Back in the early nineties when carbon nanotubes (CNTs) were first discovered, they were a big deal. Materials scientists and nanotechnologists and electrical engineers were clamoring for them. Even the general public recognized how cool they were. But we're a fickle bunch, the public, and it was only a matter of time before some new material stole our hearts: enter graphene. Graphene was to the 00s what nanotubes were to the 90s, and then some. Nanotubes, however, haven't gone anywhere and are still just as interesting as they were 20 years ago. We just need to be reminded. So without further ado, here are six exciting applications that prove carbon nanotubes have still got it.
1. They can weigh single atoms
Single atoms! That is a seriously sensitive mass sensor (say that three times fast). How is it done? Using nanoresonators. In one set-up, a carbon nanotube is placed across a trench and clamped on either side with an electrode. A voltage is then applied across the electrodes so that the nanotube is vibrating at a particular resonance frequency and the current is measured at one electrode. When a mass is added to the resonating nanotube, that frequency shifts and the measured current changes. How does this lead to such spectacular sensitvity? The carbon nanotube is cooled to 6K such that it behaves as a single electron transistor. That is, electrons enter the tube one at a time. So even minute shifts in the current are detectable. The result: my first ever encounter with the prefix "zepto". A zeptogram is 1 x 10-24 grams and is actually about 10 times larger than the mass sensitivity achieved in these type of devices.
2. They can improve hydrogen fuel cells
Polymer electrolyte membrane fuel cells are probably one of the most feasible designs for practical fuel cell applications like motor vehicles and home energy supplies. They are already used in Mercedes' new F-cell class cars and are also being considered for future iterations of the space shuttle. One of their downsides though is the need for platinum catalysts to facilitate hydrogen splitting. Platinum is not cheap — a platinum record has sold even more copies than a gold record. Generally, the Pt nanoparticles are deposited on carbon particles and blended into a polymer for incorporation into the cell. This results in a lot of platinum being so embedded that it's unavailable for catalysis. An alternative is to replace the carbon particles with carbon nanotubes which can be placed in an ordered array designed to maximize Pt availability. If you check out the paper below, you'll see they've already shown better performance with equivalent Pt loading.
Image: Car and Driver
3. They monitor the health of infrastructure
Structural health monitoring is a big deal in engineering. Engineers work very hard to monitor the state of our infrastructure, because the results are so public and devastating when they fail. Those of us in Canada probably recall the Quebec overpass collapse a few years back as one tragic example. Well nanotubes have a role to play here too. When mixed with polymer, nanotubes can be processed into what scientists refer to as artificial neurons. The "neurons" act as sensors by changing their electronic properties in response to strain. These sensitive strands can be spread in a grid over buildings or bridges in the same way neurons are distributed in our bodies and can sense and locate undue strain in the structures. Carbon nanotubes: keeping your family safe.
4. They are the primary component ofall-carbon solar cells
This is a pretty brand new idea, so these all-carbon cells aren't performing (even close to) on par with more established solar cells yet, but there are plenty of reasons why they deserve more attention. They don't require rare and expensive materials like platinum or indium. They can be easily fabricated using a 'spray-paint' technique without the need for any high-grade semiconductor processing equipment. And they don't rely on dye for photoconversion and thus cannot photobleach. The cells have one electrode made up of mixed carbon nanotubes and graphite and another made up of (ideally) exclusively semiconducting nanotubes with an electrolyte in between. This is a truly 'emerging' application and it will be interesting to see if it gathers any steam in the next decade.
Read more: Carbon Nanotube Solar Cells
5. They can treat cancer
In an ideal cancer treatment, drugs are delivered efficiently to only the site of the tumour. This will increase drug efficacy while minimizing unwanted side effects. It is fortunately not as impossible as it sounds due to unique tumour tissues that can allow for the accumulation of nanostructures. Researchers have found that linking treatments to nanotubes before setting them loose in the bloodstream is a very effective method of drug delivery. Furthermore, while human tissues tend to be transparent to near infrared light, nanotubes absorb strongly in that region. Which means that even after dropping off the attached pharmaceuticals, the nanotubes are still of use. Infrared light can be shone on the tumor site causing localized heating of the nanotubes and subsequent tumour cell death.
Image: NCI Visuals Online
6. Last year marked the very first report of an all carbon nanotube computer
With all the talk about graphene as the new and better silicon, graphene actually isn't all that great for transistors due to its inherent lack of a band gap — it has no off state. It's pretty hard to work in binary when you don't have a 0. Nanotubes, on the other hand, are semiconductors with a bandgap proportional to their diameter. They make excellent transistors, as has now been shown with the development of this computer. This prototype is able to run a simple operating system with the ability to multitask and implement a variety of instructions. While obviously not approaching the complexity of the silicon-based computer, CNT based computers have the potential to improve energy efficiency by an order of magnitude and thus seem more than worth a look.
See what I mean? Nanotubes are full of exciting surprises, and I haven't even touched on smart yarns or bionic plants. Graphene may be hogging the spotlight right now, but carbon nanotubes are still out there changing the world, one fuel cell or cancer cell at a time.