Acoustoelectronic nanotweezers manipulate miniscule particles

The approach from Duke University, North Carolina is claimed to provide a label-free, dynamically controllable method of moving and trapping nanoparticles over a large area. The technology holds promise for applications in the fields ranging from condensed matter physics to biomedicine.

The team’s findings are published online in Nature Communications.

Precisely controlling nanoparticles is a crucial ability for many emerging technologies; separating exosomes and other tiny biological molecules from blood could lead to new types of diagnostic tests for the early detection of tumours and neurodegenerative diseases. Similarly, placing engineered nanoparticles in a specific pattern before fixing them in place can help create new types of materials with highly tuneable properties.

Sound tunnels into rewritable lab-on-a-chip devices

For over a decade, Tony Jun Huang, the William Bevan Distinguished Professor of Mechanical Engineering and Materials Science at Duke, has pursued acoustic tweezer systems to manipulate particles, an endeavour that becomes more challenging when their profile drops below that of the smallest viruses.

"Although we're still fundamentally using sound, our acoustoelectronic nanotweezers use a very different mechanism than these previous technologies," said Joseph Rufo, a graduate student in Huang's laboratory. "Now we're not only exploiting acoustic waves, but electric fields with the properties of acoustic waves."

Instead of using sound waves to directly move the nanoparticles, Huang, Rufo and Peiran Zhang, a postdoc in Huang's laboratory, use sound waves to create electric fields that provide the push. The new acoustoelectronic tweezer approach works by placing a piezoelectric substrate beneath a small chamber filled with liquid. Four transducers are aligned on the chamber's sides, which send sound waves into the piezoelectric substrate.

According to Duke, these sound waves bounce around and interact with one another to create a stable pattern, and because the sound waves are creating stresses within the piezoelectric substrate, they also create electrical fields. These couple with the acoustic waves in a way that creates electric field patterns within the chamber above.

"The vibrations of the sound waves also make the electric field dynamically alternate between positive and negative charges," Zhang said in a statement. "This alternating electric field polarises the nanoparticles in liquid, which serves as a handle to manipulate them."

Because the acoustoelectronic nanotweezers induce an electromagnetic response in the nanomaterials, the nanoparticles do not need to be conductive on their own or tagged with any sort of modifier. Similarly, patterns are created with sound waves so their positions and properties can be quickly and easily modified to create a variety of options.

In the prototype, the researchers show nanoparticles placed into striped and checkerboard patterns. They can push individual particles around in an arbitrary manner dynamically, spelling out letters such as D, U, K and E. The researchers then demonstrated that these aligned nano-patterns can be transferred onto dry films using nanoparticles such as carbon nanotubes, 3.5nm proteins and 1.4nm dextran used in biomedical research. They have shown also that this can be accomplished on a working area that is tens to hundreds of times larger than current nanotweezing technologies.