architecture of metallic nanostructures: synthesis...

Published: February 08, 2011

r 2011 American Chemical Society 3513 dx.doi.org/10.1021/jp108403r | J. Phys. Chem. C 2011, 115, 3513–3527

FEATURE ARTICLE

pubs.acs.org/JPCC

Architecture of Metallic Nanostructures: Synthesis Strategy andSpecific ApplicationsHao Ming Chen and Ru-Shi Liu*

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan

ABSTRACT: Metallic nanoparticles are emerging as key materials use incatalysis, plasmonics, sensing, and spectroscopy. To approach theseapplications, the control of nanostructures provides increasing selectivityand functionality. This feature article highlights our recent researchprogress in this emerging field. This article discusses control of the shapeof metallic nanostructures and their physical/chemical nature. The numer-ous approaches for synthesizing multishaped nanostructures include (i)the use of hard templates such as anodic aluminumoxide, (ii) the use of softtemplates such as cetyltrimethylammonium bromide (CTAB), and (iii)the use of sacrificial templates. Finally, the use of such nanostructures incatalysis, sensing, and photothermal therapy are demonstrated. Generalstrategies for these methods are discussed and related recent researchdirections will also be addressed in this feature article.

1. INTRODUCTION

Over the past decade, nanomaterials have been the subject ofextensive 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 elec-tronic applications.18,19 Nanomaterials are designed at the mo-lecular (nanometer) level to take advantage of their small size andhave novel properties that are typically not observed in theirconventional, bulk counterparts. Nanomaterials have a muchlarger surface area to volume ratio than their bulk counterparts,which is the basis of their novel physical-chemical propertiesexhibited by these nanomaterials.20 The emergence of novelproperties on the nanoscale is attributable to the lack ofsymmetry at the interface or to confinement of electrons thatdoes not scale linearly with size. Accordingly, on the nanometerscale (1-100 nm), collections of atoms or molecules haveproperties that are neither those of the individual constituentsnor those of the bulk. The behavior of nanomaterials is alsoexplained by surfaces and interfaces on the nanoscale. In bulkmaterials, only a relatively small percentage of atoms are at ornear the surface or interface (as in a crystal grain boundary). Innanomaterials, the small features ensure that various atoms,perhaps half or more in some cases, are near interfaces. Surfaceproperties such as energy levels, electronic structure, and reac-tivity can differ markedly from those in interior states.20 Sinceproperties depend in this way on size, rather than on thematerial,reliable and continuous change can be achieved using a singlematerial.

Nanoscale particles of any material with a wide range ofproperties can be prepared. Their range of possible applicationsappears to be correspondingly wide, from extraordinarily tinyelectronic devices to biomedical uses. Nanomaterials include

metals, ceramics, polymeric materials, or composite materials inthe form of particles on the nanoscale. The spectroscopic andmagnetic properties of quantum-size semiconductors can thus beexploited,21-24 and polymer- or ligand-stabilized metal nanopar-ticles can be synthesized.25-34 Inorganic nanoparticles with anapproximately spherical shape have been of interest to the broadscientific community for decades. Much attention has been paidto the preparation and properties of bimetallic nanoparticlesowing to the possibility that the catalytic properties and electro-nic structures of such nanomaterials can be tuned by varying theircompositions and structures.5,35-40 Quantum dots of CdSe ofvarious sizes exhibit various maximum emissions across the entirevisible region. The light that is absorbed and emitted bysemiconductor nanoparticles can be tuned by controlling theirdiameter because the photogenerated electron-hole pair has anexciton diameter that is on the 1-10 nm scale.41-43 For metallicnanoparticles, interesting optical and electronic effects are ex-pected on a scale of approximately 1-10 nm since the mean freepath of an electron in a metal is in this range of 1-10 nm.2 Suchmetal 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, thenthey are commonly referred to as “clusters”. Ligand-stabilizedAu55 nanoparticles, proposed by Schmid and co-workers,44 areregarded as one of the most impressive examples of a “cluster”, inwhich the arrangement of metal atoms as well as ligandmoleculescan be specified. In the case of polymer-stabilized metal nano-particles, the structure and arrangement of the polymers are not

Received: September 2, 2010Revised: November 26, 2010

3514 dx.doi.org/10.1021/jp108403r |J. Phys. Chem. C 2011, 115, 3513–3527

The Journal of Physical Chemistry C FEATURE ARTICLE

known precisely, but those of the metal atoms can be ratherprecisely analyzed. Recently, the chemistry and physics ofnanoparticles have been vigorously developed although the fieldof 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 nonidealsituation.

Nanoparticles can be prepared by physical and chemicalmethods. The physical methods, which frequently involve vapordeposition, depend on the principle of subdividing bulk pre-cursors into nanoparticles. The chemical approaches involvereducing metal ions to metal atoms in the presence of stabilizers,followed by the controlled aggregation of atoms (Figure 1). Thelatter method of preparation is more effective for small anduniform nanoparticles than the former. In the latter method,controlling atomic aggregation is the most important task incontrolling the size and uniformity of the metal nanoparticles.The chemical method is more effective than the physical one formass-producing of metal nanoparticles.

Noble metal nanoclusters in the nanometer size regime dis-play numerous interesting optical, electronic, and chemicalproperties that depend on size. Such nanoscale materials havepotential applications in the development of biological nanosen-sors and optoelectronic nanodevices. A burst of research activityhas occurred in recent years in the field of the synthesis andfunctionalization of metal nanoparticles. The nanoparticles ofnoble metals exhibit increased photochemical activity because oftheir high surface/volume ratio and unusual electronic proper-ties. Silver or gold nanostructures have attracted numerousattention owing to their extensive range of applications in, forexample, catalysis, photonics, electronics, optoelectronics, andbiological sensing.1,13,14,17,45-47 Hollow nanostructures are aparticularly interesting class of materials that have unusualchemical and physical properties that are determined by theirshape and composition; they have potential in the developmentof novel and potentially useful sensing and drug-deliveryapplications.48 Hollow nanospheres, cubes, rods, tubes, andtriangles have been successfully synthesized.49-58 Processingmetal nanostructures into hollow ones can improve their per-formance because of their relatively lower densities and highersurface areas than the solid counterparts. Hollow Pd spheresexhibit strong catalytic activity in Suzuki cross-coupling reactionsand can be reused numerous times without degradation.4

Furthermore, the field of bimetallic nanoparticles has attracted

various interests recently. Bimetallic nanoparticles, which arecomprised of two metal elements, are of greater interest thanmonometallic ones from both scientific and technologicalperspectives as they often exhibit improved catalytic pro-perties.59-61 Bimetallization can improve catalytic properties ofthe 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 ofmetallic nanomaterials. In this article, we highlight our ownefforts to explore the synthesis, growth mechanism, and opticalproperties. These unusual nanomaterials are fascinating oppor-tunities for catalysis, targeting, and thermal therapy. In thefollowing sections, will discuss basic concepts concerning thegeneral architecture method of controlling the shape of metallicnanomaterials: (i) hard template, such as anodic alumina oxide,(ii) soft template, such as CTAB, and (iii) sacrificial templatenanostructures. We aim to understand the mechanism of multi-shaped nanoparticles rather than spherical ones to provide asynthetic way in this field. Even the optical and thermal proper-ties of unusually shaped metallic nanomaterials are of interest tothe biomedical community; the activity and bioapplication of thedeveloped nanomaterials will also be discussed in this featurearticle.

