pubs.acs.org/Langmuir © 2009 American Chemical Society
Gold Colloids with Unconventional Angled Shapes Ana Sanchez-Iglesias,†,‡ Marek Grzelczak,†,‡ Benito Rodrı´ guez-Gonzalez,‡ ‡ Ramon A. Alvarez-Puebla, Luis M. Liz-Marzan,*,‡ and Nicholas A. Kotov*,† †
Departments of Chemical Engineering, Materials Science and Engineering, Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, and ‡Departamento de Quı´mica-Fı´sica and Unidad Asociada CSIC-Universidade de Vigo, 36310 Vigo, Spain Received May 4, 2009
We report the formation of porous gold nanoparticles with unusual, angled shapes (such as nanocheckmarks) through spontaneous transformation of tellurium sacrificial templates by gradual galvanic replacement. High-resolution electron microscopy studies of intermediate stages reveal interesting information regarding the replacement mechanism, which involves initial “gold island growth” at the edges, and gradual branching to engulf the entire particle templates, resulting in a highly porous structure. Additionally, the high porosity of these novel nanostructures with unusual shapes is demonstrated to provide very high enhancement of the Raman scattering signal from adsorbed molecules.
Introduction Control over the overall shape of nanostructures is one of the most challenging and fundamental problems in today’s science. It is required by multiple technological areas, such as optics, catalysis, sensing, electronics, and, in fact, it is difficult to find advanced technologies that would not benefit from it. Noble metals hold special importance in nanoscale structures because of their unique optical as well as electronic properties, which have given rise to the emerging field of Plasmonics.1 Gold and other noble metals have been produced in a wide variety of shapes: fairly spherical nanoparticles (NPs),2,3 platonic solids,4,5 nanorods,6-8 and nanoplates,9,10 among others. On occasion, even spikes can form on the surface of noble metal NPs;11,12 however, the overall nanocolloids retain an approximately rotational symmetry. At the same time, nanocolloids from gold and other noble metals with decreased symmetry, e.g., containing angled shapes, are of great interest because they are expected to display unique nonlinear optical properties,13,14 plasmon localization,15-17 shape-dependent *To whom correspondence should be addressed. E-mail:
[email protected] (L.M.L.-M.);
[email protected] (N.A.K.). (1) Ozbay, E. Science 2006, 311, 189–193. (2) Park, J.; Joo, J.; Kwon, S.; Jang, Y.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 4630–4660. (3) Liz-Marzan, L. M. Langmuir 2006, 22, 32–41. (4) Tao, A.; Habas, S.; Yang, P. Small 2008, 4, 310–325. (5) Skrabalak, S.; Chen, J.; Sun, Y.; Lu, X.; Au, L.; Cobley, C.; Xia, Y. Acc. Chem. Res. 2008, 41, 1587–1595. (6) Chen, J.; Wiley, B.; Xia, Y. Langmuir 2007, 23, 4120–4129. (7) Murphy, C. J.; Sau, T.; Gole, A.; Orendorff, C.; Gao, J.; Gou, L.; Hunyadi, S.; Li, T. J. Phys. Chem. B 2005, 109, 13857–13870. (8) Perez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Mulvaney, P. Coord. Chem. Rev. 2005, 249, 1870–1901. (9) Millstone, J.; Hurst, S. J.; Traux, G. S. M.; Cutler, J. I.; Mirkin, C. A. Small 2009, 5, 646–664. (10) Pastoriza-Santos, I.; Liz-Marzan, L. M. J. Mater. Chem. 2008, 18, 1724– 1737. (11) Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Chem. Soc. Rev. 2008, 37, 1783–1791. (12) Pastoriza-Santos, I.; Liz-Marzan, L. M. Adv. Funct. Mater. 2009, 19, 679– 688. (13) Kujala, S.; Canfield, B. K.; Kauranen, M.; Svirko, Y.; Turunen, J. Opt. Express 2008, 16, 17196–17208. (14) Drozdowicz-Tomsia, K.; Xie, F.; Calander, N.; Gryczynski, I.; Gryczynski, K.; Goldys, E. M. Chem. Phys. Lett. 2009, 468, 69–74. (15) Tabor, C.; Murali, R.; Mahmoud, M.; El-Sayed, M. J. Phys. Chem. A 2009, 113, 1946–1953. (16) Ali Umar, A.; Oyama, M. Cryst. Growth Des. 2009, 9, 1146–1152. (17) Nehl, C.; Hafner, J. J. Mater. Chem. 2008, 18, 2415–2419.
