How would you react if a tiny hole in a piece of foil let through more light after you had covered it – or painted the foil a different colour?

With surprise, probably, like the physicists who discovered that this is just what happens with some very small holes. Both findings could lead to light-based transistors and other components for high-speed optical computers.

Conventional optics forbids light from passing through holes that are much smaller than its wavelength, which for visible light means less than around 400 nanometres wide. But in 1998, Thomas Ebbesen at the University of Strasbourg, France, reported that some wavelengths of visible light stream through holes in gold foil that are less than 300 nanometres wide.

It turns out that this is due to ripples known as plasmons that are found on the surface of metals and formed by the oscillation of electrons.

Total eclipse

If the frequency of light hitting the surface of a metal happens to match the oscillation of that metal’s surface electrons, the plasmons grab the photons, guide them through the holes and release them on the other side. The plasmons on gold surfaces, for example, are particularly adept at interacting with visible light.

Now a team led by at the Institute for Molecular Science in Okazaki, Japan, have found another way to coax photons through tiny holes – paradoxically, by obscuring the hole with a gold disc.

The team was shining light down an optical fibre that tapered to a 100-nanometre-wide aperture. At first, barely any light made it through the aperture; instead, it was reflected back up the fibre. But when the researchers placed a small gold disc very close to the aperture, so that it completely eclipsed the hole without actually touching it, the light started streaming through (see graphic, right).

They suspect that plasmons from the gold disc are leaping up through the hole, grabbing the photons stuck inside the fibre and dragging them through. These photons then stream around the edges of the disc.

Dye enhancement

Okamoto’s team found that if the disc touched the hole, the effect did not work; widening the disc, however, caused still more light to come through. “When we observed that the larger disc gives higher transmission, we were really surprised,” Okamoto says.

This ability to open or block a hole to light could be useful when building components for optical computers, which transmit signals using light instead of electrons.

“The novelty is in controlling this transmission with various ‘caps’,” says , a nanophotonics researcher at Bath University in the UK.

Light transmission through a tiny hole can also be controlled with dyes, as Ebbesen and his colleague James Hutchison, also at the University of Strasbourg, recently found.

Plasmon passing

Normally, when white light is shone onto a piece of gold foil pierced with tiny holes, only the wavelengths of green light pass through. But Ebbesen and Hutchison found that coating the foil with a thin layer of green dye allowed red light to pass through as well; indeed, more red than green started to come through.

This was a shock, as green dye should absorb all light except green. “One certainly doesn’t expect a sample to become transparent at the wavelengths where the molecule absorbs,” says Hutchison.

The researchers suspect that the dye molecules absorb the red light but then “pass” it to the plasmons underneath the dye, which are not of the right frequency to interact with red light directly.

Hutchison says that holes painted with various dyes could also be useful in optical computing components.

Journal references: ;

GOOD old-fashioned cash is to go down the electronic route, now that it is possible to stamp simple electronic circuits directly onto banknotes.

Modern banknotes contain up to 50 anti-counterfeiting features, but adding electronic circuits programmed to confirm the note’s authenticity is perhaps the ultimate deterrent, and would also help to simplify banknote tracking.

Silicon-based electronic circuits are clearly too thick to be incorporated into thin and fragile banknotes, but semiconducting organic molecules might be a viable alternative.

A team of German and Japanese researchers created arrays of thin-film transistors (TFTs) by carefully depositing gold, aluminium oxide and organic molecules directly onto the notes through a patterned mask, building up the TFTs layer by layer.

All this is done “without aggressive chemicals or high temperatures, both of which might have damaged the surface of the banknotes”, says team member Ute Zschieschang from the Max Planck Institute for Solid State Research in Stuttgart, Germany.

The result is an undamaged banknote containing around 100 organic TFTs, each of which is less than 250 nanometres thick and can be operated with voltages of just 3V. Such small voltages could be transmitted wirelessly by an external reader, such as the kind that communicates with the RFID tags found on many products (Advanced Materials; ).

The team’s technique has been tested on US dollars, Swiss francs, Japanese yen and Euro notes. Although the researchers have yet to work out how the organic electronics could be harnessed as an anti-counterfeit measure, the circuits are able to perform simple computing operations.