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 infollowing section. Hard template approach provides an effectiveroute to one-dimensional nanowires. In this method, the tem-plate simply serves as a scaffold in which a different material isshaped into a wirelike nanostructure. The hard template includeinorganic porous materials such as polymer membranes, anodicalumina oxide (AAO), carbon nanotubes, and so forth. Inparticular, the fabrication of metal nanowires has recentlyattracted considerable interest because they have unique mag-netic properties and potential technological applications.62 Hardtemplating draws the attention of scientists and becomes aneffective approach to obtain the nanowires of desired materials.Porous polymer membranes are prepared by the track-etchmethod.63,64 This method involves bombarding a foil of desiredmaterial to create damage tracks in this sheet and then chemicaletching upon these tracks into pores. A broad range of porediameters (below 10 nm) is available, and pore densitiesapproaching 109 pores/cm2 can be obtained.64 Because of therandom nature of the pore-production process, the pores have tiltwith respect to the surface normal and many pores may intersectinside the membranes, which lead to the decreased usefulness inpreparation of one-dimensional nanomaterials. Porous AAOmembranes are fabricated via an anodization of aluminum foilin acidic solutions, which has been studied for last five decades.The most commonly utilized hard templates are porous anodicoxide films that are formed from aluminum plates and have beenutilized to form products that have a range of morphologies.65-67

Employing two-step or self-organization anodization, porousAAO membrane has a regular hexagonal pore structure.68-72

Aluminum anodizing technology has been developed sufficientlyso that the dimensions and distribution of pores in an aluminafilm can be artificially controlled by preindentation.68,70,73,74 Thepore density as high as 1011 pores/cm2 can be obtained and poresinside the membrane have no tilt with respect to the surface

Figure 1. Schematic illustration of preparative methods of metalnanoparticles.



normal resulting in a nonconnecting pore structure.71,72 Anodi-cally oxidized alumina films have the advantages over other filmsof chemical stability and ease of control of their volume fractionor size of nanopores.64,69 The AAO membranes have been usedas an important template material owing to the regular hexagonalpore structures, higher pore density, and high thermal andchemical stability.To fill desired materials into the porous membranes, electro-

chemical deposition is a simple and versatile method for synth-esis of one-dimensional nanostructural material.75-84 Thismethod has been used to prepare a variety of nanowires includingmetal and semiconductor.85-88 In general, the synthesis ofnanomaterials within the pores of the template involves threemajor steps. First, the electrochemical deposition procedure isaccomplished by coating one side of the membrane with a metalfilm used as an electrode for electroplating. Second, ions ofdesired materials were cathodically deposited on the metalsurface at the pore bottom from the solution. Finally, the AAOtemplates were dissolved in acid or base. The free-standingnanowires on desired materials can be obtained. Figure 2apresents a common template, comprised of anodic aluminumoxide (AAO) membranes. One-dimensional channels with con-trollable diameters and lengths can be filled with desired materi-als. One-dimensional nanowires are obtained when AAO isremoved after the channels have been filled with materials. TheAAO template was removed by dissolving it in 1 M aqueousNaOH for 30 min. It was then washed several times usingdistilled water and ethanol. Figure 2b displays top-view FE-SEM micrographs of the anodic alumina oxide templates follow-ing chemical etching using phosphoric acid solution.89 Thehexagonal close-packed arrays in Figure 2b have identical porediameters of around 60 nm with a standard deviation of 5 nm.The diameter of and gaps among the pores are governed byapplied voltage.64,69 The applied voltage can be adjusted andvarious electrolytes can be used to prepare AAO templates ofvarious diameters and lengths. Figure 2c displays a magnified FE-SEM image of the as-prepared metallic nanowires that wereobtained using this electrochemical method in an AAO templatewith a diameter of about 60 nm.89 The nanowires were contin-uous and orientated roughly parallel to one another, all with auniform diameter of approximately 50 nm, which was slightly lessthan the diameter of the pores in the template. The shrinkage

that is caused by the densification and the removal of water isresponsible for this result.89 Some broken nanowires wereobserved after the AAO template was removed, because of thehigh rate of the electrochemical deposition. A high growth rateand the availability of abundant metal cations resulted in crystalgrowth that was much less directionally selective and thereforeled to the formation of very many structural defects. Differentfrom the soft template discussion in the following section, metalnanowires synthesized using the template-directed approach areusually characterized by polycrystalline structure. Thus, analumina porous template can also be used as protective templatebecause it is chemically and physically stable. Since the magneticnature of nanomaterials depends strongly on their local struc-tures and crystallinity, postannealing is generally carried out toimprove their magnetic properties. AAO can provide a stableenvironment in which to isolate each nanowire and maintain itsone-dimensional structure; this point was elucidated in ourrecent study of CoPt3 nanowires (Figure 3).

89 CoPt3 nanowireswere codeposited on an amorphous gold film on the back ofAAO. In the initial stage of CoPt3 growth, the nuclei wererandomly orientated and a newly coalesced compact depositexhibited a perfectly random orientation. The confinement of aporous nanostructure in the AAO templates facilitated theformation of the nanowires. The codeposition of Co and Ptcaused the as-prepared CoPt3 nanowires with less degree ofcrystallinity to exhibit density defects and intrinsic stress. Theanisotropy that was induced by stress may have directly com-peted with the shape anisotropy, reducing coercivity and

