Properties and Applications of Colloidal Non Spherical

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Properties and Applications of Colloidal NonsphericalNoble Metal Nanoparticles

By Tapan K. Sau,* Andrey L. Rogach,* Frank Jackel, Thomas A. Klar, and

Jochen Feldmann

Nanoparticles of noble metals belong to the most extensively studied colloidal

systems in the field of nanoscience and nanotechnology. Due to continuing

progress in the synthesis of nanoparticles with controlled morphologies, the

exploration of unique morphology-dependent properties has gained

momentum. Anisotropic features in nonspherical nanoparticles make them

ideal candidates for enhanced chemical, catalytic, and local field related

applications. Nonspherical plasmon resonant nanoparticles offer favorable

properties for their use as analytical tools, or as diagnostic and therapeutic

agents. This Review highlights morphology-dependent properties of non-

spherical noble metal nanoparticles with a focus on localized surface plasmon

resonance and local field enhancement, as well as their applications in various

fields including Raman spectroscopy, fluorescence enhancement, analytics

and sensing, photothermal therapy, (bio-)diagnostics, and imaging.

1. Introduction

Metal nanoparticles have a long history in terms of preparation,characterization, and applications. Understanding the propertiesof noble metal nanoparticles (NMNPs) and exploring theirapplication potential are two major driving forces behind the

[*] Prof. T. K. Sau,[+] Prof. A. L. Rogach,[++] Dr. F. Jackel, Prof. T. A. Klar,Prof. J. FeldmannCenter for NanoScience (CeNS)Ludwig-Maximilians-Universitat MunchenSchellingstr. 4, 80799 Munich (Germany)E-mail: [email protected]@cityu.edu.hk

Prof. T. K. Sau,[+] Prof. A. L. Rogach,[++] Dr. F. Jackel, Prof. J. FeldmannPhotonics and Optoelectronics Group, Department of PhysicsLudwig-Maximilians-Universitat MunchenAmalienstr. 54, 80799 Munich (Germany)

Prof. T. A. KlarInstitute of Physics, Technische Universitat IlmenauUnterporlitzer Str. 38, 98693 Ilmenau (Germany)

Prof. T. A. KlarInstitute of Micro- and Nanotechnologies, Technische UniversitatIlmenauUnterporlitzer Str. 38, 98693 Ilmenau (Germany)

[+] Present address: International Institute of Information Technology,Hyderabad 500 032, India

[++] Present address: Department of Physics and Materials Science, CityUniversity of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong

DOI: 10.1002/adma.200902557

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synthesis of a large variety of nanomaterials.Many properties of nanoparticles arise fromtheir large surface-area-to-volume ratio andthe spatial confinement of electrons, pho-nons, and electric fields in and around theseparticles. The large surface-area-to-volumeratio in nanoparticles may cause deviationsfrom the usual bulk atomic arrangements.The surface of a nanoparticle may beunstable due to the high surface energyand large surface curvature. A uniquecharacteristic of nanoparticle surfaces isthat they bear a high fraction of edgelike andcornerlike curved regions.[1] Edges andcorners have more coordinatively unsatu-rated atoms, i.e., dangling bonds, than flatsurfaces. Large fractions of under-coordinated surface-, corner- and

edge-atoms in a nanoparticle can affect its chemical reactivityand surface bonding properties. The electron confinement effectin a nanoparticle modifies its spectral properties via shifting ofquantum levels and change in transition probabilities.[2] Someproperties, such as particle–particle or particle–environmentinteractions, are affected by the large surface area:volume ratio aswell as confinement phenomena. Researchers have started todevelop an understanding of how the shape of a nanoparticleinfluences its properties. Nonspherical nanoparticles withmetastable structures are essentially in kinetically frozen states,[3]

and the geometric confinement via morphology control results infurther modifications of the internal structures, surface char-acteristics, and orientational confinement.

Due to continued progress in the synthesis of nanoparticleswith controlled morphologies over the last decades, exploration ofunique morphology-dependent properties and their applicationshave gained momentum. In this review, we discuss variousmorphology-dependent physical and chemical properties ofcolloidal nonspherical NMNPs and summarize some of theirmost important applications. The review is organized as follows:after a brief discussion of the self-assembly of nonsphericalNMNPs in Section 2, we review their chemical and catalyticproperties in Sections 3 and 4. In Section 5, localized surfaceplasmon resonance (LSPR) properties of rod- and prism-shaped,polyhedral, and branched Ag and Au nanoparticles, includinginvestigations of single-particle LSPR and local refractive indexsensitivity of the LSPR, are summarized. Effects of particlemorphology on local field enhancement, surface-enhancedRaman scattering (SERS), and fluorescence of fluorophores in

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Tapan K. Sau received hisPh.D. (1998) in chemistry fromthe Indian Institute of

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the close vicinity of NMNPs are reviewed in Sections 6, 7, and 8,respectively. We discuss several examples of applications ofnonspherical NMNPs in Section 9 and present conclusions andan outlook in Section 10.

Technology-Kharagpur. Heworked as a postdoctoralfellow at the University ofSouth Caroline-Columbia andClarkson University, NY, and asan assistant professor atPanjab University, Chandigarh,India. He was an Alexander-von-Humboldt Research Fellow

(2007–2009) at the Ludwig-Maximilians-UniversitatMunchen. Presently, he is appointed as an associateprofessor at the International Institute of InformationTechnology, Hyderabad, India.

Andrey L. Rogach received hisPh.D. (1995) in chemistry fromthe Belarusian State University,and completed his Habilitationin experimental physics (2009)at the Ludwig-Maximilians-Universitat Munchen. Afterseveral research positions atthe University of Hamburg(1995–2002), he was a leadstaff scientist at the PhotonicsandOptoelectronics group of theLudwig-Maximilians-Universitat

Munchen (2002–2009). Since 2008, he has been anadjunct professor at the Center for Research on AdaptiveNanostructures and Nanodevices (CRANN) in Dublin,Ireland. Presently, he is appointed as professor at the CityUniversity of Hong Kong.

2. Self-Assembly

The term self-assembly is used to describe the spontaneousformation of highly ordered assemblies or patterns fromdisordered components by noncovalent interactions. Duringthe self-assembly, the system minimizes its free energy therebyevolving towards equilibrium. It balances attractive and repulsivecolloidal and intermolecular forces that include van der Waalsinteractions, electrostatic interactions, hydrophobic/hydrophilicinteractions, surface tension, capillary forces, steric forces, andhydrogen bonds such as the hybridization of DNA.[4–14]

Self-assembly of nanoparticles into long-range ordered super-lattices requires narrow size distribution and uniform shape ofthe particles.[15–17] Therefore, particle morphology is an impor-tant factor in governing the geometrical packing in organizedstructures.[5,6,12,13,16,18] For example, Demortiere et al. observeddifferent nucleation and growth kinetics for superlattices oftruncated and perfect (i.e., sharp corners and edges) Ptnanocubes.[16] Self-assembly of perfect nanocubes generated aprimitive cubic lattice via homogeneous nucleation, whereastruncated nanocubes formed a face-centered cubic (fcc) lattice viaa heterogeneous nucleation process (Fig. 1). This was attributedto small differences in the interparticle attraction forces betweentwo different types of nanocubes during the solvent evaporation.Particle morphology will thus influence the collective propertiesof the assembly structure through various kinds of translationaland orientational ordering, and the interaction and coupling ofthe individual nanoparticle building blocks. Assembling, pattern-ing, and integration of nanoparticles in functional and orderednetworks (on suitable surfaces) is of great importance for thefabrication of efficient electronic, photonic, or sensor devices.[19]

3. Chemical Reactivities

Researchers have recently started to explore the effects of metalnanoparticle morphology on chemical reactivity. Nanoparticleshave several inherent features that change their chemistrycompared to their bulk counterparts or constituent atoms andmolecules, since adsorption and reactivity are highly structure-sensitive properties. The large surface of a nanoparticle may bestructurally and compositionally different from the bulk due tosurface relaxation and reconstruction, the presence of adsorbedlayers of reaction by-products and stabilizing molecules, etc.[20,21]

Additionally, the surface structure changes as a function of size,shape, and number of components of a particle.[3,22] The largeparticle surface area results in increased interaction with theirenvironment. These effects may lead to a radical alteration inchemical reactivity.

Long ago, Jang et al. pointed out that the surface geometryinfluences the photochemical reaction of 1,10-phenanthrolineadsorbed on Ag nanoparticles.[23] Anisotropic chemical reactiv-ities of spheroidal Au nanoparticles were reported during cyanide

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dissolution and reaction with persulfate and were attributed to thestructural metastability of the spheroids.[24–26] It has beenreported that chemical reactions started at ridges, apexes, orparticular facets during galvanic replacement reactions betweensilver nanoparticles and HAuCl4, depending on the morphologyof the template nanoparticles and reaction conditions.[27,28] Brusand co-workers reported an interesting physico-chemical con-version of sodium-citrate-stabilized colloidal Ag nanocrystals tonanoprisms.[29] According to them, irreversible ‘‘hot hole’’photo-oxidation of citrate anions adsorbed on Ag nanoparticlesbuilds up a photovoltage under visible excitation. This in turncauses the photochemical conversion of spherical 8 nm Ag seedsinto 70 nm single-crystal disk prisms under light irradiation.Recently, Tsung et al. reported selective shortening by mildoxidative dissolution of single-crystalline gold nanorods preparedusing the silver-ion-assisted seed-mediated method.[30] Theoxidation was carried out by bubbling O2 in the presence ofHCl and a high concentration of cetyltrimethylammoniumbromide (CTAB) maintained at certain temperatures.

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Figure 1. Self-assembly of perfect and truncated cubic Pt nanoparticlesshowing the effects of particle morphology on self-assembly. A) TEM imageand B) scheme of self-assembled cubic superlattices formed by perfectnanocubes. C) TEM image and D) scheme of self-assembled fcc super-lattices formed by truncated nanocubes. Reproduced with permission fromref. [16]. Copyright 2008 American Chemical Society.

Figure 2. Oxidation of Au nanorods by O2 in 1M HCl and concentratedcetyltrimethylammonium bromide (CTAB) at 70 8C. a) UV–vis spectraacquired at every 1min interval during oxidation reaction of nanorods.At the beginning of the reaction, the spectra exhibit two extinction peaks, ahigher-energy transverse plasmon band (TPB) and a lower-energy longi-tudinal plasmon band (LPB), characteristic of Au nanorod suspension. TheTPB stays at �520 nm and decreases in intensity while the LPB blue-shiftsand decreases in intensity with time. Upon further oxidation, the LPBdisappears, indicating the conversion of nanorods into nanospheres.Finally, the TPB extinction peak at �520 nm vanishes, suggesting theoxidation of Au nanospheres into ionic Au species. b–d) Typical TEMimages of Au nanoparticles formed at various stages of Au nanorodoxidation. TEM images of as-synthesized Au nanorods with longitudinalplasmon wavelength (LPW)¼ 785 nm (b), of shortened Au nanorods with

Transmission electron microscopy (TEM) studies showed that Aunanorods decreased in length whereas the diameter remainedalmost constant during oxidation (Fig. 2). Upon further oxidation,Au nanospheres form, which ultimately dissolve into ionic goldspecies. This implies that the oxidation starts at the ends of the Aunanorods. The nanorods stay single-crystalline during oxidation,suggesting that twinning is not involved in the oxidativeshortening process. This approach allows post-synthetic controlof the aspect ratio of Au nanorods.