“The system could synchronise the release of drugs with movement of the inflamed muscle”

Unyong Jeong’s team at Yonsei University in Seoul, South Korea, covered a flexible rubber film with a sheet of corrugated microporous polystyrene, with gutters around 3 micrometres wide and 1 micrometre deep. The gutters were then filled with a liquid and sealed with another rubber film. Finally, the first rubber film was peeled away to expose the underside of the liquid-filled polystyrene gutters. Flexing the patch distorts the polystyrene tunnels enough to reduce their volume, squeezing the solution out through the pores in the plastic. Once the strain is removed, the tunnels spring back into shape, ready for the next use ().

Jeong and his team demonstrated the mechanism with a dye solution, but they are now moving on to therapeutic applications.

He envisages the first practical use will be skin patches for treating muscle pain and rheumatism. “Current [skin patches] are designed to just continuously release the active agents,” he says. “If we can control the release rate responding to the motion of our muscles, it will make the patches more effective and prolong the time of use.” He is also hoping to develop biodegradable strain-release patches to heal organs and damaged muscles inside the body.

of the Methodist Hospital Research Institute in Houston, Texas, says the idea is clever. “I’ve never seen anything like it,” he adds.

Winter is just beginning in the northern hemisphere, but pieces of equipment at Harvard University are already covered in ice. It seems the clear solid can be used as a mask to build transistors more precisely – in a process that’s being dubbed “ice lithography”.

and his colleagues placed single-walled carbon nanotubes onto a silicon wafer, cooled it to about -163 °C and sprayed it with water, causing an 80-nanometre-thick layer of ice to form. Then the researchers used an electron beam to carve away two squares of ice, exposing the tops of some nanotubes, and deposited a layer of palladium on top of this ice mask.

Dipping the structure in alcohol melted the ice. The palladium layer above it then fell off, except in the two squares where the metal had stuck directly to the nanotubes. The resulting cluster of nanotubes, fused to two palladium electrodes, acted as a transistor.

The process resembles how computer chips are made, but in conventional, electron beam lithography, a chemical such as polymethyl methacrylate is used as the mask. Using ice makes the technique cleaner, cheaper and gentler on the nanotubes.

Clear control

Another advantage of ice lithography is that ice is transparent, so researchers could see where to remove sections of the mask so that the electrodes ended up precisely aligned with the nanotubes below.

In future this capability could also lead to the creation of more complex structures. “It could potentially fabricate devices which might be very difficult to fabricate using standard lithographic procedures,” says Branton.

“It’s a fascinating paper,” says Sandy Dasgupta, a chemist at the University of Texas, Arlington, who pioneered an environmentally friendly, ice-based version of a commonly used technique for separating the compounds in complex mixtures, called . He is also interested in the potential to create more complicated nanoscale structures using ice.

Branton and his colleagues came up with the idea for ice lithography around five years ago, but say they have only now refined the technology to the point where they can produce working nanoscale devices. “We’re trying to see what works and what doesn’t work,” he says.

Journal reference: Nano Letters, DOI: 10.1021/nl050405n

The motor’s rotor is a long, coal-derived molecule called anthracene, which spins around an axle composed of two ethynyl units. Each end of this axle is connected to an electrode, and a third electrode – called the gate – is located slightly below the axle.

Applying an alternating current to this gate electrode sets up an oscillating electric field that surrounds the molecular motor and, according to the researchers’ calculations, should cause the anthracene rotor to turn.

Singled out

That’s because anthracene possesses what is known as a dipole moment – its negatively charged electrons tend to congregate at one end of the molecule, making the other end positively charged. These charged ends then move in different directions under the influence of the oscillating electric field.

Tiny motors that are powered by light or magnetic fields have already been developed, but electricity carries advantages, says team member Jos Seldenthuis at the Delft University of Technology in the Netherlands.

“Even the most narrowly focused beam of light is still a few hundred times larger than a single molecule,” he says, making it difficult to control individual motors. An electric current can be directly applied to single motors for applications requiring the operation of several independent devices.

Make it real

Electricity also gives the nanotechnologists an easy way to detect that the anthracene rotor is actually turning, which can be difficult at such tiny scales. This is because the electrical conductivity of the motor, as measured by the two electrodes on either side of it, should change in a regular way as the rotor turns.

“It is an interesting concept,” says , a materials scientist at Rice University in Houston, Texas, who has developed molecular machines including a nanocar with a light-powered motor. He warns that such machines are much easier to design than to build, though.

Seldenthuis and his colleagues are now well on their way to doing just that, however. “In collaboration with other groups, we are now working hard on the experimental realisation of this concept,” he says. “Individual aspects of the design have already been verified experimentally.”