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

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



squareness. Accordingly, the magnetic domains (indicated byarrows) were randomly orientated and small. Thermal treatmentincreased coercivity and squareness by structural relaxation andreducing the number of defects. Moreover, the magnetostaticvector is well-known to lie preferentially along the wire axis toreduce the magnetostatic energy. Hence, the direction of mag-netization of the nanoarrays was along the axis of the nanowires,causing the Hc of the parallel field to exceed that of theperpendicular field. Following annealing at 400 �C, CoPt3nanowires had a “cluster-in-cluster structure”, which was asso-ciated with the ferromagnetic cobalt nanoclusters at this stage.The platinum has a particular role in determining the magneticbehavior. Platinum atoms isolated each Co cluster and facilitatedthe formation of Co nanoclusters with a single magnetic domain.As the annealing temperature was increased (500 �C), the CoPt3nanowires retained their cluster-in-cluster structure. Co and Ptatoms began to migrate by “interdiffusion”, forming a chemicallyordered structure (L12), and causing strong ferromagneticcoupling between Co and Pt atoms by hybridization of Co 3dand Pt 5d orbitals and spin polarization of Pt atoms.90,91 Both theCo cluster (blue arrows) and the CoPt3 ordered structure(yellow arrows) contributed to the magnetic properties of thenanowires (a 3-fold increase in Hc). Finally, following annealingabove 700 �C, most of the CoPt3 nanowires had a long-rangeordered structure, and the reduction in the number of defects andthe number of growing CoPt3 grains substantially increased the

coercivity and squareness. Because of its controllability in preci-sion fabrication, this template-based scheme has been extensivelyutilized in the fabrication of one-dimensional nanomaterials.Significant progress in the preparation of transition metalnanowires has been made using porous alumina templates. Suchprocedures have been developed for iron, cobalt, nickel, and alloynanowires.75-84 Nanowires of magnetic materials (Fe, Co, Ni,FePt, CoPt, and others) attract particular attention because theyare potentially useful in ultrahigh-density magnetic data storagedevices.b. Soft Templates: Growth under Selective Adsorbates.

Templating is the most common method for fabricating nano-materials. The soft templates generally refer to surfactant assem-blies such as liquid crystals, micelles, vesicles, and so forth. Unlikehard templates, soft templates are typically organic-based, andthey include ligands, surfactants, and polymers. Recent studieshave established that solution-phase methods have the potentialto grow metal nanostructures with a variety of well-definedmorphologies. In the case of silver and gold, nanorods andnanowires with controllable diameters and aspect ratios couldbe synthesized with soft templates, such as rod-shaped micellesself-assembled from cetyltrimethylammonium bromide(CTAB)92,93 or liquid crystalline phases made of sodium bis(2-ethylhexyl) sulfosuccinate (AOT), p-xylene, and water.94 Theoctylamine/water bilayer system has been demonstrated asanother soft template capable of producing silver nanoplates.95

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 0.004mM silver ions; (3) (g-i) in the presence of 0.04 mM silver ions. (Reproduced from ref 104.)



Silver nanodisks have also been synthesized by sonicatingAgNO3 and hydrazine in the presence of reverse micelles self-assembled from an AOT/isooctane/water system.96 Polyolsynthesis was a simple and versatile route to colloidal particlesmade of metals and alloys with typical examples including Ag, Au,Cu, Co, Ir, Ni, Pd, Pt, Ru, CoNi, and FeNi.97 Reasons for thepopularity and versatility of this synthesis include the ability forpolyols to dissolve many precursor salts and their highly tem-perature-dependent reducing power. Xia’s group demonstrated apolyol synthesis method to control silver nanostructures byreducing silver nitrate with ethylene glycol in the presence ofpoly(vinylpyrrolidone) (PVP).51-58,72 This process involves thereduction of an inorganic salt (silver nitrate) by polyol at anelevated temperature. The temperature-dependent property ofpolyols leads to the synthesis of colloidal particles over a broadrange of sizes, which provides an ability to control the rate ofnucleation and growth processes with careful control of tem-perature. At elevated temperatures, ethylene glycol can reduceAgþ ions into Ag atoms and thus induce the nucleation andgrowth of silver nanostructures in the solution phase. PVP plays acritical role in producing silver nanostructures with good stabilityand size/shape uniformity.52,54,98,99 PVP is commonly added as astabilizer to prevent agglomeration of the colloidal particles. It isbelieved that PVP kinetically control the growth rates of variousfaces of silver by adsorption and desorption effects, suggestingthat there seems to exist a selectivity capping of specific faces forthe function group. Consequently, the growth rates of somesurfaces would be greatly decreased, leading to a highly aniso-tropic growth for silver nanostructures. On the other hand, aseed-mediated method for preparing gold nanostructures in thepresence of CTAB has been proposed, which involved severalreaction steps.92,93,100-102 First, metal salts are reduced with astrong reducing agent (sodium borohydride) to yield sphericalparticles as seeds. These seeds have a faceted surface and can becapped with a variety of surface groups that could be presentduring the reaction. Growth solutions containing metal ions(Au3þ), structure-directing agent (CTAB), and a weak reducingagent (ascorbic acid) are prepared in a separate flask. The weakreducing agent can be capable of fully reducing the metal salt tothe elemental metal at room temperature; it is just a bit slowerthan that in the presence of CTAB. The presence of the CTAB(structure-directing agent) is crucial to prepared nanorods/wiresin this method.Interestingly, the resulting products usually are characteristic

of polycrystalline nature owing the rapid growth rate. In the caseof soft template, unlike hard template method, nucleation andgrowth reaction are operated under gentle condition. As a result,rates of nucleation and growth are practically controlled, andspecific facets on the surface of nanoparticles can be welldeveloped. Surface-energy considerations are crucial in under-standing and predicting the morphology of nanocrystals. Surfaceenergy, defined as the excess free energy per unit area for aparticular crystallographic face, largely determines the facetingand crystal growth observed for particles.103 The resultingparticle shape is a perfectly symmetric sphere if the surfaceenergy approaches minimum value. Noble metals, which adopt aface-centered cubic (fcc) lattice, possess different surface en-ergies for different crystal planes. This anisotropy results in stablemorphologies in which free energy is minimized by particlesbound and low-index planes, and the low-index crystal planesexhibit closest atomic packing. Some theoretical results wereobtained the single crystal of Cu, Ni, Pd, Pt, and Au, predicting

the instability and subsequent reconstruction of all high-index fcccrystal planes.103 Therefore, noble-metal nanocrystals are com-posed of the lowest-index crystal planes, and morphologies ofresulting products strongly depend on the experimental condi-tion (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 displaysproducts that are prepared in the absence of silver ions. Figure 4ademonstrates that the major products were spherical particleswhen the growth solution was initially added to the seed solution,but a few short nanorods were also formed because a few goldions were available. The second and third volumes of addedgrowth solution were used to grow the nanorods with meanlengths of around 59 and 570 nm, respectively. When silver ions(0.004 mM) were introduced into the reaction system, theproduct changed from one-dimensional (1D) nanorods tobipyramids (as shown in Figure 4d,e). A comparison withFigure 4a reveals that various bipyramids formed upon the firstaddition of growth solution, revealing that silver ions are essentialto their growth of bipyramids. The TEM image (Figure 4e)indicates that the product that grew following the secondaddition comprised bipyramids and irregularly faceted particles,and that the yield of the bipyramids was about one-third, whilethat of the irregularly faceted particles was approximately two-thirds. The formation of the irregularly faceted particlesmay havebeen caused by the blocking of silver on particular facets.Following the third addition of growth solution, nanorods andsome irregularly faceted particles were observed, and the meanlength of the nanorods was ∼550 nm. The aspect ratio of thenanorods that were prepared without silver ions (∼22.5) ex-ceeded that of those prepared with silver (∼17). Therefore, theaspect ratio of the gold nanorods was slightly lower. Notably, thepresence of silver not only caused the formation of irregularnanoparticles but also inhibited the anisotropic growth of gold.The resulting product became dramatically altered as more silverions were introduced into the reaction system. Following the firstaddition of growth solution to grow the seeds, the product thusformed contained irregularly faceted particles and bipyramids(Figure 4g). In particular, the morphology changed with theinclusion of irregularly faceted particles and bipyramids. Thesurfaces of the irregularly faceted particles and bipyramidsbecame very rough. The already nucleating small gold clusterdeveloped on an otherwise smooth facet, and some small tips andislands were formed on the surfaces of irregular particles andbipyramids. Whenmore growth solution was introduced into thesystem, these small tips served as nucleation sites for thesubsequent growth of gold. Accordingly, multipod-shaped (T-shaped and branched-shaped) and star-shaped nanoparticleswere observed (Figure 4h,i). Briefly, the results of the additionof silver ions at higher concentrations probably follow from theinteraction of the bromide counterions of the surfactantmonomers.104 A modified method that maintains the [growthsolution]/[seed] ratio constant throughout the reaction has beendemonstrated, in which the total volume of growth solutionadded is 10 times that of the final added volume. Notably, thelength of the nanorods was increased to∼2 μm and their aspectratios were increased to ∼70 (Figure 5).105

