Nanotechnology

Molecular Manufacturing
A machine built out of DNA

Context: Deep in every cell, miniature biological machines called ribosomes forge proteins. Following instructions written in our genetic code, ribosomes weld together amino acids to form the enzymes that modulate body chemistry and the structural materials, like collagen, that hold the body together. As good as engineers are at building machines and structures on the scale of people, they have few tools for building on the scale of molecules, as a ribosome does.

Researchers from New York University have taken a landmark step toward the goal of imitating the ribosome, building a programmable, nanoscale machine that can weld together DNA molecules.

Methods and Results: NYU chemists Shiping Liao and Nadrian Seeman twisted and bent DNA to build a structure that is approximately 110 nanometers long, 30 nanometers high, and 2 nanometers thick, roughly the same size as a ribosome, though not as complex. Just as the ribosome can be programmed to weld amino acids together in a prescribed sequence, this DNA machine can be instructed to select specific small molecules of DNA for concatenation. The DNA machine can swivel into four geometric positions and can be locked into any one of them by another fragment of DNA, the “instructions.” Locking the machine into position dials in the sequence of very short DNA strands that it will recognize and position for welding. The welding itself is performed by an enzyme that links DNA molecules. In the absence of the machine, this enzyme would create many different combinations of DNA strands; in its presence, only a single, preprogrammed combination results.

Why it Matters: The evolution of the ribosome transformed terrestrial biology, enabling cells to manufacture any protein of any size or shape as needed. Nanotechnologists seek a similar watershed, the development of a machine that could make improved pharmaceuticals or biomaterials. Liao and Seeman’s approach could also be extended to the manufacture of nonbiological products, with the goal of producing materials that are impossible to construct using conventional chemistry. Measured against these ambitions, their current nanodevice appears crude, but it is likely to be followed by more sophisticated successors.

Source: Liao, S., and N. C. Seeman. 2004. Translation of DNA signals into polymer assembly instructions, Science 306:2072–2074.

Twice as Light
Double layering boosts organic-solar-cell efficiency

Context: Solar cells offer the promise of clean, renewable energy. But most cells in use today use silicon to absorb light, which makes them brittle and so expensive to manufacture that it takes years before they produce enough electricity to recoup their cost. Enter organic solar cells, which use carbon-based molecules such as “buckyballs” rather than silicon. In theory, cells made from these cheap, flexible materials could be rolled out in sheets of translucent plastic and stuck on everything from cell phones to clothing. But while commercial silicon-based cells can convert about 20 percent of the sunlight that strikes them into electricity, organic cells typically convert only 1 or 2 percent. Stephen Forrest’s team at Princeton University has developed a new way to boost that average.

Methods and Results: When light interacts with a solar cell, different wavelengths deliver more or less energy at different depths. By varying the composition of their organic solar cells, the Princeton researchers tuned some to more efficiently absorb reddish light and others to absorb bluish light. They then layered the red cells onto the blue, to capture those wavelengths of light where they are most intense. The result: an efficiency of almost 6 percent, the highest published for such cells to date. The researchers speculate that with a third layer (say, for infrared wavelengths), the cell could reach double-digit efficiency.

Why it Matters: Organic solar cells may be cheap and widely applicable, but with their current efficiencies, even plastering the side of a skyscraper would not yield enough power to be practical. The jump in efficiency reported by the Princeton team—plus innovations in manufacturing and integration—makes the prospect of generating municipal power from these renewable solar sources more likely.

Source: Xue, J., et al. 2004. Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions. Applied Physics Letters 85:5757–5759.