Nanoparticles dendrimer-encapsulated metal

Crooks, M. et al., Dendrimer-encapsulated metal nanoparticles synthesis, characterization, and applications to catalysis, Acc. Chem. Res., 34, 181, 2001. 

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). 

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. 

Synthesis and Characterization of Dendrimer-Encapsulated Metal Nanoparticles... 

Two classes of catalysts account for most contemporary research. The first class includes transition-metal nanoparticles (e.g., Pd, Pt), their oxides (e.g., RUO2), and bimetallic materials (e.g., Pt/Ni, Pt/Ru) [104,132-134]. The second class, usually referred to as molecular catalysts, includes all transition-metal complexes, such as metalloporphyrins, in which the metal centers can assume multiple oxidation states [ 135 -137]. Previous studies have not only yielded a wealth of information about the preparation and catalytic properties of these materials, but they have also revealed shortcomings where further research is needed. Here we summarize the main barriers to progress in the field of metal-particle-based catalysis and discuss how dendrimer-encapsulated metal nanoparticles might provide a means for addressing some of the problems. 

Fig. 22. Schematic illustration of the approach used to carry out fluorous biphasic catalysis using dendrimer-encapsulated metal nanoparticles modified on their exterior with perfluoroether ponytails. Note that the ponytails can be attached by either electrostatic or covalent means. Reprinted with permission from Ref. 103 Copyright 2000 American Chemical Society...

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). 

Fig. 2 Schematic representations of metallodendritic architectures according to the metal (catalyst) location A at the periphery of a dendrimer or of a dendron B at the core of a dendrimer or at the focal point of a dendron C at branching points of a dendrimer or of a dendron D dendrimer-encapsulated metal nanoparticles (DEMNs)...

For metal ions that do not form either covalent bonds or strong complexes with the interior amine groups of PAMAM dendrimers, a new method for the synthesis of dendrimer-encapsulated metal nanoparticles has been reported [26]. For example, when G6 dendrimer (with surface hydroxyl group) -encapsulated copper is exposed to a solution containing ions more noble than copper,... 

RM Crooks, M Zhao, L Sun, V Chechik, LK Yeung. Dendrimer-Encapsulated Metal Nanoparticles Synthesis, Characterization, and Applications to Catalysis. Acc Chem Res 34 181-190, 2001. 

FI G U RE 9.4 A schematic representation of intradendrimer redox displacement of Cu to form zerovalent dendrimer-metal nanocomposites. Further displacement of Ag can also occur by Au, Pt, or Pd. Reprinted with permission from R.M. Crooks, M. Zhao, L. Sun, V. Chechik, L.K. Yeung, Dendrimer-Encapsulated Metal Nanoparticles Synthesis, Characterization, and Applications to Catalysis Accounts of Chemical Rsearch 34 (2001) 181. Copyright 2001 American Chemical Society. 

Y. Niu, R.M. Crooks, Dendrimer-Encapsulated Metal Nanoparticles and their Applications to Catalysis, Comptes Rendus Chimie 6, 1049, 2003. 

R.W.J. Scott, O.M. Wilson, R.M. Crooks, Titania-Supported Au and Pd Composites Synthesized from Dendrimer-Encapsulated Metal Nanoparticle Precursors, Chemistry of Materials 16, 5682, 2004. 

Niu Y, Crooks RM. Dendrimer-encapsulated metal nanoparticles and their applications to catalysis. C R Chimie 2003 6 1049-59. 

Dendrimer-Encapsulated Metal Nanoparticles Synthesis and Application in Catalysis... 

Fig. 4.1 Synthesis of dendrimer-encapsulated metal nanoparticles and the subsequent immobilization of the nanoparticles on mesoporous SBA-15 support. Reprinted with permission from ref. [52], Copyright 2008 American Chemical Society...

Niu YH, Crooks RM (2003) Preparation of dendrimer-encapsulated metal nanoparticles using organic solvents. Chem Mater 15 3463... 

---