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.

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

The creation of core–shell particles is attracting a great deal of interest because of the diverse applicability of these colloidal particles; e.g., as building blocks for photonic crystals, in multi-enzyme biocatalysis, and in drug delivery. This review presents the state-of-the-art in strategies for engineering particle surfaces, such as the layer-by-layer deposition process (see Figure), which allows fine control over shell thickness and composition.

Nanoengineering of particle surfaces.

The concept of a general attractive interaction between neutral atoms was first proposed by van der Surface Thermodynamics of (Lewis) Waals in 1873, to account for certain properties of Acid-Base (AB) Interactions nonideal gases and liquids.’ Three different but nevThe Young-Dupr6 Equation 933 ertheless related phenomena were subsequently shown to contribute to these “van der Waals” interactions: (1) randomly orienting dipole-dipole (or orientation) inPositive and Negative Interfacial

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Interfacial Lifshitz-van der Waals and polar interactions in macroscopic systems

Introduction PART I: THEORY Lifshitz-van der Waals (LW) Interactions Relation Between the Hamaker Constant and the Apolar Surface Tension Component Polar or Lewis Acid-Base Interactions Electrical Double Layer Interactions Brownian Movement Forces-Osmotic Interactions of Polymers Rate of Decay with Distance PART II: INTERFACIAL PROPERTIES AND STRUCTURE OF LIQUID WATER Lifshitz-van der Waals and Lewis Acid-Base Properties of Liquid Water-Physical and Physico-Chemical Effects Role of Water in Hydrophobic Attraction Role of Water in Hydrophilic Repulsion The Water-Air Interface PART III: EXPERIMENTAL MEASUREMENT METHODS Contact Angle and Surface Tension Determination and Preparation of Solid Surfaces Interfacial Tension Determination-Influence of Macroscopic- and Microscopic-Scale Interactions Different Approaches for Interpreting Contact Angles and Determining the Surface Tension and Surface Tension Components of Solids Electrokinetic Methods Direct Measurement Methods, Treating the Force Balance in Particular PART IV: ASSOCIATED PHENOMENA AND APPLICATIONS Surface Tension Components and Parameters of Liquids and Solids Attractive LW- and AB-Forces: Hydrophobic Interactions-Osmotic Pressures of PEO Solutions Repulsive AB-Forces: Hydrophilic Interactions The Primary and Secondary Interactions Phase Separation in Polymer Solutions Coacervation and Complex Coacervation Solubility of Polymers and Other Solutes Cell and Particle Stability Adsorption and Adhesion in Aqueous Media, Including Ligand-Receptor Interactions Kinetics and Energetics of Protein Adsorption onto Metal Oxide Surfaces List of Symbols Used References Index

Interfacial Forces in Aqueous Media

Polymeric stabilization of colloidal dispersions

A model, together with a comparison with previously published experimental data, is presented for initiator efficiency in seeded styrene emulsion polymerization systems in the absence of secondary particle formation. The data had shown that a number of previous models are inapplicable, viz., those assuming that the rate-determining step for free-radical entry into a particle is either diffusional capture, surfactant displacement, or colloidal entry. The data support the supposition that the rate-determining step for free-radical capture by latex particles is aqueous-phase propagation to a critical degree of polymerization, whereupon capture (irreversible adsorption) of the resulting oligomeric free radical by a particle is essentially instantaneous. Mutual aqueous-phase termination of smaller species also occurs. When account is taken of the fact that the rate coefficients for (a) the first aqueous-phase propagation step and (b) aqueous-phase termination are both in the diffusion limit, this model is in qualitative and quantitative accord with the experimental dependences of the entry rate coefficient on the concentrations of initiator, of surfactant, of aqueous-phase monomer, and of latex particles as well as on particle size and on ionic strength. For styrene emulsion polymerization initiated by persulfate, the critical oligomer size for entry was found to be dimeric.

