Synopsis: Nanotechnology

Low-Power Organics
Better insulation helps plastic circuits flex toward the market

Context: Computer chips contain millions of transistors made of silicon. But silicon is too brittle to be used in applications such as flexible displays and smart fabrics that monitor vital signs. Such products will require new materials, ideally plastics. But all “organic transistors” so far have required voltages too high and consumed power too quickly to be viable. Researchers from Siemens spin-off Infineon, the University of Stuttgart, and MIT have created organic transistors that seem to surmount these barriers.

Methods and Results: A transistor is an electronic switch that flips when voltage is applied to a wire on top of a semiconductor. However, if the wire comes in contact with the semiconductor, the circuit will leak, or waste, electrical current. So transistors are insulated from the wires that flip them by a layer of material called a “gate dielectric.” Fabricating these layers is perhaps the most challenging aspect of building transistors: thinner layers allow operation at lower voltages but are prone to pinhole-like defects through which current can leak. Up to now, organic transistors used dielectrics more than 100 nanometers thick and required more than 20 volts to operate. Infineon’s Marcus Halik and his team grew a pinhole-free film only 2.5 nanometers thick from molecules that “self-assemble.” Transistors placed atop this film switched using less than two volts and drew even less current than transistors in conventional silicon chips.

Why it Matters: Organic transistors could be used to build displays that bend like paper, cloth that computes, and cheap electronic bar codes. By demonstrating low-voltage organic transistors, researchers may have eliminated a barrier to the commercialization of such devices.

Source: Halik, M. et al. 2004. Low-voltage organic transistors with an amorphous molecular gate dielectric. Nature 431:963-966.

Super-sensitive Screen
Better virus detection through nanowires

Context: Health-care providers would benefit greatly from a ­simple, inexpensive method for determining whether a patient’s runny nose or upset stomach signifies a virus – or something else. A medi­cal lab can identify an infection from a tissue swab, but only after the time-consuming and expensive process of sample preparation and analysis.

Far better would be a device that could interact with a virus at the cellular level, producing an electrical signal that could be interpreted by computer chips and other electronics. Such a system would, in theory at least, be cheaper than existing diagnostic technolo­gies; it could also potentially screen numerous viruses at once and determine the presence or absence of any of them almost instantly. Silicon nanowires are one promising candidate as the detection technology, because they are about the same size as biological particles and could respond to their presence with great sensitivity.

Now, seminal work in the laboratories of Charles Lieber, a chemist and pioneer in the field of nanotechnology, and Xiaowei Zhuang at Harvard University has demonstrated that such a nanowire system can be built and can detect single virus particles – a milestone in the development of a new generation of ultrasensitive nanosensors.

Methods and Results: Silicon nano­wires were “decorated” with virus-specific antibodies. When a virus bonded with one of the antibodies, the charged proteins on the virus’s surface changed the nano­wire’s conductivity, in much the way that an electrical charge can turn a transistor on or off. Viruses could be detected in seconds or minutes, and one type of virus would show up clearly even in the presence of another. The process worked even with samples that had not been extensively purified, though the researchers did not attempt to directly test samples of fluids such as plasma.

Why it Matters: The Harvard researchers’ work is a breakthrough in the application of nanotechnology to the improvement of biosensors. The first use of such a virus detection system would likely come in a pharmaceutical laboratory: replacing the antibodies with potential drug molecules could help identify antiviral drugs. Such a system could make drug discovery faster and more efficient.

Outside of the lab, the technique could yield a simple biochip test that might be performed in any doctor’s office, day care center, or private home. Of course, nano­wires are not necessarily the only means of building such a chip, and other technologies may yet prove to be more robust or less expensive. But the idea of selling a biochip every time a toddler sneezes is enough to ensure that, somewhere, a venture capitalist is smiling.

Source: Patolsky, F. et al. 2004. Electrical detection of single viruses. Proceedings of the National Academy of Sciences 101:14017-14022.

Trip the Light Fantastic
Teaching old optics new tricks could lead to novel sensors and interfaces

Context: Thousands of kilometers of optical fibers have been laid under roads and even oceans, where they quietly and competently transmit telephone calls, Internet downloads, and television shows in the form of light pulses. Traditionally, these glass fibers have been simple conduits; the less they interact with the rest of the world, the better they perform. Now researchers in the laboratory of Yoel Fink at MIT have turned that notion on its head, creating a fiber that detects as well as transmits light and demonstrating a much more economical way to make large-scale light sensors.

Methods and Results: In the conventional ­fiber-drawing process, a relatively thick slug, or preform, of glass is heated in a furnace and carefully stretched. To create their light-sensing fiber, Fink and Mehmet Bayindir’s team used a complicated preform with three main active ingredients, including a semiconductor whose conductivity improves dramatically when it is illuminated; tin wires, which help conduct electricity; and a nanostructured mirror, which selectively confines light of a specific wavelength to the fiber’s core. The original preforms were approximately 30 millimeters across, and they were drawn out to form fibers a millimeter or less in diameter.

A light shone on the resulting fiber at any point along its length can be detected as a change in electrical conductivity more than ten meters away. The researchers wove a collection of these fibers together to create a two-dimensional grid, 30 by 30 centimeters square, which tracked the position of a light beam shone onto its surface.

Why it Matters: A typical light sensor is an array of many individual detectors. As a consequence, it’s limited in size: combining thousands or millions of detectors can be prohibitively expensive. By weaving together lines of fibers instead of assembling individual dots, the MIT researchers have found a way to drastically reduce the cost of spanning large areas with light detectors.
That could lead to the development of completely new kinds of optical devices and interfaces, such as a projection screen that can respond to a user’s laser pointer rather than requiring a conventional computer mouse.

Source: Bayindir, M. et al. 2004. Metal-insulator-semiconductor optoelectronic fibres. Nature 431:826-829.