Nano-sized protractor measures molecule movement
Life 23 April 2013
By Jacob Aron
Need to get a handle on a minuscule angle? All you need is your trusty plasmon nano-protractor, a new tool that can be used to do things like measure how DNA twists as it replicates.
Created by Gennady Shvets and colleagues at the University of Texas at Austin, the device is a gold sphere just 150 nanometres wide with an adjacent gold rod that is 22 by 180 nanometres. Shining a light on the sphere causes photons to scatter in a particular way, producing a distinctive, measurable spectrum.
The sphere sits on a flat surface, and is nudged towards the rod until one end is alongside. Ripples of electrons on the surfaces of the two objects, known as plasmons, then become coupled. That modifies the scattered light in a way that creates a dip in the spectrum, which is linked to the alignment between the sphere and the rod.
Placing it on, say, a DNA molecule will change the angle between the rod and the sphere, changing the position of the dip. You use the protractor by varying the light's orientation until the dip is most pronounced and the new angle is revealed.
Tiny toolbox
"The plasmonic nano-protractor is a very nice addition to our collective plasmonic nano-toolbox," says Ryan Hill of Duke University in Durham, North Carolina, who last year created a plasmon ruler.
For now, the nano-protractor works in two dimensions, rather like the plastic semicircle found in school students' pencil cases. But Shvets hopes it will soon be able to measure angles in three dimensions. The north-south angle would be determined by how far the rod is from the sphere's "equator", while the east-west angle would be measured by the direction in which the rod is pointing.
This could be done by placing tiny chunks of semiconductor called quantum dots on a larger sphere. As with the nano-rod, plasmons on the dots would become coupled to those on the sphere, creating multiple axes in the spectrum of the scattered light.
Dots sticking out from the surface of a sphere could then be attached to DNA at different places, like the sensors on a motion-capture suit. This version of the protractor would let Shvets record how the protein responsible for transcribing RNA travels along DNA's double helix, a basic part of the DNA-replication process that for now remains mysterious.
Journal reference: Nature Photonics, doi.org/mbc
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