionally for the other quantities of added gold. The 3.2 nm increase for Au/Ag=0.333 is clearly higher than what is actually observed. One reason for this is that at the Au/Ag ratio of 0.333 for sample F not all of the gold is being deposited at (56) Novo, C.; Mulvaney, P. Nano Lett. 2007, 7, 520.
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the edges. A look at Figure 5F reveals some gold nanorings that have some material growing from the nanoring in toward the center. Indeed, this is not completely unexpected if galvanic replacement of the silver within the gold nanoring is taking place. Another possible contributing factor to the lower-than-expected diameter increase is the presence of spherical gold nanoparticles mentioned above. An overview of the structural evolution of the nanoprisms with the addition of gold under the conditions utilized in this article is illustrated schematically in Scheme 1. On the basis of the UV-vis spectra and TEM data, the optimum sample is sample D. This sample displays a high degree of stability toward etching in 10 mM NaCl with very little change in the optical properties and morphology of the nanoprisms. Significantly, this is achieved with a thin layer of gold (∼0.4 nm). Indeed, there are more than 10 silver atoms for each gold atom that is added. Figure 8 shows TEM images of gold-coated silver nanoprisms from sample D after exposure to 10 mM NaCl. The vast majority of the nanoprisms are intact, with the darker goldcoated edge clearly visible. To confirm the stability of the gold-protected silver nanoprisms from sample D in 10 mM NaCl, we have carried out a statistical analysis of TEM images of gold-protected silver nanoprisms that have been soaking in 10 mM NaCl for 1 day. Histograms of the diameter and thickness measurements are presented in the top half of Figure 9. In the bottom half of Figure 9, data points corresponding to the average diameter and thickness of goldprotected silver nanoprisms from sample D, before and after soaking in 10 mM NaCl, are presented. The nanoprisms appear to have undergone a slight decrease in thickness and a slight increase in diameter upon exposure to the NaCl solution, but these changes are not statistically significant. A HRTEM image of a gold-coated nanoprism from sample D is shown in Figure 10. Lattice fringes with 2.50 A˚ spacing are clearly visible. These spacings, normally observed in flat-lying nanoprisms, are commonly attributed to 1/3 {422} formally forbidden reflections,57-60 but an alternative explanation that these arise from a local defect-induced hcp structure that has also been put forward.33,61 The lattice spacings are the same in the silver center as on the gold edge, so the gold layer adopts the same defect structure as the silver nanoprism template, consistent with epitaxial deposition of gold upon the edges of the silver nano(57) Germain, V.; Li, J.; Ingert, D.; Wang, Z. L.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 8717. (58) Lofton, C.; Sigmund, W. Adv. Funct. Mater. 2005, 15, 1197. (59) Elechiguerra, J. L.; Reyes-Gasga, J.; Yacaman, M. J. J. Mater. Chem. 2006, 16, 3906. (60) Rodrı´ guez-Gonzalez, B.; Pastoriza-Santos, I.; Liz-Marzan, L. M. J. Phys. Chem. B 2006, 110, 11796. (61) Rocha, T. C. R.; Zanchet, D. J. Phys. Chem. C 2007, 111, 6989.
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Figure 9. (Top) Histograms of thickness measurements (left) and diameter measurements (right) for gold-protected silver nanoprisms from sample D after soaking in 10 mM NaCl for 1 day. (Bottom) Data points corresponding to average thickness and diameter data for gold-protected silver nanoprisms from sample D before and after NaCl exposure. The red error bars represent the uncertainties in the data points, and the black error bars represent the width of the distributions of the diameter and thickness data for each sample.
Figure 8. TEM images of gold-coated silver nanoprisms from sample D cast from a 10 mM NaCl solution.
prisms. The definite contrast between the edges and the centers of the coated nanoprisms suggests a distinct gold layer, but more significantly, recent publications have established that the low temperatures involved in the synthesis here are not high enough to form an alloy, with the higher temperatures associated with refluxing conditions being necessary for effective alloying between gold and silver.62,63 Some alloying at the interface of the gold and silver is possible, but this is unlikely to have a negative effect on the stability of the nanoprisms toward etching. When a gold concentration of approximately 50% in Au-Ag alloy nanostructures is reached, even powerful etchants such as Fe(NO3)3 are not able to etch the silver.64 (62) Zhang, Q.; Xie, J.; Lee, J. Y.; Zhang, J.; Boothroyd, C. Small 2008, 4, 1067. (63) Wang, C.; Peng, S.; Chan, R.; Sun, S. Small 2009, 5, 567. (64) Lu, X.; Au, L.; McLellan, J.; Li, Z.-Y.; Marquez, M.; Xia, Y. Nano Lett. 2007, 7, 1764.
