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Summer 2003 Crooks Group Research Highlight |
| Preparation
and Characterization of Dendrimer-Encapsulated In 1998 we reported a strategy for preparing
metal nanoparticles within the interior of poly(amidoamine)
(PAMAM) dendrimers (JACS,
1998, 120, 4877, Acc. Chem. Res. 2001, 34,
181). Typical synthetic procedures
involve first sequestering metal ions, such as Cu, Pd, and Pt, within
the dendrimer, and then chemically reducing the resulting inorganic/organic
nanocomposite. These dendrimer-encapsulated
nanoparticles (DENs) are very stable and in some cases have good catalytic
activity. Hydroxyl-terminated
PAMAM dendrimers are usually used as templates and stabilizers for these
nanoparticles, because dendrimers having metal-complexing ligands, such
as amine groups, on their periphery crosslink and precipitate in the
presence of metal ions. However,
the synthesis of Au nanoparticles with hydroxyl-terminated PAMAM dendrimers
is complicated by the fact that the Au complex used to prepare these
materials (AuCl4-) is prematurely reduced by the
peripheral hydroxyl groups. Recently, research group members Yong-Gu
Kim and Dr. Sang-Keun Oh prepared Au nanoparticles having very narrow
size distribution in the interior of ammonium chloride-modified PAMAM
dendrimers (Gn-Qp, where n is the generation of the dendrimer
and p is the number of quaternized peripheral
groups, Figure 1). This new type of dendrimer makes it possible to prepare
Au DENs having narrow size distribution while maintaining a substantial
fraction of the reactive amine peripheral groups. The latter is important, because these unquaternized
amines can be used for linking DENs to surfaces, biomolecules, and other
types of polymers. Figure 2 shows high resolution transmission
electron microscopic (HRTEM) images and size-distribution histograms
for G6-Q116 hosting 55-atom Au nanoparticles (G6-Q116(Au55)) and 140-atom Au nanoparticles
(G6-Q116(Au140)). From the data, it is clear that the size of
the nanoparticles can be controlled by changing the [HAuCl4] : [dendrimer] ratio in the DEN precursor solution. Using this approach, Au nanoparticles having
sizes of 1.3 ± 0.3 nm and 1.6
± 0.3 nm were
observed in the HRTEM images when 55 and 140 equivalents, respectively,
of HAuCl4 per dendrimer were used.
For unpurified Au nanoparticles in this size range, this represents
an improvement in monodispersity of about 300%
compared to previous reports of Au nanoparticles prepared by other methods,
such as ligand capping (monolayer protected clusters), micelle templating,
or polymer encapsulation. The remarkably high size monodispersity can
be attributed to three factors. First,
this general methodology for preparing metal nanoparticles is based
on templating, and therefore difficult-to-control factors that affect
particle size and size dispersity, such as mass transfer, rate of nucleation,
and ligand adsorption, are not important.
Second, the high positive charge on the periphery of the quaternized
dendrimers prevents agglomeration of dendrimers and their Au nanoparticle
guests. Third, we used “magic numbers” of metal atoms
(55 and 140 Au atoms per dendrimer) to prepare these materials. Magic numbers result in formation of low-energy
structural motifs, which reduces the extent of polydispersity. The results described here will be submitted
for publication by the end of July.
Our next step is to study the electrochemical properties of these
highly monodisperse Au nanoparticles by incorporating them into thin
films on electrode surfaces. |
