Architecture of Metallic Nanostructures: Synthesis Strategy and

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Architecture of Metallic Nanostructures: Synthesis Strategy and Specific Applications Hao Ming Chen and Ru-Shi Liu* Department of Chemistry, National Taiwan University, Taipei 106, Taiwan ABSTRACT: Metallic nanoparticles are emerging as key materials use in catalysis, plasmonics, sensing, and spectroscopy. To approach these applications, the control of nanostructures provides increasing selectivity and functionality. This feature article highlights our recent research progress in this emerging field. This article discusses control of the shape of metallic nanostructures and their physical/chemical nature. The numerous approaches for synthesizing multishaped nanostructures include (i) the use of hard templates such as anodic aluminum oxide, (ii) the use of soft templates such as cetyltrimethylammonium bromide (CTAB), and (iii) the use of sacrificial templates. Finally, the use of such nanostructures in catalysis, sensing, and photothermal therapy are demonstrated. General strategies for these methods are discussed and related recent research directions will also be addressed in this feature article.

1. INTRODUCTION Over the past decade, nanomaterials have been the subject of extensive interest because of their extremely small feature size, and their potential usefulness in a wide range of industrial,1,2 catalysis,3-9 information storage,10,11 biomedical,12-17 and electronic applications.18,19 Nanomaterials are designed at the molecular (nanometer) level to take advantage of their small size and have novel properties that are typically not observed in their conventional, bulk counterparts. Nanomaterials have a much larger surface area to volume ratio than their bulk counterparts, which is the basis of their novel physical-chemical properties exhibited by these nanomaterials.20 The emergence of novel properties on the nanoscale is attributable to the lack of symmetry at the interface or to confinement of electrons that does not scale linearly with size. Accordingly, on the nanometer scale (1-100 nm), collections of atoms or molecules have properties that are neither those of the individual constituents nor those of the bulk. The behavior of nanomaterials is also explained by surfaces and interfaces on the nanoscale. In bulk materials, only a relatively small percentage of atoms are at or near the surface or interface (as in a crystal grain boundary). In nanomaterials, the small features ensure that various atoms, perhaps half or more in some cases, are near interfaces. Surface properties such as energy levels, electronic structure, and reactivity can differ markedly from those in interior states.20 Since properties depend in this way on size, rather than on the material, reliable and continuous change can be achieved using a single material. Nanoscale particles of any material with a wide range of properties can be prepared. Their range of possible applications appears to be correspondingly wide, from extraordinarily tiny electronic devices to biomedical uses. Nanomaterials include r 2011 American Chemical Society

metals, ceramics, polymeric materials, or composite materials in the form of particles on the nanoscale. The spectroscopic and magnetic properties of quantum-size semiconductors can thus be exploited,21-24 and polymer- or ligand-stabilized metal nanoparticles can be synthesized.25-34 Inorganic nanoparticles with an approximately spherical shape have been of interest to the broad scientific community for decades. Much attention has been paid to the preparation and properties of bimetallic nanoparticles owing to the possibility that the catalytic properties and electronic structures of such nanomaterials can be tuned by varying their compositions and structures.5,35-40 Quantum dots of CdSe of various sizes exhibit various maximum emissions across the entire visible region. The light that is absorbed and emitted by semiconductor nanoparticles can be tuned by controlling their diameter because the photogenerated electron-hole pair has an exciton diameter that is on the 1-10 nm scale.41-43 For metallic nanoparticles, interesting optical and electronic effects are expected on a scale of approximately 1-10 nm since the mean free path of an electron in a metal is in this range of 1-10 nm.2 Such metal nanoparticles can be reasonably called an “atom assembly” or “molecule” rather than the correct and classical term “colloid”. If the structures of these nanoparticles are well-controlled, then they are commonly referred to as “clusters”. Ligand-stabilized Au55 nanoparticles, proposed by Schmid and co-workers,44 are regarded as one of the most impressive examples of a “cluster”, in which the arrangement of metal atoms as well as ligand molecules can be specified. In the case of polymer-stabilized metal nanoparticles, the structure and arrangement of the polymers are not Received: September 2, 2010 Revised: November 26, 2010 Published: February 08, 2011 3513

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Figure 1. Schematic illustration of preparative methods of metal nanoparticles.

known precisely, but those of the metal atoms can be rather precisely analyzed. Recently, the chemistry and physics of nanoparticles have been vigorously developed although the field of nanoparticles has been neglected by chemists for a long time, probably because of their polydispersity or nonuniformity. Perfectly monodispersed metal nanoparticles are, of course, ideal, but special properties can be expected even if in a nonideal situation. Nanoparticles can be prepared by physical and chemical methods. The physical methods, which frequently involve vapor deposition, depend on the principle of subdividing bulk precursors into nanoparticles. The chemical approaches involve reducing metal ions to metal atoms in the presence of stabilizers, followed by the controlled aggregation of atoms (Figure 1). The latter method of preparation is more effective for small and uniform nanoparticles than the former. In the latter method, controlling atomic aggregation is the most important task in controlling the size and uniformity of the metal nanoparticles. The chemical method is more effective than the physical one for mass-producing of metal nanoparticles. Noble metal nanoclusters in the nanometer size regime display numerous interesting optical, electronic, and chemical properties that depend on size. Such nanoscale materials have potential applications in the development of biological nanosensors and optoelectronic nanodevices. A burst of research activity has occurred in recent years in the field of the synthesis and functionalization of metal nanoparticles. The nanoparticles of noble metals exhibit increased photochemical activity because of their high surface/volume ratio and unusual electronic properties. Silver or gold nanostructures have attracted numerous attention owing to their extensive range of applications in, for example, catalysis, photonics, electronics, optoelectronics, and biological sensing.1,13,14,17,45-47 Hollow nanostructures are a particularly interesting class of materials that have unusual chemical and physical properties that are determined by their shape and composition; they have potential in the development of novel and potentially useful sensing and drug-delivery applications.48 Hollow nanospheres, cubes, rods, tubes, and triangles have been successfully synthesized.49-58 Processing metal nanostructures into hollow ones can improve their performance because of their relatively lower densities and higher surface areas than the solid counterparts. Hollow Pd spheres exhibit strong catalytic activity in Suzuki cross-coupling reactions and can be reused numerous times without degradation.4 Furthermore, the field of bimetallic nanoparticles has attracted

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various interests recently. Bimetallic nanoparticles, which are comprised of two metal elements, are of greater interest than monometallic ones from both scientific and technological perspectives as they often exhibit improved catalytic properties.59-61 Bimetallization can improve catalytic properties of the original single-metal catalysts and create a new property, which may not be achieved by monometallic catalysts. Enormous progress has been developed in the synthesis of metallic nanomaterials. In this article, we highlight our own efforts to explore the synthesis, growth mechanism, and optical properties. These unusual nanomaterials are fascinating opportunities for catalysis, targeting, and thermal therapy. In the following sections, will discuss basic concepts concerning the general architecture method of controlling the shape of metallic nanomaterials: (i) hard template, such as anodic alumina oxide, (ii) soft template, such as CTAB, and (iii) sacrificial template nanostructures. We aim to understand the mechanism of multishaped nanoparticles rather than spherical ones to provide a synthetic way in this field. Even the optical and thermal properties of unusually shaped metallic nanomaterials are of interest to the biomedical community; the activity and bioapplication of the developed nanomaterials will also be discussed in this feature article.

