A simple and compact alternative to conventional optical trapping has been developed by researchers at the Swiss Federal Institute of Technology in Lausanne. The technique, which uses the near-field effects associated with plasmonics rather than a focused laser beam is said to be ideal for trapping and manipulating small objects in lab-on-a-chip applications and can also be integrated with other optical components such as biosensors for point-of-care applications (Optics Express 17 6018).

"We've reported the first optofluidic device that combines optical plasmonic trapping and microfluidics," Lina Huang, a researcher from the institute's nanophotonics and metrology laboratory, told optics.org. "The technique enables cell immobilization without the complex optics required in conventional optical tweezers."

Plasmonic trapping involves shining light onto a nanoscale metallic object. Incident photons excite localized electrons found at the surface of the metal. It is these excited electrons that form waves of energy, known as plasmons. "Surface plasmons produce very strong and localized electromagnetic fields, which can trap particles even more strongly than a tightly focused laser beam," said Huang.

In the Swiss set-up, a series of 100 nm diameter, 40 nm thick gold disks are deposited along a glass substrate. A microfluidic channel, which carries small objects such as cells, is placed above the disks. By illuminating the device with 608 nm light, a strong-field enhancement is induced at the edges of each disk. The highly localized field surrounding each disk produces strong field gradients, which pull particles from the microfluidic channel toward the centre of the disk - effectively trapping the particle.

While the current device can only trap particles, the Swiss group is working on broadening its function to particle sorting and transporting. "By trapping one type of particle in the optofluidic channel and collecting another type at the end of the channel, one can easily sort a mixture of two types of particles," concluded Huang. "It is also possible to design further structures for particle transportation."