Thermal conductivities of two kinds of Au nanoparticles were measured in water and toluene media. The water soluble particles, 10-20 nm in mean diameter, made with citrate stabilization showed thermal conductivity enhancement of 5%-21% in the temperature range of 30-60°C at a loading of 0.000 26 ͑by volume͒. The effect was 7%-14% for Au particles stabilized with a monolayer of octadecanethiol even for a loading of 0.011%. Comparatively lower thermal conductivity enhancement was observed for larger diameter Ag particles for significantly higher loading. Effective enhancement of 9%, even at vanishing concentrations, points to additional factors in the thermal conductivity mechanism in nanofluids. Results also point to important chemical factors such as the need for direct contact of the metal surface with the solvent medium to improve enhancement.

We report a one-step route for the synthesis of Au@TiO2, Au@ZrO2, Ag@TiO2, and Ag@ZrO2 particles in nanometer dimensions, with controllable shell thickness. This scalable procedure leads to stable and freely dispersible particles, and bulk nanocomposite materials have been made this way. The procedure leads to particles of various morphologies, with a crystalline core in the size range of 30−60 nm diameter and an amorphous shell of ∼3 nm thickness in a typical synthesis. The core diameter and shell thickness (in the range of 1−10 nm) can be varied, leading to different absorption maxima. The material has been characterized with microscopic, diffraction, and spectroscopic techniques. The metal particle growth occurs by the carbamic acid reduction route followed by hydrolysis of the metal oxide precursor, resulting in the oxide cover. The particles could be precipitated and redispersed. The shell, upon thermal treatment, gets converted to crystalline oxides. Cyclic voltammetric studies confirm the core−shell structure. The E 1/2 value is 0.250 V (ΔE ≈ 180 mV) for the quasi-reversible Ag m /Ag m + couple and 0.320 V (ΔE ≈ 100 mV) for the Au n /Au n + couple for Ag and Au particles, respectively. Adsorption on the oxide surface blocks electron transfer partially. Nonlinear optical measurements in solutions show that these materials are strong optical limiters with a high laser damage threshold.

The nanoparticles of gold and silver in solution state and supported over activated alumina are effective systems for the quantitative removal of chlorpyrifos and malathion, two common pesticides found in surface waters of developing nations, from water. In the solution phase, these pesticides adsorb onto the nanoparticles' surfaces and upon interaction for a long time, the nanoparticles with adsorbed pesticides precipitate. In contrast, complete removal of these pesticides occurs when contaminated water is passed over nanoparticles supported on alumina. A prototype of an on-line filter was made using a column of activated alumina powder loaded with silver nanoparticles and the device was used for pesticide removal for extended periods. We believe that the method has great technological potential in drinking water purification, especially using silver nanoparticles.

Temperature-dependent dynamics of monolayer-protected Au and Ag nanoclusters and silver thiolates have been investigated with quasielastic neutron scattering. The simplest motion in these systems is the uniaxial rotation of the chain, which evolves slowly with temperature. While longer chain monolayers (above C 8 ) on Au clusters are rotationally frozen at room temperature, dynamic freedom exists in lower chain lengths. In the superlattice solids of Ag clusters, the dynamics evolve slowly, and at superlattice melting, all the chains are dynamic. The data are consistent with a structure in which the monolayers form bundles on the planes of metal clusters and such bundles interdigitate, forming the cluster assemblies. In thiolates, the dynamics is distinctly different in long-and short-chain systems. It arises abruptly at the melting temperature in C 12 but a bit sluggishly in C 18 , whereas in C 6 and C 8 , it evolves with temperature. The data are correlated with temperature-dependent infrared spectroscopy, which preserves some of the progression bands even after the bulk melting temperature, but loses them completely above 498 K, suggesting a possible partially ordered phase in this temperature window. Our studies have established the fact that (a) no rotational freedom exists in several of the alkyl chain monolayers on metal cluster solids at room temperature, (b) simple uniaxial rotation explains the dynamics of these systems, (c) the dynamics evolves slowly, and (d) such motions arise abruptly in long-chain layered thiolates which are similar to planar thiolates. We find that longer chains can possess conformational defects at higher temperatures, which slow the rotational dynamics.