Dendrimer Encapsulated Nanoparticles DENs

In the typical nomenclature for DENs, the dendrimer is designated by Gx-R where x is the dendrimer generation and R is the surface group (typically -OH or -NH2, see Fig. 7.1). The stoichiometry between the dendrimer and complexed ions or reduced encapsulated nanoparticles is denoted in parentheses after the dendrimer description, e.g. (M )n or (M ). For bimetallic DENs, the metahmetal stoichiometry is typically included, e.g. G5-OH (PtigAuig). 

The Pt-Au systemhasbeen a valuable system to test DENs potential forpreparing NP systems of interest to the heterogeneous catalysis community. It is synthetically challenging, characterized by a wide bulk miscibility gap (18 to 98% Pt), and bimetallic NPs within this gap are unavailable by traditional routes. Nuzzo and coworkers have shown that bulk phase diagrams may not necessarily hold true for NPs and results with the Pt-Au system support this conclusion provided appropriate syntheses are available. Utilizing Cu displacement syntheses, these substantial 

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The possibility of using electrostatic charge attraction has been exploited in the preparation of gold dendrimer encapsulated nanoparticles (DENs), which under appropriate conditions can be fully distributed along the surface of monodispersed MWCNTs (Fig. 3.21) [103]. 

We are developing a new method for preparing heterogeneous catalysts utilizing polyamidoamine (PAMAM) dendrimers to template metal nanoparticles. (1) In this study, generation 4 PAMAM dendrimers were used to template Pt or Au Dendrimer Encapsulated Nanoparticles (DENs) in solution. For Au nanoparticles prepared by this route, particle sizes and distributions are particularly small and narrow, with average sizes of 1.3 + 0.3 nm.(2) For Pt DENs, particle sizes were around 2 nm.(3) The DENs were deposited onto silica and Degussa P-25 titania, and conditions for dendrimer removal were examined. 

In this specific case, the colloid stabilizers are dendrimers, for instance, polyamidoamine (PAMAM), which are hyperbranched polymers that ramify from a single core and form a porous sphere [103, 104] (Scheme 17.1). Dendrimer-encapsulated nanoparticles (DENs) are synthesized by sequestering metal ions within appropriate dendrimers, and then by chemically reducing the resulting composite. They can be synthesized in various media, such as water or ethanol. The size of the nanoparticles is usually nearly monodisperse, and can be tuned by varying the metal-to-dendrimer ratio prior to reduction. Supported catalysts can then he prepared by immobilizing DENs onto a sohd support. As in the case of colloids, the last step, which consists in the removal of dendrimers hy thermal treatment, may lead to an increase in both the metal-particle size and particle-size distribution. 

Another very important category of dendrimer catalysis is based on nanoparticles synthesized directly inside the dendrimers being the active component for catalytic reactions. The current review will focus on Polyamidoamine (PAMAM) dendrimer-encapsulated nanoparticles (DENs). This topic has been reviewed several times [22-24,26,42]. In this review, the techniques developed for the synthesis of mono- and bi-metallic DENs will only be briefly overviewed. The focus of this review is on the most recent advances in understanding the structure of DENs and newly developed apphcations of DENs in catalysis. 

Abstract We review the preparation, characterization, and properties of dendrimer-templated bimetallic nanoparticles. Polyamidoamine (PAMAM) dendrimers can be used to template and stabilize a wide variety of mono- and bimetallic nanoparticles. Depending on the specific requirements of the metal system, a variety of synthetic methodologies are available for preparing nanoparticles with diameters on the order of 1-3 nm with narrow particle size distributions. The resulting dendrimer-encapsulated nanoparticles, or DENs, have been physically characterized with electron microscopy techniques, as well as UV-visible and X-ray photoelectron spectroscopies. 

PAMAM dendrimers can be used to template and stabilize reduced (zero valent) metal nanopartides in solution [22, 26]. A number of synthetic strategies to prepare dendrimer encapsulated nanopartides (DENs) are available. The primary routes are described here readers are directed to recent reviews for more thorough discussions of dendrimer mediated nanoparticle syntheses [14, 25]. 

Dendrimer-encapsulated nanoparticles have potential applications in the energy sector. For example, polyamidoamine (PAMAM) dendrimer-encapsulated platinum nanoparlicles (Pt-DENs) have been introduced as a promising cathode catalyst for air-cathode single-chamber microbial fuel cells (SCMFCs) [64], This novel catalyst led to higher power production with 129.1%, as compared to cathodes with electrodeposited Pt. 

Dendrimer encapsulated Pt nanoparticles (DENs) were prepared via literature methods (1, 11). PtCl42 and dendrimer solutions (20 1 Pt2+ dendrimer molar ratio) were mixed and stirred under N2 at room temperature for 3 days. After reduction with 30 equivalents of BH4 overnight, dialysis of the resulted light brown solution (2 days) yielded Pt2o nanoparticle stock solution. The stock solution was filtered through a fine frit and Pt concentration was determined with Atomic Absorption Spectroscopy (11). Details on catalyst characterization and activity measurements have been published previously (11). 

Fig. 21. Schematic illustration of phase-transfer catalysis using an amine-terminated den-drimer-encapsulated nanoparticle complexed with a fatty acid (present in the organic phase). The fatty acid surrounds the dendrimer, yielding a monodisperse inverted micelle which is soluble in the organic phase. After catalysis, the catalyst can be reclaimed by changing the pH of the aqueous phase...

Since the first report of dendrimer-encapsulated Cu nanoparticles [15], several types of mono and bi-metallic DENs have been prepared. DEN synthesis has been recently reviewed [9,16], so only the synthesis of bimetallic DENs is described here. Bimetallic DENs can be prepared by one of three methods co-complexation of metal salts, galvanic displacement, and sequential reduction. Several bimetallic systems have already been prepared inside PAMAM dendrimers Table 1 summarizes the current literature and synthetic methods employed. 

Fig. 4.13 Depiction of the two nanoparticle synthesis techniques used and the initial reactivity results for electrophilic catalysis, (a) In the top scheme, Pt ions are loaded onto a PAMAM den-drimer and reduced to form a dendrimer-encapsulated NP. Sonication deposits the NPs on the mesoporous silica, SBA-15, to generate the NP catalysts. In the bottom scheme, polyvinylpyrrolidone (PVP) encapsulates the NP. Deposition on SBA-15 follows to produce the catalyst. In both cases, the NPs are synthesized before loading onto SBA-15. (b) Hydroalkoxylation of 1 with Pt NPs. To obtain electrophilic activity from the Pt NPs, treatment with the mild oxidant PhlClj is required. Pt4o/G40H/SBA-15 NPs must be further reduced under H atmosphere at 100 °C for 24 h before reaction. This treatment generates catalytically active NPs that activate the jc-bond in 1, resulting in hydroalkoxylation to benzofuran 2. Yields were determined by comparing peaks in NMR against an internal standard. Reprinted with permission from ref. [100]. Copyright 2009 Nature Publishing Group...

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