C O V E R of P R L

An analogy between a romantic dinner and science
How to control the energy transfer between molecules

A team of scientists from the MESA+ Institute for Nanotechnology at the University of Twente, the FOM Institute AMOLF in Amsterdam, and the Technical University of Denmark in Lyngby, has settled a long-standing scientific debate whether the energy transfer between two molecules can be controlled via the nearby environment or not. Their research shows that the energy transfer efficiency can indeed be controlled via the nearby environment. The results are being published in the leading scientific journal Physical Review Letters.

D:\PhD\Articles\2011-01 FRET_LDOS\Press\GettyImages_73091685_watermark.jpgThe lead author Christian Blum uses an analogy to explain the problem: “When you fancy someone, inviting him or her out for dinner is a great idea. The romantic environment may help to fall in love. One may wonder if the romantic environment is the reason for falling in love or if it only helps the affection to show. These matters of the heart are notoriously difficult to disentangle and measure. But for the analogous situation of energy transfer between molecules we managed to do just that.”

The much debated question was whether one can influence the rate at which energy is transferred between a couple of two closely spaced molecules by changing the nanophotonic environment. It is known that the spontaneous emission from light sources such as molecules depends on their nanophotonic environment. This can be manipulated by so-called metamaterials like photonic crystals or mirror-like interfaces. Some previous studies report strong effects of the nanophotonic environment on the energy transfer rate, others report weak effects, while even others predict that there is no such effect.

Previous studies suffered from various challenges. Either the nanophotonic environment was not precisely defined. Or the distance between the couple of donor and acceptor molecules was not controlled. Or the couples were not well separated from each other. Or combinations of the aforementioned issues arose. “The analogous situation is going out for dinner with your loved one, not knowing the kind of restaurant and whether you will sit at the same table, or at a huge table with hundreds of other couples” outlines Blum.

Back to the molecules, the researchers solved these problems in an elegant way by attaching energy donor and acceptor molecules to both ends of a piece of DNA of exactly defined length. And they made sure to minimize contact between different energy transfer couples. To achieve precise and quantitative control of the nanophotonic environment the energy transfer couples were positioned with nanometer precision very close, in fact at distances smaller than the wavelength of light, to a metallic mirror (see artist impression in illustration).

The researchers found a surprising outcome: the energy transfer rate is not at all influenced by changing the nanophotonic environment. Simultaneously, the efficiency of energy transfer can be quantitatively and predictively increased or decreased, essentially between zero and 100%, by purely changing the nanophotonic environment without changing the energy transfer system itself.

Blum summarizes, “Our research does not allow us to draw conclusions about romantic couples. But the analogy would be that lovers love each other just as much whether in a romantic environment or not. A romantic environment, just makes it more apparent that they have fallen in love.”

Energy transfer is at the basis of natural photosynthesis and plays a role in modern research and advanced high-tech applications. The researchers believe that this new control of energy transfer can be exploited for example in measuring molecular distances and in photovoltaics.

The team:

The research has been performed by dr. Christian Blum, Niels Zijlstra MSc, and prof. Vinod Subramaniam from the Nanobiophysics chair in collaboration with prof. Allard Mosk and prof. Willem Vos from the Complex Photonic Systems (COPS) chair. Both chairs are part of the MESA+ Institute for Nanotechnology, University of Twente, the Netherlands. The Nanobiophysics group is also part of the MIRA Institute for Biomedical Technology and Technical Medicine at the University of Twente. They were joined by prof. Ad Lagendijk, Photon Scattering group, FOM Institute AMOLF, Amsterdam, the Netherlands and by associate prof. Martijn Wubs, Department of Photonics Engineering, Technical University of Denmark (DTU), Lyngby, Denmark.

The research has been funded by the Dutch “Stichting voor Fundamenteel Onderzoek der Materie (FOM)”, by the “de Stichting voor de Technische Wetenschappen (STW)”, by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)”, and by the European Research Council.

Note for the press:

URL: http://link.aps.org/doi/10.1103/PhysRevLett.109.203601

DOI: 10.1103/PhysRevLett.109.203601

A preprint of the paper entitled “Nanophotonic control of the Förster resonance energy transfer efficiency” can be found at http://arxiv.org/abs/1207.2272. For further information you can contact dr. Christian Blum (+31-(0)53-489 3067) or press officer Joost Bruysters (+31-(0)53-489 2773 / +31- (0)6 1048 8228).