, or SWNTs, are capable of producing light. Metallic nanotubes actually inhibit the brightness of their fluorescent neighbors. But it has been very difficult to separate the strongly fluorescent SWNTs from all the rest in large quantities.
Nanotube soot is insoluble in water. So researchers routinely mix it with special soap and give it a dose of ultrasound to break apart clumps of nanotubes and force them to dissolve. The result is a dark liquid that is routinely put into an ultracentrifuge that subjects them to forces a few thousand times that of gravity. Centrifuging separates out a number of gross impurities.
Hertel’s team discovered that if they remove the most buoyant layer from the centrifuge, let it set for a while and then put it back in the ultracentrifuge for another 12 hours, the liquid separates into a number of distinct layers. The topmost layer has a purple color and, when analyzed, proves to contain a surprisingly uniform population of the brightest nanotubes.
The researchers had expected this approach to boost the quantum efficiency by five to ten times. The fact that the improvement was considerably larger – 20 to 100 times – came as a pleasant surprise.
“Quantum efficiency is critical, but there are several other factors that make nanotubes particularly well suited for use in living systems,” says Hertel. These factors include:
- Nanotubes emit light in a very narrow range of wavelengths, or colors. This makes it easier to pick out their signal against background noise. Furthermore, they produce light in a part of the spectrum – the near infrared where skin and other tissue is transparent – that allows the nanotube light to stand out.
- Nanotubes are made entirely from graphitic carbon, which is non-toxic and, at least so far, experiments that have been done indicate that they do not damage living cells. By comparison, quantum dots, which are a popular alternative fluorescent tagging t
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