Nanotweezers Use Electric Fields from Acoustic Waves to Manipulate Bio Molecules

What You’ll Learn:

How piezoelectric materials are used to create a defined acoustic field.
How this field, in turn, is used to develop modulate an electric field.
How the combined acoustic and electric fields can function as nanotweezers to manipulate biological particles such as DNA.

You’re likely familiar with optical tweezers that use a focused laser beam and a gradient lens to move and manipulate tiny molecules such as DNA strands (see Reference below). These optical tweezers—a process formally called optical “trapping”—were conceived, analyzed, constructed and tested in the 1980s by a team led by Arthur Ashkin at Bell Labs. His work resulted in his receiving the Nobel Prize in Physics in 2018 (the reason for the delay is a mystery).

Since their development, they have been refined and enhanced, and are now an indispensable tool with a major impact on science and study of molecular motors, DNA and other biological molecules.

However, situations arise in which this laser-based tool isn’t suitable. Alternative approaches include use of pulsed piezoelectric materials to create acoustic radiation forces that trap particles in acoustic pressure nodes or antinodes. However, this approach is limited by “acoustic streaming,” an unavoidable characteristic of the wave propagationthat counteracts the acoustic radiation forces. This streaming prevents stable trapping and “patterning” (controlled motion), especially when the particle diameter scales down below 100 nm.

The result is that the various optical-tweezer alternatives including acoustic nanotrapping and manipulation cannot provide enough force for accurate nanomanipulation without also adding excessive background noise and disturbances. This is especially the case when trying to do so for particles under 100 nm in size and over large area.

Acoustoelectronic Nanotweezers

To overcome these obstacles, a team at Duke University led by Tony Jun Huang, the William Bevan Distinguished Professor of Mechanical Engineering and Materials Science, has devised a new approach using sound-induced electric fields that they call “acoustoelectronic nanotweezers” (AENT for short). Noted Joseph Rufo, a graduate student on the project team, “Although we’re still fundamentally using sound, our acoustoelectronic nanotweezers use a very different mechanism than these previous technologies. Now we’re not only exploiting acoustic waves, but electric fields with the properties of acoustic waves.”

Their approach uses a small chamber filled with liquid. Four interdigital transducers (IDTs) are fabricated on a piezoelectric substrate and aligned with the chamber’s sides (Fig. 1). Exciting these transducers sends sound waves into the piezoelectric substrate and these, in turn, generate local elastic deformations. The deformations propagate as acoustic waves along the surface and interfere with each other and the boundaries. Since the sound waves create stresses within the piezoelectric substrate, they also produce electrical fields and eventually establish standing-wave electric fields that can be manipulated and shifted.

1. Working principles of acoustoelectronic nanotweezers (AENT): (a) Acoustoelectronic fields are generated via dynamic acoustic-wave interactions. These acoustic waves have minimal out-of-plane vibrations and associated acoustic attenuation losses in a fluid. F is the surface electric potential. (b) Schematic side-view of the electric-field distribution and trapping positions for particles with different polarizabilities relative to the medium (red sphere: high polarizability; green sphere: low polarizability). (c) Schematic mechanism of AENT on manipulating nanoparticles with lower or higher polarizability than the medium in 3D space by tuning the phases and amplitudes of the acoustic waves. Δφ₁ indicates the phase variation of interdigital transducer IDT₁. ΔA1₂ indicates the amplitudes variation of IDT₁ and IDT₂. (d) Candidate excitation configurations based on nine potential single-crystal piezoelectric materials for AENT. κ_AET is the acoustoelectronic efficiency, which is defined as the ratio between the surface electric potential and the excitation voltage on the transducer in a standing-wave mode; u_fluid is the acoustic streaming speed under consistent excitation amplitudes on different crystals. (e) Macroscopic materials with pre-designed nanotextures fabricated by AENT. The insets show microscopic images of PDMS films containing aligned carbon nanotubes and 100-nm polystyrene (PS) beads, and PEG hydrogels containing textured proteins (66 kDa and 3 kDa; kDa is kilodaltons, a measure of protein mass). Scale bar: 60 μm.

