Abu Dhabi researcher closes in on elusive blue glow

Pance Naumov at New York University Abu Dhabi hopes for breakthrough in the field of sensor technology.

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ABU DHABI // As Pance Naumov settles into his new job at New York University Abu Dhabi this week, the task ahead of him sounds simple: to make a powder that glows blue when you squeeze it and fire a laser at it.

First, though, he has a bigger question to answer. Why, exactly, do the powders he works with glow under certain circumstances?

And when they do, what is it about them that makes them glow yellow, for example, rather than green?

The answers to these questions could have useful implications in the highly specialised field of sensor technology.

Dr Naumov, who is originally from the Republic of Macedonia, had already been working on them for some years in his previous post at Osaka University in Japan.

For a long time, he and his colleagues, led by José Lopez-de-Luzuriaga from the University of la Rioja, and Antonio Laguna from the University of Zaragoza, have been able to make metal-organic compounds that, when excited with a laser and put under pressure, give out green and red light.

Now, with research published in September's Journal of the American Chemical Society, they have yellow, too.

That means a third colour that can be used as a signal, either to show how much pressure is being applied to the material, or whether a particular gas is present.

Pressure sensors are all around us, controlling and monitoring thousands of everyday applications - in gears, vehicles or in packaging materials.

They can also be used to measure indirectly other variables, such as the flow of fluids or gases, speed, water levels and altitude.

When different pressures are applied, the luminescent material lights up a different colour; the broader the palette of available colours, the more precise information the sensor can indicate.

"This gives more control and more possibilities for application in the real world," said Dr Naumov.

The researchers were able to tune the luminescent colour of their materials by reassembling and altering the chemical bonds between tiny bimetallic clusters, each consisting of two atoms of gold and two of silver.

Using a property of gold known as aurophilicity - the tendency of gold atoms to clump together with weak gold-to-gold bonds - and another phenomenon called halogen bonding - the interaction between a halogen atom (such as fluorine, chlorine, bromine and occasionally iodine) and a "Lewis base" such as gold or silver to assemble the bimetallic clusters into linear chains.

By controlling the length of the chains, they were able to control the way they responded to pressure and gas. A longer chain gives off light with a longer wavelength - meaning light further towards the red end of the spectrum.

"Think of it as sewing beads on a thread - each bead is one gold-silver cluster, each of which consists of two atoms of gold and two atoms of silver," said Dr Naumov.

Previously, the researchers had managed to make single clusters, giving green light, or many clusters within the same chain, giving red.

Now they have managed to modify the ligand - the organic, halogen-containing substance that donates charge to the metal, which is what allows it to light up.

By using a ligand with a differently positioned halogen atom, they made a third structure - two clusters joined together, like a cartoon dumbbell - that emits a different colour of light from the other two.

Their usefulness in sensors comes from a curious property: under simple mechanical force - grinding, for example - or exposure to an organic vapour such as acetonitrile, the substances can be made to flip between the three structures.

Different structure, different coloured glow - so looking at the colour tells you what pressure it is under, or whether a particular gas is present. That gives it a wide range of potential applications; it could be used, for example, to detect pollutants near a factory after a chemical spill.

It would be built into sensor devices, as a tablet-shaped node of powder with a laser pointed at it. When the laser is switched on, if organic vapours are present the tablet would glow different colours depending on the amount of a gas present.

There is much work to be done, though, and Dr Naumov is still unclear exactly what is happening when the material is excited. Nor does he know why the different structures glow different colours.

He will try to get his answer by shining ultrashort pulses of laser light on the materials, using X-ray pulses to get high-resolution images of what is happening to the atomic structure of the material as it changes hue. The hope is that knowledge will lead Dr Naumov to his holy grail: blue. Blue light is often tricky; it has a shorter wavelength and is more energetic than red or green. Once researchers work out what's going on in the compounds, the hope is he will know how to get it to shine blue.

"We would like to have sensors that emit as many colours as possible," he said. "The more colours, the more possibilities."