The morphology of the products was extremely sensitive tothe temperature of the growth solution that was adopted in theexperiments. Figure 6 depicts the evolution of the shape of thegold nanostructures at various temperatures. At high tempera-tures, AuCl4

- ions are reduced sufficiently rapidly to supply gold



nuclei or gold seed particles. The interactions of CTAB with goldseed particles at high temperature may favor the growth of goldnanoplates. Various investigations have elucidated this kineticallycontrolled growth of nanoplates.106-109 Gold nanoplates andicosahedral nanocrystals are formed by kinetically controlledgold growth procedures. As the rate of reduction increases,however, the nucleation and growth become kinetically con-trolled and the product can have a range of shapes (nanoplates).Increasing the high rate of gold reduction results in the super-saturation of gold atoms on the growing surfaces and sopromotes the formation of kinetically controlled shapes(nanoplates). In contrast, if the rate of reduction is excessivethese seeds with defects can evolve into structures other thannanoplates. A higher reaction temperature also corresponds to ahigher surface diffusion, causing Oswald ripening and yieldingthermodynamically favored shapes (icosahedral nanocrystals).Figure 6 shows the corresponding TEM images of the

products obtained as the reaction temperature is increased, andthe yield of nanorods correspondingly declines. Figure 6 clearlypresents triangular plates and hexagonal and spherical particles.

A significant number of triangular plates were observed at areaction temperature of ∼60 �C. The sides of the particles wereslanted, suggesting that rather than being flat prisms, they were,more accurately, tetrahedral with a truncated corner.104 Furtherincreasing the reaction temperature increased the number ofspherical particles, and reduced the number of triangular plates.TEM analysis revealed that over 90% of the particles had aprojected hexagonal shape with a mean size of 42 ( 9 nm whenthe reaction temperature was increased to 90 �C. A high-resolution image of a single particle revealed twinning, indicatingthat the particle comprised multiple crystal domains.104 Twin-ning is one of the most common planar defects in nanocrystals,and it is often observed in face-centered cubic metallic nano-crystals, which typically have {111} twins. Most of the productswere icosahedral. Twinning is the mechanism of formation ofthese particles, possibly because of smaller surface and volumeenergy.The fundamentals of gold growth are extremely important and

understanding them can promote the design of new materialsand more sophisticated synthetic methods. X-ray absorptionspectroscopy (XAS) can be adopted to investigate the growthof gold and propose a growth mechanism. X-ray absorption hasclearly established that gold ions that are evolved from an Au-Clcomplex as Au rods.110 The theoretical simulation of X-rayabsorption spectra further indicates the evolution of ultrafinesmall gold clusters (Au13) after a reducing agent (ascorbic acid) isadded to the growth solution. After seeds were introduced intothe reaction system and ascorbic acid was added, these Au13clusters formed and grew on the surfaces of seeds, revealingepitaxial growth in the system. An “autocatalytic growth mechan-ism” may explain this observation.105 The surfactant serves as aone-dimensional director of the growth of gold because its formsrodlike micelles. Since the growth of the seed particles into rod-shaped particles ends upon depletion of the gold atoms in thesolution, we posit that the length of nanorods could be furtherincreased if the growth of gold were not terminated. A redesignedseed-mediated growth method was reported. Growth solutionwas serially added in volumes to support the continuous forma-tion of goal. The mixture of rods was then grown by the serialaddition of growth solution into the existing seed solution in thepresence of reducing agent (ascorbic acid). The length and widthof the gold nanorods/wires increased gradually with the volumeof the growth solution (Figure 7). The average length and widthof the product after 10 mL of growth solution was added were

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

Figure 7. Plot showing the dependence of the length and width onvolume of growth solution. (Reproduced from ref 110.)

Figure 5. TEM image of nanorods after introduction of five growthsolutions into seed solution. (Reproduced from ref 105.)



58.1 ( 9.2 nm and 25.1 ( 0.9 nm, respectively. When 1200 mLof growth solution was used to grow 1D gold nanomaterials, thelength of the gold rods was increased to approximately 1700 nm(aspect ratio ∼35). Serial addition of growth solution wasadopted to maintain the presence of gold, and 1D gold nano-rods/wires with a tunable size up to the order of micrometerswere successfully prepared. CTAB is necessary for controllingthe gold and silver nanoparticle shape, since it is highly water-soluble, has bromide counterions that can chemisorb on thosemetal surfaces, a sufficiently large headgroup to help direct whichface of the crystal grows, and a sufficiently long tail to make astable bilayer on the metal surface. Surface chemistry here iscritical for future applications, thus capping agent (CTAB) has tobe taken into account for further development.c. Sacrificial Template: Growth into Hollow Interior Outer

the Surface of Template. Over the past decade, the synthesisand characterization of nanomaterials with a hollow interior havereceived great attention for the development of nanoscience andnanotechnology due to their potential morphology-dependentapplication. A number of approaches for hollow inorganicnanomaterials have been developed recently; these includeOstwald ripening, Kirkendall effect, and sacrificial template forsynthesizing desired materials.111-115 The Kirkendall effect wasfound as a diffusion phenomenon in general metallurgy, whichmeans comparative diffusive migrations among different atomicspecies in metals and alloys under heating conditions. Theporosity may result from differential solid-state diffusion ratesof the reactants in an alloying or oxidation reaction. Smigelkasand Kirkendall reported themovement of the interface between adiffusion couple (copper and zinc), which result from thedifferent diffusion rates of these two species at an elevatedtemperature.116 This phenomenon, now called the KirkendallEffect, was the first experimental proof that atomic diffusionoccurs through vacancy exchange and not by the direct inter-change of atoms. The net directional flow of matter is balancedby an opposite flow of vacancies, which can condense into poresor annihilate at dislocations. Although void formation in alloysand solders may not be a desirable process for metallurgicalmanufacturing, the physical phenomenon may provide possibi-lities for fabrication of new nanomaterials with hollow interiorsconsidering the directional matter flow and consequential vacan-cy accumulation in Kirkendall type diffusion. In general, Ostwald

