Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

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

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Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

Fundamentals of Plasmonics.- Electromagnetics of Metals.- Surface Plasmon Polaritons at Metal / Insulator Interfaces.- Excitation of Surface Plasmon Polaritons at Planar Interfaces.- Imaging Surface Plasmon Polariton Propagation.- Localized Surface Plasmons.- Electromagnetic Surface Modes at Low Frequencies.- Applications.- Plasmon Waveguides.- Transmission of Radiation Through Apertures and Films.- Enhancement of Emissive Processes and Nonlinearities.- Spectroscopy and Sensing.- Metamaterials and Imaging with Surface Plasmon Polaritons.- Concluding Remarks.

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Plasmonics: Fundamentals and Applications

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.

http://www.df.uba.ar/users/bragas/Optica%20en%20la%20nanoescala/Papers/1998_Oldenburg_ChemPhysLett_NanoengineeringOpticalResonances.pdf

Nanoengineering of optical resonances

This paper reports a systematic investigation of the growth and attachment of small gold nanoparticles to the functionalized surfaces of larger silica nanoparticles. Dilution of the gold nanoparticles in mixtures of water and ethanol led to the self-assembly of gold nanoparticles in aggregates of regular size and shape attached to the surfaces of the silica nanoparticles. Functionalization of the surfaces of silica nanoparticles with different terminal groups had a profound influence over the coverage of gold nanoparticles and clusters. While the hydrophilic functional groups NH2 and SH bound the gold nanoparticles, hydrophobic functional groups such as CH3 and PPh2 did not. The coverage of the gold nanoparticles and clusters on the surfaces of the silica nanoparticles was evaluated using transmission electron microscopy.

Formation and Adsorption of Clusters of Gold Nanoparticles onto Functionalized Silica Nanoparticle Surfaces

A metal nanoshell consists of a nanometer-scale dielectric core surrounded by a thin metallic shell. The plasmon resonance of metal nanoshells displays a geometric tunability controlled by the ratio of the core radius to the total radius. For gold-coated Au2S this ratio varies from 0.6 to 0.9, yielding a plasmon resonance tunable from 600 to greater than 1000 nm. Mie scattering theory for the nanoshell geometry quantitatively accounts for the observed plasmon resonance shifts and linewidths. In addition, the plasmon linewidth is shown to be dominated by electron surface scattering.

Linear optical properties of gold nanoshells

Gold nanoshells, nanoparticles consisting of a silica core coated with a thin gold shell, exhibit a strong optical resonance that depends sensitively on their core radius and shell thickness. Gold nanoshells have been fabricated with a peak optical extinction that can be varied across the near-infrared region of the spectrum (800 nm–2.2 μm). Multipolar plasmon resonances are clearly resolvable in the extinction spectra and agree well with electromagnetic theory. Additional resonances due to particle aggregation are also observed. The frequency agile infrared properties of these nanoparticles make them particularly attractive for a range of technologically important applications.

Infrared extinction properties of gold nanoshells

Systematic variation of the internal geometry of a dielectric core-metal shell nanoparticle allows the local electromagnetic field at the nanoparticle surface to be precisely controlled. The strength of the field as a function of core and shell dimension is measured by monitoring the surface enhanced Raman scattering (SERS) response of nonresonant molecular adsorbates (para-mercaptoaniline) bound to the nanoparticle surface. The SERS enhancement appears to be directly and exclusively due to nanoparticle geometry. Effective SERS enhancements of 106 are observable in aqueous solution, which correspond to absolute enhancements of 1012 when reabsorption of Raman emission by nearby nanoparticles is taken into account.

S. L. Westcott

Papers

Nanoengineering of optical resonances

Formation and Adsorption of Clusters of Gold Nanoparticles onto Functionalized Silica Nanoparticle Surfaces

Linear optical properties of gold nanoshells

Infrared extinction properties of gold nanoshells

Controlling the surface enhanced Raman effect via the nanoshell geometry