LPW¼ 595 nm (c), and of final oxidation products showing spherical Aunanoparticles (d). e) The changes of Au nanorod lengths (~) anddiameters (!) versus LPWs. f) The relationship between LPWs and aspectratios. The line is a linear fit. Error bars in (e) and (f) represent standarddeviations. Reproduced with permission from ref. [30]. Copyright 2006American Chemical Society.

4. Catalysis

The use of palladium and platinum particles as catalysts innumerous chemical reactions is well known.[31] Due to the finelydispersed states of nanoscale systems, one can obtain largesurface areas for a given quantity of materials. Nonsphericalparticles provide ample corners, vertices, defects, kinks, andsteps.[1] Exposure of different crystallographic facets, togetherwith the increased number of edges, corners, and faces, is ofcritical importance in controlling the catalytic activity andselectivity of metal nanoparticles. Therefore, nanoparticles ofdifferent sizes and shapes are highly desirable as catalysts in fuelcells, waste reduction, bioprocessing, and the chemical industry.

Effects of NMNP size on the catalytic activity are welldocumented.[31–34] On the contrary, knowledge about theinfluence of metallic nanoparticle shapes on the catalytic activityhas started to develop only recently. Studies on the catalyticactivities of platinum nanoparticles showed distinctly differentcatalytic activities for different morphologies.[35–37] Fukuoka et al.

Adv. Mater. 2010, 22, 1805–1825 � 2010 WILEY-VCH Verlag G

compared the catalytic properties of spherical Pt nanoparticlesand Pt nanowires formed inmesopores of FSM-16 (amesoporoussilica) respectively.[38] Pt nanowires showed substantially higheractivity in the hydrogenolysis of butane than spherical particles, inspite of a smaller total surface area of Pt nanowires. This wasattributed to the presence of more electron-deficient sites orpreferential exposure of {110} planes. The morphology of Ptmetal nanoparticles significantly affected the catalytic activity interms of product selectivity too. Pt-Rh nanowires that weresynthesized in a similar way by co-impregnating RhCl3 withH2PtCl6 also showed similar activities. Telkar et al. observed thatcubic Pd nanoparticles had higher turnover frequenciescompared to those of spherical particles for the hydrogenationof butyne-1,4-diol and of styrene oxide.[39] Further, cubic particles

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Figure 3. Effects of particlemorphology on the catalytic properties of Pt nanoparticles for theelectron transfer reaction between hexacyanoferrate(III) ions and thiosulfate ions. TEMimages of dominantly tetrahedral (a), cubic (b), and near spherical (c) Pt nanoparticles usedas catalysts and the corresponding activation energies (Ea) are given in the top panel. d) Plotof the average rate constant versus surface atom fraction on edges and corners of thetetrahedral, cubic, and near spherical Pt nanoparticles. The rate constant increases expo-nentially as the surface atom fraction on corners and edges increases. e) Plot of ln A versus Eaof the Arrhenius equation demonstrates that as the activation energy increases, thepre-exponential factor (A) also increases (compensation effect) in Pt nanoparticle catalysis.The activation energies and pre-exponential factors are those obtained when using thetetrahedral, cubic, and near spherical Pt nanoparticles to catalyze the electron-transferreaction in the first 40min. Reproduced with permission from ref. [43]. Copyright 2004American Chemical Society.

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gave higher selectivity than spherical particles forthe hydrogenation of butyne-1,4-diol intobut-2-ene-1,4-diol. Xu et al. compared the catalyticactivity of truncated triangular silver nanoparticleswith that of cubic and near-spherical silvernanoparticles in the oxidation of styrene incolloidal solution.[40] It was found that the rateof the reaction over the nanocubes was more than14 times higher than that on nanoplates and fourtimes higher than that on near-spherical nano-particles. This has been ascribed to the differentcrystal faces of the silver nanoparticles. Chimentaoet al. prepared silver nanoparticles of differentmorphologies by using the polyol process andobserved that catalytic activity and selectivity forthe selective oxidation of styrene were stronglymorphology-dependent.[41,42] El-Sayed and co-workers have investigated catalytic properties ofa number of particle morphologies.[43–46] From acomparative study of tetrahedral, spherical, andcubic shaped platinum nanoparticles catalyzingthe electron-transfer reaction between hexa-cyanoferrate(III) ions and thiosulfate ions, theseauthors reported a catalytic activity that followedthe order: cubes< spheres< tetrahedral, at theearly stages of the reaction (Fig. 3). The kineticparameters were found to correlate with thecalculated fraction of surface atoms located onthe corners and edges in each size and shape. Thehigher catalytic activity of the tetrahedral nano-particles is believed to arise from their exposed(111) facets and higher fraction of corner and edgesites. The lower catalytic activity of cubic nano-particles can be ascribed to their (100) facets and avery small fraction of surface atoms on edges and
corners. The spherical nanoparticles, which have a combinationofmany (111) and (100) facets withmany edges at their interfaces,show an activity intermediate to that of the tetrahedral and cubicnanoparticles. However, during the course of the reaction,distortions occur at the corners and edges of the tetrahedral andcubic types of nanoparticles, causing them to lose some of theircatalytic activities.Balint et al. have also observed a conversion ofthe low-index facets of the cubic Pt nanocrystals to higher-indexplanes during the reduction of NOwith C3H6, causing substantialchanges in the catalytic activity and selectivity to reactionproducts.[47] Tsung et al. reported that for pyrrole hydrogenation,Pt nanocubes enhanced ring-opening ability and thus showed ahigher selectivity to n-butylamine as compared to nanopolyhe-dra.[48] However, ethylene hydrogenation rates were independentof both size and shape of Pt nanocrystals.
Recently, researchers have been able to synthesize particleswith even more complex morphologies and have examined theircatalytic properties. For example, Mahmoud et al. prepared multi-armed Pt nanostars from tetrahedral seed nanoparticles andobserved that Pt nanostars were catalytically more active than thetetrahedral Pt nanoparticles for the reduction reaction offerrricyanide by thiosulfate.[49] This was attributed to the presenceof multiple arms with more edges and corners as well as thepresence of high-index sites on the nanostar.[49] Platinum


nanoparticles of unusual tetrahexahedral shape prepared by anelectrochemical method showed catalytic activity superior to thatof the spherical Pt nanoparticles for the oxidation of small organicmolecules such as formic acid and ethanol.[36] Lee et al. reported asynthesis of binary Pt/Pd nanoparticles by localized overgrowthof Pd on cubic Pt seeds and investigated electrocatalytic formicacid oxidation.[50] The binary Pt/Pd nanoparticles exhibited muchless self-poisoning and a lower activation energy relative to Ptnanocubes. Recently, Novo et al. studied single Au nanoparticlecatalyzed reactions by following the change in optical properties ofsingle Au nanoparticles via dark-field microscopy.[51] This dark-fieldmicroscopy-based method may allow one to study the effects ofdifferent crystal geometries and crystal facets of plasmon resonantnanoparticles on the rates of catalysis.It appears that NMNPs withcontrolledmorphologies have outstanding potential as catalysts andwill continue to enrich the area of nanocatalysis.

5. Localized Surface Plasmon Resonance

Light in the UV–vis–near IR (NIR) range impinging on aninterface between a metal and a dielectric excites, if fulfilling allboundary conditions, collective oscillations of the conductionelectrons, so-called surface plasmon polaritons or short surface


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plasmons. Associated with the excitation of surface plasmons arestrongly enhanced and highly localized electromagnetic fields(i.e., local field enhancement). Surface plasmon can either bepropagating, for instance at planar bulk metal surfaces, orlocalized as in the case of NMNPs. In both cases, theelectromagnetic field is spatially confined at the interface; i.e.,it can be described by evanescent waves in the directionperpendicular to the interface. The optical properties of colloidalNMNPs in the UV–vis–NIR spectral range aremainly determinedby localized surface plasmons that give rise to so-called localizedsurface plasmon resonances—LSPRs observable in their extinc-tion, i.e. absorption and scattering, spectra.[2,52,53] The electro-magnetic fields are not evenly distributed around nonsphericalNMNPs, which gives rise to shape-dependent LSPR spectra. Agand Au nanoparticles are of particular interest in the context ofLSPR.

The optical features of the LSPR (e.g., peak absorption,linewidth) depend on the size, shape, composition of the NMNP,its surface charge, surface-adsorbed species, interparticle inter-actions, and the refractive index of the surrounding medium.[54]

LSPR is particularly pronounced if the imaginary part of thedielectric constant is small and the real part of the dielectricconstant equals twice the negative of the dielectric constant of thesurroundings.[55,56] Silver and gold fulfill these criteria and arecommonly used as plasmonic metals. On the other hand,colloidal platinum or palladium exhibit only broad absorptioncontinua extending throughout the near UV and visible range.

