PAMAM nanoparticles

Spherical vaterite crystals were obtained with 4-mercaptobenzoic acid protected gold nanoparticles as the nucleation template by the carbonate diffusion method [51]. The crystallization of calcium carbonate in the absence of the 4-MBA capped gold nanoparticles resulted in calcite crystals. This indicates that the polymorphs of CaCOj were controlled by the acid-terminated gold nanoparticles. This result indicates that the rigid carboxylic acid structures can play a role in initiating the nucleation of vaterite as in the case of the G4.5 PAMAM dendrimer described above. 

The team of Crooks is involved in the synthesis and the use of dendrimers and, more particularly, poly(amidoamine) dendrimers (PAMAM), for the preparation of dendrimer-encapsulated mono- or bimetallic nanoparticles of various metals (Pt, Pd, Cu, Au, Ag, Ni, etc.) [55, 56]. The dendrimers were used as nanocatalysts for the hydrogenation of allyl alcohol and N-isopropylacrylamide or other alkenes under different reaction conditions (water, organic solvents, biphasic fluorous/or-ganic solvents or supercritical COz). The hydrogenation reaction rate is dependent on dendrimer generation, as higher-generation dendrimers are more sterically... 

The emission of the metal particles may thus originate from a band-to-band transition in the metal particle, which occurs at about 516 nm for gold [60, 119]. As stated above, the nature of the interaction of the dendrimer (PAMAM) host is still uncertain, there could be very strong electrostatic interactions that may play a part in the enhancement of the metal particles quantum efficiency for emission. However, one would expect that this enhancement would result in slightly distorted emission spectra, different from what was observed for the gold dendrimer nanocomposite. Further work is necessary to completely characterize the manner in which the dendrimer encapsulation enhances the emission of the metal nanoparticles. With further synthetic work in preparation of different size nanoparticles (in other words elongated and nonspherical shape particles, including nanorods) it may be possible to develop the accurate description of a... 

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas " and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

Figure 1.3 (A) Chemresistor containing gold nanoparticles and PPL (Reprinted with permission 2002 American Chemical Society.) (B) Response of the Au nanoparticle film to toluene, 1-propanol and water (a) Au/PPh (b) Au/PPI (c) Au/PAMAM. (Reprinted with permission 2003 Elsevier.) (Reproduced with permission from Ref [25] 2004 Wiley-VCH Verlag GmbH. Co. KGaA.)...

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... 

The first studies of dendrimer-encapsulated metal nanoparticles focused on Cu [82]. This is because Cu + complexes with PAMAM and PPI dendrimers are very well behaved and have easily interpretable UV-vis and EPR spectra. For example, Fig. 4a shows absorption spectra for Cu + coordinated to different ligands. In the absence of dendrimer and in aqueous solutions Cu + exists primarily as [Cu(H20)g] +, which gives rise to a broad, weak absorption band centered at 810 nm. This corresponds to the well-known d-d transition for Cu in a tetra-gonally distorted octahedral or square-planar ligand field. 

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...

Using an approach similar to that discussed previously, Pd nanoparticles were prepared within amine-terminated PAMAM dendrimers. To prevent coordination of Pd + to the primary amine groups of the dendrimers, the solution pH was adjusted to around 2, which preferentially protonates the exterior amines to a greater extent than the interior tertiary amines. Accordingly, Pd + binds preferentially to the interior tertiary amines and upon reduction Pd particles form only within the dendrimer interior. G4-NH2 dendrimer-encapsulated nanoparticles can then be quantitatively transported from an aqueous phase into toluene by addition of 10-20% of dodecanoic acid to the organic phase (Fig. 21) [19]. This transition is readily visualized by the color change the brown aqueous solution of Pd nanoparticles becomes clear after addition of the acid, while the toluene layer turns brown. Our studies have shown that this is a consequence of... 

Indeed, recent results from our laboratory indicate that dendrimer-encapsulated CdS QDs can be prepared by either of two methods [192]. The first approach is analogous to the methodology described earlier for preparing dendrimer-encapsulated metal particles. First, Cd and S salts are added to an aqueous or methanolic PAMAM dendrimer solution. This yields a mixture of intradendrimer (templated) and interdendrimer particles. The smaller, dendrimer-encapsulated nanoparticles may then be separated via size-selective photo etching [193], dendrimer modification and extraction into a nonpolar phase [19], or by washing with solvent in which the dendrimer-encapsulated particles are preferentially soluble. An alternative, higher-yield method relies on sequential addition of very small aliquots of Cd + and S " to alcoholic dendrimer solutions. 

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). 

Using a similar approach, Chechik and Crooks (73), modified the PAMAM dendrimer-encapsulated palladium nanoparticles with perfluoropolyether tails utilizing non-covalent ion-pair interactions. The catalytic hydrogenation of six substrates under biphasic conditions (toluene/ perfluoro-2-butyltetrahydrofuran FC-75) was investigated. Allyl alcohol, methyl acrylate, vinyl isopropenyl ether, and... 

Fig. 15. Schematic representation of the formation of an inverse micelle from a PAMAM dendrimer-encapsulated palladium nanoparticle.

Figure 6.2 (a) Gold nanoparticles assembled using PAMAM dendrimers and (b) small angle... 

Srivastava S, Frankamp BL, Rotello VM. Controlled plasmon resonance of gold nanoparticles self-assembled with PAMAM dendrimers. Chem Mater 2005 17 487-490. 

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. 

Bielinska et al. define DNCs as hybrid nanoparticles formed by the dispersion and immobilization of guest atoms or small clusters in dendritic polymer matrices. These authors describe the synthesis of 5-25 nm DNCs of Au NPs and G5-PAMAM dendrimers through UV decomposition of the PAMAM-HAuCf precursor. These nanoparticles have been imaged by in both in vitro and in vivo conditions [135]. 

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