New mobile nanotweezers that can capture and release sub-micron-sized particles in a fluid more quickly and efficiently than ever before have been constructed by researchers at the Indian Institute of Science in Bangalore. The devices, which are based on ferromagnetic helical nanostructures integrated with silver nanoparticles that produce mechanical force in response to light, can be used to manipulate objects such as bacteria, colloidal beads and fluorescent nanodiamonds. They could find use in applications as diverse as lab-on-a-chip technology, in microfluidics and nanoscale assembly to name a few.
Being able to manipulate nanoscale objects in liquid environments is one of the main goals of modern nanotechnology. Researchers usually do this by trapping particles with optical, acoustic, magnetic, electric or flow fields, and such technologies have led to breakthroughs in biophysics and microfluidics in recent years. Since the trapping force decreases with the size of the object, it is difficult to control objects that are sub-micron in size using these techniques, however.
Plasmonic tweezers (which work by exploiting the localized electromagnetic fields near metallic nanostructures) are a good alternative here but their drawback is that they can only pick up and move nanoobjects very slowly since they are fixed in space. Another strategy is to make use of microrobots. These are micro-to nanosized motile particles driven by chemical reactions or externally applied magnetic fields, and although they can carry and push objects very quickly, again they cannot be used to manipulate nanoscale objects.
Combining plasmonic tweezers and microbots
A team led by Ambarish Ghosh says that it has now combined these two technologies. “As well as being able to carry small objects to various spots on a microfluidic device, our nanotweezers can also be localized with high spatial resolution and can be removed from a device if necessary,” explains Ghosh. “This should open up new avenues in nanoscale assembly that did not exist before.”
The microbots made by the researchers are based on ferromagnetic, screw-shaped ferromagnetic nanostructures. They grew the structures using electron-beam evaporation of silicon dioxide on a pre-patterned substrate that is kept at an extreme angle to the incoming vapor flux and rotated slowly to achieve the helical shape. Finally, plasmonic silver nanoparticles are also placed on certain regions of the helical body.
Optical trapping and microbot motion
“The plasmonic islands work as nanoscale antennas,” explains Ghosh, “so that when we illuminate them with a light beam, a strong trapping force is generated that can pull in nearby colloidal nanoparticles. This trapping force vanishes once the light source is switched off, which means that we can tune our trapping and releasing mechanism by turning the incident illumination on and off.”
The microbots can be manoeuvred in space using a rotating, homogenous magnetic field and their motion is very similar to way that some microorganisms translate by rotating their helical flagella.
Applications in microfluidics and nanoscale assembly
According to the researchers, the microbots might find use in microfluidics for positioning, sorting, transporting and collecting single parties as small as 100 nm, something that was not possible before. “Especially interesting is being able to trap and carry live bacteria, since this technique makes use of a very low light intensity beam (that does not damage the microorganisms),” says Ghosh.
“They might also be used in nanoscale assembly to place very small objects such as nanodiamonds and quantum dots on specific positions on a device. Being able to do this could be important for next-generation quantum technologies, sensing devices, nanolasers and many more.”
The team, reporting its work in Science Robotics DOI: 10.1126/scirobotics.aaq0076, is now busy making multiple nanotweezers that work in parallel. “This will allow us to scale up our technology and will surely have some commercial impact,” Ghosh tells nanotechweb.org. “Indeed, initial results are promising.”
