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.

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

Gold nanorods with aspect ratios of 4.6 ± 1.2, 13 ± 2, and 18 ± 2.5 (all with 16 ± 3 nm short axis) are prepared by a seeding growth approach in the presence of an aqueous miceller template. Citrate-capped 3.5 nm diameter gold particles, prepared by the reduction of HAuCl4 with borohydride, are used as the seed. The aspect ratio of the nanorods is controlled by varying the ratio of seed to metal salt. The long rods are isolated from spherical particles by centrifugation.

Wet Chemical Synthesis of High Aspect Ratio Cylindrical Gold Nanorods

the cross-sectional SEM image, the contact angle is estimated to be about 85 . When we insert into Equation 1 the surface tension (~30 dyne/cm) and density (0.95 g/cm) of SBS and L of 300 nm, a maximum height of 1.87 m is obtained, which indicates that the height of the patterned polymer can be made quite large. In fact, we were able to fabricate polymer structures with a step height as high as 5 lm for an 80 lm line-and-space pattern. If we take into account the interaction at the polymer/substrate interface and the solid-like properties of the polymer melt, the theoretical maximum height would be reduced. For a more accurate model, further study would be required. The time it takes for the polymer melt to fill up the void space between the mold and the polymer film can be estimated. If we neglect the effect of gravity, then the surface tension and viscosity of the polymer melt and the size of the capillary determine the rate of flow such that the time is given by t  2 z 2

Seed‐Mediated Growth Approach for Shape‐Controlled Synthesis of Spheroidal and Rod‐like Gold Nanoparticles Using a Surfactant Template

Following a seeding growth approach, gold nanoparticles of diameters 5−40 nm were prepared with 10−15% standard deviation in diameter from 3.5 ± 0.7 nm gold particle seeds. Particle size can be controlled by varying the ratio of seed to metal salt, and thus any size in the range 5−40 nm can be prepared. The method can also be scaled up to produce 10−100 mg of gold nanoparticles.

Seeding Growth for Size Control of 5−40 nm Diameter Gold Nanoparticles

Semiconductors and metals exhibit unusual optical, electronic, and magnetic properties on the nanometer scale. Chemists and materials scientists are developing methods to make non-spherical shapes of semiconductors and metals on the nanometer scale. We use a seed-mediated growth approach to make metallic nanorods and nanowires in homogeneous solution. Control of the ratio of metallic spherical seeds to metal salt in the reduction reaction contyrols the aspect ratio of the resulting rods and wires.

Nikhil R. Jana

Papers

Wet Chemical Synthesis of High Aspect Ratio Cylindrical Gold Nanorods

Seed‐Mediated Growth Approach for Shape‐Controlled Synthesis of Spheroidal and Rod‐like Gold Nanoparticles Using a Surfactant Template

Seeding Growth for Size Control of 5−40 nm Diameter Gold Nanoparticles

Controlling the aspect ratio of inorganic nanorods and nanowires

Wet chemical synthesis of silver nanorods and nanowiresof controllable aspect ratio