A laser beam focused by a microscope can trap and manipulate a range of minute objects, including viruses and cells. But such ‘optical tweezers’ do not work well for particles smaller than about 10 nanometres, because the laser light needed to hold such minuscule objects can easily damage them.
To circumvent this problem, Justus Ndukaife and his colleagues at Vanderbilt University in Nashville, Tennessee, fashioned a gold film patterned with tiny holes and placed it inside a sample chamber that they then filled with fluid. Next, the team shone a laser on the film and applied an alternating electric field to it, creating two opposing flows in the fluid.
The researchers captured a protein molecule roughly 7 nanometres wide in an island of motionless fluid between these flows. The team could move the trapped molecule by shifting the position of the laser.
Crucially, objects trapped in this way are held a few micrometres away from the focus of the laser beam, limiting their exposure to heat and light. The approach should allow researchers to grasp and study individual biological objects, and even sort them by size.