HIGH SURFACE PLASMON TUNABILITY OF TRIANGULAR BIMETALLIC AU-

AG NANOPRISMS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Application No. 61/088,855, filed August 14, 2008, the disclosure of which is incorporated by reference in its entirety herein.

STATEMENT OF U.S. GOVERNMENT INTEREST

[0002] This invention was made with U.S. government support under Office of Naval Research Grant No. N00014-06-1-0079 and National Science Foundation Grant No. EEC- 0647560. The U.S. government has certain rights in this invention.

BACKGROUND

[0003] Nanomaterials composed of both gold (Au) and silver (Ag) have complex optical features spanning the visible and near infrared spectra, are generally considered to be low in toxicity, and exhibit facile surface functionalization chemistry (1). These properties make the nanomaterials excellent testbeds for both fundamental studies and technological advances in fields ranging from nano-biodiagnostics (2) to catalysis (3). A composite nanomaterial containing these two elements is attractive because it could provide significant plasmon tunability, hybrid chemical and surface modification properties, and insight into the growth of anisotropic nanostructures (4-6). Additionally, it is well known that multimetallic alloy and core-shell nanostructures exhibit unusual catalytic, electronic, and magnetic properties (7). However, while significant advances have been made in developing synthetic methods to prepare bimetallic pseudo-spherical particles (4, 7a) and anisotropic nanorods (5), only a few examples of crystalline triangular Au/ Ag bimetallic nanostructures currently exist (6), in spite of the fact that these structures are ideal candidates for understanding both the evolution and consequences of nanoparticle anisotropy in core-shell systems. For example, a recent report of the photo-induced preparation of Au _core -Ag _she ii nanoprisms has provided significant insight into the mechanism underlying the photochemical growth of silver nanoprisms from spherical metal seeds (6a).

SUMMARY

[0004] Disclosed herein are bimetallic nanoprisms and methods of preparing bimetallic nanoprisms. The disclosed bimetallic nanoprisms can have a bifrustum shape.

[0005] Thus, one aspect provides methods of preparing bimetallic nanoprisms comprising admixing a gold nanoprism, a silver ion source, a reducing agent, a base, and a surfactant to form the bimetallic nanoprism having a silver shell on the gold nanoprism, wherein when the

thickness of the silver shell is less than 4 nm, the bimetallic nanoprism has a plasmon resonance of 1000 nm or greater, and when the thickness of the silver shell is 4 nm or greater, the bimetallic nanoprism has a plasmon resonance of less than 1000 nm. In some specific embodiments, the admixing comprises a serial addition of the reagents, where the reducing agent is added to the gold nanoprism then the silver source then the base.

[0006] In some embodiments, the silver shell has a thickness of 1 to 3 nm. In specific embodiments, the bimetallic nanoprism has a plasmon resonance of 1100 to 1400 nm. In various embodiments, the silver shell has a thickness of 4 to 20 nm, or 4 to 10 nm. In specific embodiments, the bimetallic nanoprism has a plasmon resonance of 800 to 950 nm.

[0007] In various embodiments, the silver ion source comprises silver nitrate. In some cases, the reducing agent is ascorbic acid, sodium borohydride, 2-mercaptoethanol, dithiothreitol (DTT), hydrazine, lithium aluminum hydride, diisobutylaluminum hydride, oxalic acid, Lindlar catalyst, a sulfite compound, a stannous compound, a ferrous compound, sodium amalgam, or a mixture thereof. In various cases, the surfactant is an alkyl ammonium salt or an alkyl pyridium salt. In some embodiments, the surfactant is tetrabutylammonium bromide, dodecyldimethylamonium bromide, cetyltrimethylammonium bromide, or mixtures thereof.

[0008] Another aspect provides bifrustum nanoprisms. In some embodiments, the bifrustum nanoprisms comprise a gold triangular nanoprism and a silver layer on one or both { 111 } facets of the gold triangular nanoprism. In some embodiments, the bifrustum nanoprisms are prepared by the methods disclosed herein.