ripening commonly means the solution has a process that “thegrowth of larger crystals from those of smaller size which have ahigher solubility than the larger ones”.115 Because of the sizedifference of the forming crystals, concentrations of solutesacross the solution vary. This concentration gradient will even-tually eliminate crystallites of smaller sizes while the growth ofthe large ones proceeds. According to this nature of matterrelocation, this process can be used as a method for generation ofhollow interiors for nanomaterials if one can control the sizedistribution and aggregation patterns of as-formed primarycrystals. In the case of colloidal aggregates, some interior spacewould be eventually generated due to inhomogeneous size anddistribution of crystallites. In this process, large crystallites areessentially immobile while the smaller ones are undergoing massrelocation through dissolving and regrowing, which creates theinterior space within the original aggregates. This ripeningprocess has been recently employed to prepare a range ofdifferent hollow oxide nanostructures.111,114,117 For example,anatase TiO2 nanospheres with hollow interier were synthesizedfrom a solution route via Ostwald ripening.111 The startinganatase TiO2 nanocrystallites in spherical aggregates wereformed 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 hollownanostructures by consuming the more reactive component.The most important reaction in this method involves a sacrificialtemplate (more reactive component) and a reactant (inertcomponent). The replacement reaction between these twocomponents occurs upon the surface of template; the inert partis deposited on the surface of the template and inner part diffusesout through pin holes. The hollow shape thus formed dependsstrongly on the shape of the sacrificial template; the hollowproduct has approximately the samemorphology as the template.Gold and silver are the most widely used in operating as thesacrificial templating reaction because of their specific opticalproperties.51-58 Since the standard reduction potential of theAuCl4

-/Au pair (0.99 V vs standard hydrogen electrode, SHE)exceeds that of the Agþ/Ag pair (0.80 V vs SHE), silver isoxidized to Agþ when a silver sacrificial template and AuCl4

- are

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.)



mixed in an aqueous solution. This galvanic replacement processwas elucidated in relation to our recent fabrication of nanorattles,which comprised Au cores and Au shells, by reacting AucoreAgshellnanoparticles with HAuCl4 in an aqueous solution (Figure 8a).

118

The first step was the deposition of a uniform layer of pure silver onthe surface of Au nanoparticles. AucoreAgshell nanoparticles havebeen prepared by directly depositing Ag atoms on the surface ofAu nanoparticles using ascorbic acid as a reducing agent. Aunanoparticles were mixed directly with an aqueous solution ofsodium citrate and AgNO3. Silver ions were reduced by sodiumcitrate, yielding silver atoms, which nucleated and grew on thesurfaces of gold nanoparticles. The similarities between theatomic radii (Au = 1.44 Å; Ag = 1.45 Å) of silver and gold andbetween their lattice parameters (Au lattice constant a = 4.069 ÅJCPDS file no. 01-1172; Ag lattice constant a = 4.079 Å JCPDSfile no. 01-1164) cause the silver atoms to nucleate preferentiallyand grow on the surfaces of gold nanoparticles, rather thanhomogeneously nucleating and growing from silver nuclei. Thesecond step was the galvanic replacement reaction betweenAucoreAgshell colloids with an aqueous HAuCl4 solution at roomtemperature. This chemical reaction is given by 3Ag þ AuCl4

- f3Agþ þ 4Cl- þ Au. Galvanic replacement transformed aconformal coating of pure silver into a shell of gold. Thenanostructures that participate in each reaction step were fullycharacterized by TEM. As presented in Figure 8b, gold nano-particles as small as 8.1 nm were obtained. Figure 8c shows atypical TEM image of AucoreAgshell nanoparticles. When thesilver-coated nanoparticles underwent the galvanic replacementreaction with a solution of gold ions, the pure silver layers weretransformed into gold nanoshells. Figure 8d displays a typicalTEM image of the nanorattle. The image clearly indicated thatthe product was characterized by a rattle-like nanostructure.Notably, the formation of silver chloride should be consideredhere. Silver chloride, which is formed in the replacement reac-tion, is completely soluble in water at high temperature. Theformation of AgCl solid promoted the removal of Agþ ions andprevented the gold nanoshells from being contaminated by Ag.In the synthesis herein, the nanorattles still contained Ag and Clafter they were centrifuged numerous times to remove thesuspended AgCl solid.118 Therefore, much of the AgCl solidadhered to the surfaces of the nanorattles. When the reaction wasperformed under similar conditions but with the temperatureraised from 25 to 95 �C, the larger solubility product of AgClsolid caused it to be less contaminated. Accordingly, the shellcomponent of the nanorattles was controlled by varying thereaction temperature. The shell material and chemical reactivityof these newly synthesized nanorattles will be important in futureapplications. The sacrificial template method operated via galva-nic replacement is an effective method for preparing metallichollow nanostructures, because galvanic reaction can occurunder room temperature and be applied to various metalelements.

3. APPLICATIONS FOR SHAPED METALLICNANOMATERIALS

a. Hollow Spheres for Catalysis. The properties of noblemetal nanocrystals make them ideal materials for application incatalysis, where reaction yield and selectivity are dependent onthe nature of the cat alyst surface. Generally, catalytic perfor-mance of nanocrystals can be finely tuned either by theircomposition, which mediates electronic structure, or by their

shape, which determines surface atomic arrangement and co-ordination. Fundamental studies of single-crystal surfaces of bulkPt have shown that high-index planes generally exhibit muchhigher catalytic activity than that of the most common stableplanes, such as {111}, {100}, and even {110}, because the high-index planes have a high density of atomic steps, ledges, andkinks, which usually serve as active sites for breaking chemicalbonds.119 The bulk Pt(410) surface exhibits unusual activity forcatalytic decomposition of NO, a major pollutant of automobileexhaust.120 Tetrahexahedral Pt nanocrystals prepared by electro-chemical method are bounded by 24 facets of high-index planes∼{730} and vicinal planes such as {210} and {310}, which showenhanced catalytic activity in electro-oxidation of small organicfuels of formic acid and ethanol.121 Hollow nanostructures are aparticularly interesting class of materials that have unusualchemical and physical properties that are determined by theirshape and composition; they exhibit catalytic activities differentfrom their solid counterparts with the advantages of low density,saving of material, and high active sites. As a result, an effectiveand 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 modifiedgalvanic replacement reaction, and their use as electrocataly-sts.122 Various nature of catalyst are controlled through presentapproach, including morphology, composition, and crystallites,which may also provide various facets for specific catalysis. Thisreplacement scheme was carried out at room temperature,making the fabrication of such catalysts effortless. Figure 9apresents the complete procedure for the synthesis of porous Pthollow spheres. First, uniform silver nanospheres were synthe-sized. The second step was the galvanic replacement reactionbetween Ag nanoparticles with an aqueous H2PtCl6 solution atroom temperature, which is expressed as 4Agþ PtCl6