Journal reference:

Hydrothermal vents on the deep ocean floor are believed by many to be the cradle for early life. Now a team led by Ryuhei Nakamura at the University of Tokyo in Japan have uncovered evidence that such vents can generate electrical currents. They say these currents could have helped generate the complex carbon-based molecules that came together to produce life, as well as provide it with a handy power supply.

Vents bring minerals containing iron, copper and sulphur from deep inside the Earth’s crust to the seabed. The minerals possess an excess of electrons, so Nakamura’s team wanted to find out whether these electrons could generate an electric current in the vent.

To do this, they carried out the first lab-based electrical experiments on a type of sulphur-rich chimney known as a black smoker. The chimney was extracted from a hydrothermal vent in the middle of the South Pacific Ocean.

First, the team passed a current through the chimney wall to show that it could conduct electricity. Next, they simulated the conditions at a hydrothermal vent by pumping hot, sulphur-rich water past one side of a chimney wall, and cold, salty water past the other. This generated a weak but steady electrical current across the chimney wall (Angewandte Chemie, DOI: ).

“Hot, sulphur-rich water flowing through a sea-floor vent generates a weak electrical current”

The team thinks that the chimney walls catalyse the conversion of sulphides into elemental sulphur as the hot vent fluid travels through them. The reaction releases electrons which pass through the wall to the salt water outside, where they convert dissolved oxygen into hydrogen peroxide. Nakamura postulates that this electrical current could provide a source of energy for bacteria.

Nick Lane, a biochemist who studies hydrothermal vents at University College London, says the findings are “interesting and curious”, but points out that there was hardly any oxygen around in the primordial ocean to sustain the current.

Nakamura suggests that carbon dioxide took the place of oxygen. If this was the case, then the CO₂ would have been converted directly into carbon-based molecules, making complex organic molecules on the early Earth’s sea floors – perhaps the chemical precursors of life. The next step, they say, is to confirm that black smokers generate electricity when they are at the bottom of the ocean, not just in the lab.

Nanoparticles able to make basic decisions about whether to release their contents offer the prospect of delivering drugs exactly when and where they are needed, say chemists.

Their particles only respond to two distinct and simultaneous stimuli, acting like an “AND” computer that only produces an output signal if it receives two input signals.

“Our dream is to be able to use our mechanised nanoparticles for anti-cancer drug delivery,” lead researcher at the University of California, Los Angeles, told ��è����Ƶ.

Tumour targeting

“A localised, concentrated dose could be delivered at the site of the disease and the rest of the body would be spared,” he explains, by programming the particles to respond to specific conditions within the body.

Electronic logic gates are the cornerstone of modern computing. But logic gates have also been created from DNA and used to solve mathematical problems in the lab, and even inside living cells.

By developing the first drug delivery nanoparticle that is only triggered by the combination of two separate stimuli, Zink has now created a totally new kind of . And his approach is likely to be easier to perfect for clinical use than the logic-based DNA “doctors” some researchers are planning.

Paddle power

Zink’s silica nanoparticles are 400 nanometres in diameter and covered with tiny 2-nm pores. In trials, these particles were filled with fluorescent molecules rather than drug molecules, so any movement could be easily observed. But anything could be put inside.

The walls of the pores are lined with light-responsive, paddle-shaped molecules known as , while another set of molecular structures called pseudorotaxanes plug the pore entrances.

Shining blue light onto the particles causes the paddle-shaped azobenzenes to wave back and forth, wafting the payload molecules towards the exterior.

Unless conditions are just right, though, the pseudorotaxane plugs stay firmly in place. It takes a second action brought about by a change in pH level to cause them to change shape and unblock the pore entrances – finally allowing the azobenzenes to pump out the cargo.

So, just like an electronic logic gate, the particles only release their payload in response to both signals: light and pH. If either of those is lacking, the payload will not be released.

‘Exciting’ work

Zink and at Northwestern University in Illinois, who helped design the molecular structures, are now creating versions that respond to a range of other chemical and physical inputs, with the aim of creating even more complex control mechanisms.

, professor and chairman of the department of nanomedicine and biomedical engineering at the University of Texas, says the work is “exciting”. But he warns that getting the nanoparticles to work in the human body could be tricky.

“If that can be accomplished, however, there are very many situations in medicine where AND targeting could be of extraordinary importance,” he says.

Journal reference: Journal of the American Chemical Society (DOI: )