BRIEF DESCRIPTION OF THE FIGURES

[0009] Figure 1. (A) Image of Au _CO re-Ag _s heii triangular bifrustum nanocrystals; Inset: Electron diffraction pattern of the top of a nanocrystal; (B), (C) Images of the nanocrystals. The bright bands in the center of each nanocrystal indicate the core Au nanoprism; (D) X-ray energy dispersive spectroscopy (EDS) line profile showing Au and Ag content along the nanocrystal edge. Arrows indicate truncation of Ag shell at the edges.

[0010] Figure 2. Normalized UV-vis-NIR spectra of Au _core -Ag _she ii triangular bifrustum nanocrystal colloids after adding (a) 0 μL, (b) 1 μL, (c) 2 μL, (d) 3 μL, (e) 5 μL, (f) 8 μL, (g) 10 μL of 10 mM silver nitrate (AgNO ₃ ) solution, 50 μL of 100 mM L-ascorbic acid, and 75 μL of 100 mM sodium hydroxide (NaOH).

[0011] Figure 3. Experimental (dotted line) and theoretical (DDA calculation - solid line) extinction spectra of Au _core -Ag _s h _e ii nanoprisms; Inset: structural model of Au _core -Ag _s h _e ii nanocrystal for the DDA calculation.

[0012] Figure 4. Elemental analysis of Au _CO re-Ag _s heii nanoprisms probed by X-ray energy dispersive spectroscopy (EDS) at two different positions (A) and (B) on a single prism.

[0013] Figure 5. (a) Image of of Au _core -Ag _she ii nanoprisms after addition of 10 μL of 10 mM AgNO ₃ solution, 50 μL of 100 mM L-ascorbic acid, and 75 μL of 100 mM NaOH to the triangular Au seeds. (Scale bar in (A) represents 100 nm). The Au _cor e-Ag _s heii nanoprisms have an average edge length of 150 + 20 nm; (b) Thickness distributions of nanoprisms measured in AFM experiments upon the addition of Ag (5 μL or 10 μL each of 10 mM AgNO ₃ solution). Error bars represent 10 separate experiments.

[0014] Figure 6. X-ray photoelectron spectroscopy (XPS) analysis of an Au _cor e-Ag _s heii nanoprism. (a) Survey scan of the surface of an Au _core -Ag _she ii nanoprism before argon (Ar ⁺ ) sputtering. The atomic ratio of Ag/ Au is about 2.5, indicating that the surface is dominated by deposited Ag. The higher binding energy shoulder of the Ag peak in the inset is due to the oxidation of Ag. (b) Atomic ratio of Ag/ Au on the nanoprism surface decreases with sequential Ar ⁺ sputtering, confirming the TEM observations that the Ag exists only on the outmost layers of the nanoprism.

[0015] Figure 7. UV-vis-NIR spectra of Au _cor e-Ag _s heii nanoprisms formed after adding (a) 0 μL; (b) lμL; (c) 2μL; (d) 3μL; (e) 5μL; (f) 8μL; (g) 10 μL of 10 mM AgNO ₃ solution, 50 μL of 100 mM ascorbic acid, and 75 μL of 100 mM NaOH.

[0016] Figure 8. Images of Au _core -Ag _she ii nanoprisms after (a) 5 and (b) 10 repeated additions of silver growth solution (each aliquot of silver growth solution includes 50 μL of 100 mM ascorbic acid, 10 μL of 10 mM AgNO ₃ , and 75 μL of 100 mM NaOH). Scale bars in (a) and (b) represent 200 and 100 nm, respectively. Bimetallic pseudo-spherical and hexagonal nanoprisms are due to the reduction of Ag ⁺ on Au pseudo-spherical nanostructures and Au hexagonal nanoplates (as opposed to triangular nanoplates) both of which are known to form as byproducts during Au nanoprism synthesis, (c) Normalized UV-vis-NIR spectra of Au _core -Ag _she ii nanoprisms (dipole surface plasmon resonances) after 5 repeated additions (solid line) and 1 addition (dotted line) of silver growth solution.

[0017] Figure 9. (a) Theoretical extinction spectra (DDA calculation) of Au _CO re-Ag _s heii nanoprisms (A), Alloy structure (with mixed dielectric constant of Au and Ag) (B), pure Au

nanoprisms (C), and pure Ag nanoprisms (D); (b) Structural model of each nanoprism used in the DDA calculations.

