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April 23, 2003 Study of Amphiphilic Gold-Dendrimer Nanocomposite MonolayersY.S. Seo1, K.S. Kim1, K. Shin1, H. White1, M. Rafailovich1, J. Sokolov1, B. Lin2, H.J. Kim3, C. Zhang4, and L. Balough4 Dendrimer-based nanocomposites are novel organic-inorganic hybrid molecular structures that are synthesized by dispersing very small inorganic molecules in nano-scaled branched polymeric networks and could be applied in catalysis and drug delivery. Since these nanocmomposites are often used both in aqueous media as well as coatings on solid substrates, we tried to determine how they respond to changes in their physical environment by using x-ray reflectivity. We find that the behavior of these structures significantly changes between aqueous and attractive solid substrates and that these structures are very sensitive to whether the solution is acidic or basic, so that they can be used either as sensors or drug delivery agents.
One well-studied dendrimer is poly(amidoamine) (PAMAM), which could be used as a building block for molecular-sized medical devices, because it contains beta-alanine repeat units (which are amino acids, or basic units of proteins), and thus can be considered as spherical artificial proteins. The interaction of PAMAM with a biological object may be adjusted by modifying the dendrimer’s surface properties. But it is extremely difficult to observe PAMAM dendrimers directly in cells or in tissue, because they do not easily give rise to color in organic substances.
We synthesized amphiphilic DNCs made of gold and PAMAM by dispersing gold atoms without covalent bonds inside dendrimer molecules (figure 1). We investigated the properties of the DNC monolayer at the air/water interface at NSLS beamline X19C (figure 2-a) and on a solid substrate made of a Langmuir-Blodgett film (monomolecular layer containing molecules that are both water-loving and water-repelling) at NSLS beamline X10B (figure 2-b) using x-ray reflectivity.
Because of the small amount of gold present in the DNCs and the refractive index contrast between the gold domains and the water substrate, we successfully conducted in situ x-ray reflectivity measurements of the DNC. We found that the dendrimer layer is hydrated and the gold is uniformly distributed within the dendrimer body. The second-generation dendrimer was spherical on the water surface, while the fourth-generation had a pancake-like, or oblate, structure at high pressures. We also found that the shape of the DNCs was very sensitive to the acidity of the solution, indicating that these DNC could be used as detectors of biological activity or drug delivery systems. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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A dendrimer is a macromolecular structure containing connectors
and branching units built around a small molecule or a linear
polymer core, creating stepwise attachment of layers, called
generations, that are nearly spherical. This highly symmetrical
structure contains a large number of regularly spaced internal and
external functional groups that interact with the molecular
environment. 
Instead, another type of dendrimer, called dendrimer
nanocomposite (DNC), can easily be observed by transmission electron
microscopy and by other optical methods, so they are promising
materials for biomedical nanotechnology. 