2. SHAPE CONTROL a. Hard Template: Growth Inside the Restricted Void. There are mainly two kinds of templates, that is, “hard template” and “soft template”. The soft template will be discussed in following section. Hard template approach provides an effective route to one-dimensional nanowires. In this method, the template simply serves as a scaffold in which a different material is shaped into a wirelike nanostructure. The hard template include inorganic porous materials such as polymer membranes, anodic alumina oxide (AAO), carbon nanotubes, and so forth. In particular, the fabrication of metal nanowires has recently attracted considerable interest because they have unique magnetic properties and potential technological applications.62 Hard templating draws the attention of scientists and becomes an effective approach to obtain the nanowires of desired materials. Porous polymer membranes are prepared by the track-etch method.63,64 This method involves bombarding a foil of desired material to create damage tracks in this sheet and then chemical etching upon these tracks into pores. A broad range of pore diameters (below 10 nm) is available, and pore densities approaching 109 pores/cm2 can be obtained.64 Because of the random nature of the pore-production process, the pores have tilt with respect to the surface normal and many pores may intersect inside the membranes, which lead to the decreased usefulness in preparation of one-dimensional nanomaterials. Porous AAO membranes are fabricated via an anodization of aluminum foil in acidic solutions, which has been studied for last five decades. The most commonly utilized hard templates are porous anodic oxide films that are formed from aluminum plates and have been utilized to form products that have a range of morphologies.65-67 Employing two-step or self-organization anodization, porous AAO membrane has a regular hexagonal pore structure.68-72 Aluminum anodizing technology has been developed sufficiently so that the dimensions and distribution of pores in an alumina film can be artificially controlled by preindentation.68,70,73,74 The pore density as high as 1011 pores/cm2 can be obtained and pores inside the membrane have no tilt with respect to the surface 3514

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Figure 2. (a) Illustration of hard templates. (b) SEM top-view images of anodic alumina oxide template. (c) SEM image of CoPt3 nanowires fabricated via hard template method. (Reproduced from ref 89).

normal resulting in a nonconnecting pore structure.71,72 Anodically oxidized alumina films have the advantages over other films of chemical stability and ease of control of their volume fraction or size of nanopores.64,69 The AAO membranes have been used as an important template material owing to the regular hexagonal pore structures, higher pore density, and high thermal and chemical stability. To fill desired materials into the porous membranes, electrochemical deposition is a simple and versatile method for synthesis of one-dimensional nanostructural material.75-84 This method has been used to prepare a variety of nanowires including metal and semiconductor.85-88 In general, the synthesis of nanomaterials within the pores of the template involves three major steps. First, the electrochemical deposition procedure is accomplished by coating one side of the membrane with a metal film used as an electrode for electroplating. Second, ions of desired materials were cathodically deposited on the metal surface at the pore bottom from the solution. Finally, the AAO templates were dissolved in acid or base. The free-standing nanowires on desired materials can be obtained. Figure 2a presents a common template, comprised of anodic aluminum oxide (AAO) membranes. One-dimensional channels with controllable diameters and lengths can be filled with desired materials. One-dimensional nanowires are obtained when AAO is removed after the channels have been filled with materials. The AAO template was removed by dissolving it in 1 M aqueous NaOH for 30 min. It was then washed several times using distilled water and ethanol. Figure 2b displays top-view FESEM micrographs of the anodic alumina oxide templates following chemical etching using phosphoric acid solution.89 The hexagonal close-packed arrays in Figure 2b have identical pore diameters of around 60 nm with a standard deviation of 5 nm. The diameter of and gaps among the pores are governed by applied voltage.64,69 The applied voltage can be adjusted and various electrolytes can be used to prepare AAO templates of various diameters and lengths. Figure 2c displays a magnified FESEM image of the as-prepared metallic nanowires that were obtained using this electrochemical method in an AAO template with a diameter of about 60 nm.89 The nanowires were continuous and orientated roughly parallel to one another, all with a uniform diameter of approximately 50 nm, which was slightly less than the diameter of the pores in the template. The shrinkage

Figure 3. Phase transition of CoPt3 nanowires. (Reproduced from ref 89).

that is caused by the densification and the removal of water is responsible for this result.89 Some broken nanowires were observed after the AAO template was removed, because of the high rate of the electrochemical deposition. A high growth rate and the availability of abundant metal cations resulted in crystal growth that was much less directionally selective and therefore led to the formation of very many structural defects. Different from the soft template discussion in the following section, metal nanowires synthesized using the template-directed approach are usually characterized by polycrystalline structure. Thus, an alumina porous template can also be used as protective template because it is chemically and physically stable. Since the magnetic nature of nanomaterials depends strongly on their local structures and crystallinity, postannealing is generally carried out to improve their magnetic properties. AAO can provide a stable environment in which to isolate each nanowire and maintain its one-dimensional structure; this point was elucidated in our recent study of CoPt3 nanowires (Figure 3).89 CoPt3 nanowires were codeposited on an amorphous gold film on the back of AAO. In the initial stage of CoPt3 growth, the nuclei were randomly orientated and a newly coalesced compact deposit exhibited a perfectly random orientation. The confinement of a porous nanostructure in the AAO templates facilitated the formation of the nanowires. The codeposition of Co and Pt caused the as-prepared CoPt3 nanowires with less degree of crystallinity to exhibit density defects and intrinsic stress. The anisotropy that was induced by stress may have directly competed with the shape anisotropy, reducing coercivity and 3515

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Figure 4. TEM images of gold products synthesized under three conditions. (1) (a-c) in the absence of silver ions; (2) (d-f) in the presence of 0.004 mM silver ions; (3) (g-i) in the presence of 0.04 mM silver ions. (Reproduced from ref 104.)

squareness. Accordingly, the magnetic domains (indicated by arrows) were randomly orientated and small. Thermal treatment increased coercivity and squareness by structural relaxation and reducing the number of defects. Moreover, the magnetostatic vector is well-known to lie preferentially along the wire axis to reduce the magnetostatic energy. Hence, the direction of magnetization of the nanoarrays was along the axis of the nanowires, causing the Hc of the parallel field to exceed that of the perpendicular field. Following annealing at 400 °C, CoPt3 nanowires had a “cluster-in-cluster structure”, which was associated with the ferromagnetic cobalt nanoclusters at this stage. The platinum has a particular role in determining the magnetic behavior. Platinum atoms isolated each Co cluster and facilitated the formation of Co nanoclusters with a single magnetic domain. As the annealing temperature was increased (500 °C), the CoPt3 nanowires retained their cluster-in-cluster structure. Co and Pt atoms began to migrate by “interdiffusion”, forming a chemically ordered structure (L12), and causing strong ferromagnetic coupling between Co and Pt atoms by hybridization of Co 3d and Pt 5d orbitals and spin polarization of Pt atoms.90,91 Both the Co cluster (blue arrows) and the CoPt3 ordered structure (yellow arrows) contributed to the magnetic properties of the nanowires (a 3-fold increase in Hc). Finally, following annealing above 700 °C, most of the CoPt3 nanowires had a long-range ordered structure, and the reduction in the number of defects and the number of growing CoPt3 grains substantially increased the