[0018] Figure 10. (a) Structural model of the truncated Au _CO re-Ag _s heii nanoprisms (A), and other hypothetical structural models that were used for DDA calculations including (B): Au _core -Ag _she ii nanoprisms with Ag layer omitted from the Au { 112} facet; (C) prism structure with 4 nm layer of Ag around entire Au prism; (D) pure Au nanoprism with the same total thickness and edge length as (C); (b) Wavelengths associated with extinction maximum for the dipole and quadrupole surface plasmon resonances (SPR) from theoretical (A-D) and experimental (E) extinction spectra.

DETAILED DESCRIPTION

[0019] Disclosed herein is the synthesis of a new class of two component triangular nanoprisms (8), which comprise a triangular gold (Au) core with its broad { 111 } facets covered with a silver (Ag) layer of controllable thickness, and optionally its { 112} facets covered with a Ag layer, termed a silver "shell" herein. If present, the silver layer on the { 112} facets of the gold core can be as thick as, but are typically thinner (i.e., less thick) than, the silver layer on the { 111 } facets of the gold core. More importantly, plasmon tunability of these nanoprisms is disclosed. The plasmon resonances are tunable as the amount of Ag deposited and the thickness of the bimetallic nanoprism control SPRs of the resulting Au _core - Agsheii nanoprisms.

[0020] A method for preparation of nanoprisms as disclosed herein comprises admixing Au nanoprisms, with a silver ion source, reducing agent, base, and a surfactant to form Au _core - Ag _she ii nanoprisms, where the silver can be on both the { 111 } and the { 112} facets of the Au nanoprism, and the thickness of the silver on the { 112} facet, if present, is less than the thickness of the silver layer on the { 111 } facet.

[0021] The term "nanoprism" as used herein refers to a metal composition that exhibits prismatic properties. The nanoprisms disclosed herein are bimetallic, with a gold core and a silver shell, where the silver covers the broad faces of the gold core (e.g., the { 111 } facets). Prismatic properties can be detected using known techniques. Prismatic properties include, but are not limited to, characteristic resonances, such as surface plasmon dipole and quadrupole resonances. In cases where the nanoprism comprises two metals, such as a core metal and a shell metal, the surface plasmon resonances can be related to the thickness of the shell metal of the nanoprisms. Thus, some nanoprisms disclosed herein have plasmon

resonances that have been tailored to specific wavelengths by controlling thickness of the silver shell.

[0022] The nanoprism can have an edge length of about 25 nm to about 500 nm, about 50 nm to about 400 nm, and/or about 100 nm to about 300 nm. The nanoprism can have an edge length, for example, of 25, 30, 35, 40, 45, 50, 55, 60, 65, 60, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, and 300 nm. The nanoprisms can have a total thickness of about 10 nm to about 100 nm, such as about 12 nm to about 75 nm, or about 15 nm to about 50 nm.

[0023] The growth of Ag on a Au nanoprism surfaces displays a unique growth pattern, resulting in the first preparation of Au _CO re-Ag _s heii nanoprisms. The dipole and quadrupole surface plasmon resonances (SPRs) in these structures are highly tunable based upon the thickness of the outer Ag shell. The theoretical calculations suggest that it is the geometry and not the second chemical component that is primarily responsible for the large observed shifts. Many of these plasmon wavelengths are not accessible with pure Au triangular nanoprisms. Moreover, the Au _CO re-Ag _s heii nanoprisms, which can be in a triangular bifrustum shape (see shape (A) in Figure 9(b)), show stronger surface plasmon resonances (higher extinction) than pure Au nanoprisms, making this material useful for surface enhanced Raman scattering (16). The use of Ag to modulate the geometry of Au seeds is a facile way to extend the utility of these structures in many areas where plasmonic wavelengths are important, including diagnostic labels (2), energy harvesting (17), optical transport (18), and therapeutics (19).

