Dendrimer-Metal Nanocomposites
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scientists, because control of the critical length means control of new macroscopic properties (1). Nanoscopic size (1-100 nm) domains of materials are composed of a few atoms to up to several hundred atoms grouped together by physical forces. The small size results in extremely high surfaces of the nanoparticles, which in turn may lead to unique physical (solubility, optical, electronic, magnetic and mechanical strength) and/or chemical behavior. Nanoparticles, being much smaller than the wavelength of visible light, usually have a dispersion spectrum, (which is a function of size and material composition) in addition to an absorption spectrum. Particles that are small enough are transparent and may not absorb in the visible region. Dendrimers (2) are monodisperse macromolecules built with the highest level of synthetic control. The dendritic architecture consists of connectors and branching units built around a small molecule or a linear polymer (core), which strategy leads to spherical dendrimers or rod-like macromolecules (3), respectively. The high level of synthetic control makes it possible to synthesize a monomodal distribution of well-defined and highly symmetrical polymer molecules having a large number of equally spaced internal and external functional groups. Many unique properties of dendritic polymers originate from this fact. For example, the interior and exterior of a dendritic polymer can be very different, depending on the chemistry of terminal groups. Either the interior or the exterior can be hydrophilic (cationic, anionic, neutral) or hydrophobic (4). Dendrimers, due to the extremely high local concentration of surface groups, may interact strongly and selectively with different surfaces. These dendrimer surfaces can be compatibilized with many organic or inorganic materials while the interior space may be used for nanolevel storage and/or delivery. Since the first reports (5, 6) in 1985, it has been known that dendrimers
are able to form complexes with a great variety of ions and compounds (7-9). Dendrimers can encapsulate or physically trap organic (10) or inorganic (11) molecules. 69 Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
The concept of dendrimer nanocomposites (11-13) is based on the immobilization of preorganized metal ions. For the first time, this concept has been demonstrated on the example of metal sulfide dendrimer hybrid nanocomposites (11) and it has quickly produced further examples (13-16). Dendrimer templated nanoparticles display unique physical and chemical properties because of the atomic/molecular level dispersion of inorganic guest(s) within a dendrimer host. Dendrimer metal nanocomposites are made by reactive encapsulation, where metal ions are preorganized by the dendrimer template, then are subsequently immobilized in/on the polymer molecule. Size, shape, size distribution and surface functionality of these nanoparticles are determined and controlled by the dendritic macromolecules as well as by the chemistry of the preorganization and imm
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