| RESEARCH HIGHLIGHTS |


deposits aqueous metal ions on to nickel foam at mild voltages. After optimizing the uniformity and adhesion of their multiscale coatings, they tested their material in water contaminated by a ‘Congo red’ dye pollutant. Within half an hour, the water became almost colorless, with over 90 per cent of the dye attached to the special coating.


Close-up views of the coating’s nanostructure


using scanning electron microscopy revealed that elongated, fin-like protrusions were key to recovering active surface area for high-perfor- mance pollutant removal. “Even though these coatings have some of the highest capacities ever reported, they are only operating at a fraction of


their theoretical capacity,” says Chiam. “We are really excited about tapping their potential.”


1. Liu, J., Wong, L. M., Gurudayal, Wong, L. H., Chiam, S. Y. et al. Immobilization of dye pollutants on iron hydroxide coated substrates: Kinetics, efficiency, and the adsorption mechanism. Journal of Materials Chemistry A 4, 13280–13288 (2016).


Imaging


SPIKY NANOSTRUC- TURES CAPTURE LIFE’S FINE DETAILS


ASSEMBLING NANORODS INTO COMPLEXES SHAPED LIKE SEA URCHINS MAY ENABLE REAL-TIME IMAGING OF CELL COMPONENTS, INCLUDING DNA


Optical microscopes that use lenses to bounce photons off objects have trouble distinguishing nanometer-scale objects smaller than the imaging beam’s wavelength, such as proteins and DNA. An innovative ‘hyperlens’ designed at A*STAR can overcome optical diffraction limits by capturing high-resolution information held by short-lived or evanescent waves lurking near a target’s surface1. Hyperlens devices — composed of thin


stacks of alternate metal and plastic layers — have raised prospects for capturing living biological processes in action with high-speed optics. Key to their operation are oscillating electrons, known as surface plasmons, that resonate with and enhance evanescent waves that appear when photons strike a solid object. The narrow wavelengths of evanescent beams give nanoscale resolution to images when


30 A*STAR RESEARCH


the hyperlens propagates the images to a standard microscope. Mass production of current hyperlenses has


stalled however because of their intricate fab- rication — up to 18 different layer depositions may be required, each with stringent require- ments to avoid signal degradation. “For perfect imaging, these layers need precisely controlled thickness and purity,” says Linda Wu from the A*STAR Singapore Institute of Manufacturing Technology. “Otherwise, it’s hard to magnify the object sufficiently for a conventional microscope to pick up.” Wu and her co-workers proposed a different


type of hyperlens that eliminates the need for multiple interfaces in the light propagation direction — a major source of energy loss and image distortion. The team’s concept embeds a hemispherical array of nanorods into a central


A hemispherical hypersphere designed at A*STAR can capture nanometer-scale details in optical images thanks to its sea-urchin-shaped geometry.


insulating core, giving the hyperlens a shape similar to a thorny sea urchin. This geometry enables more efficient harvesting of evanescent waves, as well as improved image projection. “For the sea-urchin geometry, the nanosized


metallic structures align in the same direction as the light propagation direction, and they are much smaller than the wavelength of applied infrared light,” explains Wu. “Therefore, the light doesn’t ‘see’ any obstacles, and propagates effectively and naturally, without loss.” The researchers’ simulations revealed the


spiky hyperlens could separate the complex wave information into its component frequencies and then transmit this data to the microscope as an intense, easy-to-spot band. This approach was also efficient — it proved capable of resolving intricate objects, 50 to 100 nanometers wide, without the need for image post-processing.


ISSUE 6 | JANUARY – MARCH 2017


Adapted from Ref. 1 with permission of The Royal Society of Chemistry


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