[0024] In a typical preparation, Au _cor e-Ag _s heii triangular nanoprisms were synthesized by reduction of Ag ⁺ ions onto Au triangular nanoprism seeds (prepared according to literature procedures). ⁹ The Au nanoprism solution can purified, for example by centrifugation, and resuspended in a cationic surfactant solution (cetyltrimethylammonium bromide, CTAB). The bimetallic nanoprism then can be prepared by addition of a reducing agent (e.g., L- ascorbic acid), a silver ion source (e.g., AgNO ₃ solution), and a base (e.g., NaOH) into the existing Au nanoprism solution. These components can be added all together or individually, in a serial manner.

[0025] Cationic surfactants that can be used in the disclosed methods include, but are not limited to, ammonium salts having a least one and up to four alkyl substituents and alkyl pyridinium salts. Specific surfactant examples contemplated include tetrabutylammonium

bromide (TBAB), dodecyldimethylammonium bromide (DDAB), cetyltrimethylammonium bromide (CTAB). The counterion of the ammonium or alkyl pyridinium salt can be acetate, halide, pivolate, glycolate, lactate, and the like.

[0026] Bases for use in the disclosed method include, but are not limited to, sodium hydroxide, potassium hydroxide, ammonium hydroxide, magnesium hydroxide, sodium carbonate, sodium bicarbonate, and the like. A preferred base is sodium hydroxide.

[0027] Reducing agents for use in the disclosed methods include, but are not limited to, ascorbic acid, sodium borohydride, 2-mercaptoethanol, dithiothreitol (DTT), hydrazine, lithium aluminum hydride, diisobutylaluminum hydride, oxalic acid, Lindlar catalyst, sulfite compounds, stannous compounds, ferrous compounds, sodium amalgam, and the like.

[0028] Silver ion sources can be any silver salt, such as, for example, silver nitrate, silver chloride, and silver phosphate. A preferred silver ion source is silver nitrate. The concentration of silver ion in the reaction controls the thickness of the silver shell over the gold core. Au nanoprism concentration is measured by the optical density of the nanoprisms in solution at a particular wavelength. For the examples presented herein, the Au nanoprisms had a concentration of 0.8 O.D. in 1.5 mL volume. The silver ion concentrations of less than 0.05 mmol/1.5 mL of reaction volume (i.e., a silver ion concentration of less than 0.033 M) results in bimetallic nanoprisms having silver shells of less than 4 nm, and surface plasmon resonances of 1000 nm or greater (see Figure 7 a-d). Silver ion concentrations of 0.05 mmol/1.5 mL of reaction volume or greater (i.e., a silver ion concentration of 0.033 M or greater) results in bimetallic nanoprisms having silver shells of 4 nm or greater and surface plasmon resonances of less than 1000 nm (see Figure 7 e-g). Thus, the thicker the silver shell, the lower the surface plasmon resonance. Stated another way, silver ion (mol/L) to gold nanoprism (OD at 1230 nm in 1.5 mL) ratios of less than 0.033 : 0.8, or 0.4 : 1 provide silver shells having a thickness of less than 4 nm and surface plasmon resonances of 1000 nm or greater. Ratios of 0.4:1 or greater provide silver shells having a thickness of 4 nm or greater and surface plasmon resonances of less than 1000 nm.

[0029] The silver shell about the { 111 } facet of the gold core can have a thickness of about 1 nm to about 20 nm, such as about 1 to about 15nm, about 2 to about 10 nm, and about 4 to about 8 nm. Specific silver shell thicknesses contemplated include about 1, about 2, about 3, about 4, about 5, about 6, about 7, about 8, about 9, about 10, about 11, about 12, about 13, about 14, about 15, about 16, about 17, about 18, about 19, and about 20 nm. The silver shell thickness refers to the thickness of each shell on each facet of the gold core. Thus, a silver

shell thickness of 3 nm and a gold core thickness of 8 nm results in a nanoprism having a total thickness of 14 nm.

[0030] The silver shell about the { 112} facet of the gold core, if present, can have a thickness of about 1 nm to about 20 nm, such as about 1 to about 10 nm, about 1 to about 5 nm, about 1 to about 4 nm, about 1 to about 3 nm, and about 1 to about 2 nm.

