Colloidal Ag and Au nanoparticles have been intensively studied due to their potential and real applications varying from optics and catalysis to biomedicine.1-3 Many methods exist for the synthesis of Ag and Au nanoparticles, but most processes result in low nanoparticle concentrations due to aggregation into large particles.4.5 Surfactants, thiols or clays as capping agents or stabilizing reagents have been used effectively to prevent the growth and aggregation of nanoparticles. In particular, high concentrations of Ag colloidal sol have been obtained using synthetic layered clay, laponite as a steric barrier to prevent aggregation of Ag nanoparticles. The use of clay for high concentrations of noble metal nanoparticles generates nanoparticle/clay composites that are believed to be important materials with a large variety of applications in functionalized ceramics, adsorbents, ion exchanger and catalysts.7 Noble metal nanoparticles in layered clays are distributed into central layer spaces and/or on the external surfaces. The adsorption of Ag nanoparticles on the external surfaces and edges of talc in Ag-talc nanocomposites were suggested based on X-ray diffraction results that showed little variation of basal d 001 -spacing.8 In the case of kaolinite and montmorillonite, intercallation of small Ag nanoparticles or Ag clusters in the interlamellar space of layered clay minerals was reported, as well as the formation of larger Ag nanoparticles on the exterior surface of these clays.9-12 However, exfoliated noble metal-clay nanocomposites are rare in spite of expected high performance due to large surface areas of clays to allow interactions with nanometals.We have been interested in a colloidal synthesis procedure and found a simple method for very stable (> 24 months) Ag nanoparticle sols with laponite. The natural hectorite of the smectites, laponite is utilized as a stabilizing reagent in this synthesis and forms the basis for production of highly concentrated solutions of Ag nanoparticles. Laponite is a synthetic polycrystalline clay and the empirical composition is Na 0.7 [(Si 8 Mg 5.5 Li 0.3 )O 20 (OH) 4 ].6 The structure of laponite consists of layers formed by the condensation of two outer layers of linked Si(O,OH) 4 tetrahedra with a central layer of linked M(OH) 6 octahedra, where M is either an Mg or Li ion. Part of the magnesium ions in the central layer are substituted by lithium ions, resulting in a net negative charge of the layer, which is balanced by sodium ions located between adjacent layers in a stack. The sodium ions in laponite are exchangeable, and in aqueous dispersions, these ions diffuse into the water, and plate-like particles with negatively charged faces are formed. This work reports on Ag nanoparticles-laponite composites prepared by a freezedrying method from resulting Ag nanoparticle sols. The adsorption behavior of Ag nanoparticles into laponite is investigated.As the concentration of AgNO 3 increased, the intensity of the absorption peak at about 397~407 nm in UV-vis spectra of...
