Metal–organic frameworks: A conductive guest
Metal–organic frameworks (MOFs) are nanoporous crystalline materials made from metal ions connected to organic linkers, and are typically of interest in applications such as gas storage, catalysis and sensing. Because of the insulating nature of the linker molecules, and the fact that there is little orbital overlap between the linkers and the metal ions, MOFs are usually poor electrical conductors. Alec Talin, Mark Allendorf and colleagues at Sandia National Laboratories, the US National Institute of Standards and Technology, and the University of Maryland have now shown that conducting MOFs can be created by adding conjugated guest molecules to the pores of the material.
The researchers began by growing thin films of a MOF known as HKUST-1, which is made from copper ions and the organic molecule benzene-1,3,5-tricarboxylic acid, on a patterned electrode surface. The films were then infiltrated with the redox-active molecule 7,7,8,8-tetracyanoquinododimethane (TCNQ), which was found to significantly increase the conductivity of the films. By altering the TCNQ exposure time, the conductivity of the thin-film devices could be varied over six orders of magnitude and conductivities as high as 7 siemens per metre could be achieved.
With the help of spectroscopic data and modelling, Allendorf and colleagues determined that the conductivity is probably due to the TCNQ guest molecules binding to copper subunits in the MOF and forming a strong electronic coupling between them. They also suggest that the conducting MOFs could have applications in electronic devices and reconfigurable electronics. OV
Nanomedicine: Bursting malaria's bubble
Proc. Natl Acad. Sci. USA http://doi.org/q5n (2014)
At present, malaria is diagnosed using techniques that rely on invasive blood sampling, specific reagents and expert knowledge. Dmitri Lapotko and colleagues at Rice University and the John Hopkins Bloomberg School of Public Health have now developed a non-invasive, reagent-free method to diagnose the disease, which uses a laser to generate nanoscale vapour bubbles within malaria parasites in a patient's body and then detects the subsequent collapse of the bubbles.
Malaria parasites in the blood are known to digest haemoglobin from red blood cells and form nanoparticles called hemozoin. Vapour bubbles as small as 100 nm can be generated from hemozoin by using a near-infrared laser pulse. The heat from the laser rapidly expands the volume of the bubble until it 'pops', generating a signal for acoustic and optical detection. Using infected human blood samples, the researchers were able to distinguish between early- and mature-stage parasites based on the traces generated by the nanobubbles. They were also able to detect the subsequent destruction of the hemozoin-containing blood cell; healthy blood cells were unaffected.
Lapotko and colleagues also showed that infection levels as small as 0.00034% could be detected in malaria-infected mice by placing a laser probe against their ears and delivering 400 laser pulses in 20 seconds and then detecting the acoustic response. SB
Superamphiphobic surfaces: Keep them straight
The superhydrophobicity of lotus leaves has inspired scientists to recreate similar surfaces for self-cleaning and anti-fouling purposes, and to extend the super-repellent properties to nonpolar solvents. However, such superamphiphobic surfaces are flat or only slightly curved. Hans-Jürgen Butt and co-workers at the Max Planck Institute for Polymer Research in Mainz have now explored why this is the case and set physical limitations to the curvature of a surface for superamphiphobicity.
The researchers first coated a microsphere with soot-based nanoparticles to make it superamphiphobic. The coated microparticle was then attached to a cantilever and brought into contact with a liquid surface. With this set-up, the force to detach the microparticle from different solutions could be measured; this adhesion force contains information about the interaction between the two surfaces.
Under the experimental conditions, the coating fails to repel oil, even though the same nanostructuring would make a flat surface superoleophobic. Through modelling of the experimental results, the researchers are able to explain that the capillary pressure exerted by the liquid as the particle moves into it is greater than the pressure of the entrapped air at the liquid/air interface, causing the oil to wet the microparticle. Because the capillary pressure depends on the curvature of the microparticle, there is a critical radius below which superoleophobicity, and more generally superamphiphobicity, is lost. AM
Semiconductors: Infrared silicon
Photodetectors sensitive to short-wavelength (1.4–3 μm) infrared light are employed in applications such as long-distance telecommunications. Using silicon for infrared photodetectors would have the advantages of low cost and compatibility with existing electronic device technology; however, silicon has an electronic bandgap of 1.12 eV and therefore it is transparent to light with wavelengths longer than 1.1 μm. Sub-bandgap absorption in silicon at low temperatures has been achieved by introducing structural defects that modify its electronic properties, but this approach does not work at temperatures relevant for applications. Jonathan Mailoa and Tonio Buonassisi at Massachusetts Institute of Technology and colleagues in the US and Australia have now demonstrated room-temperature infrared photoresponse in a heavily doped silicon photodiode.
By introducing a supersaturated concentration of gold impurities in a 150-nm-thin silicon single crystal, the researchers fabricate a planar photodiode that shows photoresponse up to wavelengths of 2.2 μm. Photocurrent generation is mediated by mid-gap states introduced by the gold dopants: electrons are excited from the valence band to the conduction band via the dopant states. The detection efficiency is relatively small, but it could be improved up to a factor of 100 by increased sub-bandgap light absorption and device design optimization. Furthermore, the use of other impurity elements could enable spectral tuning of the photoresponse. ED
Written by Sarah Brown, Elisa De Ranieri, Alberto Moscatelli and Owain Vaughan.