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biological properties,18-20 and biosensing capabilities.21-26 Recently, plasmonic NPs have been proposed as ideal candidates for the development of novel materials with anomalous dielectric properties (such as negative index of refraction materials, NIMs), currently known as metamaterials.27-31 Angled NPs are particularly appealing for the design of NIMs in the ultraviolet-visiblenear-infrared (UV-vis-NIR) wavelength range, holding the key for overcoming diffraction limits in optics, and hence, fundamental advances in electronics and information processing.32,33 These shape requirements for NPs within the interesting nanometer scale have proven difficult to achieve.34,35 Apart from obvious scientific and technological value, asymmetric angled shapes for noble metals are elusive because of the highly symmetrical face-centered cubic (fcc) crystal lattice in which they crystallize, typically resulting in equally symmetrical NPs. Notably, Au NP synthesis within templates, which might have given an opportunity to produce more complex shapes by removing the intrinsic crystallization tendencies, also revealed the challenging nature of angled shapes. Most of the examples of (18) Wang, S.; Lu, W.; Tovmachenko, O.; Rai, U. S.; Yu, H.; Ray, P. C. Chem. Phys. Lett. 2008, 463, 145–149. (19) Sonavane, G.; Tomoda, K.; Makino, K. Colloid Surf. B 2008, 66, 274–280. (20) Murphy, C.; Gole, A.; Stone, J.; Sisco, P.; Alkilany, A.; Goldsmith, E.; Baxter, S. Acc. Chem. Res. 2008, 41, 1721–1730. (21) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042–6108. (22) Hu, M.; Chen, J.; Li, Z.; Au, L.; Hartland, G.; Li, X.; Marquez, M.; Xia, Y. Chem. Soc. Rev. 2006, 35, 1084–1094. (23) Anker, J.; Hall, W.; Lyandres, O.; Shah, N.; Zhao, J.; Van Duyne, R. Nat. Mater. 2008, 7, 442–453. (24) De, M.; Ghosh, P.; Rotello, V. Adv. Mater. 2008, 20, 4225–4241. (25) Jain, P.; Huang, X.; El-Sayed, I.; El-Sayed, M. Acc. Chem. Res. 2008, 41, 1578–1586. (26) Stewart, M.; Anderton, C.; Thompson, L.; Maria, J.; Gray, S.; Rogers, J.; Nuzzo, R. Chem. Rev. 2008, 108, 494–521. (27) Veselago, V. G. Sov. Phys. USPEKHI-USSR 1968, 10, 509–514. (28) Soukoulis, C.; Kafesaki, M.; Economou, E. Adv. Mater. 2006, 18, 1941– 1952. (29) Smith, D.; Pendry, J. B.; Wiltshire, M. C. K. Science 2004, 305, 788–792. (30) Shalaev, V. M. Nat. Photonics 2007, 1, 41–48. (31) Yao, J.; Liu, Z.; Liu, Y.; Wang, Y.; Sun, C.; Bartal, G.; Stacy, A. M.; Zhang, X. Science 2008, 321, 930. (32) Pendry, J. B.; Smith, D. R. Phys. Today 2004, 57, 37–43. (33) Valentine, J.; Zhang, S.; Zentgraf, T.; Ulin-Avila, E.; Genov, D.; Bartal, G.; Zhang, X. Nature 2008, 455, 376–379. (34) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60–103. (35) Matsubara, S.; Hayakawa, T.; Yang, Y.; Nogami, M.; Okamoto, S.; Koshikawa, N. J. Phys. Chem. C 2008, 112, 13917–13921.
