Dendrimer-templated synthesis

PAMAM dendrimer-templated synthesis of metal nanoparticles has been demonstrated for many metals, such as Cu, Pt, Pd, Au, Rh, and Ru [42]. Most of the syntheses were carried out in aqueous solution at an appropriate pH. For amine-terminated PAMAM dendrimers, the pH of the solution needs to be adjusted to 3 to protonate the terminal amine groups and prevent dendrimer cross-linking [61, 62]. Metal ions cannot bind to protonated amine groups instead, they bind to the tertiary amines inside the PAMAM dendrimer. 

Dendrimer-templated synthesis of metal nanoparticles is a powerful technique and many metal DENs can be synthesized. However, the approach cannot be used to synthesize metal nanoparticles whose precursors (metal ions) have weak interactions with the dendrimers. For certain metal ions, such as Ag% another technique was developed based on galvanic redox displacement [68]. Using the galvanic redox displacement technique, Cu DENs were mixed with Ag. Since the standard reduction potential of Ag+ is more positive than Cu, Ag will be reduced to form Ag(0), and Cu(0) will be oxidized to Cu. Therefore, Cu DENs will be converted to Ag DENs using this technique. The conversion of Cu DENs to Au, Pt, or Pd DENs has also been demonstrated using the galvanic redox displacement technique [68]. 

Since the development of the dendrimer-templated synthesis of metal nanoparticles, there have been many fnndamental questions raised about the synthesis processes and final structures of DENs. Many efforts have been devoted to answering these questions, which are the fonndation of these dendrimer-based synthesis techniques. The answers to these questions could also explain the observed catalytic properties of DENs. In this section, we discuss several recent reports that provide deeper understanding of the synthesis and structure of DENs. 

Huang W, Kuhn IN, Tsung C-K, Zhang Y, Habas SE, Yang P, Somorjai GA (2008) Dendrimer templated synthesis of one nanometer Rh and Pt particles supported on mesoporous silica catalytic activity for ethylene and pyrrole hydrogenation. Nano Lett 8 2027... 

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

In this review, we tried to cover all the supramolecular species that maybe classified as rotaxane dendrimers. We classified them by their structures - where in dendrimer rotaxane-hke features are introduced. Several different types of macrocycles have been employed as a ring component in the templated synthesis of rotaxane dendrimers. While the synthesis of Type I and II rotaxanes dendrimers is relatively straightforward, that of well-defined Type III rotaxane dendrimers, particularly those of second and higher generations, is still challenging. 

Scheme 9.4 Schematic of metal nanoparticles synthesis within dendrimer templates. (Reprinted from [54] copyright 2002, Marcel Dekker.)...

M. C. Rogers, B. Adisa, and D. A. Bruce, Synthesis and characterization of dendrimer-templated mesoporous oxidation catalysts, Catal Lett. 98,29-36 (2004). 

Section II is about the new structure and understanding of nanocatalysts. Chapters 4 and 5 provide insight for understanding the structure and reactivity of gold catalyst. Chapters 6 and 7 disclose new methods for making nanoparticle catalysts in a control way by using the sol-gel technique and dendrimer template, respectively. Chapter 8 reviews the synthesis, structure, and applications of tungsten oxide nanorods. 

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...

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]. 

H. Lang, S. Maldonado, K.J. Stevenson, B.D. Chandler, Synthesis and Characterization of Dendrimer-Templated Supported Bimetallic Pt-Au Nanoparticles, Journal of the American Chemical Society 126, 12949, 2004. 

Knecht MR, et al. Structural rearrangement of bimetallic alloy Pd-Au nanoparticles within dendrimer templates to yield core/shell configurations. Chem Mater 2008 20 1019-28. Yamamoto D, Watanabe S, Miyahara MT. Coordination and reduction processes in the synthesis of dendrimer-encapsulated Pt nanoparticles. Langmuir 2010 26 2339-45. 

Balogh L, Tomaha DA (1998) Poly(amidoamine) dendrimer-templated nanocomposites. 1. Synthesis of zerovalent copper nanoclusters. J Am Chem Soc 120 7355... 

Chung YM, Rhee HK (2004) Synthesis and catalytic applications of dendrimer-templated bimetallic nanoparticles. Catal Surv Asia 8 211... 

Dendrimers are a special class of arborescent monodisperse nanometer sized molecules that have been used in the synthesis of Au NPs as surface stabilizers or nanoreactor/templates for nanoparticle growth. Moreover, these hybrid nanomaterials have great potential for application in different fields such as sensors, imaging in cells, electrooptical devices, catalysis, drug delivery agents, and so on. 

Goodman and co-workers177-3 have approached the divergent synthesis differently. In their syntheses of collagen models consisting of three copies of homomeric peptide chains, solid-phase schemes begin with monomers 44 and then the template, the Kemp tricarboxylic acid (KTA) 10,[80 is used to link three monomers, also on resin, to form the peptide dendrimers 46 via 45 (Scheme 12). 

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