December 3, 2009 – Researchers at the California Institute of Technology (Caltech) have combined the self-assembly ability of DNA with electronic properties of carbon nanotubes (CNT) to address the problem of organizing CNTs into nanoscale electronic circuits.

(a) Single-wall carbon nanotubes labeled with "red" and "blue" DNA sequences attach to anti-red and anti-blue strands on a DNA origami, resulting in a self-assembled electronic switch. (b) An AFM image of one such structure. The blue nanotube appear brighter because it is on top of the origami; the red nanotube sits below. Scale bar is 50nm. (c) A diagrammatic view of the structure shown in b. The gray rectangle is the DNA origami. A self-assembled DNA ribbon attached to the origami improves structural stability and ease of handling. (Credit: Paul W.K. Rothemund, Hareem Maune, and Si-ping Han/Caltech/Nature Nanotechnology)

DNA origami is a type of self-assembled structure that can be programmed to form various shapes and patterns (e.g. smiley-faces, maps, even electrical diagrams). The structures are created from long single strands of viral DNA mixed with shorter synthetic DNA strands that bind them into desired shapes, generally 100nm on a side. Meanwhile, single-wall CNTs have intriguing electrical, strength, and heat conductive properties, but are problematic to arrange into desired patterns.

The solution they came up with was to soak the CNTs and DNA molecules in salt water, allowing the DNA to stick to the CNTs (protecting portions of them, to create a "handle" for recognition). Two batches were made ("blue" and "red"), and observed that single-strand DNA molecules with complementary sequences (e.g., "blue" and "antiblue") wrapped around to form a double helix. Next was building 100nm × 100nm "breadboards" in which DNA origami sequences are designed so that specific nanotubes will attach in preassigned positions — e.g., red-labeled CNTs crossing perpendicular to blue CNTs, to build a field-effect transistor (FET).

The systems were removed from solution and placed on a surface, and leads attached to measure electrical properties — and it indeed behaved like a FET, they claim. "One carbon, nanotube can switch the conductivity of the other due only to the electric field that forms when a voltage is applied to it," explained Paul W. K. Rothemund, Caltech senior research associate, in a statement. It didn’t work perfectly, noted Erik Winfree, Caltech associate professor of computer science, computation and neural systems, and bioengineering, but "it was sufficient to demonstrate the controlled construction of a simple device, a cross-junction of a pair of carbon nanotubes."

From their paper abstract:

We synthesize rectangular origami templates (75nm × 95nm) that display two lines of single-stranded DNA ‘hooks’ in a cross pattern with 6nm resolution. The perpendicular lines of hooks serve as sequence-specific binding sites for two types of nanotubes, each functionalized non-covalently with a distinct DNA linker molecule. The hook-binding domain of each linker is protected to ensure efficient hybridization. When origami templates and DNA-functionalized nanotubes are mixed, strand displacement-mediated deprotection and binding aligns the nanotubes into cross-junctions. Of several cross-junctions synthesized by this method, one demonstrated stable field-effect transistor-like behavior. In such organizations of electronic components, DNA origami serves as a programmable nanobreadboard; thus, DNA origami may allow the rapid prototyping of complex nanotube-based structures.

The group expects to improve their approach to more reliably build complex circuits involving not only CNTs but also other elements including electrodes and wiring. And the self-assembly approach is scalable to make multiple devices at a time, they note — e.g., enabling design of logic units for millions or billions of units self-assembling in parallel.