PAMAM metal-dendrimer nanocomposites

However, for the dendrimer nanocomposite metallic systems this change in shape was not observed. Again, due to the high stability to intense laser pulses, the anisotropy value of the gold dendrimer nanocomposite, which can be viewed as a measure of the symmetry of the particle, did not change after several repeated cycles of measurements. It is possible that the initial optical pumping of the electron-phonon modes of the metal particles is partially absorbed by the encapsulating PAMAM dendrimer. 

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... 

Starburst dendrimers have received considerable attention in the area of heterogeneous catalyst synthesis in the last decade. Although Tomalia et al. and Bosman et al. originally discovered these hyperbranched macromolecules and their host-guest properties in the mid-1980s. Crooks and coworkers were the first to demonstrate the ability of poly(amidoamine) (PAMAM) starburst dendrimers to act as metal nanoparticle stabilizers that could potentially aid in the synthesis of supported metal catalysts. The benchmark work of Crooks et al. and subsequent literature reports have underscored the advantages of successfully utilizing PAMAM dendrimer-nanocomposite precursors over conventional catalyst preparation methods. 

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). 

The synthesis of PAMAM dendrimers is relatively straightforward. Michael addition of methylacrylate to an ethylenediame molecule is followed by amination by four additional ethylenediamine species. This process is repeated and yields the regular, hyperbranched stmcture of PAMAM dendrimers. However, control of the degree to which the dendrimer grows (i.e., the dendrimer generation) is a challenging process. Thus, commercially available dendrimers have been frequently used for the synthesis of dendrimer-metal nanocomposites (DMNs) in most catalyst synthesis studies. 

The first two stages of the synthesis of catalysts prepared by dendrimers are inextricably linked. Proper incorporation of the metal precursor within the PAMAM dendrimer is essential for the formation of dendrimer-metal nanocomposites and, eventually, nanoparticles with controlled particle sizes. Complications in the complexation stage, such as incomplete or inadequate incorporation of the metal precursor, will leave free metal cations or colloidal particles in the impregnating solution, resulting in the formation of supported catalysts that exhibit wide particle size distributions. In the case of bimetallic catalysts, the loss is twofold In addition to an array of metal particle sizes, there will also be a significant loss of compositional control in the active phase. In short, if the complexation step is not tightly controlled, the dendrimer-prepared catalyst will not differ substantially from a catalyst prepared by wet impregnation. 

Since their host-guest properties were discovered, PAMAM dendrimers have been used in the synthesis of dendrimer-metal nanocomposites containing various metals, including Cu, Pt, Pd, Au, Ag, Rh, Ru, Ni, Fe, Mn, Co, and Sn. - The first... 

REMOVAL OF PAMAM DENDRIMERS FROM SUPPORTED DENDRIMER-METAL NANOCOMPOSITES... 

FIGURE 9.7 FTIR spectra of a G40H PAMAM dendrimer supported on y-alumina obtained at different temperatures in flowing Hj. From D.S. Deutsch, A. Siani, C.T. Williams, and M.D. Amiridis, FT-IR Investigation of the Thermal Decomposition fo Poly(amidoamide) (PAMAM) Dendrimers and Dendrimer-Metal Nanocomposites Supported on AI2O3 and Z1O2, Journal of Physical Chemistry B, Submitted 2006. 

Preparation of stable, zero-valent metallic copper solutions were demonstrated in either water or methanol [89]. After complexation within various surface modified poly(amidoamine) (PAMAM) dendrimers, copper(II) ions were reduced to zero valence metallic copper thus providing a bronze, transparent dendrimer-metal nanocomposite soluble in water. Solubility of the metal domains is determined by the surface properties of the dendrimer host molecules, however, their solutions still display characteristic optical properties associated with metal domains. Both aqueous and methanolic solutions of copper clusters were stable for several months in the absence of oxygen. Similar work and results were also reported by Crooks et al. [133]. 

Goodson III T (2001) Optical effects manifested by PAMAM dendiimer metal nanocomposites. In Fr6chet JMJ, Tomalia DA (eds) Dendrimers and other dendritic polymers. Wiley, Chichester, pp 515-541... 

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