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From the Labs: Nanotechnology

New publications, experiments, and breakthroughs in nanotechnology–and what they mean.
November 1, 2006

Nanowire Computing
A practical method for nanowire-based CMOS circuits

This nanowire-based CMOS circuit (the nanowires are too small to see) could lead to smaller, more powerful computers. (Courtesy of Dunwei Wang)

Source: “Complementary Symmetry Silicon Nanowire Logic: Power-Efficient Inverters with Gain”
Dunwei Wang et al.
Small 2(10): 1153-1158

Results: Caltech researchers have made silicon-nanowire-based logic circuits similar to the complementary metal-oxide semiconductor circuits used in computer chips. Such circuits combine two kinds of transistors that respond in opposite ways to electronic signals–a useful arrangement for energy-efficient chips. Because the new method can produce both types of transistors on a single surface, it could be suitable for mass production.

Why it matters: Because of their small size and excellent electronic properties, silicon nanowires could enable ultrasensitive handheld sensors for detecting cancer or identifying biological hazards. What’s more, the nanowires could lead to more powerful, energy-efficient computer chips. But previous prototypes of nanowire-based circuits were made using techniques that don’t lend themselves to batch processing. The new methods could make nano­wire circuits practical to manufacture.

Methods: To make p- and n-type transistors, the two types needed in CMOS circuits, researchers first created a checkerboard pattern of the p- and n-type silicon: they doped adjacent squares with different dopants, using photolithography-produced masks. Then, using a method they’d previously developed, the researchers selectively etched away silicon to form orderly arrays of nanowires. Finally, they connected these nanowires using e-beam lithography to form transistors and a fundamental type of logic circuit called an inverter.

Next steps: For mass production, the researchers will replace the e-beam lithography with the faster method of photolithography. They also need to demonstrate that an experimental process for making batches of nanowire arrays, called nano imprinting, will work in large-scale manufacturing.

Smart Nanosize Containers
Nanoparticles could signal when they are inside specific types of cells, leading to new diagnostic and treatment methods

Source: “Toward Intelligent Nanosize Bioreactors: A pH-Switchable, Channel-Equipped, Functional Polymer Nanocontainer”
Pavel Broz et al.
Nano Letters 6(10): 2349-2353

Results: Researchers in Switzerland have made 200-nanometer-wide containers dotted with pores whose walls are made of bacterial proteins. They demonstrated that these nano containers can control the location and duration of a fluorescent signal–lighting up only when the acidity of their environment matches that inside cell structures called lysosomes, which digest foreign materials that enter a cell.

Why it matters: The work shows that nanoparticles using active pores can respond to environmental cues, such as acidity, to perform useful functions. In one application, pH-sensitive nano carriers would light up only once they encountered lysosomes, ensuring that they’d reached the inside of cells. The researchers earlier showed that the carriers can latch onto particular types of cells, such as macrophages, suggesting that such a system could be used to identify specific cells in a lab sample. With some modifications, it could also be used to release a drug only inside targeted cells, making drug treatment more effective and reducing side effects by protecting nearby tissue.

Methods: Specially designed polymers combined with bacterial proteins self-assemble to form the containers, while added enzymes that break down certain compounds, causing them to fluoresce, are trapped inside. The pores’ size prevents the enzymes from escaping but lets compounds gradually enter the container to be broken down, creating a long-lasting signal that is confined to the containers. The pH sensitivity is a result of two factors: the enzymes work best at lysosomal acidities, and the pores, which are open in most conditions, close at acid concentrations that are too high.

Next Steps: The research requires further tests to confirm that the nanoparticles work in living subjects. For potential drug-delivery applications, the researchers will pair drugs with specific cellular targets and develop a release mechanism; it could be based on synthetic pores that stay closed in neutral and alkaline environments as well as highly acidic ones, opening only in the particular pH range of the inside of a lysosome.

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