Even the choice of piezoelectric material affects performance, and assessing the material was a part of the project. The reason is that surface acoustoelectronic efficiency and the acoustic streaming disturbances are the key factors in determining the performance of AENT. Higher acoustoelectronic efficiency enables more efficient electrical manipulation, while lower acoustic streaming potentials reduce hydrodynamic disturbances (noise) and improve the stability of the manipulation capabilities.

To quantify the options, they tested nine different single-crystal piezoelectric materials having good potential to generate in-plane vibrations, to characterize the frequency domain with respect to these two considerations. As a result of these tests, they selected a specific crystalline version of LiNbO₃ as the primary piezoelectric material.

Testing and Results

To verify that what they constructed was actually performing as intended, they first ran one-dimension tests on 100-nm polystyrene particles as well as larger and smaller beads. After successful validation, they moved on to two-dimension experiments and varied (or tuned) parameters including piezo-drive frequency, amplitude, phase, position and time to synthetize complex acoustoelectronic fields that trapped the particles. By adjusting these parameters, they were able to move the fields and, thus, move the particles as well, with individual particles moved to desired locations to form larger patterns (Fig. 2).

2. AENT enables nanomanipulation with single-particle precision: (a) 2D lattices containing single 400-nm polystyrene beads in several trapping wells. (b) Principle of 2D nanomanipulation by tuning the phases of the orthogonal standing waves. (c) Stacked fluorescence images showing that a 400-nm polystyrene particle can be translated along complex paths as letters “D”, “U”, “K” and “E.” (d, e) Reversible pairing of single 400-nm beads using 2D electric fields. (f) Schematic force analysis and levitation of a single 400-nm bead as the amplitude of the standing wave increases. F_AENT = acoustoelectronic force; F_Buoy = buoyant force; F_Grav = gravitational force; F_Brow = Brownian forces; DA = displacement antinode. The label “s” with bidirectional arrows indicates the direction of standing waves. (g, h) Deflection of single nanoparticles in continuous flow using narrow acoustoelectronic wave beams. (g) Deflecting a 400-nm particle in a bifurcated channel. A single 400-nm particle flows to the lower outlet by default (AENT off, red pseudo-color) or can be deflected to the upper channel when AENT is on (green pseudo-color). Gray shadings: PDMS wall. (h) Deflection of single 110-nm polystyrene bead, and single exosome particle in continuous flow. The time-elapse trajectories are plotted over the composite image, where the color scales indicate the elapsed time. Time intervals: 100 ms. Scale bars: a: 60 μm; c–f: 15 μm; g, h: 20 μm.

The authors conclude that their AENT approach, with its combination of reconfigurable acoustic waves and coupled electric fields, and simultaneous minimization of acoustic streaming, enables large actuation forces (in this context, “large” means on the order of femtonewtons to piconewtons) on nanoparticles without introducing significant hydrodynamic disturbances. Going further, other functions such pattern translation, rotation, transformation, interconnection, particle pairing, levitation, concentration and sorting were demonstrated based on the dynamic, reconfigurable nature of the acoustoelectronic fields.

The research was supported by the National Institutes of Health, the United States Army Medical Research Acquisition Activity, and the National Science Foundation). The project is described in full detail in their paper “Acoustoelectronic nanotweezers enable dynamic and large-scale control of nanomaterials” published in Nature Communications, along with comprehensive Supplementary Materials, which discuss the setup, test, and evaluation of the different piezo materials, among other aspects of the project.

The paper also links to seven short videos (with their own descriptive page) showing particles being manipulated individually and as a group (personal comment: the videos of white dots/particles moving silently on a dark background are somewhat eerie, and the ones with synchronized clusters resemble a swarm of tightly controlled, choreographed drones).