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Optical tweezers for chemists

13 October 2018 - Physics

opticaltweezersPerez2018.PNGPart of the 2018 Nobel Prize in Physics was awarded to Arthur Ashkin for his invention of optical tweezers in 1986. If you want to know what these are look no further than Youtube University for example with Styropyro demonstrating levitating nano-diamonds in a laser (link) or Phils Musical Snippets and Interesting Projects with just a laser and a felt tip pen or Atom and Sporks explaining some of the theory behind it (link). The phenomenon is surreal, in the felt tip pen clip, the effect of holding the tip of the pen in the focal point of the laser beam is obvious: the tip is briefly set on fire resulting in smoke and debris. The debris in the path of the laser is clearly visible as a bright hotspot. What happens next makes all the difference, if the laser is moved sideways the hotspot moves as well, the debris is trapped in the laser beam and since when can light push around matter?

This of course gets physicists very excited with a lot of furiously scribbled mathematical equations but chemists have been quick to adopt optical tweezers for all sorts of practical experimentation. As just one example take some very recently published work from the Spanisch IMDEA Nanoscience Institute. In it (link) Emilio Perez et al. try to determine just how much force it takes to move a macrocycle along a thread in a molecular shuttle (coincidently the stuff of the 2016 chemistry Nobel). To do so they attached one end of the thread via a DNA linker to a stationary pipette bead and the macrocycle also via a DNA link to a polystyrene bead trapped in a optical tweezers. A force is exerted on the macrocycle by moving away the laser beam from the pippete with a rate of 200 nm per second. The force required for the macrocycle to overcome a station is in the order of 8 pN comparable to the force required the cancel a DNA hairpin conformation. With a constant force applied it was also possible to build a picture of all the shuttling events taking place.