Photons, prions, skutterudites and sperm

Sperm get sorted

Credit: ©2003 American Chemical Society

A microscale integrated microfluidic device is unexpectedly coming to the rescue of infertile men. One of the most advanced fertilization techniques is based on injecting single sperm into a healthy egg. But selecting viable sperm by hand is a time consuming and inefficient procedure, and an automated alternative for sorting sperm was hitherto unavailable. Now, a group led by Shuichi Takayama at the University of Michigan have developed a simple, disposable microfluidic device to separate motile from non-motile sperm (Analytical Chemistry 75, 1671–1675; 2003). Their device uses a passive pumping system, based solely on gravity and surface tension, to provide a steady flow of liquid through the microchannels, without the need for an external power source (see image). Motile sperm exit the system through a special outlet owing to their ability to cross streamlines in a laminar fluid stream. The strongest swimmers win in vitro as well as in vivo.

Nanotube–vesicle networks

A method for forming nanotube–vesicle networks suitable for microfluidic devices has been developed by Orwar and colleagues (Langmuir; http://dx.doi.org/10.1021/1a026947r). They use a micropipette to deliver a biotin-containing soyabean lipid vesicle of ∼3 mm onto a silica surface functionalized with spots of the biotin-binding NeutrAvidin molecule. After the vesicle is placed on one 'binding' spot, the pipette is moved to deliver a vesicle to the next spot, forming a lipid nanotube — up to 100s of micrometres long —between the two vesicles. In this way a functionalized network of well-defined geometry, size and connectivity can be formed. The vesicles are prevented from spreading over the whole surface by covering the silica with a bilayer of another lipid (phosphatidylcholine). Combining this method with microelectrofusion — using d.c.-voltage pulses from a fine carbon-fibre microelectrode to fuse two or more vesicles onto one spot — could also enable more complex networks to be formed. The authors imagine potential applications such as fluidic devices for chemical analysis, or for observing chemical kinetics or membrane mechanics.

Single-crystal nanotubes

The production of crystalline solids based on high-quality nanotubes generally results in a multilayered crystal structure. Nonetheless, recent progress in the synthesis of tubular nanostructures has led to the creation of nanotube solids without these multilayers, albeit with a polycrystalline or amorphous structure. Peidong Yang and colleagues at the University of California, Berkeley, report in Nature (422, 599–602; 2003) an approach to making single-crystal gallium nitride (GaN) nanotubes, with uniform lengths of 2–5 micrometres and inner diameter of 30–200 nanometres. Their approach, which should be easy to extend to other inorganic systems, uses solid zinc oxide (ZnO) nanowires as templates for the epitaxial growth of GaN layers by chemical vapour deposition. The ZnO nanowires are subsequently removed, leaving behind ordered arrays of GaN nanotubes. Because of their useful mechanical, optical and electrical properties, these semiconducting GaN nanotubes are enormously attractive for technological applications in nanoscale electronics and in nanocapillary electrophoresis.

Playing with prions

The ability of prions to resist damage, which makes these proteins so deadly in mammals infected with spongiform encephalopathies, also makes them attractive to scientists looking for templates to build molecular wires for nanoscale electronics. Susan Lindquist and colleagues in Chicago (Proceedings of the National Academy of Sciences USA; http://dx.doi.org/10.1073/pnas.0431081100) have now exploited the properties of prion-based fibres to construct highly stable silver and gold nanowires with low resistance to electrical currents. Previous attempts to harness the natural ability of biomolecules to self-assemble into wire-like structures have suffered from the poor physical and chemical stability of most DNA and protein templates. Lindquist and colleagues use a fragment of a yeast prion that will rapidly self-assemble into amyloid fibres up to several hundred micrometres long. These amyloid fibres are natural insulators, so the researchers genetically modified the prion fragment to help the fibres bond to gold nanoparticles. In the final step, more silver and gold was deposited on the fibres to fill any gaps between the nanoparticles, and to convert the fibres into conducting wires.

Skittery skutterudites

Thermoelectric materials must be good electrical conductors but poor thermal conductors if they are to convert heat into electricity efficiently. A loose cage formed around freely vibrating atoms provides one way to minimize thermal conductivity, while hopefully preserving electrical conduction through the cage lattice. Crystal compounds known as skutterudites (M₄Sb₁₂, where M is a transition metal) offer a metallic cage of 12 Sb atoms, into which rare-earth atoms can be inserted. Fernande Grandjean and colleagues report in Physical Review Letters (90, 135505; 2003) the results of their investigation into the properties of such 'filled' skutterudites using inelastic neutron scattering and heat-capacity measurements. Unlike previous studies that used compounds with different transition metals or rare-earth atoms, the researchers compared data for the unfilled skutterudite Co₄Sb₁₂ and almost identical compounds containing thallium atoms. They identify an unambiguous signature associated with the random motion of the thallium atoms, confirming their independent vibrating behaviour and the possibility of using these compounds in thermoelectric applications, such as tiny solid-state refrigerators.

A new source of single photons

Successful schemes for photon-based quantum information processing will require a source of single photons. Many of these schemes also require the single photons to be identical — that is, their wave packets should overlap perfectly. Last year (Nature 419, 594–597; 2002), Charles Santori and colleagues showed that consecutive photons emitted from a single InAs quantum dot inside a microcavity were almost indistinguishable. The optical microcavity was used to significantly shorten the radiative lifetime of the quantum dot, thus reducing the likelihood of scattering and allowing the emitted light to remain coherent. J. M. Gerard and colleagues have now demonstrated single-photon emission from individual GaAs quantum dots without the need for a microcavity (Applied Physics Letters 82, 2206–2208; 2003). This result was achieved by using GaAs quantum dots with a very short radiative lifetime, which should, in theory, produce photon wave packets that are fully coherent. In practise, the photons are not yet fully coherent, but the authors suggest that emission of identical single photons from GaAs may be achieved in the future by resonant excitation of a particular quantum dot transition.