The effect of deviation from spherical geometry on theproperties of NMNPs is well documented through their opticalproperty studies. For nonspherical nanoparticles, such as rods,disks, and triangular prisms, the LSPRs are typically split intodistinctive dipole and quadrupole plasmon modes.[57–60] Opticalproperties of NMNPs can be theoretically predicted for variousshapes. The simplest form of a nonspherical shape is a spheroid.For spheroids, a classical quasistatic approach serves well todescribe the spectral position, width, and strength of the dipolarplasmon resonance.[55,59] Based on classical approaches, for ametal nanoparticle sufficiently small compared to the wavelengthof light l, there is a resonance condition associated with each axisj of a spheroid:

" ¼ "m 1� 1

Lj

� �(1)

where e is the complex wavelength-dependent dielectric function

of the metal, Lj is the geometrical depolarization factor along the

axis j, and em is the dielectric function of the surrounding

medium.[54,60] L1 ¼ L2 ¼ L3 ¼ 1=3 for spherical particles. For

prolate spheroidal particles (L1¼ L2< L3), the dipole resonance

splits into two absorption bands—longitudinal and transverse

modes—where the induced dipole oscillates along and perpen-

dicular to the long axis of the spheroid, respectively.[61] The

longitudinal resonance band shifts towards longer wavelengths

(red-shift) and increases in absorption cross-section as the aspect

ratio of the spheroid increases, whereas the transverse resonance

band remains near the wavelengths of the spherical particle

absorption band.[55,61,62] This example of the simplest nonspherical


shape demonstrates that nonspherical nanoparticles will exhibit

multiple optical extinction bands due to their anisotropy. For

particles with shapes more complex than spherical or spheroid,

numerical methods, such as the discrete dipole approximation

(DDA), the T-matrix method, finite-difference time-domain

(FDTD) simulations, finite element calculations, the modified

long wavelength approximation (MLWA), the multiple multipole

method, and spectral representation, are used for the calculations

of spectral characteristics.[53,58,63–73] Kuwata et al. have suggested

an empirical extension of the quasistatic (analytical) approach to

particles of arbitrary shapes, which can quantitatively predict

resonant light scattering from metal nanoparticles of arbitrary

shape, whose sizes are too large for the Rayleigh approximation to

be applicable.[74]

5.1. LSPR of Rod-Shaped Nanoparticles

Nonspherical NMNPs offer the possibility of tuning the opticalproperties over a broad spectral range. LSPR of gold nanorods areone of the most widely studied topics in the field of nanoparticleoptical properties. Short gold nanorods exhibit two bands inthe vis–NIR spectral range. Based on theoretical studies andoptical polarization measurements, the band near 530 nm hasbeen assigned to a transverse LSPR, which is polarized across(corresponding to electron oscillation perpendicular to) the longaxis of the nanorod, and the other one, appearing at a longerwavelength, has been assigned to a longitudinal LSPR mode,which is polarized along (parallel to) the long axis. Thelongitudinal LSPR peak is very sensitive to the aspect ratio(length/diameter) of the nanorod and shifts to longer wavelength(red-shift) with increasing aspect ratio.[62,75–77] From DDAsimulations, Brioude et al. found that thinner nanorods givelarger lmax shifts than thicker ones for the same difference intheir rod lengths.[77] The morphological anisotropy of goldnanorods can influence the width of the absorption band.[78] Huet al. synthesized tadpole-shaped gold nanoparticles, showing abroad peak at �583 nm with a full width at half-maximum(FWHM) of �150 nm.[79] These authors suggest that the electronoscillation corresponding to the plasmon absorption along thelong axis is retarded on a reflective path. This is attributed to alarge disparity in size from the tip of the head through the tail ofthe tadpole. The tadpole-shaped nanoparticles have yet anotherinteresting property: the tails of the tadpoles contain a highernegative charge than their heads unlike the usual case of uniformcharge distribution on the surfaces of colloidal nanoparticles, asdemonstrated by electrophoresis experiments.

For small metal particles with dimensions far less than thewavelength, the conduction electrons are all excited in-phase withthe incident electromagnetic field, yielding a dipolar oscillation.However, in larger particles, the field across the particle becomesnonuniform and multipolar excitations, such as the quadrupolarand octupolar, can be observed. This gives rise to multiplepeaks in the optical spectrum. Such multipole resonanceshave been detected experimentally and theoretically from goldnanorods/nanowires and nanoshells, and silver and goldnanoprisms.[65,72,73,80–85] The dipolar LSPR is always situated


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at longer wavelengths with respect to the multipolar ones and isred-shifted by the electric field generated by higher multipolarcharge oscillations.[52] In the case of spherical NMNPs, the LSPRcan be tuned by increasing the particle size. However, the dipolarLSPR band becomes significantly broadened due to radiationdamping.[86] The LSPR can be tuned more elegantly withoutsacrificing the linewidth of the resonance through changing thenanoparticle geometry.

Figure 4. Optical extinction spectra of polyhedral silver nanoparticles.Symmetric truncation is performed on a cube-shaped nanoparticle (withsix faces) to create various truncated nanocubes (TCs, with 14 faces), thenicosahedron (Ih, with 20 faces) and finally spherical shape (with a qua-si-infinite number of faces). The degree of truncation (r) in TCs is shown inparentheses. TCs are obtained by truncating the eight corners of the cubeby r � l, where l is the length of the cube’s side and 0< r� 1/2. Acuboctahedron (CO) is obtained for r¼ 1/2. The optical response below325 nm is independent of the particle morphology. Reproduced withpermission from ref. [52]. Copyright 2007 American Chemical Society.

5.2. LSPR of Nanoprisms

Metal nanoprisms display a range of optical features dependingon their geometry, i.e., lateral size, thickness, and degree ofcorner truncation.[65] The optical spectra of silver nanoprismsshow two in-plane resonance bands (one each due to dipole andquadrupole oscillations): an out-of-plane quadrupole resonanceband and a small shoulder due to out-of-plane dipole resonance.Their optical spectral positions show high sensitivity towards thephysical dimension and sharpness of the tips and the change inrefractive index of the surrounding medium, and therefore,nanoprisms can find applications in sensing.[65,85,87–91] Germainet al. compared the experimental absorption spectra of silvernanodisks of different sizes with those simulated using the DDAmethod.[70] For smaller nanodisk sizes, simulated spectra ofspheroidally shaped particles closely relate to the experimentalspectra. That is, the specific shape of nanodisks can be neglectedfor smaller nanodisk sizes. However, for larger nanodisks, onehas to consider the precise geometries represented by snip andaspect ratio parameters of the disks for good agreement betweenexperiments and simulations. Aherne et al. demonstrated that themain LSPR (in-plane dipole) increasingly red-shifts by increasingin the silver nanoprism edge length.[92] Furthermore, the width ofthemain LSPR band narrows with increasing nanoprism volume,as the resonance energy decreases.[87,92,93] This has been attributedto a smaller nonradiative plasmon damping with decreasingplasmon resonance energy overcompensating the increasedradiation damping at larger nanoparticle volume.

5.3. LSPR of Polyhedral Nanoparticles

Noguez and co-workers have theoretically studied the influence ofmorphology on the LSPR of metal nanoparticles.[52,94] Forpolyhedral (cube, octahedra, cuboctahedra, icosahedra, decahe-dra, with different degrees of truncation) silver nanoparticles, thenumber of LSPR peaks (dipolar and quadrupolar) decreased andthe main LSPR peak position blue-shifted as the number of facesincreased or the nanoparticles became more symmetric. This isexemplified in Figure 4. It shows how the optical extinctionspectra of silver nanoparticles change, when symmetric trunca-tions are performed on a cube to create various truncatednanocubes, then an icosahedrons, and finally a spherical shapewith a quasi-infinite number of faces. A dramatic change in LSPRfeature even for small truncations clearly shows the sensitivenature of LSPR to particle morphology. These authors have alsoestablished similar trends for LSPRs in terms of the faces,vertices, and truncations of other morphologies, such as regular,rounded, truncated (e.g., Marks-type and star-shaped) decahedral


or pentagonal bipyramid-shaped nanoparticles.[52,94,95] Alekseevaet al. demonstrated that the cubic gold nanoparticles have anextinction spectrum peaking at a wavelength of 570 nm.[96] Jianget al. showed that a �512 nm peak arises partially due to thedipolar resonance of underdeveloped nanocubes, and the otherpeak around 570 nm corresponds to that of well-facetedcubes.[96,97] McLellan et al. reported that the truncation orrounding of the corners and edges of cubes lead to a sharpeningas well as blue-shifting for the LSPR bands.[98]

In addition to geometries, such as rods, prisms, and polyhedradiscussed above, LSPR of nanoparticles with more intricatemorphologies (e.g., nanocage, branched, multipod, star-shaped,lumpy) have also been explored. Au–Ag nanocages andnanoboxes can be prepared by galvanic replacement in Agnanocubes.[99,100] Chen et al. have shown that the extinctionspectra of the Au–Ag nanoboxes can be tuned from the visible tothe NIR region by controlling the molar ratio of Ag to HAuCl4 inthe galvanic replacement reaction.[99] However, the asymmetricshapes and varying tip geometries of branched nanoparticleensembles sometimes give rise to featureless broad LSPR.[101]

Recently, Nehl et al., Kumar et al. and Khoury and Vo-Dinhdemonstrated that Au nanostars/multipods show well-definedLSPR extinction spectra comprising a short and a longwavelength plasmon band, with the latter becoming increasinglybroad and red-shifted with enlarging nanostars.[102–105] Kumaret al. applied the boundary element method (BEM) to ageometrical model of the nanostar consisting of a central spherewith either one or two (located at opposite sides of the sphere)pseudoconical caps in order to quantify the precise opticalresponse of these complex nanoparticles.[102] Their calculationsdemonstrated that plasmon oscillations associated with the tipsgive rise to the main plasmon band and dominate the overalloptical response. The number of tips is of minor importance for


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Figure 5. TEM images of an ensemble of nanorods (a) and a singlenanosphere (b). c) Light-scattering spectra from a gold nanorod (lightpolarized along the long rod axis) and a 60 nm gold nanosphere measuredunder identical conditions. The resonance energies Eres and linewidths G

are indicated. d) Polar plot of the intensity from the long-axis plasmonresonance of a nanorod as a function of polarization angle of the excitinglight (circles: experimental data; line: dipole characteristics). e) TEM imageof a representative gold nanostar, and f) polarization-dependent white lightRayleigh scattering spectra of an individual gold nanostar (208 steps inpolarization, and spectra vertically offset for clarity). a–d) Reproduced withpermission from ref. [115]. Copyright 2002 The American Physical Society.e–f) Reproduced with permission from ref. [107]. Copyright 2009 AmericanInstitute of Physics.

5.4. Single-Particle LSPR

We have mentioned earlier that optical properties depend onconcentration, size, shape, spatial arrangement, and configura-tion of the nanoparticles. With the availability of improved colloidchemical synthetic methods, it has been possible to producenanoparticles with less heterogeneity in shape and size, andcompare their optical properties with particle morphology.[106]

Although many applications require assembly of nanoparticles,several future applications will make use of the properties of theindividual nanoparticles (sensors, medical diagnostics, etc.). Inaddition to the study of their collective properties, single-particlestudies are also important for determining structure-sensitiveproperties as well as the direct relationship between particlemorphology and the resulting properties. One inherent propertyof nonspherical metal nanoparticles is their polarization-dependent response to incident light (Fig. 5). Unlike nano-spheres, the LSPR wavelength of nonspherical nanoparticlesdepends on the orientation of the incident light relative to theparticle. However, when the nonspherical nanoparticles arerandomly oriented in a medium, one observes an orientation-averaged property. Furthermore, the yield of branched particleshas often been poor and therefore the optical spectra of theensemble are dominated by the LSPR of spherical particles. Aclear structure–property relationship in such low-yield cases canbe established by single-particle measurements. The asymmetricshapes and varying tip-geometries of branched nanoparticleensembles sometimes give rise to featureless broad LSPR.[101]

Therefore, the impact of the nanoparticle morphology on theLSPR is not easily inferred from ensemble spectra for complexbranched nanoparticles. As a matter of fact, single-particleinvestigations are becoming increasingly important to studymore complex structures. For example, Nehl et al. reportedexperimental and theoretical scattering spectra of individual goldnanostars by single-particle spectroscopy.[104] Unlike the onemajor peak in bulk solution spectra, single gold nanostars showedmultiple peak scattering spectra in the visible and NIR region(Fig. 5).[104,107] The nanostar, being an example of an extremelyanisotropic nanoparticle, shows polarization dependent, tipspecific spectral peaks. An analysis of the near-field enhance-ments also revealed that the observed resonances are localizedaround the tips of a nanostar.[102,105,108]

A number of experimental methods have been developed tostudy the plasmonic properties of isolated nanoparticles includ-ing white light Rayleigh scattering in a dark-field microscope,scanning near field optical microscopy, two photon photolumi-nescence, photoelectron emission microscopy, and electronenergy-loss spectroscopy (EELS).[57,93,100,109–119] From a systema-tic study of the effect of size and shape on the spectral response ofindividual silver nanoparticles, Mock et al. concluded that specificgeometrical shapes cause distinct spectral responses and subtlechanges in the particle morphology give rise to shifts inthe individual particle spectra.[111] Effects of the substrate onthe LSPR of a nanoparticle with a specific focus on the


nanoparticle shape and size have been investigated usingsingle-particle experiments.[52,120,121] The LSPRs are red-shifteddepending on the dielectric constant of the substrate and theparticle–substrate separation distance.When a particle is in contactwith the substrate, the contact area of the particle influences themagnitude of the red-shift. By a combined single-particlemeasurement and theoretical study, Sherry et al. have shown thatin addition to a red-shifted LSPR peak, a new blue-shifted peakarises, when a silver nanocube interacts with a glass substrate.[120]

The sharp blue peak is more advantageous for chemical sensingapplications in comparison to broader spectral features stemmingfrom plasmon resonances of other particle shapes.

