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 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.

Monodisperse samples of silver nanocubes were synthesized in large quantities by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). These cubes were single crystals and were characterized by a slightly truncated shape bounded by {100}, {110}, and {111} facets. The presence of PVP and its molar ratio (in terms of repeating unit) relative to silver nitrate both played important roles in determining the geometric shape and size of the product. The silver cubes could serve as sacrificial templates to generate single-crystalline nanoboxes of gold: hollow polyhedra bounded by six {100} and eight {111} facets. Controlling the size, shape, and structure of metal nanoparticles is technologically important because of the strong correlation between these parameters and optical, electrical, and catalytic properties.

http://science.sciencemag.org/content/sci/298/5601/2176.full.pdf

Shape-Controlled Synthesis of Gold and Silver Nanoparticles

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?

Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties1,2 that depend sensitively on both size and shape3,4, and are of both fundamental and technological interest In contrast to the syntheses of zero-dimensional systems, existing preparations of one-dimensional systems often yield networks of tubes or rods which are difficult to separate5,6,7,8,9,10,11,12 And, in the case of optically active II–VI and III–V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects6,8,9,10 Thus, except for some metal nanocrystals13, there are no methods of preparation that yield soluble and monodisperse particles that are quantum-confined in two of their dimensions For semiconductors, a benchmark preparation is the growth of nearly spherical II–VI and III–V nanocrystals by injection of precursor molecules into a hot surfactant14,15 Here we demonstrate that control of the growth kinetics of the II–VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one This method should be useful, not only for testing theories of quantum confinement, but also for obtaining particles with spectroscopic properties that could prove advantageous in biological labelling experiments16,17 and as chromophores in light-emitting diodes18,19

Shape control of CdSe nanocrystals

The shapes and sizes of platinum nanoparticles were controlled by changes in the ratio of the concentration of the capping polymer material to the concentration of the platinum cations used in the reductive synthesis of colloidal particles in solution at room temperature. Tetrahedral, cubic, irregular-prismatic, icosahedral, and cubo-octahedral particle shapes were observed, whose distribution was dependent on the concentration ratio of the capping polymer material to the platinum cation. Controlling the shape of platinum nanoparticles is potentially important in the field of catalysis.

https://science.sciencemag.org/content/sci/272/5270/1924.full.pdf

Shape-Controlled Synthesis of Colloidal Platinum Nanoparticles

Colloidal gold nanoparticles with an average radius of 15 nm have a surface plasmon absorption band at 530 nm. Excitation by laser pulses of 450 fs duration, and wavelength of 600 or 380 nm “bleached” the plasmon band and produced a transient absorption at the wings of the “bleach” spectrum. The transient absorption was found to have a similar temporal behavior at different wavelengths. Analysis of their temporal behavior showed two time constants:  2.5 ps, and a slower component of >50 ps. Laser excitation close to the plasmon band at 600 nm leads to the formation of “hot” non-Fermi electronic distribution within the colloidal particles. Transient absorption from these “hot” electrons led to different absorptions from that of the plasmon absorption of “cold” electrons. The “hot” electrons relax via electron−phonon coupling in 2.5 ps, and the phonon−phonon relaxation of the lattice occurs in >50 ps. At 380 nm excitation, the amplitude of the blue wing becomes smaller, and the slow component becomes longer...

Picosecond Dynamics of Colloidal Gold Nanoparticles

The electronic dynamics of gold nanocrystals, passivated by a monolayer of alkylthiol(ate) groups, were studied by transient spectroscopy after excitation with subpicosecond laser pulses. Three solution-phase gold samples with average particle size of 1.9, 2.6, and 3.2 nm with size distribution less than 10% were used. The photoexcitation in the intraband (surface plasmon region) leads to the heating of the conduction electron gas and its subsequent thermalization through electron−electron and electron−phonon interaction. The results are analyzed in terms of the contribution of the equilibrated “hot” electrons to the surface plasmon resonance of gold. A different spectral response was observed for different sizes of gold nanoparticles. The results were compared to the dynamics of the large (30 nm diameter) gold nanocrystals colloidal solution. The size-dependent spectral changes are attributed to the reduction of the density of states for small nanoparticles. The observed variation in the kinetics of the ...

Electron Dynamics of Passivated Gold Nanocrystals Probed by Subpicosecond Transient Absorption Spectroscopy

Cubic platinum nanoparticles (4−18 nm) have been synthesized for the first time in solution by the controlled reduction of K2PtCl4 with hydrogen gas in the presence of sodium polyacrylate as a capping material. The nanoparticles are found to have fcc structures, similar to the bulk metal with {100} facets.

Temer S. Ahmadi

Papers

Shape-Controlled Synthesis of Colloidal Platinum Nanoparticles

Picosecond Dynamics of Colloidal Gold Nanoparticles

Electron Dynamics of Passivated Gold Nanocrystals Probed by Subpicosecond Transient Absorption Spectroscopy

“Cubic” Colloidal Platinum Nanoparticles

Hot electron and phonon dynamics of gold nanoparticles embedded in a gel matrix