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Abstract:
Researchers at the Centre for Nano Science and Engineering (CeNSE), IISc, have developed a technique to trap and move nano-sized particles in a fluidic medium using only light.

Trapping and moving tiny particles using light

Bengaluru, India | Posted on September 24th, 2019

In a recent study, PhD student Souvik Ghosh along with Prof. Ambarish Ghosh, at CeNSE have demonstrated a novel nanotweezer technology using focused laser beam to trap and manoeuvre a nano-sized silver disk, which in turn can attract and ensnare nanoparticles when light is shined on it.

The study was published in Nature Communications.

Tools that trap and manipulate microscopic objects using light — a Nobel Prize-winning advancement — have led to significant breakthroughs in diverse fields, from atomic physics to biology. These “optical tweezers”, however, are not efficient to trap particles that are nano-sized. This could recently be overcome with the invention of “plasmonic tweezers”, which can trap much smaller particles, such as viruses or quantum dots, at lower light intensities. They use metallic nanostructures such as gold or silver that generate a strong electromagnetic field around themselves when light hits them, which attracts and traps nanoparticles.

Plasmonic tweezers, however, have a limitation: unlike optical tweezers they are typically fixed at a spot and are only able to trap particles close to them. As a result, dynamic control over nanoscale objects in fluids remains challenging. In an earlier study published in Science Robotics, Ghosh and Ghosh managed to transport nanoscale cargoes with plasmonic tweezers integrated to magnetic nano-robots. However, due to this hybrid approach, those tweezers were not applicable for certain type of colloids such as magnetic nanoparticles. Additionally, the spatial resolution of the manipulation experiments was limited to the Brownian fluctuation of the nano-robot itself.

In the present study, the same team have come up with an advanced nanomanipulation technique that works on optical forces alone and therefore versatile in nature. The researchers have shown manipulation with magnetic colloids and even in biological buffer solution solutions. In their experiment, Ghosh et al have used a nanodisk made of silver as a plasmonic tweezer, and manoeuvred it using a focused laser beam that acted as the optical tweezer.

Earlier attempts to trap metallic nanoparticles in an optical tweezer needed high-intensity beams to hold the disk in place inside the colloidal medium. To overcome this challenge, the team fixed the silver nanodisk on top of a glass microrod to reduce its random movement. A low-intensity laser beam was then sufficient to trap and move the disk-rod hybrid inside the colloidal medium, capturing and carrying nanoparticles as small as 40 nm, along the way.

“The approach combines the strengths of two powerful tools called optical and plasmonic tweezers” says Souvik Ghosh. This unique “tweezer in a tweezer” approach could be used to precisely capture, transport and release particles such as nano-diamonds or quantum dots. As it uses low-intensity light, the approach would also enable non-invasive manipulation of fragile biological specimens such as bacteria, viruses and proteins, the researchers say.

“What we have achieved is the capability of manipulating very, very small particles, with much lower light intensity. This is important for things that can be damaged, such as living cells, or even non-living things where high-intensity beams can heat up the material,” says Ambarish Ghosh, an associate professor at CeNSE, IISc.

The demonstrated technology also showed manipulation of a collection of particles within the same colloidal medium. In addition, the researchers were able to simultaneously manipulate individual nanoparticles at different locations of the fluid and release them independently at desired places inside the fluidic chamber, a functionality that was not demonstrated before in the context of optical nanomanipulation.

The simplicity of the approach would allow the plasmonic tweezers to be integrated with advanced optical tweezer systems for large-scale manipulation and assembly of nanomaterials such as fluorescent nanodiamonds, quantum dot, nanocrystals etc. in standard lab-on-chip devices , they suggest.

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Contacts:
Ambarish Ghosh
Associate Professor
Centre for Nano Science and Engineering (CeNSE)
Indian Institute of Science (IISc)

Ph: 080-2293 2442

Souvik Ghosh
PhD student
Centre for Nano Science and Engineering (CeNSE)
Indian Institute of Science (IISc)

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