Contents 1. General Introduction 4798 2. Optical Properties of Metal Clusters in a Close-Packed Assembly: Interparticle Coupling Effect on the Surface Plasmon Resonance 4800 2.1. Cluster Size Effect 4800 2.2. Creation of Surface Plasmon Oscillation in Metal Nanoparticles 4801 2.3. Optical Properties of Isolated Metal Nanoparticles: The Mie Theory 4801 2.3.1. Assumptions of Mie Theory 4802 2.3.2. Quasistatic Response of a Spherical Metal Nanoparticle to an Electric Field 4802 2.3.3. External Size Effect of Metal Nanoparticles on the Optical Response 4802 2.3.4. Deviation of the Particle Shape from Sphericity 4804 2.3.5. Limitations of Mie Theory 4805 2.4. While the Metallic Particles in the Colloidal Solution Are Not Isolated: Maxwell Garnett Effective Medium Theory 4805 2.4.1. Assumptions of Effective-Medium Model 4806 2.4.2. Quasistatic Response of Aggregates of Metal Spheres to an Electric Field: The Effective-Medium Theory 4806 2.4.3. Explicit Particle Methods 4808 2.4.4. Discrete Dipole Approximation (DDA) 4808 2.4.5. Finite-Difference Time-Domain (FDTD) Method 4809 2.4.6. Isotropic vs Anisotropic Colloidal Assemblies 4810 3. Synthetic Strategies in Making Nanoscale Gold Assemblies 4811 3.1. In Situ Formation of Nanoscale Gold Aggregates 4811 3.1.1. Biologically Programmed Nanoparticle Assembly 4811 3.1.2. Organic Ligand Modified Nanoparticle Building Blocks 4813 3.1.3. Self-Assembly Generated by Inorganic Ligands 4817 3.1.4. Surfactant-Mediated Nanoparticle Aggregates 4819 3.1.5. Polymers as Architectures of Nanoscale Assemblies 4823 3.2. Induced Aggregation among the Already Formed Gold Particles 4825 3.2.1. Electrolyte-Induced Aggregation

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Silver and gold nanoparticles have been grown on calcium alginate gel beads using a green photochemical approach. The gel served as both a reductant and a stabilizer. The nanoparticles were characterized using UV-visible spectroscopy, X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), energy dispersive X-ray (EDS), and selected area electron diffraction (SAED) analyses. The particles are spherical, crystalline, and the size ranges for both Ag and Au nanoparticles are <10 nm. It is noticed from the sorption experiment that the loading of gold on calcium alginate beads is much more compared to that of Ag. The effectiveness of the as-prepared dried alginate-stabilized Ag and Au nanoparticles as a solid phase heterogeneous catalyst has been evaluated, for the first time, on the well-known 4-nitrophenol (4-NP) reduction to 4-aminophenol (4-AP) in the presence of excess borohydride. The reduction was very efficient and followed zero-order kinetics for both Ag and Au nanocomposites. The effects of borohydride, initial 4-NP concentration, and catalyst dose were evaluated. The catalyst efficiency was examined on the basis of turnover frequency (TOF) and recyclability. The catalytic efficiency of alginate-based Ag catalyst was much more compared to that of the Au catalyst. The as-prepared new solid-phase biopolymer-based catalysts are very efficient, stable, easy to prepare, eco-friendly, and cost-effective, and they have the potential for industrial applications.

