When molecules have to concentrate –
For their optical trap the scientists of Munich’s Ludwig-Maximilians-Universität (LMU) utilized thermal effects to transport molecules in a fluid. To start with, the so-called thermophoresis causes molecules in a temperature gradient to migrate from warm to cold areas. Their brownian motion, a seemingly random movement whose speed increases with temperature, is responsible for this behaviour: The faster molecules in the warm region hit each other more often and thus experience an impulse towards the cold zone. Additionally, the scientists use a thermoviscous fluid pump, which is based on the correlation between viscosity and temperature: Warm fluids have lower viscosity, exerting less attraction between their molecules, which in turn drift towards the colder area.
By means of both effects Braun and Weigert constructed a conveyor for molecules. An infrared laser creates hot spots at the bottom of the fluid container and the resulting temperature gradient causes the molecules to drift upwards. Moving the laser beam toward the container center activates the thermovisous pump, which transports the fluid to the container edge. Thus an optically driven conveyor is formed, which runs outward at the container bottom and inward on its surface. The molecules at the surface are transported to the center and accumulate there. This optical method enables scientists to investigate molecules undisturbed in their "natural habitat", namely a fluid.
Moreover, the optical conveyer works well even for extremely small molecules. The researchers could, for instance, accumulate DNA fragments of only five bases length within three seconds to an over hundredfold concentration. The conveyor also acts as a selective tool – for instance to investigate certain molecular bonds. In the start up enterprise NanoTemper GmbH, the Ph.D. students Stefan Duhr and Philipp Baaske already employ optic-thermal methods to verify bindings between pharmaceutical substances and biological molecules such as proteins. The optical molecule trap provides them with yet another means of characterising bio molecules. (CR/suwe)
"An Optical Conveyor for Molekules"
Franz M. Weinert and Dieter Braun,
Nano Letters, Oktober 2009