Dendrimer-nanoparticle composites

Scheme 3.20 Dendrimer-gold nanoparticle composite prepared from G5-PAMAM dendrimer and Au NPs bearing —COOH groups.

Thus, particles with a very narrow distribution were observed - in the case of a second generation dendrimer, nanoparticles with a monodisperse nucleus of 2.4 0.2 nm. It is important that not only individual dendrimers can be used for metallopolymer preparation, but also their dispersed mixtures with polymer matrices, which form new types of polymer-inorganic nanomaterials. For example, the highest poly(amidoamide) generations in water were put in swollen polymeric patterns of poly(2-hydroxyethylmethacrylate) Cu, Au or ions of a complex bound to dendrimer [90] were added to such a composition. Reduction of the metal ions resulted in new types of inorganic hybrid materials. 

From the points of view of composition and structure, composite nanoparticles can be thought of as a class existing in between two classes that are being heavily researched internationally "soft" dendrimer nanoparticles and "hard" nanocrystalline gold particles. "Soft" dendrimer nanoparticles are heavily studied, as witnessed by more than 7,000 papers and patents, hundreds of books and reviews. "Hard" nanoparticles are most acknowledged because of their catalytic activity. Potential... 

There is currently considerable interest in processing polymeric composite materials filled with nanosized rigid particles. This class of material called "nanocomposites" describes two-phase materials where one of the phases has at least one dimension lower than 100 nm [13]. Because the building blocks of nanocomposites are of nanoscale, they have an enormous interface area. Due to this there are a lot of interfaces between two intermixed phases compared to usual microcomposites. In addition to this, the mean distance between the particles is also smaller due to their small size which favors filler-filler interactions [14]. Nanomaterials not only include metallic, bimetallic and metal oxide but also polymeric nanoparticles as well as advanced materials like carbon nanotubes and dendrimers. However considering environmetal hazards, research has been focused on various means which form the basis of green nanotechnology. 

Dendrimer interior functional groups and cavities can retain guest molecules selectively, depending on the nature of the guest and the dendritic endoreceptors, the cavity size, the structure, and the chemical composition of the peripheric groups. Two main methods are known for the synthesis of metal nanoparticles inside dendrimers. The first method consists of the direct reduction of dendrimer-encapsulated metal ions (Scheme 9.4) the second method corresponds to the displacement of less-noble metal clusters with more noble elements [54]. 

R. W. J. Scott, O. M. Wilson, and R. M. Crooks, Titania-supported Au and Pd composites synthesized from dendrimer-encapsulated metal nanoparticle precursors, Chem. Mater. 16, 5682-5688 (2004). 

This chapter describes composite materials composed of dendrimers and metals or semiconductors. Three types of dendrimer/metal-ion composites are discussed dendrimers containing structural metal ions, nonstructimal exterior metal ions, and nonstructiu al interior metal ions. Nonstructural interior metal ions can be reduced to yield dendrimer-encapsulated metal and semiconductor nanoparticles. These materials are the principal focus of this chapter. Poly(amidoamine) (PAMAM) and poly(propylene imine) dendrimers, which are the two commercially available families of dendrimers, are in many cases monodisperse in size. Accordingly, they have a generation-dependent munber of interior tertiary amines. These are able to complex a range of metal ions including Pd +, and Pt +. The maximmn munber... 

Fig. 3. Schematic illustration of the synthesis of metal nanoparticles within dendrimer templates. The composites are prepared by mixing of the dendrimer and metal ion, and subsequent chemical reduction. These materials can be immobilized on electrode surfaces where they serve as electrocatalysts or dissolved in essentially any solvent (after appropriate end-group functionalization) as homogeneous catalysts for hydrogenation and other reactions...

The approach for preparing dendrimer-encapsulated Pt metal particles is similar to that used for preparation of the Cu composites chemical reduction of an aqueous solution of G4-OH(Pt +)n yields dendrimer-encapsulated Pt nanoparticles (G4-OH(Ptn)). A spectrum of G4-OH(Pt6o) is shown in Fig. 12 a it displays a much higher absorbance than G4-OH(Pt +)6o throughout the wavelength range displayed. This change results from the interband transition of the encapsulated zero-valent Pt metal particles. 