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DOI: 10.1021/la901590s
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templated gold nanostructures reported so far deal with thin membranes,36-39 rod-shaped,40-43 and hollow sphere morphologies.37 For these reasons, we decided to develop a new approach toward the preparation of angled Au NPs, which is based on the use of angled Te particles,44 which are more readily obtained, given the crystallization of this material in the trigonal crystal lattice, which has lower symmetry than fcc Au. Chemical replacement reactions45,46 between transition and noble metals can be used for this purpose. Despite many similarities with sulfur and other members of Group VI in the periodic table of elements, elemental tellurium has many properties that are characteristic of metals. It can, for example, be easily replaced by a noble metal, thus providing an opportunity in the synthesis of complex metal structures. The instability of metallic Te in solutions containing gold salts was already observed at the beginning of the 20th century.47 Intensive recent studies on colloidal Te have demonstrated the possibility of achieving galvanic replacement between tellurium and gold in a controllable fashion48-50 at the nanometer scale. In this report we present a synthetic route for the preparation of unusual gold nanostructures with X, Y, or V shapes, which are particularly important for NIM optics and sensing. The resulting gold NPs retain the original shape of Te NPs, while exhibiting a high degree of porosity. While its significance for nonlinear optics, NIMs, and biological properties remains to be established, these particles definitely possess advantages for applications in biosensing, since intrinsic “hot spots” are formed over the gaps in their porous structure, with sizes of a few nanometers, which is largely desirable for detection of small molecules by means of surface enhanced Raman scattering (SERS).51,52
Experimental Section Chemicals. Ethylenediaminetetraacetic acid dipotassium salt dehydrate (EDTA), tellurium, sodium borohydride, cadmium perchlorate hydrate, L-cysteine, sulfuric acid, gold chloride trihydrate, sodium hydroxide, and poly(diallyldimethylammonium chloride) (PDDA; Mw ∼ 400 000-500 000) were purchased from Aldrich and used as received. Milli-Q water with a resistivity higher than 18.2 MΩ cm was used in all of the preparations. Synthesis of CdTe NPs. The synthesis of water-soluble Lcysteine-capped CdTe NPs was carried out by means of the process optimized by Rogach et al.53 In brief, NaHTe gas (generated by the reaction of 15 mL of 0.5 M H2SO4 with an aged mixture of 2 mL water containing 0.175 g Te, and 0.519 g NaBH4 under N2 (36) Erlebacher, J.; Aziz, M. J.; Karma, A.; Dimitrov, N.; Sieradzki, K. Nature 2001, 410, 450–453. (37) Biener, J.; Nyce, G.; Hodge, A.; Biener, M.; Hamza, A.; Maier, S. Adv. Mater. 2008, 20, 1211–1217. (38) Hakamada, M.; Mabuchi, M. Nano Lett. 2006, 6, 882–885. (39) Huang, W.; Wang, M.; Zheng, J.; Li, Z. J. Phys. Chem. C 2009, 113, 1800–1805. (40) Liu, Z.; Searson, P. J. Phys. Chem. B 2006, 110, 4318–4322. (41) Laocharoensuk, R.; Sattayasamitsathit, S.; Burdick, J.; Kanatharana, P.; Thavarungkul, P.; Wang, J. ACS Nano 2007, 1, 403–408. (42) Bok, H. M.; Shuford, K. L.; Kim, S.; Kim, S. K.; Park, S. Nano Lett. 2008, 8, 2265–2670. (43) Ji, C.; Searson, P. J. Phys. Chem. B 2003, 107, 4494–4499. (44) Shanbhag, S.; Tang, Z.; Kotov, N. A. ACS Nano 2007, 1, 126–132. (45) Lee, W.; Kim, M.; Choi, J.; Park, J.; Ko, S.; Oh, S.; Cheon, J. J. Am. Chem. Soc. 2005, 127, 16090–16097. (46) Talapin, D. V.; Yu, H.; Shevchenko, E. V.; Lobo, A.; Murray, C. B. J. Phys. Chem. C 2007, 11, 14049–14054. (47) Hall, R.; Lenher, V. J. Am. Chem. Soc. 1902, 24, 918–927. (48) Lin, Z.; Lin, Y.; Lee, K.; Chang, H. J. Mater. Chem. 2008, 18, 2569–2572. (49) Lin, Z.; Chang, H. Langmuir 2008, 24, 365–367. (50) Wang, Y.; Tang, Z.; Podsiadlo, P.; Elkasabi, Y.; Lahann, J.; Kotov, N. A. Adv. Mater. 2006, 18, 518–522. (51) Camden, J. P.; Dieringer, J. A.; Wang, Y.; Masiello, D. J.; Marks, L. D.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2008, 130, 12616–12617. (52) Kneipp, K.; Kneipp, H.; Kneipp, J. Acc. Chem. Res. 2006, 39, 443–450. (53) Rogach, A.; Franzl, T.; Klar, T.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmuller, A.; Rakovich, Y.; Donegan, J. J. Phys. Chem. C 2007, 111, 14628–14637.
