NIST researchers’ new approach to trapping nanoparticles uses a control and feedback system that nudges them only when needed, lowering the average intensity of the beam and increasing the lifetime of the nanoparticles while reducing their tendency to wander. In the picture on the left, 100-nanometer gold nanoparticles quickly escape from a static trap while gold nanoparticles trapped using the NIST method remained strongly confined.
Scientists routinely trap and move nanoparticles in a solution with "optical tweezers"—a laser focused to a very small point. The tiny dot of laser light creates a strong electric field, or potential well, that attracts particles to the center of the beam. Although the particles are attracted into the field, the molecules of the fluid they are suspended in tend to push them out of the well. This effect only gets worse as particle size decreases because the laser's influence over a particle's movement gets weaker as the particle gets smaller. One can always turn up the power of the laser to generate a stronger electric field, but doing that can fry the nanoparticles too quickly to do anything meaningful with them—if it can hold them at all.
Using a refined technique for trapping and manipulating nanoparticles, researchers at the National Institute of Standards and Technology (NIST) have extended the trapped particles' useful life more than tenfold.* This new approach, which one researcher likens to "attracting moths," promises to give experimenters the trapping time they need to build nanoscale structures and may open the way to working with nanoparticles inside biological cells without damaging the cells with intense laser light.