Spiky germ killers

Gecko feet, lotus leaves and certain weevil scales are all examples of biological structures covered with nanoscale patterns that evolved to give their owners a specific advantage in the struggle for life – in these examples, sticky feet, self-cleaning properties and iridescence.

Another example are the wings of the cicada Psaltoda claripennis, which have arrays of nanopillars. A group of researchers based in Australia and Spain reported last year that these wings are particularly good at killing bacteria. They suspected that the bacteria are killed by the spikiness of the surface, which lethally deforms the bacteria, rather than by biochemical interactions.

Now they have more evidence for this effect, having found that wings from the dragonfly Diplacodes bipunctata, which also has a nanopillar surface, are even deadlier and kill most types of bacteria. A similar surface from an artificial nanomaterial called black silicon does the same. Black silicon is thoroughly etched and absorbs most light – hence the name.

The bactericidal activity of these different materials appears to be the result of mechanical piercing with nanospikes. The work could inspire the design of antibacterial materials.

Fishy sensors

Mercury, a metal that is liquid at room temperature, has long been regarded as a wonder material and has proved its everyday usefulness in household products such as thermometers, batteries, fluorescent light bulbs and dental fillings. However, we also know that mercury is toxic and can cause neurological disorders.

Although much has been done to reduce the level of mercury in household products, it has ended up in wastewater and as a result in the environment. Most worryingly, it has accumulated in sea life and so has entered the food chain. There are already several methods to detect levels of mercury and other metal pollutants but there is still a need for sensors that selectively detect mercury and are flexible enough to be used in various types of sample.

Scientists based in South Korea and at MIT in the US have designed a particularly sensitive mercury sensor. It is essentially an electronic transistor that incorporates graphene – a sheet of carbon atoms one atom thick. The graphene layer is adorned with molecules that specifically attach to mercury and when they do, a rapid change in electrical current can be detected.

The researchers tested their device on a solution made from mussels caught near South Korea, and showed that it is at least 100 times more sensitive than previous transistor-based mercury sensors.

Breezy energy

Around us are sources of energy such as light, heat and air flow that could potentially be harvested, for example to recharge batteries or to power gadgets. Researchers from Chongqing and Wuhan in China have designed a tiny generator that turns light breezes into useful amounts of power. It consists of a plastic film between two metal electrodes, with one end fixed and the other free. When air is flowing, the film flaps up and down, electrically charging and discharging as it rubs against the metal layers, alternating between the top and bottom electrode. This produces a stable electrical current large enough to feed an array of 46 LEDs.

The researchers added a solar cell to the device so that energy can be harvested continuously, day and night.

Biodegradable electrode

In the future, monitoring our health and medical treatments could involve swallowing tiny electrical sensors that are broken down safely in the body when they have fulfilled their purpose. A major difficulty in making such gadgets is designing the battery, which is often the biggest part of the device and contains materials that may be harmful.

Researchers from Carnegie Mellon University and the University of Oregon have designed a battery with a fully biodegrable electrode made from a molecule called eumelanin, which is found in skin, hair and eye pigments. Eumelanin forms one of the electrodes – the anode. The researchers have shown that their battery can provide electrical power for a small device for several hours. It uses mainly biodegrable materials – apart from the other electrode, the cathode.

Further work may lead to fully biocompatible medical devices that can be injected, implanted or ingested, for example to monitor a patient's condition during surgery.

A sponge to soak up carbon monoxide

Carbon monoxide is an essential resource for making many useful plastic materials. It is a side-product of various industrial processes, but is difficult to separate from other gases and often goes to waste, producing huge amounts of the greenhouse gas carbon dioxide. There are materials that can selectively absorb carbon monoxide molecules, but the challenge is to make them release the gas again, without the need to heat them to high temperatures.

Now scientists in Japan have created a sponge-like material with nanoscale pores that becomes more efficient at absorbing carbon monoxide, the more is taken up. This "self-acceleration" process is the result of interactions between copper ions and carbon monoxide molecules which open up the sponge-like structure as more molecules are captured. The gas can then be released again by heating the material to just 80C.