2-f Ptþ4AgCl þ 2 Cl-. Pure silver was converted into a shell of Ag-Ptalloy by galvanic replacement. Since the simultaneous formationof AgCl may disrupt the epitaxial deposition of gold atoms on thesurface of a silver template at room temperature, the presence ofAgCl roughened the surface of the hollow Pt shells, which wasexploited to create increase the surface area of catalyticreaction.118 The final step was the removal of the Ag and AgClfrom the templates by treating the Ag-Pt alloy shells withaqueous ammonia and HNO3 to generate nanopores on the

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



hollow Pt shells. Figure 9b shows a TEM image, in which thecenters of the spheres are brighter than their edges. The shells ofthe Pt hollow nanospheres seemed to be smooth and theirthickness was ∼4.5 nm. Figures 9c displays a TEM image ofthe hollow shells after AgCl and Ag had been removed. Theseimages clearly reveal that the products had a porous nanostruc-ture (indicated by arrows) with pore diameters of approximately0.91 nm. Notably, this entire nanostructure became segmentedby chemical etching. Most interestingly, the thickness of thehollow shell was specifying degree of significance lower afterchemical etching considerably reduced the thickness of thehollow shell, implying that the silver atoms had been removedfrom the hollow structures. This process yields Pt hollow nano-spheres with a high surface area (41 m2/g). Figure 10 plots thespecific activity in oxygen reduction reaction (ORR) of acommercial Pt catalyst, hollow Ag-Pt shells (18 m2/g), andhollow Pt shells with nanochannels (41 m2/g). The activity ofhollow Ag-Pt shell was much lower than that of the othermaterials because it had a smaller surface area. At potentials from0.9 to 0.8 V, the activity of the hollow Pt shell with nanochannelsexceeded that of the commercial catalyst, suggesting that theporous hollow shells indeed accelerated the reduction of oxygen.The ORR onset potentials of these two surfaces are similar. Twomajor factors are responsible for the higher improved chemicalperformance observed here: (i) nanochannels were formed uponchemical etching, which may have resulted in the formation of anactive site in the interior surface of the hollow shell; (ii) arelatively high density of defects, particularly vacancies, wereformed on the surface of the shell and may have promoted theformation of a product with a rougher surface. The nanochannelsmay have activated the inner surface and provided a route for thetransport of reactant and product. The incomplete shell of thehollow nanospheres may have provided an interior surface for thecatalytic reaction, and the higher surface area of the Pt nano-spheres may have increased catalytic activity. These metallichollow nanoparticles may be useful in industrial applications,including catalytic nanoreactors and related process.b. Nanorods for Antibody Sensing. Recently a great ad-

vancement was observed in utilizing metal nanoparticles, espe-cially gold, for biomedical applications, due to their uniqueshape/size-dependent properties, strong absorption/scatteringof light, stability, and nontoxic nature.2,123-128 Among all, gold

nanorods are found to be more popular and useful for potentialapplications such as biochemical sensing, biomedical diagnostics,and therapeutics due to possible tuning of their surface plasmonresonance (SPR) in the visible and near-infrared (NIR) region,which is the potential window of the electromagnetic spectrumfor in vivo applications. The strong light scattering properties ofgold nanorods have been applied mainly for optical microscopicimaging of cancer cells, and the absorption properties in the NIRregion causing a localized hypothermic effect have been utilizedfor therapeutic purposes.124,125,127 However, changes in localizedSPR due to alteration in the conditions of the surroundingenvironment of gold nanorods have been employed to analyzedifferent biorecognition events at the molecular level.129-131 Itwas observed that the longitudinal band is more sensitive to thechanges in the environment of the gold nanorods as compared tothe transverse band, making them ideal candidates for sensingand imaging applications.The goal of this part is to investigate the biosensing properties

of bioconjugated gold nanorods with a view to future biomedicalapplications. In this investigation, bioconjugated nontoxic goldnanorods were adopted as molecular probes for detecting goatIgG using the localized surface plasmon resonance method.132

The surfaces of the CTAB-stabilized gold nanorods were mod-ified by using poly(styrenesulfonate) (PSS) to reduce the toxicityof as-synthesized gold nanorods that are the result of an excess of

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.)

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



CTAB. Surface-modified gold nanorods were functionalizedusing anti-g-IgG antibodies, which were further used in the rapidand sensitive detection of g-IgG by localized surface plasmonabsorption. The gold nanorods were synthesized using a seed-mediated growth method.104 The as-prepared gold nanorodswith a mean aspect ratio of ∼3 were fairly uniform in shape andhighly dispersed in water without aggregation. Figure 11 presentsa typical procedure for preparing gold nanorods molecularprobes. The positively charged surfaces of the gold nanorodsbecame negatively charged upon exposure to PSS solution.Thereafter, the precipitate of the gold nanorods was redispersedin phosphate-buffered saline (PBS; pH = 7.4). Anionic polymerPSS coating on the gold nanorods, surrounded by positivelycharged CTAB molecules, not only reduces the cytotoxicity ofCTAB but stabilizes it against the attachment of goat anti-g-IgGto form gold nanorods molecular probes. PSS-capped goldnanorods were mixed with goat anti-g-IgG solution, with whichthey conjugated. They were then centrifuged and redispersedinto PBS solution to remove unbound antibodies. The resultinggold nanorod probes were incubated in a bovine serum albumin(BSA) blocking solution to prevent nonspecific binding and werethen utilized to detect the g-IgG.When the gold nanorod molecular probes bound to g-IgG,

aggregation of the nanorods, which were preferentially orientedin a lateral (side-to-side and/or end-to-end) manner, was drivenby a particular antibody-antigen binding process (Figure 12a).Therefore, the position, intensity, and shape of the SPR of thelongitudinal band depended on both the local refractive indexand the aggregation of the gold nanorods. The SPR bandresponded in a way that was sensitive to both changes in therefractive index, which were caused by molecular interactions inthe surroundings of the gold nanorods, and the aggregation ofgold nanorods, which was driven by biorecognition. Moreimportantly, PSS-capped gold nanorods had a larger surface area(because of their flat sides and tips) for the adsorption of proteinsthan did spherical gold nanoparticles with similar diameters.Furthermore, the aggregation of gold nanorods, which werepreferentially oriented in a lateral (side-to-side and/or end-to-end) fashion, by a specific antibody-antigen binding process wasobserved, causing a substantial wavelength shift, a reduction in

