Researchers at the Center for Nano Science and Engineering at the Indian Institute of Science (IISc), Bengaluru, developed a technique for trapping and transferring small objects at the nanoscale with optical light-using tweezers. It is a tool that can be used to collect and transfer small suspended particles, even in cells. The study was published in Nature communication.
Optical tweezers have been known for about thirty years and are used in biology to store and manipulate molecules; however, they have limitations when it comes to nanosize particles. This was partly solved by developing a "plasmonic tweezers", which works on the principle that when a precious metal disk, like gold, is illuminated with light, it creates an electromagnetic field around it. This field can attract and hold small particles.
Plasmonic tweezers are limited by the fact that they are fixed in space and therefore can only trap objects that are approaching them. Although the team in previous work has shown that such plasmonic tweezers can be maneuvered with a combination of light and magnetic fields, they could not apply this technique to some types of colloids.
Tweezers in the tweezers
In this paper, they overcame this limitation by developing a method using only optical power. They combine a silver nanodisc with a microduct made of glass, and the combination can be manipulated using only laser beams. This tweezer-tweezer approach can entrap objects around 40 nanometers in size with a single laser beam. This is typical virus or DNA size. "Optical tweezers hold plasmonic tweezers, and plasmonic tweezers capture our target nanoparticles, and therefore (plasmonic) tweezers in the (optical) tweezers," says Souvik Ghosh, the first author of the article.
When the size of any colloidal particle decreases, for example from microscale to nano-scale, the movement associated with Brownian motion or random fluctuation increases. "That is why holding a single silver nanodisc with a focused laser beam (optical tweezers) is difficult and requires a high laser intensity to generate enough strength to overcome fluctuations," explains Ghosh. If the disk size is reduced to reduce the required laser intensity, the plasmonic properties will be lost. Therefore, the team connected a dielectric microdode made of glass, which, while maintaining plasmonic properties, reduces thermal fluctuations by an order of magnitude. "The required intensity is about 100 times lower than what a normal optical tweezer usually uses to hold an object of similar dimensions," he explains.
"This technique is ready for real-world applications" – says prof. Ambarish Ghosh, whose laboratory was tested. "Simplicity and ease of implementation are the largest USP for this device. It is patented and we are already talking with the company about licensing. "