Focus on research: chemists Wybren Jan Buma en Fred Brouwer

Wybren Jan Buma and Fred Brouwer from the Van ‘t Hoff Institute for Molecular Sciences recently won the Descartes prize, also known as the "European Nobel Prize" for their research into light and molecules. Their research focuses on getting molecules to move in a controlled fashion by shining light on them.

Buma and Brouwer look into everything that has to do with light and molecules: Molecular Photonics. This is a broad research area that ranges from creating molecules for a wide range of purposes to investigating their behaviour under the influence of light. As the scale on which the processes take place is too small to be observed using a microscope, it takes a range of clever tricks to "see" what is happening. Brouwer's research focuses mainly on creating and studying molecules on a somewhat larger scale, while Buma's interests lie more in the fundamental processes. "Wybren Jan tries to isolate molecules in the gas phase, and I prefer to insert them in something," says Brouwer.

The two gentlemen therefore take two different approaches. "Wybren Jan likes to focus on the details of a single molecule. I try to approach a similar problem in a different way by changing the molecules' environment a little more each time," says Brouwer. Buma adds: ‘The great thing about our different approaches is that they complement each other. Both of us are looking at the same molecule, just from two points of view. This results in information that is more than the sum of two separate research projects.

Little spies

Brouwer, with the cooperation of the Dutch Polymer Institute and the Van der Waals-Zeeman Institute, creates molecules that give information about their environment through their colour. ‘These molecules absorb light and then, as they return to their stationary state, can emit light of different wavelengths and therefore of a slightly different colour. We create the molecules in such a way that the colour of light they emit depends on the viscosity, the polarity or the temperature of the medium they are in.'

‘This means these molecules could be used as molecular sensors, rather like little spies," says Brouwer. ‘The challenge lies in creating molecules that change their colour enough to truly tell about what it is you want to know. The Dutch Polymer Institute's project deals with a very small colour change, but it is still one we can measure very well. We still need to wait and see if the Van der Waals-Zeeman Institute's experiment will go as well.' For this experiment, Brouwer is creating molecules that can sense when pressure is applied to them and that react by changing colour. The group hopes in this way to measure the mutual forces between spheres of around one millimetre in diameter. ‘This is extremely exciting research, because we have no idea if it will work,' says Brouwer.

‘We probably won't get it right first time,' adds Buma, laughing. ‘It's a long process: first you have to develop the molecule on paper and then synthesise it in the laboratory. But you can be sure that the molecule will have some properties that you don't want, or you try to optimise particular properties. And that means going back to the drawing board once more."

Vibrating springs

Many people see molecules as rather static structures composed of tiny spheres. In reality, they are all vibrating and moving. To prevent these movements from disrupting his observations, Buma cools the molecules to almost zero degrees Kelvin. The molecules still move, but all in the same way. Buma irradiates the molecules with a laser, and by studying which colours of light the molecules absorb and which colours of light are emitted, he knows what the molecules look like. ‘You could think of the connections between atoms as little springs,' Buma explains. "A weak spring vibrates more slowly than a strong spring under the influence of light. We can measure the frequencies of these vibrations and therefore see how strong the connections are.'

The measurements create a spectrum, a chart which clearly shows the energy transfers specific to that molecule. At first sight, the charts seem ordinary, but for Buma and Brouwer, they create an almost tangible image. ‘When we look at a spectrum, we see the molecule, its structure and its movement,' Brouwer says.

Brouwer studies molecules using a different technique. He makes a thin layer of plastic made of long polymer chains. He then inserts his spy molecules, which looks like meatballs on a plate of spaghetti. The spies all have approximately the same environment but some differences are observable upon closer inspection. By shining a beam of light over the sample and looking at the colour and intensity of the light emitted, Brouwer can precisely determine the molecule's location to the nanometre and can describe how the molecule is influenced by its environment depending on what kind of spy molecule it is. For example, you could design the molecule so that when it is folded in two, it emits a more reddish light.

Molecular motor

Buma and Brouwer often work with a class of molecules called rotaxanes. They consist of a rod-like structure with a ring around it (rota = wheel, axis = axis). By working with groups all over Europe, they succeeded in developing a molecular motor. Buma holds up a jar of whitish powder. ‘These are the rotaxanes.' He could just as well have shown a completely random jar because they look like any other white powder. They are, however, very important molecules as the ring over the rod shifts under the influence of light. This research won them the Descartes Prize, and the sum of 1.36 million euros. "Unfortunately not all of it is ours," says Buma. "After dividing it out amongst different research projects and groups, we will probably be left with about €60,000. And that's nice of course, but since most equipment costs at least €100,000, it's mostly about the honour."

Brouwer researched how the speed of the ring's movement over the rod depends on the solvent in which the molecules are dissolved. He also looked at the influence of the temperature of the solvent and at the effect of small adjustments of the molecular structure on the speed of the ring's movements. This work constitutes an important contribution to the research into molecular motors.

Nano robots

"The ring's movement over the axis is comparable to that of a piston in a car engine," says Buma. It is possible to get a lot of molecular motors to make the same movement simultaneously and in this way get a droplet of liquid to move over a surface. But as for nano robots that swim through our bloodstreams and carry out repairs, both scientists believe it's still too early.

The research of the Molecular Photonics group sounds somewhat futuristic. Even so, colleague Dr Sander Woutersen is developing new techniques to observe molecule movements that occur in just a few thousandths of a millionth of a millionth of a second. Dr René Williams conducts fundamental studies into one of most important processes of artificial photosynthesis: water splitting. And Dr Hong Zhang, with the cooperation of AMC, investigates the use of nanoparticles for both labelling and removing tumours.

Chicken

Although Brouwer and Buma's interests lie with fundamental research, their science could also have interesting practical applications. A spin-off company of a group with which they work together in several European projects is studying how to record information using a layer of rotaxanes. These molecules could also be used as small ‘temperature history meters'. The spy molecules on the label on the chicken in the freezer would let us know if the chicken has been out of the freezer for too long.

Preventing salmonella poisoning or even fighting tumours sounds quite wonderful but people still fear nanotechnology. ‘If nanotechnology means that you're going to put a load of particles in consumer products or food, then you must of course be careful,' says Brouwer. Buma continues: ‘In the past, scientific information was not always reported responsibly and that can create fear. That is why it is our duty as scientists to communicate the possible advantages and disadvantages as clearly as we can to the public.' Brouwer responds: ‘We don't actually know all the advantages yet, but the potential is huge.'

Published by  Faculty of Science

4 September 2012