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Figure 5. Reversible aggregation of gold nanoparticles caused by the environmentally triggered phase transition of adsorbed ELP: (A) Au-ELP in the hydrophilic phase with spectra typical of isolated gold colloid and (B) aggregates which exhibit a redshift in their absorbance spectra compared to that of individual colloid in (A). Reprinted with permission from [235], N. Nath et al., J. Am. Chem. Soc. 123, 8197 (2001). © 2001. American Chemical Society.

number of the dendrimer is equal to the number of repetition cycles performed and may be easily determined by counting the number of branch points as one proceeds from the core to the periphery [236-243].

Dendrimer stabilized nanocomposite materials have unique potential for applications such as catalysis which require stable colloidal particles having surfaces that are not completely passivated by a stabilizing adsorbate. Because of the low mass density within the dendrimer interior, such monolayers are highly permeable to small molecules [244]. Thus, dendrimers can serve the dual purpose of acting as a "nanofilter" that passes small and specifically charged species, prevents other molecules from interacting with the colloid, and can function as a stabilizer.

Dendrimers are also recognized as monodispersed nanoreactors, possessing architectures that allow the preor-ganization of metal ions within their interiors [245]. Several noble metal nanoparticles have been prepared in the presence of dendrimers, in which the size of the nanoparticle can be controlled by the size of the dendrimers [246-250].

Two molecules of this type that are frequently used are polyamidoamine (PAMAM) dendrimers and polypropylene imine dendrimers. Au colloids in the 2-3 nm size regime have been prepared by one-phase in-situ reduction of an aqueous solution of HAuCl4 in the presence of either second- or fourth-generation (G2 or G4, respectively) PAMAM dendrimers using NaBH4 as the reducing agent. The dendrimers encapsulated the colloids, imparting stability to the aqueous colloidal solutions. The driving force for the interaction of the colloids with the dendrimers was an association of Au with the primary amine terminal groups (and perhaps the interior secondary and tertiary amines) of the dendrimer. The dendrimer generation used in such syntheses controlled the size of the resultant colloids: lower generation dendrimers gave rise to larger, less monodisperse and more aggregated colloids than those prepared using higher generation dendrimers, for a constant ratio of primary amine groups to AuCl4-. Also the stability of nanopar-ticles decreased sharply when either the solvent or the excess free dendrimer was removed from the solution [247, 251].

Sugar-persubstituted PAMAM dendrimers (sugar balls) were synthesized by Aoi et al. [252]. Since sugar balls have highly ordered structure with arranged saccharides on the peripheries of the dendrimers and terminal hydroxyl residues that can operate as reductants, sugar balls were used to act as protective agents for gold nanoparticles, against flocculation. When Au3+ ions are reduced with the hydroxyl groups of the sugar balls, the hydroxyl groups are oxidized to carbonyl groups. Thus it is found that sugar balls can act as protective agents as well as reductants for the preparation of gold nanoparticles [253].

Fourth-generation PAMAM dendrimers [254] having terminal groups partially or fully functionalized with thiol groups have also been explored for the functionalization and synthesis of gold nanoparticle [255]. Diluted solutions of thi-olated dendrimers and tetrachloroauric acid can be mixed and reduced with NaBH4 to produce stable dendrimer func-tionalized gold nanoparticles. The enhanced stability with thiolated dendrimers compared to amine-stabilized materials is due to the low concentration of dendrimer required to prepare the nanocrystals.

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