Dendritic metal nanoparticles

Metal nanoparticles exhibit interesting properties as a result of their intermediate size between bulk metal and ionic or atomic species. These properties make them attractive media for use in optical devices, medical technology, nanotechnology and, in particular, 

SQUID magnetometry demonstrated that these crystalline materials exhibited superparamagnetism at room temperature whilst NMR spectroscopic relaxation studies revealed very high T1 and T2 relaxation times. These characteristics render these iron oxide/PAMAM dendrimer-based nanoparticles as promising prospects as a new class of MRI contrast agents. 

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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]. 

Dendrimer synthesis, 26 787-788 breakthrough approaches in, 26 788 Dendritic box, 26 790 Dendritic metallic silver, 19 367—368 Dendritic nanoparticles, water-soluble, 26 796... 

Electron microscopy TEM metal-rich dendrimers, dendritically stabilised nanoparticles contrasting otherwise necessary... 

Huskens et al. exploited host-guest interactions between dendritic guest molecules and CD-modified nanoparticles for the formation of organic/metal nanoparticle multilayers on a PDMS stamp (Fig. 13.15).88 The multilayer stacks were transferred to a complementary host surface, while no materials remained on the protruding areas of the PDMS stamp. These multilayers showed a well-defined thickness control of 2 nm per bilayer. 

Kramer and coworkers recently reported on water-soluble dendritic coreshell architectures and studied the influence of the attached carbohydrate shell on the formation and stabilization of metal nanoparticles in water. For this purpose, they used hyperbranched poly(ethylenimine) (PEI) as core molecules and covalently attached different carbohydrates as shells, i.e., glycidol, gluconolactone and lactobionic acid, to obtain the corresponding PEI-glycol, PEI-gluconamide and PEI-lactobionamide. Different molecular weights of PEIX (x = 0.8, 5, 21 or 25 with different Mw = xlO3) were employed [81]. 

M. Kramer, N. Perignon, R. Haag, J. D. Marty, R. Thomann, N. Lauth-de Viguerie, and C. Mingotaud, Water-soluble dendritic architectures with carbohydrate shells for the temptation and stabilization of catalytically active metal nanoparticles. Macromolecules, 38 (2005) 8308-8315. 

Isolated metal nanoparticles yield nanopores, which can have solid or mesoporous sidewalls. Similarly as for dendrites, the conditions can be tuned to form pSi or PS (Tsujino and Matsumura 2005 Chattier et al. 2008 Lee et al. 2008 Yae et al. 2007). 

Deposition time and precursor concentration can control the resulting metal morphology (Peng and Zhu 2004 Yae et al. 2007). Initially, metal nanoparticles nucleate on the silicon surface. As time progresses, the nanoparticles grow in size eventually becoming dendrites or metal films (initially discontinuous and then continuous) (Chattier et al. 2008 Peng et al. 2003 Yae et al. 2003). The density of nanoparticles depends on the metal employed, its concentration, and the state of the silicon surface (Yae et al. 2007). 

Metallic copper nanoparticles within covalently bonded multilayered dendritic ultrathin films made of pamam, using supercritical CO2 as a processing medium, were described by Puniredd and Srinivasan . The nanoparticles were obtained in higher yield, in a denser and more stable distribution, and showed greater stability towards polar solvent attack than the analogous products of liquid solvent processes, for example, in tetrahydrofuran, which was explained by the facile solvent separation and transport. 

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). 

Selvan and coworkers167168 utilized a block copolymer micelle of polystyrene-block-poly(2-vinylpyridine) in toluene exposed to tetrachloroauric acid that was selectively adsorbed by the micelle structure. On exposure of this solution to pyrrole monomer, doped PPy was obtained concurrently with the formation of metallic gold nanoparticles. The product formed consisted of a monodispersed (7-9 nm) gold core surrounded by a PPy shell. Dendritic nanoaggregate structures were also reported... 

Besides clay-based nanocomposites, there has been huge discussion on the metallic and semiconductor-based hybrid materials. The ability of polymer materials to assemble into nanostructures describes the use of polymers providing exquisite order to nanoparticles. Finally, a discussion on potential applications of polymer—nanoparticle composites with a special focus on the use of dendrite polymers and nanoparticles for catalysis should follow (Polymer-Nanoparticle Composites Part 1 (Nanotechnology), 2010) (Figure 1.15). 

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