This chapter describes composite materials composed of dendrimers and metals or semiconductors. Three types of dendrimer/metal-ion composites are discussed: dendrimers containing structural metal ions, nonstructural exterior metal ions, and nonstructural interior metal ions. Nonstructural interior metal ions can be reduced to yield dendrimer-encapsulated metal and semiconductor nanoparticles. These materials are the principal focus of this chapter. Poly(amidoamine) (PAMAM) and poly(propylene imine) dendrimers, which are the two commercially available families of dendrimers, are in many cases monodisperse in size. Accordingly, they have a generation-dependent number of interior tertiary amines. These are able to complex a range of metal ions including Cu 2+ , Pd 2+ , and Pt 2+ . The maximum number of metal ions that can be sorbed within the dendrimer interior depends on the metal ion, the dendrimer type, and the dendrimer generation. For example, a generation six PAMAM dendrimer can contain up to 64 Cu 2+ ions. Nonstructural interior ions can be chemically reduced to yield dendrimer-encapsulated metal nanoparticles. Because each dendrimer contains a specific number of ions, the resulting metal nanoparticles are in many cases of nearly monodisperse size. Nanoparticles within dendrimers are stabilized by the dendrimer framework; that is, the dendrimer first acts as a molecular template to prepare the metal nanoparticles and then as a stabilizer to prevent agglomeration. These composites are useful for a range of catalytic applications including hydrogenations and Heck chemistry. The unique properties of the interior dendrimer microenvironment can result in formation of products not observed in the absence of the dendrimer. Moreover the exterior dendrimer branches act as a selective gate that controls access to the interior nanoparticle, which results in selective catalysis. In addition to single-metal nanoparticles, it is also possible to prepare bimetallic nanoclusters and dendrimer-encapsulated semiconductor nanoparticles, such as CdS, using this same general approach.

The recent emergence of advanced technological applications for colloidal gold suspensions and related particle assemblies and interfaces has created a demand for new chemical and physical techniques with which to characterize them. For macroscopic samples/interfaces, coherent second harmonic generation (SHG) has proven itself a useful characterization tool due, at least in part, to metal-based plasmon enhancement. In an effort to defeat or bypass the size restrictions inherent to SHG, we have utilized a related incoherent methodology, hyper-Rayleigh scattering (HRS), to interrogate aqueous colloidal suspensions of 13 nm diameter gold particles. The nanoscale particles have proven to be remarkably efficient scatterers; when evaluated in terms of the first hyperpolarizability ( ), HRS signals from the gold particles substantially surpass those observable from the best available molecular chromophores. Moreover, the present experiments indicate that is highly sensitive to colloid aggregation and imply that HRS is an effective tool for the characterization of symmetry-reducing perturbations of nanoscale interfaces.The intentional miniaturization of metal/solution interfaces to the point where colloidal stabilization of suspensions of solvated metal particles occurs has been possible, at least at a rudimentary level, for more than a century. Arguably, however, only over the past decade has the concept generated widespread excitement among chemists and materials scientists. Fueling the contemporary interest are demonstrated and emerging applications involving medical screening, 1 chemical sensing, 2 singleelectron conduction (Coulomb blockade behavior), 3 optical frequency tripling, 4 and new materials-assembly schemes. [5][6][7] Accompanying the new applications chemistry is a need for interface characterization. Among the more attractive methodologies for macroscopic metal/solution interface characterization is optical second harmonic generation (SHG). 8 For otherwise centrosymmetric metal structures, interface formation creates an asymmetry providing for interface localized signal generation. SHG has been utilized to interrogate surface symmetry, 9,10 surface charge, 11 adsorbate coverage, 12 and/or adsorbate orientation 13 on gold surfaces as well as to interrogate gold particles at the liquid/air interface. 14 However, the coherent nature of conventional SHG and the necessity of utilizing a finite wavelength of radiation place a practical lower limit on sample size. 15 On the other hand, implementation of incoherent second harmonic generation or hyper-Rayleigh scattering (HRS) methods 16 should circumvent the interface size limitation (albeit at the expense of much smaller signal intensity). 17,18 On the basis of a successful application of HRS to nanoscale silica/water interfaces (surface acidity characterization) 19 and on the basis of a brief report of HRS from silver colloid samples (molecular adsorption effects), 20 we reasoned that the methodology might be applicable to nanoscale gold/water interfaces...

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