coercivity and squareness. Because of its controllability in precision fabrication, this template-based scheme has been extensively utilized in the fabrication of one-dimensional nanomaterials. Significant progress in the preparation of transition metal nanowires has been made using porous alumina templates. Such procedures have been developed for iron, cobalt, nickel, and alloy nanowires.75-84 Nanowires of magnetic materials (Fe, Co, Ni, FePt, CoPt, and others) attract particular attention because they are potentially useful in ultrahigh-density magnetic data storage devices. b. Soft Templates: Growth under Selective Adsorbates. Templating is the most common method for fabricating nanomaterials. The soft templates generally refer to surfactant assemblies such as liquid crystals, micelles, vesicles, and so forth. Unlike hard templates, soft templates are typically organic-based, and they include ligands, surfactants, and polymers. Recent studies have established that solution-phase methods have the potential to grow metal nanostructures with a variety of well-defined morphologies. In the case of silver and gold, nanorods and nanowires with controllable diameters and aspect ratios could be synthesized with soft templates, such as rod-shaped micelles self-assembled from cetyltrimethylammonium bromide (CTAB)92,93 or liquid crystalline phases made of sodium bis(2ethylhexyl) sulfosuccinate (AOT), p-xylene, and water.94 The octylamine/water bilayer system has been demonstrated as another soft template capable of producing silver nanoplates.95 3516

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The Journal of Physical Chemistry C Silver nanodisks have also been synthesized by sonicating AgNO3 and hydrazine in the presence of reverse micelles selfassembled from an AOT/isooctane/water system.96 Polyol synthesis was a simple and versatile route to colloidal particles made of metals and alloys with typical examples including Ag, Au, Cu, Co, Ir, Ni, Pd, Pt, Ru, CoNi, and FeNi.97 Reasons for the popularity and versatility of this synthesis include the ability for polyols to dissolve many precursor salts and their highly temperature-dependent reducing power. Xia’s group demonstrated a polyol synthesis method to control silver nanostructures by reducing silver nitrate with ethylene glycol in the presence of poly(vinylpyrrolidone) (PVP).51-58,72 This process involves the reduction of an inorganic salt (silver nitrate) by polyol at an elevated temperature. The temperature-dependent property of polyols leads to the synthesis of colloidal particles over a broad range of sizes, which provides an ability to control the rate of nucleation and growth processes with careful control of temperature. At elevated temperatures, ethylene glycol can reduce Agþ ions into Ag atoms and thus induce the nucleation and growth of silver nanostructures in the solution phase. PVP plays a critical role in producing silver nanostructures with good stability and size/shape uniformity.52,54,98,99 PVP is commonly added as a stabilizer to prevent agglomeration of the colloidal particles. It is believed that PVP kinetically control the growth rates of various faces of silver by adsorption and desorption effects, suggesting that there seems to exist a selectivity capping of specific faces for the function group. Consequently, the growth rates of some surfaces would be greatly decreased, leading to a highly anisotropic growth for silver nanostructures. On the other hand, a seed-mediated method for preparing gold nanostructures in the presence of CTAB has been proposed, which involved several reaction steps.92,93,100-102 First, metal salts are reduced with a strong reducing agent (sodium borohydride) to yield spherical particles as seeds. These seeds have a faceted surface and can be capped with a variety of surface groups that could be present during the reaction. Growth solutions containing metal ions (Au3þ), structure-directing agent (CTAB), and a weak reducing agent (ascorbic acid) are prepared in a separate flask. The weak reducing agent can be capable of fully reducing the metal salt to the elemental metal at room temperature; it is just a bit slower than that in the presence of CTAB. The presence of the CTAB (structure-directing agent) is crucial to prepared nanorods/wires in this method. Interestingly, the resulting products usually are characteristic of polycrystalline nature owing the rapid growth rate. In the case of soft template, unlike hard template method, nucleation and growth reaction are operated under gentle condition. As a result, rates of nucleation and growth are practically controlled, and specific facets on the surface of nanoparticles can be well developed. Surface-energy considerations are crucial in understanding and predicting the morphology of nanocrystals. Surface energy, defined as the excess free energy per unit area for a particular crystallographic face, largely determines the faceting and crystal growth observed for particles.103 The resulting particle shape is a perfectly symmetric sphere if the surface energy approaches minimum value. Noble metals, which adopt a face-centered cubic (fcc) lattice, possess different surface energies for different crystal planes. This anisotropy results in stable morphologies in which free energy is minimized by particles bound and low-index planes, and the low-index crystal planes exhibit closest atomic packing. Some theoretical results were obtained the single crystal of Cu, Ni, Pd, Pt, and Au, predicting

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the instability and subsequent reconstruction of all high-index fcc crystal planes.103 Therefore, noble-metal nanocrystals are composed of the lowest-index crystal planes, and morphologies of resulting products strongly depend on the experimental condition (such as reaction temperature and foreign ions). Figure 4 shows serial products prepared via seed-mediated growth method in the presence of CTAB. Figure 4a-c displays products that are prepared in the absence of silver ions. Figure 4a demonstrates that the major products were spherical particles when the growth solution was initially added to the seed solution, but a few short nanorods were also formed because a few gold ions were available. The second and third volumes of added growth solution were used to grow the nanorods with mean lengths of around 59 and 570 nm, respectively. When silver ions (0.004 mM) were introduced into the reaction system, the product changed from one-dimensional (1D) nanorods to bipyramids (as shown in Figure 4d,e). A comparison with Figure 4a reveals that various bipyramids formed upon the first addition of growth solution, revealing that silver ions are essential to their growth of bipyramids. The TEM image (Figure 4e) indicates that the product that grew following the second addition comprised bipyramids and irregularly faceted particles, and that the yield of the bipyramids was about one-third, while that of the irregularly faceted particles was approximately twothirds. The formation of the irregularly faceted particles may have been caused by the blocking of silver on particular facets. Following the third addition of growth solution, nanorods and some irregularly faceted particles were observed, and the mean length of the nanorods was ∼550 nm. The aspect ratio of the nanorods that were prepared without silver ions (∼22.5) exceeded that of those prepared with silver (∼17). Therefore, the aspect ratio of the gold nanorods was slightly lower. Notably, the presence of silver not only caused the formation of irregular nanoparticles but also inhibited the anisotropic growth of gold. The resulting product became dramatically altered as more silver ions were introduced into the reaction system. Following the first addition of growth solution to grow the seeds, the product thus formed contained irregularly faceted particles and bipyramids (Figure 4g). In particular, the morphology changed with the inclusion of irregularly faceted particles and bipyramids. The surfaces of the irregularly faceted particles and bipyramids became very rough. The already nucleating small gold cluster developed on an otherwise smooth facet, and some small tips and islands were formed on the surfaces of irregular particles and bipyramids. When more growth solution was introduced into the system, these small tips served as nucleation sites for the subsequent growth of gold. Accordingly, multipod-shaped (Tshaped and branched-shaped) and star-shaped nanoparticles were observed (Figure 4h,i). Briefly, the results of the addition of silver ions at higher concentrations probably follow from the interaction of the bromide counterions of the surfactant monomers.104 A modified method that maintains the [growth solution]/[seed] ratio constant throughout the reaction has been demonstrated, in which the total volume of growth solution added is 10 times that of the final added volume. Notably, the length of the nanorods was increased to ∼2 μm and their aspect ratios were increased to ∼70 (Figure 5).105 The morphology of the products was extremely sensitive to the temperature of the growth solution that was adopted in the experiments. Figure 6 depicts the evolution of the shape of the gold nanostructures at various temperatures. At high temperatures, AuCl4- ions are reduced sufficiently rapidly to supply gold 3517

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Figure 7. Plot showing the dependence of the length and width on volume of growth solution. (Reproduced from ref 110.) Figure 5. TEM image of nanorods after introduction of five growth solutions into seed solution. (Reproduced from ref 105.)

