The optical properties of metal nanoparticles have long been of interest in physical chemistry, starting with Faraday's investigations of colloidal gold in the middle 1800s. More recently, new lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes, and dielectric environments. In this feature article, we describe recent progress in the theory of nanoparticle optical properties, particularly methods for solving Maxwell's equations for light scattering from particles of arbitrary shape in a complex environment. Included is a description of the qualitative features of dipole and quadrupole plasmon resonances for spherical particles; a discussion of analytical and numerical methods for calculating extinction and scattering cross-sections, local fields, and other optical properties for nonspherical particles; and a survey of applications to problems of recent interest involving triangula...

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The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

A method is used for preparing gold NRs with aspect ratios ranging from 1.5 to 10 for which the surface plasmon absorption maxima are between 600 and 1300 nm. This method has been adapted from a previously published seed-mediated growth method (Jana et al. Adv. Mater. 2001, 13, 1389). The disadvantages and limitations of the earlier method (i.e., formation of noncylindrical NRs, φ-shaped particles, and formation of a large fraction of spherical particles) have been overcome by use of a hexadecyltrimethylammonium bromide (CTAB)-capped seed instead of a citrate-capped one. In a single-component surfactant system, the silver content of the growth solution was used to grow NRs to a desired length. This results in reproducible formation of NRs with aspect ratios ranging from 1.5 to 4.5. To grow longer NRs with aspect ratios ranging from 4.6 to 10, a binary surfactant mixture composed of benzyldimethylhexadecylammoniumchloride (BDAC) and CTAB was used. NRs are grown in this mixture either by aging or by additio...

Preparation and Growth Mechanism of Gold Nanorods (NRs) Using Seed-Mediated Growth Method

The selection of nanoparticles for achieving efficient contrast for biological and cell imaging applications, as well as for photothermal therapeutic applications, is based on the optical properties of the nanoparticles. We use Mie theory and discrete dipole approximation method to calculate absorption and scattering efficiencies and optical resonance wavelengths for three commonly used classes of nanoparticles:  gold nanospheres, silica−gold nanoshells, and gold nanorods. The calculated spectra clearly reflect the well-known dependence of nanoparticle optical properties viz. the resonance wavelength, the extinction cross-section, and the ratio of scattering to absorption, on the nanoparticle dimensions. A systematic quantitative study of the various trends is presented. By increasing the size of gold nanospheres from 20 to 80 nm, the magnitude of extinction as well as the relative contribution of scattering to the extinction rapidly increases. Gold nanospheres in the size range commonly employed (∼40 nm)...

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Calculated Absorption and Scattering Properties of Gold Nanoparticles of Different Size, Shape, and Composition: Applications in Biological Imaging and Biomedicine

Aqueous solutions containing a high yield of suspended gold nanorods have been successfully synthesized via an electrochemical method. The control of preparing gold nanorods with different aspect ratios can be attained. Their absorption spectral features show a dominant surface plasma band corresponding to the longitudinal resonance, SPl, and its λmax shifts markedly to the red as the aspect ratio is increased. Meanwhile, the dependence of λmax for longitudinal resonance on the mean aspect ratio shows a deviation from the classical electrostatic model prediction at mean aspect ratios around 4 ± 1, where it limits the validity of the classical electrostatic approximation.

Gold Nanorods: Electrochemical Synthesis and Optical Properties

We report a revised synthetic procedure based on an electrochemical method for preparing an aqueous solution containing suspended Au nanorods. The mean aspect ratios of the Au nanorods can be experimentally adjusted between 1 and 7. The evolution of the longitudinal surface plasmon bands shows an eminently sensitive dependence on the aspect ratios of the nanorods. Their dependence is accordingly described by classical-electrostatic-model predictions. The shape transition of the nanorod particles has been studied by varying some key influencing factors such as the wavelength, the laser fluence, and matrix effects. The nanorods were exposed to laser lines at 532 and 1064 nm, frequencies which correspond closely to the short- and long-axis plasmon resonances, respectively. A photon-induced shape transition process was evidenced, and the corresponding rod-to-sphere conversion contributed by a photoannealing process was observed in both cases. Meanwhile, we observed a new type of “φ-shaped” Au nanostructure in...

The Shape Transition of Gold Nanorods

The feasibility of employing capillary zone electrophoresis to separate nanostructured gold particles was investigated. The optimal experimental conditions of rinsing procedure, type and concentration of cationic surfactant, buffer concentration, and pH value were identified for the separations. Under the optimal conditions — included rinsing capillary with 0.5 N NaOH, water, and buffer for 3 min in sequence, and running the experiment with 7.5 mM TTAB, 12.5 mM tetraborate, and pH 9.0 of buffer solution at 20 kV applied voltage — nearly resolved separation (Rs = 1.0) of gold particles with mean diameter differences as small as 1 nm was achieved.

Separation of Nanostructured Gold Particles by Capillary Zone Electrophoresis

The construction of the Taiwan Photon Source (TPS) storage ring is in progress, to be completed in mid 2012. A new helium cryogenic system, to be provided by Linde Company, will be installed after the TPS storage ring is completed. Superconducting radio-frequency (SRF) modules are required to maintain the electron energy of the storage ring and are operated in a refrigeration mode such that cold gaseous helium from the cavity cryostat is returned to the refrigerator. A distribution valve box and individual segments of multichannel transfer lines are required to supply liquids helium and nitrogen to the SRF cavities and to recover the gaseous helium and nitrogen to the cryogenic system. Here we discuss the configuration and design features of the LHe transfer system, and present also calculations of the heat load and pressure drop of the transfer system.

Design of a liquid-helium transfer system for the tps project

Colloidal multilayers of octadecanethiol-capped Au nanorods were prepared using the Langmuir-Blodgett technique. Uniform growth of the films with increasing number of deposition has been observed. Vertical transfer efficiency indicates efficient transfer for immersion of hydrophobic substrates and poor transfer for emersion of such substrates. The vertically transferred film has an order parameter S = 0.027, which suggests that the orientation of the nanorods in the film is somewhat isotropic about the dipping direction. Horizontal transfer of the Langmuir monolayer on Formvar/carbon-coated Cu grids, however, gave scattered domains of Au nanorods aligned in parallel stacks. Such behavior is probably related to the rigidity of the Langmuir monolayer.

Ser-Sing Chang

Papers

Gold Nanorods: Electrochemical Synthesis and Optical Properties

The Shape Transition of Gold Nanorods

Separation of Nanostructured Gold Particles by Capillary Zone Electrophoresis

Design of a liquid-helium transfer system for the tps project

Langmuir-Blodgett Films of Alkanethiolate Gold Nanorods