Plastic bullets

Angew. Chem. Int. Ed. doi:10.1002/anie.201002764 (2010)

An elegant way to make monodisperse polymeric microparticles of various shapes and sizes is by combining micromoulding techniques and the concepts of wetting and surface tension, report Lee and colleagues. Using a cylindrical mould but changing the addition sequence of the reagents, either bullet-like particles with convex tops, or regular cylindrical particles with flat tops can be made. To make the convex-topped particles, a photocurable polymer, polyethylene glycol diacrylate (PEG DA) and then a non-photocurable wetting solution, hexadecane, are added to the mould. The curvature results from the interfacial tension between the PEG DA in the microwells and hexadecane on the surface. Following photopolymerization of PEG DA, removal of hexadecane and bending of the mould to free the particles, individual, highly uniform convex-topped particles are isolated in a high-throughput manner. If the order of the reagents is switched, flat-topped particles are made because of the nature of the interface between PEG DA and air. Furthermore, the two sequences could be combined to yield anisotropic Janus particles composed of two distinct polymeric compartments containing different fluorescent dyes (pictured).

Cation exchange

Nano Lett. doi:10.1021/nl102560b (2010)

Owing to their large surface-to-volume ratio, cation exchange reactions in nanostructures can be very rapid compared with bulk solids. These reactions can be used to modify and tune the material composition and properties of nanostructures that can be difficult to achieve with other synthetic approaches. August Dorn and colleagues now show that measuring the conductivity of nanowires during cation exchange can prove useful in monitoring the transition from CdSe to Ag2Se in situ. The increase in conductivity during the transition is found to be well correlated with changes in elemental composition and optical absorption spectra. Electron microscopy of the same nanowires before and after cation exchange reveal that the reaction is topotaxial, and that the morphology and shape is completely preserved. The authors suggest that both the structural integrity and extreme sensitivity to changes in materials composition could be of interest for controlling dopant concentration in semiconductors, and for applications in battery electrodes and ion sensing.

Topological thermoelectrics

Appl. Phys. Lett. 97, 073108 (2010)

Topological insulators exhibit protected metallic surfaces despite being insulators in the bulk. The most widely studied topological insulators are also among the most efficient thermoelectrics that can convert wasted heat to electric voltage. The efficiency of thermoelectric conversion is expressed by the figure of merit ZT, which at room temperature is usually 1 or less. The topological properties could, however, enhance ZT. Oleg Tretiakov and colleagues explored this possibility theoretically. For a high ZT, the electrical conductivity should be maximized while the thermal one should be at its lowest possible value. Ran et al. had previously predicted (Nature Phys. 5, 298; 2009) that screw dislocations in Bi0.9Sb0.1 were in fact one-dimensional topologically protected metallic states. Increasing the dislocation density should therefore raise the electrical conductivity. On the other hand, the higher disorder should reduce the thermal conductivity. The team found that the combined effect could lead to ZT close to 10 at room temperature. The results provide an encouraging new perspective for thermoelectricity and its role in the management of wasted heat.

Oocytes as sensors

Proc. Natl Acad. Sci. USA 107, 15340–15344 (2010)

Credit: © 2010 PNAS

Xenopus laevis oocytes are cells found in the ovarian lobes of adult female frogs. Owing to their large size (1 mm in diameter), these oocytes have been widely used in electrophysiology experiments, and as a model system for the expression of membrane proteins in vivo via RNA injection from other organisms. Now, Takeuchi and colleagues have exploited these advantages by trapping oocytes in a fluidic device to build an odorant sensor capable of detecting chemicals in solution with the sensitivity of a few parts per billion. They used genetically modified oocytes that express olfactory receptors of moths or flies. On recognition of pheromones, the receptors trigger changes in the flow of ions through plasma membranes. By means of electrodes that measure changes in membrane potential, the biohybrid sensor can simultaneously detect multiple chemicals that have only a slight difference in chemical formula or isomerism. It can also be programmed to trigger the actuation of a motor in a robotic system, for potential use in monitoring applications in health, food and the environment.

Silicon memories

Nano Lett. doi:10.1021/nl102255r (2010)

Resistive switching memory devices are currently being developed by several companies as promising non-volatile computer memories. Typically, resistive switching devices are made from titanium dioxide or related oxides, where local electrochemical changes result in potentially very small device sizes, down to only a few nanometres. Moreover, because the switching process requires only electrical voltages, the device design is rather straightforward. Jun Yao and colleagues from Rice University have now uncovered a promising new resistive switching effect in silicon oxide. On application of an electrical voltage, a reduction process leads to the formation of conductive silicon nanocrystals within the insulating oxide matrix. These create a conductive pathway for electrons hopping along the nanocrystals. At higher voltages, heating effects lead to oxidation of the nanocrystals, and the material becomes insulating again. Both states are stable, and the devices studied can be switched more than 10,000 times with a remarkable on/off ratio in electrical conductance of 100,000. Being fully compatible with existing silicon technology, this approach represents a promising path towards highly scalable non-volatile memory devices.