Figure 6. Schematic illustration of shaped evolution with increase of reaction temperature. (Reproduced from ref 104.)

nuclei or gold seed particles. The interactions of CTAB with gold seed particles at high temperature may favor the growth of gold nanoplates. Various investigations have elucidated this kinetically controlled growth of nanoplates.106-109 Gold nanoplates and icosahedral nanocrystals are formed by kinetically controlled gold growth procedures. As the rate of reduction increases, however, the nucleation and growth become kinetically controlled and the product can have a range of shapes (nanoplates). Increasing the high rate of gold reduction results in the supersaturation of gold atoms on the growing surfaces and so promotes the formation of kinetically controlled shapes (nanoplates). In contrast, if the rate of reduction is excessive these seeds with defects can evolve into structures other than nanoplates. A higher reaction temperature also corresponds to a higher surface diffusion, causing Oswald ripening and yielding thermodynamically favored shapes (icosahedral nanocrystals). Figure 6 shows the corresponding TEM images of the products obtained as the reaction temperature is increased, and the yield of nanorods correspondingly declines. Figure 6 clearly presents triangular plates and hexagonal and spherical particles.

A significant number of triangular plates were observed at a reaction temperature of ∼60 °C. The sides of the particles were slanted, suggesting that rather than being flat prisms, they were, more accurately, tetrahedral with a truncated corner.104 Further increasing the reaction temperature increased the number of spherical particles, and reduced the number of triangular plates. TEM analysis revealed that over 90% of the particles had a projected hexagonal shape with a mean size of 42 ( 9 nm when the reaction temperature was increased to 90 °C. A highresolution image of a single particle revealed twinning, indicating that the particle comprised multiple crystal domains.104 Twinning is one of the most common planar defects in nanocrystals, and it is often observed in face-centered cubic metallic nanocrystals, which typically have {111} twins. Most of the products were icosahedral. Twinning is the mechanism of formation of these particles, possibly because of smaller surface and volume energy. The fundamentals of gold growth are extremely important and understanding them can promote the design of new materials and more sophisticated synthetic methods. X-ray absorption spectroscopy (XAS) can be adopted to investigate the growth of gold and propose a growth mechanism. X-ray absorption has clearly established that gold ions that are evolved from an Au-Cl complex as Au rods.110 The theoretical simulation of X-ray absorption spectra further indicates the evolution of ultrafine small gold clusters (Au13) after a reducing agent (ascorbic acid) is added to the growth solution. After seeds were introduced into the reaction system and ascorbic acid was added, these Au13 clusters formed and grew on the surfaces of seeds, revealing epitaxial growth in the system. An “autocatalytic growth mechanism” may explain this observation.105 The surfactant serves as a one-dimensional director of the growth of gold because its forms rodlike micelles. Since the growth of the seed particles into rodshaped particles ends upon depletion of the gold atoms in the solution, we posit that the length of nanorods could be further increased if the growth of gold were not terminated. A redesigned seed-mediated growth method was reported. Growth solution was serially added in volumes to support the continuous formation of goal. The mixture of rods was then grown by the serial addition of growth solution into the existing seed solution in the presence of reducing agent (ascorbic acid). The length and width of the gold nanorods/wires increased gradually with the volume of the growth solution (Figure 7). The average length and width of the product after 10 mL of growth solution was added were 3518

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Figure 8. (a) Synthetic routes for nanorattles of Au, involving galvanic replacement. TEM images of Au nanoparticles (b), AucoreAgshell nanoparticles (c), and Au nanorattles (d). (Reproduced from ref 118.)

58.1 ( 9.2 nm and 25.1 ( 0.9 nm, respectively. When 1200 mL of growth solution was used to grow 1D gold nanomaterials, the length of the gold rods was increased to approximately 1700 nm (aspect ratio ∼35). Serial addition of growth solution was adopted to maintain the presence of gold, and 1D gold nanorods/wires with a tunable size up to the order of micrometers were successfully prepared. CTAB is necessary for controlling the gold and silver nanoparticle shape, since it is highly watersoluble, has bromide counterions that can chemisorb on those metal surfaces, a sufficiently large headgroup to help direct which face of the crystal grows, and a sufficiently long tail to make a stable bilayer on the metal surface. Surface chemistry here is critical for future applications, thus capping agent (CTAB) has to be taken into account for further development. c. Sacrificial Template: Growth into Hollow Interior Outer the Surface of Template. Over the past decade, the synthesis and characterization of nanomaterials with a hollow interior have received great attention for the development of nanoscience and nanotechnology due to their potential morphology-dependent application. A number of approaches for hollow inorganic nanomaterials have been developed recently; these include Ostwald ripening, Kirkendall effect, and sacrificial template for synthesizing desired materials.111-115 The Kirkendall effect was found as a diffusion phenomenon in general metallurgy, which means comparative diffusive migrations among different atomic species in metals and alloys under heating conditions. The porosity may result from differential solid-state diffusion rates of the reactants in an alloying or oxidation reaction. Smigelkas and Kirkendall reported the movement of the interface between a diffusion couple (copper and zinc), which result from the different diffusion rates of these two species at an elevated temperature.116 This phenomenon, now called the Kirkendall Effect, was the first experimental proof that atomic diffusion occurs through vacancy exchange and not by the direct interchange of atoms. The net directional flow of matter is balanced by an opposite flow of vacancies, which can condense into pores or annihilate at dislocations. Although void formation in alloys and solders may not be a desirable process for metallurgical manufacturing, the physical phenomenon may provide possibilities for fabrication of new nanomaterials with hollow interiors considering the directional matter flow and consequential vacancy accumulation in Kirkendall type diffusion. In general, Ostwald

ripening commonly means the solution has a process that “the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones”.115 Because of the size difference of the forming crystals, concentrations of solutes across the solution vary. This concentration gradient will eventually eliminate crystallites of smaller sizes while the growth of the large ones proceeds. According to this nature of matter relocation, this process can be used as a method for generation of hollow interiors for nanomaterials if one can control the size distribution and aggregation patterns of as-formed primary crystals. In the case of colloidal aggregates, some interior space would be eventually generated due to inhomogeneous size and distribution of crystallites. In this process, large crystallites are essentially immobile while the smaller ones are undergoing mass relocation through dissolving and regrowing, which creates the interior space within the original aggregates. This ripening process has been recently employed to prepare a range of different hollow oxide nanostructures.111,114,117 For example, anatase TiO2 nanospheres with hollow interier were synthesized from a solution route via Ostwald ripening.111 The starting anatase TiO2 nanocrystallites in spherical aggregates were formed with hydrolysis of a low-concentration TiF4 solution. Nanocrystal in the central part of the aggregate was to be smaller, as they could be evacuated preferentially with long time of aging, leaving a vacant space for aggregated spheres. The sacrificial template method has been demonstrated to be a general and effective method for preparing metallic hollow nanostructures by consuming the more reactive component. The most important reaction in this method involves a sacrificial template (more reactive component) and a reactant (inert component). The replacement reaction between these two components occurs upon the surface of template; the inert part is deposited on the surface of the template and inner part diffuses out through pin holes. The hollow shape thus formed depends strongly on the shape of the sacrificial template; the hollow product has approximately the same morphology as the template. Gold and silver are the most widely used in operating as the sacrificial templating reaction because of their specific optical properties.51-58 Since the standard reduction potential of the AuCl4-/Au pair (0.99 V vs standard hydrogen electrode, SHE) exceeds that of the Agþ/Ag pair (0.80 V vs SHE), silver is oxidized to Agþ when a silver sacrificial template and AuCl4- are 3519