Single-particle spectroscopy can provide useful information onmultisegment nanorods and nanowires with alternating metal


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Figure 6. Scanning near-field transmission microscopy images of a singlegold nanorod. a) The transmission image at �530 nm (resonant with thetransverse LSPR) shows a uniform reduction of transmission intensity(dark regions indicate less transmission) along the long axis of thenanorod. b) The transmission image taken at 780 nm (resonant withthe longitudinal LSPR) shows a characteristic spatially oscillatory patternalong the long axis of the nanorod. The images in (a) and (b) correspond tothe squared amplitude mapping of the wave functions for the transverseand longitudinal LSPs, respectively, as the near-field transmission at a givenposition of the sample reflects the optical transition probability, which isproportional to the square modulus of the localized surface plasmon wavefunction (or the photonic LDOS) resonant with the observed wavelength.The longitudinal-plasmon image in (b) with a spatially oscillating structurewas assigned to a localized surface plasmon mode having a node at thecenter (mL¼ 2, next to the dipolar mode). c) The solid curve shows atransmission spectrum taken at a point (marked with T) on the nanorod(inset in (d)), and the dashed curve gives calculated DOS. d) Polarizationmeasurements show that the band at 532 nm is effectively excited when thelight is polarized perpendicular to the long axis of the nanorod, and theband 780 nm is excited by the polarization parallel to the long axis of thenanorod. Reproduced with permission from ref. [114]. Copyright 2004American Chemical Society.

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composition. Mock et al. have synthesized homogeneous andmultisegment silver, gold, and nickel nanowires, which exhibitunique subsection-dependent surface plasmon scattering oflight.[122] These multisegment plasmon resonant nanowires canfind applications as nanoscale plasmon resonant codes or labelsin biological assays. Experimental techniques, such as scanningnear-field optical microscopy (SNOM) and EELS, are tools to mapthe localized plasmonic excitations over a single nanoparticle withhigh resolution.[57,114,118,123–125] Imura et al. have carried outnear-field optical transmission-spectral measurements (mea-sured at different points on the sample surface) on single goldnanorods and have imaged the optical local density of states(LDOS) and spatial features of the plasmon wave functions(Fig. 6).[114,125] These authors have found the oscillating behaviorof the LDOS. The longer the rod length is, the larger the numberof oscillations, which is in agreement with the theoreticalpredictions by Weber et al.[126] By combining optical near-fieldmicroscopy with a time-resolved technique, they investigated theultrafast temporal responses of single gold nanorods.Interestingly, the spatiotemporal response measurementsrevealed a position-dependent energy dissipation (electron–phonon relaxation) process. It was found that the collectivemotion of electrons at the center of the rod was different from thatat the ends. The dispersion relation of a plasmon travelling alonga metal nanowire has been investigated by Schider et al.[127]

Sanders et al. have reported interesting optical properties ofsingle and overlapping silver nanowires, in which the lengthexceeds the wavelength of the incident light.[128] When one end ofa metallic nanowire is irradiated with a focused laser, the laserlight excites plasmons that propagate along the wire and couple tofree-space photons at the other end. Plasmons can be launchedfrom either end of the nanowire; however, plasmon modes werenot observed to be launched when the laser was focused on themidsection of the nanowire. Interestingly, in the case ofoverlapping nanowires, if one nanowire is excited, then radiationis emitted at the intersection and visible light is emitted at theends of the non-illuminated overlapping nanowire. Theseobservations indicate that the intersection couples plasmons inthe excited wire to both photons as well as to plasmons in thecrossing wire. These characteristics of metallic nanowires showthat they can serve as effective wave-guides in opticalnanodevices.[129,130]

5.5. Refractive Index Sensitivity of LSPR

The refractive index sensitivity of the LSPR depends on themorphology of the nanoparticles.[130,131] Longitudinal andtransverse resonances of nanorods show different sensitivitiestowards the change in dielectric environment.[132] The long-itudinal plasmon mode is highly sensitive to the change in therefractive index of the environment, and the sensitivity increasesas the aspect ratio of the nanorods increases.[133–137]

Single-particle spectroscopy has been used to show how differentmorphologies induce different sensitivities to the change in localrefractive index.[110,138] The LSPR from each of the nanoparticlesgenerally undergoes a red-shift as the local refractive index isincreased.[138] The amount of red-shift per unit of refractive indexincrease varies depending on the morphology of the

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 1805–1825

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nanoparticle.[104,131,139] By comparing the refractive indexsensitivities of a number of Au nanoparticles, Chen et al.observed an increase in the index sensitivities when Aunanoparticles become elongated or their apexes becomessharper.[131] Accordingly, Au nanospheres exhibit the smallestrefractive index sensitivity of 44 nm/RIU (refractive index unit),and branched Au nanoparticles exhibit the largest indexsensitivity of 703 nm/RIU (Table 1). These authors also foundthat the sensitivities of Au nanobipyramids were higher thanthose of the corresponding Au nanorods with increasinglongitudinal plasmon wavelengths. This suggests that thenanoparticle morphology is an important factor in determiningthe refractive index sensitivity. However, Miller and Lazarideshave theoretically predicted that for any gold nanoparticle ofmoderate size in a homogeneous dielectric environment, therefractive index sensitivity is a function of only the LSPR bandposition, and independent of particle shape, size, or composi-tion.[133] That is, the refractive index sensitivity of all arbitrarilyshaped nanoparticles in a homogeneous dielectric environmentwill be the same if they have the same plasmon band location. Itshould be pointed out here that nanoparticles are often present inan inhomogeneous dielectric environment due to asymmetricsurface adsorption or substrate interaction.

6. Local Field Enhancement

Plasmon resonant particles are surrounded by strongly enhancedand highly localized electromagnetic fields when excited at theirLSPR. These fields are the direct result of the polarizationassociated with the collective electron oscillation in the metalparticles. The local field (Eloc) on or near the nanoparticle will be asum of the incident field (E) and the induced field from theoscillating electrons; i.e., Eloc is usually higher than the originalfield E.[54,140] The local field enhancement factor, f(v), is definedby f(v)¼Eloc/E. Earlier electrodynamic calculations have shownthat the local field enhancement factor depends on the particleeccentricity.[68,141–144] The field enhancement at the tip of aspheroid is larger than the one created by a resonant sphere ofsimilar dimensions.[141–144]

Table 1. Sizes, plasmon wavelengths, and refractive index sensitivities (RIU: refparentheses following values are standard deviations. Reproduced with perm

Au nanoparticles Length

[nm] [a]

Diameter

[nm] [b]

Aspect

ratio [c]

Nanospheres 15(1)

Nanocubes 44(2)

Branched nanoparticles 80(14)

Nanorods 40(6) 17(2) 2.4(0.3)

Nanorods 55(7) 16)(2) 3.4(0.5)

Nanorods 74(6) 17(2) 4.6(0.8)

Nanobipyramids 27(4) 19(7) 1.5(0.3)

Nanobipyramids 50(6) 18(1) 2.7(0.2)

Nanobipyramids 103(7) 26(2) 3.9(0.2)

Nanobipyramids 189(9) 40(2) 4.7(0.2)

[a] Length for nanocubes is the edge length, and for branched nanoparticles, it is the distan

is the central width. [c] The ratio between the length and diameter. [d] Plasmon wavelength

the longer plasmon wavelength; for nanorods and nanobipyramids, it is the longitudinal

longer-wavelength plasmon peak, and for nanorods and nanobipyramids, it is that of t


It should be pointed out here that there are factors, such assurface-induced relaxation (surface scattering) and radiativedecay (radiation damping) of LSPRs, which impose upper limitsto the strength of the field enhancement in metal nanoparti-cles.[141] Surface scattering is a function of the surface-to-volumeratio; hence, it scales with 1/r where r is the typical dimension ofthe particle, the radius in the case of a sphere. Thus, the degree ofplasmon damping is largely influenced by the nanoparticle shape.The radiation damping can in principle be reduced by decreasingthe particle volume. However, in the case of very small particles,the plasmon is damped due to surface scattering. In the lattercase, electrons can be scattered at the particle surface, when thedimensions of the particle become comparable to or smaller thanthe free-electron mean free path. The surface electron scatteringdepends on both the particle size and shape.[145] As the radiationdamping is larger at larger size and the surface scattering isstronger at smaller size, there will be an optimum volume forwhich the highest enhancement is achieved, depending on theparticle shape.[141]

In addition to radiative damping and surface scattering, thereare two important size-independent damping mechanisms.Plasmon polaritons may either decay via single-electron excita-tions where electrons of the sp band are excited to empty statesin the sp band (intraband excitation), or d band electrons areexcited to empty states in the sp band (interband excitation).[54]

The latter is a particularly severe damping mechanism of Cunanoparticles and still disturbs the plasmon resonance ofspherical gold nanoparticles. Yet, any nanoparticle morphologywhich shifts the plasmon resonance away from the onset ofinterband excitation automatically leads to a narrowing of theplasmon spectra. A narrow plasmon spectrum (or equivalently,a long plasmon lifetime) is beneficial for all effects that relyon a huge field enhancement. For instance, the third-ordersusceptibility of metal nanoparticles[146] and the effective SERScross-section[147] are proportional to the fourth power of theinverse of the homogeneous linewidth Ghom of the plasmonresonance. Knowledge of the damping mechanisms is thereforevital for a deep understanding of the performance of metalnanostructures.

ractive index unit) of variously shaped Au nanoparticles. The numbers in theission from ref. [131]. Copyright 2008 American Chemical Society.

