Dendrimers as nanoreactors

Rhee and coworkers published the synthesis of bimetallic Pt-Pd nanoparticles [57] or Pd-Rh nanoparticles [58] within dendrimers as nanoreactors. These nanocatalysts showed a promising catalytic activity in the partial hydrogenation of 1,3-cyclooctadiene. The reaction was carried out in an ethanol/water mixture at 20 °C under dihydrogen at atmospheric pressure. The dendrimer-encapsulated nanoclusters could be reused, without significant loss of activity. 

Rhee et al. have described the synthesis of bimetaUic Pd-Rh nanoparticles within dendrimers as nanoreactors [56]. The resulting nanodusters effidently promoted the partial hydrogenation of l,3mild conditions with promising catalytic activity (Scheme 11.3). The dendrimer-encapsulated Pd-Rh bimetalUc nanocatalysts could be reused without significant loss of catalytic activity. 

These P-CD/adamantyl pseudorotaxane-terminated dendrimers can be used as nanoreactors in the preparation of gold and platinum nanoparticles in water... 

Kaneda et al. reported substrate-specific hydrogenation of olefins using the tri-ethoxybenzamide-terminated polypropylene imine) dendrimers (PPI) as nanoreactors encapsulating Pd nanoparticles (mean diameter 2-3 nm) [59]. The catalytic tests were performed in toluene at 30 °C under dihydrogen at atmospheric pressure (Table 9.3). The hydrogenation rates were seen to decrease with increasing generation of dendrimers, from G3 to G5. 

Functionalised dendrimers permit exploitation of the advantages of homogeneous and heterogeneous catalysis. Their spheroid architecture facilitates better recovery than comparable catalysts on polymer supports. They are distinctly easier to separate from the reaction mixture because the catalytic dendrimer molecules are larger than those of the resulting product. This attribute makes the dendrimers attractive as nanoreactors . 

In a sense, PAMAM dendrimers can be thought of as nanoreactors for preparing bimetallic nanoparticles. Several synthetic methodologies are now available for preparing a wide variety of bimetallic nanoparticles on the order of 1-3 nm. Beyond well-mixed nanoparticles, it is also possible to selectively... 

A dendrimer has a better-defined shape than ordinary polymers or surfactants and possesses dynamic inner cavities. That makes them capable of encapsulating of guests and acting as nanoreactors and nanoporous stabilizers [2,17],... 

FIGURE 6.25 (a) Dendrimer 1 as nanoreactor/ligand for CuAAC catalysis, (b) Comparison of... 

Hybrid structures of dendrimers with other entities have been synthesized for their collective properties. The most common is PEGylation of dendrimers (Kojima et al. 2010) to increase their circulation in the blood and to avoid immune clearance. Lipids have also been attached on the surface of PAMAM dendrimers to reduce toxicity and increase cellular uptake (Jevprasesphant et al. 2003). More interesting architectures have been created such as attachment of C60 fullerene balls (Jensen et al. 2005) on the surface of PAMAM dendrimers that act as nanoreactors by the generation of singlet oxygen species. In another instance, dendrimer-based micelles have also been prepared for anticancer therapy (Jang et al. 2005). Table 85.2 lists some of these hybrid structures. 

Dendrimers are a special class of arborescent monodisperse nanometer sized molecules that have been used in the synthesis of Au NPs as surface stabilizers or nanoreactor/templates for nanoparticle growth. Moreover, these hybrid nanomaterials have great potential for application in different fields such as sensors, imaging in cells, electrooptical devices, catalysis, drug delivery agents, and so on. 

Nanoparticles consisting of noble metals have recently attracted much attention because such particles exhibit properties differing strongly from the properties of the bulk metal [1,2], Thus, such nanoparticles are interesting for their application as catalysts [3-5], sensors [6, 7], and in electronics. However, the metallic nanoparticles must be stabilized in solution to prevent aggregation. In principle, suitable carrier systems, such as microgels [8-11], dendrimers [12, 13], block copolymer micelles [14], and latex particles [15, 16], may be used as a nanoreactors in which the metal nanoparticles can be immobilized and used for the purpose at hand. 

Since most precursors for solution-phase nanostructural growth are ionic metal salts, a typical micelle would not be effective since the precursor would not be confined to the interior of the microemulsion. Hence, reverse micelles (or inverse micelles, Figure 6.34) are used to confine the precursor ions to the aqueous interior, which effectively serves as a nanoreactor for subsequent reduction, oxidation, etc. en route to the final nanostructure. Not surprisingly, either PAMAMOS dendrimers (Chapter 5) or dodecyl-terminated (hydrophobic) PAMAM dendrimers (Figure 6.35) have been recently employed for this application. 

Recently, dendrimers loaded with Cu ions were used as template for silica formation [68]. As is well known,poly (propylene)imine dendrimer (DAB-Am-64) serves as a nanoreactor for well controlled nanoparticle formation [69]. However, when such dendrimers alone are used as silica templates [68], it does not lead to high porosity (BET surface area is 290 m /g) and does not provide ordering (Fig. 8). The latter factor is important for optical properties, but of no concern for catalytic applications. However, as silica cast over Cu-DAB-Am-64 lacks interpenetrating pore system and easy access to CuO particles, this destroys any hopes for catalytic applications as well. 

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