Visualising the nanoworld (Leiden University/LION)


(By: Leiden University/LION) Visualising cell proteins without invasive techniques is possible with the help of fluorescence. During a lecture of the Natuurwetenschappelijk Gezelschap Leiden on 18 January, winner of the Spinoza Prize 2017 and founder of the field of single molecule optics Michel Orrit explained how this works.

Making molecules light-up

Michel Orrit, professor Spectroscopy of molecules in condensed matter, begins his lecture for the Natuurwetenschappelijk Gezelschap Leiden with a simple experiment. With his laser pointer, he shines on a pink demonstration fluid. Not only a scattering of the green laser light can be seen, an orange light ray also appears: fluorescence. Due to the high energy laser light, the pink molecules in the liquid become excited and subsequently emitting fluorescent light. The same thing happens with a light pink dilution of the liquid when he point his laser at it. Would it still work if the pink molecules are not visible with the naked eye? He shines his laser on a colourless dilution. Here, too, the orange glow appears. With this, he shows that invisible molecules can be made visible via fluorescence.

A flashing light

Orrit was the first person in 1990 to detect loose molecules with fluorescence and therefore became the founder of single molecule optics. For this discovery he made use of the blinking behaviour of fluorescent molecules. Orrit explains how this works: a molecule in an excited state sends out light on and off, like a flashing light but at random intervals. This property makes it easy to see a single molecule. With more molecules together, however, this does not work, because the molecules are never all on or off at the same time.

Helpful golden nanoparticles

Some molecules send out too little or no fluorescent light individually. In order to be able to detect such a molecule, Orrit has devised a way to make them detectable: via golden nanoparticles. He has given these particles a certain shape and charge distribution so that resonance occurs at a specific light frequency. This creates a perceptible signal. The signal of these gold particles depends on their shape, so that they can be used to visualize their environment that influences the shape. For example, a protein affects the charge of the particle and the frequency changes slightly, which is then measured. In this way, these gold particles can make invisible individual molecules visible and strengthen their signal.

Exploring the nanoworld

Being able to observe individual molecules open doors in the world of nanoscience. For example, life science researchers can now study individual proteins and determine more precise DNA sequences. This increases the understanding of diseases and health, paving the way for the development of new, more efficient medicines.


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