Entrapped dendrimers

The removal of the poly(ethylene oxide) fiber material can be achieved by extraction with water. Eventually, poly(amido amine) dendrimers that are entrapped in the poly(p-xylylene) tubes are obtained. These entrapped dendrimers exhibit a high catalytic activity as reusable organocatalysts [108]. 

Dendrimers have distinctive properties, such as the ability to entrap small molecules in their core region and very low intrinsic viscosities in solution. Such properties require molecules to have achieved a particular size, and not all molecules with branches radiating from a core are large enough to develop the characteristic properties of true dendrimers. Branched molecules below this critical size are called dendrons and are the equivalent in dendrimer chemistry of oligomers in polymer chemistry. 

Dendrimer micelles of this type have been formulated as drug delivery vehicles. Dendrimers with a hydrophobic interior have been used to entrap a hydrophobic drug such as indomethacin. This is retained because of the hydrophilic periphery containing ethylene glycol functional groups, and is released slowly because of the collapsed configuration of the interior, through which molecular diffusion is obstructed. 

Shi, X., Wang, S., Sun, H., and Baker Jr., J.R. (2007) Improved biocompatibility of surface functionalized dendrimer-entrapped gold nanoparticles. Soft Matter 3, 71-74. 

Although it is not possible to prepare Ag particles inside Gn-OH by direct reduction of interior ions, stable, dendrimer-encapsulated Ag particles can be prepared by a metal exchange reaction. In this approach, dendrimer-encapsulated Cu nanoclusters are prepared as described in a previous section [82], and then upon exposure to Ag+ the Cu particles oxidize to Cu + ions, which stay entrapped within the dendrimer at pH values higher than 5.5, and Ag+ is reduced to yield a dendrimer-encapsulated Ag nanoparticle (Fig. 15). 

The hyperbranched poly( acrylic acid) graft films -C02H-rich interface on polyethylene can be modified by noncovalent methods just like CO2H-rich interfaces of PAA/Au grafts. This was shown by treating deprotonated 3-PAA/PE films with cationic polyelectrolytes like poly-D-lysine, and amine terminated PAMAM dendrimers at pH 7 [31]. Equation 10 illustrates the entrapment of PAMAM dendrimers in a 3-poly(sodium acrylate)/PE film. In these cases, polyvalent entrapment of the cationic electrolyte was evidenced in the ATR-IR spectriun by the appearance of amide C = O and N - H peaks of the guest dendrimer that were not present in the host 3-poly(sodium acrylate)/PE film. 

Even newer generations of nanomaterials are based on carbon nanotubes using the bottom-up approach. The materials are still very expensive, but the technology is evolving rapidly. Another type of nanotube has been prepared based on self-assembly of specific molecules such as chitosan-based nanoparticles of polypeptides, DNA or synthetic polymers. Phospholipids or dendrimer-coated particles are suitable for the entrapment of actives in very small vesicles. The current materials are still lacking in selectivity and yield (costs). 

Scheme 4.8. Poly(propylene imine) dendrimers that have been terminally functionalized with rert-butoxycarbonyl protected phenylalanine units. These macromolecules were described as dendritic boxes due to their ability to entrap guests. R = benzyl (20) hydrogen atoms are omitted in the structures.

Some special nanomaterials are of great interest due to their unique properties. Dendrimers are versatile, well-defined, nanosized monodispersing macromolecules which are hyperbranched synthesized polymers constructed by repetitive monomer units. They are perfect nanoarchitectures with size from lnm to more than 10nm depending on the synthesis generation. Drugs can be entrapped into the branches... 

Dendrimers have various useful properties. The number of branches increases with the step munber (the dendrimer generation). The branches are crowded at the outer surface while the inner part of the dendrimer has more empty space. Therefore, the dendrimer can behave like a capsule. Size-matched functional guest molecules become entrapped in this nanometer-scale capsule. 

Zhang GD, Harada A, Nishiyama N, Jiang DL, Koyama H, Aida T, Kataoka K (2003a) Polyiori complex micelles entrapping cadonic dendrimer porphyrin Effecdve photosensidzer for photodynamic dierapy of cancer. J Control Release 93 141—150. 

Jansen, de Brabander-van Den Berg, and Meijer describe the entrapment of molecules in a dendritic box,f ° based on poly(propyleneimine) dendrimers with a chiral shell of protected amino acids (Fig. 7). The resulting dendritic structure, 5 nm in size, possesses a dense shell (as a result of bulky surface... 

The 6-nm dendrimers under study also adsorbed to an extent onto the actin fibrils, thus further reducing diffusion. Superimposed on the photomicrograph are nanoparticles to illustrate the problems in diffusion through such a medium adsorption, obstruction, and entrapment. The overall effect is to markedly reduce diffusion at a certain particle radius diffusion virtually ceases. These few descriptions simply underline the supreme importance of nanoparticle diameter. This is one reason why to generahze about nanoparticles is somewhat difficult when not only are the substance of nanoparticles and their surface coatings different but the fundamental property of size varies over such a wide range. This in turn means that dependent properties such as surface area and volume vary even more. 

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