Dendrimers containing Pt " or Pt-metal nanoparticles are easily attached to Au and other surfaces by immersion in a dilute aqueous solution of the composite for 20 h, followed by careful rinsing and drying [59,129]. Therefore it is possible to use X-ray photoelectron spectroscopy (XPS) to determine the elemental composition and the oxidation states of Pt within dendrimers. For example, Pt(4f7/2) and Pt(4f5/2) peaks are present at 72.8 eV and 75.7 eV, respectively, prior to reduction, but after reduction they shift to 71.3 eV and 74.4 eV, respectively, which is consistent with the change in oxidation state from -i-2 to 0 (Fig. 13 a]. 

Just as DENs particle sizes have some distribution (albeit relatively narrow), there is surely some distribution in particle compositions for bimetallic DENs. This is a fundamentally important aspect of DENs, particularly with regard to their catalytic properties however, there are presently no reliable characterization methods for evaluating particle composition distributions. One method that has been applied to PdAu [21] and PtPd [19] DENs, as well as dendrimer-templated PtAu [24] is to collect single particle EDS spectra from several (15-20) nanoparticles. These experiments indicate that individual particle composition distributions may vary widely, but the difficulty in obtaining data from the smallest particles may skew the results somewhat. EDS spectra collected over large areas, which sample tens or hundreds of particles, generally agree well with the bulk composition measurements [24] and with stoichiometries set in nanoparticle synthesis [19,21,24]. 

The chemical reactivity of nanoparticle surfaces, presents interesting additional opportunities for evaluating nanoparticle surface composition. Some noble metal particles (Pd and Au in particular) can be extracted from the PAMAM dendrimer interiors into organic solution with long-chain thiols [37]. The resulting nanoparticles, referred to as Monolayer Protected Clusters (MPCs), retain the size distributions and spectroscopic characteristics of the original DENs and allow for recycling the expensive dendrimer [16]. 

Fig. 5 Dependence of the catalytic activity of the dendrimer-encapsulated PdRh bimetallic nanoparticles on its composition in partial hydrogenation of 1,3-cyclooctadiene. Reprinted with permission from J Mol Catal, A 2003, 206, 291-298. Copyright 2003 Elsevier...

Evaluating dendrimer templated nanoparticles in the absence of the dendrimer provides opportunities for insights into these new materials. In order to pursue these investigations, it is first necessary to immobilize DENs onto an appropriate substrate and to gently remove the dendrimer shell see Scheme 5. Opportunities for controlling nanoparticle size and composition make DENs potentially important precursors for heterogeneous catalysts and electrocatalysts, and DEN deposition and thermolysis are similarly critically important steps in pursuing these applications [45]. 

A complementary study evaluated composition effects on dendrimer-templated PtCu nanoparticles [23]. Although Cu-CO bands were not observed1, a similar red shift in the Pt-CO stretching frequency to the PtAu system was observed, indicating the presence of well-mixed bimetallic nanoparticles throughout the composition range. Infrared spectroscopy of CO adsorbed on both the PtAu and PtCu catalysts showed that the shifts in the CO stretching frequency upon Cu or Au incorporation were small relative... 

Possible Mechanisms and Key Characteristics of Nanomaterials. A nanoparticle/nanomaterial is generally defined as a particle/ material having a physicochemical structure greater than typical atomic/molecular dimensions but at least one dimension smaller than lOOnm. It includes particles/ materials engineered or manufactured by humans on the nanoscale with specific physicochemical composition and structure to exploit properties and functions associated with its dimensions. Some of the common nanoparticle types are (1) carbon-based materials (e.g., nanotubes, fullerenes), (2) metal-based materials (e.g., nanogold, nanosilver, quantum dots, metal oxides), and (3) dendrimers (e.g., dendritic forms of ceramics). 

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