CW laser-induced photothermal conversion and shape transformation of gold nanodogbones in hydrated chitosan films

We investigate the photothermal conversion and transformation of gold nanoparticles with an initial dogbone shape after dispersion in hydrated chitosan films, which is a representative model of biological tissue, and excitation by a CW diode laser for 1 min. Gold nanodogbones are observed to undergo a distinct modification above a sharp threshold of ~11 W cm⁻² and 110 °C. Surprisingly, the very same modification is achieved up to at least 36 W cm⁻² and 250 °C. We use an analytical model derived from Gans theory to associate the change in color of the films with the change in shape statistics of these gold nanoparticles. This model proves both convenient and dependable. We interpret the photothermal transformation as a rearrangement of particles with a dogbone shape and an aspect ratio of 4.1 into rods with an aspect ratio of 2.5, where material from the end lobes of the dogbones may relocate to the waists of the rods. In turn, additional transitions to stable gold nanospheres may exhibit fairly slower kinetics.     

Parallel or classical kinetic resolution of a planar chiral ferrocenylketone through asymmetric reductions

Racemic 1-acetyl-2-methoxymethylferrocene, (±)-1 was subjected to asymmetric reduction with two different methodologies and complementary results were obtained. When the reduction of (±)-1 was carried out in the presence of CBS-oxazaborolidine catalyst and BH₃·Me₂S as the hydrogen source, both enantiomers of the substrate were converted with comparable reaction rates and selectivities. The corresponding diastereoisomeric ferrocenylalcohols 3a and 3b were obtained in a 1:1 ratio and >90% enantiomeric excess; this reaction profile being related with a parallel kinetic resolution with high ds₁ and ds₂ diastereofacial selectivities. On the contrary, the transfer hydrogenation of (±)-1 with HCOOH/triethylamine in the presence of (R,R)-Noyori’s catalyst proceeded via classical kinetic resolution, so that the formed (?)-3b or unreacted (+)-1 could be obtained in highly enantiopure form before or beyond 50% of the substrate conversion, respectively. Alcohol 3b with an (1R_p,S)- or (1S_p,R)-configuration is not easily accessible by the diastereoselective metallation/electrophilic quenching sequence routinely applied in the synthesis of planar chiral ferrocenes. As a result, the described procedures provide a valuable access to this useful starting material for the synthesis of homochiral related derivatives.     

Carbon composite electrodes: surface and electrochemical properties

Electrodes based on particulate carbon-epoxy or silicone composites have been formed and characterised using electrochemical methods, scanning electron microscopy and scanning electrochemical microscopy. These composites are rigid, exhibit high electrical conductivity and are stable in organic solvents for prolonged periods. The bulk resistance of the Araldite-M and Araldite-CW2215 based electrodes is low, 130+/-12 and 185+/-15 ohms, respectively. In contrast, the bulk resistance of the silicone based electrodes is 1480+/-112 ohms. The uncompensated resistance of electrochemical cells where the composites act as working electrodes is significantly larger than that expected on the basis of solution resistance alone, i.e., up to 7.5 kohms in the case of the silicone composites. These results are interpreted in terms of the presence of pores within the composite material. The response times of the composite electrodes to changes in the applied potential is between 3.1 and 7.2 ms which, although almost an order of magnitude longer than a comparable glassy carbon electrode, is sufficiently rapid to give useful voltammetric data for scan rates of several V s(-1). Close to ideal reversible cyclic voltammetry is observed for ferrocene under semi-infinite diffusion control for scan rates between 0.01 and 0.1 V s(-1) at the Araldite composites. In contrast, the large resistance associated with the silicone based materials causes quasi-reversible responses to be observed over this range of scan rate. Scan rate dependent cyclic voltammetry and time resolved chronoamperometry responses observed for ferrocene in solution are consistent with those expected for a random array of microelectrodes. Scanning electron microscopy and scanning electrochemical microscopy has been used to image the shape, size and electrochemical activity of the electroactive zones. In the case of Araldite-M, the quality of the electrode surface has been probed by comparing the rate of heterogeneous electron transfer at a composite microelectrode with that found for a carbon fibre electrode. The standard heterogeneous electron transfer rate constant, k degrees , is 6.0+/-0.1 x 10(-3) cm s(-1) for the composite compared to 1.5+/-0.1 x 10(-1) cm s(-1) for the carbon fibre electrode. While the smaller rate constant found for the composite suggests a less pristine surface, k degrees is sufficiently large to support reversible, electron transfer under typical electroanalytical conditions. These fundamental measurements will underpin the development of enzyme based biosensors for use in organic solvents.     