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The Journal of Physical Chemistry C mixed in an aqueous solution. This galvanic replacement process was elucidated in relation to our recent fabrication of nanorattles, which comprised Au cores and Au shells, by reacting AucoreAgshell nanoparticles with HAuCl4 in an aqueous solution (Figure 8a).118 The first step was the deposition of a uniform layer of pure silver on the surface of Au nanoparticles. AucoreAgshell nanoparticles have been prepared by directly depositing Ag atoms on the surface of Au nanoparticles using ascorbic acid as a reducing agent. Au nanoparticles were mixed directly with an aqueous solution of sodium citrate and AgNO3. Silver ions were reduced by sodium citrate, yielding silver atoms, which nucleated and grew on the surfaces of gold nanoparticles. The similarities between the atomic radii (Au = 1.44 Å; Ag = 1.45 Å) of silver and gold and between their lattice parameters (Au lattice constant a = 4.069 Å JCPDS file no. 01-1172; Ag lattice constant a = 4.079 Å JCPDS file no. 01-1164) cause the silver atoms to nucleate preferentially and grow on the surfaces of gold nanoparticles, rather than homogeneously nucleating and growing from silver nuclei. The second step was the galvanic replacement reaction between AucoreAgshell colloids with an aqueous HAuCl4 solution at room temperature. This chemical reaction is given by 3Ag þ AuCl4- f 3Agþ þ 4Cl- þ Au. Galvanic replacement transformed a conformal coating of pure silver into a shell of gold. The nanostructures that participate in each reaction step were fully characterized by TEM. As presented in Figure 8b, gold nanoparticles as small as 8.1 nm were obtained. Figure 8c shows a typical TEM image of AucoreAgshell nanoparticles. When the silver-coated nanoparticles underwent the galvanic replacement reaction with a solution of gold ions, the pure silver layers were transformed into gold nanoshells. Figure 8d displays a typical TEM image of the nanorattle. The image clearly indicated that the product was characterized by a rattle-like nanostructure. Notably, the formation of silver chloride should be considered here. Silver chloride, which is formed in the replacement reaction, is completely soluble in water at high temperature. The formation of AgCl solid promoted the removal of Agþ ions and prevented the gold nanoshells from being contaminated by Ag. In the synthesis herein, the nanorattles still contained Ag and Cl after they were centrifuged numerous times to remove the suspended AgCl solid.118 Therefore, much of the AgCl solid adhered to the surfaces of the nanorattles. When the reaction was performed under similar conditions but with the temperature raised from 25 to 95 °C, the larger solubility product of AgCl solid caused it to be less contaminated. Accordingly, the shell component of the nanorattles was controlled by varying the reaction temperature. The shell material and chemical reactivity of these newly synthesized nanorattles will be important in future applications. The sacrificial template method operated via galvanic replacement is an effective method for preparing metallic hollow nanostructures, because galvanic reaction can occur under room temperature and be applied to various metal elements.

3. APPLICATIONS FOR SHAPED METALLIC NANOMATERIALS a. Hollow Spheres for Catalysis. The properties of noble metal nanocrystals make them ideal materials for application in catalysis, where reaction yield and selectivity are dependent on the nature of the cat alyst surface. Generally, catalytic performance of nanocrystals can be finely tuned either by their composition, which mediates electronic structure, or by their

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Figure 9. (a) Synthetic routes of porous hollow Pt nanospheres. TEM images of (b) hollow Ag-Pt nanospheres and (c) hollow Pt nanospheres with nanochannels. (Reproduced from ref 122.)

shape, which determines surface atomic arrangement and coordination. Fundamental studies of single-crystal surfaces of bulk Pt have shown that high-index planes generally exhibit much higher catalytic activity than that of the most common stable planes, such as {111}, {100}, and even {110}, because the highindex planes have a high density of atomic steps, ledges, and kinks, which usually serve as active sites for breaking chemical bonds.119 The bulk Pt(410) surface exhibits unusual activity for catalytic decomposition of NO, a major pollutant of automobile exhaust.120 Tetrahexahedral Pt nanocrystals prepared by electrochemical method are bounded by 24 facets of high-index planes ∼{730} and vicinal planes such as {210} and {310}, which show enhanced catalytic activity in electro-oxidation of small organic fuels of formic acid and ethanol.121 Hollow nanostructures are a particularly interesting class of materials that have unusual chemical and physical properties that are determined by their shape and composition; they exhibit catalytic activities different from their solid counterparts with the advantages of low density, saving of material, and high active sites. As a result, an effective and facile method has to be developed for preparation of catalyst. We recently described the fabrication of hollow spheres with nanochannels that are composed of platinum using a modified galvanic replacement reaction, and their use as electrocatalysts.122 Various nature of catalyst are controlled through present approach, including morphology, composition, and crystallites, which may also provide various facets for specific catalysis. This replacement scheme was carried out at room temperature, making the fabrication of such catalysts effortless. Figure 9a presents the complete procedure for the synthesis of porous Pt hollow spheres. First, uniform silver nanospheres were synthesized. The second step was the galvanic replacement reaction between Ag nanoparticles with an aqueous H2PtCl6 solution at room temperature, which is expressed as 4Ag þ PtCl62- f Pt þ 4AgCl þ 2 Cl-. Pure silver was converted into a shell of Ag-Pt alloy by galvanic replacement. Since the simultaneous formation of AgCl may disrupt the epitaxial deposition of gold atoms on the surface of a silver template at room temperature, the presence of AgCl roughened the surface of the hollow Pt shells, which was exploited to create increase the surface area of catalytic reaction.118 The final step was the removal of the Ag and AgCl from the templates by treating the Ag-Pt alloy shells with aqueous ammonia and HNO3 to generate nanopores on the 3520

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Figure 10. (a) Polarization curve of ORR on commercial Pt catalyst, hollow Ag-Pt shell, and hollow Pt shell with nanochannels in 0.1 M HClO4. (b) Comparison of mass activity of commercial Pt and hollow Pt shell with nanochannels at 0.85 and 0.8 V. (Reproduced from ref 122.)