Plasmon wavelength

[nm] [d]

Index sensitivity

[nm/RIU] [e]

Figure of merit

527 44(3) 0.6

538 83(2) 1.5

1141 703(19) 0.8

653 195(7) 2.6

728 224(4) 2.1

846 288(8) 1.7

645 150(5) 1.7

735 212(6) 2.8

886 392(7) 4.2

1096 540(6) 4.5

ce from the center to the branch tip of the particle. [b] The diameter for nanobipyramid

of Au nanoparticles dispersed in aqueous solutions. For branched nanoparticles, it is

plasmon wavelengths. [e] For nanobranches, it is the refractive index sensitivity of the

he longitudinal plasmon resonance peaks.


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Figure 7. Contour plots showing relative local field enhancements forexcitation of the a) 804, b) 717, and c) 594 nm nanostar plasmonresonances. The polarization angles are indicated by the insets to theright of the panels. The maximum field enhancements are indicated on topof each panel. Reproduced with permission from ref. [105]. Copyright 2007American Chemical Society.

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The main obstacle for measurements of Ghom or, alternatively,the plasmon lifetime T2, is the inhomogeneous broadening of theplasmon resonances due to broad size and shape distributions inensembles of colloidal nanoparticle dispersions. In order toovercome the inhomogeneous broadening due to the shapedistribution and different crystalline structures, several line widthmeasurements on single metallic nanoparticles have beenperformed in recent years, reporting a dephasing time T2 inthe range of a few femtoseconds (i.e., only a few cycles of thedipolar electronic oscillation occurs).[93,100,115,118,148–151]

Sonnichsen et al. investigated the dephasing of particle plasmonsin single gold nanoparticles as a function of particle size andshape using light-scattering spectroscopy.[115] The line widths forRayleigh scattering spectra of single gold nanorods were muchnarrower than the line widths obtained from the ensemblespectrum, and close to the value expected from the dielectricconstants of bulk gold.[115] Novo et al. claim that the line width ofthe longitudinal surface plasmon mode depends critically on thewidth of the rod, and the line width broadening behavior isessentially the same as that of gold spheres.[149] The line widthsobserved in the scattering spectra of the Ag-Au nanobox particleswere narrower than those observed for Au spheres, but muchbroader than those of either the pure Au nanorods or the Au–Agcore–shell nanorods.[115,150,151] A comparison with theoreticallycalculated spectra indicated that both surface scattering ofelectrons at the particle–solution interface and radiation dampingwere responsible for line width broadening. According toSonnichsen et al., the dephasing rate of nanospheres increasesfor more red-shifted (i.e., larger) particles.[115] This was attributedto the increased radiation damping with increased particlevolume. On the contrary, the dephasing rate of nanorodsdecreased drastically with increasing red-shift, i.e., for increasingnanorod aspect ratios. The reduction of the plasmon dephasingrate in nanorods was shown to be due to the suppression ofinterband damping and weaker radiation damping as comparedto nanospheres.

In addition to LSPR spectra, analytical as well as numericalmethods including DDA, finite element, and FDTD methodshave been used to predict the field enhancements in thevicinity of single metal nanoparticles of a variety ofshapes.[53,58,74,84,105,152,153] Numerical calculations by Jain et al.showed that gold nanorods have higher local field enhancementfactors than nanoshells and nanospheres.[153] Li et al. performedFDTD calculations on similarly sized Au nanorods and Au/Agnanoshuttles, and concluded that the maximum field enhance-ment of the Au/Ag nanoshuttle is about 5.1 times that of the Aunanorod.[152] Simulating triangular nanoparticles, Kottmann et al.have shown that sub-wavelength nonspherical metal nanoparti-cles exhibit multiple resonances and the field amplitudesassociated with these resonances can be extremely large, up toseveral hundred times the incoming field amplitude.[84] Theselarge electromagnetic fields are strongly localized at particularpositions on the particle surface (Fig. 7). Therefore, nonsphericalnanoparticles can be very useful for applications where extremelylarge electromagnetic fields at different wavelengths are required.However, it is interesting to note that according to DDA-basedcalculations of Hao and Schatz on silver particles, the largest fieldenhancement values were very similar for different shapes, suchas triangular prisms, oblate spheroids, or cylindrical rods.[58]


Further, for nanoparticle dimers, larger enhancements weregenerally observed for dimers with longer-wavelength dipoleplasmon resonances. However, structural properties of the dimer(such as local curvature) were found to play a less important rolein determining field enhancements.

Nonspherical metal nanoparticles may show another kind offield enhancement, termed ‘‘lightning-rod effect.’’ The term owesits origin to the sharply pointed conductor poles that are used toprotect buildings from the destroying power of lightning. Innanoparticle systems, the term refers to an enhanced chargedensity localization at a tip or vertex of a nanoparticle. When anelectromagnetic field (e.g., laser light) excites the free electrons ofa metallic tip, a highly localized, strong electric field develops atsurfaces of large curvatures (such as a tip or vertex) of thenanoparticle, leading to large field enhancement in those regionswithout the need of nanoplasmonic resonances.[143,154] Recently,based on experimental and theoretical studies on triangular silvernanoprisms, Rang et al. claimed that particle morphology and theassociated LSPR do not uniquely reflect the details of the local


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field distribution.[155] According to their report, the tips of thetriangles in general do not represent regions of highest local fieldenhancement.

Experimental determinations of field enhancements have beenperformed using two-photon photoluminescence (TPPL) fromgold bowtie nanoantennas, and a comparison with the TPPLemission from smooth gold films was made.[119] The determina-tion of field enhancements from SERS appears to be moredifficult since the total Raman enhancement factors can alsoinclude nonelectromagnetic contributions, as discussed below.

The optical local field enhancement provides the basis formany applications in molecular detection, sensing, and nano-photonic devices. A molecule residing on or near a metalnanoparticle may experience a light intensity far stronger thanjust the intensity of the incident light due to the enhanced localfields.[156] This has been exploited for a variety of applications,such as the above-mentioned SERS, surface-enhanced fluores-cence (SEF), SNOM, etc.[157–164] Nonspherical nanoparticles ofsilver and gold are attractive as SERS and SEF substrates, becausethe LSPR wavelengths of these particles can be tuned over a broadrange in the UV–vis–NIR region.[159,160,165]

7. NMNPs and Surface-Enhanced RamanScattering

Raman spectroscopy reveals the inelastic scattering of photons bymolecules or solids. The energy difference between the incidentand scattered photons, detected via observation of the red-shiftedinelastically scattered photons, is used to excite vibrational (Raman)modes of the molecule in the IR spectral region. This spectroscopictechnique is non-invasive, and provides rich structural informa-tion leading to chemically highly specific and sensitive detectionand identification of analyte molecules. However, it suffers fromlow scattering intensity since Raman scattering cross-sectionsof molecules are usually extremely small, typically10�30–10�25 cm2/molecule, compared to fluorescence cross-sections, which are in the order of �10�16 cm2/molecule forhigh-quantum-yield fluorophores.[166] In the presence of plas-monic nanoparticles or rough metal surfaces, the Ramanintensity is greatly enhanced under certain conditions.[167,168]

This phenomenon is known as surface-enhanced Ramanscattering—SERS.[169–171] In surface-enhanced resonanceRaman scattering (SERRS), the same approach is taken toachieve surface enhancement, but additional enhancement isobtained through the excitation of electronic transitions in theanalyte by using a laser source with an appropriate excitationfrequency.[167,168] Similar to fluorescence, SE(R)RS can achievesingle-molecule sensitivity and still maintain its chemicalspecificity.[172–174]

The intensity enhancement of SERS from an analyte hasbeen shown to occur through mainly two mechanisms:electromagnetic[175–178] and chemical enhancement.[168,179,180]

The electromagnetic enhancement in SERS results from thecoupling of the LSPR of nanoparticles (individual or aggregated)with Raman excitation and emission. Under plasmonic resonantexcitation condition, the enhanced local field at the location of theanalyte molecule near the metal nanoparticle amplifies both theexcitation field as well as the Raman scattered field. The scattered


Raman intensity is thereby enhanced by the square of the productof the field (amplitude) enhancement factors at the excitationfrequency nL and Stokes Raman frequency nS.

[141,148,166,176] WhennL and nS are near the LSPR frequency, the observed Ramanscattering intensity is approximately proportional to the fourthpower of the local electric field enhancement.[181] Even a modestlocal field enhancement near the metal particle leads tosubstantial electromagnetic enhancements of Raman scatteringintensities. The chemical enhancement mechanism arises due tospecific, localized analyte–substrate interactions upon analyteadsorption. These interactions include the electronic resonance-charge transfer between the molecule and metal surface, andadsorption-induced changes in the analyte polarizability.[168,179]

The Stokes Raman signal is proportional to the Ramancross-section of themolecule s, the excitation laser intensity, I(nL),and the number of molecules N which are involved in theprocess.[166] The surface-enhanced Stokes Raman signal ISERS canbe written as follows:

ISERSðnSÞ ¼ N � IðnLÞ � f ðnLÞj j2� f ðnSÞj j2�sads (2)

where sads describes the increased Raman cross-section of the

adsorbed molecule (this includes chemical enhancement), and

f(nL) and f(nS) are the field enhancement factors at the laser and

Stokes frequency, respectively.[166] SERS enhancement due to the

resonance of the incoming radiation with the LSPR is sometimes

distinguished from the nonresonant lightning-rod effect.

We have discussed above that the optical polarization of the

anisotropic metal nanoparticles with highly curved, sharp surface

features will show lightning-rod effects at surface protrusions. A

molecule situated in the vicinity of protrusions will experience an

enhanced localized electromagnetic field, which gives rise to an

enhanced Raman scattering.[108,143,154] Further, the surface

protrusions can also play a role in the chemical enhancement

mechanism, since they may act as preferred binding sites for

analyte molecules.[143,182,183]

All enhancement mechanisms discussed above can beconsidered as functions of the nanoparticle morphology, sinceit determines the LSPR, local curvature, local field enhancement,surface characteristics, and bonding nature. Researchers havebeen trying to find out, both theoretically and experimentally,which particle shapes and/or assemblies would result in thestrongest SERS enhancement.[144] The electromagnetic enhance-ment is greatest when the LSPR lmax falls between the excitationwavelength and the wavelength of the scattered photon.[181] As thenanoparticles with nonspherical morphology offer tunable LSPRwavelengths, they are ideal candidates for SERS substrates. Peakenhancement values, which are important for single-moleculeSERS, have been found to be significantly larger for spheroidsand nanoprisms compared to spheres due in part to red-shiftedplasmon excitation and to sharp points that produce strongerlightning-rod effects.[58,141,184,185] El-Sayed and co-workers com-pared the Raman spectra of a few molecules adsorbed on goldnanospheres and nanorods, using an off-plasmon resonanceexcitation condition.[183] Enhancement factors of the order of104–105 were observed for the adsorbed molecules on thenanorods, whereas no such enhancement was observed on


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Figure 8. a) Sketch of the combined dark-field scattering and Ramanmicroscope used for single-particle spectroscopy. b) Scanning force micros-copy height image of an individual gold nanostar deposited on a glasscoverslip and coated with self-assembled monolayers of 4-mercaptobenzoicacid. c) Raman spectrum of a self-assembled monolayer (SAM) of4-mercaptobenzoic acid on the single gold nanostar shown in (b) (60 sintegration time and 240 kW/cm2 at the sample). Reproduced with per-mission from ref. [107]. Copyright 2009 American Institute of Physics.