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Figure 10. HRTEM image of a silver nanoprism with a coating of gold around the edges. The inset shows a Fourier transform of the image.
So just why does the gold coating start with gold deposition at the edges and not on the flat {111} faces? A likely explanation for this rests upon the fact that the lamellar defects in the silver nanoprisms are exposed to the solution at the edges. Accordingly, DOI: 10.1021/la9009493
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these edges are defect-rich and as such are of higher energy and thus support a higher growth rate, leading to the 2D growth that is characteristic of silver nanoprisms.33 Similarly, during gold coating, the defects could lead to a much faster rate of gold deposition at the edges than on the flat {111} faces. This explanation requires the diffusion of add-atoms to be fast compared to the rate of addition of add-atoms to the nanoparticle surface so that the concentration of add-atoms in solution is the same at all crystal faces, leaving the kinetics of growth on each crystal face to determine the shape. However, another possible explanation can be found by considering the gold deposition to be a diffusionlimited reaction, where the diffusion of an add-atom from solution to a growing crystal is slow compared to the rate at which the addatom is absorbed. In this situation, a concentration gradient of add-atoms develops that increases with distance from the nanoparticle. Any anisotropic structural feature that extends out into the solution, such as the edges of the nanoprisms, would be exposed to a higher concentration of add-atoms, leading to faster growth on that feature.65,66 The fact that a coating on the edges of the silver nanoprisms is enough to protect against attack by chloride is instructive. It has been shown that low concentrations of chloride can induce a morphological change from triangular to disk through etching of the sides of the silver nanoprisms.38 The results reported here provide further evidence that etching of silver nanoprisms by chloride occurs via attack at the edges. The edges are where the higher-energy {100} crystal faces and lamellar defects are exposed to the external environment,33 whereas the flat face of the silver nanoprisms is bounded by the {111} face only. Because the {111} face of silver is left exposed in our gold-coated samples and etching does not occur when exposed to 10 mM chloride, we can therefore conclude that the chloride ions do not etch the large, flat {111} crystal face of silver nanoprisms. A key reason for this is the absence of defects on this face compared to the high density of defects that are present at the edges. If defects on the flat {111} face are present then etching could occur, and this likely explains the significant further red shift, broadening, and decrease in intensity of the LSPR of sample F in 10 mM NaCl. As the {111} surface is etched away during galvanic replacement, defects and etch pits could appear, which might be suitable sites for attack by chloride. Etching away of the silver would then leave a gold nanoring, thus explaining the behavior of the LSPR of sample F upon exposure to chloride. The coating of the edges of the silver nanoprisms with gold can be extended to much larger nanoprisms with the same resulting protection against etching by chloride. In Figure 11, TEM images of gold-coated nanoprisms of a wide range of sizes are presented. A thin coating of gold at the edges is indicated by the faint but distinct darker region that can be seen at the edges of many of the nanoprisms. Figure 12 shows the UV-vis spectra of the uncoated, gold-coated, and 10 mM NaCl-exposed gold-coated nanoprisms. There is a consistent, small red shift upon coating with gold with very little change upon storing the samples in 10 mM NaCl solution. The suitability of the gold-coated silver nanoprisms for refractive index sensing has been tested by measuring the response of the main LSPR to changes in the refractive index of the solvent for samples A-D of various sizes as shown in Figures 11 and 12. The sensitivity (wavelength shift per refractive index unit (RIU)), Δλmax RIU-1, was then computed by plotting the LSPR peak wavelength shifts, Δλmax, against the corresponding solutions’ refractive indices. In each sample, the main LSPR shows a steady red shift in response to increasing refractive index unit (RIU) as (65) Chernov, A. Sov. Phys.-Crystallogr. 1972, 16, 734. (66) Herricks, T.; Chen, J.; Xia, Y. Nano Lett. 2004, 4, 2367.