hollow Pt shells. Figure 9b shows a TEM image, in which the centers of the spheres are brighter than their edges. The shells of the Pt hollow nanospheres seemed to be smooth and their thickness was ∼4.5 nm. Figures 9c displays a TEM image of the hollow shells after AgCl and Ag had been removed. These images clearly reveal that the products had a porous nanostructure (indicated by arrows) with pore diameters of approximately 0.91 nm. Notably, this entire nanostructure became segmented by chemical etching. Most interestingly, the thickness of the hollow shell was specifying degree of significance lower after chemical etching considerably reduced the thickness of the hollow shell, implying that the silver atoms had been removed from the hollow structures. This process yields Pt hollow nanospheres with a high surface area (41 m2/g). Figure 10 plots the specific activity in oxygen reduction reaction (ORR) of a commercial Pt catalyst, hollow Ag-Pt shells (18 m2/g), and hollow Pt shells with nanochannels (41 m2/g). The activity of hollow Ag-Pt shell was much lower than that of the other materials because it had a smaller surface area. At potentials from 0.9 to 0.8 V, the activity of the hollow Pt shell with nanochannels exceeded that of the commercial catalyst, suggesting that the porous hollow shells indeed accelerated the reduction of oxygen. The ORR onset potentials of these two surfaces are similar. Two major factors are responsible for the higher improved chemical performance observed here: (i) nanochannels were formed upon chemical etching, which may have resulted in the formation of an active site in the interior surface of the hollow shell; (ii) a relatively high density of defects, particularly vacancies, were formed on the surface of the shell and may have promoted the formation of a product with a rougher surface. The nanochannels may have activated the inner surface and provided a route for the transport of reactant and product. The incomplete shell of the hollow nanospheres may have provided an interior surface for the catalytic reaction, and the higher surface area of the Pt nanospheres may have increased catalytic activity. These metallic hollow nanoparticles may be useful in industrial applications, including catalytic nanoreactors and related process. b. Nanorods for Antibody Sensing. Recently a great advancement was observed in utilizing metal nanoparticles, especially gold, for biomedical applications, due to their unique shape/size-dependent properties, strong absorption/scattering of light, stability, and nontoxic nature.2,123-128 Among all, gold

Figure 11. Schematic representation of detection of g-IgG through the assembly of gold nanorods based on antibody-antigen recognition.

nanorods are found to be more popular and useful for potential applications such as biochemical sensing, biomedical diagnostics, and therapeutics due to possible tuning of their surface plasmon resonance (SPR) in the visible and near-infrared (NIR) region, which is the potential window of the electromagnetic spectrum for in vivo applications. The strong light scattering properties of gold nanorods have been applied mainly for optical microscopic imaging of cancer cells, and the absorption properties in the NIR region causing a localized hypothermic effect have been utilized for therapeutic purposes.124,125,127 However, changes in localized SPR due to alteration in the conditions of the surrounding environment of gold nanorods have been employed to analyze different biorecognition events at the molecular level.129-131 It was observed that the longitudinal band is more sensitive to the changes in the environment of the gold nanorods as compared to the transverse band, making them ideal candidates for sensing and imaging applications. The goal of this part is to investigate the biosensing properties of bioconjugated gold nanorods with a view to future biomedical applications. In this investigation, bioconjugated nontoxic gold nanorods were adopted as molecular probes for detecting goat IgG using the localized surface plasmon resonance method.132 The surfaces of the CTAB-stabilized gold nanorods were modified by using poly(styrenesulfonate) (PSS) to reduce the toxicity of as-synthesized gold nanorods that are the result of an excess of 3521

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Figure 12. (a) TEM images of binding gold nanorods molecular probes to g-IgG. (b) UV/vis absorption spectra of as-prepared gold nanorods, PSScoated gold nanorods, PSS-coated gold nanorods/goat anti-h-IgG conjugates, those interacted with BSA prior to exposure to h-IgG and those interacted with h-IgG. (Reproduced from ref 132.)

CTAB. Surface-modified gold nanorods were functionalized using anti-g-IgG antibodies, which were further used in the rapid and sensitive detection of g-IgG by localized surface plasmon absorption. The gold nanorods were synthesized using a seedmediated growth method.104 The as-prepared gold nanorods with a mean aspect ratio of ∼3 were fairly uniform in shape and highly dispersed in water without aggregation. Figure 11 presents a typical procedure for preparing gold nanorods molecular probes. The positively charged surfaces of the gold nanorods became negatively charged upon exposure to PSS solution. Thereafter, the precipitate of the gold nanorods was redispersed in phosphate-buffered saline (PBS; pH = 7.4). Anionic polymer PSS coating on the gold nanorods, surrounded by positively charged CTAB molecules, not only reduces the cytotoxicity of CTAB but stabilizes it against the attachment of goat anti-g-IgG to form gold nanorods molecular probes. PSS-capped gold nanorods were mixed with goat anti-g-IgG solution, with which they conjugated. They were then centrifuged and redispersed into PBS solution to remove unbound antibodies. The resulting gold nanorod probes were incubated in a bovine serum albumin (BSA) blocking solution to prevent nonspecific binding and were then utilized to detect the g-IgG. When the gold nanorod molecular probes bound to g-IgG, aggregation of the nanorods, which were preferentially oriented in a lateral (side-to-side and/or end-to-end) manner, was driven by a particular antibody-antigen binding process (Figure 12a). Therefore, the position, intensity, and shape of the SPR of the longitudinal band depended on both the local refractive index and the aggregation of the gold nanorods. The SPR band responded in a way that was sensitive to both changes in the refractive index, which were caused by molecular interactions in the surroundings of the gold nanorods, and the aggregation of gold nanorods, which was driven by biorecognition. More importantly, PSS-capped gold nanorods had a larger surface area (because of their flat sides and tips) for the adsorption of proteins than did spherical gold nanoparticles with similar diameters. Furthermore, the aggregation of gold nanorods, which were preferentially oriented in a lateral (side-to-side and/or end-toend) fashion, by a specific antibody-antigen binding process was observed, causing a substantial wavelength shift, a reduction in

intensity, and a significant widening of the plasmon for rapid and sensitive detection. Figure 12b presents the absorption spectra of as-prepared gold nanorods, PSS-coated gold nanorods, PSScoated gold nanorods/goat anti-g-IgG conjugates, following interaction with BSA before exposure to g-IgG and after exposure to g-IgG. The SPR band of the as-prepared gold nanorods was at 701 nm. When these gold nanorods were capped with a layer of PSS, this band was broadened and red shifted to 716 nm, reflecting the variation in the local dielectric function of the PSS-coated GNRs. A red shift (719 nm) was observed upon exposure to excess goat anti-g-IgG solution for 1 h. However, the transverse bands of the gold nanorods did not apparently undergo a wavelength shift, because the SPR longitudinal band responded more strongly than the SPR transverse band to binding to the target biomolecules . Nonspecific binding of the uncovered gold nanorods surfaces during the immunoassay did not occur, as evidenced by the absence of a significant change in the wavelength when 50 mg/L BSA was used in PBS buffer solution for 30 min as the blocking solution, suggesting that the coverage of the surface with goat anti-g-IgG reached saturation. Adding 1 mg of g-IgG to 0.5 mL of gold nanorod molecular probe solution dramatically reduced the intensity and substantially red shifted the peak (to 763 nm). This large shift was associated with the preferential assembly of gold nanorods, which is driven by side-to-side and/or end-to-end molecular binding. The side-toside and side-to-end aggregation of gold nanorods is caused by the target-specific binding events to an extent that increases with the duration of molecular recognition of goat anti-g-IgG and g-IgG. Absorption measurements revealed a simple form of molecular sensing that is based on changes in the surrounding medium that are associated with the binding of analyte molecules to the gold nanorods, which causes a shift in the plasmonic extinction peak. Coating the surfaces of nanorods with PSS markedly increased cell viability and facilitated the intracellular uptake of the nanorods, suggesting their possible use in various biomedical applications. This rapid, sensitive, and label-free scheme of detecting molecular binding events using surfacemodified gold nanorods may support a novel optical multiplex biosensor platform and have broad potential applications in immunoassay and disease diagnosis. 3522

