Optical tweezers have been used by biophysicists since their invention at Bell Labs in the 1980s, and are typically used to study cellular components. But they have a few drawbacks, not least of which are overheating and inefficiency.
So engineers at Harvard have been working on a next-gen model they call plasmonic nanotweezers to solve those and other issues with traditional optical tweezers so that tiny particles such as viruses can be isolated, observed, and manipulated.
Back at Bell Labs, scientists had shined a laser through a microscope lens to focus it tightly. They found that light, made of electromagnetic waves, creates a gradient force at the point of focus that is capable of attracting a tiny particle and holding it in that beam of light until random motion or some other force knocks it out.
The basic limitation of this approach is that a lens cannot focus that beam beyond half the wavelength of light, so if the particle the researchers hope to trap is smaller than the focal spot, they might have trouble trapping it.
Meanwhile, that focal size limit also places an upper limit on the gradient force generated, and yet a stronger force is required to trap nanoscale particles. So for a conventional optical tweezer to capture nanoscale particles, a high-powered laser is required.
The solution to this conundrum turns out to live in nanoscale gold disks. A few years ago, researchers discovered that they could actually enhance the trapping field by focusing the laser onto an array of gold disks, exciting the electrons at the surface of the metal to create plasma oscillations (rapid waves of electromagnetic charge). This results in hot spots of enhanced fields along the edges of the gold disks.
In other work, researchers have lined sheets of glass with these tiny gold disks and submerged the setup in water. But the brightest hot spots were at the base of the pillars, partially in the glass--a tough place to trap particles. Even more problematic: if they didn't keep the laser point very low, the water would boil.
The Harvard engineers say they have solved both problems by replacing the glass with silicon, which they coated with copper and gold, and with raised gold pillars. (Because they're more thermally conductive than those sheets of glass, they act as a heat sink.) This reduces the water heating by 100-fold, and it moves those hot spots to the top edges of the pillars, where they were able to trap particles as small as 110 nanometers.
The potential applications in biophysics for trapping and manipulating nanoparticles are extensive, but one challenge is whether the researchers will be able to detect and quantify the motion of these particles.
"It's going to be harder and harder to precisely track the center of the particle when we do these manipulations," says principal investigator Ken Crozier, an associate professor of electrical engineering, in a news release. "Progress in the realm of sensing tools will need to keep up."