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Figure 1. (a) Evolution of UV-vis-NIR spectra of tellurium NPs, with transition bands indicated by arrows, upon HAuCl4 addition. Each spectrum was recorded after sequential addition of HAuCl4 in 5 min intervals. (b) The increasing absorbance in the NIR region (above the isosbestic point at 1140 nm) corresponds to the longitudinal plasmon resonance mode of Au NPs, with a maximum around 2650 nm, as indicated from the spectrum of the particles dried on a glass slide. atmosphere) was passed through to a nitrogen saturated Cd(ClO4)2 6H2O aqueous solution (0.025 M, 125 mL) at pH 11.2 (adjusted with 1 M NaOH), in the presence of L-cysteine (0.691 g) as a stabilizer. The reaction mixture was refluxed under N2 for 2 h to obtain CdTe nanocrystals with an average diameter of 3.5 nm. Checkmarks Formation. Tellurium nanocheckmarks were synthesized by a slight modification of the method reported by Tang et al.54 Briefly, to 5 mL of L-cysteine-capped CdTe nanocrystals was added 5 mL of 2-propanol. After centrifugation at 5000 rpm for 10 min, a precipitate of CdTe NPs was obtained, which was redispersed in 0.2 mL of clean water. To this solution, 5 mL of 4 mM EDTA (pH 8-9, adjusted with 1 M NaOH) was added dropwise under sonication. The resulting solution was aged inside a desiccator, and covered in aluminum foil to avoid interaction with light and oxidation of the formed Te NPs. Galvanic Replacement. After 24 h of aging, the sample was diluted with water up to 10 mL, followed by sequential addition of 0.01 mL of 0.05 M aqueous HAuCl4 at 5 min intervals, under magnetic stirring. Gold salt addition was continued until the absorbance by Te completely disappeared, giving rise to absorbance by metallic gold (see Figure 1a), which was accompanied by (54) Tang, Z.; Wang, Y.; Shanbhag, S.; Giersig, M.; Kotov, N. A. J. Am. Chem. Soc. 2006, 128, 6730–6736.