intensity, and a significant widening of the plasmon for rapid andsensitive detection. Figure 12b presents the absorption spectra ofas-prepared gold nanorods, PSS-coated gold nanorods, PSS-coated gold nanorods/goat anti-g-IgG conjugates, followinginteraction with BSA before exposure to g-IgG and after exposureto g-IgG. The SPR band of the as-prepared gold nanorods was at701 nm. When these gold nanorods were capped with a layer ofPSS, this band was broadened and red shifted to 716 nm,reflecting the variation in the local dielectric function of thePSS-coated GNRs. A red shift (719 nm) was observed uponexposure to excess goat anti-g-IgG solution for 1 h. However, thetransverse bands of the gold nanorods did not apparentlyundergo a wavelength shift, because the SPR longitudinal bandresponded more strongly than the SPR transverse band tobinding to the target biomolecules . Nonspecific binding of theuncovered gold nanorods surfaces during the immunoassay didnot occur, as evidenced by the absence of a significant change inthe wavelength when 50 mg/L BSA was used in PBS buffersolution for 30 min as the blocking solution, suggesting that thecoverage of the surface with goat anti-g-IgG reached saturation.Adding 1mg of g-IgG to 0.5mL of gold nanorodmolecular probesolution dramatically reduced the intensity and substantially redshifted the peak (to 763 nm). This large shift was associated withthe preferential assembly of gold nanorods, which is driven byside-to-side and/or end-to-end molecular binding. The side-to-side and side-to-end aggregation of gold nanorods is caused bythe target-specific binding events to an extent that increases withthe duration of molecular recognition of goat anti-g-IgG andg-IgG. Absorption measurements revealed a simple form ofmolecular sensing that is based on changes in the surroundingmedium that are associated with the binding of analyte moleculesto the gold nanorods, which causes a shift in the plasmonicextinction peak. Coating the surfaces of nanorods with PSSmarkedly increased cell viability and facilitated the intracellularuptake of the nanorods, suggesting their possible use in variousbiomedical applications. This rapid, sensitive, and label-freescheme of detecting molecular binding events using surface-modified gold nanorods may support a novel optical multiplexbiosensor platform and have broad potential applications inimmunoassay and disease diagnosis.

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



c. Multipod-Shaped Nanomaterials for PhotothermalTherapy. Multipod-shaped and branched gold nanostructureshave recently been described.104,105,133-135 These structures haveSPR properties that are similar to those of nanorods because oftheir multiple branches and multidirectional anisotropy.Branched nanoparticles have two SPR bands,104,134 the trans-verse band (which corresponds to the oscillation of electronsperpendicular to the axis of branches) and the longitudinal band(which corresponds to the oscillation of electrons along the longaxis of branches). The longitudinal band can be tuned fromvisible to near-infrared by increasing the aspect ratio of thebranches and the relative intensity can be tuned by controllingthe amount of branches per particle. The most widely usedmethod is directional growth in the presence of the commonsurfactant CTAB and silver ions (Agþ).104,105,136 The preferen-tial adsorption of CTAB and silver atoms on the various crystalfaces of gold inhibits growth perpendicular to them.104,137 Thegrowth mechanism of gold nanorods in the presence of CTABand the influence of various experimental parameters on thatmechanism has been extensively explored. However, on ourrecent study excess CTAB was found to be toxic and the surfacemodification of as-synthesized gold nanoparticles was required toimprove biocompatibility while retaining stable dispersion in anaqueous or buffer medium.132 Numerous surface-protectingreagents have been proposed, including polyethyleneglycol(PEG),138,139 polystyrene sulfonate (PSS),132,140 andchitosan.141 A negatively charged polymer such as PSS can beutilized to form a protective shell over the CTAB layer byelectrostatic interaction, but a double layer of PSS-CTAB isapplied to the surface of Au in bioanalysis to reduce the risk ofreleasing CTAB into the solution. The biopolymer chitosan hasattracted substantial attention in relation to drug delivery becauseof its high stability, low toxicity, simplicity of coating, andfunctionalization procedure. Pure chitosan nanoparticles arewidely used in drug delivery and cell imaging, and their cellularuptake has been repeatedly proven.142-144 Although the proper-ties of surface-modified gold nanoparticles must be understood,the stability of surface-capping layers and their effects followingin vivo administration for clinical purposes. The SPR response ofmultipod-shaped Au nanoparticles depends less on the spatialorientation of the nanoparticles than that of gold nanorods, asevidenced by the strong dependence of the photothermal effecton the light polarization.138 Additionally, the much larger surfacearea of multipod-shaped nanobranches improves the functiona-lization ratio in drug delivery and release applications.Spherical nanoparticles have been found to be the most

suitable for endocitosis through cell membranes whereas goldnanorodes are the most efficient light energy-to-heat convertersrelative to mass of gold compared to the Au nanoparticles withother shapes.145-147 The surface plasmon resonance of Aunanomaterials can be exploited to promote light-to-heat conver-sion.The photothermal effect of plasmonic Au nanoparticles in

living tissues has been widely investigated in the fields ofbiomedicine, drug delivery, and cancer science as having thepotential to trigger the precise drug release from the particlesurface into the medium or directly destroying cancer cells vialight-to-heat conversion. In our investigation, multipod-shapedAu nanoparticles are regarded as an effective medium in theconversion of light to heat owing to their strong SPR absorptionin the near-infrared region.104 To lower its cytotoxicity andextend bioapplication in vivo/in vitro, the CTAB surfactant thatis used to cap multipod-shaped Au nanoparticles is replaced withbiomolecular FITC-labeled chitosan. Chitosan is a highly bio-comparible/biodegradable biomolecular and natural materialthat found in various animals and plants. In particular, it is aprimary component in animal/plant shells. Figure 13 presents allof the processes associated with photothermal therapy to killcancer cells. The chitosan-modified multipod-shaped Au nano-particles are incubated with J5 cells (liver cancer) overnight toensure the internalization of nanoparticles by endocytosis. Near-infrared (NIR) light that is introduced into cells is absorbed andconverted to heat by the Au nanomaterials, and the process isenhanced by the SPR effect. The effect of heat on the cells isobserved only when Au nanomaterials are internalized, indicat-ing 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, capturedby the three-dimensional technique of confocal microscopy,which avoids the collection of surface fluorescence signals bythe detector. In Figure 14b,c, the green and blue fluorescenceclearly reveals the position of the FITC-labeled chitosan-Aumultipod-shaped nanoparticles and the nucleus stained withDAPI, respectively. However, in the control experiment thatinvolved the J5 cell that has not been treated with chitosanmultipod-shaped Au nanoparticles, no green fluorescence signalis obtained, but the blue fluorescence signal associated DAPI isobtained, indicating the all of the green fluorescence signal isfrom FITC labeled chitosan-Au multipod-shaped nanoparticles(as shown in Figure 14b).Merging of the bright (Figure 14a) andfluorescence images, as displayed in Figure 14c reveals the greenfluorescence signal near the nucleus of the J5 cell and overlaid bythe bright image, revealing that the nanoparticles had beensuccessfully internalized. The internalization of nanoparticlesdirectly affects the targeted J5 cells, improving efficiency ofphototherapy. The NIR laser with a long wavelength stronglypenetrates human skin, which effect is exploited in in vivophotothermal therapy. Accordingly, in the present investigationthe 800 nm NIR laser is applied as a source for the multipod-shaped Au nanoparticles with strong SPR absorption at 800 nmwavelength. After the internalization of chitosan-modified multi-pod-shaped Au nanoparticles in J5 cells, the cells were scannedseveral times with the NIR laser and impaged in situ. Tounderstand the change of temperature caused by exposure toNIR, the J5 cells were stained with Calcein AM, which was