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Figure 11. (A-D) TEM images of gold-coated silver nanoprisms of increasing size; Au/Ag = 0.083. The darker gold-coated edges are visible on many of the nanoprisms in each sample.
Figure 12. UV-vis spectra of samples of various sizes as shown in Figure 11: uncoated silver nanoprisms (-); gold-coated silver nanoprisms, Au/Ag = 0.083 (--); gold-coated silver nanoprisms in 10 mM NaCl for 1 day ( 3 3 3 ).
shown in Figure 13. The slope of the best fit to each set of data has been obtained to determine a value for the sensitivity for each sample. These sensitivity values are plotted in Figure 14, and it is clear that there is a steady increase in the refractive index sensitivity of the nanoprisms with the position of the main LSPR, as expected on the basis of results from recent theoretical25,67 and experimental1,68 work. Remarkably, the range of values for the sensitivity of the goldcoated silver nanoprisms is higher than the sensitivity values determined for single-particle measurements of silver nanoprisms.28 Also, they compare very well with values for a recent report on highly sensitive gold nanorings.1 The smallest gold (67) Khlebstov, B. N.; Khlebstov, N. G. J. Phys. Chem. C 2007, 111, 11516. (68) Wang, H.; Brandl, D. W.; Le, F.; Nordlander, P.; Halas, N. J. Nano Lett. 2006, 6, 827.
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silver nanoprisms presented here have a λmax of about 940 nm and a sensitivity of about 470 nm RIU-1. Thus, the gold-coated silver nanoprisms are as sensitive as any other nanoparticle with a similar LSPR position published so far.
4. Conclusions
Figure 13. Shift in the peak of the main LSPR (Δλmax) as a function of the refractive index of solution for samples A-D of increasing size. (Inset) LSPR spectra for sample C. There is a clear red shift of the LSPR as the refractive index of the solution is increased.
In this article, we have investigated the coating of silver nanoprisms with gold in an attempt to protect them against etching by the chloride that is present under physiological conditions. Remarkably, we have achieved the stabilization of silver nanoprisms against etching with a thin layer of gold, which the data strongly suggest is located mostly at the edges. Furthermore, this protective coating of gold is deposited with little change to the optical properties and without the structural damage associated with galvanic replacement. Further studies of the gold-protected silver nanoprisms are required to determine the exact nature of the gold layer. Energyfiltered TEM imaging, energy-dispersive X-ray spectroscopy (EDS) maps, or Z-contrast scanning tunnelling electron microscopy (STEM) imaging would provide a much clearer picture of the gold distribution. This would be of great assistance in gaining a greater understanding of the mechanism of gold deposition and the resulting stabilization against etching. We have studied the sensitivity of the LSPR to changes in the refractive index of the local environment and have elucidated sensitivity values higher than have been published so far for silver nanoprisms. This, combined with the newly achieved resistance to etching by chloride, makes these nanoparticles ideal candidates for the development of refractive index-based biosensors. Our future research will involve further optimization of the sensitivity of these silver/gold nanoprisms and their utilization in biosensing of important biological species. Acknowledgment. We are grateful to Prof. Werner Blau for the use of equipment in his laboratory. We also thank Mr. Neal Leddy at the Centre for Microscopy and Analysis, TCD, for assistance with electron microscopy. Financial support for a postdoctoral fellowship for D.A. provided by the Irish Research Council on Science and Engineering Technology (IRCSET) is gratefully acknowledged.
Figure 14. Plot of sensitivity data, obtained from best fits to the data in Figure 13, against the position of the main LSPR. The straight line is a best fit to the data.
nanorings in that report have a λmax of about 950 nm and a sensitivity of about 520 nm RIU-1, whereas the largest gold-coated
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Supporting Information Available: Shifts in position of the main LSPR of each sample. Photographs of samples A-F. TEM images illustrating that ascorbic acid rounds the tips of silver nanoprisms. This material is available free of charge via the Internet at http://pubs.acs.org.
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