Entry of free radicals into latex particles in emulsion polymerization

An approximate theory of depletion stabilization and depletion flocculation (i.e., stabilization and flocculation of colloidal particles by polymers in free solution) is presented. This is based upon the rotational isomeric state-Monte Carlo procedures for calculating the conformation of macromolecules between two particles and the Flory-Huggins theory of polymer solutions. It is shown that reasonable agreement between theory and experiment can be achieved for the critical volume fractions of polymer required to induce both depletion flocculation and depletion stabilization. The presence of adsorbed or attached polymer at the interface may have a significant effect on the onset of both flocculation and stabilization; this is especially true if the molecular weight of the attached chains is comparable to or greater than that of the free polymer. The effects of the steric chains, however, are reduced if the free polymer is significantly higher in molecular weight. Free energy minimization procedures are then necessary to calculate the polymer distribution between the plates. The agreement obtained between theory and experiment suggests that the theory incorporates the dominant factors that control depletion flocculation and stabilization.

Depletion stabilization and depletion flocculation

A detailed theory is presented for nucleation kinetics in emulsion polymerization systems based on the coagulation of precursor particles (which may themselves be formed by either homogeneous nucleation or micellar entry). These precursor particles differ from true latex particles by a slower rate of polymerization and the lack of stability against coagulation. The coagulative nucleation theory combines extended Muller-Smoluchowski coagulation kinetics with DLVO theory. Expressions are provided for the time evolutions of the nucleation rate, particle number, and particle size distribution (PSD). With physically reasonable values for the parameters for the coagulation kinetics, agreement is obtained with data for styrene emulsion polymerization systems. In particular, excellent accord is obtained with the early-time evolution of the PSD, such data being especially sensitive to assumptions as to the nucleation mechanism. In addition, agreement is obtained with data on the dependence of particle number on surfactant and initiator concentrations.

Coagulative nucleation and particle size distributions in emulsion polymerization

A complete set of rate equations describing the kinetics of free-radical polymerization is deduced, in which termination rate coefficients are allowed to depend on chain length. The rate equations are applicable to bulk, solution, and heterogeneous (e.g., emulsion) polymerization systems. They incorporate the following kinetic processes: initiation, propagation, transfer, chain-length-dependent termination, and, for emulsion systems, exit and reentry (i.e., aqueous-phase kinetic processes are taken into account). It is shown that, despite their apparent complexity, the full set of population balance equations can, to a good approximation, be reduced to a single first-order differential equation. This equation and that for the overall rate of polymerization form a pair of coupled differential equations that can be solved easily and are thus suitable for routine modeling of experiment. The key parameters required in the model are the rate coefficients for transfer and propagation and the diffusion coefficient of the monomer as a function of the polymer weight fraction (conversion), this variation being used in expressions for the dependence of the termination rate coefficient on the length of a growing chain. In accord with previous experiment and theory (Adams, M. E.; Russell, G. T.; Casey, B. S.; Napper, D. H.; Gilbert, R. G.; Sangster, D. F. Macromolecules 1990, 23, 4624), the present theory indicates that, at intermediate conversions, termination is dominated by interactions between short chains formed by transfer and entangled long chains. In the regime in which propagation is diffusion-controlled, most termination events involve two highly entangled chains, whose ends move by the "reaction-diffusion" process (Russell, G. T.; Napper, D. H.; Gilbert, R. G. Macromolecules 1988, 21, 2133). The mathematical treatments of this paper are of very general applicability and should inter alia be useful in addressing practical problems such as minimizing the residual monomer content of polymer products.

Donald H. Napper

Papers

Polymeric stabilization of colloidal dispersions

Entry of free radicals into latex particles in emulsion polymerization

Depletion stabilization and depletion flocculation

Coagulative nucleation and particle size distributions in emulsion polymerization

Chain-length-dependent termination rate processes in free-radical polymerizations. 1. Theory