G1 dendrimers-mediated evolution of silver nanostructures from nanoparticles to solid spheres.
With the control of G1 poly(amidoamine) (PAMAM), an evolutionary course of stable colloidal silver from discrete nanoparticles to solid spheres through ultraviolet irradiation reduction of silver nitrate solutions was observed by transmission electron microscopy (TEM). The morphologies of the products depend on the Ag+ concentration. A mechanism of globular assembly was put forward to interpret the evolution of the nanostructures. Powder X-ray diffraction (XRD), electron diffraction (ED) patterns, and X-ray photoelectron spectroscopy (XPS) indicate the presence of cubic symmetry silver. XPS and Fourier transform infrared (FT-IR) spectroscopy confirm that dendrimers have participated in the stabilization and control of Ag nanostructures. In the UV-vis spectra, the intense surface plasmons are centered at 425 and 430 nm corresponding to the shapes of dots and solid spheres, respectively. The solid spheres exhibit excellent catalytic efficiency on the reduction of 2,7-dicholoroflurescein (DCF).
Photophysical investigation of starburst dendrimers and their interactions with anionic and cationic surfactants
Fluorescence spectroscopy has been used to characterize the structure of a unique class of anionic macromolecules: starburst dendrimers (SBDs) possessing an external anionic surface. Pyrene was used as a photoluminescence probe to sense various hydrophobic sites in the microheterogeneous architecture offered by poly(amidoamine) starburst dendrimers possessing sodium carboxylated surfaces. A series of IO different "half-generations, n.5" starburst dendrimers, which differ systematically in molecular weight, size, and surface charge density, have been studied. The probe method provides experimental evidence for a structural surface transition between generations 3.5 and 4.5. The same probe method was applied to study how the structural properties of the starburst dendrimers determine their interactions with small ionic amphiphilic molecules. Whereas starburst dendrimers do not noticeably affect the micellization process of an anionic surfactant (sodium dodecyl sulfate, SDS), the association process with a cationic surfactant (dodecyltetrammonium bromide, DTAB) leads to the formation of two different types of SBD-templated surfactant aggregates. Addition of DTAB to aqueous solutions containing the earlier generation (0.5-3.5) SBD leads to the formation of SBD-templated surfactant aggregates that result from noncooperative, random condensation of surfactant molecules on the anionic dendrimer surface. Addition of DTAB to aqueous solutions containing later generation (4.5-9.5) SBDs leads to the formation of SBD-templated surfactant aggregates resulting from initial, noncooperative, random condensation, followed by cooperative condensation of surfactant molecules on the anionic dendrimer surface. The results are shown to be consistent with a change in the morphology of SBDs from an open, branched structure for generations 0.5-3.5 to a closed, increasingly compact surface for generations 4.5-9.5.
Chromophore-Labeled Dendrons as Light Harvesting Antennae
A novel series of polyether dendrimer segments (dendrons) end-capped with pyrenyl, naphthyl, or methyl groups has been prepared by a convergent growth method. Steady-state fluorescence measurements indicate the absence of intramolecular naphthalene excimer in the naphthyl-capped dendrons. However, in the pyrenyl-capped dendrons, excimer emission predominates. Fluorescence from both the naphthyl monomer and pyrenyl excimer are quenched when a suitable electron donor (e.g., a 3-[dimethylamino]phenoxy group) is covalently attached at the dendron focal point. No sensitized emission from the dendron backbone is observed in the chromophore-labeled dendrons, although the control methyl-capped dendron fluoresces weakly at 310 nm when excited at 284 nm. Absorption and fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes for the chromophore-labeled dendrons are reported.