DNA Nanotubes To Generate Ultrasmall Electronic and Biomedical Innovations

Scientists are now designing and assembling intricate structures on a scale nearly incomprehensibly small. Their medium is the double-helical DNA molecule, a resourceful building material offering near limitless building potential.

In the January 2, 2009, issue of the journal Science, Drs. Yan and Liu, researchers from Arizona State University's (ASU) Biodesign Institute and faculty in the department of chemistry and biochemistry (Tempe, USA), demonstrated for the first time the three-dimensional characterization of DNA nanotubules, rings, and spirals, each a few hundred thousandths the diameter of a human hair. These DNA nanotubes and other synthetic nanostructures may soon help in providing a new generation of ultrasmall electronic and biomedical innovations.

Dr. Yan and Liu are working in the rapidly growing field of structural DNA nanotechnology. By copying natural structures, they discovered that they could capitalize on the DNA molecule's extraordinary properties of self-assembly. When ribbonlike strands of the molecule are brought together, they fasten to each other like strips of Velcro, according to simple rules governing the pairing of their four chemical bases (labeled A, C, T, and G). From this small alphabet, nature has provided an astounding variety of forms. DNA accomplishes this through the cellular synthesis of structural proteins coded for by specific sequences of the bases. Such proteins are essential constituents of living matter: forming cell walls, vessels, tissues, and organs. However, DNA itself can also form stable architectural structures, and it may be artificially persuaded into doing so.

In his research, Dr. Yan has been much inspired by nanoscale ingenuity in the natural world. "Unicellular creatures like oceanic diatoms,” he pointed out, "contain self-assembled protein architectures.” These diverse forms of enormous delicacy and practicality are frequently the result of the controlled self-assembly of both organic and inorganic material.

Scientists in the field of structural DNA nanotechnology, including Dr. Yan's group, have previously demonstrated that prefabricated DNA elements could be induced to self-assemble, forming useful nanostructural platforms or "tiles.” Such tiles are able to snap together, through base pairings, forming larger arrays.

This study responds to one of the fundamental challenges in nanotechnology and materials science, the construction of molecular-level forms in three dimensions. To accomplish this, the team used gold nanoparticles, which can be placed on single-stranded DNA, compelling these flexible molecular tile arrays to bend away from the nanoparticles, curling into closed loops or forming spring-like spirals or nested rings, approximately 30 to 180 nanometers in diameter.

The gold nanoparticles, which coerce DNA strands to arc back on themselves, produce a force known as steric hindrance, whose magnitude depends on the size of particle used. Using this steric hindrance, Drs. Yan and Liu have shown for the first time that DNA nanotubules can be specifically directed to curl into closed rings with high yield.

When 5 nanometer gold particles were used, a milder steric hindrance directed the DNA tiles to curl up and join complementary neighboring segments, often forming spirals of varying diameter in addition to closed rings. A 10-nanometer gold particle, however, exerted greater steric hindrance, directing a more tightly constrained curling, which produced mostly closed tubules. Dr. Yan stressed that the particle not only participates in the self-assembly process as the directed material, but also as an active agent, inducing and guiding formation of the nanotube.

With the assistance of Dr. Anchi Cheng and Jonathan Brownell at the Scripps Research Institute (La Jolla, CA, USA), they have used an imaging technique known as electron cryotomography to provide the first glimpses of the elusive three-dimensional (3D) architecture of DNA nanotubules. "You quickly freeze the sample in vitreous ice,” Dr. Yan explained, describing the process. "This will preserve the native conformation of the structure.” Subsequent imaging at various tilted angles allows the reconstruction of the 3D nanostructure with the gold particles providing enough electron density for clear visualization.

DNA nanotubules will soon be ready to join their carbon nanotube family, providing flexible, resilient, and manipulatable structures at the molecular level, according to the researchers. Extending control over 3D architectures will lay the foundation for future applications in photometry, photovoltaics, touch screen, and flexible displays, as well as for far-reaching biomedical advancements. "The ability to build three-dimensional structures through self-assembly is really exciting,” Dr. Yan said. "It's massively parallel. You can simultaneously produce millions or trillions of copies.”

The investigators believe that controlled tubular nanostructures bearing nanoparticles may be applied to the design of electrical channels for cell-cell communication or used in the construction of various nanoelectrical devices.

Related Links:
Arizona State University's Biodesign Institute