Network synthesis under survivability constraints

Telecommunication networks are subject to link and equipment failures. Since failures cannot be entirely avoided, networks have to be designed so as to survive failure situations. In this paper, we are interested in designing low cost survivable networks. Given point-to-point traffic demands and a cost/capacity function for each link, we aim at finding the minimum cost capacities satisfying the given demands and survivability requirements. A survivability model that reroutes interrupted traffic using all the available capacities on the network is presented and studied. In the proposed model, capacity and flow assignments for each network operating state are jointly optimized. We prove the -hardness of the optimisation problem defined by dual constraints. Then, we propose a polynomial relaxation along with a fast heuristic to compute a feasible solution of the problem from its relaxed optimal solution. Our solution approaches are tested on a set of problem instances.Received: September 2002, Revised: July 2003, AMS classification: 90C05     

Metal-chelating nanoparticles as selective fluorescent sensor for Cu2+

A fluorescent sensor for Cu(2+) at the nanomolar level in water has been designed by associating a BODIPY fluorophore and a selective ligand (cyclam) in ultrafine polymer nanoparticles.     

Easy access to diastereomerically pure platinacycles

The synthesis of the first examples of diastereomerically pure platinacycles having simultaneously a chiral carbon and a sigma[Pt-C(sp(2), ferrocene)] bond is described.     

Theoretical Study of Aerosol Filtration by Nucleopore Filters: The Intermediate Crossover Regime of Brownian Diffusion and Direct Interception

We study the problem of aerosol filtration by formulating a unified approach that incorporates the dominant mechanisms of particle capture in cylindrical pores. The theoretical approach presented here takes into account the effects of flow slip at the pore wall and predicts an enhanced efficiency in the intermediate crossover regime between Brownian diffusion and direct interception. We also suggest how the results obtained for cylindrical pores can be used to estimate the efficiency of granular ceramic filters in the region of the most penetrating particle size, where the enhanced efficiency effects are strongly amplified by the large number of pores, or more generally unit bed elements, acting in series. Copyright 2001 Academic Press.     

A biofunctional polymeric coating for microcantilever molecular recognition

An innovative route to activate silicon microcantilevers (MCs) for label free molecular recognition is presented. The method consists in coating the underivatized MCs with a functional ter-polymer based on N,N-dimethylacrylamide (DMA) bearing N-acryloyloxysuccinimide (NAS) and 3-(trimethoxysilyl)propyl-methacrylate (MAPS), two functional monomers that confer to the polymer the ability to react with nucleophilic species on biomolecules and with glass silanols, respectively. The polymer was deposited onto MCs by dip coating. Polymer coated MCs were tested in both static and dynamic modes of actuation, featuring detection of DNA hybridization as well as protein/protein interaction. In the dynamic experiments, focused on protein detection, the MCs showed an average mass responsivity of 0.4 Hz/pg for the first resonant mode and of 2.5 Hz/pg for the second resonant mode. The results of the static experiments, dedicated to DNA hybridization detection, allowed for direct estimation of the DNA duplex formation energetics, which resulted fully consistent with the nominal expected values. These results, together with easiness and cheapness, high versatility, and excellent stability of the recognition signal, make the presented route a reliable alternative to standard SAM functionalization (for microcantilevers (MCs) and for micro-electro-mechanical systems (MEMS) in general).