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



adopted as an indicator of viability, since it only emits greenfluorescence in live cell cytoplasma. Restated, heating to a hightemperature induced the death of J5 cells, and the disappearanceof green fluorescence was observed. The series of fluorescenceimages in Figure 15 reveal the effect of photothermal therapy onthe nanoparticles that were treated J5 cells and exposure to an800 nm NIR laser. Numerous scans (1.5 s per scan) wereperformed to examine the evolution of J5 cell death. In thecontrol experiment that involved J5 cells that had not

internalized chitosan-modified multipod-shaped Au nanoparti-cles (Figure 15b), no cell death was observed between the initialscan and the 50th scan of the NIR laser, as revealed by themaintainance of green fluorescence of the initial scan. However,significant disappearance of green fluorescence after the 30thscan demonstrated the death of J5 cells by heat. However, by the50th scan, almost 70% of cells had been killed by heat. Hence, theinternalization of chitosan-modified Au multipod-shaped nano-particles can have an important role in the conversion of an NIR

Figure 14. (a) Bright and (b,c) fluorescence images of J5 cell treated with multibranched Au nanoparticless, captured by confocal microscopy. Greenand 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.



laser to heat by the SPR effect, which can be exploited in thedevelopment and improvement of local therapy, especially againstcancer cells that have low tolerance of high temperatures. Theresults indicate that the chitosan-modified Au multipod-shapednanoparticles have potential as a photothermal therapy agent, butin vivo thermal therapy still depends on the specific targeting ofcancer cells. The biomolecular modification of chitosan Au multi-pod-shaped nanoparticles must be performed to develop localphotothermal therapy against targeted cancer cells.

4. CONCLUDING REMARKS

This article presented general guiding principles for thesynthesis of well-defined shapes, including one-dimensionalnanowires (using a hard template), the spherical core-shellstructure, the nanostructure with interior space (by sacrificialreplacement), nanorods/wires with various aspect ratios, trian-gular nanoplates, and branched nanoparticless (using a softtemplate). The anodic alumina oxide template is often appliedin the preparation of one-dimensional nanomaterials by electro-chemical deposition, and especially in the preparation of transi-tion-metal nanowires that are difficult to prepare by other means.The use of soft templates reveals a versatile route for synthesizingmultishaped gold nanoparticles (such as spherical nanoparticles,bipyramids, nanorods, nanowires, T- and star-shaped nanopar-ticles, and triangular nanoplates) that can be tuned by controllinga few factors. The optical performance and surface chemistry canbe varied by controlling the shape of the gold nanostructures,which can thus be utilized in numerous fields. Hollow nanos-tructures are a particularly interesting class of materials that haveunusual chemical and physical characteristics that are determinedby their shape and composition. They have great potential in thedevelopment of novel and potentially useful sensing and drug-delivery applications. Transforming metal nanostructures intohollow structures improves their performance because doing soreduces their densities are and increases their surface areas abovethose of their solid counterparts. The galvanic replacementreaction has been exploited as a powerful method of preparinghollow metal nanostructures, which have various interestingoptical characteristics. The incomplete shell of the hollow nano-spheres may provide interior surface for the catalytic reaction,and the high surface area of Pt nanospheres is associated withtheir high catalytic activity. These metallic hollow nanoparticlesmay be useful in industrial applications, including catalyticnanoreactors and related fields. Measurements of absorptionby a gold nanostructure revealed a simple form of molecularsensing of changes in the surrounding medium associated withthe binding of analyte molecules to the gold nanorods, whichcauses a shift in the plasmonic extinction peak. This rapid,sensitive, and label-free scheme for detecting molecular bindingevents using gold nanorodsmay support a novel optical multiplexbiosensor platform in immunoassay and disease diagnosis. Thesurfaces of as-prepared Au nanoparticles were modified usingchitosan to improve biocompatibility and eliminate cytotoxicCTAB. A photothermal therapy experiment on J5 cells using apulsed NIR laser source indicated that branched Au nanoparti-cles can effectively transform NIR light into heat. NIR laserirradiation with a wavelength of 800 nm and a power density of60 mW cm-2 sufficed to cause rapid J5 cell hyperthermia when 4μg/μL Au nanobranches was loaded in the incubation solution,revealing that chitosan-capped Au NPs have great potential asthermal therapeutic applications.

Although this feature article considers only multiformmetallicnanomaterials, in view of its extensive applicability it is of greatpotential to the preparation of numerous other optical andotherwise functional materials in various forms by methods.Control of surface chemistry is crucial to such applications assurface catalysis and biomedical diagnostics or photothermaltherapy. The chemical and physical characteristics of noblemetallic nanoparticles make them variously useful in photonics,chemical, biochemical sensing, and imaging applications. Thisarticle focused on several possibilities, but as the ability tomanufacture 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 atNational Taiwan University. He received his M.S. degree in 2004and his Ph.D. in Chemistry from National Taiwan University in2008, where he worked on the synthesis of nanocrystals forparticular applications. His current research interests includesynthesis of nanomaterials, metallic nanocrystals for bioapplica-tions, and semiconductor nanomaterials for solar energyconversion.

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



the National Tsing Hua University (Taiwan) in 1983. Heobtained two Ph.D. degrees in chemistry, one from NationalTsing Hua University in 1990 and the other from the Universityof Cambridge in 1992. He worked at Materials ResearchLaboratories at the Industrial Technology Research Institutefrom 1983 to 1985. He was an Associate Professor at theDepartment of Chemistry of National Taiwan University from1995 to 1999, when he was promoted to a professorship in 1999.Professor Liu’s research concern is in the field the MaterialsChemistry. He is the author and coauthor of more than 350publications in scientific international journals. He has also beengranted more than 80 patents.

’ACKNOWLEDGMENT

The authors would like to thank the National Science Councilof the Republic of China, Taiwan, for financially supporting thisresearch under Contract Nos. NSC 97-2113-M-002-012-MY3and NSC 99-2120-M-002-012.

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