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spherical gold nanoparticles under similar conditions. Theseenhancement factors were two orders of magnitude larger thanthe calculated values. This was, therefore, attributed to thechemical enhancement contributions on the gold nanorodsurface. Murphy and co-workers reported the effects of aspectratios of silver and gold nanorods on SERS in colloidalsolution.[186] The optical properties of these nanorods have beentailored to have variable degrees of overlap of their longitudinalplasmon mode with the excitation source in the absence ofinterparticle plasmon coupling effects. According to theseauthors, silver and gold nanorods of an aspect ratio of 10 and1.7, respectively, show total Raman enhancement factors in therange of 103–107, when the plasmon resonance overlaps with theexcitation frequency. These enhancement factor are 10–100 timesgreater than the ones of particles with aspect ratios that lack thespectral overlap between their plasmon resonance and theexcitation wavelength.[186] Similarly, Xia and co-workers demon-strated that for a given size, the sharper cubes gave strongerRaman peaks than truncated ones.[98] The difference inenhancement factor was attributed to the variation in overlapbetween the particle LSPR and the laser frequency.

Kottmann et al. studied the relation between the nonregularcross-sectional shape of a nanowire, its resonance spectrum, andthe resulting field enhancement by numerical calculations.[187]

According to them, the maximum field and Raman enhance-ments depend on the cross-sectional shapes of silver nanowires,among others. The maximum Raman enhancement shouldexceed 106 for a triangular cross-section; for a square, it is about105, whereas it is below 104 for the hexagonal, pentagonal, andcircular cross-sections. Calculations of Xu et al. indicate that themaximum enhancement factor achievable through electromag-netic mechanisms is on the order of 1011.[144] This can be realizedonly under special circumstances, such as for strongly coupledstructures (such as dimer configurations) or at locations close tosharp surface protrusions.[102,107,144] From the extensive literaturethat reports estimates and measurements of Raman enhance-ment factors, we limit ourselves to mentioning a few: Tian et al.have reported that platinum nanothorns prepared by anelectrochemical method exhibit considerably higher SERS activitythan smooth spherical platinum nanoparticles possibly due to thelightning-rod effect.[188] The SERS signal significantly decreaseswith the decrease in nanothorn tip sharpness and nanothorndensity. Murphy and co-workers reported surface enhancementfactors in the range of 107–109 for 4-mercaptobenzoic acid(4-MBA) molecules sandwiched between a smooth gold substrateand gold nanoparticles of several different morphologies.[182]

Recently, star-shaped gold nanoparticles received increasingattention due to the potentially large field enhancements in thevicinity of their tips.[105] Khoury and Vo-Dinh have reported anenhancement factor �5� 103 for SERS of 4-MBA molecules ongold nanoparticles with such sharp surface protrusions insolution ensemble measurements.[103] Nalbant-Esenturk et al.estimated the Raman enhancement factors of gold nanostars toexceed 105 using 2-mercaptopyridine and crystal violet asanalytes.[189] Rodriguez-Lorenzo et al. recently demonstrated thata SERS signal amplification of�1010 can be achieved in junctionsbetween a gold nanostar tip and a gold surface for1,5-naphtalenedithiol which allows zeptomolar analyte detec-tion.[108] Hrelescu et al. reported Raman scattering from single


gold nanostars coated with self-assembled monolayers of 4-MBA(Fig. 8) and estimated a total Raman enhancement factor of �107

for individual nanostars.[107] This wide range of enhancementfactors reported for rather similar structures demonstrate thedifficulty in determining reliable estimates for total Ramanenhancement factors. The difficulties may include changinganalyte–particle interactions if different analytes are employed,different experimental situations (i.e., ensemble versus singleobjects, or geometry), or potentially different amounts of analytedue to different initial surface functionalization, which hinders aprecise determination of the number ofmolecules contributing tothe signal. Recently, Uji-i et al. demonstrated remote excitation ofSERS by forming a hotspot between a silver nanoparticle and asilver nanowire. SERS in the hotspot region could be observedafter launching a propagating plasmon in the wire through laserexcitation several micrometers away from the hotspot.[190]

According to Xu et al., single spherical or regular crystalshaped nanoparticles are likely inefficient as substrates forsingle-molecule SERS due to their comparatively weak electro-magnetic enhancement effect.[144] Strongly coupled structures,such as dimer configurations or particles with sharp surfaceprotrusions, may be suitable for single-molecule SERS due tolarge and spatially confined electromagnetic enhancementeffects.[144,191,192] It should be noted however, that in dimerstructures at very small interparticle distances (<1 nm) electro-magnetic enhancements will be limited by quantum mechanicaleffects such as interparticle electron tunneling.[193] Thoughelectromagnetic enhancement is likely to produce the dominantcontribution to single-molecule sensitivity in SERS in these cases,an additional chemical or electronical resonant enhancement


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effect has to be invoked in order to explain the SERSenhancement factors of the order 1014 reported in theliterature.[144,172–174,180,191] Based on DDA calculations of thefield enhancement factors for various different silverparticle sizes, shapes, orientations, and interparticle spacings,Hao and Schatz also concluded that not all of the single-moleculeSERS enhancement factor of 1012 can be ascribed to purelyelectromagnetic effects.[58] Wokaun et al. have shown thatradiation damping limits the values of the attainable electricfield intensity at the tip of small metal rods and hence limits theenhancement factors found in SERS.[141] According to them, thedifference between various shapes is virtually eliminated forlarger volumes due to radiation damping.

8. NMNPs and Fluorescence

Fluorescence, the spontaneous radiative decay of electronicallyexcited molecules, is widely used as a signal transduction tool forthe detection of trace levels of analytes, imaging, andsingle-molecule studies of biological systems. Metal nanoparti-cles have been found to either quench or enhance fluorescenceintensity of a fluorophore.[156,162–164,194–196] The quenching andenhancement are determined by interactions between fluoro-phores and metal surfaces depending on the location of thefluorophore around the particle, the orientation of its transitiondipole moment relative to the particle surface, and theparticle–fluorophore distance.[156,162–164,194–198] Quenching mayoccur by Forster resonant energy transfer (FRET) to the surfaceplasmon absorption of the metal particle. FRET is a dipole–dipoleinteraction mechanism in which a donor fluorophore in itsexcited state transfers its energy radiationless to an acceptor.FRET has been widely used tomeasure the distance between sitesor biomolecules due to its 1/R6 distance dependence (R is thedistance between the dipoles concerned).[199–201] When donorsand acceptors are present in proximity with sub-wavelengthmetal particles, the rates of energy transfer increase at distancesup to 70 nm, which is �10-fold larger than typical Forsterdistances.[199,200]

Enhancement occurs through the increase of the strength ofthe incident light field which increases the rate of excitation, andan increase in the radiative decay rate of the fluorophore.[194] Theresonantly enhanced local field of metal nanoparticles canincrease or suppress the rate of excitation. Metal particles cangreatly enhance local excitation intensities, since the lightintensity is proportional to the square of the field. Anotherimportant effect of metal particles is that they can modify theintrinsic radiative decay rate of a fluorophore. An appropriateproximity and orientation of fluorophores to metal particlesmodifies the photonic mode density around the fluorophore andwill lead to a radiative rate outranging the rate of losses underfavorable circumstances.[194,197] Theoretical calculations show anincrease of the radiative decay rate of up to two orders ofmagnitude for a fluorophore near a prolate silver nanoparticle,unlike the radiative rate increase in case of a spherically or anonresonantly shaped nanoparticle.[202]

The shape of a nanoparticle exerts its influence on thefluorescence of a fluorophore depending on the location ofthe fluorophore around the particle and the orientation of its dipole


moment relative to the particle surface.[159,203,204] For example, thefluorescence intensity depends on whether a fluorophore is locatednear the tip of an ellipsoid (on the major axis), or along the minoraxis of the ellipsoid. The fluorophore will experience two differentmodes of the LSPR. Further, there is a difference whether thefluorophore dipole moment is perpendicular or parallel to theparticle surface. Fluorescence is either enhanced or quenchedsince the molecular transition dipole is enhanced or canceled bythe corresponding image dipole induced in the metal.[203]

Depending on the aspect ratio, a radiative rate enhancement bya factor of 103 or greater can be found for a fluorophore (transitionmoment) orientation perpendicular to the surface of anellipsoid.[203] Therefore, quenching or enhancement of thefluorescence and changes of fluorescence lifetime are dependenton the morphology of the metal particles.

Gold nanoparticles have also been observed to show weakintrinsic photoemission via an interband transition mechan-ism.[205] This effect can be enhanced by many orders ofmagnitude in the presence of plasmon resonances.For example,El-Sayed and co-workers have shown that the photoluminescencequantum efficiency from gold nanorods can be enhanced by 6 to 7orders of magnitude compared to bulk metal.[206] Li et al. havedemonstrated that sufficiently long gold nanorods (aspectratio> 13) exhibit intense fluorescence.[160] The emissionobserved in gold nanorods of different aspect ratios is a functionof local field enhancement factors and is therefore enhanced inthe spectral vicinity of the LSPR.[160,206–208] Enhanced emission ofthe rod is more than 1 order of magnitude larger compared to thatfor the nanosphere.[207] Under pulsed laser excitation, Aunanorods were observed to emit a strong TPPL, which is manytimes brighter than the TPPL from single dye molecules.[209,210]

The strong TPPL is thought to arise from the plasmon-enhancedtwo-photon absorption cross-section due to coupling of weakelectronic transitions in themetal to the particle plasmons.[210,211]

TPPL has been recently used as a nonlinear optical imagingmode, which will be discussed below.