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Sanchez-Iglesias et al. a color change from dark-blue to brown. After the absorbance spectrum was stabilized, the sample was centrifuged (4500 rpm, 20 min) and redispersed in water. Film Preparation for Optical Measurements in Air. Microscope glass slides were used as substrates, to facilitate measurements of UV-vis-NIR spectra. The substrates were sonicated for 10 min in water and then in ethanol, subsequently thoroughly cleaned using piranha solution (7:3 H2SO4/H2O2) at 80 °C for 30 min (Warning: piranha solution can react violently with organic substance, operate with extreme caution!), rinsed with deionized water, and dried under an air stream. For the deposition of tellurium and gold NPs, the slides were first immersed in an aqueous solution of positively charged PDDA (1 mg/mL in 0.5 M NaCl aqueous solution) for 20 min. The slides were then positively charged, favoring the electrostatic interaction with negatively charged particles. The pretreated slides were immersed into the corresponding colloid overnight. Finally, after adsorption the substrates were rinsed with water and dried under an air stream. Characterization. Optical characterization was carried out by UV-vis-NIR spectroscopy with a Cary 5000 spectrophotometer, using 1 mm path length quartz cuvettes or after deposition of particles on glass slides. Transmission electron microscopy (TEM) images were obtained with a JEOL JEM 1010 transmission electron microscope operating at an acceleration voltage of 100 kV, while high-resolution TEM (HRTEM) and analytical scanning TEM (STEM) studies were performed with a JEOL JEM 2010 FEG-TEM operating at an acceleration voltage of 200 kV. Energy-dispersive X-ray spectra (EDS) were acquired using an Inca Energy 200 TEM system from Oxford Instruments, and elemental maps were acquired coupling the X-ray spectrometer to an STEM unit, equipped with a high-angle annular darkfield detector (HAADF). Mapping was performed with 0.7 nm probe size and 40 cm camera length. Background subtraction was carried out prior to mapping overlap. Zeta potential was measured from electrophoretic mobility data using a Zetasizer Nano S (Malvern Instruments, Malvern, U.K.). Raman spectra were collected in backscattering geometry with a Renishaw Invia Reflex system equipped with a charge-coupled device (CCD) detector and a confocal Leica microscope. The spectrograph uses a high-resolution grating with additional bandpass filter optics. Excitation of the sample was carried out with a 785 nm laser line, with acquisition times of 50 s and power at the sample of about 1 μW. The laser was focused onto the sample with a 100 objective, providing a spatial resolution of ca. 500 nm2. SERS maps were collected by using the Renishaw StreamLine accessory with a stepsize of 200 nm.
Results and Discussion The preparation of the porous gold colloids with unusual angled shapes starts from an aqueous dispersion of 3.5 nm CdTe NPs, stabilized by L-cysteine, with absorbance and emission maxima located at 575 and 618 nm, respectively, (see Figure S1 in the Supporting Information (SI)) with an estimated particle size of 3.5 nm, according to the method reported in ref 53. These CdTe NPs were subsequently used as building blocks for the construction of Te checkmarks through oriented assembly driven by stabilizer-depletion with EDTA.54,55 The formation of Te NPs can be easily monitored through changes in the color of solution, from orange to dark-blue after 24 h. Subsequently, addition of HAuCl4 leads to a gradual oxidation of Te and reduction of Au, which eventually results in a colloidal dispersion of porous Au NPs. This process can again be monitored using UV-vis-NIR spectroscopy. The initial Te particles display two absorption bands, centered at 270 and 617 nm, (Figure 1), which represent transitions from the p-bonding valence band (VB2) to the (55) Tang, Z.; Wang, Y.; Sun, K.; Kotov, N. A. Adv. Mater. 2005, 17, 358–363.
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Figure 2. TEM images of initial tellurium NPs (wires and checkmarks), before (a) and after (b) galvanic replacement with gold. (c) Higher magnification TEM image of a single gold nanocheckmark manifesting a rough surface and porous structure.
p-antibonding conduction band (CB1) and from p-lone-pair (VB3) to p-antibonding (CB1), respectively.56 During galvanic replacement, these bands gradually damped while the absorbance at higher wavelengths rose as a result of the formation of Au NPs, which was also revealed through the appearance of a surface plasmon resonance band at 524 nm, which is typical for either spherical Au NPs or transverse modes in Au rods or other anisotropic Au NPs.57 Because of the high absorption by water in the NIR, the optical features of the resulting colloid could not be measured above 1400 nm. Therefore, UV-vis-NIR spectra were measured from a glass slide on which the colloid was dried. The resulting spectra revealed a broad and intense band, presumably a surface plasmon resonance band, with a maximum located at ca. 2650 nm, which would correspond to highly anisotropic gold NPs with high aspect ratio (Figure 1b). The structural evolution of the NPs within the colloid during this process was studied by TEM (Figure 2). The TEM images of the Te colloid (Figure 2a) reveal the presence of thin