Hydroxyl-terminated PAMAM dendrimers

In addition to hydroxyl-terminated PAMAM dendrimers, we also investigated the binding ability of amine-terminated G4-NH2. Figure 7 a shows the d-d... 

Fig. 19. Turn-over frequencies for hydrogenation of allyl alcohol and N-isopropyl acrylamide obtained in water using dendrimer-encapsulated Pd nanoparticles of constant average size. The hydroxyl-terminated PAMAM dendrimer generation varies from G4 to G8...

Zhao et al (70) developed a method for the synthesis of dendrimer-encapsulated metal nanoparticles based on sorbing metal ions into (modified) PAMAM dendrimers followed by a reduction. Dendrimers encapsulating copper, palladium, and platinum nanoparticles have been prepared. Hydroxyl-terminated PAMAM dendrimers were used to prepare encapsulated palladium (PAMAM generations 4, 6, and 8) and platinum (PAMAM generations 4 and 6) nanoparticles. The dendrimer-encapsulated palladium and platinum nanocomposites catalyzed the hydrogenation reaction of allyl alcohol and N-isopropyl acrylamide in water 71). 

Properties of Hydroxyl-Terminated PAMAM Dendrimers by Generation... 

While they are not strictly redox-containing dendrimers. Crooks and Zhao have encapsulated platinum and other metallic nanoparticles inside PAMAM dendrimers. The catalytic properties of these materials have been examined using cyclic voltammetry, and it was found that a gold electrode, modified with large hydroxyl-terminated PAMAM dendrimers containing clusters of 60 platinum atoms, was readily able to catalyze O2 reduction [98]. Similar results have been reported for palladium clusters encapsulated in the same hydroxyl-terminated PAMAM dendrimers [99]. 

As early as 1989, Tomalia et al. [137] demonstrated with carbon-13, spin-lattice relaxation (Ti) techniques that small molecules such as acetylsalicyclic acid or 2,4-dichlorophenoxyacetic acid could be encapsulated within the interior of carboxymethyl ester terminated PAMAM dendrimers exhibiting guestihost stoichiometries of 4 1 by weight or 3 1 on a molar basis. These encapsulations appear to be driven by simple ion pairing of the acidic guest molecules with the tertiary amine sites located within the PAMAM dendrimer host. More recently, extensive work by Twyman et al. [138,139] and others [140] has shown that dendrimers may function as very versatile host molecules for drug delivery systems. Figure 18 illustrates the incorporation of benzoic acid within the interior of a hydroxyl terminated PAMAM dendrimer. 

Figure 8.3 The complexation of a benzoic acid guest in a hydroxyl terminated PAMAM dendrimer that...

Crooks group prepared monodisperse (1.7 0.2nm) palladium nanoparh-cles within the interiors of three different generations of hydroxyl-terminated PAMAM dendrimers [41]. This process involved encapsulation of the nonselec-tive catalyst (the Pd nanoparticle) within a selective nanoporous cage (the dendr-imer). These dendrimer-encapsulated palladium nanoparticles were used as catalysts to hydrogenate allyl alcohol and four R-substituted derivatives in a metha-nol/water mixture. The results showed that higher-generation dendrimer encapsulated catalysts (DECs) or larger substrates resulted in lower turnover frequencies. 

Lesniak et al. (2005) also describe the preparation and use of similar dendrimer/silver nanoclusters using G-5 PAMAM dendrimers terminated with either amino, hydroxyl, or... 

Starburst PAMAM dendrimers (Fig. 1), a specific class of commercially available dendrimers that have repeating amine/amide branching units, have drawn considerable interest in recent years due to their potential applications in medicine, nanotechnology, and catalysis [3-7]. These dendrimers are readily functionalized to terminate in diverse moieties such as primary amines, carboxylates, hydroxyls, or hydrophobic alkyl chains. Because dendrimer size and end groups can be varied, they are typically named by their generation (Gl, G2, etc.) and exterior functionality (- NH2, - OH). 

Another type of dendrimer, that has been used to encapsulate Au32 particles is hydroxyl-terminated fifth-generation dendrimers (G5-PAMAM). The particles were adsorbed on silica (0.3wt.% Au),108 and after washing and removal of the dendrimer under oxygen at 573 K, followed by reduction under hydrogen at 573 K, the supported gold particles were even larger than above (14.5 nm), probably because silica is not a support that easily leads to small particles. 

An important class of water-soluble dendrimers - the PAMAM dendrimers - have also been investigated as drug delivery systems. Twyman and co-workers have demonstrated that hydroxyl terminated water-soluble PAMAM dendrimer systems were capable of encapsulating small acidic hydrophobic molecules such as benzoic acid (Figure 8.3). The encapsulation capabilities of the PAMAM materials was first confined to benzoic acids and... 

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