[0031] The surface plasmon resonance is related to the thickness of the silver shell and can be about 800 to about 1400 nm, with thicker silver shells (e.g., about 4 nm or greater) having surface plasmon resonances less than 1000 nm, and thinner silver shells (e.g., less than 4 nm) having surface plasmon resonances of 1000 nm or greater.

[0032] Both TEM (Transmission Electron Microscopy) and STEM (Scanning Transmission Electron Microscopy) images of the Au _cor e-Ag _s heii nanoprism structures obtained after addition of 10 μL of a 10 mM AgNO ₃ solution to the triangular Au seeds show the formation of the triangular bifrustum nanoprisms (Figure 1). The Au _CO re-Ag _s heii triangular bifrustum nanoprisms have an average edge length of 150 ± 20 nm (60 nanoprisms evaluated, Figure 5A). Individual particles exhibit a hexagonal electron diffraction pattern, indicating that the particle is a single crystal with broad faces consisting of { 111 } lattice planes (Figure IA inset). The diffraction pattern was indexed as the { 111 } zone axis of an fee structure. The lattice parameters of Ag (a = 0.4058 nm) and Au (a = 0.4079 nm) are close, and therefore the diffraction spots are likely overlapped). The plate-like triangular nanoprism seeds, as well as other plate-like structures, share this common structural motif (10-13). Although the chemical identity of the Ag coating is not immediately apparent from the contrast in the image of the { 111 } lattice planes, elemental analysis of these faces by X-ray energy dispersive spectroscopy (EDS) indicates the presence of both Ag and Au (Figure 6).

[0033] Subsequent microscopy analysis of the edges of these particles shows that they are better described as core- shell rather than alloy structures. Figure IB and 1C show STEM edge-on images of Au _core -Ag _she ii triangular bifrustum nanoprisms (after adding 10 μL of 10 mM AgNO ₃ solution). The STEM images show Z-contrast (the heavier the element, the brighter the contrast), and were collected using a high-angle annular dark field detector. Thus, in these images, the core Au nanoprism was easily observed as a bright band because the Au scatters electrons more effectively than Ag. An EDS line profile of the Ag and Au composition traversing the edge of an Au _core -Ag _she ii nanoprism suggests that the Ag covers the entire surface of the Au nanoprism. Gold is also observed on the edge, with a clear

interface between the Au and Ag regions (14). The average thickness of each nanoprism, as determined by STEM at the center of the { 111 } faces, was about 17 nm.

[0034] The thickness of the triangular bifrustum nanoprisms can be controlled by the amount of Ag deposited on the Au triangular seed (Figure 5B). For example, the average thickness of the triangular bifrustum nanoprisms changed from about 4 to 8 nm, respectively, depending upon whether the Au seeds were treated with 5 or 10 μL of 10 mM AgNO ₃ solution in a 1.5 mL reaction volume. For any of these core-shell triangular bifrustum nanoprisms, as they are sputtered with Ar ⁺ ions, the ratio of Ag/ Au decreases, which reinforces the conclusion that the Ag is in the outermost layer of the nanoprism (Figure 7).

[0035] UV-vis-NIR spectroscopy allows one to easily track the growth of the core-shell triangular bifrustum nanoprisms (Figure 2). The dipole plasmon resonance is sensitive to the amount of Ag deposited on the triangular Au seeds. Note that in the spectrum of the triangular Au seeds, the bands at 1230 and 790 nm are assigned to the dipole and quadrupole surface plasmon resonances (SPRs), respectively (Figure 2 and Figure 8, spectrum a) (9a). Significantly, a gradual blue- shift and strong enhancement in the both of these resonances is observed as a function of Ag deposition and increased triangular bifrustum nanoprism thickness (Figure 2 and Figure 8, spectra b-g,).

[0036] With monometallic triangular nanoprism systems, plasmon tunability is possible via size control, by either changing nanoprism thickness or edge length (10, 1 Ia). However, the class of nanoprisms disclosed herein exhibits a much greater range of plasmon tunability. In addition, one can realize structures with SPRs as low as 800 nm, which have not been observed with well-formed Au triangular nanoprisms (9, 11).