nanowires in a relatively high concentration, together with somewhat bigger V-, X-, and Y-shaped NPs in a global concentration around 35%. The original shape distribution is preserved after galvanic exchange, resulting in gold reduction and the formation of porous gold V-, X-, and Y-shaped nanostructures (Figure 2b). The size of either tellurium or Au V-shaped particles, or checkmarks, remains similar, with arm lengths around 219 ( 60 nm, and diameters of 40 ( 11 nm. All experiments consistently manifested the absence of nucleation of gold NPs outside of the tellurium substrates during the galvanic replacement reaction, even though the process of gold reduction was rather fast. A higher magnification TEM image of a single Au checkmark reveals a rough surface and nonuniform contrast, suggesting the “island-like” reduction of gold on the Te template (Figure 2c). A similar observation has been recently reported, for the reduction of Au by tellurium nanowires in the presence of cetyltrimethylammonium bromide (CTAB) at high pH.48 In order to better understand the crystalline nature, growth mode, surface homogeneity, as well as future synthetic possibilities for the obtained colloidal noble metal nanostructures, we carried out a more detailed characterization by HRTEM and STEM imaging of the particles at the initial, intermediate, and final stages of the process. This study showed that, when an insufficient amount of the Au salt was added to the solution of Te nanoscale checkmarks, small gold islands, with a diameter of ∼9 nm (Figure 3a,b), were formed at the edges of the Te checkmarks. HRTEM observation of these islands clearly revealed the presence of grain boundaries, suggesting a polycrystalline morphology. The different crystalline motifs between Au and Te (56) Isomaki, H. M.; Boehm, J. Phys. Scr. 1982, 25, 801–803. (57) Myroshnychenko, V.; Rodrı´ guez-Fernandez, J.; Pastoriza-Santos, I.; Funston, A. M.; Novo, C.; Mulvaney, P.; Liz-Marzan, L. M.; Garcı´ a, F. J. Chem. Soc. Rev. 2008, 37, 1792–1805.
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Figure 3. Partially (a,b) and fully (c) replaced tellurium by gold. Dark field STEM image (a) of an individual V-shaped NP (checkmark) with bright Au spots distributed on the lateral parts. (b) HRTEM image of a single polycrystalline Au NP (9 nm) on the monocrystalline edge of a Te checkmark. (c) HRTEM image of porous and polycrystalline structure of a Au checkmark. (d) SAED pattern from a single gold checkmark, showing the presence of polycrystalline fcc metallic gold.
in these hybrid nanoscale checkmarks was clearly distinguished by fast Fourier transform (FFT) filtering and inverse reconstruction (Figure S2), undoubtedly showing a single-crystal structure for the Te template, in contrast with the polycrystalline nature of the newly formed Au particle. Similarly, when the galvanic replacement of Te by Au is allowed to complete by simply adding a sufficient amount of Au(III), full transformation into metallic gold was observed (Figure 3c). The final Au NPs were found to be polycrystalline and highly porous, as indicated by a nonuniform contrast. Figure 3c shows a high-resolution image of a fragment from a gold nanoscale checkmark, where holes are visible, as well as the polycrystalline structure of gold within these structures. Selected area electron diffraction (SAED) analysis revealed that the crystalline structure could be indexed to the expected cubic (fcc) crystal lattice of Au, with characteristic (111), (020), (220), (113), (222), and (313) reflections, showing that Te from the starting particle has been totally replaced by Au. Confirmation of the edge-to-center progression of Au deposition on Te was obtained from EDS and STEM-X-ray energy dispersive spectroscopy (STEM-XEDS) elemental mapping (green was assigned to the Te LR emission, while red was assigned to the Au LR emission) on a single checkmark at the various reaction stages, from initial tellurium, Te-Au hybrids, and the final gold particles (Figure 4). Indeed, the images and spectra clearly indicate that the starting Te NPs (Figure 4a and spectrum a) initially undergo replacement by Au mostly at the edges (Figure 4b, spectrum b), followed by complete replacement and dissolution of Te template (Figure 4c, spectrum c). Interestingly, we are still able to observe the rough and highly porous morphology of the resulting Au NPs in the elemental mapping image. Thus, the initial stages of Au reduction are mainly determined by the shape of the Te particle templates, which are flat and have a high curvature, both at the tips and the lateral parts. In particular, 11434 DOI: 10.1021/la901590s
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Figure 4. Colored maps of single particles obtained by STEMXEDS analysis (upper panel) and the corresponding EDS spectra (lower panel) showing the relative elemental distribution (Te = green; Au=red). (a) Initial Te checkmark. (b) Intermediate stage with a small amount of Au, showing initial replacement at the edges (red spots). (c) Fully converted Au checkmarks. Differences in the surface roughness between the initial Te and the final Au NPs are clearly visible. (The energy regions in EDS spectra of Te and Au are highlighted by their corresponding colors to facilitate comparison. Note that the intensities of peaks should be taken as approximate.)