9. Applications of Nonspherical NMNPs

9.1. Analytical Applications

NMNPs, and in particular gold nanoparticles, possess largescattering cross-sections, high degree of biocompatibility, richsurface functionalization chemistry, and high photostability.Therefore, these particles have been shown to be very useful forbiomolecular manipulation, labeling, and detection.[212–214]

Further, for several (in vivo) applications, nonspherical goldnanoparticles are also advantageous because their LSPR can betuned to the NIR wavelength range, the so-called biologicalwindow, where the absorption, scattering, and fluorescence fromendogenous biological chromophores are minimal.[215]

Sensitivity and specificity are two critical issues in modernbiomedical diagnosis research. Yguerabide and Yguerabidedemonstrated that single NMNP can be used as optical reportersin clinical and biological applications by using the strong elasticlight scattering properties of these particles.[86,117,216] Plasmonresonant particles have very high scattering cross-sectionscompared to other optical labeling entities under the same


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illumination conditions. For example, the cross-section for elasticlight scattering from an �80 nm plasmon resonant particle isequivalent to the absorption cross-section of 500 000 individualfluorescein molecules or >105-fold that from typical semicon-ductor quantum dots.[217] The impressive signal of ultrabrightplasmon resonant particles allows them to be individuallyidentified and counted relatively easily, pushing the analytedetection limit to a dramatically low value. This, being aided bythe high photostability and absence of blinking, promises anultrasensitive assay format based on single-target moleculedetection. NMNPs can replace or complement the colorimetric,fluorescent, or radioactive labels routinely used in immunoassaysand cellular imaging.[86,216–219]

Single or aggregated NMNPs can also act as transducers thatconvert the changes in the interparticle distance or local refractiveindex into spectral shifts in their absorption and scatteringspectra. Thus the plasmon resonant nanoparticles have verypromising applications as label-free biosensors.[109,137,220–223]

This sensing property uses the spectral change or the LSPRbrought in by the biomolecular event occurring in the vicinity ofthe plasmon resonant nanoparticles. The biomolecular event caneither change the interparticle distance and thereby the couplingof surface plasmons between the particles or the local refractiveindex of the nano-environment of the nanoparticle. Whenthe plasmon resonant particles are very close to each other, theplasmon oscillations from adjacent particles can couple givingrise to hybridized plasmon modes derived from the modes of theindividual particles.[224–226] The occurrence of a stronglydistance-dependent red-shifted mode has been utilized byresearchers to develop colorimetric sensors for the detection ofDNA as well as insertions, deletions, and mismatches at asingle-base resolution.[227–229] Similarly, Dujardin et al. reportedself-assembly of oligonucleotide-functionalized short gold nanor-ods.[14] As discussed earlier in Section 5.5, the LSPR peak positionand intensity are sensitive to the local refractive indexsurrounding the nanoparticle. The binding of biomoleculartargets to receptor-functionalized nanoparticles results in achange in the local refractive index and hence spectrally shiftsthe LSPR of nanoparticles. These changes enable monitoring ofmolecular binding in real time with high sensitivity by simplespectrophotometric techniques.[109,230,231] Chen et al. have shownthat for gold nanorods the longitudinal LSPR band sensing modeexhibits much higher sensitivity and linearity to the concentrationof the target biomolecules compared to that of the goldnanospheres.[137] Marinakos et al. have also observed that thenanorod-based sensors have advantages over gold nanosphere-based sensors.[220,232,233] In addition to a significantly lowerdetection limit, the signal of the nanorod sensor provides theinternal self-reference due to the measurement of wavelengthshift of the plasmon bands.[220,232,233] The peak wavelength shiftis solely determined by changes in the dielectric properties (i.e.,refractive indices) in the immediate vicinity of the nanoparticles,and concentration change has minimal or no effect on thisparameter. Spectral changes induced by changes in the refractiveindex in the vicinity of individual gold nanorods due to targetbinding have been utilized by Yu and Irudayaraj for multiplexbiosensing by using the longitundinal plasmonmode of nanorodsof different length functionalized with different specific analyteacceptors.[234] Recently, Nusz et al. reported a single-gold nanorod


LSPR biosensor for the binding of streptavidin to biotin.[235] Thelowest streptavidin concentration that was experimentally mea-sured was 1nM, which was substantially lower than that detectedby biotinylated single gold nanospheres.[109,235,236]

Sonnichsen and Alivisatos reported that gold nanorods couldbe exploited for the study of rotational motion in biomole-cules.[237] Murphy and co-workers demonstrated the use of opticalpatterns, produced by resonant Rayleigh scattering from goldnanorods, as markers by which local deformations of stretchablepolymers could be measured using image correlation techniques.These authors proposed that modified gold nanorods could beused for examiningmechanical effects in biological tissues.[238] Liet al. suggested the use of relatively intense fluorescence ofsufficiently long gold nanorods (aspect ratio >13) as probes influorescence-based microarray assays and optical biosensordevelopment, and demonstrated the application of this conceptby monitoring DNA hybridization events by measuring theconcomitant change in fluorescence intensity.[160] Nishizawa et al.have demonstrated that hollow Au nanotubules supportselective ion transport analogous to that observed in ion-exchangepolymers, when the inside radius of the tubule is small relative tothe thickness of the electrical double layer.[239] The metalnanotubule membranes can be cation-permselective, anion-permselective, or nonselective, depending on the potentialapplied to the membrane. Metal nanotubule membranes havepotential for applications as universal ion-exchange membranes,and in chemical separations, such as biomedical separations orindustrial gas separations.[240]

NMNPs have been used for the detection of inorganic ions,organic molecules, and biomolecules. One of the majormechanisms of detection involves the change in the LSPR dueto interaction between the particle and adsorbed analyte. Sudeepet al. reported selective detection of micromolar concentrations ofcysteine and glutathione in the presence of various other a-aminoacids by exploiting the interparticle plasmon coupling in Aunanorods.[241] Rex et al. have demonstrated the analytical potentialof Au nanorods for monitoring Hg in water samples withoutstanding selectivity and sensitivity based on amalgamationwithout previous separation or preconcentration of the originalsample.[242]

9.2. Photothermal Therapy

Large absorption cross-sections and the decay of plasmons intoelectronic excitations with subsequent electron–phonon relaxa-tion make metal nanoparticles very efficient heat sources. Metalparticles normally show rather poor light emitting ability.Therefore, they can efficiently convert the absorbed light intoheat via nonradiative electron relaxation dynamics, known asphotothermal effect, and serve as nanometric local heatsources.[243–248] As the light absorption cross-section is greatlyenhanced under plasmonic resonance conditions (the spectrumof the heat generation rate demonstrates a typical plasmon peak atthe LSPR frequency), the magnitude of generated heat willdepend on the particle size and shape, in addition to the nature ofthe metal and the number of nanoparticles present in the system.

As optical and thermal responses are closely related, metalnanoparticles can be used as nanometric heat sources and probes


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Figure 9. Photothermal destruction of SK-BR-3 breast cancer cells (invitro) treated with immunogold nanocages (45 nm size) and irradiatedby an 810 nm laser at a power density of 1.5W/cm2 for 5min. A well-defined circular zone of dead cells is revealed by A) calcein AM assay, andB) ethidium homodimer-1 (EthD-1) assay. In the control experiment, cellsirradiated under the same conditions, but without immunogold nanocagetreatment maintained viability, as indicated by C) calcein fluorescenceassay and D) the lack of intracellular EthD-1 uptake. Reproduced withpermission from ref. [274]. Copyright 2007 American Chemical Society.

for local temperature variations via their optical beha-vior.[229,243,249,250] For example, heat-induced changes in opticalproperties (e.g., refractive index) of the surrounding inhomoge-neous medium can be recorded in photothermal imaging.[251]

The heat generated in metal nanoparticles may also induce phasetransformations in their surrounding matrix and can be used torelease drugs from polymer capsules or to release the contents ofsmall containers inside living cells.[252–255] The importance ofnanoparticle shape in the photothermal effect has beendemonstrated in photothermal therapy.[256,257] In these applica-tions, after attaching to tumor cells, the nanoparticles areirradiated by short laser pulses and the absorbed light energy isquickly transformed into heat. The heat generated may reachthresholds which result in desired target damage throughthermal denaturation and coagulation or through mechanicalstress caused by microbubble formation, acoustic, and shockwave generation.[258,259] The photothermal therapy is relativelysimple to perform and has the potential of treating tumorsembedded in vital regions without surgical resection. In order toeffectively absorb light in biological medium, the metallicnanoparticles should show a LSPR in the biological window(�650–900nm). LSPR of several kinds of nonspherical Ag and Auparticles, such as nanorods, nanoshells, nanocages, andnanostars, can be tailored to this range.[107,153,260] Suitableselection of the experimental parameters, therefore, promiseshighly localized and controlled destruction or treatment of thetargets varying from a few nanometer to tens of micrometers(such as from bacteria to (cancer) cells).[256,257,261–266]

Experimentally, it has been observed that 30–40 nm particlesare most effective for photothermal therapy.[267] According tosome calculations, silica core–gold nanoshells (core diameter:50–100 nm, shell thickness: 3–8 nm) and gold nanorods (aspectratio¼ {15–20 nm}/{50–70 nm}) aremore efficient photothermallabels and sensitizers than solid single gold spheres of equivalentvolume.[268] Gold nanoparticles, especially nanorods, are emer-ging as one of the most promising candidates for photothermalapplications since they are strong absorbers, biocompatible, easilyconjugated to antibodies or proteins, and have adjustable opticalproperties.[153,269] Owing to the large plasmon resonanceabsorption efficiency of nanorods, the threshold energy densityfor photothermolysis can be sufficiently small.[265] Further, due totheir large scattering cross-section, nanorods can be usedsimultaneously for imaging and photothermal therapy.[261,270–273]

Gold nanoparticles of other morphologies such as nanocages andnanoshells, whose optical properties can be tuned to the vis–NIRspectral region, have also been tested for their photothermaltherapeutic properties (Fig. 9).[266,274] Chen et al. reported thatimmunogold nanocages had lower power density threshold of1.5W cm�2 for selective destruction of cancer cells than thosereported for gold nanorods (10W cm�2) or gold nanoshells(35W cm�2).[257,261] However, in addition to the morphologyeffect, other parameters such as the density of nanocages on eachcell could also contribute to the lowering of the thermal damagethreshold.

