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.

This review focuses on the synthesis, protection, functionalization, and application of magnetic nanoparticles, as well as the magnetic properties of nanostructured systems. Substantial progress in the size and shape control of magnetic nanoparticles has been made by developing methods such as co-precipitation, thermal decomposition and/or reduction, micelle synthesis, and hydrothermal synthesis. A major challenge still is protection against corrosion, and therefore suitable protection strategies will be emphasized, for example, surfactant/polymer coating, silica coating and carbon coating of magnetic nanoparticles or embedding them in a matrix/support. Properly protected magnetic nanoparticles can be used as building blocks for the fabrication of various functional systems, and their application in catalysis and biotechnology will be briefly reviewed. Finally, some future trends and perspectives in these research areas will be outlined.

Magnetic nanoparticles: Synthesis, protection, functionalization, and application

1. Introduction 20642. Synthesis of Magnetic Nanoparticles 20662.1. Classical Synthesis by Coprecipitation 20662.2. Reactions in Constrained Environments 20682.3. Hydrothermal and High-TemperatureReactions20692.4. Sol-Gel Reactions 20702.5. Polyol Methods 20712.6. Flow Injection Syntheses 20712.7. Electrochemical Methods 20712.8. Aerosol/Vapor Methods 20712.9. Sonolysis 20723. Stabilization of Magnetic Particles 20723.1. Monomeric Stabilizers 20723.1.1. Carboxylates 20733.1.2. Phosphates 20733.2. Inorganic Materials 20733.2.1. Silica 20733.2.2. Gold 20743.3. Polymer Stabilizers 20743.3.1. Dextran 20743.3.2. Polyethylene Glycol (PEG) 20753.3.3. Polyvinyl Alcohol (PVA) 20753.3.4. Alginate 20753.3.5. Chitosan 20753.3.6. Other Polymers 20753.4. Other Strategies for Stabilization 20764. Methods of Vectorization of the Particles 20765. Structural and Physicochemical Characterization 20785.1. Size, Polydispersity, Shape, and SurfaceCharacterization20795.2. Structure of Ferro- or FerrimagneticNanoparticles20805.2.1. Ferro- and Ferrimagnetic Nanoparticles 20805.3. Use of Nanoparticles as Contrast Agents forMRI20825.3.1. High Anisotropy Model 20845.3.2. Small Crystal and Low Anisotropy EnergyLimit20855.3.3. Practical Interests of Magnetic NuclearRelaxation for the Characterization ofSuperparamagnetic Colloid20855.3.4. Relaxation of Agglomerated Systems 20856. Applications 20866.1. MRI: Cellular Labeling, Molecular Imaging(Inﬂammation, Apoptose, etc.)20866.2.

Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization, Physicochemical Characterizations, and Biological Applications

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?

In the past decade, colloidal solutions have been assumed to be very efficient templates for controlling particle size and shape A large number of groups have used reverse micelles to control the size of spherical nanoparticles This makes it possible to determine the various parameters involved in such processes, and demonstrates that nanoparticles can be considered to be efficient nanoreactors However, some discrepancies arise There are few reports concerning the control of particle shape, and it is still rather difficult to determine the key parameters, such as the adsorption of salts and other molecules, and the synthesis procedure Here, we discuss these controls of the size and shape of inorganic nanomaterials

The role of soft colloidal templates in controlling the size and shape of inorganic nanocrystals

In this feature article, syntheses of nanosized particles by using colloidal assemblies as a template are described We asked ourselves the following question:  What parameters play a role in the control of the size, shape, and polydispersity? We know that parameters such as the shape of colloidal assemblies, the hydration of the head polar group, the water molecules bounded to the interface, etc play a major role However, there are a number of exceptions preventing any generalization It is shown that the chemical mechanism in nanoparticles production in colloidal assemblies can differ from those usually observed in homogeneous solution This shows that the solution chemistry cannot always be transferred to colloidal systems It is possible to select the size and markedly reduce the polydispersity by a surface treatment of the nanoparticles This favors formation of mono- and multilayers made of nanoparticles, and it is found that these particles form crystals organized in a three-dimensional face-cent

Nanosized Particles Made in Colloidal Assemblies

In this feature article, the methods of obtaining various mesostructures made of nanocrystals are described. With silver and silver sulfide, the nanocrystals are able to self organize in 2D and 3D super lattices to form “supra” crystals. With cobalt and ferrites nanocrystals, it has been possible to make ribbons, dots, or labyrinths. These mesostructures present new physical properties differing from those of the isolated nanocrystal and from those of the bulk phase. Collective optical, magnetic, and transport properties are demonstrated. By applying a magnetic field during the deposition process of nanocrystals, the easy axes of the particles are oriented along the direction of the applied field, inducing again the collective magnetic properties.

Nanocrystal Self-Assemblies: Fabrication and Collective Properties

Silver nanosized crystallites have been synthesized in AOT reverse micelles. By using a functionalized surfactant, silver sulfosuccinate (Ag(AOT)), a reactant, a high concentration of monodisperse and stable nanosize particles can be obtained even in the presence of air. This study focused on the various parameters controlling the nanosize of silver particles such as micellar structure and nature of the reducing agent or of the solvent. At low water content, a precursor such as Ag 4 2+ can be stabilized. The changes in the size of the crystallites with the number of initiators per micelle and with the micellar exchange processes can be explained from a kinetic model

In situ synthesis of silver nanocluster in AOT reverse micelles

A simple method is used to prepare highly monodispersed silver nanoparticles in the liquid phase, which starts from an initial synthesis in functionalized AOT reverse micelles. To narrow the particle size distribution from 43% to 12.5% in dispersion, the particles are extracted from the micellar solution. The size-selected precipitation method is used. The decrease in polydispersity of the silver nanoparticles is followed by transmission electron microscopy, by UV−vis spectroscopy, and by small-angle X-ray scattering. The nanocrystallites dispersed in hexane are deposited on a support. A monolayer made of nanoparticles with spontaneous hexagonal organization is observed. The immersion of the support on the solution yields to the formation of organized multilayers arranged as microcrystal in a face-centered cubic structure.

Marie-Paule Pileni

Papers

The role of soft colloidal templates in controlling the size and shape of inorganic nanocrystals

Nanosized Particles Made in Colloidal Assemblies

Nanocrystal Self-Assemblies: Fabrication and Collective Properties

In situ synthesis of silver nanocluster in AOT reverse micelles

Synthesis of Highly Monodisperse Silver Nanoparticles from AOT Reverse Micelles: A Way to 2D and 3D Self-Organization