the edges with [100] surfaces54 possess higher surface energy and therefore are readily oxidized when exposed to Au(III) in solution (Figure 4b). Additionally, the crystalline mismatch between trigonal Te and cubic Au impedes the growth of a thin layer of Au covering the Te surface in a uniform fashion.46,48,49 Further addition of Au salt leads to a gradual size increase of Au islands, not only at the edges but also on the flat areas of the particles, and these nuclei grow until Te is totally depleted by oxidation. Because of the preferential nucleation at the edges, these seem to be more uniform in the final nanostructures (see Figure S3), thus providing sufficient stability against fragmentation despite their highly porous nature. The process can take place according to the following redox reaction: 4HAuCl4 þ 3Te þ 9H2 O f 4Au þ 3H2 TeO3 þ 16HCl Te is oxidized by Au(III) forming tellurous acid or TeO32- which is soluble in either acidic or basic environment.50 The standard redox potentials for the TeO32-/Te and AuCl4-/Au pairs are -0.57 V and 1.002 V,58 respectively, which indeed suggests spontaneous gold reduction by Te NPs. In the present work, the initial CdTe NPs were stabilized by L-cysteine, which was partially removed to induce the formation of metallic Te, meaning that gold formation takes place on 54 L-cysteine-stabilized Te. The final angled NPs from Au (after centrifugation) were found to possess a zeta potential of -56 mV, which is likely to arise from cystine groups on the surface, which are negatively charged at pH above 5.02.59 It is well-known that (58) Electrochemical series. In CRC Handbook of Chemistry and Physics, Internet Version 2005, Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2005; http:// www.hbcpnetbase.com. (59) Mandal, S.; Gole, A.; Lala, N.; Gonnade, R.; Ganvir, V.; Sastry, M. Langmuir 2001, 17, 6262–6268.
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Figure 5. (A) Optical and (C) SERS mapping (1072 cm-1, ring breathing, (B)) of a single gold checkmark after exposure to BT in the vapor phase.
the thiol group of cysteine can undergo dimerization to form cystine via the oxidation of the two thiol groups, which then chemically adsorb onto the gold surface.60 Either thiols or disulfide groups from cystine dimers and similar molecular constructs can thus promote the observed initial “island” growth of Au on Te, due to the strong affinity of the thiol functional group toward Au surfaces. Finally, a demonstration of the excellent plasmonic properties of these rough and porous gold nanostructures is provided by their high SERS activity. The enhancement of the Raman signal has been shown to require the presence of metallic surfaces able to sustain localized surface plasmon resonances,61 with the close interaction between two or more particles being an essential condition for the formation of the so-called hot spots, where extremely high electric fields are created.62 The surface dimensions required to create hot spots are on the order of a few nanometers, which basically coincides with the pore size in our particles. Therefore, it can be expected that single checkmarks may be used as efficient SERS enhancers, with “intrinsic” hot spots. A simple experiment was carried out in this direction, comprising the retention of benzenethiol (BT) molecules from the gas phase onto a sample of the obtained gold particles, previously dried onto a glass slide. Because of the presence of the thiol group in BT, retention is efficient, and the use of a gas sample avoids any changes arising from close contact with a solution. Upon BT adsorption, the sample was imaged within a confocal Raman system and high-quality, intense SERS spectra for BT, characterized by the CH bending (1022 cm-1) and the ring breathings (999 and 1072 cm-1), could be readily acquired (Figure 5b). This indicates that the checkmark surfaces are excellent SERS substrates. Additionally, careful scanning of the sample with a step size of 200 nm (resolution is ca. 500 nm) allowed us to obtain a rough reproduction of the checkmark shape for a selected particle (Figure 5a), from the measured SERS signal (Figure 5c). This high-quality mapping of a single particle is a clear piece of evidence that surfaces of checkmarks behave as a dense collection of highly efficient hot spots. This may be attributed not only to the roughness of the surface per se but also to the convenient topology of the surface, the typical size of gaps between the channels in the Au checkmarks being ∼3 nm. As established theoretically,63 this matches very well with the gap size required for efficient (60) Ma, Z.; Han, H. Colloids Surf., A 2008, 317, 229–233. (61) Stiles, P.; Dieringer, J.; Shah, N. C.; Van Duyne, R. P. Annu. Rev. Anal. Chem. 2008, 1, 601–626. (62) Etchegoin, P.; Cohen, L. F.; Hartigan, H.; Brown, R. J. C.; Milton, M. J. T.; Gallop, J. C. J. Chem. Phys. 2003, 119, 5281–5289. (63) Xu, H.; Bjerneld, E. J.; K€all, M.; Borjesson, L. Phys. Rev. Lett. 1999, 83, 4357–4360.
