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Mar 15, 2012
Carbon nanotubes add biosensing to their list of tricks
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A carbon nanotube treated with a capture agent, in yellow, can bind with and detect the purple-colored target protein -- this changes the electrical resistance of the nanotube and creates a sensing device (Graphic courtesy of Oregon State University)Last week, 'lab on a chip' researchers at Oregon State University reported the use of carbon nanotubes as a new form of biosensing technology. Their research is motivated by the drive to create an affordable and portable medical diagnostic 'lab' by shrinking sensors small enough to fit on a miniature modern silicon chip. This research highlights the role of carbon nanostructures in our culture's next era of technological miniaturization, and why biological technologies are likely to be on the forefront.

'Lab on a chip' is a suggestive metaphor for the next step in what has long been among the ultimate goal of electronic technological development: miniaturization. For nearly half a century, the silicon chip has been synonymous with the revolutionary miniaturization offered by silicon-based electronics. Early pre-silicon computer technology was made possible by vacuum tube amplifiers and switches that made up computer circuitry. Vacuum tubes were large, expensive and unreliable -- many parents will eagerly tell stories of computers the size of buildings while using their palm-size smartphones. Silicon's special semi-conducting properties are responsible for such dramatic technological improvements over the span of one generation's lifetime. These properties meant that scientists could construct the amplifiers and switches needed to make computer circuits on increasingly smaller scales. The miniaturization of silicon chip electronics is essentially only limited by the cost of manufacturing circuits on these scales.

Still, silicon is likely to soon be supplanted for two reasons. First, silicon technology could encounter a fundamental limit on the size to which circuits can shrink. Such a limit may well exist, but remains ill-defined as it is one that researchers continue to push. The second reason, exacerbated by the increasing cost of pushing that limit, is that another technology could offer a cheaper and easier road to further miniaturization.

The first millimeter-scale computer system -- one millimeter is one million nanometers -- roughly the size of one million carbon nanotubes (Photo courtesy of Gyouhu Kim from University of Michigan)At the moment, the most likely path is cleared by silicon's little brother -- carbon -- sitting just above it in the same chemical family on the periodic table. In the past decade researchers have begun to recognize the unique electronic properties of the structures formed by carbon atoms. The versatility of carbon is obvious in the difference between a diamond and the graphite of a pencil. Both are pure compounds of carbon, but their different structures give rise to dramatically different properties. As 

far as miniaturization goes, researchers are particularly interested in tube-like structures formed by carbon, known as nanotubes. Interestingly, nanotubes also exhibit semiconducting properties that allow for the creation of simple circuit elements that enable the creation of transistors and eventually computers. Because of their size, developed nanotube electronics could eventually offer much smaller, cheaper integrated circuits than silicon-based electronics. For the moment, the widespread application of carbon nanotubes in electronics is limited by the high cost of mass production and their relatively rapid degradation when exposed to oxygen.

Still, the OSU group is not interested in nanotubes solely for their electrical properties, but also for their organic ones. They are interested in electrical structures of carbon because of their fundamental role in life as we understand it. There is a reason life is known as carbon-based: properties of carbon allow the assembly of stable complex organic molecules that form the basis of life. Carbon is unique among atoms in the periodic table not because it offers four electrons to be 'shared' with other atoms (in a process known as 'covalent' bonding, a relatively common way that molecules are held together), but because all four electrons remain close enough to the nucleus to form four unusually strong versions of these bonds. Atoms like hydrogen with fewer exposed electrons can form a similarly strong bond, but only one bond per atom, which is not enough to form complex structures. There are also atoms with many more electrons to share, but because these atoms are much bigger, the electrons are much more loosely bound and their molecular bonds will be weaker. Carbon bonding, both with other carbon molecules and with a wide variety of other atoms, is responsible for the building blocks of life as well as the unique structure and properties of carbon nanotubes.

Spinning carbon nanotubeIn short, the match between the electronics of nanotubes and the innate ability to interact with the carbon-based molecules of life is natural. The OSU group was able to show how this match can help them achieve their goal of shrinking an entire medical lab down to the size of a computer chip. When complex molecules such as proteins interact with nanotubes, the nanotube's electric properties will change. Using nanotubes, the researchers can detect and quantify the presence of proteins. Effectively, they have shown that we can use nanotubes as nanoscopic biological sensors. If we custom fabricate nanoscopic sensors for every kind of protein medical professionals might want to detect, we could someday have an entire suite of diagnostic capabilities on a single chip.

Carbon nanostructures offer a wide variety applications that will change the way we think about and use many technologies. Aside from the biological uses discussed, carbon nanostructure-based materials find widespread usage because of their unusual strength at remarkably low weights. Nanotube electronics may well usher in a new era of computer miniaturization, and may lead to unexpected breakthroughs like those in nanotube-based solar technologies. The OSU research reminds us of the intimate connection with organic molecules, but more importantly it reiterates an increasingly common message: carbon nanostructures are likely to be an important part of our technological future, and researchers are just beginning to scratch the surface.

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