Core-shell nanocomposites (R-Au) bearing well-defined gold nanoparticles as surface atoms of variable sizes (8-55 nm) have been synthesized exploiting polystyrene-based commercial anion exchangers. Immobilization of gold nanoparticles, prepared by the Frens method, onto the resin beads in the chloride form is possible by the ready exchange of the citrate-capped negatively charged gold particles. The difficulty of nanoparticle loading, avoiding aggregation, has been solved by stepwise operation. Analysis of the gold particles after immobilization and successive elution confirm the unaltered particle morphology while compared to those of the citrate-capped gold particles in colloidal dispersion. It was observed that the rate of the reaction increases with the increase in catalyst loading, which suggests the catalytic behavior of the gold nanoparticles for the reduction of the aromatic nitrocompounds. The rate constant, k, was found to be proportional to the total surface area of the nanoparticles in the system. Kinetic study for the reduction of a series of aromatic nitrocompounds reveals that the aromatic nitrocompound exclusively adsorbs to atop sites of gold particles and that the rate of the reduction reaction increases as the particle size decreases. Similar reaction kinetics was observed involving gold sol of variable size (homogeneous catalysis) as catalyst. The induction time and the activation energy of the reaction decreases with decrease in particle size indicating the decrease in activation energy for the smaller particles, which also speaks for the increase of surface roughness with decrease in particle size. The observed rate dependence, in relation to particle size, is attributed to a higher reactivity of the coordinatively unsaturated surface atoms in small particles compared to low-index surface atoms prevalent in larger particles.

Nitrophenol reduction to aminophenol with a reducing agent is conveniently carried out in aqueous medium mainly with a metal or metal oxide catalyst. This reduction is presently considered as a benchmark reaction to test a catalyst nanoparticle. Thousands of original reports have enriched this field of nanoparticle catalyzed reaction. Synthesis of different metal and metal oxide nanoparticles and their composites along with their role as catalysts for nitrophenol reduction with varying reducing agents have been elucidated here. The progress of the reaction is conveniently monitored by UV-visible spectrophotometry and hence it becomes a universally accepted model reaction. In this review we have discussed the reaction kinetics considering its elegance and importance enlightening the long known Langmuir-Hinshelwood mechanism and Eley-Rideal mechanism at length, along with a few other mechanisms recently reported. A brief description of the synthetic procedures of various nanoparticles and their respective catalytic behaviour towards nitroarene reduction has also been accounted here.

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Cetylpyridinium chloride(CPC)-stabilized gold organosol in toluene has been prepared by using a two-phase (water-toluene) extraction of AuCl 4followed by its reduction with sodium borohydride in the presence of the surfactant, CPC. The surfactant-stabilized gold nanoparticles were exploited to examine their optical properties when exposed to various solvent systems and ligands by measuring the changes in the localized surface plasmon resonance (LSPR) spectrum. It was seen that the position of the surface plasmon band of metal nanoparticles is greatly influenced by the solvents and the ligands under consideration. The surface plasmon absorption maxima modulates/varies between 520 and 550 nm for gold nanoparticles, depending on the refractive index of the solvent. The significant discovery presented here is that λ max of the LSPR shifts to the blue by 3 nm for the increase of one carbon atom in the alcohol chain. Cationic and anionic surfactants of different chain lengths induce changes in the optical properties of gold nanoparticles, whereas zwitterionic amino acid molecules do not incite remarkable changes in the LSPR spectrum. The λ max of the LSPR gradually shifts to the red with the increase in chain length for both the cationic and anionic surfactants indicating specific binding of the surfactant molecules around the gold particles. Binding of three model compounds (1-dodecylamine, 1-dodecanol, and 1-dodecanethiol) indicates their relative affinity toward the gold surface that corroborate the HSAB (Hard-Soft Acid-Base) principle.

Beta-cyclodextrin (β-CD) in alkaline solution has been observed to produce mono-and bimetallic nanoparticles of silver and gold and to provide in-house stability to both types of particles. Thus, the weak reducing capability of the β-CD molecule (oxidation occurs at +1.33 V vs Ag/AgCl) and its unique kinetic control over the evolution of both normal and inverted core-shell bimetallic architectures have been established. The structure and composition of the bimetallic particles were characterized by UV-visible spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, electron dispersive spectroscopy, and X-ray photoelectron spectroscopy. Bimetallic core-shell particles containing silver shells have been shown to provide an elegant SERS-active substrate compared to the corresponding monometallic nanoparticles, and therefore, they highlight the importance of electronic ligand effects on the enhancement of the Raman signals of molecular probes on nanostructured metallic surfaces.