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Article
stimulation of Raman scattering in molecules. For the sake of completion, the SERS activity of these novel nanostructures was compared with conventional substrates,64 such as spherical, citratereduced gold colloids, which were measured under the same conditions (Figure S4). First, we observed that both checkmarks and wires present in the sample display at least 2-fold higher SERS activity than standard colloids when these are extremely aggregated, and around 10-fold higher when they are only slightly aggregated. As a result of these properties, these nanocheckmarks can be used as local nanometer scale reporters of the concentration of multiple target analytes. Being externally interrogated with a laser beam, each checkmark is independent from each other and can resolve local conditions with nanometer scale resolution, which is much needed in codification strategies for tissue analysis and multiplex high-throughput screening with microfluidics.
Conclusions Spontaneous transformation of tellurium checkmarks into gold, preserving their initial shape, can be achieved by gradual galvanic replacement. Interestingly, cysteine-stabilized initial “gold island growth” was observed only at checkmark edges, which gradually engulfed the entire surface. Advantage can be taken of the high porosity of these unusually shaped NPs and we have demonstrated the potential use of these particles as enhancers in SERS spectroscopy. This synthetic procedure can be readily extended to other types of metals, which could lead to formation of angled porous particles with enhanced catalytic properties (Pt, Pd), particles with magnetic anisotropy or with strong gradient of temperature or near electric field between the arms (Ag). Also, an example that is particularly interesting for NPs originating in angled shapes is the use of single particle spectroscopy in biosensing. Note, however, that angled gold particles with low symmetry can be a challenge for optical modeling, which has been successful for simpler shapes.57 Nevertheless, new geometrical models with described shapes is a fundamental factor determining successful development of new optical devices that span both the areas of plasmonics and metamaterials. Acknowledgment. The authors thank Zhiyong Tang and Alexey Shavel for helpful discussions during quantum dots synthesis and checkmarks formation. N.A.K. thanks NSF, AFOSR, NAVY, DARPA, Dow Chemicals, and Nico Technologies Corporation for partial support of this research. L.M.L.-M. acknowledges funding from the Spanish Ministerio de Ciencia e Innovaci on (MAT2007-62696). Supporting Information Available: Absorbance and emission spectra of CdTe NPs stabilized by L-cysteine used as seeds for the growth of checkmarks; FFT analysis from a gold nucleus (intermedium sample) grown at the edge of a tellurium checkmark arm; TEM image showing a fragment of an arm from a gold nanocheckmark; SERS spectra of BT acquired on checkmarks, as compared to extremely and slightly aggregated citrate-reduced gold spherical colloids; SERS maps and optical images for the extremely and slightly aggregated citrate-reduced gold spherical colloids. This material is available free of charge via the Internet at http:// pubs.acs.org. (64) Aroca, R. F.; Alvarez-Puebla, R. A.; Pieczonka, N.; Sanchez-Cortez, S.; Garcia-Ramos, J. V. Adv. Colloid Interface Sci. 2005, 116, 45–61.
DOI: 10.1021/la901590s
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