Optical ’Tweezers’ Trap Microparticles

A simple technique for trapping suspended microparticles with a laser-illuminated array of gold microdots is demonstrated by Romain Quidant and colleagues in Nature Physics. The technique could prove useful for manipulating living cells in ’lab-on-a-chip’ microfluidic systems.

According to Science Daily, until now, the most promising approach to controlling cells and other micrometre-sized particles suspended in a liquid has been with so-called optical tweezers. These devices exploit the fact that such particles are attracted to and can be trapped by a focused light field. But one of their drawbacks is that they are bulky and complex devices to set up.

The technique demonstrated by Quidant and colleagues is much simpler. It relies on the fact that when a gold microdot or other metallic microstructure is illuminated with light, it will concentrate the optical field in its vicinity, similar in effect to the focusing power of a lens. By setting out an array of gold microdots on a glass slide and illuminating it with a laser, the authors effectively create an array of optical tweezers, which they use to trap microparticles suspended in a fluid droplet placed on the slide. Moreover, they show that they can use the array to selectively trap particles of a specific size from a collection of two different sizes, by controlling the size of the gold dots.