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Figure 13. Phototherapy using chitosan-multibranched Au nanoparticles exposed to 800 nm NIR laser.

c. Multipod-Shaped Nanomaterials for Photothermal Therapy. Multipod-shaped and branched gold nanostructures

have recently been described.104,105,133-135 These structures have SPR properties that are similar to those of nanorods because of their multiple branches and multidirectional anisotropy. Branched nanoparticles have two SPR bands,104,134 the transverse band (which corresponds to the oscillation of electrons perpendicular to the axis of branches) and the longitudinal band (which corresponds to the oscillation of electrons along the long axis of branches). The longitudinal band can be tuned from visible to near-infrared by increasing the aspect ratio of the branches and the relative intensity can be tuned by controlling the amount of branches per particle. The most widely used method is directional growth in the presence of the common surfactant CTAB and silver ions (Agþ).104,105,136 The preferential adsorption of CTAB and silver atoms on the various crystal faces of gold inhibits growth perpendicular to them.104,137 The growth mechanism of gold nanorods in the presence of CTAB and the influence of various experimental parameters on that mechanism has been extensively explored. However, on our recent study excess CTAB was found to be toxic and the surface modification of as-synthesized gold nanoparticles was required to improve biocompatibility while retaining stable dispersion in an aqueous or buffer medium.132 Numerous surface-protecting reagents have been proposed, including polyethyleneglycol (PEG),138,139 polystyrene sulfonate (PSS),132,140 and chitosan.141 A negatively charged polymer such as PSS can be utilized to form a protective shell over the CTAB layer by electrostatic interaction, but a double layer of PSS-CTAB is applied to the surface of Au in bioanalysis to reduce the risk of releasing CTAB into the solution. The biopolymer chitosan has attracted substantial attention in relation to drug delivery because of its high stability, low toxicity, simplicity of coating, and functionalization procedure. Pure chitosan nanoparticles are widely used in drug delivery and cell imaging, and their cellular uptake has been repeatedly proven.142-144 Although the properties of surface-modified gold nanoparticles must be understood, the stability of surface-capping layers and their effects following in vivo administration for clinical purposes. The SPR response of multipod-shaped Au nanoparticles depends less on the spatial orientation of the nanoparticles than that of gold nanorods, as evidenced by the strong dependence of the photothermal effect on the light polarization.138 Additionally, the much larger surface area of multipod-shaped nanobranches improves the functionalization ratio in drug delivery and release applications. Spherical nanoparticles have been found to be the most suitable for endocitosis through cell membranes whereas gold nanorodes are the most efficient light energy-to-heat converters relative to mass of gold compared to the Au nanoparticles with other shapes.145-147 The surface plasmon resonance of Au nanomaterials can be exploited to promote light-to-heat conversion.The photothermal effect of plasmonic Au nanoparticles in

living tissues has been widely investigated in the fields of biomedicine, drug delivery, and cancer science as having the potential to trigger the precise drug release from the particle surface into the medium or directly destroying cancer cells via light-to-heat conversion. In our investigation, multipod-shaped Au nanoparticles are regarded as an effective medium in the conversion of light to heat owing to their strong SPR absorption in the near-infrared region.104 To lower its cytotoxicity and extend bioapplication in vivo/in vitro, the CTAB surfactant that is used to cap multipod-shaped Au nanoparticles is replaced with biomolecular FITC-labeled chitosan. Chitosan is a highly biocomparible/biodegradable biomolecular and natural material that found in various animals and plants. In particular, it is a primary component in animal/plant shells. Figure 13 presents all of the processes associated with photothermal therapy to kill cancer cells. The chitosan-modified multipod-shaped Au nanoparticles are incubated with J5 cells (liver cancer) overnight to ensure the internalization of nanoparticles by endocytosis. Nearinfrared (NIR) light that is introduced into cells is absorbed and converted to heat by the Au nanomaterials, and the process is enhanced by the SPR effect. The effect of heat on the cells is observed only when Au nanomaterials are internalized, indicating the importance of “localization of therapy”, which means “focused” therapy rather than “dispersed” therapy. In Figure 14, the fluorescence images show a cross-section of J5 cells, captured by the three-dimensional technique of confocal microscopy, which avoids the collection of surface fluorescence signals by the detector. In Figure 14b,c, the green and blue fluorescence clearly reveals the position of the FITC-labeled chitosan-Au multipod-shaped nanoparticles and the nucleus stained with DAPI, respectively. However, in the control experiment that involved the J5 cell that has not been treated with chitosan multipod-shaped Au nanoparticles, no green fluorescence signal is obtained, but the blue fluorescence signal associated DAPI is obtained, indicating the all of the green fluorescence signal is from FITC labeled chitosan-Au multipod-shaped nanoparticles (as shown in Figure 14b). Merging of the bright (Figure 14a) and fluorescence images, as displayed in Figure 14c reveals the green fluorescence signal near the nucleus of the J5 cell and overlaid by the bright image, revealing that the nanoparticles had been successfully internalized. The internalization of nanoparticles directly affects the targeted J5 cells, improving efficiency of phototherapy. The NIR laser with a long wavelength strongly penetrates human skin, which effect is exploited in in vivo photothermal therapy. Accordingly, in the present investigation the 800 nm NIR laser is applied as a source for the multipodshaped Au nanoparticles with strong SPR absorption at 800 nm wavelength. After the internalization of chitosan-modified multipod-shaped Au nanoparticles in J5 cells, the cells were scanned several times with the NIR laser and impaged in situ. To understand the change of temperature caused by exposure to NIR, the J5 cells were stained with Calcein AM, which was 3523

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Figure 14. (a) Bright and (b,c) fluorescence images of J5 cell treated with multibranched Au nanoparticless, captured by confocal microscopy. Green and blue luminescence colors indicate locations of FITC-labeled chitosan-multibranched Au nanoparticles and DAPI stained cell nucleus. (d) and (e,f) bright and fluorescence images of control J5 cell, respectively.

Figure 15. (a) Top layer: thermal therapies based on treatment of J5 cell with chitosan-multibranched Au nanoparticles under 800 nm NIR laser. Images of J5 cells are recorded in situ following 30th scan and 50th scan of NIR laser. (b) Bottom layer: the control experiment on J5 cells for comparsion.

adopted as an indicator of viability, since it only emits green fluorescence in live cell cytoplasma. Restated, heating to a high temperature induced the death of J5 cells, and the disappearance of green fluorescence was observed. The series of fluorescence images in Figure 15 reveal the effect of photothermal therapy on the nanoparticles that were treated J5 cells and exposure to an 800 nm NIR laser. Numerous scans (1.5 s per scan) were performed to examine the evolution of J5 cell death. In the control experiment that involved J5 cells that had not

internalized chitosan-modified multipod-shaped Au nanoparticles (Figure 15b), no cell death was observed between the initial scan and the 50th scan of the NIR laser, as revealed by the maintainance of green fluorescence of the initial scan. However, significant disappearance of green fluorescence after the 30th scan demonstrated the death of J5 cells by heat. However, by the 50th scan, almost 70% of cells had been killed by heat. Hence, the internalization of chitosan-modified Au multipod-shaped nanoparticles can have an important role in the conversion of an NIR 3524

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The Journal of Physical Chemistry C laser to heat by the SPR effect, which can be exploited in the development and improvement of local therapy, especially against cancer cells that have low tolerance of high temperatures. The results indicate that the chitosan-modified Au multipod-shaped nanoparticles have potential as a photothermal therapy agent, but in vivo thermal therapy still depends on the specific targeting of cancer cells. The biomolecular modification of chitosan Au multipod-shaped nanoparticles must be performed to develop local photothermal therapy against targeted cancer cells.

