The rapid reduction of Na(2)PdCl(4) by ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP) has recently been demonstrated as a convenient method of generating Pd cubooctahedra and twinned nanoparticles. Here we describe a new procedure where Pd triangular or hexagonal nanoplates could be selectively synthesized by manipulating the reduction kinetics of the polyol process. More specifically, the reduction rate was substantially reduced through the introduction of Fe(III) species and the O(2)/Cl(-) pair, two wet etchants for Pd(0). The etching power of the O(2)/Cl(-) pair could be further enhanced by adding an acid to lower the pH of the reaction solution. Unlike the previously reported synthesis of Ag and Au nanoplates, light was found to have no indispensable role in the formation of Pd nanoplates. Both triangular and hexagonal nanoplates of Pd exhibited surface plasmon resonance (SPR) peaks in the visible region, and their positions matched with the results of discrete dipole approximation (DDA) calculation. Thanks to their sharp corners and edges, these Pd nanoplates could serve as active substrates for surface-enhanced Raman scattering (SERS).

Silver nanobars with rectangular side facets and an average aspect ratio of 2.7 have been synthesized by modifying the concentration of bromide added to a polyol synthesis. Subsequent rounding of nanobars transformed them into nanorice. Due to their anisotropy, nanobars and nanorice exhibit two plasmon resonance peaks, scattering light both in the visible and in the near-infrared regions. With a combination of discrete-dipole approximation calculations and single-nanoparticle spectroscopy, we explored the effect of nanostructure aspect ratio and corner sharpness on the frequency of plasmon resonance. Near-field calculations and surface-enhanced Raman scattering measurements on single particles were performed to show how local field enhancement changes with both the wavelength and polarization of incident light.

Silver nanocubes dispersed in water were transformed into Pd-Ag or Pt-Ag nanoboxes by adding either Na(2)PdCl(4) or Na(2)PtCl(4). By controlling the amount of noble metal salt added, and therefore the molar ratio of Na(2)PdCl(4) or Na(2)PtCl(4) to Ag, we could tune the surface plasmon resonance peak of the nanostructures across the entire visible spectrum, from 440 to 730 nm. Replacement of Ag with Pd resulted in the formation of a nanobox composed of a Pd-Ag alloy single crystal, but the nanobox formed after replacement of Ag with Pt was instead composed of distinct Pt nanoparticles. DDA calculations suggest that both nanoboxes absorb light strongly, with Q(abs)/Q(sca) approximately 5. After galvanic replacement, Pd-Ag and Pt-Ag nanostructures remain SERS active, suggesting their use as a SERS probe for studying the dependence of interfacial chemistry on composition.

Feature ArticleDewetting of colloidal suspensions across countered surfaces has been combined with capillary forces to assemble spherical colloids into complex and well-controlled structures that include circular rings, polygonal and polyhedral clusters, and linear, zigzag, and spiral chains. It is also possible to generate hetero-aggregates in the configuration of HF and H 2 O molecules that contain spherical colloids of different sizes, compositions, densities, functions, or a combination of these features.

Gold-silver alloy nanocages with controllable pores on the surface have been synthesized via galvanic replacement reaction between truncated Ag nanocubes and aqueous HAuCl 4 . Unlike the previous studies, the initiation of replacement reaction started in a controllable way, simultaneously from eight corners of the truncated Ag nanocubes where {111} facets were exposed. The formation of cubic nanocages with pores at all the corners was determined by the capping agent, poly(vinyl pyrrolidone) (PVP), which preferentially covered the {100} facets of a truncated Ag nanocube.Nanocages 1 --nanostructures with hollow interiors and porous walls --are useful in applications such as encapsulation and site-specific delivery of chemicals. Nanocages made of noble metals are particularly attractive because of their strong scattering and absorption peaks in the near-infrared region known as surface plasmon resonance (SPR). 2 Although metallic nanocages can be easily prepared by depositing thin shells of a metal on silica or polymer beads followed by selective removal of the beads, 3 the polycrystalline walls of such nanostructures often lead to poorly defined pore size/shape and weak mechanical strength. We have recently developed a new method based on galvanic replacement reaction between Ag nanostructures and Au, Pt, or Pd compounds to fabricate nanocages characterized by singlecrystal walls. 4 The same strategy has also been extended by a number of research groups to several other systems. 5 While small pores often appear on the walls of final product via a dealloying process, they tended to be polydispersed in size/shape and randomly scattered across the surface. Here we demonstrated, for the first time, that the pores can be made uniform in size and shape, and confined to the corners when Ag nanocubes with truncated corners were used as the sacrificial template. Figure 1 summarizes all major steps involved in the formation of Au-Ag nanocages with wellcontrolled pores at the corners. A key feature of this synthetic procedure is the use of Ag nanocubes whose sharp corners have been truncated in advance via a thermal annealing process. 6 The mechanism of truncation is in agreement with our previous observation that poly (vinyl pyrrolidone) (PVP, a capping polymer commonly used in the polyol synthesis of Ag nanostructures) tends to preferentially cover the {100} rather than {111} facets. 7 If Ag nanocubes with sharp corners are thermally aged in ethylene glycol in the presence of a small amount of PVP, {111} facets poorly capped by PVP will develop at the corners, leading to truncation of all corners. When added into an aqueous solution of HAuCl 4 , the corner regions xia@chem.washington.edu. unprotected by PVP will serve as primary sites for the dissolution of silver and eventually become well-defined pores at all corners of a nanobox. NIH Public AccessAs a control experiment, we started with Ag nanocubes bearing sharp corners. Figure 2A shows both SEM and TEM (the inset) images of such nanocubes that were synthesized...

