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).
Synthesis, characterization, and catalytic activity for hybrids of multi-walled carbon nanotube and amphiphilic poly(propyleneimine) dendrimer immobilized with silver and palladium nanoparticle.
Four types of new multi-walled carbon nanotube (MWCNT) based nanohybrid catalysts were prepared through simple methods. Initially, MWCNT was functionalized with carboxyl group and subsequently bonded with amphiphilic poly(propyleneimine) dendrimer (APPI) having generation (G2) and (G3). They are abbreviated as MWCNT-APPI (G2) and MWCNT-APPI (G3). Silver nanoparticles (AgNPs) and palladium nanoparticles (PdNPs) were separately immobilized to each hybrid to obtain four types of MWCNT based nanohybrid catalysts. The pseudo-first order rate constants for reduction of 4-nitrophenol revealed that among the four types of MWCNT nanohybrid catalysts, MWCNT-APPI (G3)-PdNPs was more efficient than the other catalysts with five-fold higher efficiency than the homogeneous PPI (G3)-PdNPs. For the same reaction with MWCNTs-APPI (G3)-PdNPs catalyst, the variation of [catalyst] and [NaBH4] was directly proportional to kobs. The study of reusability of MWCNT-APPI (G3)-PdNPs proved no loss in activity even after recycling the catalyst for five times.
Characterization of Starburst Dendrimers by Electron Paramagnetic Resonance. 2. Positively Charged Nitroxide Radicals of Variable Chain Length Used as Spin Probes
Electron paramagnetic resonance (EPR) spectroscopy has been used to investigate the structure and binding ability of a novel class of anionic macromolecules: half-generation poly(amidoamine) starburst dendrimers (n.5SBDs), which differ systematically in size (generation), and which are terminated by sodium carboxylated surfaces. The half-generations in the range 1.5-7.5 have been investigated. To some extent n.5-SBDs mimic both anionic micelles, in their shape and extemal surface, and biomacromolecules, such as proteins and enzymes, in their internal structure. Positively charged nitroxide radicals attached to carbon chains of different lengths were used to probe hydrophilic and hydrophobic binding sites of the SBD structure. Mobility and polarity parameters were evaluated by means of a computer-aided analysis of the EPR spectra of the interacting radicals. The study of these parameters as a function of pH demonstrated the role of electrostatic interactions in promoting binding at the extemal SBD/ water interface. Such interactions are found to be weaker than those occumng with anionic micelles, due to the greater hydrophilic character of the internal SBD structure. Analysis of the EPR parameters as a function of radical chain length, generation, and carboxylate concentration provided evidence of both hydrophilic and hydrophobic interactions between the probe radicals and n.5-SBDs. The results are consistent with the hypothesis that the radical chain may enter the SBD intemal structure and interact with hydrophobic sites. Analysis of EPR spectra at different temperatures provided the activation energies for the rotational motion of the interacting probe.