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spectra. HRTEM data confirm these generation-dependent increases in size of the QDs. As for dendrimer-encapsulated metal nanoparticles, the size of the CdS nanoparticles is related to the number of tertiary amine groups in the dendrimerâ€™s outer shell.
VI. SUMMARY AND CONCLUSIONS This chapter has comprehensively reviewed new composite materials called dendrimer-encapsulated nanoparticles. We first reported these interesting inorganic/organic hybrid materials in 1998 (57), and within a matter of months our results were confirmed by others (60). Between the time of the first report and the present, we have shown that dendrimers can template a vast array of metal nanoparticles, including Cu, Pt, Pd, Ru, Ag, Ni, and Au. Metal ions that bind directly to ligands on the interior of the dendrimer are prepared by direct reduction of the composite, but it is also possible to prepare intradendrimer metal nanoparticles from nonbinding ions by displacing a less noble metal such as Cu. We have also shown that bimetallic, and presumably multimetallic, dendrimer-encapsulated metal nanoparticles can be prepared and that even semiconductor quantum dots are accessible. There are a number of desirable aspects of this synthetic approach and of the resulting materials. First, because the dendrimer can act as a template for preparing the particles, and because dendrimers are commercially available in different sizes (generations), nearly monodisperse particles ranging in size from 1 nm to perhaps 5 or 6 nm in diameter can be prepared. Second, once synthesized, the nanoparticles are stabilized by the dendrimer host. Therefore, the particles do not agglomerate even after repeated drying and resolvation cycles. Third, the dendrimer acts as a nanofilter that selectively allows particular substrates to encounter the encapsulated particle. As we showed, this attribute allows the composites to perform size-selective catalysis. Moreover, because the particles are contained within the dendrimer primarily by steric considerations (e.g., in contrast to only ligand stabilization), the nanoparticle surface is sufficiently accessible that catalytic reactions proceed at reasonable rates. Fourth, because the particle is confined within the dendrimer, functional groups on the dendrimer surface can be used to control solubility, as a synthetic handle for stabilization of the nanocomposite in polymer coatings, and as a means for enabling direct surface immobilization of dendrimers on metal, semiconducting, and insulating surfaces. Thus, the approach described here for preparing dendrimer-encapsulated nanoparticles takes advantage of each of the unique aspects of dendrimer structure: the chemistry of the terminal groups, the generation-dependent size, the three-dimensional structure, the low-density region near the core, and the endoreceptors present within the dendrimer interior.
Chemistry of metal nanoparticles