4. CONCLUDING REMARKS This article presented general guiding principles for the synthesis of well-defined shapes, including one-dimensional nanowires (using a hard template), the spherical core-shell structure, the nanostructure with interior space (by sacrificial replacement), nanorods/wires with various aspect ratios, triangular nanoplates, and branched nanoparticless (using a soft template). The anodic alumina oxide template is often applied in the preparation of one-dimensional nanomaterials by electrochemical deposition, and especially in the preparation of transition-metal nanowires that are difficult to prepare by other means. The use of soft templates reveals a versatile route for synthesizing multishaped gold nanoparticles (such as spherical nanoparticles, bipyramids, nanorods, nanowires, T- and star-shaped nanoparticles, and triangular nanoplates) that can be tuned by controlling a few factors. The optical performance and surface chemistry can be varied by controlling the shape of the gold nanostructures, which can thus be utilized in numerous fields. Hollow nanostructures are a particularly interesting class of materials that have unusual chemical and physical characteristics that are determined by their shape and composition. They have great potential in the development of novel and potentially useful sensing and drugdelivery applications. Transforming metal nanostructures into hollow structures improves their performance because doing so reduces their densities are and increases their surface areas above those of their solid counterparts. The galvanic replacement reaction has been exploited as a powerful method of preparing hollow metal nanostructures, which have various interesting optical characteristics. The incomplete shell of the hollow nanospheres may provide interior surface for the catalytic reaction, and the high surface area of Pt nanospheres is associated with their high catalytic activity. These metallic hollow nanoparticles may be useful in industrial applications, including catalytic nanoreactors and related fields. Measurements of absorption by a gold nanostructure revealed a simple form of molecular sensing of changes in the surrounding medium associated with the binding of analyte molecules to the gold nanorods, which causes a shift in the plasmonic extinction peak. This rapid, sensitive, and label-free scheme for detecting molecular binding events using gold nanorods may support a novel optical multiplex biosensor platform in immunoassay and disease diagnosis. The surfaces of as-prepared Au nanoparticles were modified using chitosan to improve biocompatibility and eliminate cytotoxic CTAB. A photothermal therapy experiment on J5 cells using a pulsed NIR laser source indicated that branched Au nanoparticles can effectively transform NIR light into heat. NIR laser irradiation with a wavelength of 800 nm and a power density of 60 mW cm-2 sufficed to cause rapid J5 cell hyperthermia when 4 μg/μL Au nanobranches was loaded in the incubation solution, revealing that chitosan-capped Au NPs have great potential as thermal therapeutic applications.

FEATURE ARTICLE

Although this feature article considers only multiform metallic nanomaterials, in view of its extensive applicability it is of great potential to the preparation of numerous other optical and otherwise functional materials in various forms by methods. Control of surface chemistry is crucial to such applications as surface catalysis and biomedical diagnostics or photothermal therapy. The chemical and physical characteristics of noble metallic nanoparticles make them variously useful in photonics, chemical, biochemical sensing, and imaging applications. This article focused on several possibilities, but as the ability to manufacture and modify nanocrystals continues to improve, the creative uses of these materials are sure to increase.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ BIOGRAPHIES

Hao Ming Chen is currently a postdoctoral research fellow at National Taiwan University. He received his M.S. degree in 2004 and his Ph.D. in Chemistry from National Taiwan University in 2008, where he worked on the synthesis of nanocrystals for particular applications. His current research interests include synthesis of nanomaterials, metallic nanocrystals for bioapplications, and semiconductor nanomaterials for solar energy conversion.

Ru-Shi Liu is currently a professor at the Department of Chemistry, National Taiwan University. He received his Bachelor’s Degree in Chemistry from Shoochow University (Taiwan) in 1981. He received his Master’s Degree in nuclear science from 3525

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The Journal of Physical Chemistry C the National Tsing Hua University (Taiwan) in 1983. He obtained two Ph.D. degrees in chemistry, one from National Tsing Hua University in 1990 and the other from the University of Cambridge in 1992. He worked at Materials Research Laboratories at the Industrial Technology Research Institute from 1983 to 1985. He was an Associate Professor at the Department of Chemistry of National Taiwan University from 1995 to 1999, when he was promoted to a professorship in 1999. Professor Liu’s research concern is in the field the Materials Chemistry. He is the author and coauthor of more than 350 publications in scientific international journals. He has also been granted more than 80 patents.

’ ACKNOWLEDGMENT The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract Nos. NSC 97-2113-M-002-012-MY3 and NSC 99-2120-M-002-012. ’ REFERENCES (1) Chen, S.; Yang, Y. J. Am. Chem. Soc. 2002, 124, 5280. (2) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (3) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (4) Kim, S.-W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (5) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (6) Sinha, A. K.; Seelan, S.; Tsubota, S.; Haruta, M. Angew. Chem., Int. Ed. 2004, 43, 1546. (7) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (8) Zhou, S.; Mcllwrath, K.; Jackson, G.; Eichhorn, B. J. Am. Chem. Soc. 2006, 128, 1780. (9) Chen, H. M.; Chen, C. K.; Chang, Y.-C.; Tsai, C.-W.; Liu, R.-S.; Hu, S.-F.; Chang, W.-S.; Chen, K.-H. Angew. Chem., Int. Ed. 2010, 49, 5966. (10) Chen, H. M.; Liu, R.-S.; Li, H.; Zeng, H. C. Angew. Chem., Int. Ed. 2006, 45, 2713. (11) Peyser, L. A.; Vinson, A. E.; Bartko, A. P.; Dickson, R. M. Science 2001, 291, 103. (12) Charles Cao, Y. W.; Jin, R.; Mirkin, C. A. Science 2002, 297, 1536. (13) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (14) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (15) Park, S.-J.; Taton, T. A.; Mirkin, C. A. Science 2002, 295, 1503. (16) Rosi, N. L.; Giljohann, D. A.; Thaxton, C. S.; Lytton-Jean, A. K. R.; Han, M. S.; Mirkin, C. A. Science 2006, 312, 1027. (17) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 1757. (18) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (19) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554. (20) Optical Properties of Metal Clusters; Kreibig, U., Vollmer, M., Eds.; Springer: New York, 1995. (21) Henglein, A. Top. Curr. Chem. 1988, 143, 113. (22) Henglein, A. Chem. Rev. 1989, 89, 1861. (23) Weller, H. Angew. Chem., Int. Ed. 1993, 105, 43. (24) Weller, H. Adv. Mater. 1993, 5, 88. (25) Bradley, J. S.; Hill, E. W.; Behal, S.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1992, 4, 1234. (26) Bradley, J. S.; Millar, J. M.; Hill, M.; Hall, E. W. J. Am. Chem. Soc. 1991, 113, 4016. (27) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795.

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