Can be used in microfluidics and in biomedicine.

A team from Indian Institute of Science, Bengaluru, has succeeded in designing a new class of mobile nanotweezers that can pick up, hold and move tiny cargo, the size of molecules, in a fluid. The work by Souvik Ghosh and Ambarish Ghosh of Centre for Nanoscience and Engineering, IISc, overcomes the earlier limitation of nanotweezers that were only able to trap and hold the molecules. Apart from nanoscale assembly – where tiny objects such as nanodiamonds or quantum dots need to be picked up and moved to a desired location — this has applications also in microfluidics, where live bacteria need to be manipulated and in biomedicine.

Limitations

Picking up and moving molecules suspended in a fluid, such as a colloid, is a busy area of research. Plasmonic nanotweezers — nanosized tweezers made of noble metals, which have been studied so far to trap cargo, have the limitation that they are fixed in position. When they are illuminated by light, they develop a ‘potential gradient’ around them. This is like a slope, and nearby particles get attracted to the potential’s minimum just as things roll down a slope. However, the limitation is that it can only capture particles that are within the range of the field.

In the new work, the robotic, mobile nanotweezer can pick up tiny particles and move them over a short distance when the microrobots are subjected to an external magnetic field. “We can tune our trapping and releasing mechanism by subsequently turning the incident illumination on and off. To move these nanotweezers, we use a rotating magnetic field which rotates the helix [of the nanotweezer] and [moves it] like a cork-screw,” says Souvik Ghosh, first author of the study published in Science Robotics.

Thermal fluctuations

The colloidal particles move due to thermal fluctuations, therefore it is very difficult to manipulate the nanoparticles. Also as the size of the particles decreases, so, too, does the trapping force. The researchers’ main challenge was to overcome this and generate sufficient trapping force using a small amount of laser power. To achieve this, small helical structures are grown on a pre-patterned substrate by electron beam evaporation of silicon dioxide (made of mostly glass). “The substrate is kept at an extreme angle to the incoming vapour flux and rotated slowly to achieve the helical shape. Apart from glass, we also combine silver (plasmonic properties) and iron (magnetic properties) nanostructures at appropriate places on the helical body,” explains Souvik Ghosh, in an email to The Hindu.

As a next step, the team is working on parallelising the process. Thus, a series of microrobots will work together like an assembly line. “This will allow us to scale up our technology and will surely have commercial impact, and initial results are promising,” says Mr Ghosh.