Salem et al. have demonstrated a new nonviral gene therapyapproach by using multisegment metal nanorods.[275,276]

Differential molecular binding strategies were used to attachplasmids and a cell-targeting protein to spatially separated regionsof the delivery system. This approach can be extended to include


other components that allow additional functionalities to beintroduced (such as an extra segment that binds to anendosomolytic agent) to create a versatile synthetic gene deliverysystem. Takahashi et al. have reported that Au nanorods can beused as carriers for a phototriggered gene delivery, with 1064 nmlaser irradiation triggering the release of plasmid DNA fromphosphatidylcholine-modified nanorod–DNA (PC-NR–DNA) com-plexes via morphological transformation of Au nanorods intospherical nanoparticles.[277] Kitagawa et al. have reported controlledrelease of myoglobin protein from the myoglobin–gold nanorodcomplexes by NIR (pulsed) laser irradiation.[278]

9.3. Cellular Uptake and Imaging

Chithrani et al. have studied the size- and shape-dependence ofthe uptake of gold nanoparticles into mammalian cells.[279] Theseauthors found that 50 nm spheres were taken up more effectivelyby the cells compared to both smaller and larger spheres in the10–100 nm size range. On the other hand, uptake of lower-aspect-ratio (1:3) nanorods was greater than that of those withhigher aspect ratio (1:5). Interestingly, nanospheres were takenup more efficiently than nanorods with dimensions in the10–100 nm range. However, the difference in surface chemistrycould be partly responsible for the difference in uptake betweenspherical and rod-shaped NMNPs. Still very little is known about


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the independent role of particle size, shape, and surfacechemistry on cellular uptake.[280]

Strong optical response, coupled with photo- and chemicalstability, biocompatibility, and easy surface conjugation chem-istry, make NIR-absorbing nonspherical Au nanoparticlesattractive contrast agents compared to molecular dyes for NIRimaging applications.[281] A number of optical imaging techni-ques, such as dark-field microscopy (DFM), TPPL microscopy,and optical coherence tomography (OCT), are used to imageliving tissue with subcellular resolution. From numericalcalculations, El-Sayed and co-workers have concluded that goldnanorods offer the superior NIR absorption and scattering atmuch smaller particle sizes compared to nanospheres andnanoshells.[153] Their calculations further showed that Aunanorods with a large aspect ratio along with a small effectiveradius were the best photoabsorbing nanoparticles, whereas thelargest scattering contrast could be obtained from nanorods ofhigh aspect ratio with a larger effective radius.[153] Gold nanorodshave been demonstrated to be promising contrast agents fordark-field imaging studies.[261,281–285] Dark-field microscopy hasbeen the most common means for plasmon resonant nanopar-ticle imaging. However, strong background signal in the case ofintracellular imaging often impairs the detection of lowconcentrations of plasmon resonant particles by this technique.TPPL microscopy is an alternative imaging option under suchconditions. There are a few advantages associated with TPPLimaging. For example, in TPPL NIR frequencies in the biologicalwindow can be used at power densities well below the damagethreshold of biological tissue. Further, TPPL can provide 3Dspatial resolution avoiding tissue autofluorescence. TPPL ofnonspherical Au nanoparticles has been used to image thelocation of these particles both in in-vitro and in-vivo targetcells.[123,210,286–288] Nonspherical Au nanoparticles have also beenreported to improve the imaging performance when they are usedas exogenous contrast agents in OCT and photoacoustictomography (PAT), two other noninvasive imaging tech-niques.[99,289–298] OCT detects the depth of reflections of lightof low coherence. It can produce real-time, high-resolution(typically 1–15mm) images in vivo of biological tissues with aspatial resolution that is many times greater than that producedby ultrasound imaging or magnetic resonance imaging.[299] InPAT, photoinduced thermalization of conduction electrons in theNIR-active particles results in a temperature increase of hundredsof degrees.[300] This leads to plasma formation, microbubbleexpansion, and collapse. The expansion and collapse of cavitationbubbles produce an acoustic shockwave which can be detected byultrasonic transducers. PAT has the advantage over pure opticalimaging. PAT combines the intrinsic optical contrast character-istics with the capability of the diffraction-limited high spatialresolution of ultrasound.[301,302]

9.4. Other Applications

The range of applications of nonspherical NMNPs encompassesseveral fields, from science and engineering to technology, and isexpanding rapidly. Progress in nanoparticle synthesis andmodification has fueled enormous advances in applicationsrecently. In the following, we mention several other interesting


physical and chemical properties of nonspherical NMNPs, whichshow their promising application potentials.

Silver has been known for a long time for its bactericidalproperties. In the recent past, silver nanoparticles had beenreported to show antibacterial properties that find immediateapplications in biomedical science and household appliances.Chemisorbed Agþ ions on the nanoparticle surface are thought tobe responsible for the antibacterial action of silver nanoparticles.Recently, Pal et al. reported a particle morphology-dependentantibacterial activity for a number of silver nanoparticles.[303]

According to this report, truncated silver nanotriangles showedbactericidal properties at much lower total silver content thanspherical nanoparticles, which in turn showed better bactericidalproperties at lower total silver concentration than silver nanorods.The degree of antibacterial activity is believed to depend on theavailability of atom-dense {111} faces on the surface of theparticles. Berry et al. have reported that CTAB-capped Aunanorods can be deposited on the surface of gram-positivebacteria more easily than peptide and nucleic-acid-cappednanoparticles.[304] The strong electrostatic interaction betweenthe rod and bacterium surface results in bending of the nanorodsand formation of a percolating–conducting network on thebacterium surface. This gives an enhancement of four orders ofmagnitude, in the conductivity of the rod network compared to anetwork of nanospheres. The authors believe that the highconductivity at only �10% coverage (which is well below thepercolation threshold of 45%) can open the possibility offabricating electronic circuitry on bacteria without suffocatingthe microorganism.

Laicer et al. have reported that alkanethiol-functionalized goldnanorods can act as morphological seeds that specificallytemplate the growth and direction of large, uniform, cylindri-cal-phase domains from a polystyrene-block-polyisoprene(PS-b-PI) copolymer solution, whereas spherical gold nanopar-ticles and other materials, which are commonly used asheterogeneous nucleants in crystallization, fail to promotesingle-crystalline domain growth in the same material.[305]

Sanders et al. and Dickson and Lyon have demonstrated thathigh-aspect-ratio metal nanostructures, such as Au and Agnanorods/nanowires, can be used to transport optical signals overdistances of several micrometers.[128,306] When plasmon propa-gation is launched parallel to the long axis of nanorods/wires, theincident optical energy passes down the length as a surface-boundplasmon mode and re-emerges from the end as a photon (viaplasmon scattering). By constructing a composite nanowire withtwo discrete segments of Au and Ag, they have demonstrated thata unidirectional plasmon transmission through the compositenanowire can be obtained; i.e., an electro-optic heterojunction canbe constructed by controlling rod orientation, composition, andincident wavelength. With much higher bandwidths thanelectronic devices and smaller spatial constraints than conven-tional light guides, plasmon-mediated transport of opticalinformation on the scale of tens of micrometers and controllingthe directional flow of optical information show great promise forfuture optical devices, high-density optical information proces-sing, and nanoscale optical sensing methods. Zijlstra et al.recently demonstrated the use of gold nanorods for five-dimensional data storage based on thermal reshaping ofnanorods by pulsed laser irradiation for writing and TPPL


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imaging for reading.[307] Due to the strongly anisotropicplasmonic response of the nanorods only a small subpopulationis thermally reshaped for a given laser wavelength andpolarization. The use of five wavelengths and two polarizationorientations, therefore, allowed the authors to write and read18 images in three layers spaced by 10mm (Fig. 10). Theyestimated that the storage density can reach �1 Tbit cm�3 whichwould result in a capacity of about 1.6 Tbyte for a DVD-sized disk.Pan et al. have studied the optical limiting properties of a numberof metal (Cu, Co, Ni, Pd, Pt, and Ag) nanowires.[308] They havefound that the nanowires have broadband optical limitingcapability and that the optical limiting performances of somemetal wires are comparable to or better than those of carbon
nanotubes. Thus, metal nanowires can findapplications as cheap and efficient opticallimiters. Recently, Selhuber-Unkel et al. haveshown that gold nanorods can be used asefficient optical handles in nanoscale experi-ments.[309] These authors anticipate that goldnanorods can be employed as force sensorsin future in vitro and in vivo studies as wellas force transducers in single-moleculeexperiments.
Figure 10. Gold nanorod surface plasmon-mediated optical recording and readout. a) A sche-matic illustration of the patterning mechanism. The patterning is achieved by the photothermalreshaping of the gold nanorods in the focal volume of the focusing objective. The nanorodreshaping is selective in terms of aspect ratio and orientation of the nanorod. A linear polarizedlaser pulse will only be absorbed by gold nanorods that are aligned to the laser light polarizationand exhibit an absorption cross-section that matches the laser wavelength. Top panel of (a):s-polarized laser light with a wavelength of 840 nm will only affect the nanorods with anintermediate aspect ratio that are aligned to the laser polarization (encircled spheres indicaterod reshaping). Bottom panel of (a): p-polarized laser light with a wavelength of 980 nm onlyreshapes the high-aspect-ratio gold nanorods aligned with the laser light polarization.b) Normalized two-photon photoluminescence (TPPL) raster scans of 18 patterns encodedin the same area using two laser light polarizations and three different laser wavelengths. Patternswere written in three layers spaced by 10mm. The recording laser pulse properties are indicated(wavelength at left, polarization at bottom). The recordings were retrieved by detecting the TPPLexcited with the same wavelength and polarization as employed for the recording. The size of allimages is 100mm� 100mm, and the patterns are 75 pixels �75 pixels. Reproduced withpermission from ref. [307]. Copyright 2009Macmillan Publishers Limited.

10. Conclusions and Outlook

Understanding the properties and exploringapplications of nonspherical NMNPs havemade great progress in recent years.Advances in theory and computational tech-niques for quantitative understanding of themorphology-dependent properties have beensatisfactory. Scientists around the world haveshown tremendous interest in a variety ofapplications of NMNPs ranging from efficientcatalysis to improved methods for diagnosticsand the treatment of diseases. This reviewattempted to highlight properties and applica-tions of chemically synthesized colloidalNMNPs with nonsperical morphologies.Both physical and chemical properties ofnanoparticles are affected by subtle aspectsof particle morphology. This gives an oppor-tunity to generate novel properties, which inturn offers numerous innovative possibilitiesfor technological applications. Anisotropicfeatures in nonspherical nanoparticles makethem ideal candidates for enhanced chemical,catalytic, and local field related applications.Nonspherical plasmon resonant nanoparticlesoffer favorable properties for their use as (invivo) analytical tools, transport vehicles, as wellas diagnostic and therapeutic agents.

New applications of NMNPs are bound toevolve rapidly as more and more particles withwell-controlled morphologies become avail-able. One of the major trends in furtherdevelopment of nonspherical nanoparticles


will be to make them multifunctional and to control theirproperties via the local environment or external stimuli. This isessential for construction of future nanodevices. Looking furtherto future potential applications, one can expect that the NMNPs ofcontrolled morphologies will find applications as in vivo forcesensors; morphological seeds for aligned block copolymer/inorganic nanocomposites with anisotropic properties; smartcatalysts (for fuel cells, waste treatment, bioprocessing, etc);enhancers in photovoltaics and other energy conversion devices;better therapeutic and imaging agents; probes for single-moleculesensing of drugs, toxins, and environmental pollutants; and keycomponents in energy transport and novel opto-electronicdevices.


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Acknowledgements

We acknowledge that there are many more important papers publishedthan those reviewed in this contribution. However, space restriction forcedus to execute a somewhat arbitrary selection of reports to be included. Wealso acknowledge contributions of our co-workers and collaborators. Wethank the German Research Foundation (DFG) for financial supportthrough the Excellence Cluster ‘‘Nanosystems Initiative Munich’’ (NIM)and the LMUexcellent program, and the Alexander-von-HumboldtFoundation for the research fellowship to T.K.S.

Received: July 30, 2009

Published online: January 4, 2010

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