This paper describes a two-step procedure for generating cubic nanocages and nanoframes. In the first step, Au/Ag alloy nanoboxes were synthesized through the galvanic replacement reaction between Ag nanocubes and an aqueous HAuCl 4 solution. The second step involved the selective removal (or dealloying) of Ag from the alloy nanoboxes with an aqueous etchant based on Fe (NO 3 ) 3 or NH 4 OH. The use of a wet etchant other than HAuCl 4 for the dealloying process allows one to better control the wall thickness and porosity of resultant nanocages because there is no concurrent deposition of Au. By increasing the amount of Fe(NO 3 ) 3 or NH 4 OH added to the dealloying process, nanoboxes derived from 50-nm Ag nanocubes could be converted into nanocages and then cubic nanoframes with surface plasmon resonance (SPR) peaks continuously shifted from the visible region to 1200 nm. It is also possible to obtain nanocages with relatively narrow SPR peaks (with an FWHM as small as 180 nm) by controlling the amount of HAuCl 4 used for the galvanic replacement reaction and thus optimization of the percentage of Au in the alloy nanoboxes.Hollow nanoparticles of noble metals have unique physical and chemical characteristics that differentiate them from many other types of nanostructured materials. 1 They have also been recognized for a range of applications, including catalysis, 2 optical sensing, 3 drug delivery, 4 biomedical imaging, 5 as well as photothermal cancer treatment. 6 Among various synthetic routes, the one based on galvanic replacement reaction has proven to be the most effective in generating hollow nanostructures from a number of noble metals. 1,7 It has been demonstrated that bimetallic alloy hollow nanostructures composed of Ag and Au, Pd, or Pt could be conveniently prepared by reacting Ag nanostructures with a compound containing a less reactive noble metal such as Au, Pd, or Pt. In particular, Au/Ag nanoboxes (nanostructures with hollow interiors) and nanocages (nanostructures with hollow interiors and porous walls) with tunable surface plasmon resonance (SPR) peaks can be routinely synthesized by reacting Ag nanocubes with HAuCl 4 in an aqueous solution. By increasing the amount of HAuCl 4 added to a suspension of Ag nanocubes, the nanocubes can be controlled to evolve from solid objects into cubic nanoboxes and then nanocages. As a well-established example, the SPR peaks of *Corresponding author. E-mail: xia@chem.washington.edu. gold hollow nanoboxes and nanocages can be readily tuned from the visible to the near-infrared (NIR) region by varying the wall thickness relative to the overall dimension. 8 NIH Public AccessAlthough the protocol based on the galvanic replacement reaction with HAuCl 4 works well for Ag nanostructures of a variety of shapes, it has a drawback that limits our ability to achieve a tight control over the wall thickness and porosity for the resultant nanocages. In the early stage of the reaction (or when a relatively small amount of HAuCl 4 is added), Ag atoms are dissolved fr...

Silver nanocubes with sharp or truncated corners were synthesized, deposited on silicon substrates, and functionalized with Raman-active thiols for surface-enhanced Raman scattering (SERS) studies. The use of substrates with registration marks allowed us to correlate the SERS spectra from individual nanocubes to their physical parameters revealed by high-resolution SEM imaging. We observed dramatic variations in SERS intensity when the nanocubes were oriented at different angles relative to the polarization of excitation laser. This angular dependence was less significant when the nanocubes were truncated and became nearly spherical in profile. Numerical calculations were employed to confirm our observations, and to attribute the source of variation to the difference in near-field distribution between different laser polarizations.

This paper describes the fastest route to monodispersed silver nanocubes. By adding a trace amount of sodium sulfide (Na(2)S) or sodium hydrosulfide (NaHS) to the conventional polyol synthesis, the reaction time was significantly shortened from 16-26 hours to 3-8 minutes. By merely adjusting the reaction time, monodispersed silver nanocubes of 25-45 nm in edge length were rapidly and routinely produced on relatively large scales. These small nanocubes are of great interest for biomedical applications by way of generating gold nanocages with plasmon resonance peaks tunable to the near-infrared region through a galvanic replacement reaction.