[0037] It has been shown that the reduction of Au ions on seed Au nanoprisms predominately occurs on the Au{ 112} facet, and leads to Au nanoprisms with larger edge lengths (lla). As disclosed herein, Ag ⁺ ion is reduced and deposited on both the Au{ 111 } and Au{ 112} facets, and forms Ag shells with notable changes in nanoprism thickness and geometry. Without being bound by theory, it is believed that the interaction of Ag ⁺ and Br ^" ion (from CTAB) on the metallic surface may affect the crystal growth process. Groups have studied the mechanism of Ag nanoplate formation by varying the concentration of components including CTAB, and demonstrated a key interaction between a CTAB molecular layer adsorbed on the metallic surface and Ag ⁺ ions (12).

[0038] Interestingly, the Ag shells created in the disclosed methods show truncation on the edges (Figure 1), which leads to the observed triangular bifrustum structure. While mixed

samples of monometallic Ag triangular bifrustums have been prepared using a different approach, ^12a the Au/ Ag bimetallic triangular bifrustum structure has not previously been synthesized. The truncation is retained after subsequent additions of growth solutions to give bimetallic nanoprisms which can be described as thicker triangular bifrustums or complete bipyramidal structures. Control over morphology of bimetallic nanoprisms can be affected by changing the amount of Ag ⁺ added (Figure 8).

[0039] Discrete dipole approximation (DDA) calculations based on the AFM and STEM experiments were carried out to simulate the optical features of the Au _cor e-Ag _s heii nanoprisms (Figure 3) (15). For the simulations, an average of the layered structure with truncations was used. Because the Ag layer on the Au{ 112} facet is not clear from the TEM images (Figure 1), the average edge length of Au seed nanoprisms (144 nm) has been used without modification (9a). The locations of the calculated dipole and quadrupole plasmon peaks do not exactly match the experimentally measured spectrum. However, the resonance wavelengths are extremely sensitive to fine details of the structures (particularly to the degree of rounding of the tips, and thickness of the Ag and Au layers) so small deviations are expected. The experimental spectrum also shows broader SPR peaks than those in the DDA calculation due to polydispersity of bimetallic nanoprisms in solution. In addition to the model in Figure 3, the alloy and single component models (Au and Ag each) as well as other hypothetical models have been simulated for comparison purposes (Figure 9, 10). Significantly, the results show that the component change (i.e. Ag versus Au) is not crucial to obtain the observed blue shifts in SPRs because the wavelengths of maximum extinction in each model are very close to each other. (Figure 9). Therefore, it is the geometry of the structure, which is tuned by the amount of Ag added and the corresponding increase in silver shell thickness, that leads to high SPR tunability in this class of nanoprism structures.

[0040] The resulting bimetallic nanoprisms can be further purified to separate smaller nanoparticles and unreacted materials from the nanoprisms having the desired dimensions. In one example, this purification is performed by filtration. Typically, the filtration is accomplished using an aluminum oxide filter having 100 nm pores, for example (Whatman, Florham Park, NJ USA). Other means of purification or removal of small nanoparticles from the reaction solution can be employed. A nonlimiting example includes centrifugation.

[0041] Additional aspects and details of the invention will be apparent from the following examples, which are intended to be illustrative rather than limiting.

EXAMPLES

[0042] Cetyltrimethylammonium bromide (CTAB, 95%), hydrogen tetrachloroaurate trihydrate (HAuCl ₄ *3H ₂ O, 99.9%), silver nitrate (AgNO ₃ , 99%), sodium borohydride (NaBH ₄ , 99.995%), sodium hydroxide (NaOH, 99.998%), L-ascorbic acid (99%), trisodium citrate (99%), sodium iodide (NaI, 99%) were obtained from Sigma-Aldrich and used as received. All stock solutions were freshly prepared before each reaction. Prior to use, all glassware was washed with Aqua Regia (3:1 ratio by volume of HCl and HNO ₃ ) and rinsed thoroughly with Nanopure™ water (Barnstead, 18.2 Mω). The original Au nanoprism solution was prepared via literature methods (see Millstone, et al. /. Am. Chem. Soc. 127:5312-5313 (2005); and Millstone, et al. Nano Lett. 8:2526-2529 (2008)). The Au nanoprism solution was purified by centrifugation (3 min at 8,000 rpm) and resuspended in CTAB solution (50 mM).

