Extraction of Au nanoparticles having narrow size distributions from within dendrimer templates

Here, we show that Au nanoparticles having diameters of less than 2.2 nm can be extracted from within the interior of PAMAM dendrimers using n-alkanethiol extractants. Extraction proceeds quickly, regardless of the size of the nanoparticle, the dendrimer generation, or the peripheral functionalization of the dendrimer. The extraction rate is fastest for the lowest generation dendrimers, the smallest nanoparticles, and the shortest chain-length n-alkanethiols. Other important results of this study include the following. First, within the accuracy of absorbance spectroscopy, the extraction yield is quantitative. Second, NMR and FT-IR spectroscopy indicate that after extraction the dendrimer remains in the aqueous phase and can be used to template additional metal particles. Third, the size and optical characteristics of the extracted nanoparticles are the same as the precursor dendrimer-encapsulated nanoparticles (DENs). Fourth, a 100-fold excess of n-alkanethiol molecules is required to prevent aggregation of DENs during extraction.     

Synthesis of bicyclo[4.n.1]alkanones

Cyclic beta-keto ester monoanions react with 1,4-dihalobutenes to give C-alkylated products which subsequently undergo a stereoselective SN2' O-alkylation reaction to yield functionalized enol ethers. When the starting material was ethyl cyclopentanone carboxylate, the C-alkylated product, treated with a base, directly afforded the functionalized bicyclo[4.2.1]nonanone. The enol ethers were submitted to a flash vacuum thermolysis (FVT) reaction to readily afford functionalized bicyclo[4.n.1]alkanones (n = 3, 4). This reaction sequence was applied to the synthesis of a functionalized tricyclo[7.4.1.0(1,5)]tetradecanone, which represents an analogue to the tricyclic core of ingenol.     

Effects of delayed recovery and nonuniform transmission on the spreading of diseases in complex networks

We investigate the effects of delaying the time to recovery (delayed recovery) and of nonuniform transmission on the propagation of diseases on structured populations. Through a mean-field approximation and large-scale numerical simulations, we find that postponing the transition from the infectious to the recovered states can largely reduce the epidemic threshold, therefore promoting the outbreak of epidemics. On the other hand, if we consider nonuniform transmission among individuals, the epidemic threshold increases, thus inhibiting the spreading process. When both mechanisms are at work, the latter might prevail, hence resulting in an increase of the epidemic threshold with respect to the standard case, in which both ingredients are absent. Our findings are of interest for a better understanding of how diseases propagate on structured populations and to a further design of efficient immunization strategies.     

Volume approximation of smooth convex bodies by three-polytopes of restricted number of edges

For a given convex body K in \Bbb R³{\Bbb R}^3 with C ² boundary, let P ^c _n be the circumscribed polytope of minimal volume with at most n edges, and let P ⁱ _n be the inscribed polytope of maximal volume with at most n edges. Besides presenting an asymptotic formula for the volume difference as n tends to infinity in both cases, we prove that the typical faces of P ^c _n and P ⁱ _n are asymptotically regular triangles and squares, respectively, in a suitable sense.     

Sequential extraction--spectrofluorometric determination of lead and cadmium using cryptands.

A spectrofluorimetric method for the determination of ultratrace amounts of lead and cadmium is described based on the sequential extraction of the ternary ion-association complexes formed between the cation, a cryptand as the ligand and eosin as a counter ion. A linear working range from the detection limit (0.5 ng ml(-1) to 250 ng ml(-1) of lead and to 1 50 ng ml(-1) of cadmium was obtained. The relative standard deviation was 2-4%. The proposed method has been applied successfully to the determination of lead and cadmium in zinc metals and soft drinks.     

Compression elastic modulus of neutral,ionic, and amphoteric hydrogels based on N‐vinylimidazole

Several hydrogels of N‐vinylimidazole and sodium styrenesulfonate have been prepared by radical cross‐linking copolymerization in aqueous solution, using N,N′‐methylene‐bisacrylamide as crosslinker. Depending on composition, these hydrogels were neutral, amphoteric, cationic or anionic. Compression‐strain measurements were performed on samples as‐synthesized and swollen in deionized water or in acid aqueous solutions, with and without salt. It was thus found that the cross‐linking densities determined by compression measurements on as‐synthesized samples are in good accordance with those calculated by means of the model of polymer networks with pendant vinyl groups. A non‐Gaussian parameter (β) was introduced to explain that the elastic moduli (G) of samples swollen at equilibrium are larger than predicted by the Gaussian model. The β values of the neutral or ionized systems increase with swelling and fall into a single curve, which denotes a common behavior. Swelling has two opposite effects on G; on the one hand G decreases because the polymer volume fraction diminish and the system shifts from the affine limit to the phantom one; on the other, β increases and contributes to increasing G. The balance of those two opposite effects determines the variation of G with swelling. The possible contribution of ionic crosslinks to ν_e for the polyampholyte and for the polycation wearing divalent counteranions was discussed. A peculiar system is poly(sodium styrenesulfonate), whose cross‐linking density is much lower than expected. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1078–1087, 2009     

Dido's problem in the plane for domains with fixed diameter

We find the connected compact domains in the closed half-plane, with fixed area and diameter, which minimize the relative perimeter.This work has been partially supported by the DGICYT (Spain) Grant no. PB 91-0324.     

Regioselective Synthesis of Polyheterohalogenated Naphthalenes via the Benzannulation of Haloalkynes

Independent control of halide substitution at six of the seven naphthalene positions of 2‐arylnaphthalenes is achieved through the regioselective benzannulation of chloro‐, bromo‐, and iodoalkynes. The modularity of this approach is demonstrated through the preparation of 44 polyheterohalogenated naphthalene products, most of which are difficult to access through known naphthalene syntheses. The outstanding regioselectivity of the reaction is both predictable and proven unambiguously by single‐crystal X‐ray diffraction for many examples. This synthetic method enables the rapid preparation of complex aromatic systems poised for further derivatization using established cross‐coupling methods. The power and versatility of this approach makes substituted naphthalenes highly attractive building blocks for new organic materials and diversity‐oriented synthesis.     

Comparison of fluorescence-enhancing reagents and optimization of laser fluorimetric technique for the determination of dissolved uranium

Results from tests aimed at optimizing an instrumental procedure for the direct and fast determination of uranium in solution by laser fluorescence are presented. A comparison of sample fluorescence measured using different fluorescence enhancing reagents was performed: sodium pyrophosphate, orthophosphoric acid, sulphuric acid and a commercially available fluorescence enhancer were tested for the determination of uranium. From the experimental results, 0.01 M Na₄P₂O₇·10H₂O showed the best performance. Effects of reagent pH, different matrices, different concentrations of dissolved Th, and sample volume were investigated. Applications of the improved procedure for the determination of uranium in samples arising from UO₂-based high level nuclear waste dissolution studies are described.     

Carleson measures for weighted holomorphic Besov spaces

We obtain characterizations of positive Borel measures μ on B ⁿ so that some weighted holomorphic Besov spaces B _s ^p (ω,B ⁿ ) are embedded in L ^p (d μ).