[0043] The Au _core -Ag _she ii triangular bifrustum nanoprisms were grown by serial addition of Ag ⁺ ions into the existing nanoprism solution in the presence of excess L-ascorbic acid and NaOH. The growth step included adding L-ascorbic acid (50 μL, 100 mM), AgNO ₃ (1-10 μL, 10 mM), and NaOH (75 μL, 100 mM) to a 1.5 mL aliquot of Au nanoprism solution (concentration of Au nanoprism seed solution was determined by optical density; 0.8 O. D. at 1230 nm). The addition order (L-ascorbic acid(aq) -> AgNO ₃ (aq) -> NaOH(aq)) was important to reduce independent Ag particle nucleation. The reaction mixture was agitated for 30 seconds after every addition of each solution by shaking. The resulting Au _core -Ag _she ii nanoprisms were allowed to fully react for one additional hour after the last growth solution addition, after which point no further growth was observed (as determined by UV-vis-NIR spectra). By introducing subsequent growth additions, the Au _cor e-Ag _s heii nanoprisms were grown to thicker triangular bifrustums or complete bipyramidal structures (e. g., Figure 8). In all cases, growth solutions were added step-by-step in order to mitigate the occurrence of separate Ag nucleation. Each aliquot of the growth solution includes 50 μL of 100 mM L- ascorbic acid(aq), 10 μL of 10 mM AgNO ₃ (aq), and 75 μL of 100 mM NaOH(aq). For a given number of additions, if equivalent amounts of Ag ⁺ and other reagents were added as a single addition, an increase in the population of the pseudo spherical Ag nanoparticles was observed.

[0044] The resulting bimetallic nanoprisms were characterized by UV-vis-NIR spectroscopy using a Cary 5000 spectrophotometer and imaged using a Hitachi-8100 transmission electron microscope at 200 kV. TEM samples were prepared by concentrating the nanoprism mixture using centrifugation (centrifuged twice for 3 min at 8000 rpm and

resuspending pellets in 100 μL of NanopureTM water) and immobilizing 10 μL of the solution on a Formvar-coated Cu grid. High-resolution TEM, STEM, and EDS images were taken using a JEOL 2100 transmission electron microscope at 200 kV. The STEM images were collected using high-angle annular dark field detector, which shows Z-contrast. Scanning electron microscopy (SEM) images were taken on a Leo 1525. Tapping mode AFM (TM-AFM) images were taken with a NanoMan AFM system (Veeco Instruments). For XPS, samples were transferred to an analysis chamber equipped with an X-ray photoelectron spectrometer (Omicron). An Al Ka (1486.5 eV) anode with a power of 200 W (20 kV) was used. XPS spectra were gathered using a hemispherical energy analyzer operated at pass energy of 70.0 eV for survey scan and 20.0 eV for elemental analysis. Binding energies were referenced to the Au(4f) peak at 84.0 eV for pure Au. All DDA calculations were performed using DDSCAT7.0 (see Draine, et al., "User Guide for the Discrete Dipole Approximation Code DDSCAT 7.0" to be found under http://arxiv.org/abs/0809.0337v4 (2008)). The media effect was included where the refractive index of water is 1.331. The gold dielectric constants were from Johnson, et al., Phys. Rev. B, 6:4370-4379 (1972), and the silver dielectric constants were from Lynch, et al., in Handbook of Optical Constants of Solids. (Eds: E. D. Palik), Academic press, New York, 1985, p 350. The alloy dielectric constants are a volume average of their components. The grid spacing is 1 nm in all calculations. All calculated extinction spectra were averaged by different orientations of particles relative to incident lights.

[0045] The foregoing describes and exemplifies the invention but is not intended to limit the invention defined by the claims which follow. All of the methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the materials and methods of this invention have been described in terms of specific embodiments, it will be apparent to those of skill in the art that variations may be applied to the materials and/or methods and in the steps or in the sequence of steps of the methods described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents which are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved.

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